Functional differences between memory and naive CD8 T cells

Survival and division fate programs are preserved but retuned during the naïve to memory CD8+ T-cell transition

Memory T cells are generated from naïve precursors undergoing proliferation during the initial immune response. Both naïve and memory T cells are maintained in a resting, quiescent state and respond to activation with a controlled proliferative burst and differentiation into effector cells. This similarity in the maintenance and response dynamics points to the preservation of key cellular fate programs; however, whether memory T cells have acquired intrinsic changes in these programs that may contribute to the enhanced immune protection in a recall response is not fully understood. Here we used a quantitative model-based analysis of proliferation and survival kinetics of in vitro-stimulated murine naïve and memory CD8+ T cells in response to homeostatic and activating signals to establish intrinsic similarities or differences within these cell types. We show that resting memory T cells display heightened sensitivity to homeostatic cytokines, responding to interleukin (IL)-2 in addition to IL-7 and IL-15. The proliferative response to αCD3 was equal in size and kinetics, demonstrating that memory T cells undergo the same controlled division burst and automated return to quiescence as naïve T cells. However, perhaps surprisingly, we observed reduced expansion of αCD3-stimulated memory T cells in response to activating signals αCD28 and IL-2 compared with naïve T cells. Overall, we demonstrate that although sensitivities to cytokine and costimulatory signals have shifted, fate programs regulating the scale of the division burst are conserved in memory T cells.

Sublethal whole-body irradiation induces permanent loss and dysfunction in pathogen-specific circulating memory CD8 T cell populations

The increasing use of nuclear energy sources inevitably raises the risk of accidental or deliberate radiation exposure and associated immune dysfunction. However, the extent to which radiation exposure impacts memory CD8 T cells, potent mediators of immunity to recurring intracellular infections and malignancies, remains understudied. Using P14 CD8 T cell chimeric mice (P14 chimeras) with an lymphocytic choriomeningitis virus (LCMV) infection model, we observed that sublethal (5Gy) whole-body irradiation (WBI) induced a rapid decline in the number of naive (TN) and P14 circulating memory CD8 T cells (TCIRCM), with the former being more susceptible to radiation-induced numeric loss. While TN cell numbers rapidly recovered, as previously described, the number of P14 TCIRCM cells remained low at least 9 mo after radiation exposure. Additionally, the remaining P14 TCIRCM in irradiated hosts exhibited an inefficient transition to a central memory (CD62L+) phenotype compared to nonirradiated P14 chimeras. WBI also resulted in long-lasting T cell intrinsic deficits in memory CD8 T cells, including diminished cytokine and chemokine production along with impaired secondary expansion upon cognate Ag reencounter. Irradiated P14 chimeras displayed significantly higher bacterial burden after challenge with Listeria monocytogenes expressing the LCMV GP33-41 epitope relative to nonirradiated controls, likely due to radiation-induced numerical and functional impairments. Taken together, our findings suggest that sublethal radiation exposure caused a long-term numerical, impaired differentiation, and functional dysregulation in preexisting TCIRCM, rendering previously protected hosts susceptible to reinfection.

Memory CD8+ T cells upregulate glycolysis and effector functions under limiting oxygen conditions

Memory CD8+ T cells are indispensable for maintaining long-term immunity against intracellular pathogens and tumors. Despite their presence at oxygen-deprived infected tissue sites or in tumors, the impact of local oxygen pressure on memory CD8+ T cells remains largely unclear. We sought to elucidate how oxygen pressure impacts memory CD8+ T cells arising after infection with Listeria monocytogenes-OVA. Our data revealed that reduced oxygen pressure during in vitro culture switched CD8+ T cell metabolism from oxidative phosphorylation to a glycolytic phenotype. Quantitative proteomic analysis showed that limiting oxygen conditions increased the expression of glucose transporters and components of the glycolytic pathway, while decreasing TCA cycle and mitochondrial respiratory chain proteins. The altered CD8+ T cell metabolism did not affect the expansion potential, but enhanced the granzyme B and IFN-γ production capacity. In vivo, memory CD8+ T cells cultured under low oxygen pressure provided protection against bacterial rechallenge. Taken together, our study indicates that strategies of cellular immune therapy may benefit from reducing oxygen during culture to develop memory CD8+ T cells with superior effector functions.

Sepsis-induced changes in differentiation, maintenance, and function of memory CD8 T cell subsets

Formation of long-lasting memory lymphocytes is one of the foundational characteristics of adaptive immunity and the basis of many vaccination strategies. Following the rapid expansion and contraction of effector CD8 T cells, the surviving antigen (Ag)-specific cells give rise to the memory CD8 T cells that persist for a long time and are phenotypically and functionally distinct from their naïve counterparts. Significant heterogeneity exists within the memory CD8 T cell pool, as different subsets display distinct tissue localization preferences, cytotoxic ability, and proliferative capacity, but all memory CD8 T cells are equipped to mount an enhanced immune response upon Ag re-encounter. Memory CD8 T cells demonstrate numerical stability under homeostatic conditions, but sepsis causes a significant decline in the number of memory CD8 T cells and diminishes their Ag-dependent and -independent functions. Sepsis also rewires the transcriptional profile of memory CD8 T cells, which profoundly impacts memory CD8 T cell differentiation and, ultimately, the protective capacity of memory CD8 T cells upon subsequent stimulation. This review delves into different aspects of memory CD8 T cell subsets as well as the immediate and long-term impact of sepsis on memory CD8 T cell biology.

Characterization of the effect of histone deacetylation inhibitors on CD8+ T cells in the context of aging

BACKGROUND

Posttranslational protein modifications regulate essential cellular processes, including the immune cell activation. Despite known age-related alterations of the phenotype, composition and cytokine profiles of immune cells, the role of acetylation in the aging process of the immune system was not broadly investigated. Therefore, in the current study the effect of acetylation on the protein expression profiles and function of CD8⁺ T cells from donors of distinct age was analyzed using histone deacetylase inhibitors (HDACi).

METHODS

CD8⁺ T cells isolated from peripheral blood mononuclear cells of 30 young (< 30 years) and 30 old (> 60 years) healthy donors were activated with anti-CD3/anti-CD28 antibodies in the presence and absence of a cocktail of HDACi. The protein expression profiles of untreated and HDACi-treated CD8⁺ T cells were analyzed using two-dimensional gel electrophoresis. Proteins with a differential expression level (less than 0.66-fold decrease or more than 1.5-fold increase) between CD8⁺ T cells of young and old donors were identified by matrix-associated laser desorption ionization-time of flight mass spectrometry. Functional enrichment analysis of proteins identified was performed using the online tool STRING. The function of CD8⁺ T cells was assessed by analyses of cytokine secretion, surface expression of activation markers, proliferative capacity and apoptosis rate.

RESULTS

The HDACi treatment of CD8⁺ T cells increased in an age-independent manner the intracellular acetylation of proteins, in particular cytoskeleton components and chaperones. Despite a strong similarity between the protein expression profiles of both age groups, the functional activity of CD8⁺ T cells significantly differed with an age-dependent increase in cytokine secretion and expression of activation markers for CD8⁺ T cells from old donors, which was maintained after HDACi treatment. The proliferation and apoptosis rate of CD8⁺ T cells after HDACi treatment was equal between both age groups.

