Homologous restriction factor: effect on complement C8 and C9 uptake and lysis

Area of Science:

• Immunology
• Cell Biology
• Complement System

Background:

• Homologous restriction factor (HRF) is a regulatory protein on human erythrocytes.
• HRF inhibits the membrane attack complex (MAC) of the complement system.
• The MAC, composed of C5b-9, causes cell lysis.

Purpose of the Study:

• To investigate the inhibitory mechanism of HRF on complement-mediated lysis.
• To determine HRF's effect on C5b-8 and C5b-9 complexes.
• To quantify HRF's impact on C9 binding to target cells.

Main Methods:

• Utilized 125I-labelled HRF for insertion into erythrocyte targets.
• Assessed lysis reduction by C5b-8 and C5b-9 complexes.
• Employed antibodies against HRF to block its function.
• Quantified C9 and C7 molecule binding to target cells.

Main Results:

• HRF insertion reduced lysis by C5b-8 by approximately 80%.
• Antibody-mediated blocking of HRF increased C5b-8 lysis without affecting C8 binding.
• HRF significantly reduced C9 binding from ~6000 to ~500 molecules per cell.
• HRF limited C9 binding to the level of C7 bound, allowing only one C9 per C5b-8 complex.

Conclusions:

• HRF effectively inhibits complement-mediated lysis by both C5b-8 and MAC (C5b-9).
• HRF acts by limiting C9 recruitment and incorporation into the C5b-8 complex.
• HRF plays a crucial role in protecting cells from complement-induced damage.