title: Inhibition of the alternative C3 convertase and classical C5 convertase of complement by group A streptococcal M protein.
abstract: When Streptococcus pyogenes group A type 3 strain C203 (M+) and its M-protein-lacking derivative, strain C203S (M-), were treated with normal human serum in the presence of magnesium-EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid], virulent M+ bacteria bound only 10 to 30% as much C3 and factors B and P as did avirulent M- bacteria. After treatment of M+ bacteria with trypsin, which inactivates M protein, their binding of these substances was similar to that of M- bacteria. Pretreatment of M+ bacteria with the Fab fragment of rabbit immunoglobulin G anti-M antibody also increased their binding of C3 in the absence of Ca2+. Therefore, M protein inhibits the alternative C3 convertase. In contrast, in the presence of Ca2+ and Mg2+, M+ bacteria bound 75% as much C3 as M- bacteria. This binding was mostly mediated by classical pathway activation, because M+ bacteria bound much smaller amounts of factors B and P than did M- bacteria but consumed amounts of C4 and C2 comparable to those consumed by M- bacteria. On the other hand, the amount of C5 bound to M+ bacteria was much less than that bound to M- bacteria, and the consumption of C5 and C8 by M+ bacteria was also much less than that by M- bacteria. Therefore, M protein does not inhibit the classical C3 convertase but does inhibit the classical C5 convertase. When M+ and M- streptococci were incubated with normal human serum containing radiolabeled C3 in the presence of Ca2+ and Mg2+, more than 85% of the C3 bound to either type of streptococcus was extractable by sodium dodecyl sulfate and alkali treatment. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the C3 extracted from both strains showed that it was mostly C3b and iC3b. The proportions of C3b and iC3b, respectively, were 7.5 and 71.9% on M+ bacteria and 18.9 and 58.4% on M- bacteria. These results support and extend previous findings that the antiphagocytic activity of streptococcal M protein may be due to complement inhibition mediated by the binding of factor H.
authors: Department of Bacteriology, Osaka University Medical School, Japan.
authors: K
authors: K
authors: Hong
authors: T
authors: T
authors: Kinoshita
authors: J
authors: J
authors: Takeda
authors: H
authors: H
authors: Kozono
authors: P
authors: P
authors: Pramoonjago
authors: Y U
authors: YU
authors: Kim
authors: K
authors: K
authors: Inoue
mesh_terms: D000818:Animals; D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002352:Carrier Proteins; D003166:Complement Activating Enzymes / Q000037:antagonists & inhibitors*; D050577:Complement C3-C5 Convertases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D003180:Complement C3b Inactivator Proteins / Q000378:metabolism; D011415:Complement Factor B / Q000378:metabolism; D017242:Complement Factor H; D003170:Complement Pathway, Alternative / Q000187:drug effects; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D011817:Rabbits; D013297:Streptococcus pyogenes / Q000276:immunology
chemical_list: D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; C493495:CFH protein, human; D002352:Carrier Proteins; D003180:Complement C3b Inactivator Proteins; C014689:streptococcal M protein; D017242:Complement Factor H; D003166:Complement Activating Enzymes; D050577:Complement C3-C5 Convertases; D011415:Complement Factor B; D008274:Magnesium; D002118:Calcium