Regulation of the human alternative complement pathway: formation of a ternary complex between factor h, surface-bound c3b and chemical groups on nonactivating surfaces

Abstract

Sephadex [alpha(1----6) cross-linked dextran] activates the human alternative pathway of complement. Substitution of hydroxyl groups of Sephadex with carboxymethyl groups (CM) results in a dose-dependent decrease of the activating capacity of the polymer in normal human serum. Sephadex bearing one CM group/glycosyl unit (CM-Seph 0.95) exhibited no activating capacity. CM groups did not interfere with the ability of the polymer to covalently bind C3b in the presence of purified alternative pathway proteins nor with the capacity of bound-C3b to form a C3 convertase in the absence of regulatory proteins. C3b that was bound to CM-Seph 0.95 was more susceptible to inactivation by factors H and I in serum than C3b bound to Sephadex. Binding studies using 125I-labeled H demonstrated that H bound with a similar affinity to the activating particle Sephadex, to Sephadex bearing C3b and to the nonactivating particle CM-Seph 0.95. However, factor H bound with a 5- to 7-fold higher affinity to CM-Seph 0.95 bearing C3b. These results demonstrate a requirement for both CM groups and C3b molecules in order for H to bind with high affinity to C3b on the non-activating surface, and indicate that H formed a ternary complex with surface-bound C3b and CM groups on CM-Seph 0.95. Using a chemically defined model system, the present study provides a molecular basis for the enhanced interaction between surface-bound C3b and factor H on nonactivators of the human alternative pathway.

Discrimination between activators and nonactivators of the alternative pathway of complement: regulation via a sialic acid/polyanion binding site on factor H.

“…The simplest is that attachment of C3b to polysaccharides or proteins causes, through occupation of a restriction site, at most a 10-fold reduction in the affinity for factor H. Whether this is due to a conformational change in C3b, steric hinderance, or partial competition for the factor H site on C3b is unknown. Reports of neoantigenic sites on bound C3b not present on fluid-phase C3b favor a conformational change (46 (47). A two-point attachment model where factor H binds with higher affinity because it binds to C3b and to surface anions can be ruled out sincefluid-phase polyanions enhance C3b-H binding on cells and in the fluid phase.…”

His-384 Allotypic Variant of Factor H Associated with Age-related Macular Degeneration Has Different Heparin Binding Properties from the Non-disease-associated Form

“…Factor H is believed to discriminate self from non-self by recognizing polyanionic structures on the former, such as sialic acid and the glycosaminoglycan (GAG) chains of proteoglycans (e.g. heparan sulfate (HS) and dermatan sulfate (DS)) and thus inhibits complement activation on host surfaces (11,12). Factor H has been shown to be present in retinal blood vessels in the choroid (5) and is associated with the drusen of AMD patients (2, 13).…”

A flow cytometric immunoassay to quantify adsorption of complement activation products (iC3b, C3d, SC5b-9) on artificial surfaces

“…The surface concentration on agarose beads was twice as high for polystyrene beads at 2 h, not a surprising result given that polysaccharide surfaces such as zymosan, cellulose, and Sephadex (crosslinked dextran) are recognized as having high capacity to activate complement. 19,20 Further, the amount absorbed represented a significant fraction of the percentage generated. A similar significant fraction has also been reported for Silastic™, polyethylene, and polyvinyl alcohol hydrogel (PVA) surfaces after quantification of SC5b-9 following desorption with 0.05% sodium dodecyl sulfate (SDS) using an enzyme-linked immunoflow assay (ELIFA) to quantitate bulk and surface levels.…”