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Double staining of bone marrow cells for CD13 and CD33 leucocyte differentiatian antigens and for DNA content has allowed us to evaluate the proliferative capacity of myelopoiesis in patients with myelodysplastic syndromes (MDS) using flow cytometry. By analysing 39 patients (15 RA/RAS, 14 RAEB and 10 RAEB-t) and eight normal controls, we found significant differences in both the percentage of cells positive for these immature myeloid antigens between the FAB groups as well as in the fractions of CD13 and CD33 positive cells in S or S-G2M phase of the cell cycle. Moreover, a clear decrease in the immature myeloid cell proliferative activity upon progression within the FAB groups was evident. Finally, we found a significant negative association between the percentage of myeloblasts in the bone marrow and the proliferative activity of the immature myeloid cells, indicating that the block in differentiation in MDS patients might be coupled to a simultaneous block in proliferation, especially in advanced stages. These data suggest that the use of double parameter assays in the longitudinal follow-up of MDS patients might yield new information about the biology of MDS.

Evaluating the proliferative activity of the immature erythro- and myelopoiesis as well as the mature myelopoiesis in 21 MDS patients and 14 healthy controls by simultaneously staining bone marrow cells for surface phenotype and DNA content, we found the percentages of proliferating S-phase cells in the early stage of MDS were higher. With disease progression evaluated by the FAB classification this parameter decreased significantly for both the immature myelo- and erythropoiesis. Evaluation of the proliferative activity of the mature myelopoiesis defined by the CD66 antigen revealed no difference between the normal controls and the MDS patients. Using another assay simultaneously labelling bone marrow cells for three leucocyte differentiation antigens during treatment with GM-CSF and low-dose AraC the cells clearly differentiated in one case. In another patient the disease seemed to progress as evaluated by cells only expressing immature antigens. The above mentioned immunophenotypic changes persisted at least one month after termination of treatment. In conclusion, the evaluation of proliferation and differentiation of leucocyte subsets using multiparameter flowcytometric assays in myelodysplastic patients from different FAB groups before as well as during treatment with haemopoietic growth factors may prove valuable in the future.

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