Endothelial CD47 promotes vascular endothelial-cadherin tyrosine phosphorylation and participates in T cell recruitment at sites of inflammation in vivo

At sites of inflammation, endothelial adhesion molecules bind leukocytes and transmit signals required for transendothelial migration (TEM). We previously reported that adhesive interactions between endothelial cell CD47 and leukocyte signal regulatory protein g (SIRPg) regulate human T cell TEM. The role of endothelial CD47 in T cell TEM in vivo, however, has not been explored. In this study, CD47^-/- mice showed reduced recruitment of blood T cells as well as neutrophils and monocytes in a dermal air pouch model of TNF-α-induced inflammation. Reconstitution of CD47^-/- mice with wild-type bone marrow cells did not restore leukocyte recruitment to the air pouch, indicating a role for endothelial CD47. The defect in leukocyte TEM in the CD47^-/- endothelium was corroborated by intravital microscopy of inflamed cremaster muscle microcirculation in bone marrow chimera mice. In an in vitro human system, CD47 on both HUVEC and T cells was required for TEM. Although previous studies showed CD47-dependent signaling required G _αi-coupled pathways, this was not the case for endothelial CD47 because pertussis toxin, which inactivates Gai, had no inhibitory effect, whereas G_αi was required by the T cell for TEM. We next investigated the endothelial CD47-dependent signaling events that accompany leukocyte TEM. Ab-induced cross-linking of CD47 revealed robust actin cytoskeleton reorganization and Src- and Pyk-2-kinase dependent tyrosine phosphorylation of the vascular endothelialcadherin cytoplasmic tail. This signaling was pertussis toxin insensitive, suggesting that endothelial CD47 signaling is independent of G_αi. These findings suggest that engagement of endothelial CD47 by its ligands triggers outside-in signals in endothelium that facilitate leukocyte TEM.