Vav1 Regulates T-Cell Activation through a Feedback Mechanism and Crosstalk between the T-Cell Receptor and CD28

Vav1, a Rac/Rho guanine nucleotide exchange factor and a critical component of the T-cell receptor (TCR) signaling cascade is tyrosine phosphorylated rapidly in response to T-cell activation. Vav1 has established roles in proliferation, cytokine secretion, Ca2+ responses, and actin cytoskeleton regulation; however, its function in the regulation of phosphorylation of TCR components, including the zeta chain, the CD3 delta, epsilon, gamma chains, and the associated kinases Lck and ZAP-70, is not well established. To obtain a more comprehensive picture of the role of Vavl in receptor proximal sighaling, we performed a wide-scale characterization of Vavl-dependent tyrosine phosphorylation events using quantitative phosphoproteomic analysis of VavI-deficient T cells across a time course of TCR stimulation. Importantly, this study revealed a new function for Vav1 in the negative feedback regulation of the phosphorylation of immunoreceptor tyrosine-based activation motifs within the chains, CD3 delta, epsilon, gamma chains, as well as activation sites on the critical T cell tyrosine kinases Itk, Lck, and ZAP-70. Our study also uncovered a previously unappreciated role for Vavl in crosstalk between the CD28 and TCR signaling pathways.