Role of dendritic cells in enhancement of herpes simplex virus type 1 latency and reactivation in vaccinated mice

The anti-apoptotic function of HSV-1 LAT in neuronal cell cultures but not its function during reactivation correlates with expression of two small non-coding RNAs, sncRNA1&2

Multiple functions are associated with HSV-1 latency associated transcript (LAT), including establishment of latency, virus reactivation, and antiapoptotic activity. LAT encodes two sncRNAs that are not miRNAs and previously it was shown that they have antiapoptotic activity in vitro. To determine if we can separate the antiapoptotic function of LAT from its latency-reactivation function, we deleted sncRNA1 and sncRNA2 sequences in HSV-1 strain McKrae, creating ΔsncRNA1&2 recombinant virus. Deletion of the sncRNA1&2 in ΔsncRNA1&2 virus was confirmed by complete sequencing of ΔsncRNA1&2 virus and its parental virus. Replication of ΔsncRNA1&2 virus in tissue culture or in the eyes of WT infected mice was similar to that of HSV-1 strain McKrae (LAT-plus) and dLAT2903 (LAT-minus) viruses. The levels of gB DNA in trigeminal ganglia (TG) of mice latently infected with ΔsncRNA1&2 virus was intermediate to that of dLAT2903 and McKrae infected mice, while levels of LAT in TG of latently infected ΔsncRNA1&2 mice was significantly higher than in McKrae infected mice. Similarly, the levels of LAT expression in Neuro-2A cells infected with ΔsncRNA1&2 virus was significantly higher than in McKrae infected cells. Reactivation in TG of ΔsncRNA1&2 infected mice was similar to that of McKrae and time of reactivation in both groups were significantly faster than dLAT2903 infected mice. However, levels of apoptosis in Neuro-2A cells infected with ΔsncRNA1&2 virus was similar to that of dLAT2903 and significantly higher than that of McKrae infected cells. Our results suggest that the antiapoptotic function of LAT resides within the two sncRNAs, which works independently of its latency-reactivation function and it has suppressive effect on LAT expression in vivo and in vitro.

Role of PD-1 and immune cells in HSV infection and latency

A large proportion of the world's population is infected with HSV-1. Antiviral CD8+ T cells and CD8α+ dendritic cells are closely related to HSV-1 infection and latency. Latency-associated transcript of HSV-1 and PD-1 are involved in the regulation of latency and reactivation of HSV-1. Here, the role of latency-associated transcript, PD-1, CD8+ T cells and CD8α+ dendritic cells in HSV-1 infection, the inter-relationships between them and how these interactions lead to latency are discussed, possibly providing new ideas for the treatment of HSV-1 infection.

Binding of herpesvirus entry mediator (HVEM) and HSV-1 gD affect reactivation but not latency levels

Previously we reported that the HSV-1 latency associated transcript (LAT) specifically upregulates the cellular herpesvirus entry mediator (HVEM) but no other known HSV-1 receptors. HSV-1 glycoprotein D (gD) binds to HVEM but the effect of this interaction on latency-reactivation is not known. We found that the levels of latent viral genomes were not affected by the absence of gD binding to HVEM. However, reactivation of latent virus in trigeminal ganglia explant cultures was blocked in the absence of gD binding to HVEM. Neither differential HSV-1 replication and spread in the eye nor levels of latency influenced reactivation. Despite similar levels of latency, reactivation in the absence of gD binding to HVEM correlated with reduced T cell exhaustion. Our results indicate that HVEM-gD signaling plays a significant role in HSV-1 reactivation but not in ocular virus replication or levels of latency. The results presented here identify gD binding to HVEM as an important target that influences reactivation and survival of ganglion resident T cells but not levels of latency. This concept may also apply to other herpesviruses that engages HVEM.

Stacking the odds: Multiple sites for HSV-1 latency

A hallmark of herpes simplex virus (HSV) infection is the establishment of latent virus in peripheral sensory ganglia of the latently infected host. We and others originally reported that the latency-associated transcript (LAT) is the only abundantly expressed viral gene in neurons within trigeminal ganglia (TG) of a latently infected host. Here, we investigated the possible contribution of various cells [i.e., B cells, dendritic cells (DCs), fibroblasts, glial cells, innate lymphoid cells (ILCs), macrophages, microglia, monocytes, natural killer cells, neurons, neutrophils, and T cells] isolated from TG of latently infected mice. Our results demonstrated that all of these cell types contain LAT, with DCs, neurons, and ILCs having the most LAT⁺ cells. These results suggest that HSV-1 can establish a quiescent/latent infection in a subset of nonneuronal cells, which enhances the chances that the virus will survive in its host.

Knockout of signal peptide peptidase in the eye reduces HSV-1 replication and eye disease in ocularly infected mice

We previously reported that knocking out signal peptide peptidase (SPP), a glycoprotein K (gK) binding partner, in mouse peripheral sensory neurons reduced latency-reactivation in infected mice without affecting primary virus replication or eye disease. Since virus replication in the eye plays an essential role in eye disease, we generated a conditional knockout mouse lacking SPP expression in the eye by crossing Pax6 (paired box 6)-Cre mice that have intact Pax6 expression with SPPflox/flox mice. Significantly less SPP protein expression was detected in the eyes of Pax6-SPP-/- mice than in WT control mice. HSV-1 replication in the eyes of Pax6-SPP-/- mice was significantly lower than in WT control mice. Levels of gB, gK, and ICP0 transcripts in corneas, but not trigeminal ganglia (TG), of Pax6-SPP-/- infected mice were also significantly lower than in WT mice. Corneal scarring and angiogenesis were significantly lower in Pax6-SPP-/- mice than in WT control mice, while corneal sensitivity was significantly higher in Pax6-SPP-/- mice compared with WT control mice. During acute viral infection, absence of SPP in the eye did not affect CD4 expression but did affect CD8α and IFNγ expression in the eye. However, in the absence of SPP, latency-reactivation was similar in Pax6-SPP-/- and WT control groups. Overall, our results showed that deleting SPP expression in the eyes reduced primary virus replication in the eyes, reduced CD8α and IFNγ mRNA expression, reduced eye disease and reduced angiogenesis but did not alter corneal sensitivity or latency reactivation to HSV-1 infection. Thus, blocking gK binding to SPP in the eye may have therapeutic potential by reducing both virus replication in the eye and eye disease associated with virus replication.

Fraternal Twins: The Enigmatic Role of the Immune System in Alphaherpesvirus Pathogenesis and Latency and Its Impacts on Vaccine Efficacy

Although the establishment, maintenance and reactivation from alphaherpesvirus latency is far from fully understood, some things are now manifestly clear: Alphaherpesvirus latency occurs in neurons of the peripheral nervous system and control of the process is multifactorial and complex. This includes components of the immune system, contributions from non-neuronal cells surrounding neurons in ganglia, specialized nucleic acids and modifications to the viral DNA to name some of the most important. Efficacious vaccines have been developed to control both acute varicella and zoster, the outcome of reactivation, but despite considerable effort vaccines for acute herpes simplex virus (HSV) infection or reactivated lesions have thus far failed to materialize despite considerable effort. Given the relevance of the immune system to establish and maintain HSV latency, a vaccine designed to tailor the HSV response to maximize the activity of components most critical for controlling reactivated infection might limit the severity of recurrences and hence reduce viral transmission. In this review, we discuss the current understanding of immunological factors that contribute to HSV and VZV latency, identify differences between varicella-zoster virus (VZV) and HSV that could explain why vaccines have been valuable at controlling VZV disease but not HSV, and finish by outlining possible strategies for developing effective HSV vaccines.

Small Noncoding RNA (sncRNA1) within the Latency-Associated Transcript Modulates Herpes Simplex Virus 1 Virulence and the Host Immune Response during Acute but Not Latent Infection

HSV-1 latency-associated transcript (LAT) plays a major role in establishing latency and reactivation; however, the mechanism by which LAT controls these processes is largely unknown. In this study, we sought to establish the role of the small noncoding RNA1 (sncRNA1) encoded within LAT during HSV-1 ocular infection. Our results suggest that sncRNA1 has a protective role during acute ocular infection by modulating the innate immune response to infection.

