T-cell help permits memory CD8(+) T-cell inflation during cytomegalovirus latency

Mucosal T-cell responses to chronic viral infections: Implications for vaccine design

Mucosal surfaces that line the respiratory, gastrointestinal and genitourinary tracts are the major interfaces between the immune system and the environment. Their unique immunological landscape is characterized by the necessity of balancing tolerance to commensal microorganisms and other innocuous exposures against protection from pathogenic threats such as viruses. Numerous pathogenic viruses, including herpesviruses and retroviruses, exploit this environment to establish chronic infection. Effector and regulatory T-cell populations, including effector and resident memory T cells, play instrumental roles in mediating the transition from acute to chronic infection, where a degree of viral replication is tolerated to minimize immunopathology. Persistent antigen exposure during chronic viral infection leads to the evolution and divergence of these responses. In this review, we discuss advances in the understanding of mucosal T-cell immunity during chronic viral infections and how features of T-cell responses develop in different chronic viral infections of the mucosa. We consider how insights into T-cell immunity at mucosal surfaces could inform vaccine strategies: not only to protect hosts from chronic viral infections but also to exploit viruses that can persist within mucosal surfaces as vaccine vectors.

Memory inflation: Beyond the acute phase of viral infection

Memory inflation is confirmed as the most commonly dysregulation of host immunity with antigen-independent manner in mammals after viral infection. By generating large numbers of effector/memory and terminal differentiated effector memory CD8+ T cells with diminished naïve subsets, memory inflation is believed to play critical roles in connecting the viral infection and the onset of multiple diseases. Here, we reviewed the current understanding of memory inflated CD8+ T cells in their distinct phenotypic features that different from exhausted subsets; the intrinsic and extrinsic roles in regulating the formation of memory inflation; and the key proteins in maintaining the expansion and proliferation of inflationary populations. More importantly, based on the evidences from both clinic and animal models, we summarized the potential mechanisms of memory inflation to trigger autoimmune neuropathies, such as Guillain-Barré syndrome and multiple sclerosis; the correlations of memory inflation between tumorigenesis and resistance of tumour immunotherapies; as well as the effects of memory inflation to facilitate vascular disease progression. To sum up, better understanding of memory inflation could provide us an opportunity to beyond the acute phase of viral infection, and shed a light on the long-term influences of CD8+ T cell heterogeneity in dampen host immune homeostasis.

Latent CMV infection of Lymphatic endothelial cells is sufficient to drive CD8 T cell memory inflation

CMV, a ubiquitous herpesvirus, elicits an extraordinarily large T cell response that is sustained or increases over time, a phenomenon termed 'memory inflation.' Remarkably, even latent, non-productive infection can drive memory inflation. Despite intense research on this phenomenon, the infected cell type(s) involved are unknown. To identify the responsible cell type(s), we designed a Cre-lox murine CMV (MCMV) system, where a spread-deficient (ΔgL) virus expresses recombinant SIINFEKL only in Cre+ host cells. We found that latent infection of endothelial cells (ECs), but not dendritic cells (DCs) or hepatocytes, was sufficient to drive CD8 T cell memory inflation. Infection of Lyve-1-Cre and Prox1-CreERT2 mice revealed that amongst EC subsets, infection of lymphatic ECs was sufficient. Genetic ablation of β2m on lymphatic ECs did not prevent inflation, suggesting another unidentified cell type can also present antigen to CD8 T cells during latency. This novel system definitively shows that antigen presentation by lymphatic ECs drives robust CD8 T cell memory inflation.

Hematopoietic cell-mediated dissemination of murine cytomegalovirus is regulated by NK cells and immune evasion

Cytomegalovirus (CMV) causes clinically important diseases in immune compromised and immune immature individuals. Based largely on work in the mouse model of murine (M)CMV, there is a consensus that myeloid cells are important for disseminating CMV from the site of infection. In theory, such dissemination should expose CMV to cell-mediated immunity and thus necessitate evasion of T cells and NK cells. However, this hypothesis remains untested. We constructed a recombinant MCMV encoding target sites for the hematopoietic specific miRNA miR-142-3p in the essential viral gene IE3. This virus disseminated poorly to the salivary gland following intranasal or footpad infections but not following intraperitoneal infection in C57BL/6 mice, demonstrating that dissemination by hematopoietic cells is essential for specific routes of infection. Remarkably, depletion of NK cells or T cells restored dissemination of this virus in C57BL/6 mice after intranasal infection, while dissemination occurred normally in BALB/c mice, which lack strong NK cell control of MCMV. These data show that cell-mediated immunity is responsible for restricting MCMV to hematopoietic cell-mediated dissemination. Infected hematopoietic cells avoided cell-mediated immunity via three immune evasion genes that modulate class I MHC and NKG2D ligands (m04, m06 and m152). MCMV lacking these 3 genes spread poorly to the salivary gland unless NK cells were depleted, but also failed to replicate persistently in either the nasal mucosa or salivary gland unless CD8⁺ T cells were depleted. Surprisingly, CD8⁺ T cells primed after intranasal infection required CD4⁺ T cell help to expand and become functional. Together, our data suggest that MCMV can use both hematopoietic cell-dependent and -independent means of dissemination after intranasal infection and that cell mediated immune responses restrict dissemination to infected hematopoietic cells, which are protected from NK cells during dissemination by viral immune evasion. In contrast, viral replication within mucosal tissues depends on evasion of T cells.

