Binding of complement inhibitor C4b-binding protein to a highly virulent S. pyogenes M1 strain is mediated by protein H and enhances adhesion to and invasion of endothelial cells.
Streptococcus pyogenes AP1, a strain of the highly virulent M1 serotype, uses exclusively protein H to bind the complement inhibitor C4b-binding protein (C4BP). We found a strong correlation between the ability of AP1 and its isogenic mutants lacking protein H to inhibit opsonisation with complement C3b and binding of C4BP. C4BP bound to immobilized protein H or AP1 bacteria retained its cofactor