Essential role of M1 macrophages in blocking cytokine storm and pathology associated with murine HSV-1 infection

Ocular HSV-1 infection is a major cause of eye disease and innate and adaptive immunity both play a role in protection and pathology associated with ocular infection. Previously we have shown that M1-type macrophages are the major and earliest infiltrates into the cornea of infected mice. We also showed that HSV-1 infectivity in the presence and absence of M2-macrophages was similar to wild-type (WT) control mice. However, it is not clear whether the absence of M1 macrophages plays a role in protection and disease in HSV-1 infected mice. To explore the role of M1 macrophages in HSV-1 infection, we used mice lacking M1 activation (M1^-/- mice). Our results showed that macrophages from M1^-/- mice were more susceptible to HSV-1 infection in vitro than were macrophages from WT mice. M1^-/- mice were highly susceptible to ocular infection with virulent HSV-1 strain McKrae, while WT mice were refractory to infection. In addition, M1^-/- mice had higher virus titers in the eyes than did WT mice. Adoptive transfer of M1 macrophages from WT mice to M1^-/- mice reduced death and rescued virus replication in the eyes of infected mice. Infection of M1^-/- mice with avirulent HSV-1 strain KOS also increased ocular virus replication and eye disease but did not affect latency-reactivation seen in WT control mice. Severity of virus replication and eye disease correlated with significantly higher inflammatory responses leading to a cytokine storm in the eyes of M1^-/- infected mice that was not seen in WT mice. Thus, for the first time, our study illustrates the importance of M1 macrophages specifically in primary HSV-1 infection, eye disease, and survival but not in latency-reactivation.

Macrophages circulating in the blood or present in different tissues constitute an important barrier against infection. We previously showed that the absence of M2 macrophages does not impact HSV-1 infectivity in vivo. However, in this study we demonstrated an essential role of M1 macrophages in protection from primary HSV-1 replication, death, and eye disease but not in latency-reactivation.

Suppression of CD80 expression by ICP22 affect HSV-1 replication and CD8+IFNγ+ infiltrates in the eye of infected mice but not latency-reactivation

Previously, we reported that herpes simplex virus type 1 (HSV-1) ICP22 binds to the CD80 promoter and suppresses its expression in vitro and in vivo. To better understand the impact of ICP22 binding to CD80 on HSV-1 infectivity and pathogenicity, we mapped the region of ICP22 required to bind the CD80 promoter to a 40-amino-acid (aa) region of ICP22. We constructed a recombinant HSV-1 expressing a truncated form of ICP22 that lacks these 40 aa, which does not bind to the CD80 promoter (KOS-ICP22Δ40) and retains the ability to replicate efficiently in rabbit skin cells, in contrast to ICP22-null virus. The replication of this recombinant virus in vitro and in vivo was higher than that of the ICP22-null virus, but virus replication kinetics were lower than those of the wild-type (WT) control virus. Similar to ICP22-null virus, the KOS-ICP22Δ40 mutant virus increased CD80 expression in dendritic cells (DCs) and interferon gamma (IFN-γ) expression in CD8⁺ T cells but not CD4⁺ T cells in infected mouse corneas. In contrast to the significantly reduced virus replication in the eyes of ocularly infected mice, the levels of latency reactivation were similar between KOS-ICP22Δ40 virus and WT virus. Thus, blocking ICP22 binding to the CD80 promoter using a recombinant virus expressing a truncated ICP22 that lacks CD80 promoter binding appears to reduce virus replication and enhance CD8⁺IFN-γ⁺ infiltrates in corneas of infected mice, with no effect on latency reactivation.

IMPORTANCE Direct binding of HSV-1 ICP22 to the CD80 promoter downregulates the expression of the costimulatory molecule CD80 but not CD86. In this study, we fine mapped the region of ICP22 required for binding to the CD80 promoter and constructed a recombinant virus containing a deletion in ICP22 that failed to bind to the CD80 promoter. This recombinant virus replicated less efficiently in vitro and in vivo than did the WT control virus, although CD80-expressing CD11c⁺ cells and IFN-γ-expressing CD8⁺ T cells were increased. Interestingly, the levels of latency and reactivation in the two viruses were similar despite lower virus replication in the eyes of infected mice. Therefore, blocking the interaction of ICP22 with the CD80 promoter could be used to temper the immune response.

Crosstalk Between Epithelial Cells, Neurons and Immune Mediators in HSV-1 Skin Infection

Herpes simplex virus type 1 (HSV-1) infection is highly prevalent in humans, with approximately two-thirds of the world population living with this virus. However, only a fraction of those carrying HSV-1, which elicits lifelong infections, are symptomatic. HSV-1 mainly causes lesions in the skin and mucosae but reaches the termini of sensory neurons innervating these tissues and travels in a retrograde manner to the neuron cell body where it establishes persistent infection and remains in a latent state until reactivated by different stimuli. When productive reactivations occur, the virus travels back along axons to the primary infection site, where new rounds of replication are initiated in the skin, in recurrent or secondary infections. During this process, new neuron infections occur. Noteworthy, the mechanisms underlying viral reactivations and the exit of latency are somewhat poorly understood and may be regulated by a crosstalk between the infected neurons and components of the immune system. Here, we review and discuss the immune responses that occur at the skin during primary and recurrent infections by HSV-1, as well as at the interphase of latently-infected neurons. Moreover, we discuss the implications of neuronal signals over the priming and migration of immune cells in the context of HSV-1 infection.

Restoring Herpesvirus Entry Mediator (HVEM) Immune Function in HVEM-/- Mice Rescues Herpes Simplex Virus 1 Latency and Reactivation Independently of Binding to Glycoprotein D

The immune modulatory protein herpes virus entry mediator (HVEM) is one of several cellular receptors used by herpes simplex virus 1 (HSV-1) for cell entry. HVEM binds to HSV-1 glycoprotein D (gD) but is not necessary for HSV-1 replication in vitro or in vivo Previously, we showed that although HSV-1 replication was similar in wild-type (WT) control and HVEM-/- mice, HSV-1 does not establish latency or reactivate effectively in mice lacking HVEM, suggesting that HVEM is important for these functions. It is not known whether HVEM immunomodulatory functions contribute to latency and reactivation or whether its binding to gD is necessary. We used HVEM-/- mice to establish three transgenic mouse lines that express either human WT HVEM or human or mouse HVEM with a point mutation that ablates its ability to bind to gD. Here, we show that HVEM immune function, not its ability to bind gD, is required for WT levels of latency and reactivation. We further show that HVEM binding to gD does not affect expression of the HVEM ligands BTLA, CD160, or LIGHT. Interestingly, our results suggest that binding of HVEM to gD may contribute to efficient upregulation of CD8α but not PD1, TIM-3, CTLA4, or interleukin 2 (IL-2). Together, our results establish that HVEM immune function, not binding to gD, mediates establishment of latency and reactivation.IMPORTANCE HSV-1 is a common cause of ocular infections worldwide and a significant cause of preventable blindness. Corneal scarring and blindness are consequences of the immune response induced by repeated reactivation events. Therefore, HSV-1 therapeutic approaches should focus on preventing latency and reactivation. Our data suggest that the immune function of HVEM plays an important role in the HSV-1 latency and reactivation cycle that is independent of HVEM binding to gD.

Role of TH17 Responses in Increasing Herpetic Keratitis in the Eyes of Mice Infected with HSV-1

Purpose

T_H17 cells play an important role in host defense and autoimmunity yet very little is known about the role of IL17 in herpes simplex virus (HSV)-1 infectivity. To better understand the relationship between IL17 and HSV-1 infection, we assessed the relative impact of IL17A-deficiency and deficiency of its receptors on HSV-1 responses in vivo.

Methods

We generated IL17RA^−/− and IL17RA^−/−RC^−/− mice in-house and infected them along with IL17A^−/− and IL17RC^−/− mice in the eyes with 2 × 10⁵ PFU/eye of wild type (WT) HSV-1 strain McKrae. WT C57BL/6 mice were used as control. Virus replication in the eye, survival, corneal scarring (CS), angiogenesis, levels of latency-reactivation, and levels of CD8 and exhaustion markers (PD1, TIM3, LAG3, CTLA4, CD244, and CD39) in the trigeminal ganglia (TG) of infected mice were determined on day 28 postinfection.

