A dominant role for the immunoproteasome in CD8+ T cell responses to murine cytomegalovirus

Modulation of cytomegalovirus immune evasion identifies direct antigen presentation as the predominant mode of CD8 T-cell priming during immune reconstitution after hematopoietic cell transplantation

Cytomegalovirus (CMV) infection is the most critical infectious complication in recipients of hematopoietic cell transplantation (HCT) in the period between a therapeutic hematoablative treatment and the hematopoietic reconstitution of the immune system. Clinical investigation as well as the mouse model of experimental HCT have consistently shown that timely reconstitution of antiviral CD8 T cells is critical for preventing CMV disease in HCT recipients. Reconstitution of cells of the T-cell lineage generates naïve CD8 T cells with random specificities among which CMV-specific cells need to be primed by presentation of viral antigen for antigen-specific clonal expansion and generation of protective antiviral effector CD8 T cells. For CD8 T-cell priming two pathways are discussed: "direct antigen presentation" by infected professional antigen-presenting cells (pAPCs) and "antigen cross-presentation" by uninfected pAPCs that take up antigenic material derived from infected tissue cells. Current view in CMV immunology favors the cross-priming hypothesis with the argument that viral immune evasion proteins, known to interfere with the MHC class-I pathway of direct antigen presentation by infected cells, would inhibit the CD8 T-cell response. While the mode of antigen presentation in the mouse model of CMV infection has been studied in the immunocompetent host under genetic or experimental conditions excluding either pathway of antigen presentation, we are not aware of any study addressing the medically relevant question of how newly generated naïve CD8 T cells become primed in the phase of lympho-hematopoietic reconstitution after HCT. Here we used the well-established mouse model of experimental HCT and infection with murine CMV (mCMV) and pursued the recently described approach of up- or down-modulating direct antigen presentation by using recombinant viruses lacking or overexpressing the central immune evasion protein m152 of mCMV, respectively. Our data reveal that the magnitude of the CD8 T-cell response directly reflects the level of direct antigen presentation.

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

Introduction

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

Methods

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

Results

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

Conclusions

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

'Stem-like' precursors are the fount to sustain persistent CD8+ T cell responses

Virus-specific CD8⁺ T cells that differentiate in the context of resolved versus persisting infections exhibit divergent phenotypic and functional characteristics, which suggests that their differentiation trajectories are governed by distinct cellular dynamics, developmental pathways and molecular mechanisms. For acute infection, it is long known that antigen-specific T cell populations contain terminally differentiated effector T cells, known as short-lived effector T cells, and proliferation-competent and differentiation-competent memory precursor T cells. More recently, it was identified that a similar functional segregation occurs in chronic infections. A failure to generate proliferation-competent precursor cells in chronic infections and tumors results in the collapse of the T cell response. Thus, these precursor cells are major therapeutic and prophylactic targets of immune interventions. These observations suggest substantial commonality between T cell responses in acute and chronic infections but there are also critical differences. We are therefore reviewing the common features and peculiarities of precursor cells in acute infections, different types of persistent infection and cancer.

Molecular biology of autoinflammatory diseases

The long battle between humans and various physical, chemical, and biological insults that cause cell injury (e.g., products of tissue damage, metabolites, and/or infections) have led to the evolution of various adaptive responses. These responses are triggered by recognition of damage-associated molecular patterns (DAMPs) and/or pathogen-associated molecular patterns (PAMPs), usually by cells of the innate immune system. DAMPs and PAMPs are recognized by pattern recognition receptors (PRRs) expressed by innate immune cells; this recognition triggers inflammation. Autoinflammatory diseases are strongly associated with dysregulation of PRR interactomes, which include inflammasomes, NF-κB-activating signalosomes, type I interferon-inducing signalosomes, and immuno-proteasome; disruptions of regulation of these interactomes leads to inflammasomopathies, relopathies, interferonopathies, and proteasome-associated autoinflammatory syndromes, respectively. In this review, we discuss the currently accepted molecular mechanisms underlying several autoinflammatory diseases.

Direct Evidence for Viral Antigen Presentation during Latent Cytomegalovirus Infection

Murine models of cytomegalovirus (CMV) infection have revealed an immunological phenomenon known as “memory inflation” (MI). After a peak of a primary CD8⁺ T-cell response, the pool of epitope-specific cells contracts in parallel to the resolution of productive infection and the establishment of a latent infection, referred to as “latency.” CMV latency is associated with an increase in the number of cells specific for certain viral epitopes over time. The inflationary subset was identified as effector-memory T cells (iTEM) characterized by the cell surface phenotype KLRG1⁺CD127⁻CD62L⁻. As we have shown recently, latent viral genomes are not transcriptionally silent. Rather, viral genes are sporadically desilenced in a stochastic fashion. The current hypothesis proposes MI to be driven by presented viral antigenic peptides encoded by the corresponding, stochastically expressed viral genes. Although this mechanism suggests itself, independent evidence for antigen presentation during viral latency is pending. Here we fill this gap by showing that T cell-receptor transgenic OT-I cells that are specific for peptide SIINFEKL proliferate upon adoptive cell transfer in C57BL/6 recipients latently infected with murine CMV encoding SIINFEKL (mCMV-SIINFEKL), but not in those latently infected with mCMV-SIINFEKA, in which antigenicity is lost by mutation L8A of the C-terminal amino acid residue.

Influenza- and MCMV-induced memory CD8 T cells control respiratory vaccinia virus infection despite residence in distinct anatomical niches

Induction of memory CD8 T cells residing in peripheral tissues is of interest for T cell-based vaccines as these cells are located at mucosal and barrier sites and can immediately exert effector functions, thus providing protection in case of local pathogen encounter. Different memory CD8 T cell subsets patrol peripheral tissues, but it is unclear which subset is superior in providing protection upon secondary infections. We used influenza virus to induce predominantly tissue resident memory T cells or cytomegalovirus to elicit a large pool of effector-like memory cells in the lungs and determined their early protective capacity and mechanism of reactivation. Both memory CD8 T cell pools have unique characteristics with respect to their phenotype, localization, and maintenance. However, these distinct features do not translate into different capacities to control a respiratory vaccinia virus challenge in an antigen-specific manner, although differential activation mechanisms are utilized. While influenza-induced memory CD8 T cells respond to antigen by local proliferation, MCMV-induced memory CD8 T cells relocate from the vasculature into the tissue in an antigen-independent and partially chemokine-driven manner. Together these results bear relevance for the development of vaccines aimed at eliciting a protective memory CD8 T cell pool at mucosal sites.

