IL-21-mediated expansion of Vγ9Vδ2 T cells is limited by the Tim-3 pathway.

Vγ9Vδ2 T cells are the main γδ T subset in the peripheral blood and lymphoid organs. Previous studies have shown that Vγ9Vδ2 T cells could expand in the presence of phosphoantigens and IL-2 and exert antitumor functions. However, their potency was limited because sustained proliferation could not be achieved, possibly due to exhaustion caused by prolonged antigenic stimulation. In this study, we examined the proliferative response of Vγ9Vδ2 T cells to IL-21, a cytokine previously shown to promote NK cell and CD8 T cell cytotoxicity. We found that IL-21 could significantly improve the proliferation of phosphoantigen-stimulated Vγ9Vδ2 T cells in a dose-dependent manner. However, in acute myeloid leukemia (AML) patients, the efficacy of IL-21 was significantly reduced. Vγ9Vδ2 T cells from AML patients exhibited lower expression of IL-21R, and required higher levels of IL-21 for expansion. IL-21-treated Vγ9Vδ2 T cells from AML patients presented lower increase in STAT1 phosphorylation than Vγ9Vδ2 T cells from healthy volunteers. Interestingly, AML Vγ9Vδ2 T cells presented significantly higher Tim-3 expression than healthy Vγ9Vδ2 T cells. IL-21 treatment further induced Tim-3 upregulation. Blocking Tim-3 increased the proliferation and the STAT phosphorylation in Vγ9Vδ2 T cells in response to IL-21. Together, these results demonstrated that IL-21 could significantly expand the Vγ9Vδ2 T cells, but its efficacy was limited since it also increased the expression of checkpoint molecule Tim-3.