Results

No significant differences in virus replication in the eye, survival, latency, reactivation, and exhaustion markers were detected among IL17A^−/−, IL17RA^−/−, IL17RC^−/−, IL17RA^−/−RC^−/−, and WT mice. However, mice lacking IL17 had significantly less CS and angiogenesis than WT mice. In addition, angiogenesis levels in the absence of IL17RC and irrespective of the absence of IL17RA were significantly less than in IL17A- or IL17RA-deficient mice.

Conclusions

Our results suggest that the absence of IL17 protects against HSV-1-induced eye disease, but has no role in protecting against virus replication, latency, or reactivation. In addition, our data provide rationale for blocking IL17RC function rather than IL17A or IL17RA function as a key driver of HSV-1-induced eye disease.

Expression of Murine CD80 by Herpes Simplex Virus 1 in Place of Latency-Associated Transcript (LAT) Can Compensate for Latency Reactivation and Anti-apoptotic Functions of LAT

High rates of wild-type (WT) herpes simplex virus 1 (HSV-1) latency reactivation depend on the anti-apoptotic activities of latency-associated transcript (LAT). Replacing LAT with the baculovirus inhibitor of apoptosis protein (cpIAP) or cellular FLIP (FLICE-like inhibitory protein) gene restored the WT latency reactivation phenotype to that of a LAT-minus [LAT(-)] virus, while similar recombinant viruses expressing interleukin-4 (IL-4) or interferon gamma (IFN-γ) did not. However, HSV-1 recombinant virus expressing cpIAP did not restore all LAT functions. Recently, we reported that a similar recombinant virus expressing CD80 in place of LAT had higher latency reactivation than a LAT-null virus. The present study was designed to determine if this CD80-expressing recombinant virus can restore all LAT functions as observed with WT virus. Our results suggest that overexpression of CD80 fully rescues LAT function in latency reactivation, apoptosis, and immune exhaustion, suggesting that LAT and CD80 have multiple overlapping functions.IMPORTANCE Recurring ocular infections caused by HSV-1 can cause corneal scarring and blindness. A major function of the HSV-1 latency-associated transcript (LAT) is to establish high levels of latency and reactivation, thus contributing to the development of eye disease. Here, we show that the host CD80 T cell costimulatory molecule functions similarly to LAT and can restore the ability of LAT to establish latency, reactivation, and immune exhaustion as well as induce the expression of caspase 3, caspase 8, caspase 9, and Bcl2. Our results suggest that, in contrast to several other previously tested genes, CD80-expressing virus can completely compensate for all known and tested LAT functions.

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.

Roles of Type 1, 2, and 3 Innate Lymphoid Cells in Herpes Simplex Virus 1 Infection In Vitro and In Vivo

Innate lymphoid cells (ILCs) play important roles in host defense and inflammation. They are classified into three distinct groups based on their cytokine and chemokine secretion patterns and transcriptome profiles. Here, we show that ILCs isolated from mice can be infected with herpes simplex virus 1 (HSV-1) but that subsequent replication of the virus is compromised. After infection, type 2 ILCs expressed significantly higher levels of granulocyte colony-stimulating factor (G-CSF), interleukin 1α (IL-1α), IL-6, IL-9, RANTES, tumor necrosis factor alpha (TNF-α), CXCL1, CXCL2, CXCL10, CCL3, and CCL4 than infected type 1 or type 3 ILCs. Transcriptome-sequencing (RNA-seq) analysis of the ILCs 24 h after HSV-1 infection revealed that 77 herpesvirus genes were detected in the infected type 3 ILCs, whereas only 11 herpesvirus genes were detected in infected type 1 ILCs and 27 in infected type 2 ILCs. Compared with uninfected cells, significant upregulation of over 4,000 genes was seen in the HSV-1-infected type 3 ILCs, whereas 414 were upregulated in the infected type 1 ILCs and 128 in the infected type 2 ILCs. In contrast, in all three cell types, only a limited number of genes were significantly downregulated. Type 1, type 2, and type 3 ILC-deficient mice were used to gain insights into the effects of the ILCs on the outcome of ocular HSV-1 infection. No significant differences were found on comparison with similarly infected wild-type mice or on comparison of the three strains of deficient mice in terms of virus replication in the eyes, levels of corneal scarring, latency-reactivation in the trigeminal ganglia, or T-cell exhaustion. Although there were no significant differences in the survival rates of infected ILC-deficient mice and wild-type mice, there was significantly reduced survival of the infected type 1 or type 3 ILC-deficient mice compared with type 2 ILC-deficient mice. Adoptive transfer of wild-type T cells did not alter survival or any other parameters tested in the infected mice. Our results indicate that type 1, 2, and 3 ILCs respond differently to HSV-1 infection in vitro and that the absence of type 1 or type 3, but not type 2, ILCs affects the survival of ocularly infected mice.IMPORTANCE In this study, we investigated for the first time what roles, if any, innate lymphoid cells (ILCs) play in HSV-1 infection. Analysis of isolated ILCs in vitro revealed that all three subtypes could be infected with HSV-1 but that they were resistant to replication. The expression profiles of HSV-1-induced cytokines/chemokines and cellular and viral genes differed among the infected type 1, 2, and 3 ILCs in vitro While ILCs play no role or a redundant role in the outcomes of latency-reactivation in infected mice, absence of type 1 and type 3, but not type 2, ILCs affects the survival of infected mice.

The Chemokine Receptor CXCR4 Mediates Recruitment of CD11c+ Conventional Dendritic Cells Into the Inflamed Murine Cornea

Purpose

The cornea contains distinct populations of antigen-presenting cells (APCs), including conventional dendritic cells (cDCs). Little is known about the molecular mechanisms involved in cDCs homing and recruitment into the naïve and inflamed cornea. The purpose of this study was to investigate the presence of CXCR4 and its ligand CXCL12 in the murine cornea and its role in cDC migration during corneal inflammation.

Methods

The expression of CXCR4 and CXCL12 in naïve and suture-inflamed murine corneas was assessed by whole-mount staining, flow cytometry, and quantitative PCR. The role of CXCR4 in recruitment into inflamed corneas was investigated using adoptive transfer of cDCs blocked with neutralizing antibody against CXCR4.

Results

We show the chemokine receptor CXCR4 to be expressed on 51.7% and 64.8% of total corneal CD11c⁺ cDCs, equating to 98.6 ± 12.5 cells/mm² in the peripheral and 64.7 ± 10.6 cells/mm² in the central naïve cornea, respectively. Along with a 4.5-fold increase in CXCL12 expression during inflammation (P < 0.05), infiltrating cDCs also expressed CXCR4 in both the peripheral (222.6 ± 33.3 cells/mm²; P < 0.001) and central cornea (161.9 ± 23.8 cells/mm²; P = 0.001), representing a decrease to 31.0% and 37.3% in the cornea, respectively. Further, ex vivo blockade (390.1 ± 40.1 vs. 612.1 ± 78.3; P = 0.008) and local blockade (263.5 ± 27.1 vs. 807.5 ± 179.5, P < 0.001) with anti-CXCR4 neutralizing antibody resulted in a decrease in cDCs homing into the cornea compared with cells pretreated with isotype controls.

