LAUSR

Mutations in the KRAS gene are among the most common driver mutations in cancer. Targeting KRAS G12C mutations has shown some promise in clinical studies, though acquired resistance mutations pose a challenge for small molecule-based monotherapies. Vaccines targeting KRAS mutations remain highly desirable for durable tumor control and clinical benefit to patients with solid tumors. While cytotoxic CD8 T cells are critical to tumor control and clearance, CD4 T cell activation is a key component of a durable anti-tumor response. Cancer neoepitopes with homology to infectious diseases antigens (mimotopes) were associated with long-term clinical benefit following CTLA-4 blockade. In this study, we screened healthy donor PBMC samples via ex vivo interferon-gamma (IFNγ) ELISpot and identified KRAS G12C-specific T cell responses associated with cross-reactive TCRs recognizing a bacterial lipoprotein peptide sequence with homology to KRAS G12C. Healthy donors with T cell responses to both KRAS G12C and mimotope peptides shared common HLA-DR alleles, and KRAS G12C-specific T cell responses were CD4 dependent via depletion assays. KRAS G12C-reactive CD4 healthy donor T cells were polyfunctional as assessed by intracellular cytokine staining, and killing of class II target cells presenting G12C peptides assessed by Incucyte assay was CD4-dependent. We further identified vaccine-induced KRAS G12C specific CD4 T cell responses in a patient with KRAS G12C-positive non-small cell lung cancer (NSCLC) receiving a shared neoantigen-targeting cancer vaccine consisting of Chimpanzee adenovirus prime and self-amplifying mRNA (samRNA) boost vaccinations in combination with nivolumab 480 mg IV (NCT03953235). This patient had previously progressed on prior checkpoint inhibitor therapy and showed signs of clinical benefit with molecular response (e.g., ctDNA reduction) and tumor shrinkage following vaccination. Patient PBMCs were analyzed by ex vivo IFNγ ELISpot, and G12C-specific T cell responses were detectable at baseline and increased following vaccination. T cell receptor (TCR) and transcriptome single cell sequencing analyses showed a large clonal population of CD4 effector memory cells in the post-vaccination samples. Functional analyses of patient-derived TCR clonotypes from post-vaccination samples via recombinant TCR screening assays identified two functional clonotypes recognizing KRAS G12C epitopes in the context of HLA-DR on target cells. Studies to further elucidate KRAS G12C vaccine-induced TCR functionality in primary cells and transgenic mouse models are ongoing. This study provides important insight into both naturally occurring bacterial mimotope-based and vaccine-induced T cell responses against KRAS G12C neoepitopes that informs future therapeutic approaches targeting KRAS G12C tumors. Citation Format: Christine D. Palmer, Meghan G. Hart, Sonia Kounlavouth, Harshni Venkatraman, Lauren D. Kraemer, Martina Marrali, Calixto Dominguez, Fatema Z. Chowdhury, Jason R. Jaroslavsky, Lindsey Arcebuche, Lorenzo Hernandez, Bukola Adeoye, Severino Cuison, Amy R. Rappaport, Greg Boucher, Monica Lane, Melissa Rotunno, Kenneth Avocetien, Leiliane Sousa, Chris Puccia, Molly Likes, Rahul Vegesna, Rita Zhou, Alexis Mantilla, Matthew J. Davis, Ankur Dhanik, Melissa Johnson, Andrew R. Ferguson, Karin Jooss. HLA-DR-restricted CD4 T cell responses to KRAS G12C in healthy donors linked to bacterial mimotope: lessons for KRAS neoantigen vaccines in cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2267.