Effect of complement-protein-C3b density on the binding of complement factor H to surface-bound C3b

Area of Science:

• Immunology
• Biochemistry
• Complement System

Background:

• The complement system is crucial for innate immunity.
• The alternative pathway of complement activation can be initiated on surfaces.
• Regulation of complement activation is essential to prevent self-damage.

Purpose of the Study:

• To investigate the effect of complement C3b fragment density on the binding of complement proteins B and H.
• To elucidate the mechanism behind the observed density-dependent binding of factor H.
• To explore the regulatory role of C3b density in alternative pathway activation.

Main Methods:

• Coupling of complement C3b activation fragments to Sepharose 4B and thiopropyl-Sepharose 6B at varying densities.
• Assaying the binding of radioactively labeled complement proteins B and H to C3b-coated particles.
• Utilizing a model system to quantify the relationship between C3b density and factor H/B binding.

Main Results:

• Factor H binding, but not factor B binding, decreased significantly with lower densities of solid-phase C3b.
• A sigmoidal relationship was observed between C3b density and factor H binding.
• High C3b densities on surfaces create high-affinity binding sites for factor H, potentially involving dimeric factor H.

Conclusions:

• Surface C3b density is a critical factor influencing factor H binding.
• The accumulation of C3b fragments promotes factor H interaction, suggesting a regulatory mechanism.
• This density-dependent regulation may control the extent of alternative pathway activation on surfaces.