KLRK1 and rheumatoid arthritis: However, there are no reports on the relationship between MDSCs and NK cells in RA, and Nausch et al. (60) found that in RMA-S tumor mice, M-MDSCs could express retinoic acid early inducible-1 (RAE-1) ligand to interact with NKG2D ligands on NK cells and activate NK cells to produce large amounts of IFN-γ.