The C5a/C5aR1 Axis Promotes Migration of Tolerogenic Dendritic Cells to Lymph Nodes, Impairing the Anticancer Immune Response.

The precise mechanisms by which the complement system contributes to the establishment of an immunosuppressive tumor microenvironment and promotes tumor progression remain unclear. In this study, we investigated the expression and function of complement C5a receptor 1 (C5aR1) in human and mouse cancer-associated dendritic cells (DC). First, we observed an overexpression of C5aR1 in tumor-infiltrating DCs, compared with DCs from the blood or spleen. C5aR1 expression was restricted to type 2 conventional DCs and monocyte-derived DCs, which displayed a tolerogenic phenotype capable of inhibiting T-cell activation and promoting tumor growth. C5aR1 engagement in DCs drove their migration from tumors to tumor-draining lymph nodes, where C5a levels were higher. We used this knowledge to optimize an anticancer therapy aimed at enhancing DC activity. In three syngeneic tumor models, C5aR1 inhibition significantly enhanced the efficacy of poly I:C, a Toll-like receptor 3 agonist, in combination with PD-1/PD-L1 blockade. The contribution of C5aR1 inhibition to the antitumor activity of the combination treatment relied on type 1 conventional DCs and antigen-specific CD8+ T cells, required lymphocyte egress from secondary lymphoid organs, and was associated with an increase in IFNγ signaling. In conclusion, our study highlights the importance of the C5a/C5aR1 axis in the biology of cancer-associated DCs and provides compelling evidence for the therapeutic potential of modulating the complement system to enhance DC-mediated immune responses against tumors.