Ocular herpes simplex: changing epidemiology, emerging disease patterns, and the potential of vaccine prevention and therapy

PURPOSE

To review the changing epidemiology of herpes simplex virus infection, emerging patterns of herpetic ocular disease, and the challenges and promise of herpes simplex virus vaccine therapy.

DESIGN

Perspective.

METHODS

Literature review.

RESULTS

An epidemic increase in genital herpes simplex type 2 infection is reflected in a 30% increase in HSV-2 antibodies in the United States since 1976. Approximately one in four people in the United States over age 30 is infected with HSV-2. Primary acquisition of herpes simplex type 1 is becoming progressively delayed in many industrialized countries, in contrast to developing nations where the virus is acquired early in life and is ubiquitous. Changes in sexual behavior among young adults have been associated with a recent increase in genital HSV-1 infection, resulting from oral-genital rather than genital-genital contact. Clinical trials of HSV vaccines using selected herpes simplex virus type 2 proteins mixed in adjuvant have shown limited efficacy in seronegative women, but not in men.

CONCLUSIONS

The recent epidemic of genital herpes simplex type 2 infection is likely to result in an increase in neonatal ocular herpes and in delayed cases of acute retinal necrosis syndrome. The increase in genital HSV-1 may lead to industry production of vaccines that contain components of both HSV-1 and HSV-2 targeted toward limiting genital disease and transmission. As newer herpes simplex vaccines become available, ophthalmologists must be vigilant that a boost in immunity against HSV does not have a paradoxical effect in exacerbating break-through cases that develop immune-mediated herpes simplex stromal keratitis.

Efficacy of a recombinant glycoprotein D subunit vaccine on the development of primary and recurrent ocular infection with herpes simplex virus type 1 in mice

The protective efficacy of a glycoprotein D subunit vaccine (gD2 SB AS4) was evaluated in a mouse model of human recurrent herpetic stromal keratitis (HSK). When administered before primary infection, gD2 SB AS4 protected mice against corneal pathology, mortality, and latency resulting from ocular viral challenge with herpes simplex virus type 1 (HSV-1) McKrae strain. In addition, gD2 SB AS4 significantly decreased postreactivation corneal disease. A control vaccine, gD2 alum, protected against acute ocular infection only. When administered after primary infection, gD2 SB AS4 vaccination decreased postreactivation ocular shedding but had no other significant effects. Vaccination with gD2 SB AS4 was associated with high anti-gD antibody responses and low delayed-type hypersensitivity responses. These results have identified a prophylactic vaccine, gD2 SB AS4, with activity against acute and recurrent HSK in mice and emphasize the need for vaccine evaluation in both primary and recurrent ocular herpetic disease models.

Therapeutic immunization with a virion host shutoff-defective, replication-incompetent herpes simplex virus type 1 strain limits recurrent herpetic ocular infection

Immunization of mice with herpes simplex virus type 1 (HSV-1) mutant viruses containing deletions in the gene for virion host shutoff (vhs) protein diminishes primary and recurrent corneal infection with wild-type HSV-1. vhs mutant viruses are severely attenuated in vivo but establish latent infections in sensory neurons. A safer HSV-1 mutant vaccine strain, Delta41Delta29, has combined vhs and replication (ICP8-) deficits and protects BALB/c mice against primary corneal infection equivalent to a vhs- strain (BGS41). Here, we tested the hypothesis that Delta41Delta29 can protect as well as BGS41 in a therapeutic setting. Because immune response induction varies with the mouse and virus strains studied, we first determined the effect of prophylactic Delta41Delta29 vaccination on primary ocular infection of NIH inbred mice with HSV-1 McKrae, a model system used to evaluate therapeutic vaccines. In a dose-dependent fashion, prophylactic Delta41Delta29 vaccination decreased postchallenge tear film virus titers and ocular disease incidence and severity while eliciting high levels of HSV-specific antibodies. Adoptive transfer studies demonstrated a dominant role for immune serum and a lesser role for immune cells in mediating prophylactic protection. Therapeutically, vaccination with Delta41Delta29 effectively reduced the incidence of UV-B-induced recurrent virus shedding in latently infected mice. Therapeutic Delta41Delta29 and BGS41 vaccination decreased corneal opacity and delayed-type hypersensitivity responses while elevating antibody titers, compared to controls. These data indicate that replication is not a prerequisite for generation of therapeutic immunity by live HSV mutant virus vaccines and raise the possibility that genetically tailored replication-defective viruses may make effective and safe therapeutic vaccines.

