Herpes simplex virus vaccines

Herpes simplex virus vaccines

SECTION THREE: Vaccines in development and new vaccine strategies 50 Herpes simplex virus vaccines Lawrence R. Stanberry Robert B. Belshe O'er ladi...

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SECTION THREE: Vaccines in development and new vaccine strategies

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Herpes simplex virus vaccines Lawrence R. Stanberry Robert B. Belshe

O'er ladies’ lips, who straight on kisses dream,Which oft the angry Mab with blisters plagues.

William Shakespeare in Romeo and Juliet

Over 2,500 years ago Hippocrates used the Greek term herpes, meaning “to creep”, to describe spreading skin lesions.1 Since that time, mucocutaneous herpes simplex virus (HSV) infections have been viewed largely as nuisance diseases because most are self-limiting. This rather uninformed view fails to consider that HSV infections can be life threatening in the newborn, the severely malnourished, and the immunocompromised host; leave survivors severely neurologically impaired; result in blindness; trigger bouts of erythema multiforme; and that genital herpes increases threefold the risk of human immunodeficiency virus (HIV) acquisition.2

Background Clinical description The clinical illness caused by HSV is determined by the portal of entry, the immune status of the host, and whether the infection is primary or recurrent. Genital herpes classically presents with vesiculoulcerative skin lesions that appear on mucosal and keratinized epithelium as well as around the anal opening, buttocks, and/or thighs. Mild infections lack, instead, classic findings and may not be recognized as genital herpes3 but may have nonpathognomonic findings, including tingling, burning, itching, erythematous patches, and small skin fissures. Greater than 80% of primary genital HSV infections are asymptomatic or unrecognized,4 and asymptomatic recurrent infection or asymptomatic viral shedding is also more common than clinically recognized recurrence and occur both in those with a history of symptomatic infections and those lacking such history.5 Although HSV-1 and HSV-2 can cause clinically indistinguishable primary infections, they differ in their ability to cause recurrent infections, with HSV-1 causing significantly fewer episodes of recurrent genital infection than HSV-2,6 while the converse is true for oral-labial HSV infections. Common oral-facial infections are generally caused by HSV-1.7 Herpes gingivostomatitis is characterized by oral pain, fever (40° C-40.6° C), and vesiculoulcerative lesions throughout the mouth and pharynx. Herpes pharyngitis is less severe than gingivostomatitis, with lesions generally limited to the tonsils and pharynx.8 Herpes labialis is the most common recurrent oralfacial HSV infection and often is heralded by prodromal symptoms, including tingling, itching, burning, or pain before the

development of the herpetic lesions, which typically develop on the vermilion border of the lip, although lesions sometimes occur on the nose, chin, cheek, or oral mucosa.9 HSV can also cause other skin infections, including herpes gladiatorum or scrum pox, herpes whitlow, and potentially lifethreatening HSV infections among persons with skin disorders.10,11 HSV infection of the eye may be primary or recurrent and is usually unilateral and may involve the conjunctiva, cornea, or retina.12 Conjunctivitis is the most common ocular HSV infection. The distinguishing feature of corneal infection is dendritic or geographic ulcers. Beyond the newborn period, HSV encephalitis almost always results from HSV-1 and typically involves the frontal and/or temporal cortex. HSV is the most common cause of recurrent aseptic (Mollaret) meningitis, and aseptic meningitis is seen in up to 15% of cases of primary genital herpes.13,14 Perinatal HSV infections are life threatening and most commonly result from mother-to-infant transmission.15 The clinical manifestations are variable but broadly fall into one of three patterns of disease: (1) disease localized to the skin, eyes, or mouth (SEM); (2) encephalitis; or (3) disseminated infection. HSV infections in the immunocompromised host, particularly those with compromised cellular immunity such as seen with acquired immunodeficiency disease (AIDS), may be severe and life threatening.16,17

Complications There are myriad complications associated with HSV infection. Acute genital herpes infection may be associated with urologic and neurologic complications, while recurrent genital herpes can cause intermittent and persistent neurologic signs and symptoms, significant psychologic morbidity,18 and significantly increases the risk of HIV acquisition and transmission.2 Cutaneous infections can result in erythema multiforme. Ocular HSV infections can result in blindness. Patients surviving HSV encephalitis may have severe neurologic sequelae, including blindness, deafness, cerebral palsy, and profound mental retardation. Death can result from infections in patients who are immunocompromised, those with immature immune systems, and those with skin disorders. There is also growing evidence that HSV neuronal infection may play a role in the pathogenesis of Alzheimer disease.19

