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Of the several families of adhesion receptors involved in leukocyte-endothelial cell interactions, only the selectins have been shown to initiate leukocyte interaction under physiologic shear; indeed, beta 2 (CD18) intergrins responsible for neutrophil arrest are unable to engage without prior selectin-mediated rolling. In contrast, alpha 4 (CD49d) integrins are shown here to initiate lymphocyte contract ("tethering") in vitro under shear and in the absence of a selectin contribution. The alpha 4 integrin ligands MAdCAM-1 and VCAM-1 support loose reversible interactions including rolling, as well as rapid sticking and arrest that is favored following integrin activation. Moreover, alpha 4 beta 7 mediates L-selectin (CD62L)-independent attachment of blood-borne lymphocytes to lamina propria venules in situ. Scanning electron microscopy of alpha 4 beta 7hi lymphoid cells reveals that, like L-selectin, alpha 4 beta 7 is highly concentrated on microvillous sites of initial cellular contact, whereas the beta 2 integrin LFA-1 is excluded from villi. Thus, alpha 4 but not beta 2 integrins can initiate leukocyte adhesion under flow, a capacity that may be in part a function of topographic presentation on microvilli.

The mucosal vascular addressin, MAdCAM-1, is an immunoglobulin superfamily adhesion molecule for lymphocytes that is expressed by mucosal venules and helps direct lymphocyte traffic into Peyer's patches (PP) and the intestinal lamina propria. We demonstrate that the lymphocyte integrin alpha 4 beta 7, also implicated in homing to PP, is a receptor for MAdCAM-1. Certain antibodies to alpha 4 and beta 7 integrin chains but not to the beta 2 integrin LFA-1 inhibit lymphocyte binding to purified MAdCAM-1 and to MAdCAM-1 transfectants. Lymph node lymphocytes, alpha 4 beta 7+ TK1 lymphoma cells, and a beta 7-transfected variant of an alpha 4+ B cell line, 38C13, bind constitutively to MAdCAM-1. Binding is enhanced by Mn(++)-induced integrin activation. The related integrin alpha 4 beta 1 supports efficient binding to VCAM-1 but not to MAdCAM-1, even after integrin activation, indicating that MAdCAM-1 is a preferential ligand for alpha 4 beta 7. Alpha 4 beta 7 can also bind VCAM-1, but this requires greater integrin activation than binding to MAdCAM-1. The findings imply a selective role for the interaction of alpha 4 beta 7 and MAdCAM-1 lymphocyte in homing to mucosal sites.

LECAM-1 (leukocyte-endothelial cell adhesion molecule 1), the lymphocyte lectin ("selectin") homing receptor for peripheral lymph nodes (PLNs), participates in the earliest interactions of polymorphonuclear neutrophilic leukocytes (PMNs) with inflamed venules. Here, we present evidence that LECAM-1 mediates this function through a novel mechanism--presentation of oligosaccharide ligands to the inducible vascular selectins endothelial-leukocyte adhesion molecule (ELAM-1) and granule membrane protein 140 (GMP-140). PMN, but not lymphocyte, LECAM-1 is modified with the vascular selectin ligand sialyl Lewis x (sLex) and specifically binds ELAM-1-transfected cells. Although only a small fraction of total cell surface sLex, LECAM-1-associated sLex appears to play a prominent role in PMN interactions with cell-associated ELAM-1 and GMP-140, as anti-LECAM-1 monoclonal antibodies or selective removal of cell surface LECAM-1 inhibits PMN binding to vascular selectin transfectants by up to 70%. The enhanced function of LECAM-1-associated sLex may reflect the striking concentration, shown here, of LECAM-1 on PMN surface microvilli, the site of initial cellular contact.