1 6481 123 TOX IS EXPRESSED BY EXHAUSTED AND POLYFUNCTIONAL HUMAN EFFECTOR MEMORY CD8(+) T CELLS. CD8(+) T CELL EXHAUSTION IS A HALLMARK OF MANY CANCERS AND CHRONIC INFECTIONS. IN MICE, T CELL FACTOR 1 (TCF-1) MAINTAINS EXHAUSTED CD8(+) T CELL RESPONSES, WHEREAS THYMOCYTE SELECTION-ASSOCIATED HMG BOX (TOX) IS REQUIRED FOR THE EPIGENETIC REMODELING AND SURVIVAL OF EXHAUSTED CD8(+) T CELLS. HOWEVER, IT HAS REMAINED UNCLEAR TO WHAT EXTENT THESE TRANSCRIPTION FACTORS PLAY ANALOGOUS ROLES IN HUMANS. IN THIS STUDY, WE MAPPED THE EXPRESSION OF TOX AND TCF-1 AS A FUNCTION OF DIFFERENTIATION AND SPECIFICITY IN THE HUMAN CD8(+) T CELL LANDSCAPE. HERE, WE DEMONSTRATE THAT CIRCULATING TOX(+) CD8(+) T CELLS EXIST IN MOST HUMANS, BUT THAT TOX IS NOT EXCLUSIVELY ASSOCIATED WITH EXHAUSTION. EFFECTOR MEMORY CD8(+) T CELLS GENERALLY EXPRESSED TOX, WHEREAS NAIVE AND EARLY-DIFFERENTIATED MEMORY CD8(+) T CELLS GENERALLY EXPRESSED TCF-1. CYTOLYTIC GENE AND PROTEIN EXPRESSION SIGNATURES WERE ALSO DEFINED BY THE EXPRESSION OF TOX. IN THE CONTEXT OF A RELENTLESS IMMUNE CHALLENGE, EXHAUSTED HIV-SPECIFIC CD8(+) T CELLS COMMONLY EXPRESSED TOX, OFTEN IN CLUSTERS WITH VARIOUS ACTIVATION MARKERS AND INHIBITORY RECEPTORS, AND EXPRESSED LESS TCF-1. HOWEVER, POLYFUNCTIONAL MEMORY CD8(+) T CELLS SPECIFIC FOR CYTOMEGALOVIRUS (CMV) OR EPSTEIN-BARR VIRUS (EBV) ALSO EXPRESSED TOX, EITHER WITH OR WITHOUT TCF-1. A SIMILAR PHENOTYPE WAS OBSERVED AMONG HIV-SPECIFIC CD8(+) T CELLS FROM INDIVIDUALS WHO MAINTAINED EXCEPTIONAL IMMUNE CONTROL OF VIRAL REPLICATION. COLLECTIVELY, THESE DATA DEMONSTRATE THAT TOX IS EXPRESSED BY MOST CIRCULATING EFFECTOR MEMORY CD8(+) T CELL SUBSETS AND NOT EXCLUSIVELY LINKED TO EXHAUSTION. 2020 2 6522 38 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 3 559 32 BACH2 ENFORCES THE TRANSCRIPTIONAL AND EPIGENETIC PROGRAMS OF STEM-LIKE CD8(+) T CELLS. DURING CHRONIC INFECTION AND CANCER, A SELF-RENEWING CD8(+) T CELL SUBSET MAINTAINS LONG-TERM IMMUNITY AND IS CRITICAL TO THE EFFECTIVENESS OF IMMUNOTHERAPY. THESE STEM-LIKE CD8(+) T CELLS DIVERGE FROM OTHER CD8(+) SUBSETS EARLY AFTER CHRONIC VIRAL INFECTION. HOWEVER, PATHWAYS GUARDING STEM-LIKE CD8(+) T CELLS AGAINST TERMINAL EXHAUSTION REMAIN UNCLEAR. HERE, WE SHOW THAT THE GENE ENCODING TRANSCRIPTIONAL REPRESSOR BACH2 IS TRANSCRIPTIONALLY AND EPIGENETICALLY ACTIVE IN STEM-LIKE CD8(+) T CELLS BUT NOT TERMINALLY EXHAUSTED CELLS EARLY AFTER INFECTION. BACH2 OVEREXPRESSION ENFORCED STEM-LIKE CELL FATE, WHEREAS BACH2 DEFICIENCY IMPAIRED STEM-LIKE CD8(+) T CELL DIFFERENTIATION. SINGLE-CELL TRANSCRIPTOMIC