Stochastic Episodes of Latent Cytomegalovirus Transcription Drive CD8 T-Cell "Memory Inflation" and Avoid Immune Evasion

Acute infection with murine cytomegalovirus (mCMV) is controlled by CD8+ T cells and develops into a state of latent infection, referred to as latency, which is defined by lifelong maintenance of viral genomes but absence of infectious virus in latently infected cell types. Latency is associated with an increase in numbers of viral epitope-specific CD8+ T cells over time, a phenomenon known as "memory inflation" (MI). The "inflationary" subset of CD8+ T cells has been phenotyped as KLRG1+CD62L- effector-memory T cells (iTEM). It is agreed upon that proliferation of iTEM requires repeated episodes of antigen presentation, which implies that antigen-encoding viral genes must be transcribed during latency. Evidence for this has been provided previously for the genes encoding the MI-driving antigenic peptides IE1-YPHFMPTNL and m164-AGPPRYSRI of mCMV in the H-2d haplotype. There exist two competing hypotheses for explaining MI-driving viral transcription. The "reactivation hypothesis" proposes frequent events of productive virus reactivation from latency. Reactivation involves a coordinated gene expression cascade from immediate-early (IE) to early (E) and late phase (L) transcripts, eventually leading to assembly and release of infectious virus. In contrast, the "stochastic transcription hypothesis" proposes that viral genes become transiently de-silenced in latent viral genomes in a stochastic fashion, not following the canonical IE-E-L temporal cascade of reactivation. The reactivation hypothesis, however, is incompatible with the finding that productive virus reactivation is exceedingly rare in immunocompetent mice and observed only under conditions of compromised immunity. In addition, the reactivation hypothesis fails to explain why immune evasion genes, which are regularly expressed during reactivation in the same cells in which epitope-encoding genes are expressed, do not prevent antigen presentation and thus MI. Here we show that IE, E, and L genes are transcribed during latency, though stochastically, not following the IE-E-L temporal cascade. Importantly, transcripts that encode MI-driving antigenic peptides rarely coincide with those that encode immune evasion proteins. As immune evasion can operate only in cis, that is, in a cell that simultaneously expresses antigenic peptides, the stochastic transcription hypothesis explains why immune evasion is not operative in latently infected cells and, therefore, does not interfere with MI.

Role of immunoproteasomes and thymoproteasomes in health and disease

The proteasome is a multisubunit protease that degrades intracellular proteins into small peptides. Besides playing a pivotal role in many cellular processes indispensable for survival, it is involved in the production of peptides presented by major histocompatibility complex class I molecules. In addition to the standard proteasome shared in all eukaryotes, jawed vertebrates have two specialized forms of proteasome known as immunoproteasomes and thymoproteasomes. The immunoproteasome, which contains cytokine-inducible catalytic subunits with distinct cleavage specificities, produces peptides presented by class I molecules more efficiently than the standard proteasome. The thymoproteasome, which contains a unique catalytic subunit β5t, is a tissue-specific proteasome expressed exclusively in cortical thymic epithelial cells. It plays a critical role in CD8⁺ cytotoxic T cell development via positive selection. This review provides a brief overview on the structure and function of these specialized forms of proteasome and their involvement in human disease.

The Ubiquitin-Proteasome System in Immune Cells

The ubiquitin–proteasome system (UPS) is the major intracellular and non-lysosomal protein degradation system. Thanks to its unique capacity of eliminating old, damaged, misfolded, and/or regulatory proteins in a highly specific manner, the UPS is virtually involved in almost all aspects of eukaryotic life. The critical importance of the UPS is particularly visible in immune cells which undergo a rapid and profound functional remodelling upon pathogen recognition. Innate and/or adaptive immune activation is indeed characterized by a number of substantial changes impacting various cellular processes including protein homeostasis, signal transduction, cell proliferation, and antigen processing which are all tightly regulated by the UPS. In this review, we summarize and discuss recent progress in our understanding of the molecular mechanisms by which the UPS contributes to the generation of an adequate immune response. In this regard, we also discuss the consequences of UPS dysfunction and its role in the pathogenesis of recently described immune disorders including cancer and auto-inflammatory diseases.

The avid competitors of memory inflation

Cytomegaloviruses (CMV) coevolve with their hosts and latently persist in the vast majority of adult mammals. Therefore, persistent T-cell responses to CMV antigens during virus latency offer a fascinating perspective on the evolution of the T-cell repertoire in natural settings. We addressed here the life-long interactions between CMV antigens presented on MHC-I molecules and the CD8 T-cell response. We present the mechanistic evidence from the murine model of CMV infection and put it in context of clinical laboratory results. We will highlight the remarkable parallels in T-cell responses between the two biological systems, and focus in particular on memory inflation as a result of competitive processes, both between viral antigenic peptides and between T-cell receptors on the host's cytotoxic lymphocytes.

Impact of multiple hits with cognate antigen on memory CD8+ T-cell fate

Antigen-driven activation of CD8+ T cells results in the development of a robust anti-pathogen response and ultimately leads to the establishment of long-lived memory T cells. During the primary response, CD8+ T cells interact multiple times with cognate antigen on distinct types of antigen-presenting cells. The timing, location and context of these antigen encounters significantly impact the differentiation programs initiated in the cells. Moderate re-activation in the periphery promotes the establishment of the tissue-resident memory T cells that serve as sentinels at the portal of pathogen entry. Under some circumstances, moderate re-activation of T cells in the periphery can result in the excessive expansion and accumulation of circulatory memory T cells, a process called memory inflation. In contrast, excessive re-activation stimuli generally impede conventional T-cell differentiation programs and can result in T-cell exhaustion. However, these conditions can also elicit a small population of exhausted T cells with a memory-like signature and self-renewal capability that are capable of responding to immunotherapy, and restoration of functional activity. Although it is clear that antigen re-encounter during the primary immune response has a significant impact on memory T-cell development, we still do not understand the molecular details that drive these fate decisions. Here, we review our understanding of how antigen encounters and re-activation events impact the array of memory CD8+ T-cell subsets subsequently generated. Identification of the molecular programs that drive memory T-cell generation will advance the development of new vaccine strategies that elicit high-quality CD8+ T-cell memory.

Cytomegalovirus inhibition of extrinsic apoptosis determines fitness and resistance to cytotoxic CD8 T cells

Viral immune evasion is currently understood to focus on deflecting CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) prevents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoevasins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mechanism through which vICA acts, demonstrating the central contribution of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.

