Regulation of NKG2D⁺CD8⁺ T-cell-mediated antitumor immune surveillance: Identification of a novel CD28 activation-mediated, STAT3 phosphorylation-dependent mechanism

The natural killer (NK) group 2D (NKG2D) receptor, which displays on mouse and human NK cells, activates CD8⁺ T cells and small subsets of other T cells. NKG2D⁺CD8⁺ T cells play critical roles in both innate and adaptive immunity upon engagement with NKG2D ligands to eliminate tumor and infected cells. Despite the important role of NKG2D⁺CD8⁺ T cells in immune surveillance, the mechanisms of how NKG2D expression on CD8⁺ T cells is regulated remain poorly defined. We treated mouse and human CD8⁺ T cells with CD80 recombinant protein, plus a pharmacologic model with small molecular inhibitors to determine which signaling pathway leads to NKG2D regulation on CD8⁺T cells. This study revealed that CD28 activation gives rise to sustained NKG2D expression on both mouse and human CD8⁺ T cells in a signal transducer and activator of transcription 3 (STAT3) phosphorylation-dependent manner. Further, we found that CD28 activation stimulated sustained activation of the tyrosine kinase Lck, which recruits and triggers Janus kinase/STAT3 signaling to phosphorylate STAT3, and in turn increases NKG2D expression. Moreover, NKG2D induction on CD8⁺ T cells exerts cytolytic activity against target tumor cells in vitro, as well as significantly improves the antitumor therapeutic effects in vivo in an NKG2D-dependent manner. Taken together, these results elucidated a novel mechanism of NKG2D regulation by phosphorylated STAT3 (pSTAT3) on CD8⁺ T cells upon CD28 activation. This mechanism may shed light on the effectiveness of CD80-based, NKG2D-dependent antitumor immunotherapy.