Demonstration of specific C5a receptor on intact human polymorphonuclear leukocytes.

Abstract

Human C5a [fragment a of complement component 5], a complement-derived anaphylatoxin, is a potent mediator of human leukocyte chemotaxis. Using a homogeneous preparation of C5a that was 125I-labeled, the presence of a specific cellular receptor for this glycoportein on intact human polymorphonuclear leukocytes was demonstrated. Cellular uptake of the radiolabeled ligand occurred rapidly, and the rate of dissociation was extremely slow. Cellular binding was saturable with respect to 125I-labeled C5a, and half-saturation occurred at a concentration of 3-7 .times. 10-9 M. The number of C5a binding sites per cell was estimated as 1-3 .times. 105. The ligand (C5a) displays specific structural features that are required for binding because analogs of C5a e.g., C5ades Arg or a yeast carboxypeptidase-digested C5a derivative C5a-(I-69) inhibited the binding, but C3a anaphylatoxin, which resembles C5a chemically, did not. C5a-mediated leukocyte chemotaxis and C5a-induced lysosomal enzyme release from cytochalasin B-treated cells closely paralleled uptake of the ligand, clearly indicating that it is a receptor-C5a interaction that leads to stimulation of these cellular responses.