1 6522 114 TRANSCRIPTIONAL AND EPIGENETIC REGULATION OF T CELL HYPORESPONSIVENESS. NAIVE CD8(+) T CELLS DIFFERENTIATE INTO EFFECTOR AND MEMORY CYTOLYTIC T CELLS (CTLS) DURING AN ACUTE INFECTION. IN CONTRAST, IN SCENARIOS OF PERSISTENT ANTIGEN STIMULATION, SUCH AS CHRONIC INFECTIONS AND CANCER, ANTIGEN-SPECIFIC CTLS SHOW A GRADUAL DECREASE IN EFFECTOR FUNCTION, A PHENOMENON THAT HAS BEEN TERMED CD8(+) T CELL "EXHAUSTION" OR "DYSFUNCTION." ANOTHER HYPORESPONSIVE STATE, TERMED "ANERGY", IS OBSERVED WHEN T CELLS ARE ACTIVATED IN THE ABSENCE OF POSITIVE COSTIMULATORY SIGNALS. AMONG THE MANY NEGATIVE REGULATORS INDUCED IN HYPORESPONSIVE T CELLS ARE INHIBITORY CELL-SURFACE RECEPTORS, SUCH AS PD-1, LAG-3, CTLA-4, AND TIM-3; "CHECKPOINT BLOCKADE" THERAPIES THAT INVOLVE TREATMENT OF PATIENTS WITH CANCER WITH BLOCKING ANTIBODIES TO THOSE RECEPTORS SHOW CONSIDERABLE PROMISE IN THE CLINIC BECAUSE THE BLOCKING ANTIBODIES CAN MITIGATE HYPORESPONSIVENESS AND PROMOTE TUMOR REJECTION. IN THIS REVIEW, WE DESCRIBE RECENT ADVANCES IN OUR MOLECULAR UNDERSTANDING OF THESE HYPORESPONSIVE STATES. WE REVIEW EVIDENCE FOR THE INVOLVEMENT OF DIVERSE TRANSCRIPTION FACTORS, METABOLIC PROGRAMS, AND CHROMATIN ACCESSIBILITY CHANGES IN HYPORESPONSIVE T CELLS, AND WE DISCUSS HOW CHECKPOINT BLOCKADE THERAPIES AFFECT THE MOLECULAR PROGRAM OF CD8(+) T CELL EXHAUSTION. 2017 2 6851 42 [MOLECULAR PROFILES OF EXHAUSTED T CELLS AND THEIR IMPACT ON RESPONSE TO IMMUNE CHECKPOINT BLOCKADE]. T CELL EXHAUSTION IS INDUCED IN THE CONTEXT OF CHRONIC VIRUS INFECTION AND TUMOR MICROENVIRONMENT, IN WHICH CYTOTOXIC T CELLS ARE REPEATEDLY EXPOSED TO THE TARGET ANTIGEN AND DEPRIVED OF THEIR EFFECTOR FUNCTIONS. MULTIPLE STUDIES HAVE ALREADY SHOWN THE SIGNIFICANT IMPACT OF IMMUNE CHECKPOINT MOLECULES SUCH AS PD1 ON FUNCTIONAL PROPERTIES OF EXHAUSTED T CELLS. IN ADDITION TO THESE SIGNALS, EXHAUSTED T CELLS POSSESS DISTINCT TRANSCRIPTIONAL AND EPIGENETIC PROFILES COMPARED WITH CONVENTIONAL EFFECTOR AND MEMORY T CELLS. IMPORTANTLY, MOST OF THESE FEATURES ARE NOT AFFECTED BY IMMUNE CHECKPOINT BLOCKADE, SUGGESTING THAT GENETIC AND EPIGENETIC REMODELING OF T CELLS IS AN UNDERLYING MOLECULAR MECHANISM ESSENTIAL FOR T CELL EXHAUSTION. MOREOVER, IT HAS NOW BEEN EVIDENT THAT EXHAUSTED T CELLS ARE A HETEROGENEOUS CELL POPULATION COMPOSED OF DISTINCT T CELL SUBSETS, AND THESE FUNCTIONAL DIFFERENCES PROFOUNDLY AFFECT THERAPEUTIC EFFICACY OF CANCER IMMUNOTHERAPY. IN THIS REVIEW, I WILL DISCUSS RECENT STUDIES INVESTIGATING MOLECULAR MECHANISMS OF T CELL EXHAUSTION, INCLUDING NOVEL KEY MOLECULES ESSENTIALLY ASSOCIATED WITH T CELL EXHAUSTION. THESE FINDINGS ARE POTENTIALLY APPLICABLE TO REINVIGORATE EFFECTOR FUNCTIONS OF EXHAUSTED T CELLS. 2022 3 2879 49 FUNDAMENTALS TO THERAPEUTICS: EPIGENETIC MODULATION OF CD8(+) T CELL EXHAUSTION IN THE TUMOR MICROENVIRONMENT. IN THE SETTING OF CHRONIC ANTIGEN EXPOSURE IN THE TUMOR MICROENVIRONMENT (TME), CYTOTOXIC CD8(+) T CELLS (CTLS) LOSE THEIR IMMUNE SURVEILLANCE CAPABILITIES AND ABILITY TO CLEAR TUMOR CELLS AS A RESULT OF THEIR DIFFERENTIATION INTO TERMINALLY EXHAUSTED CD8(+) T CELLS. IMMUNE CHECKPOINT BLOCKADE (ICB) THERAPIES REINVIGORATE EXHAUSTED CD8(+) T CELLS BY TARGETING