Cas-L Is Required for β1 Integrin-Mediated Costimulation in Human T Cells

Abstract

β1 integrins provide a costimulus for TCR/CD3-driven T cell activation and IL-2 production in human peripheral T cells. However, this β1 integrin-mediated costimulation is impaired in a human T lymphoblastic line, Jurkat. We studied the molecular basis of this impaired costimulation and found that Cas-L, a 105-kDa docking protein, is marginally expressed in Jurkat T cells, whereas Cas-L is well expressed in peripheral T cells. Cas-L is a binding protein and a substrate for focal adhesion kinase and is tyrosine phosphorylated by β1 integrin stimulation. We here show that the transfection of wild-type Cas-L in Jurkat T cells restores β1 integrin-mediated costimulation. However, Cas-L transfection had no effect on CD28-mediated costimulation, indicating that Cas-L is specifically involved in the β1 integrin-mediated signaling pathway. Furthermore, transfection of the Cas-LΔSH3 mutant failed to restore β1 integrin-mediated costimulation in Jurkat cells. Cas-LΔSH3 mutant lacks the binding site for focal adhesion kinase and is not tyrosine phosphorylated after β1 integrin stimulation. These findings strongly suggest that the tyrosine phosphorylation of Cas-L plays a key role in the signal transduction in the β1 integrin-mediated T cell costimulation.