HIMF and Stem Cells Figure 3. Both chronic hypoxia and pulmonary HIMF gene transfer recruit BMD cells to the pulmonary vasculature. (A–C, J–M) Light micrograph of fluorescence images to show blood vessel structure. Frozen sections from normoxic (20.8% O2) (D, N), hypoxic (10.0% O2) (E, O), and AAV-HIMF treated (2.561010 VP) (F, P, Q) lungs were stained with a rabbit anti-GFP polyclonal antibody that was visualized by an FITC-conjugated goat anti-rabbit IgG antibody (green). (G–I, R–U): Differential interference contrast images of light and fluorescence images to show structure. A–L, N– P, R–T: Scale bar: 50mm. M, Q, U: Scale bar: 20mm. doi:10.1371/journal.pone.0011251.g003 and CD34. This profile indicates that the cells are likely mesenchymal in origin and have the capacity to differentiate into myofibroblast-like cells and possibly vascular smooth muscle cells. We also show that HIMF has a direct effect on HMSCs by increasing cell migration in a PI-3K-dependent manner. Together, these findings demonstrate that HIMF/FIZZ1/RELMa plays an important role in the recruitment and engraftment of BMD cells in pulmonary vascular remodeling. We have previously described HIMF expression in proliferating cell nuclear antigen (PCNA)-positive cells, vascular smooth muscle cells, and endothelial cells of the remodeling vasculature in animals with chronic hypoxia-induced PH [3]. We have also demonstrated that AAV-HIMF pulmonary gene transfer leads to HIMF expression in the vascular smooth muscle and endothelium of lung vessels (all sizes), bronchial epithelial cells, and alveolar type II cells [7]. The HIMF expression pattern in bone marrow transplant recipients used in this study was consistent with our previous work that showed increased HIMF in the actively dividing pulmonary vascular endothelial and smooth muscle cells following hypoxic exposure or pulmonary AAV-HIMF treatment [3,7]. We also have recently demonstrated that HIMF plays a key role in hypoxia-induced pulmonary vascular remodeling; we can induce pulmonary vascular remodeling and the hemodynamic and cardiac hypertrophic changes of PH by pulmonary gene transfer of HIMF [7]. In the current study, we demonstrate that hypoxia and intranasal instillation of AAV-HIMF induce the same pattern of muscularization of small pulmonary arteries. It is largely unknown whether the pulmonary vascular remodeling involves the proliferation of resident vascular cells, transition of other resident lung cells to stem cells critical to remodeling, or recruitment of BMD cells and stem cells to the lung. This is an area of considerable controversy. Many recent studies have shown that BMD cells are involved in tissue remodeling and repair of several organs, including the lung [20]. These cells have also been shown to be localized in atherosclerotic lesions of the vasculature [21,22]. In fact, human patients who PLoS ONE | www.plosone.org 6 June 2010 | Volume 5 | Issue 6 | e11251