Complement protein C1q directs macrophage polarization and limits inflammasome activity during the uptake of apoptotic cells.

Deficiency in C1q, the recognition component of the classical complement cascade and a pattern recognition receptor involved in apoptotic cell clearance, leads to lupus-like autoimmune diseases characterized by auto-antibodies to self proteins and aberrant innate immune cell activation likely due to impaired clearance of apoptotic cells. In this study, we developed an autologous system using primary human lymphocytes and human monocyte-derived macrophages (HMDMs) to characterize the effect of C1q on macrophage gene expression profiles during the uptake of apoptotic cells. C1q bound to autologous apoptotic lymphocytes modulated expression of genes associated with JAK/STAT signaling, chemotaxis, immunoregulation, and NLRP3 inflammasome activation in LPS-stimulated HMDMs. Specifically, C1q sequentially induced type I IFNs, IL-27, and IL-10 in LPS-stimulated HMDMs and IL-27 in HMDMs when incubated with apoptotic lymphocyte conditioned media. Coincubation with C1q tails prevented the induction of type I IFNs and IL-27 in a dose-dependent manner, and neutralization of type I IFNs partially prevented IL-27 induction by C1q. Finally, C1q decreased procaspase-1 cleavage and caspase-1-dependent cleavage of IL-1β suggesting a potent inhibitory effect of C1q on inflammasome activation. These results identify specific molecular pathways induced by C1q to suppress macrophage inflammation and provide potential therapeutic targets to control macrophage polarization and thus inflammation and autoimmunity.