Inhibition of classical C5 convertase in the complement system by factor H

Area of Science:

• Immunology
• Complement System Biology

Background:

• The classical complement pathway is crucial for immune defense.
• C5 convertase activity is a key step in complement-mediated cell lysis.
• Factor H is a regulator of the alternative complement pathway.

Purpose of the Study:

• To investigate the influence of Factor H on the haemolytic activity of the classical C5 convertase.
• To determine how Factor H affects the interaction between C5 and complement-coated cells.

Main Methods:

• Assessing the interaction of C5 with EAC1,4b,2a,3b cells bearing varying numbers of C3b sites.
• Measuring C5 consumption in the fluid phase.

Main Results:

• Factor H showed minimal effect on C5 interaction with cells having low C3b sites.
• Factor H significantly inhibited C5 interaction with cells possessing high C3b sites.
• The degree of inhibition by Factor H correlated positively with the number of C3b sites.
• Factor H inhibited C5 consumption, indicating an effect on C5 convertase activity.

Conclusions:

• Factor H regulates the classical C5 convertase activity.
• Factor H's inhibitory effect is dependent on the density of C3b sites on target cells.
• Factor H primarily inhibits the enzymatic activity of C5 convertase, not C5 binding to cells.