Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates

mTOR Signaling pathway as a master regulator of memory CD8+ T-cells, Th17, and NK cells development and their functional properties

The mammalian target of rapamycin (mTOR) is a member of the evolutionary phosphatidylinositol kinase-related kinases (PIKKs). mTOR plays a pivotal role in the regulation of diverse aspects of cellular physiology such as body metabolism, cell growth, protein synthesis, cell size, autophagy, and cell differentiation. Immunologically, mTOR has a fundamental part in controlling and shaping diverse functions of innate and adaptive immune cells, in particular, T-cell subsets differentiation, survival, and metabolic reprogramming to ultimately regulate the fate of diverse immune cell types. Researchers report that rapamycin, a selective mTOR inhibitor, and immunosuppressive agent, has surprising immunostimulatory effects on inducing both quantitative and qualitative aspects of virus-specific memory CD8⁺ T-cells differentiation and homeostasis in a T-cell-intrinsic manner. The mTOR signaling pathway also plays a critical role in dictating the outcome of regulatory T cells (Treg), T helper 17 (Th17) cells, and natural killer (NK) cells proliferation and maturation, as well as the effector functions and cytotoxic properties of NK cells. Manipulation of mTOR activity is a critical therapeutic approach for pharmacological agents that seek to inhibit mTOR. This approach should enhance specific memory CD8 ⁺ T-cells responses and induce fully functional effector properties of NK cells to provoke their antitumor and antiviral activities.

Expression of the DNA-Binding Factor TOX Promotes the Encephalitogenic Potential of Microbe-Induced Autoreactive CD8+ T Cells

Infections are thought to trigger CD8⁺ cytotoxic T lymphocyte (CTL) responses during autoimmunity. However, the transcriptional programs governing the tissue-destructive potential of CTLs remain poorly defined. In a model of central nervous system (CNS) inflammation, we found that infection with lymphocytic choriomeningitis virus (LCMV), but not Listeria monocytogenes (Lm), drove autoimmunity. The DNA-binding factor TOX was induced in CTLs during LCMV infection and was essential for their encephalitogenic properties, and its expression was inhibited by interleukin-12 during Lm infection. TOX repressed the activity of several transcription factors (including Id2, TCF-1, and Notch) that are known to drive CTL differentiation. TOX also reduced immune checkpoint sensitivity by restraining the expression of the inhibitory checkpoint receptor CD244 on the surface of CTLs, leading to increased CTL-mediated damage in the CNS. Our results identify TOX as a transcriptional regulator of tissue-destructive CTLs in autoimmunity, offering a potential mechanistic link to microbial triggers.

Checkpoint Blockade Immunotherapy Induces Dynamic Changes in PD-1-CD8+ Tumor-Infiltrating T Cells

An improved understanding of the anti-tumor CD8⁺ T cell response after checkpoint blockade would enable more informed and effective therapeutic strategies. Here we examined the dynamics of the effector response of CD8⁺ tumor-infiltrating lymphocytes (TILs) after checkpoint blockade therapy. Bulk and single-cell RNA profiles of CD8⁺ TILs after combined Tim-3+PD-1 blockade in preclinical models revealed significant changes in the transcriptional profile of PD-1^- TILs. These cells could be divided into subsets bearing characterstics of naive-, effector-, and memory-precursor-like cells. Effector- and memory-precursor-like TILs contained tumor-antigen-specific cells, exhibited proliferative and effector capacity, and expanded in response to different checkpoint blockade therapies across different tumor models. The memory-precursor-like subset shared features with CD8⁺ T cells associated with response to checkpoint blockade in patients and was compromised in the absence of Tcf7. Expression of Tcf7/Tcf1 was requisite for the efficacy of diverse immunotherapies, highlighting the importance of this transcriptional regulator in the development of effective CD8⁺ T cell responses upon immunotherapy.

Cytokine-Mediated Regulation of CD8 T-Cell Responses During Acute and Chronic Viral Infection

The common γ-chain cytokines, interleukin (IL)-2, IL-7, and IL-15, regulate critical aspects of antiviral CD8 T-cell responses. During acute infections, IL-2 controls expansion and differentiation of antiviral CD8 T cells, whereas IL-7 and IL-15 are key cytokines to maintain memory CD8 T cells long term in an antigen-independent manner. On the other hand, during chronic infections, in which T-cell exhaustion is established, precise roles of these cytokines in regulation of antiviral CD8 T-cell responses are not well defined. Nonetheless, administration of IL-2, IL-7, or IL-15 can increase function of exhausted CD8 T cells, and thus can be an attractive therapeutic approach. A new subset of stem-cell-like CD8 T cells, which provides a proliferative burst after programmed cell death (PD)-1 therapy, has been recently described during chronic viral infection. Further understanding of cytokine-mediated regulation of this CD8 T-cell subset will improve cytokine therapies to treat chronic infections and cancer in combination with immune checkpoint inhibitors.

Regulation of Effector and Memory CD8 T Cell Differentiation by IL-2-A Balancing Act

Interleukin-2 (IL-2) regulates key aspects of CD8 T cell biology–signaling through distinct pathways IL-2 triggers critical metabolic and transcriptional changes that lead to a spectrum of physiological outcomes such as cell survival, proliferation, and effector differentiation. In addition to driving effector differentiation, IL-2 signals are also critical for formation of long-lived CD8 T cell memory. This review discusses a model of rheostatic control of CD8 T cell effector and memory differentiation by IL-2, wherein the timing, duration, dose, and source of IL-2 signals are considered in fine-tuning the balance of key transcriptional regulators of cell fate.

Transcriptional and Epigenetic Regulation of Effector and Memory CD8 T Cell Differentiation

Immune protection and lasting memory are accomplished through the generation of phenotypically and functionally distinct CD8 T cell subsets. Understanding how these effector and memory T cells are formed is the first step in eventually manipulating the immune system for therapeutic benefit. In this review, we will summarize the current understanding of CD8 T cell differentiation upon acute infection, with a focus on the transcriptional and epigenetic regulation of cell fate decision and memory formation. Moreover, we will highlight the importance of high throughput sequencing approaches and single cell technologies in providing insight into genome-wide investigations and the heterogeneity of individual CD8 T cells.

