Latency of alpha-herpes viruses is accompanied by a chronic inflammation in human trigeminal ganglia but not in dorsal root ganglia

Relevance of tissue-resident memory CD8 T cells in the onset of Parkinson's disease and examination of its possible etiologies: infectious or autoimmune?

Tissue-resident memory CD8 T cells are responsible for local immune surveillance in different tissues, including the brain. They constitute the first line of defense against pathogens and cancer cells and play a role in autoimmunity. A recently published study demonstrated that CD8 T cells with markers of residency containing distinct granzymes and interferon-γ infiltrate the parenchyma of the substantia nigra and contact dopaminergic neurons in an early premotor stage of Parkinson's disease. This infiltration precedes α-synuclein aggregation and neuronal loss in the substantia nigra, suggesting a relevant role for CD8 T cells in the onset of the disease. To date, the nature of the antigen that initiates the adaptive immune response remains unknown. This review will discuss the role of tissue-resident memory CD8 T cells in brain immune homeostasis and in the onset of Parkinson's disease and other neurological diseases. We also discuss how aging and genetic factors can affect the CD8 T cell immune response and how animal models can be misleading when studying human-related immune response. Finally, we speculate about a possible infectious or autoimmune origin of Parkinson's disease.

Varicella-zoster virus proteome-wide T-cell screening demonstrates low prevalence of virus-specific CD8 T-cells in latently infected human trigeminal ganglia

BACKGROUND

Trigeminal ganglia (TG) neurons are an important site of lifelong latent varicella-zoster virus (VZV) infection. Although VZV-specific T-cells are considered pivotal to control virus reactivation, their protective role at the site of latency remains uncharacterized.

METHODS

Paired blood and TG specimens were obtained from ten latent VZV-infected adults, of which nine were co-infected with herpes simplex virus type 1 (HSV-1). Short-term TG-derived T-cell lines (TG-TCL), generated by mitogenic stimulation of TG-derived T-cells, were probed for HSV-1- and VZV-specific T-cells using flow cytometry. We also performed VZV proteome-wide screening of TG-TCL to determine the fine antigenic specificity of VZV reactive T-cells. Finally, the relationship between T-cells and latent HSV-1 and VZV infections in TG was analyzed by reverse transcription quantitative PCR (RT-qPCR) and in situ analysis for T-cell proteins and latent viral transcripts.

RESULTS

VZV proteome-wide analysis of ten TG-TCL identified two VZV antigens recognized by CD8 T-cells in two separate subjects. The first was an HSV-1/VZV cross-reactive CD8 T-cell epitope, whereas the second TG harbored CD8 T-cells reactive with VZV specifically and not the homologous peptide in HSV-1. In silico analysis showed that HSV-1/VZV cross reactivity of TG-derived CD8 T-cells reactive with ten previously identified HSV-1 epitopes was unlikely, suggesting that HSV-1/VZV cross-reactive T-cells are not a common feature in dually infected TG. Finally, no association was detected between T-cell infiltration and VZV latency transcript abundance in TG by RT-qPCR or in situ analyses.

CONCLUSIONS

The low presence of VZV- compared to HSV-1-specific CD8 T-cells in human TG suggests that VZV reactive CD8 T-cells play a limited role in maintaining VZV latency.

Recent Issues in Varicella-Zoster Virus Latency

Varicella-zoster virus (VZV) is a human herpes virus which causes varicella (chicken pox) as a primary infection, and, following a variable period of latency in neurons in the peripheral ganglia, may reactivate to cause herpes zoster (shingles) as well as a variety of neurological syndromes. In this overview we consider some recent issues in alphaherpesvirus latency with special focus on VZV ganglionic latency. A key question is the nature and extent of viral gene transcription during viral latency. While it is known that this is highly restricted, it is only recently that the very high degree of that restriction has been clarified, with both VZV gene 63-encoded transcripts and discovery of a novel VZV transcript (VLT) that maps antisense to the viral transactivator gene 61. It has also emerged in recent years that there is significant epigenetic regulation of VZV gene transcription, and the mechanisms underlying this are complex and being unraveled. The last few years has also seen an increased interest in the immunological aspects of VZV latency and reactivation, in particular from the perspective of inborn errors of host immunity that predispose to different VZV reactivation syndromes.

Critical Role of Regulatory T Cells in the Latency and Stress-Induced Reactivation of HSV-1

Herpes simplex virus 1 (HSV-1) spreads in populations through a latency entry and reactivation cycle. The role of host immune-suppressive factor regulatory T cells (Treg cells) in controlling latency establishment and reactivation is not completely understood. Here, using an HSV-1 ocular infection murine model, we observe a positive correlation between the level of Treg cells and viral infectivity and demonstrate the requirement for Treg cells in latency establishment. Furthermore, we show that host stress leads to HSV-1 reactivation via increased Treg cell control of CD8⁺ T cells, permitting viral replication under diminished immune surveillance. Together, we propose that Treg cell regulation may serve as a key target for controlling HSV infection.

The Herpes Simplex Virus Latency-Associated Transcript Gene Is Associated with a Broader Repertoire of Virus-Specific Exhausted CD8+ T Cells Retained within the Trigeminal Ganglia of Latently Infected HLA Transgenic Rabbits