Cytomegalovirus (CMV) is a common cause of disease in immune compromised individuals as well as a common cause of congenital infections leading to disease in newborns. The virus is thought to enter primarily via mucosal barrier tissues, such as the oral and nasal mucosa. However, it is not clear how the virus escapes these barrier tissues to reach distant sites. In this study, we used a mouse model of CMV infection. Our data illustrate a complex balance between the immune system and viral infection of “myeloid cells”, which are most commonly thought to carry the virus around the body after infection. In particular, our data suggest that robust immune responses at the site of infection force the virus to rely on myeloid cells to escape the site of infection. Moreover, viral genes designed to evade these immune responses were needed to protect the virus during and after its spread to distant sites. Together, this work sheds light on the mechanisms of immune control and viral survival during CMV infection of mucosal tissues and spread to distant sites of the body.

The CD4+ T Cell Response to Human Cytomegalovirus in Healthy and Immunocompromised People

While CD8+ T cells specific for human cytomegalovirus (HCMV) have been extensively studied in both healthy HCMV seropositive carriers and patients undergoing immunosuppression, studies on the CD4+ T cell response to HCMV had lagged behind. However, over the last few years there has been a significant advance in our understanding of the importance and contribution that CMV-specific CD4+ T cells make, not only to anti-viral immunity but also in the potential maintenance of latently infected cells. During primary infection with HCMV in adults, CD4+ T cells are important for the resolution of symptomatic disease, while persistent shedding of HCMV into urine and saliva is associated with a lack of HCMV specific CD4+ T cell response in young children. In immunosuppressed solid organ transplant recipients, a delayed appearance of HCMV-specific CD4+ T cells is associated with prolonged viremia and more severe clinical disease, while in haematopoietic stem cell transplant recipients, it has been suggested that HCMV-specific CD4+ T cells are required for HCMV-specific CD8+ T cells to exert their anti-viral effects. In addition, adoptive T-cell immunotherapy in transplant patients has shown that the presence of HCMV-specific CD4+ T cells is required for the maintenance of HCMV-specific CD8+ T cells. HCMV is a paradigm for immune evasion. The presence of viral genes that down-regulate MHC class II molecules and the expression of viral IL-10 both limit antigen presentation to CD4+ T cells, underlining the important role that this T cell subset has in antiviral immunity. This review will discuss the antigen specificity, effector function, phenotype and direct anti-viral properties of HCMV specific CD4+ T cells, as well as reviewing our understanding of the importance of this T cell subset in primary infection and long-term carriage in healthy individuals. In addition, their role and importance in congenital HCMV infection and during immunosuppression in both solid organ and haemopoietic stem cell transplantation is considered.

Investigating the Dynamics of MCMV-Specific CD8+ T Cell Responses in Individual Hosts

Infection by Cytomegalovirus (CMV) is characterized by the massive expansion and continued maintenance of CMV-specific CD8⁺ T cells for certain CMV-derived peptides. This phenomenon called “memory inflation" has made CMV a primary target for the generation of T cell based vaccine vectors against various diseases. However, many aspects concerning the generation and maintenance of the inflationary CD8⁺ T cell response still remain to be resolved. In this study, we combined experimental data and mathematical models to analyze the dynamics of circulatory inflationary CD8⁺ T cells within individual mice infected by MCMV. Obtaining frequent measurements on the number and frequency of CMV-specific CD8⁺ T cells up to 70 days post infection, we find that mathematical models assuming differing viral stimuli during acute infection and the inflationary phase provide a better description for the observed dynamics than models relying on similar viral stimuli during both phases. In addition, our analysis allowed a detailed quantification of the different phases of memory inflation within individual mice (1^st-expansion - contraction - 2^nd expansion/maintenance) indicating remarkable consistency of the timing of these phases across mice, but considerable variation in the size of the individual responses between mice. Our analysis provides a first step toward generating a mechanistic framework for analyzing the generation and maintenance of inflationary CD8⁺ T cells while accounting for individual heterogeneity. Extending these analyses by incorporating measurements from additional compartments and more prolonged sampling will help to obtain a systematic and quantitative understanding of the factors regulating the process of memory inflation.

Association of Cytomegalovirus DNA and Immunologic Markers of Cardiovascular Disease

BACKGROUND

Persons living with human immunodeficiency virus (HIV) (PLWH) with high cytomegalovirus (CMV)-specific interferon (IFN) γ response have increased numbers of endothelium homing receptor (CX3CR1)+-expressing cells that are associated with cardiovascular disease. The current study was performed to investigate the effect of cellular levels of CMV DNA on these markers.

METHODS

Eighty paired peripheral blood mononuclear cell samples were collected ≥12 months apart from 40 CMV-seropositive PLWH with suppressed HIV RNA, who started antiretroviral therapy at median of 3-months of infection. The samples were assessed for CMV-specific IFN-γ response by means of enzyme-linked immunospot assay, and participants were classified as low responders (LRs) or high responders (HRs) based on IFN-γ production (≤100 or >100 spot-forming units [SFUs]/105 cells).

RESULTS

Of the 40 participants, 26 (65%) were HRs and 14 (35%) LRs at baseline, which did not change over time or by CMV levels (median at first/second time points, 383/308 SFUs/106 cells for HRs vs 21/41 SFUs/106 for LRs). A decrease in IFN-γ over time was associated with higher CMV DNA levels (P < .01). High CMV response was also associated with increased CD28+CD27-CD4+ T cells expressing CX3CR1 (P < .001). Similarly, increased IFN-γ production was associated with increased CMV-specific CX3CR1+CD28+CD27-CD4+ and CD8+ T cells (P < .001).

CONCLUSIONS

These findings demonstrate that levels of CMV-specific IFN-γ response in PLWH are stable over time, and that HRs have increased circulating T cells expressing CX3CR1 that may put them at increased risk of cardiovascular disease and other inflammatory diseases.

