T cell memory in the context of persistent herpes viral infections

Virus-Specific T Cells for the Treatment of Systemic Infections Following Allogeneic Hematopoietic Cell and Solid Organ Transplantation

Viral infections are a major source of morbidity and mortality in the context of immune deficiency and immunosuppression following allogeneic hematopoietic cell (allo-HCT) and solid organ transplantation (SOT). The pharmacological treatment of viral infections is challenging and often complicated by limited efficacy, the development of resistance, and intolerable side effects. A promising strategy to rapidly restore antiviral immunity is the adoptive transfer of virus-specific T cells (VST). This therapy involves the isolation and ex vivo expansion or direct selection of antigen-specific T cells from healthy seropositive donors, followed by infusion into the patient. This article provides a practical guide to VST therapy by reviewing manufacturing techniques, donor selection, and treatment indications. The safety and efficacy data of VSTs gathered in clinical trials over nearly 30 years is summarized. Current challenges and limitations are discussed, as well as opportunities for further research and development.

Overall avidity declines in TCR repertoires during latent CMV but not EBV infection

Introduction

The avidity of the T-cell receptor (TCR) for antigenic peptides presented by the MHC (pMHC) on cells is an essential parameter for efficient T cell-mediated immunity. Yet, whether the TCR-ligand avidity can drive the clonal evolution of virus antigen-specific CD8 T cells, and how this process is determined in latent Cytomegalovirus (CMV)- against Epstein-Barr virus (EBV)-mediated infection remains largely unknown.

Methods

To address these issues, we quantified monomeric TCR-pMHC dissociation rates on CMV- and EBV-specific individual TCRαβ clonotypes and polyclonal CD8 T cell populations in healthy donors over a follow-up time of 15-18 years. The parameters involved during the long-term persistence of virus-specific T cell clonotypes were further evaluated by gene expression profiling, phenotype and functional analyses.

Results

Within CMV/pp65-specific T cell repertoires, a progressive contraction of clonotypes with high TCR-pMHC avidity and low CD8 binding dependency was observed, leading to an overall avidity decline during long-term antigen exposure. We identified a unique transcriptional signature preferentially expressed by high-avidity CMV/pp65-specific T cell clonotypes, including the inhibitory receptor LILRB1. Interestingly, T cell clonotypes of high-avidity showed higher LILRB1 expression than the low-avidity ones and LILRB1 blockade moderately increased T cell proliferation. Similar findings were made for CD8 T cell repertoires specific for the CMV/IE-1 epitope. There was a gradual in vivo loss of high-avidity T cells with time for both CMV specificities, corresponding to virus-specific CD8 T cells expressing enhanced LILRB1 levels. In sharp contrast, the EBV/BMFL1-specific T cell clonal composition and distribution, once established, displayed an exceptional stability, unrelated to TCR-pMHC binding avidity or LILRB1 expression.

Conclusions

These findings reveal an overall long-term avidity decline of CMV- but not EBV-specific T cell clonal repertoires, highlighting the differing role played by TCR-ligand avidity over the course of these two latent herpesvirus infections. Our data further suggest that the inhibitor receptor LILRB1 potentially restricts the clonal expansion of high-avidity CMV-specific T cell clonotypes during latent infection. We propose that the mechanisms regulating the long-term outcome of CMV- and EBV-specific memory CD8 T cell clonotypes in humans are distinct.

T cell exhaustion is associated with cognitive status and amyloid accumulation in Alzheimer's disease

Studies over the last 100 years have suggested a link between inflammation, infectious disease, and Alzheimer's Disease (AD). Understanding how the immune system changes during the development of AD may facilitate new treatments. Here, we studied an aging cohort who had been assessed for AD pathology with amyloid positron emission tomography and cognitive testing, and conducted high dimensional flow cytometry on peripheral blood mononuclear and cerebrospinal fluid cells. Participants were assigned a classification of being amyloid negative cognitively normal, amyloid positive cognitively normal (APCN), or amyloid positive mild cognitive impairment (APMCI), an early stage of AD. We observed major alterations in the peripheral innate immune system including increased myeloid and plasmacytoid dendritic cells in the blood of APMCI participants. When the adaptive immune system was examined, amyloid positive participants, regardless of cognitive status, had increased CD3+ T cells. Further analyses of CD4+ and CD8+ T cells revealed that APMCI participants had an increase in more differentiated phenotype T cells, such as effector memory and effector memory CD45RA expressing (TEMRA), compared to those with normal cognition. When T cell function was measured, we observed that T cells from APCN participants had increased IFNγ+GzB- producing cells compared to the other participants. In contrast, we demonstrate that APMCI participants had a major increase in T cells that lacked cytokine production following restimulation and expressed increased levels of PD-1 and Tox, suggesting these are exhausted cells. Rejuvenation of these cells may provide a potential treatment for AD.

