KLRK1 and rheumatoid arthritis: Researchers found that fibroblast-like synoviocytes in RA (RA-FLS) expressed ligands for NK cell receptors and stimulated degranulation of the human NK cell line Nishi cells towards RA-FLS, in which NKG2D, DNAM-1, NKp46, and NKp44 were the key activating receptors involved (28).