PTGS2 and neoplasm: In addition to TGF-β, there are soluble immunosuppressive metabolites, such as lactic acid, kynurenine and PGE2, in tumor microenvironments; therapeutic strategies that target these metabolites, through inhibition of lactate dehydrogenase, indoleamine 2,3-dioxygenases, and cyclooxygenase 2, respectively, may invigorate NK cell-based tumor immunosurveillance (99, 136, 137).