Investigation of the Interactions Between Complement Factor H, C3b, and Amyloid β

Abstract

Purpose: : Amyloid β (Aβ), complement component C3b and complement factor H have been identified in drusen. We studied the effects of Aβ1-40 and 1-42, in different aggregation states, on the cofactor activity of purified, allotypic variant-specific isoforms of factor H in the factor I - mediated proteolytic cleavage of C3b. We then determined whether this modulation was due to direct C3b-Aβ interaction or to factor H-Aβ binding.

Methods: : Factor H was purified from plasma from individuals homozygous for the double variants: I44/Y384 (aka I62/Y402), V44/Y384 and V44/H384. Fluid-phase and cell-based assays of the cofactor activity of factor H were used to establish the effect on the kinetics of the reaction when Aβ, in various aggregation states, was introduced to the assay. The binding of Aβ on a solid surface to variant-specific factor H was investigated. The direct effect of Aβ on C3b was evaluated using silver-stained gels and Westerns, and aggregation was visualized using C3b-coated sheep erythrocytes. The ability of factor H to influence this reaction was also investigated.

Results: : Monomeric Aβ1-40 increased the rate of proteolytic cleavage of C3b by factor I and factor H in both the fluid-phase and cell-based assays. This modulation was not variant specific. Oligomeric Aβ1-42 decreases the rate of this reaction. Purified, variant-specific factor H was added to Aβ of different lengths and aggregation states (mono-, oligomeric or fibrillar) on a plastic plate. Oligomeric Aβ1-42 exhibited the highest binding overall with all three forms of factor H. Oligomeric and, to a lesser extent, fibrillar Aβ 1-42 aggregate C3b at certain concentrations and this aggregation could be partially inhibited by the presence of factor H.

Keywords: age-related macular degeneration • inflammation • drusen