Conclusions

Our results demonstrate that corneal CXCL12 plays a direct role in CXCR4⁺ cDC recruitment into the cornea. The CXCR4/CXCL12 axis is therefore a potential target to modulate corneal inflammatory responses.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

CCR2+ migratory macrophages with M1 status are the early-responders in the cornea of HSV-1 infected mice

Complex interactions between HSV-1 and infiltrating immune cells play important roles in establishing localized, acute virus replication as well as chronic latent infection. The extent and duration of initial virus replication are the key determinants of subsequent pathologic inflammatory responses and therefore, the accumulation of immune cell populations at this time point is a key target for prevention. Therefore, we evaluated the role of various immune cell infiltrates between 1 h and 28 days post-infection (PI) using mice infected with virulent HSV-1 strain McKrae without corneal scarification. The effect of corneal scarification on immune cell infiltrates was also determined. We first determined the activation status and origin of macrophage infiltrates as early as 1 h PI. We found a sharp increase in the total macrophage population after 12 h PI, that was primarily due to infiltration of CCR2⁺ migratory macrophages, mostly in M1 status (MHC II⁺). The number of CCR2^- resident macrophages, mostly unpolarized (M0), increased gradually over time and peaked at 48 h PI. Interestingly, some of the resident macrophages gained an M2-like phenotype (CD206^Low), which peaked at 12 h PI, concurrent with M1 macrophage infiltration. From 1–7 days PI, infiltration of various immune cells correlated strongly with HSV-1 replication, with neutrophils showing the biggest increase, and NKT cells the biggest decrease, after infection. The presence of geographical ulcer did not correlate with increased infiltration, while mice with corneal scarring had significantly more immune cell infiltration than those without corneal scarring. Overall, we showed time-dependent infiltration of various immune cells in the eye of HSV-1 infected mice. Initial infiltration of macrophages followed by infiltration of T cells at later times PI demonstrates the importance of targeting macrophages rather than other immune cells type, for therapeutic treatment of HSV-1.

TSLP Protects Corneas From Pseudomonas aeruginosa Infection by Regulating Dendritic Cells and IL-23-IL-17 Pathway

Purpose

We sought to determine the role of epithelium-produced thymic stromal lymphopoietin (TSLP) and its underlying mechanisms in corneal innate immune defense against Pseudomonas (P.) aeruginosa keratitis.

Methods

The expression of TSLP and TSLPR in cultured human corneal epithelial cells (HCECs) and mouse corneas was determined by PCR, Western, and/or ELISA. Cellular localization of TSLP receptor (TSLPR) was determined by whole mount confocal microscopy. TSLP-TSLPR signaling was downregulated by neutralizing antibodies and/or small interfering (si)RNA; their effects on the severity of P. aeruginosa–keratitis and cytokine expression were assessed using clinical scoring, bacterial counting, PMN infiltration, and real-time PCR. The role of dendritic cells (DCs) in corneal innate immunity was determined by local DC depletion using CD11c-DTR mice.

Results

P. aeruginosa–infection induced the expression of TSLP and TSLPR in both cultured primary HCECs and in C57BL/6 mouse corneas. While TSLP was mostly expressed by epithelial cells, CD11c-positive cells were positive for TSLPR. Targeting TSLP or TSLPR with neutralizing antibodies or TSLPR with siRNA resulted in more severe keratitis, attributable to an increase in bacterial burden and PMN infiltration. TSLPR neutralization significantly suppressed infection-induced TSLP and interleukin (IL)-17C expression and augmented the expression of IL-23 and IL-17A. Local depletion of DCs markedly increased the severity of keratitis and exhibited no effects on TSLP and IL-23 expression while suppressing IL-17A and C expression in P. aeruginosa–infected corneas.

Conclusions

The epithelium-expressed TSLP plays a protective role in P. aeruginosa keratitis through targeting of DCs and in an IL-23/IL-17 signaling pathway-related manner.

Herpes Simplex Virus 1 Latency and the Kinetics of Reactivation Are Regulated by a Complex Network of Interactions between the Herpesvirus Entry Mediator, Its Ligands (gD, BTLA, LIGHT, and CD160), and the Latency-Associated Transcript

Recently, we reported that the herpesvirus entry mediator (HVEM; also called TNFRSF14 or CD270) is upregulated by the latency-associated transcript (LAT) of herpes simplex virus 1 (HSV-1) and that the absence of HVEM affects latency reactivation but not primary infection in ocularly infected mice. gD has been shown to bind to HVEM. LIGHT (TNFSF14), CD160, and BTLA (B- and T-lymphocyte attenuator) also interact with HVEM and can interfere with HSV gD binding. It was not known if LIGHT, CD160, or BTLA affected the level of latency reactivation in the trigeminal ganglia (TG) of latently infected mice. To address this issue, we ocularly infected LIGHT^-/-, CD160^-/-, and BTLA^-/- mice with LAT(+) and LAT(-) viruses, using similarly infected wild-type (WT) and HVEM^-/- mice as controls. The amount of latency, as determined by the levels of gB DNA in the TG of the LIGHT^-/-, CD160^-/-, and BTLA^-/- mice infected with either LAT(+) or LAT(-) viruses, was lower than that in WT mice infected with LAT(+) virus and was similar in WT mice infected with LAT(-) virus. The levels of LAT RNA in HVEM^-/-, LIGHT^-/-, CD160^-/-, and BTLA^-/- mice infected with LAT(+) virus were similar and were lower than the levels of LAT RNA in WT mice. However, LIGHT^-/-, CD160^-/-, and BTLA^-/- mice, independent of the presence of LAT, had levels of reactivation similar to those of WT mice infected with LAT(+) virus. Faster reactivation correlated with the upregulation of HVEM transcript. The LIGHT^-/-, CD160^-/-, and BTLA^-/- mice had higher levels of HVEM expression, and this, along with the absence of BTLA, LIGHT, or CD160, may contribute to faster reactivation, while the absence of each molecule, independent of LAT, may have contributed to lower latency. This study suggests that, in the absence of competition with gD for binding to HVEM, LAT RNA is important for WT levels of latency but not for WT levels of reactivation.IMPORTANCE The effects of BTLA, LIGHT, and CD160 on latency reactivation are not known. We show here that in BTLA, LIGHT, or CD160 null mice, latency is reduced; however, HVEM expression is upregulated compared to that of WT mice, and this upregulation is associated with higher reactivation that is independent of LAT but dependent on gD expression. Thus, one of the mechanisms by which BTLA, LIGHT, and CD160 null mice enhance reactivation appears to be the increased expression of HVEM in the presence of gD. Thus, our results suggest that blockade of HVEM-LIGHT-BTLA-CD160 contributes to reduced HSV-1 latency and reactivation.

An M2 Rather than a TH2 Response Contributes to Better Protection against Latency Reactivation following Ocular Infection of Naive Mice with a Recombinant Herpes Simplex Virus 1 Expressing Murine Interleukin-4

We found previously that altering macrophage polarization toward M2 responses by injection of colony-stimulating factor 1 (CSF-1) was more effective in reducing both primary and latent infections in mice ocularly infected with herpes simplex virus 1 (HSV-1) than M1 polarization by gamma interferon (IFN-γ) injection. Cytokines can coordinately regulate macrophage and T helper (T_H) responses, with interleukin-4 (IL-4) inducing type 2 T_H (T_H2) as well as M2 responses and IFN-γ inducing T_H1 as well as M1 responses. We have now differentiated the contributions of these immune compartments to protection against latency reactivation and corneal scarring by comparing the effects of infection with recombinant HSV-1 in which the latency-associated transcript (LAT) gene was replaced with either the IL-4 (HSV-IL-4) or IFN-γ (HSV-IFN-γ) gene using infection with the parental (LAT-negative) virus as a control. Analysis of peritoneal macrophages in vitro established that the replacement of LAT with the IL-4 or IFN-γ gene did not affect virus infectivity and promoted polarization appropriately. Protection against corneal scarring was significantly higher in mice ocularly infected with HSV-IL-4 than in those infected with HSV-IFN-γ or parental virus. Levels of primary virus replication in the eyes and trigeminal ganglia (TG) were similar in the three groups of mice, but the numbers of gC⁺ cells were lower on day 5 postinfection in the eyes of HSV-IL-4-infected mice than in those infected with HSV-IFN-γ or parental virus. Latency and explant reactivation were lower in both HSV-IL-4- and HSV-IFN-γ-infected mice than in those infected with parental virus, with the lowest level of latency being associated with HSV-IL-4 infection. Higher latency correlated with higher levels of CD8, PD-1, and IFN-γ mRNA, while reduced latency and T-cell exhaustion correlated with lower gC⁺ expression in the TG. Depletion of macrophages increased the levels of latency in all ocularly infected mice compared with their undepleted counterparts, with macrophage depletion increasing latency in the HSV-IL-4 group greater than 3,000-fold. Our results suggest that shifting the innate macrophage immune responses toward M2, rather than M1, responses in HSV-1 infection would improve protection against establishment of latency, reactivation, and eye disease.IMPORTANCE Ocular HSV-1 infections are among the most frequent serious viral eye infections in the United States and a major cause of virus-induced blindness. As establishment of a latent infection in the trigeminal ganglia results in recurrent infection and is associated with corneal scarring, prevention of latency reactivation is a major therapeutic goal. It is well established that absence of latency-associated transcripts (LATs) reduces latency reactivation. Here we demonstrate that recombinant HSV-1 expressing IL-4 (an inducer of T_H2/M2 responses) or IFN-γ (an inducer of T_H1/M1 responses) in place of LAT further reduced latency, with HSV-IL-4 showing the highest overall protective efficacy. In naive mice, this higher protective efficacy was mediated by innate rather than adaptive immune responses. Although both M1 and M2 macrophage responses were protective, shifting macrophages toward an M2 response through expression of IL-4 was more effective in curtailing ocular HSV-1 latency reactivation.

