Abstract
Specialized epithelial cells with a tuft of apical microvilli (“brush cells”) sense luminal content and initiate protective reflexes in response to potentially harmful substances. They utilize the canonical taste transduction cascade to detect “bitter” substances such as bacterial quorum-sensing molecules. In the respiratory tract, most of these cells are cholinergic and are approached by cholinoceptive sensory nerve fibers. Utilizing two different reporter mouse strains for the expression of choline acetyltransferase (ChAT), we observed intense labeling of a subset of thymic medullary cells. ChAT expression was confirmed by in situ hybridization. These cells showed expression of villin, a brush cell marker protein, and ultrastructurally exhibited lateral microvilli. They did not express neuroendocrine (chromogranin A, PGP9.5) or thymocyte (CD3) markers but rather thymic epithelial (CK8, CK18) markers and were immunoreactive for components of the taste transduction cascade such as Gα-gustducin, transient receptor potential melastatin-like subtype 5 channel (TRPM5), and phospholipase Cβ2. Reverse transcription and polymerase chain reaction confirmed the expression of Gα-gustducin, TRPM5, and phospholipase Cβ2. Thymic “cholinergic chemosensory cells” were often in direct contact with medullary epithelial cells expressing the nicotinic acetylcholine receptor subunit α3. These cells have recently been identified as terminally differentiated epithelial cells (Hassall’s corpuscle-like structures in mice). Contacts with nerve fibers (identified by PGP9.5 and CGRP antibodies), however, were not observed. Our data identify, in the thymus, a previously unrecognized presumptive chemosensitive cell that probably utilizes acetylcholine for paracrine signaling. This cell might participate in intrathymic infection-sensing mechanisms.
Highlights
The thymus is the site of maturation of naive T cells from immature thymocytes, which are derived from progenitors recruited from the bone marrow
Intense ChATBac-enhanced green fluorescent protein (eGFP) fluorescence was observed in scattered cells in the thymic medulla, partly forming loose clusters, with a preference for the outer medulla (Fig. 1a, b)
The present study describes a hitherto unrecognized epithelial cell phenotype that resides in the medulla of the mouse thymus and that shares several hallmark features with chemosensory cells of the airway mucosa: 1) villincontaining microvilli, 2) expression of the downstream components of the canonical bitter and sweet/umami taste transduction cascade, i.e., Gα-gustducin, phospholipase Cβ2 (PLCβ2), and transient receptor potential melastatin-like subtype 5 channel (TRPM5), and 3) choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme
Summary
The thymus is the site of maturation of naive T cells from immature thymocytes, which are derived from progenitors recruited from the bone marrow. Thymocytes are subjected to positive and negative selection processes in the cortex and medulla, respectively, for which specialized subsets of thymic epithelial cells are indispensable. Naive mature thymocytes are released into the circulation and are recruited to secondary lymphatic tissues. These processes are influenced by cholinergic signaling mechanisms, as low-dose nicotine arrests thymocyte maturation at the double-positive stage in fetal murine thymus organ culture (Middlebrook et al 2002). Whereas the extent and relevance of the cholinergic innervation of the thymus have been controversially discussed (Fatani et al 1986; Singh et al 1987; Dorko et al 2011), ACh synthesis has been ascribed to subsets of thymic epithelial cells and lymphocytes (Tria et al 1992; Rinner et al 1999; Kawashima and Fujii 2004), the definitive in situ identification of ACh producing cells in the thymus has not been unequivocally achieved as yet
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