Pathology of Kidney Transplantation

Pathology of Kidney Transplantation

CHAPTER 26 Pathology of Kidney Transplantation Alton B. Farris, III  •  Lynn D. Cornell  •  Robert B. Colvin CHAPTER OUTLINE RENAL ALLOGRAFT BIOPSY ...

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Pathology of Kidney Transplantation Alton B. Farris, III  •  Lynn D. Cornell  •  Robert B. Colvin



Optimal Tissue Microscopy


CLASSIFICATION OF PATHOLOGIC DIAGNOSES IN THE RENAL ALLOGRAFT DONOR KIDNEY BIOPSY HYPERACUTE REJECTION ACUTE RENAL ALLOGRAFT REJECTION Acute T-Cell-Mediated Rejection Tubulointerstitial Rejection (Type I) Endarteritis (Type II Rejection) Glomerular Lesions Atypical Rejection Syndromes Differential Diagnosis Acute Antibody-Mediated Rejection Diagnostic Criteria Pathologic Features C4d Interpretation Differential Diagnosis Classification Systems

LATE GRAFT DISEASES Chronic Antibody-Mediated Rejection Peritubular Capillary and Tubulointerstitial Lesions Transplant Arteriopathy Accommodation C4d-Negative Antibody-Mediated Rejection Chronic T-Cell-Mediated Rejection Chronic Allograft Arteriopathy Sequence of Arterial Lesions Differential Diagnosis of Late Biopsies Transplant Glomerulopathy

RENAL ALLOGRAFT BIOPSY Renal biopsy remains the “gold standard” for the diagnosis of episodes of graft dysfunction that occur ­ commonly in patients after transplantation.410 Studies have indicated that the results of a renal allograft biopsy change the clinical diagnosis in 30–42% and therapy in

Acute CNI Toxicity Toxic Tubulopathy Acute Arteriolar Toxicity and Thrombotic Microangiopathy Differential Diagnosis Chronic CNI Toxicity CNI Arteriolopathy Glomerular Lesions Tubules and Interstitium Differential Diagnosis


MAJOR RENAL VASCULAR DISEASE DE NOVO GLOMERULAR DISEASE Membranous Glomerulonephritis Anti-GBM Nephritis De Novo Podocytopathy in Congenital Nephrosis Focal Segmental Glomerulosclerosis

RECURRENT RENAL DISEASE Posttransplant Lymphoproliferative Disease


38–83% of patients, even after the first year.183,185,295,410 Most importantly, unnecessary immunosuppression was avoided in 19% of patients.295 The biopsy is also a gold mine of information on pathogenetic mechanisms, a generator of hypotheses that can be tested in experimental animal studies and in clinical trials. Finally, the biopsy serves, in turn, to validate the hypothesis tested in 377


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such trials. Renal biopsy interpretation currently relies primarily on histopathology complemented by immunological molecular probes, and perhaps in the future, with quantitative gene expression. This chapter describes the relevant light, immunofluorescence, and electron microscopy (EM) findings of the most common lesions affecting the renal allograft and their differential diagnosis, citing references largely limited to human pathological studies after 1990. The discussion is broadly divided into allograft rejection and non-rejection pathology, with an emphasis on differential diagnosis of acute and chronic allograft dysfunction. Grading systems of acute and chronic rejection are discussed further in those sections.

Optimal Tissue At least seven non-sclerotic glomeruli and two arteries (bigger than arterioles) are recommended for adequate evaluation.59,366 Using these criteria, the sensitivity of a single core is approximately 90%, and the predicted sensitivity of two cores is about 99%.59 However, adequacy depends entirely on the lesions seen in the biopsy: one artery with endarteritis is sufficient for the diagnosis of acute cellular rejection (ACR), even if no glomerulus is present; similarly, immunofluorescence or EM of one glomerulus is adequate to diagnose membranous glomerulonephritis (MGN). In contrast, a large portion of cortex with a minimal infiltrate does not exclude rejection. Subcapsular cortex often shows inflammation and fibrosis and is not representative. Diagnosis of certain diseases is even possible with only medulla (acute humoral rejection (AHR), polyomavirus tubulointerstitial nephritis (PTN)). However, a normal medulla does not rule out rejection.403 Frozen sections for light microscopy are of limited value; the diagnostic accuracy of frozen sections was 89% when compared with paraffin sections.50 Rapid processing in formalin/paraffin processing is preferred.

Microscopy The biopsy is examined for glomerular, tubular, vascular, and interstitial pathology, including: (1) transplant glomerulitis, glomerulopathy, and de novo or recurrent glomerulonephritis; (2) tubular injury, isometric vacuolization, tubulitis, atrophy, or intranuclear viral inclusions; (3) endarteritis, fibrinoid necrosis, thrombi, myocyte necrosis, nodular medial hyalinosis, or chronic allograft arteriopathy; (4) interstitial infiltrates of activated ­mononuclear cells, edema, or neutrophils, fibrosis, and scarring. Arteries and arterioles are particularly scrutinized, as well as ­peritubular capillaries (PTCs), since the diagnostic lesions often lie in the vessels. Our standard immunofluorescence panel detects IgG, IgA, IgM, C3, C4d, albumin, and fibrin in cryostat sections. C4d, a complement fragment, is used to identify antibody-mediated rejection (AMR); the other stains are primarily for recurrent or de novo glomerulonephritis.53 Immunohistochemistry in paraffin sections may be used for C4d. EM is valuable when de novo or recurrent glomerular disease is suspected and to evaluate PTC basement membranes.164

Classification of Pathologic Diagnoses in the Renal Allograft The ideal diagnostic classification of renal allograft pathology should be based on pathogenesis, have therapeutic relevance, and be reproducible. The current classification, based on Banff and other systems (Table 26-1), meets these criteria.61,237

TABLE 26-1  Pathological Classification of Renal Allograft Disease I. Immunological rejection A. Hyperacute rejection B. Acute rejection 1. Acute T-cell-mediated rejection (acute cellular ­rejection, C4d−) a. Tubulointerstitial (Banff type I) b. Endarteritis (Banff type II) c. Arterial fibrinoid necrosis/transmural inflammation (Banff type III) d. Glomerular (transplant glomerulitis; no Banff type) 2. Acute antibody-mediated rejection (acute humoral rejection, C4d+) a. Tubular injury b. Capillaritis/thrombotic microangiopathy c. Arterial fibrinoid necrosis C. Chronic rejection 1. Chronic T-cell-mediated rejection (with T-cell activity) 2. Chronic antibody-mediated rejection (with antibody activity, C4d+) II. Alloantibody/autoantibody-mediated diseases of allografts A. Anti-GBM disease in Alport’s syndrome B. Nephrotic syndrome in nephrin-deficient recipients C. Anti-TBM disease in TBM antigen-deficient recipients D. De novo membranous glomerulonephritis E. Anti-angiotensin II receptor autoantibody syndrome III. Non-rejection injury A. Acute ischemic injury (acute tubular necrosis) B. Drug toxicity 1. Calcineurin inhibitor (cyclosporine, tacrolimus) 2. mTOR inhibitors (sirolimus, everolimus, rapamycin) C. Acute tubulointerstitial nephritis (drug allergy) D. Infection (viral, bacterial, fungal) E. Major artery/vein thrombosis F. Mechanical 1. Obstruction 2. Urine leak G. Renal artery stenosis H. Arteriosclerosis I. De novo glomerular disease J. Posttransplant lymphoproliferative disease K. Chronic allograft nephropathy, not otherwise classified (interstitial fibrosis and tubular atrophy) IV. Recurrent primary disease A. Immunological (e.g., IgA nephropathy, lupus nephritis, anti-GBM disease) B. Metabolic (e.g., amyloidosis, diabetes, oxalosis) C. Unknown (e.g., dense deposit disease, focal segmental glomerulosclerosis) mTOR, mammalian target of rapamycin; GBM, glomerular ­basement membrane; TBM, tubular basement membrane. From Colvin RB, Nickeleit V. Renal transplant pathology. In: Jennette JC, Olson JL, Schwartz MM, et al., editors. Heptinstall’s pathology of the kidney. Philadelphia: Lippincott-Raven, 2006. p. 1347.

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Biopsy of the deceased donor kidney is sometimes used to determine the suitability of the kidney for transplantation. Objective pathologic criteria based on outcome that could be applied to the renal biopsy as a screening test have not been established, as donor biopsies are not routinely performed and controlled trials have not been done. One of the major problems in assessing the donor kidney is that this is usually done with cryostat sections, often by nonrenal pathologists in the middle of the night. Even though many other studies try to correlate fibrosis or vascular disease, reproducibility of scoring these lesions, even on permanent sections by expert renal pathologists in broad daylight, is notoriously poor.105 Arbitrary criteria risk that kidneys will be discarded needlessly. In two large studies, the outcome at 1–5 years was not measurably correlated with pathological lesions.44,284 However, as rejection and patient death from complications diminish, the influence of the quality of the graft is likely to increase. Glomerulosclerosis can be readily assessed in frozen section, by the most casual observers, and has been a popular parameter for judging the quality of the donor kidney. Glomerulosclerosis >20% correlates with poor graft outcome in some, but not all, studies.89,109,310 The odds ratio remained significant after adjustment for donor age, rejection episodes, or panel-reactive antibody.310 However, other large studies have failed to detect a major effect of glomerulosclerosis >20%, if adjusted for the age of the donor301 or renal function.84 At least 25 glomeruli are needed to correlate with outcome.402 A wedge biopsy may not be representative, since it includes mostly outer cortex, the zone where glomerulosclerosis and fibrosis due to vascular disease are most severe; therefore a needle biopsy is recommended. Other lesions may cause the transplant surgeon or pathologist to argue against use of the graft. Arterial intimal fibrosis increases the risk of delayed graft function (DGF)172 and has a slight effect on 2-year graft survival (6% decrease).383 Thrombotic microangiopathy (TMA), with widespread but less than 50% glomerular thrombi, increases the likelihood of DGF and primary non-function,301 but is compatible with unaltered 2-year graft survival.225 Reversal of diabetic glomerulosclerosis1 and IgA nephropathy have been reported,169 as well as MGN,261 lupus nephritis,204 membranoproliferative glomerulonephritis (MPGN),39 and endothelialosis due to pre-eclampsia (personal observation). Mathematically combined scores of pathological lesions have been proposed as a guide,314 including, most recently, the Maryland Aggregate Pathology Index.254 In addition to glomerulosclerosis, the score includes interstitial fibrosis, glomerular size, periglomerular fibrosis, arterial wall-to-lumen ratio, and arteriolar hyalinosis. Further, whether this will provide an efficient separation of beneficial organs will depend on prospective validation studies. At this time histological evaluation is recommended in donors with any evidence of renal dysfunction, a family history of renal disease, or those whose age is >60 years. Histological selection of kidneys from donors over 60 years can result in a graft survival rate similar to that of grafts from younger patients.314

Hyperacute rejection refers to immediate rejection (typically within 10 minutes to an hour) of the kidney upon perfusion with recipient blood, where the recipient is presensitized to alloantigens on the surface of the graft endothelium. During surgery, the graft kidney becomes soft, flabby and livid, mottled, purple or cyanotic in color; urine output ceases. The kidney subsequently swells and widespread hemorrhagic cortical necrosis and medullary congestion appear. The large vessels are sometimes thrombosed. Early lesions show marked accumulation of platelets in glomerular capillary lumens that appear as amorphous pale pink, finely granular masses in hematoxylin and ­eosin-stained slides (negative on periodic acid–Schiff (PAS) stains). Neutrophil and platelet margination then occurs over the next hour or so along damaged endothelium of small arteries, arterioles, glomeruli, and PTCs and the capillaries fill with sludged (compacted) red cells and fibrin.409 The larger arteries are usually spared. The neutrophils do not infiltrate initially but form “chain-like” figures in the PTCs without obvious thrombi.409 The endothelium is stripped off the underlying basal lamina, and the interstitium becomes edematous and hemorrhagic. Intravascular coagulation occurs and cortical necrosis ensues over 12–24 hours. The medulla is relatively spared, but is ultimately affected as the whole kidney becomes necrotic.184 Widespread microthrombi are usually found in the arterioles and glomeruli and can be detected even in totally necrotic samples. The small arteries may show fibrinoid necrosis. Mononuclear infiltrates are typically sparse. A T-cell component may be present, as judged by CD3+ cells in the adventitia of small arteries.108 By EM, neutrophils attach to injured glomerular endothelial cells.409 The endothelium is swollen, separated from the glomerular basement membrane (GBM) by a lucent space. Capillary loops and PTCs are often bare of endothelium. Platelet, fibrin thrombi, and trapped erythrocytes occlude capillaries.61 The site of antibody and complement deposition is determined by the site of the target endothelial alloantigens. Hyperacute rejection due to pre-existing antihuman leukocyte antigen (HLA) class I antibodies may show C3, C4d, and fibrin throughout the microvasculature.138 ABO antibodies (primarily IgM) also deposit in all vascular endothelium. Cases with anticlass II antibodies may have IgG/IgM primarily in glomerular and PTCs, where class II is normally conspicuous.3 In the antiendothelial monocyte antigen cases, IgG is primarily in PTCs, rather than glomeruli or arteries.297 Often antibodies cannot be detected in the vessels,352 even though they can be eluted from the kidney.211,241 In these cases C4d should be positive in PTCs53 and more useful than immunoglobulin stains. Occasional cases, particularly intraoperative biopsies, may be negative for C4d, perhaps related to focally decreased perfusion or insufficient time to generate substantial C4d amounts. The differential diagnosis of hyperacute rejection includes ischemia and major vascular thrombosis. The major diagnostic feature of hyperacute rejection is C4d deposition in PTCs and the prominence of neutrophils


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in capillaries. While the finding of antibody and C4d deposition in PTCs is diagnostic when present, negative immunofluorescence stains do not exclude hyperacute rejection. Exogenous antibody (rabbit or horse antilymphocyte serum) can cause severe endothelial injury, sometimes with C4d deposition mimicking hyperacute rejection.54 Hyperacute rejection typically has more hemorrhage, necrosis, and neutrophil accumulation in glomeruli and PTCs than acute tubular necrosis (ATN), although glomerular neutrophils alone are associated with ischemia.107 Major arterial thrombosis has predominant necrosis with little hemorrhage or microthrombi and PTC neutrophils are not that prominent. Renal vein thrombosis shows marked congestion and relatively little neutrophil response.

