CD28 Utilizes Vav-1 to Enhance TCR-Proximal Signaling and NF-AT Activation

Abstract

The mechanism through which CD28 costimulation potentiates TCR-driven gene expression is still not clearly defined. Vav-1, an exchange factor for Rho GTPases thought to regulate, mainly through Rac-1, various signaling components leading to cytokine gene expression, is tyrosine phosphorylated upon CD28 engagement. Here, we provide evidence for a key role of Vav-1 in CD28-mediated signaling. Overexpression of Vav-1 in Jurkat cells in combination with CD28 ligation strongly reduced the concentration of staphylococcus enterotoxin E/MHC required for TCR-induced NF-AT activation. Surprisingly, upon Vav-1 overexpression CD28 ligation sufficed to activate NF-AT in the absence of TCR engagement. This effect was not mediated by overexpression of ZAP-70 nor of SLP-76 but necessitated the intracellular tail of CD28, the intactness of the TCR-proximal signaling cascade, the Src-homology domain 2 (SH2) domain of Vav-1, and SLP-76 phosphorylation, an event which was favored by Vav-1 itself. Cells overexpressing Vav-1 formed lamellipodia and microspikes reminiscent of Rac-1 and Cdc42 activation, respectively, for which the SH2 domain of Vav-1 was dispensable. Together, these data suggest that CD28 engagement activates Vav-1 to boost TCR signals through a synergistic cooperation between Vav-1 and SLP-76 and probably via cortical actin changes to facilitate the organization of a signaling zone.

Vav exchange factor counteracts the HIV‐1 Nef‐mediated decrease of plasma membrane GM1 and NF‐AT activity in T cells

“…Moreover, Vav overexpression not only recovered Nef-mediated inhibition of NF-AT activity induced by SEE (Fig. 7A), but also strongly enhanced NF-AT activation [42]. Similar results were obtained in Jurkat cells transiently transfected with Vav together with CD8 or CD8-Nef (Fig.…”

“…These cells do not have any morphological and functional similarity with T cells. Moreover, also in absence of Nef, the Jurkat clone Vav (CL9) already displays cellular structures like lamellipodia and microspikes being typically associated with the Rho GTPases [42]. Therefore, the Vav-mediated hyperactivation of the actin cytoskeleton rearrangement and the induction of exocytic pathways could counterbalance the effect of Nef on GM1 in our system.…”

“…It is necessary for a polarized cortical actin change beneath the TCR/CD28 engaged receptors [38] and for optimal calcium mobilization [49]. Moreover, CD28-delivered costimulatory signals and Vav are required for inducing NF-AT activation [42]. Therefore, a defect in Vav functions mediated by Nef may represent a possible cause of the differential activation of transcription factors.…”

Glucose Uptake Is Limiting in T Cell Activation and Requires CD28-Mediated Akt-Dependent and Independent Pathways

“…Another signaling pathway that may link CD28 to increased Glut1 protein levels may be the small GTPase GEF, Vav1 (38). Vav1 associates with the membrane in a PI3K-independent mechanism (39) and is known to potently induce activation of NFAT, NF-B, and AP1 (40). Each of these transcription factors could be responsible for induction of Glut1 protein expression downstream of CD28 and explain how CD3 signals alone, when strong enough, are capable of inducing increased Glut1 expression.…”

T-Cell Receptor-Induced NF-κB Activation Is Negatively Regulated by E3 Ubiquitin Ligase Cbl-b

“…The NF-AT family of transcription factors has been implicated in the activation of the interleukin-2 gene. Whereas NF-AT proteins are phosphorylated and reside in the cytoplasm in resting cells, these proteins are dephosphorylated and translocate rapidly to the nucleus upon stimulation of cells with calcium-mobilizing agents (32,34). To investigate whether the loss of Cbl-b also affects the activation of NF-AT, both WT and Cbl-b Ϫ/Ϫ T cells were stimulated with anti-CD3 or anti-CD3 plus anti-CD28 Ab.…”