An attenuated herpesvirus vectored vaccine candidate induces T-cell responses against highly conserved porcine reproductive and respiratory syndrome virus M and NSP5 proteins that are unable to control infection

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a leading cause of economic loss in pig farming worldwide. Existing commercial vaccines, all based on modified live or inactivated PRRSV, fail to provide effective immunity against the highly diverse circulating strains of both PRRSV-1 and PRRSV-2. Therefore, there is an urgent need to develop more effective and broadly active PRRSV vaccines. In the absence of neutralizing antibodies, T cells are thought to play a central role in controlling PRRSV infection. Herpesvirus-based vectors are novel vaccine platforms capable of inducing high levels of T cells against encoded heterologous antigens. Therefore, the aim of this study was to assess the immunogenicity and efficacy of an attenuated herpesvirus-based vector (bovine herpesvirus-4; BoHV-4) expressing a fusion protein comprising two well-characterized PRRSV-1 T-cell antigens (M and NSP5). Prime-boost immunization of pigs with BoHV-4 expressing the M and NSP5 fusion protein (vector designated BoHV-4-M-NSP5) induced strong IFN-γ responses, as assessed by ELISpot assays of peripheral blood mononuclear cells (PBMC) stimulated with a pool of peptides representing PRRSV-1 M and NSP5. The responses were closely mirrored by spontaneous IFN-γ release from unstimulated cells, albeit at lower levels. A lower frequency of M and NSP5 specific IFN-γ responding cells was induced following a single dose of BoHV-4-M-NSP5 vector. Restimulation using M and NSP5 peptides from PRRSV-2 demonstrated a high level of cross-reactivity. Vaccination with BoHV-4-M-NSP5 did not affect viral loads in either the blood or lungs following challenge with the two heterologous PRRSV-1 strains. However, the BoHV-4-M-NSP5 prime-boost vaccination showed a marked trend toward reduced lung pathology following PRRSV-1 challenge. The limited effect of T cells on PRRSV-1 viral load was further examined by analyzing local and circulating T-cell responses using intracellular cytokine staining and proliferation assays. The results from this study suggest that vaccine-primed T-cell responses may have helped in the control of PRRSV-1 associated tissue damage, but had a minimal, if any, effect on controlling PRRSV-1 viral loads. Together, these results indicate that future efforts to develop effective PRRSV vaccines should focus on achieving a balanced T-cell and antibody response.

The innate and T-cell mediated immune response during acute and chronic gammaherpesvirus infection

Immediately after entry into host cells, viruses are sensed by the innate immune system, leading to the activation of innate antiviral effector mechanisms including the type I interferon (IFN) response and natural killer (NK) cells. This innate immune response helps to shape an effective adaptive T cell immune response mediated by cytotoxic T cells and CD4+ T helper cells and is also critical for the maintenance of protective T cells during chronic infection. The human gammaherpesvirus Epstein-Barr virus (EBV) is a highly prevalent lymphotropic oncovirus that establishes chronic lifelong infections in the vast majority of the adult population. Although acute EBV infection is controlled in an immunocompetent host, chronic EBV infection can lead to severe complications in immunosuppressed patients. Given that EBV is strictly host-specific, its murine homolog murid herpesvirus 4 or MHV68 is a widely used model to obtain in vivo insights into the interaction between gammaherpesviruses and their host. Despite the fact that EBV and MHV68 have developed strategies to evade the innate and adaptive immune response, innate antiviral effector mechanisms still play a vital role in not only controlling the acute infection but also shaping an efficient long-lasting adaptive immune response. Here, we summarize the current knowledge about the innate immune response mediated by the type I IFN system and NK cells, and the adaptive T cell-mediated response during EBV and MHV68 infection. Investigating the fine-tuned interplay between the innate immune and T cell response will provide valuable insights which may be exploited to design better therapeutic strategies to vanquish chronic herpesviral infection.

ThymicPeptides Reverse Immune Exhaustion in Patients with Reactivated Human Alphaherpesvirus1 Infections