Lack of Sprouty 1 and 2 enhances survival of effector CD8+ T cells and yields more protective memory cells

Identifying novel pathways that promote robust function and longevity of cytotoxic T cells has promising potential for immunotherapeutic strategies to combat cancer and chronic infections. We show that sprouty 1 and 2 (Spry1/2) molecules regulate the survival and function of memory CD8⁺ T cells. Spry1/2 double-knockout (DKO) ovalbumin (OVA)-specific CD8⁺ T cells (OT-I cells) mounted more vigorous autoimmune diabetes than WT OT-I cells when transferred to mice expressing OVA in their pancreatic β-islets. To determine the consequence of Spry1/2 deletion on effector and memory CD8⁺ T cell development and function, we used systemic infection with lymphocytic choriomeningitis virus (LCMV) Armstrong. Spry1/2 DKO LCMV gp33-specific P14 CD8⁺ T cells survive contraction better than WT cells and generate significantly more polyfunctional memory T cells. The larger number of Spry1/2 DKO memory T cells displayed enhanced infiltration into infected tissue, demonstrating that absence of Spry1/2 can result in increased recall capacity. Upon adoptive transfer into naive hosts, Spry1/2 DKO memory T cells controlled Listeria monocytogenes infection better than WT cells. The enhanced formation of more functional Spry1/2 DKO memory T cells was associated with significantly reduced mTORC1 activity and glucose uptake. Reduced p-AKT, p-FoxO1/3a, and T-bet expression was also consistent with enhanced survival and memory accrual. Collectively, loss of Spry1/2 enhances the survival of effector CD8⁺ T cells and results in the formation of more protective memory cells. Deleting Spry1/2 in antigen-specific CD8⁺ T cells may have therapeutic potential for enhancing the survival and functionality of effector and memory CD8⁺ T cells in vivo.

Genealogy, Dendritic Cell Priming, and Differentiation of Tissue-Resident Memory CD8+ T Cells

Tissue-resident memory CD8⁺ T (Trm) cells define a distinct non-recirculating subset. Trm cells constitute a first line of defense against local infections in barrier tissues, but they are also found in non-barrier tissues and play a role in antitumor immunity. Their differentiation in tissues and their phenotypical, transcriptional, and functional characteristics are the object of active research. Herein, we will discuss the potential existence of committed CD8⁺ Trm precursors and the genealogy of memory CD8⁺ T cell subsets. In addition to the priming of naive T cells, there is some plasticity of antigen-experienced effector and memory T cell subsets to generate Trm precursors. Local inflammation, antigen presentation, and cytokines drive Trm differentiation. It is of prime interest how specific dendritic cell subsets modulate priming and differentiation of Trm cells, as well as their reactivation within tissues. Research on how we can manipulate generation of memory T cells subsets is key for improved vaccination strategies.

Cytokines and CD8 T cell immunity during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection and hospitalization in infants. In spite of the enormous clinical burden caused by RSV infections, there remains no efficacious RSV vaccine. CD8 T cells mediate viral clearance as well as provide protection against a secondary RSV infection. However, RSV-specific CD8 T cells may also induce immunopathology leading to exacerbated morbidity and mortality. Many of the crucial functions performed by CD8 T cells are mediated by the cytokines they produce. IFN-γ and TNF are produced by CD8 T cells following RSV infection and contribute to both the acceleration of viral clearance and the induction of immunopathology. To prevent immunopathology, regulatory mechanisms are in place within the immune system to inhibit CD8 T cell effector functions after the infection has been cleared. The actions of a variety of cytokines, including IL-10 and IL-4, play a critical role in the regulation of CD8 T cell effector activity. Herein, we review the current literature on CD8 T cell responses and the functions of the cytokines they produce following RSV infection. Additionally, we discuss the regulation of CD8 T cell activation and effector functions through the actions of various cytokines.

Effector CD4 T cells with progenitor potential mediate chronic intestinal inflammation

Effector CD4 T cells with progenitor properties are present during chronic intestinal inflammation, and these cells support the maintenance of disease. The expression of the glycosyltransferase ST6Gal-I by these cells promotes cell survival and TCF1 levels.

Dysregulated CD4 T cell responses are causally linked to autoimmune and chronic inflammatory disorders, yet the cellular attributes responsible for maintaining the disease remain poorly understood. Herein, we identify a discrete population of effector CD4 T cells that is able to both sustain and confer intestinal inflammation. This subset of pathogenic CD4 T cells possesses a unique gene signature consistent with self-renewing T cells and hematopoietic progenitor cells, exhibits enhanced survival, and continually seeds the terminally differentiated IFNγ-producing cells in the inflamed intestine. Mechanistically, this population selectively expresses the glycosyltransferase ST6Gal-I, which is required for optimal expression of the stemness-associated molecule TCF1 by effector CD4 T cells. Our findings indicate that the chronicity of T cell–mediated inflammation is perpetuated by specific effector CD4 T cells with stem-like properties.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

Developmental Origin Governs CD8+ T Cell Fate Decisions during Infection

Heterogeneity is a hallmark feature of the adaptive immune system in vertebrates. Following infection, naive T cells differentiate into various subsets of effector and memory T cells, which help to eliminate pathogens and maintain long-term immunity. The current model suggests there is a single lineage of naive T cells that give rise to different populations of effector and memory T cells depending on the type and amounts of stimulation they encounter during infection. Here, we have discovered that multiple sub-populations of cells exist in the naive CD8⁺ T cell pool that are distinguished by their developmental origin, unique transcriptional profiles, distinct chromatin landscapes, and different kinetics and phenotypes after microbial challenge. These data demonstrate that the naive CD8⁺ T cell pool is not as homogeneous as previously thought and offers a new framework for explaining the remarkable heterogeneity in the effector and memory T cell subsets that arise after infection.

Transplantation preferentially induces a KLRG-1lo CD127hi differentiation program in antigen-specific CD8+ T cells

Models of infection have shaped our understanding of programmed memory T cell differentiation, yet whether these models apply to memory programming in the context of transplantation has yet to be defined. Previous work has identified differences in the response of antigen-specific CD8⁺ T cells to cognate antigen based on the environment in which the antigen is presented. Thus, we hypothesized that programming of antigen specific CD8⁺ T cells responding to graft and pathogen may be dissimilar. Here we find that antigen-specific CD8⁺ T cells primed by a skin graft contract faster than those primed by gammaherpesvirus (gHV), yet are able to expand more rapidly upon rechallenge. Moreover, graft-primed antigen-specific CD8⁺ T cells exhibited higher frequencies of cells secreting IL-2 and demonstrate lower expression of KLRG-1, which are qualities suggestive of increased recall potential. Additionally, the expression of CD127 at a memory time point suggests graft-elicited CD8⁺ antigen specific T cells are maintained in a less terminally-differentiated state compared to gHV-elicited CD8⁺ antigen specific T cells, despite fewer cells being present at that time point. Taken together, our findings suggest that the surface marker expression and functional profiles of T cells depends on the priming conditions and may be used to predict immunologic risk following transplantation after traditional allosensitization or heterologous immune priming.

