C-reactive protein increases cytokine responses to Streptococcus pneumoniae through interactions with Fc gamma receptors.

Streptococcus pneumoniae is the most common organism responsible for community acquired pneumonia and meningitis. In pneumococcal pneumonia, a strong local inflammatory cytokine response reduces the frequency of bacteremia and increases survival. The initiation of this cytokine response by innate recognition of bacterial cell wall components through TLR has been described, but the role of soluble innate mediators has received limited attention. C-reactive protein (CRP) is an acute phase protein that binds phosphocholine residues on S. pneumoniae cell walls. CRP interacts with phagocytic cells through FcgammaRI and FcgammaRII and activates the classical complement pathway. CRP is protective in mouse pneumococcal bacteremia by increasing complement-dependent clearance and killing of bacteria. We studied the cytokine response of PBMC stimulated with CRP-opsonized S. pneumoniae to determine the effect of CRP interaction with FcgammaR. CRP dramatically increased the production of TNF-alpha and IL-1beta in response to S. pneumoniae. These increases were blocked by phosphocholine, which inhibits CRP binding to S. pneumoniae, by inhibitors of FcgammaR signaling, and by mAb to FcgammaRI and FcgammaRII. A mutated rCRP with decreased FcgammaR binding had a decreased ability to stimulate TNF-alpha release, compared with wild-type CRP. Individuals who were homozygous for the R-131 allele of FcgammaRIIA, which has a higher affinity for CRP, showed higher responses to CRP-opsonized bacteria than did individuals homozygous for the H-131 allele, further implicating this receptor. The results indicate that CRP recognition of S. pneumoniae and binding to FcgammaR may enhance the early protective cytokine response to infection.