Latent virus influences the generation and maintenance of CD8+ T cell memory

Herpes Simplex Virus 1 (HSV-1) 0ΔNLS Live-Attenuated Vaccine Protects against Ocular HSV-1 Infection in the Absence of Neutralizing Antibody in HSV-1 gB T Cell Receptor-Specific Transgenic Mice

The contribution of T cell and antibody responses following vaccination in resistance to herpes simplex virus 1 (HSV-1) infection continues to be rigorously investigated. In the present article, we explore the contribution of CD8+ T cells specific for the major antigenic epitope for HSV-1 glycoprotein B (gB498-505, gB) in C57BL/6 mice using a transgenic mouse (gBT-I.1) model vaccinated with HSV-1 0ΔNLS. gBT-I.1-vaccinated mice did not generate a robust neutralization antibody titer in comparison to the HSV-1 0ΔNLS-vaccinated wild-type C57BL/6 counterpart. Nevertheless, the vaccinated gBT-I.1 mice were resistant to ocular challenge with HSV-1 compared to vehicle-vaccinated animals based on survival and reduced corneal neovascularization but displayed similar levels of corneal opacity. Whereas there was no difference in the virus titer recovered from the cornea comparing vaccinated mice, HSV-1 0ΔNLS-vaccinated animals possessed significantly less infectious virus during acute infection in the trigeminal ganglia (TG) and brain stem compared to the control-vaccinated group. These results correlated with a significant increase in gB-elicited interferon-γ (IFN-γ), granzyme B, and CD107a and a reduction in lymphocyte activation gene 3 (LAG-3), programmed cell death 1 (PD-1), and T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3) expressed by TG infiltrating gB-specific CD8+ T cells from the HSV-1 0ΔNLS-vaccinated group. Antibody depletion of CD8+ T cells in HSV-1 0ΔNLS-vaccinated mice rendered animals highly susceptible to virus-mediated mortality similar to control-vaccinated mice. Collectively, the HSV-1 0ΔNLS vaccine is effective against ocular HSV-1 challenge, reducing ocular neovascularization and suppressing peripheral nerve virus replication in the near absence of neutralizing antibody in this unique mouse model.IMPORTANCE The role of CD8+ T cells in antiviral efficacy using a live-attenuated virus as the vaccine is complicated by the humoral immune response. In the case of the herpes simplex virus 1 (HSV-1) 0ΔNLS vaccine, the correlate of protection has been defined to be primarily antibody driven. The current study shows that in the near absence of anti-HSV-1 antibody, vaccinated mice are protected from subsequent challenge with wild-type HSV-1 as measured by survival. The efficacy is lost following depletion of CD8+ T cells. Whereas increased survival and reduction in virus replication were observed in vaccinated mice challenged with HSV-1, cornea pathology was mixed with a reduction in neovascularization but no change in opacity. Collectively, the study suggests CD8+ T cells significantly contribute to the host adaptive immune response to HSV-1 challenge following vaccination with an attenuated virus, but multiple factors are involved in cornea pathology in response to ocular virus challenge.

Herpes Simplex Virus 1-Specific CD8+ T Cell Priming and Latent Ganglionic Retention Are Shaped by Viral Epitope Promoter Kinetics

Reactivation of herpes simplex virus 1 (HSV-1) from neurons in sensory ganglia such as the trigeminal ganglia (TG) is influenced by virus-specific CD8⁺ T cells that infiltrate the ganglia at the onset of latency and contract to a stable activated tissue-resident memory population. In C57BL/6 mice, half of HSV-specific CD8⁺ T cells (gB-CD8s) recognize one dominant epitope (residues 498 to 505) on glycoprotein B (gB_498-505), while the remainder (non-gB-CD8s) recognize 19 subdominant epitopes from 12 viral proteins. To address how expression by HSV-1 influences the formation and ganglionic retention of CD8⁺ T cell populations, we developed recombinant HSV-1 with the native immunodominant gB epitope disrupted but then expressed ectopically from different viral promoters. In mice, the epitope expressed from the gB promoter restored full gB-CD8 immunodominance to 50%. Intriguingly, earlier expression from constitutive, immediate-early, and early promoters did not significantly increase immunodominance, indicating that these promoters cannot elicit more than half of the CD8 compartment. Epitope expressed from candidate viral promoters of "true late" HSV-1 genes either delayed or reduced the priming efficiency of gB-CD8s and their levels in the TG at early times. HSV expressing the epitope from the full latency-associated transcript promoter did not efficiently prime gB-CD8s; however, gB-CD8s primed by a concurrent wild-type flank infection infiltrated the TG and were retained long term, suggesting that latent epitope expression is sufficient to retain gB-CD8s. Taken together, the data indicate that viral promoters shape latent HSV-1-specific CD8⁺ T cell populations and should be an important consideration in future vaccine design.IMPORTANCE Latency of HSV-1 in host neurons enables long-term persistence from which reactivation may occur to cause recurrent diseases, such as blinding herpetic stromal keratitis. Latency is not antigenically silent, and viral proteins are sporadically expressed at low levels without full virion production. This protein expression is recognized by ganglion-resident HSV-1-specific CD8⁺ T cells that maintain a protective resident population. Since these T cells can influence lytic/latent decisions in reactivating neurons, we argue that improving their ganglionic retention and function may offer a strategy in vaccine design to reduce reactivation and recurrent disease. To understand factors driving the infiltration and retention of ganglionic CD8s, we examined several HSV recombinants that have different viral promoters driving expression of the immunodominant gB epitope. We show that the selection of epitope promoter influences CD8⁺ T cell population hierarchies and their function.