Tcf1+ cells are required to maintain the inflationary T cell pool upon MCMV infection

Cytomegalovirus-based vaccine vectors offer interesting opportunities for T cell-based vaccination purposes as CMV infection induces large numbers of functional effector-like cells that accumulate in peripheral tissues, a process termed memory inflation. Maintenance of high numbers of peripheral CD8 T cells requires continuous replenishment of the inflationary T cell pool. Here, we show that the inflationary T cell population contains a small subset of cells expressing the transcription factor Tcf1. These Tcf1⁺ cells resemble central memory T cells and are proliferation competent. Upon sensing viral reactivation events, Tcf1⁺ cells feed into the pool of peripheral Tcf1⁻ cells and depletion of Tcf1⁺ cells hampers memory inflation. TCR repertoires of Tcf1⁺ and Tcf1⁻ populations largely overlap, with the Tcf1⁺ population showing higher clonal diversity. These data show that Tcf1⁺ cells are necessary for sustaining the inflationary T cell response, and upholding this subset is likely critical for the success of CMV-based vaccination approaches.

Upon infection with cytomegalovirus, CD8⁺ T cells undergo prolific expansion in a process known as memory inflation. Here the authors define a population of Tcf1 expressing cells within the inflationary pool that is critical in fuelling this process.

Vaccine Vectors Harnessing the Power of Cytomegaloviruses

Cytomegalovirus (CMV) species have been gaining attention as experimental vaccine vectors inducing cellular immune responses of unparalleled strength and protection. This review outline the strengths and the restrictions of CMV-based vectors, in light of the known aspects of CMV infection, pathogenicity and immunity. We discuss aspects to be considered when optimizing CMV based vaccines, including the innate immune response, the adaptive humoral immunity and the T-cell responses. We also discuss the antigenic epitopes presented by unconventional major histocompatibility complex (MHC) molecules in some CMV delivery systems and considerations about routes for delivery for the induction of systemic or mucosal immune responses. With the first clinical trials initiating, CMV-based vaccine vectors are entering a mature phase of development. This impetus needs to be maintained by scientific advances that feed the progress of this technological platform.

Divergent memory responses driven by adenoviral vectors are impacted by epitope competition

Adenoviral vectors induce robust epitope-specific CD8+ T cell responses. Within the repertoire of responses generated both conventional memory evolution and the phenomenon of memory inflation are seen. The rules governing which epitopes inflate are not fully known, but may include a role for both antigen processing and competition. To investigate this, we looked at memory generated from vectors targeting the Gp33-41 (KAVYNFATC/K9C) epitope from the gp of lymphocytic choriomeningitis virus (LCMV) in mice. This well-described epitope has both the Gp33-41 and Gp34-41 epitopes embedded within it. Vaccination with a full-length gp or a minigene Ad-Gp33/K9C vector-induced conventional memory responses against the immunodominant Gp33/K9C epitope but a strong inflationary response against the Gp34/A8C epitope. These responses showed sustained in vivo function, with complete protection against LCMV infectious challenge. Given the unexpected competition between epitopes seen in the minigene model, we further tested epitope competition using the full-length Ad-LacZ (β-galactosidase) model. Generation of an Ad-LacZ vector with a single amino acid disruption of the inflationary β-gal96-103 /D8V epitope transformed the β-gal497-504 /I8V epitope from conventional to inflationary memory. This work collectively demonstrates the importance of epitope competition within adenoviral vector inserts and is of relevance to future studies using adenoviral vectored immunogens.

Age-Dependent Effects of Immunoproteasome Deficiency on Mouse Adenovirus Type 1 Pathogenesis

Acute respiratory infection with mouse adenovirus type 1 (MAV-1) induces activity of the immunoproteasome, an inducible form of the proteasome that shapes CD8 T cell responses by enhancing peptide presentation by major histocompatibility complex (MHC) class I. We used mice deficient in all three immunoproteasome subunits (triple-knockout [TKO] mice) to determine whether immunoproteasome activity is essential for control of MAV-1 replication or inflammatory responses to acute infection. Complete immunoproteasome deficiency in adult TKO mice had no effect on MAV-1 replication, virus-induced lung inflammation, or adaptive immunity compared to C57BL/6 (B6) controls. In contrast, immunoproteasome deficiency in neonatal TKO mice was associated with decreased survival and decreased lung gamma interferon (IFN-γ) expression compared to B6 controls, although without substantial effects on viral replication, histological evidence of inflammation, or expression of the proinflammatory cytokines tumor necrosis factor alpha and interleukin-1β in lungs or other organs. T cell recruitment and IFN-γ production was similar in lungs of infected B6 and TKO mice. In lungs of uninfected B6 mice, we detected low levels of immunoproteasome subunit mRNA and protein that increased with age. Immunoproteasome subunit expression was lower in lungs of adult IFN-γ-deficient mice compared to B6 controls. Together, these results demonstrate developmental regulation of the immunoproteasome that is associated with age-dependent differences in MAV-1 pathogenesis.IMPORTANCE MAV-1 infection is a useful model to study the pathogenesis of an adenovirus in its natural host. Host factors that control MAV-1 replication and contribute to inflammation and disease are not fully understood. The immunoproteasome is an inducible component of the ubiquitin proteasome system that shapes the repertoire of peptides presented by MHC class I to CD8 T cells, influences other aspects of T cell survival and activation, and promotes production of proinflammatory cytokines. We found that immunoproteasome activity is dispensable in adult mice. However, immunoproteasome deficiency in neonatal mice increased mortality and impaired IFN-γ responses in the lungs. Baseline immunoproteasome subunit expression in lungs of uninfected mice increased with age. Our findings suggest the existence of developmental regulation of the immunoproteasome, like other aspects of host immune function, and indicate that immunoproteasome activity is a critical protective factor early in life.