AND EPIGENOMIC APPROACHES REVEALED THAT BACH2 ESTABLISHED THE TRANSCRIPTIONAL AND EPIGENETIC PROGRAMS OF STEM-LIKE CD8(+) T CELLS. IN ADDITION, BACH2 SUPPRESSED THE MOLECULAR PROGRAM DRIVING TERMINAL EXHAUSTION THROUGH TRANSCRIPTIONAL REPRESSION AND EPIGENETIC SILENCING. THUS, OUR STUDY REVEALS A NEW PATHWAY THAT ENFORCES COMMITMENT TO STEM-LIKE CD8(+) LINEAGE AND PREVENTS AN ALTERNATIVE TERMINALLY EXHAUSTED CELL FATE. 2021 4 2409 36 EPIGENETIC SCARRING OF EXHAUSTED T CELLS HINDERS MEMORY DIFFERENTIATION UPON ELIMINATING CHRONIC ANTIGENIC STIMULATION. EXHAUSTED CD8 T CELLS (T(EX)) ARE A DISTINCT STATE OF T CELL DIFFERENTIATION ASSOCIATED WITH FAILURE TO CLEAR CHRONIC VIRUSES AND CANCER. IMMUNOTHERAPIES SUCH AS PD-1 BLOCKADE CAN REINVIGORATE T(EX) CELLS, BUT REINVIGORATION IS NOT DURABLE. A MAJOR UNANSWERED QUESTION IS WHETHER T(EX) CELLS DIFFERENTIATE INTO FUNCTIONAL DURABLE MEMORY T CELLS (T(MEM)) UPON ANTIGEN CLEARANCE. HERE, USING A MOUSE MODEL, WE FOUND THAT UPON ELIMINATING CHRONIC ANTIGENIC STIMULATION, T(EX) CELLS PARTIALLY (RE)ACQUIRE PHENOTYPIC AND TRANSCRIPTIONAL FEATURES OF T(MEM) CELLS. THESE 'RECOVERING' T(EX) CELLS ORIGINATED FROM THE T CELL FACTOR (TCF-1(+)) T(EX) PROGENITOR SUBSET. NEVERTHELESS, THE RECALL CAPACITY OF THESE RECOVERING T(EX) CELLS REMAINED COMPROMISED AS COMPARED TO T(MEM) CELLS. CHROMATIN-ACCESSIBILITY PROFILING REVEALED A FAILURE TO RECOVER CORE MEMORY EPIGENETIC CIRCUITS AND MAINTENANCE OF A LARGELY EXHAUSTED OPEN CHROMATIN LANDSCAPE. THUS, DESPITE SOME PHENOTYPIC AND TRANSCRIPTIONAL RECOVERY UPON ANTIGEN CLEARANCE, EXHAUSTION LEAVES DURABLE EPIGENETIC SCARS CONSTRAINING FUTURE IMMUNE RESPONSES. THESE RESULTS SUPPORT EPIGENETIC REMODELING INTERVENTIONS FOR T(EX) CELL-TARGETED IMMUNOTHERAPIES. 2021 5 2242 29 EPIGENETIC MODULATION OF CD8(+) T CELL FUNCTION IN LENTIVIRUS INFECTIONS: A REVIEW. CD8(+) T CELLS ARE CRITICAL FOR CONTROLLING VIREMIA DURING HUMAN IMMUNODEFICIENCY VIRUS (HIV) INFECTION. THESE CELLS PRODUCE CYTOLYTIC FACTORS AND ANTIVIRAL CYTOKINES THAT ELIMINATE VIRALLY- INFECTED CELLS. DURING THE CHRONIC PHASE OF HIV INFECTION, CD8(+) T CELLS PROGRESSIVELY LOSE THEIR PROLIFERATIVE CAPACITY AND ANTIVIRAL FUNCTIONS. THESE DYSFUNCTIONAL CELLS ARE UNABLE TO CLEAR THE PRODUCTIVELY INFECTED AND REACTIVATED CELLS, REPRESENTING A ROADBLOCK IN HIV CURE. THEREFORE, MECHANISMS TO UNDERSTAND CD8(+) T CELL DYSFUNCTION AND STRATEGIES TO BOOST CD8(+) T CELL FUNCTION NEED TO BE INVESTIGATED. USING THE FELINE IMMUNODEFICIENCY VIRUS (FIV) MODEL FOR LENTIVIRAL PERSISTENCE, WE HAVE DEMONSTRATED THAT CD8(+) T