CD28-mediated up-regulation of β₁-integrin adhesion involves phosphatidylinositol 3-kinase

Cross-linking of the CD28 Ag on T cells results in increased β₁- integrin-mediated adhesion to fibronectin. Chimeric contracts containing the CD28 cytoplasmic domain fused to the extracellular and transmembrane regions of CD2 were expressed in HL60 cells to investigate CD28-mediated regulation of adhesion. Ab cross-linking of the CD2/28 chimera resulted in increased β₁-dependent adhesion of HL60 transfectants to fibronectin. Induced binding was completely inhibited by the phosphatidylinositol 3-kinase (PI 3-K) inhibitor wortmannin. Cross-linking of the CD2/28 chimera also induced association of the p85 subunit of PI 3-K with the CD2/28 cytoplasmic domain. In contrast, cross-linking of a CD2/28 chimera containing a tyrosine to phenylalanine substitution in the YMNM motif did not result in increased adhesion to fibronectin and did not lead to association of the chimera with PI 3-K. These results directly implicate the YMNM motif and PI 3-K in the regulation of β₁-integrin activity by the CD28 Ag.