Thrombopoietin-independent effect of interferon-γ on the proliferation of human megakaryocyte progenitors

Flow cytometric study revealed that almost all CD34⁺ cells in human umbilical cord blood expressed interferon-γ receptor (IFN-γR). To clarify the precise functional roles of IFN-γR in human CD34⁺ cells, we examined the effect of IFN-γ alone and in combination with various cytokines on the growth of haemopoietic progenitor cells in CD34⁺ cells using a serum-free clonal culture. Surprisingly, IFN-γ alone supported only megakaryocyte (MK)colonies in a dose-dependent manner with a plateau level at 1000 U/ml of IFN-γ. IFN-γ at 1000 U/ml induced 10 ± 1.2 MK colonies from 1 x 10³ CD34⁺ cells, whereas thrombopoietin (TPO), interleukin [IL)-3, stem cell factor (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-γ monoclonal antibody (mAb) to the IFN-γ culture completely abrogated MK colony formation, whereas the mAb had no effect on TPO-dependent production of MK colonies. In contrast, although anti-TPO polyclonal Ab almost-completely blocked TPO-dependent MK colony formation, it failed to inhibit the generation of MK colonies induced by IFN-γ, suggesting that the observed effect of IFN-γ on the proliferation of human MK progenitor cells is independent of TPO. The addition of IFN-γ to culture with TPO or SCF significantly augmented the development of MK colonies, whereas it did not affect IL-3-dependent MK colony formation. Additionally, IFN-γ induced the increase of DNA content of cultured glycoprotein IIb/IIIa-positive megakaryocytes. These results suggest that IFN-γ may have regulatory roles inhuman megakaryocytopoiesis.