Spontaneous megakaryocyte colony formation in myeloproliferative disorders is not neutralizable by antibodies against IL3, IL6 and GM-CSF.

Megakaryocyte progenitor growth in 42 patients with myeloproliferative disorders (MPD), including 23 essential thrombocythaemia (ET), eight polycythaemia vera (PV), six chronic myelogenous leukaemia (CML) and five primary myelofibrosis (PMF), was studied in vitro using plasma clot assay and serum-free agar culture. Spontaneous megakaryocyte colonies (CFU-MK) were found in 34/40 (80%) blood and 14/18 (77.8%) bone marrow plasma clot cultures, and also observed in 27/35 (77.1%) blood and 10/18 (55.6%) bone marrow serum-free agar cultures. In the blood of 27 patients with MPD (15 ET, four PV, four CML and four PMF) and the bone marrow of 10 patients (five ET, four CML and one PV), spontaneous colony formation was observed in both plasma clot and serum-free agar cultures. However, spontaneous CFU-MK was only found in plasma clot culture, but not in agar culture in two blood (one ET and one CML) and four bone marrow cultures (one ET, two PV, one CML). The colony numbers were greatly increased in the presence of aplastic anaemia serum (AAS) under both conditions. In 17 patients (12 ET, two CML and three PV) with spontaneous megakaryocyte colonies, anti-cytokine antibody neutralizing experiments were carried out in blood cultures. Anti-IL3, anti-IL6 and anti-GM-CSF antibody, alone or in combination, at different concentrations (1, 5 and 10 micrograms/ml), were added into plasma clot or agar cultures without exogenous stimulating growth factors. The results showed that the numbers of spontaneous megakaryocyte colonies were not significantly decreased in the presence of these monoclonal antibodies in the cultures. The data indicated that the megakaryocyte progenitor growth in MPD under in vitro conditions was heterogenous, and independent of exogenous stimulatory factors in most patients and that optimal megakaryocyte colony development in MPD still requires exogenous growth factors. Three possibilities are discussed with regard to the phenomenon that the spontaneous colony formation was not decreased with the addition of anti-IL3, anti-IL6 and anti-GM-CSF antibodies.