Mechanism of Collaborative Enhancement of Binding of Paired Antibodies to Distinct Epitopes of Platelet Endothelial Cell Adhesion Molecule-1

Monoclonal antibodies (mAbs) directed to extracellular epitopes of human and mouse Platelet Endothelial Cell Adhesion Molecule-1 (CD31 or PECAM-1) stimulate binding of other mAbs to distinct adjacent PECAM-1 epitopes. This effect, dubbed Collaborative Enhancement of Paired Affinity Ligands, or CEPAL, has been shown to enhance delivery of mAb-targeted drugs and nanoparticles to the vascular endothelium. Here we report new insights into the mechanism underlying this effect, which demonstrates equivalent amplitude in the following models: i) cells expressing a full length PECAM-1 and mutant form of PECAM-1 unable to form homodimers; ii) isolated fractions of cellular membranes; and, iii) immobilized recombinant PECAM-1. These results indicate that CEPAL is mediated not by interference in cellular functions or homophilic PECAM-1 interactions, but rather by conformational changes within the cell adhesion molecule induced by ligand binding. This mechanism, mediated by exposure of partially occult epitopes, is likely to occur in molecules other than PECAM-1 and may represent a generalizable phenomenon with valuable practical applications.