Abstract 4157: Tumor context dictates reliance on TCF1 for response to immunotherapy
Authors: Escobar Giulia , Tooley Katherine , Oliveras Joan Pages , Huang Linglin , Cheng Hanning , Xue Chang , Mangani Davide , Hazel Natanael , Rutigliani Carola , Biasco Luca , Anderson Ana C. 
Published: April 2023

Abstract
Stem-like CD8 T cells are regulated by the transcription factor TCF1 and are key players in the response to immune checkpoint blockade (ICB). However, recent findings indicate that the dependance on TCF1+ stem-like T cells for ICB efficacy may not be equal across patients or in different tumor contexts. Here we leveraged TCF1 conditional knock-out (TCF1 cKO) mice to investigate how TCF1 instructs the early fate and functions of CD8 T cells upon ICB therapy in tumors that differ for immunogenicity and levels of tumor antigen expression. Strikingly, we discovered that TCF1 expression in CD8 T cells is required for ICB efficacy in poorly immunogenic B16OVA melanomas but is dispensable in highly immunogenic MC38OVA colorectal tumors. Single-cell RNA sequencing and immunophenotyping in the tumor draining lymph node (TDLN) revealed defective priming and expansion of tumor-specific TCF1 cKO T cells in B16OVA- but not MC38OVA-bearing mice treated with ICB. Conversely, ICB therapy efficiently expanded tumor-specific TCF1 WT T cells in the TDLN of both tumor models. In vitro, we found defective proliferation, reduced PD1 and CD28 up-regulation and reduced phosphorylation of key molecules downstream the T cell receptor pathway in TCF1 cKO T cells stimulated with low but not high TCR signals. These data indicate that TCF1 poises T cells for optimal responsiveness in suboptimal priming conditions such as those found in low antigen expressing tumors. Furthermore, in the absence of TCF1 we found the accumulation in the TDLN of a subset of tumor-specific naïve T cells poised to give rise to short-lived effectors, which are less suited to sustain anti-tumor responses in poorly immunogenic tumors where expansion of T cells retaining memory potential is required for durable responses. Single-cell RNA sequencing and immunophenotyping within the tumor microenvironment showed that in MC38OVA tumors both WT and TCF1 cKO mice expanded a CD8 subset sharing a signature with transitory effectors, which were shown to mediate ICB efficacy in chronic viral infection models. Expansion of cytotoxic CD8 T cells likely accounted for the strong anti-tumor response observed in both WT and TCF1 cKO mice. Conversely, B16OVA tumors failed to expand transitory effectors and accumulated Tox+ dysfunctional CD8 T cells. Importantly, in the absence of TCF1 dysfunctional T cells became destabilized, failed to persist and shared features with CD8 T cells found in patients that fail ICB. Altogether, the reduced priming of stem-like T cells in the TDLN combined with a destabilized dysfunctional T cell state in the tumor contributed to the failure of TCF1 cKO mice to sustain effective ICB responses in poorly immunogenic tumors. Our study highlights a role for TCF1 during the priming and early stages of the anti-tumor CD8 T cell response with important implications for guiding optimal therapeutic interventions in cancers with low frequency of TCF1+ CD8 T cells and low neoantigen expression.
Citation Format: Giulia Escobar, Katherine Tooley, Joan Pages Oliveras, Linglin Huang, Hanning Cheng, Chang Xue, Davide Mangani, Natanael Hazel, Carola Rutigliani, Luca Biasco, Ana C. Anderson. Tumor context dictates reliance on TCF1 for response to immunotherapy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4157.