Fuel and brake of memory T cell inflation

Memory T cell inflation is a process in which a large number of effector memory T cells accumulates in peripheral tissues. This phenomenon is observed upon certain low level persistent virus infections, but it is most commonly described upon infection with the β-herpesvirus Cytomegalovirus. Due to the induction of this large pool of functional effector CD8 T cells in peripheral tissues, the interest in using CMV-based vaccine vectors for vaccination purposes is rising. However, the exact mechanisms of memory T cell inflation are not yet fully understood. It is clear that repetitive exposure to antigen is a key determinant for memory inflation, and therefore the viral inoculum dose and the subsequent number of viral reactivation events strongly impact on the magnitude of the inflationary T cell pool. In addition, the number of CMV-specific CD8 T cells that is able to sense these reactivation events affects the size of the inflationary T cell pool. In the following, we will discuss factors that either promote or limit T cell inflation from both the virus and host perspective. These factors mostly operate by influencing the amount of available antigen or by affecting the T cell pool that is able to respond to the antigen. Furthermore, we will discuss the recent use of CMV-based vaccines in pre-clinical experimental settings, where these vectors have shown promising results by inducing prolonged effector memory T cell responses to foreign-introduced epitopes and thereby provided protection from subsequent virus or tumour challenges.

TCR repertoire evolution during maintenance of CMV-specific T-cell populations

During infections and cancer, the composition of the T-cell receptor (TCR) repertoire of antigen-specific CD8⁺ T cells changes over time. TCR avidity is thought to be a major driver of this process, thereby interacting with several additional regulators of T-cell responses to form a composite immune response architecture. Infections with latent viruses, such as cytomegalovirus (CMV), can lead to large T-cell responses characterized by an oligoclonal TCR repertoire. Here, we review the current status of experimental studies and theoretical models of TCR repertoire evolution during CMV infection. We will particularly discuss the degree to which this process may be determined through structural TCR avidity. As engineered TCR-redirected T cells have moved into the spotlight for providing more effective immunotherapies, it is essential to understand how the key features of a given TCR influence T-cell expansion and maintenance in settings of infection or malignancy. Deeper insights into these mechanisms will improve our basic understanding of T-cell immunology and help to identify optimal TCRs for immunotherapy.

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.

IL-33 Augments Virus-Specific Memory T Cell Inflation and Potentiates the Efficacy of an Attenuated Cytomegalovirus-Based Vaccine

Candidate vaccines designed to generate T cell-based immunity are typically vectored by nonpersistent viruses, which largely fail to elicit durable effector memory T cell responses. This limitation can be overcome using recombinant strains of CMV. Proof-of-principle studies have demonstrated the potential benefits of this approach, most notably in the SIV model, but safety concerns require the development of nonreplicating alternatives with comparable immunogenicity. In this study, we show that IL-33 promotes the accumulation and recall kinetics of circulating and tissue-resident memory T cells in mice infected with murine CMV. Using a replication-deficient murine CMV vector, we further show that exogenous IL-33 boosts vaccine-induced memory T cell responses, which protect against subsequent heterologous viral challenge. These data suggest that IL-33 could serve as a useful adjuvant to improve the efficacy of vaccines based on attenuated derivatives of CMV.

Memory inflation following adenoviral vaccination depends on IL-21

Cytomegalovirus (CMV) and non-replicating adenoviral vectors can induce expanded, sustained effector-memory CD8⁺ T-cell responses, termed “memory inflation”. During murine CMV (MCMV) infection, CD4⁺ Tcells maintain inflationary virus-specific CD8⁺ T-cell responses via IL-2 but not IL-21. Adenovirus vector vaccination can induce phenotypically, functionally and transcriptionally similar inflationary responses, but it is not known how IL-21 influences the inflating memory response to adenoviral vaccination. Here, we show that IL-21 is an absolute requirement for induction and maintenance of vaccine-derived inflationary CD8⁺ T-cell responses. These data indicate that the induction pathway of inflationary Ad-LacZ T-cells is distinct from inflationary MCMV-specific T-cells and is highly dependent on IL-21. Our observations highlight a fundamental difference in the mechanism by which adenovirus vectors and MCMV drive inflationary T-cell responses.

The impact of inflationary cytomegalovirus-specific memory T cells on anti-tumour immune responses in patients with cancer

Human cytomegalovirus (CMV) is a ubiquitous, persistent beta herpesvirus. CMV infection contributes to the accumulation of functional antigen-specific CD8⁺ T-cell pools with an effector-memory phenotype and enrichment of these immune cells in peripheral organs. We review here this 'memory T-cell inflation' phenomenon and associated factors including age and sex. 'Collateral damage' due to CMV-directed immune reactivity may occur in later stages of life - arising from CMV-specific immune responses that were beneficial in earlier life. CMV may be considered an age-dependent immunomodulator and a double-edged sword in editing anti-tumour immune responses. Emerging evidence suggests that CMV is highly prevalent in patients with a variety of cancers, particularly glioblastoma. A better understanding of CMV-associated immune responses and its implications for immune senescence, especially in patients with cancer, may aid in the design of more clinically relevant and tailored, personalized treatment regimens. 'Memory T-cell inflation' could be applied in vaccine development strategies to enrich for immune reactivity where long-term immunological memory is needed, e.g. in long-term immune memory formation directed against transformed cells.

