Forced expression of human macrophage colony-stimulating factor in CD34⁺ cells promotes monocyte differentiation in vitro and in vivo but blunts osteoclastogenesis in vitro

Objectives: Here, we tested the hypothesis that human M-CSF (hM-CSF) overexpressed in cord blood (CB) CD34⁺ cells would induce differentiation and survival of monocytes and osteoclasts in vitro and in vivo. Methods: Human M-CSF was overexpressed in cord blood CD34⁺ cells using a lentiviral vector. Results: We show that LV-hM-CSF-transduced CB CD34⁺ cells expand 3.6- and 8.5-fold more with one or two exposures to the hM-CSF-expressing vector, respectively, when compared to control cells. Likewise, LV-hM-CSF-transduced CB CD34⁺ cells show significantly higher levels of monocytes. In addition, these cells produced high levels of hM-CSF. Furthermore, they are able to differentiate into functional bone-resorbing osteoclasts in vitro. However, osteoclast differentiation and bone resorption were blunted compared to control CD34⁺ cells receiving exogenous hM-CSF. NSG mice engrafted with LV-hM-CSF-transduced CB CD34⁺ cells have physiological levels of hM-CSF production that result in an increase in the percentage of human monocytes in peripheral blood and bone marrow as well as in the spleen, lung and liver. Conclusion: In summary, ectopic production of human M-CSF in CD34⁺ cells promotes cellular expansion and monocyte differentiation in vitro and in vivo and allows for the formation of functional osteoclasts, albeit at reduced levels, without an exogenous source of M-CSF, in vitro.