Inhibition of antimicrobial peptides by group A streptococci: SIC and DRS

Area of Science:

• Microbiology
• Immunology
• Protein Biochemistry

Background:

• Group A Streptococcus (GAS) is a significant human pathogen.
• Certain GAS strains, particularly M1, secrete the highly virulent streptococcal inhibitor of complement (SIC) protein.
• SIC was initially identified as a complement inhibitor but is now recognized as a broader inhibitor of the innate mucosal immune response.

Purpose of the Study:

• To investigate the polyfunctional inhibitory activities of the SIC protein.
• To delineate the roles of different SIC domains (SRR, LRR, PRR) in inhibiting various components of the innate immune system.
• To compare the inhibitory functions of SIC with a related protein, DRS, secreted by M12 GAS strains.

Main Methods:

• Recombinant expression and purification of SIC protein domains.
• In vitro assays to assess the inhibitory activity of SIC and its domains against antimicrobial peptides (lysozyme, SLPI, LL-37, hNP-1, human beta-defensins) and complement.
• Sequence analysis and functional comparison of SIC and the distantly related to SIC (DRS) protein.

Main Results:

• SIC inhibits a broad spectrum of antimicrobial peptides and proteins, including lysozyme, SLPI, LL-37, hNP-1, and human beta-defensins 1-3.
• The N-terminal SRR domain of SIC is responsible for complement inhibition.
• The SRR+LRR domains are required for binding and inhibition of lysozyme, human beta-defensin-3, and SLPI.
• DRS, a related protein from M12 GAS, inhibits human beta-defensin-3 but not lysozyme, SLPI, or complement, suggesting domain-specific functions.

Conclusions:

• SIC is a multifunctional virulence factor of GAS that effectively neutralizes key components of the innate mucosal immune system.
• The distinct domains of SIC contribute to its ability to evade different arms of the host defense.
• Understanding SIC's inhibitory mechanisms provides insights into GAS pathogenesis and potential therapeutic targets.