CD4(+) and CD8(+) cells are key participants in the development of recurrent herpetic stromal keratitis in mice

Ocular herpes simplex virus (HSV) infection results in an immune-mediated inflammation of the corneal stroma known as herpetic stromal keratitis (HSK). Recurrent HSK is a common cause of virus-induced corneal blindness in humans. The role of CD4(+) and CD8(+) T cell subsets in the disease pathogenesis is ill defined and varies with the virus strain and host genetic background. To examine the contribution of T cell subsets to corneal disease, we studied the development of recurrent HSK in CD4 or CD8 gene knockout (KO) mice ocularly infected with HSV-1 McKrae strain. Following UV-B induced viral reactivation, corneal opacity in latently infected BALB/c (HSV sensitive) CD4 and CD8 KO mice was reduced compared to infected BALB/c mice with normal genotype. In contrast, opacity in C57BL/6 (HSV resistant) CD4 and CD8 KO latent mice did not differ from genetically normal latent mice. Virus-induced corneal opacity was not demonstrable in C57BL/6 CD4/CD8 double KO mice. Increased viral shedding, measured by reactivation rate, days shedding or viral titers, occurred in CD4 KO mice of both strains. Our findings indicate that both CD4(+) and CD8(+) cells play a role in the immunopathogenesis of recurrent HSK, and their role is dependent upon the host genetic profile.

Therapeutic vaccination with vhs(-) herpes simplex virus reduces the severity of recurrent herpetic stromal keratitis in mice

Virion host shutoff (vhs)-deficient herpes simplex virus (HSV) was tested as a therapeutic vaccine in a mouse model of UV light-induced recurrent herpetic stromal keratitis. Four weeks after primary corneal infection, mice were vaccinated intraperitoneally with vhs(-) vaccine or control. Four weeks after vaccination, the eyes of latently infected mice were UV-B irradiated to induce recurrent virus shedding and disease. Post-irradiation corneal opacity in latently infected, vhs(-)-vaccinated mice was significantly reduced compared to control-vaccinated mice (P=0.007 to 0.035). The incidence and duration of recurrent virus shedding were the same in both groups. Antibody titres were increased (P=0.05) and delayed type hypersensitive responses were unaffected by vhs(-) vaccination. Combined with studies using different vaccination timing and vhs(-) genotypes, these data suggest that deletion of vhs is a useful strategy in the development of a therapeutic HSV vaccine, and that temporal and genetic factors influence vaccination outcome.

Mechanism of exercise-induced augmentation of lymphokine activated killer (LAK) cell activity in the horse

Intense exercise affects various parameters of the immune system. The overall effect of exercise on immune function is dependent upon the physical condition of the subject, the intensity and duration of the exercise period, and the immune parameter assessed. Unconditioned horses subjected to a single bout of intensive exercise exhibit multiple alterations in immune function, including an augmentation of lymphokine activated killer (LAK) cell function. This increase in LAK cell activity is not due to an increase in circulating LAK precursors. While peripheral blood mononuclear cells from exercising horses exhibit greater responsiveness to IL-2, this is not due to an increase in IL-2 receptor expression. LAK cell generation in vitro is augmented by those catecholamines and neuropeptides which are produced during exercise, suggesting a direct effect of these compounds on LAK cell generation at a step post IL-2 receptor binding.

Cytokine expression in murine corneas during recurrent herpetic stromal keratitis

PURPOSE

Recurrent herpetic stromal keratitis (HSK) is a potentially blinding, immune-mediated disease. To better understand the immunopathology of recurrent HSK, we examined the cytokine profile of mouse corneas with the condition.

METHODS

The eyes of latently infected mice were examined for corneal pathology and cytokine content following UV-B-stimulated herpes simplex virus (HSV) reactivation.

RESULTS

Peak HSV-induced corneal disease, manifested by stromal opacification, occurred 7-14 days after viral reactivation in latently infected mice. In qualitative RT-PCR analyses, IFNgamma, IL-10, IL-4, and IL-12 p40 mRNA were simultaneously expressed before and during recurrent HSK. Competitive, semi-quantitative RT-PCR evaluation of cytokine mRNA revealed highest IFNgamma expression before and during clinical disease with a decline thereafter. IL-4 levels peaked and declined before day 14, while IL-10 peaked on days 7 or 14 and paralleled IFNgamma at lower levels. Small amounts of IL-12 p40 mRNA were detected late in the disease course. ELISA evaluation of corneal extracts demonstrated similar results, featuring early expression of Th2 cytokines relative to disease.

