Human vascular adhesion protein-1 (VAP-1) plays a critical role in lymphocyte-endothelial cell adhesion cascade under shear.

Lymphocyte binding to vascular endothelium is a prerequisite for the movement of immune cells from the blood into lymphoid tissues and into sites of inflammation. Human vascular adhesion protein-1 (VAP-1) is an endothelial glycoprotein involved in this interaction. It also displays an enzymatic (monoamine oxidase) activity. Here we examined how recombinant human VAP-1 mediates lymphocyte binding using rotatory and flow chamber binding assays. VAP-1 cDNA transfected into an endothelial cell line, which does not bind lymphocytes, renders the cell line capable of binding lymphocytes in a shear-dependent manner. VAP-1 transfectants bound lymphocytes 5 times better than monocytes with a preference for T killer cells, and no specific granulocyte adherence was detectable. The binding is partially inhibited by anti-VAP-1 monoclonal antibodies or by blocking lymphocyte L-selectin and CD18 integrins, but not by inhibition of several other homing-associated molecules. In contrast, CD44 ligation on lymphocytes markedly upregulates their VAP-1-dependent adhesion, suggesting that the VAP-1 counterreceptor can be activated via CD44. The transfectant model also allowed us to perform detailed structure-function analyses of VAP-1. We show that the exposed integrin-binding motif RGD or the enzymatic activity is not indispensable for VAP-1-dependent adhesion. Together, these data show that VAP-1 can reconstitute the lymphocyte-endothelial adhesion cascade under shear and propose a critical role for VAP-1 in lymphocyte emigration from the blood.