Mechanisms of complement activation by crystalline cholesterol

Area of Science:

• Immunology
• Biochemistry
• Complement System

Background:

• Cholesterol crystals are implicated in inflammatory diseases.
• The complement system plays a crucial role in innate immunity and inflammation.
• Understanding how cholesterol crystals interact with the complement system is vital for diseasepathogenesis research.

Purpose of the Study:

• To elucidate the mechanism by which cholesterol crystals activate complement in human serum.
• To identify the specific complement convertase formed on cholesterol crystals.
• To determine the nature of the interaction between cholesterol crystals and complement components.

Main Methods:

• Incubation of cholesterol crystals with human serum.
• Washing and analysis of treated crystals for complement convertase activity.
• Investigation of cation and factor dependency.
• Immunofluorescence and antisera to detect C3 fragments.
• Hydroxylaminolysis to assess bond stability.

Main Results:

• Cholesterol crystals form a fixed alternative C3/C5 convertase (C3bBb(P)) in human serum.
• Convertase formation is dependent on divalent cations and factor B.
• A C3 fragment, likely C3b, is covalently bound to cholesterol via an ester bond.
• Cholesterol acetate does not activate complement.
• Cholesterol crystals bind factor I, potentially amplifying convertase activity.

Conclusions:

• Cholesterol crystals predominantly activate the alternative complement pathway.
• A covalent ester bond links activated C3 to cholesterol, stabilizing the convertase.
• This interaction may contribute to inflammation in conditions associated with cholesterol crystal deposition.