Abstract 1170: An off-the-shelf vaccine activates mutant KRAS-specific T cells in patients with resected pancreatic cancer

Abstract Background: Oncogenic mutations in KRAS (mKRAS) are expressed in up to 90% of pancreatic ductal adenocarcinomas (PDAC) therefore representing a promising immunotherapeutic target. We developed a pooled mKRAS peptide vaccine targeting the 6 most common mutations in PDAC: G12V, G12A, G12C, G12R, G12D, or G13D (NCT04117087). In a phase I study, we evaluated mKRAS-specific T cell responses raised by the vaccine when given in combination with immune checkpoint inhibitors to PDAC patients following resection and adjuvant chemotherapy. Materials and Methods: Patients received the pooled mKRAS peptide vaccine (0.3mg/peptide and 0.5mg poly-ICLC weekly for 4 doses in combination with ipilimumab and nivolumab followed by boosters every 8 weeks with nivolumab. To detect vaccine-induced mKRAS-specific T cells, pre- and post-vaccine peripheral blood mononuclear cells (PBMCs) were restimulated with control or individual mKRAS peptides and IFNγ release was measured by ELISPOT. mKRAS-specific T cell activation, proliferation, memory, and exhaustion profiles were assessed by CyTOF and cytokine secretion was determined by ELISA. For a subset of patients, mKRAS-specific T cells were expanded in vitro and T cell receptor (TCR) β chain sequencing and scRNA/TCRseq were used to identify the KRAS mutation-specific TCR repertoires and define their phenotypes in circulation. Results: mKRAS-specific T cells were detected by IFNγ ELISPOT post-vaccination for all patients, although magnitude and mutation-specific responses varied between patients. CyTOF analysis identified mKRAS-specific Th1 CD4+ central memory (cm, CCR7+IFNγ+) and effector memory (em, CCR7-IFNγ +IL2+ TNFɑ+) responses as well as CD8+ effector (eff, CD137+GZMBhiKi67hi) and effector memory (CCR7-CD137+GZMB+) responses. mKRAS-specific T cells were also detectable in the extended booster phase of treatment (>1 year post initial vaccination). In vitro expanded mKRAS-specific T cells mapped to CD4+ Tcm/em and CD8+ Tem populations within single cell datasets of unstimulated PBMCs collected post-vaccine. 5-25% of the mKRAS-specific T cells significantly expanded to more than one mKRAS antigen suggesting potential cross-reactive T cell clonotypes. In addition, shared public mKRAS-specific T cell clonotypes were identified across patients. Conclusions: This study demonstrates mKRAS vaccine induction of de novo, high quality mKRAS-specific polyfunctional CD4+ and CD8+ T cells. mKRAS-specific TCR analysis identified shared public TCRs across vaccinated patients that have important implications for future “off the shelf” adoptive therapy approaches. Ongoing studies are aimed to validate antigen specificity and map HLA-restriction of mKRAS-specific T cells. Overall, this study demonstrates a positive immunogenic signal of this pooled mutant KRAS vaccine that may be amenable for interception vaccination strategies against PDAC. Citation Format: Amanda L. Huff, Alex Girgis, Saurav D. Haldar, Emily Davis-Marcisak, Thatcher Heumann, Gabriella Longway, Lalitya Andaloori, Alexei Hernandez, Maximilian F. Konig, Brian Mog, Ludmila Danilova, Luciane T. Kagohara, Julie M. Nauroth, Amy M. Thomas, Elana J. Fertig, Won Jin Ho, Elizabeth M. Jaffee, Nilofer Azad, Neeha Zaidi. An off-the-shelf vaccine activates mutant KRAS-specific T cells in patients with resected pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1170.