Increasing antigen presentation on HSV-1-infected cells increases lesion size but does not alter neural infection or latency

CD8⁺ T cells have a role in the control of acute herpes simplex virus (HSV) infection and may also be important in the maintenance of latency. In this study we have explored the consequences of boosting the efficacy of CD8⁺ T cells against HSV by increasing the amount of an MHC I-presented epitope on the surface of infected cells. To do this we used HSVs engineered to express an extra copy of the immunodominant CD8⁺ T cell epitope in C57Bl/6 mice, namely gB₄₉₈ (SSIEFARL). Despite greater presentation of gB₄₉₈ on infected cells, CD8⁺ T cell responses to these viruses in mice were similar to those elicited by a control virus. Further, the expression of extra gB₄₉₈ did not significantly alter the extent or stability of latency in our mouse model, and virus loads in skin and sensory ganglia of infected mice were not affected. Surprisingly, mice infected with these viruses developed significantly larger skin lesions than those infected with control viruses and notably, this phenotype was dependent on MHC haplotype. Therefore increasing the visibility of HSV-infected cells to CD8⁺ T cell attack did not impact neural infection or latency, but rather enhanced pathology in the skin.

Influence of an immunodominant herpes simplex virus type 1 CD8+ T cell epitope on the target hierarchy and function of subdominant CD8+ T cells

Herpes simplex virus type 1 (HSV-1) latency in sensory ganglia such as trigeminal ganglia (TG) is associated with a persistent immune infiltrate that includes effector memory CD8+ T cells that can influence HSV-1 reactivation. In C57BL/6 mice, HSV-1 induces a highly skewed CD8+ T cell repertoire, in which half of CD8+ T cells (gB-CD8s) recognize a single epitope on glycoprotein B (gB498-505), while the remainder (non-gB-CD8s) recognize, in varying proportions, 19 subdominant epitopes on 12 viral proteins. The gB-CD8s remain functional in TG throughout latency, while non-gB-CD8s exhibit varying degrees of functional compromise. To understand how dominance hierarchies relate to CD8+ T cell function during latency, we characterized the TG-associated CD8+ T cells following corneal infection with a recombinant HSV-1 lacking the immunodominant gB498-505 epitope (S1L). S1L induced a numerically equivalent CD8+ T cell infiltrate in the TG that was HSV-specific, but lacked specificity for gB498-505. Instead, there was a general increase of non-gB-CD8s with specific subdominant epitopes arising to codominance. In a latent S1L infection, non-gB-CD8s in the TG showed a hierarchy targeting different epitopes at latency compared to at acute times, and these cells retained an increased functionality at latency. In a latent S1L infection, these non-gB-CD8s also display an equivalent ability to block HSV reactivation in ex vivo ganglionic cultures compared to TG infected with wild type HSV-1. These data indicate that loss of the immunodominant gB498-505 epitope alters the dominance hierarchy and reduces functional compromise of CD8+ T cells specific for subdominant HSV-1 epitopes during viral latency.

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.

Immunological control of herpes simplex virus infections

Herpes simplex virus type 1 (HSV-1) is capable of causing a latent infection in sensory neurons that lasts for the lifetime of the host. The primary infection is resolved following the induction of the innate immune response that controls replication of the virus until the adaptive immune response can clear the active infection. HSV-1-specific CD8⁺ T cells survey the ganglionic regions containing latently infected neurons and participate in preventing reactivation of HSV from latency. The long-term residence and migration dynamics of the T cells in the trigeminal ganglia appear to distinguish them from the traditional memory T cell subsets. Recently described tissue resident memory (T_RM) T cells establish residence and survive for long periods in peripheral tissue compartments following antigen exposure. This review focuses on the immune system response to HSV-1 infection. Particular emphasis is placed on the evidence pointing to the HSV-1-specific CD8⁺ T cells in the trigeminal belonging to the T_RM class of memory T cells and the role of T_RM cells in virus infection, pathogenesis, latency, and disease.

