Abstract IA03: Therapeutically-active neoantigen-specific cancer vaccines

Our past work focused on defining the ability of of the natural immune system to control and shape developing tumors, a process we termed cancer immunoediting. We showed that in its most complex form, cancer immunoediting occurs in three phases: elimination, equilibrium and escape. We subsequently used a combination of exome sequencing and epitope prediction algorithms to show that mutant proteins in highly immunogenic tumor cells derived from methylcholanthrene treated immunodeficient mice represent immunodominant, tumor specific antigens for CD8+ T cells and that immunoselection was a major mechanism underlying the immunoediting process. We next asked whether our approach could be extended into the area of immunotherapy by exploring whether we could identify antigens in progressively growing established tumors that rendered them susceptible to checkpoint blockade immunotherapy. T cell lines generated from anti-PD-1 treated mice that rejected T3 progressor sarcoma cells displayed restriction to H-2Kb but not to H-2Db. We then identified expressed nonsynonymous mutations in T3 cells using exome sequencing and generated a prioritized list of potential H-2Kb binding neoepitopes. This analysis predicted two unequivocal “best candidates”—a mutant form of Laminin α subunit 4 (mLama4) and a mutant glucosyltransferase (mAlg8). When tested in vitro, the two neoepitopes were the only ones among the 62 top predicted H-2Kb binding sequences that stimulated T3 sarcoma specific T cells. These findings were validated by showing that: a) the two mutant neoepitopes could be detected bound to H-2Kb on intact T3 sarcoma cells; b) CTLs expressing TCRs for mLama4 and mAlg8 accumulated in T3 tumors in anti-PD-1-treated, tumor-bearing mice; c) vaccination of naive WT mice with mutant but not WT forms of Lama4 or Alg8 peptides induced strong CD8+ T cell responses; and d) naive mice vaccinated either prophylactically or therapeutically with synthetic long peptides containing the mLama4 plus mAlg8 neoepitopes controlled outgrowth of T3 tumors. We have since gone on to explore whether MHC-I restricted neoepitopes were sufficient to drive therapeutic responses to tumors. For this work, we used oncogene driven sarcomas derived from mice expressing a mutant activated form of Kras and lacking p53 (KP sarcomas). KP sarcomas were not immunogenic and their outgrowth could not be controlled in mice treated either with the combination of anti-CTLA-4 plus anti-PD-1 or using a tumor challenge and re-challenge protocol. Exome sequencing of KP sarcomas revealed that they expressed an extremely limited number of nonsynonomous mutations and that none of the mutations were predicted to form epitopes for T cells. Analysis of tumor bearing mice revealed that they did not develop either tumor reactive CD4+ or CD8+ T cells. Enforced expression of strong MHC-I or MHC-II epitopes into KP sarcomas did not increase tumor immunogenicity. In contrast, ectopic expression of combinations of both MHC-I plus MHC-II epitopes in KP tumors rendered the tumor cells highly immunogenic resulting in their rejection. These data thus demonstrate that protective immune responses to tumors requires the presence of both MHC-I and MHC-II restricted epitopes and thus provide additional insights into the requirements for effective personalized cancer vaccines. Citation Format: Robert D. Schreiber. Therapeutically-active neoantigen-specific cancer vaccines [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr IA03.