Impact of epitope escape on PD-1 expression and CD8 T-cell exhaustion during chronic infection

Replication-attenuated r3LCMV vectors potentiate tumor control via IFN-I

Viral vectors are being used for the treatment of cancer. Yet their efficacy varies among tumors and their use poses challenges in immunosuppressed patients, underscoring the need for alternatives. We report striking antitumoral effects by a nonlytic viral vector based on attenuated lymphocytic choriomeningitis virus (r3LCMV). We show in multiple tumor models that injection of tumor-bearing mice with this novel vector results in improved tumor control and survival. Importantly, r3LCMV also improved tumor control in immunodeficient Rag1-/- mice. Single cell RNA-Seq analyses, antibody blockade experiments, and KO models revealed a critical role for host IFN-I in the antitumoral efficacy of r3LCMV vectors. Collectively, these data demonstrate potent antitumoral effects by a replication-attenuated LCMV vector and unveil mechanisms underlying its antitumoral efficacy.

CD8+ T cells in the cancer-immunity cycle

CD8+ T cells are end effectors of cancer immunity. Most forms of effective cancer immunotherapy involve CD8+ T cell effector function. Here, we review the current understanding of T cell function in cancer, focusing on key CD8+ T cell subtypes and states. We discuss factors that influence CD8+ T cell differentiation and function in cancer through a framework that incorporates the classic three-signal model and a fourth signal-metabolism-and also consider the impact of the tumor microenvironment from a T cell perspective. We argue for the notion of immunotherapies as "pro-drugs" that act to augment or modulate T cells, which ultimately serve as the drug in vivo, and for the importance of overall immune health in cancer treatment and prevention. The progress in understanding T cell function in cancer has and will continue to improve harnessing of the immune system across broader tumor types to benefit more patients.

In Vitro Modeling of CD8 T Cell Exhaustion Enables CRISPR Screening to Reveal a Role for BHLHE40

Identifying novel molecular mechanisms of exhausted CD8 T cells (T ex ) is a key goal of improving immunotherapy of cancer and other diseases. However, high-throughput interrogation of in vivo T ex can be costly and inefficient. In vitro models of T ex are easily customizable and quickly generate high cellular yield, offering an opportunity to perform CRISPR screening and other high-throughput assays. We established an in vitro model of chronic stimulation and benchmarked key phenotypic, functional, transcriptional, and epigenetic features against bona fide in vivo T ex . We leveraged this model of in vitro chronic stimulation in combination with pooled CRISPR screening to uncover transcriptional regulators of T cell exhaustion. This approach identified several transcription factors, including BHLHE40. In vitro and in vivo validation defined a role for BHLHE40 in regulating a key differentiation checkpoint between progenitor and intermediate subsets of T ex . By developing and benchmarking an in vitro model of T ex , we demonstrate the utility of mechanistically annotated in vitro models of T ex , in combination with high-throughput approaches, as a discovery pipeline to uncover novel T ex biology.

Longitudinal Analysis of the Phenotype, Transcriptional Profile, and Anatomic Location of Memory CD8 T Cell Subsets after Acute Viral Infection

Increased demand for novel, highly effective vaccination strategies necessitates a better understanding of long-lived memory CD8 T cell differentiation. To achieve this understanding, we used the mouse model of acute lymphocytic choriomeningitis virus (LCMV) infection. We reexamined classical memory CD8 T cell subsets and performed in-depth, longitudinal analysis of their phenotype, transcriptional programming, and anatomic location within the spleen. All analyses were performed at multiple time points from 8 days to 1 year postinfection. Memory subsets are conventionally defined by their expression of KLRG1 and IL-7Rα, as follows: KLRG1⁺IL-7Rα^- terminal effectors (TEs) and KLRG1^-IL-7Rα⁺ memory precursors (MPs). But we also characterized a third KLRG1⁺IL-7Rα⁺ subset which we refer to as KLRG1⁺ MPs. In these analyses, we defined a comprehensive memory phenotype that is associated with higher levels of CD28 expression. We also demonstrated that MPs, KLRG1⁺ MPs, and TEs have distinct localization programs within the spleen. We found that MPs became preferentially enriched in the white pulp as early as 1 to 2 weeks postinfection, and their predominance in the white pulp was maintained throughout the course of a year. On the other hand, KLRG1⁺ MPs and TEs localized to the red pulp just as early, and they consistently localized to the red pulp thereafter. These findings indicate that location may be crucial for memory formation and that white pulp-derived signals may contribute to long-term memory survival. Achieving robust memory responses following vaccination may require more deliberate consideration of which memory phenotypes are induced, as well as where they traffic, as these factors could impact their longevity. IMPORTANCE CD8 T cells play a critical role in viral immunity and it is important to understand how memory cells are formed and what processes lead to their long-term maintenance. Here, we use a mouse model of acute infection to perform an in-depth, longitudinal analysis of memory CD8 T cell differentiation, examining the phenotype and location of memory cells out to 1 year postinfection.

Anti-coronavirus vaccines will not accelerate the transition of humanity to a non-pandemic period, but the pandemic will take fewer victims

The vaccination rate worldwide has reached enormous proportions, and it is likely that at least 75% of the world's population will be vaccinated. The controversy is that, while people aged 65 and older suffer a significantly higher mortality rate from COVID-19, plans are being made to vaccinate young people under the age of 20. Equally thorny is the question of vaccinating people who already have antibodies to SARS-CoV-2, as well as B and T memory cells, because they contracted and survived the virus. The possible consequences of large-scale vaccination are difficult to predict, when some people do not have access to the vaccine at all and others have already received 3 doses of the vaccine. SARS-CoV-2 will circulate through the human population forever and continue to mutate, as viruses do. Therefore, in the coming years, the need to develop and use effective vaccines and medicines for the prevention and treatment of COVID-19 will remain urgent in view of the high mortality rate from this disease. To date, three vaccine platforms have been most used: adenoviral vector, inactivated, and mRNA. There is some concern about the side effects that occur after vaccination. Whether modern anti-coronavirus vaccines can raise the safety threshold, only time will answer. It is obvious that the pandemic will end, but the virus will remain in the human population, leaving behind invaluable experience and tens of millions of victims. This article is based on search retrieves in research articles devoted to COVID-19 mainly published in 2020-2021 and examines the possible consequences of the worldwide vaccination against SARS-CoV-2 and suggests that, while anti-coronavirus vaccines will not magically transport humanity to a non-pandemic world, they may greatly reduce the number of victims of the pandemic and help us learn how to live with COVID-19.

Brd4 Regulates the Homeostasis of CD8+ T-Lymphocytes and Their Proliferation in Response to Antigen Stimulation

CD8+ T cells are major components of adaptive immunity and confer robust protective cellular immunity, which requires adequate T-cell numbers, targeted migration, and efficient T-cell proliferation. Altered CD8+ T-cell homeostasis and impaired proliferation result in dysfunctional immune response to infection or tumorigenesis. However, intrinsic factors controlling CD8+ T-cell homeostasis and immunity remain largely elusive. Here, we demonstrate the prominent role of Brd4 on CD8+ T cell homeostasis and immune response. By upregulating Myc and GLUT1 expression, Brd4 facilitates glucose uptake and energy production in mitochondria, subsequently supporting naïve CD8+ T-cell survival. Besides, Brd4 promotes the trafficking of naïve CD8+ T cells partially through maintaining the expression of homing receptors (CD62L and LFA-1). Furthermore, Brd4 is required for CD8+ T cell response to antigen stimulation, as Brd4 deficiency leads to a severe defect in clonal expansion and terminal differentiation by decreasing glycolysis. Importantly, as JQ1, a pan-BRD inhibitor, severely dampens CD8+ T-cell immune response, its usage as an anti-tumor agent or latency-reversing agent for human immunodeficiency virus type I (HIV-1) should be more cautious. Collectively, our study identifies a previously-unexpected role of Brd4 in the metabolic regulation of CD8+ T cell-mediated immune surveillance and also provides a potential immunomodulation target.