CELLS EXHIBIT EPIGENETIC CHANGES SUCH AS DNA DEMETHYLATION DURING THE COURSE OF INFECTION AS COMPARED TO UNINFECTED CATS. WE HAVE ALSO DEMONSTRATED THAT LENTIVIRUS-ACTIVATED CD4(+)CD25(+) T REGULATORY CELLS INDUCE FORKHEAD BOX P3 (FOXP3) EXPRESSION IN VIRUS-SPECIFIC CD8(+) T CELL TARGETS, WHICH BINDS THE INTERLEUKIN (IL)-2, TUMOR NECROSIS FACTOR (TNF)-Α, AND INTERFERON (IFN)-Γ PROMOTERS IN THESE CD8(+) T CELLS. FINALLY, WE HAVE REPORTED THAT EPIGENETIC MODULATION REDUCES FOXP3 BINDING TO THESE PROMOTER REGIONS. THIS REVIEW COMPARES AND CONTRASTS OUR CURRENT UNDERSTANDING OF CD8(+) T CELL EPIGENETICS AND MECHANISMS OF LYMPHOCYTE SUPPRESSION DURING THE COURSE OF LENTIVIRAL INFECTION FOR TWO ANIMAL MODELS, FIV AND SIMIAN IMMUNODEFICIENCY VIRUS (SIV). 2018 6 2421 42 EPIGENETIC SIGNATURE OF PD-1+ TCF1+ CD8 T CELLS THAT ACT AS RESOURCE CELLS DURING CHRONIC VIRAL INFECTION AND RESPOND TO PD-1 BLOCKADE. WE HAVE RECENTLY DEFINED A NOVEL POPULATION OF PD-1 (PROGRAMMED CELL DEATH 1)+ TCF1 (T CELL FACTOR 1)+ VIRUS-SPECIFIC CD8 T CELLS THAT FUNCTION AS RESOURCE CELLS DURING CHRONIC LCMV INFECTION AND PROVIDE THE PROLIFERATIVE BURST SEEN AFTER PD-1 BLOCKADE. SUCH CD8 T CELLS HAVE BEEN FOUND IN OTHER CHRONIC INFECTIONS AND ALSO IN CANCER IN MICE AND HUMANS. THESE CD8 T CELLS EXHIBIT STEM-LIKE PROPERTIES UNDERGOING SELF-RENEWAL AND ALSO DIFFERENTIATING INTO THE TERMINALLY EXHAUSTED CD8 T CELLS. HERE WE COMPARED THE EPIGENETIC SIGNATURE OF STEM-LIKE CD8 T CELLS WITH EXHAUSTED CD8 T CELLS. ATAC-SEQ ANALYSIS SHOWED THAT STEM-LIKE CD8 T CELLS HAD A UNIQUE SIGNATURE IMPLICATING ACTIVITY OF HMG (TCF) AND RHD (NF-KAPPAB) TRANSCRIPTION FACTOR FAMILY MEMBERS IN CONTRAST TO HIGHER ACCESSIBILITY TO ETS AND RUNX MOTIFS IN EXHAUSTED CD8 T CELLS. IN ADDITION, REGULATORY REGIONS OF THE TRANSCRIPTION FACTORS TCF7 AND ID3 WERE MORE ACCESSIBLE IN STEM-LIKE CELLS WHEREAS PRDM1 AND ID2 WERE MORE ACCESSIBLE IN EXHAUSTED CD8 T CELLS. WE ALSO COMPARED THE EPIGENETIC SIGNATURES OF THE 2 CD8 T CELL SUBSETS FROM CHRONICALLY INFECTED MICE WITH EFFECTOR AND MEMORY CD8 T CELLS GENERATED AFTER AN ACUTE LCMV INFECTION. BOTH CD8 T CELL SUBSETS GENERATED DURING CHRONIC INFECTION WERE STRIKINGLY DIFFERENT FROM CD8 T CELL SUBSETS FROM ACUTE INFECTION. INTERESTINGLY, THE STEM-LIKE CD8 T CELL SUBSET FROM CHRONIC INFECTION, DESPITE SHARING KEY FUNCTIONAL PROPERTIES WITH MEMORY CD8 T CELLS, HAD A VERY DISTINCT EPIGENETIC PROGRAM. THESE RESULTS SHOW THAT THE CHRONIC STEM-LIKE CD8 T CELL PROGRAM REPRESENTS A SPECIFIC ADAPTATION OF THE T CELL RESPONSE TO PERSISTENT ANTIGENIC STIMULATION. 