Expression of HLA molecules on hepatocytes: Does ursodeoxycholic acid prevent immunologic injurY?

Expression of HLA molecules on hepatocytes: Does ursodeoxycholic acid prevent immunologic injurY?

August 1990 single open reading frame that is similar to the amino acid sequence motif thought to encode RNA-dependent RNA polymerase and can be foun...

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August 1990

single open reading frame that is similar to the amino acid sequence motif thought to encode RNA-dependent RNA polymerase and can be found in all positive-strand RNA viruses. A 7.6-kb RNA transcript, detected in infected macaque liver only, hybridized with ET1.l; oligonucleotide probe studies suggested that the HEV genome is a positive, single-stranded RNA. A similarity to the caliciviruses is suggested. Comment. A decade ago, application of serological testing for HAV and hepatitis B virus (HBV) revealed the existence of waterborne, entericaIly trahsmitted outbreaks of hepatitis that could not , be attributed to either agent (Lancet 1980;2:876-879; Am J Med 1980:68:818-824). Subsequently, spherical viruslike particles were visualized by electron microscopy in fecal specimens of affected patienfs; these particles could be shown by immune electron microscopy to react with acute-phase patient sera (Intervirology 1983;20:23-31; J Gen Virol 1984;65:1005-1007). Similar particles were identified in stool samples after experimental transmission studies in human volunteers and nonhuman primates. Marmosets, cynomolgus monkeys (Proc Nat1 Acad Sci USA 1987;84:6277-6281), chimpanzees (Lancet 1988;1:550-553), and rhesus monkeys (Hepatology 1989;10:466-472) were shown to be susceptible to infection. While morb than one agent may ultimately be shown to be responsible for enterically transmitted non-A, non-B hepatitis, current information, supported by the data in this report, strongly suggests that HEV is the major etiologic player. Although only a handful of viral hepatitis cases have been attributed to imported HEV infections in the United States, largescale outbreaks of hepatitis E have been identified in diverse geographic regions in both the old and new worlds. The precise contribution of HEV to endemic hepatitis morbidity has been poorly understood in the absence of a simple, specific, sensitive, and inexpensive serological test. The techniques used thus far, namely immune electron microscopy and immunofluorescence staining (J Infect Dis 1989;159:1042-1049). remain cumbersome and are not widely available. Nonetheless, clinical experience indicates that HEV may be the principal etiologic agent of water-borne hepatitis in the third world and that it is responsible for the high frequency of fatal, fulminant hepatitis observed in pregnant women in affected parts of the world. Case fatality rates in pregnant women may reach and possibly exceed 20%. At this time there is no convincing evidence that HEV infections persist in nature, i.e., a carrier state has yet to be demonstrated in human beings, and chronic hepatitis, cirrhosis, and hepatocellular carcinoma do not appear to be established sequelae. This landmark work not only establishes HEV as an RNA virus, but it shouti provide the foundation for the development of diagnostic serological assays for HEV infection. Hepatitis E virus noti joins the three other known RNA-containing hepatitis virusesHAV. HCV, and HDV @)-but HAV hybridization probes have failed to reveal homology of cDNA from HAV with HEV. A genetic relationship to HCV and HDV also would seem unlikely. Evidence for more than one serological strain of HEV is not yet available and seems unlikely because second attacks of enterically transmitted non-A, non-B hepatitis are thought to be rare. Although HEV appears to resemble the caliciviruses in some characteristics, more work will be needed to determine whether HEV is a member of the calicivirus family. Hepatitis E virus has yet to be propagated in tissue culture systems, and evidence of cytopathogenicity has not been found after inoculation of the following cell lines with HEV: primary buffalo or African green monkey kidney, primary Rhesus monkey kidney, continuous fetal rhesus kidney, and continuous hunian embryonic kidney cell lines (JAMA 1984:252:3140-3145; J Gen Virol lQ84;65:1005-1007). These observations are consistent with limited morphological evaluations which suggest that hepatic

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necrosis in HEV infection is immunologically mediated and is not a direct cytopathic effect (Liver 1989:9:135-145). With the foundation established by this work, a wide array of investigations will certainly be undertaken in the near future. It should be possible to define the duration of fecal shedding of HEV with reasonable precision, to determine whether viremia occurs and for how long if so, and to determine whether HEV is present in body fluids other than bile, intestinal contents, and feces. Diagnostic assays should permit definition of the serological events underlying HEV infection and characterization of the protective, neutralizing antibodies. Hepatitis workers will remain busy! R. S. KOFF, M.D.