From Vaccine Vector to Oncomodulation: Understanding the Complex Interplay between CMV and Cancer

Cytomegalovirus (CMV) is a herpesvirus that establishes a persistent, but generally asymptomatic, infection in most people in the world. However, CMV drives and sustains extremely large numbers of antigen-specific T cells and is, therefore, emerging as an exciting platform for vaccines against infectious diseases and cancer. Indeed, pre-clinical data strongly suggest that CMV-based vaccines can sustain protective CD8+ T cell and antibody responses. In the context of vaccines for infectious diseases, substantial pre-clinical studies have elucidated the efficacy and protective mechanisms of CMV-based vaccines, including in non-human primate models of various infections. In the context of cancer vaccines, however, much less is known and only very early studies in mice have been conducted. To develop CMV-based cancer vaccines further, it will be critical to better understand the complex interaction of CMV and cancer. An array of evidence suggests that naturally-acquired human (H)CMV can be detected in cancers, and it has been proposed that HCMV may promote tumor growth. This would obviously be a concern for any therapeutic cancer vaccines. In experimental models, CMV has been shown to play both positive and negative roles in tumor progression, depending on the model studied. However, the mechanisms are still largely unknown. Thus, more studies assessing the interaction of CMV with the tumor microenvironment are needed. This review will summarize the existing literature and major open questions about CMV-based vaccines for cancer, and discuss our hypothesis that the balance between pro-tumor and anti-tumor effects driven by CMV depends on the location and the activity of the virus in the lesion.

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.

Caspase-8-dependent control of NK- and T cell responses during cytomegalovirus infection

Caspase-8 (CASP8) impacts antiviral immunity in expected as well as unexpected ways. Mice with combined deficiency in CASP8 and RIPK3 cannot support extrinsic apoptosis or RIPK3-dependent programmed necrosis, enabling studies of CASP8 function without complications of unleashed necroptosis. These extrinsic cell death pathways are naturally targeted by murine cytomegalovirus (MCMV)-encoded cell death suppressors, showing they are key to cell-autonomous host defense. Remarkably, Casp8^-/-Ripk3^-/-, Ripk1^-/-Casp8^-/-Ripk3^-/- and Casp8^-/-Ripk3^K51A/K51A mice mount robust antiviral T cell responses to control MCMV infection. Studies in Casp8^-/-Ripk3^-/- mice show that CASP8 restrains expansion of MCMV-specific natural killer (NK) and CD8 T cells without compromising contraction or immune memory. Infected Casp8^-/-Ripk3^-/- or Casp8^-/-Ripk3^K51A/K51A mice have higher levels of virus-specific NK cells and CD8 T cells compared to matched RIPK3-deficient littermates or WT mice. CASP8, likely acting downstream of Fas death receptor, dampens proliferation of CD8 T cells during expansion. Importantly, contraction proceeds unimpaired in the absence of extrinsic death pathways owing to intact Bim-dependent (intrinsic) apoptosis. CD8 T cell memory develops in Casp8^-/-Ripk3^-/- mice, but memory inflation characteristic of MCMV infection is not sustained in the absence of CASP8 function. Despite this, Casp8^-/-Ripk3^-/- mice are immune to secondary challenge. Interferon (IFN)γ is recognized as a key cytokine for adaptive immune control of MCMV. Ifngr^-/-Casp8^-/-Ripk3^-/- mice exhibit increased lifelong persistence in salivary glands as well as lungs compared to Ifngr^-/- and Casp8^-/-Ripk3^-/- mice. Thus, mice deficient in CASP8 and RIPK3 are more dependent on IFNγ mechanisms for sustained T cell immune control of MCMV. Overall, appropriate NK- and T cell immunity to MCMV is dependent on host CASP8 function independent of RIPK3-regulated pathways.

Early primed KLRG1- CMV-specific T cells determine the size of the inflationary T cell pool

Memory T cell inflation is a process in which a subset of cytomegalovirus (CMV) specific CD8 T cells continuously expands mainly during latent infection and establishes a large and stable population of effector memory cells in peripheral tissues. Here we set out to identify in vivo parameters that promote and limit CD8 T cell inflation in the context of MCMV infection. We found that the inflationary T cell pool comprised mainly high avidity CD8 T cells, outcompeting lower avidity CD8 T cells. Furthermore, the size of the inflationary T cell pool was not restricted by the availability of specific tissue niches, but it was directly related to the number of virus-specific CD8 T cells that were activated during priming. In particular, the amount of early-primed KLRG1^- cells and the number of inflationary cells with a central memory phenotype were a critical determinant for the overall magnitude of the inflationary T cell pool. Inflationary memory CD8 T cells provided protection from a Vaccinia virus challenge and this protection directly correlated with the size of the inflationary memory T cell pool in peripheral tissues. These results highlight the remarkable protective potential of inflationary CD8 T cells that can be harnessed for CMV-based T cell vaccine approaches.

Cytomegalovirus induces a lifelong infection in the majority of the world's population, due to the ability of the virus to establish latency. Upon CMV infection, large numbers of effector memory T cells are induced in peripheral tissues, a process that is termed memory inflation. As inflationary T cells are highly functional, CMV-based vaccines have gained substantial interest for vaccination purposes. Here we examine factors that promote and limit memory T cell inflation. We found that there were no constraints on the availability of specific niches for inflationary T cells in tissues and that high avidity T cells predominately contribute to the inflationary T cell population in the beginning of infection. Moreover, the number of early primed KLRG1^- CMV-specific T cells in the acute phase of infection set the limit for memory T cell inflation. Furthermore, we show that inflationary T cells provided protection from a pathogenic challenge in peripheral tissues such as the ovaries. Thus, inflationary T cells comprise a population of T cells that can protect peripheral tissues from pathogenic infections and their efficacy can be regulated by balancing the number of KLRG1^- CMV-specific cells during priming.

Cytomegalovirus memory inflation and immune protection

Cytomegalovirus (CMV) infection induces powerful and sustained T-cell responses against a few selected immunodominant antigenic epitopes. This immune response was named memory inflation, because it does not contract in the long term, and may even expand over months and years of virus latency. It is by now understood that memory inflation does not occur at the expense of the naïve T-cell pool, but rather as a competitive selection process within the effector pool, where viral antigens with higher avidity of TCR binding and with earlier expression patterns outcompete those that are expressed later and bind TCRs less efficiently. It is also understood that inflationary epitopes require processing by the constitutive proteasome in non-hematopoietic cells, and this likely implies that memory inflation is fuelled by direct low-level antigenic expression in latently infected cells. This review proposes that these conditions make inflationary epitopes the optimal candidates for adoptive immunotherapy of CMV disease in the immunocompromised host. At present, functional target CMV epitopes have been defined only for the most common HLA haplotypes. Mapping the uncharacterized inflationary epitopes in less frequent HLAs may, thus, be a strategy for the identification of optimal immunotherapeutic targets in patients with uncommon haplotypes.

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.

