Generation of complement anaphylatoxins and C5b-9 by crystalline cholesterol oxidation derivatives depends on hydroxyl group number and position.
Cholesterol crystals activate the human alternative complement pathway. Loss of Factor B hemolytic activity in C2-deficient serum was comparable to that in normal human serum after incubation with cholesterol crystals. Consumption of Factor B hemolytic activity in normal serum incubated with cholesterol occurred in a time- and dose-dependent manner. The reduced capacity of crystals-absorbed serum to activate C2, but not Factor B, on fresh crystals, indicated that cholesterol mediates antibody-dependent classical pathway activation in addition to alternative pathway activation in whole serum. Cholestane triol, an oxidation derivative of cholesterol which bears three hydroxyl groups, cleaved 5-fold more C3 than cholesterol in normal human serum. Three cholesterol derivatives, each bearing two hydroxyl groups, were intermediate activators between cholesterol and cholestane triol. The compounds differed, however, in their complement-activating ability, indicating that hydroxyl position as well as number exerts an influence on complement activation. Measurements of C3adesArg and C5adesArg antigens in cholesterol crystal treated serum revealed that approx. 10% of total serum C3 was cleaved and that, on a molar basis, only 3% C5 cleavage occurred relative to C3 cleavage. For 1 mole of C5a generated, 0.1 moles of fluid-phase C5b-9 was detected. Although the extent of C3 cleavage varied with each cholesterol derivative depending on the position and number of hydroxyl groups, the relative coupling efficiency of C3 and C5 cleavage and C5a and C5b-9 generation was similar for all compounds. The ability of cholesterol and its oxidation products to generate anaphylatoxins and C5b-9 complexes may be of importance in mediating inflammatory processes involved in atherogenesis.