Recurrent infection with human alphaherpesvirus 1 (HHV-1) may be associated with immune exhaustion that impairs virus elimination. Thymic peptides enhance immune function and thus could overcome immune exhaustion. In this study, we investigated whether reactivation of herpes infections was associated with immune exhaustion. Moreover, we examined the impact of treatment with thymostimulin on the expression of programmed cell death protein 1 (PD-1) and its ligand (PD-L1) on T and B lymphocytes in patients suffering from recurrent HHV-1 reactivation. We also assessed the effector function of peripheral blood mononuclear cells (PBMCs) after stimulation with thymic peptides. We enrolled 50 women with reactivated HHV-1 infections and healthy volunteers. We measured the expression of various activation and exhaustion markers on the surface of PBMCs using flow cytometry. In ex vivo experiments, we measured the secretion of inflammatory cytokines by PBMCs cultured with thymostimulin. Compared with controls, patients with reactivated HHV-1 infections had increased percentages of CD3+ co-expressing CD25, an activation marker (p < 0.001). Moreover, these patients had increased percentages of CD4+ and CD8+ cells co-expressing the inhibitory markers PD-1 and PD-L1. In cultures of PBMCs from the patients, thymostimulin increased the secretion of interferon gamma (p < 0.001) and interleukin (IL)-2 (p = 0.023), but not IL-4 or IL-10.Two-month thymostimulin therapy resulted in no reactivation of HHV-1 infection during this period and the reduction of PD-1 and PD-L1 expression on the surface of T and B lymphocytes (p < 0.001). In conclusion, reactivation of herpes infection is associated with immune exhaustion, which could be reversed by treatment with thymic peptides.

Frequent Recurrences of Genital Herpes Are Associated with Enhanced Systemic HSV-Specific T Cell Response

Objectives

Genital herpes simplex virus (HSV) infection is controlled by HSV-specific T cells in the genital tract, and the role of systemic T cell responses is not fully understood. Thus, we analysed T cell responses in patients with recurrent genital herpes (GH).

Methods

T cell responses to HSV-1 and HSV-2 native antigens and the expression of HLA-DR and CD38 molecules on circulating CD8+ T cells were analysed in adults with high frequency of GH recurrences (19 patients) and low frequency of GH recurrences (7 patients) and 12 HSV-2 seronegative healthy controls. The study utilized the interferon-γ Elispot assay for measurement of spot-forming cells (SFC) after ex vivo stimulation with HSV antigens and flow cytometry for analysis of the expression of activation markers in unstimulated T cells.

Results

The patients with high frequency of GH recurrences (mean number of recurrences of 13.3 per year) had significantly enhanced HSV-specific T cell responses than the HSV-2 seronegative healthy controls. Moreover, a trend of higher numbers of SFC was observed in these patients when compared with those with low frequency of GH recurrences (mean number of recurrences of 3.3 per year). Additionally, no differences in CD38 and HLA-DR expression on circulating CD8+ T cells were found among the study groups.

Conclusions

Frequency of GH recurrences positively correlates with high numbers of systemic HSV-specific T cells.

Do cytomegalovirus-specific memory T cells interfere with new immune responses in lymphoid tissues?

In both mice and humans, the CD8 T cell compartment is expanded with age in the presence of a cytomegalovirus (CMV) infection due to an absolute increase in the CD8+ T cell effector memory (TEM) cells. It has been hypothesized that in CMV+ subjects, such accumulated TEM cells could interfere with responses to new infection by competing for space/resources or could inhibit new responses by other, undefined, means. Here we present evidence against this hypothesis. We show that MCMV-specific CD8 T cells accumulate in blood and bone marrow, but not lymph nodes (frequent sites of immune response initiation), in either persistent lifelong CMV infection or following reactivation. Moreover, adoptive transfer of effector memory T cells from MCMV positive mice into naïve animals did not interfere with either humoral or cellular response to West Nile virus or Listeria monocytogenes infection in recipient mice. We conclude that MCMV infection is unlikely to inhibit new immune responses in old animals through direct interference of MCMV-specific CD8 T cells with the priming.

Development of T cell immunity to norovirus and rotavirus in children under five years of age

Most of the research effort to understand protective immunity against norovirus (NoV) has focused on humoral immunity, whereas immunity against another major pediatric enteric virus, rotavirus (RV), has been studied more thoroughly. The aim of this study was to investigate development of cell-mediated immunity to NoV in early childhood. Immune responses to NoV GI.3 and GII.4 virus-like particles and RV VP6 were determined in longitudinal blood samples of 10 healthy children from three months to four years of age. Serum IgG antibodies were measured using enzyme-linked immunosorbent assay and production of interferon-gamma by peripheral blood T cells was analyzed by enzyme-linked immunospot assay. NoV-specific T cells were detected in eight of 10 children by the age of four, with some individual variation. T cell responses to NoV GII.4 were higher than those to GI.3, but these responses were generally lower than responses to RV VP6. In contrast to NoV-specific antibodies, T cell responses were transient in nature. No correlation between cell-mediated and antibody responses was observed. NoV exposure induces vigorous T cell responses in children under five years of age, similar to RV. A role of T cells in protection from NoV infection in early childhood warrants further investigation.