Single-cell profiling identifies impaired adaptive NK cells expanded after HCMV reactivation in haploidentical HSCT

Haploidentical hematopoietic stem cell transplantation (h-HSCT) represents an efficient curative approach for patients affected by hematologic malignancies in which the reduced intensity conditioning induces a state of immunologic tolerance between donor and recipient. However, opportunistic viral infections greatly affect h-HSCT clinical outcomes. NK cells are the first lymphocytes that recover after transplant and provide a prompt defense against human cytomegalovirus (HCMV) infection/reactivation. By undertaking a longitudinal single-cell computational profiling of multiparametric flow cytometry, we show that HCMV accelerates NK cell immune reconstitution together with the expansion of CD158b1b2j^pos/NKG2A^neg/NKG2C^pos/NKp30^lo NK cells. The frequency of this subset correlates with HCMV viremia, further increases in recipients experiencing multiple episodes of viral reactivations, and persists for months after the infection. The transcriptional profile of FACS-sorted CD158b1b2j^pos NK cells confirmed the ability of HCMV to deregulate NKG2C, NKG2A, and NKp30 gene expression, thus inducing the expansion of NK cells with adaptive traits. These NK cells are characterized by the downmodulation of several gene pathways associated with cell migration, the cell cycle, and effector-functions, as well as by a state of metabolic/cellular exhaustion. This profile reflects the functional impairments of adaptive NK cells to produce IFN-γ, a phenomenon also due to the viral-induced expression of lymphocyte-activation gene 3 (LAG-3) and programmed cell death protein 1 (PD-1) checkpoint inhibitors.

Characterization of CD8 T Cell-Mediated Mutations in the Immunodominant Epitope GP33-41 of Lymphocytic Choriomeningitis Virus

Cytotoxic T lymphocytes (CTLs) represent key immune effectors of the host response against chronic viruses, due to their cytotoxic response to virus-infected cells. In response to this selection pressure, viruses may accumulate escape mutations that evade CTL-mediated control. To study the emergence of CTL escape mutations, we employed the murine chronic infection model of lymphocytic choriomeningitis virus (LCMV). We developed an amplicon-based next-generation sequencing pipeline to detect low frequency mutations in the viral genome and identified non-synonymous mutations in the immunodominant LCMV CTL epitope, GP33-41, in infected wildtype mice. Infected Rag2-deficient mice lacking CTLs did not contain such viral mutations. By using transgenic mice with T cell receptors specific to GP33-41, we characterized the emergence of viral mutations in this epitope under varying selection pressure. We investigated the two most abundant viral mutations by employing reverse genetically engineered viral mutants encoding the respective mutations. These experiments provided evidence that these mutations prevent activation and expansion of epitope-specific CD8 T cells. Our findings on the mutational dynamics of CTL escape mutations in a widely-studied viral infection model contributes to our understanding of how chronic viruses interact with their host and evade the immune response. This may guide the development of future treatments and vaccines against chronic infections.

Reversing T-cell exhaustion in immunotherapy: a review on current approaches and limitations

Introduction:T cell functions are altered during chronic viral infections and tumor development. This is mainly manifested by significant changes in T cells' epigenetic and metabolic landscapes, pushing them into an 'exhausted' state. Reversing this T cell exhaustion has been emerging as a 'game-changing' therapeutic approach against cancer and chronic viral infection.Areas covered:This review discusses the cellular pathways related to T cell exhaustion, and the clinical development and possible cellular targets that can be exploited therapeutically to reverse this exhaustion. We searched various databases (e.g. Google Scholar, PubMed, Elsevier, and other scientific database sites) using the keywords T cell exhaustion, T cell activation, co-inhibitory receptors, and reversing T cell exhaustion.Expert opinion:The discovery of the immune checkpoints pathways represents a significant milestone toward understanding and reversing T cell exhaustion. Antibodies that target these pathways have already demonstrated promising activities in reversing T cell exhaustion. Nevertheless, there are still many associated limitations. In this context, next-generation alternatives are on the horizon. This includes the use of small molecules to block the immune checkpoints' receptors, combining them with other treatments, and identifying novel, safer and more effective immunotherapeutic targets.

In vivo CD8+ T cell CRISPR screening reveals control by Fli1 in infection and cancer

Improving effector activity of antigen-specific T cells is a major goal in cancer immunotherapy. Despite the identification of several effector T cell (TEFF)-driving transcription factors (TFs), the transcriptional coordination of TEFF biology remains poorly understood. We developed an in vivo T cell CRISPR screening platform and identified a key mechanism restraining TEFF biology through the ETS family TF, Fli1. Genetic deletion of Fli1 enhanced TEFF responses without compromising memory or exhaustion precursors. Fli1 restrained TEFF lineage differentiation by binding to cis-regulatory elements of effector-associated genes. Loss of Fli1 increased chromatin accessibility at ETS:RUNX motifs, allowing more efficient Runx3-driven TEFF biology. CD8+ T cells lacking Fli1 provided substantially better protection against multiple infections and tumors. These data indicate that Fli1 safeguards the developing CD8+ T cell transcriptional landscape from excessive ETS:RUNX-driven TEFF cell differentiation. Moreover, genetic deletion of Fli1 improves TEFF differentiation and protective immunity in infections and cancer.

Cancer and HIV-1 Infection: Patterns of Chronic Antigen Exposure

The main role of the human immune system is to eliminate cells presenting foreign antigens and abnormal patterns, while maintaining self-tolerance. However, when facing highly variable pathogens or antigens very similar to self-antigens, this system can fail in completely eliminating the anomalies, leading to the establishment of chronic pathologies. Prototypical examples of immune system defeat are cancer and Human Immunodeficiency Virus-1 (HIV-1) infection. In both conditions, the immune system is persistently exposed to antigens leading to systemic inflammation, lack of generation of long-term memory and exhaustion of effector cells. This triggers a negative feedback loop where effector cells are unable to resolve the pathology and cannot be replaced due to the lack of a pool of undifferentiated, self-renewing memory T cells. In addition, in an attempt to reduce tissue damage due to chronic inflammation, antigen presenting cells and myeloid components of the immune system activate systemic regulatory and tolerogenic programs. Beside these homologies shared between cancer and HIV-1 infection, the immune system can be shaped differently depending on the type and distribution of the eliciting antigens with ultimate consequences at the phenotypic and functional level of immune exhaustion. T cell differentiation, functionality, cytotoxic potential and proliferation reserve, immune-cell polarization, upregulation of negative regulators (immune checkpoint molecules) are indeed directly linked to the quantitative and qualitative differences in priming and recalling conditions. Better understanding of distinct mechanisms and functional consequences underlying disease-specific immune cell dysfunction will contribute to further improve and personalize immunotherapy. In the present review, we describe relevant players of immune cell exhaustion in cancer and HIV-1 infection, and enumerate the best-defined hallmarks of T cell dysfunction. Moreover, we highlight shared and divergent aspects of T cell exhaustion and T cell activation to the best of current knowledge.