CONCLUSIONS

Despite a comparable effect of HDACi treatment on the protein signature of CD8⁺ T cells from donors of different ages, an initial higher functionality of CD8⁺ T cells from old donors when compared to CD8⁺ T cells from young donors was detected, which might have clinical relevance.

Mechanistic and pharmacodynamic studies of DuoBody-CD3x5T4 in preclinical tumor models

CD3 bispecific antibodies (bsAbs) show great promise as anticancer therapeutics. Here, we show in-depth mechanistic studies of a CD3 bsAb in solid cancer, using DuoBody-CD3x5T4. Cross-linking T cells with tumor cells expressing the oncofetal antigen 5T4 was required to induce cytotoxicity. Naive and memory CD4+ and CD8+ T cells were equally effective at mediating cytotoxicity, and DuoBody-CD3x5T4 induced partial differentiation of naive T-cell subsets into memory-like cells. Tumor cell kill was associated with T-cell activation, proliferation, and production of cytokines, granzyme B, and perforin. Genetic knockout of FAS or IFNGR1 in 5T4+ tumor cells abrogated tumor cell kill. In the presence of 5T4+ tumor cells, bystander kill of 5T4− but not of 5T4−IFNGR1− tumor cells was observed. In humanized xenograft models, DuoBody-CD3x5T4 antitumor activity was associated with intratumoral and peripheral blood T-cell activation. Lastly, in dissociated patient-derived tumor samples, DuoBody-CD3x5T4 activated tumor-infiltrating lymphocytes and induced tumor-cell cytotoxicity, even when most tumor-infiltrating lymphocytes expressed PD-1. These data provide an in-depth view on the mechanism of action of a CD3 bsAb in preclinical models of solid cancer.

Persistent CAD activity in memory CD8+ T cells supports rRNA synthesis and ribosomal biogenesis required at rechallenge

Memory CD8⁺ T cells are characterized by their ability to persist long after the initial antigen encounter and their capacity to generate a rapid recall response. Recent studies have identified a role for metabolic reprogramming and mitochondrial function in promoting the longevity of memory T cells. However, detailed mechanisms involved in promoting their rapid recall response are incompletely understood. Here, we identify a role for the initial and continued activation of the trifunctional rate-limiting enzyme of the de novo pyrimidine synthesis pathway CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase) as critical in promoting the rapid recall response of previously activated CD8⁺ T cells. We found that CAD was rapidly phosphorylated upon naïve T cell activation in an mTORC1-dependent manner, yet remained phosphorylated long after initial activation. Previously activated CD8⁺ T cells displayed continued de novo pyrimidine synthesis in the absence of mitogenic signals, and interfering with this pathway diminished the speed and magnitude of cytokine production upon rechallenge. Inhibition of CAD did not affect cytokine transcript levels but diminished available pre-rRNA (ribosomal RNA), the polycistronic rRNA precursor whose synthesis is the rate-limiting step in ribosomal biogenesis. CAD inhibition additionally decreased levels of detectable ribosomal proteins in previously activated CD8⁺ T cells. Conversely, overexpression of CAD improved both the cytokine response and proliferation of memory T cells. Overall, our studies reveal a critical role for CAD-induced pyrimidine synthesis and ribosomal biogenesis in promoting the rapid recall response characteristic of memory T cells.

The CD8α-PILRα interaction maintains CD8+ T cell quiescence

T cell quiescence is essential for maintaining a broad repertoire against a large pool of diverse antigens from microbes and tumors, but the underlying molecular mechanisms remain largely unknown. We show here that CD8α is critical for the maintenance of CD8⁺ T cells in a physiologically quiescent state in peripheral lymphoid organs. Upon inducible deletion of CD8α, both naïve and memory CD8⁺ T cells spontaneously acquired activation phenotypes and subsequently died without exposure to specific antigens. PILRα was identified as a ligand for CD8α in both mice and humans, and disruption of this interaction was able to break CD8⁺ T cell quiescence. Thus, peripheral T cell pool size is actively maintained by the CD8α-PILRα interaction in the absence of antigen exposure.

The ER-Mitochondria Interface as a Dynamic Hub for T Cell Efficacy in Solid Tumors

The endoplasmic reticulum (ER) is a large continuous membranous organelle that plays a central role as the hub of protein and lipid synthesis while the mitochondria is the principal location for energy production. T cells are an immune subset exhibiting robust dependence on ER and mitochondrial function based on the need for protein synthesis and secretion and metabolic dexterity associated with foreign antigen recognition and cytotoxic effector response. Intimate connections exist at mitochondrial-ER contact sites (MERCs) that serve as the structural and biochemical platforms for cellular metabolic homeostasis through regulation of fission and fusion as well as glucose, Ca²⁺, and lipid exchange. Work in the tumor immunotherapy field indicates that the complex interplay of nutrient deprivation and tumor antigen stimulation in the tumor microenvironment places stress on the ER and mitochondria, causing dysfunction in organellar structure and loss of metabolic homeostasis. Here, we assess prior literature that establishes how the structural interface of these two organelles is impacted by the stress of solid tumors along with recent advances in the manipulation of organelle homeostasis at MERCs in T cells. These findings provide strong evidence for increased tumor immunity using unique therapeutic avenues that recharge cellular metabolic homeostasis in T cells.

The Interplay Between Epigenetic Regulation and CD8+ T Cell Differentiation/Exhaustion for T Cell Immunotherapy

Effective immunotherapy treats cancers by eradicating tumourigenic cells by activated tumour antigen-specific and bystander CD8⁺ T-cells. However, T-cells can gradually lose cytotoxicity in the tumour microenvironment, known as exhaustion. Recently, DNA methylation, histone modification, and chromatin architecture have provided novel insights into epigenetic regulations of T-cell differentiation/exhaustion, thereby controlling the translational potential of the T-cells. Thus, developing strategies to govern epigenetic switches of T-cells dynamically is critical to maintaining the effector function of antigen-specific T-cells. In this mini-review, we 1) describe the correlation between epigenetic states and T cell phenotypes; 2) discuss the enzymatic factors and intracellular/extracellular microRNA imprinting T-cell epigenomes that drive T-cell exhaustion; 3) highlight recent advances in epigenetic interventions to rescue CD8⁺ T-cell functions from exhaustion. Finally, we express our perspective that regulating the interplay between epigenetic changes and transcriptional programs provides translational implications of current immunotherapy for cancer treatments.

Cell Cycle Entry Control in Naïve and Memory CD8+ T Cells

CD8+ T cells play important roles in immunity and immuno-oncology. Upon antigen recognition and co-stimulation, naïve CD8+ T cells escape from dormancy to engage in a complex programme of cellular growth, cell cycle entry and differentiation, resulting in rapid proliferation cycles that has the net effect of producing clonally expanded, antigen-specific cytotoxic T lymphocytes (CTLs). A fraction of activated T cells will re-enter dormancy by differentiating into memory T cells, which have essential roles in adaptive immunity. In this review, we discuss the current understanding of cell cycle entry control in CD8+ T cells and crosstalk between these mechanisms and pathways regulating immunological phenotypes.