2019 7 2443 32 EPIGENETIC STABILITY OF EXHAUSTED T CELLS LIMITS DURABILITY OF REINVIGORATION BY PD-1 BLOCKADE. BLOCKING PROGRAMMED DEATH-1 (PD-1) CAN REINVIGORATE EXHAUSTED CD8 T CELLS (T(EX)) AND IMPROVE CONTROL OF CHRONIC INFECTIONS AND CANCER. HOWEVER, WHETHER BLOCKING PD-1 CAN REPROGRAM T(EX) INTO DURABLE MEMORY T CELLS (T(MEM)) IS UNCLEAR. WE FOUND THAT REINVIGORATION OF T(EX) IN MICE BY PD-L1 BLOCKADE CAUSED MINIMAL MEMORY DEVELOPMENT. AFTER BLOCKADE, REINVIGORATED T(EX) BECAME REEXHAUSTED IF ANTIGEN CONCENTRATION REMAINED HIGH AND FAILED TO BECOME T(MEM) UPON ANTIGEN CLEARANCE. T(EX) ACQUIRED AN EPIGENETIC PROFILE DISTINCT FROM THAT OF EFFECTOR T CELLS (T(EFF)) AND T(MEM) CELLS THAT WAS MINIMALLY REMODELED AFTER PD-L1 BLOCKADE. THIS FINDING SUGGESTS THAT T(EX) ARE A DISTINCT LINEAGE OF CD8 T CELLS. NEVERTHELESS, PD-1 PATHWAY BLOCKADE RESULTED IN TRANSCRIPTIONAL REWIRING AND REENGAGEMENT OF EFFECTOR CIRCUITRY IN THE T(EX) EPIGENETIC LANDSCAPE. THESE DATA INDICATE THAT EPIGENETIC FATE INFLEXIBILITY MAY LIMIT CURRENT IMMUNOTHERAPIES. 2016 8 568 36 BATF REGULATES PROGENITOR TO CYTOLYTIC EFFECTOR CD8(+) T CELL TRANSITION DURING CHRONIC VIRAL INFECTION. DURING CHRONIC VIRAL INFECTION, CD8(+) T CELLS DEVELOP INTO THREE MAJOR PHENOTYPICALLY AND FUNCTIONALLY DISTINCT SUBSETS: LY108(+)TCF-1(+) PROGENITORS, LY108(-)CX(3)CR1(-) TERMINALLY EXHAUSTED CELLS AND THE RECENTLY IDENTIFIED CX(3)CR1(+) CYTOTOXIC EFFECTOR CELLS. NEVERTHELESS, HOW CX(3)CR1(+) EFFECTOR CELL DIFFERENTIATION IS TRANSCRIPTIONALLY AND EPIGENETICALLY REGULATED REMAINS ELUSIVE. HERE, WE IDENTIFY DISTINCT GENE REGULATORY NETWORKS AND EPIGENETIC LANDSCAPES UNDERPINNING THE FORMATION OF THESE SUBSETS. NOTABLY, OUR DATA DEMONSTRATE THAT CX(3)CR1(+) EFFECTOR CELLS BEAR A STRIKING SIMILARITY TO SHORT-LIVED EFFECTOR CELLS DURING ACUTE INFECTION. GENETIC DELETION OF TBX21 SIGNIFICANTLY DIMINISHED FORMATION OF THE CX(3)CR1(+) SUBSET. IMPORTANTLY, WE FURTHER IDENTIFY A PREVIOUSLY UNAPPRECIATED ROLE FOR THE TRANSCRIPTION FACTOR BATF IN MAINTAINING A PERMISSIVE CHROMATIN STRUCTURE THAT ALLOWS THE TRANSITION FROM TCF-1(+) PROGENITORS TO CX(3)CR1(+) EFFECTOR CELLS. BATF DIRECTLY BOUND TO REGULATORY REGIONS NEAR TBX21 AND KLF2, MODULATING THEIR ENHANCER ACCESSIBILITY TO FACILITATE THE TRANSITION. THESE MECHANISTIC INSIGHTS CAN POTENTIALLY BE HARNESSED TO OVERCOME T CELL EXHAUSTION DURING CHRONIC INFECTION AND CANCER. 