Glycogen synthase 3 (GSK-3) regulation of PD-1 expression and and its therapeutic implications

The past few years have witnessed exciting progress in the application of immune check-point blockade (ICB) for the treatment of various human cancers. ICB was first used against cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) to demonstrate durable anti-tumor responses followed by ICB against programmed cell death-1 (PD-1) or its ligand, PD-L1. Present approaches involve the use of combinations of blocking antibodies against CTLA-4, PD-1 and other inhibitory receptors (IRs) such as TIM3, TIGIT and LAG3. Despite this success, most patients are not cured by ICB therapy and there are limitations to the use of antibodies including cost, tumor penetration, the accessibility of receptors, and clearance from the cell surface as well as inflammatory and autoimmune complications. Recently, we demonstrated that the down-regulation or inhibition of glycogen synthase kinase 3 (GSK-3) down-regulates PD-1 expression in infectious diseases and cancer (Taylor et al., 2016 Immunity 44, 274-86; 2018 Cancer Research 78, 706-717; Krueger and Rudd 2018 Immunity 46, 529-531). In this Review, we outline the use of small molecule inhibitors (SMIs) that target intracellular pathways for co-receptor blockade in cancer immunotherapy.

iCa2+ Flux, ROS and IL-10 Determines Cytotoxic, and Suppressor T Cell Functions in Chronic Human Viral Infections

Exhaustion of CD8⁺ T cells and increased IL-10 production is well-known in chronic viral infections but mechanisms leading to loss of their cytotoxic capabilities and consequent exhaustion remain unclear. Exhausted CD8⁺T cells also called T suppressors are highly immune suppressive with altered T cell receptor signaling characteristics that mark it exclusively from their cytotoxic counterparts. Our study found that iCa²⁺ flux is reduced following T cell receptor activation in T suppressor cells when compared to their effector counterpart. Importantly chronic activation of murine cytotoxic CD8⁺ T cells lead to reduced iCa²⁺ influx, decreased IFN-γ and enhanced IL-10 production and this profile is mimicked in Tc1 cells upon reduction of iCa²⁺ flux by extracellular calcium channel inhibitors. Further reduced iCa²⁺ flux induced ROS which lead to IFN-γ reduction and increased IL-10 producing T suppressors through the STAT3—STAT5 axis. The above findings were substantiated by our human data where reduced iCa²⁺ flux in chronic Hepatitis infections displayed CD8⁺ T cells with low IFN-γ and increased IL-10 production. Importantly treatment with an antioxidant led to increased IFN-γ and reduced IL-10 production in human chronic Hep-B/C samples suggesting overall a proximal regulatory role for iCa²⁺ influx, ROS, and IL-10 in determining the effector/ suppressive axis of CD8⁺ T cells.

Antigen-specific immune decoys intercept and exhaust autoimmunity to prevent disease

Relapsing-remitting patterns of many autoimmune diseases such as multiple sclerosis (MS) are perpetuated by a recurring circuit of adaptive immune cells that amplify in secondary lymphoid organs (SLOs) and traffic to compartments where antigen is abundant to elicit damage. Some of the most effective immunotherapies impede the migration of immune cells through this circuit, however, broadly suppressing immune cell migration can introduce life-threatening risks for patients. We developed antigen-specific immune decoys (ASIDs) to mimic tissues targeted in autoimmunity and selectively intercept autoimmune cells to preserve host tissue. Using Experimental Autoimmune Encephalomyelitis (EAE) as a model, we conjugated autoantigen PLP_139-151 to a microporous collagen scaffold. By subcutaneously implanting ASIDs after induction but prior to the onset of symptoms, mice were protected from paralysis. ASID implants were rich with autoimmune cells, however, reactivity to cognate antigen was substantially diminished and apoptosis was prevalent. ASID-implanted mice consistently exhibited engorged spleens when disease normally peaked. In addition, splenocyte antigen-presenting cells were highly activated in response to PLP rechallenge, but CD3⁺ and CD19 ⁺ effector subsets were significantly decreased, suggesting exhaustion. ASID-implanted mice never developed EAE relapse symptoms even though the ASID material had long since degraded, suggesting exhausted autoimmune cells did not recover functionality. Together, data suggested ASIDs were able to sequester and exhaust immune cells in an antigen-specific fashion, thus offering a compelling approach to inhibit the migration circuit underlying autoimmunity.

A dynamical motif comprising the interactions between antigens and CD8 T cells may underlie the outcomes of viral infections

Some viral infections culminate in very different outcomes in different individuals. They can be rapidly cleared in some, cause persistent infection in others, and cause mortality from immunopathology in yet others. The conventional view is that the different outcomes arise as a consequence of the complex interactions between a large number of different factors (virus, different immune cells, and cytokines). Here, we identify a simple dynamical motif comprising the essential interactions between antigens and CD8 T cells and posit it as predominantly determining the outcomes. Viral antigen can activate CD8 T cells, which in turn, can kill infected cells. Sustained antigen stimulation, however, can cause CD8 T-cell exhaustion, compromising effector function. Using mathematical modeling, we show that the motif comprising these interactions recapitulates all of the outcomes observed. The motif presents a conceptual framework to understand the variable outcomes of infection. It also explains a number of confounding experimental observations, including the variation in outcomes with the viral inoculum size, the evolutionary advantage of exhaustion in preventing lethal pathology, the ability of natural killer (NK) cells to act as rheostats tuning outcomes, and the role of the innate immune response in the spontaneous clearance of hepatitis C. Interventions that modulate the interactions in the motif may present routes to clear persistent infections or limit immunopathology.

TOX transcriptionally and epigenetically programs CD8+ T cell exhaustion

Exhausted CD8+ T (Tex) cells in chronic infections and cancer have limited effector function, high co-expression of inhibitory receptors and extensive transcriptional changes compared with effector (Teff) or memory (Tmem) CD8+ T cells. Tex cells are important clinical targets of checkpoint blockade and other immunotherapies. Epigenetically, Tex cells are a distinct immune subset, with a unique chromatin landscape compared with Teff and Tmem cells. However, the mechanisms that govern the transcriptional and epigenetic development of Tex cells remain unknown. Here we identify the HMG-box transcription factor TOX as a central regulator of Tex cells in mice. TOX is largely dispensable for the formation of Teff and Tmem cells, but it is critical for exhaustion: in the absence of TOX, Tex cells do not form. TOX is induced by calcineurin and NFAT2, and operates in a feed-forward loop in which it becomes calcineurin-independent and sustained in Tex cells. Robust expression of TOX therefore results in commitment to Tex cells by translating persistent stimulation into a distinct Tex cell transcriptional and epigenetic developmental program.

The PD-1/PD-L1 Axis and Virus Infections: A Delicate Balance

Programmed cell death protein (PD-1) and its ligands play a fundamental role in the evasion of tumor cells from antitumor immunity. Less well appreciated is the fact that the PD-1/PD-L1 axis also regulates antiviral immune responses and is therefore modulated by a number of viruses. Upregulation of PD-1 and its ligands PD-L1 and PD-L2 is observed during acute virus infection and after infection with persistent viruses including important human pathogens such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and hepatitis B virus (HBV). Experimental evidence suggests that insufficient signaling through the PD-1 pathway promotes immunopathology during acute infection by exaggerating primary T cell responses. If chronic infection is established, however, high levels of PD-1 expression can have unfavorable immunological consequences. Exhaustion and suppression of antiviral immune responses can result in viral immune evasion. The role of the PD-1/PD-L1 axis during viral infections is further complicated by evidence that PD-L1 also mediates inflammatory effects in the acute phase of an immune response. In this review, we discuss the intricate interplay between viruses and the PD-1/PD-L1 axis.

