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

Applications of Anti-Cytomegalovirus T Cells for Cancer (Immuno)Therapy

Infection with cytomegalovirus (CMV) is highly prevalent in the general population and largely controlled by CD8pos T cells. Intriguingly, anti-CMV T cells accumulate over time to extraordinarily high numbers, are frequently present as tumor-resident 'bystander' T cells, and remain functional in cancer patients. Consequently, various strategies for redirecting anti-CMV CD8pos T cells to eliminate cancer cells are currently being developed. Here, we provide an overview of these strategies including immunogenic CMV peptide-loading onto endogenous HLA complexes on cancer cells and the use of tumor-directed fusion proteins containing a preassembled CMV peptide/HLA-I complex. Additionally, we discuss conveying the advantageous characteristics of anti-CMV T cells in adoptive cell therapy. Utilization of anti-CMV CD8pos T cells to generate CAR T cells promotes their in vivo persistence and expansion due to appropriate co-stimulation through the endogenous (CMV-)TCR signaling complex. Designing TCR-engineered T cells is more challenging, as the artificial and endogenous TCR compete for expression. Moreover, the use of expanded/reactivated anti-CMV T cells to target CMV peptide-expressing glioblastomas is discussed. This review highlights the most important findings and compares the benefits, disadvantages, and challenges of each strategy. Finally, we discuss how anti-CMV T cell therapies can be further improved to enhance treatment efficacy.

The Viral G-Protein-Coupled Receptor Homologs M33 and US28 Promote Cardiac Dysfunction during Murine Cytomegalovirus Infection

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects the majority of the world population and causes lifelong latent infection. HCMV has been shown to exacerbate cardiovascular diseases, including myocarditis, vascular sclerosis, and transplant vasculopathy. Recently, we have shown that murine CMV (MCMV) recapitulates the cardiovascular dysfunction observed in patients with HCMV-induced myocarditis. To understand the viral mechanisms involved in CMV-induced heart dysfunction, we further characterized cardiac function in response to MCMV and examined virally encoded G-protein-coupled receptor homologs (vGPCRs) US28 and M33 as potential factors that promote infection in the heart. We hypothesized that the CMV-encoded vGPCRs could exacerbate cardiovascular damage and dysfunction. Three viruses were used to evaluate the role of vGPCRs in cardiac dysfunction: wild-type MCMV, a M33-deficient virus (∆M33), and a virus with the M33 open reading frame (ORF) replaced with US28, an HCMV vGPCR (i.e., US28+). Our in vivo studies revealed that M33 plays a role in promoting cardiac dysfunction by increasing viral load and heart rate during acute infection. During latency, ΔM33-infected mice demonstrated reduced calcification, altered cellular gene expression, and less cardiac hypertrophy compared with wild-type MCMV-infected mice. Ex vivo viral reactivation from hearts was less efficient in ΔM33-infected animals. HCMV protein US28 expression restored the ability of the M33-deficient virus to reactivate from the heart. US28+ MCMV infection caused damage to the heart comparable with wild-type MCMV infection, suggesting that the US28 protein is sufficient to complement the function of M33 in the heart. Altogether, these data suggest a role for vGPCRs in viral pathogenesis in the heart and thus suggest that vGPCRs promote long-term cardiac damage and dysfunction.

Cellular senescence is a key mediator of lung aging and susceptibility to infection

Aging results in systemic changes that leave older adults at much higher risk for adverse outcomes following respiratory infections. Much work has been done over the years to characterize and describe the varied changes that occur with aging from the molecular/cellular up to the organismal level. In recent years, the systemic accumulation of senescent cells has emerged as a key mediator of many age-related declines and diseases of aging. Many of these age-related changes can impair the normal function of the respiratory system and its capability to respond appropriately to potential pathogens that are encountered daily. In this review, we aim to establish the effects of cellular senescence on the disruption of normal lung function with aging and describe how these effects compound to leave an aged respiratory system at great risk when exposed to a pathogen. We will also discuss the role cellular senescence may play in the inability of most vaccines to confer protection against respiratory infections when administered to older adults. We posit that cellular senescence may be the point of convergence of many age-related immunological declines. Enhanced investigation into this area could provide much needed insight to understand the aging immune system and how to effectively ameliorate responses to pathogens that continue to disproportionately harm this vulnerable population.

Immunity to acute virus infections with advanced age

New infections in general, and new viral infections amongst them, represent a serious challenge to an older organism. This review discusses the age-related alterations in responsiveness to infection from the standpoint of virus:host relationship and the host physiological whole-organism and specific immune response to the virus. Changes with age in the innate and adaptive immune system homeostasis and function are reviewed briefly. This is followed by a review of specific alterations and defects in the response of older organisms (chiefly mice and humans) to acute (particularly emerging and re-emerging) viral infections, with a very brief summary of the response to latent persistent infections. Finally, we provide a brief summary of the perspectives for possible interventions to enhance antiviral immunity.

The hallmarks of CMV-specific CD8 T-cell differentiation

Upon cytomegalovirus (CMV) infection, large T-cell responses are elicited that remain high or even increase over time, a phenomenon named memory T-cell inflation. Besides, the maintained robust T-cell response, CMV-specific T cells seem to have a distinctive phenotype, characterized by an advanced differentiation state. Here, we will review this "special" differentiation status by discussing the cellular phenotype based on the expression of CD45 isoforms, costimulatory, inhibitory and natural killer receptors, adhesion and lymphocyte homing molecules, transcription factors, cytokines and cytotoxic molecules. In addition, we focus on whether the differentiation state of CMV-specific CD8 T cells is unique in comparison with other chronic viruses and we will discuss the possible impact of factors such as antigen exposure and aging on the advanced differentiation status of CMV-specific CD8 T cells.