2021 9 3373 32 HISTONE MODULATION BLOCKS TREG-INDUCED FOXP3 BINDING TO THE IL-2 PROMOTER OF VIRUS-SPECIFIC CD8(+) T CELLS FROM FELINE IMMUNODEFICIENCY VIRUS-INFECTED CATS. CD8(+) T CELLS ARE CRITICAL FOR CONTROLLING HIV INFECTION. DURING THE CHRONIC PHASE OF LENTIVIRAL INFECTION, CD8(+) T CELLS LOSE THEIR PROLIFERATIVE CAPACITY AND EXHIBIT IMPAIRED ANTIVIRAL FUNCTION. THIS LOSS OF CD8(+) T CELL FUNCTION IS DUE, IN PART, TO CD4(+)CD25(+) T REGULATORY (TREG) CELL-MEDIATED SUPPRESSION. OUR RESEARCH GROUP HAS DEMONSTRATED THAT LENTIVIRUS-ACTIVATED CD4(+)CD25(+) TREG CELLS INDUCE THE REPRESSIVE TRANSCRIPTION FACTOR FORKHEAD BOX P3 (FOXP3) IN AUTOLOGOUS CD8(+) T CELLS FOLLOWING CO-CULTURE. WE HAVE RECENTLY REPORTED THAT TREG-INDUCED FOXP3 BINDS THE INTERLEUKIN-2 (IL-2), INTERFERON-GAMMA (IFN- GAMMA), AND TUMOR NECROSIS FACTOR-ALPHA (TNF-ALPHA) PROMOTERS IN VIRUS-SPECIFIC CD8(+) T CELLS. THESE DATA SUGGEST AN IMPORTANT ROLE OF FOXP3-MEDIATED CD8(+) T CELL DYSFUNCTION IN LENTIVIRAL INFECTION. TO ELUCIDATE THE MECHANISM OF THIS SUPPRESSION, WE PREVIOUSLY REPORTED THAT DECREASED METHYLATION FACILITATES FOXP3 BINDING IN MITOGEN-ACTIVATED CD8(+) T CELLS FROM FELINE IMMUNODEFICIENCY VIRUS (FIV)-INFECTED CATS. WE DEMONSTRATED THE REDUCED BINDING OF FOXP3 TO THE IL-2 PROMOTER BY INCREASING METHYLATION OF CD8(+) T CELLS. IN THE STUDIES PRESENTED HERE, WE ASK IF ANOTHER FORM OF EPIGENETIC MODULATION MIGHT ALLEVIATE FOXP3-MEDIATED SUPPRESSION IN CD8(+) T CELLS. WE HYPOTHESIZED THAT DECREASING HISTONE ACETYLATION IN VIRUS-SPECIFIC CD8(+) T CELLS WOULD DECREASE TREG-INDUCED FOXP3 BINDING TO THE IL-2 PROMOTER. INDEED, USING ANACARDIC ACID (AA), A KNOWN HISTONE ACETYL TRANSFERASE (HAT) INHIBITOR, WE DEMONSTRATE A REDUCTION IN FOXP3 BINDING TO THE IL-2 PROMOTER IN VIRUS-SPECIFIC CD8(+) T CELLS CO-CULTURED WITH AUTOLOGOUS TREG CELLS. THESE DATA IDENTIFY A NOVEL MECHANISM OF FOXP3-MEDIATED CD8(+) T CELL DYSFUNCTION DURING LENTIVIRAL INFECTION. 2018 10 2879 41 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 11 6482 39 TOX TRANSCRIPTIONALLY AND EPIGENETICALLY PROGRAMS CD8(+) T CELL EXHAUSTION. EXHAUSTED CD8(+) T (T(EX)) CELLS IN CHRONIC INFECTIONS AND CANCER HAVE LIMITED EFFECTOR FUNCTION, HIGH CO-EXPRESSION OF INHIBITORY RECEPTORS AND EXTENSIVE TRANSCRIPTIONAL CHANGES COMPARED WITH EFFECTOR (T(EFF)) OR MEMORY (T(MEM)) CD8(+) T CELLS. T(EX) CELLS ARE IMPORTANT CLINICAL TARGETS OF CHECKPOINT BLOCKADE AND OTHER IMMUNOTHERAPIES. EPIGENETICALLY, T(EX) CELLS ARE A DISTINCT IMMUNE SUBSET, WITH A UNIQUE CHROMATIN LANDSCAPE COMPARED WITH T(EFF) AND T(MEM) CELLS. HOWEVER, THE MECHANISMS THAT GOVERN THE TRANSCRIPTIONAL AND EPIGENETIC DEVELOPMENT OF T(EX) CELLS REMAIN UNKNOWN. HERE WE IDENTIFY THE HMG-BOX TRANSCRIPTION FACTOR