An Alternative Leukocyte Homotypic Adhesion Mechanism, LFA-1/ICAM-l-independent, Triggered through the Human VLA-4 Integrin

The VLA-4 (CD49d/CD29) integrin is the only member of the VLA family expressed by resting lymphoid cells that has been involved in cell-cell adhesive interactions. We here describe the triggering of homotypic cell aggregation of peripheral blood T lymphocytes and myelomonocytic cells by mAbs specific for certain epitopes of the human VLAo~4 subunit. This anti-VLA-4-induced cell adhesion is isotype and Fc independent. Similar to phorbol ester-induced homotypic adhesion, cell aggregation triggered through VLA-4 requires the presence of divalent cations, integrity of cytoskeleton and active metabolism. However, both adhesion phenomena differed at their kinetics and temperature requirements. Moreover, cell adhesion triggered through VLA-4 cannot be inhibited by cell preincubation with anti-LFA-lc¢ (CDIla), LFA-I~ (CD18), or ICAM-1 (CD54) mAb as opposed to that mediated by phorbol esters, indicating that it is a LFA-1/ICAM-1 independent process. Antibodies specific for CD2 or LFA-3 (CD58) did not affect the VLA-4-mediated cell adhesion. The ability to inhibit this aggregation by other anti-VLA-4-specific antibodies recognizing epitopes on either the VLAa4 (CD49d) or/3 (CD29) chains suggests that VLA-4 is directly involved in the adhesion process. Furthermore, the simultaneous binding of a pair of aggregation-inducing mAbs specific for distinct antigenic sites on the 0~4 chain resulted in the abrogation of cell aggregation. These results indicate that VLA-4mediated aggregation may constitute a novel leukocyte adhesion pathway. T HE integrin superfamily includes receptors for extracellular matrix (ECM) ~ components as well as receptors involved in cell-cell adhesive interactions (19, 38). Structurally, the integrin molecules are ot,:~ heterodimers, in which the c~ subunit is noncovalently associated with the/3 chain. Three subfamilies of integrins have been classically described, each one defined by a common/3 subunit (designated as/31, ~2, and/33) that shares multiple c~ subunits (19, 38). The/32 integrins make up the leukocyte LFA-1, Mac-l, and p150,95 family (23, 34). Mac-1 and p150,95 function as complement receptors and they are also involved in the adhesion of monocytes and granulocytes to endothelial cells and other substrates. The LFA-I member is implicated in cell-cell interactions, thus being essential in the adhesive function of all leukocyte cell types (23, 39). The/33 integrins are ECM protein receptors and include the vitronectin receptor and platelet glycoprotein IIb/IIIa (20, 29). Finally, the/31 integrin subfamily, also known as VLA proteins, contains at least six different a chains (VLA-1-VLA-6). Among the VLA proteins, VLA-2, VLA-5, and VLA-6 funcI. Abbreviations used in this paper: ECM, extracellular matrix; ICAM-I, intracellular adhesion molecule-l; LFA, lymphocyte function-associated antigen; VLA, very late activation antigen. tion as receptors for ECM proteins such as collagen, fibronectin, and laminin, respectively, whereas VLA-3 binds both fibronectin and laminin (5, 12, 13, 32, 38). Recently, a role has been suggested for the VLA-4 integrin in intercellular leukocyte interactions (16, 40). Alternatively, VLA-4 can also function as a receptor for ECM components and has recently been shown to mediate T lymphocyte attachment to human fibronectin (43). The VLA-4 integrin is expressed on thymocytes, resting and activated peripheral blood lymphocytes, monocytes, T and B cell lines and myelomonocytic cell lines (9, 13, 15, 35). The association between c~4 and B chains is weaker than other VLA c~/~ heterodimers and it can be readily broken by detergents or high ionic concentrations (15, 35). Recently, cDNA clones for the a4 subunit have been isolated and sequenced (40). Within the VLA family, VLA-4 is atypical because it is the only VLA heterodimer reported to participate in cell-cell interactions. Thus, VLA-4 has been previously shown to be involved in effector-target cell association during cytolytic T lymphocyte-mediated killing based on the inhibitory effect exerted by anti-VLA-4 mAb on this activity (7, 40). Similarly, an anti-VLA-4 antibody was also suggested to block heterotypic interaction between helper and suppressor cells (21). The mouse homologue of VLA-4, designated as © The Rockefeller University Press, 0021-9525/90/06/2157/9 $2.00 The Journal of Cell Biology, Volume 110, June 199