Impact of CMV upon immune aging: facts and fiction

Aging is accompanied by significant defects in immunity and compromised responses to new, previously unencountered microbial pathogens. Most humans carry several persistent or latent viruses as they age, interacting with the host immune systems for years. In that context maybe the most studied persistent virus is Cytomegalovirus, infamous for its ability to recruit very large T cell responses which increase with age and to simultaneously evade elimination by the immune system. Here we will address how lifelong CMV infection and the immunological burden of its control might affect immune reactivity and health of the host over time.

Cytomegalovirus-Driven Adaption of Natural Killer Cells in NKG2Cnull Human Immunodeficiency Virus-Infected Individuals

Expansion of natural killer (NK) cells expressing NKG2C occurs following human cytomegalovirus (HCMV) infection and is amplified by human immunodeficiency virus (HIV) co-infection. These NKG2C-expressing NK cells demonstrate enhanced CD16-dependent cytokine production and downregulate FcεRIγ and promyelocytic leukemia zinc finger protein (PLZF). Lacking NKG2C diminishes resistance to HIV infection, but whether this affects NK cell acquisition of superior antibody-dependent function is unclear. Therefore, our objective was to investigate whether HCMV-driven NK cell differentiation is impaired in NKG2C^null HIV-infected individuals. Phenotypic (CD2, CD16, CD57, NKG2A, FcεRIγ, and PLZF expression) and functional (cytokine induction and cytotoxicity) properties were compared between HIV⁻infected NKG2C^null and NKG2C-expressing groups. Cytokine production was compared following stimulation through natural cytotoxicity receptors or through CD16. Cytotoxicity was measured by anti-CD16-redirected lysis and by classical antibody-dependent cell-mediated cytotoxicity (ADCC) against anti-class I human leukocyte antigen (HLA) antibody-coated cells. Our data indicate highly similar HCMV-driven NK cell differentiation in HIV infection with or without NKG2C. While the fraction of mature (CD57^pos) NK cells expressing CD2 (p = 0.009) or co-expressing CD2 and CD16 (p = 0.03) was significantly higher in NKG2C^null HIV-infected individuals, there were no significant differences in NKG2A, FcεRIγ, or PLZF expression. The general phenotypic and functional equivalency observed suggests NKG2C-independent routes of HCMV-driven NK cell differentiation, which may involve increased CD2 expression.

Memory CD8 T cell inflation vs tissue-resident memory T cells: Same patrollers, same controllers?

The induction of long-lived populations of memory T cells residing in peripheral tissues is of considerable interest for T cell-based vaccines, as they can execute immediate effector functions and thus provide protection in case of pathogen encounter at mucosal and barrier sites. Cytomegalovirus (CMV)-based vaccines support the induction and accumulation of a large population of effector memory CD8 T cells in peripheral tissues, in a process called memory inflation. Tissue-resident memory (T_RM ) T cells, induced by various infections and vaccination regimens, constitute another subset of memory cells that take long-term residence in peripheral tissues. Both memory T cell subsets have evoked substantial interest in exploitation for vaccine purposes. However, a direct comparison between these two peripheral tissue-localizing memory T cell subsets with respect to their short- and long-term ability to provide protection against heterologous challenge is pending. Here, we discuss communalities and differences between T_RM and inflationary CD8 T cells with respect to their development, maintenance, function, and protective capacity. In addition, we discuss differences and similarities between the transcriptional profiles of T_RM and inflationary T cells, supporting the notion that they are distinct memory T cell populations.

Increased senescent CD8+ T cells in the peripheral blood mononuclear cells of Behçet's disease patients

Behçet's disease (BD) is a chronic inflammatory disease characterized by recurrent mucocutaneous, ocular, and skin lesions. Immunosenescence is associated with increased susceptibility to infection and chronic low grade inflammation. This study aimed to investigate the differences in the frequencies of immunosenescent cells in the peripheral blood mononuclear cells (PBMCs) of patients with BD. PBMCs were isolated from age-matched patients with active BD (n = 19), inactive BD (n = 20), disease controls (DCs, n = 15) and healthy controls (HCs, n = 15). The frequencies of senescent CD4+ T cells (CD3+ CD4+ CD27- CD28- cells), CD8+ T cells (CD3+ CD8+ CD27- CD28- cells) and B cells (CD19+ CD27- IgD- cells) were analyzed using flow cytometry. Senescence-associated β galactosidase activity was also measured in CD8+ T cells using flow cytometry with 5-dodecanoylaminofluorescein di-β-D-galactopyranoside. Frequencies of senescent CD4+ and CD19+ cells were not significantly different between the groups. The frequency of senescent CD8+ T cells was significantly higher in active BD than in DCs and HCs. C-reactive protein and erythrocyte sedimentation rate levels, which indicate disease activity, did not correlate with increased frequencies of immunosenescent cells. Steroid treatment, specific organ involvement, and HLA-B51 status did not have a significant influence on the frequencies of immunosenescent cells. Frequencies of senescence-associated β galactosidase+ CD8+ T cells were significantly higher in active BD and inactive BD compared to DCs and HCs. There was an increased frequency of senescent CD8+ T cells in the PBMCs of patients with BD.