Memory Inflation Drives Tissue-Resident Memory CD8+ T Cell Maintenance in the Lung After Intranasal Vaccination With Murine Cytomegalovirus

Tissue-resident memory T (T_RM) cells provide first-line defense against invading pathogens encountered at barrier sites. In the lungs, T_RM cells protect against respiratory infections, but wane more quickly than T_RM cells in other tissues. This lack of a sustained T_RM population in the lung parenchyma explains, at least in part, why infections with some pathogens, such as influenza virus and respiratory syncytial virus (RSV), recur throughout life. Intranasal (IN) vaccination with a murine cytomegalovirus (MCMV) vector expressing the M protein of RSV (MCMV-M) has been shown to elicit robust populations of CD8⁺ T_RM cells that accumulate over time and mediate early viral clearance. To extend this finding, we compared the inflationary CD8⁺ T cell population elicited by MCMV-M vaccination with a conventional CD8⁺ T cell population elicited by an MCMV vector expressing the M2 protein of RSV (MCMV-M2). Vaccination with MCMV-M2 induced a population of M2-specific CD8⁺ T_RM cells that waned rapidly, akin to the M2-specific CD8⁺ T_RM cell population elicited by infection with RSV. In contrast to the natural immunodominance profile, however, coadministration of MCMV-M and MCMV-M2 did not suppress the M-specific CD8⁺ T cell response, suggesting that progressive expansion was driven by continuous antigen presentation, irrespective of the competitive or regulatory effects of M2-specific CD8⁺ T cells. Moreover, effective viral clearance mediated by M-specific CD8⁺ T_RM cells was not affected by the coinduction of M2-specific CD8⁺ T cells. These data show that memory inflation is required for the maintenance of CD8⁺ T_RM cells in the lungs after IN vaccination with MCMV.

The (gradual) rise of memory inflation

Memory inflation, as a term, has been used for 15 years now to describe the longitudinal development of stable, expanded CD8+ T memory pools with a distinct phenotype and functional profile which emerge in specific infection and vaccine settings. These settings have in common the persistence of antigen, especially cytomegalovirus infection but also more recently adenoviral vector vaccination. However, in contrast to chronic infections which lead to "exhaustion" the repeated antigen encounters experienced by CD8+ T cells lead to development of a robust T-cell population structure which maintains functionality and size. In this review, I will discuss how the ideas around this form of memory have evolved over time and some new models which can help explain how these populations are induced and sustained. These models are relevant to immunity against persistent viruses, to novel vaccine strategies and to concepts about aging.

Novel viral vectors in infectious diseases

Since the development of vaccinia virus as a vaccine vector in 1984, the utility of numerous viruses in vaccination strategies has been explored. In recent years, key improvements to existing vectors such as those based on adenovirus have led to significant improvements in immunogenicity and efficacy. Furthermore, exciting new vectors that exploit viruses such as cytomegalovirus (CMV) and vesicular stomatitis virus (VSV) have emerged. Herein, we summarize these recent developments in viral vector technologies, focusing on novel vectors based on CMV, VSV, measles and modified adenovirus. We discuss the potential utility of these exciting approaches in eliciting protection against infectious diseases.

Exhaustion and Inflation at Antipodes of T Cell Responses to Chronic Virus Infection

Viruses that have coevolved with their host establish chronic infections that are well tolerated by the host. Other viruses, that are partly adapted to their host, may induce chronic infections where persistent replication and viral antigen expression occur. The former induce highly functional and resilient CD8T cell responses called memory inflation. The latter induce dysfunctional and exhausted responses. The reasons compelling T cell responses towards inflationary or exhausted responses are only partly understood. In this review we compare the two conditions and describe mechanistic similarities and differences. We also provide a list of potential reasons why exhaustion or inflation occur in different virus infections. We propose that T cell-mediated transcriptional repression of viral gene expression provides a critical feature of inflation that allows peaceful virus and host coexistence. The virus is controlled, but its genome is not eradicated. If this mechanism is not available, as in the case of RNA viruses, the virus and the host are compelled to an arms race. If virus proliferation and spread proceed uncontrolled for too long, T cells are forced to strike a balance between viral control and tissue destruction, losing antiviral potency and facilitating virus persistence.

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.

Enforced OX40 Stimulation Empowers Booster Vaccines to Induce Effective CD4+ and CD8+ T Cell Responses against Mouse Cytomegalovirus Infection

There is an imperative need for effective preventive vaccines against human cytomegalovirus as it poses a significant threat to the immunologically immature, causing congenital disease, and to the immune compromised including transplant recipients. In this study, we examined the efficacy of synthetic long peptides (SLPs) as a CD4⁺ and CD8⁺ T cell-eliciting preventive vaccine approach against mouse CMV (MCMV) infection. In addition, the use of agonistic OX40 antibodies to enhance vaccine efficacy was explored. Immunocompetent C57BL/6 mice were vaccinated in a prime-boost vaccination regiment with SLPs comprising various MHC class I- and II-restricted peptide epitopes of MCMV-encoded antigens. Enforced OX40 stimulation resulted in superior MCMV-specific CD4⁺ as CD8⁺ T cell responses when applied during booster SLP vaccination. Vaccination with a mixture of SLPs containing MHC class II epitopes and OX40 agonistic antibodies resulted in a moderate reduction of the viral titers after challenge with lytic MCMV infection. Markedly, the combination of SLP vaccines containing both MHC class I and II epitopes plus OX40 activation during booster vaccination resulted in polyfunctional (i.e., IFN-γ⁺, TNF⁺, IL-2⁺) CD4⁺ and CD8⁺ T cell responses that were even higher in magnitude when compared to those induced by the virus, and this resulted in the best containment of virus dissemination. Our results show that the induction of strong T cell responses can be a fundamental component in the design of vaccines against persistent viral infections.

