Individually distinct Ig homology domains in PECAM-1 regulate homophilic binding and modulate receptor affinity

PECAM-1 (CD31) is a 130-kDa member of the immunoglobulin (Ig) gene superfamily that is constitutively expressed at high concentration at endothelial cell intercellular junctions and at moderate density on the surface of circulating leukocytes and platelets. Recent in vitro and in vivo studies have shown the PECAM-1 plays a central role in mediating the extravasation of leukocytes from the vessel wall in response to inflammatory mediators. To study the binding characteristics of PECAM-1, phospholipid vesicles were prepared and examined by flow cytometry and immunofluorescence microscopy for their ability to associate with each other and with cells. Proteoliposomes containing high concentrations of PECAM-1 interacted homophilically with each other, forming large self-aggregates. PECAM-1 proteoliposomes, as well as soluble bivalent PECAM-1 in the form of a PECAM-1/IgG immunoadhesin, associated homophilically with cells expressing human, but not murine, PECAM-1. This binding could be completely inhibited by monoclonal antibody Fab fragments specific for Ig homology Domain 1 or Domains 1 + 2. Binding studies using cells expressing human PECAM-1 deletion mutants and murine/human chimeras confirmed that both Ig Domains 1 and 2 were both necessary and sufficient for homophilic binding. In contrast, engagement of membrane-proximal Domain 6 with monoclonal antibody Fab fragments had the opposite effect and augmented the binding of PECAM-1 proteoliposomes to cells. Thus, PECAM-1, like certain integrins, appears to be capable of antibody-induced conformational changes that alter affinity for its ligand. Similar changes induced by physiologic stimuli could be important in regulating the function of PECAM-1 in vascular cells.