TOX AS A CENTRAL REGULATOR OF T(EX) CELLS IN MICE. TOX IS LARGELY DISPENSABLE FOR THE FORMATION OF T(EFF) AND T(MEM) CELLS, BUT IT IS CRITICAL FOR EXHAUSTION: IN THE ABSENCE OF TOX, T(EX) CELLS DO NOT FORM. TOX IS INDUCED BY CALCINEURIN AND NFAT2, AND OPERATES IN A FEED-FORWARD LOOP IN WHICH IT BECOMES CALCINEURIN-INDEPENDENT AND SUSTAINED IN T(EX) CELLS. ROBUST EXPRESSION OF TOX THEREFORE RESULTS IN COMMITMENT TO T(EX) CELLS BY TRANSLATING PERSISTENT STIMULATION INTO A DISTINCT T(EX) CELL TRANSCRIPTIONAL AND EPIGENETIC DEVELOPMENTAL PROGRAM. 2019 12 5704 41 SINGLE-CELL RNA-SEQ REVEALS TOX AS A KEY REGULATOR OF CD8(+) T CELL PERSISTENCE IN CHRONIC INFECTION. PROGENITOR-LIKE CD8(+) T CELLS MEDIATE LONG-TERM IMMUNITY TO CHRONIC INFECTION AND CANCER AND RESPOND POTENTLY TO IMMUNE CHECKPOINT BLOCKADE. THESE CELLS SHARE TRANSCRIPTIONAL REGULATORS WITH MEMORY PRECURSOR CELLS, INCLUDING T CELL-SPECIFIC TRANSCRIPTION FACTOR 1 (TCF1), BUT IT IS UNCLEAR WHETHER THEY ADOPT DISTINCT PROGRAMS TO ADAPT TO THE IMMUNOSUPPRESSIVE ENVIRONMENT. BY COMPARING THE SINGLE-CELL TRANSCRIPTOMES AND EPIGENETIC PROFILES OF CD8(+) T CELLS RESPONDING TO ACUTE AND CHRONIC VIRAL INFECTIONS, WE FOUND THAT PROGENITOR-LIKE CD8(+) T CELLS BECAME DISTINCT FROM MEMORY PRECURSOR CELLS BEFORE THE PEAK OF THE T CELL RESPONSE. WE DISCOVERED A COEXPRESSION GENE MODULE CONTAINING TOX THAT EXHIBITED HIGHER TRANSCRIPTIONAL ACTIVITY ASSOCIATED WITH MORE ABUNDANT ACTIVE HISTONE MARKS IN PROGENITOR-LIKE CELLS THAN MEMORY PRECURSOR CELLS. MOREOVER, THYMOCYTE SELECTION-ASSOCIATED HIGH MOBILITY GROUP BOX PROTEIN TOX (TOX) PROMOTED THE PERSISTENCE OF ANTIVIRAL CD8(+) T CELLS AND WAS REQUIRED FOR THE PROGRAMMING OF PROGENITOR-LIKE CD8(+) T CELLS. THUS, LONG-TERM CD8(+) T CELL IMMUNITY TO CHRONIC VIRAL INFECTION REQUIRES UNIQUE TRANSCRIPTIONAL AND EPIGENETIC PROGRAMS ASSOCIATED WITH THE TRANSCRIPTION FACTOR TOX. 2019 13 6121 31 THE EPIGENETIC LANDSCAPE OF T CELL EXHAUSTION. EXHAUSTED T CELLS IN CANCER AND CHRONIC VIRAL INFECTION EXPRESS DISTINCTIVE PATTERNS OF GENES, INCLUDING SUSTAINED EXPRESSION OF PROGRAMMED CELL DEATH PROTEIN 1 (PD-1). HOWEVER, THE REGULATION OF GENE EXPRESSION IN EXHAUSTED T CELLS IS POORLY UNDERSTOOD. HERE, WE DEFINE THE ACCESSIBLE CHROMATIN LANDSCAPE IN EXHAUSTED CD8(+) T CELLS AND SHOW THAT IT IS DISTINCT FROM FUNCTIONAL MEMORY CD8(+) T CELLS. EXHAUSTED CD8(+) T CELLS IN HUMANS AND A MOUSE MODEL OF CHRONIC VIRAL INFECTION ACQUIRE A STATE-SPECIFIC EPIGENETIC LANDSCAPE ORGANIZED INTO FUNCTIONAL MODULES OF ENHANCERS. GENOME EDITING SHOWS THAT PD-1 EXPRESSION IS REGULATED IN PART BY AN EXHAUSTION-SPECIFIC ENHANCER THAT CONTAINS ESSENTIAL RAR, T-BET, AND SOX3 MOTIFS. FUNCTIONAL ENHANCER MAPS MAY OFFER TARGETS FOR GENOME EDITING THAT ALTER GENE EXPRESSION PREFERENTIALLY IN EXHAUSTED CD8(+) T CELLS. 