Fundamentals of T Cell Metabolism and Strategies to Enhance Cancer Immunotherapy

Emerging reports show that metabolic pathways can be targeted to enhance T cell-mediated immunity to tumors. Yet, tumors consume key metabolites in the host to survive, thus robbing T cells of these nutrients to function and thrive. T cells are often deprived of basic building blocks for energy in the tumor, including glucose and amino acids needed to proliferate or produce cytotoxic molecules against tumors. Immunosuppressive molecules in the host further compromise the lytic capacity of T cells. Moreover, checkpoint receptors inhibit T cell responses by impairing their bioenergetic potential within tumors. In this review, we discuss the fundamental metabolic pathways involved in T cell activation, differentiation and response against tumors. We then address ways to target metabolic pathways to improve the next generation of immunotherapies for cancer patients.

Single Cell Analysis of Blood Mononuclear Cells Stimulated Through Either LPS or Anti-CD3 and Anti-CD28

Immune cell activation assays have been widely used for immune monitoring and for understanding disease mechanisms. However, these assays are typically limited in scope. A holistic study of circulating immune cell responses to different activators is lacking. Here we developed a cost-effective high-throughput multiplexed single-cell RNA-seq combined with epitope tagging (CITE-seq) to determine how classic activators of T cells (anti-CD3 coupled with anti-CD28) or monocytes (LPS) alter the cell composition and transcriptional profiles of peripheral blood mononuclear cells (PBMCs) from healthy human donors. Anti-CD3/CD28 treatment activated all classes of lymphocytes either directly (T cells) or indirectly (B and NK cells) but reduced monocyte numbers. Activated T and NK cells expressed senescence and effector molecules, whereas activated B cells transcriptionally resembled autoimmune disease- or age-associated B cells (e.g., CD11c, T-bet). In contrast, LPS specifically targeted monocytes and induced two main states: early activation characterized by the expression of chemoattractants and a later pro-inflammatory state characterized by expression of effector molecules. These data provide a foundation for future immune activation studies with single cell technologies (https://czi-pbmc-cite-seq.jax.org/).

Area under Immunosurveillance: Dedicated Roles of Memory CD8 T-Cell Subsets

Immunological memory, defined as the ability to respond in an enhanced manner upon secondary encounter with the same pathogen, can provide substantial protection against infectious disease. The improved protection is mediated in part by different populations of memory CD8 T cells that are retained after primary infection. Memory cells persist in the absence of pathogen-derived antigens and enable secondary CD8 T-cell responses with accelerated kinetics and of larger magnitude after reencounter with the same pathogen. At least three subsets of memory T cells have been defined that are referred to as central memory CD8 T cells (Tcm), effector memory CD8 T cells (Tem), and tissue-resident memory CD8 T cells (Trm). Tcm and Tem are circulating memory T cells that mediate bodywide immune surveillance in search of invading pathogens. In contrast, Trm permanently reside in peripheral barrier tissues, where they form a stationary defensive line of sentinels that alert the immune system upon pathogen reencounter. The characterization of these different subsets has been instrumental in our understanding of the strategies that memory T cells employ to counter invading pathogens. It is clear that memory T cells not only have a numerical advantage over naive T cells resulting in improved protection in secondary responses, but also acquire distinct sets of competencies that assist in pathogen clearance. Nevertheless, inherent challenges are associated with the allocation of memory T cells to a limited number of subsets. The classification of memory T cells into Tcm, Tem, and Trm may not take into account the full extent of the heterogeneity that is observed in the memory population. Therefore, in this review, we will revisit the current classification of memory subsets, elaborate on functional and migratory properties attributed to Tcm, Tem, and Trm, and discuss how potential heterogeneity within these populations arises and persists.

Memory CD4+ T Cells in Immunity and Autoimmune Diseases

CD4⁺ T helper (Th) cells play central roles in immunity in health and disease. While much is known about the effector function of Th cells in combating pathogens and promoting autoimmune diseases, the roles and biology of memory CD4⁺ Th cells are complex and less well understood. In human autoimmune diseases such as multiple sclerosis (MS), there is a critical need to better understand the function and biology of memory T cells. In this review article we summarize current concepts in the field of CD4⁺ T cell memory, including natural history, developmental pathways, subsets, and functions. Furthermore, we discuss advancements in the field of the newly-described CD4⁺ tissue-resident memory T cells and of CD4⁺ memory T cells in autoimmune diseases, two major areas of important unresolved questions in need of answering to advance new vaccine design and development of novel treatments for CD4⁺ T cell-mediated autoimmune diseases.

WASp Is Essential for Effector-to-Memory conversion and for Maintenance of CD8+T Cell Memory

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency characterized by recurrent infections, micro thrombocytopenia, eczema, and a high incidence of autoimmunity and malignancy. A defect in the T cell compartment is thought to be a major cause of immunodeficiency in patients with WAS; However, whether the antigen specific T memory cell is altered has not been extensively studied. Here, we examined the expansion/contraction kinetics of CD8⁺ memory T cells and their maintenance in WASp^−/− mice. The results showed that WAS protein (WASp) is not required for differentiation of CD8⁺ effector T cells; however, CD8⁺ T cells from WASp^−/− mice were hyperactive, resulting in increased cytokine production. The number of CD8⁺ T memory cells decreased as mice aged, and CD8⁺ T cell recall responses and protective immunity were impaired. WASp-deficient CD8⁺ T cells in bone marrow chimeric mice underwent clonal expansion, but the resulting effector cells failed to survive and differentiate into CD8⁺ memory T cells. Taken together, these findings indicate that WASp plays an intrinsic role in differentiation of CD8⁺ memory T cells.

Striking a Balance-Cellular and Molecular Drivers of Memory T Cell Development and Responses to Chronic Stimulation

Effective adaptive immune responses are characterized by stages of development and maturation of T and B cell populations that respond to disturbances in the host homeostasis in cases of both infections and cancer. For the T cell compartment, this begins with recognition of specific peptides by naïve, antigen-inexperienced T cells that results in their activation, proliferation, and differentiation, which generates an effector population that clears the antigen. Loss of stimulation eventually returns the host to a homeostatic state, with a heterogeneous memory T cell population that persists in the absence of antigen and is primed for rapid responses to a repeat antigen exposure. However, in chronic infections and cancers, continued antigen persistence impedes a successful adaptive immune response and the formation of a stereotypical memory population of T cells is compromised. With repeated antigen stimulation, responding T cells proceed down an altered path of differentiation that allows for antigen persistence, but much less is known regarding the heterogeneity of these cells and the extent to which they can become “memory-like,” with a capacity for self-renewal and recall responses that are characteristic of bona fide memory cells. This review focuses on the differentiation of CD4⁺ and CD8⁺ T cells in the context of chronic antigen stimulation, highlighting the central observations in both human and mouse studies regarding the differentiation of memory or “memory-like” T cells. The importance of both the cellular and molecular drivers of memory T cell development are emphasized to better understand the consequences of persisting antigen on T cell fates. Integrating what is known and is common across model systems and patients can instruct future studies aimed at further understanding T cell differentiation and development, with the goal of developing novel methods to direct T cells toward the generation of effective memory populations.

