The traditionally understood roles of complement in innate immunity take place in the extracellular environment, directly opsonizing and even lysing some pathogens via the action of C3 deposition and formation of the terminal membrane attack complex 1 but also acting as a danger sensing mechanism, transmitting signals to cells via cell-surface receptors of activated complement components. 2 While these functions have largely been attributed to circulating complement proteins secreted mainly from the liver, it has been recognized that local expression of complement proteins can also have an important role in tissues, 3 for example, in infection 4,5 and in complement-dependent pathologies. [6][7][8] The discovery that some cell types express multiple complement proteins required for complete activation pathways, leading to spontaneous activation, also opened the possibility for autocrine complement activation playing roles in cellular homeostasis, activation, and inflammation. 9,10 For example, more recently, cell-intrinsic C3 expression in fibroblasts has been shown to play an important role in priming tissue sites for inflammation. 11 In autoinflammatory disease, repeated attacks can increase in severity. It was shown that mice injected at the same site with identical doses of sterile inflammatory danger-or pathogenassociated molecular patterns (DAMPs or PAMPs) suffered increasing levels of inflammation, while injection at different sites did not. This was not dependent on the adaptive immune system, as shown using SCID and RAG1-KO mice, but was dependent on C3, as shown using C3-KO mice. In addition, adoptive transfer of exposed fibroblasts from WT but not C3-KO mice "primed" the recipient site for increased inflammation, indicating the importance of fibroblastexpressed C3 in this priming process. This detailed and clear study indicates a role for local, or cell-intrinsic C3 in fibroblasts in sterile inflammation. The exact function of C3 was not however defined,