Virology and viral antigens HSV contains a double-stranded DNA genome of ≈152 kilobases that encodes at least 84 proteins. The viral genome resides within an icosadeltahedral capsid that is surrounded by an outer

Herpes simplex virus vaccines Herpes virus Tegument Lipid bilayer DNA core Capsid Envelope glycoprotein spikes (peplomers; gB–gN)

Figure 50-1 Herpes simplex virus structure.

envelope composed of a lipid bilayer containing at least 12 viral glycoproteins (Figure 50–1). The envelope glycoproteins are the major targets for humoral immunity; gB, gC, gD, or gH/gL antibodies inhibit the absorption or neutralize HSV in vitro, and antibodies to gB or gD inhibit neuron-to-keratinocyte spread in a cell culture system.20 CD4 and CD8 cellular responses can be induced by a wide array of HSV proteins, including glycoproteins (gB, gC, gD, gE, and gH), structural capsid and tegument proteins (gene products of UL7, UL21, UL25, UL26, UL29, UL47, UL48, UL49, and the proteins VP5, VP11/12, VP13/14, VP16, VP22, and ICP8), and nonstructural proteins (ICP0, ICP4, and ICP27 and the gene product of UL5020). HSV-1 and HSV-2 have a similar genetic composition, genome organization, and extensive deoxyribonucleic acid (DNA) and protein homology. One notable difference exists between the HSV-1 and HSV—two glycoprotein G genes. This difference forms the basis for the current type-specific serologic tests that are used to discriminate accurately whether a person has been infected with HSV-1, HSV-2, or both.21

Pathogenesis as it relates to prevention The pathogenesis of mucocutaneous HSV infections involves replication of virus at the portal of entry and uptake and spread via nerve fibers to regional sensory ganglia, where further viral replication occurs and where latent infection is established.22 Animal studies indicate that challenge with low doses of virus is sufficient to cause infection but that higher doses are required to cause clinically apparent illness.23 During the initial infection, innate and adaptive humoral and cellular immune responses limit viral replication and allow control of the acute disease. Despite these responses, the latent virus can periodically reactivate in sensory ganglia to produce progeny viruses, which are transported to the periphery, where further replication occurs. This peripheral replication can result in symptomatic recurrent infection or asymptomatic shedding. It is important to note that hematogenous spread does not play an important role in the pathogenesis of infection, except in the host with an immature or compromised immune system.16 In considering the development of vaccines to control HSV infections, an understanding of the pathogenesis provides four insights: first, the lack of hematogenous viral dissemination in healthy persons means that an HSV vaccine cannot target prevention of viral spread in the bloodstream, a step which has proved crucial for other pathogens; second, higher doses of virus are required to cause disease than subclinical infection, suggesting that if a vaccine cannot prevent infection, it might be able to shift symptomatic disease to subclinical infection; third, the ability of the virus to reactivate and cause

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recurrent infections, despite the host engendering a variety of virus-specific adaptive responses, suggests that development of a vaccine cannot simply be based upon determining the dominant immune responses that result from natural infection, a strategy that has successfully led to the development of other vaccines; and fourth, HSV vaccines may need to target the portal of entry (such as the genital or oral mucosa) or possibly protect the nerve and ganglia against infection.

Diagnosis HSV infections should be suspected based upon the history of exposure and clinical findings, particularly vesicular lesions; however, the clinical diagnosis of genital herpes has been shown to be remarkably unreliable.24 Suspicion of an HSV infection requires laboratory confirmation. Virus culture is the gold standard for mucocutaneous HSV infections. Virus typing should be done in conjunction with culture for suspected genital herpes because virus type is important in predicting the frequency of subsequent recurrent infections. Although not as sensitive as culture, direct detection of HSV antigens in clinical specimens can be done rapidly and has very good specificity.25 The use of a polymerase chain reaction (PCR)-based assay for detection of HSV DNA is highly sensitive and specific, and, although of somewhat limited availability, it is emerging in routine clinical use and is the test of choice in examining cerebrospinal fluid in cases of suspected HSV encephalitis or meningitis.26 Serologic tests can be used to support the clinical diagnosis of HSV infection in patients who were diagnosed too late in the course of an acute illness for viral culture or viral antigen or DNA assays to be accurate. IgM assays are unreliable, and their interpretation is made difficult because recurrent infections can trigger an anamnestic IgM response that can be misinterpreted as indicating an acute primary infection. To be helpful, IgG tests should be type specific21 and their results interpreted in the context of the disease manifestations and clinical history.