Bromodomain inhibitor JQ1 reversibly blocks IFN-γ production

As a class, ‘BET’ inhibitors disrupt binding of bromodomain and extra-terminal motif (BET) proteins, BRD2, BRD3, BRD4 and BRDT, to acetylated histones preventing recruitment of RNA polymerase 2 to enhancers and promoters, especially super-enhancers, to inhibit gene transcription. As such, BET inhibitors may be useful therapeutics for treatment of cancer and inflammatory disease. For example, the small molecule BET inhibitor, JQ1, selectively represses MYC, an important oncogene regulated by a super-enhancer. IFN-γ, a critical cytokine for both innate and adaptive immune responses, is also regulated by a super-enhancer. Here, we show that JQ1 represses IFN-γ expression in TH1 polarized PBMC cultures, CD4+ memory T cells, and NK cells. JQ1 treatment does not reduce activating chromatin marks at the IFNG locus, but displaces RNA polymerase II from the locus. Further, IFN-γ expression recovers in polarized TH1 cultures following removal of JQ1. Our results show that JQ1 abrogates IFN-γ expression, but repression is reversible. Thus, BET inhibitors may disrupt the normal functions of the innate and adaptive immune response.

Cutting Edge: Synapse Propensity of Human Memory CD8 T Cells Confers Competitive Advantage over Naive Counterparts

Microcontact printing can be used to mimic spatially limiting Ag presentation.

High synapse propensity of human memory CD8 T cells prevents naive cell recruitment.

Memory T cells are endowed with multiple functional features that enable them to be more protective than naive T cells against infectious threats. It is not known if memory cells have a higher synapse propensity (SP; i.e., increased probability to form immature immunological synapses that then provide an entry into different modes of durable interaction with APCs). In this study, we show that only human memory CD8 T cells have remarkably high SP compared with naive counterparts. Such a dichotomy between naive and memory cells is not observed within the human CD4 or murine CD8 T cell population. Higher SP in human memory CD8 T cells allows them to outcompete and prevent naive CD8 T cells from getting recruited to the response. This observation has implications for original antigenic sin and aging of the immune system in humans.

NF-κB/mTOR/MYC Axis Drives PRMT5 Protein Induction After T Cell Activation via Transcriptional and Non-transcriptional Mechanisms

Multiple sclerosis is an autoimmune disease of the central nervous system (CNS) mediated by CD4+ T cells and modeled via experimental autoimmune encephalomyelitis (EAE). Inhibition of PRMT5, the major Type II arginine methyltransferase, suppresses pathogenic T cell responses and EAE. PRMT5 is transiently induced in proliferating memory inflammatory Th1 cells and during EAE. However, the mechanisms driving PRMT5 protein induction and repression as T cells expand and return to resting is currently unknown. Here, we used naive mouse and memory mouse and human Th1/Th2 cells as models to identify mechanisms controlling PRMT5 protein expression in initial and recall T cell activation. Initial activation of naive mouse T cells resulted in NF-κB-dependent transient Prmt5 transcription and NF-κB, mTOR and MYC-dependent PRMT5 protein induction. In murine memory Th cells, transcription and miRNA loss supported PRMT5 induction to a lesser extent than in naive T cells. In contrast, NF-κB/MYC/mTOR-dependent non-transcriptional PRMT5 induction played a major role. These results highlight the importance of the NF-κB/mTOR/MYC axis in PRMT5-driven pathogenic T cell expansion and may guide targeted therapeutic strategies for MS.

Allograft dendritic cell p40 homodimers activate donor-reactive memory CD8+ T cells

Recipient endogenous memory T cells with donor reactivity pose an important barrier to successful transplantation and costimulatory blockade-induced graft tolerance. Longer ischemic storage times prior to organ transplantation increase early posttransplant inflammation and negatively impact early graft function and long-term graft outcome. Little is known about the mechanisms enhancing endogenous memory T cell activation to mediate tissue injury within the increased inflammatory environment of allografts subjected to prolonged cold ischemic storage (CIS). Endogenous memory CD4+ and CD8+ T cell activation is markedly increased within complete MHC-mismatched cardiac allografts subjected to prolonged versus minimal CIS, and the memory CD8+ T cells directly mediate CTLA-4Ig-resistant allograft rejection. Memory CD8+ T cell activation within allografts subjected to prolonged CIS requires memory CD4+ T cell stimulation of graft DCs to produce p40 homodimers, but not IL-12 p40/p35 heterodimers. Targeting p40 abrogates memory CD8+ T cell proliferation within the allografts and their ability to mediate CTLA-4Ig-resistant allograft rejection. These findings indicate a critical role for memory CD4+ T cell-graft DC interactions to increase the intensity of endogenous memory CD8+ T cell activation needed to mediate rejection of higher-risk allografts subjected to increased CIS.

Comparative analysis of murine T-cell receptor repertoires

For understanding the rules and laws of adaptive immunity, high-throughput profiling of T-cell receptor (TCR) repertoires becomes a powerful tool. The structure of TCR repertoires is instructive even before the antigen specificity of each particular receptor becomes available. It embodies information about the thymic and peripheral selection of T cells; the readiness of an adaptive immunity to withstand new challenges; the character, magnitude and memory of immune responses; and the aetiological and functional proximity of T-cell subsets. Here, we describe our current analytical approaches for the comparative analysis of murine TCR repertoires, and show several examples of how these approaches can be applied for particular experimental settings. We analyse the efficiency of different metrics used for estimation of repertoire diversity, repertoire overlap, V-gene and J-gene segments usage similarity, and amino acid composition of CDR3. We discuss basic differences of these metrics and their advantages and limitations in different experimental models, and we provide guidelines for choosing an efficient way to lead a comparative analysis of TCR repertoires. Applied to the various known and newly developed mouse models, such analysis should allow us to disentangle multiple sophisticated puzzles in adaptive immunity.

CD45-mediated control of TCR tuning in naïve and memory CD8+ T cells

Continuous contact with self-major histocompatibility complex (MHC) ligands is essential for survival of naïve T cells but not memory cells. This surprising finding implies that T cell subsets may vary in their relative T-cell receptor (TCR) sensitivity. Here we show that in CD8⁺T cells TCR sensitivity correlates inversely with levels of CD5, a marker for strong self-MHC reactivity. We also show that TCR sensitivity is lower in memory CD8⁺ T cells than naïve cells. In both situations, TCR hypo-responsiveness applies only to short-term TCR signalling events and not to proliferation, and correlates directly with increased expression of a phosphatase, CD45 and reciprocal decreased expression of activated LCK. Inhibition by high CD45 on CD8⁺ T cells may protect against overt TCR auto-MHC reactivity, while enhanced sensitivity to cytokines ensures strong responses to foreign antigens.

Naïve T cells establish self-tolerance via negative selection of cells with strong reactivity for self-peptide/MHC complexes, but undergo T-cell receptor (TCR) desensitisation when leaving the thymus. Here, Cho et al. show that TCR desensitisation correlates with cell-surface density of the phosphatase CD45.

