Complement activation promoted by the lectin pathway mediates C3aR-dependent sarcoma progression and immunosuppression

Complement has emerged as a component of tumor-promoting inflammation. We conducted a systematic assessment of the role of complement activation and effector pathways in sarcomas. C3 −/− , MBL1/2 −/− and C4 −/− mice showed reduced susceptibility to 3-methylcholanthrene sarcomagenesis and transplanted sarcomas, whereas C1q and factor B deficiency had marginal effects. Complement 3a receptor (C3aR), but not C5aR1 and C5aR2, deficiency mirrored the phenotype of C3 −/− mice. C3 and C3aR deficiency were associated with reduced accumulation and functional skewing of tumor-associated macrophages, increased T-cell activation and response to anti-PD-1 therapy. Transcriptional profiling of sarcoma-infiltrating macrophages and monocytes revealed the enrichment of the major histocompatibility complex II–dependent antigen presentation pathway in C3-deficient cells. In patients, C3aR expression correlated with a macrophage population signature, and C3-deficiency-associated signatures predicted better clinical outcome. These results suggest that the lectin pathway and C3a–C3aR axis are key components of complement and macrophage-mediated sarcoma promotion and immunosuppression.