UBM scaffold material prevented bleomycin-induced pulmonary fibrosis regardless of the form of the material (particulate or digested). UBM treatment significantly reduced the histologic presentation of fibrosis, such that there was no significant difference between the histologic appearance of the lungs in animals treated with bleomycin and UBM as compared to those treated with the saline alone. More impressively, UBM treatment significantly prevented the development of bleomycin injury even when administered 1 day after bleomycin exposure in mice. Less than 1% of bleomycin intratracheally instilled in the lungs of mice is present 24
h after injection.33
Thus, UBM can attenuate the severity of fibrosis even when administered after the bleomycin injury has occurred. Therefore, these novel findings demonstrate the immense potential for the use of UBM as a novel therapy for pulmonary fibrosis.
It is clear that the attenuation of fibrosis in response to UBM was not due to neutralization of the bleomycin when co-administered with these compounds as cell culture studies showed that UBM did not prevent cell death. Further, animals treated with bleomycin and ECM products had the same increase in the number of inflammatory cells and similar cellular composition in the BALF as animals treated with bleomycin alone. These results suggest that UBM limited bleomycin-induced fibrogenesis and did not do so by interfering with the direct injurious effects of bleomycin.
The digested form of UBM had the same protective effect as the lyophilized particulate form in vivo
. This suggests that the composition of the UBM and its degradation products may be more important to the repair process than the specific ultrastructure of the particulate form of UBM.25
UBM is known to possess basement membrane structure that is preserved through the process of making a powder.11
The presence of a basement membrane structure has been shown to promote healing in various organs, including the lungs, by providing guidance for re-epithelialization and separating the epithelium from the interstitial connective tissue.34
Chemotaxis and wound healing assays performed in the present study showed that the digested form of UBM could promote migration of airway epithelial cells and also could stimulate wound healing in the presence of bleomycin. It is possible that small peptides from the degradation of laminin and collagen IV in the basement membrane increase the migration of epithelial cells.
Many studies have noted that degradation of the UBM is an important component of the host response that leads to site-appropriate tissue remodeling. Previous studies have shown that UBM degradation products recruit progenitor cells to the site of remodeling,13,15,35
and provide bacteriostasis.14
On the other hand, studies have found that the oxidative fragmentation of ECM components, such as heparan sulfate, collagen, syndecan-1, and hyaluronan, influences the development of fibrosis.31,36–38
As the UBM scaffold material is a mixture of ECM proteins and other substances, such as growth factors,39
it is likely that this complex composition of ECM promotes healing that outweighs the detrimental effects of the proteoglycans and glycosoaminoglycans degradation products in these ECM-based materials.
Although this study clearly illustrates that UBM attenuates bleomycin-induced pulmonary fibrosis, the mechanism by which it does so is still unclear. Current evidence is presented to suggest that the UBM plays a role in epithelial repair, but additional studies are warranted to fully understand this phenomenon. The current work also shows that treatment of UBM with or after bleomycin showed an elevated number of immune cells, predominantly macrophages. In previous studies in which UBM scaffolds were used to bridge a defect in the rat abdominal wall, site-specific tissue repair was associated with a predominately M2 macrophage phenotype during the first month after surgery.18–20
Within the lung, the M2 macrophage phenotype has been reported to be associated with the onset of fibrosis.40,41
Therefore, it is likely important to fully characterize the immune response to UBM in this model as it could provide evidence that the ability for the UBM to modulate the immune response may be tissue dependent. The phenotype of the macrophages could also play a role in the matrix metalloproteases that are present within the lung to degrade the instilled particulate ECM and to participate in remodeling of the lung parenchyma.
Finally, UBM was selected for investigation because it contains a basement membrane component and has been shown to be beneficial for other airway applications. Organ-specific ECM derived from the trachea and lung has recently been explored for airway repair.42–48
In the lung models, the acellular lung matrix with its three-dimensional architecture and composition largely intact promoted site-specific cell differentiation and retained similar mechanical characteristics.42,45–47
It is possible that an airway-derived ECM may be even more effective for prevention of fibrosis than a heterotopic ECM source due to the organ-specific signaling molecules that would be present within the ECM.