Regulation of effector and memory CD8(+) T cell function by inflammatory cytokines

Cells communicate with each other through the production and secretion of cytokines, which are integral to the host response to infection. Once recognized by specific cytokine receptors expressed on the cell surface, these exogenous signals direct the biological function of a cell in order to adapt to their microenvironment. CD8(+) T cells are critical immune cells that play an important role in the control and elimination of intracellular pathogens. Current findings have demonstrated that cytokines influence all aspects of the CD8(+) T cell response to infection or immunization. The cytokine milieu induced at the time of activation impacts the overall magnitude and function of the effector CD8(+) T cell response and the generation of functional memory CD8(+) T cells. This review will focus on the impact of inflammatory cytokines on different aspects of CD8(+) T cell biology.

Dynamic regulation of permissive histone modifications and GATA3 binding underpin acquisition of granzyme A expression by virus-specific CD8(+) T cells

Numerous studies have focused on the molecular regulation of perforin (PFP) and granzyme B (GZMB) expression by activated cytotoxic T lymphocytes (CTLs), but little is known about the molecular factors that underpin granzyme A (GZMA) expression. In vitro activation of naïve CD8(+) T cells, in the presence of IL-4, enhanced STAT6-dependent GZMA expression and was associated with GATA3 binding and enrichment of transcriptionally permissive histone posttranslational modifications (PTMs) across the Gzma gene locus. While GZMA expression by effector influenza A virus specific CTLs was also associated with a similar permissive epigenetic signature, memory CTL lacked enrichment of permissive histone PTMs at the Gzma locus, although this was restored within recalled secondary effector CTLs. Importantly, GZMA expression by virus-specific CTLs was associated with GATA3 binding at the Gzma locus, and independent of STAT6-mediated signaling. This suggests regulation of GZMA expression is underpinned by differentiation-dependent regulation of chromatin composition at the Gzma locus and that, given GATA3 is key for CTL differentiation in response to infection, GATA3 expression is regulated by a distinct, IL-4 independent, signaling pathway. Overall, this study provides insights into the molecular mechanisms that control transcription of Gzma during virus-induced CD8(+) T-cell differentiation.

Transcriptional Enhancers in the Regulation of T Cell Differentiation

The changes in phenotype and function that characterize the differentiation of naïve T cells to effector and memory states are underscored by large-scale, coordinated, and stable changes in gene expression. In turn, these changes are choreographed by the interplay between transcription factors and epigenetic regulators that act to restructure the genome, ultimately ensuring lineage-appropriate gene expression. Here, we focus on the mechanisms that control T cell differentiation, with a particular focus on the role of regulatory elements encoded within the genome, known as transcriptional enhancers (TEs). We discuss the central role of TEs in regulating T cell differentiation, both in health and disease.

Comparison of naïve and central memory derived CD8+ effector cell engraftment fitness and function following adoptive transfer

Human CD8⁺ effector T cells derived from CD45RO⁺CD62L⁺ precursors enriched for central memory (T_CM) precursors retain the capacity to engraft and reconstitute functional memory upon adoptive transfer, whereas effectors derived from CD45RO⁺CD62L^- precursors enriched for effector memory precursors do not. Here we sought to compare the engraftment fitness and function of CD8⁺ effector T cells derived from CD45RA⁺CD62L⁺ precursors enriched for naïve and stem cell memory precursors (T_N/SCM) with that of T_CM. We found that cytotoxic T cells (CTLs) derived from T_CM transcribed higher levels of CD28, FOS, INFγ, Eomesodermin (Eomes), and lower levels of BCL2L11, maintained higher levels of phosphorylated AKT, and displayed enhanced sensitivity to the proliferative and anti-apoptotic effects of γ-chain cytokines compared to CTLs derived from T_N/SCM. Higher frequencies of CTLs derived from T_CM retained CD28 expression and upon activation secreted higher levels of IL-2. In NOD/Scid IL-2RγC^null mice, CD8⁺ T_CM derived CTLs engrafted to higher frequencies in response to human IL-15 and mounted robust proliferative responses to an immunostimulatory vaccine. Similarly, CD8⁺ T_CM derived CD19CAR⁺ CTLs exhibited superior antitumor potency following adoptive transfer compared to their CD8⁺ T_N/SCM derived counterparts. These studies support the use of T_CM enriched cell products for adoptive therapy of cancer.

Inflammatory IL-15 is required for optimal memory T cell responses

Due to their ability to rapidly proliferate and produce effector cytokines, memory CD8+ T cells increase protection following reexposure to a pathogen. However, low inflammatory immunizations do not provide memory CD8+ T cells with a proliferation advantage over naive CD8+ T cells, suggesting that cell-extrinsic factors enhance memory CD8+ T cell proliferation in vivo. Herein, we demonstrate that inflammatory signals are critical for the rapid proliferation of memory CD8+ T cells following infection. Using murine models of viral infection and antigen exposure, we found that type I IFN-driven expression of IL-15 in response to viral infection prepares memory CD8+ T cells for rapid division independently of antigen reexposure by transiently inducing cell-cycle progression via a pathway dependent on mTOR complex-1 (mTORC1). Moreover, exposure to IL-15 allowed more rapid division of memory CD8+ T cells following antigen encounter and enhanced their protective capacity against viral infection. Together, these data reveal that inflammatory IL-15 promotes optimal responses by memory CD8+ T cells.

Chitosan hydrogel vaccine generates protective CD8 T cell memory against mouse melanoma

CD8(+) T cells are important in the control of viral infections and cancers because of their cytolytic activity. A vaccine able to generate these cells could be beneficial in the prevention or treatment of these diseases. Chitosan hydrogel is a promising vaccine formulation that has previously been shown to generate effector CD8(+) T cells in a mouse model. This vaccine promotes sustained release of antigen and adjuvant, which generates a robust effector response. For longer lasting immunity, a memory population of these CD8(+) T cells is required to control further disease. We found that vaccination with chitosan hydrogel or dendritic cells using ovalbumin protein as a model antigen and Quil-A adjuvant provided protection in a subcutaneous melanoma challenge 30 days later. Ovalbumin-specific memory CD8(+) T cells were detectable following vaccination with the chitosan hydrogel but not the dendritic cell vaccine and an in vivo cytotoxicity assay demonstrated specific lysis of target cells in chitosan hydrogel vaccinated mice but not those receiving dendritic cell vaccination. These results demonstrate that vaccination with chitosan hydrogel is equally effective as dendritic cell vaccination in tumour protection but has more readily detectable immune correlates of protection. This may be advantageous in predetermining protection in vaccinated individuals.

CCDC88B is a novel regulator of maturation and effector functions of T cells during pathological inflammation

Kennedy et al. identify a mutation in coiled-coil domain containing protein 88b (Ccdc88b) that confers protection against lethal neuroinflammation during experimental cerebral malaria. CCDC88B is expressed in immune cells and regulates T cell maturation and effector functions. In humans, the CCDC88B gene maps to a locus associated with susceptibility to several inflammatory and autoimmune disorders.

We used a genome-wide screen in mutagenized mice to identify genes which inactivation protects against lethal neuroinflammation during experimental cerebral malaria (ECM). We identified an ECM-protective mutation in coiled-coil domain containing protein 88b (Ccdc88b), a poorly annotated gene that is found expressed specifically in spleen, bone marrow, lymph nodes, and thymus. The CCDC88B protein is abundantly expressed in immune cells, including both CD4⁺ and CD8⁺ T lymphocytes, and in myeloid cells, and loss of CCDC88B protein expression has pleiotropic effects on T lymphocyte functions, including impaired maturation in vivo, significantly reduced activation, reduced cell division as well as impaired cytokine production (IFN-γ and TNF) in response to T cell receptor engagement, or to nonspecific stimuli in vitro, and during the course of P. berghei infection in vivo. This identifies CCDC88B as a novel and important regulator of T cell function. The human CCDC88B gene maps to the 11q13 locus that is associated with susceptibility to several inflammatory and auto-immune disorders. Our findings strongly suggest that CCDC88B is the morbid gene underlying the pleiotropic effect of the 11q13 locus on inflammation.

