Autocrine Regulation of Interferon γ in Mesenchymal Stem Cells Plays a Role in Early Osteoblastogenesis

Interferon (IFN)gamma is a strong inhibitor of osteoclast differentiation and activity. However, its role in osteoblastogenesis has not been carefully examined. Using microarray expression analysis, we found that several IFN gamma-inducible genes were upregulated during early phases of osteoblast differentiation of human mesenchymal stem cells (hMSCs). We therefore hypothesized that IFN gamma may play a role in this process. We first observed a strong and transient increase in IFN gamma production following hMSC induction to differentiate into osteoblasts. We next blocked this endogenous production using a knockdown approach with small interfering RNA and observed a strong inhibition of hMSC differentiation into osteoblasts with a concomitant decrease in Runx2, a factor indispensable for osteoblast development. Additionally, exogenous addition of IFN gamma accelerated hMSC differentiation into osteoblasts in a dose-dependent manner and induced higher levels of Runx2 expression during the early phase of differentiation. We next examined IFN gamma signaling in vivo in IFN gamma receptor 1 knockout (IFN gamma R1(-/-)) mice. Compared with their wildtype littermates, IFN gamma R1(-/-) mice exhibited a reduction in bone mineral density. As in the in vitro experiments, MSCs obtained from IFN gamma R1(-/-) mice showed a lower capacity to differentiate into osteoblasts. In summary, we demonstrate that the presence of IFN gamma plays an important role during the commitment of MSCs into the osteoblastic lineage both in vitro and in vivo, and that this process can be accelerated by exogenous addition of IFN gamma. These data therefore support a new role for IFN gamma as an autocrine regulator of hMSC differentiation and as a potential new target of bone-forming cells in vivo. STEM CELLS 2009; 27: 550-558