The infections with herpes simplex virus type 1 and type 2 (HSV-1 & HSV-2) have been prevalent since the ancient Greek times. To this day, they still affect a staggering number of over a half billion individuals worldwide. HSV-2 infections cause painful genital herpes, encephalitis, and death in newborns. HSV-1 infections are more prevalent than HSV-2 infections and cause potentially blinding ocular herpes, oro-facial herpes and encephalitis. While genital herpes in mainly caused by HSV-2 infections, in recent years, there is an increase in the proportion of genital herpes caused by HSV-1 infections in young adults, which reach 50% in some western societies. While prophylactic and therapeutic HSV vaccines remain urgently needed for centuries their development has been notoriously difficult. During the most recent National Institute of Health (NIH) workshop titled "Next Generation Herpes Simplex Virus Vaccines: The Challenges and Opportunities", basic researchers, funding agencies, and pharmaceutical representatives gathered: (i) to assess the status of herpes vaccine research; and (ii) to identify the gaps and propose alternative approaches in developing a safe and efficient herpes vaccine. One “common denominator” among previously failed clinical herpes vaccine trials is that they either used a whole virus or whole viral proteins, which contain both pathogenic “symptomatic” and protective “asymptomatic” antigens/epitopes. In this report, we continue to advocate that using an “asymptomatic” epitope-based vaccine strategy that selectively incorporates protective epitopes which: (i) are exclusively recognized, in vitro, by effector memory CD4+ and CD8+ TEM cells from “naturally” protected seropositive asymptomatic individuals; and (ii) protect, in vivo, human leukocyte antigen (HLA) transgenic animal models from ocular and genital herpes infections and diseases, could be the answer to many of the scientific challenges facing HSV vaccine development. We review the role of animal models in herpes vaccine development and discuss its current status, challenges, and prospects.

Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen that infects over 3.72 billion individuals worldwide and can cause potentially blinding recurrent corneal herpetic disease. HSV-1 establishes latency within sensory neurons of trigeminal ganglia (TG) and TG-resident CD8+ T cells play a critical role in preventing its reactivation. The repertoire, phenotype and function of protective CD8+ T cells are unknown. Bolstering the apparent feeble numbers of CD8+ T cells in TG remains a challenge for immunotherapeutic strategies. In this study, a comprehensive panel of 467 HLA-A*0201-restricted CD8+ T cell epitopes were predicted from the entire HSV-1 genome. CD8+ T cell responses to these genome-wide epitopes were compared in HSV-1 seropositive symptomatic (SYMP) individuals (with a history of numerous episodes of recurrent herpetic disease) vs. asymptomatic (ASYMP) individuals (who are infected but never experienced any recurrent herpetic disease). Frequent polyfunctional HSV-specific effector memory IFN-γ+CD107a/b+CD44highCD62LlowCD8+ TEM cells were detected in ASYMP individuals and were mainly directed against three “ASYMP” epitopes. In contrast, SYMP individuals have more mono-functional central memory CD44highCD62LhighCD8+ TCM cells. Furthermore, therapeutic immunization with an innovative prime/pull vaccine, based on priming with multiple “ASYMP” epitopes (prime) and neurotropic TG delivery of the T-cell attracting chemokine CXCL-10 (pull), boosted the number and function of CD44highCD62LlowCD8+ TEM and tissue-resident CD103highCD8+ TRM cells in TG of latently infected HLA-A*0201 Tg mice and reduced recurrent ocular herpes following UV-B induced reactivation. These findings have profound implications in the development of T-cell-based immunotherapeutic strategies to treat blinding recurrent herpes infection and disease.

A significantly larger repertoire of dysfunctional (exhausted) HSV-specific CD8(+)T cells were found in the TG of HLA transgenic rabbits latently infected with wild-type HSV-1 or with LAT-rescued mutant (i.e., LAT(+)TG) than in a more restricted repertoire of functional HSV-specific CD8(+)T cells in the TG of HLA transgenic rabbits latently infected with LAT-null mutant (i.e., LAT(-)TG). These findings suggest that the HSV-1 LAT locus interferes with the host cellular immune response by shaping a broader repertoire of exhausted HSV-specific CD8(+)T cells within the latency/reactivation TG site.

