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].