Binding and activation of the first complement component by soluble immune complexes: effect of complex size and...

Area of Science:

• Immunology
• Biochemistry

Background:

• Soluble immune complexes (SICs) are crucial in immune responses.
• The first component of complement (C1) initiates the classical complement pathway.
• Understanding SIC interaction with C1 is key to immune regulation.

Purpose of the Study:

• To investigate how SIC characteristics affect C1 binding and activation.
• To determine the influence of antibody/antigen (Ab/Ag) ratio and complex size on C1 activity.

Main Methods:

• Preparation of SICs using bovine thyroglobulin (BTg) or tetanus toxoid (TT) with specific IgG antibodies.
• Assessing C1 binding by measuring inhibition of C1 hemolytic activity.
• Measuring C1 activation by quantifying C4 consumption.
• Utilizing ultracentrifugation to fractionate complexes by size.

Main Results:

• High Ab/Ag ratios significantly enhanced C1 binding and activation for BTg-anti-BTg complexes.
• The Ab/Ag ratio had a limited effect on TT-anti-TT complex activity.
• Increasing SIC size markedly increased C1 binding and activation for both antigen types.
• Antigen type influences the impact of Ab/Ag ratio on C1 activation.

Conclusions:

• Both SIC size and Ab/Ag ratio are critical factors in activating the classical complement pathway.
• The influence of Ab/Ag ratio on C1 activation is antigen-dependent.
• These findings provide insights into the molecular mechanisms of complement activation by immune complexes.