SPECIFIC INHIBITORY RECEPTORS, THUS PROMOTING THEIR CYTOLYTIC ACTIVITY TOWARDS TUMOR CELLS. DESPITE EXCITING RESULTS WITH ICB THERAPIES, MANY PATIENTS WITH SOLID TUMORS STILL FAIL TO RESPOND TO SUCH THERAPIES AND PATIENTS WHO INITIALLY RESPOND CAN DEVELOP RESISTANCE. RECENTLY, THROUGH NEW SEQUENCING TECHNOLOGIES SUCH AS THE ASSAY FOR TRANSPOSASE-ACCESSIBLE CHROMATIN WITH SEQUENCING (ATAC-SEQ), EPIGENETICS HAS BEEN APPRECIATED AS A CONTRIBUTING FACTOR THAT ENFORCES T CELL DIFFERENTIATION TOWARD EXHAUSTION IN THE TME. IMPORTANTLY, SPECIFIC EPIGENETIC ALTERATIONS AND EPIGENETIC FACTORS HAVE BEEN FOUND TO CONTROL CD8(+) T CELL EXHAUSTION PHENOTYPES. IN THIS REVIEW, WE WILL EXPLAIN THE BACKGROUND OF T CELL DIFFERENTIATION AND VARIOUS EXHAUSTION STATES AND DISCUSS HOW EPIGENETICS PLAY AN IMPORTANT ROLE IN THESE PROCESSES. THEN WE WILL OUTLINE SPECIFIC EPIGENETIC CHANGES AND CERTAIN EPIGENETIC AND TRANSCRIPTION FACTORS THAT ARE KNOWN TO CONTRIBUTE TO CD8(+) T CELL EXHAUSTION. WE WILL ALSO DISCUSS THE MOST RECENT METHODOLOGIES THAT ARE USED TO STUDY AND DISCOVER SUCH EPIGENETIC MODULATIONS. FINALLY, WE WILL EXPLAIN HOW EPIGENETIC REPROGRAMMING IS A PROMISING APPROACH THAT MIGHT FACILITATE THE DEVELOPMENT OF NOVEL EXHAUSTED T CELL-TARGETING IMMUNOTHERAPIES. 2022 4 771 47 CD8(+) T CELL EXHAUSTION. CD8(+) T CELLS ARE IMPORTANT FOR THE PROTECTIVE IMMUNITY AGAINST INTRACELLULAR PATHOGENS AND TUMOR. IN THE CASE OF CHRONIC INFECTION OR CANCER, CD8(+) T CELLS ARE EXPOSED TO PERSISTENT ANTIGEN AND/OR INFLAMMATORY SIGNALS. THIS EXCESSIVE AMOUNT OF SIGNALS OFTEN LEADS CD8(+) T CELLS TO GRADUAL DETERIORATION OF T CELL FUNCTION, A STATE CALLED "EXHAUSTION." EXHAUSTED T CELLS ARE CHARACTERIZED BY PROGRESSIVE LOSS OF EFFECTOR FUNCTIONS (CYTOKINE PRODUCTION AND KILLING FUNCTION), EXPRESSION OF MULTIPLE INHIBITORY RECEPTORS (SUCH AS PD-1 AND LAG3), DYSREGULATED METABOLISM, POOR MEMORY RECALL RESPONSE, AND HOMEOSTATIC PROLIFERATION. THESE ALTERED FUNCTIONS ARE CLOSELY RELATED WITH ALTERED TRANSCRIPTIONAL PROGRAM AND EPIGENETIC LANDSCAPE THAT CLEARLY DISTINGUISH EXHAUSTED T CELLS FROM NORMAL EFFECTOR AND MEMORY T CELLS. T CELL EXHAUSTION IS OFTEN ASSOCIATED WITH INEFFICIENT CONTROL OF PERSISTING INFECTIONS AND CANCERS, BUT RE-INVIGORATION OF EXHAUSTED T CELLS WITH INHIBITORY RECEPTOR BLOCKADE CAN PROMOTE IMPROVED IMMUNITY AND DISEASE OUTCOME. ACCUMULATING EVIDENCES SUPPORT THE THERAPEUTIC POTENTIAL OF TARGETING EXHAUSTED T CELLS. HOWEVER, EXHAUSTED T CELLS COMPRISE HETEROGENOUS CELL POPULATION WITH DISTINCT RESPONSIVENESS TO INTERVENTION. UNDERSTANDING MOLECULAR MECHANISM OF T CELL EXHAUSTION IS ESSENTIAL TO ESTABLISH RATIONAL IMMUNOTHERAPEUTIC INTERVENTIONS. 2019 5 2718 45 EXHAUSTED T CELLS AND EPIGENETIC STATUS. EXHAUSTED T CELLS ARE A GROUP OF DYSFUNCTIONAL T CELLS, WHICH ARE PRESENT IN CHRONIC INFECTIONS OR TUMORS. THE MOST SIGNIFICANT CHARACTERISTICS OF EXHAUSTED T CELLS ARE ATTENUATED EFFECTOR CYTOTOXICITY, REDUCED CYTOKINE PRODUCTION, AND UPREGULATION OF MULTIPLE INHIBITORY MOLECULAR RECEPTORS (E.G., PD-1, TIM-3, AND LAG-3). THE INTRACELLULAR METABOLIC CHANGES, ALTERED EXPRESSION OF TRANSCRIPTION FACTORS, AND A UNIQUE EPIGENETIC LANDSCAPE CONSTITUTE THE EXHAUSTION