Transcriptional regulation of hypofunction in tumor-specific CD8+ T cells

Abstract

Functional CD8+ tumor-infiltrating lymphocytes (TIL) positively correlate with improved cancer patient survival. However, mechanisms of self-tolerance often result in TIL with a hypofunctional phenotype incapable of tumor clearance. Using a solid tumor model in mice, we found that CD8+ T cells became tolerized in less than 24 hours in an established tumor environment. To define the collective impact of pathways suppressing TIL function, we compared genome-wide mRNA expression of tumor-specific CD8+ T cells from the tumor (hypofunctional T cells) and periphery (functional T cells). Notably, gene expression and hypofunction of TIL more closely resembled self-tolerance than viral-exhaustion. Differential gene expression was refined to identify a core set of genes that defined hypofunctional TIL; these data comprise the first "molecular profile" of tumor-specific TIL. The molecular profile of TIL was further dissected to determine the extent of overlap and distinction between pathways that collectively restrict T cell functions. As suggested by the molecular profile of TIL, protein expression of inhibitory receptor LAG-3 was differentially regulated throughout prolonged late-G1/early-S phase of the cell cycle. These data may accelerate efficient identification of combination therapies to unleash anti-tumor immunity of TIL specifically against tumor cells. The molecular profile of TIL predicted that inhibitory pathways converge at one transcription factor, E2F1, to regulate gene expression in late-G1/early-S phase and self-tolerance of CD8+ T cells. Methyltransferase PRMT1 posttranslationally directs E2F1 to drive cell cycle inhibition and apoptosis. PRMT1 mRNA was overexpressed by TIL. PRMT1 knockdown increased proliferation of tumor-specific CD8+ T cells in the periphery of tumor-bearing hosts and trended towards increased accumulation as TIL. Although subtle, these data suggest that anti-tumor immunity of TIL would be increased by restricting PRMT1 modification of E2F1 in tumor-specific CD8+ T cells. Advances in technology augment identification and therapeutic use of intracellular targets, such as E2F1, to boost anti-tumor function of TIL. Within these platforms, the molecular profile of TIL provides an invaluable resource for identification of novel transcriptional regulators of TIL hypofunction.