1 5620 137 SCHRODINGER'S T CELLS: MOLECULAR INSIGHTS INTO STEMNESS AND EXHAUSTION. T CELL STEMNESS AND EXHAUSTION COEXIST AS TWO KEY CONTRASTING PHENOMENA DURING CHRONIC ANTIGEN STIMULATION, SUCH AS INFECTION, TRANSPLANT, CANCER, AND AUTOIMMUNITY. T CELL EXHAUSTION REFERS TO THE PROGRESSIVE LOSS OF EFFECTOR FUNCTION CAUSED BY CHRONIC ANTIGEN EXPOSURE. EXHAUSTED T (T(EX)) CELLS HIGHLY EXPRESS MULTIPLE INHIBITORY RECEPTORS AND EXHIBIT SEVERE DEFECTS IN CELL PROLIFERATION AND CYTOKINE PRODUCTION. THE TERM T CELL STEMNESS DESCRIBES THE STEM CELL-LIKE BEHAVIORS OF T CELLS, INCLUDING SELF-RENEWAL, MULTIPOTENCY, AND FUNCTIONAL PERSISTENCE. IT IS WELL ACCEPTED THAT NAIVE AND SOME MEMORY T CELL SUBSETS HAVE STEM CELL-LIKE PROPERTIES. WHEN INVESTIGATING THE EXHAUSTIVE DIFFERENTIATION OF T CELLS IN CHRONIC INFECTION AND CANCER, RECENT STUDIES HIGHLIGHTED THE STEMNESS OF "PRECURSORS OF EXHAUSTED" T (T(PEX)) CELLS PRIOR TO THEIR TERMINAL DIFFERENTIATION TO T(EX) CELLS. CLINICALLY SUCCESSFUL CHECKPOINT BLOCKADES FOR CANCER TREATMENT APPEAR TO INVIGORATE ANTITUMOR T(PEX) CELLS BUT NOT T(EX) CELLS. HERE WE DISCUSS THE TRANSCRIPTIONAL AND EPIGENETIC REGULATIONS OF T CELL STEMNESS AND EXHAUSTION, WITH A FOCUS ON HOW SYSTEMS IMMUNOLOGY WAS AND WILL BE UTILIZED TO DEFINE THE MOLECULAR BASIS UNDERLYING THE TRANSITION OF T(PEX) TO T(EX) CELLS. WE SUGGEST A "STEPWISE MODEL" OF T CELL STEMNESS AND EXHAUSTION, IN WHICH LOSS OF STEMNESS AND EXHAUSTION PROGRESSION ARE GRADUAL MULTI-STEP PROCESSES. WE PROVIDE PERSPECTIVES ON THE RESEARCH NEEDED TO DEFINE T CELL STEMNESS AND EXHAUSTION IN THE TRANSPLANTATION SETTING, IN WHICH ALLOGENIC T CELLS ARE ALSO CHRONICALLY EXPOSED TO ALLOANTIGENS. A BETTER UNDERSTANDING OF T CELL STEMNESS AND EXHAUSTION WILL SHED LIGHT ON DEVELOPING NOVEL STRATEGIES FOR IMMUNOTHERAPIES. 2021 2 5944 37 TARGETING SMALL MOLECULE TYROSINE KINASES BY POLYPHENOLS: NEW MOVE TOWARDS ANTI-TUMOR DRUG DISCOVERY. BACKGROUND: CANCER IS A COMPLEX DISEASE INVOLVING GENETIC AND EPIGENETIC ALTERATION THAT ALLOWS CELLS TO ESCAPE NORMAL HOMEOSTASIS. KINASES PLAY A CRUCIAL ROLE IN SIGNALING PATHWAYS THAT REGULATE CELL FUNCTIONS. DEREGULATION OF KINASES LEADS TO A VARIETY OF PATHOLOGICAL CHANGES, ACTIVATING CANCER CELL PROLIFERATION AND METASTASES. THE MOLECULAR MECHANISM OF CANCER IS COMPLEX AND THE DYSREGULATION OF TYROSINE KINASES LIKE ANAPLASTIC LYMPHOMA KINASE (ALK), BCR-ABL (FUSION GENE FOUND IN PATIENT WITH CHRONIC MYELOGENOUS LEUKEMIA (CML), JAK (JANUS ACTIVATED KINASE), SRC FAMILY KINASES (SFKS), ALK (ANAPLASTIC LYMPHOMA KINASE), C-MET (MESENCHYMAL- EPITHELIAL TRANSITION), EGFR (EPIDERMAL GROWTH FACTOR RECEPTOR), PDGFR (PLATELET-DERIVED GROWTH FACTOR RECEPTOR), RET (REARRANGED DURING TRANSFECTION) AND VEGFR (VASCULAR ENDOTHELIAL GROWTH FACTOR RECEPTOR) PLAYS MAJOR ROLE IN THE PROCESS OF CARCINOGENESIS. RECENTLY, KINASE INHIBITORS HAVE OVERCOME MANY PROBLEMS OF TRADITIONAL CANCER CHEMOTHERAPY AS THEY EFFECTIVELY SEPARATE OUT NORMAL, NON-CANCER CELLS AS WELL AS RAPIDLY MULTIPLYING CANCER CELLS. METHODS: ELECTRONIC DATABASES WERE SEARCHED TO EXPLORE THE SMALL MOLECULE TYROSINE KINASES BY POLYPHENOLS WITH THE HELP OF DOCKING STUDY (GLIDE-7.6 PROGRAM INTERFACED WITH MAESTRO-V11.3 OF SCHRODINGER 2017) TO SHOW THE BINDING ENERGIES OF POLYPHENOLS INHIBITOR WITH DIFFERENT TYROSINE KINASES IN ORDER TO DIFFERENTIATE BETWEEN THE TARGETS. RESULTS: FROM THE LITERATURE SURVEY, IT WAS OBSERVED THAT THE NUMBER OF POLYPHENOLS DERIVED FROM NATURAL SOURCES ALTERS THE EXPRESSION AND SIGNALING CASCADE OF TYROSINE KINASE IN VARIOUS TUMOR MODELS. THEREFORE, THE DEVELOPMENT OF POLYPHENOLS AS A TYROSINE KINASE INHIBITOR AGAINST TARGETED PROTEINS IS REGARDED AS AN UPCOMING TREND FOR CHEMOPREVENTION. CONCLUSION: IN THIS REVIEW, WE HAVE DISCUSSED THE ROLE OF POLYPHENOLS AS CHEMORECEPTIVE WHICH WILL HELP IN FUTURE FOR THE DEVELOPMENT AND DISCOVERY OF NOVEL SEMISYNTHETIC ANTICANCER AGENTS COUPLED WITH POLYPHENOLS. 2020 3 2409 35 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 4 6482 44 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 5 2443 29 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 6 5671 51 SHARED AND DISTINCT BIOLOGICAL CIRCUITS IN EFFECTOR, MEMORY AND EXHAUSTED CD8(+) T CELLS REVEALED BY TEMPORAL SINGLE-CELL TRANSCRIPTOMICS AND EPIGENETICS. NAIVE CD8(+) T CELLS CAN DIFFERENTIATE INTO EFFECTOR (T(EFF)), MEMORY (T(MEM)) OR EXHAUSTED (T(EX)) T CELLS. THESE DEVELOPMENTAL PATHWAYS ARE ASSOCIATED WITH DISTINCT TRANSCRIPTIONAL AND EPIGENETIC CHANGES THAT ENDOW CELLS WITH DIFFERENT FUNCTIONAL CAPACITIES AND THEREFORE THERAPEUTIC POTENTIAL. THE MOLECULAR CIRCUITRY UNDERLYING THESE DEVELOPMENTAL TRAJECTORIES AND THE EXTENT OF HETEROGENEITY WITHIN T(EFF), T(MEM) AND T(EX) POPULATIONS REMAIN POORLY UNDERSTOOD. HERE, WE USED THE