Abstract A14: Irreversible electroporation is an “in situ vaccine” and induces antitumor immune responses in pancreatic cancer

The pancreatic cancer (PC) microenvironment is notoriously immunosuppressive and responds poorly to conventional checkpoint blockade immunotherapy, necessitating a need to overcome this challenge. Irreversible electroporation (IRE) is a nonthermal method of inducing cell death that is currently being used clinically for selected patients with locally advanced PC. Using a robust syngeneic mouse model of PC using cells established from a tumor arising in an LSL-KrasG12D/+; LSL-Trp53R172H/+; PDX1Cre/+; LSL-ROSA26 Luc/+ mouse (KPC), we have demonstrated that IRE alone can serve as an “in situ vaccine” and induce systemic immune responses that prevent secondary tumor growth (prophylactic immunity). We have furthermore demonstrated that these effects are augmented by combination with innate immune activators and PD-1 checkpoint blockade. We observed a significant decrease in immunosuppressive myeloid-derived suppressor cells (MDSCs) (CD11b+, CD11c-, GR1+) at one week in the IRE and drug alone groups with an additive effect in the combination groups. We also saw a profound shift in the ratio of immunostimulatory M1 (MHC II+, CD206 lo): immunosuppressive M2 (MHC II -, CD206 hi) macrophages in the combination groups and a modest increase in CD8+ T cells with IRE alone that was almost doubled in the combination groups with a concomitant decrease in T regulatory cells. Using exome and RNA sequencing of the KPC tumors expressed, nonsynonymous mutations in these tumors were identified and 101 long peptides (20 mer) were generated harboring these mutations at positions 9 and 15. When the CD8+ T-cells post-IRE were incubated along with these mutant peptides and bone marrow-derived dendritic cells, we were able to observe that these T cells were reactive (Interferon gamma secretion) to these neoantigenic peptide pools. Further, we were able to identify specific mutant peptides against which T-cell response was stimulated post IRE that were similar or even better than vaccinating the mice with irradiated KPC cells, suggesting that IRE induced tumor neoantigen-specific prophylactic immune response. Specifically, local delivery of toll-like receptor 7 (TLR7) agonist (1V270) and agonistic CD40 antibody resulted in complete regression of treated primary tumors in 7/10 mice in the experimental group. Increased presence of CD8+ dendritic cells (CD45+ CD11C+) in the draining lymph node suggested that the agonistic CD40 antibody could have triggered increased dendritic cell matura