Recent clinical successes of PD-1/L1 and CTLA-4 blockade have demonstrated the potential of immunotherapies for the treatment of cancer. Research suggests the clinical efficacy of these checkpoint blockade therapies requires the presence of tumor-reactive T cells prior to therapy. It is thought that these tumor-reactive T cells primarily recognize nonsynonymous mutations (neoepitopes) uniquely expressed by a patient’s tumor. Unfortunately, at present the majority of patients fail to respond to checkpoint blockade, and it is hypothesized that one can improve their efficacy by combining them with vaccines targeting neoepitopes. Using the MC38 murine colon cancer model, we previously showed that an effective antitumor immunity could be generated when combining either a peptide or recombinant adenoviral-based vaccine targeting neoepitopes with three rationally selected immune-oncology agents. We utilized an IL-15 superagonist (N-803) to promote the generation and expansion of T cells and a tumor-targeted IL12 (NHS-IL12) and PD-L1 blockade to facilitate T-cell expansion and function within the tumor microenvironment. This multifaceted treatment regimen was effective at inducing tumor clearance in the majority of animals treated. In the present study, we examine the immune response in animals that fail to respond to our treatment regimen. By analyzing the spread of immunity to neoepitopes expressed by the MC38 tumor, but not contained within the vaccine, we were able to correlate treatment failures to less diversity in the antitumor immune response. To examine this observation more closely, tumor-bearing mice were treated with different combinations of neoepitope vaccine, N-803, PD-L1 blockade, and NHS-IL12, to ascertain which components of the combination therapy facilitated epitope spreading. We also examined which cell populations were able to cooperate with tumor-reactive CD8+ T cells to promote and/or inhibit epitope spread. Our observations demonstrate the importance of a diverse antitumor immune response and offers insights on how one might increase the diversity of the antitumor immune response following treatment with immune-oncology agents. These studies were performed in collaboration with NantOmics, NantBio and ImmunityBio, and EMD Serono as a part of Collaborative Research and Development Agreements (CRADA) with the NCI.

Citation Format: Karin L. Lee, Claudia Palena, Jeffrey Schlom, Duane H. Hamilton. The importance of epitope spreading in the generation of effective antitumor immunity in mice treated with combination immunotherapy [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B29.