4-1BB regulates NKG2D costimulation in human cord blood CD8⁺ T cells

Ligation of NKG2D, a potent costimulatory receptor, can be either beneficial or detrimental to CD8⁺ cytotoxic T cell (CTL) responses. Factors for these diverse NKG2D effects remain elusive. In this study, we demonstrate that 4-1BB, another costimulatory receptor, is an essential regulator of NKG2D in CD8⁺ T cells. Costimulation of NKG2D caused down-modulation of NKG2D, but induced 4-1BB expression on the cell surface, even in the presence of TGF-β1, which inhibits 4-1BB expression. Resulting NKG2D^-4-1BB⁺ cells were activated but still in an immature state with low cytotoxic activity. However, subsequent 4-1BB costimulation induced cytotoxic activity and restored down-modulated NKG2D. The cytotoxic activity and NKG2D expression induced by 4-1BB on NKG2D⁺4-1BB ⁺ cells were refractory to TGF-β1 down-modulation. Such 4-1BB effects were enhanced by IL-12. In contrast, in the presence of IL-4, 4-1BB effects were abolished because IL-4 down-modulated NKG2D and 4-1BB expression in cooperation with TGF-β1, generating another CD8⁺ T-cell type lacking both NKG2D and 4-1BB. These NKG2D^-4-1BB^- cells were inert and unable to gain cytotoxic activity. Our results suggest that 4-1BB plays a critical role in protecting NKG2D from TGF-β1-mediated down-modulation. Coexpression of NKG2D and 4-1BB may represent an important biomarker for defining competency of tumor infiltrating CD8⁺ T cells.