Evasion of Phagocytosis through Cooperation between Two Ligand-binding Regions in Streptococcus pyogenes M Protein

Abstract

The M protein of Streptococcus pyogenes is a major bacterial virulence factor that confers resistance to phagocytosis. To analyze how M protein allows evasion of phagocytosis, we used the M22 protein, which has features typical of many M proteins and has two well-characterized regions binding human plasma proteins: the hypervariable NH2-terminal region binds C4b-binding protein (C4BP), which inhibits the classical pathway of complement activation; and an adjacent semivariable region binds IgA-Fc. Characterization of chromosomal S. pyogenes mutants demonstrated that each of the ligand-binding regions contributed to phagocytosis resistance, which could be fully explained as cooperation between the two regions. Deposition of complement on S. pyogenes occurred almost exclusively via the classical pathway, even under nonimmune conditions, but was down-regulated by bacteria-bound C4BP, providing an explanation for the ability of bound C4BP to inhibit phagocytosis. Different opsonizing antisera shared the ability to block binding of both C4BP and IgA, suggesting that the two regions in M22 play important roles also under immune conditions, as targets for protective antibodies. These data indicate that M22 and similar M proteins confer resistance to phagocytosis through ability to bind two components of the human immune system.

Host–pathogen interactions in Streptococcus pyogenes infections, with special reference to puerperal fever and a comment on vaccine development

“…1). The antiphagocytic property of this M protein may be explained as cooperation between two variable ligand-binding regions [13]. The HVR binds human C4BP and an adjacent semi-variable region (SVR) binds human IgA-Fc.…”

“…The HVR binds human C4BP and an adjacent semi-variable region (SVR) binds human IgA-Fc. The HVR makes an important contribution to phagocytosis resistance because bacteria-bound C4BP retains its ability to inhibit complement activation, thereby preventing opsonization due to complement deposition [13,14]. Surprisingly, S. pyogenes lacking M protein activates the classical complement pathway even in normal human serum.…”

“…Surprisingly, S. pyogenes lacking M protein activates the classical complement pathway even in normal human serum. This may explain why S. pyogenes exploits human C4BP for protection against complement activation [13].…”

“…The SVR, which binds human IgA-Fc [15], also makes an important contribution to phagocytosis resistance [13], but the mechanism by which this region contributes to virulence is not clear. Possibly, the IgA-binding region acts by inhibiting the binding of IgA-Fc to the human IgA-receptor CD89 on phagocytes, thereby interfering with IgA effector functions [16].…”

“…and IgA, while non-protective antibodies do not have this property [13]. Thus, the key property of a protective antibody may be its ability to inhibit binding of ligand (s) readily be studied in soluble form and different ligand-binding regions can be studied as synthetic peptides [9,18].…”