Persistent pathogens, such as herpes simplex virus 1 (HSV-1), have evolved a variety of immune evasion strategies to avoid being detected and destroyed by the host's immune system. A dynamic cross talk appears to occur between the HSV-1 latency-associated transcript (LAT), the only viral gene that is abundantly transcribed during latency, and the CD8(+)T cells that reside in HSV-1 latently infected human and rabbit trigeminal ganglia (TG). The reactivation phenotype of TG that are latently infected with wild-type HSV-1 or with LAT-rescued mutant (i.e., LAT(+)TG) is significantly higher than TG latently infected with LAT-null mutant (i.e., LAT(-)TG). Whether LAT promotes virus reactivation by selectively shaping a unique repertoire of HSV-specific CD8(+)T cells in LAT(+)TG is unknown. In the present study, we assessed the frequency, function, and exhaustion status of TG-resident CD8(+)T cells specific to 40 epitopes derived from HSV-1 gB, gD, VP11/12, and VP13/14 proteins, in human leukocyte antigen (HLA-A*0201) transgenic rabbits infected ocularly with LAT(+)versus LAT(-)virus. Compared to CD8(+)T cells from LAT(-)TG, CD8(+)T cells from LAT(+)TG (i) recognized a broader selection of nonoverlapping HSV-1 epitopes, (ii) expressed higher levels of PD-1, TIM-3, and CTLA-4 markers of exhaustion, and (iii) produced less tumor necrosis factor alpha, gamma interferon, and granzyme B. These results suggest a novel immune evasion mechanism by which the HSV-1 LAT may contribute to the shaping of a broader repertoire of exhausted HSV-specific CD8(+)T cells in latently infected TG, thus allowing for increased viral reactivation.

IMPORTANCE

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

Adaptive Immune Responses in a Multiple Sclerosis Patient with Acute Varicella-Zoster Virus Reactivation during Treatment with Fingolimod

Fingolimod, an oral sphingosine 1-phosphate (S1P) receptor modulator, is approved for the treatment of relapsing forms of multiple sclerosis (MS). The interference with S1P signaling leads to retention particularly of chemokine receptor-7 (CCR7) expressing T cells in lymph nodes. The immunological basis of varicella zoster virus (VZV) infections during fingolimod treatment is unclear. Here, we studied the dynamics of systemic and intrathecal immune responses associated with symptomatic VZV reactivation including cessation of fingolimod and initiation of antiviral therapy. Key features in peripheral blood were an about two-fold increase of VZV-specific IgG at diagnosis of VZV reactivation as compared to the previous months, a relative enrichment of effector CD4+ T cells (36% versus mean 12% in controls), and an accelerated reconstitution of absolute lymphocytes counts including a normalized CD4+/CD8+ ratio and reappearance of CCR7+ T cells. In cerebrospinal fluid (CSF) the lymphocytic pleocytosis and CD4+/CD8+ ratios at diagnosis of reactivation and after nine days of fingolimod discontinuation remained unchanged. During this time CCR7+ T cells were not observed in CSF. Further research into fingolimod-associated VZV reactivation and immune reconstitution is mandatory to prevent morbidity and mortality associated with this potentially life-threatening condition.

Diffusion tensor magnetic resonance imaging of trigeminal nerves in relapsing herpetic keratouveitis

Background

Corneal hypoesthesia is the landmark of HSV and VZV keratitis and can lead to neurotrophic keratitis. Diffusion tensor imaging (DTI) is a new magnetic resonance imaging (MRI) derived technique, which offers possibilities to study axonal architecture. We aimed at assessing the potential impact of recurrent HSV or VZV-related keratitis on the axonal architecture of trigeminal nerves using DTI.

Design

Prospective non-interventional study.

Participants

Twelve patients and 24 controls.

Methods

DTI using MRI of the trigeminal fibers and corneal esthesiometry using the Cochet-Bonnet esthesiometer were acquired for patients affected by unilateral and recurrent HSV or VZV-related keratitis (3 months after the last corneal inflammatory event), and control subjects with no history of ocular or neuronal disease affecting the trigeminal pathways.

Main Outcome Measures

Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were compared between the 2 eyes of both patients and controls, and correlated with corneal esthesiometry.

Results

FA was lower in the trigeminal fibers ipsilateral to the affected eye compared to the non-affected side (0.39±0.02 versus 0.46±0.04, P=0.03). This difference was more important than the intra-individual variability observed in controls. Concomitantly, the asymmetry in ADC results was significantly correlated with the loss of corneal sensitivity in the affected eye.

Conclusions

Corneal hypoesthesia related to HSV and VZV keratitis is associated with persistent modifications in the architecture and functionality of the trigeminal fibers. These results add further explanation to the pathogenesis of HSV and VZV-induced neurotrophic keratitis, which may occur despite an apparent quiescence of the disease.

The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

Analysis of T cell responses during active varicella-zoster virus reactivation in human ganglia

Varicella-zoster virus (VZV) is responsible for both varicella (chickenpox) and herpes zoster (shingles). During varicella, the virus establishes latency within the sensory ganglia and can reactivate to cause herpes zoster, but the immune responses that occur in ganglia during herpes zoster have not previously been defined. We examined ganglia obtained from individuals who, at the time of death, had active herpes zoster. Ganglia innervating the site of the cutaneous herpes zoster rash showed evidence of necrosis, secondary to vasculitis, or localized hemorrhage. Despite this, there was limited evidence of VZV antigen expression, although a large inflammatory infiltrate was observed. Characterization of the infiltrating T cells showed a large number of infiltrating CD4(+) T cells and cytolytic CD8(+) T cells. Many of the infiltrating T cells were closely associated with neurons within the reactivated ganglia, yet there was little evidence of T cell-induced neuronal apoptosis. Notably, an upregulation in the expression of major histocompatibility complex class I (MHC-I) and MHC-II molecules was observed on satellite glial cells, implying these cells play an active role in directing the immune response during herpes zoster. This is the first detailed characterization of the interaction between T cells and neuronal cells within ganglia obtained from patients suffering herpes zoster at the time of death and provides evidence that CD4(+) and cytolytic CD8(+) T cell responses play an important role in controlling VZV replication in ganglia during active herpes zoster.

IMPORTANCE

VZV is responsible for both varicella (chickenpox) and herpes zoster (shingles). During varicella, the virus establishes a life-long dormant infection within the sensory ganglia and can reawaken to cause herpes zoster, but the immune responses that occur in ganglia during herpes zoster have not previously been defined. We examined ganglia obtained from individuals who, at the time of death, had active herpes zoster. We found that specific T cell subsets are likely to play an important role in controlling VZV replication in ganglia during active herpes zoster.