Treatment and prevention with antivirals All symptomatic primary HSV infections probably warrant treatment. Acyclovir, valacyclovir, and famciclovir are the principal drugs used in the management of HSV infections. Early initiation of therapy results in maximum therapeutic benefit but does not prevent the establishment of the latent infection. Likewise, treatment of recurrent infections does not eradicate the latent virus infection. Less commonly used drugs include foscarnet, cidofovir, trifluorothymidine, brivudin, and idoxuridine. A landmark study determined that the chronic use of valacyclovir by patients with HSV-2 genital herpes could reduce the risk of transmission of infection to a susceptible heterosexual partner by 48%.27

Epidemiology Seroprevalence HSV-1 infection is generally spread through contact with contaminated oral secretions. HSV-1 seroprevalence rises rapidly in the first decade of life, with 50% of European and 70% of American 10-year-old children being seropositive.28 By the sixth and seventh decades, 70% to 80% of adult Americans and Europeans are HSV-1 seropositive. Although data on HSV-1 seroprevalence in the developing world are limited, studies suggest greater than 90% of adults in Africa and Asia are HSV-1 seropositive.29 In contrast to HSV-1, HSV-2 seroprevalence is essentially zero in the first decade of life, rising sharply with the

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SECTION THREE • Vaccines in development and new vaccine strategies

onset of sexual activity during adolescence, thus r­eflecting the primary mode of transmission. HSV-2 seroprevalence rates tend to increase steadily throughout the third and fourth decades of life among Europeans and Americans, reaching a peak of about 20% in Europe and 30% in the United States among generally low-risk populations but with peak rates of approximately 45% and 60% for high-risk populations in Europe and the United States, respectively.28 In general, Central and South America have HSV-2 seroprevalence rates similar to the United States and Europe, while parts of Africa generally have much higher rates, and Asia tends to be lower; however, in any geographic area among different populations, there are great variations in HSV-2 seroprevalence rates.29 Two American longitudinal studies provide some estimate of the rate of HSV-2 acquisition. In a vaccine trial involving high-risk volunteers ­followed prospectively, gender was shown to influence acquisition of HSV-2 infection, with a rate of 3.8 cases per 100 person-years of ­follow-up for men compared with 5.8 for women.30 In this study, preexisting immunity to HSV-1 did not influence the rate of acquisition of HSV-2 infection but did increase the proportion of infections that were asymptomatic. A study of adolescent girls found an acquisition rate of 4.4 cases per 100 person-years of sexual activity for HSV-2 infection and reported that preexisting HSV-1 immunity afforded girls protection against both HSV-2 infection and disease.31 The impact of preexisting HSV-1 immunity on HSV-2 acquisition is controversial but needs to be considered in designing vaccine trials aimed at preventing HSV-2 infection or disease. Incidence data are limited and underrepresent the magnitude of HSV infections because most HSV infections are asymptomatic.32 Clinical studies of patients presenting with HSV disease indicate that most oral and nongenital skin infections, as well as central nervous system infections outside the neonatal period, are caused by HSV-1.32

Active immunization History of vaccine development The history of HSV vaccine development dates from the 1930s.33 The initial work focused on therapeutic vaccines intended for the control of recurrent mucocutaneous HSV infections. Clinical assessment of prophylactic vaccines did not begin until the late 1940s.34 Two major problems hampered early research: the lack of defined vaccine products and fatally flawed design of clinical trials. Early vaccines were crude preparations exemplified by the 1938 vaccine developed by Frank,35 which consisted of a formalin-treated HSV-2–infected brain emulsion prepared from rabbits dying of encephalitis. These products were evaluated in open trials involving patients with oral, ocular, or genital HSV infections and using subjective patient assessment of improvement. The importance of randomized, blinded, placebocontrolled clinical trials was not appreciated until the landmark study of Kern and Schiff in 1964,36 which demonstrated comparable self-reported improvement in the frequency of recurrences for both vaccine and placebo recipients. In the 1940s and 1950s vaccines were made from virus grown in embryonated eggs and inactivated by physical means. By the 1960s vaccines were prepared by chemical or physical inactivation of virus grown in cell cultures. Between 1946 and 1982, more than two dozen clinical trials of experimental vaccines were reported, but no vaccine was convincingly proven to be effective.37 The modern era of HSV vaccine development, begun in the 1980s and 1990s, has been largely unsuccessful (summarized in Table 50-1). However, two studies conducted in the 1990s were successful in establishing the feasibility of developing therapeutic and prophylactic HSV vaccines.