2016 14 6319 40 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 15 770 35 CD8(+) T CELL DYSFUNCTION BY TOX INTOXICATION: A PROTUMORIGENIC EVENT IN THE TUMOR MICROENVIRONMENT. ACCUMULATING EVIDENCE SUGGESTS THE ROLE OF CELLULAR COMPONENTS IN ACHIEVING ANTITUMOR TO PROTUMOR MICROENVIRONMENTS. AMONG THE VARIOUS TYPES OF CELLS WITHIN THE TUMOR NICHE, THE STATE OF CD8(+) T CELLS APPARENTLY CHANGES FROM CYTOTOXIC T EFFECTOR CELLS AND MEMORY T CELLS TO EXHAUSTED CD8(+) T CELLS. THESE CHANGES IN THE PHENOTYPE OF CD8(+) T CELLS PROMOTE THE PROTUMOR MICROENVIRONMENT. RECENTLY, COMPREHENSIVE EXPERIMENTAL DATA DELINEATED THE ROLE OF THYMOCYTE SELECTION-ASSOCIATED HIGH-MOBILITY GROUP-BOX PROTEIN (TOX), WHICH REGULATES THE TRANSCRIPTIONAL PROCESS AND EPIGENETIC REMODELING, WITH IMPLICATIONS IN TUMOR AND CHRONIC VIRAL INFECTIONS. THIS PERSPECTIVE SUMMARIZES THE MOLECULAR MECHANISMS THAT LINK CD8(+) T CELLS, TOX, AND TRANSCRIPTIONAL AND EPIGENETIC REPROGRAMMING AS WELL AS FUTURE DIRECTIONS FOR DETERMINING NEW AVENUES OF CANCER THERAPEUTICS. 2021 16 1262 38 CUTTING EDGE: PROLONGED EXPOSURE TO HIV REINFORCES A POISED EPIGENETIC PROGRAM FOR PD-1 EXPRESSION IN VIRUS-SPECIFIC CD8 T CELLS. AG-SPECIFIC CD8 T CELLS PLAY A CRITICAL ROLE IN CONTROLLING HIV INFECTION BUT EVENTUALLY LOSE ANTIVIRAL FUNCTIONS IN PART BECAUSE OF EXPRESSION AND SIGNALING THROUGH THE INHIBITORY PROGRAMMED DEATH-1 (PD-1) RECEPTOR. TO BETTER UNDERSTAND THE IMPACT OF PROLONGED TCR LIGATION ON REGULATION OF PD-1 EXPRESSION IN HIV-SPECIFIC CD8 T CELLS, WE INVESTIGATED THE CAPACITY OF VIRUS-SPECIFIC CD8 T CELLS TO MODIFY THE PD-1 EPIGENETIC PROGRAM AFTER REDUCTION IN VIRAL LOAD. WE OBSERVED THAT THE TRANSCRIPTIONAL REGULATORY REGION WAS UNMETHYLATED IN THE PD-1(HI) HIV-SPECIFIC CD8 T CELLS, WHEREAS IT REMAINED METHYLATED IN DONOR-MATCHED NAIVE CELLS AT ACUTE AND CHRONIC STAGES OF INFECTION. SURPRISINGLY, THE PD-1 PROMOTER REMAINED UNMETHYLATED IN HIV-SPECIFIC CD8 T CELLS FROM SUBJECTS WITH A VIRAL LOAD CONTROLLED BY ANTIVIRAL THERAPY FOR >2 Y OR FROM ELITE CONTROLLERS. TOGETHER, THESE DATA DEMONSTRATE THAT THE EPIGENETIC PROGRAM AT THE PD-1 LOCUS BECOMES FIXED AFTER PROLONGED EXPOSURE TO HIV VIRUS. 