Background Non-melanoma skin cancer, which includes cutaneous squamous cell carcinoma (cSCC), is the most common cancer. In the US, there are more than a million cases of cSCC diagnosed annually. Although most early stage cSCC are successfully treated with excision, cSCC results in significant morbidity, and approximately 4% of cSCC patients develop metastases and 2% die. T cell-mediated immunotherapy with immune checkpoint inhibitors improves survival in cSCC patients, but about one half of patients do not respond. To improve immunotherapeutic approaches, there is a need to better understand T cell-mediated control of cSCC.
Methods To investigate the neoantigen-specific T cell responses that control cSCC, we generated a panel of transplantable cSCC clonal cell lines from solar simulated light-induced tumors in BALB/c mice.1 Whole exome and RNA sequencing was performed on the cSCC cell lines. Mice lacking T cells and T cell depleting antibodies were used to study the role of T cells. MHC class I neoantigens anticipated to elicit T cell responses were prioritized based on predicted MHC class I binding affinity, presentation on MHC class I, and mRNA variant allele-specific expression.ELISPOT assays were used to quantify IFN-γ secreting T cells specific for each predicted neoantigen. To test the ability of vaccination to constrain cSCC, mice were vaccinated with irradiated tumor cells or an immunogenic neoantigen.
Results The cSCC cell lines constitutively express MHC class I, and thus, can be targeted for destruction by CD8 T cells. CSCC tumors grow faster in athymic mice, lacking mature T cells, compared to wild-type mice, supporting that T cells constrain cSCC growth. In vivo depletion of CD8 and CD4 T cells demonstrates a major role for CD8 T cells and a contributing role for CD4 T cells in controlling cSCC growth. Vaccination with irradiated cSCC cells completely protects from tumor challenge, and this response is dependent on CD8 T cells. Vaccination with irradiated tumor cells from another cSCC cell line does not protect from tumor challenge, demonstrating that vaccination-induced protection is cell line specific. IFN-γ-secreting CD8 T cells that recognize a mutated Kars peptide are present within cSCC tumors. Prophylactic vaccination with the mutated Kars peptide decreases cSCC growth, demonstrating that an immune response to mutated Kars controls tumor growth.
Conclusions Neoantigen-specific CD8 T cells constrain cSCC growth in a clinically relevant mouse model. Neoantigen-specific approaches may improve immunotherapy outcomes for cSCC patients.
Acknowledgements This work was supported by the American Skin Association (A.C.A), NIH (T32CA009213, A.C.A; F30CA257622, A.C.A; F30CA281056, E.S.B; P30CA023074, D.J.R; P01CA229112, K.T.H), and Training Oncology Physician Scientists Award from the University of Arizona Cancer Center (E.S.B).
Adams AC, Macy AM, Saboda K, et al. Solar Simulated Light Induces Cutaneous Squamous Cell Carcinoma in Inbred Mice: A Clinically Relevant Model to Investigate T-Cell Responses. J Invest Dermatol. 2021;141(12):2990–2993.e2996.
Ethics Approval This study was approved by the University of Arizona’s Institutional Animal Care and Use Committee (Protocol #13–469).
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