C-reactive protein (CRP) is an acute phase reactant protein considered to be the prototypic marker for inflammation and its associated diseases. However, little is known about how CRP affects the immune system. In this study, we investigated the effect of CRP on dendritic cell (DC) differentiation, activation and biological functions. CD14+ monocytes were purified from PBMC and differentiated into DC in vitro. CRP (10 microg/mL) substantially down-regulated expression of DC-SIGN (CD209) and the costimulatory molecules CD40 and CD86 during DC differentiation. This inhibitory effect was more pronounced when CRP was added at the early stage (0-2 days) of DC differentiation. The inhibitory effect of CRP could be specifically blocked by an anti-CD32 Ab. In addition, CRP dramatically down-regulated expression of the antigen-uptake molecules CD205 and CD206, resulting in reduced DC endocytosis. Furthermore, CRP down-regulated expression of the costimulatory molecules CD40, CD80 and CD86 as well as the DC maturation marker CD83 after lipopolysaccharide-induced DC maturation. CRP-treated DC also showed an inhibitory effect on allogeneic T cell proliferation in a mixed leukocyte reaction. CRP treatment of activated DC preferentially decreased production of the proinflammatory and inflammatory cytokines IL-6, IL-8, IL-12, TNF-alpha, MIP-1alpha, MIP-1beta and MCP-1. This work reveals a new role for CRP in modulating the immune system by inhibiting DC differentiation, maturation and functions mainly through FcgammaRIIa/CD32.