Soluble domain 1 of PECAM (CD31) is sufficient to block transendothelial migration (TEM) in vitro and in vivo

PECAM, a cell adhesion molecule expressed on both leukocytes and endothelium, is uniquely involved in leukocyte diapedesis (TEM) across the endothelium. Our studies using mAbs bound to monocyte PECAM suggested that this function is mediated by amino terminal domain(s) of PECAM (domain 1 or 2 on leukocyte). To study which domains of endothelial PECAM are involved in TEM, we generated soluble PECAM-human IgG fusion proteins as decoys to mimick endothelial PECAM as presented to leukocytes. In a quantitative in vitro assay, we show that full-length PECAM-IgG blocks monocyte TEM as efficiently as several mAbs against domain 1 or 2 (i.e., by ≥70%). In addition, a soluble truncated protein, containing only the first domain, blocks TEM equally efficiently as the full-length reagent. These observations were further evaluated in a mouse thioglycollate peritonitis model by testing the corresponding murine soluble reagents, and similar results were obtained: Soluble domain 1-IgG functions as well as full-length reagent and as well as a blocking mAb in preventing PMN and monocyte migration into the peritoneal cavity. The block is at the level of TEM, resembling the block in vitro. Interestingly, unlike the full-length reagent, the soluble domain 1 does not bind to endothelial PECAM tightly enough to be detected by flow cytometry. This is the first demonstration that a truncated soluble cell adhesion molecule with no stable binding activity itself can block inflammation in vivo. Our data support the hypothesis that the same amino terminal domains of PECAM-1 on both monocyte and endothelial cell interact with each other in a homophilic manner during TEM.