TCR Signal Quality Modulates Fate Decisions of Single CD4+ T Cells in a Probabilistic Manner

To what extent the lineage decisions of activated CD4⁺ T cells are determined by the quality of T cell receptor (TCR) ligation is incompletely understood. Here, we show that individual T cells expressing identical TCRs take highly variable fate decisions despite binding the same ligand. We identify a mathematical model that correctly captures this probabilistic behavior and allows one to formalize changes in TCR signal quality-due to cognate versus altered peptide ligation-as changes of lineage-specific proliferation and differentiation rates. We show that recall responses also adhere to this probabilistic framework requiring recruitment of multiple memory clones to provide reliable differentiation patterns. By extending our framework to simulate hypothetical TCRs of distinct binding strength, we reconstruct primary and secondary response patterns emerging from a polyclonal TCR repertoire in silico. Collectively, these data suggest that individual T cells harboring distinct TCRs generate overlapping primary differentiation patterns that segregate only upon repetitive immunization.

Epigenetic control of CD8+ T cell differentiation

Upon stimulation, small numbers of naive CD8+ T cells proliferate and differentiate into a variety of memory and effector cell types. CD8+ T cells can persist for years and kill tumour cells and virally infected cells. The functional and phenotypic changes that occur during CD8+ T cell differentiation are well characterized, but the epigenetic states that underlie these changes are incompletely understood. Here, we review the epigenetic processes that direct CD8+ T cell differentiation and function. We focus on epigenetic modification of DNA and associated histones at genes and their regulatory elements. We also describe structural changes in chromatin organization that affect gene expression. Finally, we examine the translational potential of epigenetic interventions to improve CD8+ T cell function in individuals with chronic infections and cancer.

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.

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.

Foxo transcription factors in T cell biology and tumor immunity

The evolutionally conserved forkhead box O (Foxo) family of transcription factors is pivotal in the control of nutrient sensing and stress responses. Recent studies have revealed that the Foxo proteins have been rewired to regulate highly specialized T cell activities. Here, we review the latest advances in the understanding of how Foxo transcription factors control T cell biology, including T cell trafficking, naive T cell homeostasis, effector and memory responses, as well as the differentiation and function of regulatory T cells. We also discuss the emerging evidence on Foxo-mediated regulation in antitumor immunity. Future work will further explore how the Foxo-dependent programs in T cells can be exploited for cancer immunotherapy.

The Respiratory Environment Diverts the Development of Antiviral Memory CD8 T Cells

Our understanding of memory CD8⁺ T cells has been largely derived from acute, systemic infection models. However, memory CD8⁺ T cells generated from mucosal infection exhibit unique properties and, following respiratory infection, are not maintained in the lung long term. To better understand how infection route modifies memory differentiation, we compared murine CD8⁺ T cell responses to a vesicular stomatitis virus (VSV) challenge generated intranasally (i.n.) or i.v. The i.n. infection resulted in greater peak expansion of VSV-specific CD8⁺ T cells. However, this numerical advantage was rapidly lost during the contraction phase of the immune response, resulting in memory CD8⁺ T cell numerical deficiencies when compared with i.v. infection. Interestingly, the antiviral CD8⁺ T cells generated in response to i.n. VSV exhibited a biased and sustained proportion of early effector cells (CD127^loKLRG1^lo) akin to the developmental program favored after i.n. influenza infection, suggesting that respiratory infection broadly favors an incomplete memory differentiation program. Correspondingly, i.n. VSV infection resulted in lower CD122 expression and eomesodermin levels by VSV-specific CD8⁺ T cells, further indicative of an inferior transition to bona fide memory. These results may be due to distinct (CD103⁺CD11b⁺) dendritic cell subsets in the i.n. versus i.v. T cell priming environments, which express molecules that regulate T cell signaling and the balance between tolerance and immunity. Therefore, we propose that distinct immunization routes modulate both the quality and quantity of antiviral effector and memory CD8⁺ T cells in response to an identical pathogen and should be considered in CD8⁺ T cell-based vaccine design.

G-Protein Coupled Receptor 18 Contributes to Establishment of the CD8 Effector T Cell Compartment

The requirements for effector and memory CD8 T cell development are incompletely understood. Recent work has revealed a role for G-protein coupled receptor 18 (GPR18) in establishment of the intestinal CD8αα intraepithelial lymphocyte compartment. Here, we report that GPR18 is also functionally expressed in conventional CD8αβ T cells. When the receptor is lacking, mice develop fewer CD8⁺ KLRG1⁺ Granzyme B⁺ effector-memory cells. Bone marrow chimera studies show that the GPR18 requirement is CD8 T cell intrinsic. GPR18 is not required for T-bet expression in KLRG1⁺ CD8 T cells. Gene transduction experiments confirm the functional activity of GPR18 in CD8 T cells. In summary, we describe a novel GPCR requirement for establishment or maintenance of the CD8 KLRG1⁺ effector-memory T cell compartment. These findings have implications for methods to augment CD8 effector cell numbers.

KLRG1+ Effector CD8+ T Cells Lose KLRG1, Differentiate into All Memory T Cell Lineages, and Convey Enhanced Protective Immunity

Protective immunity against pathogens depends on the efficient generation of functionally diverse effector and memory T lymphocytes. However, whether plasticity during effector-to-memory CD8⁺ T cell differentiation affects memory lineage specification and functional versatility remains unclear. Using genetic fate mapping analysis of highly cytotoxic KLRG1⁺ effector CD8⁺ T cells, we demonstrated that KLRG1⁺ cells receiving intermediate amounts of activating and inflammatory signals downregulated KLRG1 during the contraction phase in a Bach2-dependent manner and differentiated into all memory T cell linages, including CX₃CR1^int peripheral memory cells and tissue-resident memory cells. "ExKLRG1" memory cells retained high cytotoxic and proliferative capacity distinct from other populations, which contributed to effective anti-influenza and anti-tumor immunity. Our work demonstrates that developmental plasticity of KLRG1⁺ effector CD8⁺ T cells is important in promoting functionally versatile memory cells and long-term protective immunity.

Transcriptional programming of tissue-resident memory CD8+ T cells

Tissue-resident memory CD8⁺ T cells (T_RM) are localized in non-lymphoid tissues throughout the body where they mediate long-lived protective immunity at common sites of pathogen exposure. As the signals controlling T_RM differentiation are uncovered, it is becoming apparent that the dynamic activities of numerous transcription factors are intricately involved in T_RM formation. Here, we highlight known transcriptional regulators of T_RM differentiation and discuss how understanding the transcriptional programming of CD8⁺ T cell residency in non-lymphoid tissues can be leveraged to prevent or treat disease.

