Soluble HLA-I-mediated secretion of TGF-β1 by human NK cells and consequent down-regulation of anti-tumor cytolytic activity

Soluble HLA class I (sHLA-I) molecules can regulate survival of NK cells and their antitumor killing activity. Herein, we have analysed whether interaction of sHLA-I with CD8 and/or different isoforms of killer Ig-like receptors (KIR) induced secretion of transforming growth factor (TGF)-beta 1. CD8(+)KIR(-) NK cell clones secreted TGF-beta 1 upon the interaction of sHLA-I with CD8 molecule. sHLA-Cw4 or sHLA-Cw3 alleles engaging inhibitory isoforms of KIR, namely KIR2DL1 or KIR2DL2, strongly downregulated TGF-beta 1 production elicited through CD8. On the other hand, sHLA-Cw4 or sHLA-Cw3 alleles induced secretion of TGF-beta 1 by ligation of stimulatory KIR2DS1 or KIR2DS2 isoforms. TGF-beta 1 strongly reduced NK cell-mediated tumor cell lysis and production of pro-inflammatory cytokines such as TNF-alpha and IFN-gamma. Also, TGF-beta 1 inhibited NK cell cytolysis induced by the engagement of stimulatory receptors including NKG2D, DNAM1, 2B4, CD69, NKp30, NKp44 and NKp46. The IL-2-dependent surface upregulation of some of these receptors was prevented by TGF-beta 1. Furthermore, TGF-beta 1 hampered IL-2-induced NK cell proliferation but not IL-2-mediated rescue from apoptosis of NK cells. Depletion of TGF-beta 1 restored all the NK cell-mediated functional activities analysed. Taken together these findings suggest that sHLA-I antigens may downregulate the NK cell-mediated innate response by inducing TGF-beta 1 release.