Human mesenchymal stem cells as a source for lung epithelial regeneration

Lung diseases are leading causes of death worldwide and, for many of them, epithelial injury is a crucial pathogenetic step. For those diseases that require lung transplant as ultimate therapy, such as cystic fibrosis, stem cell therapy has been recently suggested as alternative approach for lung epithelial regeneration. Nevertheless, this goal remains far from being realized due to the lack of understanding the identity and the role of resident stem cell populations involved in this process, as well as the finely tuned mechanisms controlling lung tissue regeneration. In addition, the potential usefulness of the administration of exogenous stem cell populations, such as mesenchymal stem cell (MSC), has been tested with contradictory and unsatisfactory results in terms of lung epithelial differentiation. Our work is based on the evidence that human MSC (hMSC) have been found not only in the bone marrow (BM-hMSC), but also in large variety of other organs including the lung; in addition, different reports suggest that the tissue of origin could be responsible of the specific differentiation properties of MSC. Thus, our aim was to verify if resident hMSC, as characterized by comparative analysis with BM-hMSC, can be isolated from the lung and induced to differentiate in vitro into respiratory epithelial cells. Our results show that human lung MSC-like populations (Lung-hMSC ) can be isolated and expanded in vitro and display the typical biological properties of all hMSC, such as immunophenotype, multilineage differentiation potential, self-renewal capacity, clonogenicity and immune regulatory mechanisms. Moreover, Lung-hMSC display similar stemness gene profile as compared to BM-hMSC, but express significative higher levels of the stemness marker Nestin both at mRNA and protein levels. Nevertheless, we found that Lung-hMSC , after retinoic acid (RA) treatment to induce epithelial differentiation, resulted more prone to undergo mesenchymal to epithelial transition (MET) as compared to BM-hMSC. This property was highlighted by the change from spindel shape to cobblestone morphology and by the statistically significant upregulation of specific epithelial genes (cytokeratin 18, tight junction protein and occludin) together with the downregulation of mesenchymal genes (vimentin, snai2 and thy1), as assessed by qRT-PCR. In vitro MET is limited to a small percentage of hMSC and the transition to a fully differentiated epithelial phenotype is not complete, as confirmed by the presence of cells coexpressing both epithelial and mesenchymal proteins. Nevertheless, the treatment with RA is capable of inducing also a partial functional epithelial polarization, more evident in Lung-hMSC than in BM-hMSC, as assessed by trans-epithelial electric resistance (TEER). All these data suggest that Lung-hMSC can be efficiently induced to undergo epithelial differentiation more easily than BM-hMSC, and this may represent the first step to plan regenerative strategies aimed to the regeneration of lung injuries.