Platelet-endothelial cell adhesion molecule (PECAM)-1 has been implicated in leukocyte migration through the perivascular basement membrane (PBM) though the mechanisms involved are unclear. The present results demonstrate that the ability of α₆ integrins to mediate neutrophil migration through the PBM is PECAM-1 dependent, a response associated with PECAM-1–mediated increased expression of α₆β₁ on transmigrating neutrophils in vivo. An anti-α₆ integrins mAb (GoH3) inhibited (78%, P < 0.001) neutrophil migration through interleukin (IL)-1β–stimulated cremasteric venules, primarily at the level of the PBM, as analyzed by intravital and electron microscopy. In PECAM-1–deficient mice (KO), a reduced level of neutrophil transmigration elicited by IL-1β (4-h reaction) was observed in both the cremaster muscle (55% inhibition, P < 0.05) and in the peritoneum (57% inhibition, P < 0.01) but GoH3 had no additional inhibitory effect on these responses. FACS^® analysis of neutrophils demonstrated increased expression of α₆β₁ on transmigrated peritoneal neutrophils, as compared with blood neutrophils, in wild-type but not KO mice even though neutrophils from both strains of mice exhibited comparable levels of intracellular expression of α₆ as observed by immunofluorescent staining and confocal microscopy. Furthermore, mice deficient in either leukocyte or endothelial cell PECAM-1, as developed by bone marrow transplantation, demonstrated a similar level of reduced neutrophil transmigration and expression of α₆β₁ on transmigrated neutrophils as that detected in KO mice.

The results demonstrate a role for PECAM-1 homophilic interaction in neutrophil transmigration and increased expression of α₆β₁ on the cell surface of transmigrated neutrophils in vivo, a response that could contribute to the mechanism of PECAM-1–mediated neutrophil migration through the PBM.