Membrane-bound C4b interacts endogenously with complement receptor CR1 of human red cells

Area of Science:

• Immunology
• Complement System Biology

Background:

• The classical complement pathway, crucial for immune response, can inadvertently damage autologous cells.
• C4b deposition on cell membranes and C3 convertase assembly amplify the complement cascade.
• Irreversible inactivation of C4b is essential to prevent self-damage.

Purpose of the Study:

• To investigate the mechanism of C4b inactivation on human red blood cells.
• To elucidate the role of endogenous complement receptor type 1 (CR1) in C4b regulation.

Main Methods:

• Utilizing a bifunctional crosslinking reagent to identify CR1-C4b complexes on red blood cell membranes.
• Assessing the cofactor activity of CR1 in factor I-mediated cleavage of C4b.
• Investigating the influence of CR1 and C4b concentrations on cleavage kinetics.

Main Results:

• Deposited C4b interacts with endogenous CR1 on the same red blood cell, forming stable complexes.
• CR1's cofactor activity is essential for factor I to cleave and inactivate C4b.
• Endogenous CR1, not exogenous, is the primary target for factor I-mediated C4b inactivation.

Conclusions:

• Complement receptor type 1 (CR1) plays a critical role in the endogenous inactivation of C4b on red blood cells.
• CR1 and factor I act concertedly to irreversibly inactivate C4b, preventing complement-mediated cell damage.
• This mechanism prevents the amplification of the complement cascade and protects autologous cells.