Chemokines: Critical Regulators of Memory T Cell Development, Maintenance, and Function

Memory T cells are central to orchestrating antigen-specific recall responses in vivo. Compared to naïve T cells, memory T cells respond more quickly to cognate peptide:MHC with a shorter lag time for entering the cell cycle and exerting effector functions. However, it is now well established that this enhanced responsiveness is not the only mechanism whereby memory T cells are better equipped than naïve T cells to rapidly and robustly induce inflammation. In contrast to naïve T cells, memory T cells are composed of distinct subsets with unique trafficking patterns and localizations. Tissue-resident memory T cells persist in previously inflamed tissue and function as first responders to cognate antigen reexposure. In addition, a heterogeneous group of circulating memory T cells augment inflammation by either rapidly migrating to inflamed tissue or responding to cognate antigen within secondary lymphoid organs and producing additional effector T cells. Defining the mechanisms regulating T cell positioning and trafficking and how this influences the development, maintenance, and function of memory T cell subsets is essential to improving vaccine design as well as treatment of immune-mediated diseases. In this chapter, we will review our current knowledge of how chemokines, critical regulators of cell positioning and migration, govern memory T cell biology in vivo. In addition, we discuss areas of uncertainty and future directions for further delineating how T cell localization influences memory T cell biology.

Influence of Inflammation in the Process of T Lymphocyte Differentiation: Proliferative, Metabolic, and Oxidative Changes

T lymphocytes, from their first encounter with their specific antigen as naïve cell until the last stages of their differentiation, in a replicative state of senescence, go through a series of phases. In several of these stages, T lymphocytes are subjected to exponential growth in successive encounters with the same antigen. This entire process occurs throughout the life of a human individual and, earlier, in patients with chronic infections/pathologies through inflammatory mediators, first acutely and later in a chronic form. This process plays a fundamental role in amplifying the activating signals on T lymphocytes and directing their clonal proliferation. The mechanisms that control cell growth are high levels of telomerase activity and maintenance of telomeric length that are far superior to other cell types, as well as metabolic adaptation and redox control. Large numbers of highly differentiated memory cells are accumulated in the immunological niches where they will contribute in a significant way to increase the levels of inflammatory mediators that will perpetuate the new state at the systemic level. These levels of inflammation greatly influence the process of T lymphocyte differentiation from naïve T lymphocyte, even before, until the arrival of exhaustion or cell death. The changes observed during lymphocyte differentiation are correlated with changes in cellular metabolism and these in turn are influenced by the inflammatory state of the environment where the cell is located. Reactive oxygen species (ROS) exert a dual action in the population of T lymphocytes. Exposure to high levels of ROS decreases the capacity of activation and T lymphocyte proliferation; however, intermediate levels of oxidation are necessary for the lymphocyte activation, differentiation, and effector functions. In conclusion, we can affirm that the inflammatory levels in the environment greatly influence the differentiation and activity of T lymphocyte populations. However, little is known about the mechanisms involved in these processes. The elucidation of these mechanisms would be of great help in the advance of improvements in pathologies with a large inflammatory base such as rheumatoid arthritis, intestinal inflammatory diseases, several infectious diseases and even, cancerous processes.

Sustained Id2 regulation of E proteins is required for terminal differentiation of effector CD8+ T cells

CD8⁺ T cells responding to infection differentiate into short-lived effector cells destined to die or memory cells that provide long-lived protection. Omilusik et al. demonstrate that commitment to an effector cell fate is not necessarily terminal and that sustained transcriptional regulation is required to maintain subset-specific properties.

CD8⁺ T cells responding to infection differentiate into a heterogeneous population composed of progeny that are short-lived and participate in the immediate, acute response and those that provide long-lasting host protection. Although it is appreciated that distinct functional and phenotypic CD8⁺ T cell subsets persist, it is unclear whether there is plasticity among subsets and what mechanisms maintain subset-specific differences. Here, we show that continued Id2 regulation of E-protein activity is required to maintain the KLRG1^hi CD8⁺ T cell population after lymphocytic choriomeningitis virus infection. Induced deletion of Id2 phenotypically and transcriptionally transformed the KLRG1^hi “terminal” effector/effector-memory CD8⁺ T cell population into a KLRG1^lo memory-like population, promoting a gene-expression program that resembled that of central memory T cells. Our results question the idea that KLRG1^hi CD8⁺ T cells are necessarily terminally programmed and suggest that sustained regulation is required to maintain distinct CD8⁺ T cell states.

Unexpected positive control of NFκB and miR-155 by DGKα and ζ ensures effector and memory CD8+ T cell differentiation

Signals from the T-cell receptor (TCR) and γ-chain cytokine receptors play crucial roles in initiating activation and effector/memory differentiation of CD8 T-cells. We report here that simultaneous deletion of both diacylglycerol kinase (DGK) α and ζ (DKO) severely impaired expansion of CD8 effector T cells and formation of memory CD8 T-cells after Listeria monocytogenes infection. Moreover, ablation of both DGKα and ζ in preformed memory CD8 T-cells triggered death and impaired homeostatic proliferation of these cells. DKO CD8 T-cells were impaired in priming due to decreased expression of chemokine receptors and migration to the draining lymph nodes. Moreover, DKO CD8 T-cells were unexpectedly defective in NFκB-mediated miR-155 transcript, leading to excessive SOCS1 expression and impaired γ-chain cytokine signaling. Our data identified a DGK-NFκB-miR-155-SOCS1 axis that bridges TCR and γ-chain cytokine signaling for robust CD8 T-cell primary and memory responses to bacterial infection.

Effector CD8 T cells dedifferentiate into long-lived memory cells

Memory CD8 T cells that circulate in the blood and are present in lymphoid organs are an essential component of long-lived T cell immunity. These memory CD8 T cells remain poised to rapidly elaborate effector functions upon re-exposure to pathogens, but also have many properties in common with naive cells, including pluripotency and the ability to migrate to the lymph nodes and spleen. Thus, memory cells embody features of both naive and effector cells, fuelling a long-standing debate centred on whether memory T cells develop from effector cells or directly from naive cells. Here we show that long-lived memory CD8 T cells are derived from a subset of effector T cells through a process of dedifferentiation. To assess the developmental origin of memory CD8 T cells, we investigated changes in DNA methylation programming at naive and effector cell-associated genes in virus-specific CD8 T cells during acute lymphocytic choriomeningitis virus infection in mice. Methylation profiling of terminal effector versus memory-precursor CD8 T cell subsets showed that, rather than retaining a naive epigenetic state, the subset of cells that gives rise to memory cells acquired de novo DNA methylation programs at naive-associated genes and became demethylated at the loci of classically defined effector molecules. Conditional deletion of the de novo methyltransferase Dnmt3a at an early stage of effector differentiation resulted in reduced methylation and faster re-expression of naive-associated genes, thereby accelerating the development of memory cells. Longitudinal phenotypic and epigenetic characterization of the memory-precursor effector subset of virus-specific CD8 T cells transferred into antigen-free mice revealed that differentiation to memory cells was coupled to erasure of de novo methylation programs and re-expression of naive-associated genes. Thus, epigenetic repression of naive-associated genes in effector CD8 T cells can be reversed in cells that develop into long-lived memory CD8 T cells while key effector genes remain demethylated, demonstrating that memory T cells arise from a subset of fate-permissive effector T cells.

