ROLE OF INTEGRIN ALPHA-4-BETA-7/ALPHA-4-BETA-P IN LYMPHOCYTE ADHERENCE TO FIBRONECTIN AND VCAM-1 AND IN HOMOTYPIC CELL CLUSTERING

Integrins are heterodimeric cell surface proteins that mediate both cell-cell and cell-extracellular matrix interactions. We and others recently identified cDNAs encoding a novel integrin beta-subunit, beta-7, in lymphocytes. We have now detected beta-7 mRNA in mouse TK-1 T lymphoma cells, which are known to express the putative Peyer's patch homing receptor alpha-4-beta-P. We used an anti-peptide antiserum and a novel mAb against the beta-7 subunit to show that TK-1 cells express beta-7 as the only subunit associated with alpha-4. We conclude that beta-7 and beta-P are identical. We also show that activated peripheral blood T cells express alpha-4-beta-7. We studied the function of alpha-4-beta-7/alpha-4-beta-P in TK-1 cells, which do not express very late antigen (VLA)-4 (alpha-4-beta-1). Cells adhered to intact fibronectin and to a fibronectin fragment containing the CS-1 region, but not to a fragment containing the RGD sequence. Adhesion to fibronectin was inhibited by antibodies to alpha-4. suggesting that alpha-4-beta-7 is a fibronectin receptor. We confirmed that alpha-4-beta-7 binds to the CS-1 region of fibronectin using affinity chromatography. TK-1 cell adhesion to the vascular cell adhesion molecule VCAM-1 was also inhibited by antibodies to alpha-4, implying that alpha-4-beta-7 also plays a role in the adherence of lymphocytes to endothelial cells. TK-1 cell binding to fibronectin and VCAM-1 is markedly increased by brief PMA stimulation. We also found that mAbs against alpha-4 and beta-7 induce homotypic clustering of TK-1 cells. Taken together these results suggest that alpha-4-beta-7/alpha-4-beta-P recognizes some or all of the same widely distributed ligands recognized by VLA-4 (alpha-4-beta-1) and that the role of alpha-4-beta-7/alpha-4-beta-P may not be restricted to lymphocyte homing.