Invited review: human air-liquid-interface organotypic airway tissue models derived from primary tracheobronchial epithelial cells-overview and perspectives

Abstract

The lung is an organ that is directly exposed to the external environment. Given the large surface area and extensive ventilation of the lung, it is prone to exposure to airborne substances, such as pathogens, allergens, chemicals, and particulate matter. Highly elaborate and effective mechanisms have evolved to protect and maintain homeostasis in the lung. Despite these sophisticated defense mechanisms, the respiratory system remains highly susceptible to environmental challenges. Because of the impact of respiratory exposure on human health and disease, there has been considerable interest in developing reliable and predictive in vitro model systems for respiratory toxicology and basic research. Human air-liquid-interface (ALI) organotypic airway tissue models derived from primary tracheobronchial epithelial cells have in vivo-like structure and functions when they are fully differentiated. The presence of the air-facing surface allows conducting in vitro exposures that mimic human respiratory exposures. Exposures can be conducted using particulates, aerosols, gases, vapors generated from volatile and semi-volatile substances, and respiratory pathogens. Toxicity data have been generated using nanomaterials, cigarette smoke, e-cigarette vapors, environmental airborne chemicals, drugs given by inhalation, and respiratory viruses and bacteria. Although toxicity evaluations using human airway ALI models require further standardization and validation, this approach shows promise in supplementing or replacing in vivo animal models for conducting research on respiratory toxicants and pathogens.

Keywords: Air-liquid-interface (ALI) airway cultures; Exposure system; Inhalation toxicology; Pathogen-host interaction; Pulmonary drug testing.

Figure 1.

Human respiratory tract is lined with region-specific cell types. Both the upper and lower respiratory tracts are shown in this illustration. The lower respiratory comprises the conducting zone for air passage and respiratory zone for gas exchange between the lung and pulmonary capillaries. The conducting airway consists of the trachea, bronchi, and bronchiole; the respiratory airway consists of the respiratory bronchioles as well as the alveolar ducts and sacs. Each region of the respiratory tract is lined with region-specific cell types as illustrated in this figure. Human airway has 23 generations of dichotomous branching, starting from the trachea as generation 0. The ALI airway tissue models discussed in this review are derived from epithelial cells harvested from generations 0 to 2 (labeled in the graph). Minor modification of the drawing published by LeMessurier et al. (2020) Front Immunol 11: 3. Copyright© 2020 LeMessurier, Tiwary, Morin, and Samarasinghe.

Figure 2.

A schematic diagram of the human ALI airway tissue model. The procedure of establishing the ALI airway tissue model is schematically illustrated on the left panel. NHBE cells are seeded onto the microporous membrane pre-coated with extracellular matrix proteins, such as collagen (stage 1). Primary cells continue to proliferate under the submerged condition until they reach complete confluence (stage 2). Differentiation is initiated by lifting NHBE cells to the air-liquid interface and feeding with differentiation medium from the basolateral side (stage 3). NHBE cells are differentiated into a pseudostratified phenotype on a microporous membrane around 4 weeks after air-lift. The apical side of the ALI culture is exposed to air; the basolateral side takes up nutrients through microporous membrane. (a) Ciliated cell; (b) mucus-producing goblet cells; (c) basal cells.

Figure 3.

Representative in vitro exposure system from Vitrocell®. (A) Vitrocell® cloud system. Configurations with single or double aerosol chambers and various insert sizes are available. The setup with double chambers (on the right) allows conducting vehicle exposures simultaneously with chemical exposures. (B) Vitrocell® Spiking System.