Broadening the repertoire of functional herpes simplex virus type 1-specific CD8+ T cells reduces viral reactivation from latency in sensory ganglia

A large proportion of the world population harbors HSV type 1 (HSV-1) in a latent state in their trigeminal ganglia (TG). TG-resident CD8(+) T cells appear important for preventing HSV-1 reactivation from latency and recurrent herpetic disease. In C57BL/6J mice, half of these cells are specific for an immunodominant epitope on HSV-1 glycoprotein B, whereas the other half are specific for 18 subdominant epitopes. In this study, we show that the CD8(+) T cell dominance hierarchy in the TG established during acute infection is maintained during latency. However, CD8(+) T cells specific for subdominant epitopes lose functionality, whereas those specific for the immunodominant epitope exhibit increased functionality in latently infected TG. Furthermore, we show that IL-10 produced by 16.4 ± 2.8% of TG-resident CD4(+) T cells maintains the immunodominance hierarchy in part through selective inhibition of subdominant CD8(+) T cell proliferation. Upon systemic anti-IL-10R Ab treatment, we observed a significant expansion of functional subdominant CD8(+) T cells, resulting in significantly improved protection from viral reactivation. In fact, systemic anti-IL-10R Ab treatment prevented viral reactivation in up to 50% of treated mice. Our results not only demonstrate that HSV-1 reactivation from latency can be prevented by expanding the repertoire of functional TG-resident CD8(+) T cells, but also that IL-10R blockade might have therapeutic potential to reduce or eliminate recurrent herpetic disease.

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.

γ-Herpesvirus reactivation differentially stimulates epitope-specific CD8 T cell responses

The γ-herpesviruses are characterized by their ability to establish lifelong latency. Subsequent immune suppression leads to viral reactivation from latency and the onset of a variety of pathologies, including lymphoproliferative disease and cancers. CD8 T cells play a key role in preventing reactivation of latent virus. Therefore, to develop effective therapeutic immune strategies, it is essential to understand the maintenance of CD8 T cell responses during latency. Because the γ-herpesviruses are highly species-specific and mice cannot be infected with the human pathogens, EBV or Kaposi's sarcoma-associated herpesvirus, we have used a natural rodent γ-herpesvirus experimental infection model, γ-herpesvirus-68. In this report, we show that during long-term latent infection, naive CD8 T cells are recruited into the ongoing immune response in an epitope-specific manner. When virus reactivation is induced in vivo, the recruitment of CD8 T cells for some, but not all, epitopes is enhanced. The variation in recruitment is not due to differences in epitope presentation. We also show that CD8 T cells that are newly stimulated during reactivation are functionally impaired compared with acutely stimulated cells in terms of cytokine production. Thus, our results demonstrate unexpected complexity in the response of CD8 T cells specific for different viral epitopes that were stimulated during acute infection, quiescent latency, and reactivation.

Maintenance of T cell function in the face of chronic antigen stimulation and repeated reactivation for a latent virus infection

Persisting infections are often associated with chronic T cell activation. For certain pathogens, this can lead to T cell exhaustion and survival of what is otherwise a cleared infection. In contrast, for herpesviruses, T cells never eliminate infection once it is established. Instead, effective immunity appears to maintain these pathogens in a state of latency. We used infection with HSV to examine whether effector-type T cells undergoing chronic stimulation retained functional and proliferative capacity during latency and subsequent reactivation. We found that latency-associated T cells exhibited a polyfunctional phenotype and could secrete a range of effector cytokines. These T cells were also capable of mounting a recall proliferative response on HSV reactivation and could do so repeatedly. Thus, for this latent infection, T cells subjected to chronic Ag stimulation and periodic reactivation retain the ability to respond to local virus challenge.

Behavior and function of tissue-resident memory T cells

Our understanding of memory T cell function in mice and men is to date in large part restricted to the behavior of circulating memory T cells. Emerging evidence, however, suggests that in addition to such systemic memory T cell populations, a separate population of locally confined memory T cells is generated at former sites of antigen encounter. Here, we discuss the potential function of these long-term tissue-resident memory T cells (T(TRM)), how such local T cell memory can be maintained for prolonged periods of time, and how the induction of long-term tissue-resident memory T cells may potentially be exploited during vaccination.

Persistent viral infections and immune aging

Immunosenescence comprises a set of dynamic changes occurring to both, the innate as well as the adaptive immune system that accompany human aging and result in complex manifestations of still poorly defined deficiencies in the elderly population. One of the most prominent alterations during aging is the continuous involution of the thymus gland which is almost complete by the age of 50. Consequently, the output of naïve T cells is greatly diminished in elderly individuals which puts pressure on homeostatic forces to maintain a steady T cell pool for most of adulthood. In a great proportion of the human population, this fragile balance is challenged by persistent viral infections, especially Cytomegalovirus (CMV), that oblige certain T cell clones to monoclonally expand repeatedly over a lifetime which then occupy space within the T cell pool. Eventually, these inflated memory T cell clones become exhausted and their extensive accumulation accelerates the age-dependent decline of the diversity of the T cell pool. As a consequence, infectious diseases are more frequent and severe in elderly persons and immunological protection following vaccination is reduced. This review therefore aims to shed light on how various types of persistent viral infections, especially CMV, influence the aging of the immune system and highlight potential measures to prevent the age-related decline in immune function.

