B7 costimulation molecules encoded by replication-defective, vhs-deficient HSV-1 improve vaccine-induced protection against corneal disease

The immunobiology of corneal HSV-1 infection and herpetic stromal keratitis

SUMMARYHuman alphaherpesvirus 1 (HSV-1) is a highly successful neurotropic pathogen that primarily infects the epithelial cells lining the orofacial mucosa. After primary lytic replication in the oral, ocular, and nasal mucosal epithelial cells, HSV-1 establishes life-long latency in neurons within the trigeminal ganglion. Patients with compromised immune systems experience frequent reactivation of HSV-1 from latency, leading to virus entry in the sensory neurons, followed by anterograde transport and lytic replication at the innervated mucosal epithelial surface. Although recurrent infection of the corneal mucosal surface is rare, it can result in a chronic immuno-inflammatory condition called herpetic stromal keratitis (HSK). HSK leads to gradual vision loss and can cause permanent blindness in severe untreated cases. Currently, there is no cure or successful vaccine to prevent latent or recurrent HSV-1 infections, posing a significant clinical challenge to managing HSK and preventing vision loss. The conventional clinical management of HSK primarily relies on anti-virals to suppress HSV-1 replication, anti-inflammatory drugs (such as corticosteroids) to provide symptomatic relief from pain and inflammation, and surgical interventions in more severe cases to replace damaged cornea. However, each clinical treatment strategy has limitations, such as local and systemic drug toxicities and the emergence of anti-viral-resistant HSV-1 strains. In this review, we summarize the factors and immune cells involved in HSK pathogenesis and highlight alternate therapeutic strategies for successful clinical management of HSK. We also discuss the therapeutic potential of immunoregulatory cytokines and immunometabolism modulators as promising HSK therapies against emerging anti-viral-resistant HSV-1 strains.

ICP8-vhs- HSV-2 Vaccine Expressing B7 Costimulation Molecules Optimizes Safety and Efficacy against HSV-2 Infection in Mice

Herpes simplex virus 2 (HSV-2) causes most sexually transmitted genital ulcerative disease. No effective prophylactic vaccine is currently available. Replication-defective (ICP8-) HSV stimulates immune responses in animals without producing progeny virus, making it potentially useful as a safe form of a live vaccine against HSV. We previously demonstrated that mice generate a stronger response to ICP8- virus encoding B7-2 costimulation molecules than to the parental replication-defective virus. We have also demonstrated enhanced immunogenicity of an ICP8-, virion host shutoff (vhs)- virus which can no longer destabilize viral and host mRNAs. Here, we constructed a triple mutant, ICP8-vhs-B7-2+ strain, and compared it to both double mutant viruses. Immunization of mice with a single dose of ICP8-B7-2+ or ICP8-vhs-B7-2+ virus decreased challenge virus replication in the vaginal mucosa, genital disease, and mortality more effectively than immunization with the ICP8-vhs- virus. Immunization with ICP8-B7-2+ or ICP8-vhs-B7-2+ virus also effectively suppressed subsequent HSV-2 infection of the nervous system compared to immunization with the ICP8-vhs- virus. ICP8-B7-2+ and ICP8-vhs-B7-2+ strains induced more IFN gamma-producing CD8 T cells and memory CD8 T cells than did ICP8-vhs- virus, potentially explaining the enhanced protective effects. Thus, B7 costimulation molecules expressed from a replication-defective vaccine can enhance vaccine efficacy, even in an immunocompetent host.