PROGRAM. RECENTLY, RESEARCHERS HAVE MADE PROGRESS IN UNDERSTANDING EXHAUSTED T CELLS, WITH THE DEFINITION AND IDENTIFICATION OF EXHAUSTED T CELLS CHANGING FROM PHENOTYPE-BASED TO BEING CLASSIFIED AT THE TRANSCRIPTIONAL AND EPIGENETIC LEVELS. RECENT STUDIES HAVE REVEALED THAT EXHAUSTED T CELLS CAN BE SEPARATED INTO TWO SUBGROUPS, NAMELY TCF1(+)PD-1(+) PROGENITOR-LIKE PRECURSOR EXHAUSTED CELLS AND TCF1(-)PD-1(+) TERMINALLY DIFFERENTIATED EXHAUSTED T CELLS. MOREOVER, THE PROGENITOR-LIKE PRECURSOR CELL POPULATION MAY BE A SUBSET OF T CELLS THAT CAN RESPOND TO IMMUNOTHERAPY. STUDIES HAVE ALSO FOUND THAT TOX INITIATES AND DOMINATES THE DEVELOPMENT OF EXHAUSTED T CELLS AT THE TRANSCRIPTIONAL AND EPIGENETIC LEVELS. TOX ALSO MAINTAINS T CELL SURVIVAL AND MAY AFFECT DECISIONS REGARDING TREATMENT STRATEGIES. IN THIS REVIEW, WE DISCUSS THE LATEST DEVELOPMENTS IN T CELL EXHAUSTION IN REGARDS TO DEFINITIONS, SUBPOPULATIONS, DEVELOPMENT MECHANISMS, DIFFERENCES IN DIVERSE DISEASES, AND TREATMENT PROSPECTS FOR EXHAUSTED T CELLS. FURTHERMORE, WE HYPOTHESIZE THAT THE EPIGENETIC STATE REGULATED BY TOX MIGHT BE THE KEY POINT, WHICH CAN DETERMINE THE REVERSIBILITY OF EXHAUSTION AND THE EFFICACY OF IMMUNOTHERAPY. 2020 6 6060 42 THE DEVELOPMENT OF CD8 T-CELL EXHAUSTION HETEROGENEITY AND THE THERAPEUTIC POTENTIALS IN CANCER. CD8(+) T CELLS ARE ESSENTIAL LYMPHOCYTES WITH CYTOTOXIC PROPERTIES FOR ANTITUMOR IMMUNOTHERAPY. HOWEVER, DURING CHRONIC INFECTION OR TUMORIGENESIS, THESE CELLS OFTEN BECOME DYSFUNCTIONAL WITH A GRADUALLY DEPLETED ABILITY TO RELEASE CYTOKINES AND THE EXHIBITION OF REDUCED CYTOTOXICITY, THE STATE REFERRED TO AS "T-CELL EXHAUSTION" (TEX). THIS UNIQUE STATE WAS CHARACTERIZED BY THE INCREASING EXPRESSION OF INHIBITORY CHECKPOINT RECEPTORS, AND INTERVENTIONS TARGETING IMMUNE CHECKPOINT BLOCKADES (ICBS) HAVE BEEN CONSIDERED AS A PROMISING STRATEGY TO STIMULATE T-CELL KILLING. RECENT INVESTIGATIONS HAVE DEMONSTRATED THAT EXHAUSTED T CELLS NOT ONLY DISPLAY FUNCTIONAL, METABOLIC, TRANSCRIPTIONAL, AND EPIGENETIC DIFFERENCES BUT ALSO COMPRISE A HETEROGENEOUS GROUP OF CELLS. IN THIS REVIEW, WE SUMMARIZE THE CURRENT FINDINGS ON DYNAMIC DIFFERENTIATION PROCESS DURING TEX HETEROGENEITY DEVELOPMENT IN CANCER AND CHRONIC INFECTION. WE DISCUSS HOW THE RESPONSES TO IMMUNOTHERAPY ARE DETERMINED BY THESE DISTINCT SUBSETS AND HIGHLIGHT PROSPECTIVE APPROACHES FOR IMPROVING THE EFFICACY OF ICB THERAPY FOR CANCER BY LEVERAGING THE HETEROGENEITY OF T CELLS. 2023 7 2145 29 EPIGENETIC MAINTENANCE OF ACQUIRED GENE EXPRESSION PROGRAMS DURING MEMORY CD8 T CELL HOMEOSTASIS. MEMORY CD8 T CELLS HAVE A UNIQUE ABILITY TO PROVIDE LIFELONG IMMUNITY AGAINST PATHOGENS CONTAINING THEIR COGNATE EPITOPE. BECAUSE OF THEIR ABILITY TO PROVIDE LIFELONG PROTECTION, THE GENERATION OF MEMORY T CELLS IS NOW A MAJOR FOCUS FOR CURRENT VACCINATION OR ADOPTIVE CELL THERAPY APPROACHES TO TREAT CHRONIC VIRAL INFECTIONS AND CANCER. IT IS NOW CLEAR THAT MAINTENANCE OF MEMORY CD8 T CELLS OCCURS THROUGH A PROCESS OF ANTIGEN-INDEPENDENT HOMEOSTATIC PROLIFERATION, WHICH IS REGULATED IN PART BY THE GAMMA CHAIN CYTOKINES IL-7 AND IL-15. HERE, WE WILL DESCRIBE THE ROLE OF THESE CYTOKINES IN THE SURVIVAL AND SELF-RENEWAL OF MEMORY CD8 T CELLS. FURTHER, WE WILL DESCRIBE THE ROLE OF EPIGENETICS IN THE MAINTENANCE OF ACQUIRED FUNCTIONS AMONG MEMORY CD8 T CELLS DURING HOMEOSTATIC PROLIFERATION. 