Regional and mucosal memory T cells

After infection, most antigen-specific memory T cells reside in nonlymphoid tissues. Tissue-specific programming during priming leads to directed migration of T cells to the appropriate tissue, which promotes the development of tissue-resident memory in organs such as intestinal mucosa and skin. Mechanisms that regulate the retention of tissue-resident memory T cells include transforming growth factor-β (TGF-β)-mediated induction of the E-cadherin receptor CD103 and downregulation of the chemokine receptor CCR7. These pathways enhance protection in internal organs, such as the nervous system, and in the barrier tissues--the mucosa and skin. Memory T cells that reside at these surfaces provide a first line of defense against subsequent infection, and defining the factors that regulate their development is critical to understanding organ-based immunity.

Circulating herpes simplex type 1 (HSV-1)-specific CD8+ T cells do not access HSV-1 latently infected trigeminal ganglia

Background

Therapeutic vaccines can be designed to enhance existing T cell memory populations for increased protection against re-infection. In the case of herpes simplex virus type 1, recurrent disease results from reactivation of latent virus in sensory ganglia, which is controlled in part by a ganglia-resident HSV-specific memory CD8⁺ T cell population. Thus, an important goal of a therapeutic HSV-1 vaccine would be to enhance this population.

Methods

HSV-1-infected mice were treated with TAK-779 to block CCR5- and CXCR3-mediated CD8⁺ T cell migration during both acute and latent infections. Additionally, HSV-1-specific CD8⁺ T cells were transferred into HSV-1 latently infected mice to mimic the effect of a therapeutic vaccine, and their migration into trigeminal ganglia (TG) was traced during steady-state latency, or during recovery of the TG-resident memory CD8⁺ T cell population following stress-, and corticosterone-induced depletion and HSV-1 reactivation from latency. Bromodeoxy uridine (BrdU) incorporation measured cell proliferation in vivo.

Results

TAK-779 treatment during acute HSV-1 infection reduced the number of infiltrating CD8⁺ T cells but did not alter the number of viral genome copies. TAK-779 treatment during HSV latency did not affect the size of the TG-resident memory CD8⁺ T cell population. Transferred HSV-specific CD8⁺ T cells failed to access latently infected TG during steady-state latency, or during recovery of the TG resident HSV-specific CD8⁺ T cell population following exposure of latently infected mice to stress and corticosterone. Recovery of the HSV-specific CD8⁺ T cell population after stress and corticosterone treatment occurred with homeostatic levels of cell division and did not require CD4⁺ T cell help.

Conclusions

Our findings are consistent with the notion that the CD8⁺ T cells in latently infected TG are a tissue-resident memory (Trm) population that is maintained without replenishment from the periphery, and that when this population is disrupted, it recovers without proliferation or detectable recruitment of HSV-specific CD8⁺ T cells from the blood. The compartmentalization of the HSV-specific CD8⁺ memory T cell population in latently infected TG will complicate the design of therapeutic vaccines.

T-cell receptor signals direct the composition and function of the memory CD8+ T-cell pool

SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) nucleates a signaling complex critical for T-cell receptor (TCR) signal propagation. Mutations in the tyrosines of SLP-76 result in graded defects in TCR-induced signals depending on the tyrosine(s) affected. Here we use 2 strains of genomic knock-in mice expressing tyrosine to phenylalanine mutations to examine the role of TCR signals in the differentiation of effector and memory CD8(+) T cells in response to infection in vivo. Our data support a model in which altered TCR signals can determine the rate of memory versus effector cell differentiation independent of initial T-cell expansion. Furthermore, we show that TCR signals sufficient to promote CD8(+) T-cell differentiation are different from those required to elicit inflammatory cytokine production.

Herpes simplex virus glycoprotein D interferes with binding of herpesvirus entry mediator to its ligands through downregulation and direct competition

To initiate membrane fusion and virus entry, herpes simplex virus (HSV) gD binds to a cellular receptor such as herpesvirus entry mediator (HVEM). HVEM is a tumor necrosis factor (TNF) receptor family member with four natural ligands that either stimulate (LIGHT and LTα) or inhibit (BTLA and CD160) T cell function. We hypothesized that the interaction of gD with HVEM affects the binding of natural ligands, thereby modulating the immune response during infection. Here, we investigated the effect that gD has on the interaction of HVEM with its natural ligands. First, HSV gD on virions or cells downregulates HVEM from the cell surface. Similarly, trans-interaction with BTLA or LIGHT also downregulates HVEM from the cell surface, suggesting that HSV may subvert a natural mechanism for regulating HVEM activity. Second, we showed that wild-type gD had the lowest affinity for HVEM compared with the four natural ligands. Moreover, gD directly competed for binding to HVEM with BTLA but not LTα or LIGHT, indicating the possibility that gD selectively controls HVEM signals. On the other hand, natural ligands influence the use of HVEM by HSV. For instance, soluble BTLA, LTα, and LIGHT inhibited the binding of wild-type gD to HVEM, and soluble BTLA and LTα blocked HSV infection of HVEM-expressing cells. Thus, gD is at the center of the interplay between HVEM and its ligands. It can interfere with HVEM function in two ways, by competing with the natural ligands and by downregulating HVEM from the cell surface.