Efficacy of an HSV-1 Neuro-Attenuated Vaccine in Mice Is Reduced by Preventing Viral DNA Replication

We previously isolated an HSV-1 mutant, KOS-NA, that contains two non-synonymous mutations in UL39. One of the mutations, resulting in an R950H amino acid substitution in ICP6, renders KOS-NA severely neuro-attenuated and significantly reduces HSV-1 latency. Vaccination of mice with KOS-NA prior to corneal challenge provides significant protection against HSV-1-mediated eye diseases even at a very low immunizing dose, indicating its utility as a vaccine scaffold. Because KOS-NA contains a neuro-attenuating mutation in a single gene, we sought to improve its safety by deleting a portion of the UL29 gene whose protein product, ICP8, is essential for viral DNA replication. Whereas KOS-NA reduced replication of HSV-1 challenge virus in the corneal epithelium and protected mice against blepharitis and keratitis induced by the challenge virus, KOS-NA/8- and an ICP8- virus were significantly less efficacious except at higher doses. Our results suggest that the capacity to replicate, even at significantly reduced levels compared with wild-type HSV-1, may be an important feature of an effective vaccine. Means to improve safety of attenuated viruses as vaccines without compromising efficacy should be sought.

A HSV1 mutant leads to an attenuated phenotype and induces immunity with a protective effect

Herpes simplex virus type 1 (HSV1) is a complicated structural agent with a sophisticated transcription process and a high infection rate. A vaccine against HSV1 is urgently needed. As multiple viral-encoded proteins, including structural and nonstructural proteins, contribute to immune response stimulation, an attenuated or deficient HSV1 vaccine may be relatively reliable. Advances in genomic modification technologies provide reliable means of constructing various HSV vaccine candidates. Based on our previous work, an M6 mutant with mutations in the UL7, UL41, LAT, Us3, Us11 and Us12 genes was established. The mutant exhibited low proliferation in cells and an attenuated phenotype in an animal model. Furthermore, in mice and rhesus monkeys, the mutant can induce remarkable serum neutralizing antibody titers and T cell activation and protect against HSV1 challenge by impeding viral replication, dissemination and pathogenesis.

CD80 Plays a Critical Role in Increased Inflammatory Responses in Herpes Simplex Virus 1-Infected Mouse Corneas

We recently reported that herpes simplex virus 1 (HSV-1) infection suppresses CD80 but not CD86 expression in vitro and in vivo This suppression required the HSV-1 ICP22 gene. We also reported that overexpression of CD80 by HSV-1 exacerbated corneal scarring in BALB/c mice. We now show that this recombinant virus (HSV-CD80) expressed high levels of CD80 both in vitro in cultured rabbit skin cells and in vivo in infected mouse corneas. CD80 protein was detected on the surface of infected cells. The virulence of the recombinant HSV-CD80 virus was similar to that of the parental strain, and the replication of HSV-CD80 was similar to that of control virus in vitro and in vivo Transcriptome analysis detected 75 known HSV-1 genes in the corneas of mice infected with HSV-CD80 or parental virus on day 4 postinfection. Except for significantly higher CD80 expression in HSV-CD80-infected mice, levels of HSV-1 gene expression were similar in corneas from HSV-CD80-infected and parental virus-infected mice. The number of CD8+ T cells was higher, and the number of CD4+ T cells was lower, in the corneas of HSV-CD80-infected mice than in mice infected with parental virus. HSV-CD80-infected mice displayed a transient increase in dendritic cells. Transcriptome analysis revealed mild differences in dendritic cell maturation and interleukin-1 signaling pathways and increased expression of interferon-induced protein with tetratricopeptide repeats 2 (Ifit2). Together, these results suggest that increased CD80 levels promote increased CD8+ T cells, leading to exacerbated eye disease in HSV-1-infected mice.IMPORTANCE HSV-1 ocular infections are the leading cause of corneal blindness. Eye disease is the result of a prolonged immune response to the replicating virus. HSV-1, on the other hand, has evolved several mechanisms to evade clearance by the host immune system. We describe a novel mechanism of HSV-1 immune evasion via ICP22-dependent downregulation of the host T cell costimulatory molecule CD80. However, the exact role of CD80 in HSV-1 immune pathology is not clear. In this study, we show that eye disease is independent of the level of HSV-1 replication and that viral expression of CD80 has a detrimental role in corneal scarring, likely by increasing CD8+ T cell recruitment and activation.