CD8+ T cell exhaustion

CD8⁺ T cells are important for the protective immunity against intracellular pathogens and tumor. In the case of chronic infection or cancer, CD8⁺ T cells are exposed to persistent antigen and/or inflammatory signals. This excessive amount of signals often leads CD8⁺ T cells to gradual deterioration of T cell function, a state called "exhaustion." Exhausted T cells are characterized by progressive loss of effector functions (cytokine production and killing function), expression of multiple inhibitory receptors (such as PD-1 and LAG3), dysregulated metabolism, poor memory recall response, and homeostatic proliferation. These altered functions are closely related with altered transcriptional program and epigenetic landscape that clearly distinguish exhausted T cells from normal effector and memory T cells. T cell exhaustion is often associated with inefficient control of persisting infections and cancers, but re-invigoration of exhausted T cells with inhibitory receptor blockade can promote improved immunity and disease outcome. Accumulating evidences support the therapeutic potential of targeting exhausted T cells. However, exhausted T cells comprise heterogenous cell population with distinct responsiveness to intervention. Understanding molecular mechanism of T cell exhaustion is essential to establish rational immunotherapeutic interventions.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

Multifactorial heterogeneity of virus-specific T cells and association with the progression of human chronic hepatitis B infection

Associations between chronic antigen stimulation, T cell dysfunction, and the expression of various inhibitory receptors are well characterized in several mouse and human systems. During chronic hepatitis B virus (HBV) infection (CHB), T cell responses are blunted with low frequencies of virus-specific T cells observed, making these parameters difficult to study. Here, using mass cytometry and a highly multiplexed combinatorial peptide–major histocompatibility complex (pMHC) tetramer strategy that allows for the detection of rare antigen-specific T cells, we simultaneously probed 484 unique HLA-A*1101–restricted epitopes spanning the entire HBV genome on T cells from patients at various stages of CHB. Numerous HBV-specific T cell populations were detected, validated, and profiled. T cells specific for two epitopes (HBVpol387and HBVcore169) displayed differing and complex heterogeneities that were associated with the disease progression, and the expression of inhibitory receptors on these cells was not linearly related with their extent of T cell dysfunction. For HBVcore169-specific CD8+T cells, we found cellular markers associated with long-term memory, polyfunctionality, and the presence of several previously unidentified public TCR clones that correlated with viral control. Using high-dimensional trajectory analysis of these cellular phenotypes, a pseudo-time metric was constructed that fit with the status of viral infection in corresponding patients. This was validated in a longitudinal cohort of patients undergoing antiviral therapy. Our study uncovers complex relationships of inhibitory receptors between the profiles of antigen-specific T cells and the status of CHB with implications for new strategies of therapeutic intervention.

HCV-Specific T Cell Responses During and After Chronic HCV Infection

Hepatitis C virus (HCV)-specific T cell responses are closely linked to the clinical course of infection. While T cell responses in self-limiting infection are typically broad and multi-specific, they display several distinct features of functional impairment in the chronic phase. Moreover, HCV readily adapts to immune pressure by developing escape mutations within epitopes targeted by T cells. Much of our current knowledge on HCV-specific T cell responses has been gathered under the assumption that this might eventually pave the way for a therapeutic vaccine. However, with the development of highly efficient direct acting antivirals (DAAs), there is less interest in the development of a therapeutic vaccine for HCV and the scope of T cell research has shifted. Indeed, the possibility to rapidly eradicate an antigen that has persisted over years or decades, and has led to T cell exhaustion and dysfunction, provides the unique opportunity to study potential T cell recovery after antigen cessation in a human in vivo setting. Findings from such studies not only improve our basic understanding of T cell immunity but may also advance immunotherapeutic approaches in cancer or chronic hepatitis B and D infection. Moreover, in order to edge closer to the WHO goal of HCV elimination by 2030, a prophylactic vaccine is clearly required. Thus, in this review, we will summarize our current knowledge on HCV-specific T cell responses and also provide an outlook on the open questions that require answers in this field.

T cell receptor sequencing of activated CD8 T cells in the blood identifies tumor-infiltrating clones that expand after PD-1 therapy and radiation in a melanoma patient

PD-1-targeted therapy has dramatically changed advanced cancer treatment. However, many questions remain, including specificity of T cells activated by PD-1 therapy and how peripheral blood analysis correlates to effects at tumor sites. In this study, we utilized TCR sequencing to dissect the composition of peripheral blood CD8 T cells activated upon therapy, comparing it with tumor-infiltrating lymphocytes. We report on a nonagenarian melanoma patient who showed a prominent increase in peripheral blood Ki-67 + CD8 T cells following brain stereotactic radiation and anti-PD-1 immunotherapy. Proliferating CD8 T cells exhibited an effector-like phenotype with expression of CD38, HLA-DR and Granzyme B, as well as expression of the positive costimulatory molecules CD28 and CD27. TCR sequencing of peripheral blood CD8 T cells revealed a highly oligoclonal repertoire at baseline with one clonotype accounting for 30%. However, the majority of dominant clones-including a previously identified cytomegalovirus-reactive clone-did not expand following treatment. In contrast, expanding clones were present at low frequencies in the peripheral blood but were enriched in a previously resected liver metastasis. The patient has so far remained recurrence-free for 36 months, and several CD8 T cell clones that expanded after treatment were maintained at elevated levels for at least 8 months. Our data show that even in a nonagenarian individual with oligoclonal expansion of CD8 T cells, we can identify activation of tumor-infiltrating CD8 T cell clones in peripheral blood following anti-PD-1-based immunotherapies.

Comparative Proliferation Capacity of Gag-C-Specific Naive and Memory CD4+ and CD8+ T Lymphocytes in Rapid, Viremic Slow, and Slow Progressors During Human Immunodeficiency Virus Infection

The exact cause of altered dynamics in T cells compartment during HIV infection remains elusive to date. In this longitudinal study, the proliferation frequency of different T cell subsets was investigated in untreated HIV-1-infected Indian individuals stratified as rapid (R), viremic slow (VS), slow (S) progressors, and healthy controls. Ten healthy and 20 treatment-naive HIV-1-infected individuals were enrolled. Expression of Ki67 nuclear antigen was examined on HIV-specific T cell subsets in peripheral blood lymphocytes. Upon stimulation with HIV-1 Gag-C peptide pools, effector memory (EM) CD4 T cells (R vs. S, EM CD4, p < 0.05) of R progressors proliferated significantly compared with those of S progressors at baseline. However, central memory (CM) CD8 T cell subsets proliferated significantly in VS and S progressors compared with those in R progressors, wherein highest proliferation frequency of EM CD8 T cells was observed. At follow-up visit, the proliferation frequency of naive CD8 T cells was significantly higher in R progressors than S progressors (R vs. S naive CD8, p < 0.05). The findings suggest altered dynamics of different CD4+ and CD8+ T cell subsets in R, VS, and S progressors. The increase in CM T cell proliferation in VS and S progressors could be attributed to slower progression of the HIV infection. Hence, treatment strategies must be focused on restoring the homeostatic balance to restore T cell functionality.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

Long-Term Persistence of Exhausted CD8 T Cells in Chronic Infection Is Regulated by MicroRNA-155

Persistent viral infections and tumors drive development of exhausted T (TEX) cells. In these settings, TEX cells establish an important host-pathogen or host-tumor stalemate. However, TEX cells erode over time, leading to loss of pathogen or cancer containment. We identified microRNA (miR)-155 as a key regulator of sustained TEX cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection. Genetic deficiency of miR-155 ablated CD8 T cell responses during chronic infection. Conversely, enhanced miR-155 expression promoted expansion and long-term persistence of TEX cells. However, rather than strictly antagonizing exhaustion, miR-155 promoted a terminal TEX cell subset. Transcriptional profiling identified coordinated control of cell signaling and transcription factor pathways, including the key AP-1 family member Fosl2. Overexpression of Fosl2 reversed the miR-155 effects, identifying a link between miR-155 and the AP-1 transcriptional program in regulating TEX cells. Thus, we identify a mechanism of miR-155 regulation of TEX cells and a key role for Fosl2 in T cell exhaustion.