On the role of the immunoproteasome in transplant rejection

The immunoproteasome is expressed in cells of hematopoietic origin and is induced during inflammation by IFN-γ. Targeting the immunoproteasome with selective inhibitors has been shown to be therapeutically effective in pre-clinical models for autoimmune diseases, colitis-associated cancer formation, and transplantation. Immunoproteasome inhibition prevents activation and proliferation of lymphocytes, lowers MHC class I cell surface expression, reduces the expression of cytokines of activated immune cells, and curtails  T helper 1 and 17 cell differentiation. This might explain the in vivo efficacy of immunoproteasome inhibition in different pre-clinical disease models for autoimmunity, cancer, and transplantation. In this review, we summarize the effect of immunoproteasome inhibition in different animal models for transplantation.

Dysregulation of immunoproteasomes in autoinflammatory syndromes

Immunoproteasomes degrade ubiquitin-coupled proteins and play a role in creating peptides for presentation by MHC class I proteins. Studies of gene-deficient mice, in which each immunoproteasomal subunit was affected, have demonstrated that dysfunction of immunoproteasomes leads to immunodeficiency, i.e. reduced expression of MHC class I and attenuation of CD8 T-cell responses. Recent studies, however, have uncovered a new type of autoinflammatory syndrome characterized by fever, nodular erythema and progressive partial lipodystrophy that is caused by genetic mutations in immunoproteasome subunits. These mutations disturbed the assembly of immunoproteasomes, which led to reduced proteasomal activity and thus accumulation of ubiquitin-coupled proteins. Those findings suggest that immunoproteasomes function as anti-inflammatory machinery in humans. The discovery of a new type of autoinflammatory syndrome caused by dysregulated immunoproteasomes provides novel insights into the important roles of immunoproteasomes in inflammation as well as the spectrum of autoinflammatory diseases.

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.

Computational comparison of availability in CTL/gag epitopes among patients with acute and chronic HIV-1 infection

BACKGROUND

Recent studies indicate that there is selection bias for transmission of viral polymorphisms associated with higher viral fitness. Furthermore, after transmission and before a specific immune response is mounted in the recipient, the virus undergoes a number of reversions which allow an increase in their replicative capacity. These aspects, and others, affect the viral population characteristic of early acute infection.

METHODS

160 singlegag-gene amplifications were obtained by limiting-dilution RT-PCR from plasma samples of 8 ARV-naïve patients with early acute infection (<30 days, 22 days average) and 8 ARV-naive patients with approximately a year of infection (10 amplicons per patient). Sanger sequencing and NGS SMRT technology (Pacific Biosciences) were implemented to sequence the amplicons. Phylogenetic analysis was performed by using MEGA 6.06. HLA-I (A and B) typing was performed by SSOP-PCR method. The chromatograms were analyzed with Sequencher 4.10. Epitopes and immune-proteosomal cleavages prediction was performed with CBS prediction server for the 30 HLA-A and -B alleles most prevalent in our population with peptide lengths from 8 to 14 mer. Cytotoxic response prediction was performed by using IEDB Analysis Resource.

RESULTS

After implementing epitope prediction analysis, we identified a total number of 325 possible viral epitopes present in two or more acute or chronic patients. 60.3% (n = 196) of them were present only in acute infection (prevalent acute epitopes) while 39.7% (n = 129) were present only in chronic infection (prevalent chronic epitopes). Within p24, the difference was equally dramatic with 59.4% (79/133) being acute epitopes (p < 0.05). This is consistent with progressive viral adaptation to immune response in time and further supported by the fact that cytotoxic responses prediction showed that acute epitopes are more likely to generate immune response than chronic epitopes. Interestingly, only 27.5% of acute epitopes match the population-level consensus sequence of the virus.

CONCLUSIONS

Our results indicate that certain non-consensus viral residues might be transmitted more frequently than consensus-residues when located in immunological relevant positions (epitopes). This observation might be relevant to the rationale behind development of an effective vaccineto reduce viral reservoir and induce functional cure of HIV infection based in prevalent acute epitopes.

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.

Tissue maintenance of CMV-specific inflationary memory T cells by IL-15

Cytomegalovirus (CMV) infection induces an atypical CD8 T cell response, termed inflationary, that is characterised by accumulation and maintenance of high numbers of effector memory like cells in circulation and peripheral tissues—a feature being successfully harnessed for vaccine purposes. Although stability of this population depends on recurrent antigen encounter, the requirements for prolonged survival in peripheral tissues remain unknown. Here, we reveal that murine CMV-specific inflationary CD8 T cells are maintained in an antigen-independent manner and have a half-life of 12 weeks in the lung tissue. This half-life is drastically longer than the one of phenotypically comparable inflationary effector cells. IL-15 alone, and none of other common γ-cytokines, was crucial for survival of inflationary cells in peripheral organs. IL-15, mainly produced by non-hematopoietic cells in lung tissue and being trans-presented, promoted inflationary T cell survival by increasing expression of Bcl-2. These results indicate that inflationary CD8 T cells are not just simply effector-like cells, rather they share properties of both effector and memory CD8 T cells and they appear to be long-lived cells compared to the effector cells from acute virus infections.

A majority of the human population is infected with cytomegalovirus (CMV), which results in lifelong persistence due to viral latency. CMV induces remarkably strong and sustained effector memory-like CD8 T cell responses in circulation and peripheral tissues, also referred to as memory CD8 T cell "inflation". In tissues, these effector memory-like cells contribute to immunosurveillance and early control of CMV reactivation events. Due to the high numbers and effector-like functional properties of inflationary CD8 T cells in peripheral tissues, CMV-based vectors are gaining substantial interest in the context of T cell based vaccines that protect peripheral tissues against infections or tumors. Here, we investigated how the stable peripheral pool of inflationary CD8 T cells is maintained and show that inflationary CD8 T cells are long-lived T cells and have a markedly prolonged half-life compared to effector CD8 T cells. In peripheral organs such as lung, IL-15 cytokine is pivotal in promoting maintenance of inflationary cells by inducing expression of the anti-apoptotic molecule Bcl-2. We show that IL-15 is mainly expressed by non-hematopoietic cells in lung tissue and that IL-15 is trans-presented to the inflationary CD8 T cells in vivo. Thus, CMV-driven inflationary CD8 T cell responses represent a unique T cell subset in peripheral tissues that is regulated differently compared to T_RM CD8 T cells emerging after vaccination or acute infections.

