INTERACTIONS BETWEEN HUMAN-COMPLEMENT COMPONENTS FACTOR-H, FACTOR-I AND C3B

Using a microtitre plate assay, direct binding between complement fact ors I and H was demonstrated, and ligand blotting indicated that facto r H interacts with the heavy chain of factor I. Similarly, direct C3(N H3)-factor I and C3(NH3)-factor H binding was characterized [where C3( NK3) is a form of C3 that is cleaved by factor I in the presence of fa ctor H]. Both factor H and factor I interacted with both chains of C3( NH3) and blotting. Binding reactions between all three pairs of compon ents were highly dependent on ionic strength, and showed similar pH op tima. Binding assays with all three components present led to the foll owing conclusions. (a) Binding sites for C3(NH3) and factor I on facto r H do not overlap, and binding of factor I and C3(NH3) to soluble fac tor H promotes the weak factor I-C3(NH3) interaction. (b) Anomalies ar ise with immobilized factor H, which may be artefactual or may reflect the physiological situation. (c) Similarly, binding sites on factor I for C3(NH3) factor H do not overlap, and binding of factor H and C3(N H3)r I promotes direct factor H-C3(NH3) interactions. Based on these r esults, a model of the interactions between factor H, factor I and C3( NH3) leading to the processing of C3(NH3) is proposed.