Mechanisms of bone marrow progenitor cell apoptosis in aplastic anaemia and the effect of anti-thymocyte globulin: examination of the role of the Fas-Fas-L interaction

The mechanism of action of anti-thymocyte globulin (ATG) in aplastic anaemi a (AA) is complex. Bone marrow (BM) CD34(+) cells in AA have been shown to be more apoptotic and have a higher expression of Fas antigen (Fas-ag) than in normal donors. The aims of this study were to delineate further the mec hanism for increased bone marrow progenitor cell apoptosis in AA and invest igate the effects of ATG on apoptosis and Fas-ag expression. BM was obtaine d from six normal donors and 10 untreated AA patients. We confirmed that AA BM CD34(+) cells were more apoptotic than normal donor cells (P = 0.002). Following treatment with ATG, the mean percentage reduction of apoptosis wa s 34% (9.2-65.9%). BM from 30 AA and 10 normal donors was then stained for CD34, Fas-ag and 7-AminoActinomycin D. The proportion of CD34(+) Fas(+) cel ls was higher in untreated AA (P = 0.0001) than in normal donors. Results a lso showed that the majority of CD34(+) Fas(+) cells were apoptotic/dead in normal donors (mean 81%) and AA (88%), indicating that Fas is involved in apoptosis of CD34(+) cells. In contrast, the majority of CD34(+) Fas(-) cel ls in normal donors were live (mean 91%), while two patterns emerged in unt reated AA. In seven patients, the majority of cells were live, however, in the remaining eight patients, the majority of cells were apoptotic/dead, su ggesting an alternative mechanism for apoptosis in addition to Fas-ag. Fina lly, we have shown that in vivo ATG treatment reduced the expression of Fas -ag on AA BM CD34(+) cells.