Immunoproteasome Subunits Are Required for CD8+ T Cell Function and Host Resistance to Brucella abortus Infection in Mice

The immunoproteasome is a specific proteasome isoform composed of three subunits, termed β1i, β2i, and β5i. Its proteolytic activity enhances the quantity and quality of peptides to be presented by major histocompatibility complex class I (MHC-I) molecules to CD8⁺ T cells. However, the role of the combined deficiency of the three immunoproteasome subunits in protective immunity against bacterial pathogens has not been investigated. In this study, we addressed the role of the immunoproteasome during infection by Brucella abortus, an intracellular bacterium that requires CD8⁺ T cell responses for the control of infection. Here, we demonstrate that immunoproteasome triple-knockout (TKO) mice were more susceptible to Brucella infection. This observed susceptibility was accompanied by reduced interferon gamma (IFN-γ) production by mouse CD4⁺ and CD8⁺ T lymphocytes. Moreover, the absence of the immunoproteasome had an impact on MHC-I surface expression and antigen presentation by dendritic cells. CD8⁺ T cell function, which plays a pivotal role in B. abortus immunity, also presented a partial impairment of granzyme B expression and, consequently, reduced cytotoxic activity. In conclusion, these results strongly suggest that immunoproteasome subunits are important components in host resistance to B. abortus infection by impacting both the magnitude and quality of CD8⁺ T cell responses.

Features of Effective T Cell-Inducing Vaccines against Chronic Viral Infections

For many years, the focus of prophylactic vaccines was to elicit neutralizing antibodies, but it has become increasingly evident that T cell-mediated immunity plays a central role in controlling persistent viral infections such as with human immunodeficiency virus, cytomegalovirus, and hepatitis C virus. Currently, various promising prophylactic vaccines, capable of inducing substantial vaccine-specific T cell responses, are investigated in preclinical and clinical studies. There is compelling evidence that protection by T cells is related to the magnitude and breadth of the T cell response, the type and homing properties of the memory T cell subsets, and their cytokine polyfunctionality and metabolic fitness. In this review, we evaluated these key factors that determine the qualitative and quantitative properties of CD4+ and CD8+ T cell responses in the context of chronic viral disease and prophylactic vaccine development. Elucidation of the mechanisms underlying T cell-mediated protection against chronic viral pathogens will facilitate the development of more potent, durable and safe prophylactic T cell-based vaccines.

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.

The immunoproteasome subunit LMP7 is required in the murine thymus for filling up a hole in the T cell repertoire

Cells of hematopoietic origin express high levels of the immunoproteasome, a cytokine-inducible variant of the proteasome which has been implicated in regulating inflammatory responses and antigen presentation. In the thymus, medullary thymic epithelial cells (mTECs) and cortical thymic epithelial cells (cTECs) do express different proteasome subunits exerting chymotrypsin-like activities suggesting distinct functions in thymic T cell selection. Employing the lymphocytic choriomeningitis virus (LCMV) infection model, we could show that the immunoproteasome subunit LMP7 was absolutely required for the generation of LCMV GP_118-125 -specific T cells although the class I mediated presentation of GP_118-125 was not dependent on LMP7. Using bone marrow chimeras and adoptive transfer of LMP7-deficient CD8⁺ T cells into RAG1-deficient mice we show that LMP7-deficient mice lacked GP_118-125 -specific T cell precursors and that LMP7 was required in radioresistant cells - most likely thymic epithelial cells - to enable their selection. Since LMP7 is strongly expressed in negatively selecting mTECs but barely in positively selecting cTECs our data suggest that LMP7 was required to avoid excessive negative selection of GP_118-125 -specific T cell precursors. Taken together, this study demonstrates that the immunoproteasome is a crucial factor for filling up holes within the cytotoxic T cell repertoire.

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.

CMV immune evasion and manipulation of the immune system with aging

Human cytomegalovirus (HCMV) encodes numerous proteins and microRNAs that function to evade the immune response and allow the virus to replicate and disseminate in the face of a competent innate and acquired immune system. The establishment of a latent infection by CMV, which if completely quiescent at the level of viral gene expression would represent an ultimate in immune evasion strategies, is not sufficient for lifelong persistence and dissemination of the virus. CMV needs to reactivate and replicate in a lytic cycle of infection in order to disseminate further, which occurs in the face of a fully primed secondary immune response. Without reactivation, latency itself would be redundant for the virus. It is also becoming clear that latency is not a totally quiescent state, but is characterized by limited viral gene expression. Therefore, the virus also needs immune evasion strategies during latency. An effective immune response to CMV is required or viral replication will cause morbidity and ultimately mortality in the host. There is clearly a complex balance between virus immune evasion and host immune recognition over a lifetime. This poses the important question of whether long-term evasion or manipulation of the immune response driven by CMV is detrimental to health. In this meeting report, three groups used the murine model of CMV (MCMV) to examine if the contribution of the virus to immune senescence is set by the (i) initial viral inoculum, (ii) inflation of T cell responses, (iii) or the balance between functionally distinct effector CD4+ T cells. The work of other groups studying the CMV response in humans is discussed. Their work asks whether the ability to make immune responses to new antigens is compromised by (i) age and HCMV carriage, (ii) long-term exposure to HCMV giving rise to an overall immunosuppressive environment and increased levels of latent virus, or (iii) adapted virus mutants (used as potential vaccines) that have the capacity to elicit conventional and unconventional T cell responses.

Intratumoral infection by CMV may change the tumor environment by directly interacting with tumor-associated macrophages to promote cancer immunity

Cytomegalovirus (CMV) is a herpesvirus that induces an extremely robust and sustained immune response. For this reason, CMV has been proposed as a vaccine vector to promote immunity to both pathogens and cancer. However, exploration of CMV as a vaccine vector is at an early stage and there are many questions. Using a mouse melanoma model, we recently found that a CMV-based vaccine induced large populations of melanoma-specific T cells, but was not effective at slowing tumor growth unless it was injected directly into the tumor. These surprising results have led us to hypothesize that CMV may be adept at modulating the tumor micro-environment through its infection of macrophages. Importantly, injection of CMV into the growing tumor synergized with blockade of the PD-1 checkpoint to clear well-established tumors. Here, we discuss our results in the context of CMV-based vaccines for pathogens and cancer.

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.

