INDIVIDUALLY DISTINCT IG HOMOLOGY DOMAINS IN PECAM-1 REGULATE HEMOPHILIC BINDING AND MODULATE RECEPTOR AFFINITY

PECAM-1 (CD31) is a 130-kDa member of the immunoglobulin (Ig) gene sup erfamily that is constitutively expressed at high concentration at end othelial cell intercellular junctions and at moderate density on the s urface of circulating leukocytes and platelets, Recent in vitro and in vivo studies have shown that PECAM-1 plays a central, role in mediati ng 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 concentrat ions of PECAM-1 interacted homophilically with each other, forming lar ge self-aggregates. PECAM-1 proteoliposomes, as well as soluble bivale nt PECAM-1 in the form of a PECAM-1/IgG immunoadhesin, associated homo philically with cells expressing human, but not murine, PECAM-1. This binding could be completely inhibited by monoclonal antibody Fab fragm ents specific for Ig homology Domain 1 or Domains 1 + 2. Binding studi es using cells expressing human PECAM-1 deletion mutants and murine/hu man chimeras confirmed that both Ig Domains 1 and 2 were both necessar y and sufficient for hemophilic binding. In contrast, engagement of me mbrane-proximal Domain 6 with monoclonal antibody Fab fragments had th e opposite effect and augmented the binding of PECAM-1 proteoliposomes to cells. Thus, PECAM-1, like certain integrins, appears to be capabl e of antibody-induced conformational changes that alter affinity for i ts ligand. Similar changes induced by physiologic stimuli could be imp ortant in regulating the function of PECAM-1 in vascular cells.