Therapeutic genital herpes vaccine Burke and colleagues46 at Chiron (Emeryville, CA) developed a subunit vaccine consisting of recombinant HSV-2 glycoprotein D (gD2) produced by genetic engineering in Chinese hamster ovary (CHO) cells combined with aluminum hydroxide (alum) as an adjuvant. Based on animal studies supporting the concept of a therapeutic HSV vaccine,47 Straus and ­colleagues48 ­ conducted a double-blind, placebo-controlled clinical trial investigating the effect of the vaccine on the ­frequency of symptomatic outbreaks in patients with genital herpes. Ninety-eight patients with documented genital herpes, who reported 4 to 14 recurrences per year, were randomized to receive deltoid injections of either 100 μg gD2 in alum or alum alone (­placebo) at 0 and 2 months. Clinically recognizable recurrences were documented for 1 year. The vaccinees experienced fewer monthly recurrences than placebo recipients (mean, 0.42 ± 0.05 ­ versus 0.55 ± 0.05, respectively), had significantly fewer virologically confirmed monthly recurrences (mean, 0.18 ± 0.03 versus 0.28 ± 0.03, respectively), and had a significantly lower median number of recurrences for the study year (4 [range, 0–17] v­ ersus 6 [0–15], respectively). The overall effect appeared greater during the first 4 months of the trial. By the nature of the study, all p ­ articipants were HSV-2 seropositive, but the gD2 v­ accine boosted neutralizing HSV-2 antibodies fourfold and gD2-­ specific titers sevenfold over baseline levels. Although the overall effect was transient and modest, the results did establish the feasibility of treating an established HSV infection with a therapeutic vaccine. A follow-up Chiron study using recombinant HSV-2 glycoproteins B and D combined with MF59, a squalene oil-inwater emulsion adjuvant, failed to demonstrate a therapeutic benefit in patients with recurrent genital herpes.44 It is possible that the failure resulted from use of lower amounts of the HSV glycoproteins in the second trial (10 μg each of gB2 and gD2) compared to the first trial (100 μg gD2); however, it has also been postulated that the failure of the s­ econd vaccine might have been due to the selection of the MF59 adjuvant, which principally a­ ugments antibody responses.49 Important questions remain unanswered, including what were the key immunologic mediators of the effect (eg, cellular versus antibody responses); what was the critical site of the effect (eg, genital skin or latently infected sensory ganglia), and could ­therapeutic vaccines impact asymptomatic shedding and transmission as well as symptomatic recurrences. Animal studies indicate that induction or enhancement of HSV-specific cellular immune responses are required for a therapeutic effect and that therapeutic vaccines can reduce clinically recognizable recurrences as well as viral shedding.50,51

Prophylactic genital herpes vaccines In the 1980s, Chiron developed and tested a prophylactic HSV-2 vaccine consisting of truncated HSV-2 glycoproteins gD2 and gB2 with the adjuvant MF59. The vaccine was shown to be highly immunogenic but was ineffective in two randomized, double-blind, placebo-controlled studies.30 The first study (Partners Trial) enrolled 531 HSV-2–seronegative sexual partners of HSV-2–infected persons, while the second study (STI Clinic Trial) enrolled 1862 high-risk persons from sexually transmitted infection (STI) clinics. The trials were placebo controlled, with a 0, 1, 6 month immunization schedule and an 18-month study duration. The primary outcome measure was acquisition of HSV-2 infection as assessed by seroconversion to HSV-2 antigens not present in the vaccine. The data from the two studies were combined for the analysis, and although the overall efficacy of the vaccine was only 9% during the entirety of the studies, in the first 5 months, the rate of HSV-2 acquisition was 50% lower among vaccine recipients compared with

Herpes simplex virus vaccines

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Table 50-1 HSV Vaccines Not Proven Effective in Clinical Trials Developer or manufacturer

Application

Comment

Reference

Cell culture–derived inactivated HSV-1

Skinner

Prophylaxis genital herpes

Clinical trials used historic controls, efficacy not established

38, 39

Cell culture–derived inactivated HSV-1

Skinner

Therapeutic genital herpes

No clinical benefit in controlled trials

40

Egg-derived inactivated HSV-1 (Lupidon H) or HSV-2 (Lupidon G)

Bruschettini

Therapeutic mucocutaneous herpes

Patients with oral and genital herpes included in same study, making interpretation of results impossible

41

HSV-2 deletion mutant

Sanofi-Pasteur

Prophylaxis genital herpes

Minimally immunogenic in phase 1 trial

42

Cell culture–derived HSV-2 glycoproteins

Merck

Prophylaxis genital herpes

Shown to be ineffective in a well-designed clinical study

43

Recombinant HSV-2 glycoproteins B and D with MF59

Chiron (now Novartis Vaccines)