Profiling of Protein O-GlcNAcylation in Murine CD8+ Effector- and Memory-like T Cells

During an acute infection, antigenic stimulation leads to activation, expansion, and differentiation of naïve CD8⁺ T cells, first into cytotoxic effector cells and eventually into long-lived memory cells. T cell antigen receptors (TCRs) detect antigens on antigen-presenting cells (APCs) in the form of antigenic peptides bound to major histocompatibility complex I (MHC-I)-encoded molecules and initiate TCR signal transduction network. This process is mediated by phosphorylation of many intracellular signaling proteins. Protein O-GlcNAc modification is another post-translational modification involved in this process, which often has either reciprocal or synergistic roles with phosphorylation. In this study, using a chemoenzymatic glycan labeling technique and proteomics analysis, we compared protein O-GlcNAcylation of murine effector and memory-like CD8⁺ T cells differentiated in vitro. By quantitative proteomics analysis, we identified 445 proteins that are significantly regulated in either effector- or memory-like T cell subsets. Furthermore, qualitative and quantitative analysis identified highly regulated protein clusters that suggest involvement of this post-translational modification in specific cellular processes. In effector-like T cells, protein O-GlcNAcylation is heavily involved in transcriptional and translational processes that drive fast effector T cells proliferation. During the formation of memory-like T cells, protein O-GlcNAcylation is involved in a more specific, perhaps more targeted regulation of transcription, mRNA processing, and translation. Significantly, O-GlcNAc plays a critical role as part of the "histone code" in both CD8⁺ T cells subgroups.

Origin and differentiation of human memory CD8 T cells after vaccination

The differentiation of human memory CD8 T cells is not well understood. Here we address this issue using the live yellow fever virus (YFV) vaccine, which induces long-term immunity in humans. We used in vivo deuterium labelling to mark CD8 T cells that proliferated in response to the virus and then assessed cellular turnover and longevity by quantifying deuterium dilution kinetics in YFV-specific CD8 T cells using mass spectrometry. This longitudinal analysis showed that the memory pool originates from CD8 T cells that divided extensively during the first two weeks after infection and is maintained by quiescent cells that divide less than once every year (doubling time of over 450 days). Although these long-lived YFV-specific memory CD8 T cells did not express effector molecules, their epigenetic landscape resembled that of effector CD8 T cells. This open chromatin profile at effector genes was maintained in memory CD8 T cells isolated even a decade after vaccination, indicating that these cells retain an epigenetic fingerprint of their effector history and remain poised to respond rapidly upon re-exposure to the pathogen.

Novel Concepts for HIV Vaccine Vector Design

The unprecedented challenges of developing effective vaccines against intracellular pathogens such as HIV, malaria, and tuberculosis have resulted in more rational approaches to vaccine development. Apart from the recent advances in the design and selection of improved epitopes and adjuvants, there are also ongoing efforts to optimize delivery platforms.

The unprecedented challenges of developing effective vaccines against intracellular pathogens such as HIV, malaria, and tuberculosis have resulted in more rational approaches to vaccine development. Apart from the recent advances in the design and selection of improved epitopes and adjuvants, there are also ongoing efforts to optimize delivery platforms. Viral vectors are the best-characterized delivery tools because of their intrinsic adjuvant capability, unique cellular tropism, and ability to trigger robust adaptive immune responses. However, a known limitation of viral vectors is preexisting immunity, and ongoing efforts are aimed at developing novel vector platforms with lower seroprevalence. It is also becoming increasingly clear that different vectors, even those derived from phylogenetically similar viruses, can elicit substantially distinct immune responses, in terms of quantity, quality, and location, which can ultimately affect immune protection. This review provides a summary of the status of viral vector development for HIV vaccines, with a particular focus on novel viral vectors and the types of adaptive immune responses that they induce.

The Loss of TET2 Promotes CD8+ T Cell Memory Differentiation

T cell differentiation requires appropriate regulation of DNA methylation. In this article, we demonstrate that the methylcytosine dioxygenase ten-eleven translocation (TET)2 regulates CD8⁺ T cell differentiation. In a murine model of acute viral infection, TET2 loss promotes early acquisition of a memory CD8⁺ T cell fate in a cell-intrinsic manner without disrupting Ag-driven cell expansion or effector function. Upon secondary recall, TET2-deficient memory CD8⁺ T cells demonstrate superior pathogen control. Genome-wide methylation analysis identified a number of differentially methylated regions in TET2-deficient versus wild-type CD8⁺ T cells. These differentially methylated regions did not occur at the loci of differentially expressed memory markers; rather, several hypermethylated regions were identified in known transcriptional regulators of CD8⁺ T cell memory fate. Together, these data demonstrate that TET2 is an important regulator of CD8⁺ T cell fate decisions.

Mycobacterium tuberculosis infection modulates adipose tissue biology

Mycobacterium tuberculosis (Mtb) primarily resides in the lung but can also persist in extrapulmonary sites. Macrophages are considered the prime cellular habitat in all tissues. Here we demonstrate that Mtb resides inside adipocytes of fat tissue where it expresses stress-related genes. Moreover, perigonadal fat of Mtb-infected mice disseminated the infection when transferred to uninfected animals. Adipose tissue harbors leukocytes in addition to adipocytes and other cell types and we observed that Mtb infection induces changes in adipose tissue biology depending on stage of infection. Mice infected via aerosol showed infiltration of inducible nitric oxide synthase (iNOS) or arginase 1 (Arg1)-negative F4/80⁺ cells, despite recruitment of CD3⁺, CD4⁺ and CD8⁺ T cells. Gene expression analysis of adipose tissue of aerosol Mtb-infected mice provided evidence for upregulated expression of genes associated with T cells and NK cells at 28 days post-infection. Strikingly, IFN-γ-producing NK cells and Mtb-specific CD8⁺ T cells were identified in perigonadal fat, specifically CD8⁺CD44^-CD69⁺ and CD8⁺CD44^-CD103⁺ subpopulations. Gene expression analysis of these cells revealed that they expressed IFN-γ and the lectin-like receptor Klrg1 and down-regulated CD27 and CD62L, consistent with an effector phenotype of Mtb-specific CD8⁺ T cells. Sorted NK cells expressed higher abundance of Klrg1 upon infection, as well. Our results reveal the ability of Mtb to persist in adipose tissue in a stressed state, and that NK cells and Mtb-specific CD8⁺ T cells infiltrate infected adipose tissue where they produce IFN-γ and assume an effector phenotype. We conclude that adipose tissue is a potential niche for Mtb and that due to infection CD8⁺ T cells and NK cells are attracted to this tissue.