Blocking Virus Replication during Acute Murine Cytomegalovirus Infection Paradoxically Prolongs Antigen Presentation and Increases the CD8+ T Cell Response by Preventing Type I IFN-Dependent Depletion of Dendritic Cells

Increasing amounts of pathogen replication usually lead to a proportionate increase in size and effector differentiation of the CD8⁺ T cell response, which is attributed to increased Ag and inflammation. Using a murine CMV that is highly sensitive to the antiviral drug famciclovir to modulate virus replication, we found that increased virus replication drove increased effector CD8⁺ T cell differentiation, as expected. Paradoxically, however, increased virus replication dramatically decreased the size of the CD8⁺ T cell response to two immunodominant epitopes. The decreased response was due to type I IFN-dependent depletion of conventional dendritic cells and could be reproduced by specific depletion of dendritic cells from day 2 postinfection or by sterile induction of type I IFN. Increased virus replication and type I IFN specifically inhibited the response to two immunodominant epitopes that are known to be dependent on Ag cross-presented by DCs, but they did not inhibit the response to "inflationary" epitopes whose responses can be sustained by infected nonhematopoietic cells. Our results show that type I IFN can suppress CD8⁺ T cell responses to cross-presented Ag by depleting cross-presenting conventional dendritic cells.

BPTF Depletion Enhances T-cell-Mediated Antitumor Immunity

Genetic studies in fruit flies have implicated the chromatin remodeling complex nucleosome remodeling factor (NURF) in immunity, but it has yet to be studied in mammals. Here we show that its targeting in mice enhances antitumor immunity in two syngeneic models of cancer. NURF was disabled by silencing of bromodomain PHD-finger containing transcription factor (BPTF), the largest and essential subunit of NURF. We found that both CD8⁺ and CD4⁺ T cells were necessary for enhanced antitumor activity, with elevated numbers of activated CD8⁺ T cells observed in BPTF-deficient tumors. Enhanced cytolytic activity was observed for CD8⁺ T cells cocultured with BPTF-silenced cells. Similar effects were not produced with T-cell receptor transgenic CD8⁺ T cells, implicating the involvement of novel antigens. Accordingly, enhanced activity was observed for individual CD8⁺ T-cell clones from mice bearing BPTF-silenced tumors. Mechanistic investigations revealed that NURF directly regulated the expression of genes encoding immunoproteasome subunits Psmb8 and Psmb9 and the antigen transporter genes Tap1 and Tap2 The PSMB8 inhibitor ONX-0914 reversed the effects of BPTF ablation, consistent with a critical role for the immunoproteasome in improving tumor immunogenicity. Thus, NURF normally suppresses tumor antigenicity and its depletion improves antigen processing, CD8 T-cell cytotoxicity, and antitumor immunity, identifying NURF as a candidate therapeutic target to enhance antitumor immunity. Cancer Res; 76(21); 6183-92. ©2016 AACR.

CMV-Specific CD8 T Cell Differentiation and Localization: Implications for Adoptive Therapies

Human cytomegalovirus (HCMV) is a ubiquitous virus that causes chronic infection and, thus, is one of the most common infectious complications of immune suppression. Adoptive transfer of HCMV-specific T cells has emerged as an effective method to reduce the risk for HCMV infection and/or reactivation by restoring immunity in transplant recipients. However, the CMV-specific CD8⁺ T cell response is comprised of a heterogenous mixture of subsets with distinct functions and localization, and it is not clear if current adoptive immunotherapy protocols can reconstitute the full spectrum of CD8⁺ T cell immunity. The aim of this review is to briefly summarize the role of these T cell subsets in CMV immunity and to describe how current adoptive immunotherapy practices might affect their reconstitution in patients. The bulk of the CMV-specific CD8⁺ T cell population is made up of terminally differentiated effector T cells with immediate effector function and a short life span. Self-renewing memory T cells within the CMV-specific population retain the capacity to expand and differentiate upon challenge and are important for the long-term persistence of the CD8⁺ T cell response. Finally, mucosal organs, which are frequent sites of CMV reactivation, are primarily inhabited by tissue-resident memory T cells, which do not recirculate. Future work on adoptive transfer strategies may need to focus on striking a balance between the formation of these subsets to ensure the development of long lasting and protective immune responses that can access the organs affected by CMV disease.

Interferon-dependent immunoproteasome activity during mouse adenovirus type 1 infection

The immunoproteasome is an inducible host mechanism that aids in the clearance of damaged proteins. The immunoproteasome also influences immune function by enhancing peptide presentation by MHC class I and promotes inflammation via IκB degradation and activation of NF-κB. We used mouse adenovirus type 1 (MAV-1) to characterize the role of the immunoproteasome in adenovirus pathogenesis. Following intranasal infection of mice, immunoproteasome activity in the heart and lung was significantly increased in an IFN-γ-dependent manner. Absence of the β5i immunoproteasome subunit and pharmacological inhibition of β5i activity had minimal effects on viral replication, virus-induced cellular inflammation, or induction of cytokine expression. Likewise, the establishment of protective immunity following primary infection was not significantly altered by β5i deficiency. Thus, although immunoproteasome activity is robustly induced during acute infection with MAV-1, our data suggest that other mechanisms are capable of compensating for immunoproteasome activity to maintain antiviral immunity and appropriate inflammatory responses.

Proteasome function shapes innate and adaptive immune responses

The proteasome system degrades more than 80% of intracellular proteins into small peptides. Accordingly, the proteasome is involved in many essential cellular functions, such as protein quality control, transcription, immune responses, cell signaling, and apoptosis. Moreover, degradation products are loaded onto major histocompatibility class I molecules to communicate the intracellular protein composition to the immune system. The standard 20S proteasome core complex contains three distinct catalytic active sites that are exchanged upon stimulation with inflammatory cytokines to form the so-called immunoproteasome. Immunoproteasomes are constitutively expressed in immune cells and have different proteolytic activities compared with standard proteasomes. They are rapidly induced in parenchymal cells upon intracellular pathogen infection and are crucial for priming effective CD8(+) T-cell-mediated immune responses against infected cells. Beyond shaping these adaptive immune reactions, immunoproteasomes also regulate the function of immune cells by degradation of inflammatory and immune mediators. Accordingly, they emerge as novel regulators of innate immune responses. The recently unraveled impairment of immunoproteasome function by environmental challenges and by genetic variations of immunoproteasome genes might represent a currently underestimated risk factor for the development and progression of lung diseases. In particular, immunoproteasome dysfunction will dampen resolution of infections, thereby promoting exacerbations, may foster autoimmunity in chronic lung diseases, and possibly contributes to immune evasion of tumor cells. Novel pharmacological tools, such as site-specific inhibitors of the immunoproteasome, as well as activity-based probes, however, hold promises as innovative therapeutic drugs for respiratory diseases and biomarker profiling, respectively.

