Effects of antithymocyte globulin on bone marrow CD34+cells in aplastic anaemia and myelodysplasia

The mechanism of action of antithymocyte globulin (ATG) in the treatment of aplastic anaemia (AA) and myelodysplastic syndromes (MDS) is poorly unders tood and may involve many different mechanisms. The aim of this in vitro st udy was to investigate further the effect of ATG on haemopoietic progenitor cells. A total of 16 patients (10 AA and 6 MDS) and 12 normal control subj ects were studied. Purified bone marrow (BM) CD34+ cells were cultured in c ommitted progenitor assay in the presence of ATG and autologous serum, then scored on day 14 for granulocyte-monocyte colony-forming units (CFU-GM) an d erythroid colonies. ATG was found to be inhibitory to haemopoietic progen itor cells at high concentrations (1000 mu g/ml and 100 mu g/ml). This was confirmed by CD34-FITC and 7AAD staining of purified normal CD34+ cells aft er overnight incubation with ATG. In contrast, at lower doses (0.1-10 mu g/ ml), ATG produced an increase in colony growth in most normal, MDS and AA B M CD34+ cells. The greatest effect was in patients with non-severe AA, in w hom the greatest increase in CFU-GM was seen at 0.5 mu g/ml (P < 0.02) and 0.1 mu g/ml (P = 0.02) and erythroid colonies at 0.1 mu g/ml (P < 0.05). Se rum ATG levels peaked during infusion to levels that were found to be toxic to haemopoietic progenitor cells in vitro and fell thereafter to levels th at were associated with the highest colony numbers (0.1 and 0.5 mu g/ml) in vitro. These results suggest that an increase in haemopoietic progenitor c ells by ATG may be one of several important mechanisms for haematological r ecovery in AA and MDS.