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Stem-like CD8 T cells that are regulated by the transcription factor TCF1 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 and raises the question of what determines reliance on TCF1 for effective responses. Here we leveraged TCF1 conditional knock-out (TCF1-cKO) mice to investigate how TCF1 instructs the early fate and functions of CD8 cells upon ICB therapy in tumors that differ for immunogenicity and levels of tumor antigens 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 revealed defective priming and expansion of tumor-specific TCF1-cKO T cells in the tumor draining lymph-node (TDLN) of B16OVA- but not MC38OVA-bearing mice treated with ICB. In vitro, we found defective proliferation, reduced PD-1 and CD28 up-regulation and reduced phosphorylation of key molecules downstream the T cell receptor pathway when TCF1-cKO T cells were stimulated with low doses of antigens but not when stimulated with strong TCR signals. These data indicate that TCF1 poises T cells for optimal antigen responsiveness. Furthermore, transcriptional profiling of T cells in the TDLN revealed the accumulation of a subset of tumor-specific naïve T cells poised to give rise to short-lived effectors in TCF1-cKO mice and thus is less suited to sustain anti-tumor responses in poorly immunogenic tumors where expansion of T cells retaining stem-like potential is required for durable anti-tumor responses. In the MC38OVA tumor, single-cell-RNA sequencing and immunophenotyping showed that both WT and TCF1-cKO mice displayed an expanded CD8 subset sharing a signature with transitory effector cells that mediate ICB efficacy in chronic viral infection models. These cells likely accounted for the strong anti-tumor response observed in both WT and TCF1-cKO mice. Conversely, B16-OVA tumors retained a higher frequency of stem-like cells, failed to expand transitory effectors and accumulated Tox+ CD8 T cells, sharing a signature with CD8 cells expanded in non-responders to ICB. Importantly, loss of TCF1 was associated with reduced maintenance and proliferation of stem-like precursors and reduced expression of the Tox gene which was required for the survival of the dysfunctional T cells. Altogether, these data indicate that the reduced priming and maintenance of stem-like T cells combined with a destabilized dysfunctional T cell state in the tumor microenvironment contributed to the failure of TCF1-cKO mice to sustain effective ICB responses in poorly immunogenic tumors. Our study highlights a role for TCF1 in the early stages of the CD8 T cell response with important implications for guiding the choice of optimal therapeutic interventions in tumors expressing low neoantigen levels. Citation Format: Giulia Escobar, Katherine Tooley, Joan Pagès Oliveras, Cheng Hanning, Xue Chang, Davide Mangani, Hazel Natanael, Carola Rutigliani, Luca Biasco, Ana Carrizosa Anderson. Tumor context dictates reliance on TCF1 for response to immunotherapy [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B35.

The fourth “Melanoma Bridge Meeting” took place in Naples, December 5 to 8th, 2013. The four topics discussed at this meeting were: Diagnosis and New Procedures, Molecular Advances and Combination Therapies, News in Immunotherapy, and Tumor Microenvironment and Biomarkers. Until recently systemic therapy for metastatic melanoma patients was ineffective, but recent research in tumor biology and immunology has led to the development of new targeted and immunotherapeutic agents that prolong progression-free survival (PFS) and overall survival (OS). New therapies, such as mitogen-activated protein kinase (MAPK) pathway inhibitors, like BRAF and MEK inhibitors, as well as other signaling pathways inhibitors, are being tested in metastatic melanoma either as monotherapy or in combination, and have yielded promising results. Improved survival rates have also been observed with immune therapy for patients with metastatic melanoma. Immune-modulating antibodies came to the forefront with anti-CTLA-4, programmed cell death-1 (PD-1) and PD-1 ligand 1 (PD-L1) pathway blocking antibodies that result in durable responses in a subset of melanoma patients. Agents targeting other immune inhibitory (e.g., Tim-3) or immune stimulating (e.g., CD137) receptors and other approaches such as adoptive cell transfer demonstrate clinical benefit in melanoma as well. This meeting’s specific focus was on advances in targeted therapy and immunotherapy. Both combination targeted therapy approaches and different immunotherapies were discussed. Similarly to the previous meetings, the importance of biomarkers for clinical application as markers for diagnosis, prognosis and prediction of treatment response was an integral part of the meeting. Significant consideration was given to issues surrounding the development of novel therapeutic targets as further study of patterns of resistance to both immunologic and targeted drugs are paramount to future drug development to guide existing and future therapies. The overall emphasis on biomarkers supports novel concepts toward integrating biomarkers into contemporary clinical management of patients with melanoma across the entire spectrum of disease stage. Translation of the knowledge gained from the biology of tumor microenvironment across different tumors represents a bridge to impact on prognosis and response to therapy in melanoma.