Structural difference in the complement activation site of human IgG1 and IgG3

Area of Science:

• Immunology
• Structural Biology
• Complement System

Background:

• The C1q binding site on IgG molecules, crucial for complement activation, involves specific residues in the C(H)2 domain.
• Human IgG1 and IgG3 are typically the most effective IgG subclasses in activating the complement system.
• Understanding structural differences between IgG subclasses is key to elucidating their distinct roles in immune responses.

Purpose of the Study:

• To investigate the structural requirements for complement activation by human IgG1 and IgG3 subclasses.
• To identify specific amino acid residues responsible for differential complement activation between IgG1 and IgG3.
• To explore the functional implications of these structural differences on antibody-dependent complement-mediated lysis (ADCML).

Main Methods:

• Generation of NIP-specific IgG1 and IgG3 antibodies with targeted mutations in the C1q binding site.
• Assessment of ADCML using target cells with varying antigen concentrations and different complement sources (human, rabbit, guinea pig).
• ELISA methods were employed to quantify complement activation, and hinge region truncation was used to assess its impact.

Main Results:

• Mutations at Asp(270), Leu(334), and Leu(335) in the C1q binding site revealed significant differences between IgG1 and IgG3.
• Alanine substitution at Asp(270) heavily reduced complement activation in IgG1, while only moderately reducing it in IgG3.
• These observed differences were independent of the IgG3 hinge region, indicating intrinsic C(H)2 domain variations.

Conclusions:

• Human IgG1 and IgG3 possess distinct structural features within their C1q binding sites.
• These structural specializations contribute to the differential efficiency of IgG1 and IgG3 in activating the complement system.
• The findings provide insights into the functional divergence of IgG subclasses in immune effector mechanisms.