Herpes Simplex Virus 1 Replication, Ocular Disease, and Reactivations from Latency Are Restricted Unilaterally after Inoculation of Virus into the Lip

Ocular herpes simplex keratitis (HSK) is a consequence of viral reactivations from trigeminal ganglia (TG) and occurs almost exclusively in the same eye in humans. In our murine oro-ocular (OO) model, herpes simplex virus 1 (HSV-1) inoculation in one side of the lip propagates virus to infect the ipsilateral TG. Replication here allows infection of the brainstem and infection of the contralateral TG. Interestingly, HSK was observed in our OO model only from the eye ipsilateral to the site of lip infection. Thus, unilateral restriction of HSV-1 may be due to differential kinetics of virus arrival in the ipsilateral versus contralateral TG. We inoculated mice with HSV-1 reporter viruses and then superinfected them to monitor changes in acute- and latent-phase gene expression in TG after superinfection compared to the control (single inoculation). Delaying superinfection by 4 days after initial right lip inoculation elicited failed superinfecting-virus gene expression and eliminated clinical signs of disease. Initial inoculation with thymidine kinase-deficient HSV-1 (TK_del) completely abolished reactivation of wild-type (WT) superinfecting virus from TG during the latent stage. In light of these seemingly failed infections, viral genome was detected in both TG. Our data demonstrate that inoculation of HSV-1 in the lip propagates virus to both TG, but with delay in reaching the TG contralateral to the side of lip infection. This delay is responsible for restricting viral replication to the ipsilateral TG, which abrogates ocular disease and viral reactivations from the contralateral side. These observations may help to understand why HSK is observed unilaterally in humans, and they provide insight into vaccine strategies to protect against HSK.IMPORTANCE Herpetic keratitis (HK) is the leading cause of blindness by an infectious agent in the developed world. This disease can occur after reactivation of herpes simplex virus 1 in the trigeminal ganglia, leading to dissemination of virus to, and infection of, the cornea. A clinical paradox is evidenced by the bilateral presence of latent viral genomes in both trigeminal ganglia, while for any given patient the disease is unilateral with recurrences in a single eye. Our study links the kinetics of early infection to unilateral disease phenomenon and demonstrates protection against viral reactivation when kinetics are exploited. Our results have direct implications in the understanding of human disease pathogenesis and immunotherapeutic strategies for the treatment of HK and viral reactivations.

CD28 Costimulation Is Required for Development of Herpetic Stromal Keratitis but Does Not Prevent Establishment of Latency

Corneal infection with herpes simplex virus 1 (HSV-1) leads to infection of trigeminal ganglia (TG), typically followed by the establishment of latency in the infected neurons. When latency is disrupted, the virus reactivates and migrates back to the cornea, where it restimulates the immune response, leading to lesions in a disease called herpetic stromal keratitis (HSK). HSK requires T cell activation, as in the absence of T cells there is no disease. We decided to determine if CD28 costimulation of T cells was required in HSK. The results indicated that C57BL/6 CD28^-/- and BALB/c CD28^-/- mice failed to develop recurrent HSK, while their wild-type counterparts did. In order to better understand the dynamics of TG infection in these mice, we evaluated the amount of virus in infected TG and the number of individual neurons harboring latent virus. The results indicated that CD28^-/- mice possessed significantly increased genome levels in their TG but many fewer LAT-positive cells than wild-type mice from day 7 to day 30 but that after day 30 these differences became nonsignificant. We next evaluated total and antigen-specific CD8⁺ T cells in TG. The results indicated that there were significantly fewer CD8 T cells in TG from day 10 to day 25 but that after that the differences were not significant. Taken together, these data suggest that CD28 costimulation is required for HSK but that while initial infection of TG is greater in CD28^-/- mice, this begins to normalize with time and this normalization is concurrent with the delayed development of antigen-specific CD8⁺ T cells.IMPORTANCE We study the pathogenesis of herpes simplex virus-mediated corneal disease. T cells play a critical role both in disease and in the maintenance of latency in neurons. Consequently, the focus of this study was to evaluate the role that T cell costimulation plays both in corneal disease and in controlling the ability of the virus to maintain a stable infection of the ganglia that innervate the cornea. We demonstrate that in the absence of costimulation with CD28, corneal disease does not take place. However, this costimulation does not prevent the ability of CD8⁺ T cells to develop and, thus, control latent infection of neurons. We conclude from these studies that CD28 costimulation is required for corneal destructive immune responses but that CD8⁺ T cells develop over time and help to maintain latency.

