Human NK Cells Downregulate Zap70 and Syk in Response to Prolonged Activation or DNA Damage

The extent of NK cell activity during the innate immune response affects downstream immune functions and, ultimately, the outcome of infectious or malignant disease. However, the mechanisms that terminate human NK cell responses have yet to be defined. When activation receptors expressed on NK cell surfaces bind to ligands on diseased cells, they initiate a signal that is propagated by a number of intracellular kinases, including Zap70 and Syk, eventually leading to NK cell activation. We assayed Zap70 and Syk content in NK cells from healthy human donors and identified a subset of NK cells with unusually low levels of these two kinases. We found that this Zap70^lowSyk^low subset consisted of NK cells expressing a range of surface markers, including CD56^hi and CD56^low NK cells. Upon in vitro stimulation with target cells, Zap70^lowSyk^low NK cells failed to produce IFN-γ and lysed target cells at one third the capacity of Zap70^hiSyk^hi NK cells. We determined two independent in vitro conditions that induce the Zap70^lowSyk^low phenotype in NK cells: continuous stimulation with activation beads and DNA damage. The expression of inhibitory receptors, including NKG2A and inhibitory killer Ig-like receptors (KIRs), was negatively correlated with the Zap70^lowSyk^low phenotype. Moreover, expression of multiple KIRs reduced the likelihood of Zap70 downregulation during continuous activation, regardless of whether NK cells had been educated through KIR-HLA interactions in vivo. Our findings show that human NK cells are able to terminate their functional activity without the aid of other immune cells through the downregulation of activation kinases.