IN LEUKEMIC HEMATOPOIESIS CD34 ANTIGEN DOES NOT HAVE THE SAME SIGNIFICANCE AS IT DOES NORMAL HEMATOPOIESIS

The aim of the present study was to analyze whether or not leukemic cl onogenic cells are restricted to the CD34+ cell fraction and to invest igate the effect of IL-3 and G-CSF on blast cell populations dissected according to their CD34 reactivity. For this purpose 34 patients were studied. Patients were classified into th ree groups according to CD3 4 antigen expression: (1) cases in which all blast cells (100%) were p ositive for the CD34 Ag (n = 9); (2) cases in which all blast cells la cked the expression of this antigen (n = 10); and (3) patients in whom both, CD34 positive and negative blast cell subsets coexisted (n = 15 ). in 15 cases immunomagnetic cell selection was performed and two sub populations were separated: one, phenotypically more immature (CD34+), and another, theoretically more differentiated (CD34-/33+). in additi on, in three cases both CD34+ and CD34- blast cell subpopulations were sorted using a FACStar flow cytometer. Blast colony assays were perfo rmed using 0.9% methylcellulose and two different recombinant human he matopoietic growth factors (HGFs), IL-3 and G-CSF, were used as growth stimulants. Either, a single or a combination of the growth facto rs was added to cultures. Colony formation was observed in both 100% posi tive or 100% negative cases for the CD34 antigen as well as in the CD3 4+ and CD34- cell fractions separated by immunomagnetic selection or f low cytometry. The effect of G-CSF and IL-3 on both cell fractions was as follows: cases with a uniform population according to CD34 express ion (100% positive or negative) showed a better growth response with I L-3 especially for the CD34+ cases (87% vs 40% of CD34+ and CD34- case s, respectively). Within the CD34-/33+ selected fractions, IL-3 tended to induce a higher proliferative response than G-CSF while the opposi te was found within the CD34+ cell selected fractions. In contrast it was observed that both IL-3 and G-CSF induced a higher PE on the CD34- blast cells (both selected and 100% negative), although the differenc e was not statistically significant. The existence of a possible syner gistic effect (SE) between HGFs was also explored. Overall, a synergis tic growth was observed in nine out of the 13 selected cases studied a nd this effect could be seen in both CD34- or CD34+ blast cell fractio ns. The analysis of the complete phenotypic characteristics of these c ells revealed that cell fractions showing SE were more immature accord ing to the expression of CD15 and HLA-DR antigens. We can conclude tha t in leukemic hematopoiesis, CD34 antigen expression does not have the same significance as it does in normal hematopoiesis since clonogenic cells are not restricted to the CD34+ acute myeloid leukemia (AML) bl ast cell fraction. Moreover, our study shows that the heterogeneous re sponse to HGFs observed in AML patients may be associated with the exi stence of immunophenotypically different blast cell subsets. (C) 1997 Elsevier Science Ltd.