ACUTE RENAL ALLOGRAFT REJECTION Acute rejection typically develops in the first 2–6 weeks after transplantation, but can arise in a normally functioning kidney from 3 days to 10 years or more, or in a graft affected by other conditions, such as ATN, calcineurin inhibitor toxicity (CNIT), or chronic rejection. Acute rejection may be caused by T cell or antibody injuring the graft, acting separately or together (Table 26-1). Only since 1999 has the distinction between the two pathogenetic pathways been clearly made in the literature and the criteria continue to be refined. Since these respond to different immunosuppressive therapies, the distinction is of considerable clinical importance.

Acute T-Cell-Mediated Rejection Acute T-cell-mediated rejection, also known as ACR, is caused by T cells reacting to donor histocompatibility antigens expressed in the tubules, interstitium, vessels, and glomeruli, separately or in combination (Table 26-2). The donor ureter is also affected, but rarely sampled.106 Tubulointerstitial Rejection (Type I) The prominent microscopic feature of ACR is a pleiomorphic interstitial infiltrate of mononuclear cells, accompanied by interstitial edema and sometimes hemorrhage (Figure 26-1). The infiltrate is typically patchy, both in the cortex and medulla. The infiltrating cells are primarily T cells and macrophages. Activated T cells (lymphoblasts) with increased basophilic cytoplasm, nucleoli, and occasional mitotic figures indicate increased synthetic and proliferative activity.182 Granulocytes are not uncommonly present but rarely prominent. When neutrophils are conspicuous, the possibility of AMR or pyelonephritis should be considered. Eosinophils are present in about 30% of biopsies with rejection and can be abundant, but are rarely more than 2–3% of the infiltrate.8,274 Abundant eosinophils (10% of infiltrate) are associated with endarteritis (Banff type II).233 Mast cells increase, as judged by tryptase content, and correlate with edema.72 Acute rejection with abundant plasma cells has been described as early as the first month, associated with poor graft survival.4,48,228 Infiltrating T cells express cytotoxic molecules, namely perforin,175,294 FasL,5,294 granzyme A and

TABLE 26-2  Banff/Types of Acute T-­Cell-Mediated Rejection* Borderline/Suspicious

Type I A B Type II A B Type III

Any tubulitis (1 cell/tubule or more (t1, t2, or t3)) + infiltrate 0–25% (i0 or i1) or mild tubulitis (1–4 cells/tubule) (t1) and infiltrate >25% (i2 or i3) Tubulitis >4 cells/tubule + infiltrate >25% With 5–10 cells/tubule (t2) With >10 cells/tubule (t3) Mononuclear cells under arterial endothelium <25% luminal area (v1) ≥25% luminal area (v2) Transmural arterial inflammation, or fibrinoid arterial necrosis with accompanying lymphocytic inflammation (v3)†

*All cases should be analyzed for C4d deposition. If C4d is ­present, an additional diagnosis of concurrent antibody-mediated rejection is made. † Cases with these features are often due to alloantibody. To use as a category of T-cell-mediated rejection requires C4d in peritubular capillaries to be negative. From Colvin RB, Nickeleit V. Renal transplant pathology. In: Jennette JC, Olson JL, Schwartz MM, et al., editors. Heptinstall’s pathology of the kidney. Philadelphia: Lippincott-Raven, 2006. p. 1347.

B,191,236,294,324 TIA-1/GMP-17,230,236 and tumor necrosis factor (TNF)-β (lymphotoxin).281 Mononuclear cells invade tubules and insinuate between tubular epithelial cells, a process termed “tubulitis” (Figure 26-1B), which is best appreciated in sections stained with PAS or a silver stain to delineate the tubular basement membrane (TBM). All cortical tubules (proximal and distal) as well as the medullary tubules and the collecting ducts may be affected. Tubular cell apoptosis occurs,15,162,230,282 which correlates with the number of cytotoxic cells and macrophages in the infiltrate.230,282 Tubular epithelial cells express HLA-DR, intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion molecule (VCAM)-1 in increased amounts in ACR21,29,35,93,101–103,269,289,401 and express the costimulatory molecules CD80 and CD86.276 Tubules also synthesize TNF-α,252 transforming growth factor-β1, interleukin (IL)-15, osteopontin, and vascular endothelial growth factor.7,290,411 Increased expression of S100A4 and smooth-muscle α-actin signals a differentiation toward mesenchymal cells, a process sometimes termed epithelial to mesenchymal transition.190,319 This does not seem to be accompanied by migration of the cells out of the tubule.91,237 Not all events in the graft rejection are causing injury. Some tubular cell-derived molecules have the potential to inhibit acute rejection, such as protease inhibitor-9, the only known inhibitor of granzyme B324 and IL-15, which inhibits expression of perforin.411 T cells that downregulate the immune response are also present, as judged by their expressioin of the transcription factor Foxp3.399 Foxp3 cells are also prominent in accepted grafts in humans176 and mice62,248 and may help distinguish harmful from beneficial infiltrates.27,28,126 CD8+ and CD4+ cells invade tubules.392 Intratubular T cells with cytotoxic granules,230 and CD4 + FOXP3+ cells398

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FIGURE 26-1  ■  Acute cellular rejection, type I. (A) Mononuclear cells, composed of activated lymphocytes and macrophages, infiltrate the edematous interstitium and invade tubules. Tubulitis affects proximal and other tubules, where mononuclear cells are interposed between the tubular epithelial cells (B). The invading mononuclear cells appear dark with scant cytoplasm, which distinguishes them from tubular epithelial cells. The tubular basement membranes are stained red by the periodic acid–Schiff stain, which is useful to delineate the boundary between the tubule and the interstitium.

accumulate selectively in the tubules, compared with the interstitial infiltrate. T cells proliferate once i­nside the tubule, as judged by the marker Ki67 (MIB-1), which contributes to their concentration within tubules, in addition to selective invasion.230,320 Increased tubular HLA-DR,29,102 TNF-α,252 interferon-γ (IFN-γ) ­receptor,281 IL-2 receptor,187 and IL-8 are detectable by immunoperoxidase study in ACR. Several adhesion molecules are increased on tubular cells during rejection, including ICAM-1 (CD54) and VCAM-1, that correlate with the degree of T-cell infiltration.35 Signs of tubular cell injury can be detected by TdTuridine-nick end label (TUNEL) apoptosis assay. Increased numbers of TUNEL +  tubular cells are present in acute rejection, compared with normal kidneys.162,230 The frequency was significantly lower in cyclosporine toxicity or ATN.230 The degree of apoptosis correlates with the cytotoxic cells in the infiltrate, consistent with a pathogenetic relationship.230 Prominent apoptosis of the infiltrating T cells has also been detected at a frequency comparable to that in the normal thymus (1.8% of cells).230 Apoptosis probably occurs in infiltrating T cells as a result of activation-induced cell death and would thereby serve to limit the immune reaction.230 Little, if any, immunoglobulin deposition is found by immunofluorescence in ACR, which is characterized primarily by extravascular fibrin accumulation in the interstitium and not uncommonly increased C3 along the TBM. The C3 is largely derived from tubular cells.11 C3 may have a role in the pathogenesis of acute rejection, since C3-deficient mouse kidneys have prolonged survival.305 Gene expression studies of graft tissue have revealed that transcripts for proteins of cytotoxic T lymphocytes (CTLs), such as granzyme B, perforin, and Fas ligand76,152,205,347,376,377,381 and the master transcription factor for CTLs, T-bet, are characteristic of ACR.152 Graft CTL-associated transcripts (CATs) precede tubulitis in

mouse kidney grafts.86 Treatment of rejection is followed by a measurable decrease in CATs.377 However, knockout of either granzyme or perforin does not prevent acute rejection, suggesting they are not essential.85 Other genes associated with acute rejection are IFN-γ, TNF-β, TNF-α, RANTES, and macrophage inflammatory protein (MIP)-1α.152 Endarteritis (Type II Rejection) Infiltration of mononuclear cells under arterial and arteriolar endothelium is the defining lesion of type II ACR (Figure 26-2). Many terms have been used for this process, including “endothelialitis,” “endothelitis,” “endovasculitis,” “intimal arteritis,” or “endarteritis.” We prefer endarteritis, which emphasizes the type of vessel (artery versus vein) involved and the site of inflammation. Mononuclear cells, that are sometimes attached to the endothelial surface, are insufficient for the diagnosis of endarteritis; however, they probably represent the early phase of this lesion. Endarteritis in ACR must not be confused with fibrinoid necrosis of arteries. The latter is characteristic of acute AMR and can also be seen in thrombotic vasculopathy. Endarteritis has been reported historically in 35–56% of renal biopsies with ACR22,59,188,274,338 and it may be more common in patients on belatacept.400 Many do not find the lesion as often; this may possibly be ascribed to inadequate sampling, overdiagnosis of rejection (increasing the denominator), patient population with respect to medication adherence (severity of rejection), or the timing of the biopsy with respect to antirejection therapy. Endarteritis lesions affect arteries of all sizes, including the arteriole, although the lesions affect larger vessels preferentially. For example, in a detailed analysis, 27% of the artery cross-sections were affected, versus 13% of the arterioles.274 A sample of four arteries would have an estimated sensitivity of about 75% in the detection of


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FIGURE 26-2  ■ Acute cellular rejection, type II. (A) Endarteritis in a medium-sized artery. The endothelium is lifted by undermining mononuclear cells, without involvement of the media. (B) Subendothelial infiltration in a small artery with underlying arteriosclerosis (donor disease). This acute process should be distinguished from chronic transplant arteriopathy.

type II rejection.274 Thus a sample may not be considered adequate to rule out endarteritis unless several arteries are included. “Arteriolitis” has the same significance as endarteritis.23 Endarteritis can occur in cases with little or no interstitial infiltrate or tubulitis, arguing that it has a distinct pathogenetic mechanism.61 Recent studies have suggested that endarteritis may also be related to antidonor antibodies,197 and such lesions can be produced in mice by adoptive transfer of donor-specific antibodies.149 Endothelial cells are typically reactive with increased cytoplasmic volume and basophilia. The endothelium shows disruption and lifting from supporting stroma by infiltrating inflammatory cells.10 Occasionally endothelial cells are necrotic or absent; however, thrombosis is rare. Endothelial apoptosis occurs162,230 and increased numbers of endothelial cells appear in the circulation.413 The medium usually shows little change. In severe cases a transmural mononuclear infiltrate may be seen (termed type III rejection). The cells infiltrating the endothelium and intima are T cells and monocytes, but not B cells.10 Both CD8+ and CD4+ cells invade the intima in early grafts, but later CD8+ cells predominate,392 suggesting that class I antigens are the primary target.230 Vascular endothelial cell apoptosis can be detected in sites of endarteritis.162,230 Normal arterial endothelial cells express class I antigens, weak ICAM-1, and little or no class II antigens, or VCAM-1. During acute rejection the endothelium of arteries expresses increased HLA-DR101,392 and ICAM-1 and VCAM-1.36,93 This adhesion molecule upregulation occurs in association with CD3+35 and CD25+103 infiltrating mononuclear cells. Endothelial cells also have decreased endothelin expression in rejection with endarteritis, but not in tubulointerstitial rejection.404 Glomerular Lesions In most ACR cases, glomeruli are spared or show minor changes, typically a few scattered mononuclear cells (T cells and monocytes) and occasionally segmental

endothelial damage (as in Figure 26-3A).390 A severe form of this glomerular injury, termed “transplant glomerulitis” or “acute allograft glomerulopathy,” develops in a minority of cases (<5%), manifested by hypercellularity, injury, and enlargement of endothelial cells, infiltration of glomeruli by mononuclear cells and by webs of PAS-positive material.317 Crescents and thrombi are rare. The glomeruli contain numerous CD3+ and CD8+ T cells and monocytes.145,392 Fibrin and scant immunoglobulin and complement deposits are found in glomeruli. Endarteritis often accompanies the transplant glomerulitis.240 Glomerulitis is more often related to AMR, and evidence for this (C4d, donor-specific antibodies) should be sought in these cases. Atypical Rejection Syndromes Unique patterns of rejection have been observed under novel immunosuppression regimens. For example, pronounced lymphocyte depletion results from alemtuzumab (Campath-1H) and ACR in this setting has a prominent monocyte population (i.e., an acute monocytic rejection).115,181,182 Much of the interstitial rejection infiltrate stains for CD68, correlating with renal dysfunction and tubular stress, shown by HLA-DR staining of the tubules. Under these conditions, T cells did not correlate with renal dysfunction or HLA-DR staining.115 Studies have recently included simultaneous bone marrow and kidney transplantation protocols in an attempt to induce tolerance to the transplanted organ. In these studies, HLA-mismatched renal transplants have been performed; withdrawal of maintenance immunosuppression has been accomplished in some of the patients with relatively preserved renal function.176 In several of these patients, a capillary leak or engraftment syndrome has been observed around 10 days after a simultaneous kidney/bone marrow transplant preceded by a non-myeloablative conditioning regimen. In this “engraftment syndrome,” acute tubular injury is accompanied by congested PTCs containing mononuclear cells

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FIGURE 26-3  ■ Acute humoral rejection. (A) Low power shows mild interstitial inflammation, focal hemorrhage, neutrophils, and thrombi in glomerular capillaries and dilated peritubular capillaries with leukocytes. (B) At high power neutrophils can be seen in the peritubular capillaries with little tubulitis. Periodic acid–Schiff stain. (C) Acute transplant glomerulitis is prominent in this case of acute humoral rejection. Glomerular endothelial cells are swollen and the capillaries are filled with mononuclear cells, probably mostly macrophages. Periodic acid–Schiff stain. (D) C4d stain of a case of acute humoral rejection shows prominent, diffuse staining of dilated peritubular capillaries, sometimes containing inflammatory cells, and linear staining along the glomerular basement membrane. Immunohistochemistry with a polyclonal anti-C4d rabbit antibody.

