Binding of human C4BP to the hypervariable region of M protein: a molecular mechanism of phagocytosis resistance in Streptococcus pyogenes

The amino-terminal hypervariable region (HVR) of streptococcal M protein is required for the ability of this virulence factor to confer phagocytosis r esistance. The function of the HVR has remained unknown, but the finding th at many HVRs with extremely divergent sequences bind the human complement r egulator C4b-binding protein (C4BP) has suggested that this ligand may play a role in phagocytosis resistance. We used the M22 system to study the fun ction of bound C4BP and provide several lines of evidence that C4BP indeed contributes to phagocytosis resistance. First, the ability of anti-HVR anti bodies to cause opsonization correlated with their ability to inhibit bindi ng of C4BP. Secondly, a short deletion in the HVR eliminated C4BP binding a nd also reduced the ability of M22 to confer phagocytosis resistance. Third ly, the addition of an excess of pure C4BP to a phagocytosis system almost completely blocked the effect of opsonizing anti-HVR antibodies. Together, our data indicate that binding of C4BP to the HVR of M22 plays an important role in phagocytosis resistance, but other properties of M22 also contribu te. This study provides the first molecular insight into the mechanisms by which the HVR of an M protein confers phagocytosis resistance.