Complement activation by C-reactive protein is essential for protection against pneumococcal infection

Abstract

Abstract C-reactive protein (CRP), a component of the innate immune system, is an antimicrobial plasma protein. The primary ligand-binding specificity of CRP is for phosphocholine-containing substances and ligand-complexed human CRP activates the complement system in both human and mouse sera. It has been shown that human CRP protects mice against infection with lethal doses of Streptococcus pneumoniae; the protection is due to CRP-dependent decrease in bacteremia and increase in survival time. In this study, we tested a decades old hypothesis that the complement-activating function of ligand-complexed CRP contributes to protection of mice against pneumococcal infection. We employed site-directed mutagenesis of CRP, guided by its three-dimensional structure, and identified a mutant H38R which, unlike wild-type CRP, did not activate C3 in both human and mouse sera. The calcium-dependent binding to phosphocholine on pneumococci, the half-life in mouse circulation, and the overall pentameric structure of H38R mutant CRP were similar to that of wild-type CRP. Employing a murine model of pneumococcal infection, we found that passively administered H38R mutant CRP did not protect mice from infection. Infected mice injected with H38R mutant CRP showed no reduction in bacteremia and did not survive longer, as opposed to infected mice receiving wild-type CRP. Thus, the hypothesis that complement activation by ligand-complexed CRP is an antimicrobial effector function is true. We conclude that complement activation by ligand-complexed CRP is essential for CRP-mediated protection against pneumococcal infection. (NIH AI117730)