Streptococcus pyogenes (GAS) M protein is the primary virulence factor enabling immune evasion. Its antiphagocytic mechanism primarily involves:

• A Cleaving IgA molecules with the IgA1 protease, preventing mucosal opsonization
• B Producing streptokinase that plasminogen activates and dissolves fibrin-opsonin deposits
• C Inhibiting TLR2 signaling by binding MyD88 adaptor protein
• D Binding serum factor H and FHL-1 to the bacterial surface, thereby inhibiting complement activation by displacing C3b and accelerating C3b degradation ✓

Explanation

M protein is a surface-anchored alpha-helical coiled-coil protein that binds complement regulatory proteins factor H and factor H-like protein 1 (FHL-1) to the GAS surface. These host proteins act as cofactors for factor I-mediated cleavage of C3b to inactive iC3b, preventing opsonophagocytosis. This allows GAS to evade complement-mediated killing. IgA1 protease is a virulence factor of Neisseria and Streptococcus pneumoniae, not GAS. Streptokinase is involved in virulence but fibrinolysis is a separate mechanism. TLR2/MyD88 inhibition is not M protein's mechanism.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.