Application of nanotechnologies for improved immune response against infectious diseases in the developing world

There is an urgent need for new strategies to combat infectious diseases in developing countries. Many pathogens have evolved to elude immunity and this has limited the utility of current therapies. Additionally, the emergence of co-infections and drug resistant pathogens has increased the need for advanced therapeutic and diagnostic strategies. These challenges can be addressed with therapies that boost the quality and magnitude of an immune response in a predictable, designable fashion that can be applied for wide-spread use. Here, we discuss how biomaterials and specifically nanoscale delivery vehicles can be used to modify and improve the immune system response against infectious diseases. Immunotherapy of infectious disease is the enhancement or modulation of the immune system response to more effectively prevent or clear pathogen infection. Nanoscale vehicles are particularly adept at facilitating immunotherapeutic approaches because they can be engineered to have different physical properties, encapsulated agents, and surface ligands. Additionally, nanoscaled point-of-care diagnostics offer new alternatives for portable and sensitive health monitoring that can guide the use of nanoscale immunotherapies. By exploiting the unique tunability of nanoscale biomaterials to activate, shape, and detect immune system effector function, it may be possible in the near future to generate practical strategies for the prevention and treatment of infectious diseases in the developing world.

17-beta estradiol promotion of herpes simplex virus type 1 reactivation is estrogen receptor dependent

Correlations between estrogen and herpes simplex virus (HSV) reactivation from latency have been suggested by numerous clinical reports, but causal associations are not well delineated. In a murine HSV-1 corneal infection model, we establish 17-beta estradiol (17-betaE) treatment of latently infected ovariectomized mice induces viral reactivation, as demonstrated by increased viral load and increased immediate-early viral gene expression in the latently infected trigeminal ganglia (TG). Interestingly, the increased HSV reactivation occurred in the absence of inhibition of viral specific CD8(+) T-cell effector function. 17-betaE administration increased HSV reactivation in CD45(+) cell-depleted TG explant cultures, providing further support that leukocyte-independent effects on latently infected neurons were responsible for the increased reactivation. The drug-induced increases in HSV copy number were not recapitulated upon in vivo treatment of latently infected estrogen receptor alpha-deficient mice, evidence that HSV reactivation promoted by 17-betaE was estrogen receptor dependent. These findings provide additional framework for the emerging conceptualization of HSV latency as a dynamic process maintained by complex interactions among multiple cooperative and competing host, viral, and environmental forces. Additional research is needed to confirm whether pregnancy or hormonal contraceptives containing 17-betaE also promote HSV reactivation from latency in an estrogen receptor-dependent manner.

Memory CD8+ T cell differentiation

In response to infection or effective vaccination, naive antigen-specific CD8+ T cells undergo a dramatic highly orchestrated activation process. Initial encounter with an appropriately activated antigen-presenting cell leads to blastogenesis and an exponential increase in antigen-specific CD8+ T cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in formation of both primary effector and long-lived memory cells. Current findings have emphasized the heterogeneity of effector and memory cell populations with the description of multiple cellular subsets based on phenotype, function, and anatomic location. Yet, only recently have we begun to dissect the underlying factors mediating the temporal control of the development of distinct effector and memory CD8+ T cell sublineages. In this review we will focus on the requirements for mounting an effective CD8+ T cell response and highlight the elements regulating the differentiation of effector and memory subsets.

Heterologous CD8 T cell immune response to HSV induced by toll like receptor ligands

A memory response is established following primary antigen exposure that stays more or less constant. It appears to adopt a set-point in magnitude but upon re-exposure the response is quicker and better and there is an upward shift in memory frequency that varies with individuals based on the exposure pattern to other microbes or its components. Our investigations were designed to test such differences of non-specific stimulation by PAMPs in lowering the threshold of activation. Neonatal mice were pre-exposed to TLR-ligands intermittently and later analyzed for its resilience to challenge with virus during adult-life. Secondly, adult mice with pre-existing memory to virus were exposed to various TLR-ligands and analyzed for their quality of memory response. The TLR-ligands exposed animals were better responders to a new agent exposure compared to the animals kept in sterile surroundings. Moreover, immune memory recall and the viral specific CD8(+) T cells response with TLR-ligands were comparable to the recall response with the cognate antigen. The results provide insights into the role of hyper-sanitized environment versus PAMPs mediated signaling in adaptive immunity and long-term immune memory.

