Chronic overexpression of membrane‐bound flt3 ligand by T lymphocytes in severe aplastic anaemia

Summary. Aplastic anaemia (AA) is an immune-mediated bone marrow failure associated with high serum levels of flt3 ligand (FL). We examined expression of the membranebound isoform of FL in peripheral blood and bone marrow cells from AA patients at diagnosis (na 16) and after immunosuppressive (IS) treatment (na 36). Flow cytometry demonstrated strongly increased FL levels on the cell surface of T lymphocytes in AA relative to normal controls (P , 0·0001). T-cell-specific expression of membranebound FL was confirmed by confocal microscopy. FL mRNA and total cellular FL protein levels were increased about threefold. Overexpression of FL in AA was observed for up to 20 years after IS treatment. FL levels correlated inversely with CD34 1 cell numbers and the colony-forming ability of AA bone marrow (Ra 20·68 and 20·85 respectively). Histological examination of spleen specimens and bone marrow biopsies gave no evidence of degeneration or fibrosis due to prolonged exposure to high FL. Levels of membrane-bound FL were not increased in autoimmune diseases (na 23), including rheumatoid arthritis and lupus erythematosus, nor in graft-versus-host disease (na 8). Chronic overexpression of FL on the surface of T lymphocytes in AA, but not in other T-cell-mediated disorders, suggests that membrane-bound FL plays a role in cell‐cell interactions in bone marrow failure and may be important for long-term haemopoietic recovery.