Prophylaxis genital herpes

Shown to be ineffective in a well-designed clinical study

30

Recombinant HSV-2 glycoproteins B and D with MF59

Chiron (now Novartis Vaccines)

Therapeutic genital herpes

Shown to be ineffective in a well designed clinical study

44

Glycoprotein H–deleted HSV-2 mutant

GlaxoWellcome/ Cantab

Therapeutic genital herpes

Shown to be ineffective in a well-designed clinical study

45

Vaccine

placebo recipients. A post hoc analysis of the data suggested there might be a difference in efficacy by gender, with an efficacy rate of 26% in women compared with 4% in men. It may be worth noting that there appeared to be a trend toward protection in the Partners Trial in the vaccinated women compared with the placebo recipients (4.0 vs. 8.8 cases per 100 personyears, respectively); however, the same trend was not observed in women enrolled from STI clinics (5.5 cases for vaccine recipients vs. 5.4 cases per 100 person-years for placebo recipients) (Table 50–2). The reasons why the adjuvanted subunit vaccine failed, despite inducing high titers of HSV-2–specific antibody, are unclear, and likewise, the explanation for transient protection in women is uncertain. The apparent trend toward protection in vaccinated women in the Partners Trial, but not the STI Clinic Trial, suggests that these two populations of women may respond to vaccines differently and that women in HSV-2–discordant relationships who are repetitively exposed to subinfectious doses of virus may be immunologically primed and hence respond more favorably to immunization. In parallel with the Chiron HSV vaccine program, GlaxoSmithKline Biologicals (Wavre, Belgium) was developing a subunit vaccine containing a CHO cell–derived truncated gD2 antigen (20 μg) combined with alum (500 μg) and the potent adjuvant 3-de-O-acylated monophosphoryl lipid A (3D-MPL) (50 μg). In a phase 1 evaluation, the vaccine was shown to be well tolerated and capable of inducing humoral and cellular immune responses superior to gD2 and alum alone.52 In the 1990s the vaccine was evaluated in two multinational double-blind, randomized, placebo-controlled consort studies.53 The first study (007 Trial) enrolled 847 HSV-1– and HSV-2–seronegative partners (268 women) of HSV-2–infected persons, and the second study (017 Trial) enrolled 1867 HSV-2– seronegative partners (710 women) of HSV-2–infected persons. The vaccine or control injection (alum-MPL in first study and alum in the second study) was administered intramuscularly at 0, 1, and 6 months. The study duration was 19 months. The primary outcome measure was protection against HSV-2 disease; in the first study the primary outcome was assessed in

all participants (men and women), while in the second study, the primary outcome was assessed in women only, with assessment in men being a secondary outcome measure. In the first study, the HSV-2–infected partners were required to abstain from using suppressive antiherpes therapy but were allowed to do so in the second study. Overall, in both studies, the protection against HSV-2 disease offered by the vaccine and alum control were similar: the efficacy of the vaccine in the first study (007 Trial) was 38% in HSV-1–and HSV-2–seronegative men and women, while in the second study (017 Trial), it was 42% in HSV-2–seronegative women participants (see Table 50–2). In both studies, there was a nonsignificant trend toward protection of HSV-seronegative women against HSV-2 infection (vaccine efficacy of 46% in the first study and 39% in the second study; P = .08 in each study). However, the gD2–alum-MPL vaccine afforded significant protection (vaccine efficacy of 73%-74%) against genital HSV-2 disease in women who had no preexisting antibody to either HSV-1 or HSV-2. There was no evidence that the vaccine was protective in women who were initially HSV-1 seropositive or in men. In the phase 3 trials, the vaccine was safe, generally well tolerated, and induced gD-specific neutralizing antibodies and a T-helper (Th1) cell-mediated immune profile. Because neither study met its predetermined primary outcome (ie, for study one, prevention of genital herpes disease in HSV-2–seronegative men and women; for study 2, prevention of genital herpes disease in HSV-2–seronegative women), a third study (the HERPEVAC Trial for women) was planned and undertaken. During the course of the HERPEVAC study, the results of a previously initiated multinational safety and immunogenicity trial became available. The study, involving 7460 volunteers (4968 in the vaccine group and 2492 in the placebo group), found the vaccine to be generally safe but did find significantly more local reactions, especially soreness, compared with placebo.54 The reactions tended to be mild to moderate in severity, lasting less than 3 days, although severe local reactions were noted in 7.0% of vaccine doses and 1.2% of placebo doses.