Mutual Interference between Cytomegalovirus and Reconstitution of Protective Immunity after Hematopoietic Cell Transplantation

Hematopoietic cell transplantation (HCT) is a therapy option for aggressive forms of hematopoietic malignancies that are resistant to standard antitumoral therapies. Hematoablative treatment preceding HCT, however, opens a “window of opportunity” for latent Cytomegalovirus (CMV) by releasing it from immune control with the consequence of reactivation of productive viral gene expression and recurrence of infectious virus. A “window of opportunity” for the virus represents a “window of risk” for the patient. In the interim between HCT and reconstitution of antiviral immunity, primarily mediated by CD8⁺ T cells, initially low amounts of reactivated virus can expand exponentially, disseminate to essentially all organs, and cause multiple organ CMV disease, with interstitial pneumonia (CMV-IP) representing the most severe clinical manifestation. Here, I will review predictions originally made in the mouse model of experimental HCT and murine CMV infection, some of which have already paved the way to translational preclinical research and promising clinical trials of a preemptive cytoimmunotherapy of human CMV disease. Specifically, the mouse model has been pivotal in providing “proof of concept” for preventing CMV disease after HCT by adoptive transfer of preselected, virus epitope-specific effector and memory CD8⁺ T cells bridging the critical interim. However, CMV is not a “passive antigen” but is a pathogen that actively interferes with the reconstitution of protective immunity by infecting bone marrow (BM) stromal cells that otherwise form niches for hematopoiesis by providing the structural microenvironment and by producing hematopoietically active cytokines, the hemopoietins. Depending on the precise conditions of HCT, reduced homing of transplanted hematopoietic stem- and progenitor cells to infected BM stroma and impaired colony growth and lineage differentiation can lead to “graft failure.” In consequence, uncontrolled virus spread causes morbidity and mortality. In the race between viral BM pathology and reconstitution of antiviral immunity following HCT, exogenous reconstitution of virus-specific CD8⁺ T cells by adoptive cell transfer as an interventional strategy can turn the balance toward control of CMV.

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.

The immune response to cytomegalovirus in allogeneic hematopoietic stem cell transplant recipients

Approximately, up to 70 % of the human population is infected with cytomegalovirus (CMV) that persists for life in a latent state. In healthy people, CMV reactivation induces the expansion of CMV-specific T cells up to 10 % of the entire T cell repertoire. On the contrary, CMV infection is a major opportunistic viral pathogen that remains a leading cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation. Due to the delayed CMV-specific immune recovery, the incidence of CMV reactivation during post-transplant period is very high. Several methods are currently available for the monitoring of CMV-specific responses that help in clinical monitoring. In this review, essential aspects in the immune recovery against CMV are discussed to improve the better understanding of the immune system relying on CMV infection and, thereby, helping the avoidance of CMV disease or reactivation following hematopoietic stem cell transplantation with severe consequences for the transplanted patients.

Human cytomegalovirus (HCMV)-specific CD4+ and CD8+ T cells are both required for prevention of HCMV disease in seropositive solid-organ transplant recipients

In solid-organ transplant recipients (SOTR) the protective role of human cytomegalovirus (HCMV)-specific CD4+, CD8+ and γδ T-cells vs. HCMV reactivation requires better definition. The aim of this study was to investigate the relevant role of HCMV-specific CD4+, CD8+ and γδ T-cells in different clinical presentations during the post-transplant period. Thirty-nine SOTR underwent virologic and immunologic follow-up for about 1 year after transplantation. Viral load was determined by real-time PCR, while immunologic monitoring was performed by measuring HCMV-specific CD4+ and CD8+ T cells (following stimulation with autologous HCMV-infected dendritic cells) and γδ T-cells by flow cytometry. Seven patients had no infection and 14 had a controlled infection, while both groups maintained CD4+ T-cell numbers above the established cut-off (0.4 cell/µL blood). Of the remaining patients, 9 controlled the infection temporarily in the presence of HCMV-specific CD8+ only, until CD4+ T-cell appearance; while 9 had to be treated preemptively due to a viral load greater than the established cut-off (3×10(5) DNA copies/mL blood) in the absence of specific CD4+ T-cells. Polyfunctional CD8+ T-cells as well as Vδ2- γδ T-cells were not associated with control of infection. In conclusion, in the absence of HCMV-specific CD4+ T-cells, no long-term protection is conferred to SOTR by either HCMV-specific CD8+ T-cells alone or Vδ2- γδ T-cell expansion.

Deciphering the role of DC subsets in MCMV infection to better understand immune protection against viral infections

Infection of mice with murine cytomegalovirus (MCMV) recapitulates many physiopathological characteristics of human CMV infection and enables studying the interactions between a virus and its natural host. Dendritic cells (DC) are mononuclear phagocytes linking innate and adaptive immunity which are both necessary for MCMV control. DC are critical for the induction of cellular immunity because they are uniquely efficient for the activation of naïve T cells during their first encounter with a pathogen. DC are equipped with a variety of innate immune recognition receptors (I2R2) allowing them to detect pathogens or infections and to engulf molecules, microorganisms or cellular debris. The combinatorial engagement of I2R2 during infections controls DC maturation and shapes their response in terms of cytokine production, activation of natural killer (NK) cells and functional polarization of T cells. Several DC subsets exist which express different arrays of I2R2 and are specialized in distinct functions. The study of MCMV infection helped deciphering the physiological roles of DC subsets and their molecular regulation. It allowed the identification and first in vivo studies of mouse plasmacytoid DC which produce high level of interferons-α/β early after infection. Despite its ability to infect DC and dampen their functions, MCMV induces very robust, efficient and long-lasting CD8 T cell responses. Their priming may rely on the unique ability of uninfected XCR1⁺ DC to cross-present engulfed viral antigens and thus to counter MCMV interference with antigen presentation. A balance appears to have been reached during co-evolution, allowing controlled replication of the virus for horizontal spread without pathological consequences for the immunocompetent host. We will discuss the role of the interplay between the virus and DC in setting this balance, and how advancing this knowledge further could help develop better vaccines against other intracellular infectious agents.

