Streptococcal β Protein Has Separate Binding Sites for Human Factor H and IgA-Fc

The group B streptococcus (GBS) is the most important cause of life-threatening bacterial infections in newborn infants. Protective immunity to GBS infection is elicited by several surface proteins, one of which, the beta protein, is known to bind human IgA-Fc. Here, we show that the beta protein also binds human factor H (FH), a negative regulator of complement activation. Absorption experiments with whole human plasma demonstrated binding of FH to a GBS strain expressing beta protein but not to an isogenic beta-negative mutant. This binding was due to a direct interaction between beta and FH, as shown by experiments with purified proteins. Inhibition tests and studies with beta fragments demonstrated that FH and IgA-Fc bind to separate and nonoverlapping regions in beta. Heparin, a known ligand for FH, specifically inhibited the binding between beta and FH, suggesting that FH has overlapping binding sites for beta and heparin. Bacteria-bound FH retained its complement regulatory activity, implying that beta-expressing GBS may use bound FH to evade complement attack. The finding that beta protein binds FH adds to a growing list of interactions between human pathogens and complement regulatory proteins, supporting the notion that these interactions are of general importance in bacterial pathogenesis.