Constitutive expression of GM-CSF mRNA by CML blast cells is correlated with endogenous megakaryocytic colony...

Purified blast cells from peripheral blood of 12 patients, with chronic myelogenous leukemia (CML) in chronic phase, were analyzed for the constitutive expression of the granulocyte-macrophage colony-stimulating factor (GM-CSF) transcript. Seven out 12 patients exhibited the specific 1.0 kb GM-CSF mRNA. Six from these patients presented an increased level of spontaneous megakaryocytic colony formation. Using an immunocytochemical procedure, the presence of GM-CSF was detected in a large proportion (52% to 72%) of the blast cells of the three patients studied, who were selected for the high expression of GM-CSF mRNA transcripts. Because the role of GM-CSF in the regulation of human megakaryocytopoiesis is well documented, we investigated the inhibiting effect of anti-GM-CSF antibodies on the spontaneous megakaryocytic colony growth of three of the patients expressing the GM-CSF transcript. Addition of anti-GM-CSF had a high neutralizing effect ranging from 60% to 70% inhibition of endogenous megakaryocytic colony growth. As the GM-CSF synthesis by leukemic cells is often induced by interleukin-1 (IL-1), we also investigated the effect of anti-IL-1 antibody on the spontaneous megakaryocytic colony growth of the same three patients. No significant inhibiting effect was observed, showing that the role of GM-CSF in the spontaneous colony formation is not mediated by IL-1. In addition, patients who constitutively expressed the GM-CSF transcript and showed endogenous megakaryocytic colony growth were those having a significantly higher platelet count than the group of patients without GM-CSF transcript and with no endogenous megakaryocytic colonies. In conclusion, these results suggest that GM-CSF, but not IL-1, participates in the production of spontaneous megakaryocytic colony formation observed in some CML patients. The true autocrine or paracrine nature of this stimulation remains to be established.