KLRK1 and neoplasm: IL-33-stimulated macrophages highly produce MMP-9, which proteolytically trims activating receptor natural killer group 2, member D (NKG2D) and its ligands MHC class I polypeptide-related sequence A/B (MICA/B) on the surface of tumor-infiltrating lymphocytes and melanoma cells, and thus impedes the immune surveillance of tumor-infiltrating lymphocytes (197).