Defining the regulation and regulatory mechanisms of TCF-1 in CD8+ T cell differentiation

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PROJECT SUMMARY CD8 T cells are critical for immune surveillance of both intracellular pathogens and cancer. Chronic and acute antigen exposure entails distinct differentiation trajectories of CD8 T cells. In acute infection, naïve CD8 T cells that recognize antigen undergo clonal expansion and differentiate into either cytotoxic effectors or quiescent central memory cells (Tcm) that mediate rapid responses to subsequent infections. In settings of chronic antigen exposure, such as cancer, CD8 T cells instead differentiate into to terminally exhausted (TEX) cells. TEX have attenuated proliferative, cytokine-secretory, and cytolytic functions relative to the effector cells elicited by acute infection. Although they still exert a significant level of viral control, TEX are unable to sustain durable responses and require constant replenishment from a pool of quiescent, stem-like T progenitor exhausted (TPEX). Importantly, immune checkpoint blockade therapy acts on TPEX to mobilize T cell responses to tumor antigens. Despite the therapeutic imperative to elicit Tcm in response to vaccination and TPEX in responses to cancer, the molecular mechanisms by which these T cell subsets develop are incompletely understood. It is known that the transcription factor (TF) TCF-1 is required for both Tcm and TPEX populations and is downregulated during the differentiation of effector and exhausted CD8 T cells. Although pro-inflammatory cytokines such as IL-2 and IL-12 are known to induce the downregulation of TCF-1 (encoded by Tcf7), it is unclear how these extracellular cues are integrated to regulate TCF-1 expression during the fate decisions of effector and TEX differentiation. It is also incompletely understood how TCF-1 contributes to the establishment of unique transcriptional profiles of TPEX and Tcm despite the dependency of both cell subsets on this TF. The proposed study seeks to address these gaps in knowledge by testing the central hypotheses (1) that self- amplification of IL-2 receptor signaling through upregulation of the IL-2R alpha chain (Il2ra) promotes downregulation of TCF-1 by direct repression of Tcf7 by the IL-2-inducible TFs AP4 and Blimp1 and (2) that differential engagement of TLE family co-repressors and intrinsic HDAC activity by TCF-1 in TPEX and Tcm result in subset-specific transcriptional regulation by TCF-1. The research project will interrogate the necessity of IL- 2R self-amplification in terminal differentiation by ablating IL-2R signaling-responsive cis-regulatory elements in the Il2ra and Tcf7 loci in mouse models of chronic and acute infections. The requirements of TLE co-repressor recruitment and HDAC activity in TPEX and Tcm will be interrogated by complementing Tcf7-deficient CD8 T cells with Tcf7 mutants deficient in either HDAC or TLE co-repressor binding activity and infecting mice with chronic or acute LCMV. The proposed research project is expected to advance the long-term objective of determining the molecular basis by which Tcm and TPEX fate decisions are made, with the ultimate goal of engineering durable CD8 T cell responses to infection and cancer.