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SECTION THREE • Vaccines in development and new vaccine strategies Table 50-2 HSV-2 Attack Rates and Vaccine Efficacy in Four HSV Vaccine Trials Chiron gB/gD-MF59 vaccine Partners Trial

Overall attack rate

GSK gD-AS04 vaccine

STI Clinic Trial

Control

Vaccine

Control

Vaccine

Control

Vaccine

Control

3.4

4.6

4.4

4.6

3.5 (1.7 to 5.3)*

6.0 (3.7 to 8.5)

3.0 (1.8 to 4.2)

3.6 (2.3 to 4.9)

38% (− 18 to 68) 2.8

1.1

4.1

4.3

3.7 (1.5-6.0)

11% (− 20 to 34) 3.7 (1.4-5.9)

− 11% (− 161 to 53)

Vaccine efficacy in men Attack rate in women

017 Trial†

Vaccine

Overall vaccine efficacy Attack rate in men

007 Trial*

4.0

8.8

5.5

5.4

Vaccine efficacy in women

3.1 (0.1 to 6.1)

2.8 (1.4 to 4.2)

2.7 (1.3 to 4.1)

− 10% (− 127 to 47) 11.6 (5.9 to 17.4)

73% (19 to 91)

3.3 (1.1 to 5.4)

5.0 (2.6 to 7.5)

42% (− 31 to 74)

Attack rate in HSV-1–negative participants

3.5

6.3

5.1

4.4

Same as overall

Same as overall

3.4 (1.2 to 5.6)

8.3 (4.8 to 11.8)

Attack rate in HSV-1–positive participants

3.4

3.5

4.1

4.7

NA

NA

ND

ND

Attack rate in HSV-1–negative women

ND

ND

ND

ND

As above for women

As above for women

3.5 (0 to 7.4)

13.3 (6.3 to 20.3)

Vaccine efficacy in HSV-1–negative women Attack rate in HSV-1–negative men

As above for women

ND

ND

ND

ND

Vaccine efficacy in HSV-1– negative men

As above for men As above for men

74% (9 to 93)

As above for men

3.3 (0.7 to 6.0)

5.3 (1.7 to 8.9)

32% (− 95 to 76)

*007 Trial: participants HSV-2 and HSV-1 seronegative. † 017 Trial: participants HSV-2 seronegative but could be HSV-1 seropositive or seronegative. Chiron trials: primary outcome measure—prevention of HSV-2 infection. GlaxoSmithKline (GSK) trials: primary outcome measure—prevention of HSV-2 disease. Attack rates are per 100 person-years. Values in parentheses are the 95% confidence intervals; confidence interval data not given for Chiron trials. HSV, Herpes simplex virus; NA, not applicable; ND, not determined; STI, sexually transmitted infection. Data from Corey L, Langenberg AG, Ashley R, et al. Recombinant glycoprotein vaccine for the prevention of genital HSV-2 infection: two randomized controlled trials. JAMA 282:331–340, 1999; Stanberry LR, Spruance SL, Cunningham AL, et al. Glycoprotein-D-adjuvant vaccine to prevent genital herpes. N Engl J Med 347:1652–1661, 2002.

The rate of local reactions did not increase with subsequent vaccine administrations or in subjects with preexisting HSV immunity. Of interest, women receiving either the vaccine or placebo reported significantly more local and systemic adverse reactions than did men. Regarding immunogenicity, the vaccine induced higher titers of HSV gD–specific antibody than occurs after HSV infection. The third phase 3 trial (the HERPEVAC study) was cosponsored by GlaxoSmithKline (GSK) and the National Institutes of Health (NIH) and differed from the previous two phase 3 trials in that the participants were young HSV–seronegative women who were more representative of the general population compared with participants in the previous trials and were at risk of genital HSV-2 infection because they were in a relationship with

a sexual partner who had a history of recurrent genital herpes.74 The randomized, double-blinded trial was conducted with 8,323 women aged 18 to 30 years. The primary endpoint was reduction in genital disease caused by either type 1 or type 2 HSV; but the vaccine did not meet this endpoint. Overall vaccine efficacy was 20% confidence interval ([CI], − 29 to 50) against genital herpes disease; however, type-specific efficacy was found against HSV-1 but not HSV-2. After three doses of vaccine, the vaccine efficacy against culture-positive genital HSV-1 disease was 82% CI (35–95). Vaccine efficacy against HSV-1 infection (with or without disease) was 35% CI (13–52), but efficacy against HSV-2 infection was not observed (− 8% CI [− 59 to 26]). A substudy examined whether immunized women who became infected had comparable patterns of ­ asymptomatic