Cytomegalovirus-Specific T Cells Restricted by HLA-Cw*0702 Increase Markedly with Age and Dominate the CD8+ T-Cell Repertoire in Older People

Cytomegalovirus (CMV) infection elicits a strong T-cell immune response, which increases further during aging in a process termed "memory inflation." CMV downregulates the expression of HLA-A and HLA-B on the surface of infected cells to limit presentation of viral peptides to T-cells although HLA-C is relatively spared as it also engages with inhibitory killer immunoglobulin receptor receptors and therefore reduces lysis by natural killer cells. We investigated the magnitude and functional properties of CMV-specific CD8+ T-cells specific for 10 peptides restricted by HLA-C in a cohort of 53 donors between the age of 23 and 91 years. This was achieved via peptide stimulation of PBMCs followed by multicolor flow cytometry. Three peptides, derived from proteins generated in the immediate-early period of viral replication and restricted by HLA-Cw*0702, elicited strong immune responses, which increased substantially with age such that the average aggregate response represented 37% of the CD8+ T-cell pool within donors above 70 years of age. Remarkably, a single response represented 70% of the total CD8+ T-cell pool within a 91-year-old donor. HLA-Cw*0702-restricted CD8+ T-cell responses were immunodominant over HLA-A and HLA-B-restricted CMV-specific responses and did not show features of exhaustion such as PD-1 or CD39 expression. Indeed, such CTL exhibit a polyfunctional cytokine profile with co-expression of IFN-γ and TNF-α and a strong cytotoxic phenotype with intracellular expression of perforin and granzymeB. Functionally, HLA-Cw*0702-restricted CTL show exceptionally high avidity for cognate peptide-HLA and demonstrate very early and efficient recognition of virally infected cells. These observations indicate that CD8+ T-cells restricted by HLA-C play an important role in the control of persistent CMV infection and could represent a novel opportunity for CD8+ T-cell therapy of viral infection within immunosuppressed patients. In addition, the findings provide further evidence for the importance of HLA-C-restricted T-cells in the control of chronic viral infection.

Altered populations of natural killer cells, cytotoxic T lymphocytes, and regulatory T cells in major depressive disorder: Association with sleep disturbance

A subset of individuals with major depressive disorder (MDD) have impaired adaptive immunity characterized by a greater vulnerability to viral infection and a deficient response to vaccination along with a decrease in the number and/or activity of T cells and natural killer cells (NKC). Nevertheless, it remains unclear which specific subsets of lymphocytes are altered in MDD, a shortcoming we address here by utilizing an advanced fluorescence-activated cell sorting (FACS) method that allows for the differentiation of important functionally-distinct lymphocyte sub-populations. Furthermore, despite evidence that sleep disturbance, which is a core symptom of MDD, is itself associated with alterations in lymphocyte distributions, there is a paucity of studies examining the contribution of sleep disturbance on lymphocyte populations in MDD populations. Here, we measured differences in the percentages of 13 different lymphocytes and 6 different leukocytes in 54 unmedicated MDD patients (partially remitted to moderate) and 56 age and sex-matched healthy controls (HC). The relationship between self-reported sleep disturbance and cell counts was evaluated in the MDD group using the Pittsburgh Sleep Quality Index (PSQI). The MDD group showed a significantly increased percentage of CD127^low/CCR4⁺ T_reg cells, and memory T_reg cells, as well as a reduction in CD56⁺CD16^- (putative immunoregulatory) NKC counts, the latter, prior to correction for body mass index. There also was a trend for higher effector memory CD8⁺ cell counts in the MDD group versus the HC group. Further, within the MDD group, self-reported sleep disturbance was associated with an increased percentage of effector memory CD8⁺ cells but with a lower percentage of CD56⁺CD16^- NKC. These results provide important new insights into the immune pathways involved in MDD, and provide novel evidence that MDD and associated sleep disturbance increase effector memory CD8⁺ and T_reg pathways. Targeting sleep disturbance may have implications as a therapeutic strategy to normalize NKC and memory CD8⁺ cells in MDD.

Cytomegalovirus (CMV) Epitope-Specific CD4+ T Cells Are Inflated in HIV+ CMV+ Subjects

Select CMV epitopes drive life-long CD8⁺ T cell memory inflation, but the extent of CD4 memory inflation is poorly studied. CD4⁺ T cells specific for human CMV (HCMV) are elevated in HIV⁺ HCMV⁺ subjects. To determine whether HCMV epitope-specific CD4⁺ T cell memory inflation occurs during HIV infection, we used HLA-DR7 (DRB1*07:01) tetramers loaded with the glycoprotein B DYSNTHSTRYV (DYS) epitope to characterize circulating CD4⁺ T cells in coinfected HLA-DR7⁺ long-term nonprogressor HIV subjects with undetectable HCMV plasma viremia. DYS-specific CD4⁺ T cells were inflated among these HIV⁺ subjects compared with those from an HIV^- HCMV⁺ HLA-DR7⁺ cohort or with HLA-DR7-restricted CD4⁺ T cells from the HIV-coinfected cohort that were specific for epitopes of HCMV phosphoprotein-65, tetanus toxoid precursor, EBV nuclear Ag 2, or HIV gag protein. Inflated DYS-specific CD4⁺ T cells consisted of effector memory or effector memory-RA⁺ subsets with restricted TCRβ usage and nearly monoclonal CDR3 containing novel conserved amino acids. Expression of this near-monoclonal TCR in a Jurkat cell-transfection system validated fine DYS specificity. Inflated cells were polyfunctional, not senescent, and displayed high ex vivo levels of granzyme B, CX₃CR1, CD38, or HLA-DR but less often coexpressed CD38⁺ and HLA-DR⁺ The inflation mechanism did not involve apoptosis suppression, increased proliferation, or HIV gag cross-reactivity. Instead, the findings suggest that intermittent or chronic expression of epitopes, such as DYS, drive inflation of activated CD4⁺ T cells that home to endothelial cells and have the potential to mediate cytotoxicity and vascular disease.

Cerebrospinal fluid cyto-/chemokine profile during acute herpes simplex virus induced anti-N-methyl-d-aspartate receptor encephalitis and in chronic neurological sequelae

AIM

To examine the cytokine/chemokine profile of cerebrospinal fluid (CSF) during acute herpes simplex virus-induced N-methyl-d-aspartate receptor (NMDAR) autoimmunity and in chronic/relapsing post-herpes simplex virus encephalitis (HSE) neurological syndromes.