Herpes Simplex Virus 1 ICP22 Suppresses CD80 Expression by Murine Dendritic Cells

Herpes simplex virus type 1 (HSV-1) has the ability to delay its clearance from the eye during ocular infection. Here, we show that ocular infection of mice with HSV-1 suppressed expression of the costimulatory molecule CD80 but not CD86 in the cornea. The presence of neutralizing anti-HSV-1 antibodies did not alleviate this suppression. At the cellular level, HSV-1 consistently downregulated the expression of CD80 by dendritic cells (DCs) but not by other antigen-presenting cells. Furthermore, flow cytometric analysis of HSV-1-infected corneal cells during a 7-day period reduced CD80 expression in DCs but not in B cells, macrophages, or monocytes. This suppression was associated with the presence of virus. Similar results were obtained using infected or transfected spleen cells or bone marrow-derived DCs. A combination of roscovitine treatment, transfection with immediate early genes (IE), and infection with a recombinant HSV-1 lacking the ICP22 gene shows the importance of ICP22 in downregulation of the CD80 promoter but not the CD86 promoter in vitro and in vivo At the mechanistic level, we show that the HSV-1 immediate early gene ICP22 binds the CD80 promoter and that this interaction is required for HSV-1-mediated suppression of CD80 expression. Conversely, forced expression of CD80 by ocular infection of mice with a recombinant HSV-1 exacerbated corneal scarring in infected mice. Taken together, these studies identify ICP22-mediated suppression of CD80 expression in dendritic cells as central to delayed clearance of the virus and limitation of the cytopathological response to primary infection in the eye.IMPORTANCE HSV-1-induced eye disease is a major public health problem. Eye disease is associated closely with immune responses to the virus and is exacerbated by delayed clearance of the primary infection. The immune system relies on antigen-presenting cells of the innate immune system to activate the T cell response. We found that HSV-1 utilizes a robust and finely targeted mechanism of local immune evasion. It downregulates the expression of the costimulatory molecule CD80 but not CD86 on resident dendritic cells irrespective of the presence of anti-HSV-1 antibodies. The effect is mediated by direct binding of HSV-1 ICP22, the product of an immediate early gene of HSV-1, to the promoter of CD80. This immune evasion mechanism dampens the host immune response and, thus, reduces eye disease in ocularly infected mice. Therefore, ICP22 may be a novel inhibitor of CD80 that could be used to modulate the immune response.

Attenuated Herpes Simplex Virus 1 (HSV-1) Expressing a Mutant Form of ICP6 Stimulates a Strong Immune Response That Protects Mice against HSV-1-Induced Corneal Disease