In 2015, tuberculosis (TB) affected 10.4 million individuals causing 1.8 million deaths per year. Yet, a much larger group– 2 billion people–harbors latent TB infection (LTBI) without clinical symptoms, but at lifelong risk of reactivation. The physiological niches of Mycobacterium tuberculosis (Mtb) persistence remain incompletely defined and both pulmonary and extrapulmonary sites have been proposed. Adipose tissue constitutes 15–25% of total body mass and is an active production site for hormones and inflammatory mediators. The increasing prevalence of obesity, has led to greater incidence of type 2 diabetes. These patients suffer from three times higher risk of developing TB, pointing to a potential link between adipose tissue and TB pathogenesis. In individuals with LTBI, Mtb survives in a stressed, non-replicating state with low metabolic activity and resting macrophages serve as preferred habitat and become effectors after appropriate stimulation. Here we demonstrate that Mtb can infect and persist within adipocytes where it upregulates stress-related genes. In vivo, relative proportions of leukocyte subsets infiltrating adipose tissue varied under different conditions of infection. During natural aerosol Mtb infection, distinct leukocyte subsets, including mononuclear phagocytes, Mtb-specific CD8⁺ T cells and NK cells infiltrated adipose tissue and became activated. Thus, our study shows that adipose tissue is not only a potential reservoir for this pathogen but also undergoes significant alteration during TB infection.

FOXO1 opposition of CD8+ T cell effector programming confers early memory properties and phenotypic diversity

The factors and steps controlling postinfection CD8⁺ T cell terminal effector versus memory differentiation are incompletely understood. Whereas we found that naive TCF7 (alias "Tcf-1") expression is FOXO1 independent, early postinfection we report bimodal, FOXO1-dependent expression of the memory-essential transcription factor TCF7 in pathogen-specific CD8⁺ T cells. We determined the early postinfection TCF7^high population is marked by low TIM3 expression and bears memory signature hallmarks before the appearance of established memory precursor marker CD127 (IL-7R). These cells exhibit diminished TBET, GZMB, mTOR signaling, and cell cycle progression. Day 5 postinfection, TCF7^high cells express higher memory-associated BCL2 and EOMES, as well as increased accumulation potential and capacity to differentiate into memory phenotype cells. TCF7 retroviral transduction opposes GZMB expression and the formation of KLRG1^pos phenotype cells, demonstrating an active role for TCF7 in extinguishing the effector program and forestalling terminal differentiation. Past the peak of the cellular immune response, we report a gradient of FOXO1 and TCF7 expression, which functions to oppose TBET and orchestrate a continuum of effector-to-memory phenotypes.

Interleukin 10 enhanced CD8+ T cell activity and reduced CD8+ T cell apoptosis in patients with diffuse large B cell lymphoma

The pleiotropic cytokine interleukin (IL)-10 is best characterized by its ability to downregulate inflammation and promote peripheral tolerance. On the other hand, IL-10 was also found to maintain the effector response of CD8⁺ T cells and promote the expansion of tumor-resident CD8⁺ T cells. In diffuse large B cell lymphoma (DLBCL), the role of IL-10 has been characterized in tumor cells but not in CD8⁺ T cells. We found that CD8⁺ T cells in DLBCL presented robust interferon (IFN)-γ expression early during TCR-activation but could not maintain this response later on, which was characterized by significantly lower CD8⁺ T cell degranulation and higher apoptosis. These observations were associated with higher PD-1 expression in DLBCL CD8⁺ T cells. Furthermore, the PD-1⁺ cells were strongly enriched in the IFN-γ⁺, but not the IFN-γ^-, fraction. Interestingly, exogenous IL-10 significantly improved the survival of DLBCL CD8⁺ T cells, and resulted in significantly higher IFN-γ, ganzyme A and granzyme B expression in the absence of CD19⁺ tumor cells, and significantly improved CD8⁺ T cell-mediated specific lysis of CD19⁺ tumor cells. IL-10 did not alter the expression of PD-1 in DLBCL CD8⁺ T cells, but curiously, IL-10-treated DLBCL CD8⁺ T cells were less susceptible to PD-L1-mediated apoptosis. We then demonstrated that IL-10 treatment significantly elevated the expression of pro-survival factor Bcl-2. Blocking IL-10 resulted in higher apoptosis, fewer IFN-γ⁺ CD8⁺ T cells, and lower Bcl-2 expression. IL-10 also significantly increased STAT3, but not STAT1, phosphorylation in CD8⁺ T cells. Together, these results suggested that IL-10 could enhance CD8⁺ T cell inflammation in DLBCL patients.

Identification of a novel population of highly cytotoxic c-Met-expressing CD8+ T lymphocytes

CD8⁺ cytotoxic T lymphocytes (CTLs) are critical mediators of anti-tumor immunity, and controlling the mechanisms that govern CTL functions could be crucial for enhancing patient outcome. Previously, we reported that hepatocyte growth factor (HGF) limits effective murine CTL responses via antigen-presenting cells. Here, we show that a fraction of murine effector CTLs expresses the HGF receptor c-Met (c-Met⁺ CTLs). Phenotypic and functional analysis of c-Met⁺ CTLs reveals that they display enhanced cytolytic capacities compared to their c-Met^- CTL counterparts. Furthermore, HGF directly restrains the cytolytic function of c-Met⁺ CTLs in cell-mediated cytotoxicity reactions in vitro and in vivo and abrogates T-cell responses against metastatic melanoma in vivo Finally, we establish in three murine tumor settings and in human melanoma tissues that c-Met⁺ CTLs are a naturally occurring CD8⁺ T-cell population. Together, our findings suggest that the HGF/c-Met pathway could be exploited to control CD8⁺ T-cell-mediated anti-tumor immunity.

Translation is actively regulated during the differentiation of CD8+ effector T cells

Translation is a critical process in protein synthesis, but translational regulation in antigen-specific T cells in vivo has not been well defined. Here we have characterized the translatome of virus-specific effector CD8⁺ T cells during acute LCMV infection of mice. Antigen-specific T cells exerted dynamic translational control of gene expression that correlated with cell proliferation and T cell antigen receptor (TCR) stimulation. Translation of mRNAs that encode translation machinery including ribosomal protein mRNAs was upregulated during the T cell expansion phase, followed by translational inhibition of these transcripts when the effector CD8⁺ T cells stopped dividing just prior to the contraction phase. This translational suppression was more pronounced in terminal effector cells compared to memory precursor cells, and was regulated by antigenic stimulation and mTOR signals. Our studies show that translational activity of transcripts encoding ribosomal proteins is regulated during effector CD8⁺ T cell differentiation and may play a role in fate decisions involved in the formation of memory cells.