Inflation and long-term maintenance of CD8 T cells responding to a latent herpesvirus depend upon establishment of latency and presence of viral antigens

Following the priming and contraction phases of the T cell response, latent persistent herpesviruses lead to an accumulation of large pools of virus-specific CD8 T cells, also known as memory inflation (MI). The mechanism of this inflation is incompletely understood, largely because the molecular reactivation of these viruses in vivo and its impact upon T cell biology have not been resolved in mice, and because the relevant observations in humans remain, by necessity, correlative. Understanding these processes is essential from the standpoint of the proposed critical role for latent herpesviruses in aging of the immune system. We studied the causes of memory CD8 T cell accumulation following systemic HSV-1 administration as a model of widespread latent viral infection in humans. A direct role of viral latency and Ag-specific restimulation in driving the accumulation and maintenance of inflated CD8 T cells and a strongly suggested role of viral reactivation in that process were shown by the following: 1) lack of MI in the absence of established latency; 2) prevention or delay of MI with drugs that curtail viral replication; and 3) abrogation of MI by the transfer of inflated T cells into a virus-free environment. These results strongly suggest that periodic, subclinical reactivations of a latent persistent virus cause dysregulation of memory CD8 T cell homeostasis, similar to the one in humans. Moreover, results with antiviral drugs suggest that this approach could be considered as a treatment modality for maintaining T cell diversity and/or function in old age.

Early CD4(+) T cell help prevents partial CD8(+) T cell exhaustion and promotes maintenance of Herpes Simplex Virus 1 latency

HSV-specific CD8(+) T cells provide constant immunosurveillance of HSV-1 latently infected neurons in sensory ganglia, and their functional properties are influenced by the presence of latent virus. In this study, we show that ganglionic HSV-specific CD8(+) T cells exhibit a higher functional avidity (ability to respond to low epitope density) than their counterparts in noninfected lungs, satisfying a need for memory effector cells that can respond to low densities of viral epitopes on latently infected neurons. We further show that lack of CD4(+) T cell help during priming leads to a transient inability to control latent virus, which was associated with a PD-1/PD-L1 mediated reduced functional avidity of ganglionic HSV-specific CD8(+) T cells. CD4(+) T cells are not needed to maintain CD8(+) T cell memory through 34 d after infection, nor do they have a direct involvement in the maintenance of HSV-1 latency.

Chronic human infection with Trypanosoma cruzi drives CD4+ T cells to immune senescence

Previously we found that the frequency of IFN-gamma-producing CD8(+) T cells specific for Trypanosoma cruzi inversely correlates with disease severity in chronic human Chagas disease along with low levels of IL-2-secreting CD8(+) T cells in all clinical stages. This impairment of the parasite-specific T cell responses was associated with phenotypic features of immune senescence of the CD8(+) T cell compartment. These data prompted us to address the question of whether the CD4(+) T cell compartment also experiences signs of exhaustion. Thus, we performed a functional and phenotypical characterization of T. cruzi-specific and overall CD4(+) T cells in chronically infected subjects with different degrees of cardiac dysfunction. The results show an inverse association between disease severity and the frequency of T. cruzi-specific IFN-gamma-producing CD4(+) T cells. The high expression of CD27 and CD28 with a relative low expression of CD57 found on CD4(+)IFN-gamma(+) T cells suggests that the effector T cell pool in chronic T. cruzi infection includes a high proportion of newly recruited T cells, but a low frequency of long-term memory cells. The total CD4(+) T cell compartment shows signs of senescence and later stages of differentiation associated with more severe stages of the disease. These findings support the hypothesis that long-term T. cruzi infection in humans might exhaust long-lived memory T cells.

The dendritic and T cell responses to herpes simplex virus-1 are modulated by dietary vitamin E

Previous studies from our laboratory have shown that dietary alpha-tocopherol (vitamin E, or VE) is essential for regulating the cytokine and chemokine response in the brain to herpes simplex virus-1 (HSV-1) infection. The timing of T cell infiltration is critical to the resolution of central nervous system HSV-1 infections. Specifically, the appearance of "neuroprotective" CD8(+)IFN-gamma(+) T cells is crucial. During CNS infection, CD8(+) T cell priming and expansion in the draining lymph node, followed by recruitment and expansion, occurs in the spleen with subsequent accumulation in the brain. Weanling male BALB/cByJ mice were placed on VE-deficient (Def) or -adequate diets for 4 weeks followed by intranasal infection with HSV-1. VE-Def mice had fewer CD8(+)IFN-gamma(+) T cells trafficking to the brain despite increased CD8(+)IFN-gamma(+) T cells and activated dendritic cells in the periphery. VE-Def mice had increased T regulatory cells (Tregs) in the periphery and brain, and the increase in Tregs decreased CD8(+) T cell numbers in the brain. Our results demonstrate that adequate levels of VE are important for trafficking antigen-specific T cells to the brain, and dietary VE levels modulate T regulatory and dendritic cells in the periphery.