The function and dysfunction of memory CD8+ T cells in tumor immunity

The generation and maintenance of CD8+ T cell memory is crucial to long-term host survival, yet the basic tenets of CD8+ T cell immunity are still being established. Recent work has led to the discovery of tissue-resident memory cells and refined our understanding of the transcriptional and epigenetic basis of CD8+ T cell differentiation and dysregulation. In parallel, the unprecedented clinical success of immunotherapy has galvanized an intense, global research effort to decipher and de-repress the anti-tumor response. However, the progress of immunotherapy is at a critical juncture, since the efficacy of immuno-oncology agents remains confined to a fraction of patients and often fails to provide durable benefit. Unlocking the potential of immunotherapy requires the design of strategies that both induce a potent effector response and reliably forge stable, functional memory T cell pools capable of protecting from recurrence or relapse. It is therefore essential that basic and emerging concepts of memory T cell biology are rapidly and faithfully transposed to advance therapeutic development in cancer immunotherapy. This review highlights seminal and recent reports in CD8+ T cell memory and tumor immunology, and evaluates recent data from solid cancer specimens in the context of the key paradigms from preclinical models. We elucidate the potential significance of circulating effector cells poised downstream of neoantigen recognition and upstream of T cell dysfunction and propose that cells in this immunological 'sweet spot' may be key anti-tumor effectors.

Role of PD-1 during effector CD8 T cell differentiation

PD-1 (programmed cell death-1) is the central inhibitory receptor regulating CD8 T cell exhaustion during chronic viral infection and cancer. Interestingly, PD-1 is also expressed transiently by activated CD8 T cells during acute viral infection, but the role of PD-1 in modulating T cell effector differentiation and function is not well defined. To address this question, we examined the expression kinetics and role of PD-1 during acute lymphocytic choriomeningitis virus (LCMV) infection of mice. PD-1 was rapidly up-regulated in vivo upon activation of naive virus-specific CD8 T cells within 24 h after LCMV infection and in less than 4 h after peptide injection, well before any cell division had occurred. This rapid PD-1 expression by CD8 T cells was driven predominantly by antigen receptor signaling since infection with a LCMV strain with a mutation in the CD8 T cell epitope did not result in the increase of PD-1 on antigen-specific CD8 T cells. Blockade of the PD-1 pathway using anti-PD-L1 or anti-PD-1 antibodies during the early phase of acute LCMV infection increased mTOR signaling and granzyme B expression in virus-specific CD8 T cells and resulted in faster clearance of the infection. These results show that PD-1 plays an inhibitory role during the naive-to-effector CD8 T cell transition and that the PD-1 pathway can also be modulated at this stage of T cell differentiation. These findings have implications for developing therapeutic vaccination strategies in combination with PD-1 blockade.

Clonally diverse CD38+HLA-DR+CD8+ T cells persist during fatal H7N9 disease

Severe influenza A virus (IAV) infection is associated with immune dysfunction. Here, we show circulating CD8+ T-cell profiles from patients hospitalized with avian H7N9, seasonal IAV, and influenza vaccinees. Patient survival reflects an early, transient prevalence of highly activated CD38+HLA-DR+PD-1+ CD8+ T cells, whereas the prolonged persistence of this set is found in ultimately fatal cases. Single-cell T cell receptor (TCR)-αβ analyses of activated CD38+HLA-DR+CD8+ T cells show similar TCRαβ diversity but differential clonal expansion kinetics in surviving and fatal H7N9 patients. Delayed clonal expansion associated with an early dichotomy at a transcriptome level (as detected by single-cell RNAseq) is found in CD38+HLA-DR+CD8+ T cells from patients who succumbed to the disease, suggesting a divergent differentiation pathway of CD38+HLA-DR+CD8+ T cells from the outset during fatal disease. Our study proposes that effective expansion of cross-reactive influenza-specific TCRαβ clonotypes with appropriate transcriptome signatures is needed for early protection against severe influenza disease.

Nivolumab Enhances In Vitro Effector Functions of PD-1+ T-Lymphocytes and Leishmania-Infected Human Myeloid Cells in a Host Cell-Dependent Manner

Functional impairment of T-cells and a concomitant augmented expression of programmed death-1 (PD-1) have been observed in visceral leishmaniasis patients, as well as in experimental models for visceral and cutaneous leishmaniasis. The PD-1/PD-1-ligand (PD-1/PD-L) interaction negatively regulates T-cell effector functions, which are required for parasite control during leishmaniasis. The aim of this study was to elucidate the impact of the PD-1/PD-L axis in a human primary in vitro infection model of Leishmania major (Lm). Blocking the PD-1/PD-L interaction with nivolumab increased T-cell proliferation and release of the proinflammatory cytokines TNFα and IFNγ during the cocultivation of Lm-infected human monocyte-derived macrophages (hMDMs) or dendritic cells (hMDDC) with autologous PD-1⁺-lymphocytes. As a consequence Lm infection decreased, being the most pronounced in hMDDC, compared to proinflammatory hMDM1 and anti-inflammatory hMDM2. Focusing on hMDDC, we could partially reverse effects mediated by PD-1 blockade by neutralizing TNFα but not by neutralizing IFNγ. Furthermore, PD-1 blockade increased intracellular expression of perforin, granulysin, and granzymes in proliferating CD4⁺-T-cells, which might be implicated in reduction of Lm-infected cells. In all, our data describe an important role for the PD-1/PD-L axis upon Lm infection using a human primary cell system. These data contribute to a better understanding of the PD-1-induced T-cell impairment during disease and its influence on immune effector mechanisms to combat Lm infection.

Immune checkpoints in chronic obstructive pulmonary disease

Cell-mediated immune responses are vital to the body's defence against infection and play a key role in tumour immunity. T-cell activation and cytotoxic function is tightly regulated by a series of immune-regulatory receptor-ligand interactions or immune checkpoints. These controls limit immune-mediated damage, particularly in the context of chronic infection. However, prolonged signalling through these axes can lead to progressive loss of T-cell function, termed exhaustion.Understanding of the biology of checkpoints and that exhaustion is reversible has been key to the development of new therapies directed at reversing the dysfunctional status of T-cells, which are dramatically improving outcomes of cancer treatment.Emerging data suggest that immune checkpoint axes are dysregulated in chronic obstructive pulmonary disease (COPD). T-cells from diseased lungs express the key receptor programmed death (PD)1 and demonstrate loss of cytotoxic function. However, the picture is complex with evidence of downregulation of the associated ligand PDL1 on alveolar macrophages. The resulting impact may be excessive T-cell inflammation as a consequence of acute infection, which may contribute to the pattern of exacerbation and lung damage characteristic of COPD. More work is needed to understand these immune controls in COPD before the therapeutic advances seen in lung cancer can be explored.

PD-1 and cancer: molecular mechanisms and polymorphisms

The programmed cell death protein 1 (PD-1) is expressed by activated T cells that act as an immunoregulatory molecule, and are responsible for the negative regulation of T cell activation and peripheral tolerance. The PD-1 gene also encodes an inhibitory cell surface receptor involved in the regulation of T cell functions during immune responses/tolerance. Beyond potent inhibitory effects on T cells, PD-1 also has a role in regulating B cell and monocyte responses. An overexpression of PD-1 has been reported to contribute to immune system avoidance in different cancers. In particular, PD-1 over-expression influences tumor-specific T cell immunity in a cancer microenvironment. Blocking the PD-1/PD-1 ligand (PD-L1) pathway could potentially augment endogenous antitumor responses. Along these lines, the use of PD-1/PD-L1 inhibitors has been applied in clinical trials against diverse forms of cancer. It was believed that antibodies targeting PD-1/PD-L1 might synergize with other treatments that enhance endogenous antitumor immunity by blocking inhibitory receptor-ligand interactions. However, in all cases, the host genetic status (as well as that of the tumor) is likely to have an impact on the expected outcomes. Various investigations have evaluated the association between PD-1 polymorphisms and the risk of various types of cancer. Frequently studied PD-1 polymorphisms, PD-1.1 (rs36084323), PD-1.3 (rs11568821), PD-1.5 (rs2227981), PD-1.9 (rs2227982), and PD-1 rs7421861, and their associations in the risk of susceptibility to different types of cancer are mentioned in this review, as are studies highlighting the significance of conducting genetic association studies in different ethnic populations.

