TCF1^lo CD8 T cells proliferate and persist autonomously in tumors

Cancers develop in humans over months to years, and tumor-specific CD8 T cells (TST) can interact with cancer cells throughout tumorigenesis. Nevertheless, the long-term population dynamics of TST, especially within progressing tumors, are not well understood. A paradigm first established in chronic viral infection and applied to tumors describes a population hierarchy among exhausted T cells. Progenitor/stem-like exhausted T cells, which express the transcription factor T cell factor 1 (TCF1), maintain the population through self-renewal and by giving rise to terminally differentiated TCF1^lo progeny. This has led to a focus on TCF1^hi T cells, and though TCF1^lo CD8 T cells are the predominant tumor-infiltrating/tumor-reactive subtype in patients, they have been largely overlooked. We leveraged our autochthonous liver cancer model to analyze TST differentiation and proliferation throughout tumorigenesis. Dual EdU/BrdU labeling studies revealed that throughout tumorigenesis, a subset of TCF1^lo TST in the liver stochastically entered and exited cell cycle, and at later time points there was no evidence of a TCF1^hi progenitor-like population. Moreover, TCF1-knockout TST proliferated and persisted robustly in tumors. Using liver cancer and melanoma models, we showed that tumor-resident TCF1^lo TST proliferate and persist autonomously, even when new TST influx into tumors is inhibited. The prevailing notion is that only TCF1^hi TST self-renew but we now demonstrate, using a clinically relevant mouse cancer model, that TCF1^lo TST stochastically proliferate to achieve long-term population maintenance. Future studies to understand and harness this mechanism to improve T cell persistence in tumors could lead to novel immunotherapies for patients with cancer.