Elucidation of Vav function in T lymphocyte development and activation

The Vav family of Rho-guanine nucleotide exchange factors (GEF) is thought to orchestrate signaling events downstream of lymphocyte antigen receptors. Elucidation of Vav function has been obscured thus far by the expression of three highly related family members. To address this issue, we generated mice lacking all Vav family proteins (VavNULL) and found that Vav NULL mice show a severe impairment in T cell development, activation, and function. While the expression of Vav1 alone is sufficient for normal lymphocyte development, our data reveal lineage-specific roles for Vav2 and Vav3, with the first demonstration that Vav3 plays an important compensatory function in T cells. Mechanistically, we show that the Vav family is crucial for antigen receptor-induced Ca2+ and MAPK signaling and the mobilization of the actin cytoskeleton at TCR contact sites. Taken together, these data define an essential role for the entire Vav protein family in lymphocyte development and activation and establish the limits of functional redundancy both within this family, and between Vav and other Rho GEFs.; While the exact mechanism of Vav function in lymphoid cells remains obscure, a particular outstanding issue is whether or not GEF activity is essential for Vav function in lymphocytes. It is also not known how Vav function in T cells is regulated by tyrosine phosphorylation. To address these issues, we used a novel VavNULL-hematopoietic stem cell complementation approach to generate lymphocytes in which endogenous Vav is replaced with mutated Vav proteins. We found that Vav assembles into signaling microclusters at TCR contact sites and is indispensable for TCR-initiated actin polymerization. Surprisingly, however, Vav function(s) in these TCR signaling events does not require the intrinsic GEF activity. We also reveal an unexpected complexity of the regulation of Vav function by tyrosine residues in the N-terminal "acidic" region and identify tyrosine 174 as a crucial regulatory site in vivo . We propose here a model for Vav function as a critical linker for TCR activation of T cells.