Herpes simplex virus vaccines

viral shedding to those observed in unimmunized HSV– infected women or whether they shed less virus, as has been shown in experimental animal studies.55 The issue of asymptomatic shedding has important public health implications.56 The substudy enrolled 43 participants (30 HSV vaccine recipients and 13 control subjects) who acquired HSV-2 infection during the trial. The women collected anogenital swabs on 60 consecutive days for quantitation of HSV-2 genome, beginning 3 to 6 months after disease onset (24 women, 15 in the HSV vaccine group, and 9 in the control group), or seroconversion (19 women, 15 in the HSV vaccine group and 4 in the control group). Although the rate of viral shedding was higher among the HSV vaccine recipients compared with controls, 29% versus 17%, respectively, (relative risk [RR] = 1.55, 95% CI [1.281.86]), the mean quantity of HSV DNA on days with shedding did not differ between the two groups. The lack of vaccine efficacy against HSV-2 in the HEPEVAC trial is puzzling in view of the previous two gD2 vaccine studies in discordant couples that found efficacy against HSV-2. The distinguishing feature of discordant couples is that they are a highly select group in which one uninfected partner is potentially repeatedly exposed to HSV by an infected partner. Attack rates of HSV-2 genital disease in the prior GSK gD2 vaccine studies in discordant couples were high in the uninfected women (13.9% for 19 months or 8.4% per year) and were reduced significantly by vaccine (VE (vaccine efficacy)= 73% and 74% in the two trials; P <.05 in each case).53 Likewise, the Chiron gB2/ gD2 vaccine appeared to have some effect in vaccinated women in the discordant couples (partners) trial but not in women who were recruited from STI clinics.30 Potential reasons why the gD2 vaccine might protect women who were in a long-term relationship with an HSV-2–infected male partner (mean duration of relationship was 23 months) but not women in other highrisk relationships might include (1) an undefined immunologic priming event from chronic sexual exposure to HSV-2 antigens from the infected partner,57 which could be boosted by the subunit vaccine; (2) selection bias for a population of relatively HSV-2–resistant women who developed enhanced resistance because of the gD2 vaccine; or (3) that women in a long-term relationship have less frequent58 and perhaps less intense or less traumatic exposure to HSV-2 because of the long-term nature of the relationship. Vaccine efficacy against genital HSV-1 infections was demonstrated in the HERPEVAC trial but not in the original two GSK discordant couple studies because too few cases of HSV-1 genital disease occurred in the original studies. As the HERPEVAC study recruited women from the general population, it was anticipated that approximately 30% of the incident cases of symptomatic genital herpes would be due to HSV-1. The gD2 antigen is derived from HSV-2 but shares significant homology with HSV-1 gD, which may explain the protection against HSV-1. The finding that the gD2 vaccine protected women recruited from the general population against HSV-1 genital infection but not HSV-2 genital infection might indicate some important difference in the immunobiology of the two viruses and type-specific immune responses to vaccine antigen may reveal differences in antibody activity versus HSV-1 or HSV-2 or whether HSV-1 is more easily neutralized by vaccine-induced antibodies. It is also possible that the vaccine efficacy is influenced by the mode of virus transmission. It is generally believed that most cases of genital HSV-1 infection result from oral-genital sex rather than penile-vaginal sex.59 Perhaps oral-genital sex results in exposure to lower concentrations of virus; possibly microabrasions associated with penile-vaginal sex facilitates transmission; or perhaps the cervical epithelium reached during penile-vaginal sex is more susceptible to HSV than the cells of the vaginal introitus, the cells most generally exposed during oral-genital sex. At this time, we can only speculate on the biologic basis for

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the efficacy against HSV-1 genital infection but not against HSV-2 genital infection. In comparing the results of the Chiron and GlaxoSmithKline vaccine programs, it may be worth noting that a major difference in the two vaccines was the adjuvant. The Chiron vaccine contained MF59, while the GlaxoSmithKline vaccine contained alum and MPL. The partial effectiveness of this glycoprotein vaccine compared with the failure of the Chiron glycoprotein vaccine suggests that adjuvant may be critical in facilitating the induction of important protective immune responses. More research is needed to understand the adjuvant effects as well as the unexpected gender-specific protection seen with the gD2-alum-MPL vaccine. Another important unanswered question is whether a vaccine that can protect against genital herpes can afford protection against nongenital HSV diseases.60

