Clustering the adhesion molecules VLA-4 (CD49d/CD29) in Jurkat T cells or VCAM-1 (CD106) in endothelial (ECV 304) cells activates the phosphoinositide pathway and triggers Ca2+ mobilization

Ligation of very late antigen (VLA)-4 (α₄β₁ integrin) with a cross-linked anti-α₄ subunit monoclonal antibody (mAb) triggered a biphasic Ca²⁺ response in Jurkat cell populations and in peripheral human lymphocytes. Cross-linking vascular cell adhesion molecule (VCAM)-1 (the counter-receptor of VLA-4) in ECV 304 endothelial cells generated a biphasic Ca²⁺ response. Tumor necrosis factor-α-primed human umbilical cord vascular endothelial cells also responded to the cross-linked mAb with a biphasic Ca²⁺ profile. Ligated VLA-4 (Jurkat cells) or VCAM-1 (ECV 304) stimulated the production of myo-inositol 1,4,5-trisphosphate. ECV 304 cells induced a biphasic Ca²⁺ response in Fura2-loaded Jurkat cells, whereas a transient response was observed when Jurkat cells were added to Fura2-loaded ECV 304 cells. The Ca²⁺ responses in these experiments involved VLA-4/VCAM-1 interactions since they were significantly reduced (～ 80%) by prior treatment of the target cells with the relevant noncross-linked mAb. Close contact between the cells triggered mutual Ca²⁺ signaling as shown by spectrofluorimetric and confocal microscopy time-dependent recordings. Fibronectin and its CS-1 fragment (V25) triggered a sustained Ca²⁺ response in Jurkat cells (confocal microscopy). Our results suggest that the VLA-4 and VCAM-1 adhesion molecules can transduce a signal that involves activation of the phosphoinositide pathway and the mobilization of Ca²⁺.