Interactions between streptococcal M proteins and human plasma proteins
Abstract: Streptococcus pyogenes is a common human pathogen, which causes a variety of diseases. A major virulence factor of S. pyogenes is the surface-associated M protein, which mediates resistance to phagocytosis. Several studies indicate that M proteins inhibit complement deposition and thereby phagocytosis. It is often assumed that S. pyogenes lacking M protein activates complement via the alternative pathway and that M protein inhibits complement deposition via this pathway. However, recent evidence indicates that many S. pyogenes strains actually activate the classical pathway, and that the M proteins in these strains inhibit deposition via this pathway. For this purpose, the bacteria recruit the human complement inhibitor C4b-binding protein (C4BP), which specifically inhibits the classical pathway. The two first papers in this thesis are focused on the biological properties of M proteins that do not bind C4BP. These M proteins bind the human plasma protein fibrinogen (Fg). In the first paper, we studied the role of bound Fg under non-immune conditions, i.e. in the absence of opsonic antibodies, and found that bacteria-bound Fg inhibits complement deposition via the classical pathway, and hence phagocytosis. Thus, the available data indicate that M proteins may recruit either C4BP or Fg to inhibit complement deposition, which occurs via the classical pathway even under non-immune conditions. These findings allow us to propose a unifying mechanism by which M proteins recruit a human plasma protein to interfere with complement deposition and phagocytosis. The second paper analyzes the role of M protein under immune conditions, i.e. in the presence of anti-M antibodies. It is known that antibodies to the N-terminal hypervariable region (HVR) are opsonic, but the role of antibodies to other regions in M protein have remained unclear. We used the M5 protein to study this problem. In initial studies we showed that only the Fg-binding B-repeat region of this M protein is required for phagocytosis resistance under non-immune conditions. However, only antibodies to the HVR were able to opsonize M5 bacteria. This apparent paradox could be explained by the demonstration that Fg and albumin specifically inhibited binding of antibodies to the B- and C-repeat regions, respectively, in the M5 protein. Thus, non-opsonizing epitopes in M5 correspond to regions that are covered by human plasma proteins, Fg and albumin, that block binding of anti-M antibodies, while opsonic epitopes allow binding of antibodies. These data provide a molecular explanation for the difference between opsonic and non-opsonic epitopes in an M protein. The third part of this thesis is focused on the study of an M protein-derived peptide, which binds human serum IgA, and the use of this peptide to develop a one-step purification method to isolate IgA from serum.
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