1.4. Novel, rare airway epithelial cells:
Novel epithelial cell types are being identified using single cell RNA sequencing that might play a role in chronic airway disease. Neuroendocrine cells are strategically located at the branch of the airway where allergens and other harmful substances accumulate [16]. They serve as airway chemoreceptors that monitor airway status and release calcitonin gene–related peptide, which activates group 2 innate lymphoid cells (ILC2s) that further promote Th2 allergic responses [17, 18].
Tuft cells and M cells are highly specialized cell types within the bronchial epithelium. Tuft cells are involved in chemo-sensing of luminal signals and the initiation/regulation of immune responses [19]. In contrast, M cells sample antigenic structures and enable their transfer to lymphoid structures in the airways [20].
Ionocytes express high levels of the cystic fibrosis transmembrane conductance regulator (CFTR), which regulates TJ assembly and epithelial cell differentiation [21]. Ionocytes are the major source of CFTR expression suggesting its crucial role in regulating epithelial barrier function.
Above all, the airway epithelium is not only a physical barrier, it is also a key sensor and integrator of the surrounding environment that undergoes precise and strict regulation. It maintains the integrity of the immune system and the steady state of the airway microenvironment by well-balanced and coordinated activities of its cellular and biochemical components [11].