C1q binding to surface-bound IgG is stabilized by C1r₂s₂ proteases

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Complement system activation is crucial for antibody-mediated clearance of pathogens and tumor cells.
• The C1 complex (C1qr2s2), comprising C1q and C1r2s2 proteases, initiates the classical complement pathway upon binding to antibody Fc domains.
• Current understanding suggests C1q globular heads mediate C1-IgG interactions, but the role of C1r2s2 proteases is less defined.

Purpose of the Study:

• To investigate the impact of C1r2s2 proteases on the stability of C1q-IgG complexes.
• To explore the influence of IgG subclasses on C1q-IgG binding.
• To assess the therapeutic potential of engineered antibodies with enhanced C1q-IgG interactions.

Main Methods:

• Studied C1q-IgG complex formation on DNP-coated surfaces and Staphylococcus aureus.
• Utilized antibody engineering to introduce hexamer-enhancing mutations into monoclonal IgG.
• Assessed complement-dependent phagocytosis by human neutrophils using engineered IgGs.

Main Results:

• C1r2s2 proteases significantly affect C1q-IgG complex stability, with variations across human IgG subclasses.
• Hexamer-enhancing mutations in IgG improve C1q-IgG stability independently of C1r2s2.
• Engineered, hexamer-enhanced IgGs targeting S. aureus demonstrated improved complement-dependent phagocytosis.

Conclusions:

• The C1r2s2 proteases play a direct role in modulating C1q binding to surface-bound IgG.
• Antibody engineering strategies that promote hexamerization can enhance complement activation and effector functions.
• These findings offer insights for designing more effective antibody-based therapies.