Intracellular Fas ligand is elevated in T lymphocytes in severe aplastic anaemia

Increased expression of Fas receptor by haemopoietic progenitors in aplastic anaemia (AA) suggests that excessive apoptosis contributes to multilineage bone marrow (BM) failure. To investigate the role of Fas ligand (FasL) in triggering progenitor cell death, we examined FasL levels in T lymphocytes of patients with severe untreated AA (n = 8). Expression of FasL on the surface of CD3+ cells was not detectable. However, flow cytometric analysis of saponin-permeabilized cells demonstrated higher levels of intracellular FasL in AA than in normal T cells (P < 0.005), both prior to and following activation with phytohaemagglutinin. Confocal microscopy revealed that FasL-specific signals overlapped with cathepsin D staining, indicating that intracellular FasL is stored in lysosomal granules. Levels of intracellular FasL in patients examined 1 month after immunosuppression with antilymphocyte globulin and cyclosporin A were lower than prior to treatment. The caspase inhibitors, DEVD and zVAD, enhanced colony formation and prolonged survival of AA BM cells in liquid cultures by about 10-fold (P < 0.05). Taken together, these data provide further evidence that apoptosis by the Fas/FasL system plays a role in the depletion of stem cells in AA.