Therapeutic efficacy of combined vaccination against tumor pericyte-associated antigens DLK1 and DLK2 in mice

When compared with vascular cells in normal tissues, pericytes and vascular endothelial cells (VEC) in tumor blood vessels exhibit altered morphology and epigenetic programming that leads to the expression of unique antigens that allow for differential recognition by CD8⁺ T cells. We have previously shown that the Notch antagonist delta-like homolog 1 (DLK1) is a tumor pericyte-associated antigen expressed in setting of melanoma and a range of carcinomas. In this report, we show that therapeutic vaccination against DLK1 in murine models results in slowed tumor growth, but also to the compensatory expression of the DLK1 homolog, DLK2, by tumor-associated pericytes. Vaccines targeting both DLK1 and DLK2 resulted in superior antitumor benefits in association with improved activation and recruitment of antigen-specific Type 1 CD8⁺ T cells, reduced presence of myeloid-derived suppressive cells, T regulatory cell and tumor vascular normalization. The antitumor efficacy of vaccines coordinately targeting DLK1 and DLK2 was further improved by inclusion of PD-L1 blockade, thus defining a combination immunotherapy theoretically suitable for the treatment of a broad range of solid (vascularized) cancers.

Decreased PD-1 Expression on CD8 Lymphocyte Subsets and Increase in CD8 Tscm Cells in Children with HIV Receiving Raltegravir

We investigated the effect of combination antiretroviral therapy (cART) on immune recovery, particularly on the percentages of PD-1-positive cells within the major leukocyte subsets. Cryopreserved peripheral blood mononuclear cells and plasma samples collected longitudinally from a subset of 13 children and adolescents (between 9.7 and 18.2 years old) who were enrolled in the International Maternal Pediatric Adolescent AIDS Clinical Trials (IMPAACT) P1066 were used for this study. Immunophenotyping by flow cytometry was performed to determine the effect of raltegravir-containing cART regimen on the distribution of leukocyte populations, on the expression of PD-1 on T cell subpopulations, and on the expression of well-established markers of T cell activation (CD38 and HLA-DR) on CD8 T cells. C reactive protein (CRP), lipopolysaccharide (LPS), IL-6, and soluble CD163 were assayed in plasma samples by an enzyme-linked immunosorbent assay. Plasma viral loads were decreased in all subjects (by an average of 2.9 log units). The cART regimen, including raltegravir, induced changes in CD8 T cell subsets, consistent with an effective antiretroviral outcome and improved immunologic status, including increased percentages of CD8 stem cell memory T cells (Tscm). The percentages of CD8 PD-1-positive cells decreased significantly as compared with baseline levels. Among the proinflammatory markers measured in plasma, sCD163 showed a decline that was associated with cART. cART therapy, including raltegravir, over 48 weeks in children is associated with immune restoration, consistent with effective antiretroviral therapy, namely decreased percentages of PD-1⁺ CD8⁺ T cells, an increase in CD8 Tscm cells, and decreased levels of sCD163.

'Off-the-shelf' immunotherapy with iPSC-derived rejuvenated cytotoxic T lymphocytes

Adoptive T-cell therapy to target and kill tumor cells shows promise and induces durable remissions in selected malignancies. However, for most cancers, clinical utility is limited. Cytotoxic T lymphocytes continuously exposed to viral or tumor antigens, with long-term expansion, may become unable to proliferate ("exhausted"). To exploit fully rejuvenated induced pluripotent stem cell (iPSC)-derived antigen-specific cytotoxic T lymphocytes is a potentially powerful approach. We review recent progress in engineering iPSC-derived T cells and prospects for clinical translation. We also describe the importance of introducing a suicide gene safeguard system into adoptive T-cell therapy, including iPSC-derived T-cell therapy, to protect from unexpected events in first-in-humans clinical trials.

Epigenetic stability of exhausted T cells limits durability of reinvigoration by PD-1 blockade

Blocking Programmed Death-1 (PD-1) can reinvigorate exhausted CD8 T cells (T_EX) and improve control of chronic infections and cancer. However, whether blocking PD-1 can reprogram T_EX into durable memory T cells (T_MEM) is unclear. We found that reinvigoration of T_EX in mice by PD-L1 blockade caused minimal memory development. After blockade, reinvigorated T_EX became reexhausted if antigen concentration remained high and failed to become T_MEM upon antigen clearance. T_EX acquired an epigenetic profile distinct from that of effector T cells (T_EFF) and T_MEM cells that was minimally remodeled after PD-L1 blockade. This finding suggests that T_EX are a distinct lineage of CD8 T cells. Nevertheless, PD-1 pathway blockade resulted in transcriptional rewiring and reengagement of effector circuitry in the T_EX epigenetic landscape. These data indicate that epigenetic fate inflexibility may limit current immunotherapies.

Impaired Subset Progression and Polyfunctionality of T Cells in Mice Exposed to Methamphetamine during Chronic LCMV Infection

Methamphetamine (METH) is a widely used psychostimulant that severely impacts the host’s innate and adaptive immune systems and has profound immunological implications. T cells play a critical role in orchestrating immune responses. We have shown recently how chronic exposure to METH affects T cell activation using a murine model of lymphocytic choriomeningitis virus (LCMV) infection. Using the TriCOM (trinary state combinations) feature of GemStone^™ to study the polyfunctionality of T cells, we have analyzed how METH affected the cytokine production pattern over the course of chronic LCMV infection. Furthermore, we have studied in detail the effects of METH on splenic T cell functions, such as cytokine production and degranulation, and how they regulate each other. We used the Probability State Modeling (PSM) program to visualize the differentiation of effector/memory T cell subsets during LCMV infection and analyze the effects of METH on T cell subset progression. We recently demonstrated that METH increased PD-1 expression on T cells during viral infection. In this study, we further analyzed the impact of PD-1 expression on T cell functional markers as well as its expression in the effector/memory subsets. Overall, our study indicates that analyzing polyfunctionality of T cells can provide additional insight into T cell effector functions. Analysis of T cell heterogeneity is important to highlight changes in the evolution of memory/effector functions during chronic viral infections. Our study also highlights the impact of METH on PD-1 expression and its consequences on T cell responses.

Chronic hepatitis B: immune pathogenesis and emerging immunotherapeutics

Hepatitis B virus (HBV) evades, subverts, activates and regulates host immune components, thereby impacting its natural history and disease pathogenesis. Recent advances in our understanding of immune interactions in chronic viral infection and tumor therapy are applicable to chronic hepatitis B (CHB). With recent successes of tumor immunotherapy, there is a renewed interest in exploring immunotherapeutics in achieving sustained and functional cure of chronic hepatitis B. In this review, we discuss aspects of host innate and adaptive immune regulatory and pathogenic responses relevant for HBV infection. We also highlight several immune modulatory approaches in clinical development to treat CHB.