and red blood cells. Immunohistochemistry shows that the cells are primarily CD68 + MPO +  mononuclear cells and CD3 + CD8+ T cells, the latter with a high proliferation index (Ki67+). XY chromosome fluorescence in situ hybridization has been used to demonstrate that the PTC cells are recipient-derived, correlating with chimerism studies showing a simultaneous decline in circulating donor cells and recovery of recipient circulating cells. PTC endothelial injury can also be seen on EM in these cases.92,176 Similar pathologic features have been observed in recipients of autologous mesenchymal stem cells (Remuzzi et al., personal communication). The etiology of the syndrome remains undefined, and others have performed combined kidney and bone marrow transplants without observing this phenomenon.201 Differential Diagnosis Interstitial mononuclear inflammation and tubulitis occur in a variety of diseases other than acute rejection. Tubulitis has been documented in renal transplants with dysfunction due to lymphoceles (obstruction) and in urine leaks, possibilities that need to be considered and excluded by other techniques.71 Acute obstruction typically has some dilation of the collecting tubules, especially in the outer cortex. Edema and a mild mononuclear infiltrate are also common.59,71,216 When eosinophils are more abundant

than usual for rejection and eosinophils invading tubules are identified, then drug allergy may be favored over rejection. The presence of endarteritis permits a definitive diagnosis of active rejection.274 Lymphocytes commonly surround vessels (without medial involvement), a nonspecific feature, and must not be confused with endarteritis. Tubulitis is often present in atrophic tubules and does not indicate acute rejection. The diagnosis of acute pyelonephritis should be raised when active inflammation and abundant intratubular neutrophils are present. A note of caution, though: in AHR, neutrophilic tubulitis with neutrophil casts can be seen222; a C4d stain and urine culture will help in distinguishing between these.61 Prominent tubulitis does favor acute rejection over ATN, particularly in the proximal tubules.216 The usual diagnostic feature of polyomavirus interstitial nephritis (BK virus) is the enlarged, hyperchromatic tubular nuclei with lavender viral nuclear inclusions, often in collecting ducts. However, these may be inconspicuous, and diligent study of multiple sections may be required. Other clues are prominent apoptosis of tubular cells, abundant plasma cells, which invade tubules. Immunohistochemistry for the polyoma SV40 large T antigen or in situ hybridization for BK polyomavirus and EM (even of paraffin) will confirm the diagnosis. Sometimes BK virus infection, with its exuberant plasmacytic infiltration and activated


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TABLE 26-3  Differentiation between Acute Rejection and Acute Calcineurin Inhibitor Toxicity Acute Rejection Interstitium Infiltrate

Calcineurin Inhibitor Toxicity Absent to mild


Moderate to marked Usual

Tubules Tubular injury Vacuoles Tubulitis

Usual Occasional Prominent

Usual Common Minimal to absent

Arterioles Endotheliitis Smooth muscle

Can be present Absent

Mucoid intimal


Absent Sometimes degeneration present Sometimes thickening with present red blood cells (TMA)

Arteries Endotheliitis


Peritubular Capillaries C4d May be positive Glomeruli Mononuclear cells Thrombi

Can be present

Absent (rare mononuclear TMA) Negative




Occasionally prominent (TMA)

TMA, thrombotic microangiopathy.

immunoblasts, may be confused with the plasmacytic hyperplasia form of posttransplant lymphoproliferative disease (PTLD),61 which also should be considered in the differential diagnosis of ACR. Rarer infections, including cytomegalovirus and microsporidia, should also be considered in biopsies with interstitial inflammation. Patients with CNIT typically have a minimal mononuclear cell infiltrate (Table 26-3). Endarteritis or C4d + is found extremely rarely, if ever, in CNIT and if either is present, is the most discriminating feature for acute rejection.265,353,384

Acute Antibody-Mediated Rejection Acute AMR (also known as AHR) is due to damage by circulating antibodies that react to donor alloantigens on endothelium. These antigens include most commonly HLA class I and class II antigens53,137,344 and in ABOincompatible grafts the ABO blood group antigens.96 Other proposed antigens on the endothelium (allo- or autoantigens) may also contribute. The clearest evidence of non-major histocompatibility complex (MHC) alloantigens comes from the rare observation of AHR in HLAidentical grafts.52,125 AHR may occur in the absence of

evidence for T-cell-mediated injury, particularly in positive-crossmatch transplants42,199,346; however, it is not uncommon for both to be present, particularly in the later posttransplant period (months to years).53 The main risk factors for the development of antiHLA antibodies are blood transfusion, pregnancy, and prior transplant.208 Donor-specific antibody (DSA), which typically refers to anti-HLA antibody, may arise de novo in the posttransplant period, or alloantibody may be present prior to transplantation in the case of positive-crossmatch or ABO blood group-incompatible transplants with preconditioning regimens to lower the alloantibody level prior to transplantation. AHR typically presents with clinically severe acute rejection137 1–3 weeks after transplantation, but also can arise months to years later, associated with decreased immunosuppression or non-compliance.382 With current therapy, approximately 5–7% of recipients develop an episode of AHR, and about 25% of biopsies taken for acute rejection have pathological evidence of an AHR component.61 The main risk factor is presensitization by blood transfusion, pregnancy, or prior transplant208; however, the majority have a negative cross-match at the time of transplantation.61 Traditionally, identification of AHR in biopsies is difficult since none of the histologic features is diagnostic and immunoglobulin deposition was usually not detectable in the graft.221,270,323 Techniques for demonstrating C4d in PTCs, pioneered by Feucht et al.,95 substantially improved detection of this condition53,69,222,306,323; most studies since 1999 have employed it as a criterion for AHR. New evidence points to AHR with little or no C4d deposition (discussed below). Serologic testing for DSA has become more sensitive in the past decade due to the widespread use of solid-phase assays over the older cell-based assays.47,113 These assays can be used prior to transplantation and for posttransplant monitoring for DSA. These more sensitive methods of detecting DSA have brought to light the spectrum of alloantibody-mediated damage (e.g., capillaritis) that may not have been recognized in previous studies.65 Diagnostic Criteria The three diagnostic criteria for AHR are: (1) histologic evidence of acute injury (neutrophils in capillaries, acute tubular injury, fibrinoid necrosis); (2) evidence of antibody interaction with tissue (typically C4d in PTCs); and (3) serologic evidence of circulating antibodies to antigens expressed by donor endothelium (typically HLA).222,306 If only two of the three major criteria are established (for example, when antibody is negative or not done), the diagnosis is considered suspicious for AHR. Biopsies meeting criteria for both AHR and ACR type I or II are considered to have both forms of rejection. Some biopsies show prominent capillaritis and glomerulitis in association with DSA but little or no C4d. These C4d-negative cases also are “suspicious” for active AMR by the current definitions, but may be mediated by antibodies via cellular rather than complement mechanisms. This rarely occurs in the acute setting, and is discussed further in the chronic AMR section.

26  Pathology of Kidney Transplantation 385

Pathologic Features Histologic findings are typically scant to moderate mononuclear interstitial infiltrates, sometimes with prominent neutrophils142,222,313,390 and increased numbers of macrophages212 (Figure 26-3B). The extent of mononuclear infiltration often does not meet the criteria for ACR.313 PTCs have neutrophils in about 50% of cases and are classically dilated (Figure 26-3B). Interstitial edema and hemorrhage can be prominent. Glomeruli have accumulations of macrophages (~50% of cases) and neutrophils (~25% of cases) (Figure 26-3C)222,275,313,390 and occasionally fibrin thrombi or segmental necrosis.137,222,390 Acute tubular injury, sometimes severe, can be identified in many cases and may be the only initial manifestation of AHR. Focal necrosis of whole tubular cross-sections, similar to cortical necrosis, has been reported; 38–70% of AHR cases may have patchy infarction.206,390 Little mononuclear cell tubulitis is found, although a neutrophilic tubulitis with or without neutrophil casts may be prominent,390 resembling acute pyelonephritis. Plasma cells can be abundant in AHR, either early4 or late75,300 after transplantation, sometimes associated with severe edema and increased IFN-γ production in the graft.75 B cells can be also present, but have no apparent diagnostic value.61 In about 15% of cases small arteries show fibrinoid necrosis, with little mononuclear infiltrate in the intima or adventitia but with neutrophils and karyorrhexic debris (Figure 26-4).206,390 Arterial thrombosis can be found in 10% and a pattern resembling TMA has also been reported.206 Around 75% of cases with fibrinoid necrosis are C4d-positive.142,222,275,390 Presumably the C4d-negative cases had T-cell-mediated rejection or TMA. Antibodies to the angiotensin II type 1 receptor have been detected in a few cases with arterial fibrinoid necrosis, in the absence of capillary C4d deposition.83 The presence of mononuclear endarteritis in cases of AHR strongly suggests a component of T-cell-mediated rejection.390

FIGURE 26-4  ■ Fibrinoid arterial necrosis: an arteriole with destruction of the medial wall smooth-muscle cells by fibrinoid necrosis. Some neutrophils are present underneath the reactive and swollen endothelium. This vascular change is distinctly different from endarteritis (compare with Figure 26-2) and can be seen in both acute humoral rejection and type III acute rejection. This case had positive C4d.

By EM the PTCs are dilated, containing neutrophils. The endothelium is reactive and shows loss of fenestrations. The glomerular endothelium is separated from the GBM by a widened lucent space with endothelial cell swelling390 and loss of endothelial fenestrations, indicative of injury. Platelets, fibrin, and neutrophils are found in glomerular and PTCs. The small arteries with fibrinoid necrosis show marked endothelial injury and loss, smooth-muscle necrosis, and deposition of fibrin. 61 C4d Interpretation Feucht and colleagues first drew attention to C4d as a possible marker of an antibody-mediated component of severe rejection.95 C4d, a fragment of complement component C4, is released during activation of the classical complement pathway by antigen–antibody interaction. C4d binds to tissue components via a thioester bond at the local site of activation. The covalent linkage explains why C4d remains for several days after alloantibody disappears, since antibody binds to cell surface antigens that can be lost by modulation, shedding, or cell death. C4d deposition can precede histological evidence of AHR by 5–34 days.133 C4d in 1-week protocol biopsies was followed by clinical acute rejection in 82% of cases380 and was associated with donor-reactive antibodies.186 Although immunoglobulin deposition is found in only a minority of cases, C4d is characteristically detected in a widespread, uniform ring-like distribution in the PTCs by immunofluorescence in cryostat sections53,95. Deposition occurs in both the cortex and medulla. In a comparison of methods for C4d, the triple-layer immunofluorescence technique53 proved the most sensitive, although the difference with immunohistochemistry in paraffin-embedded tissue was small.259 Using immunohistochemistry in formalin-fixed, paraffin-embedded tissue, C4d has a similar pattern (Figure 26-3D), although the intensity is variable. With fixed tissue, plasma in the capillaries and interstitium may stain for C4d, which interferes with interpretation.61 “Plasma staining” is a fixation artifact of C4d immunohistochemistry, and so PTCs must show clear circumferential staining to be called positive by this technique. Glomerular capillary staining also occurs, but is hard to distinguish from C4d normally found in the mesangium in frozen sections stained by immunofluorescence. Formalin fixation eliminates this background staining and demonstrates glomerular C4d in about 30% of AHR cases.313 PTC C4d deposition is associated with concurrent circulating antibodies to donor HLA class I or II antigens in 88–95% of recipients with acute rejection.33,132,222 Falsenegative antibody assays are probably due most often to absorption by the graft, as shown by elution from rejected grafts in patients who had no detectable circulating antibody.217 Alternatively, non-HLA antigens may be the target.52,125 Other components of the complement system have been sought. C3d, a degradation product of C3, was found in PTCs in 39–60% of biopsies from HLAmismatched grafts with diffuse C4d.132,142,193,380 C3d was usually,132 but not always,193 associated with C4d. C3d correlated with AHR in all studies, and was associated


Kidney Transplantation: Principles and Practice

with increased risk of graft loss in two series, compared to C3d-negative cases, but C3d provided no convincing additional risk compared with C4d+. The interpretation of C3d stains is complicated by the common presence of C3d along the TBM.132 Even though C3d should indicate more complete complement activation, it added no diagnostic value to C4d in grafts showing histologic features of AHR, except in the setting of ABO-incompatible grafts.132 Other complement components, such as C1q, C5b-9, and C-reactive protein, are not conspicuous in PTCs in acute rejection.166,277 Lectin pathway components, which activate C4 by binding to microbial carbohydrates, are sometimes detected.159,380 Natural killer (NK) cells have been the focus of recent research in antibody-mediated graft injury.6,148 Microarray analysis has indicated that several DSA-specific gene transcripts show high expression in NK cells, and immunohistochemistry also shows prominent numbers of PTC NK cells in these cases.144 Depletion of NK cells with antiNK1.1 significantly reduced DSA-induced chronic allograft vasculopathy in a murine cardiac allograft model.148 The prognosis of AHR is uniformly worse than ACR,53,137,188,206,390,417 but outcome has generally improved with early recognition and vigorous therapy. In one series, 75% of the 1-year graft losses from acute rejection were in the C4d+ AHR group.222 However, some of those who recover from the acute episode of AHR have a similar long-term outcome,390 suggesting that the pathogenetic humoral response can be transient if treated effectively. While most approaches for treatment or prevention of AHR involve removing alloantibody from the circulation (by plasmapheresis) or decreasing production of alloantibody (e.g., by antiplasma cell drugs), another technique to prevent graft damage by antibody is by inhibiting complement. Eculizumab, a humanized monoclonal antibody directed against the terminal complement component C5, is now being applied in renal transplantation, particularly in sensitized (positive-crossmatch) patients at a high risk for early AHR. C5 is downstream of C4d in the complement cascade; thus, with DSA activation of complement, diffuse C4d deposition would be expected even with effective C5 inhibition. Early surveillance biopsies in eculizumab-treated patients have shown diffuse C4d deposition but absent morphological signs of AHR, including a lack of endothelial cell activation by EM. The absence of respective pathology suggests endothelial protection by eculizumab, and moreover supports the notion that most cases of early AHR are complement-mediated. However, AHR was observed in a few patients despite eculizumab therapy, and may be due to IgM DSA not detected by the usual DSA testing methods.24 Notably, a subset of patients still developed features of chronic humoral rejection (CHR), including transplant glomerulopathy (TG), although the time of treatment with eculizumab varied between patients, from 1 month to 1 year.370 While effective in preventing early AHR in positive-crossmatch transplants, it appears that complement inhibition alone does not entirely prevent chronic, antibody-mediated microcirculation injury. Furthermore, the limited diagnostic reliability for AHR of C4d and serum DSA is apparent in this setting, suggesting that diagnostic criteria refinements are needed.