A human immunodeficiency caused by mutations in the PIK3R1 gene

Recently, patient mutations that activate PI3K signaling have been linked to a primary antibody deficiency. Here, we used whole-exome sequencing and characterized the molecular defects in 4 patients from 3 unrelated families diagnosed with hypogammaglobulinemia and recurrent infections. We identified 2 different heterozygous splice site mutations that affect the same splice site in PIK3R1, which encodes the p85α subunit of PI3K. The resulting deletion of exon 10 produced a shortened p85α protein that lacks part of the PI3K p110-binding domain. The hypothetical loss of p85α-mediated inhibition of p110 activity was supported by elevated phosphorylation of the known downstream signaling kinase AKT in patient T cell blasts. Analysis of patient blood revealed that naive T and memory B cell counts were low, and T cell blasts displayed enhanced activation-induced cell death, which was corrected by addition of the PI3Kδ inhibitor IC87114. Furthermore, B lymphocytes proliferated weakly in response to activation via the B cell receptor and TLR9, indicating a B cell defect. The phenotype exhibited by patients carrying the PIK3R1 splice site mutation is similar to that of patients carrying gain-of-function mutations in PIK3CD. Our results suggest that PI3K activity is tightly regulated in T and B lymphocytes and that various defects in the PI3K-triggered pathway can cause primary immunodeficiencies.

IL-15 regulates memory CD8+ T cell O-glycan synthesis and affects trafficking

Memory and naive CD8+ T cells exhibit distinct trafficking patterns. Specifically, memory but not naive CD8+ T cells are recruited to inflamed tissues in an antigen-independent manner. However, the molecular mechanisms that regulate memory CD8+ T cell trafficking are largely unknown. Here, using murine models of infection and T cell transfer, we found that memory but not naive CD8+ T cells dynamically regulate expression of core 2 O-glycans, which interact with P- and E-selectins to modulate trafficking to inflamed tissues. Following infection, antigen-specific effector CD8+ T cells strongly expressed core 2 O-glycans, but this glycosylation pattern was lost by most memory CD8+ T cells. After unrelated infection or inflammatory challenge, memory CD8+ T cells synthesized core 2 O-glycans independently of antigen restimulation. The presence of core 2 O-glycans subsequently directed these cells to inflamed tissue. Memory and naive CD8+ T cells exhibited the opposite pattern of epigenetic modifications at the Gcnt1 locus, which encodes the enzyme that initiates core 2 O-glycan synthesis. The open chromatin configuration in memory CD8+ T cells permitted de novo generation of core 2 O-glycans in a TCR-independent, but IL-15-dependent, manner. Thus, IL-15 stimulation promotes antigen-experienced memory CD8+ T cells to generate core 2 O-glycans, which subsequently localize them to inflamed tissues. These findings suggest that CD8+ memory T cell trafficking potentially can be manipulated to improve host defense and immunotherapy.

Influenza infection results in local expansion of memory CD8(+) T cells with antigen non-specific phenotype and function

Primary viral infections induce activation of CD8(+) T cells responsible for effective resistance. We sought to characterize the nature of the CD8(+) T cell expansion observed after primary viral infection with influenza. Infection of naive mice with different strains of influenza resulted in the rapid expansion of memory CD8(+) T cells exhibiting a unique bystander phenotype with significant up-regulation of natural killer group 2D (NKG2D), but not CD25, on the CD44(high) CD8(+) T cells, suggesting an antigen non-specific phenotype. We further confirmed the non-specificity of this phenotype on ovalbumin-specific (OT-I) CD8(+) T cells, which are not specific to influenza. These non-specific CD8(+) T cells also displayed increased lytic capabilities and were observed primarily in the lung. Thus, influenza infection was shown to induce a rapid, antigen non-specific memory T cell expansion which is restricted to the specific site of inflammation. In contrast, CD8(+) T cells of a similar phenotype could be observed in other organs following administration of systemic agonistic anti-CD40 and interleukin-2 immunotherapy, demonstrating that bystander expansion in multiple sites is possible depending on whether the nature of activation is either acute or systemic. Finally, intranasal blockade of NKG2D resulted in a significant increase in viral replication early during the course of infection, suggesting that NKG2D is a critical mediator of anti-influenza responses prior to the initiation of adaptive immunity. These results characterize further the local bystander expansion of tissue-resident, memory CD8(+) T cells which, due to their early induction, may play an important NKG2D-mediated, antigen non-specific role during the early stages of viral infection.

Activation of the TCR complex by small chemical compounds

Small chemical compounds and certain metal ions can activate T cells, resulting in drug hypersensitivity reactions that are a main problem in pharmacology. Mostly, the drugs generate new antigenic epitopes on peptide-major histocompatibility complex (MHC) molecules that are recognized by the T-cell antigen receptor (TCR). In this review we discuss the molecular mechanisms of how the drugs alter self-peptide-MHC, so that neo-antigens are produced. This includes (1) haptens covalently bound to peptides presented by MHC, (2) metal ions and drugs that non-covalently bridge self-pMHC to the TCR, and (3) drugs that allow self-peptides to be presented by MHCs that otherwise are not presented. We also briefly discuss how a second signal-next to the TCR-that naïve T cells require to become activated is generated in the drug hypersensitivity reactions.

Proximity of TCR and its CD8 coreceptor controls sensitivity of T cells

Spatial organisation of T cell receptor (TCR) and its coreceptor CD8 on the surface of live naïve and Ag-experienced CD8(+) T cells was resolved by fluorescence lifetime cross-correlation microscopy. We found that exposure of naïve CD8(+) T cells to antigen (Ag) causes formation of [TCR, CD8] functional ensembles on the cell surface which correlated with significantly enhanced sensitivity of these cells. In contrast, TCR and CD8 are randomly distributed on the surface of naïve cells. Our model suggests that close proximity of TCR and CD8 can increase Ag sensitivity of T cells by significant accelerating the TCR-peptide-major histocompatibility complex (pMHC) binding rate and stabilisation of this complex. We suggest that the proximity of these primary signalling molecules contributes to the mechanism of functional avidity maturation of CD8(+) T cells by switching them from a low to high sensitivity mode.

Immunogenicity of a recombinant measles HIV-1 subtype C vaccine

The HIV epidemic is greatest in Sub-Saharan Africa and India where HIV-1 subtype C is predominant. To control the spread of HIV in these parts of the world a preventive HIV-1 subtype C vaccine is urgently required. Here we report the immunogenicity of a candidate HIV-1 subtype C vaccine delivered by a recombinant measles vector carrying an insert encoding HIV-1 subtype C Gag, RT and Nef (MV1-F4), in MHC-typed non-human primates. HIV-1 specific cytokine secreting CD4+ and CD8+ T cell responses were detected in 15 out of 16 vaccinees. These HIV-specific T cell responses persisted in lymphoid tissues. Anti-HIV-1 antibody responses were detected in 15 out of 16 vaccinees and titres were boosted by a second immunisation carried out 84 days later. These findings support further exploration of the MV1-F4 vector as a candidate HIV-1 subtype C vaccine or as part of a wider vaccine strategy.