Defense against HSV-1 in a murine model is mediated by iNOS and orchestrated by the activation of TLR2 and TLR9 in trigeminal ganglia

Background

Herpes simplex 1 (HSV-1) causes various human clinical manifestations, ranging from simple cold sores to encephalitis. Innate immune cells recognize pathogens through Toll-like receptors (TLRs), thus initiating the immune response. Previously, we demonstrated that the immune response against HSV-1 is dependent on TLR2 and TLR9 expression and on IFN gamma production in the trigeminal ganglia (TG) of infected mice. In this work, we further investigated the cells, molecules, and mechanisms of HSV-1 infection control, especially those that are TLR-dependent.

Methods

C57BL/6 wild-type (WT), TLR2^−/−, TLR9^−/−, and TLR2/9^−/− mice were intranasally infected with HSV-1. On the viral peak day, the TG and brains were collected from mice and TLR expression was measured in the TG and brain and inducible nitric oxide synthase (iNOS) expression was measured in the TG by real-time PCR. Immunofluorescence assays were performed in mice TG to detect iNOS production by F4/80⁺ cells. Intraperitoneal macrophages nitric oxide (NO) production was evaluated by the Griess assay. WT, CD8^−/−, RAG^−/−, and iNOS^−/− mice were intranasally infected in a survival assay, and their cytokine expression was measured in the TG by real-time PCR.

Results

Infected WT mice exhibited significantly increased TLR expression, compared with their respective controls, in the TG but not in the brain. TLR-deficient mice had moderately increased TLR expression in the TG and brain in compare with the non-infected animals. iNOS expression in the WT infected mice TG was higher than in the other groups with increased production by macrophages in the WT infected mice, which did not occur in the TLR2/9^−/− mice. Additionally, the intraperitoneal macrophages of the WT mice had a higher production of NO compared with those of the TLR-deficient mice. The CD8^−/−, RAG^−/−, and iNOS^−/− mice had 100% mortality after the HSV-1 infection compared with 10% of the WT mice. Cytokines were overexpressed in the iNOS^−/− infected mice, while the RAG^−/− mice were nearly unresponsive to the virus.

Conclusion

TLRs efficiently orchestrate the innate immune cells, eliciting macrophage response (with NO production by the macrophages), thereby controlling the HSV-1 infection through the immune response in the TG of mice.

Three-dimensional normal human neural progenitor tissue-like assemblies: a model of persistent varicella-zoster virus infection

Varicella-zoster virus (VZV) is a neurotropic human alphaherpesvirus that causes varicella upon primary infection, establishes latency in multiple ganglionic neurons, and can reactivate to cause zoster. Live attenuated VZV vaccines are available; however, they can also establish latent infections and reactivate. Studies of VZV latency have been limited to the analyses of human ganglia removed at autopsy, as the virus is strictly a human pathogen. Recently, terminally differentiated human neurons have received much attention as a means to study the interaction between VZV and human neurons; however, the short life-span of these cells in culture has limited their application. Herein, we describe the construction of a model of normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs), which can be successfully maintained for at least 180 days in three-dimensional (3D) culture, and exhibit an expression profile similar to that of human trigeminal ganglia. Infection of NHNP TLAs with cell-free VZV resulted in a persistent infection that was maintained for three months, during which the virus genome remained stable. Immediate-early, early and late VZV genes were transcribed, and low-levels of infectious VZV were recurrently detected in the culture supernatant. Our data suggest that NHNP TLAs are an effective system to investigate long-term interactions of VZV with complex assemblies of human neuronal cells.

Varicella-zoster virus (VZV), the alphaherpesvirus that typically causes childhood chickenpox and shingles in adults, becomes latent in neurons, thus remaining in the body for a lifetime. Unfortunately, few models are available to study the establishment of VZV latency since the virus infects only humans and establishes persistent infections and latency only in neurons, a slowly proliferating, short-lived cell in culture. We have successfully maintained normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs) in 3-dimensional (3D) cultures for up to 6 months. The 3D NHNP TLAs show some characteristics as those found in the human trigeminal ganglia, the site of VZV latency. NHNP TLAs infected with VZV remain viable for 3 months during which time VZV DNA replicates and remains genetically stable, virus genes are transcribed, and infectious VZV is sporadically released. The ability to maintain VZV infected NHNP cells in culture for extended times provides the unique opportunity to study the molecular interactions between this important human pathogen and neuronal tissue to an extent previously unattainable.

T-cell immunity to human alphaherpesviruses

Human alphaherpesviruses (αHHV) - herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus (VZV) - infect mucosal epithelial cells, establish a lifelong latent infection of sensory neurons, and reactivate intermittingly to cause recrudescent disease. Although chronic αHHV infections co-exist with brisk T-cell responses, T-cell immune suppression is associated with worsened recurrent infection. Induction of αHHV-specific T-cell immunity is complex and results in poly-specific CD4 and CD8 T-cell responses in peripheral blood. Specific T-cells are localized to ganglia during the chronic phase of HSV infection and to several infected areas during recurrences, and persist long after viral clearance. These recent advances hold promise in the design of new vaccine candidates.

The host immune response to varicella zoster virus

Varicella zoster virus (VZV) is a highly successful human pathogen, which is never completely eliminated from the host. VZV causes two clinically distinct diseases, varicella (chickenpox) during primary infection and herpes zoster (shingles) following virus reactivation from latency. Throughout its lifecycle the virus encounters the innate and adaptive immune response, and in order to prevent eradication it has developed many mechanisms to evade and overcome these responses. This review will provide a comprehensive overview of the host immune response to VZV infection, during the multiple stages of the virus lifecycle and at key sites of VZV infection. We will also briefly describe some of the strategies employed by the virus to overcome the host immune response and the ongoing challenges in further elucidating the interplay between VZV and the host immune response in an attempt to lead to better therapies and a ‘second generation’ vaccine for VZV 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.

