Regulatory effects of C5a on IL-17A, IL-17F, and IL-23.

The complement anaphylatoxin, C5a, through binding to its receptors (C5aR or C5L2), has important biological properties for recruitment and activation of phagocytes. C5a has been identified as a powerful modulator of Toll-like receptor-induced cytokine and chemokine production by macrophages. Both the complement system and the interleukin (IL)-17 cytokine family protect against extracellular pathogens by enhancing innate immune functions. On the basis of its concentration, C5a can either positively or negatively modulate the production by macrophages of IL-17 family members as well as IL-23 via the phosphatidylinositol 3-kinase/Akt signaling cascade. C5a can also affect the production and maintenance of IL-17-producing T cells. Using C5a, C5aR, or C5L2 deficiency or blockade, IL-17/IL-23 production and/or IL-17-dependent disease progression has been shown to be substantially modified. The contributions of C5a interaction with its receptors in the production of IL-17/IL-23 and promotion of IL-17-dependent immune responses are reviewed.