CD8 memory T cells have a bioenergetic advantage that underlies their rapid recall ability

A characteristic of memory T (TM) cells is their ability to mount faster and stronger responses to reinfection than naïve T (TN) cells do in response to an initial infection. However, the mechanisms that allow this rapid recall are not completely understood. We found that CD8 TM cells have more mitochondrial mass than CD8 TN cells and, that upon activation, the resulting secondary effector T (TE) cells proliferate more quickly, produce more cytokines, and maintain greater ATP levels than primary effector T cells. We also found that after activation, TM cells increase oxidative phosphorylation and aerobic glycolysis and sustain this increase to a greater extent than TN cells, suggesting that greater mitochondrial mass in TM cells not only promotes oxidative capacity, but also glycolytic capacity. We show that mitochondrial ATP is essential for the rapid induction of glycolysis in response to activation and the initiation of proliferation of both TN and TM cells. We also found that fatty acid oxidation is needed for TM cells to rapidly respond upon restimulation. Finally, we show that dissociation of the glycolysis enzyme hexokinase from mitochondria impairs proliferation and blocks the rapid induction of glycolysis upon T-cell receptor stimulation in TM cells. Our results demonstrate that greater mitochondrial mass endows TM cells with a bioenergetic advantage that underlies their ability to rapidly recall in response to reinfection.

Defining the molecular blueprint that drives CD8(+) T cell differentiation in response to infection

A cardinal feature of adaptive, cytotoxic T lymphocyte (CTL)-mediated immunity is the ability of naïve CTLs to undergo a program of differentiation and proliferation upon activation resulting in the acquisition of lineage-specific T cell functions and eventual establishment of immunological memory. In this review, we examine the molecular factors that shape both the acquisition and maintenance of lineage-specific effector function in virus-specific CTL during both the effector and memory phases of immunity.

Immunogenicity of a recombinant measles-HIV-1 clade B candidate vaccine

Live attenuated measles virus is one of the most efficient and safest vaccines available, making it an attractive candidate vector for a HIV/AIDS vaccine aimed at eliciting cell-mediated immune responses (CMI). Here we have characterized the potency of CMI responses generated in mice and non-human primates after intramuscular immunisation with a candidate recombinant measles vaccine carrying an HIV-1 insert encoding Clade B Gag, RT and Nef (MV1-F4). Eight Mauritian derived, MHC-typed cynomolgus macaques were immunised with 10⁵ TCID₅₀ of MV1-F4, four of which were boosted 28 days later with the same vaccine. F4 and measles virus (MV)-specific cytokine producing T cell responses were detected in 6 and 7 out of 8 vaccinees, respectively. Vaccinees with either M6 or recombinant MHC haplotypes demonstrated the strongest cytokine responses to F4 peptides. Polyfunctional analysis revealed a pattern of TNFα and IL-2 responses by CD4+ T cells and TNFα and IFNγ responses by CD8+ T cells to F4 peptides. HIV-specific CD4+ and CD8+ T cells expressing cytokines waned in peripheral blood lymphocytes by day 84, but CD8+ T cell responses to F4 peptides could still be detected in lymphoid tissues more than 3 months after vaccination. Anti-F4 and anti-MV antibody responses were detected in 6 and 8 out of 8 vaccinees, respectively. Titres of anti-F4 and MV antibodies were boosted in vaccinees that received a second immunisation. MV1-F4 carrying HIV-1 Clade B inserts induces robust boostable immunity in non-human primates. These results support further exploration of the MV1-F4 vector modality in vaccination strategies that may limit HIV-1 infectivity.

Allospecific CD4(+) effector memory T cells do not induce graft-versus-host disease in mice

We studied whether allospecific CD4(+) effector memory T cells (T(EM)) could induce graft-versus-host disease (GVHD) using a novel GVHD model induced solely by CD4(+) T cell receptor transgenic TEa cells. Allospecific T(EM) generated in a lymphopenic host bore a typical memory phenotype. Moreover, these cells were able to elicit a faster and more effective proliferative response on challenge with alloantigen in vitro and to mediate "second-set" skin graft rejection in vivo. However, these allospecific T(EM) were unable to induce GVHD. Allospecific T(EM) recipients became tolerant to alloantigen as a result of clonal deletion. Even though allospecific T(EM) were able to respond to alloantigen initially, the expansion of these cells and inflammatory cytokine production during GVHD were dramatically decreased. The inability of allospecific T(EM) to sustain the alloresponse may be a result of enhanced activation-induced cell death. These observations provide insight into how allospecific CD4(+) T(EM) respond to alloantigen during GVHD and underscore the fundamental differences in alloresponses mediated by allospecific T(EM) in graft rejection and GVHD settings.

Delineation of antigen-specific and antigen-nonspecific CD8(+) memory T-cell responses after cytokine-based cancer immunotherapy

Memory T cells exhibit tremendous antigen specificity within the immune system and accumulate with age. Our studies reveal an antigen-independent expansion of memory, but not naive, CD8(+) T cells after several immunotherapeutic regimens for cancer resulting in a distinctive phenotype. Signaling through T-cell receptors (TCRs) or CD3 in both mouse and human memory CD8(+) T cells markedly up-regulated programmed death-1 (PD-1) and CD25 (IL-2 receptor α chain), and led to antigen-specific tumor cell killing. In contrast, exposure to cytokine alone in vitro or with immunotherapy in vivo did not up-regulate these markers but resulted in expanded memory CD8(+) T cells expressing NKG2D, granzyme B, and possessing broadly lytic capabilities. Blockade of NKG2D in mice also resulted in significantly diminished antitumor effects after immunotherapy. Treatment of TCR-transgenic mice bearing nonantigen expressing tumors with immunotherapy still resulted in significant antitumor effects. Human melanoma tissue biopsies obtained from patients after topically applied immunodulatory treatment resulted in increased numbers of these CD8(+) CD25(-) cells within the tumor site. These findings demonstrate that memory CD8(+) T cells can express differential phenotypes indicative of adaptive or innate effectors based on the nature of the stimuli in a process conserved across species.

Increased sensitivity of antigen-experienced T cells through the enrichment of oligomeric T cell receptor complexes

Although memory T cells respond more vigorously to stimulation and they are more sensitive to low doses of antigen than naive T cells, the molecular basis of this increased sensitivity remains unclear. We have previously shown that the T cell receptor (TCR) exists as different-sized oligomers on the surface of resting T cells and that large oligomers are preferentially activated in response to low antigen doses. Through biochemistry and electron microscopy, we now showed that previously stimulated and memory T cells have more and larger TCR oligomers at the cell surface than their naive counterparts. Reconstitution of cells and mice with a point mutant of the CD3ζ subunit, which impairs TCR oligomer formation, demonstrated that the increased size of TCR oligomers was directly responsible for the increased sensitivity of antigen-experienced T cells. Thus, we propose that an "avidity maturation" mechanism underlies T cell antigenic memory.