The herpes simplex virus type 1 latency-associated transcript inhibits phenotypic and functional maturation of dendritic cells

We recently found that the herpes simplex virus-1 (HSV-1) latency-associated transcript (LAT) results in exhaustion of virus-specific CD8⁺ T cells in latently-infected trigeminal ganglia (TG). In this study we sought to determine if this impairment may involve LAT directly and/or indirectly interfering with DC maturation. We found that a small number of HSV-1 antigen-positive DCs are present in the TG of latently-infected CD11c/eYFP mice; however, this does not imply that these DCs are acutely or latently infected. Some CD8⁺ T cells are adjacent to DCs, suggesting possible interactions. It has previously been shown that wild-type HSV-1 interferes with DC maturation. Here we show for the first time that this is associated with LAT expression, since compared to LAT⁻ virus: (1) LAT⁺ virus interfered with expression of MHC class I and the co-stimulatory molecules CD80 and CD86 on the surface of DCs; (2) LAT⁺ virus impaired DC production of the proinflammatory cytokines IL-6, IL-12, and TNF-α; and (3) DCs infected in vitro with LAT⁺ virus had significantly reduced the ability to stimulate HSV-specific CD8⁺ T cells. While a similar number of DCs was found in LAT⁺ and LAT⁻ latently-infected TG of CD11c/eYFP transgenic mice, more HSV-1 Ag-positive DCs and more exhausted CD8 T cells were seen with LAT⁺ virus. Consistent with these findings, HSV-specific cytotoxic CD8⁺ T cells in the TG of mice latently-infected with LAT⁺ virus produced less IFN-γ and TNF-α than those from TG of LAT⁻-infected mice. Together, these results suggest a novel immune-evasion mechanism whereby the HSV-1 LAT increases the number of HSV-1 Ag-positive DCs in latently-infected TG, and interferes with DC phenotypic and functional maturation. The effect of LAT on TG-resident DCs may contribute to the reduced function of HSV-specific CD8⁺ T cells in the TG of mice latently infected with LAT⁺ virus.

Investigation of varicella-zoster virus neurotropism and neurovirulence using SCID mouse-human DRG xenografts

Varicella-zoster virus (VZV) is a medically important human alphaherpesvirus. Investigating pathogenic mechanisms that contribute to VZV neurovirulence are made difficult by a marked host restriction. Our approach to investigating VZV neurotropism and neurovirulence has been to develop a mouse-human xenograft model in which human dorsal root ganglia (DRG) are maintained in severe compromised immunodeficient (SCID) mice. In this review, we will describe our key findings using this model in which we have demonstrated that VZV infection of SCID DRG xenograft results in rapid and efficient spread, enabled by satellite cell infection and polykaryon formation, which facilitates robust viral replication and release of infectious virus. In neurons that persist following this acute replicative phase, VZV genomes are present at low frequency with limited gene transcription and no protein synthesis, a state that resembles VZV latency in the natural human host. VZV glycoprotein I and interaction between glycoprotein I and glycoprotein E are critical for neurovirulence. Our work demonstrates that the DRG model can reveal characteristics about VZV replication and long-term persistence of latent VZV genomes in human neuronal tissues, in vivo, in an experimental system that may contribute to our knowledge of VZV neuropathogenesis.

Control of HSV-1 latency in human trigeminal ganglia--current overview

Although recurrent Herpes simplex virus type 1 (HSV-1) infections are quite common in humans, little is known about the exact molecular mechanisms involved in latency and reactivation of the virus from its stronghold, the trigeminal ganglion. After primary infection, HSV-1 establishes latency in sensory neurons, a state that lasts for the life of the host. Reactivation of the virus leads to recurrent disease, ranging from relatively harmless cold sores to ocular herpes. If herpes encephalitis-often a devastating disease-is also caused by reactivation or a new infection, is still a matter of debate. It is widely accepted that CD8(+) T cells as well as host cellular factors play a crucial role in maintaining latency. At least in the animal model, IFNγ and Granzyme B secretion of T cells were shown to be important for control of viral latency. Furthermore, the virus itself expresses factors that regulate its own latency-reactivation cycle. In this regard, the latency associated transcript, immediate-early proteins, and viral miRNAs seem to be the key players that control latency and reactivation on the viral side. This review focuses on HSV-1 latency in humans in the light of mechanisms learned from animal models.

Apparent expression of varicella-zoster virus proteins in latency resulting from reactivity of murine and rabbit antibodies with human blood group a determinants in sensory neurons

Analyses of varicella-zoster virus (VZV) protein expression during latency have been discordant, with rare to many positive neurons detected. We show that ascites-derived murine and rabbit antibodies specific for VZV proteins in vitro contain endogenous antibodies that react with human blood type A antigens in neurons. Apparent VZV neuronal staining and blood type A were strongly associated (by a χ² test, α = 0.0003). Adsorption of ascites-derived monoclonal antibodies or antiserum with type A erythrocytes or the use of in vitro-derived VZV monoclonal antibodies eliminated apparent VZV staining. Animal-derived antibodies must be screened for anti-blood type A reactivity to avoid misidentification of viral proteins in the neurons of the 30 to 40% of individuals who are blood type A.

Review: The neurobiology of varicella zoster virus infection

Varicella zoster virus (VZV) is a neurotropic herpesvirus that infects nearly all humans. Primary infection usually causes chickenpox (varicella), after which virus becomes latent in cranial nerve ganglia, dorsal root ganglia and autonomic ganglia along the entire neuraxis. Although VZV cannot be isolated from human ganglia, nucleic acid hybridization and, later, polymerase chain reaction proved that VZV is latent in ganglia. Declining VZV-specific host immunity decades after primary infection allows virus to reactivate spontaneously, resulting in shingles (zoster) characterized by pain and rash restricted to one to three dermatomes. Multiple other serious neurological and ocular disorders also result from VZV reactivation. This review summarizes the current state of knowledge of the clinical and pathological complications of neurological and ocular disease produced by VZV reactivation, molecular aspects of VZV latency, VZV virology and VZV-specific immunity, the role of apoptosis in VZV-induced cell death and the development of an animal model provided by simian varicella virus infection of monkeys.

