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Inhibition of the alternative C3 convertase and classical C5 convertase of complement by group A streptococcal M protein. Paper-13976. 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. Paper-13976.
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.
Therefore, M protein does not inhibit the classical C3 convertase but does inhibit the classical C5 convertase. Paper-13976.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the C3 extracted from both strains showed that it was mostly C3b and iC3b.
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+. Paper-13976.
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. Paper-13976.
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. Paper-13976.
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. Paper-13976.
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. Paper-13976.
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. Paper-13976.
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