CONCLUSIONS

The presence of Th2 cytokines during early stages of recurrent herpetic corneal lesions indicate the presence of a mixed Th1 and Th2 cell infiltrate, which is likely associated with a memory response to viral antigens. These data suggest that disease resolution in corneas with recurrent HSK may depend upon the balance between destructive and protective cytokines at individual sites of viral recurrence.

Interferon gamma is not required for recurrent herpetic stromal keratitis

The role that interferon-gamma (IFNgamma) plays during herpetic stromal keratitis (HSK) has not been definitively determined. In primary HSK most reports suggest that IFNgamma may help control viral replication and contribute to corneal pathology. However, its role in recurrent HSK has not been directly addressed. The present study addresses its role in recurrent HSK by comparing HSK in latently infected normal and IFNgamma gene knockout (GKO) on the C57BL/6 background. We initially evaluated HSK following primary infection and observed that GKO mice had higher tear film virus titers, but virtually identical ocular disease as normal mice. In contrast, following reactivation of latent virus, GKO mice had a greater incidence and severity of opacity, neovascularization, and blepharitis. Interestingly, the incidence of reactivation after UV-B exposure was equivalent in GKO and normal mice, but virus shedding was increased in the GKO groups. We also observed diminished delayed-type hypersensitivity responses in GKO mice, as expected. These data indicate that IFNgamma is important for the control of virus replication in both primary and recurrent ocular HSV infection in C57BL/6 mice. The enhanced recurrent disease seen in GKO mice may be the result of increased viral titers and persistence in these mice which act to prolong the stimulation of an inflammatory response.

The effects of aminoguanidine on primary and recurrent ocular herpes simplex virus infection

In primary ocular herpes simplex virus (HSV) infection, nitric oxide may function to control viral replication and herpetic stromal keratitis (HSK) lesions. Recurrent HSK, manifested as corneal opacity and neovascularization, is the potentially blinding sequel to primary infection. Here, we assess the effects of nitric oxide synthase inhibition on a mouse model of recurrent HSK. In preliminary primary infection experiments, NIH inbred mice treated with aminoguanidine, an inhibitor of inducible nitric oxide synthase (iNOS), experienced no changes in post-infection tear, brain, or ganglia virus titers, but encephalitis-related mortality was elevated. After UV-B stimulated viral reactivation, iNOS inhibition did not affect virus shedding or clinical disease. In contrast to primary HSK, there was no exacerbation of mortality in recurrent disease. Our findings indicate that nitric oxide can be neuroprotective without antiviral effects in primary HSK, and does not play a significant role in the pathogenesis of recurrent HSK. Compared with data from other mouse strains, this work suggests that there may be a genetic component to the importance of NO in controlling ocular HSV infection.

CD137 costimulation is associated with reduced herpetic stromal keratitis and with developing normal CD8+ T cells in trigeminal ganglia

Costimulatory interactions can be critical in developing immune responses to infectious agents. We recently reported that herpes simplex type 1 (HSV-1) infections of the cornea require a functional CD28-CD80/86 interaction to not only reduce the likelihood of encephalitis, but also to mediate herpetic stromal keratitis (HSK) following viral reactivation. In this same spirit we decided to determine the role that CD137 costimulation plays during HSK. Using both B6-CD137L^-/- mice, as well as antagonistic and agonistic antibodies to CD137 we characterize the immune response and to what extent CD137 plays an important role during this disease. Immune responses were measured in both the cornea and in the trigeminal ganglia where the virus forms a latent infection. We demonstrate that CD137 costimulation leads to reduced corneal disease. Interestingly, we observed that lack of CD137 costimulation resulted in significantly reduced CD8⁺ T expansion and function in the trigeminal ganglia. Finally, we showed that viruses that have been genetically altered to express CD137 display significantly reduced corneal disease, though they did present similar levels of trigeminal infection and peripheral virus production following reactivation of a latent infection. CD137 interactions lead to reduced HSK and are necessary to develop robust trigeminal CD8⁺ T cell responses.

Increased frequency of recognition of delipidated versus intact CNS myelin in multiple sclerosis and control subjects

The proliferative response of mononuclear cells from MS patients and normal control subjects to intact and delipidated myelin membranes was examined. The mean frequency of recognition in both groups of human subjects was greater for delipidated myelin than for intact myelin. Human T cell lines established using intact or delipidated myelin as the antigen were highly heterogeneous in response, and were each able to recognize myelin basic protein and myelin proteolipid protein peptides. However, there was no difference in the frequency of recognition of either form of myelin membrane when MS patients were compared to control subjects. Our results suggest that the presentation of delipidated forms of membrane proteins might enhance the response to myelin antigens in vivo, and be relevant to demyelinating diseases.