Differential Diagnosis For differential diagnosis, it is helpful that both ATN323,389 and TMA in native kidneys are C4d-negative. Among 26 cases of TMA/hemolytic uremic syndrome in native kidneys, none had positive C4d, including cases with lupus anticoagulant and antiphospholipid antibodies.323 In five cases of recurrent hemolytic uremic syndrome in transplant recipients, C4d was also negative.14 Among native kidney diseases, only lupus nephritis198,323 and endocarditis198 have been reported to have PTC C4d. Glomerular C4d deposits, of course, are not specific, since they occur in many forms of immune complex glomerulonephritis in native kidneys. Arterial intimal fibrosis often stains for C4d, even in native kidneys, and should not be taken as evidence of AMR.323 The comparative features of “pure” humoral and ACR are given in Table 26-4. In AHR neutrophils are the predominant inflammatory cells in PTCs, glomeruli, tubules, and the interstitium, with or without accompanying fibrinoid necrosis. The vascular lesion of AHR, if present, is fibrinoid necrosis of the wall while in ACR endarteritis is the usual lesion. C4d deposition in PTCs (immunofluorescence microscopy) is typically only present in AHR, but not in ACR.61

Classification Systems The most widely used system currently is the “Banff working schema” (“Banff ” for short). Banff started as an international collaborative effort led by Kim Solez, Lorraine Racusen, and Philip Halloran to achieve a consensus that would be useful for drug trials and routine TABLE 26-4  Differentiation between Acute Humoral Rejection and Acute Cellular Rejection Acute Humoral Rejection

Acute Cellular Rejection



Moderate to severe Present Mononuclear cells Negative

Tubules Acute tubular necrosis Tubulitis

Can be present

Usually absent


Mononuclear cells

Vessels Endarteritis Fibrinoid necrosis

Can be present Typically present

Present in type II Present in type III

Interstitium Infiltrate Edema Peritubular capillaries C4d*

Present Neutrophils

Glomeruli Inflammatory Neutrophils cells Fibrinoid necrosis Can be present

Mononuclear cells Typically absent

*C4d staining in peritubular capillaries indicates activation of the classical complement pathway by humoral antibody (monoclonal antibody, immunofluorescence microscopy).

26  Pathology of Kidney Transplantation 387

diagnosis.237,365,366 Banff is still growing and remodeling, undergoing revisions based on data presented and debated at the biennial Banff meeting. These included restructuring that separated the category of endarteritis, according to the National Institutes of Health Cooperative Clinical Trials in Transplantation criteria,59,307 the addition of acute306 and chronic AMR,367 and the birth366 and death367 of “chronic allograft nephropathy (CAN).”367 Banff scores three elements to assess acute rejection: tubulitis (t); the extent of cortical mononuclear infiltrate (i); and vascular inflammation (intimal arteritis or transmural inflammation) (v). Mononuclear cell glomerulitis (g) is scored but not yet part of the classification of rejection. Banff recognizes three major categories of acute T-cell-mediated rejection (tubulointerstitial, endarteritis, and arterial fibrinoid necrosis) (Table 26-2). The threshold for type I (tubulointerstitial) ACR is >25% cortical mononuclear inflammation in the non-atrophic areas, provided tubulitis of at least 5–10 cells/tubule is present.307 Cases with no tubulitis, regardless of the extent of infiltrate, are not considered ACR. Biopsies with C4d +  PTCs are considered to have an additional component of AMR, which occurs in 20–30% of cases.143 Cases with tubulitis are termed “suspicious for rejection” or “borderline” in the current Banff system. Many, but not all, of these cases are early or mild acute rejection: 75–88% of patients with suspicious/borderline category and graft dysfunction improve renal function with increased immunosuppression,332,342 comparable to the response rate in type I rejection (86%).332 A minority (28%) of untreated suspicious/borderline cases progress to frank acute rejection in 40 days.232 Almost all with suspicious/borderline findings do well, provided there is no element of concurrent AMR, which commonly has a suspicious/borderline pattern, although care must be taken not to misinterpret peritubular capillaritis as interstitial inflammation.65,313 The suspicious category is not counted as acute rejection in most clinical trials – a major omission in our opinion. The interobserver reproducibility of the present Banff classification is sufficient but needs improvement. In a Canadian study, the agreement rate for rejection was 74%, but there was only 43% agreement on the suspicious/borderline cases,122 similar to a European series.397 Among a group of 21 European pathologists, the agreement rate was poor for all of the acute Banff scores (t, i, v, g) in transplant biopsy slides (all kappa scores <0.4).105 Agreement for t and v scores improved significantly when participants were asked to grade a lesion in a photograph (kappa scores of 0.61 and 0.69, respectively), arguing that the challenge is primarily finding the lesion in the glass slide. Lack of improvement in the other categories (g, i) argues that the definitions are faulty. Despite these considerations, Banff is fully accepted as a scoring system of drug trials and is used widely in clinical practice (although not necessarily with an individual score report).61

LATE GRAFT DISEASES Although acute rejection has diminished in clinical importance in the last decade, allografts are still lost by slow, progressive diseases that cause a 3–5% annual attrition

rate. The specific causes of this are many and sometimes difficult to ascertain, particularly if only an end-stage kidney is examined. Unfortunately two terms, “chronic rejection” and “CAN,” have been used in past literature to lump together these myriad diseases. The role of the pathologist in interpreting the biopsy is to provide the most specific diagnosis possible and indicate the activity of the process. While some have argued that the renal biopsy is not useful in analyzing graft dysfunction after 1 year, the data show that in 8–39% of patients the biopsy led to a change in management that improved renal function.185,295 Here we will discuss the criteria used to distinguish some of these diseases and those that remain idiopathic. The term “chronic rejection” is best defined as chronic injury primarily mediated by an immune reaction to donor alloantigens.

Chronic Antibody-Mediated Rejection Circulating anti-HLA antibodies have long been associated with increased risk of late graft loss.155,386 Chronic, active AMR (CHR) was first reported in 2001223 and is now recognized as a separate category in the Banff schema.367 CHR differs from AHR in the usual lack of evidence of acute inflammation (neutrophils, thrombi, necrosis) and the presence of matrix synthesis (basement membrane multilamination, fibrosis in arterial intima and the interstitium). CHR commonly arises late (>6 months after transplantation) and usually occurs in patients with or without a history of AHR, although C4d in early biopsies is a risk factor for later TG with C4d.117,119,131,312 In the setting of de novo DSA, many patients have reduced levels of immunosuppression (absorption, iatrogenic, or non-compliance).406 In these cases, a combination of CHR and AHR may be seen, along with a component of T-cell-mediated rejection.199 Patients with CHR typically present with late graft dysfunction (average of 4–5 years posttransplant), with proteinuria and circulating DSA. Most do not have a previous episode of AHR and, indeed, present with no obvious preceding clinical event. The major risk factor identified is reduced immunosuppression due to non-compliance.406 Iatrogenic and physiologic causes also contribute. The criteria of CHR are the triad of: (1) one of the following morphologic features: TG (duplication or “double contours” in GBMs), multilamination of the PTC basement membrane, PTC loss and interstitial fibrosis, or chronic arteriopathy with fibrous intimal thickening (without duplication of the internal elastica); (2) diffuse C4d deposition in PTCs; and (3) circulating DSA. If only two elements of the triad are present, the diagnosis is considered “suspicious.” Although helpful when positive, C4d deposition and serum DSA are particularly problematic in the chronic setting: they are less sensitive markers due to serum DSA level variability with time posttransplant. Two features point to ongoing immunologic activity: the presence of C4d and mononuclear cells in glomerular and PTCs. TG, defined by duplication of the GBM by light microscopy in the absence of specific de novo or recurrent glomerular disease, was one of the first lesions to be linked to DSA. TG has been associated with anti-HLA


Kidney Transplantation: Principles and Practice

antibodies (especially anti-class II), with the risk increasing if the antibodies were donor-specific.117 TG is best revealed in PAS or silver stains (Figure 26-5A, B). The glomeruli may show an increase in mesangial cells and matrix with various degrees of scarring and adhesions. In some cases mesangiolysis or webbing of the mesangium may be prominent as well as segmental or global sclerosis.

EM reveals duplication or multilamination of the GBM (Figure 26-5C), often accompanied by cellular (mononuclear or mesangial cell) interposition, widening or lucency of the subendothelial space, and a moderate increase in mesangial matrix and cells.135 Glomeruli may show focal and segmental glomerulosclerosis (FSGS), especially in more advanced TG, and some cases with collapsing



∗ E



E FIGURE 26-5  ■  Chronic allograft glomerulopathy. (A) Widespread duplication of the glomerular basement membrane (GBM) with mild mesangial hypercellularity and increased mononuclear cells in the glomerular capillaries. Periodic acid–Schiff stain. (B) GBM multilamination at high power in a silver stain. (C) Electron microscopy: high power of a glomerular capillary showing duplication of the GBM; the new or second layer of GBM (short arrow) forms underneath the endothelium (E) and is separated from the old GBM layer (long arrow) by the cellular (mononuclear or mesangial cell) interposition (*). (D) Immunohistochemistry stain for C4d in paraffin sections shows prominent C4d deposition in glomerular and peritubular capillaries. (E) Electron microscopy: high ­magnification of a peritubular capillary with multilamination (arrow) of the basement membrane. Inset: higher magnification of the area marked by arrow. E, endothelium; I, interstitium.

26  Pathology of Kidney Transplantation 389

FSGS lesions have been observed. EM detects 40% more cases of TG than light microscopy.163 The GBM typically has rarefactions, microfibrils, and cellular debris but few or no deposits.43,158,304 Endothelial cells may appear reactive with loss of fenestrae, probably undergoing “dedifferentiation.”57,158,304 Podocyte foot process effacement ranges from minimal to quite extensive,158 corresponding to the degree of proteinuria. The non-duplicated GBM may become slightly thickened, attributable to compensatory hypertrophy. Peritubular Capillary and Tubulointerstitial Lesions PTCs may be dilated and prominent, with thick basement membranes, or may altogether disappear, leaving only occasional traces of the original basement membrane behind.30,160 In a subset of patients, PTCs have prominent C4d deposition (as in Figure 26-3D), which is associated with circulating antidonor HLA class I or II reactive antibodies.223 Other allografts with CHR features may show focal or multifocal C4d staining of PTCs by immunofluorescence or immunohistochemistry, or dim C4d staining by immunofluorescence. EM reveals splitting and multilayering of the PTC basement membrane (Figure 26-5E), first described by Monga.224,250 Each ring probably represents the residue of one previous episode of endothelial injury, going from oldest (outer) to most recent (inner). Quantitation is necessary to establish diagnostic specificity. Scoring of multilamination requires EM, not always available for transplant biopsies, and quantitative assessment of the number of layers, since to distinguish from other common causes of lamination, more than six layers have to be present.163 Only in chronic rejection were three or more PTCs found with 5–6 circumferential layers or one PTC with seven or more circumferential layers.163 PTC lamination correlates with TG,163,224 C4d deposition,312 and loss of PTCs.160 Marked multilamination (5–6 layers in three capillaries or >6 in one) was found in 50% of cases with interstitial fibrosis that lacked arterial or glomerular changes, and may point to past episodes of rejection as the cause of the fibrosis.224 Transplant Arteriopathy Alloantibodies to graft class I antigens are a specific risk factor for chronic transplant arteriopathy in human renal allografts.73,168 Typically, transplant arteriopathy is recognized by thickening of the arterial intima with mononuclear inflammatory cells (CD3+ T cells or CD68+ monocytes/macrophages) within the thickened intima. In a recent study, patients with preformed DSA showed accelerated arteriosclerosis on serial biopsies.146,147 While the transplant arteriopathy lesions were attributable to DSA on serial biopsies from the same allografts, transplant arteriopathy may not be distinguishable from the arterial intimal thickening seen in hypertension.146 Careful study of the progression of arteriosclerosis in allografts has shown that DSA exacerbates the process, similar to accelerated aging of the kidney.146 Experiments in animals show that transplant arteriopathy can be initiated by passive transfer of donor-reactive MHC antibodies in recipients with no functional T cells.223,312 Transplant

arteriopathy is also believed to be due to chronic T-cellmediated injury and is further described in that section. The relative contribution of these two pathways is far from clear and no criteria exist to distinguish them at the level of the arteries. Accommodation Not all patients with circulating DSA have clinical or pathologic evidence of chronic graft injury. This paradox was observed in ABO grafts and termed “accommodation.”17 Accommodation is thought to represent a process of endothelial cell adaptation to antibody and complement over time. In accommodation, DSAs may be detectable; however, morphologic signs of tissue injury are absent. Subclinical interaction of antibody with graft endothelium (accommodation) has been revealed by the demonstration of diffuse C4d in PTCs, found in 2.0% of routine protocol biopsies,234 and a higher frequency among presensitized patients (17%) or patients with ABO-incompatible grafts (51%).96,132 The stability of such accommodation, referring to the presence of PTC C4d deposition in the absence of other evidence of antibody-mediated injury, has not been established. In renal allografts PTC C4d deposition in the absence of other evidence of antibody-mediated injury is observed without signs of acute or chronic rejection; more specifically, there is no ATN-like minimal inflammation, no glomerulitis (g0), no chronic TG (cg0), no peritubular capillaritis (ptc0), and no PTC basement membrane multilamination (<5 layers by EM). Current Banff criteria refer to this situation as “C4d deposition without evidence of active rejection.” If there are simultaneous borderline changes, the cases are considered to be indeterminate.360 Accommodation is common in the setting of ABO-incompatible allografts, with at least 80% of normal surveillance biopsies showing C4d deposition in PTCs with no apparent long-term consequences.132 However, evidence from patients406 and non-human primates362 suggests that DSA and C4d deposition (and/or capillaritis) lead to chronic graft pathology and loss and these patients should be closely monitored and efforts made to reduce DSA levels. Longitudinal Studies. CHR is a process that evolves over several years, typically beginning entirely subclinically. In non-human primates with MHC-incompatible grafts and no immunosuppression, C4d deposition predicts chronic rejection with glomerulopathy and arteriopathy and ultimate graft loss with a high degree of certainty.361 A sequence of four stages has been demonstrated in protocol biopsies. The process begins with antibody production, followed by C4d deposition, and later, morphologic and functional changes.361 A similar sequence, with the addition of capillaritis and glomerulitis, has been reported in patients.406 C4d-Negative Antibody-Mediated Rejection Attention has recently been drawn to so-called “C4dnegative” AMR. These cases have DSA and varying degrees of morphological evidence of antibody-mediated