The Absence of DHHC3 Affects Primary and Latent Herpes Simplex Virus 1 Infection

UL20, an essential herpes simplex virus 1 (HSV-1) protein, is involved in cytoplasmic envelopment of virions and virus egress. We reported recently that UL20 can bind to a host protein encoded by the zinc finger DHHC-type containing 3 (ZDHHC3) gene (also known as Golgi-specific DHHC zinc finger protein [GODZ]). Here, we show for the first time that HSV-1 replication is compromised in murine embryonic fibroblasts (MEFs) isolated from GODZ^-/- mice. The absence of GODZ resulted in blocking palmitoylation of UL20 and altered localization and expression of UL20 and glycoprotein K (gK); the expression of gB and gC; and the localization and expression of tegument and capsid proteins within HSV-1-infected MEFs. Electron microscopy revealed that the absence of GODZ limited the maturation of virions at multiple steps and affected the localization of virus and endoplasmic reticulum morphology. Virus replication in the eyes of ocularly HSV-1-infected GODZ^-/- mice was significantly lower than in HSV-1-infected wild-type (WT) mice. The levels of UL20, gK, and gB transcripts in the corneas of HSV-1-infected GODZ^-/- mice on day 5 postinfection were markedly lower than in WT mice, whereas only UL20 transcripts were reduced in trigeminal ganglia (TG). In addition, HSV-1-infected GODZ^-/- mice showed notably lower levels of corneal scarring, and HSV-1 latency reactivation was also reduced. Thus, normal HSV-1 infectivity and viral pathogenesis are critically dependent on GODZ-mediated palmitoylation of viral UL20.IMPORTANCE HSV-1 infection is widespread. Ocular infection can cause corneal blindness; however, approximately 70 to 90% of American adults exposed to the virus show no clinical symptoms. In this study, we show for the first time that the absence of a zinc finger protein called GODZ affects primary and latent infection, as well as reactivation, in ocularly infected mice. The reduced virus infectivity is due to the absence of the GODZ interaction with HSV-1 UL20. These results strongly suggest that binding of UL20 to GODZ promotes virus infectivity in vitro and viral pathogenesis in vivo.

Roles of M1 and M2 Macrophages in Herpes Simplex Virus 1 Infectivity

Macrophages are the predominant infiltrate in the corneas of mice that have been ocularly infected with herpes simplex virus 1 (HSV-1). However, very little is known about the relative roles of M1 (classically activated or polarized) and M2 (alternatively activated or polarized) macrophages in ocular HSV-1 infection. To better understand these relationships, we assessed the impact of directed M1 or M2 activation of RAW264.7 macrophages and peritoneal macrophages (PM) on subsequent HSV-1 infection. In both the RAW264.7 macrophage and PM in vitro models, HSV-1 replication in M1 macrophages was markedly lower than in M2 macrophages and unstimulated controls. The M1 macrophages expressed significantly higher levels of 28 of the 32 tested cytokines and chemokines than M2 macrophages, with HSV-1 infection significantly increasing the levels of proinflammatory cytokines and chemokines in the M1 versus the M2 macrophages. To examine the effects of shifting the immune response toward either M1 or M2 macrophages in vivo, wild-type mice were injected with gamma interferon (IFN-γ) DNA or colony-stimulating factor 1 (CSF-1) DNA prior to ocular infection with HSV-1. Virus replication in the eye, latency in trigeminal ganglia (TG), and markers of T cell exhaustion in the TG were determined. We found that injection of mice with IFN-γ DNA, which enhances the development of M1 macrophages, increased virus replication in the eye; increased latency; and also increased CD4, CD8, IFN-γ, and PD-1 transcripts in the TG of latently infected mice. Conversely, injection of mice with CSF-1 DNA, which enhances the development of M2 macrophages, was associated with reduced virus replication in the eye and reduced latency and reduced the levels of CD4, CD8, IFN-γ,and PD-1 transcripts in the TG. Collectively, these results suggest that M2 macrophages directly reduce the levels of HSV-1 latency and, thus, T-cell exhaustion in the TG of ocularly infected mice.IMPORTANCE Our findings demonstrate a novel approach to further reducing HSV-1 replication in the eye and latency in the TG by modulating immune components, specifically, by altering the phenotype of macrophages. We suggest that inclusion of CSF-1 as part of any vaccination regimen against HSV infection to coax responses of macrophages toward an M2, rather than an M1, response may further improve vaccine efficacy against ocular HSV-1 replication and latency.

Dendritic cells, macrophages, NK and CD8+ T lymphocytes play pivotal roles in controlling HSV-1 in the trigeminal ganglia by producing IL1-beta, iNOS and granzyme B

Background

Herpes simplex virus type 1 (HSV-1) cause not only mild symptoms but also blindness and encephalitis. It was previously shown that the immune response against HSV-1 occurs mainly in the trigeminal ganglia (TG) and that Toll-like receptors 2 and 9 (TLR2/9) are important in mediating this response. It was also demonstrated that iNOS (nitric oxide synthase) and interleukin 1 beta (IL-1β) play an essential role in the defense against HSV-1 infection. Importantly, the present work aimed to identify the primary cells responsible for iNOS and IL-1β production and search for other important molecules and cells that might or might not depend on TLR2/9 receptors to mediate the immune response against HSV-1.

Methods

C57BL/6 (wild type, WT) and TLR2/9^−/− mice were infected by the intranasal route with HSV-1 (1 × 10⁶ p.f.u.). Cells were obtained from the TG and spleen tissues and the profile of immune cells was determined by flow cytometry in infected and mock infected WT and knockout mice. The percentage of cells producing iNOS, IL-1β, granzyme B and perforin was also determined by flow cytometry. Chemokine monocyte chemoattractant protein-1 (MCP1) was measured by Cytometric Bead Array (CBA) in the TG, spleen and lung. Expression of type I interferons (IFNs), interleukins (IL) 5 and 10, IL-1β and granzyme B were quantified by real time PCR.

Results

The results indicate that dendritic cells (DCs) and monocytes/macrophages (Mo/Mϕ) were the main sources of IL-1β and iNOS, respectively, which, together with type I IFNs, were essential for the immune response against HSV-1. Additionally, we showed that granzyme B produced by CD8⁺ T and NK lymphocytes and MCP-1 were also important for this immune response. Moreover, our data indicate that the robust production of MCP-1 and granzyme B is either TLR-independent or down regulated by TLRs and occurs in the TG of TLR2/9^−/− infected mice.

Conclusion

Taken together, our data provide strong evidence that the responses mediated by DCs, Mo/Mϕ, NK and CD8⁺ T lymphocytes through IL-1β, iNOS and granzyme B production, respectively, together with the production of type I IFN early in the infection, are crucial to host defense against HSV-1.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0692-x) contains supplementary material, which is available to authorized users.