UV-B induced HSV-1 reactivation leads to infectious virus in the corneas of virtually all latently infected mice and requires an intact STING to develop herpetic stromal keratitis

Reactivation of latent herpes simplex type 1 results in virus returning to the cornea leading to recurrent herpetic stromal keratitis (rHSK). We compare two competing models to reactivate viruses from latency, UV-B irradiation and cyclophosphamide. Results revealed that while both result in corneal recrudescence, only UV-B irradiation results in rHSK. To better understand the dynamics of reactivation, we analyzed corneas for both the presence of infectious viruses and the dynamics of exposure to multiple reactivations using UV-B. We noted that multiple reactivations result in progressively worse corneal disease. We also noted that expression of IFNα and STING, surragate markers for the presence of virus, are induced by the presence of reactivated virus. Studies to determine the importance of STING to the development of HSK revealed that in the absence of STING, mice do not develop significant HSK and the magnitude of the infiltrate of CD45 + cells in these corneas is significantly reduced. The resulting paucity of CD45 + CD11b + GR-1 + F4/80-neutrophils, and to a lesser extent CD45 + CD11b + GR-1-F4/80 + macrophages in B6-STING KO mice following reactivation is likely the underlying cause for lack of rHSK as has been noted by ourselves and others. These results underscore the critical importance of STING's role in developing rHSK.

IL-1 and TNF-alpha are important factors in the pathogenesis of murine recurrent herpetic stromal keratitis

PURPOSE

To better understand the role of interleukin (IL)-1 and tumor necrosis factor (NF)alpha in recurrent herpetic stromal keratitis (HSK), the cytokine content and the effects of anti-cytokine antibodies on mouse corneas with the disease were examined.

METHODS

Competitive reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent analyses of IL-1alpha and TNF-alpha content were performed on corneas removed 3, 5, 7, 10, 14, and 21 days after latently infected NIH mice were irradiated with UV-B light to reactivate herpes simplex virus (HSV). In separate experiments, mice were injected with anti-IL-1 or anti-TNF-a antibodies 1 day before and 7 days after reactivation.

RESULTS

UV-B irradiation stimulated an increase in corneal IL-la mRNA in reactivated (virus shedding) mice. This increase persisted longer and was higher than in UV-B irradiated uninfected control animals. IL-1alpha and TNF-alpha protein in corneas of reactivated mice was significantly elevated on days 3 to 10 compared with day 0 levels, and exceeded levels in control corneas on the same days. Anti-IL-1 and anti-TNF-alpha antibody administration both resulted in significantly decreased virus-induced corneal opacity between 7 and 21 days after UV-B exposure.

CONCLUSIONS

IL-1alpha and TNF-alpha are upregulated in corneas in mice experiencing recurrent HSK. Abrogation of virus-induced corneal disease by anti-cytokine antibodies suggests that these cytokines play important roles in the pathogenesis of recurrent disease. Therefore, neutralization of specific proinflammatory cytokines may have potential therapeutic value.

UVB induced reactivation leads to HSV1 in the corneas of virtually all latently infected mice and requires STING to develop corneal disease

Reactivation of latent herpes simplex type 1 results in virus returning to the cornea leading to recurrent herpetic stromal keratitis (rHSK). We compare two competing models to reactivate viruses from latency, UV-B irradiation and cyclophosphamide (CP). Results revealed that while both result in corneal recrudescence, only UV-B irradiation results in rHSK. To better understand the dynamics of reactivation, we analyzed corneas for both the presence of infectious viruses and the dynamics of exposure to multiple reactivations using UV-B. We noted that multiple reactivations result in progressively worse corneal disease. We also noted that expression of IFNα and STING, surragate markers for the presence of virus, are induced by the presence of reactivated virus. Studies to determine the importance of STING to the development of HSK revealed that in the absence of STING, mice do not develop significant HSK and the magnitude of the infiltrate of CD45+ cells in these corneas is significantly reduced. The resulting paucity of CD45+CD11b+GR-1+F4/80-neutrophils, and to a lesser extent CD45+CD11b+GR-1-F4/80+ macrophages in B6-STING KO mice following reactivation is likely the underlying cause for lack of rHSK as has been noted by ourselves and others. These results underscore the critical importance of STING's role in developing rHSK.