CD40 ligation is a critical step in the activation and maturation of CD40+ antigen-presenting cells (APCs) and facilitates robust T cell priming, but the interaction of CD40 activation with innate immune sensors — particularly in combination with anti-tumor chemotherapy — is unknown. To investigate this issue, we used the KrasG12D+/-;Trp53R172H+/-;Pdx-1 Cre (KPC) genetically engineered mouse model of pancreatic ductal adenocarcinoma (PDA) to study the effect of chemotherapy with agonistic CD40 treatment. KPC mice bearing spontaneous PDA were diagnosed by ultrasound and treated with gemcitabine (Gem) and nab-paclitaxel (nP), with or without agonist CD40 antibody (FGK45). Gem/nP/CD40-treated mice exhibited a tumor response rate of 37.5% vs. 0% in the Gem/nP group; however, no regressions were observed in Gem/nP/CD40-treated KPC mice if they were first depleted of CD8 T cells, indicating T cell dependency. To understand the mechanism of Gem/nP/CD40 treatment, a PDA cell line was developed from a C57Bl/6 KPC mouse with a spontaneous tumor and injected subcutaneously into syngeneic mice. Mice bearing established tumors that were treated with Gem/nP/CD40 had increased rates of tumor regression (59.7%) compared to Gem/nP treatment (11.2%) or CD40 alone (15.3%), as well as significantly reduced tumor growth kinetics (p<0.0001), and improved overall survival (p<0.01) compared to either Gem/nP or CD40 alone. The anti-tumor effect was entirely lost in CD40 knockout (KO) mice or in IFN-γ KO mice, or in mice depleted of CD8 T cells. In the tumor site at day 12, while overall CD3+ T cell populations did not vary across cohorts, the proportion of CD4+FoxP3+ T regulatory (Treg) cells was reduced 6.9-fold after Gem/nP/CD40 and the proportion of activated CD44hiCD4+ T helper cells increased 1.8-fold compared to control treatment (p<0.01 in each case). The ratios of both CD4 T helper and CD8 T effector cells to Tregs were significantly increased (p<0.001 and p<0.05, respectively). Two days after CD40 therapy, APCs in the tumor had increased CD86 and MHCII (37% vs. 10% in control group, p<0.0001), and increased production of both TNF-α and IL-12p40. Despite this evidence for a robust CD40-dependent T cell anti-tumor response with Gem/nP/CD40, there was no impact on this phenotype when we studied KO mice deficient for classically described innate immune sensors: Gem/nP/CD40 treatment was fully effective in host mice genetically deficient in expression of MyD88, TLR4, TLR3, TRIF, IFNAR, STING, Caspase1, Caspase11, IL-1R, and TNF-α. Thus, despite the well-described role of innate immune sensors in potentiating T cell responses to tumors, agonist CD40 therapy is sufficient to override the requirement for any of these pathways. Our findings reveal that CD40 stimulation bypasses canonical mediators of immune activation, and suggests from a translational standpoint, that CD40 agonists may be critical and non-redundant pieces of a combination armamentarium for the treatment of highly immunosuppressive cancers.

Citation Format: Katelyn T. Byrne, Robert H. Vonderheide. CD40 stimulation overrides role of innate immune sensors for priming of T cells in cancer. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A076.