Correlates of protection The critical immune responses required of an effective prophylactic or therapeutic HSV vaccine are unknown, although it is widely held that they are likely to be different for the two types of products.61,62 Because HSV has evolved several strategies to evade the immune system62 and because HSV infection in one anatomic site provides, at best, limited protection against HSV infection at a second site,63,64 an evaluation of host responses to infection may not be a useful strategy for identifying protective immune responses. Animal studies suggest that neutralizing antibody is essential for prophylactic vaccination, while the results of clinical trials suggest that robust T-cell responses will also be necessary for effective prophylaxis and induction of effective immune responses will likely require some balance between innate and adaptive responses.65 It is likely that the correlates of protection for prophylactic HSV vaccines will be both neutralizing antibody and T-cell responses greater than those seen in infected persons. At this point, it is unclear whether CD4+ or CD8+ T cells will be more important in affording protection against infection. Regarding therapeutic vaccines, it is clear that antibody is not a correlate of protection, and most evidence currently suggests that HSV-specific CD8+ T-cell responses will be the key to protection. Recent studies on HSV CD8 T cells in animal ganglia suggest that latent infection may lead to CD8 T-cell exhaustion and hence compromise the ability of therapeutic vaccines to engender robust CD8 T-cell augmentation.66

Public health considerations HSV infections are a global public health problem, both as a consequence of HSV-induced disease and indirectly by enhancing the acquisition and transmission of HIV.2,29 The United States is experiencing an ongoing epidemic of genital HSV infections.4 The potential impact of the partially effective GlaxoSmithKline prophylactic vaccine will be limited by its gender specificity and its lack of efficacy in HSV-1–seropositive women, a particular problem for the developing world, where most individuals acquire nongenital HSV-1 infections in early life. Nevertheless, mathematical modeling of universal immunization of young women in the United States suggests that a partially effective vaccine could have a significant impact on the ongoing genital herpes epidemic.67

Future vaccines Currently, there is only limited clinical research directed at developing HSV vaccines. Wyeth (Madison, NJ) (now Pfizer, [New York, NY]) conducted a phase 1 trial of a DNA vaccine administered by a needle-free device; the experimental vaccine was well tolerated but appeared to be poorly immunogenic.68

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SECTION THREE • Vaccines in development and new vaccine strategies

In a phase 1 study, Antigenics (New York, NY) assessed the safety and immunogenicity of a vaccine consisting of an HLA A*0201– restricted epitope in HSV-2 gB and truncated human constitutive heat shock protein 70.69 The HSV ­peptide–heat shock chaperone vaccine appeared safe but was not i­mmunogenic. Other

s­trategies still in preclinical development include replicationimpaired and replication-competent attenuated HSV mutants, DNA vaccines, inactivated vaccines, ­prime-boost strategies, and vectored vaccines, including use of the neonatal Fc receptor for mucosal immunization.70–73

Access the complete reference list online at  http://www.expertconsult.com 2. 24. 29. 30. 53.

Freeman EE, Weiss HA, Glynn JR, et al. Herpes simplex virus 2 infection increases HIV acquisition in men and women: systematic review and metaanalysis of longitudinal studies. AIDS 2006;20:73–83. Langenberg AG, Corey L, Ashley RL, et al. A prospective study of new infections with herpes simplex virus type 1 and type 2. N Engl J Med 1999;341:1432–8. Smith JS, Robinson NJ. Age-specific prevalence of infection with herpes simplex virus types 2 and 1: a global review. J Infect Dis 2002;186(Suppl. 1):S3–28. Corey L, Langenberg AG, Ashley R, et al. Recombinant glycoprotein vaccine for the prevention of genital HSV-2 infection: two randomized controlled trials. JAMA 1999;282:331–40. Stanberry LR, Spruance SL, Cunningham AL, et al. Glycoprotein-D-adjuvant vaccine to prevent genital herpes. N Engl J Med 2002;347:1652–61.

54. Bernstein DI, Aoki FY, Tyring SK, et al. Safety and immunogenicity of glycoprotein D-adjuvant genital herpes vaccine. Clin Infect Dis 2005;40:1271–81. 56. Garnett GP, Dubin G, Slaoui M, et al. The potential epidemiological impact of a genital herpes vaccine for women. Sex Transm Infect 2004;80:24–9. 57. Posavad CM, Remington M, Mueller DE, et al. Detailed characterization of T cell responses to herpes simplex virus-2 in immune seronegative persons. J Immunol 2010;184:3250–9. 60. Stanberry LR, Cunningham AL, Mindel A, et al. Prospects for control of herpes simplex virus disease through immunization. Clin Infect Dis 2000;30:549–66. 63. Koelle DM, Corey L. Recent progress in herpes simplex virus immunobiology and vaccine research. Clin Microbiol Rev 2003;16:96–113.

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