Assessment of the interaction of human complement regulatory proteins withgroup A Streptococcus. Identification of a high-affinity group A Streptococcus binding site in FHL-1

Group A Streplococcus (GAS), the most frequent bacterial cause of suppurati ve infections in humans, expresses on the cell surface M proteins with capa city to bind factor H, FHL-1 and C4b binding protein (C4BP). This has been interpreted as a mechanism developed by this pathogen to decrease phagocyto sis by macrophages and polymorphonuclear cells. We report the analysis of t he capacity to bind factor H, FHL-1 and C4BP of 69 clinical isolates from 1 9 different serotypes. We show that strains binding complement regulators ( 30/69) belong to specific M serotypes. Of these, M18 strains are relatively frequent and interact with all three complement regulators simultaneously. However, the most virulent M1 and M3 strains did not bind complement regul ators in our assays. The relevance of the interaction between complement re gulators and S. pyogenes was analyzed using different approaches with the c onclusion that under physiological conditions only FHL-1 and C4BP bind to s treptococci. We show that FHL-1 presents a higher binding affinity for S. p yogenes than factor H because it carries a hydrophobic, high-affinity, GAS binding site in addition to the heparin binding site in SCR7. Using synthet ic peptides we provide evidence that the high-affinity GAS binding site in FHL-1 involves the hydrophobic tail (Ser-Phe-Thr-Leu) that distinguishes FH L-1 from factor H.