Delivery of Interferon-gamma by an adenovirus vector blocks herpes simplex virus Type 1 reactivation in vitro and in vivo independent of RNase L and double-stranded RNA-dependent protein kinase pathways

HSV-1 is a significant human pathogen that can result in the loss of sight as a result of episodic reactivation of latent virus from sensory ganglion neurons. In this study the potential efficacy of anti-viral cytokine expression in preventing latent virus reactivation was investigated. Both type I (IFN-beta) and type II (IFN-gamma) IFN transgene expression following transduction of trigeminal ganglion explant cultures significantly reduced the incident of HSV-1 reactivation that in the case of IFN-beta was dependent on the presence of double stranded RNA-dependent protein kinase and RNase L. In vivo, expression of the IFN-gamma but not IFN-beta transgene significantly delayed and reduced the frequency of reactivation of latent mice exposed to UV light without discernable inflammation. This result is the first report that demonstrates the ability to block reactivation using an ectopic cytokine expression system and warrants further exploration as a means to prevent HSV-1 reactivation.

Reevaluating the CD8 T-cell response to herpes simplex virus type 1: involvement of CD8 T cells reactive to subdominant epitopes

In C57BL/6 (B6) mice, most herpes simplex virus (HSV)-specific CD8 T cells recognize a strongly immunodominant epitope on glycoprotein B (gB498) and can inhibit HSV type 1 (HSV-1) reactivation from latency in trigeminal ganglia (TG). However, half of the CD8 T cells retained in latently infected TG of B6 mice are not gB498 specific and have been largely ignored. The following observations from our current study indicate that these gB498-nonspecific CD8 T cells are HSV specific and may contribute to the control of HSV-1 latency. First, following corneal infection, OVA257-specific OT-1 CD8 T cells do not infiltrate the infected TG unless mice are simultaneously immunized with OVA257 peptide, and then they are not retained. Second, 30% of CD8 T cells in acutely infected TG that produce gamma interferon in response to HSV-1 stimulation directly ex vivo are gB498 nonspecific, and these cells maintain an activation phenotype during viral latency. Finally, gB498-nonspecific CD8 T cells are expanded in ex vivo cultures of latently infected TG and inhibit HSV-1 reactivation from latency in the absence of gB498-specific CD8 T cells. We conclude that many of the CD8 T cells that infiltrate and are retained in infected TG are HSV specific and potentially contribute to maintenance of HSV-1 latency. Identification of the viral proteins recognized by these cells will contribute to a better understanding of the dynamics of HSV-1 latency.

Rapidly cleared episodes of herpes simplex virus reactivation in immunocompetent adults

BACKGROUND

Herpes simplex virus (HSV) remains latent in nerve root ganglia of infected persons and is thought to reactivate several times yearly. Recent in situ data show the localization of HSV-specific CD8(+) T cells at the dermal epidermal junction next to peripheral sensory nerve endings, suggesting that viral reactivation may occur more frequently than previously appreciated.

METHODS

Twenty-five HSV-2-seropositive and 18 HSV-1-seropositive healthy adults collected anogenital and oral swabs, respectively, 4 times per day for 60 days. Swabs were assayed for HSV, using a quantitative polymerase chain reaction assay.

RESULTS

Twenty-four percent of anogenital reactivations and 21% of oral reactivations lasted < or =6 h, and 49% of anogenital reactivations and 39% of oral reactivations lasted < or =12 h. Lesions were reported in only 3 (7%) of 44 anogenital reactivations and 1 (8%) of 13 oral reactivations lasting < or =12 h. The median HSV DNA levels at initial and last detection were 10(3.5) and 10(3.3) copies/mL, respectively, during anogenital reactivation and 10(3.7) and 10(3.0) copies/mL, respectively, during oral reactivation.

CONCLUSIONS

This high frequency of short subclinical HSV reactivation in immunocompetent hosts strongly suggests that the peripheral mucosal immune system plays a critical role in clearing HSV reactivations.

Medroxyprogesterone acetate inhibits CD8+ T cell viral-specific effector function and induces herpes simplex virus type 1 reactivation

Clinical research suggests hormonal contraceptive use is associated with increased frequencies of HSV reactivation and shedding. We examined the effects of medroxyprogesterone acetate (MPA), the compound most commonly used for injectable hormonal contraception, on HSV type 1 (HSV-1) reactivation and CD8(+) T cell function in murine trigeminal ganglia (TG). In ex vivo TG cultures, MPA dramatically inhibited canonical CD8(+) T cell effector functions, including IFN-gamma production and lytic granule release, and increased HSV-1 reactivation from latency. In vivo, MPA treatment of latently infected ovariectomized mice inhibited IFN-gamma production and lytic granule release by TG resident CD8(+) T cells stimulated directly ex vivo. RNA specific for the essential immediate early viral gene ICP4 as well as viral genome DNA copy number were increased in mice that received MPA during latency, suggesting that treatment increased in vivo reactivation. The increase in HSV-1 copy number appeared to be the result of a two-tine effect, as MPA induced higher reactivation frequencies from latently infected explanted TG neurons in the presence or absence of CD45(+) cells. Our data suggest hormonal contraceptives that contain MPA may promote increased frequency of HSV reactivation from latency through the combinatory effects of inhibiting protective CD8(+) T cell responses and by a leukocyte-independent effect on infected neurons.

