Radiotherapy and CD40 activation separately augment immunity to checkpoint blockade in cancer.

Immunotherapy in pancreatic ductal adenocarcinoma (PDA) remains a difficult clinical problem despite success in other disease types with immune checkpoint blockade (ICB) and chimeric antigen receptor T-cell therapy. Mechanisms driving immunosuppression and poor T-cell infiltration in PDA are incompletely understood. Here, we use genetically engineered mouse models of PDA that recapitulate hallmarks of human disease to demonstrate that CD40 pathway activation is required for clinical response to radiotherapy and ICB with alpha CTLA-4 and alpha PD-1. The combination of an agonist alpha CD40 antibody, radiotherapy, and dual ICB eradicated irradiated and unirradiated (i.e., abscopal) tumors, generating long-term immunity. Response required T cells and also short-lived myeloid cells and was dependent on the long noncoding RNA myeloid regulator Morrbid. Using unbiased random forest machine learning, we built unique, contextual signatures for each therapeutic component, revealing that (i) radiotherapy triggers an early proinflammatory stimulus, ablating existing intratumoral T cells and upregulating MHC class I and CD86 on antigen-presenting cells, (ii) alpha CD40 causes a systemic and intratumoral reorganization of the myeloid compartment, and (iii) ICB increases intratumoral T-cell infiltration and improves the CD8 T-cell: regulatory T-cell ratio. Thus, alpha CD40 and radiotherapy nonredundantly augment antitumor immunity in PDA, which is otherwise refractory to ICB, providing a clear rationale for clinical evaluation. Significance: Radiotherapy and alpha CD40 disrupt key links between innate and adaptive immunity, ameliorating resistance to immune checkpoint blockade in pancreatic cancer via multiple cellular mechanisms. (C) 2018 AACR.