2018 8 3288 42 HIERARCHICAL TRANSCRIPTIONAL NETWORK GOVERNING HETEROGENEOUS T CELL EXHAUSTION AND ITS IMPLICATIONS FOR IMMUNE CHECKPOINT BLOCKADE. THE FUNDAMENTAL PRINCIPLE OF IMMUNE CHECKPOINT BLOCKADE (ICB) IS TO PROTECT TUMOR-INFILTRATING T CELLS FROM BEING EXHAUSTED. DESPITE THE REMARKABLE SUCCESS ACHIEVED BY ICB TREATMENT, ONLY A SMALL GROUP OF PATIENTS BENEFIT FROM IT. CHARACTERIZED BY A HYPOFUNCTIONAL STATE WITH THE EXPRESSION OF MULTIPLE INHIBITORY RECEPTORS, EXHAUSTED T (TEX) CELLS ARE A MAJOR OBSTACLE IN IMPROVING ICB. T CELL EXHAUSTION IS A PROGRESSIVE PROCESS WHICH ADAPTS TO PERSISTENT ANTIGEN STIMULATION IN CHRONIC INFECTIONS AND CANCERS. IN THIS REVIEW, WE ELUCIDATE THE HETEROGENEITY OF TEX CELLS AND OFFER NEW INSIGHTS INTO THE HIERARCHICAL TRANSCRIPTIONAL REGULATION OF T CELL EXHAUSTION. FACTORS AND SIGNALING PATHWAYS THAT INDUCE AND PROMOTE EXHAUSTION ARE ALSO SUMMARIZED. MOREOVER, WE REVIEW THE EPIGENETIC AND METABOLIC ALTERATIONS OF TEX CELLS AND DISCUSS HOW PD-1 SIGNALING AFFECTS THE BALANCE BETWEEN T CELL ACTIVATION AND EXHAUSTION, AIMING TO PROVIDE MORE THERAPEUTIC TARGETS FOR APPLICATIONS OF COMBINATIONAL IMMUNOTHERAPIES. 2023 9 2660 33 EPITHERAPY AND IMMUNE CHECKPOINT BLOCKADE: USING EPIGENETIC REINVIGORATION OF EXHAUSTED AND DYSFUNCTIONAL T CELLS TO REIMBURSE IMMUNOTHERAPY RESPONSE. BACKGROUND: CANCER CELLS SUBVERT NATURAL IMMUNOSUPPRESSION BY UPREGULATING THE EXPRESSION OF CHECKPOINT PROTEINS AND THEIR LIGANDS. FOR EXAMPLE, TUMOR CELLS EXPRESSING PROGRAMMED DEATH-LIGAND 1 (PD-L1) INDUCE IMMUNE CELL TOLERANCE TO CANCERS, THEREBY FACILITATING TUMOR PROGRESSION. THE RECENT CLINICAL SUCCESS OF IMMUNOTHERAPY, PARTICULARLY CHECKPOINT BLOCKADE, REPRESENTS A SIGNIFICANT ADVANCE IN CANCER THERAPY. HOWEVER, MANY CANCERS DEVELOP RESISTANCE TO IMMUNOTHERAPIES, AND THE UNDERLYING MECHANISMS AND HOW THESE MIGHT BE EXPLOITED TO OVERCOME RESISTANCE STILL NEED TO BE DETERMINED. METHODS: T CELL DYSFUNCTION, IN PART CAUSED BY CHRONIC T CELL RECEPTOR STIMULATION, DIMINISHES THE CAPACITY FOR DURABLE RESPONSES TO CHECKPOINT BLOCKADE. FURTHERMORE, T CELL POPULATIONS ARE PHENOTYPICALLY AND FUNCTIONALLY HETEROGENEOUS, RESULTING IN VARYING RESPONSES TO CHECKPOINT BLOCKADE. RECENT MOLECULAR STUDIES OF T CELL HETEROGENEITY HAVE SHOWN THAT CHECKPOINT BLOCKADE ON ITS OWN DOES NOT ALTER THE EPIGENETIC LANDSCAPE OF T CELLS, DESPITE EPIGENETIC CHANGES GOVERNING T CELL PHENOTYPE. CONCLUSION: HERE WE ARGUE THAT EPIGENETIC MODIFIERS CAN BE USED TO PRIME AND SENSITIZE T CELLS TO IMMUNOTHERAPY. ADMINISTERING EPITHERAPY IN CONJUNCTION WITH CHECKPOINT BLOCKADE COULD DECREASE T CELL EXHAUSTION AND IMMUNOTHERAPY RESISTANCE IN MANY CANCER TYPES. 