METHOD

We measured longitudinal serial CSF cyto-/chemokines (n=34) and a glial marker (calcium-binding astroglial protein, S100B) in one patient during acute HSE and subsequent anti-NMDAR encephalitis, and compared the results with those from two patients with anti-NMDAR encephalitis without preceding HSE. We also compared cyto-/chemokines in cross-sectional CSF samples from three children with previous HSE who had ongoing chronic or relapsing neurological symptoms (2yr 9 mo-16y after HSE) with those in a group of children having non-inflammatory neurological conditions (n=20).

RESULTS

Acute HSE showed elevation of a broad range of all T-helper-subset-related cyto-/chemokines and S100B whereas the post-HSE anti-NMDAR encephalitis phase showed persistent elevation of two of five T-helper-1 (chemokine [C-X-C motif] ligand 9 [CXCL9], CXCL10), three of five predominantly B-cell (CXCL13, CCL19, a proliferation-inducing ligand [APRIL])-mediated cyto-/chemokines, and interferon-α. The post-HSE anti-NMDAR encephalitis inflammatory response was more pronounced than anti-NMDAR encephalitis. All three chronic post-HSE cases showed persistent elevation of CXCL9, CXCL10, and interferon-α, and there was histopathological evidence of chronic lymphocytic inflammation in one biopsied case 7 years after HSE. Two of three chronic cases showed a modest response to immune therapy.

INTERPRETATION

HSE-induced anti-NMDAR encephalitis is a complex and pronounced inflammatory syndrome. There is persistent CSF upregulation of cyto-/chemokines in chronic or relapsing post-HSE neurological symptoms, which may be modifiable with immune therapy. The elevated cyto-/chemokines may be targets of monoclonal therapies.

The association between clinical outcome and CD8+ lymphocytic infiltration in advanced stages of colorectal cancer differs by latent virus infection in tumour tissue

AIMS

In the near future, an immunoscore based on the quantification of lymphocytic populations can be expected as a fundamental supplement of colorectal cancer (CRC) classification. This study explored whether latent viral infection has an influence on prognostically relevant host immunity in CRC.

METHODS AND RESULTS

CD8⁺ lymphocytic infiltration in three tumour compartments of 121 CRC was compared with clinical data and occurrence of latent infection with herpes simplex virus (HSV1, HSV2), cytomegalovirus (CMV), human papillomavirus (HPV16 and HPV18) in the tumour tissue, which was determined by polymerase chain reaction (PCR). Intraepithelial CD8⁺ lymphocytic infiltration (IE_CD8+ ) showed a trend towards correlation with clinical stage (P = 0.073), significant differences between CRC with and without metastases (P = 0.001) and a significant correlation with overall survival (OS, P = 0.001). Each of these three clinical parameters showed a significant link to IE_CD8+ in the virus DNA-negative (P-values: 0.001-0.036), but no significant differences in the virus DNA-positive subgroup, which is consistent with a moderating effect of virus DNA on these associations. A significant correlation of CD8⁺ infiltration in the invasive margin (IM_CD8+ ) with OS (P = 0.016) was also moderated by virus DNA.

CONCLUSION

Our data suggest a possible influence of latent viral infection on the association between clinical outcome and CD8⁺ lymphocytic infiltration in CRC tissue. After confirmation of these results by large cohort studies, a potential interaction between microbial pathogens and host immunity in CRC and its impact on prognostic immunoscores and/or new therapeutic strategies should be investigated further.

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.

Peptide Processing Is Critical for T-Cell Memory Inflation and May Be Optimized to Improve Immune Protection by CMV-Based Vaccine Vectors

Cytomegalovirus (CMV) elicits long-term T-cell immunity of unparalleled strength, which has allowed the development of highly protective CMV-based vaccine vectors. Counterintuitively, experimental vaccines encoding a single MHC-I restricted epitope offered better immune protection than those expressing entire proteins, including the same epitope. To clarify this conundrum, we generated recombinant murine CMVs (MCMVs) encoding well-characterized MHC-I epitopes at different positions within viral genes and observed strong immune responses and protection against viruses and tumor growth when the epitopes were expressed at the protein C-terminus. We used the M45-encoded conventional epitope HGIRNASFI to dissect this phenomenon at the molecular level. A recombinant MCMV expressing HGIRNASFI on the C-terminus of M45, in contrast to wild-type MCMV, enabled peptide processing by the constitutive proteasome, direct antigen presentation, and an inflation of antigen-specific effector memory cells. Consequently, our results indicate that constitutive proteasome processing of antigenic epitopes in latently infected cells is required for robust inflationary responses. This insight allows utilizing the epitope positioning in the design of CMV-based vectors as a novel strategy for enhancing their efficacy.

Experimental cytomegalovirus (CMV) based vaccine vectors have provided highly encouraging results as innovative vaccine formulations against deadly virus infections, such as Ebola or AIDS. Nevertheless, it has remained incompletely understood why CMV is so efficient at stimulating T-lymphocytes, the immune cells that recognize pathogens within infected cells. We have generated an array of CMV mutants expressing the same antigen in different genes or in different parts of the same gene. This allowed us to identify that the immediate environment of the antigen, rather than properties of the antigen itself, crucially determine the immune protection conferred by CMV-based vaccines, implying that optimal immunity depends on the ability of host cells to degrade CMV proteins into peptides, short units that are recognized by T-cells. Detailed analysis revealed that strong and sustained T-cell immunity occurs only when their antigenic targets are processed by a primitive cellular machinery that is present in all cells of the body, rather than by its newly-evolved counterpart, which is present only in specialized antigen-presenting cells. Most importantly, our results provide a simple strategy to develop improved CMV vaccines by positioning the antigenic peptides at the right spot in CMV proteins.