Kidney Transplantation: Principles and Practice

injury but lack detectable C4d deposition in PTC endothelium.128–130 This has been described in two settings: late indication biopsies (with features of chronic AMR), and early protocol biopsies in stable patients who were presensitized.130,210 In these cases other signs of endothelial injury were detected (capillaritis or increased endothelial gene expression). Negative C4d might be explained by time-dependent degradation of C4d deposits in the microcirculation, complement-independent antibodymediated injury, or issues with the C4d stain itself (sensitivity, interpretation of a positive).129 Molecular studies have uncovered a subset of cases with morphological features of antibody-mediated injury and DSA showing increased endothelial cell-associated transcript expression, indicative of endothelial cell activation and stress. These data suggest that 50–60% of late AMR may lack C4d positivity. It is probably much less common in AHR: we have only rarely seen such cases (as a consult). Cases are missed by current Banff criteria due to C4d negativity.359 It would appear that there is another category of AMR that is between acute and chronic, in that neutrophils and rapid decline in graft dysfunction are absent, as well as no evidence of chronic matrix production (duplication of basement membranes, fibrosis of interstitium or intima). This is characterized by capillaritis and glomerulitis with mononuclear cells, with or without C4d, in association with DSA. This was described clearly in 3-month protocol biopsies in clinically stable presensitized patients and carried an increased risk of development of chronic lesions (TG) at 1 year.209 Eventually, this will likely be added to the Banff diagnostic armamentarium as a distinct category of AMR; however, data are still being gathered by a respective Banff Working Group regarding the significance of this entity in an attempt to provide diagnostic criteria.237 At present we suggest the term “smoldering” AMR for this condition.

Chronic T-Cell-Mediated Rejection There is no doubt that T cells can cause chronic graft injury, yet the specific criteria are not well developed and subject to refinement. Using the CHR model, the current Banff classification defines “chronic active T-cellmediated rejection” as showing morphologic features of chronicity (arterial intimal fibrosis without elastosis) combined with features indicative of ongoing T-cell activity (mononuclear cells in the intima). Interstitial fibrosis with a mononuclear infiltrate and tubulitis is in some instances also probably part of this condition, as surveillance follow-up biopsies after an episode of ACR not uncommonly show continued inflammation.110 However, at present the arterial lesions are the most definitive. It is anticipated that molecular gene expression studies will help in the future to document the activity of the infiltrate. Other non-specific features that are commonly present in association with transplant arteriopathy are loss of PTCs and interstitial fibrosis and tubular atrophy (IFTA).160 Chronic Allograft Arteriopathy Small and large arteries, as early as 1 month after transplantation, can begin to develop severe intimal proliferation and luminal narrowing.41,58,146 The intimal change is

FIGURE 26-6  ■  Chronic allograft arteriopathy: an interlobular artery with prominent intimal fibroplasia. The presence of scattered mononuclear cells in the intima and the lack of duplication of the internal elastica are characteristic of chronic rejection. This biopsy was positive for C4d.

most prominent in the larger arteries, but can be seen at all levels, from interlobular arteries to the main renal artery. The intima shows pronounced concentric fibrous thickening with invasion and proliferation of spindle-shaped myofibroblasts (Figure 26-6). This vascular change has been termed chronic transplant arteriopathy and, when combined with an infiltrate of mononuclear cells in the intima, is characteristic of chronic T-cell-mediated rejection. Subendothelial mononuclear cells are one of the most distinctive features and argue that the endothelium itself is a target. T cells (CD4+, CD8+, CD45RO+), macrophages, and dendritic cells infiltrate the intima.124,285,335 T cells express cytotoxic markers, including perforin99 and GMP-17,230 and markers of proliferation (proliferating cell nuclear antigen).124 No B cells (CD20) are detected.124 It is imagined that this is a dampened version of the endarteritis of acute rejection. The second distinctive feature is the lack of multilamination of the elastica interna (fibroelastosis), best appreciated in elastin stains. Fibroelastosis, typical of hypertensive, atrophic, and aging arterial changes provide useful differential diagnostic features from rejection. Foamy macrophages containing lipid droplets are sometimes seen along the internal elastica and can be found as early as 4 weeks after transplantation. The endothelium expresses increased adhesion molecules, notably ICAM-1 and VCAM-1. Antagonism of ICAM-1 binding/expression inhibits chronic rejection329 and in humans certain ICAM-1 genetic polymorphisms (e.g., exon 4, the Mac-1 binding site) appear to confer a higher risk factor for chronic rejection.226 The endothelium remains of donor origin157,345; however, some of the spindle-shaped cells that contribute to the intimal thickening are of recipient origin.178,285 The myointimal cells stain prominently for smooth-muscle actin, sometimes so strikingly that a “double media” seems to be formed.333 This phenomenon has also been described as the development of a new artery inside and concentric with the old,156 with elastic laminae and a muscular media, separated from the old internal elastic lamina poorly by cellular tissue. By EM,

26  Pathology of Kidney Transplantation 391

the thickened intima consists of myofibroblasts, collagen fibrils, basement membrane material, and a loose amorphous electron-lucent ground substance.302 The matrix consists of collagen, fibronectin, tenascin, proteoglycans (biglycan and decorin), and acid mucopolysaccarides.56,123,227 Fibronectin has the extra domain of cellular fibronectin, typical of embryonic or wound-­healing fibronectin.123 Several growth factors/­cytokines have been detected. Platelet-derived growth factor (PDGF) A chain protein is primarily in endothelial cells, while the B chain is in macrophages and smooth-muscle cells.9 Enhanced PDGF B-type receptor protein was found on intimal cells and on smooth-muscle cells of the proliferating vessels.94 Fibroblast growth factor-1 and its receptor are present in the thickened intima.179 TNF-α is in the smooth muscle of vessels with chronic rejection in contrast to normal kidneys.280 Sequence of Arterial Lesions T-cell-mediated arterial lesions can be divided into three stages, which probably differ in mechanism and reversibility.57 The stage I lesion is endarteritis, characteristic of type II ACR. This lesion lacks matrix formation. This acute stage is believed to be T-cell-mediated endothelial injury. Stage II lesions have intimal matrix production and accumulation of myofibroblasts forming a “neointima.” This stage also contains mononuclear cells (T cells and macrophages), believed to be active in the intimal proliferation and accumulation of matrix. Intermediate stages between stage I and II lesions are sometimes found, with lymphocytes admixed with fibrin and fibromuscular proliferation, well documented in a non-human primate model of chronic rejection.407 Secondary factors probably become increasingly important as the lesion progresses to stage III, where the intima is fibrous and inflammatory cells are scant. A fourth category, resembling natural atherosclerosis with cholesterol clefts and calcification, has also been proposed.124 A large body of experimental evidence supports the concept that the arterial lesions are immunologically mediated57: (1) the lesions do not routinely arise in isografts; (2) the target antigens can be either major or minor histocompatibility antigens2,68,331; (3) the specific initiator is probably T cells followed by antibody (antibody is necessary and sufficient for the fibrous lesion in mice); (4) the target cell is probably the endothelium, but the smooth muscle may also be affected; (5) secondary non-immunologic mechanisms analogous to those in atherosclerosis are important in the progression of the lesion; and ultimately (6) the process may be independent of specific antidonor immunological activity. T cells are sufficient to initiate cellular vascular lesions in B-cell-deficient mice, but these lesions do not readily progress to fibrosis in the absence of antibody.328 Fibrous lesions are also markedly reduced in strain combinations that fail to elicit a humoral antibody response. The best evidence for T-cell mechanisms of chronic allograft injury in humans is that subclinical or late clinical cellular rejection is associated with progressive graft fibrosis and dysfunction63,263,327 and endarteritis is associated with later transplant arteriopathy.192 As mentioned previously,

antibodies likely conspire to accelerate the process of allograft arteriopathy/arteriosclerosis.146,147

Differential Diagnosis of Late Biopsies The diagnosis of the cause of late graft dysfunction remains a challenge for several reasons: the diagnostic features may have been lost or obscured over time (e.g., loss of viral antigens or C4d) and typically more than one pathological process is present, analogous to the layers of failed civilizations in an archeological dig. The most important questions for the pathologist and clinician, and the prime reason for the biopsy, are whether the disease is active and whether the graft is potentially salvageable, so that additional therapy can be proposed. Transplant Glomerulopathy Duplication of the GBM has many other causes, such as TMA and MPGN; however these do not have C4d in PTC unless there is more than one concurrent pathologic process. Also in CHR, GBMs may show multilamination extending completely around the capillary, even between the endothelium and the mesangium, less likely to be seen in other conditions.49 With immunohistochemical techniques in paraffin sections, C4d is present along the glomerular capillary walls in about 10–30% of TG.312,354 Although officially required for diagnosis by the Banff schema, it is now recognized that approximately 30% of TG due to CHR is C4d-negative.180 Notably, while most cases of TG are due to CHR, this pattern is also seen in allografts with chronic TMA and in patients with hepatitis C virus (HCV) infection.19 Crescents or diffuse immunoglobulin deposits suggest recurrent or de novo glomerulonephritis.116,287,303 Arteriosclerosis. Transplant arteriopathy is traditionally distinguished from chronic allograft arteriopathy by the presence of fibroelastosis (increased elastic fibers in the intima) in the former. To distinguish intimal fibrosis due to hypertension from that due to chronic rejection, an elastin stain is valuable, since in hypertension, but not necessarily in rejection, the elastica interna is multilayered (“elastosis”), and in chronic rejection the elastica is not duplicated, but may be fractured. Foam cells and mononuclear cells in the intima also favor rejection. A recent study, however, suggested that some lesions of vascular intimal thickening due to alloantibody are indistinguishable from those due to hypertension.146 Chronic CNIT has been traditionally diagnosed by the presence of nodular hyaline replacement of individual smooth-muscle cells, which may form distinctive deposits on the outer side of the arteriole, as described by Mihatsch as cyclosporine arteriolopathy.243–247 Ordinary hyalinosis due to diabetes, hypertension, or aging typically is subendothelial. However, peripheral nodular hyalinosis, while more common in patients on CNI, can be seen in a substantial fraction (28%) of biopsies at 10 years in patients never exposed to CNI.364 CHR. The features that point to a component of CHR are discussed above and include most specifically the presence of C4d in PTC and/or glomeruli and capillaritis and


Kidney Transplantation: Principles and Practice

glomerulitis. Multilamination of the GBM or PTC basement membranes is also typical. In the absence of C4d in PTC, other causes of lamination of the GBM must be excluded. Polyomavirus. Demonstration of polyomavirus by immunohistochemistry in previous biopsies can point to a causal role in the late graft damage, even when the virus is no longer detectable.61 Obstruction, usually difficult to diagnose by histology, archetypically shows dilated collecting ducts, especially in the outer cortex, lymphatics filled with Tamm–Horsfall protein, occasionally ruptured tubules with granulomas, and sometimes acute tubular injury.61 Renal artery stenosis causes tubular atrophy (or even acute injury) accompanied by relatively little fibrosis or intraparenchymal arteriolar/arterial lesions.61 Recurrent and de novo glomerular diseases are generally identified by their light, immunofluorescence, and EM criteria in native kidneys.61 Interstitial Fibrosis. IFTA do not serve to distinguish rejection from other causes, such as CNI toxicity and previous BK polyomavirus infection. Some of these cases may be the end-stage of active processes in which the etiologic agent is no longer appreciable (e.g., late effects of polyomavirus or TMA). Others may represent burnedout or inactive rejection. This might be the case for TG or arteriopathy without C4d deposition. The term “CAN” was created in Banff in 1993 to draw attention to the fact that not all late graft injury was due to rejection, and that, to make the diagnosis of rejection, certain more specific features than interstitial fibrosis and tubular atrophy needed to be present (notably chronic glomerular or arterial lesions). However, the unintended consequence was that “CAN” itself became a diagnosis that inhibited the search for specific, and perhaps treatable, causes. CAN has been replaced in Banff 2005 with category 5: “IF and TA, no evidence of any specific etiology.” This now includes only those cases for which no specific etiologic features can be defined, and excludes those with pathologic features of CHR, chronic CNIT, hypertensive renal disease, PTN, obstruction, or other de novo or recurrent renal disease.367

ACUTE TUBULAR INJURY The morphologic basis of DGF is usually acute ischemic injury (also known as ATN). The most common feature histologically is loss of the brush borders of proximal tubular cells, best shown on a PAS stain with focal interstitial edema and mononuclear cell accumulation (Figure 26-7). The tubular lumen appears larger than normal and lacks the usual artifactual sloughing of the apical cytoplasm in human renal biopsies (this sloughing has occurred in vivo and was washed downstream). The other features of ATN include flattening of the cytoplasm and loss of cell nuclei due to apoptosis/death of individual tubular epithelial cells and covering of the TBM by the remaining cells. The lumen contains individual apoptotic detached cells (“anoikis”) and inflammatory cells. Reactive changes in