The herpes simplex virus 1 latency-associated transcript promotes functional exhaustion of virus-specific CD8+ T cells in latently infected trigeminal ganglia: a novel immune evasion mechanism

Following ocular herpes simplex virus 1 (HSV-1) infection of C57BL/6 mice, HSV-specific (HSV-gB(498-505) tetramer(+)) CD8(+) T cells are induced, selectively retained in latently infected trigeminal ganglia (TG), and appear to decrease HSV-1 reactivation. The HSV-1 latency-associated transcript (LAT) gene, the only viral gene that is abundantly transcribed during latency, increases reactivation. Previously we found that during latency with HSV-1 strain McKrae-derived viruses, more of the total TG resident CD8 T cells expressed markers of exhaustion with LAT(+) virus compared to LAT(-) virus. Here we extend these findings to HSV-1 strain 17syn+-derived LAT(+) and LAT(-) viruses and to a virus expressing just the first 20% of LAT. Thus, the previous findings were not an artifact of HSV-1 strain McKrae, and the LAT function involved mapped to the first 1.5 kb of LAT. Importantly, to our knowledge, we show here for the first time that during LAT(+) virus latency, most of the HSV-1-specific TG resident CD8 T cells were functionally exhausted, as judged by low cytotoxic function and decreased gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) production. This resulted in LAT(-) TG having more functional HSV-gB(498-505) tetramer(+) CD8(+) T cells compared to LAT(+) TG. In addition, LAT expression, in the absence of other HSV-1 gene products, appeared to be able to directly or indirectly upregulate both PD-L1 and major histocompatibility complex class I (MHC-I) on mouse neuroblastoma cells (Neuro2A). These findings may constitute a novel immune evasion mechanism whereby the HSV-1 LAT directly or indirectly promotes functional exhaustion (i.e., dysfunction) of HSV-specific CD8(+) T cells in latently infected TG, resulting in increased virus reactivation.

Illuminating viral infections in the nervous system

Viral infections are a major cause of human disease. Although most viruses replicate in peripheral tissues, some have developed unique strategies to move into the nervous system, where they establish acute or persistent infections. Viral infections in the central nervous system (CNS) can alter homeostasis, induce neurological dysfunction and result in serious, potentially life-threatening inflammatory diseases. This Review focuses on the strategies used by neurotropic viruses to cross the barrier systems of the CNS and on how the immune system detects and responds to viral infections in the CNS. A special emphasis is placed on immune surveillance of persistent and latent viral infections and on recent insights gained from imaging both protective and pathogenic antiviral immune responses.

Varicella zoster virus latency

Primary infection by varicella zoster virus (VZV) typically results in childhood chickenpox, at which time latency is established in the neurons of the cranial nerve, dorsal root and autonomic ganglia along the entire neuraxis. During latency, the histone-associated virus genome assumes a circular episomal configuration from which transcription is epigenetically regulated. The lack of an animal model in which VZV latency and reactivation can be studied, along with the difficulty in obtaining high-titer cell-free virus, has limited much of our understanding of VZV latency to descriptive studies of ganglia removed at autopsy and analogy to HSV-1, the prototype alphaherpesvirus. However, the lack of miRNA, detectable latency-associated transcript and T-cell surveillance during VZV latency highlight basic differences between the two neurotropic herpesviruses. This article focuses on VZV latency: establishment, maintenance and reactivation. Comparisons are made with HSV-1, with specific attention to differences that make these viruses unique human pathogens.

Experimental models to study varicella-zoster virus infection of neurons

Varicella zoster virus (VZV) infection results in the establishment of latency in human sensory neurons. Reactivation of VZV leads to herpes zoster which can be followed by persistent neuropathic pain, termed post-herpetic neuralgia (PHN). Humans are the only natural host for VZV, and the strict species specificity of the virus has restricted the development of an animal model of infection which mimics all phases of disease. In order to elucidate the mechanisms which control the establishment of latency and reactivation as well as the effect of VZV replication on neuronal function, in vitro models of neuronal infection have been developed. Currently these models involve culturing and infecting dissociated human fetal neurons, with or without their supporting cells, an intact explant fetal dorsal root ganglia (DRG) model, neuroblastoma cell lines and rodent neuronal cell models. Each of these models has distinct advantages as well as disadvantages, and all have contributed towards our understanding of VZV neuronal infection. However, as yet none have been able to recapitulate the full virus lifecycle from primary infection to latency through to reactivation. The development of such a model will be a crucial step towards advancing our understanding of the mechanisms involved in VZV replication in neuronal cells, and the design of new therapies to combat VZV-related disease.

Varicella-zoster virus human ganglionic latency: a current summary

Varicella-zoster virus (VZV) is a ubiquitous human herpes virus typically acquired in childhood when it causes varicella (chickenpox), following which the virus establishes a latent infection in trigeminal and dorsal root ganglia that lasts for the life of the individual. VZV subsequently reactivates, spontaneously or after specific triggering factors, to cause herpes zoster (shingles), which may be complicated by postherpetic neuralgia and several other neurological complications including vasculopathy. Our understanding of VZV latency lags behind our knowledge of herpes simplex virus type 1 (HSV-1) latency primarily due to the difficulty in propagating the virus to high titers in a cell-free state, and the lack of a suitable small-animal model for studying virus latency and reactivation. It is now established beyond doubt that latent VZV is predominantly located in human ganglionic neurons. Virus gene transcription during latency is epigenetically regulated, and appears to be restricted to expression of at least six genes, with expression of gene 63 being the hallmark of latency. However, viral gene transcription may be more extensive than previously thought. There is also evidence for several VZV genes being expressed at the protein level, including VZV gene 63-encoded protein, but recent evidence suggests that this may not be a common event. The nature and extent of the chronic inflammatory response in latently infected ganglia is also of current interest. There remain several questions concerning the VZV latency process that still need to be resolved unambiguously and it is likely that this will require the use of newly developed molecular technologies, such as GeXPS multiplex polymerase chain reaction (PCR) for virus transcriptional analysis and ChIP-seq to study the epigenetic of latent virus genome ( Liu et al, 2010 , BMC Biol 8: 56).

