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Antiopsonic activity of fibrinogen bound to M protein on the surface of group A streptococci

E Whitnack, E H Beachey

    The Journal of Clinical Investigation
    |April 1, 1982
    PubMed

    Insights

    Group A Streptococcus M protein hinders neutrophil uptake by binding fibrinogen. This interaction is crucial for bacterial virulence and immune evasion strategies.

    Area of Science:

    • Microbiology
    • Immunology
    • Bacteriology

    Background:

    • Virulent group A streptococci (GAS) possess M protein, a key virulence factor.
    • Neutrophil phagocytosis is a critical host defense mechanism against bacterial infections.
    • The antiopsonic effect of M protein is known to impair bacterial clearance.

    Purpose of the Study:

    • To investigate the mechanism by which M type 24 group A streptococci evade neutrophil uptake.
    • To determine the role of plasma components, specifically fibrinogen, in this evasion process.
    • To elucidate the interaction between M protein and fibrinogen during bacterial-host encounters.

    Main Methods:

    • Incubation of M type 24 GAS with neutrophils in various blood components (plasma, serum).
    • Assessment of bacterial uptake by neutrophils using light microscopy.
    • Inhibition assays using purified fibrinogen and plasma.
    • Immunofluorescent staining for complement component C3 deposition.
    • Electron microscopy to visualize bacterial surface changes after treatment with plasma/fibrinogen.

    Main Results:

    • Neutrophil uptake of GAS was minimal in plasma but prompt in serum.
    • Uptake was inhibited by plasma or fibrinogen, and by pretreating bacteria with these components.
    • Fibrinogen solutions lost inhibitory activity after absorption with M protein.
    • Complement deposition (C3) was altered, showing reduced binding on bacteria treated with plasma/fibrinogen.
    • Electron microscopy revealed changes in the bacterial surface fibrillar layer upon interaction with fibrinogen.

    Conclusions:

    • The antiopsonic activity of M protein is partly mediated by its binding to fibrinogen.
    • Fibrinogen binding to M protein alters bacterial surface properties, potentially hindering opsonophagocytosis.
    • This interaction represents a significant mechanism of immune evasion for group A Streptococcus.

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