Interrelationship of Primary Virus Replication, Level of Latency, and Time to Reactivation in the Trigeminal Ganglia of Latently Infected Mice

We sought to determine the possibility of an interrelationship between primary virus replication in the eye, the level of viral DNA in the trigeminal ganglia (TG) during latency, and the amount of virus reactivation following ocular herpes simplex virus type 1 (HSV-1) infection. Mice were infected with virulent (McKrae) or avirulent (KOS and RE) strains of HSV-1, and virus titers in the eyes and TG during primary infection, level of viral gB DNA in TG on day 28 postinfection (p.i.), and virus reactivation on day 28 p.i. as measured by explant reactivation were calculated. Our results suggest that the avirulent strains of HSV-1, even after corneal scarification, had lower virus titers in the eye, had less latency in the TG, and took a longer time to reactivate than virulent strains of HSV-1. The time to explant reactivation of avirulent strains of HSV-1 was similar to that of the virulent LAT((-)) McKrae-derived mutant. The viral dose with the McKrae strain of HSV-1 affected the level of viral DNA and time to explant reactivation. Overall, our results suggest that there is no absolute correlation between primary virus titer in the eye and TG and the level of viral DNA in latent TG and time to reactivation.

IMPORTANCE

Very little is known regarding the interrelationship between primary virus replication in the eye, the level of latency in TG, and the time to reactivate in the mouse model. This study was designed to answer these questions. Our results point to the absence of any correlation between the level of primary virus replication and the level of viral DNA during latency, and neither was an indicator of how rapidly the virus reactivated following explant TG-induced reactivation.

CD8+ T Cells Play a Bystander Role in Mice Latently Infected with Herpes Simplex Virus 1

Based on an explant reactivation model, it has been proposed that CD8(+) T cells maintain latency in trigeminal ganglia (TG) of mice latently infected with herpes simplex virus 1 (HSV-1) [T. Liu, K. M. Khanna, X. Chen, D. J. Fink, and R. L. Hendricks, J Exp Med 191:1459-1466, 2000, doi:10.1084/jem.191.9.1459; K. M. Khanna, R. H. Bonneau, P. R. Kinchington, and R. L. Hendricks, Immunity 18:593-603, 2003, doi:10.1016/S1074-7613(03)00112-2]. In those studies, BALB/c mice were ocularly infected with an avirulent HSV-1 strain (RE) after corneal scarification. However, in our studies, we typically infect mice with a virulent HSV-1 strain (McKrae) that does not require corneal scarification. Using a combination of knockout mice, adoptive transfers, and depletion studies, we recently found that CD8α(+) dendritic cells (DCs) contribute to HSV-1 latency and reactivation in TG of ocularly infected mice (K. R. Mott, S. J. Allen, M. Zandian, B. Konda, B. G. Sharifi, C. Jones, S. L. Wechsler, T. Town, and H. Ghiasi, PLoS One 9:e93444, 2014, doi:10.1371/journal.pone.0093444). This suggested that CD8(+) T cells might not be the major regulators of HSV-1 latency in the mouse TG. To investigate this iconoclastic possibility, we used a blocking CD8 antibody and CD8(+) T cells in reactivated TG explants from mice latently infected with (i) the avirulent HSV-1 strain RE following corneal scarification or (ii) the virulent HSV-1 strain McKrae without corneal scarification. Independently of the strain or approach, our results show that CD8α(+) DCs, not CD8(+) T cells, drive latency and reactivation. In addition, adoptive transfer of CD8(+) T cells from wild-type (wt) mice to CD8α(-/-) mice did not restore latency to the level for wt mice or wt virus. In the presence of latency-associated transcript (LAT((+)); wt virus), CD8(+) T cells seem to play a bystander role in the TG. These bystander T cells highly express PD-1, most likely due to the presence of CD8α(+) DCs. Collectively, these results support the notion that CD8(+) T cells do not play a major role in maintaining HSV-1 latency and reactivation.

SIGNIFICANCE

This study addresses a fundamentally important and widely debated issue in the field of HSV latency-reactivation. In this article, we directly compare the effects of anti-CD8 antibody, CD8(+) T cells, LAT, and CD8α(+) DCs in blocking explant reactivation in TG of mice latently infected with avirulent or virulent HSV-1. Our data suggest that CD8(+) T cells are not responsible for an increase or maintenance of latency in ocularly infected mice. However, they seem to play a bystander role that correlates with the presence of LAT, higher subclinical reactivation levels, and higher PD-1 expression levels.

A Dual Role for Corneal Dendritic Cells in Herpes Simplex Keratitis: Local Suppression of Corneal Damage and Promotion of Systemic Viral Dissemination

The cornea is the shield to the foreign world and thus, a primary site for peripheral infections. However, transparency and vision are incompatible with inflammation and scarring that may result from infections. Thus, the cornea is required to perform a delicate balance between fighting infections and preserving vision. To date, little is known about the specific role of antigen-presenting cells in viral keratitis. In this study, utilizing an established murine model of primary acute herpes simplex virus (HSV)-1 keratitis, we demonstrate that primary HSV keratitis results in increased conventional dendritic cells (cDCs) and macrophages within 24 hours after infection. Local depletion of cDCs in CD11c-DTR mice by subconjuntival diphtheria toxin injections, led to increased viral proliferation, and influx of inflammatory cells, resulting in increased scarring and clinical keratitis. In addition, while HSV infection resulted in significant corneal nerve destruction, local depletion of cDCs resulted in a much more severe loss of corneal nerves. Further, local cDC depletion resulted in decreased corneal nerve infection, and subsequently decreased and delayed systemic viral transmission in the trigeminal ganglion and draining lymph node, resulting in decreased mortality of mice. In contrast, sham depletion or depletion of macrophages through local injection of clodronate liposomes had neither a significant impact on the cornea, nor an effect on systemic viral transmission. In conclusion, we demonstrate that corneal cDCs may play a primary role in local corneal defense during viral keratitis and preserve vision, at the cost of inducing systemic viral dissemination, leading to increased mortality.

Mutations within the pathogenic region of herpes simplex virus 1 gK signal sequences alter cell surface expression and neurovirulence

To investigate the role of the signal sequences of herpes simplex virus 1 (HSV-1) gK on virus replication and viral pathogenesis, we constructed recombinant viruses with or without mutations within the signal sequences of gK. These recombinant viruses expressed two additional copies of the mutated (MgK) or native (NgK) form of the gK gene in place of the latency-associated transcript with a myc epitope tag to facilitate detection at their 3' ends. The replication of MgK virus was similar to that of NgK both in vitro and in vivo, as well as in the trigeminal ganglia (TG) of latently infected mice. The levels of gB and gK transcripts in the corneas, TG, and brains of infected mice on days 3 and 5 postinfection were markedly virus and time dependent, as well as tissue specific. Mutation in the signal sequence of gK in MgK virus blocked cell surface expression of gK-myc in rabbit skin cells, increased 50% lethal dose, and decreased corneal scarring in ocularly infected mice compared to the NgK or revertant (RgK) virus. MgK and NgK viruses, and not the RgK virus, showed a reduced extent of explant reactivation at the lower dose of ocular infection but not at the higher dose. However, the time of reactivation was not affected by overexpression of the different forms of gK. Taken together, these results strongly suggest that the 8mer peptide (ITAYGLVL) within the signal sequence of gK promotes cell surface expression of gK in infected cells and ocular pathogenesis in infected mice.

IMPORTANCE

In this study, we show for the first time that mutations within the signal sequence of gK blocked cell surface expression of inserted recombinant gK in vitro. Furthermore, this blockage in cell surface expression was correlated with higher 50% lethal dose and less corneal scarring in vivo. Thus, these studies point to a key role for the 8mer within the signal sequence of gK in HSV-1-induced pathogenicity.

Batf3 deficiency is not critical for the generation of CD8α⁺ dendritic cells

Recently, we have reported that CD8α(+) DCs, rather than CD8(+) T cells, are involved in the establishment and maintenance of HSV-1 latency in the trigeminal ganglia (TG) of ocularly infected mice. In the current study, we investigated whether similar results can be obtained using Batf3(-/-) mice that previously were reported to lack CD8α(+) DCs. However, our results demonstrate that Batf3(-/-) mice, without any known infection, express CD8α(+) DCs. Consequently, due to the presence of CD8α(+) DCs, no differences were detected in the level of HSV-1 latency between Batf3(-/-) mice compared with wild type control mice.

