Engineering the pulmonary epithelium "on a chip" to investigate immune responses to the inhaled external environmental stimuli

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According to the World Health Organisation (WHO), it was estimated that 90% of the worldwide population breathes polluted air and can contributed to 4.2 million premature deaths worldwide in 2016 (WHO, 2018). The pulmonary epithelium, being the first line of defence against the external insults, are subjected to mechanical forces and contact with the inhaled agents thus can be frequently injured (Whitsett & Alenghat, 2015). The pulmonary epithelium is well known to participate in the clearance of some foreign particles by trapping them in the mucus produced by mucous cells (mainly goblet cells) and concerted movement of mucus out of the lungs by a group of ciliated cells (Davies & Moores, 2010; Knight & Holgate, 2003). Together with the help of basal cells, the pulmonary epithelium is also able to engage in immune surveillance, initiate an innate immune response and regulate lung fluids and the proximate connective tissue (Deckers et al., 2017; Knight & Holgate, 2003; Loxham, Davies & Blume, 2014). Upon damage or stimulation, the bronchial epithelium can undergo remodelling such as goblet cell hyperplasia, mucus hypersecretion, airway thickening. In addition, an inflammatory response can be initiated by the secretion of proinflammatory cytokines, chemokines specific for innate immune cells and proteases (Knight & Holgate, 2003). In chronic respiratory diseases, chronic or repeated inflammation and chronic airway remodelling are associated with the progression of various respiratory diseases such as asthma and COPD. However, the exact physiological and pathological mechanisms induced by contact of these environmental stimuli in the context of airflow shear stress in human are largely underexplored.Due to the limitations of the available experimental models, a robust model with features mimicking the airflow in the airway lumen is required. Previously, Benam and others (2016) have developed a small airway-on-a-chip which consists of a differentiated alveoli epithelium exposed to flow of air as well as a layer of endothelium and a current of media representing the blood flow. Along with other publications by other groups (not listed here), their predominant research focuses on studying the basal side of the epithelium (e.g. interactions with endothelium to promote neutrophil adherence). But the research on the apical side of the epithelium has been largely ignored. In this project, a pulmonary bronchial epithelium-on-chip model with controlled airflow-based shear stress will be developed to assist in studying the effect of environmental stimuli on the pulmonary epithelium at the tissue level in vitro. The microfluid chip will consist of a differentiated bronchial epithelium supported on a membrane with a 'breathing' or oscillating airflow on its apical side and a media flow on the basal side (Benam et al., 2016). Once successfully established, this model can reliably examine the effect of different levels of shear stress in the airway channel and the effects of individual or combinations of inhaled environmental stimuli. This will provide novel mechanistic insights in the physiological and pathophysiological responses of the airway cells to stimuli such as pollutants. This will be assessed in terms of epithelial barrier permeability, mucus production, cilia function and immunological responses, such as receptor expression as well as cytokine and chemokine production. The design of the microfluidic chips also allows direct imaging to visualise any structural changes of the epithelium as well as real-time cytokine measurements. By closely mimicking the environment that the epithelium cells are exposed to in vivo. This micro-engineered model will hopefully reduce the use of animal models in this field, shed light into organ-level responses to environmental stimuli and contribute to our understanding of the development of chronic respiratory diseases.