Vaccine protection against SIVmac239 acquisition

The biological characteristics of HIV pose serious difficulties for the success of a preventive vaccine. Molecularly cloned SIVmac239 is difficult for antibodies to neutralize, and a variety of vaccine approaches have had great difficulty achieving protective immunity against it in rhesus monkey models. Here we report significant protection against i.v. acquisition of SIVmac239 using a long-lasting approach to vaccination. The vaccine regimen includes a replication-competent herpesvirus engineered to contain a near-full-length SIV genome that expresses all nine SIV gene products, assembles noninfectious SIV virion particles, and is capable of eliciting long-lasting effector-memory cellular immune responses to all nine SIV gene products. Vaccinated monkeys were significantly protected against acquisition of SIVmac239 following repeated marginal dose i.v. challenges over a 4-month period. Further work is needed to define the critical components necessary for eliciting this protective immunity, evaluate the breadth of the protection against a variety of strains, and explore how this approach may be extended to human use.

Altered CD4+ T cell immunity in nurses occupationally exposed to viral pathogens

Pathogen exposure, including but not limited to herpesviruses, moulds the shape of the immune system, both at a basal state and in response to immune challenge. However, little is known about the impact of high exposure to other viruses on baseline immune signatures and how the immune system copes with repetitive exposures to maintain a balanced functionality. Here we investigated baseline immune signatures, including detailed T cell phenotyping, antigen-specific CD4⁺ and CD8⁺ T cell responses and cytokine profile in paediatric (PED) nurses, who have high occupational exposure to viral pathogens including varicella zoster virus (VZV) and respiratory viruses, and in neonatal intensive care unit (NICU) nurses, as a control group with infrequent occupational exposure. Our results show a lower CD4⁺ T cell response to two VZV proteins (IE62 and gE) and to tetanus toxoid (TT) in PED nurses who are cytomegalovirus (CMV)-seronegative, compared to CMV-seronegative NICU nurses, and that the decline might be more pronounced the more sustained the exposure. This decline might be due to an attrition of VZV- and TT-specific T cells as a result of the continuous pressure on the CD4⁺ T cell compartment. Moreover, our data suggest that the distinct T cell phenotypes known to be associated with CMV-seropositivity might be less prominent in PED nurses compared to NICU nurses, implying a plausible attenuating effect of occupational exposure on CMV-associated immunosenescence. Overall, this pilot study reveals an impact of occupational exposure to viral pathogens on CD4⁺ T cell immunity and supports further investigation in a larger cohort.

A Paradigmatic Interplay between Human Cytomegalovirus and Host Immune System: Possible Involvement of Viral Antigen-Driven CD8+ T Cell Responses in Systemic Sclerosis

Human cytomegalovirus (HCMV) is a highly prevalent opportunistic agent in the world population, which persists as a latent virus after a primary infection. Besides the well-established role of this agent causing severe diseases in immunocompromised individuals, more recently, HCMV has been evoked as a possible factor contributing to the pathogenesis of autoimmune diseases such as systemic sclerosis (SSc). The interplay between HCMV and immune surveillance is supposed to become unbalanced in SSc patients with expanded anti-HCMV immune responses, which are likely involved in the exacerbation of inflammatory processes. In this study, blood samples from a cohort of SSc patients vs. healthy subjects were tested for anti-HCMV immune responses (IgM, IgG antibodies, and T cells to peptide pools spanning the most immunogenic HCMV proteins). Statistically significant increase of HCMV-specific CD8+ T cell responses in SSc patients vs. healthy subjects was observed. Moreover, significantly greater HCMV-specific CD8+ T cell responses were found in SSc patients with a longer disease duration and those with higher modified Rodnan skin scores. Given the known importance of T cells in the development of SSc and that this virus may contribute to chronic inflammatory diseases, these data support a relevant role of HCMV-specific CD8+ T cell responses in SSc pathogenesis.

Quantifying size and diversity of the human T cell alloresponse

Alloreactive T lymphocytes are the primary mediators of immune responses in transplantation, both in the graft-versus-host and host-versus-graft directions. While essentially all clones comprising the human T cell repertoire have been selected on self-peptide presented by self-human leukocyte antigens (self-HLAs), much remains to be understood about the nature of clones capable of responding to allo-HLA molecules. Quantitative tools to study these cells are critical to understand fundamental features of this important response; however, the large size and diversity of the alloreactive T cell repertoire in humans presents a great technical challenge. We have developed a high-throughput T cell receptor (TCR) sequencing approach to characterize the human alloresponse. We present a statistical method to model T cell clonal frequency distribution and quantify repertoire diversity. Using these approaches, we measured the diversity and frequency of distinct alloreactive CD4+ and CD8+ T cell populations in HLA-mismatched responder-stimulator pairs. Our findings indicate that the alloimmune repertoire is highly specific for a given pair of individuals, that most alloreactive clones circulate at low frequencies, and that a high proportion of TCRs is likely able to recognize alloantigens.

