Bacterial killing by complement requires direct anchoring of membrane attack complex precursor C5b-7

Area of Science:

• Immunology
• Microbiology
• Molecular Biology

Background:

• The human complement system's Membrane Attack Complex (MAC) directly kills Gram-negative bacteria.
• MAC pores form when C5b initiates assembly with C6, C7, C8, and C9 on bacterial surfaces.
• Previous work indicated bacterial killing by MAC pores necessitates local C5 conversion by surface-bound convertases.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the essential requirement for local MAC pore assembly in bacterial killing.
• To investigate why the spatial proximity of MAC component assembly is critical for effective bacterial lysis.

Main Methods:

• Utilized binding experiments with fluorescently labeled C6 to assess C5b6 release.
• Employed trypsin shaving and atomic force microscopy to evaluate C5b-7 anchoring to bacterial surfaces.
• Analyzed complement-resistant clinical *Escherichia coli* strains to identify evasion strategies.

Main Results:

• Demonstrated that rapid C7 interaction with C5b6 is crucial for forming bactericidal MAC pores on *E. coli*.
• Showed that C7 binding to C5b6 prevents the dissociation of C5b6 from the bacterial surface.
• Identified that complement-resistant *E. coli* strains interfere with C5b-7 anchoring, preventing stable MAC pore insertion and bacterial killing.

Conclusions:

• The efficient anchoring of C5b-7 to the bacterial cell envelope, mediated by rapid C7 interaction with C5b6, is essential for stable MAC pore formation and bacterial lysis.
• Bacterial pathogens can evade complement-mediated killing by disrupting the anchoring of MAC components, specifically C5b-7.
• These findings offer fundamental molecular insights into MAC pore assembly and bacterial resistance mechanisms against complement attack.