Age-related dysregulation of CD8+ T cell memory specific for a persistent virus is independent of viral replication

The immune system devotes substantial resources to the lifelong control of persistent pathogens, which were hypothesized to play an important role in immune aging. Specifically, the presence of latent herpesviruses has been correlated with immune exhaustion and shorter lifespan in octogenarians. But neither the causality nor the mechanistic link(s) were established, and the relative roles of persistent antigenic stimulation and of virus-independent homeostatic disturbances in T cell aging remain unresolved. We longitudinally analyzed expansion, contraction, and long-term maintenance of CD8(+) T cells responding to localized infection with a latent virus, HSV-1. Young mice exhibited the expected expansion and contraction of HSV-1-specific cells and the stable maintenance of memory T cells into advanced adulthood. However, upon entry into senescence, many (>40%) animals exhibited an accumulation in Ag-specific cells (memory inflation) which in some animals was comparable to that observed in acute infection. Inflation occurred to the same extent in control mice and mice continuously treated with the anti-HSV drug famciclovir, which inhibits viral replication and was able to reduce expression of the glycoprotein B. Age-related inflation was also found long after infection with an acute virus. The inflating cells largely maintained Ag-specific function, and exhibited typical central memory phenotype, with no signs of Ag-specific activation. They exhibited increased expression of CD122 and CD127, akin to the Ag-independent T cell clonal expansions found in old specific pathogen-free laboratory mice. This collectively suggests that, in this model, the inflating cells may be selected for high responsiveness to environmental cytokines largely in an Ag-independent manner.

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.

Psychological stress compromises CD8+ T cell control of latent herpes simplex virus type 1 infections

Recurrent HSV-1 ocular disease results from reactivation of latent virus in trigeminal ganglia, often following immunosuppression or exposure to a variety of psychological or physical stressors. HSV-specific CD8+ T cells can block HSV-1 reactivation from latency in ex vivo trigeminal ganglia cultures through production of IFN-gamma. In this study, we establish that either CD8+ T cell depletion or exposure to restraint stress permit HSV-1 to transiently escape from latency in vivo. Restraint stress caused a reduction of TG-resident HSV-specific CD8+ T cells and a functional compromise of those cells that survive. Together, these effects of stress resulted in an approximate 65% reduction of cells capable of producing IFN-gamma in response to reactivating virus. Our findings demonstrate persistent in vivo regulation of latent HSV-1 by CD8+ T cells, and strongly support the concept that stress induces HSV-1 reactivation from latency at least in part by compromising CD8+ T cell surveillance of latently infected neurons.

Memory generation and maintenance of CD8+ T cell function during viral persistence

During infection with viruses that establish latency, the immune system needs to maintain lifelong control of the infectious agent in the presence of persistent Ag. By using a gamma-herpesvirus (gammaHV) infection model, we demonstrate that a small number of virus-specific central-memory CD8+ T cells develop early during infection, and that virus-specific CD8+T cells maintain functional and protective capacities during chronic infection despite low-level Ag persistence. During the primary immune response, we show generation of CD8+ memory T cell precursors expressing lymphoid homing molecules (CCR7, L-selectin) and homeostatic cytokine receptors (IL-7alpha, IL-2/IL-15beta). During long-term persistent infection, central-memory cells constitute 20-50% of the virus-specific CD8+ T cell population and maintain the expression of L-selectin, CCR7, and IL-7R molecules. Functional analyses demonstrate that during viral persistence: 1) CD8+ T cells maintain TCR affinity for peptide/MHC complexes, 2) the functional avidity of CD8+ T cells measured as the capacity to produce IFN-gamma is preserved intact, and 3) virus-specific CD8+ T cells have in vivo killing capacity. Next, we demonstrate that at 8 mo post-virus inoculation, long-term CD8+ T cells are capable of mediating a protective recall response against the establishment of gammaHV68 splenic latency. These observations provide evidence that functional CD8+ memory T cells can be generated and maintained during low-load gammaHV68 persistence.

Dengue virus-specific suppressor T cells: current perspectives

Dengue virus was the first microorganism that was shown to induce generation of antigen-specific suppressor T (TS) cells in mice. The cascade of the three generations of TS cells (TS1, TS2, TS3) and their secretary products, the suppressor factors (SF1, SF2), was delineated. The TS pathway was proposed to be protective through inhibition of the production of enhancing antibody, which may enhance the severity of dengue disease. The currently second most favoured mechanism of severe dengue disease is the 'cytokine tsunami'. During the last decade, suppressor/regulatory T cells have been studied in greater detail using modern techniques in various diseases, including viral infections. This brief review discusses the role of dengue-specific suppressor T cells in protection and/or induction of severe dengue disease in view of our current understanding of suppressor/regulatory T cells.