FIGURE 26-7  ■ Acute tubular necrosis. Dilated “rigid”-appearing tubular lumens with loss of brush borders, occasional loss of nuclei, and cytoplasmic thinning. Mild edema is present but there is little inflammation. Glomeruli are normal. Periodic acid– Schiff stain.

the tubular epithelium are seen after 24–48 hours, including large basophilic nuclei with prominent nucleoli, increased cytoplasmic basophilia, and occasionally mitoses. Focal interstitial, PTC, and glomerular capillary neutrophils may be seen but are not as prominent as in AHR; and C4d is negative. Mechanical flushing of cadaveric kidneys with organ preservation fluid immediately before transplantation (as advocated by some) was associated with abnormal cellular debris within the tubules and eosinophilic proteinaceous material within Bowman’s capsule and an increased frequency of DGF.318 DGF has other causes; and if function has not recovered in 1–2 weeks, a diagnostic biopsy is recommended to ascertain the presence of occult acute rejection, found in 18% of patients with DGF at 7 days.167

CALCINEURIN INHIBITOR NEPHROTOXICITY The CNI class of drugs, including cyclosporine and tacrolimus, causes both acute and chronic nephrotoxicity that includes ischemic injury without morphologic features, vacuolar tubulopathy, acute endothelial injury (TMA), and arteriolar hyalinosis.245,247 These cause secondary pathological effects, such as tubular atrophy, interstitial fibrosis and global or segmental glomerulosclerosis. As judged by protocol biopsies, chronic CNIT is universal in renal transplants after about 5 years.263 Chronic CNIT can also damage native kidneys in patients with other organ transplants, and contributes to the 7–21% prevalence of end-stage renal disease in non-renal transplant recipients after 5 years.286

Acute CNI Toxicity Toxic Tubulopathy The biopsy features of acute toxicity are quite variable. A normal biopsy is found in “functional CNIT,” which is due to reversible vasospasm.315 In toxic tubulopathy,

26  Pathology of Kidney Transplantation 393

FIGURE 26-8 ■ Acute calcineurin inhibitor nephrotoxicity with isometric vacuolization of tubular epithelium. This change can also be seen in other causes of tubular injury, including ischemia, osmotic diuretics, and intravenous immunoglobulin.

proximal tubules show the most conspicuous morphologic changes with loss of brush borders and isometric (uniformly sized), clear, fine vacuolization (or microvacuoles) in the epithelial cells (Figure 26-8). The microvacuoles contain clear aqueous fluid rather than lipid, and are indistinguishable from those caused by osmotic diuretics or ischemia. EM shows that the vacuoles in cyclosporine toxicity are due to dilation of the endoplasmic reticulum and appear empty.246 Isometric vacuolization may begin in the straight portion of the proximal tubule,246 although it can extend to the convoluted portion. The degree of vacuolization does not correlate with drug levels; some patients with CNIT lack the vacuolar change,218 and isometric vacuoles can be found in a minority of patients with stable renal function.368 However, reduction of the CNI dosage causes disappearance of tubular vacuolization.381

The pathologic changes are believed to be an exaggeration of CNI-induced endothelial and smoothmuscle damage. The small arteries and arterioles have mucoid intimal thickening with acid mucopolysaccarides and extravasated red cells and fragments; fibrinoid necrosis and thrombi may be prominent. Apoptosis of endothelial and smooth-muscle cells is seen. The medial smooth muscle can develop a mucoid appearance with loss of a clear definition of the cells.265 The arterioles may show hypertrophy of the endothelial cells and have a “constricted” appearance.265 The vascular lumens may be partially or completely obliterated by the intimal proliferation and endothelial swelling. The vascular lesions are most severe in the interlobular and arcuatesized arteries, and can lead to cortical infarction.369 By immunofluorescence microscopy, the vessels stain with IgM, C3, and fibrin.61 The glomeruli typically have swollen bloodless capillaries with scattered fibrin-platelet thrombi (Figure 26-9A), particularly in the hilum,351 the socalled pouch lesion.242 The endothelial cells are swollen and may completely obliterate the capillary lumens. The GBM is segmentally duplicated with cellular (mononuclear or mesangial cell) interposition best seen by EM, which also shows the loss of fenestrae and swelling of the endothelial cytoplasm. Variable mesangial expansion, sclerosis, and mesangiolysis242 may be seen. Marked congestion and focal, global, or segmental necrosis can be present.394 Differential Diagnosis

Acute tubular toxicity of cyclosporine may be indistinguishable from ischemia or tubulopathy from intravenous immunoglobulin or mannitol, which all have vacuoles by light microscopy.134 By EM a coarser and more varied vacuolization is typical of ATN and the periphery of infarcts246 compared with the isometric (uniform) vacuoles of cyclosporine toxicity. The vacuoles of osmotic diuretic injury do not involve the endoplasmic reticulum, as do Acute Arteriolar Toxicity and Thrombotic those of cyclosporine toxicity.242 Necrosis of tubular cells Microangiopathy is more common in ATN (0.5% of tubules), characteristically involving whole tubular cross-sections.368 Acute Arterioles are a significant target of CNIT. The most charmedial apoptosis/degeneration in arterioles is the only acteristic acute changes include individual medial smoothdefinitive finding favoring CsA toxicity. muscle cell degeneration, necrosis/apoptosis, and loss.246 Morphology alone cannot distinguish the various The apoptotic smooth-muscle cells are later replaced by etiologies of TMA,207,279 which in renal transplants are rounded, “lumpy” protein deposits or hyalinosis, which most commonly CNI, AHR, HCV, and recurrent TMA. is the beginning of a more chronic arteriolopathy.246 Accumulation of glycogen (PAS-positive, diastase-­ Recurrence should be the first choice when the recipient’s original disease was TMA, unless associated with sensitive) in smooth-muscle cells has been described on a diarrheal illness. C4d deposition in PTCs is preshigh doses.195 Endothelial cells can have prominent vacuent in AHR but absent in CNI-associated TMA (as in olization and some swelling. Immunofluorescence miFigure 26-9B; see section on AHR). Serum should also croscopy of the vessels often shows deposits of IgM, C3 be tested for anti-HLA class I and class II, and antiendoand sometimes fibrin/fibrinogen, but these changes are thelial antibodies. HCV-positive renal allograft recipients non-specific.26 may develop TMA with associated elevation of circulatTMA due to CNI was first reported in bone marrow ing anticardiolipin antibody18; thus hepatitis serology and transplant recipients treated with cyclosporine351 and ocanticardiolipin antibody determination could help distincurs in about 1–4% of renal allograft recipients, even guish between HCV versus CNI in the etiology of TMA. with careful attention to drug levels, suggesting that it is The healing phase of TMA may leave intimal fibrosis dose-independent and probably idiosyncratic.46,151 Most that resembles chronic rejection, even with a few intimal cases present with a delayed onset and a slow loss of funcmononuclear cells.61 tion 1–5 months posttransplant.369


Kidney Transplantation: Principles and Practice



FIGURE 26-9  ■ Thrombotic microangiopathy associated with calcineurin inhibitors. (A) A glomerulus with widespread endothelial swelling, segmental glomerular basement membrane duplication, and focal collapse resembling a crescent. Arterioles show endothelial swelling and occasional peripheral hyaline nodules. Periodic acid–Schiff stain. (B) No glomerular or peritubular capillary C4d deposition is detected in this case. Immunohistochemistry for C4d in paraffin, using rabbit polyclonal anti-C4d.

Chronic CNI Toxicity

CNI Arteriolopathy

Irreversible chronic renal failure due to CNIT was first demonstrated in native kidneys of heart transplant patients who received cyclosporine for more than a year.256 Similar lesions arise in patients on tacrolimus.311 Biopsies showed IFTA, arteriolar hyalinosis, and sometimes focal glomerular scarring. These findings have been confirmed and extended in numerous other studies.26 Since many features resemble chronic rejection in the kidney, the most convincing pathology data come from non-renal transplant patients on cyclosporine.78,278

The chronic phase of CNI arteriolopathy is characterized by replacement of the degenerated medial smooth-muscle cells with hyaline-like deposits, in a beaded pattern along the peripheral, outer media (Figure 26-10A). This has been referred to as “nodular protein (hyaline) deposits”244 in a “pearl-like pattern”26 and “peripheral medial nodular hyalinosis,” and is now called “CNI arteriolopathy.” The current evidence supports the view that this type of arteriolopathy is more common but not specific for CNI,364 despite extensive historical evidence to the contrary.278 Evidence of apoptosis is sometimes found in the form of



FIGURE 26-10  ■  Calcineurin inhibitor arteriolopathy. (A) Several arterioles with peripheral nodular hyalinosis, where hyalin deposits replace necrotic/apoptotic smooth-muscle cells in the outermost media. (B) Electron microscopy: an artery that has “beads” of hyalin (*) along the outer media. L, arteriolar lumen; T, tubule. (Periodic acid–Schiff 800×; electron microscopy 2700×)

26  Pathology of Kidney Transplantation 395

karyorrhexic debris in the medium, but fibrinoid necrosis is not observed.257 In severe cases the medium is nearly devoid of smooth-muscle cells.257 EM reveals a distinctive replacement of individual smooth-muscle cells of afferent arterioles with amorphous electron-dense material, which contains cell debris and protrudes into the adventia (Figure 26-10B). This gives rise to the beaded hyalinosis distribution in the outer medium noted by light microscopy. The myocyte nuclei are sometimes condensed (apoptotic), or have two nuclei or mitotic figures.415 The cytoplasm is vacuolated, with dilated endoplasmic reticulum, and has degenerated mitochondria, lipofuscin granules, multivesicular bodies, and a disarray of microfibrils and reduced intercellular junctions. The endothelium sometimes appears “swollen,” protruding into and narrowing the lumen, and having reduced cell junctions; aggregates of platelets are rare.13,415 These findings support the view that the smooth-muscle myocyte of the afferent arteriole is a primary target of CNI injury. Immunofluorescence microscopy shows IgM and C3 in a relatively non-specific, but conspicuous, sheathing of the arterioles.26 CNI arteriolopathy begins and predominates in the afferent arterioles but may progress to the small arteries and efferent arterioles.26,415 Decreased renin immunostaining in the juxtaglomerular apparatus suggests that the prime target of CNI is the renin-producing smoothmuscle cell in the afferent arteriole.378 The frequency of arterioles affected with hyalinosis is typically small (<15%) and the lesions can easily be overlooked.379 In renal transplant patients on cyclosporine, 15% of protocol biopsies at 6 months showed CNI arteriolopathy which increased to 45% in 18-month protocol biopsies336; “nonspecific” hyalinosis showed no progressive increase. Tenyear protocol biopsies showed a 2.4-fold increased risk of peripheral nodular hyaline in patients treated with CNI compared to those never on CNI, but even 28% of the latter had the lesion.364 The arteriolar lesions also develop in native kidneys of patients who receive even low doses of cyclosporine for 2 years.299,418 Mihatsch has suggested a scoring system of CNI arteriolopathy with improved reproducibility (personal communication).358 Glomerular Lesions After 1 year on cyclosporine, glomeruli show increased numbers with global or segmental sclerosis.78,278 Focal, segmental sclerosis was more common in CNI-treated bone marrow (13%) and heart transplant (27%) recipients at autopsy than their respective CNI-free controls (0% and 14%).278 Heart transplant recipients have an increase in the heterogeneity of glomerular volume and size, with more small and large glomeruli (compensatory hypertrophy) compared with controls (living kidney donors).257 The shift to smaller glomeruli becomes more extreme with chronic renal failure and the hypertrophied glomeruli disappear.255 Thus hyperfiltration injury probably causes the progressive glomerular proteinuria and sclerosis. Bone marrow and heart transplant patients at autopsy show glomerular collapse in 59% of patients on CNI versus 8% of those not on CNI.278 This can develop into florid collapsing glomerulopathy, attributed to the severe CNI arteriolopathy.120

Immunofluorescence findings are non-specific (IgM and C3 in scarred areas). EM in cardiac and liver transplant recipients showed diffuse expansion of the mesangial matrix, with little hypercellularity, GBM, or podocyte lesions.78,257 Those with frank collapsing glomerulopathy have podocyte foot process effacement and detachment of podocytes from the GBM.120 The endothelium shows loss of its normal fenestrae, perhaps reflecting a component of TMA.61 Tubules and Interstitium IFTA was recognized as a feature of CNIT in the early studies.387 The interstitium had prominent patchy fibrosis, with a scanty infiltrate. Band-like narrow zones of IFTA (“striped fibrosis”) were once regarded as characteristic of CNIT90,322,353; however, indistinguishable “stripes” occur in patients not maintained on CNI,74 casting doubt on the specificity of that pattern. Interstitial fibrosis also develops in native kidneys in patients on CNI243,291,418,419 and remains for at least a month after discontinuing the drug.239 Thus even low doses of cyclosporine can cause significant and presumably permanent loss of renal function by inducing chronic tubulointerstitial nephritis.61 Differential Diagnosis Distinction between chronic rejection and chronic CNIT is a challenge (Table 26-5). The nodular arteriolopathy is supportive of CNIT but not decisive. The arterioles are relatively spared in chronic rejection, compared with TABLE 26-5  Differentiation between Chronic Rejection and Chronic Calcineurin Inhibitor Toxicity

Interstitium Infiltrate Fibrosis

Chronic Rejection

Calcineurin Inhibitor Toxicity

Plasma cells Patchy

Mild Patchy, “striped”

Peritubular Capillaries C4d Often positive Multilamination BM Usual

Negative Absent

Tubules Tubular atrophy Vacuoles

Usual Occasional

Usual Occasional

Arterioles Smooth muscle


External nodular


Usual degeneration Present hyalinosis

Arteries Intimal fibrosis


Mononuclear cells


Can be present but unrelated Absent intima

Glomeruli Duplication GBM Mesangial expansion

Usual Can be present

Absent Can be present

BM, basement membrane; GBM, glomerular basement membrane.