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.

Modification of Antigen Impacts on Memory Quality after Adenovirus Vaccination

The establishment of robust T cell memory is critical for the development of novel vaccines for infections and cancers. Classical memory generated by CD8(+)T cells is characterized by contracted populations homing to lymphoid organs. T cell memory inflation, as seen for example after CMV infection, is the maintenance of expanded, functional, tissue-associated effector memory cell pools. Such memory pools may also be induced after adenovirus vaccination, and we recently defined common transcriptional and phenotypic features of these populations in mice and humans. However, the rules that govern which epitopes drive memory inflation compared with classical memory are not fully defined, and thus it is not currently possible to direct this process. We used our adenoviral model of memory inflation to first investigate the role of the promoter and then the role of the epitope context in determining memory formation. Specifically, we tested the hypothesis that conventional memory could be converted to inflationary memory by simple presentation of the Ag in the form of minigene vectors. When epitopes from LacZ and murine CMV that normally induce classical memory responses were presented as minigenes, they induced clear memory inflation. These data demonstrate that, regardless of the transgene promoter, the polypeptide context of a CD8(+)T cell epitope may determine whether classical or inflating memory responses are induced. The ability to direct this process by the use of minigenes is relevant to the design of vaccines and understanding of immune responses to pathogens.

Adenoviral Vector Vaccination Induces a Conserved Program of CD8(+) T Cell Memory Differentiation in Mouse and Man

Following exposure to vaccines, antigen-specific CD8⁺ T cell responses develop as long-term memory pools. Vaccine strategies based on adenoviral vectors, e.g., those developed for HCV, are able to induce and sustain substantial CD8⁺ T cell populations. How such populations evolve following vaccination remains to be defined at a transcriptional level. We addressed the transcriptional regulation of divergent CD8⁺ T cell memory pools induced by an adenovector encoding a model antigen (beta-galactosidase). We observe transcriptional profiles that mimic those following infection with persistent pathogens, murine and human cytomegalovirus (CMV). Key transcriptional hallmarks include upregulation of homing receptors and anti-apoptotic pathways, driven by conserved networks of transcription factors, including T-bet. In humans, an adenovirus vaccine induced similar CMV-like phenotypes and transcription factor regulation. These data clarify the core features of CD8⁺ T cell memory following vaccination with adenovectors and indicate a conserved pathway for memory development shared with persistent herpesviruses.

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Adenovector vaccination induces two transcriptionally distinct CD8 memory responses

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The sustained response induced by adenovectors and CMV is closely related

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The core molecular features are shared tightly in mouse and man

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Adenovaccines in humans induce a CD8 response that recapitulates these core features

Regulation of immunoproteasome function in the lung

Impaired immune function contributes to the development of chronic obstructive pulmonary disease (COPD). Disease progression is further exacerbated by pathogen infections due to impaired immune responses. Elimination of infected cells is achieved by cytotoxic CD8⁺  T cells that are activated by MHC I-mediated presentation of pathogen-derived antigenic peptides. The immunoproteasome, a specialized form of the proteasome, improves generation of antigenic peptides for MHC I presentation thereby facilitating anti-viral immune responses. However, immunoproteasome function in the lung has not been investigated in detail yet. In this study, we comprehensively characterized the function of immunoproteasomes in the human and murine lung. Parenchymal cells of the lung express low constitutive levels of immunoproteasomes, while they are highly and specifically expressed in alveolar macrophages. Immunoproteasome expression is not altered in whole lung tissue of COPD patients. Novel activity-based probes and native gel analysis revealed that immunoproteasome activities are specifically and rapidly induced by IFNγ treatment in respiratory cells in vitro and by virus infection of the lung in mice. Our results suggest that the lung is potentially capable of mounting an immunoproteasome-mediated efficient adaptive immune response to intracellular infections.

The immunoproteasome and viral infection: a complex regulator of inflammation

During viral infection, proper regulation of immune responses is necessary to ensure successful viral clearance with minimal host tissue damage. Proteasomes play a crucial role in the generation of antigenic peptides for presentation on MHC class I molecules, and thus activation of CD8 T cells, as well as activation of the NF-κB pathway. A specialized type of proteasome called the immunoproteasome is constitutively expressed in hematopoietic cells and induced in non-immune cells during viral infection by interferon signaling. The immunoproteasome regulates CD8 T cell responses to many viral epitopes during infection. Accumulating evidence suggests that the immunoproteasome may also contribute to regulation of proinflammatory cytokine production, activation of the NF-κB pathway, and management of oxidative stress. Many viruses have mechanisms of interfering with immunoproteasome function, including prevention of transcriptional upregulation of immunoproteasome components as well as direct interaction of viral proteins with immunoproteasome subunits. A better understanding of the role of the immunoproteasome in different cell types, tissues, and hosts has the potential to improve vaccine design and facilitate the development of effective treatment strategies for viral infections.

Memory T cells specific for murine cytomegalovirus re-emerge after multiple challenges and recapitulate immunity in various adoptive transfer scenarios

Reconstitution of CMV-specific immunity after transplant remains a primary clinical objective to prevent CMV disease, and adoptive immunotherapy of CMV-specific T cells can be an effective therapeutic approach. Because of viral persistence, most CMV-specific CD8(+) T cells become terminally differentiated effector phenotype CD8(+) T cells (TEFF). A minor subset retains a memory-like phenotype (memory phenotype CD8(+) T cells [TM]), but it is unknown whether these cells retain memory function or persist over time. Interestingly, recent studies suggest that CMV-specific CD8(+) T cells with different phenotypes have different abilities to reconstitute sustained immunity after transfer. The immunology of human CMV infections is reflected in the murine CMV (MCMV) model. We found that human CMV- and MCMV-specific T cells displayed shared genetic programs, validating the MCMV model for studies of CMV-specific T cells in vivo. The MCMV-specific TM population was stable over time and retained a proliferative capacity that was vastly superior to TEFF. Strikingly, after transfer, TM established sustained and diverse T cell populations even after multiple challenges. Although both TEFF and TM could protect Rag(-/-) mice, only TM persisted after transfer into immune replete, latently infected recipients and responded if recipient immunity was lost. Interestingly, transferred TM did not expand until recipient immunity was lost, supporting that competition limits the Ag stimulation of TM. Ultimately, these data show that CMV-specific TM retain memory function during MCMV infection and can re-establish CMV immunity when necessary. Thus, TM may be a critical component for consistent, long-term adoptive immunotherapy success.