EXPRESSION OF HLA MOLECULE3 ON HEPATOCYTES: DOES URSODEOXYCHOLIC ACID PREVENT IMMUNOLOGIC IN JURY? Caimus Y, Cane P, Rouger P, Poupon R. [Service d’Hepatologie et Institut National de la Transfusion Sanguine, HBpital Saint-Antoine, 75012 Paris, France). Hepatic Expression of Class I and Class II Major Histocompatibility Complex Molecules in Primary Biliary Cirrhosis: Effect of Ursodeoxycholic Acid. Hepatology 1990;11(1):12-15. The authors studied the effect of ursodeoxycholic acid (UDCA) therapy on the expression of the major histocompatibility complex class I and II molecules in primary biliary cirrhosis (PBC). Liver biopsies were obtained in 9 untreated PBC patients, 8 UDCA-treated PBC patients (13-15 mg . kg’ - day-’ for l-3 years), and 8 patient controls who had no hepatobiliary disease. Specific monoclonal antibodies against class I or class II HLA molecules were prepared. Direct immunofluorescent techniques were used to evaluate the expression of these molecules in liver biopsy sections. In the control [normal] livers, the expression of HLA class I molecules was confined to the sinusoidal and biliary cells. In contrast, in the untreated PBC patients, the HLA class I molecules were also expressed on the hepatocytes. In the patients treated with UDCA, the expression of HLA class I molecules was significantly lower than in the untreated PBC patients. However, the abnormal expression of HLA class II molecules on the biliary cells was similar in the untreated and treated PBC patients. Comment. After having been used for a variety of gastrointestinal and hepatological disorders for many years outside of the United States, UDCA has recently been approved by the federal Food and Drug Administration for oral dissolution treatment of cholesterol gallstones. However, studies by several investigators have shown that UDCA is not only cholelitholytically effective and extraordinarily safe, but it also expresses hepatoprotective properties. Liver test abnormalities, which occur with the use of chenodeoxycholic acid alone, are not present when this bile acid is taken together with UDCA (Dig Dis Sci 1986;31:1032-1040). Leuschner et al.. who used UDCA therapy in patients who had both gallstones and chronic active hepatitis, noted that the abnormalities of liver-associated enzymes improved [Dig Dis Sci 1985;30:642-649). Subsequently, other investigators observed that UDCA has similar effects in other chronic liver diseases, such as PBC (Lancet 1987;1:834-836). According to recent data by Leuschner et al., UDCA also appears to

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GASTROENTEROLOGY

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improve the abnormal liver histology at the early stages of PBC (Gastroenterology 1989;97:1268-1274). The mechanism of the therapeutic action of UDCA in chronic liver diseases is unclear, but it has been related to the choleretic properties of this compound and to its ability to counteract the cholestatic and cyto(hepato)toxic effects of other bile acids. The present paper by Calmus et al. suggests that UDCA also reduces immunological alterations, which may be instrumental in the pathogenesis and progression of PBC. The finding that UDCA lowers the aberrant expression of HLA’ class I molecules on hepatocyte membranes may, as the authors point out, provide for a mechanism of the beneficial effect of UDCA in PBC. Because HLA class I molecules are thought to serve as a target for cytotoxic T cells, their reduction may suppress perlportal and lobular necrosis (Lancet 1984;2:1009-1013). One c’an only speculate how UDCA may decrease the expression of HLA class I molecules on hepatocyte membranes. In light of studies by other investigators in bile duct-ligated rats, Calmus et al. consider the Possibility that the abnormal expre.&on of HLA class I molecules is the direct result of cholestasis or of the accumulation of toxic bile acids._This chain of pathogenetic events would be reversed by the choleretic action of UDCA and by the change in bile acid composition that takes place during UDCA therapy. In addition to this possible mechanism of action of UDCA, we believe that this bile acid may also activate other functions that may play a role in the protection of the hepatocyte membranes against

