Rewiring T cell exhaustion with immune checkpoint blockade therapy

PROJECT SUMMARY The goal of this project is to determine whether it is possible to promote the development and/or maintenance of CD8+ T cells that are essential for anti-tumor responses to PD-1 immune checkpoint blockade (ICB) therapy by targeting PSGL-1 (P-selectin glycoprotein-1). Studies of exhausted CD8 T cells (TEX) with chronic LCMV infection identified that terminally dysfunctional TEX (TTEX) arise by the progressive differentiation of less exhausted cells with the capacity for effector functions, and that these cells derive from stem-cell like, PD-1+ self-renewing cells (TSC) expressing the transcription factor TCF-1. Although not yet as well studied in cancer, the progeny of TSC, designated precursor exhausted T cells (TPEX), are found in mouse models of melanoma and in melanoma cancer patient tumors where their frequencies predict responses to PD-1 ICB. The identification of these subsets is considered to be paradigm shifting since TEX were previously thought to minimally alter their anti-tumor responses because of epigenetic stability. However, induction of TPEX responses by PD-1 ICB causes rapid TEX differentiation and increased numbers of TTEX contributing to immunotherapy resistance. Thus, a fundamental question is whether TSC and/or TPEX can be generated/maintained to increase patient responses and progression free survival with PD-1 ICB. We identified an unexpected function of PSGL- 1 as a T cell intrinsic checkpoint inhibitor in responses to chronic viral infection and tumors. PSGL-1 acts upstream of PD-1 to promote the upregulation of multiple inhibitory receptors (IRs) and TTEX differentiation during chronic antigen stimulation. Although VISTA, a PD-L1 homologue, was shown to be a ligand for PSGL-1 in vitro, its function in vivo has not been established, and its binding to PSGL-1 may be a critical driver of TEX differentiation. We showed that PSGL-1 engagement together with TCR signaling elicits TTEX development from both human and mouse effector T cells in vitro, underscoring its integral connection to immune inhibitory pathways and relevance to human anti-tumor responses. In contrast, PSGL-1 deficiency prevents TTEX development by overcoming the inhibition of TCR signaling conferred by chronic stimulation, thereby promoting significant control of an anti-PD-1 resistant melanoma. Notably, this outcome is recapitulated with PSGL-1 blockade, the focus of this application. With PSGL-1-/- T cells, we find greater frequencies of CD8 T cells that retain effector functions and express genes which identify TSC/TPEX, including TCF-1. Using PD-1 ICB resistant melanoma models in addition to melanoma patient samples, we propose test the hypothesis that PSGL-1 blockade promotes the generation and/or persistence of TSC and TPEX by preventing engagement of VISTA or other novel ligand(s). We will determine the fate of these cells with PSGL-1 ICB, alone and together with anti- PD-1. We will test novel monoclonal anti-human PSGL-1 antibodies for their ability to support TPEX differentiation or block TTEX differentiation to enhance anti-tumor T cell responses and to corroborate the potential for clinical targeting of PSGL-1 as a novel immunotherapy.