Cytomegalovirus Reinfections Stimulate CD8 T-Memory Inflation

Cytomegalovirus (CMV) has been shown to induce large populations of CD8 T-effector memory cells that unlike central memory persist in large quantities following infection, a phenomenon commonly termed “memory inflation”. Although murine models to date have shown very large and persistent CMV-specific T-cell expansions following infection, there is considerable variability in CMV-specific T-memory responses in humans. Historically such memory inflation in humans has been assumed a consequence of reactivation events during the life of the host. Because basic information about CMV infection/re-infection and reactivation in immune competent humans is not available, we used a murine model to test how primary infection, reinfection, and reactivation stimuli influence memory inflation. We show that low titer infections induce “partial” memory inflation of both mCMV specific CD8 T-cells and antibody. We show further that reinfection with different strains can boost partial memory inflation. Finally, we show preliminary results suggesting that a single strong reactivation stimulus does not stimulate memory inflation. Altogether, our results suggest that while high titer primary infections can induce memory inflation, reinfections during the life of a host may be more important than previously appreciated.

A new perspective of the structural complexity of HCMV-specific T-cell responses

BACKGROUND

In studies exploring the effects of HCMV infection on immune system aging ('immunosenescence'), after organ transplantation or in other settings, HCMV-specific T-cell responses are often assessed with respect to purportedly 'immunodominant' protein subunits. However, the response structure in terms of recognized antigens and response hierarchies (architecture) is not well understood and actual correlates of immune protection are not known.

METHODS

We explored the distribution of T-cell response sizes and dominance hierarchies as well as response breadth in 33 HCMV responders with respect to >200 HCMV proteins.

RESULTS

At the individual responder level HCMV-specific T-cell responses were generally arranged in clear dominance hierarchies; interestingly, the number of proteins recognized by an individual correlated closely with the size of their biggest response. Target-specificity varied considerably between donors and across hierarchy levels with the presence, size, and hierarchy position of responses to purportedly 'immunodominant' targets being unpredictable.

CONCLUSIONS

Predicting protective immunity based on isolated HCMV subunit-specific T-cell responses is questionable in light of the complex architecture of the overall response. Our findings have important implications for T-cell monitoring, intervention strategies, as well as the application of animal models to the understanding of human infection.

Identification and spontaneous immune targeting of an endogenous retrovirus K envelope protein in the Indian rhesus macaque model of human disease

BACKGROUND

Endogenous retroviruses (ERVs) are remnants of ancient retroviral infections that have invaded the germ line of both humans and non-human primates. Most ERVs are functionally crippled by deletions, mutations, and hypermethylation, leading to the view that they are inert genomic fossils. However, some ERVs can produce mRNA transcripts, functional viral proteins, and even non-infectious virus particles during certain developmental and pathological processes. While there have been reports of ERV-specific immunity associated with ERV activity in humans, adaptive immune responses to ERV-encoded gene products remain poorly defined and have not been investigated in the physiologically relevant non-human primate model of human disease.

FINDINGS

Here, we identified the rhesus macaque equivalent of the biologically active human ERV-K (HML-2), simian ERV-K (SERV-K1), which retains intact open reading frames for both Gag and Env on chromosome 12 in the macaque genome. From macaque cells we isolated a spliced mRNA product encoding SERV-K1 Env, which possesses all the structural features of a canonical, functional retroviral Envelope protein. Furthermore, we identified rare, but robust T cell responses as well as frequent antibody responses targeting SERV-K1 Env in rhesus macaques.

CONCLUSIONS

These data demonstrate that SERV-K1 retains biological activity sufficient to induce cellular and humoral immune responses in rhesus macaques. As ERV-K is the youngest and most active ERV family in the human genome, the identification and characterization of the simian orthologue in rhesus macaques provides a highly relevant animal model in which to study the role of ERV-K in developmental and disease states.

Cytomegalovirus Infection and Memory T Cell Inflation

Cytomegalovirus (CMV) infection in healthy individuals is usually asymptomatic and results in latent infection. CMV reactivation occasionally occurs in healthy individuals according to their immune status over time. T cell responses to CMV are restricted to a limited number of immunodominant epitopes, as compared to responses to other chronic or persistent viruses. This response results in progressive, prolonged expansion of CMV-specific CD8⁺ T cells, termed 'memory inflation'. The expanded CMV-specific CD8⁺ T cell population is extraordinarily large and is more prominent in the elderly. CMV-specific CD8⁺ T cells possess rather similar phenotypic and functional features to those of replicative senescent T cells. In this review, we discuss the general features of CMV-specific inflationary memory T cells and the factors involved in memory inflation.

Acyclovir Therapy Reduces the CD4+ T Cell Response against the Immunodominant pp65 Protein from Cytomegalovirus in Immune Competent Individuals

Cytomegalovirus (CMV) infects the majority of the global population and leads to the development of a strong virus-specific immune response. The CMV-specific CD4+ and CD8+ T cell immune response can comprise between 10 and 50% of the T cell pool within peripheral blood and there is concern that this may impair immunity to other pathogens. Elderly individuals with the highest magnitude of CMV-specific immune response have been demonstrated to be at increased risk of mortality and there is increasing interest in interventions that may serve to moderate this. Acyclovir is an anti-viral drug with activity against a range of herpes viruses and is used as long term treatment to suppress reactivation of herpes simplex virus. We studied the immune response to CMV in patients who were taking acyclovir to assess if therapy could be used to suppress the CMV-specific immune response. The T cell reactivity against the immunodominant late viral protein pp65 was reduced by 53% in people who were taking acyclovir. This effect was seen within one year of therapy and was observed primarily within the CD4+ response. Acyclovir treatment only modestly influenced the immune response to the IE-1 target protein. These data show that low dose acyclovir treatment has the potential to modulate components of the T cell response to CMV antigen proteins and indicate that anti-viral drugs should be further investigated as a means to reduce the magnitude of CMV-specific immune response and potentially improve overall immune function.