Herpes simplex virus type 1 persists in the aged brain through hypothetical expression of accessory genes

Herpes simplex virus type 1 persists in the brain of most aged individuals and may contribute to the pathogenesis of Alzheimer's disease. The virus likely utilizes accessory genes for neural spread within the nervous system and herpes simplex virus type 1 may regulate various host responses through an array of accessory genes. This mini-review focuses on these viral accessory genes that may shed light on the potential mechanisms of this enigmatic phenomenon in the elderly brain.

Therapeutic implications of new insights into the critical role of VP16 in initiating the earliest stages of HSV reactivation from latency

Reactivation of herpes simplex virus (HSV) is a leading cause of fatal encephalitis in the USA and recurrent herpetic keratitis is a major infectious cause of blindness. There is no effective vaccine and no cure for HSV latency. While current antiviral drugs reduce viral replication, none prevent the initiation of reactivation in the nervous system and, thus, chronic inflammatory damage proceeds. The discovery that HSV VP16 is necessary for the exit from latency represents the first potential target for preventing the chronic inflammatory insult associated with HSV reactivation. Blocking VP16 transactivation would reduce the spread of the virus in the population and, importantly, presumably reduce or prevent the pathological long term chronic inflammation in the nervous system.

Expression of varicella-zoster virus immediate-early regulatory protein IE63 in neurons of latently infected human sensory ganglia

Varicella-zoster virus (VZV) causes varicella and establishes latency in sensory nerve ganglia, but the characteristics of VZV latency are not well defined. Immunohistochemical detection of the VZV immediate-early 63 (IE63) protein in ganglion neurons has been described, but there are significant discrepancies in estimates of the frequency of IE63-positive neurons, varying from a rare event to abundant expression. We examined IE63 expression in cadaver ganglia using a high-potency rabbit anti-IE63 antibody and corresponding preimmune serum. Using standard immunohistochemical techniques, we evaluated 10 ganglia that contained VZV DNA from seven individuals. These experiments showed that neuronal pigments were a confounding variable; however, by examining sections coded to prevent investigator bias and applying statistical analysis, we determined that IE63 protein, if present, is in a very small proportion of neurons (<2.8%). To refine estimates of IE63 protein abundance, we modified our protocol by incorporating a biological stain to exclude the pigment signal and evaluated 27 ganglia from 18 individuals. We identified IE63 protein in neurons within only one ganglion, in which VZV glycoprotein E and an immune cell infiltrate were also demonstrated. Antigen preservation was shown by detection of neuronal synaptophysin. These data provide evidence that the expression of IE63 protein, which has been referred to as a latency-associated protein, is rare. Refining estimates of VZV protein expression in neurons is important for developing a hypothesis about the mechanisms by which VZV latency may be maintained.

Molecular characterization of varicella zoster virus in latently infected human ganglia: physical state and abundance of VZV DNA, Quantitation of viral transcripts and detection of VZV-specific proteins

Varicella zoster virus (VZV) establishes latency in neurons of human peripheral ganglia where the virus genome is most likely maintained as a circular episome bound to histones. There is considerable variability among individuals in the number of latent VZV DNA copies. The VZV DNA burden does not appear to exceed that of herpes simplex type 1 (HSV-1). Expression of VZV genes during latency is highly restricted and is regulated epigenetically. Of the VZV open reading frames (ORFs) that have been analyzed for transcription during latency using cDNA sequencing, only ORFs 21, 29, 62, 63, and 66 have been detected. VZV ORF 63 is the most frequently and abundantly transcribed VZV gene detected in human ganglia during latency, suggesting a critical role for this gene in maintaining the latent state and perhaps the early stages of virus reactivation. The inconsistent detection and low abundance of other VZV transcripts suggest that these genes play secondary roles in latency or possibly reflect a subpopulation of neurons undergoing VZV reactivation. New technologies, such as GeXPS multiplex PCR, have the sensitivity to detect multiple low abundance transcripts and thus provide a means to elucidate the entire VZV transcriptome during latency.

Fewer latent herpes simplex virus type 1 and cytotoxic T cells occur in the ophthalmic division than in the maxillary and mandibular divisions of the human trigeminal ganglion and nerve

Following primary infection of the mouth, herpes simplex virus type 1 (HSV-1) travels retrogradely along the maxillary (V2) or mandibular (V3) nerve to the trigeminal ganglion (TG), where it establishes lifelong latency. Symptomatic HSV-1 reactivations frequently manifest as herpes labialis, while ocular HSV-1 disease is rare. We investigated whether these clinical observations are mirrored by the distribution of latent HSV-1 as well as cytotoxic T-cell infiltration around the nerve cell bodies and in the nerve fibers. The three divisions of the TG were separated by using neurofilament staining and carbocyanine dye Di-I tracing and then screened by in situ hybridization for the presence of HSV-1 latency-associated transcript (LAT). The T-cell distribution and the pattern of cytolytic molecule expression were evaluated by immunohistochemistry. The Di-I-labeled neurons were largely confined to the nerve entry zone of the traced nerve branches. Very few Di-I-labeled neurons were found in adjacent divisions due to traversing fiber bundles. LAT was abundant in the V2 and V3 divisions of all TG but was scarce or totally absent in the ophthalmic (V1) division. CD8(+) T cells were found in all three divisions of the TG and in the respective nerves, clearly clustering in V2 and V3, which is indicative of a chronic inflammation. Only T cells surrounding neurons in the V2 and V3 ganglionic divisions expressed granzyme B. In conclusion, the large accumulation of LAT and cytotoxic T cells in the V2 and V3 but not in the V1 division of the TG reflects the sites supplied by the sensory fibers and the clinical reactivation patterns.

