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Role of fibrinogen in complement inhibition by streptococcal M protein
R D Horstmann1, H J Sievertsen, M Leippe
1Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany.
Abstract:
M protein, the major virulence factor of group A streptococci, has antiopsonic activity in that it inhibits activation of the alternative complement pathway on the streptococcal surface. Two properties of M protein have been claimed to account for the inhibitory activity, namely, (i) its binding affinity for complement factor H, which is an inhibitor of alternative pathway activation, and (ii) its high binding affinity for fibrinogen. We have recently shown that fibrinogen, like M protein, inhibits alternative pathway activation by possessing binding affinity for factor H. Here we report that fibrinogen effectively competes with factor H for binding to M protein but retains its own binding affinity for factor H. The presence of fibrinogen did not significantly affect alternative pathway inhibition on the streptococcal surface.
Insights
Group A streptococci M protein inhibits complement activation. Fibrinogen competes for M protein binding but does not alter complement inhibition on the bacterial surface.
Area of Science:
- Microbiology
- Immunology
- Biochemistry
Background:
- M protein is a major virulence factor of Group A Streptococcus (GAS).
- M protein exhibits antiopsonic activity by inhibiting the alternative complement pathway on the bacterial surface.
- This inhibition is attributed to M protein's binding affinity for complement factor H and fibrinogen.
Purpose of the Study:
- To investigate the interaction between M protein, factor H, and fibrinogen.
- To determine the role of fibrinogen in M protein-mediated alternative complement pathway inhibition.
Main Methods:
- Studied the binding affinities of M protein, factor H, and fibrinogen.
- Assessed the effect of fibrinogen on factor H binding to M protein.
- Evaluated alternative complement pathway activation on the streptococcal surface in the presence of fibrinogen.
Main Results:
- Fibrinogen competes with factor H for binding to M protein.
- Fibrinogen retains its own binding affinity for factor H.
- The presence of fibrinogen did not significantly alter the inhibition of alternative complement pathway activation by M protein on the streptococcal surface.
Conclusions:
- Fibrinogen's interaction with M protein does not disrupt M protein's ability to inhibit the alternative complement pathway.
- The antiopsonic activity of M protein is maintained even in the presence of fibrinogen, suggesting a complex interplay between these factors in Group A Streptococcus virulence.