CD73 is dispensable for the regulation of inflationary CD8+ T-cells after murine cytomegalovirus infection and adenovirus immunisation

The ecto-5'-nucleotidase (CD73) is expressed by T-cell subsets, myeloid derived suppressive cells and endothelial cells. It works in conjunction with CD39 to regulate the formation and degradation of adenosine in vivo. Adenosine has previously been shown to suppress the proliferation and cytokine secretion of T-cells and recent evidence suggests that inhibition of CD73 has the potential to enhance T-cell directed therapies. Here we utilised a CD73 knockout mouse model to assess the suppressive ability of CD73 on CD8⁺ T-cell classical memory and memory “inflation”, induced by murine cytomegalovirus (MCMV) infection and adenovirus immunisation. We show that CD73 is dispensable for normal CD8⁺ T-cell differentiation and function in both models. Thus CD73 as a suppressor of CD8⁺ T-cells is unlikely to play a deterministic role in the generation and functional characteristics of antiviral memory in these settings.

Systemic hematogenous maintenance of memory inflation by MCMV infection

Several low-grade persistent viral infections induce and sustain very large numbers of virus-specific effector T cells. This was first described as a response to cytomegalovirus (CMV), a herpesvirus that establishes a life-long persistent/latent infection, and sustains the largest known effector T cell populations in healthy people. These T cells remain functional and traffic systemically, which has led to the recent exploration of CMV as a persistent vaccine vector. However, the maintenance of this remarkable response is not understood. Current models propose that reservoirs of viral antigen and/or latently infected cells in lymph nodes stimulate T cell proliferation and effector differentiation, followed by migration of progeny to non-lymphoid tissues where they control CMV reactivation. We tested this model using murine CMV (MCMV), a natural mouse pathogen and homologue of human CMV (HCMV). While T cells within draining lymph nodes divided at a higher rate than cells elsewhere, antigen-dependent proliferation of MCMV-specific effector T cells was observed systemically. Strikingly, inhibition of T cell egress from lymph nodes failed to eliminate systemic T cell division, and did not prevent the maintenance of the inflationary populations. In fact, we found that the vast majority of inflationary cells, including most cells undergoing antigen-driven division, had not migrated into the parenchyma of non-lymphoid tissues but were instead exposed to the blood supply. Indeed, the immunodominance and effector phenotype of inflationary cells, both of which are primary hallmarks of memory inflation, were largely confined to blood-localized T cells. Together these results support a new model of MCMV-driven memory inflation in which most immune surveillance occurs in circulation, and in which most inflationary effector T cells are produced in response to viral antigen presented by cells that are accessible to the blood supply.

Herpesviruses persist for the life of the host and must be continuously controlled by a robust immune surveillance effort. In the case of the cytomegalovirus (CMV), this ongoing immune surveillance promotes the accumulation of CMV-specific T cells in a process known as “memory inflation”. We and others have proposed that the ability to induce memory inflation may be an important benefit of CMV-based vaccine vectors that persist within the host and continuously boost the immune response. However, it has been difficult to determine where T cells are encountering CMV in the body, leading to many unanswered questions about the maintenance of this remarkable response. Previous models proposed that T cells encountered viral antigen within lymph nodes and then migrated to other tissues to prevent CMV reactivation. However, we found that the majority of T cells stimulated by CMV were present in circulation, where they could be sustained without the input from T cells localized to lymph nodes. In fact, two of the defining features of memory inflation - inflated numbers and an effector phenotype - were restricted to cells that were exposed to the blood. Thus, we propose that memory inflation during CMV infection is largely the result of immune surveillance that occurs in circulation.

Fever, rash, and systemic symptoms: understanding the role of virus and HLA in severe cutaneous drug allergy

Drug hypersensitivity syndromes such as abacavir hypersensitivity and the severe cutaneous adverse drug reactions have been associated with significant short- and long-term morbidity and mortality. More recently, these immunologically mediated and previously unpredictable diseases have been shown to be associated with primarily class I but also class II HLA alleles. The case of the association of HLA-B*57:01 and abacavir hypersensitivity has created a translational roadmap for how this knowledge can be used in the clinic to prevent severe reactions. Although many hurdles exist to the widespread translation of such HLA screening approaches, our understanding of how drugs interact with the major histocompatibility complex has contributed to the discovery of new models that have provided considerable insights into the immunopathogenesis of severe cutaneous adverse drug reactions and other T-cell-mediated drug hypersensitivity syndromes. Future translation of this knowledge will facilitate the development of preclinical toxicity screening to significantly improve efficacy and safety of drug development and design.

Naïve and memory CD8 T cell pool homeostasis in advanced aging: impact of age and of antigen-specific responses to cytomegalovirus

Alterations in the circulating CD8+ T cell pool, with a loss of naïve and accumulation of effector/effector memory cells, are pronounced in older adults. However, homeostatic forces that dictate such changes remain incompletely understood. This observational cross-sectional study explored the basis for variability of CD8+ T cell number and composition of its main subsets: naïve, central memory and effector memory T cells, in 131 cytomegalovirus (CMV) seropositive subjects aged over 60 years. We found great heterogeneity of CD8+ T cell numbers, which was mainly due to variability of the CD8 + CD28- T cell subset regardless of age. Analysis, by multiple regression, of distinct factors revealed that age was a predictor for the loss in absolute number of naïve T cells, but was not associated with changes in central or effector memory CD8+ T cell subsets. By contrast, the size of CD8+ T cells specific to pp65 and IE-1 antigens of CMV, predicted CD28 - CD8+ T cell, antigen-experienced CD8+ T cell, and even total CD8+ T cell numbers, but not naïve CD8+ T cell loss. These results indicate a clear dichotomy between the homeostasis of naïve and antigen-experienced subsets of CD8+ T cells which are independently affected, in human later life, by age and antigen-specific responses to CMV, respectively.