Association Between Programed Cell Death-1 and CD4+ T Cell Alterations in Different Phases of Ischemic Stroke Patients

Objective: We aimed to analyze alterations in T cell subgroups during different post-ischemic stroke (IS) phases to explore the possible mechanisms underlying stroke-induced immune depression (SIID).

Methods: Sixty-four IS patients who met the entry criteria were divided into three groups: an acute phase group, a sub-acute phase group and a stable phase group. Fourteen healthy individuals were selected as normal controls. The phenotype distribution of T cells in patient peripheral blood was analyzed, and the immune checkpoint receptors programed cell death-1 (PD-1) and T cell immunoglobulin and mucin domain 3 (Tim-3) were detected in different T cell phenotypes.

Results: Compared with the control group, the absolute number of CD4⁺ T cells and CD4⁺ T central memory (TCM) cells was significantly increased in the acute phase group but decreased in the sub-acute phase and stable phase groups compared with that in the acute phase group. PD-1 expression in CD4⁺ T cells in the stable phase group showed a significant increase compared with that in the acute phase group. The expression of PD-1 on CD4⁺ TCM cells and CD4⁺ T effector memory (TEM) cells showed significant decreases in the acute phase compared with control cells; however, in the sub-acute phase and the stable phase, PD-1 expression was significantly increased compared with that in the acute phase.

Conclusions: T cell dysfunction, especially CD4⁺ T cell dysfunction, occurred during different IS phases. PD-1 was highly expressed in CD4⁺ T cells of different phenotypes after the acute phase and was associated with alterations in CD4⁺ T cells. Particularly, PD-1 was negatively correlated with the absolute number of TCM cells among different CD4⁺ T cell phenotypes, which may be one of the possible mechanisms of SIID.

Past Life and Future Effects-How Heterologous Infections Alter Immunity to Influenza Viruses

Influenza virus frequently mutates due to its error-prone polymerase. This feature contributes to influenza virus’s ability to evade pre-existing immunity, leading to annual epidemics and periodic pandemics. T cell memory plays a key protective role in the face of an antigenically distinct influenza virus strain because T cell targets are often derived from conserved internal proteins, whereas humoral immunity targets are often sites of increased mutation rates that are tolerated by the virus. Most studies of influenza T cell memory are conducted in naive, specific pathogen free mice and do not account for repetitive influenza infection throughout a lifetime, sequential acute heterologous infections between influenza infections, or heterologous chronic co-infections. By contrast to these mouse models, humans often experience numerous influenza infections, encounter heterologous acute infections between influenza infections, and are infected with at least one chronic virus. In this review, we discuss recent advances in understanding the effects of heterologous infections on the establishment and maintenance of CD8+ T cell immunological memory. Understanding the various factors that affect immune memory can provide insights into the development of more effective vaccines and increase reproducibility of translational studies between animal models and clinical results.

Rare Control of SIVmac239 Infection in a Vaccinated Rhesus Macaque

Effector memory T cell (T_EM) responses display potent antiviral properties and have been linked to stringent control of simian immunodeficiency virus (SIV) replication. Since recurrent antigen stimulation drives the differentiation of CD8⁺ T cells toward the T_EM phenotype, in this study we incorporated a persistent herpesviral vector into a heterologous prime/boost/boost vaccine approach to maximize the induction of T_EM responses. This new regimen resulted in CD8⁺ T_EM-biased responses in four rhesus macaques, three of which controlled viral replication to <1,000 viral RNA copies/ml of plasma for more than 6 months after infection with SIVmac239. Over the course of this study, we made a series of interesting observations in one of these successful controller animals. Indeed, in vivo elimination of CD8αβ⁺ T cells using a new CD8β-depleting antibody did not abrogate virologic control in this monkey. Only after its CD8α⁺ lymphocytes were depleted did SIV rebound, suggesting that CD8αα⁺ but not CD8αβ⁺ cells were controlling viral replication. By 2 weeks postinfection (PI), the only SIV sequences that could be detected in this animal harbored a small in-frame deletion in nef affecting six amino acids. Deep sequencing of the SIVmac239 challenge stock revealed no evidence of this polymorphism. However, sequencing of the rebound virus following CD8α depletion at week 38.4 PI again revealed only the six-amino acid deletion in nef. While any role for immunological pressure on the selection of this deleted variant remains uncertain, our data provide anecdotal evidence that control of SIV replication can be maintained without an intact CD8αβ⁺ T cell compartment.