Immunotherapy Expands and Maintains the Function of High-Affinity Tumor-Infiltrating CD8 T Cells In Situ

Cancer cells harbor high-affinity tumor-associated Ags capable of eliciting potent antitumor T cell responses, yet detecting these polyclonal T cells is challenging. Therefore, surrogate markers of T cell activation such as CD69, CD44, and programmed death-1 (PD-1) have been used. We report in this study that in mice, expression of activation markers including PD-1 is insufficient in the tumor microenvironment to identify tumor Ag-specific T cells. Using the Nur77GFP T cell affinity reporter mouse, we highlight that PD-1 expression can be induced independent of TCR ligation within the tumor. Given this, we characterized the utility of the Nur77GFP model system in elucidating mechanisms of action of immunotherapies independent of PD-1 expression. Coexpression of Nur77GFP and OX40 identifies a polyclonal population of high-affinity tumor-associated Ag-specific CD8(+) T cells, which produce more IFN-γ in situ than OX40 negative and doubles in quantity with anti-OX40 and anti-CTLA4 mAb therapy but not with anti-PD-1 or programmed death ligand-1. Moreover, expansion of these high-affinity CD8 T cells prolongs survival of tumor-bearing animals. Upon chronic stimulation in tumors and after adoptive cell therapy, CD8 TCR signaling and Nur77GFP induction is impaired, and tumors progress. However, this can be reversed and overall survival significantly enhanced after adoptive cell therapy with agonist OX40 immunotherapy. Therefore, we propose that OX40 agonist immunotherapy can maintain functional TCR signaling of chronically stimulated tumor-resident CD8 T cells, thereby increasing the frequency of cytotoxic, high-affinity, tumor-associated Ag-specific cells.

Immediate Dysfunction of Vaccine-Elicited CD8+ T Cells Primed in the Absence of CD4+ T Cells

CD4(+) T cell help is critical for optimal CD8(+) T cell memory differentiation and maintenance in many experimental systems. In addition, many reports have identified reduced primary CD8(+) T cell responses in the absence of CD4(+) T cell help, which often coincides with reduced Ag or pathogen clearance. In this study, we demonstrate that absence of CD4(+) T cells at the time of adenovirus vector immunization of mice led to immediate impairments in early CD8(+) T cell functionality and differentiation. Unhelped CD8(+) T cells exhibited a reduced effector phenotype, decreased ex vivo cytotoxicity, and decreased capacity to produce cytokines. This dysfunctional state was imprinted within 3 d of immunization. Unhelped CD8(+) T cells expressed elevated levels of inhibitory receptors and exhibited transcriptomic exhaustion and anergy profiles by gene set enrichment analysis. Dysfunctional, impaired effector differentiation also occurred following immunization of CD4(+) T cell-deficient mice with a poxvirus vector. This study demonstrates that following priming with viral vectors, CD4(+) T cell help is required to promote both the expansion and acquisition of effector functions by CD8(+) T cells, which is accomplished by preventing immediate dysfunction.

Glycogen Synthase Kinase 3 Inactivation Drives T-bet-Mediated Downregulation of Co-receptor PD-1 to Enhance CD8(+) Cytolytic T Cell Responses

Despite the importance of the co-receptor PD-1 in T cell immunity, the upstream signaling pathway that regulates PD-1 expression has not been defined. Glycogen synthase kinase 3 (GSK-3, isoforms α and β) is a serine-threonine kinase implicated in cellular processes. Here, we identified GSK-3 as a key upstream kinase that regulated PD-1 expression in CD8(+) T cells. GSK-3 siRNA downregulation, or inhibition by small molecules, blocked PD-1 expression, resulting in increased CD8(+) cytotoxic T lymphocyte (CTL) function. Mechanistically, GSK-3 inactivation increased Tbx21 transcription, promoting enhanced T-bet expression and subsequent suppression of Pdcd1 (encodes PD-1) transcription in CD8(+) CTLs. Injection of GSK-3 inhibitors in mice increased in vivo CD8(+) OT-I CTL function and the clearance of murine gamma-herpesvirus 68 and lymphocytic choriomeningitis clone 13 and reversed T cell exhaustion. Our findings identify GSK-3 as a regulator of PD-1 expression and demonstrate the applicability of GSK-3 inhibitors in the modulation of PD-1 in immunotherapy.

Multiple Inhibitory Pathways Contribute to Lung CD8+ T Cell Impairment and Protect against Immunopathology during Acute Viral Respiratory Infection

Viruses are frequent causes of lower respiratory infection (LRI). Programmed cell death-1 (PD-1) signaling contributes to pulmonary CD8(+) T cell (TCD8) functional impairment during acute viral LRI, but the role of TCD8 impairment in viral clearance and immunopathology is unclear. We now find that human metapneumovirus infection induces virus-specific lung TCD8 that fail to produce effector cytokines or degranulate late postinfection, with minimally increased function even in the absence of PD-1 signaling. Impaired lung TCD8 upregulated multiple inhibitory receptors, including PD-1, lymphocyte activation gene 3 (LAG-3), T cell Ig mucin 3, and 2B4. Moreover, coexpression of these receptors continued to increase even after viral clearance, with most virus-specific lung TCD8 expressing three or more inhibitory receptors on day 14 postinfection. Viral infection also increased expression of inhibitory ligands by both airway epithelial cells and APCs, further establishing an inhibitory environment. In vitro Ab blockade revealed that multiple inhibitory receptors contribute to TCD8 impairment induced by either human metapneumovirus or influenza virus infection. In vivo blockade of T cell Ig mucin 3 signaling failed to enhance TCD8 function or reduce viral titers. However, blockade of LAG-3 in PD-1-deficient mice restored TCD8 effector functions but increased lung pathology, indicating that LAG-3 mediates lung TCD8 impairment in vivo and contributes to protection from immunopathology during viral clearance. These results demonstrate that an orchestrated network of pathways modifies lung TCD8 functionality during viral LRI, with PD-1 and LAG-3 serving prominent roles. Lung TCD8 impairment may prevent immunopathology but also contributes to recurrent lung infections.

Modulation of Innate Immune Mechanisms to Enhance Leishmania Vaccine-Induced Immunity: Role of Coinhibitory Molecules

No licensed human vaccines are currently available against any parasitic disease including leishmaniasis. Several antileishmanial vaccine formulations have been tested in various animal models, including genetically modified live-attenuated parasite vaccines. Experimental infection studies have shown that Leishmania parasites utilize a broad range of strategies to undermine effector properties of host phagocytic cells, i.e., dendritic cells (DCs) and macrophages (MΦ). Furthermore, Leishmania parasites have evolved strategies to actively inhibit T_H1 polarizing functions of DCs and to condition the infected MΦ toward anti-inflammatory/alternative/M2 phenotype. The altered phenotype of phagocytic cells is characterized by decreased production of antimicrobial reactive oxygen, nitrogen molecules, and pro-inflammatory cytokines, such as IFN-γ, IL-12, and TNF-α. These early events limit the activation of T_H1-effector cells and set the stage for pathogenesis. Furthermore, this early control of innate immunity by the virulent parasites results in substantial alteration in the adaptive immunity characterized by reduced proliferation of CD4⁺ and CD8⁺ T cells and T_H2-biased immunity that results in production of anti-inflammatory cytokines, such as TGF-β, and IL-10. More recent studies have also documented the induction of coinhibitory ligands, such as CTLA-4, PD-L1, CD200, and Tim-3, that induce exhaustion and/or non-proliferation in antigen-experienced T cells. Most of these studies focus on viral infections in chronic phase, thus limiting the direct application of these results to parasitic infections and much less to parasitic vaccines. However, these studies suggest that vaccine-induced protective immunity can be modulated using strategies that enhance the costimulation that might reduce the threshold necessary for T cell activation and conversely by strategies that reduce or block inhibitory molecules, such as PD-L1 and CD200. In this review, we will focus on the polarization of antigen-presenting cells and subsequent role of costimulatory and coinhibitory molecules in mediating vaccine-induced immunity using live-attenuated Leishmania parasites as specific examples.