2013 17 771 39 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 18 5853 37 SUBSETS OF EXHAUSTED CD8(+) T CELLS DIFFERENTIALLY MEDIATE TUMOR CONTROL AND RESPOND TO CHECKPOINT BLOCKADE. T CELL DYSFUNCTION IS A HALLMARK OF MANY CANCERS, BUT THE BASIS FOR T CELL DYSFUNCTION AND THE MECHANISMS BY WHICH ANTIBODY BLOCKADE OF THE INHIBITORY RECEPTOR PD-1 (ANTI-PD-1) REINVIGORATES T CELLS ARE NOT FULLY UNDERSTOOD. HERE WE SHOW THAT SUCH THERAPY ACTS ON A SPECIFIC SUBPOPULATION OF EXHAUSTED CD8(+) TUMOR-INFILTRATING LYMPHOCYTES (TILS). DYSFUNCTIONAL CD8(+) TILS POSSESS CANONICAL EPIGENETIC AND TRANSCRIPTIONAL FEATURES OF EXHAUSTION THAT MIRROR THOSE SEEN IN CHRONIC VIRAL INFECTION. EXHAUSTED CD8(+) TILS INCLUDE A SUBPOPULATION OF 'PROGENITOR EXHAUSTED' CELLS THAT RETAIN POLYFUNCTIONALITY, PERSIST LONG TERM AND DIFFERENTIATE INTO 'TERMINALLY EXHAUSTED' TILS. CONSEQUENTLY, PROGENITOR EXHAUSTED CD8(+) TILS ARE BETTER ABLE TO CONTROL TUMOR GROWTH THAN ARE TERMINALLY EXHAUSTED T CELLS. PROGENITOR EXHAUSTED TILS CAN RESPOND TO ANTI-PD-1 THERAPY, BUT TERMINALLY EXHAUSTED TILS CANNOT. PATIENTS WITH MELANOMA WHO HAVE A HIGHER PERCENTAGE OF PROGENITOR EXHAUSTED CELLS EXPERIENCE A LONGER DURATION OF RESPONSE TO CHECKPOINT-BLOCKADE THERAPY. THUS, APPROACHES TO EXPAND THE POPULATION OF PROGENITOR EXHAUSTED CD8(+) T CELLS MIGHT BE AN IMPORTANT COMPONENT OF IMPROVING THE RESPONSE TO CHECKPOINT BLOCKADE. 2019 19 1759 37 EARLY PRECURSOR T CELLS ESTABLISH AND PROPAGATE T CELL EXHAUSTION IN CHRONIC INFECTION. CD8(+) T CELLS RESPONDING TO CHRONIC INFECTIONS OR TUMORS ACQUIRE AN 'EXHAUSTED' STATE ASSOCIATED WITH ELEVATED EXPRESSION OF INHIBITORY RECEPTORS, INCLUDING PD-1, AND IMPAIRED CYTOKINE PRODUCTION. EXHAUSTED T CELLS ARE CONTINUOUSLY REPLENISHED BY T CELLS WITH PRECURSOR CHARACTERISTICS THAT SELF-RENEW AND DEPEND ON THE TRANSCRIPTION FACTOR TCF1; HOWEVER, THEIR DEVELOPMENTAL REQUIREMENTS ARE POORLY UNDERSTOOD. IN THE PRESENT STUDY, WE DEMONSTRATE THAT HIGH ANTIGEN LOAD PROMOTED THE DIFFERENTIATION OF PRECURSOR T CELLS, WHICH ACQUIRED HALLMARKS OF EXHAUSTION WITHIN DAYS OF INFECTION, WHEREAS EARLY EFFECTOR CELLS RETAINED POLYFUNCTIONAL FEATURES. EARLY PRECURSOR T CELLS SHOWED EPIGENETIC IMPRINTING CHARACTERISTIC OF T CELL RECEPTOR-DEPENDENT TRANSCRIPTION FACTOR BINDING AND WERE RESTRICTED TO THE GENERATION OF CELLS DISPLAYING EXHAUSTION CHARACTERISTICS. TRANSCRIPTION FACTORS BACH2 AND BATF WERE KEY REGULATORS WITH OPPOSING FUNCTIONS IN THE GENERATION OF EARLY PRECURSOR T CELLS. OVERALL, WE DEMONSTRATE THAT EXHAUSTION MANIFESTS FIRST IN TCF1(+) PRECURSOR T CELLS AND IS PROPAGATED SUBSEQUENTLY TO THE POOL OF ANTIGEN-SPECIFIC T CELLS. 2020 20 3288 31 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