Partners in Crime: The Role of CMV in Immune Dysregulation and Clinical Outcome During HIV Infection

In the current era of combination antiretroviral therapy (ART), human immunodeficiency virus (HIV)-infected individuals are living longer and healthier lives. Nevertheless, HIV-infected persons are at greater risk for age-related disorders, which have been linked to residual immune dysfunction and inflammation. HIV-infected individuals are almost universally co-infected with cytomegalovirus (CMV) and both viruses are associated with inflammation-related morbidities. Therefore, a detailed investigation of the relationship between CMV and aging-related morbidities emerging during chronic HIV infection is warranted. Here, we review the literature on how CMV co-infection affects HIV infection and host immunity and we discuss the gaps in our knowledge that need elucidation.

Cytomegalovirus driven immunosenescence-An immune phenotype with or without clinical impact?

The continuous emerging increase in life span has led to vulnerability to a number of different diseases in the elderly. Some of these risks may be attributed to specific changes in the immune system referred to as immunoscenescence. This term aims to describe decreased immune functions among elderly individuals, and is characterized to be harmful age-associated changes in the immune system that lead to its gradual immune dysfunction. An impaired function of the immune system may increase susceptibility to various diseases in the elderly population such as infections, cardiovascular diseases and cancer. Although it is unclear how this immune phenotype develops, emerging evidence suggest that it may reflect an exhaustion of the immune system, possibly caused by one or several chronic infections. The main candidate is human cytomegalovirus (CMV), which can induce immune dysfunctions observed in immunoscenescence. Although the immune system is currently considered to be exhausted in CMV positive elderly individuals, it is not known whether such dysfunction of the immune system is a main reason for increased susceptibility to other diseases, or if direct effects of the virus in disease pathogenesis reflect the increased vulnerability to them. These aspects will be discussed in this review.

T cell responses to cytomegalovirus

Human cytomegalovirus (HCMV) establishes a latent infection that generally remains asymptomatic in immune-competent hosts for decades but can cause serious illness in immune-compromised individuals. The long-term control of CMV requires considerable effort from the host immune system and has a lasting impact on the profile of the immune system. One hallmark of CMV infection is the maintenance of large populations of CMV-specific memory CD8(+) T cells - a phenomenon termed memory inflation - and emerging data suggest that memory inflation is associated with impaired immunity in the elderly. In this Review, we discuss the molecular triggers that promote memory inflation, the idea that memory inflation could be considered a natural pathway of T cell maturation that could be harnessed in vaccination, and the broader implications of CMV infection and the T cell responses it elicits.

Tissue-resident memory T cells in cytomegalovirus infection

Herpesviruses establish life-long infection in their hosts and maintain latent reservoirs for sporadic reactivation at peripheral sites, such as skin and mucosae. For herpes simplex virus infection, experimental studies in mice revealed that immediate protection against local reactivation or superinfection events in the skin relies on tissue resident memory T cells (TRM) rather than on their circulating counterparts. Recent evidence extends this notion to cytomegalovirus infection, which potently induces TRM cells in both mice and humans particularly in mucosal tissues that constitute important viral sanctuaries and are relevant entry sites for challenge and superinfections. The discovery unravels promising opportunities to exploit cytomegalovirus based vaccine vectors for the specific induction of tissue resident T cell subsets.

Type I interferon signaling enhances CD8+ T cell effector function and differentiation during murine gammaherpesvirus 68 infection

CD8(+) T cell responses are critical to the control of replication and reactivation associated with gammaherpesvirus infection. Type I interferons (IFNs) have been shown to have direct and indirect roles in supporting CD8(+) T cell development and function during viral infection; however, the role of type I interferons during latent viral infection has not been examined. Mice deficient in type I IFN signaling (IFNAR1(-/-) mice) have high levels of reactivation during infection with murine gammaherpesvirus 68 (MHV68), a murine gammaherpesvirus model for Epstein-Barr virus. We hypothesized that type I IFNs function to enhance the anti-gammaherpesvirus CD8(+) T cell response. To test this, IFNAR1(-/-) mice were infected with MHV68 and the CD8(+) T cell response was analyzed. In the absence of type I IFN signaling, there was a marked increase in short-lived effector CD8(+) T cells, and MHV68-specific CD8(+) T cells had upregulated expression of PD-1 and reduced tumor necrosis factor alpha (TNF-α), gamma IFN (IFN-γ), and interleukin-2 (IL-2) production. Suppressing MHV68 replication early in infection using the antiviral cidofovir rescued CD8(+) T cell cytokine production and reduced PD-1 expression. However, suppressing high levels of reactivation in IFNAR1(-/-) mice failed to improve CD8(+) T cell cytokine production during latency. T cell-specific abrogation of type I IFN signaling showed that the effects of type I IFNs on the CD8(+) T cell response during MHV68 infection are independent of direct type I IFN signaling on T cells. Our findings support a model in which type I IFNs likely suppress MHV68 replication, thus limiting viral antigen and facilitating an effective gammaherpesvirus-directed CD8(+) T cell response.

