FUNCTIONAL-ROLE OF ALPHA-2/BETA-1 AND ALPHA-4/BETA-1 INTEGRINS IN LEUKOCYTE INTERCELLULAR-ADHESION INDUCED THROUGH THE COMMON BETA-1-SUBUNIT

Whereas all of the integrins in the VLA protein subfamily are involved in cell-extracellular matrix interactions, only VLA-4 (through the alpha4 subunit) has been implicated in the triggering of intercellular adhesion. Here we describe that the VLA protein beta1 subunit (CD29) is also involved in the induction of homotypic cell aggregation. We have obtained three novel anti-beta1 monoclonal antibodies (mAb) with the ability to induce cell aggregation on different leukocyte cell types. These mAb recognize an antigenic site on the common beta1 chain of VLA proteins which is topographically and/or functionally distinct from other epitopes previously defined by several prototype anti-beta1 mAb. Induction of cell aggregation by anti-beta1 mAb is epitope specific, isotype and Fc independent, and displays kinetics similar to alpha4-mediated aggregation. This cell aggregation requires an intact cellular metabolism, the presence of divalent cations in the extracellular medium, and the integrity of the cytoskeleton. We also have found that the Na+/H+ antiporter may be essential for this process. For Ramos cells, which bear only the VLA alpha4/beta1 heterodimer, intercellular adhesion induced through the VLA-beta1 chain could be selectively inhibited by other anti-beta1 mAb as well as by anti-alpha4 mAb. Interestingly, anti-beta1 mAb which induced strong aggregation of VLA-alpha2- or VLA-alpha4-transfected K562 cells, had minimal effect on the alpha2- alpha4-alpha5+ K562 cell line. Furthermore, the beta1-mediated induction of cell aggregation on alpha2-K562- and alpha4-K562-transfected cells was blocked by preincubation with either anti-alpha2 or anti-alpha4 mAb, respectively, as well as by other anti-beta1 mAb. Interestingly, parental K562 cells were able to interact with both alpha2- and alpha4-transfected K562 cells, thus suggesting that counter-receptors for both integrins (VLA-2 and VLA-4) might exist on these cells. Together these results provide strong evidence supporting the involvement of alpha2/beta1 and alpha4/beta1 heterodimers in intercellular interactions and underline the pivotal role of the common beta1 chain of VLA proteins in the integrin-mediated induction of cell aggregation.