We previously isolated a herpes simplex virus 1 (HSV-1) mutant, KOS-NA, that carries two nonsynonymous mutations in UL39, resulting in L393P and R950H amino acid substitutions in infected cell protein 6 (ICP6). Our published data studying KOS-NA pathogenesis strongly suggest that one of these ICP6 substitutions expressed from KOS-NA, R950H, severely impaired acute viral replication in the eyes and trigeminal ganglia of mice after inoculation onto the cornea and consequently impaired establishment and reactivation from latency. Because of its significant neuroattenuation, we tested KOS-NA as a potential prophylactic vaccine against HSV-1 in a mouse model of corneal infection. KOS-NA stimulated stronger antibody and T cell responses than a replication-competent ICP0-null mutant and a replication-incompetent ICP8-null mutant optimized for immunogenicity. Immunizations with the ICP0^-, ICP8^-, and KOS-NA viruses all reduced replication of wild-type HSV-1 challenge virus in the corneal epithelium to similar extents. Low immunizing doses of KOS-NA and the ICP8^- virus, but not the ICP0^- virus, protected mice against eyelid disease (blepharitis). Notably, only KOS-NA protected almost completely against corneal disease (keratitis) and greatly reduced latent infection by challenge virus. Thus, vaccination of mice with KOS-NA prior to corneal challenge provides significant protection against HSV-1-mediated disease of the eye, even at a very low immunizing dose. These results suggest that KOS-NA may be the foundation of an effective prophylactic vaccine to prevent or limit HSV-1 ocular diseases.IMPORTANCE HSV-1 is a ubiquitous human pathogen that infects the majority of the world's population. Although most infections are asymptomatic, HSV-1 establishes lifelong latency in infected sensory neurons, from which it can reactivate to cause deadly encephalitis or potentially blinding eye disease. No clinically effective vaccine is available. In this study, we tested the protective potential of a neuroattenuated HSV-1 mutant (KOS-NA) as a vaccine in mice. We compared the effects of immunization with KOS-NA to those of two other attenuated viruses, a replication-competent (ICP0^-) virus and a replication-incompetent (ICP8^-) virus. Our data show that KOS-NA proved superior to the ICP0- and ICP8-null mutants in protecting mice from corneal disease and latent infection. With its significant neuroattenuation, severe impairment in establishing latency, and excellent protective effect, KOS-NA represents a significant discovery in the field of HSV-1 vaccine development.

Herpes Simplex Vaccines: Prospects of Live-attenuated HSV Vaccines to Combat Genital and Ocular infections

Herpes simplex virus type-1 (HSV-1) and its closely related type-2 (HSV-2) viruses cause important clinical manifestations in humans including acute ocular disease and genital infections. These viruses establish latency in the trigeminal ganglionic and dorsal root neurons, respectively. Both viruses are widespread among humans and can frequently reactivate from latency causing disease. Currently, there are no vaccines available against herpes simplex viral infections. However, a number of promising vaccine approaches are being explored in pre-clinical investigations with few progressing to early phase clinical trials. Consensus research findings suggest that robust humoral and cellular immune responses may partially control the frequency of reactivation episodes and reduce clinical symptoms. Live-attenuated viral vaccines have long been considered as a viable option for generating robust and protective immune responses against viral pathogens. Varicella zoster virus (VZV) belongs to the same alphaherpesvirus subfamily with herpes simplex viruses. A live-attenuated VZV vaccine has been extensively used in a prophylactic and therapeutic approach to combat primary and recurrent VZV infection indicating that a similar vaccine approach may be feasible for HSVs. In this review, we summarize pre-clinical approaches to HSV vaccine development and current efforts to test certain vaccine approaches in human clinical trials. Also, we discuss the potential advantages of using a safe, live-attenuated HSV-1 vaccine strain to protect against both HSV-1 and HSV-2 infections.

The Current State of Vaccine Development for Ocular HSV-1 Infection

HSV-1 continues to be the leading cause of infectious corneal blindness. Clinical trials for vaccines against genital HSV infection have been ongoing for more than three decades. Despite this, no approved vaccine exists, and no formal clinical trials have evaluated the impact of HSV vaccines on eye health. We review here the current state of development for an efficacious HSV-1 vaccine and call for involvement of ophthalmologists and vision researchers.

Antigenic breadth: a missing ingredient in HSV-2 subunit vaccines?

