Identification of the C3bi receptor of human monocytes and macrophages by using monoclonal antibodies.

Abstract

Four monoclonal antibodies, IB4, OKM1, OKM9 and OKM10, all directed against the C3bi [complement component 3b inhibitor] receptor of human monocytes and macrophages (M.vphi.) were obtained. Two criteria were used to determine the specificity of these antibodies. First, culture surfaces coated with the antireceptor antibodies caused specific down modulation of C3bi receptor activity on M.vphi. adherent to these substrates. Second, receptor protein purified by using IB4 or OKM1 retained the ability to bind selectively to C3bi-coated erythrocytes. Each of the antibodies recognizes a distinct epitope on the C3bi receptor; they do not compete with one another for binding to monocytes. When immobilized on a solid support, each of the antibodies binds a molecule from M.vphi. lysates that can simultaneously bind one of the other monoclonal anti-C3bi receptor antibodies. OKM10 binds and masks the ligand-binding site of the C3bi receptor, while IB4, OKM1 and OKM9 bind to sites remote from the C3bi binding site. All 4 antibodies immunoprecipitated polypeptides of MW 185,000 [daltons] and 105,000 from 125I-surface-labeled M.vphi.. IB4 also precipitates polypeptides of MW 185,000, 153,000 and 105,000. Evidently, the C3bi receptor of human M.vphi. is a complex composed of 2 polypeptides, MW 185,000 and 105,000. Monoclonal antibodies reacting with 4 distinct antigenic determinants of this complex were identified. The determinant recognized by antibody OKM10 is at or near the ligand-binding site of the receptor. The determinant recognized by antibody IB4 is shared by at least 2 other leukocyte surface proteins.