Vav1-phospholipase C-γ1 (Vav1-PLC-γ1) Pathway Initiated by T Cell Antigen Receptor (TCRγδ) Activation Is Required to Overcome Inhibition by Ubiquitin Ligase Cbl-b during γδT Cell Cytotoxicity

T cell antigen receptor gamma delta (TCR gamma delta) and natural killer group 2, member D (NKG2D) are two crucial receptors for gamma delta T cell cytotoxicity. Compelling evidences suggest that gamma delta T cell cytotoxicity is TCR gamma delta-dependent and can be co-stimulated by NKG2D. However, the molecular mechanism of underlying TCR gamma delta-dependent activation of gamma delta T cells remains unclear. In this study we demonstrated that TCR gamma delta but not NKG2D engagement induced lytic granule polarization and promoted gamma delta T cell cytotoxicity. TCR gamma delta activation alone was sufficient to trigger Vav1-dependent phospholipase C-gamma 1 signaling, resulting in lytic granule polarization and effective killing, whereas NKG2D engagement alone failed to trigger cytotoxicity-related signaling to overcome the inhibitory effect of Cbl-b; therefore, NKG2D engagement alone could not induce effective killing. However, NKG2D ligation augmented the activation of gamma delta T cell cytotoxicity through the Vav1-phospholipase C-gamma 1 pathway. Vav1 overexpression or Cbl-b knockdown not only enhanced TCR gamma delta activation-initiated killing but also enabled NKG2D activation alone to induce gamma delta T cell cytotoxicity. Taken together, our results suggest that the activation of gamma delta T cell cytotoxicity requires a strong activation signal to overcome the inhibitory effect of Cbl-b. Our finding provides new insights into the molecular mechanisms underlying the initiation of gamma delta T cell cytotoxicity and likely implications for optimizing gamma delta T cell-based cancer immunotherapy.