2020 10 6521 33 TRANSCRIPTIONAL AND EPIGENETIC REGULATION OF PD-1 EXPRESSION. PROGRAMMED CELL DEATH-1 (PD-1) IS A CO-INHIBITORY RECEPTOR THAT PLAYS IMPORTANT ROLES IN REGULATING T CELL IMMUNITY AND PERIPHERAL TOLERANCE. PD-1 SIGNALING PREVENTS T CELLS FROM OVERACTIVATION DURING ACUTE INFECTIONS, BUT IT MAINTAINS T CELL EXHAUSTION DURING CHRONIC INFECTIONS. TUMOR CELLS CAN EXPLOIT THE PD-1 SIGNALING PATHWAY TO EVADE ANTITUMOR IMMUNE RESPONSES. THE PD-1 SIGNALING PATHWAY IS ALSO ESSENTIAL FOR MAINTAINING PERIPHERAL TOLERANCE AND PREVENTION OF AUTOIMMUNITY. PD-1 EXPRESSION IS STRICTLY AND DIFFERENTIALLY REGULATED BY DIVERSE MECHANISMS IN IMMUNE CELLS. IT IS ACTIVATED AND REPRESSED BY DISTINCT TRANSCRIPTION FACTORS IN DIFFERENT CIRCUMSTANCES. MOREOVER, EPIGENETIC MECHANISMS ARE ALSO INVOLVED IN REGULATING PD-1 EXPRESSION. IN THIS REVIEW, WE SUMMARIZE THE KNOWLEDGE OF THE TRANSCRIPTIONAL AND EPIGENETIC REGULATION OF PD-1 EXPRESSION DURING DIFFERENT IMMUNE RESPONSES. 2021 11 4726 29 NOT-SO-OPPOSITE ENDS OF THE SPECTRUM: CD8(+) T CELL DYSFUNCTION ACROSS CHRONIC INFECTION, CANCER AND AUTOIMMUNITY. CD8(+) T CELLS ARE CRITICAL MEDIATORS OF CYTOTOXIC EFFECTOR FUNCTION IN INFECTION, CANCER AND AUTOIMMUNITY. IN CANCER AND CHRONIC VIRAL INFECTION, CD8(+) T CELLS UNDERGO A PROGRESSIVE LOSS OF CYTOKINE PRODUCTION AND CYTOTOXICITY, A STATE TERMED T CELL EXHAUSTION. IN AUTOIMMUNITY, AUTOREACTIVE CD8(+) T CELLS RETAIN THE CAPACITY TO EFFECTIVELY MEDIATE THE DESTRUCTION OF HOST TISSUES. ALTHOUGH THE CLINICAL OUTCOME DIFFERS IN EACH CONTEXT, CD8(+) T CELLS ARE CHRONICALLY EXPOSED TO ANTIGEN IN ALL THREE. THESE CHRONICALLY STIMULATED CD8(+) T CELLS SHARE SOME COMMON PHENOTYPIC FEATURES, AS WELL AS TRANSCRIPTIONAL AND EPIGENETIC PROGRAMMING, ACROSS DISEASE CONTEXTS. A BETTER UNDERSTANDING OF THESE CD8(+) T CELL STATES MAY REVEAL NOVEL STRATEGIES TO AUGMENT CLEARANCE OF CHRONIC VIRAL INFECTION AND CANCER AND TO MITIGATE SELF-REACTIVITY LEADING TO TISSUE DAMAGE IN AUTOIMMUNITY. 2021 12 1464 37 DISSECTING THE HETEROGENEITY OF EXHAUSTED T CELLS AT THE MOLECULAR LEVEL. OUR UNDERSTANDING OF MECHANISMS UNDERLYING T-CELL EXHAUSTION HAS BEEN REFINED BY ANALYSIS OF EXHAUSTED T CELLS AT THE MOLECULAR LEVEL. THE DEVELOPMENT AND FUNCTIONS OF EXHAUSTED T CELLS ARE REGULATED BY A NUMBER OF TRANSCRIPTION FACTORS, EPIGENETIC FACTORS AND METABOLIC ENZYMES. IN ADDITION, RECENT WORK TO DISSECT EXHAUSTED T CELLS AT THE SINGLE-CELL LEVEL HAS ENABLED US TO DISCOVER A PRECURSOR EXHAUSTED T-CELL SUBSET EQUIPPED WITH LONG-TERM SURVIVAL CAPACITY. STARTING FROM THE ANALYSIS OF MOUSE MODELS, THE EXISTENCE OF PRECURSOR EXHAUSTED T CELLS HAS ALSO BEEN DOCUMENTED IN HUMAN T CELLS IN THE CONTEXT OF CHRONIC VIRUS INFECTIONS OR TUMORS. CLINICAL DATA SUGGEST THAT EVALUATING THE QUALITY OF EXHAUSTED T CELLS ON THE BASIS OF THEIR DIFFERENTIATION STATUS MAY BE HELPFUL TO PREDICT THE THERAPEUTIC RESPONSE TO INHIBITION OF PROGRAMMED DEATH 1 (PD1). MOREOVER, BEYOND IMMUNE-CHECKPOINT BLOCKADE, NOVEL THERAPEUTIC APPROACHES TO RE-INVIGORATE EXHAUSTED T CELLS HAVE BEEN EXPLORED BASED ON MOLECULAR INSIGHTS INTO T-CELL EXHAUSTION. HERE I WILL DISCUSS KEY MOLECULAR PROFILES ASSOCIATED WITH THE DEVELOPMENT, MAINTENANCE AND DIFFERENTIATION OF EXHAUSTED T CELLS AND HOW THESE FINDINGS CAN BE APPLICABLE IN THE FIELD OF CANCER IMMUNOTHERAPY. 