(764) Novel autochthonous cancer model to investigate TCR signal strength in tumor-specific T cell differentiation

Introduction/Rationale: Immunotherapies demonstrate the potential power of CD8 T cells to eliminate cancer cells; however, these strategies only work in a subset of patients and tumor types. To design predictably effective immunotherapies, we must elucidate the mechanisms controlling tumor-specific T cell (TST) activation, dysfunction, and therapeutic response. T cell receptor (TCR) signal strength (determined by the affinity of TCR for peptide-bound major histocompatibility complex (pMHC)) is known to regulate T cell differentiation. However, its impact on anti-tumor immunity is less clear.

Methods: We developed a novel, inducible autochthonous cancer model in which we can systematically vary TCR signal strength, spanning the range observed for human tumor-infiltrating T cells: we cloned a Sleeping Beauty (SB) transposon/transposase-based vector encoding the oncogene MYC followed by an inverted inducible tumor model antigen (OVA). By single amino acid substitutions to the native OVA sequence, we generated altered peptide ligands that are recognized by OVA-specific CD8 T cells with varying TCR signal strength. SB-vectors, along with a vector silencing the tumor suppressor P53, were delivered via hydrodynamic tail vein injection into mice with inducible Cre expression, a highly-efficient method to induce liver tumors. Employing this model, we can track CD8 TST over months from tumor initiation to endpoints.

Results: We characterized CD8 TST in progressing tumors, and their response to immunotherapy. Tumors expressing high affinity epitopes induced TST expansion and TST ultimately acquired an exhausted phenotype with time. Tumors with low affinity epitopes lacked TST expansion. Strikingly, however, these TST could be activated through immunization, mediating robust anti-tumor immune responses.

Conclusion: We investigated how TCR signal strength determines T cell activation, differentiation, and anti-tumor activity, revealing interesting insights into how low-affinity TST can be therapeutically utilized in solid tumors.