Herpes simplex virus 1 (HSV-1) establishes latency within the sensory neurons of the trigeminal ganglia (TG). HSV-specific memory CD8 ϩ T cells play a critical role in preventing HSV-1 reactivation from TG and subsequent virus shedding in tears that trigger recurrent corneal herpetic disease. The CXC chemokine ligand 10 (CXCL10)/CXC chemokine receptor 3 (CXCR3) chemokine pathway promotes T cell immunity to many viral pathogens, but its importance in CD8 ϩ T cell immunity to recurrent herpes has been poorly elucidated. In this study, we determined how the CXCL10/CXCR3 pathway affects TG-and cornea-resident CD8 ϩ T cell responses to recurrent ocular herpesvirus infection and disease using a well-established murine model in which HSV-1 reactivation was induced from latently infected TG by UV-B light. Following UV-B-induced HSV-1 reactivation, a significant increase in both the number and function of HSV-specific CXCR3 ϩ CD8 ϩ T cells was detected in TG and corneas of protected C57BL/6 (B6) mice, but not in TG and corneas of nonprotected CXCL10 Ϫ/Ϫ or CXCR3 Ϫ/Ϫ deficient mice. This increase was associated with a significant reduction in both virus shedding and recurrent corneal herpetic disease. Furthermore, delivery of exogenous CXCL10 chemokine in TG of CXCL10 Ϫ/Ϫ mice, using the neurotropic adeno-associated virus type 8 (AAV8) vector, boosted the number and function of effector memory CD8 ϩ T cells (T EM ) and tissue-resident memory CD8 ϩ T cells (T RM ), but not of central memory CD8 ϩ T cells (T CM ), locally within TG, and improved protection against recurrent herpesvirus infection and disease in CXCL10 Ϫ/Ϫ deficient mice. These findings demonstrate that the CXCL10/CXCR3 chemokine pathway is critical in shaping CD8 ϩ T cell immunity, locally within latently infected tissues, which protects against recurrent herpesvirus infection and disease.IMPORTANCE We determined how the CXCL10/CXCR3 pathway affects CD8 ϩ T cell responses to recurrent ocular herpesvirus infection and disease. Using a wellestablished murine model, in which HSV-1 reactivation in latently infected trigeminal ganglia was induced by UV-B light, we demonstrated that lack of either CXCL10 chemokine or its CXCR3 receptor compromised the mobilization of functional CD8 ϩ T EM and CD8 ϩ T RM cells within latently infected trigeminal ganglia following virus reactivation. This lack of T cell mobilization was associated with an increase in recurrent ocular herpesvirus infection and disease. Inversely, augmenting the amount of

HSV type 1 (HSV-1)-specific CD8 T cells protect from herpes infection and disease. However, the nature of protective CD8 T cells in HSV-1 seropositive healthy asymptomatic (ASYMP) individuals (with no history of clinical herpes disease) remains to be determined. In this study, we compared the phenotype and function of HSV-specific CD8 T cells from HLA-A*02:01-positive ASYMP and symptomatic (SYMP) individuals (with a documented history of numerous episodes of recurrent ocular herpetic disease). We report that although SYMP and ASYMP individuals have similar frequencies of HSV-specific CD8 T cells, the "naturally" protected ASYMP individuals have a significantly higher proportion of multifunctional HSV-specific effector memory CD8 T cells (CD73CD45RACCR7CD8 effector memory RA (T) and CD73CD45RACCR7CD8 effector memory (T) as compared with SYMP individuals. Similar to humans, HSV-1-infected ASYMP B6 mice had frequent multifunctional HSV-specific CD73CD8 T cells in the cornea, as compared with SYMP mice. Moreover, in contrast to wild type B6, CD73 deficient mice infected ocularly with HSV-1 developed more recurrent corneal herpetic infection and disease. This was associated with less functional CD8 T cells in the cornea and trigeminal ganglia, the sites of acute and latent infection. The phenotypic and functional characteristics of HSV-specific circulating and in situ CD73CD8 T cells, demonstrated in both ASYMP humans and mice, suggest a positive role for effector memory CD8 T cells expressing the CD73 costimulatory molecule in the protection against ocular herpes infection and disease. These findings are important for the development of safe and effective T cell-based herpes immunotherapy.