A new model for CD8+ T cell memory inflation based upon a recombinant adenoviral vector

CD8(+) T cell memory inflation, first described in murine CMV (MCMV) infection, is characterized by the accumulation of high-frequency, functional Ag-specific CD8(+) T cell pools with an effector-memory phenotype and enrichment in peripheral organs. Although persistence of Ag is considered essential, the rules underpinning memory inflation are still unclear. The MCMV model is, however, complicated by the virus's low-level persistence and stochastic reactivation. We developed a new model of memory inflation based on a β-galactosidase (βgal)-recombinant adenovirus vector. After i.v. administration in C57BL/6 mice, we observed marked memory inflation in the βgal96 epitope, whereas a second epitope, βgal497, undergoes classical memory formation. The inflationary T cell responses show kinetics, distribution, phenotype, and functions similar to those seen in MCMV and are reproduced using alternative routes of administration. Memory inflation in this model is dependent on MHC class II. As in MCMV, only the inflating epitope showed immunoproteasome independence. These data define a new model for memory inflation, which is fully replication independent, internally controlled, and reproduces the key immunologic features of the CD8(+) T cell response. This model provides insight into the mechanisms responsible for memory inflation and, because it is based on a vaccine vector, also is relevant to novel T cell-inducing vaccines in humans.

Antiviral therapy can reverse the development of immune senescence in elderly mice with latent cytomegalovirus infection

Cytomegalovirus (CMV) infection leads to the development of adaptive and humoral immune responses that are among the largest for any pathogen, and intriguingly, the magnitude of the immune response increases with age, a phenomenon termed "memory inflation." Elevated CMV-specific immunity has been correlated with an increased mortality rate in elderly individuals and with impaired vaccination responses. The latent phase of CMV infection is characterized by intermittent episodes of subclinical viral reactivation and the production of immunogenic transcripts that may maintain memory inflation of virus-specific cytotoxic lymphocytes. However, the relative importance of CMV reactivation in the development of memory inflation is uncertain, as is the potential for antiviral treatment to reverse this effect. Here, we administered valaciclovir for up to 12 months in mice with established murine CMV (MCMV) infection. Treatment reduced the magnitude of the MCMV-specific CD8(+) T-lymphocyte response by 80%, and the residual MCMV tetramer-specific lymphocytes exhibited a less differentiated phenotype. In addition, latent MCMV infection suppressed the proportion of naïve CD8(+) T cells by 60% compared to antiviral-treated mice or MCMV-negative animals. Furthermore, treatment led to a reduction in influenza A viral loads following a challenge in elderly MCMV-infected animals and also reduced the differentiation of influenza virus-specific cytotoxic lymphocytes. These observations demonstrate that MCMV-specific memory inflation is maintained by viral replication and that therapeutic intervention could lead to improved immune function.

T cell memory in the context of persistent herpes viral infections

The generation of a functional memory T cell pool upon primary encounter with an infectious pathogen is, in combination with humoral immunity, an essential process to confer protective immunity against reencounters with the same pathogen. A prerequisite for the generation and maintenance of long-lived memory T cells is the clearance of antigen after infection, which is fulfilled upon resolution of acute viral infections. Memory T cells play also a fundamental role during persistent viral infections by contributing to relative control and immuosurveillance of active replication or viral reactivation, respectively. However, the dynamics, the phenotype, the mechanisms of maintenance and the functionality of memory T cells which develop upon acute/resolved infection as opposed to chronic/latent infection differ substantially. In this review we summarize current knowledge about memory CD8 T cell responses elicited during α-, β-, and γ-herpes viral infections with major emphasis on the induction, maintenance and function of virus-specific memory CD8 T cells during viral latency and we discuss how the peculiar features of these memory CD8 T cell responses are related to the biology of these persistently infecting viruses.

Age-related changes in CD8 T cell homeostasis and immunity to infection

Studies of CD8 T cell responses to vaccination or infection with various pathogens in both animal models and human subjects have revealed a markedly consistent array of age-related defects. In general, recent work shows that aged CD8 T cell responses are decreased in magnitude, and show poor differentiation into effector cells, with a reduced arsenal of effector functions. Here we review potential mechanisms underlying these defects. We specifically address phenotypic and numeric changes to the naïve CD8 T cell precursor pool, the impact of persistent viral infection(s) and inflammation, and contributions of the aging environment in which these cells are activated.

Memory T cell inflation: understanding cause and effect

Typically, during viral infections, T cells encounter antigen, undergo proliferative expansion and ultimately contract into a pool of memory cells. However, after infection with cytomegalovirus, a ubiquitous β-herpesvirus, T cell populations specific for certain epitopes do not contract but instead are maintained and/or accumulate at high frequencies with a characteristic effector-memory phenotype. This feature has also been noted after other infections, for example, by parvoviruses. We discuss this so-called memory T cell inflation and the factors involved in this phenomenon. Also, we consider the potential therapeutic use of memory T cell inflation as a vaccine strategy and the associated implications for immune senescence.

Sustained CD8+ T cell memory inflation after infection with a single-cycle cytomegalovirus

Cytomegalovirus (CMV) is a β-herpesvirus that establishes a lifelong latent or persistent infection. A hallmark of chronic CMV infection is the lifelong persistence of large numbers of virus-specific CD8+ effector/effector memory T cells, a phenomenon called “memory inflation”. How the virus continuously stimulates these T cells without being eradicated remains an enigma. The prevailing view is that CMV establishes a low grade “smoldering” infection characterized by tiny bursts of productive infection which are rapidly extinguished, leaving no detectable virus but replenishing the latent pool and leaving the immune system in a highly charged state. However, since abortive reactivation with limited viral gene expression is known to occur commonly, we investigated the necessity for virus reproduction in maintaining the inflationary T cell pool. We inhibited viral replication or spread in vivo using two different mutants of murine CMV (MCMV). First, famcyclovir blocked the replication of MCMV encoding the HSV Thymidine Kinase gene, but had no impact on the CD8+ T cell memory inflation once the infection was established. Second, MCMV that lacks the essential glycoprotein L, and thus is completely unable to spread from cell to cell, also drove memory inflation if the virus was administered systemically. Our data suggest that CMV which cannot spread from the cells it initially infects can repeatedly generate viral antigens to drive memory inflation without suffering eradication of the latent genome pool.

