INDUCTION OF HOMOTYPIC LYMPHOCYTE AGGREGATION - EVIDENCE FOR A NOVEL ACTIVATION STATE OF THE BETA(1) INTEGRIN

Intercellular adhesion of Jurkat lymphocytic cells was investigated by use of monoclonal antibodies 33B6 and 18D3, which bind to the beta(1) integrin receptor, 33B6 induced homotypic aggregation of Jurkat cells , whereas 18D3 inhibited this aggregation, Jurkat cells could be induc ed to aggregate at low 33B6 concentrations corresponding to 5% beta(1) integrin site occupancy, and the rate of aggregation was maximum at 3 0% occupancy. Simultaneous addition of mab 18D3 and 33B6 demonstrated that the two antibodies mediate changes in the beta(1) integrin activa tion state that are competitive in nature. Aggregation through beta(1) integrin induced by 33B6 was reversed by subsequent addition of 18D3, To farther examine the mechanism by which 33B6 and 18D3 affect cell a dhesion function, we explored the binding of monoclonal antibody (mAb) 15/7. This mAb recognizes an activation epitope of the beta(1) integr in and has been shown to sustain cell adhesion to vascular cell adhesi on molecule 1 (VCAM-1) and fibronectin. Activation of Jurkat cells wit h Mn2+ caused a 2.5-fold increase in 15/7 binding but did not increase binding of 33B6, 33B6 partially blocked 15/1 binding to beta(1) integ rin on unstimulated and Mn2+-activated Jurkat cells, 18D3 did not affe ct mAb 15/1 binding. These results indicate that 33B6 and 18D3 modulat ed homotypic aggregation by inducing a novel activation state of the v ery late activation integrin distinct from the state recognized by 15/ 1, which supports cell binding to VCAM-1 and fibronectin.