Generating long-lived CD8(+) T-cell memory: Insights from epigenetic programs

T-cell-based immunological memory has the potential to provide the host with life-long protection against pathogen reexposure and thus offers tremendous promise for the design of vaccines targeting chronic infections or cancer. In order to exploit this potential in the design of new vaccines, it is necessary to understand how and when memory T cells acquire their poised effector potential, and moreover, how they maintain these properties during homeostatic proliferation. To gain insight into the persistent nature of memory T-cell functions, investigators have turned their attention to epigenetic mechanisms. Recent efforts have revealed that many of the properties acquired among memory T cells are coupled to stable changes in DNA methylation and histone modifications. Furthermore, it has recently been reported that the delineating features among memory T cells subsets are also linked to distinct epigenetic events, such as permissive and repressive histone modifications and DNA methylation programs, providing exciting new hypotheses regarding their cellular ancestry. Here, we review recent studies focused on epigenetic programs acquired during effector and memory T-cell differentiation and discuss how these data may shed new light on the developmental path for generating long-lived CD8(+) T-cell memory.

Systems-guided forward genetic screen reveals a critical role of the replication stress response protein ETAA1 in T cell clonal expansion

T-cell immunity requires extremely rapid clonal proliferation of rare, antigen-specific T lymphocytes to form effector cells. Here we identify a critical role for ETAA1 in this process by surveying random germ line mutations in mice using exome sequencing and bioinformatic annotation to prioritize mutations in genes of unknown function with potential effects on the immune system, followed by breeding to homozygosity and testing for immune system phenotypes. Effector CD8⁺ and CD4⁺ T-cell formation following immunization, lymphocytic choriomeningitis virus (LCMV) infection, or herpes simplex virus 1 (HSV1) infection was profoundly decreased despite normal immune cell development in adult mice homozygous for two different Etaa1 mutations: an exon 2 skipping allele that deletes Gly78-Leu119, and a Cys166Stop truncating allele that eliminates most of the 877-aa protein. ETAA1 deficiency decreased clonal expansion cell autonomously within the responding T cells, causing no decrease in their division rate but increasing TP53-induced mRNAs and phosphorylation of H2AX, a marker of DNA replication stress induced by the ATM and ATR kinases. Homozygous ETAA1-deficient adult mice were otherwise normal, healthy, and fertile, although slightly smaller, and homozygotes were born at lower frequency than expected, consistent with partial lethality after embryonic day 12. Taken together with recently reported evidence in human cancer cell lines that ETAA1 activates ATR kinase through an exon 2-encoded domain, these findings reveal a surprisingly specific requirement for this ATR activator in adult mice restricted to rapidly dividing effector T cells. This specific requirement may provide new ways to suppress pathological T-cell responses in transplantation or autoimmunity.

Cyclophosphamide treatment regulates the balance of functional/exhausted tumor-specific CD8+ T cells

An important question is how chemotherapy may (re-)activate tumor-specific immunity. In this study, we provide a phenotypic, functional and genomic analysis of tumor-specific CD8⁺ T cells in tumor (P815)-bearing mice, treated or not with cyclophosphamide. Our data show that chemotherapy favors the development of effector-type lymphocytes in tumor bed, characterized by higher KLRG-1 expression, lower PD-1 expression and increased cytotoxicity. This suggests re-engagement of T lymphocytes into the effector program. IFN-I appears involved in this remodeling. Our findings provide some insight into how cyclophosphamide regulates the amplitude and quality of tumor-specific immune responses.

Frequent inoculations with radiation attenuated sporozoite is essential for inducing sterile protection that correlates with a threshold level of Plasmodia liver-stage specific CD8+ T cells

Whole sporozoite vaccine (WSV) is shown to induce sterile protection that targets Plasmodium liver-stage infection. There are many underlying issues associated with induction of effective sterile protracted protection. In this study, we have addressed how the alterations in successive vaccine regimen could possibly affect the induction of sterile protection. We have demonstrated that the pattern of vaccination with RAS (radiation attenuated sporozoites) induces varying degrees of protection among B6 mice. Animals receiving four successive doses generated 100% sterile protection. However, three successive doses, though with the same parasite inoculum as four doses, could induce sterile protection in ∼50% mice. Interestingly, mice immunized with the same 3 doses, but with longer gap, could not survive the challenge. We demonstrate that degree of protection correlates with the frequencies of IFN-γ⁺ and multifunctional (IFN-γ⁺ CD107a⁺) CD8⁺ T_EM cells present in liver. The failure to achieve protective threshold frequency of these cells in liver might make the host more vulnerable to parasite infection during infectious sporozoite challenge.

CD8 T-cell regulation by T regulatory cells and the programmed cell death protein 1 pathway

The primary function of the immune system is to protect the host from infectious microorganisms and cancers. However, a major component of the immune response involves the direct elimination of cells in the body and the induction of systemic inflammation, which may result in life-threatening immunopathology. Therefore, the immune system has developed complex mechanisms to regulate itself with a specialized subset of CD4 T lymphocytes (referred to as regulatory T cells) and immune checkpoint pathways, such as the programmed cell death protein 1 pathway. These immune regulatory mechanisms can be exploited by pathogens and tumours to establish persistence in the host, warranting a deeper understanding of how to fine-tune immune responses during these chronic diseases. Here, I discuss various features of immune regulatory pathways and what important aspects must be considered in the next generation of therapies to reverse immune exhaustion, understanding that this process is a natural mechanism to prevent the host from destroying itself.

T regulatory cells are critical for the maintenance, anamnestic expansion and protection elicited by vaccine-induced CD8 T cells

T regulatory (Treg) cells are critical for preventing autoimmunity and suppressing immune responses during cancer and chronic infection. However, the role of Treg cells in the generation of vaccine-induced immune memory remains ill-defined. Using the mouse model of lymphocytic choriomeningitis virus (LCMV) infection, we demonstrate that transient absence of Treg cells during effector to memory CD8 T-cell transition results in a permanent impairment in the maintenance, function and recall capacity of CD8 T cells. Memory CD8 T cells in mice that were transiently depleted of Treg cells exhibited defective up-regulation of memory markers with a significant decrease in polyfunctionality. However, Treg-depleted mice showed no significant change in CD4 T-cell responses, and antibody levels relative to control. Altogether, this study evaluates the role of Treg cells in the formation of immune memory and demonstrates an important role for Treg cells in promoting memory CD8 T-cell differentiation and vaccine-induced immune protection against intracellular pathogens.