Cytomegalovirus (CMV) establishes life-long, asymptomatic infections in healthy people. Ongoing immune surveillance prevents viral disease but also results in the accumulation of large numbers of virus-specific T cells. The mechanisms by which the virus persists while stimulating such strong immune responses are unknown. We and others had hypothesized that periodic viral replication and spread to neighboring cells allowed CMV to replenish the pool of infected cells while stimulating virus-specific T cells to accumulate. In this manuscript, we have tested this model by blocking the replication or spread of murine cytomegalovirus (MCMV) and found, surprisingly, that accumulation of virus-specific T cells occurs independently of viral replication. Moreover, these T cells developed the terminal differentiated phenotype that is indicative of repeated antigenic stimulation. Thus, these data suggest that CMV can remain active and continuously stimulate the immune system, while avoiding immune-mediated clearance, without the capacity to spread from cell to cell.

Functional CD8 T cell memory responding to persistent latent infection is maintained for life

Aging is associated with depressed naive T cell responses, but it is less clear whether T cell memory established early in life also becomes impaired with age. This is particularly important for T cells responding to latent persistent infection, which need to remain functional and capable of controlling the infection over the lifetime; however, repeated stimulation over the lifetime may dysregulate their maintenance or function, potentially contributing to impaired immunity in the elderly. Systemic infection with HSV-1, a persistent latent virus, is associated with memory inflation of virus-specific CD8 T cells. We tested how these inflated memory cells are maintained from adulthood into old age. We found no significant differences in the numbers (i.e., blood, spleen), ex vivo Ag-specific IFN-γ production, and in vivo recall response to HSV-1 (i.e., proliferation, IFN-γ production, cytolysis) between adult and old memory T cells. There was a discrete shift from dominantly effector memory phenotype in the adults to a central memory-like phenotype in the old mice, with fewer old cells expressing the killer cell lectin-like receptor G1 (KLRG1). Adult and old KLRG1(+) memory CD8 T cells were functionally identical: both produced IFN-γ but could minimally proliferate in response to viral challenge. Interestingly, regardless of age, KLRG1(+) cells retained the ability to proliferate and survive in response to homeostatic signals, both in vitro (culture with IL-7 and IL-15) and in vivo (expansion following transfer into lymphopenic recipients). This finding demonstrates that functional effector memory T cells, including those expressing KLRG-1, are maintained and are functional for life, despite the presence of persistent viral infection.

Infection with cytomegalovirus but not herpes simplex virus induces the accumulation of late-differentiated CD4+ and CD8+ T-cells in humans

Human cytomegalovirus (CMV) establishes persistent, usually asymptomatic, infection in healthy people. Because CMV infection is associated with the presence of lower proportions of peripheral naïve CD8+ T-cells and a higher fraction of late-differentiated CD8+ cells, commonly taken as biomarkers of age-associated compromised adaptive immunity ('immunosenescence'), we asked whether chronic exposure to any persistent virus mediates these effects. Herpes simplex virus (HSV) is also a widespread herpesvirus that establishes lifelong persistence, but, unlike CMV, its impact on the distribution of T-cell subsets has not been established. Here, we analysed T-cell subsets in 93 healthy people aged 42-81 years infected or not infected with CMV and/or HSV. Individuals harbouring CMV were confirmed to possess lower frequencies of naïve CD8+ T-cells (defined as CD45RA+CCR7+CD27+CD28+) and greater proportions of late-differentiated effector memory (CD45RA-CCR7-CD27-CD28-) and so-called TEMRA (CD45RA+CCR7-CD27-CD28-) CD4 and CD8 subsets, independent of HSV seropositivity. In CMV-seronegative donors, HSV did not affect T-cell subset distribution significantly. We conclude that these hallmarks of age-associated alterations to immune signatures are indeed observed in the general population in people infected with CMV and not those infected with a different persistent herpesvirus.

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.

Vaccine effectiveness in older individuals: what has been learned from the influenza-vaccine experience

Vaccination policies in most high-income countries attempt to reduce the adverse impact of influenza targeting people aged at least 60 years. However, while it is widely believed that the current immunization strategy saves many lives, influenza infection still remains a severe burden in aged individuals leading to a wide debate on the exact magnitude of the benefit of vaccination in this population. The first aim of the present review is to examine how effective current influenza-vaccine strategies are in aged adults, by analysing which are the most important factors modulating the interpretation of study results in this population. Furthermore, consideration will be given to how immune factors influence the measurement of vaccine efficacy/effectiveness, where advancing age leads to deleterious changes in the adaptive immune system, resulting in less than optimal responses to infectious agents and vaccination. Finally this review concludes with possible strategies to improve the ability of the senescent immune system to respond to vaccination.

CD8+ T cell differentiation in the aging immune system: until the last clone standing

A substantial deterioration of the naïve CD8(+) T cell pool occurs regularly in humans beyond the age of 65 years. While recall responses to pathogens encountered during youth or adulthood are largely uncompromised, the de novo generation of memory responses by aged naïve CD8(+) T cells is perturbed. In recent years evidence has accumulated that the diminished responsiveness of naïve CD8(+) T cells in aged humans and other mammals coincides with a progressive loss of naïve T cell receptor (TCR) repertoire diversity. In this review we focus on thymic involution and chronic latent viral infections as key factors driving the reduction in naïve TCR repertoire diversity. We present novel insights gained by studying the antigen-driven differentiation of single CD8(+) T cells in young hosts and discuss possible implications of these insights for therapeutic support of the thinned-out clonal T cell repertoire of the elderly by vaccination or adoptive cell therapy.