In a mouse model of pancreatic ductal adenocarcinoma, Rech et al. found that the combination of radiotherapy (RT), agonist CD40 (agCD40), and anti-PD-1/anti-CTLA-4 decreased both irradiated and non-irradiated tumor burden and improved survival. The antitumor effect was dependent on CD4+ and CD8+ T cells, short-lived myeloid cells, and host CD40, IFNγ, and Batf3. Random forest analysis indicated that key predictors of response were RT ablation of intratumoral CD4-/CD8- T cells, reconfiguration of myeloid compartments by agCD40, increased unirradiated intratumoral CD8+ T cells, and increased CD8+/Treg ratio at peak tumor regression.
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 (RT) and ICB with alphaCTLA-4 and alphaPD-1. The combination of an agonist alphaCD40 antibody, RT, 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 non-coding RNA myeloid regulator Morrbid. Using unbiased random forest machine learning, we built unique, contextual signatures for each therapeutic component, revealing that (i) RT triggers an early proinflammatory stimulus, ablating existing intratumoral T cells and upregulating MHC class I and CD86 on antigen presenting cells, (ii) alphaCD40 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, alphaCD40 and RT non-redundantly augment anti-tumor immunity in PDA, which is otherwise refractory to ICB, providing a clear rationale for clinical evaluation.