FAS FAS LIGAND (FASL)-DEREGULATED APOPTOSIS AND IL-6 INSENSITIVITY INHIGHLY MALIGNANT MYELOMA CELLS/

IL-6 is a growth factor which interferes in the apoptosis of malignant plasma cells. Here we explore its role in the spontaneous and Fas/Fas L-regulated apoptosis of seven myeloma cell clones (MCC). MCC-2 and -7 were constitutively defective in Pas antigen in the presence of large membrane exposure of Fast, and showed a high rate of cell proliferati on irrespective of the presence of IL-6. Cytofluorimetric analysis fol lowing propidium iodide (PI) staining revealed a minimal extent of spo ntaneous apoptosis, as in other IL-6-insensitive, though Fas-positive MCC, namely MCC-3 and -5. By contrast, a regular amplitude of apoptosi s occurred in the remaining IL-6-dependent clones. Their propensity to cell death, as well as their Fast membrane expression, were promptly down-modulated by the cytokine, whereas no substantial effect was dete cted in IL-6-independent MCC. Furthermore, we investigated the quantit ative secretion of FasL. Both [3-(4,5-dimethylthiazol-2-yl)-2,5dipheny l tetrazolium bromide] (MTT) cytotoxicity assay and PI staining of WC8 lymphoblasts from a Fas-transfected mouse lymphoma, incubated with su pernatants from MCC, showed a variable cytocidal property, thus confir ming the cellular release of Fast. However, a significant elevation of Fast secretion occurred in both Fas(-) MCC, whereas molecular cloning and sequencing of Fas revealed the presence of a splicing variant, na mely Fas Exo4,6Del, in the cDNA from both MCC-3 and -5, which were pre viously demonstrated to be unresponsive to Fas stimulation. Taken toge ther, these data provide evidence that concurrence of IL-6 insensitivi ty and deregulation of apoptosis in myeloma cells reflects a high mali gnancy grade. It is suggested that the secretion of Fas splicing varia nts in Fas(+) plasma cells, as well as the over-production of FasL in Fas(-) myelomas, are differential mechanisms by which myeloma cells es cape host immune surveillance.