Interactions between Streptococcus pyogenes and the human immune system, with special reference to C4b-binding protein

Abstract: The ability of many pathogenic microorganisms to cause infection is dependent on their ability to evade attack by the complement system of the host. One mechanism by which microorganisms may avoid complement attack is to bind complement inhibitory molecules present in the host. This thesis describes studies of the complement inhibitor C4BP and of inter-actions between C4BP and the human pathogen Streptococcus pyogenes (group A streptococci). The first paper describes structural studies of bovine C4BP, while the following three papers deal with the interactions between human C4BP and S. pyogenes. Contrary to human C4BP, bovine C4BP does not form a complex with protein S, a plasma protein with regulatory function in the coagulation system. Sequence analysis demonstrated a deletion in the putative protein S–binding subunit of bo-vine C4BP, providing an explanation for the lack of C4BP–protein S complex in bovine plasma. It is well known that most strains of S. pyogenes express one or two surface proteins that are members of the M-protein family. Many strains express a single ”M-protein”, which is an important virulence factor, by virtue of its ability to pre-vent phagocytosis. For strains that express two members of the family, the bio-logical function of those two ”M-like” proteins has been unclear. We have found that the latter type of strains usually bind C4BP, and that this property is due to one of the two molecules in the M-protein family. The bound C4BP was found to be functionally active, suggesting that it contributes to the protection against complement attack. Structural analysis showed that C4BP binds to a highly variable part of the M-like proteins. Thus, the ability to bind C4BP has been retained in evolution, although the binding region shows very extensive primary sequence variation. This finding indicates that the ability to bind C4BP is important for the strepto-cocci. To assess the functional role of the two M-like proteins expressed by many S. pyogenes strains, isogenic mutants were derived from a clinical isolate. These mutants were analyzed in phagocytosis assays. The results showed that each of the two M-like proteins of this strain have antiphagocytic function. Thus, this type of strain expresses two antiphagocytic M-proteins. The results indicate that one of these M-proteins, the one that binds C4BP, may be more important than the other for phagocytosis resistance.