The successful human papillomavirus and hepatitis B virus subunit vaccines contain single viral proteins that represent 22 and 12%, respectively, of the antigens encoded by these tiny viruses. The herpes simplex virus 2 (HSV-2) genome is >20 times larger. Thus, a single protein subunit represents 1% of HSV-2's total antigenic breadth. Antigenic breadth may explain why HSV-2 glycoprotein subunit vaccines have failed in clinical trials, and why live HSV-2 vaccines that express 99% of HSV-2's proteome may be more effective. I review the mounting evidence that live HSV-2 vaccines offer a greater opportunity to stop the spread of genital herpes, and I consider the unfounded 'safety concerns' that have kept live HSV-2 vaccines out of U.S. clinical trials for 25 years.

Herpes simplex virus 1 ICP22 but not US 1.5 is required for efficient acute replication in mice and VICE domain formation

The herpes simplex virus 1 (HSV-1) immediate-early protein, infected cell protein 22 (ICP22), is required for efficient replication in restrictive cells, for virus-induced chaperone-enriched (VICE) domain formation, and for normal expression of a subset of viral late proteins. Additionally, ICP22 is important for optimal acute viral replication in vivo. Previous studies have shown that the US1 gene that encodes ICP22, produces an in-frame, N-terminally truncated form of ICP22, known as US1.5. To date, studies conducted to characterize the functions of ICP22 have not separated its functions from those of US1.5. To determine the individual roles of ICP22 and US1.5, we made viral mutants that express either ICP22 with an M90A mutation in the US1.5 initiation codon (M90A) or US1.5 with three stop codons introduced upstream of the US1.5 start codon (3×stop). Our studies showed that, in contrast to M90A, 3×stop was unable to replicate efficiently in the eyes and trigeminal ganglia of mice during acute infection, to efficiently establish a latent infection, or to induce VICE domain formation and was only mildly reduced in its replication in restrictive HEL-299 cells and murine embryonic fibroblasts (MEFs). Both mutants enhanced the expression of the late viral proteins virion host shutoff (vhs) and glycoprotein C (gC) and inhibited viral gene expression mediated by HSV-1 infected cell protein 0 (ICP0). When we tested our mutants' sensitivity to type I interferon (beta interferon [IFN-β]) in restrictive cells, we noticed that the plating of the ICP22 null (d22) and 3×stop mutants was reduced by the addition of IFN-β. Overall, our data suggest that US1.5 partially complements the functions of ICP22.

Pan-HSV-2 IgG antibody in vaccinated mice and guinea pigs correlates with protection against herpes simplex virus 2

We lack a correlate of immunity to herpes simplex virus 2 (HSV-2) that may be used to differentiate whether a HSV-2 vaccine elicits robust or anemic protection against genital herpes. This gap in knowledge is often attributed to a failure to measure the correct component of the adaptive immune response to HSV-2. However, efforts to identify a correlate of immunity have focused on subunit vaccines that contain less than 3% of HSV-2's 40,000-amino-acid proteome. We were interested to determine if a correlate of immunity might be more readily identified if 1. animals were immunized with a polyvalent immunogen such as a live virus and/or 2. the magnitude of the vaccine-induced immune response was gauged in terms of the IgG antibody response to all of HSV-2's antigens (pan-HSV-2 IgG). Pre-challenge pan-HSV-2 IgG levels and protection against HSV-2 were compared in mice and/or guinea pigs immunized with a gD-2 subunit vaccine, wild-type HSV-2, or one of several attenuated HSV-2 ICP0⁻ viruses (0Δ254, 0Δ810, 0ΔRING, or 0ΔNLS). These six HSV-2 immunogens elicited a wide range of pan-HSV-2 IgG levels spanning an ∼500-fold range. For 5 of the 6 immunogens tested, pre-challenge levels of pan-HSV-2 IgG quantitatively correlated with reductions in HSV-2 challenge virus shedding and increased survival frequency following HSV-2 challenge. Collectively, the results suggest that pan-HSV-2 IgG levels may provide a simple and useful screening tool for evaluating the potential of a HSV-2 vaccine candidate to elicit protection against HSV-2 genital herpes.

