Expression and function of the FAS antigen in B chronic lymphocytic leukemia and hairy cell leukemia

The expression and function of the FAS antigen was analyzed in 21 patients with B chronic lymphocytic leukemia (CLL) and four with hairy cell leukemia (HCL) using a specific IgM monoclonal antibody and FACS analysis. The FAS antigen was expressed in a minority (5-41%, mean 15.6%) of the CLL cells in 10 of 21 CLL patients and this expression was not modified during spontaneous or hydrocortisone-induced apoptosis of CLL cells. In contrast, culture with gamma-interferon (gamma-IFN) upregulated the expression of FAS in all CLL patients, with 65-100% (mean 84.8%) of the cells being positive after 2 days in vitro culture. Culture with alpha-IFN induced FAS expression in 15 of 19 CLL patients tested, with 15-74% (mean 34%) of the cells being FAS+ after 2 days culture. IL-4 and IL-10, lymphokines that inhibit and promote CLL apoptosis respectively, did not modify the expression of FAS. These results from FACS analysis were consistent with FAS mRNA analysis of fresh and cultured CLL cells, using a semi-quantitative reverse transcriptase (RT)-PCR technique. Although IL-4 and IFNs prevent apoptotic cell death of CLL cells in vitro, the present results show that IFNs induce the expression of the apoptosis-inducing protein FAS. However, FAS+ CLL cells were not killed in the presence of anti-FAS monoclonal antibody (while the FAS+ Jurkat and four lymphoblastoid cell lines were killed). This resistance is not due to a mutated FAS protein, since only wild-type FAS cDNA was demonstrated in the leukemic cells of three CLL patients. In four HCL patients 34-53% (mean 44.5%) of the leukemic cells were FAS+ and they were also resistant to the anti-FAS mediated cytotoxicity. The combination of high bcl-2 protein levels and resistance to anti-FAS mediated cytotoxicity may contribute to the extended in vivo survival of CLL and HCL cells.