Clinical and molecular aspects of varicella zoster virus infection

A declining cell-mediated immunity to varicella zoster virus (VZV) with advancing age or immunosuppression results in virus reactivation from latently infected human ganglia anywhere along the neuraxis. Virus reactivation produces zoster, often followed by chronic pain (postherpetic neuralgia or PHN) as well as vasculopathy, myelopathy, retinal necrosis and cerebellitis. VZV reactivation also produces pain without rash (zoster sine herpete). Vaccination after age 60 reduces the incidence of shingles by 51%, PHN by 66% and the burden of illness by 61%. However, even if every healthy adult over age 60 years is vaccinated, there would still be about 500,000 zoster cases annually in the United States alone, about 200,000 of whom will experience PHN. Analyses of viral nucleic acid and gene expression in latently infected human ganglia and in an animal model of varicella latency in primates are serving to determine the mechanism(s) of VZV reactivation with the aim of preventing reactivation and the clinical sequelae.

Noncytotoxic lytic granule-mediated CD8+ T cell inhibition of HSV-1 reactivation from neuronal latency

Reactivation of herpes simplex virus type 1 (HSV-1) from neuronal latency is a common and potentially devastating cause of disease worldwide. CD8+ T cells can completely inhibit HSV reactivation in mice, with interferon-gamma affording a portion of this protection. We found that CD8+ T cell lytic granules are also required for the maintenance of neuronal latency both in vivo and in ex vivo ganglia cultures and that their directed release to the junction with neurons in latently infected ganglia did not induce neuronal apoptosis. Here, we describe a nonlethal mechanism of viral inactivation in which the lytic granule component, granzyme B, degrades the HSV-1 immediate early protein, ICP4, which is essential for further viral gene expression.

Development and pathogenic evaluation of recombinant herpes simplex virus type 1 expressing two fluorescent reporter genes from different lytic promoters

To develop means to explore viral gene expression in ganglia without laborious histological sectioning and staining, we created a two color fluorescent recombinant HSV-1, in which enhanced green fluorescent protein (EGFP) and red fluorescent protein (RFP) are expressed from the glycoprotein B (gB) and glycoprotein C (gC) promoters respectively. We show that this virus retained growth and pathogenic capacity both in vitro and in vivo compared to wild type HSV-1; established latent infections with similar genome copy number in trigeminal ganglia (TG); induced a similar HSV-specific CD8(+) T cell infiltrate; did not induce CD8(+) T cells reactive to EGFP or RFP; and reactivated from latency with normal kinetics in ex vivo TG cultures. Fluorescent EGFP expression in plaques surrounding neurons preceded RFP expression and provided highly sensitive detection of reactivation and different stages of infection in ex vivo TG cultures. Expression of both EGFP and RFP in neurons was readily detectable in whole mounts of TG excised during acute infection and following in vivo sodium butyrate-induced reactivation from latency. This virus constitutes a useful reagent for monitoring lytic viral promoter activity in sensory neurons in vivo and in vitro.

Effector CD4+ T-cell involvement in clearance of infectious herpes simplex virus type 1 from sensory ganglia and spinal cords

In primary infection, CD8(+) T cells are important for clearance of infectious herpes simplex virus (HSV) from sensory ganglia. In this study, evidence of CD4(+) T-cell-mediated clearance of infectious HSV type 1 (HSV-1) from neural tissues was also detected. In immunocompetent mice, HSV-specific CD4(+) T cells were present in sensory ganglia and spinal cords coincident with HSV-1 clearance from these sites and remained detectable at least 8 months postinfection. Neural CD4(+) T cells isolated at the peak of neural infection secreted gamma interferon, tumor necrosis factor alpha, interleukin-2 (IL-2), or IL-4 after stimulation with HSV antigen. HSV-1 titers in neural tissues were greatly reduced over time in CD8(+) T-cell-deficient and CD8(+) T-cell-depleted mice, suggesting that CD4(+) T cells could mediate clearance of HSV-1 from neural tissue. To examine possible mechanisms by which CD4(+) T cells resolved neural infection, CD8(+) T cells were depleted from perforin-deficient or FasL-defective mice. Clearance of infectious virus from neural tissues was not significantly different in perforin-deficient or FasL-defective mice compared to wild-type mice. Further, in spinal cords and brains after vaginal HSV-1 challenge of chimeric mice expressing both perforin and Fas or neither perforin nor Fas, virus titers were significantly lower than in control mice. Thus, perforin and Fas were not required for clearance of infectious virus from neural tissues. These results suggest that HSV-specific CD4(+) T cells are one component of a long-term immune cell presence in neural tissues following genital HSV-1 infection and play a role in clearance of infectious HSV-1 at neural sites, possibly via a nonlytic mechanism.

The prevalence of human herpesvirus 6 in human sensory ganglia and its co-occurrence with alpha-herpesviruses

Human herpesvirus 6 (HHV-6) persists in the central nervous system, but its prevalence in the peripheral nervous system, a preferred latency site for herpesviruses, has not been studied. Using nested polymerase chain reaction (PCR), the authors determined the distribution of HHV-6 in human sensory ganglia. HHV-6 was present in 30% of trigeminal, 40% of geniculate, 25% of vestibular, and 55% of dorsal root ganglia. It co-occurred with alpha-herpesviruses (herpes simplex virus type 1 or varicella-zoster virus) in 91% of the ganglia. As HHV-6 positivity did not depend on the presence of inflammatory cells, known to harbor the virus, HHV-6 probably resides in the ganglia themselves.

