Solitary chemosensory cells in the respiratory tract of man

Abstract

Objective: Specialized sensory epithelial cells sensing noxious chemicals and initiating protective reflexes are found along the mammalian respiratory tract. Studies in mice suggested the presence of at least two populations: 1) neuroendocrine cells (marker: PGP9.5), 2) solitary cholinergic chemosensory cells (SCCC) (brush or tuft cells; markers: GNAT3, PLCβ2, TRPM5, POU2F3). In humans, a positional and sensory signaling pathway focused inventory of SCCC is yet missing. Methods: Single- and multiple-labelling immunofluorescence with relevant marker antibodies was performed on human vallate papillae (positive control) and respiratory tract (nose, trachea, lung) obtained from anatomy body donors and organ donors. Pig trachea and lung were studied for comparison; TRPM5-eGFP reporter and C57BL6/J mice served as reference. Publicly available scRNAseq data were analyzed in silico. Results: PLCβ2-antisera labelled cells in human taste buds, but not in the respiratory mucosa; TRPM5-, POU2F3- and GNAT3-positive cells were not found. Accordingly, in silico-analysis revealed minimal expression of these markers in human airway epithelial cells, opposite to mice. Guided by scRNAseq data, LRMP-antibodies (lymphoid-restricted membrane protein) were used and labelled cells in taste buds and rare slender epithelial cells along the entire airways with predominance in bronchioli (0.206 cells/mm; trachea: 0.003). Multiple-immunolabeling established them as separate entity, distinct from ciliated, secretory, neuroendocrine cells and ionocytes. In pig, the distribution is similar to human. In mice, LRMP was also localized specifically to brush cells which were restricted to extrapulmonary airways. Conclusions: These data identify chemosensory cells in human and pig airways with predominant intrapulmonary (bronchioli) localization, substantially different from mice.