True grit: programmed necrosis in antiviral host defense, inflammation, and immunogenicity

Programmed necrosis mediated by receptor interacting protein kinase (RIP)3 (also called RIPK3) has emerged as an alternate death pathway triggered by TNF family death receptors, pathogen sensors, IFNRs, Ag-specific TCR activation, and genotoxic stress. Necrosis leads to cell leakage and acts as a "trap door," eliminating cells that cannot die by apoptosis because of the elaboration of pathogen-encoded caspase inhibitors. Necrotic signaling requires RIP3 binding to one of three partners-RIP1, DAI, or TRIF-via a common RIP homotypic interaction motif. Once activated, RIP3 kinase targets the pseudokinase mixed lineage kinase domain-like to drive cell lysis. Although necrotic and apoptotic death can enhance T cell cross-priming during infection, mice that lack these extrinsic programmed cell death pathways are able to produce Ag-specific T cells and control viral infection. The entwined relationship of apoptosis and necrosis evolved in response to pathogen-encoded suppressors to support host defense and contribute to inflammation.

Viral inoculum dose impacts memory T-cell inflation

Memory T-cell inflation develops during certain persistent viral infections and is characterized by the accumulation and maintenance of large numbers of effector-memory T cells, albeit with varying degrees in size and phenotype among infected hosts. The underlying mechanisms that control memory T-cell inflation are not yet fully understood. Here, we dissected CMV-specific memory T-cell formation and its connection to the initial infectious dose by varying the inoculum size. After low dose inoculum with mouse CMV, the accumulation of inflationary memory T cells was severely hampered and correlated with reduced reservoirs of latent virus in nonhematopoietic cells and diminished antigen-driven T-cell proliferation. Moreover, lowering of the initial viral dose turned the characteristic effector memory-like inflationary T cells into more central memory-like cells as evidenced by the cell-surface phenotype of CD27(high) , CD62L(+) , CD127(+) , and KLRG1(-) , and by improved secondary expansion potential. These data show the impact of the viral inoculum on the degree of memory T-cell inflation and provide a rationale for the observed variation of human CMV-specific T-cell responses in terms of magnitude and phenotype.

What we have learned from animal models of HCMV

Although human cytomegalovirus (HCMV) primary infection is generally asymptomatic, in immune-compromised patients HCMV increases morbidity and mortality. As a member of the betaherpesvirus family, in vivo studies of HCMV are limited due to its species specificity. CMVs from other species are often used as surrogates to express HCMV genes/proteins or used as models for inferring HCMV protein function in humans. Using innovative experiments, these animal models have answered important questions about CMV's life cycle, dissemination, pathogenesis, immune evasion, and host immune response. This chapter provides CMV biologists with an overview of the insights gained using these animal models. Subsequent chapters will provide details of the specifics of the experimental methods developed for each of the animal models discussed here.

An attenuated temperature-sensitive strain of cytomegalovirus (tsm5) establishes immunity without development of CD8(+) T cell memory inflation

Cytomegalovirus (CMV) is a widely prevalent herpesvirus that is well tolerated by an immune competent host yet establishes a state of chronic infection. The virus is thought to undergo frequent subclinical episodes of reactivation which leads to an unusually large accumulation of CMV-specific CD8(+) T lymphocytes in the peripheral blood, a phenomenon termed "memory inflation." The high magnitude of the CMV T cell response has been implicated in impaired immunity to heterologous pathogens such as EBV, influenza and West Nile virus. Here, using murine CMV (MCMV), we show that memory inflation of virus-specific CD8(+) T cells is avoided if mice are infected with a replication defective virus called temperature-sensitive mutant 5 (tsm5), which carries an attenuating mutation within the DNA primase gene. Mice infected with tsm5 do generate primary T cell responses towards viral proteins but these do not amass to skew the memory repertoire of CD8(+) T cells. Therefore, attenuation of the virus replication machinery may be valuable in future CMV vaccine designs because the virus remains immunogenic but does not contribute to CMV associated T cell immune senescence.

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.

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.

Immune senescence: relative contributions of age and cytomegalovirus infection

Immune senescence, defined as the age-associated dysregulation and dysfunction of the immune system, is characterised by impaired protective immunity and decreased efficacy of vaccines. Recent clinical, epidemiological and immunological studies suggest that Cytomegalovirus (CMV) infection may be associated with accelerated immune senescence, possibly by restricting the naïve T cell repertoire. However, direct evidence whether and how CMV-infection is implicated in immune senescence is still lacking. In this study, we have investigated whether latent mouse CMV (MCMV) infection with or without thymectomy (Tx) alters antiviral immunity of young and aged mice. After infection with lymphocytic choriomeningitis virus (LCMV) or Vaccinia virus, specific antiviral T cell responses were significantly reduced in old, old MCMV-infected and/or Tx mice compared to young mice. Importantly, control of LCMV replication was more profoundly impaired in aged MCMV-infected mice compared to age-matched MCMV-naïve or young mice. In addition, latent MCMV infection was associated with slightly reduced vaccination efficacy in old Tx mice. In contrast to the prevailing hypothesis of a CMV-mediated restriction of the naïve T cell repertoire, we found similar naïve T cell numbers in MCMV-infected and non-infected mice, whereas ageing and Tx clearly reduced the naïve T cell pool. Instead, MCMV-infection expanded the total CD8⁺ T cell pool by a massive accumulation of effector memory T cells. Based on these results, we propose a new model of increased competition between CMV-specific memory T cells and any ‘de novo’ immune response in aged individuals. In summary, our results directly demonstrate in a mouse model that latent CMV-infection impairs immunity in old age and propagates immune senescence.