Immunological aspects of acute and recurrent herpes simplex keratitis

Herpes simplex keratitis (HSK) belongs to the major causes of visual morbidity worldwide and available methods of treatment remain unsatisfactory. Primary infection occurs usually early in life and is often asymptomatic. Chronic visual impairment and visual loss are caused by corneal scaring, thinning, and vascularization connected with recurrent HSV infections. The pathogenesis of herpetic keratitis is complex and is still not fully understood. According to the current knowledge, corneal scarring and vascularization are the result of chronic inflammatory reaction against HSV antigens. In this review we discuss the role of innate and adaptive immunities in acute and recurrent HSV ocular infection and present the potential future targets for novel therapeutical options based on immune interventions.

Role of CD8+ T cells and lymphoid dendritic cells in protection from ocular herpes simplex virus 1 challenge in immunized mice

The development of immunization strategies to protect against ocular infection with herpes simplex virus 1 (HSV-1) must address the issue of the effects of the strategy on the establishment of latency in the trigeminal ganglia (TG). It is the reactivation of this latent virus that can cause recurrent disease and corneal scarring. CD8(+) T cells and dendritic cells (DCs) have been implicated in the establishment and maintenance of latency through several lines of inquiry. The objective of the current study was to use CD8α(-/-) and CD8β(-/-) mice to further evaluate the contributions of CD8(+) T cells and the CD8α(+) and CD8α(-) subpopulations of DCs to the protection afforded against ocular infection by immunization against HSV-1 and their potential to increase latency. Neutralizing antibody titers were similar in immunized CD8α(-/-), CD8β(-/-), and wild-type (WT) mice, as was virus replication in the eye. However, on day 3 postinfection (p.i.), the copy number of HSV-1 glycoprotein B (gB) was higher in the corneas and TG of CD8α(-/-) mice than those of WT mice, whereas on day 5 p.i. it was lower. As would be anticipated, the lack of CD8α(+) or CD8β(+) cells affected the levels of type I and type II interferon transcripts, but the effects were markedly time dependent and tissue specific. The levels of latent virus in the TG, as estimated by measurement of LAT transcripts and in vitro explant reactivation assays, were lower in the immunized, ocularly challenged CD8α(-/-) and WT mice than in their CD8β(-/-) counterparts. Immunization reduced the expression of PD-1, a marker of T-cell exhaustion, in the TG of ocularly challenged mice, and mock-immunized CD8α(-/-) mice had lower levels of PD-1 expression and latency than mock-immunized WT or CD8β(-/-) mice. The expansion of the CD8α(-) subpopulation of DCs through injection of WT mice with granulocyte-macrophage colony-stimulating factor (GM-CSF) DNA reduced the amount of latency and PD-1 expression in the TG of infected mice. In contrast, injection of FMS-like tyrosine kinase 3 ligand (Flt3L) DNA, which expanded both subpopulations, was less effective. Our results suggest that the absence of both CD8α(+) T cells and CD8α(+) DCs does not reduce vaccine efficacy, either directly or indirectly, in challenged mice and that administration of GM-CSF appears to play a beneficial role in reducing latency and T-cell exhaustion. Importance: In the past 2 decades, two large clinical HSV vaccine trials were performed, but both vaccine studies failed to reach their goals. Thus, as an alternative to conventional vaccine studies, we have used a different strategy to manipulate the host immune responses in an effort to induce greater protection against HSV infection. In lieu of the pleiotropic effect of CD8α(+) DCs in HSV-1 latency, in this report, we show that the absence of CD8α(+) T cells and CD8α(+) DCs has no adverse effect on vaccine efficacy. In line with our hypothesis, we found that pushing DC subpopulations from CD8α(+) DCs toward CD8α(-) DCs by injection of GM-CSF reduced the amount of latent virus and T-cell exhaustion in TG. While these studies point to the lack of a role for CD8α(+) T cells in vaccine efficacy, they in turn point to a role for GM-CSF in reducing HSV-1 latency.

CD8α dendritic cells drive establishment of HSV-1 latency

It is generally accepted that CD8 T cells play the key role to maintain HSV-1 latency in trigeminal ganglia of ocularly infected mice. Yet, comparably little is known about the role of innate immunity in establishment of viral latency. In the current study, we investigated whether CD8α DCs impact HSV-1 latency by examining latency in the trigeminal ganglia (TG) of wild-type (WT) C57BL/6 versus CD8α-/- (lack functional CD8 T cells and CD8α+ DCs), CD8β-/- (have functional CD8α+ T cells and CD8α+ DCs), and β2m-/- (lack functional CD8 T cells but have CD8α+ DCs) mice as well as BXH2 (have functional CD8 T cells but lack CD8α+ DCs) versus WT C3H (have functional CD8α T cells and CD8α+ DCs) mice. We also determined whether the phenotype of CD8α-/- and BXH2 mice could be restored to that of WT mice by adoptive transfer of WT CD8+ T cells or bone marrow (BM) derived CD8α+ DCs. Our results clearly demonstrate that CD8α DCs, rather than CD8 T cells, are responsible for enhanced viral latency and recurrences.

Coregulatory interactions among CD8α dendritic cells, the latency-associated transcript, and programmed death 1 contribute to higher levels of herpes simplex virus 1 latency

The latency-associated transcript (LAT) of herpes simplex virus 1 (HSV-1), CD8α(+) dendritic cells (DCs), and programmed death 1 (PD-1) have all been implicated in the HSV-1 latency-reactivation cycle. It is not known, however, whether an interaction between LAT and CD8α(+) DCs regulates latency and T-cell exhaustion. To address this question, we used LAT-expressing [LAT(+)] and LAT-negative [LAT(-)] viruses. Depletion of DCs in mice ocularly infected with LAT(+) virus resulted in a reduction in the number of T cells expressing PD-1 in the trigeminal ganglia (TG), whereas depletion of DCs in mice similarly infected with LAT(-) virus did not alter PD-1 expression. CD8α(+) DCs, but not CD4(+) DCs, infected with LAT(+) virus had higher levels of ICP0, ICP4, thymidine kinase (TK), and PD-1 ligand 1 (PD-L1) transcripts than those infected with LAT(-) virus. Coculture of infected bone marrow (BM)-derived DCs from wild-type (WT) mice, but not infected DCs from CD8α(-/-) mice, with WT naive T cells contributed to an increase in PD-1 expression. Transfer of bone marrow from WT mice but not CD8α(-/-) mice to recipient Rag1(-/-) mice increased the number of latent viral genomes in reconstituted mice infected with the LAT(+) virus. Collectively, these data indicated that a reduction in latency correlated with a decline in the levels of CD8α(+) DCs and PD-1 expression. In summary, our results demonstrate an interaction among LAT, PD-1, and CD11c CD8α(+) cells that regulates latency in the TG of HSV-1-infected mice.

IMPORTANCE

Very little is known regarding the interrelationship of LAT, PD-1, and CD8α(+) DCs and how such interactions might contribute to relative numbers of latent viral genomes. We show here that (i) in both in vivo and in vitro studies, deficiency of CD8α(+) DCs significantly reduced T-cell exhaustion in the presence of LAT(+) virus but not LAT(-) virus; (ii) HSV-1 infectivity was significantly lower in LAT(-)-infected DCs than in their LAT(+)-infected counterparts; and (iii) adoptive transfer of bone marrow (BM) from WT but not CD8α(-/-) mice to recipient Rag1(-/-) mice restored latency to the level in WT mice following infection with LAT(+) virus. These studies point to a key role for CD8α(+) DCs in T-cell exhaustion in the presence of LAT, which leads to larger numbers of latent viral genomes. Thus, altering this negative function of CD8α(+) DCs can potentially be used to generate a more effective vaccine against HSV infection.

