Regulation of NKG2DCD8 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. NKG2DCD8 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 NKG2DCD8 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 CD8T 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 , as well as significantly improves the antitumor therapeutic effects 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.