Influence of reverse signaling via membrane TNF-alpha on cytotoxicity of NK92 cells.

Membrane tumor necrosis factor-alpha (mTNF-alpha) serves as a receptor transducing signals into mTNF-alpha-bearing cells. Among human peripheral blood mononuclear cells, natural killer (NK) cells have been reported to be the only cell type constitutively expressing mTNF-alpha, which is involved in the cytotoxicity of resting NK cells. Using an IL-2-dependent human NK cell line, NK92, which constitutively expresses mTNF-alpha, we examined the effect of reverse signaling via mTNF-alpha on cellular activities. When the cells were prestimulated with soluble TNFR1 (sTNFR1) which activated mTNF-alpha-mediated reverse signaling, the cytotoxicity of NK92 cells was significantly increased. Further investigation demonstrated that prestimulation with sTNFR1 augmented exocytosis and mRNA transcription of two cytotoxic molecules, perforin and granzyme B, which could serve as underlying molecular mechanisms by which mTNF-alpha-mediated reverse signaling promoted cytotoxicity of NK cells toward K562 cells. On the other hand, pretreatment of NK92 with sTNFR1 boosted the expression of FasL and TNF-alpha, including both the secretory and membrane forms. These molecules also contribute to the NK-mediated cytotoxicity, although K562 cells are Fas-negative and sTNF-alpha-resistant. Interestingly, the mTNF-alpha reverse signaling was found to act synergistically with IL-2 on NK-mediated cytotoxicity. This synergy markedly promoted the production of secretory as well as membrane cytotoxic molecules which may be responsible for the enhanced NK92-mediated cytotoxicity. Our observations suggest that, via reverse signaling, constitutively expressed mTNF-alpha may sensitize NK cells to activating stimuli, such as IL-2, resulting in increased NK-mediated cytotoxicity through promoting the production of multiple cytotoxic effector molecules.