Cell-intrinsic complement C3 suppresses IFN-β production in macrophages

The cell-intrinsic complement system has emerged as an important orchestrator of a variety of cell-physiological processes, with complement components interacting with intracellular effector systems to regulate cellular responses to pathogens or noxious stimuli. For instance, intracellular C5 signaling through a mitochondrial C5a receptor (C5aR1) controls IL-1{beta} production in human monocytes and macrophages. Here, we investigated whether cell-intrinsic C3 similarly regulates inflammatory responses in macrophages. In LPS-stimulated C3 knockout THP-1-derived macrophages, interferon (IFN)-{beta} production was increased, accompanied by elevated expression of interferon-stimulated genes and enhanced secretion of IFN-induced cytokines and chemokines. C3-deficient cells showed increased phosphorylation of IRF3 at Ser396 and a stabilization of the interaction between IRF3 and TBK1, along with enhanced IRF3 dimerization and nuclear translocation. TBK1 phosphorylation was unaffected, indicating that C3 limits IRF3-TBK1 complex formation rather than upstream TBK1 activation. Small-molecule inhibitors of complement factors B and D restored full-length C3 abundance in LPS-stimulated primary human macrophages, consistent with inhibition of the C3 convertase. It also reduced LPS-induced IFN-{beta} production in primary human macrophages and THP-1 cells, suggesting that full-length, uncleaved C3 suppresses IFN-{beta} production. Collectively, these findings identify cell-intrinsic C3 as a suppressor of IFN-{beta} production in human macrophages, highlighting the importance of the cell-intrinsic complement system in fine-tuning inflammatory responses to pathogens.