Effect of C-reactive protein on the complement-mediated stimulated of human neutrophils by Streptococcus pneumoniae serotypes 3 and 6.

C-reactive protein (CRP) has long been known to appear in the sera of individuals with inflammatory processes, but its role in host defense against bacterial infection is unclear. We have recently demonstrated that CRP in the presence of the classical complement pathway markedly enhances the opsonization of Streptococcus pneumoniae serotype 27 by polymorphonuclear leukocytes (Edwards et al., J. Immunol. 128:2493-2496). In this report we have extended these studies to characterize the role of CRP in the opsonization of other S. pneumoniae serotypes. Two clinically important serotypes, 3 and 6, were tested along with the nonpathogenic rough strain R36a. All strains were found to bind radiolabeled CRP in the presence of calcium and to activate the classical complement pathway in normal human serum. However, the opsonophagocytic response of polymorphonuclear leukocytes to the strains, measured by chemiluminescence, was quite different. In contrast to the marked enhancement by CRP of the chemiluminescent response to serotype 27 in normal human serum, CRP had no effect on the opsonization of serotype 6 or R36a and inhibited opsonization of serotype 3 in normal serum. In serum from a hypogammaglobulinemic patient, CRP enhanced the lowered chemiluminescent response to serotype 3 and 6 organisms but did not restore the response to normal. The greater opsonic effect of CRP on serotype 27 may be related to the ability of CRP to bind to the capsule as well as to the cell wall of this serotype or to differences in the amount of CRP bound to the different strains.