Inclusion of CD80 in HSV targets the recombinant virus to PD-L1 on DCs and allows productive infection and robust immune responses

CD80 plays a critical role in stimulation of T cells and subsequent control of infection. To investigate the effect of CD80 on HSV-1 infection, we constructed a recombinant HSV-1 virus that expresses two copies of the CD80 gene in place of the latency associated transcript (LAT). This mutant virus (HSV-CD80) expressed high levels of CD80 and had similar virus replication kinetics as control viruses in rabbit skin cells. In contrast to parental virus, this CD80 expressing recombinant virus replicated efficiently in immature dendritic cells (DCs). Additionally, the susceptibility of immature DCs to HSV-CD80 infection was mediated by CD80 binding to PD-L1 on DCs. This interaction also contributed to a significant increase in T cell activation. Taken together, these results suggest that inclusion of CD80 as a vaccine adjuvant may promote increased vaccine efficacy by enhancing the immune response directly and also indirectly by targeting to DC.

Interactions between herpesvirus entry mediator (TNFRSF14) and latency-associated transcript during herpes simplex virus 1 latency

Herpesvirus entry mediator (HVEM) is one of several cell surface proteins herpes simplex virus (HSV) uses for attachment/entry. HVEM regulates cellular immune responses and can also increase cell survival. Interestingly, latency-associated transcript (LAT), the only viral gene consistently expressed during neuronal latency, enhances latency and reactivation by promoting cell survival and by helping the virus evade the host immune response. However, the mechanisms of these LAT activities are not well understood. We show here for the first time that one mechanism by which LAT enhances latency and reactivation appears to be by upregulating HVEM expression. HSV-1 latency/reactivation was significantly reduced in Hvem(-/-) mice, indicating that HVEM plays a significant role in HSV-1 latency/reactivation. Furthermore, LAT upregulated HVEM expression during latency in vivo and also when expressed in vitro in the absence of other viral factors. This study suggests a mechanism whereby LAT upregulates HVEM expression potentially through binding of two LAT small noncoding RNAs to the HVEM promoter and that the increased HVEM then leads to downregulation of immune responses in the latent microenvironment and increased survival of latently infected cells. Thus, one of the mechanisms by which LAT enhances latency/reactivation appears to be through increasing expression of HVEM.

Dendritic cell-epithelium interplay is a determinant factor for corneal epithelial wound repair

The functions of intraepithelial dendritic cells (DCs) are critical for mucosal innate and adaptive immunity, but little is known about the role of tissue-specific DCs in epithelial homeostasis and tissue repair. By using the epithelial debridement wound model and CD11c-diphtheria toxin receptor mice that express a CD11c promoter-driven diphtheria toxin receptor, we showed that DCs migrate along with the epithelial sheet to cover the wound and that local depletion of DCs resulted in a significant delay in epithelial wound closure. In response to wounding, migratory epithelia produce CXCL10, thymic stromal lymphopoietin, and IL-1β and its antagonist soluble IL-1 receptor antagonist (sIL-1Ra); depletion of corneal DCs reversed their elevated expressions to a different extent, suggesting a DC-mediated positive feedback loop in epithelial gene expression. Furthermore, both CXCL10 and thymic stromal lymphopoietin were localized in migratory epithelia, suggesting that epithelial cells play a key role in DC infiltration and activation in injured corneas. On the other hand, DC depletion resulted in suppressed epithelial AKT activation, increased cell apoptosis, and decreased polymorphonuclear leukocyte infiltration in the healing cornea. These results indicate that DCs and epithelium form a functional entity at mucosal surfaces for maintaining corneal homeostasis and for tissue repair.

Adaptive and innate transforming growth factor beta signaling impact herpes simplex virus 1 latency and reactivation

Innate and adaptive immunity play important protective roles by combating herpes simplex virus 1 (HSV-1) infection. Transforming growth factor β (TGF-β) is a key negative cytokine regulator of both innate and adaptive immune responses. Yet, it is unknown whether TGF-β signaling in either immune compartment impacts HSV-1 replication and latency. We undertook genetic approaches to address these issues by infecting two different dominant negative TGF-β receptor type II transgenic mouse lines. These mice have specific TGF-β signaling blockades in either T cells or innate cells. Mice were ocularly infected with HSV-1 to evaluate the effects of restricted innate or adaptive TGF-β signaling during acute and latent infections. Limiting innate cell but not T cell TGF-β signaling reduced virus replication in the eyes of infected mice. On the other hand, blocking TGF-β signaling in either innate cells or T cells resulted in decreased latency in the trigeminal ganglia of infected mice. Furthermore, inhibiting TGF-β signaling in T cells reduced cell lysis and leukocyte infiltration in corneas and trigeminal ganglia during primary HSV-1 infection of mice. These findings strongly suggest that TGF-β signaling, which generally functions to dampen immune responses, results in increased HSV-1 latency.

CD11c controls herpes simplex virus 1 responses to limit virus replication during primary infection

CD11c is expressed on the surface of dendritic cells (DCs) and is one of the main markers for identification of DCs. DCs are the effectors of central innate immune responses, but they also affect acquired immune responses to infection. However, how DCs influence the efficacy of adaptive immunity is poorly understood. Here, we show that CD11c(+) DCs negatively orchestrate both adaptive and innate immunity against herpes simplex virus type 1 (HSV-1) ocular infection. The effectiveness and quantity of virus-specific CD8(+) T cell responses are increased in CD11c-deficient animals. In addition, the levels of CD83, CD11b, alpha interferon (IFN-α), and IFN-β, but not IFN-γ, were significantly increased in CD11c-deficient animals. Higher levels of IFN-α, IFN-β, and CD8(+) T cells in the CD11c-deficient mice may have contributed to lower virus replication in the eye and trigeminal ganglia (TG) during the early period of infection than in wild-type mice. However, the absence of CD11c did not influence survival, severity of eye disease, or latency. Our studies provide for the first time evidence that CD11c expression may abrogate the ability to reduce primary virus replication in the eye and TG via higher activities of type 1 interferon and CD8(+) T cell responses.

IL-2 suppression of IL-12p70 by a recombinant HSV-1 expressing IL-2 induces T-cell auto-reactivity and CNS demyelination

To evaluate the role of cellular infiltrates in CNS demyelination in immunocompetent mice, we have used a model of multiple sclerosis (MS) in which different strains of mice are infected with a recombinant HSV-1 expressing IL-2. Histologic examination of the mice infected with HSV-IL-2 demonstrates that natural killer cells, dendritic cells, B cells, and CD25 (IL-2rα) do not play any role in the HSV-IL-2-induced demyelination. T cell depletion, T cell knockout and T cell adoptive transfer experiments suggest that both CD8⁺ and CD4⁺ T cells contribute to HSV-IL-2-induced CNS demyelination with CD8⁺ T cells being the primary inducers. In the adoptive transfer studies, all of the transferred T cells irrespective of their CD25 status at the time of transfer were positive for expression of FoxP3 and depletion of FoxP3 blocked CNS demyelination by HSV-IL-2. The expression levels of IL-12p35 relative to IL-12p40 differed in BM-derived macrophages infected with HSV-IL-2 from those infected with wild-type HSV-1. HSV-IL-2-induced demyelination was blocked by injecting HSV-IL-2-infected mice with IL-12p70 DNA. This study demonstrates that suppression of the IL-12p70 function of macrophages by IL-2 causes T cells to become auto-aggressive. Interruption of this immunoregulatory axis results in demyelination of the optic nerve, the spinal cord and the brain by autoreactive T cells in the HSV-IL-2 mouse model of MS.

The role of dendritic cells in alphaherpesvirus infections: archetypes and paradigms

Dendritic cells (DCs) play a critical role in orchestrating both innate and adaptive components of the immune system and are therefore of pivotal importance in the initiation of immune responses to control and eliminate viral infections. A major focus of this review is to give an overview on the recent findings that point out the importance of DCs in controlling alphaherpesvirus infections, but also indicate that these viruses have evolved several strategies to inhibit and/or exploit DC functions to delay or escape elimination by the immune system. In addition, we point out the common features and interspecies differences between DCs from man and animal, and discuss the potential use of animal alphaherpesvirus homologues to gain further insights into the interaction between alphaherpesviruses and DCs in their natural virus-host environment. Finally, recent knowledge on the potential of alphaherpesviruses as vectors for DC stimulation and their use for immunotherapy is presented.