Differentiation-dependent functional and epigenetic landscapes for cytokine genes in virus-specific CD8+ T cells

Although the simultaneous engagement of multiple effector mechanisms is thought to characterize optimal CD8(+) T-cell immunity and facilitate pathogen clearance, the differentiation pathways leading to the acquisition and maintenance of such polyfunctional activity are not well understood. Division-dependent profiles of effector molecule expression for virus-specific T cells are analyzed here by using a combination of carboxyfluorescein succinimidyl ester dilution and intracellular cytokine staining subsequent to T-cell receptor ligation. The experiments show that, although the majority of naive CD8(+) T-cell precursors are preprogrammed to produce TNF-α soon after stimulation and a proportion make both TNF-α and IL-2, the progressive acquisition of IFN-γ expression depends on continued lymphocyte proliferation. Furthermore, the extensive division characteristic of differentiation to peak effector activity is associated with the progressive dominance of IFN-γ and the concomitant loss of polyfunctional cytokine production, although this is not apparent for long-term CD8(+) T-cell memory. Such proliferation-dependent variation in cytokine production appears tied to the epigenetic signatures within the ifnG and tnfA proximal promoters. Specifically, those cytokine gene loci that are rapidly expressed following antigen stimulation at different stages of T-cell differentiation can be shown (by ChIP) to have permissive epigenetic and RNA polymerase II docking signatures. Thus, the dynamic changes in cytokine profiles for naive, effector, and memory T cells are underpinned by specific epigenetic landscapes that regulate responsiveness following T-cell receptor ligation.

Immunologic considerations for generating memory CD8 T cells through vaccination

Following infection or vaccination, naïve CD8 T cells that receive the appropriate integration of antigenic, co-stimulatory and inflammatory signals undergo a programmed series of biological changes that ultimately results in the generation of memory cells. Memory CD8 T cells, in contrast to naïve cells, more effectively limit or prevent pathogen re-infection because of both qualitative and quantitative changes that occur following their induction. Unlike vaccination strategies aimed at generating antibody production, the ability to generate protective memory CD8 T cells has proven more complicated and problematic. However, recent experimental results have revealed important principles regarding the molecular and genetic basis for memory CD8 T cell formation, as well as identified ways to manipulate their development through vaccination, resulting in potential new avenues to enhance protective immunity.

CD4+CD25+Foxp3+ regulatory T cells are dispensable for controlling CD8+ T cell-mediated lung inflammation

Every person harbors a population of potentially self-reactive lymphocytes controlled by tightly balanced tolerance mechanisms. Failures in this balance evoke immune activation and autoimmunity. In this study, we investigated the contribution of self-reactive CD8(+) T lymphocytes to chronic pulmonary inflammation and a possible role for naturally occurring CD4(+)CD25(+)Foxp3(+) regulatory T cells (nTregs) in counterbalancing this process. Using a transgenic murine model for autoimmune-mediated lung disease, we demonstrated that despite pulmonary inflammation, lung-specific CD8(+) T cells can reside quiescently in close proximity to self-antigen. Whereas self-reactive CD8(+) T cells in the inflamed lung and lung-draining lymph nodes downregulated the expression of effector molecules, those located in the spleen appeared to be partly Ag-experienced and displayed a memory-like phenotype. Because ex vivo-reisolated self-reactive CD8(+) T cells were very well capable of responding to the Ag in vitro, we investigated a possible contribution of nTregs to the immune control over autoaggressive CD8(+) T cells in the lung. Notably, CD8(+) T cell tolerance established in the lung depends only partially on the function of nTregs, because self-reactive CD8(+) T cells underwent only biased activation and did not acquire effector function after nTreg depletion. However, although transient ablation of nTregs did not expand the population of self-reactive CD8(+) T cells or exacerbate the disease, it provoked rapid accumulation of activated CD103(+)CD62L(lo) Tregs in bronchial lymph nodes, a finding suggesting an adaptive phenotypic switch in the nTreg population that acts in concert with other yet-undefined mechanisms to prevent the detrimental activation of self-reactive CD8(+) T cells.

Combination of antibodies inhibits accelerated rejection mediated by memory T cells in xenoantigen-primed mice

BACKGROUND

Donor-reactive memory T cells are known to accelerate allograft rejection; in our previous study, we reported that combined monoclonal antibodies (mAbs) could prolong islet allograft survival in alloantigen-primed mice. In this study, we examine the effects of donor-reactive memory T cells on the xenograft survival and methods to prolong the islet graft survival.

METHODS

To collect donor-reactive T cells, we performed full-thickness rat skin xenografting on BALB/c mice and isolated the T cells from the mice after 6-8 weeks. These cells were then adoptively transferred to syngenic mice 1 day before rat-to-mouse islet transplantation. Three experimental groups were established in the adoptive transfer model: recipient mice treated with isotype mAbs (isotype group); mice treated with anti-CD40L mAb (anti-CD40L group); and mice treated with anti-CD40L, anti-OX40L, and anti-CD122 mAbs (3-combined group).

RESULTS

Lewis rat islet xenografts transplanted in naïve mice showed a mean survival time (MST) of 12.8 days, while the graft rejection was accelerated if the recipient mice were treated with adoptively transferred donor-reactive T cells (MST, 8.67 days). Treatment with anti-CD40L mb could not reverse the accelerated rejection (MST, 9.3 days). However, when anti-CD40L mb was combined with anti-OX40L and anti-CD122 mAbs, there was a considerable increase in the MST, which was 72.2 days. Compared to the isotype group, the 3-combined group had significantly lesser proportion of memory T cells and greater proportion of regulatory T cells (Tregs) in the spleen. Meanwhile, in the 3-combined group, the production of anti-rat antibodies was markedly inhibited.

CONCLUSION

  Treatment with a combination of antibodies could significantly reverse the accelerated rejection mediated by donor-reactive memory T cells by inhibiting cellular and humoral immune responses.

Suppressing memory T cell activation induces islet allograft tolerance in alloantigen-primed mice

Memory T cells are known to play a key role in prevention of allograft tolerance in alloantigen-primed mice. Here, we used an adoptively transferred memory T cell model and an alloantigen-primed model to evaluate the abilities of different combinations of monoclonal antibodies (mAb) to block key signaling pathways involved in activation of effector and memory T cells. In the adoptively transferred model, the use of anti-CD134L mAb effectively prevented activation of CD4(+) memory T cells and significantly prolonged islet survival, similar to the action of anti-CD122 mAb to CD8(+) memory T cells. In the alloantigen-primed model, use of anti-CD134L and anti-CD122 mAbs in addition to co-stimulatory blockade with anti-CD154 and anti-LFA-1 prolonged secondary allograft survival and significantly reduced the proportion of memory T cells; meanwhile, this combination therapy increased the proportion of regulatory T cells (Tregs) in the spleen, inhibited lymphocyte infiltration in the graft, and suppressed alloresponse of recipient splenic T cells. However, we also detected high levels of alloantibodies in the serum which caused high levels of damage to the allogeneic spleen cells. Our results suggest that combination of four mAbs can significantly suppress the function of memory T cells and prolong allograft survival in alloantigen primed animals.