LYMPHOCYTIC CHORIOMENINGITIS VIRUS MODEL OF ACUTE-RESOLVING AND CHRONIC INFECTION TO ADDRESS THESE GAPS BY APPLYING LONGITUDINAL SINGLE-CELL RNA-SEQUENCING (SCRNA-SEQ) AND SINGLE-CELL ASSAY FOR TRANSPOSASE-ACCESSIBLE CHROMATIN SEQUENCING (SCATAC-SEQ) ANALYSES. THESE ANALYSES UNCOVERED NEW SUBSETS, INCLUDING A SUBPOPULATION OF T(EX) CELLS EXPRESSING NATURAL KILLER CELL-ASSOCIATED GENES THAT IS DEPENDENT ON THE TRANSCRIPTION FACTOR ZEB2, AS WELL AS MULTIPLE DISTINCT TCF-1(+) STEM/PROGENITOR-LIKE SUBSETS IN ACUTE AND CHRONIC INFECTION. THESE DATA ALSO REVEALED INSIGHTS INTO THE RESHAPING OF T(EX) SUBSETS FOLLOWING PROGRAMMED DEATH 1 (PD-1) PATHWAY BLOCKADE AND IDENTIFIED A KEY ROLE FOR THE CELL STRESS REGULATOR, BTG1, IN ESTABLISHING THE T(EX) POPULATION. FINALLY, THESE RESULTS HIGHLIGHTED HOW THE SAME BIOLOGICAL CIRCUITS SUCH AS CYTOTOXICITY OR STEM/PROGENITOR PATHWAYS CAN BE USED BY CD8(+) T CELL SUBSETS WITH HIGHLY DIVERGENT UNDERLYING CHROMATIN LANDSCAPES GENERATED DURING DIFFERENT INFECTIONS. 2022 7 6522 49 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 8 3617 34 IN VITRO MODELING OF CD8(+) T CELL EXHAUSTION ENABLES CRISPR SCREENING TO REVEAL A ROLE FOR BHLHE40. IDENTIFYING MOLECULAR MECHANISMS OF EXHAUSTED CD8 T CELLS (T(EX)) IS A KEY GOAL OF IMPROVING IMMUNOTHERAPY OF CANCER AND OTHER DISEASES. HOWEVER, HIGH-THROUGHPUT INTERROGATION OF IN VIVO T(EX) CAN BE COSTLY AND INEFFICIENT. IN VITRO MODELS OF T(EX) ARE EASILY CUSTOMIZABLE AND QUICKLY GENERATE HIGH CELLULAR YIELD, ENABLING CRISPR SCREENING AND OTHER HIGH-THROUGHPUT ASSAYS. WE ESTABLISHED AN IN VITRO MODEL OF CHRONIC STIMULATION AND BENCHMARKED KEY PHENOTYPIC, FUNCTIONAL, TRANSCRIPTIONAL, AND EPIGENETIC FEATURES AGAINST BONA FIDE IN VIVO T(EX). WE LEVERAGED THIS MODEL OF IN VITRO CHRONIC STIMULATION IN COMBINATION WITH CRISPR SCREENING TO IDENTIFY TRANSCRIPTIONAL REGULATORS OF T CELL EXHAUSTION. THIS APPROACH IDENTIFIED SEVERAL TRANSCRIPTION FACTORS, INCLUDING BHLHE40. IN VITRO AND IN VIVO VALIDATION DEFINED A ROLE FOR BHLHE40 IN REGULATING A KEY DIFFERENTIATION CHECKPOINT BETWEEN PROGENITOR AND INTERMEDIATE T(EX) SUBSETS. BY DEVELOPING AND BENCHMARKING AN IN VITRO MODEL OF T(EX), THEN APPLYING HIGH-THROUGHPUT CRISPR SCREENING, WE DEMONSTRATE THE UTILITY OF MECHANISTICALLY ANNOTATED IN VITRO MODELS OF T(EX). 