The immunologic basis for severe neonatal herpes disease and potential strategies for therapeutic intervention

Herpes simplex viruses types 1 and 2 (HSV-1 and HSV-2) infect a large proportion of the world's population. Infection is life-long and can cause periodic mucocutaneous symptoms, but it only rarely causes life-threatening disease among immunocompetent children and adults. However, when HSV infection occurs during the neonatal period, viral replication is poorly controlled and a large proportion of infants die or develop disability even with optimal antiviral therapy. Increasingly, specific differences are being elucidated between the immune system of newborns and those of older children and adults, which predispose to severe infections and reflect the transition from fetal to postnatal life. Studies in healthy individuals of different ages, individuals with primary or acquired immunodeficiencies, and animal models have contributed to our understanding of the mechanisms that control HSV infection and how these may be impaired during the neonatal period. This paper outlines our current understanding of innate and adaptive immunity to HSV infection, immunologic differences in early infancy that may account for the manifestations of neonatal HSV infection, and the potential of interventions to augment neonatal immune protection against HSV disease.

Recurrent herpetic stromal keratitis in mice: a model for studying human HSK

Herpes simplex virus 1 (HSV-1) infection of the cornea leads to a potentially blinding disease, termed herpetic stromal keratitis (HSK) that is characterized by lesions of an immunoinflammatory nature. In spite of the fact that HSK typically presents as a recurrent disease due to reactivation of virus which latently infects the trigeminal ganglia, most murine studies of HSK have employed a primary and not recurrent model of the disease. This report documents the several recurrent models of HSK that have been developed and how data generated from these models differs in some important aspects from data generated following primary infection of the cornea. Chief among these differences is the fact that recurrent HSK takes place in the context of an animal that has a preexisting anti-HSV immune response, while primary HSK occurs in an animal that is developing such a response. We will document both differences and similarities that derive from this fundamental difference in these models with an eye towards possible vaccines and therapies that demonstrate promise in treating HSK.

Herpes simplex virus 1 ICP0 phosphorylation site mutants are attenuated for viral replication and impaired for explant-induced reactivation

In cell culture experiments, phosphorylation appears to be a critical regulator of the herpes simplex virus 1 (HSV-1) immediate-early (IE) protein, ICP0, which is an E3 ubiquitin ligase that transactivates viral gene expression. Three major regions of phosphorylation in ICP0 (amino acids 224 to 232, 365 to 371, and 508 to 518) have been identified, and mutant viruses that block phosphorylation sites within each region (termed Phos 1, 2, and 3, respectively) have been constructed. Previous studies indicated that replication of Phos 1 is significantly reduced compared to that of wild-type virus in cell culture (C. Boutell, et al., J. Virol. 82:10647-10656, 2008). To determine the effects these phosphorylation site mutations have on the viral life cycle in vivo, mice were ocularly infected with wild-type HSV-1, the Phos mutants, or their marker rescue counterparts. Subsequently, viral replication, establishment of latency, and viral explant-induced reactivation of these viruses were examined. Relative to wild-type virus, Phos 1 eye titers were reduced as much as 7- and 18-fold on days 1 and 5 postinfection, respectively. Phos 2 eye titers showed a decrease of 6-fold on day 1 postinfection. Titers of Phos 1 and 2 trigeminal ganglia were reduced as much as 16- and 20-fold, respectively, on day 5 postinfection. Additionally, the reactivation efficiencies of Phos 1 and 2 were impaired relative to wild-type HSV-1, although both viruses established wild-type levels of latency in vivo. The acute replication, latency, and reactivation phenotypes of Phos 3 were similar to those of wild-type HSV-1. We conclude from these studies that phosphorylation is likely a key modulator of ICP0's biological activities in a mouse ocular model of HSV-1 infection.