2022 13 451 44 APPLICATION OF ATAC-SEQ IN TUMOR-SPECIFIC T CELL EXHAUSTION. RESEARCHES SHOW THAT CHRONIC VIRAL INFECTION AND PERSISTENT ANTIGEN AND/OR INFLAMMATORY SIGNAL EXPOSURE IN CANCER CAUSES THE FUNCTIONAL STATUS OF T CELLS TO BE ALTERED, MAINLY BY MAJOR CHANGES IN THE EPIGENETIC AND METABOLIC ENVIRONMENT, WHICH THEN LEADS TO T CELL EXHAUSTION. THE DISCOVERY OF THE IMMUNE CHECKPOINT PATHWAY IS AN IMPORTANT MILESTONE IN UNDERSTANDING AND REVERSING T CELL EXHAUSTION. ANTIBODIES TARGETING THESE PATHWAYS HAVE SHOWN SUPERIOR ABILITY TO REVERSE T CELL EXHAUSTION. HOWEVER, THERE ARE STILL SOME LIMITATIONS IN IMMUNE CHECKPOINT BLOCKING THERAPY, SUCH AS THE SHORT-TERM NATURE OF THERAPEUTIC EFFECTS AND HIGH INDIVIDUAL HETEROGENEITY. ASSAY FOR TRANSPOSASE-ACCESSIBLE CHROMATIN WITH SEQUENCING(ATAC-SEQ) IS A METHOD USED TO ANALYZE THE ACCESSIBILITY OF WHOLE-GENOME CHROMATIN. IT USES HYPERACTIVE TN5 TRANSPOSASE TO ASSESS CHROMATIN ACCESSIBILITY. RECENTLY, A GROWING NUMBER OF STUDIES HAVE REPORTED THAT ATAC-SEQ CAN BE USED TO CHARACTERIZE THE DYNAMIC CHANGES OF EPIGENETICS IN THE PROCESS OF T CELL EXHAUSTION. IT HAS BEEN DETERMINED THAT IMMUNE CHECKPOINT BLOCKING CAN ONLY TEMPORARILY RESTORE THE FUNCTION OF EXHAUSTED T CELLS BECAUSE OF AN IRREVERSIBLE CHANGE IN THE EPIGENETICS OF EXHAUSTED T CELLS. IN THIS STUDY, WE REVIEW THE LATEST DEVELOPMENTS, WHICH PROVIDE A CLEARER MOLECULAR UNDERSTANDING OF T CELL EXHAUSTION, REVEAL POTENTIAL NEW THERAPEUTIC TARGETS FOR PERSISTENT VIRAL INFECTION AND CANCER, AND PROVIDE NEW INSIGHTS FOR DESIGNING EFFECTIVE IMMUNOTHERAPY FOR TREATING CANCER AND CHRONIC INFECTION. 2023 14 5414 42 REGULATION OF CD8(+) T MEMORY AND EXHAUSTION BY THE MTOR SIGNALS. CD8(+) T CELLS ARE THE KEY EXECUTIONERS OF THE ADAPTIVE IMMUNE ARM, WHICH MEDIATES ANTITUMOR AND ANTIVIRAL IMMUNITY. NAIVE CD8(+) T CELLS DEVELOP IN THE THYMUS AND ARE QUICKLY ACTIVATED IN THE PERIPHERY AFTER ENCOUNTERING A COGNATE ANTIGEN, WHICH INDUCES THESE CELLS TO PROLIFERATE AND DIFFERENTIATE INTO EFFECTOR CELLS THAT FIGHT THE INITIAL INFECTION. SIMULTANEOUSLY, A FRACTION OF THESE CELLS BECOME LONG-LIVED MEMORY CD8(+) T CELLS THAT COMBAT FUTURE INFECTIONS. NOTABLY, THE GENERATION AND MAINTENANCE OF MEMORY CELLS IS PROFOUNDLY AFFECTED BY VARIOUS IN VIVO CONDITIONS, SUCH AS THE MODE OF PRIMARY ACTIVATION (E.G., ACUTE VS. CHRONIC IMMUNIZATION) OR FLUCTUATIONS IN HOST METABOLIC, INFLAMMATORY, OR AGING FACTORS. THEREFORE, MANY T CELLS MAY BE LOST OR BECOME EXHAUSTED AND NO LONGER FUNCTIONAL. COMPLICATED INTRACELLULAR SIGNALING PATHWAYS, TRANSCRIPTION FACTORS, EPIGENETIC MODIFICATIONS, AND METABOLIC PROCESSES ARE INVOLVED IN THIS PROCESS. THEREFORE, UNDERSTANDING THE CELLULAR AND MOLECULAR BASIS FOR THE GENERATION AND FATE OF MEMORY AND EXHAUSTED CD8(+) CELLS IS CENTRAL FOR HARNESSING CELLULAR IMMUNITY. IN THIS REVIEW, WE FOCUS ON MAMMALIAN TARGET OF RAPAMYCIN (MTOR), PARTICULARLY SIGNALING MEDIATED BY MTOR COMPLEX (MTORC) 2 IN MEMORY AND EXHAUSTED CD8(+) T CELLS AT THE MOLECULAR LEVEL. 2023 15 2056 37 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. 