Effects of interferon-gamma and tumour necrosis factor-alpha on CD95/Fas ligand-mediated apoptosis in human blood eosinophils.

Many cells, including eosinophils, express CD95 (Fas), a surface receptor that mediates apoptosis when ligated by specific antibodies or its natural ligand, Fas ligand (FasL). As apoptosis may play an important role in the regulation of tissue eosinophilia, factors that modulate eosinophil sensitivity to apoptosis are of great interest. It has previously been shown that interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) together increase CD95 surface expression on eosinophils. However, the functional consequences of this increase in CD95 expression have not been demonstrated in detail. We therefore investigated whether the increase in CD95 expression mediated by IFN-gamma/TNF-alpha indeed translates into increased, FasL-mediated apoptosis of eosinophils. For this purpose, purified eosinophils from normal donors were incubated with different concentrations of FasL and induction of apoptosis was assessed by annexin-V/propidium iodide assay. Unlike Jurkat cells, which became apoptotic within 2 h after incubation with FasL, an increase in eosinophil apoptosis could first be dedicated after 6 h incubation with FasL. Prestimulation with IFN-gamma/TNF-alpha for 24 h significantly enhanced FasL-induced apoptosis in eosinophils. This increase in CD95/FasL-mediated apoptosis was correlated with an IFN-gamma/TNF-alpha-mediated increase in CD95 expression. From these findings we conclude that the combination of IFN-gamma and TNF-alpha enhances CD95 expression, which results in an increase in FasL-mediated apoptosis of eosinophils in vitro.