The Journal of Experimental Medicine
Keystone Symposia
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Published online 29 September 2003 doi:10.1084/jem.20030543
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© Rockefeller University Press, 0022-1007/2003/10/1057 $5.00
The Journal of Experimental Medicine, Volume 198, Number 7, 1057-1068

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

Fredric Carlsson, Karin Berggård, Margaretha Stålhammar-Carlemalm and Gunnar Lindahl

Department of Medical Microbiology, Dermatology, and Infection, Lund University, SE-22362 Lund, Sweden

Address correspondence to Gunnar Lindahl, Dept. of Medical Microbiology, Dermatology, and Infection, Lund University, Sölvegatan 23, SE-22362 Lund, Sweden. Phone: 46-46-173244; Fax: 46-46-189117; email: gunnar.lindahl{at}mmb.lu.se

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.

Key Words: innate immunity • group A streptococcus • phagocytosis resistance • complement • IgA


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