The airway epithelium, the first barrier against pathogens, is endowed with active antimicrobial mechanisms and consists of different cell types, from the most abundant ciliated, goblet and basal cells to rare pulmonary neuroendocrine and tuft cells. Recently, different studies have revealed the existence of a new rare cell population, named ionocytes (1-2), characterized by a high expression of the CFTR chloride channel and the transcription factor Forkead Box I1 (FOXI1). In cystic fibrosis (CF), impaired CFTR function results in dehydration of airway surface, mucus accumulation and bacterial colonization. The airway epithelium is also the entry site for a variety of viruses, like SARS-CoV-2, who enters the cells by binding to the angiotensin-converting enzyme 2 (ACE2) (3). Interestingly, CF patients, who are particularly sensitive to respiratory viral infections, do not seem to be at risk of severe COVID-19.
Our general aim is to investigate the composition of the airway epithelium under health and disease. Specifically, we are investigating 1) expression and role of ionocytes in transepithelial ion transport; 2) ACE2 expression in CF and non-CF cells to understand whether a different expression explains the apparent resistance of CF patients to SARS-CoV-2.
Our molecular and functional studies are based on nasal and bronchial cells from CF patients and control individuals, freshly collected or cultured as differentiated epithelia.
We analyzed by immunofluorescence nasal cells from a broad panel of CF and non-CF patients. Ionocytes were easily detected in both sample types as FOXI1-positive cells and appeared more abundant in the nasal (3-5%) compared to bronchial epithelium. CFTR expression at the plasma membrane correlated with the type of CF mutation: patients with severe mutations (affecting CFTR synthesis or trafficking), showed absent or markedly decreased expression in the plasma membrane. In contrast, patients with milder mutations exhibited a clear CFTR signal in the apical membrane. In general, we found no enhanced abundance of ionocytes in CF individuals with severe CFTR mutations, which could be expected as a compensatory mechanism for the defect in CFTR function. We conducted similar analysis in patients with primary ciliary dyskinesia, another genetic disease with defective mucociliary clearance and susceptibility to bacterial infection. We detected no significant differences compared to control individuals.
Regarding ACE2, we found a substantial higher expression in nasal vs. bronchial cells. Interestingly, ACE2 appeared to be specifically localized on the apical membrane of ciliated cells, at the base of cilia. Furthermore, we found no different ACE2 expression between CF and non-CF samples, thus in contrast with the results of a recent study that reported a decreased ACE2 expression in CF epithelia (4).
The role of ionocytes in airway epithelia is still unclear. The high expression of CFTR may imply a prevalent role in chloride secretion. However, since other more abundant cell types in the epithelium also express CFTR, it is possible that CFTR in ionocytes have a more specialized function. The higher resistance of CF patients to severe forms of COVID19 does not correlate with lower ACE2 expression. Further studies are needed to clarify the underlying mechanism.