Human NK cells constitutively express membrane TNF-alpha (mTNF alpha) and present mTNF alpha-dependent cytotoxic activity

We have evaluated the expression and the involvement of membrane-associated TNF-alpha (mTNF-alpha) in human NK cell-mediated cytotoxicity. Results fro m FCM analysis show that peripheral blood NK cells constitutively express m TNF-alpha. In contrast, mTNF-alpha expression is undetectable on resting T cells, B cells and monocytes. Western blotting analysis confirmed that fres hly purified NK cells express the 17-kDa soluble form (sTNF-alpha) and the 26-kDa transmembrane form of TNF-alpha. Stimulation with IL-2, IL-15 and IL -18 up-regulates TNF-alpha mRNA, sTNF-alpha and mTNF-alpha expression in NK cells. The role of mTNF-alpha in the cytotoxic activity of resting NK cell s has been evaluated in in vitro cytotoxic assays using freshly purified NK cells fixed with paraformaldehyde as effector cells tin order to avoid the participation of cytotoxic soluble mediators such as perforin, granzymes o r sTNF-alpha and the TNF-alpha-sensitive Fas ligand- and TRAIL-resistant ce ll line KYM-1-D4 as target cell. Results show that fixed NK cells kill the KYM-1-D4 cells and that neutralizing anti-TNF-alpha antibodies partly preve nt this effect. In contrast to the other types of peripheral blood mononucl ear cells NK cells from adult blood constitutively express functional mTNF- alpha in the absence of prior contact with target cells or activation. Thes e data demonstrate a novel mechanism of cell-mediated cytotoxicity by non-a citvated human peripheral blood NK cells.