The effect of C4 on C1 binding and activation

Area of Science:

• Immunology
• Complement System Biology

Background:

• The complement system is a critical part of innate immunity.
• Complement component C1 initiates the classical pathway of complement activation.
• Immunoglobulin (IgG) binding to cells can trigger immune responses.

Purpose of the Study:

• To investigate the role of cell-bound complement C4 in modulating C1 interaction with IgG-bearing cells.
• To elucidate how C4 influences the enzymatic activity of C1 on C2.
• To assess the impact of protein A on C1 binding and activation in the presence and absence of C4.

Main Methods:

• Utilizing cells coated with immunoglobulin G (IgG).
• Introducing cell-bound human complement C4 to these cells.
• Assessing C1 uptake and C1-mediated C2 cleavage.
• Employing protein A from Staphylococcus aureus to block C1 binding.

Main Results:

• Cell-bound C4 significantly enhances C1 uptake by IgG-bearing cells, particularly with lower IgG concentrations.
• C4 presence dramatically promotes C1's enzymatic activity on C2.
• Protein A effectively blocks C1 binding and activation on IgG-coated cells.
• Protein A loses its inhibitory effect on C1 binding and activation when C4 is also present on the cell surface.
• C4-bearing cells show resistance to protein A-mediated inhibition of hemolysis.

Conclusions:

• Cell-bound C4 plays a key regulatory role in the classical complement pathway.
• C4 enhances C1 efficiency and modulates its interaction with IgG-coated targets.
• The presence of C4 can protect against inhibition by agents like protein A, suggesting a complex interplay in complement activation.