2023 9 3188 37 HBV-SPECIFIC CD8+ T-CELL TOLERANCE IN THE LIVER. HEPATITIS B VIRUS (HBV) REMAINS A LEADING CAUSE OF LIVER-RELATED MORBIDITY AND MORTALITY THROUGH CHRONIC HEPATITIS THAT MAY PROGRESS TO LIVER CIRRHOSIS AND CANCER. THE CENTRAL ROLE PLAYED BY HBV-SPECIFIC CD8+ T CELLS IN THE CLEARANCE OF ACUTE HBV INFECTION, AND HBV-RELATED LIVER INJURY IS NOW WELL ESTABLISHED. VIGOROUS, MULTIFUNCTIONAL CD8+ T CELL RESPONSES ARE USUALLY INDUCED IN MOST ADULT-ONSET HBV INFECTIONS, WHILE CHRONIC HEPATITIS B (CHB) IS CHARACTERIZED BY QUANTITATIVELY AND QUALITATIVELY WEAK HBV-SPECIFIC CD8+ T CELL RESPONSES. THE MOLECULAR BASIS OF THIS DICHOTOMY IS POORLY UNDERSTOOD. GENOMIC ANALYSIS OF DYSFUNCTIONAL HBV-SPECIFIC CD8+ T CELLS IN CHB PATIENTS AND VARIOUS MOUSE MODELS SUGGEST THAT MULTIFACETED MECHANISMS INCLUDING NEGATIVE SIGNALING AND METABOLIC ABNORMALITIES COOPERATIVELY ESTABLISH CD8+ T CELL DYSFUNCTION. IMMUNOREGULATORY CELL POPULATIONS IN THE LIVER, INCLUDING LIVER RESIDENT DENDRITIC CELLS (DCS), HEPATIC STELLATE CELLS (HSCS), MYELOID-DERIVED SUPPRESSOR CELLS (MDSCS), MAY CONTRIBUTE TO INTRAHEPATIC CD8+ T CELL DYSFUNCTION THROUGH THE PRODUCTION OF SOLUBLE MEDIATORS, SUCH AS ARGINASE, INDOLEAMINE 2,3-DIOXYGENASE (IDO) AND SUPPRESSIVE CYTOKINES AND THE EXPRESSION OF CO-INHIBITORY MOLECULES. A SERIES OF RECENT STUDIES WITH MOUSE MODELS OF HBV INFECTION SUGGEST THAT GENETIC AND EPIGENETIC CHANGES IN DYSFUNCTIONAL CD8+ T CELLS ARE THE MANIFESTATION OF PROLONGED ANTIGENIC STIMULATION, AS WELL AS THE ABSENCE OF CO-STIMULATORY OR CYTOKINE SIGNALING. THESE NEW FINDINGS MAY PROVIDE POTENTIAL NEW TARGETS FOR IMMUNOTHERAPY AIMING AT INVIGORATING HBV-SPECIFIC CD8+ T CELLS, WHICH HOPEFULLY CURES CHB. 2021 10 3288 41 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 11 771 43 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 12 6319 57 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 13 2660 39 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 14 6851 50 [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 15 769 46 CD8 T CELL EXHAUSTION DURING CHRONIC VIRAL INFECTION AND CANCER. EXHAUSTED CD8 T (TEX) CELLS ARE A DISTINCT CELL LINEAGE THAT ARISE DURING CHRONIC INFECTIONS AND CANCERS IN ANIMAL MODELS AND HUMANS. TEX CELLS ARE CHARACTERIZED BY PROGRESSIVE LOSS OF EFFECTOR FUNCTIONS, HIGH AND SUSTAINED INHIBITORY RECEPTOR EXPRESSION, METABOLIC DYSREGULATION, POOR MEMORY RECALL AND HOMEOSTATIC SELF-RENEWAL, AND DISTINCT TRANSCRIPTIONAL