IMPORTANCE

The murine gammaherpesvirus MHV68 has both genetic and biologic homology to the human gammaherpesvirus Epstein-Barr virus (EBV), which infects over 90% of humans. Latent EBV infection and reactivation are associated with various life-threatening diseases and malignancies. Host suppression of gammaherpesvirus latency and reactivation requires both CD8(+) T cells as well as type I interferon signaling. Type I IFNs have been shown to critically support the antiviral CD8(+) T cell response in other virus models. Here, we identify an indirect role for type I IFN signaling in enhancing gammaherpesvirus-specific CD8(+) T cell cytokine production. Further, this function of type I IFN signaling can be partially rescued by suppressing viral replication during early MHV68 infection. Our data suggest that type I IFN signaling on non-T cells can enhance CD8(+) T cell function during gammaherpesvirus infection, potentially through suppression of MHV68 replication.

Normalizing ELISPOT responses to T-cell counts: a novel approach for quantification of HCMV-specific CD4(+) and CD8(+) T-cell responses in kidney transplant recipients

BACKGROUND

Human cytomegalovirus (HCMV) is the most common opportunistic virus infection in solid organ transplant recipients. The analysis of HCMV-specific T-cell immunity after organ transplant is of relevant clinical interest.

OBJECTIVES

To analyze HCMV-specific CD4(+) and CD8(+) T-cell responses in healthy subjects and kidney transplant recipients (KTR).

STUDY DESIGN

HCMV-specific T-cell responses were evaluated by interferon-γ (IFN-γ) enzyme-linked immunospot (ELISPOT) using overlapping 15-mer peptide pools of immediate early (IE)-1, IE-2, phosphoprotein 65 (pp65) (for stimulation of both CD4(+) and CD8(+) T-cell responses) and a pool of 34 short peptides (8-12 amino acids in length, for stimulation of CD8(+) T-cell responses). ELISPOT results were normalized to T-cell subset counts and their correlations with a reported dendritic cell (DC)-based assay, which simultaneously quantifies HCMV-specific CD4(+) and CD8(+) T-cell responses, were analyzed.

RESULTS

HCMV-seropositive KTR showed higher ELISPOT responses compared to HCMV-seropositive healthy subjects. IE-1 and pp65 ELISPOT responses were mediated mainly by CD8(+) T-cells and, to a lesser extent, CD4(+) T cells; IE-2 peptides appear to stimulate CD56(+) cells (natural killer cells). In HCMV-seropositive healthy subjects, ELISPOT results (expressed either as net spots/million cells or normalized to the corresponding T-cell count) significantly correlated with the DC assay. However, in HMCV-seropositive KTR, only normalized ELISPOT responses to overlapping 15-mer peptide pools significantly correlated with DC-assay responses.

CONCLUSIONS

The normalized ELISPOT represents a novel and simple approach for quantifying and monitoring HCMV-specific CD4(+) and CD8(+) T-cell responses in KTR.

Gammaherpesvirus latency differentially impacts the generation of primary versus secondary memory CD8+ T cells during subsequent infection

Unlike laboratory animals, humans are infected with multiple pathogens, including the highly prevalent herpesviruses. The purpose of these studies was to determine the effect of gammaherpesvirus latency on T cell number and differentiation during subsequent heterologous viral infections. Mice were first infected with murine gammaherpesvirus 68 (MHV68), a model of Epstein-Barr virus (EBV) infection, and then after latency was established, they were challenged with the Armstrong strain of lymphocytic choriomeningitis virus (LCMV). The initial replication of LCMV was lower in latently infected mice, and the maturation of dendritic cells was abated. Although the number of LCMV-specific effector CD8(+) T cells was not altered, they were skewed to a memory phenotype. In contrast, LCMV-specific effector CD4(+) T cells were increased in latently infected mice compared to those in mice infected solely with LCMV. When the memory phase was reached, latently infected mice had an LCMV-specific memory T cell pool that was increased relative to that found in singly infected mice. Importantly, LCMV-specific memory CD8(+) T cells had decreased CD27 and increased killer cell lectin-like receptor G1 (KLRG1) expression. Upon secondary challenge, LCMV-specific secondary effector CD8(+) T cells expanded and cleared the infection. However, the LCMV-specific secondary memory CD8(+) T cell pool was decreased in latently infected animals, abrogating the boosting effect normally observed following rechallenge. Taken together, these results demonstrate that ongoing gammaherpesvirus latency affects the number and phenotype of primary versus secondary memory CD8(+) T cells during acute infection.