CD8 T cell control of HSV reactivation from latency is abrogated by viral inhibition of MHC class I

In humans, herpes simplex virus (HSV) establishes latency in sensory nerve ganglia from where it periodically reactivates, whereas in murine models, the virus efficiently establishes latency but rarely reactivates. HSV inhibits MHC class I antigen presentation to CD8 T cells efficiently in humans but poorly in mice, and whether this is a crucial determinant of HSV's ability to reactivate in humans remains uncertain. To test this, we generated a panel of recombinant HSVs that inhibit presentation by murine MHC class I mimicking the effect in humans. Antigen-specific CD8 T cells prevent the in vivo reactivation of wild-type HSV. Despite their presence in the ganglia of latently infected mice, CD8 T cells do not prevent the reactivation of recombinant HSVs that inhibit murine MHC class I in mice. These findings suggest that efficient inhibition of MHC class I by HSV is a key factor in its ability to reactivate in humans.

CD8 T cells and latent herpes simplex virus type 1: keeping the peace in sensory ganglia

Herpes simplex virus type 1 (HSV-1) infections represent a significant worldwide heath problem. The lack of an effective therapy to curtail reactivation of HSV-1 from a state of neuronal latency has lead to significant morbidity and mortality. Effective therapies to prevent reactivation must likely elicit a protective CD8 T-cell response that could act to prevent reactivation from sensory neurons prior to release of infectious virus at the periphery. This review focuses on the present understanding of how CD8 T cells maintain HSV-1 latency and how this knowledge could facilitate the generation of more effective therapeutic modalities.

Presence of HSV-1 immediate early genes and clonally expanded T-cells with a memory effector phenotype in human trigeminal ganglia

The latent persistence of herpes simplex virus type 1 (HSV-1) in human trigeminal ganglia (TG) is accompanied by a chronic CD8 T-cell infiltrate. The focus of the current work was to look for HSV-1 transcription activity as a potential trigger of the immune response and to characterize the immune cell infiltrates by this feature. We combined in situ hybridization, laser cutting microscopy, and single cell RT-PCR to demonstrate the expression of the HSV-1 immediate early (IE) genes ICP0 and ICP4 in human trigeminal neurons. Using CDR3 spectratyping, we showed that the infiltrating T-cells are clonally expanded, indicating an antigen-driven immune response. Moreover, the persisting CD8+ T-cells had features of the memory effector phenotype. The voltage-gated potassium channel Kv1.3, a marker of chronic activated memory effector cells, and the chemokines CCL5 and CXCL10 were expressed by a subpopulation of infiltrating cells. The corresponding chemokine receptors CCR5 and CXCR3 were co-expressed on virtually all CD8 T-cells. In addition, T-cells expressed granzymes and perforin. In contrast to animal models of HSV-1 latency, hardly any FoxP3-positive regulatory T-cells were detected in human TG. Thus, HSV-1 IE genes are expressed in human TG and the infiltrating T-cells bear several characteristics that suggest viral antigenic stimulation.

Downregulation of class I major histocompatibility complex surface expression by varicella-zoster virus involves open reading frame 66 protein kinase-dependent and -independent mechanisms

We show here that the varicella-zoster virus (VZV) open reading frame 66 (ORF66) protein kinase is one mechanism employed to reduce class I major histocompatibility complex (MHC-I) surface expression in VZV-infected cells. Cells expressing enhanced green fluorescent protein-tagged functional and inactivated ORF66 (GFP-66 and GFP-66kd) from replication-defective adenovirus vectors revealed that ORF66 reduced MHC-I surface levels in a manner dependent on kinase activity. Cells infected with recombinant VZV expressing GFP-66 exhibited a significantly greater reduction in MHC-I surface expression than that observed in cells infected with VZV disrupted in GFP-66 expression. MHC-I maturation was delayed in its transport from the endoplasmic reticulum through the Golgi in both adenovirus-transduced cells expressing only GFP-66 and in VZV-infected cells expressing high levels of GFP-66, and this was predominantly kinase dependent. MHC-I levels were reduced in VZV-infected cells, and analyses of intracellular MHC-I revealed accumulation of folded MHC-I in the Golgi region, irrespective of ORF66 expression. Thus, the ORF66 kinase is important for VZV-mediated MHC-I downregulation, but additional mechanisms also may be involved. Analyses of the VZV ORF9a protein, the ortholog of the bovine herpesvirus 1 transporter associated with antigen processing inhibitor UL49.5 revealed no effects on MHC-I. These results establish a new role for viral protein kinases in immune evasion and suggest that VZV utilizes unique mechanisms to inhibit antigen presentation.

Selective retention of herpes simplex virus-specific T cells in latently infected human trigeminal ganglia

Primary infection with herpes simplex virus 1 (HSV-1) and varicella zoster virus (VZV) results in lifelong latent infections of neurons in sensory ganglia such as the trigeminal ganglia (TG). It has been postulated that T cells retained in TG inhibit reactivation of latent virus. The acquisition of TG specimens of individuals within hours after death offered the unique opportunity to characterize the phenotype and specificity of TG-resident T cells in humans. High numbers of activated CD8(+) T cells expressing a late effector memory phenotype were found to reside in latently infected TG. The T cell infiltrate was oligoclonal, and T cells selectively clustered around HSV-1 but not VZV latently infected neurons. Neuronal damage was not observed despite granzyme B expression by the neuron-interacting CD8(+) T cells. The TG-resident T cells, mainly CD8(+) T cells, were directed against HSV-1 and not to VZV, despite neuronal expression of VZV proteins. The results implicate that herpesvirus latency in human TG is associated with a local, persistent T cell response, comprising activated late effector memory CD8(+) T cells that appear to control HSV-1 latency by noncytolytic pathways. In contrast, T cells do not seem to be directly involved in controlling VZV latency in human TG.