Cytomegalovirus (CMV) persistently infects 50–90% of the human population. After primary infection, constant immune surveillance is required to prevent CMV-related disease. During ageing, increasing T cell resources are expended to keep CMV under control. Recent human studies have suggested that this investment may come at the cost of accelerated immune senescence, a condition describing the age-associated decline of the immune system's functionality. In the present study, we have developed a mouse model to directly investigate whether and how CMV-infection might impair immunity of aged individuals. We demonstrate that old mice with long-lasting CMV-infection are more susceptible to viral infections than old mice without CMV since their virus specific T cell response is suppressed. Contrary to the prevailing hypothesis we found no indication for a CMV-associated shrinking of the naïve T cell compartment. Instead, CMV-infection precipitated a massive expansion of memory T cells. Thus, we propose an alternative mechanism of CMV-enhanced immune senescence based on T cell competition between CMV-specific memory T cells and de novo generated T cell responses. In summary, we provide the first direct evidence that CMV-infection is indeed a propagating factor for poor immunity in the elderly.

IL-10 suppression of NK/DC crosstalk leads to poor priming of MCMV-specific CD4 T cells and prolonged MCMV persistence

IL-10 is an anti-inflammatory cytokine that regulates the extent of host immunity to infection by exerting suppressive effects on different cell types. Herpes viruses induce IL-10 to modulate the virus-host balance towards their own benefit, resulting in prolonged virus persistence. To define the cellular and molecular players involved in IL-10 modulation of herpes virus-specific immunity, we studied mouse cytomegalovirus (MCMV) infection. Here we demonstrate that IL-10 specifically curtails the MCMV-specific CD4 T cell response by suppressing the bidirectional crosstalk between NK cells and myeloid dendritic cells (DCs). In absence of IL-10, NK cells licensed DCs to effectively prime MCMV-specific CD4 T cells and we defined the pro-inflammatory cytokines IL-12, IFN-γ and TNF-α as well as NK cell activating receptors NKG2D and NCR-1 to regulate this bidirectional NK/DC interplay. Consequently, markedly enhanced priming of MCMV-specific CD4 T cells in Il10^−/− mice led to faster control of lytic viral replication, but this came at the expense of TNF-α mediated immunopathology. Taken together, our data show that early induction of IL-10 during MCMV infection critically regulates the strength of the innate-adaptive immune cell crosstalk, thereby impacting beneficially on the ensuing virus-host balance for both the virus and the host.

Cytomegalovirus (CMV) infections are very widespread in mammalian hosts. Despite the fact that CMVs are usually well controlled by the immune system, they cause persistent life-long infection and have evolved a number of strategies to effectively modulate or hide from host immunity. Since the establishment of an immunosuppressive environment favors virus persistence, IL-10 is one of the host targets that CMVs actively use to tune the virus-host balance toward their own benefit, resulting in prolonged virus persistence and hence increased chance for horizontal transmission. Here, we delineate the mechanisms of how IL-10 exerts its powerful immune-suppressing function in the context of murine cytomegalovirus (MCMV) infection. We found that IL-10 specifically restrains the priming of MCMV-specific CD4 T cell responses by suppressing dendritic cell (DC) - natural killer cell (NK) crosstalk during acute MCMV infection. Target molecules mediating this bi-directional crosstalk between DCs and NK cells were the pro-inflammatory cytokines IL-12, IFN-γ and TNF-α as well as NK cell activating receptors NKG2D and NCR-1 and all of them were markedly suppressed by IL-10. A consequence resulting from this impeded DC-NK cross-talk by IL-10, leading to poor priming of MCMV-specific CD4 T cell responses was increased lytic CMV persistence and reduced development of host tissue damage. Our study indicates that early induction of IL-10 during MCMV infection critically regulates the strength of the innate-adaptive crosstalk, thereby imparting on the ensuing virus-host balance for the benefit of both the virus and the host.

From crucial to negligible: functional CD8⁺ T-cell responses and their dependence on CD4⁺ T-cell help

CD8(+) T cells play an important role in controlling pathogenic infections and are therefore key players in the immune response. It has been shown that among other factors CD4(+) T cells can shape the magnitude as well as the quality of primary and/or secondary CD8(+) T-cell responses. However, due to the complexity and the differences among diverse immunization or infection models, the overall requirement, the time points, as well as the specific mechanism(s) of CD4(+) T-cell help may differ substantially. Here, we summarize current knowledge about the differential requirement of CD4(+) T-cell help in promoting primary CD8(+) T-cell responses as well as establishing functional memory CD8(+) T cells in various experimental settings.

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.

