CD4+T cells mediate non-canonical rejection of major histocompatibility class-I deficient pancreatic tumors independently of CD8+T cells

Abstract Cytolytic CD8+ T cells are a major mediator of immune-induced tumor rejection, but many patients have primary or acquired resistance to CD8+ T cell immunity. Reduced major histocompatibility complex I (MHC I) expression contributes to resistance, including treatment-refractory pancreatic ductal adenocarcinoma (PDAC). Interrogating interventions in PDAC will reveal novel mechanisms regulating sensitivity to immunotherapy. Using a genetically engineered mouse model of PDAC, we previously reported that agonistic anti-CD40 and dual immune checkpoint blockade (ICB) induces CD8+ and a CD4+ T cell responses mediating tumor rejection. Here, we utilized CRISPR− Cas9 to disrupt the dominant MHC I (H-2Kb) allele expression in PDAC cell lines, precluding direct antigen presentation to CD8+ T cells. Injection of MHC I-deficient tumor clones revealed normal tumor growth at baseline, and significantly delayed tumor growth in response to CD40/ICB in a CD4+ T cell-dependent manner (p < 0.0001 vs. vehicle-treated mice). There was no direct contribution of tumor rejection by CD8+ T cells, despite the generation of a tumor-specific response (24.5% vs. 1.98% in vehicle-treated mice, p<0.003). CD4+ T cells upregulated IFN-γ production (52.3% vs. 11.9% in vehicle-treated mice, p<0.0001), and host IFN-γ expression was required. No changes were observed in the expression of cytotoxic molecules by CD4+ T cells after CD40/ICB, and depletion of myeloid cells did not dampen treatment efficacy. Thus, CD4+ T cells mediated PDAC rejection via non-canonical mechanisms after treatment with CD40/ICB. This study provides critical insight into novel mediators of tumor clearance, and opportunities to exploit CD4 T+ cells as regulators of tumor immunity. Funded by the Parker Institute for Cancer Immunotherapy.