C-reactive protein enhances immunity to Streptococcus pneumoniae by targeting uptake to Fc gamma R on dendritic cells.

C-reactive protein (CRP) is an acute phase reactant with roles in innate host defense, clearance of damaged cells, and regulation of the inflammatory response. These activities of CRP depend on ligand recognition, complement activation, and binding to FcgammaR. CRP binds to phosphocholine in the Streptococcus pneumoniae cell wall and provides innate defense against pneumococcal infection. These studies examine the effect of this early innate defense molecule on the development of Abs and protective immunity to S. pneumoniae. Dendritic cells (DC) initiate and direct the adaptive immune response by integrating innate stimuli with cytokine synthesis and Ag presentation. We hypothesized that CRP would direct uptake of S. pneumoniae to FcgammaR on DC and enhance Ag presentation. CRP opsonization of the R36a strain of S. pneumoniae increased the uptake of bacteria by DC. DC pulsed with untreated or CRP-opsonized R36a were transferred into recipient mice, and Ab responses were measured. In mice challenged with free R36a, CRP opsonization resulted in higher secondary and memory IgG responses to both phosphocholine and pneumococcal surface protein A. Furthermore, mice immunized with DC that had been pulsed with CRP-opsonized R36a showed increased resistance to intranasal infection with virulent S. pneumoniae. The effects of CRP on Ag uptake, Ab responses, and protection from infection all required FcR gamma-chain expression on DC. The results indicate that innate recognition by CRP enhances effective uptake and presentation of bacterial Ags through FcgammaR on DC and stimulates protective adaptive immunity.