2018 16 790 35 CELLULAR AND MOLECULAR MECHANISMS OF CD8(+) T CELL DIFFERENTIATION, DYSFUNCTION AND EXHAUSTION. T CELLS FOLLOW A TRIPHASIC DISTINCT PATHWAY OF ACTIVATION, PROLIFERATION AND DIFFERENTIATION BEFORE BECOMING FUNCTIONALLY AND PHENOTYPICALLY "EXHAUSTED" IN SETTINGS OF CHRONIC INFECTION, AUTOIMMUNITY AND IN CANCER. EXHAUSTED T CELLS PROGRESSIVELY LOSE CANONICAL EFFECTOR FUNCTIONS, EXHIBIT ALTERED TRANSCRIPTIONAL NETWORKS AND EPIGENETIC SIGNATURES AND GAIN CONSTITUTIVE EXPRESSION OF A BROAD COINHIBITORY RECEPTOR SUITE. THIS REVIEW OUTLINES RECENT ADVANCES IN OUR UNDERSTANDING OF EXHAUSTED T CELL BIOLOGY AND EXAMINES CELLULAR AND MOLECULAR MECHANISMS BY WHICH A STATE OF DYSFUNCTION OR EXHAUSTION IS ESTABLISHED, AND MECHANISMS BY WHICH EXHAUSTED T CELLS MAY STILL CONTRIBUTE TO PATHOGEN OR TUMOUR CONTROL. FURTHER, THIS REVIEW DESCRIBES OUR UNDERSTANDING OF EXHAUSTED T CELL HETEROGENEITY AND OUTLINES THE MECHANISMS BY WHICH CHECKPOINT BLOCKADE DIFFERENTIALLY ENGAGES EXHAUSTED T CELL SUBSETS TO OVERCOME EXHAUSTION AND RECOVER T CELL FUNCTION. 2020 17 5900 41 T-CELL EXHAUSTION IN ORGAN TRANSPLANTATION. EXHAUSTION OF T CELLS OCCURS IN RESPONSE TO LONG-TERM EXPOSURE TO SELF AND FOREIGN ANTIGENS. IT LIMITS T CELL CAPACITY TO PROLIFERATE AND PRODUCE CYTOKINES, LEADING TO AN IMPAIRED ABILITY TO CLEAR CHRONIC INFECTIONS OR ERADICATE TUMORS. T-CELL EXHAUSTION IS ASSOCIATED WITH A SPECIFIC TRANSCRIPTIONAL, EPIGENETIC, AND METABOLIC PROGRAM AND CHARACTERISTIC CELL SURFACE MARKERS' EXPRESSION. RECENT STUDIES HAVE BEGUN TO ELUCIDATE THE ROLE OF T-CELL EXHAUSTION IN TRANSPLANT. HIGHER LEVELS OF EXHAUSTED T CELLS HAVE BEEN ASSOCIATED WITH BETTER GRAFT FUNCTION IN KIDNEY TRANSPLANT RECIPIENTS. IN CONTRAST, REINVIGORATING EXHAUSTED T CELLS BY IMMUNE CHECKPOINT BLOCKADE THERAPIES, WHILE PROMOTING TUMOR CLEARANCE, INCREASES THE RISK OF ACUTE REJECTION. LYMPHOCYTE DEPLETION AND HIGH ALLOANTIGEN LOAD HAVE BEEN IDENTIFIED AS MAJOR DRIVERS OF T-CELL EXHAUSTION. THIS COULD ACCOUNT, AT LEAST IN PART, FOR THE REDUCED RATES OF ACUTE REJECTION IN ORGAN TRANSPLANT RECIPIENTS INDUCED WITH THYMOGLOBULIN AND FOR THE PRO-TOLEROGENIC EFFECTS OF A LARGE ORGAN SUCH AS THE LIVER. AMONG THE DRUGS THAT ARE WIDELY USED FOR MAINTENANCE IMMUNOSUPPRESSION, CALCINEURIN INHIBITORS HAVE A CONTRASTING INHIBITORY EFFECT ON EXHAUSTION OF T CELLS, WHILE THE INFLUENCE OF MTOR INHIBITORS IS STILL UNCLEAR. HARNESSING OR ENCOURAGING THE NATURAL PROCESSES OF EXHAUSTION MAY PROVIDE A NOVEL STRATEGY TO PROMOTE GRAFT SURVIVAL AND TRANSPLANTATION TOLERANCE. 2022 18 4386 40 MITOCHONDRIAL STRESS INDUCED BY CONTINUOUS STIMULATION UNDER HYPOXIA RAPIDLY DRIVES T CELL EXHAUSTION. CANCER AND CHRONIC INFECTIONS INDUCE T CELL EXHAUSTION, A HYPOFUNCTIONAL FATE CARRYING DISTINCT EPIGENETIC, TRANSCRIPTOMIC AND METABOLIC CHARACTERISTICS. HOWEVER, DRIVERS OF EXHAUSTION REMAIN POORLY UNDERSTOOD. AS INTRATUMORAL EXHAUSTED T CELLS EXPERIENCE SEVERE HYPOXIA, WE HYPOTHESIZED THAT METABOLIC STRESS ALTERS THEIR RESPONSES TO OTHER SIGNALS, SPECIFICALLY, PERSISTENT ANTIGENIC STIMULATION. IN VITRO, ALTHOUGH CD8(+) T CELLS EXPERIENCING CONTINUOUS STIMULATION OR HYPOXIA ALONE DIFFERENTIATED INTO FUNCTIONAL EFFECTORS, THE COMBINATION RAPIDLY DROVE T CELL DYSFUNCTION CONSISTENT WITH EXHAUSTION. CONTINUOUS STIMULATION PROMOTED BLIMP-1-MEDIATED REPRESSION OF PGC-1ALPHA-DEPENDENT MITOCHONDRIAL REPROGRAMMING, RENDERING CELLS POORLY RESPONSIVE TO HYPOXIA. LOSS OF MITOCHONDRIAL FUNCTION GENERATED INTOLERABLE LEVELS OF REACTIVE OXYGEN SPECIES (ROS), SUFFICIENT TO PROMOTE EXHAUSTED-LIKE STATES, IN PART THROUGH PHOSPHATASE INHIBITION AND THE CONSEQUENT ACTIVITY OF NUCLEAR FACTOR OF ACTIVATED T CELLS. REDUCING T CELL-INTRINSIC ROS AND LOWERING TUMOR HYPOXIA LIMITED T CELL EXHAUSTION, SYNERGIZING WITH IMMUNOTHERAPY. THUS, IMMUNOLOGIC AND METABOLIC SIGNALING ARE INTRINSICALLY LINKED: THROUGH MITIGATION OF METABOLIC STRESS, T CELL DIFFERENTIATION CAN BE ALTERED TO PROMOTE MORE FUNCTIONAL CELLULAR FATES. 