Introduction Post pneumonectomy lung growth

Introduction Post-pneumonectomy lung growth is a striking example of adult tissue morphogenesis. In many mammalian species, including humans (Butler et al., 2012; Kajstura et al., 2011), removal of one lung is associated with compensatory growth of the remaining lung to near-baseline levels (Hsia et al., 2004). In rodents, the growth occurs within days of pneumonectomy (Konerding et al., 2012). Recent studies have demonstrated that the growth of the remaining lung reflects not simply an increase in alveolar size, but an increase in the number of acetylcholine receptor (Fehrenbach et al., 2008). Although the consequences of this process have been well-documented (Hsia et al., 2004), the mechanism of compensatory lung growth is unknown. In many organs, regeneration depends not only upon progenitor cells, but also upon a microenvironment conducive to tissue growth (Nystul and Spradling, 2006; Scadden, 2006). These microenvironments—commonly referred to as “niches”—include the stromal structures and intercellular signals necessary to provide growth support. In the lung, a unique structural feature is that 90% of the organ volume is air. The alveolus, the gas exchange unit of the lung, is a small cavity surrounded by a thin epithelial lining and a delicate meshwork of capillaries. Because gas diffusion and exchange is sensitive to structural barriers, there are few stromal elements available to contribute to lung growth. Given the dramatic post-pneumonectomy growth in species such as the mouse, and the limited cellular diversity within the normal lung, it is likely that accessory cells, derived from the peripheral blood, contribute to the regenerative lung microenvironment. Previous work from our laboratory has used a parabiotic cross-circulation model to identify blood-borne cells potentially contributing to post-pneumonectomy lung growth. Complete cross-circulation between wild-type (WT) and green fluorescent protein (GFP) parabionts results in 50% of the blood cells expressing GFP (Chamoto et al., 2012a, 2012b; Gibney et al., 2012); these fluorescent cells can be tracked as they migrate into the remaining WT lung after pneumonectomy. This approach has demonstrated the migration and incorporation of CD34+ progenitor cells into the growing lung microcirculation (Chamoto et al., 2012b). Studies of alveolar macrophages (CD11c+, CD11b−) demonstrated few blood-derived cells—a finding consistent with local renewal (Chamoto et al., 2012a). An intriguing observation in the flow cytometry studies of both alveolar macrophages and endothelial cells was a dominant population of blood-derived CD11b+ cells within the growing lung. The CD11b+ molecule, also known as MAC-1 or CR3, is natively expressed on a variety of lung cells including monocytes, small macrophages, dendritic cells, granulocytes and natural killer cells (Gonzalez-Juarrero et al., 2003). CD11b is not typically expressed on the dominant leukocyte in the airspace (Matthews et al., 2007)—namely, alveolar macrophages. The rapid accumulation of the CD11b+ population suggested a functional contribution of these cells to post-pneumonectomy lung growth. In this report, we investigated the hypothesis that the normally scant stromal support for lung growth is augmented in post-pneumonectomy mice by blood-borne CD11b+ cells. Although the CD11b marker was empirically identified, the CD11b membrane molecule defines a population of cells implicated in a variety of reparative and regenerative processes (Melero-Martin et al., 2010; Ohki et al., 2010). Relevant to the morphogenetic changes associated with interstitial capillary growth and alveolar epithelial growth, we anticipated that these blood-borne leukocytes could have an accessory role in lung regeneration.
Methods
Results
Discussion In this report, we investigated the contribution of blood-borne cells to the regenerating adult lung. After murine pneumonectomy, we found that 1) CD11b+ cells were the dominant blood-borne population migrating into the remaining lung, 2) the migrating CD11b+ cells partitioned into two compartments: interstitial tissue and pulmonary airspace; 3) a genetic deficiency in the CD11b molecule (CD18−/− mutation) was associated with impaired lung growth, and 4) both tissue and airspace CD11b+ cells expressed genes associated with alveolar angiogenesis and ECM remodeling. Together, these data suggest that blood-borne CD11b+ cells represent an accessory cell population capable of contributing to adult lung growth.