Kidney Transplantation: Principles and Practice

chronic CNIT, and the arteries are more affected, with proliferative intimal fibrosis without elastosis.245 PTC C4d deposits or mononuclear cells in the arterial intima are the most useful signs of an active rejection process. An inflammatory infiltrate, including plasma cells, is less common in CNIT than rejection.260 Other features are not decisive. IFTA and glomerular sclerosis are found in either. GBM duplication and endothelial dedifferentiation can also be seen in either, although perhaps more commonly in chronic rejection.61

unequivocal evidence for rejection. Strong, but not absolute, evidence for a drug etiology is the invasion of multiple tubules by eosinophils, and eosinophils in tubular casts (Colvin, unpublished observation), usually attributed to prophylactic trimethaprim-sulfamethoxazole (Bactrim). We have also seen one case of severe acute interstitial nephritis and serum sickness-like syndrome secondary to the horse antithymocyte globulin.61


Many organisms can infect the transplanted kidney, ranging from mycobacteria and candida229 to herpes simplex virus355 and human herpesvirus-1.355 In addition, viruses such as cytomegalovirus and HCV can have indirect effects on the transplant, promoting rejection or immunemediated disease.66,317,392 Here we will discuss the three most important types of infections: polyomavirus, adenovirus, and bacterial pyelonephritis.

Inhibitors of the mammalian target of rapamycin (TORi) (rapamycin, everolimus, sirolimus) can cause DGF due to tubular toxicity that resembles myeloma cast nephropathy. Pathologically, in addition to acute tubular injury, eosinophilic debris and macrophages were present in tubular lumina, that mimicked myeloma casts, but the casts stain for keratin, rather than immunoglobulin light chains.363 TORi can also cause TMA, indistinguishable from that due to CNI.316 Increased proteinuria is common on patients switched from CNI to TORi because they had developed severe CNIT. In these patients, GFR improves but increased proteinuria develops in about 30%.200 CNI exposure is not necessary for the proteinuric response to TORi. Conversion from azathioprine to TORi can also cause increased proteinuria.393 Patients started on TORi without CNI had double the risk of proteinuria at 6–12 months compared with those on CNI.372 Few pathological studies have been published. One reported a variety of glomerular diseases typical of native kidneys, suggesting recurrent disease.79 A recipient begun on TORi developed 12 g/day proteinuria in the first week after transplantation, which remitted after the drug was discontinued.375 Biopsy showed no obvious glomerular disease was evident by light, immunofluorescence, or EM, suggesting the proteinuria was due to failure of tubular reabsorption. One notable case report described collapsing glomerulopathy in a patient with Kaposi’s sarcoma converted to TORi from azathioprine.165 We have seen two cases of FSGS in patients started on TORi: one had collapsing glomerulopathy (Cornell et al., unpublished). More pathology studies are clearly needed, particularly on those patients started on TORi.61

DRUG-INDUCED ACUTE TUBULOINTERSTITIAL NEPHRITIS Drug induced interstitial nephritis in the allograft is similar to that in the native kidney, and resembles tubulo­ interstitial rejection. Both are characterized by an intense mononuclear interstitial infiltrate and tubulitis, and have variable numbers of eosinophils. Acute rejection occasionally has a prominent eosinophilic infiltrate8,136,154,189,385,405; conversely, drug-induced interstitial nephritis may have no eosinophils, especially that due to non-steroidal anti-inflammatory drugs.60 Endarteritis, if present, is


Polyomavirus Tubulointerstitial Nephritis PTN has emerged since 1996 as a significant cause of early and late graft damage.80,81,219,268,272,292 Among various series of patients on tacrolimus/mycophenolate mofetil, PTN arises in about 5%, similar to the prevalence of acute rejection. The virus was originally isolated from BK, a Sudanese patient who had distal donor ureteral stenosis, 3 months after a living related transplant.111 BK virus is related to JC virus (which also inhabits the human urinary tract) and to simian virus SV40. These viruses are members of the papovavirus group, which includes the papillomaviruses. The BK virus commonly infects urothelium but rarely causes morbidity in immunocompetent individuals. However, in renal transplant recipients three lesions have been attributed to BK virus: hemorrhagic cystitis, ureteral stenosis, and interstitial nephritis.51,112,153 PTN is characterized by a patchy mononuclear infiltrate associated with tubulitis and tubular cell injury (Figure 26-11B).292 The infiltrate often contains plasma cells, which sometimes invade the tubules. Concurrent ACR may be present. Tubular cell apoptosis is prominent, as well as “dedifferentiation” of tubular epithelial cells, with loss of polarity and a spindly shape. PTN has three recognized stages: stage A has only minimal inflammation; stage B shows marked tubular injury, denudation of the TBMs and interstitial edema with a mixed, mild to marked inflammatory cell infiltrate; stage C has marked IFTA.81,82,150,271,272 The recognition of viral nuclear inclusions is the key step in diagnosis. The affected nuclei are usually enlarged with a smudgy, amorphous lavender inclusion (Figure 26-11B). Other nuclear changes found less commonly are eosinophilic, granular inclusions with or without a halo, and a vesicular variant with coarsely clumped, irregular basophilic material.268,269,273 These nuclear inclusions tend to be grouped in tubules, particularly collecting ducts in the cortex and outer medulla, and can often be spotted at low power. Immunohistochemistry and EM confirm the diagnosis. Monoclonal antibodies are commercially available which react with BK-specific

26  Pathology of Kidney Transplantation 397





FIGURE 26-11  ■  Polyoma (BK) virus infection. (A) Low-power view showing patchy mononuclear inflammation in the medulla with groups of atypical nuclei in tubular epithelium (arrows). (B) Higher power shows polyomavirus inclusion (arrow), marked tubulitis, and tubular cell apoptosis. (C) Immunohistochemistry: monoclonal antibody to SV40 large T antigen (homologous to BK, JC, and other polyoma viruses), many tubular epithelial cell nuclei appear dark brown due to immunoreactivity for polyoma virus. (D) Electron microscopy: high magnification of a tubular cell nucleus (N) containing polyoma virions (arrow), that are rounded, 30–35 nm in diameter, and organized in arrays (from Cynomolgus monkey). (From van Gorder MA, Della Pelle P, Henson JW, et al. Cynomolgus polyoma virus infection: a new member of the polyoma virus family causes interstitial nephritis, ureteritis, and enteritis in immunosuppressed cynomolgus monkeys. Am J Pathol 1999;154:1273-84.)396

determinants and with the large T antigen of several polyoma species (Figure 26-11C). EM will reveal the characteristic intranuclear paracrystalline arrays of viral particles of about 40 nm diameter (Figure 26-11D). Other tests useful for monitoring patients at risk are urine cytology (“decoy cells”) and polymerase chain reaction quantitation of virus in the blood, although these are not specific enough to make a PTN diagnosis.61 Polyomavirus infections may cause an immune complex deposition along the TBM, as described in 43% of cases in a series from Seattle, being the most common cause of IgG deposits in the TBM of transplants.34 Granular IgG, C3, and C4d are focally present by immunofluorescence and amorphous electron-dense deposits by EM. The prognostic significance is not known.61 Late graft fibrosis and scarring/“CAN” may be caused by polyomavirus, even though the virus is no longer demonstrable. The virus is cytopathic for tubular cells and leads to characteristically destructive tubular lesions, with only TBM remaining. The diagnosis is sometimes only possible by review of prior biopsies. Suspicion of PTN is heightened if tubular destruction is severe. The process may be clinically silent: protocol biopsies have shown a subclinical incidence of PTN of 1.2%.40 Furthermore, PTN can affect native kidneys of recipients of non-renal allografts; only a few cases have been reported, but this

may be in part due to a presumption of CNIT and a lack of renal biopsies in this setting.203

Adenovirus Adenovirus, most frequently serotype 11, causes hemorrhagic cystitis and also occasionally tubulointerstitial nephritis in renal allografts, which may resemble a space-occupying lesion by imaging studies.202,414 Biopsy shows necrotizing inflammation with neutrophils and tubular destruction, interstitial hemorrhage and red cell casts, granulomatous inflammation,38,161,267,357 or a zonal inflammation localized to the outer medulla.220 Tubular cells have intranuclear ground-glass inclusions with a distinct halo surrounded by a ring of marginated chromatin and glassy smudged nuclei. The diagnosis is established by immunoperoxidase stains for viral antigen in tubular cells, and EM to reveal the intranuclear crystalline arrays of 75–80-nm viral particles. Immune complexes may also contribute to the injury. Decreased immunosuppression has been followed by recovery.61

Acute Pyelonephritis Pyelonephritis is a potentially devastating complication of transplantation. Pyelonephritis can present as acute


Kidney Transplantation: Principles and Practice

renal failure114,416 and cause graft loss.139,171 Pyelonephritis arises most often 1 year or more after transplantation (80% of episodes).298 Escherichia coli was the most common organism (80%). Acute pyelonephritis is a not uncommon finding on renal biopsy, despite the expectation that the process is patchy.416 Renal biopsies are not the usual method of diagnosis; however, if neutrophils are abundant, especially if they form destructive abscesses and casts in tubules, the diagnosis should be at the top of the list. Other variants are emphysematous pyelonephritis, due to gas-producing organisms,171 xanthogranulomatous pyelonephritis,87,170 and malakoplakia.373

MAJOR RENAL VASCULAR DISEASE Most arterial thromboses develop in the early posttransplant period and produce acute infarction with microthrombi and scant inflammation.20 Evidence for underlying rejection should be sought by careful examination of the larger arteries for endarteritis. Renal artery stenosis (typically at the anastomosis site), a cause of late graft dysfunction, can be deceptive clinically and pathologically.37,356 Biopsies show tubular injury or atrophy with relatively little inflammation or fibrosis. Renal vein thrombosis causes a swollen and purple kidney, sometimes with graft rupture.334 The cortex shows severe hemorrhagic congestion, and extensive infarction and necrosis,238 sometimes with diffuse microcapillary thrombi. Intracapillary leukocytes can be a clue as in native kidneys. Late renal vein thrombosis is associated with proteinuria due to MGN or TG, sometimes with graft loss.340 Lupus anticoagulant has been detected in a few patients.215

DE NOVO GLOMERULAR DISEASE Patients without previous glomerular disease occasionally develop lesions in the allograft that resemble a primary glomerular disease, rather than the usual chronic allograft glomerulopathy. While some are no doubt coincidental, at least three are related to an alloimmune response to the allograft: MGN, anti-GBM disease in Alport’s syndrome, and recurrent nephrotic syndrome in congenital nephrosis. A fourth, relatively common de novo glomerular disease, FSGS, is believed to be related to hyperfiltration injury of the allograft or marked microvascular compromise due to CNIT.61

Membranous Glomerulonephritis De novo MGN is typically a late complication, with a prevalence of about 1–2%.70,141,251 In contrast, recurrent MGN can present early.321 The risk factors for de novo MGN include time after transplant, de novo MGN in a first graft,141 and HCV infection.70,251 Light microscopy usually shows rather mild GBM changes. Mesangial hypercellularity is found in about 33%. Mononuclear cells can be abundant in glomerular capillaries, raising the possibility of transplant glomerulitis or renal vein thrombosis.249 Immunofluorescence shows granular deposits along the GBM that stain for IgG, C3, C4d, and factor H67; about 35% are more irregular and segmental in

FIGURE 26-12 ■  De novo membranous glomerulonephritis: subepithelial electron-dense deposits (arrows) along the glo­ merular basement membrane with intervening basement membrane spikes. Podocyte (P) foot processes are effaced. C, capillary lumen; U, urinary space.

distribution than typical primary (idiopathic) MGN.249,391 By EM subepithelial electron-dense deposits are present (Figure 26-12), which are smaller and more irregular in distribution than primary MGN.249,391 Endothelial changes and GBM duplication typical of TG are present in half of the cases.249,391 Repeat biopsies have shown persistence or progression of the deposits in most cases and occasional resolution.12,249 The pathogenesis of de novo MGN has not been established. The literature supports the hypothesis that de novo MGN may be a form of AMR or directed at minor histocompatibility antigen(s) in the glomerulus, presumably on the podocyte or a special type of chronic rejection.57,389,391 The common presence of TG is consistent with this hypothesis.249,391

Anti-GBM Nephritis Patients with Alport’s syndrome or hereditary nephritis commonly develop anti-GBM alloantibodies, because they genetically lack self-tolerance to GBM collagen components. However this leads to glomerulonephritis in only a minority. Overall, de novo crescentic and necrotizing glomerulonephritis due to anti-GBM antibodies after transplantation is uncommon, seen in only 5% of male adult renal allograft recipients with typical Alport’s syndrome.173,174,16 The pathology is similar to that in native kidney with prominent crescents (not a feature of allograft rejection), segmental necrosis, and red cell casts. Second transplantation with and without recurrent anti-GBM nephritis have both been reported.77,121,395 The 5-year graft survival may be equal to that of non-Alport’s recipients.119

De Novo Podocytopathy in Congenital Nephrosis Congenital nephrotic syndrome of the Finnish type, an autosomal recessive disease due to mutations in the nephrin gene NPHS1, paradoxically leads to posttransplant nephrotic syndrome.213,283 The podocyte pathology

26  Pathology of Kidney Transplantation 399

arteriosclerosis, and interstitial fibrosis were also present. A rapid progression to renal failure occurred in 80% of the patients (2–12 months). The cause is unknown; all patients were human immunodeficiency virus-negative. Collapsing glomerulopathy can also develop in native kidneys in patients on CNI (Figure 26-13).120

resembles minimal-change disease and usually responds to cyclophosphamide.98,194 De novo “minimal-change disease” is thought to be caused by the alloantibodies to nephrin in some cases.296