2021 19 2410 30 EPIGENETIC SCARS OF CD8(+) T CELL EXHAUSTION PERSIST AFTER CURE OF CHRONIC INFECTION IN HUMANS. T CELL EXHAUSTION IS AN INDUCED STATE OF DYSFUNCTION THAT ARISES IN RESPONSE TO CHRONIC INFECTION AND CANCER. EXHAUSTED CD8(+) T CELLS ACQUIRE A DISTINCT EPIGENETIC STATE, BUT IT IS NOT KNOWN WHETHER THAT CHROMATIN LANDSCAPE IS FIXED OR PLASTIC FOLLOWING THE RESOLUTION OF A CHRONIC INFECTION. HERE WE SHOW THAT THE EPIGENETIC STATE OF EXHAUSTION IS LARGELY IRREVERSIBLE, EVEN AFTER CURATIVE THERAPY. ANALYSIS OF CHROMATIN ACCESSIBILITY IN HCV- AND HIV-SPECIFIC RESPONSES IDENTIFIES A CORE EPIGENETIC PROGRAM OF EXHAUSTION IN CD8(+) T CELLS, WHICH UNDERGOES ONLY LIMITED REMODELING BEFORE AND AFTER RESOLUTION OF INFECTION. MOREOVER, CANONICAL FEATURES OF EXHAUSTION, INCLUDING SUPER-ENHANCERS NEAR THE GENES TOX AND HIF1A, REMAIN 'EPIGENETICALLY SCARRED.' T CELL EXHAUSTION IS THEREFORE A CONSERVED EPIGENETIC STATE THAT BECOMES FIXED AND PERSISTS INDEPENDENT OF CHRONIC ANTIGEN STIMULATION AND INFLAMMATION. THERAPEUTIC EFFORTS TO REVERSE T CELL EXHAUSTION MAY REQUIRE NEW APPROACHES THAT INCREASE THE EPIGENETIC PLASTICITY OF EXHAUSTED T CELLS. 2021 20 6319 44 THE ROAD LESS TAKEN: LESS APPRECIATED PATHWAYS FOR MANIPULATING CD8(+) T CELL EXHAUSTION. EXHAUSTED CD8(+) T (TEX) CELLS ARE A DISTINCT CELL POPULATION THAT ARISE DURING PERSISTENT ANTIGEN EXPOSURE IN THE CONTEXT OF CHRONIC INFECTIONS AND CANCERS. ALTHOUGH CHARACTERIZED BY PROGRESSIVE LOSS OF EFFECTOR FUNCTIONS, HIGH AND SUSTAINED INHIBITORY RECEPTOR EXPRESSION AND DISTINCT TRANSCRIPTIONAL AND EPIGENETIC PROGRAMS, TEX CELLS ARE HETEROGENEOUS. AMONG THESE, A SELF-RENEWING TCF-1(+) TEX POPULATION, HAVING UNIQUE CHARACTERISTICS AND THE ABILITY TO RESPOND TO IMMUNE-CHECKPOINT BLOCKADE, GIVES RISE TO TCF-1(-) TERMINALLY TEX CELLS. THESE TCF-1(+) CELLS HAVE STEM CELL-LIKE PROPERTIES SIMILAR TO MEMORY T CELL POPULATIONS, BUT THE SIGNALS THAT REGULATE THE DEVELOPMENTAL PATHWAYS AND RELATIONSHIPS AMONG EXHAUSTED CELL POPULATIONS ARE STILL UNCLEAR. HERE, WE REVIEW OUR CURRENT UNDERSTANDING OF TEX CELL BIOLOGY, AND DISCUSS SOME LESS APPRECIATED MOLECULES AND PATHWAYS AFFECTING T CELL EXHAUSTION. WE HIGHLIGHT TWO CO-STIMULATORY RECEPTORS, CD226 AND CD137, AND THEIR ROLE IN INDUCING OR RESTRAINING T CELL EXHAUSTION, AS WELL AS SIGNALING PATHWAYS THAT MAY BE AMENABLE TO PHARMACOLOGICAL INHIBITION WITH A FOCUS ON PHOSPHOINOSITIDE-3 KINASE AND IL-2 PARTIAL AGONISTS. FINALLY, WE DISCUSS NOVEL METHODS THAT MAY INCREASE TCF-1(+) POPULATIONS AND THEREFORE IMPROVE IMMUNOTHERAPY RESPONSIVENESS. UNDERSTANDING FEATURES OF AND PATHWAYS TO EXHAUSTION HAS IMPORTANT IMPLICATIONS FOR THE SUCCESS OF IMMUNOTHERAPY, INCLUDING CHECKPOINT BLOCKADE AND ADOPTIVE T-CELL TRANSFER THERAPIES. 2022