cytotoxic and immunological injury. Studies in our laboratory have recently shown that UDCA increases receptor-dependent binding, uptake, and degradation of LDL in isolated hepatocytes. The mechanism of this direct effect of UDCA is unknown, but it represents yet another example of the ability of UDCA to modulate the expression of certain hepatocellular membrane-related functions. Other investigators have shown that UDCA changes the fluidity of membranes, such as that of erythrocytes (Gastroenterology 1989;97:1268-1274). Thus it appears possible that UDCA modifies the configuration of phospholipid-cholesterol layers of cell membranes and makes them more resistant to toxic insults. However, a note of caution is in order. The studies by Calmus et al. were not conducted in a controlled manner, i.e., the liver biopsies were not studied in the same patients before and after UDCA treatment. Therefore, further studies are needed to confirm the potentially very important observation by these authors. M. BORUM. M.D. H. FROMM. M.D.

EXCITING NEWS ON GASTRIC NEURONS Schemann M, ‘Wood JD. (Department of Physiology, College of Medicine, Ohio State University, Columbus, Ohio]. Electrical behavior of myenteric neurones in the gastric corpus of the guinea-pig. J Physiol (Lond) 1989;417:501-518 (September). The enteric nervous system is an independent third division of the autonomic nervous system. It controls many gastrointestinal functions: motility, absorption, secretion, and blood flow. Most electrophysiological studies of the enteric nervous system have been carried out on neurons in the small bowel: gastric intrinsic neurons have had virtually no study. Because the stomach has special functions in digestion and exhibits unique motor reflexes, the investigators believed that a study of the electrical and synaptic properties of the intramural gastric neurons was appropriate.

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Full-thickness sections of the wall of the guinea pig stomach were placed in a dish containing supporting medium, and the myenteric plexus was exposed by dissection of the mucosa and the circular muscle layer. Ganglia were visualized and immobilized with stainless steel wires. Glass electrodes, filled with 3 mol/L KCL, were used to record membrane potentials. Fast events were stored for analysis on a digital oscilloscope. All other events were recorded on videotape for subsequent analysis. The morphology of the gastric myenteric plexus was studied with a cholinesterase stain, which showed that gastric ganglia differ from intestin,al ganglia by their more stellate (rather than ovoid) shape and that each ganglion contains 10-50 nerve cell bodies. Four to six fiber tracts enter each ganglion, and the ganglia form the nodes in the meshwork of the myenterlc plexus. The morphology of individual neurons was outlined by electrophoresis of a fluorescent dye, Lucifer Yellow, into the cells using hyperpolarizing currents. This showed three cell shapes: [a) cells with a single long process and several short processes; (b) cells with two or more long processes; and (c) cells with one long process and several broad, club-shaped projections. The longest processes projected through threerow$ of ganglia. Few processes had varicosities, and these were often located on that segment of the process that was within the ganglion. By electrophysiological criteria, three types of neurons could be distinguished: gastric type I neurons accounted for half of all impaled neurons and were the most excitable ones. Injection of depolarizing currents into the cell soma triggered bursts of action potentials: with the strongest pulses, 10 or more action potentials occurred at a rate as rapid as 100 Hz. Type II neurons accounted for one third of all those impaled; their resting membrane potential of about -60 mV was similar to that of type I cells; however, the threshold for spike discharge in type II neurons was 31 mV, as compared with 39 mV for the type I neurons. The remaining impaled neurons of type III had a high resting

potential (-68 mV) and failed to discharge action potentials even when depolarized beyond a membrane potential of 0; still, their morphology and their nicotinic cholinergic input (described in the companion paper) showed that they were neurons rather than glial cells. In all neurons, the resting membrane potential could be accounted for largely by the K+ equilibration potential, with a lower conductance present in type I than in type III neurons. Action potentials in type I and type II neurons were blocked by tetrodotoxin, but some had a component of inward Ca2+ current. The hyperpolarizing after-potentials in type I and type II neurons differed from those in AH/type 2 intestinal neurons (a) in that there was no latency between the after-potential of the spike and the onset of the hyperpolarization and (b) in its much shorter duration. The study indicates that gastric intramural neurons, like their intestinal counterparts, can be differentiated into subgroups according to their electrophysiological properties. Type I gastric neurons differed from type I intestinal neurons in their higher input resistance and infrequent anodal breakthrough. Because the neural networks of the stomach control functions that differ considerably from those of the small bowel, some specialization in the gastric neurons of the enteric nervoussystem should come as no surprise.