TCR stimulation strength is inversely associated with establishment of functional brain-resident memory CD8 T cells during persistent viral infection

Establishing functional tissue-resident memory (T_RM) cells at sites of infection is a newfound objective of T cell vaccine design. To directly assess the impact of antigen stimulation strength on memory CD8 T cell formation and function during a persistent viral infection, we created a library of mouse polyomavirus (MuPyV) variants with substitutions in a subdominant CD8 T cell epitope that exhibit a broad range of efficiency in stimulating TCR transgenic CD8 T cells. By altering a subdominant epitope in a nonstructural viral protein and monitoring memory differentiation of donor monoclonal CD8 T cells in immunocompetent mice, we circumvented potentially confounding changes in viral infection levels, virus-associated inflammation, size of the immunodominant virus-specific CD8 T cell response, and shifts in TCR affinity that may accompany temporal recruitment of endogenous polyclonal cells. Using this strategy, we found that antigen stimulation strength was inversely associated with the function of memory CD8 T cells during a persistent viral infection. We further show that CD8 T_RM cells recruited to the brain following systemic infection with viruses expressing epitopes with suboptimal stimulation strength respond more efficiently to challenge CNS infection with virus expressing cognate antigen. These data demonstrate that the strength of antigenic stimulation during recruitment of CD8 T cells influences the functional integrity of T_RM cells in a persistent viral infection.

Tissue-resident memory (T_RM) cells are a subset of memory T cells that primarily reside in non-lymphoid tissues and serve as sentinels and effectors against secondary infections. T_RM cells have been extensively characterized in mucosal barriers, but much less is known about this population in non-barrier sites such as the brain. In this study, we designed a novel strategy to evaluate the impact of T cell stimulation strength on the generation and functionality of memory CD8 T cells in both lymphoid and nonlymphoid tissues. Using a mouse polyomavirus (MuPyV) library expressing variants of a subdominant epitope recognized by TCR transgenic CD8 T cells, we found that systemic infection producing weaker responses during T cell priming was sufficient for recruitment of effector cells to the brain. Furthermore, lower stimulation conferred greater functionality to memory T cells in the spleen and to brain T_RM cells. Our findings demonstrate that the strength of antigenic stimulation experienced by a naïve T cell early in infection is a determinant of memory functional integrity during viral persistence in a non-barrier organ.

Polycomb Repressive Complex 2-Mediated Chromatin Repression Guides Effector CD8+ T Cell Terminal Differentiation and Loss of Multipotency

Understanding immunological memory formation depends on elucidating how multipotent memory precursor (MP) cells maintain developmental plasticity and longevity to provide long-term immunity while other effector cells develop into terminally differentiated effector (TE) cells with limited survival. Profiling active (H3K27ac) and repressed (H3K27me3) chromatin in naive, MP, and TE CD8⁺ T cells during viral infection revealed increased H3K27me3 deposition at numerous pro-memory and pro-survival genes in TE relative to MP cells, indicative of fate restriction, but permissive chromatin at both pro-memory and pro-effector genes in MP cells, indicative of multipotency. Polycomb repressive complex 2 deficiency impaired clonal expansion and TE cell differentiation, but minimally impacted CD8⁺ memory T cell maturation. Abundant H3K27me3 deposition at pro-memory genes occurred late during TE cell development, probably from diminished transcription factor FOXO1 expression. These results outline a temporal model for loss of memory cell potential through selective epigenetic silencing of pro-memory genes in effector T cells.

Early Effector T Lymphocytes Coexpress Multiple Inhibitory Receptors in Primary Non-Small Cell Lung Cancer

Clinical efficacy of PD-1/PD-L1 targeting relies upon the reactivation of tumor-specific but functionally impaired PD-1⁺ T cells present before therapy. Thus, analyzing early-stage primary tumors may reveal the presence of T cells that are not yet functionally impaired. In this study, we report that activated (HLA-DR⁺) T cells with an effector memory (T_EM) profile are enriched in such lesions. Tumor-infiltrating lymphocytes coexpressed PD-1 with the inhibitory receptors TIM-3, CTLA-4, LAG-3, and TIGIT, but also displayed a recently activated, nonexhausted phenotype. We also identified a subset of CD8⁺PD-1⁺FOXP3⁺ T lymphocytes at the earliest phase of functional differentiation after priming, termed "early effector cells" (EEC), which also exhibited an activated nonexhausted phenotype, but was less differentiated and associated with coexpression of multiple inhibitory receptors. In response to autologous tumor, EECs upregulated CD107a, produced IL2 and IFNγ, and were competent for differentiation. The identification of EECs marked by inhibitory receptor expression at tumor sites will enable investigations of early stages of adaptive antitumor immunity, as well as support the rationale for administering immunotherapy in early-stage non-small cell lung cancer. Cancer Res; 77(4); 851-61. ©2016 AACR.

Akt signaling is critical for memory CD8+ T-cell development and tumor immune surveillance

Memory CD8+ T cells confer long-term immunity against tumors, and anticancer vaccines therefore should maximize their generation. Multiple memory CD8+ T-cell subsets with distinct functional and homing characteristics exist, but the signaling pathways that regulate their development are ill defined. Here we examined the role of the serine/threonine kinase Akt in the generation of protective immunity by CD8+ T cells. Akt is known to be activated by the T-cell antigen receptor and the cytokine IL-2, but its role in T-cell immunity in vivo has not been explored. Using CD8+ T cells from pdk1K465E/K465E knockin mice, we found that decreased Akt activity inhibited the survival of T cells during the effector-to-memory cell transition and abolished their differentiation into C-X-C chemokine receptor 3 (CXCR3)loCD43lo effector-like memory cells. Consequently, antitumor immunity by CD8+ T cells that display defective Akt signaling was substantially diminished during the memory phase. Reduced memory T-cell survival and altered memory cell differentiation were associated with up-regulation of the proapoptotic protein Bim and the T-box transcription factor eomesodermin, respectively. These findings suggest an important role for effector-like memory CD8+ T cells in tumor immune surveillance and identify Akt as a key signaling node in the development of protective memory CD8+ T-cell responses.

Armc5 deletion causes developmental defects and compromises T-cell immune responses

Armadillo repeat containing 5 (ARMC5) is a cytosolic protein with no enzymatic activities. Little is known about its function and mechanisms of action, except that gene mutations are associated with risks of primary macronodular adrenal gland hyperplasia. Here we map Armc5 expression by in situ hybridization, and generate Armc5 knockout mice, which are small in body size. Armc5 knockout mice have compromised T-cell proliferation and differentiation into Th1 and Th17 cells, increased T-cell apoptosis, reduced severity of experimental autoimmune encephalitis, and defective immune responses to lymphocytic choriomeningitis virus infection. These mice also develop adrenal gland hyperplasia in old age. Yeast 2-hybrid assays identify 16 ARMC5-binding partners. Together these data indicate that ARMC5 is crucial in fetal development, T-cell function and adrenal gland growth homeostasis, and that the functions of ARMC5 probably depend on interaction with multiple signalling pathways.

Mutations in ARMC5 are associated with risk of primary macronodular adrenal gland hyperplasia. Here the authors show that mice lacking Armc5 have adrenal gland hyperplasia and defective T-cell proliferation, differentiation, survival and in vivo T-cell-mediated immune responses.