Immunotherapy of CT26 murine tumors is characterized by an oligoclonal response of tissue-resident memory T cells against the AH1 rejection antigen.

Mice bearing CT26 tumors can be cured by administration of L19-mIL12 or F8-mTNF, two antibody fusion proteins which selectively deliver their cytokine payload to the tumor. In both settings, cancer cures crucially depended on CD8+ T cells and the AH1 peptide (derived from the gp70 protein of the murine leukemia virus) acted as the main tumor-rejection antigen, with ∼50% of CD8+ T cells in the neoplastic mass being AH1-specific after therapy. In order to characterize the clonality of the T cell response, its phenotype, and activation status, we isolated CD8+ T cells from tumors and secondary lymphoid organs and submitted them to T cell receptor (TCR) and total mRNA sequencing. We found an extremely diverse repertoire of more than 40 000 unique TCR sequences, but the ten most abundant TCRs accounted for >60% of CD8+ T-cell clones in the tumor. AH1-specific TCRs were consistently found among the most abundant sequences. AH1-specific T cells in the tumor had a tissue-resident memory phenotype. Treatment with L19-mIL12 led to overexpression of IL-12 receptor and of markers of cell activation and proliferation. These data suggest that the antitumor response driven by antibody-cytokine fusions proceeds through an oligoclonal expansion and activation of tumor-infiltrating CD8+ T cells.