American tegumentary leishmaniasis: T-cell differentiation profile of cutaneous and mucosal forms-co-infection with Trypanosoma cruzi

American tegumentary leishmaniasis displays two main clinical forms: cutaneous (CL) and mucosal (ML). ML is more resistant to treatment and displays a more severe and longer evolution. Since both forms are caused by the same Leishmania species, the immunological response of the host may be an important factor determining the evolution of the disease. Herein, we analyzed the differentiation and memory profile of peripheral CD4(+) and CD8(+) T lymphocytes of patients with CL and ML and their Leishmania-T. cruzi co-infected counterparts. We measured the expression of CD27, CD28, CD45RO, CD127, PD-1 and CD57, together with interferon-γ and perforin. A highly differentiated phenotype was reflected on both T subsets in ML and preferentially on CD8(+) T cells in CL. A positive trend toward a higher T differentiation profile was found in T. cruzi-infected CL and ML patients as compared with Leishmania single infections. Association between CD8(+) T-cell differentiation and illness duration was found within the first year of infection, with progressive increase of highly differentiated markers over time. Follow-up of patients with good response to therapy showed predominance of early differentiated CD8(+) T cells and decrease of highly differentiated cells, while patients with frequent relapses presented the opposite pattern. CD8(+) T cells showed the most striking changes in their phenotype during leishmaniasis. Patients with long-term infections showed the highest differentiated degree implying a relation between T differentiation and parasite persistence. Distinct patterns of CD8(+) T differentiation during follow-up of different clinical outcomes suggest the usefulness of this analysis in the characterization of Leishmania-infected patients.

Viral Escape Mutant Epitope Maintains TCR Affinity for Antigen yet Curtails CD8 T Cell Responses

T cells have the remarkable ability to recognize antigen with great specificity and in turn mount an appropriate and robust immune response. Critical to this process is the initial T cell antigen recognition and subsequent signal transduction events. This antigen recognition can be modulated at the site of TCR interaction with peptide:major histocompatibility (pMHC) or peptide interaction with the MHC molecule. Both events could have a range of effects on T cell fate. Though responses to antigens that bind sub-optimally to TCR, known as altered peptide ligands (APL), have been studied extensively, the impact of disrupting antigen binding to MHC has been highlighted to a lesser extent and is usually considered to result in complete loss of epitope recognition. Here we present a model of viral evasion from CD8 T cell immuno-surveillance by a lymphocytic choriomeningitis virus (LCMV) escape mutant with an epitope for which TCR affinity for pMHC remains high but where the antigenic peptide binds sub optimally to MHC. Despite high TCR affinity for variant epitope, levels of interferon regulatory factor-4 (IRF4) are not sustained in response to the variant indicating differences in perceived TCR signal strength. The CD8+ T cell response to the variant epitope is characterized by early proliferation and up-regulation of activation markers. Interestingly, this response is not maintained and is characterized by a lack in IL-2 and IFNγ production, increased apoptosis and an abrogated glycolytic response. We show that disrupting the stability of peptide in MHC can effectively disrupt TCR signal strength despite unchanged affinity for TCR and can significantly impact the CD8+ T cell response to a viral escape mutant.

Beyond adjuvants: immunomodulation strategies to enhance T cell immunity

Engagement of CD8T cells is a crucial aspect of immune responses to pathogens and in tumor surveillance. Nonetheless most vaccination strategies with common adjuvants fail to elicit long-term memory CD8T cells. Increased knowledge on the cellular and molecular requirements for CD8T cell activation has unveiled new opportunities to directly modulate CD8T cells to generate optimal responses. During chronic infections and cancer, immunomodulation strategies to enhance T cell responses may be particularly necessary to overcome the immunosuppressive microenvironment. In this review we will discuss blockade of inhibitory receptors; interleukin-2 administration; regulatory T cell modulation; and targeting of mTOR, as means to enhance CD8T cell immunity.

IL-4 Induced Innate CD8+ T Cells Control Persistent Viral Infection

Memory-like CD8⁺ T cells expressing eomesodermin are a subset of innate T cells initially identified in a number of genetically modified mice, and also exist in wild mice and human. The acquisition of memory phenotype and function by these T cells is dependent on IL–4 produced by PLZF⁺ innate T cells; however, their physiologic function is still not known. Here we found that these IL-4-induced innate CD8⁺ T cells are critical for accelerating the control of chronic virus infection. In CIITA-transgenic mice, which have a substantial population of IL-4-induced innate CD8⁺ T cells, this population facilitated rapid control of viremia and induction of functional anti-viral T-cell responses during infection with chronic form of lymphocytic choriomeningitis virus. Characteristically, anti-viral innate CD8⁺ T cells accumulated sufficiently during early phase of infection. They produced a robust amount of IFN-γ and TNF-α with enhanced expression of a degranulation marker. Furthermore, this finding was confirmed in wild-type mice. Taken together, the results from our study show that innate CD8⁺ T cells works as an early defense mechanism against chronic viral infection.

Over the course of viral infection there may be a limited time period during which the host system can eliminate the virus. When viruses are not eliminated within this period of time, virus can establish persistent infection. Here, we show that IL-4-induced innate CD8⁺ T cells are able to effectively control chronic virus infection. Innate T cells are heterogeneous population of T cells that acquire effector/memory phenotype as a result of their maturation process in thymus, unlike conventional T cells that differentiate into memory cells after antigen encounter in periphery. Previous data suggest that innate T cells might serve as a first-line of defense against certain bacterial pathogens. IL-4-induced innate CD8⁺ T cells are a unique subset of innate T cells that were recently identified in both mouse and human. We found that IL-4-induced innate CD8⁺ T cells immediately accumulated after viral infection and produced a robust amount of effector cytokines. Thereby, IL-4-induced innate CD8⁺ T cells provide an effective barrier to the establishment of persistent infection via effective virus control during the early phase of viral infection. Collectively our data show that IL-4-induced innate CD8⁺ T cells works as an early defense mechanism against chronic viral infection.

Molecular and cellular insights into T cell exhaustion

In chronic infections and cancer, T cells are exposed to persistent antigen and/or inflammatory signals. This scenario is often associated with the deterioration of T cell function: a state called 'exhaustion'. Exhausted T cells lose robust effector functions, express multiple inhibitory receptors and are defined by an altered transcriptional programme. T cell exhaustion is often associated with inefficient control of persisting infections and tumours, but revitalization of exhausted T cells can reinvigorate immunity. Here, we review recent advances that provide a clearer molecular understanding of T cell exhaustion and reveal new therapeutic targets for persisting infections and cancer.

Interaction between unrelated viruses during in vivo co-infection to limit pathology and immunity

Great progress has been made in understanding immunity to viral infection. However, infection can occur in the context of co-infection by unrelated pathogens that modulate immune responses and/or disease. We have studied immunity and disease during co-infection with two unrelated viruses: Ectromelia virus (ECTV) and Lymphocytic Choriomeningitis virus (LCMV). ECTV infection can be a lethal in mice due in part to the blockade of Type I Interferons (IFN-I). We show that ECTV/LCMV co-infection results in decreased ECTV viral load and amelioration of ECTV-induced disease, likely due to IFN-I induction by LCMV, as rescue is not observed in IFN-I receptor deficient mice. However, immune responses to LCMV in ECTV co-infected mice were also lower compared to mice infected with LCMV alone and potentially biased toward effector-memory cell generation. Thus, we provide evidence for bi-directional effects of viral co-infection that modulate disease and immunity.