Focal Segmental Glomerulosclerosis De novo FSGS has been described in adult recipients of pediatric kidneys,266,412 in which the presumed pathogenesis is hyperfiltration injury, in long-standing grafts, in which parenchymal loss due to CNIT or chronic rejection leads to hyperfiltration injury of residual glomeruli, and as the collapsing variant of FSGS, probably related to CNI arteriolopathy.231 De novo collapsing glomerulopathy presents months to years after transplantation with proteinuria (2–12 g/ day).231,258,374 Diffuse or focal, global, or segmental collapse of glomeruli was evident with prominent hyperreactive podocytes (Figure 26-13). Arteriolar hyalinosis,

RECURRENT RENAL DISEASE Recurrent disease (e.g., dense deposit in Figure 26-14) is a significant cause of allograft failure.45,97,308 The frequency and clinical significance of recurrence vary with the disease (Table 26-6). In one study, glomerular diseases, including recurrent and de novo glomerulonephritis and TG, were responsible for 37% of cases of graft loss; 14% of death-censored graft losses were due to recurrent glomerular disease.88 Recurrence of immunemediated disease may become a greater problem in the future with longer graft survival and development of tolerance protocols that require no immunosuppression. The reader is referred to a comprehensive review elsewhere for detailed information regarding specific diseases.61 Transplantation also can uniquely illuminate the early pathologic events that precede clinical signs and determine the reversibility of pre-existing lesions in the donor kidney (e.g., diabetes, IgA nephropathy). For example, early recurrent MGN – as early as 2 weeks posttransplant the glomeruli can show staining in a membranous pattern by immunofluorescence for IgG, C4d, and kappa and lambda light chains, but corresponding electrondense deposits may not be present ultrastructurally; these features can be seen on biopsy without proteinuria clinically.321 Later biopsies of the allografts show a more typical membranous pattern with subepithelial deposits by EM. Diabetic nephropathy begins with an increase in allograft glomerular volume at 6 months,288 followed by increases in mesangial volume.408 Thickening of the GBM is first evident after 2–3 years32,408 and nodular diabetic glomerulosclerosis at 5–15 years posttransplant (Figure 26-15).140 Tubulointerstitial diseases may also recur, such as with recurrent oxalate nephropathy in primary hyperoxaluria.25

FIGURE 26-13  ■ De novo collapsing glomerulopathy: collapsed glomerular capillaries and prominent podocyte proliferation, hypertrophy, and abundant reabsorption droplets. Severe arteriolar hyalinosis with peripheral nodules typical of calcineurin inhibitor arteriolopathy was present. This is a native kidney in a patient with a heart–lung transplant. Periodic acid–Schiff stain. (From Goes N, Colvin RB. Renal failure nine years after a heart-lung transplant. N Engl J Med 2006;6:671–9.)120



FIGURE 26-14  ■  Recurrent dense deposit disease. (A) Electron microscopy: widespread very electron-dense deposits that are continuous, linear, and embedded in the glomerular basement membrane proper, i.e., intramembranous (arrows). Similar deposits are also seen in the mesangium (M). C, capillary lumen; U, urinary space. (B) Immunofluorescence microscopy: staining for C3 shows broad linear ribbon-like deposits along the glomerular basement membrane and blob-like deposits in the mesangium (mesangial rings).


Kidney Transplantation: Principles and Practice

TABLE 26-6  Classification of Recurrent Renal Disease Usually Recur (>50% of Patients) Adverse effect* Primary hemolytic uremic syndrome Primary oxalosis Dense deposit disease Collapsing FSGS† Little or no adverse Immunotactoid/fibrillary effect glomerulopathy† Systemic light-chain disease† Diabetes mellitus‡ Commonly Recur (5–50%) Adverse effect FSGS Membranoproliferative GN, type I Membranous GN ANCA-related diseases Wegener’s granulomatosis Pauci-immune GN Microscopic polyarteritis Progressive systemic sclerosis Sickle cell nephropathy† Little or no adverse IgA nephropathy effect Henoch–Schönlein purpura Amyloidosis Rarely Recur (<5%) Adverse effect Little or no adverse effect Recurrence reported§

Never Recur (0%) Unique complications

No unique complications

Anti-GBM disease Systemic lupus erythematosus Fabry’s disease Cystinosis Thrombotic thrombocytopenic purpura Adenosine phosphoribosyl transferase deficiency Familial fibronectin glomerulopathy Lipoprotein glomerulopathy Malacoplakia Hereditary nephritis/Alport’s syndrome (anti-GBM disease) Congenital nephrosis (nephrotic syndrome; nephrin autoantibody) Polycystic disease (all genetic types) Osteo-onychodysplasia (nail-patella)† Acquired cystic disease Secondary hemolytic uremic syndrome (infection) Secondary FSGS Familial FSGS† Postinfectious acute glomerulonephritis†

*Adverse effect defined as graft loss of >5% (when disease recurs). † Limited experience: few cases reported (n <10). ‡ Arteriolar and glomerular lesions recur to some degree in most, if not all, cases, but nodular glomerulosclerosis delayed until >5 years. § Recurrence occurs, but too few cases are reported to classify frequency or consequences. ANCA, antineutrophil cytoplasmic antibody; FSGS, focal segmental glomerulosclerosis; GBM, glomerular basement membrane; GN, glomerulonephritis.

Posttransplant Lymphoproliferative Disease Immunosuppression leads to an increased risk of malignancy, particularly those neoplasms caused by viruses and ultraviolet radiation. These malignancies are presumptively suppressed by immune responses which recognize the viral or mutation-derived neoantigens. The major viral-related tumors are Kaposi’s sarcoma (human ­herpesvirus-8), cervical cancer (human papillomavirus), and PTLD (Epstein–Barr virus). Of these, PTLD not uncommonly affects the kidney, sometimes presenting as graft dysfunction. PTLD involving the kidney can resemble ACR, in having a widespread mononuclear infiltrate invading tubules and even vessels.228,309,337 In our experience, a useful clue that favors PTLD is when the infiltrate forms a dense sheet of monomorphic lymphoblasts without edema or granulocytes (Figure 26-16A). Serpiginous necrosis of the lymphoid cells (irregular patches) is distinctive, but not always present.309 The other features found to be helpful include nodular and expansile aggregates of immature lymphoid cells; the nuclei are enlarged and vesicular with prominent nucleoli that may be multiple. Immunohistochemistry is helpful in identifying the predominance of B cells in the infiltrate, which is never seen in rejection alone. If the cells have a monoclonal kappa or lambda phenotype the diagnosis is confirmed. The definitive diagnosis of PTLD is in situ hybridization for EBER (Epstein–Barr virus-encoded RNA) (Figure 26-16B).

PROTOCOL BIOPSIES “Protocol” or “surveillance” biopsies taken at predetermined times for evaluation of the status of the renal allograft, independent of renal function, are currently the standard of care at several leading transplant centers64,177,253,263,327,341,371 and widely used in clinical trials to evaluate efficacy.55 Protocol biopsies have the potential ability to reveal mechanisms of late graft loss and to identify active processes that might be interrupted therapeutically before irreversible injury has occurred.88 The risk of protocol biopsy is low. There were no deaths or graft losses in the Hannover series of over 1000 biopsies339 and graft loss was 0.04%.104 The current interest in protocol biopsies started with David Rush and colleagues, who made the surprising observation that 30% of biopsies from stable patients 1–3 months posttransplant showed histological rejection325 and those with these lesions show later loss of renal function.326,327 Many other studies have confirmed this result.64,177,253,263,327,341 Mononuclear inflammation that meets the Banff criteria for ACR or borderline acute rejection is found in 5–50% of protocol biopsies in the first 12 months, depending on therapy and patient populations.264 Those with inflammation have a higher risk of graft dysfunction or fibrosis at later time points.65,178,254,264 Grafts with both inflammation and fibrosis do the worst.64,214,253,350 In one study, the best predictor of allograft function 1 year after transplantation was persistent inflammation, of any type, including those patterns considered in Banff to be irrelevant to the diagnosis of acute rejection

26  Pathology of Kidney Transplantation 401



FIGURE 26-15  ■  Recurrent diabetic nephropathy 12 years after transplant. (A) Glomerulus with prominent Kimmelstiel–Wilson mesangial nodules (arrow) and arteriolar hyalinosis. Periodic acid–Schiff stain. (B) Electron microscopy of another case shows homogeneous thickening of the glomerular basement membrane up to 1100 nm. C, capillary lumen; U, urinary space.



FIGURE 26-16  ■  Posttransplant lymphoproliferative disease. (A) Dense mononuclear cell infiltrate in the interstitium that permeates between the tubules without tubulitis (although tubulitis may occur in posttransplant lymphoproliferative disease (PTLD)). The monomorphic infiltrate and the lack of edema distinguish PTLD from the usual cellular rejection. (B) In situ hybridization: nuclei of mononuclear cells stain dark, brown-black for Epstein–Barr virus-encoded RNA (EBER), which is the definitive test for the diagnosis of PTLD.

(in areas of interstitial fibrosis, around large blood vessels, in nodules, or in subcapsular areas).235 Infiltrates in areas of atrophy correlated with IFTA at 6 months and graft dysfunction at 2 years. In another study, protocol biopsies at 1 year posttransplant that showed fibrosis and inflammation predicted a worse GFR at 5 years compared to biopsies with fibrosis and no inflammation and compared to normal biopsies.293 These results suggest that these infiltrates are part of the pathogenesis of slow, progressive renal injury.55,214 Grafts in recipients that are developing tolerance also typically have graft infiltrates, sometimes termed the “acceptance reaction,”349 which spontaneously disappears and is followed by indefinite graft survival.31,330 The acceptance reaction had less infiltration by CD3+ T cells and macrophages, less T-cell activation, longlasting apoptosis of graft-infiltrating T cells, less IFN-γ and more IL-10 than rejecting grafts. What differentiates infiltrates in patients with stable and unstable graft function? In stable grafts endarteritis is found rarely (0.3% in one series)234 and can herald an impending acute episode.325 Among the interstitial infiltrates, only the diffuse pattern (rich in macrophages and granzyme B CTLs) was more common in biopsies taken for acute dysfunction.235 In contrast, nodular infiltrates (rich in B cells and activated T cells) were more

common in protocol biopsies. Similarly, infiltrates rich in activated macrophages distinguished biopsies with clinical versus subclinical acute rejection.127 Molecular studies have shown that increased levels of transcripts for T-bet (a Th1 master transcription factor), FasL (cytotoxic mediator) and CD152 (CTLA-4, an inhibitory costimulatory molecule) are associated with graft dysfunction.152 Foxp3 cells are on the list of suspects to distinguish non-aggressive infiltrates.31,348 Recent evidence shows that regulatory T cells (Treg) that express the Foxp3 transcription factor infiltrate tolerated grafts in mice treated with costimulatory blockade.196 Foxp3 cells can also be found in grafts with infiltrates interpreted as acute rejection.398 Although the significance of Foxp3+ cells has yet to be determined, high numbers of such Treg cells are likely beneficial,248 in view of the known suppressor functions of these cells. The hope of much ongoing research is the discovery of markers that predict graft acceptance in a clinical setting.100,248 The most important question is whether treatment of subclinical rejection is beneficial (and then what therapy is optimal). No study has dared to randomize treatment in patients with acute rejection on protocol biopsy. The closest to a controlled trial was that of Rush and colleagues, who found that patients with protocol biopsies, treated with steroid boluses if they had subclinical


Kidney Transplantation: Principles and Practice

rejection, had a better outcome than a group of patients who declined a renal biopsy (and were presumed to have a similar frequency of subclinical rejection).327 Other diseases revealed by the “eye of the needle” clearly benefit from altered therapy, including CNIT262,263 and polyomavirus infection.40

FUTURE DIRECTIONS IN BIOPSY ASSESSMENT Biopsy assessment will likely further improve from advances in image analysis techniques and molecular understanding. The contribution from a variety of “-omics” fields and technologies has led to improvements in allograft biopsy assessment.235,343 Molecular phenotypes have been characterized for a variety of pathologic states in renal allograft biopsies; however, the clinical utility of these molecular phenotypes will need additional validation before it is understood in what circumstances molecular assessment can be superior to histopathology. In addition, before molecular biopsy assessment is clinically feasible as an adjunct to histopathology, additional improvements are needed in molecular method turnaround time, cost, and the reporting required for high-dimensional “-omics” data. In addition, digital microscopic techniques (e.g., whole-slide scanning) are emerging which will likely improve biopsy assessment. Whole histology slide images contain highly detailed image information, allowing data mining through computer-based image analysis techniques. For example, interstitial fibrosis assessment can be automated; and automation can likely make interstitial fibrosis assessment more reproducible.91 Multiparameter staining techniques can also be coupled with digital imaging and analysis algorithms to provide more objective and quantitative assessment of molecular derangements in the renal biopsies. The advancements in technology and pathologic understanding will likely provide a more complete picture and allow enhanced patient care. Acknowledgments Many thanks to Dr. Shamila Mauiyyedi, a coauthor of the prior version, and to Dr. Paul J. Kurtin, for his useful suggestions on the manuscript. REFERENCES 1. Abouna GM, Al Adnani MS, Kremer GD, et al. Reversal of diabetic nephropathy in human cadaveric kidneys after transplantation into non-diabetic recipients. Lancet 1983;2:1274–6. 2. Adams DH, Tilney NL, Collins JJJ, et al. Experimental graft arteriosclerosis. I. The Lewis-to-F-344 allograft model. Transplantation 1992;53:1115–9. 3. Ahern AT, Artruc SB, DellaPelle P, et al. Hyperacute rejection of HLA-AB-identical renal allografts associated with B lymphocyte and endothelial reactive antibodies. Transplantation 1982;33:103–6. 4. Aiello FB, Calabrese F, Rigotti P, et al. Acute rejection and graft survival in renal transplanted patients with viral diseases. Mod Pathol 2004;17:189–96. 5. Akasaka Y, Ishikawa Y, Kato S, et al. Induction of Fas-mediated apoptosis in a human renal epithelial cell line by interferongamma: involvement of Fas-mediated apoptosis in acute renal rejection. Mod Pathol 1998;11:1107–14.

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