Introduction: Cancer immunotherapy has made waves of progress in recent years with the advent of checkpoint inhibitor therapies against several cancers. However, pancreatic cancer (PC), a disease which kills approximately 40,000 patients each year in the US, has successfully evaded immunotherapy approaches. PC hosts a notoriously immunosuppressive microenvironment comprised of an abundance of immunosuppressive tumor associated macrophages, myeloid derived suppressor cells (MDSCs) and regulatory T-cells with a scarcity of effector CD8+ T-cells. Irreversible electroporation (IRE) is a non-thermal ablation technique that induces tumor cell death without destruction of adjacent collagenous structures. Unlike thermal ablation techniques, IRE results in a gradual apoptotic cell death and is currently in clinical use for selected patients with locally advanced PC. We hypothesize that the abundant release of antigens by IRE can induce an “in situ vaccination” effect that can elicit adaptive T-cell-mediated anti-tumor immune responses.

Methods: We have developed a robust syngeneic immunocompetent mouse model of PC using a cell line established from a tumor arising in a LSL-KrasG12D/+; LSL-Trp53R172H/+; PDX1Cre/+; LSL-ROSA26 Luc/+ mouse (KPC). We have utilized the ECM 830 square wave pulse generator to deliver IRE (100 μsec pulses of electricity at 1500 V/cm using a two-needle array probe) to subcutaneous tumors measuring 5-7 mm in diameter. The effects of IRE on primary tumor growth and secondary tumor challenge were observed. Flow cytometry-based immunoprofiling studies of tumors at one week post-IRE were performed in triplicate and presented as mean difference between IRE and no treatment (standard error). *P<0.01 by student’s t-test was considered statistically significant.

Results: Vaccination studies with irradiated KPC cells confirmed that they were immunogenic in C57BL/6 mice. In preliminary studies, we identified the minimal IRE dose to induce tumor regression to be 150 pulses, with complete regression in 3 of 9 tumors (33%). Survival was prolonged in the IRE groups, particularly in mice with complete regression. This outcome was confirmed to be immune-mediated, as IRE of tumor-bearing immunodeficient Rag-1-/- mice, which lack functional T and B cell components, results in progressive outgrowth with similar kinetics as untreated mice. When immunocompetent mice with tumor regression were re-challenged two weeks later with 105 cells injected into the contralateral flank, we observed no growth in the secondary tumors, confirming adaptive immune activation post-IRE. Tumor immunoprofiling revealed a 43±2%* decrease in the number of tumor-infiltrating MDSCs (CD11b+/CD11c+/Gr-1hi) post-IRE, with a similar decrease in the respective tumor draining inguinal lymph nodes. An increase in total tumor infiltrating lymphocytes (TILs) was also observed, including an 18±2%* increase in CD8+ TILs and a concomitant large increase (2.5 fold*) in PD-1+/CD8+ double positive T-cells.

Conclusion: These results suggest that IRE is capable of triggering an antitumor immune response that contributes to the local effects of IRE and inhibits growth of secondary tumor re-challenge. IRE affects the composition of immune cells in the tumor microenvironment, including a large increase in PD-1+/CD8+ TILs. These effects may be augmented by combining IRE with checkpoint blockade, which is a focus of our ongoing studies.

Citation Format: Jayanth Shankara Narayanan, Tomoko Hayashi, Aaron M. Miller, Stephen P. Schoenberger, Rebekah R. White. Irreversible electroporation (IRE) acts as an “in situ vaccine” and induces antitumor immune responses in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr B78.