THROMBOPOIETIN-INDEPENDENT EFFECT OF INTERFERON-GAMMA ON THE PROLIFERATION OF HUMAN MEGAKARYOCYTE PROGENITORS

Flow cytometric study revealed that almost all CD34(+) cells in human umbilical cord blood expressed interferon-gamma receptor (IFN-gamma R) . To clarify the precise functional roles of IFN-gamma R in human CD34 (+) cells, we examined the effect of IFN-gamma alone and in combinatio n with various cytokines on the growth of haemopoietic progenitor cell s in CD34(+) cells using a serum-free clonal culture. Surprisingly, IF N-gamma alone supported only megakaryocyte (MK) colonies in a dose-dep endent manner with a plateau level at 1000 U/ml of IFN-gamma. IFN-gamm a at 1000 U/ml induced 10 +/- 1.2 MX colonies from 1 x 10(3) CD34(+) c ells, whereas thrombopoietin (TPO), interleukin (IL)-3, stem cell fact or (SCF) or IL-6 alone induced 22 +/- 4.0, 22 +/- 4.2, 4 +/- 0.6 and 0 MK colonies, respectively. The addition of anti-IFN-gamma monoclonal antibody (mAb) to the IFN?I culture completely abrogated MK colony for mation, whereas the mAb had no effect on TPO-dependent production of M K colonies, In contrast, although anti-TPO polyclonal Ab almost comple tely blocked TPO-dependent MK colony formation, it failed to inhibit t he generation of MK colonies induced by IFN-gamma, suggesting that the observed effect of IFN-gamma on the proliferation of human MK progeni tor cells is independent of TPO. The addition of IFN-gamma to culture with TPO or SCF significantly augmented the development of MK colonies , whereas it did not affect IL-3-dependent MK colony formation. Additi onally, IFN-gamma induced the increase of DNA content of cultured glyc oprotein IIb/IIIa-positive megakaryocytes. These results suggest that IFN-gamma may have regulatory roles in human megakaryocytopoiesis.