CD8 T cell activation in cancer is comprised of two distinct phases

Abstract The CD8 T cell response to tumors is extremely variable with heterogenous T cell subsets, a stem-like CD8 T cell (PD1+TCF1+) that sustains the CD8 response and gives rise to a terminally differentiated (TD) cytotoxic cell (TCF1-Tim3+). Although these subsets have been described, how tumor-specific CD8 T cells are activated and differentiate in tumors are not well defined. Using a prostate cancer model that expresses the LCMV-GP (TRAMPC1-GP), we studied tumor-specific CD8 T cell activation by transferring LCMV-GP specific P14 CD8 T cells into tumor-bearing mice. We found that P14s are activated in the tumor-draining LNs (TDLNs) and acquire a stem-like phenotype. These cells migrate into the tumor as stem-like CD8 T cells and only differentiate into a TD CD8 T cell in the tumor. We found that stem-like CD8 T cells need additional co-stimulation from antigen presenting cells within the tumor to fully differentiate, even though they have been previously activated in TLDNs. Similarly, stem-like CD8s from human kidney cancer require both TCR and co-stimulatory signals to divide and differentiation ex-vivo and can differentiate when co-cultured with autologous dendritic cells. The addition of IL12 with TCR alone was not sufficient to induce differentiation, but improved differentiation when co-stimulation was present. This demonstrates the necessity of additional TCR and co-stimulation once activated stem-like CD8 T cells migrate into the tumor. Overall, these data suggest two distinct phases of CD8 T cell differentiation, the first occurs in the TDLN where they are initially activated. The second occurs in the tumor, where they require additional co-stimulation to differentiate and acquire an effector phenotype.