EMBRYONIC STEM CELLS/INDUCED PLURIPOTENT STEM CELLS Autocrine Regulation of Interferon γ in Mesenchymal Stem Cells plays a Role in Early Osteoblastogenesis

Interferon gamma (IFNγ) 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 IFNγ inducible genes were upregulated during early phases of osteoblast differentiation of human mesenchymal stem cells (hMSC). We therefore hypothesized that IFNγ may play a role in this process. We first observed a strong and transient increase in IFNγ production following hMSC induction to differentiate into osteoblasts. We next blocked this endogenous production using a knockdown approach with siRNA 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 IFNγ 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 IFNγ signaling in vivo in IFNγ receptor-1 knock-out (IFNγR1) mice. Compared to their wild-type littermates, IFNγR1 mice exhibited a reduction in bone mineral density. As in the in vitro experiments, MSC obtained from IFNγR1 mice showed a lower capacity to differentiate into osteoblasts. In summary, we demonstrate that the presence of IFNγ plays an important role during the commitment of MSC into the osteoblast lineage both in vitro and in vivo, and that this process can be accelerated by exogenous addition of IFNγ. These data therefore support a new role for IFNγ as an autocrine regulator of hMSC differentiation and as a potential new target of bone forming cells in vivo.