AND EPIGENETIC PROGRAMS. THE ABILITY TO REINVIGORATE TEX CELLS THROUGH INHIBITORY RECEPTOR BLOCKADE, SUCH AS ALPHAPD-1, HIGHLIGHTS THE THERAPEUTIC POTENTIAL OF TARGETING THIS POPULATION. EMERGING INSIGHTS INTO THE MECHANISMS OF EXHAUSTION ARE INFORMING IMMUNOTHERAPIES FOR CANCER AND CHRONIC INFECTIONS. HOWEVER, LIKE OTHER IMMUNE CELLS, TEX CELLS ARE HETEROGENEOUS AND INCLUDE PROGENITOR AND TERMINAL SUBSETS WITH UNIQUE CHARACTERISTICS AND RESPONSES TO CHECKPOINT BLOCKADE. HERE, WE REVIEW OUR CURRENT UNDERSTANDING OF TEX CELL BIOLOGY, INCLUDING THE DEVELOPMENTAL PATHS, TRANSCRIPTIONAL AND EPIGENETIC FEATURES, AND CELL INTRINSIC AND EXTRINSIC FACTORS CONTRIBUTING TO EXHAUSTION AND HOW THIS KNOWLEDGE MAY INFORM THERAPEUTIC TARGETING OF TEX CELLS IN CHRONIC INFECTIONS, AUTOIMMUNITY, AND CANCER. 2019 16 2879 42 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 17 1306 51 DEFINING 'T CELL EXHAUSTION'. 'T CELL EXHAUSTION' IS A BROAD TERM THAT HAS BEEN USED TO DESCRIBE THE RESPONSE OF T CELLS TO CHRONIC ANTIGEN STIMULATION, FIRST IN THE SETTING OF CHRONIC VIRAL INFECTION BUT MORE RECENTLY IN RESPONSE TO TUMOURS. UNDERSTANDING THE FEATURES OF AND PATHWAYS TO EXHAUSTION HAS CRUCIAL IMPLICATIONS FOR THE SUCCESS OF CHECKPOINT BLOCKADE AND ADOPTIVE T CELL TRANSFER THERAPIES. IN THIS VIEWPOINT ARTICLE, 18 EXPERTS IN THE FIELD TELL US WHAT EXHAUSTION MEANS TO THEM, RANGING FROM COMPLETE LACK OF EFFECTOR FUNCTION TO ALTERED FUNCTIONALITY TO PREVENT IMMUNOPATHOLOGY, WITH POTENTIAL DIFFERENCES BETWEEN CANCER AND CHRONIC INFECTION. THEIR RESPONSES HIGHLIGHT THE DICHOTOMY BETWEEN TERMINALLY DIFFERENTIATED EXHAUSTED T CELLS THAT ARE TCF1(-) AND THE SELF-RENEWING TCF1(+) POPULATION FROM WHICH THEY DERIVE. THESE TCF1(+) CELLS ARE CONSIDERED BY SOME TO HAVE STEM CELL-LIKE PROPERTIES AKIN TO MEMORY T CELL POPULATIONS, BUT THE DEVELOPMENTAL RELATIONSHIPS ARE UNCLEAR AT PRESENT. RECENT STUDIES HAVE ALSO HIGHLIGHTED AN IMPORTANT ROLE FOR THE TRANSCRIPTIONAL REGULATOR TOX IN DRIVING THE EPIGENETIC ENFORCEMENT OF EXHAUSTION, BUT KEY QUESTIONS REMAIN ABOUT THE POTENTIAL TO REVERSE THE EPIGENETIC PROGRAMME OF EXHAUSTION AND HOW THIS MIGHT AFFECT THE PERSISTENCE OF T CELL POPULATIONS. 2019 18 6481 35 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 19 1464 46 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 20 559 28 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