Response of hairy cells to IFN-α involves induction of apoptosis through autocrine TNF-α and protection by adhesion

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Abstract
Although hairy cell leukemia is uniquely sensitive to interferon-α (IFN-α), the biologic basis for this phenomenon remains unclear. Here we examine the effects of IFN-α on cultured hairy cells (HCs), taking into account the possible modifying influence of cell adhesion. We make the novel observation that therapeutic concentrations of IFN-α kill nonadherent HCs by inducing apoptosis. In keeping with the persistence of HCs in tissues during therapy, such killing was inhibited by integrin-mediated adhesion to vitronectin or fibronectin. Exposure of HCs to IFN-α resulted in a marked increase in tumor necrosis factor-α (TNF-α) secretion. Furthermore, blocking antibodies to TNF-RI or TNF-RII protected HCs from IFN-α–induced apoptosis, demonstrating that such killing was mediated by TNF-α. In the absence of IFN-α, exogenous TNF-α did not induce HC apoptosis, showing that IFN-α sensitized HCs to the proapoptotic effect of autocrine TNF-α. This sensitization to TNF-α–induced killing was attributable to suppression of IAP (inhibitors of apoptosis) production known to be regulated by the cytoprotective nuclear factor–κB–dependent arm of TNF-α signaling. Moreover, engagement of the receptors for fibronectin or vitronectin prevented this IFN-α–induced down-regulation of IAPs. Understanding of the signals involved in the combined effects of IFN-α and TNF-α and abrogation of those induced by integrin engagement offers the possibility of sensitizing other malignant cells to IFN-α–induced killing and thereby extending the therapeutic use of this cytokine.