IMPORTANCE

CD8(+) T cells are critical for the clearance of intracellular pathogens, including viruses, certain bacteria, and tumors. However, current models for memory CD8(+) T cell differentiation are derived from pathogen-free laboratory mice challenged with a single pathogen or vaccine vector. Unlike laboratory animals, all humans are infected with multiple acute and chronic pathogens, including the highly prevalent herpesviruses Epstein-Barr virus (EBV), cytomegalovirus (CMV), herpes simplex viruses (HSV), and varicella-zoster virus (VZV). The purpose of these studies was to determine the effect of gammaherpesvirus latency on T cell number and differentiation during subsequent heterologous viral infections. We observed that ongoing gammaherpesvirus latency affects the number and phenotype of primary versus secondary memory CD8(+) T cells during acute infection. These results suggest that unlike pathogen-free laboratory mice, infection or immunization of latently infected humans may result in the generation of T cells with limited potential for long-term protection.

Autoimmune effector memory T cells: the bad and the good

Immunological memory is a hallmark of adaptive immunity, a defense mechanism endowed to vertebrates during evolution. However, an autoimmune pathogenic role of memory lymphocytes is also emerging with accumulating evidence, despite reasonable skepticism on their existence in a chronic setting of autoimmune damage. It is conceivable that autoimmune memory would be particularly harmful since memory cells would constantly "remember" and attack the body's healthy tissues. It is even more detrimental given the resistance of memory T cells to immunomodulatory therapies. In this review, we focus on self-antigen-reactive CD(+) effector memory T (TEM) cells, surveying the evidence for the role of the T(EM) compartment in autoimmune pathogenesis. We will also discuss the role of T(EM) cells in chronic and acute infectious disease settings and how they compare to their counterparts in autoimmune diseases. With their long-lasting potency, the autoimmune T(EM) cells could also play a critical role in anti-tumor immunity, which may be largely based on their reactivity to self-antigens. Therefore, although autoimmune T(EM) cells are "bad" due to their role in relentless perpetration of tissue damage in autoimmune disease settings, they are unlikely a by-product of industrial development along the modern surge of autoimmune disease prevalence. Rather, they may be a product of evolution for their "good" in clearing damaged host cells in chronic infections and malignant cells in cancer settings.

Implication of human herpesviruses in oncogenesis through immune evasion and supression

All human herpesviruses (HHVs) have been implicated in immune system evasion and suppression. Moreover, two HHV family members, i.e. EBV and KSHV, are recognised as oncogenic viruses. Our literature review summarises additional examples of possible oncogenic mechanisms that have been attributed to other HHVs. In general, HHVs affect almost every cancer-implicated branch of the immune system, namely tumour-promoting inflammation, immune evasion, and immunosuppression. Some HHVs accomplish these effects by inhibiting apoptotic pathways and by promoting proliferation. Mechanisms related to immunosupression and low grade chronic inflammation could eventually result in the initiation and progression of cancer. In this article we open a discussion on the members of Herpesviridae, their immune evasion and suppression mechanisms, and their possible role in cancer development. We conclude that discerning the mechanisms of interplay between HHV, immune system, and cancer is essential for the development of novel preventative and therapeutic approaches for cancer treatment and prophylaxis.

Persistence of EBV antigen-specific CD8 T cell clonotypes during homeostatic immune reconstitution in cancer patients

Persistent viruses are kept in check by specific lymphocytes. The clonal T cell receptor (TCR) repertoire against Epstein-Barr virus (EBV), once established following primary infection, exhibits a robust stability over time. However, the determinants contributing to this long-term persistence are still poorly characterized. Taking advantage of an in vivo clinical setting where lymphocyte homeostasis was transiently perturbed, we studied EBV antigen-specific CD8 T cells before and after non-myeloablative lympho-depleting chemotherapy of melanoma patients. Despite more advanced T cell differentiation, patients T cells showed clonal composition comparable to healthy individuals, sharing a preference for TRBV20 and TRBV29 gene segment usage and several co-dominant public TCR clonotypes. Moreover, our data revealed the presence of relatively few dominant EBV antigen-specific T cell clonotypes, which mostly persisted following transient lympho-depletion (TLD) and lymphocyte recovery, likely related to absence of EBV reactivation and de novo T cell priming in these patients. Interestingly, persisting clonotypes frequently co-expressed memory/homing-associated genes (CD27, IL7R, EOMES, CD62L/SELL and CCR5) supporting the notion that they are particularly important for long-lasting CD8 T cell responses. Nevertheless, the clonal composition of EBV-specific CD8 T cells was preserved over time with the presence of the same dominant clonotypes after non-myeloablative chemotherapy. The observed clonotype persistence demonstrates high robustness of CD8 T cell homeostasis and reconstitution.

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.