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The luminal surface of the respiratory tract is lined by a layer of epithelium that mainly consists of multi-ciliated, club, goblet, and basal stem cells1,2. The epithelial layer serves as a primary defense mechanism of the lung, acting as a biophysical barrier that protects the underlying airway tissue against inhaled pathogens, particulates, or chemical gases. It protects the airway tissue via multiple mechanisms, including intercellular tight junction formation, mucociliary clearance, and antimicrobial and antioxidant secretion3,4. The defective airway epithelium is associated with devastating respiratory diseases, such as chronic obstructive pulmonary disease (COPD)5, primary ciliary dyskinesia (PCD)6, and cystic fibrosis (CF)7.
Advances in lung-on-chip (LOC) technology represent an opportunity to study human lung development, model various lung diseases, and develop new therapeutic materials in tightly regulated in vitro environments. For example, airway epithelium and endothelium can be cultured on opposite sides of a thin, porous membrane to mimic the gas exchanging lung tissue, allowing faithful disease modeling and drug testing8. Similarly, in vitro disease models have been created to model airway diseases in vitro, such as COPD9 and cystic fibrosis10. However, a major challenge of LOC devices is recapitulating the complex three-dimensional (3D) architecture of the lung tissue and dynamic cell-tissue matrix interactions in vitro11.
Recently, innovative tissue engineering methodologies have been developed that allow manipulation of ex vivo lung tissues12. Using these methodologies, denuded allogenic or xenogeneic tissue grafts can be prepared by removing the endogenous cells from the lung tissue via chemical, physical, and mechanical treatments13. In addition, the preserved native tissue extracellular matrix (ECM) in the decellularized lung scaffolds provide the physio-mimetic structural, biochemical, and biomechanical cues for implanted cells to attach, proliferate, and differentiate14,15.
Here, an imaging-guided bioreactor system created by combining LOC and tissue engineering technologies to allow in vitro tissue manipulation and culture of explanted rat tracheal tissues is reported. Using this airway tissue bioreactor, the protocol demonstrates selective removal of the endogenous epithelial cells without disrupting the underlying subepithelial cellular and biochemical components of the airway tissue. We next show the homogenous distribution and instantaneous deposition of the newly seeded exogenous cells, such as mesenchymal stem cells (MSCs), on the denuded airway lumen by instilling the cell-loaded collagen I pre-gel solution. In addition, by using the micro-optical imaging device integrated into the bioreactor, the visualization of the trachea lumen during epithelium removal and endogenous cell delivery is also done. Further, it is shown that the trachea and newly implanted cells can be cultured in the bioreactor without noticeable cell death and tissue degradation for 4 days. We envision that the imaging-enabled bioreactor platform, the thin film-based de-epithelialization technique, and the cell delivery method used in this study can be useful for generating airway tissues for in vitro disease modeling and drug screening.
The bioreactor includes a rectangular chamber connected to a programmable syringe pump, perfusion pump, and ventilator for culturing isolated rat trachea. The bioreactor features inlets and outlets connected to the trachea or the tissue culture chamber to separately supply reagents (e.g., culture media) to the internal and external spaces of the trachea (Figure 1). A custom-built imaging system can be used to visualize the interior of the in vitro-cultured rat trachea at the cellular level (Figure 2). The endogenous epithelium of the trachea is removed via the instillation of a detergent-based decellularization solution followed by vibration-assisted airway washing (Figure 3). Hydrogel solution, such as type I collagen, is used as a delivery vehicle for seeding exogenous cells uniformly and instantaneously across the denuded trachea lumen (Figure 4). All the materials used to construct the bioreactor and conduct the experiments are provided in the Table of Materials.