The death receptor 3/TL1A pathway is essential for efficient development of antiviral CD4⁺ and CD8⁺ T-cell immunity

Death receptor 3 (DR3, TNFRSF25), the closest family relative to tumor necrosis factor receptor 1, promotes CD4(+) T-cell-driven inflammatory disease. We investigated the in vivo role of DR3 and its ligand TL1A in viral infection, by challenging DR3-deficient (DR3(KO)) mice and their DR3(WT) littermates with the β-herpesvirus murine cytomegalovirus or the poxvirus vaccinia virus. The phenotype and function of splenic T-cells were analyzed using flow cytometry and molecular biological techniques. We report surface expression of DR3 by naive CD8(+) T cells, with TCR activation increasing its levels 4-fold and altering the ratio of DR3 splice variants. T-cell responses were reduced up to 90% in DR3(KO) mice during acute infection. Adoptive transfer experiments indicated this was dependent on T-cell-restricted expression of DR3. DR3-dependent CD8(+) T-cell expansion was NK and CD4 independent and due to proliferation, not decreased cell death. Notably, impaired immunity in DR3(KO) hosts on a C57BL/6 background was associated with 4- to 7-fold increases in viral loads during the acute phase of infection, and in mice with suboptimal NK responses was essential for survival (37.5%). This is the first description of DR3 regulating virus-specific T-cell function in vivo and uncovers a critical role for DR3 in mediating antiviral immunity.

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.

Non-hematopoietic cells in lymph nodes drive memory CD8 T cell inflation during murine cytomegalovirus infection

During human and murine cytomegalovirus (MCMV) infection an exceptionally large virus-specific CD8 T cell pool is maintained in the periphery lifelong. This anomalous response is only seen for specific subsets of MCMV-specific CD8 T cells which are referred to as 'inflationary T cells'. How memory CD8 T cell inflation is induced and maintained is unclear, though their activated phenotype strongly suggests an involvement of persistent antigen encounter during MCMV latency. To dissect the cellular and molecular requirements for memory CD8 T cell inflation, we have generated a transgenic mouse expressing an MHC class I-restricted T cell receptor specific for an immunodominant inflationary epitope of MCMV. Through a series of adoptive transfer experiments we found that memory inflation was completely dependent on antigen presentation by non-hematopoietic cells, which are also the predominant site of MCMV latency. In particular, non-hematopoietic cells selectively induced robust proliferation of inflationary CD8 T cells in lymph nodes, where a majority of the inflationary CD8 T cells exhibit a central-memory phenotype, but not in peripheral tissues, where terminally differentiated inflationary T cells accumulate. These results indicate that continuous restimulation of central memory CD8 T cells in the lymph nodes by infected non-hematopoietic cells ensures the maintenance of a functional effector CD8 T pool in the periphery, providing protection against viral reactivation events.

Cytomegaloviruses (CMVs) infect the majority of the human population and persist lifelong via latency. CMV latency is thought to be a dynamic state, characterized by stochastic viral reactivation events coupled to CMV-derived antigen presentation. In support of this hypothesis is the exceptionally large CMV-specific CD8 T cell response which constitutes an integral part of immune surveillance of CMV reactivation. Conversely, it may also contribute to immune senescence as it significantly shapes the overall CD8 T cell pool in bias of CMV-specificity. In mice, only a subset of CMV-specific CD8 T cells, also called ‘inflationary CD8 T cells’, contribute to this large response. The mechanism leading to the selective accumulation and persistence of memory CD8 T cells during MCMV latency is largely unknown. Here, we unraveled the mechanisms of memory CD8 T cell inflation using a newly generated TCR transgenic mouse with specificity for an immunodominant inflationary MCMV epitope. We show that antigen presentation on non-hematopoietic cells is essential for memory inflation and that memory inflation in peripheral tissues is fueled by lymph node-resident central memory CD8 T cells, being locally reactivated by non-hematopoietic cells, inducing their local expansion and migration to peripheral tissues where they control viral reactivation events.

Absence of cross-presenting cells in the salivary gland and viral immune evasion confine cytomegalovirus immune control to effector CD4 T cells

Horizontal transmission of cytomegaloviruses (CMV) occurs via prolonged excretion from mucosal surfaces. We used murine CMV (MCMV) infection to investigate the mechanisms of immune control in secretory organs. CD4 T cells were crucial to cease MCMV replication in the salivary gland (SG) via direct secretion of IFNγ that initiated antiviral signaling on non-hematopoietic cells. In contrast, CD4 T cell helper functions for CD8 T cells or B cells were dispensable. Despite SG-resident MCMV-specific CD8 T cells being able to produce IFNγ, the absence of MHC class I molecules on infected acinar glandular epithelial cells due to viral immune evasion, and the paucity of cross-presenting antigen presenting cells (APCs) prevented their local activation. Thus, local activation of MCMV-specific T cells is confined to the CD4 subset due to exclusive presentation of MCMV-derived antigens by MHC class II molecules on bystander APCs, resulting in IFNγ secretion interfering with viral replication in cells of non-hematopoietic origin.

Cytomegaloviruses (CMVs) infect 50 to 90 % of the world's population and cause severe clinical complication in immunosuppressed individuals. An important tissue for horizontal transmission is the salivary gland (SG). CD4 T cells are crucial for viral control in this organ. However, how CD4 T cells control MCMV and why CD8 T cells, important effector cells in other organs, are inefficient in the SG, remains unclear. Here we show that CD4 T cells exert direct antiviral effector rather than helper functions by secretion of IFNγ acting on non-hematopoietic cells. Although SG-resident CD8 T cells were able to produce IFNγ and outnumbered CD4 T cells, absence of MHC class I expression on infected cells due to CMV-encoded immune evasion genes and concomitant absence of cross-presenting antigen presenting cells prohibited antigen recognition by CD8 T cells. Deletion of CMV-encoded immune evasion genes enabled CD8 T cells to control MCMV replication in the SG in absence of CD4 T cells. Hence, CMV control depends on direct antiviral functions of CD4 T cells because of exclusive MHC class II-restricted CMV antigen presentation by bystander APCs in the SG, exemplifying a strategy of effective immune evasion by which CMVs to promote their own transmission.