Homotypic leukocyte aggregation triggered by a monoclonal antibody specific for a novel epitope expressed by the integrin β1 subunit: Conversion of nonresponsive cells by transfecting human integrin α4 subunit cDNA

The monoclonal antibody 33B6 was found to be specific for the beta 1 integrin subunit. Treatment of leukocytes with this antibody induced a vigorous homotypic aggregation that had similar physiologic conditions as aggregation induced by a monoclonal antibody specific for the alpha 4 subunit. Expression of a beta 1 subunit on the cell surface was not sufficient for mAb 33B6-mediated aggregation to occur, since cells of the K562 erythroleukemia line failed to respond even though they expressed the beta 1 subunit and the 33B6 epitope. However, after transfection with cDNA encoding the alpha 4 subunit, K562 cells acquired the ability to aggregate in response to mAb 33B6 binding. By contrast, mAb 33B6 blocked cell binding to the endothelial surface protein vascular cell adhesion molecule-1 and the extracellular matrix protein fibronectin. These results suggest that the beta 1 epitope defined by mAb 33B6 may play a novel role in regulating leukocyte adhesive interactions.