CIT tumor lines: A novel series of immunogenic squamous cell skin carcinoma cell lines derived from chemical carcinogenesis

Immunotherapy is now widely used to treat advanced-stage skin cancer, but it is effective for only approximately half of skin cancer patients, including both patients with melanoma and cutaneous squamous cell carcinoma (cSCC). To overcome current barriers, preclinical mouse models that faithfully recapitulate the tumor genetics, mutation burdens, and neoantigen patterns of specific human tumor types are essential. However, while many models exist for melanoma, there are currently relatively few transplantable murine models of cSCC, which is responsible for nearly as many deaths as melanoma each year. Here we describe a novel series of 11 cSCC tumor lines, the Carcinogen-Induced Tumor (CIT) lines, syngeneic to the FVB strain, that address this need. The CIT lines were established from skin carcinomas induced by DMBA and TPA treatment and harbor genetic drivers and overall tumor mutational burdens that recapitulate those found in cSCC. Each CIT line gives rise to tumors with a consistent immune infiltration pattern, ranging from T cell-rich "hot" tumors to T cell-poor "cold" tumors. Hot CIT lines exhibit partial responses to treatment with immune checkpoint inhibitors, and we have identified two neoantigens present in an immunotherapy-responsive CIT line. The CIT lines thus provide a valuable new series of preclinical models for studying anti-tumor immune responses and developing strategies to improve immunotherapy efficacy in cSCC.