Toll-Like Receptor-Induced Expression of the NKG2D Ligand MICA on Monocytes Modulates Their Interaction with NK Cells.

Reciprocal interactions of NK cells with Dendritic Cells (DC) have been shown to influence activation of NK cells as well as maturation or lysis of DC and subsequent adaptive immune responses. However, little is known about the interaction of peripheral blood Monocytes with NK cells. Here we report that stimulation of Toll-like receptors (TLR) on Monocytes using LPS, Pam3Cys, R848 and PolyI:C induced mRNA and surface expression of MICA but no relevant changes of the further ligands of the activating immunoreceptor NKG2D, MICB or ULBP1-3. MICA upregulation upon stimulation with TLR ligands correlated with CD80 and HLA-DR upregulation and increased secretion of the pro-inflammatory cytokines IL-6, IL-8 and TNF by Monocytes. Interestingly, high MICA surface expression on activated Monocytes was not accompanied by a release of MICA in soluble form. The LPS-induced MICA upregulation by Monocytes was highly sensitive since it was observed with as little as 4 ng/ml LPS after 15 hours and was associated with IL-6 secretion and inhibition of apoptosis indicating that Monocytes were activated and functional. To determine whether upregulated MICA expression on Monocytes was detected by NK cells we took advantage of the fact that NKG2D is down-regulated after interacting with its ligands. While NKG2D levels on NK cells were not substantially changed in cultures with unstimulated Monocytes, a marked reduction of NKG2D was observed in the presence of activated MICA-expressing Monocytes. The modulation of NKG2D did not occur when NK cells and monocytes were separated by a transwell filter, which demonstrates that the numerous cytokines produced by TLR-activated Monocytes are not responsible for the NKG2D modulation and confirmes the role of MICA in Monocyte-NK cell interaction. Importantly, TLR-induced MICA expression on Monocytes stimulated IFN-γ production of NK cells, which could be reduced by addition of blocking anti-MICA/B F(ab’)₂ fragments while isotype control had no effect. This demonstrates that the observed stimulation of NK cells was in fact specifically due to NKG2D-NKG2DL interaction. Our data indicate that NKG2D-MICA interaction provides a mechanism by which NK cells and Monocytes may communicate directly during innate immune responses to infections in humans.