Diagram showing proposed signaling pathways for C3aR-mediated up-regulation of DC function in Ag presentation. Local production and activation of C3 generates extracellular C3a; binding of C3a to C3aR on DCs activates Gαi protein, which in turn inhibits the production of cAMP, and consequently modulates DC functional activation and results in the up-regulation of DC functions of Ag uptake and presentation. In addition, engagement of C3aR mediates inhibition of MAPK/P38 and activation of PI3K/AKT and MAPK/ERK1/2 pathways, which may contribute to the up-regulation of DC function in Ag presentation.