Identification and characterization of a novel intraepithelial lymphoid tissue in the gills of Atlantic salmon

TLDR

Atlantic salmon gills serve as a respiratory organ and barrier, yet no lymphoid cell accumulations analogous to mammalian mucosa have been reported. The authors identified the intraepithelial tissue by cytokeratin immunohistochemistry, electron microscopy, laser capture microdissection, and RT‑PCR. They discovered a novel intraepithelial lymphoid tissue in Atlantic salmon gills, comprising MHC II⁺ T cells, which represents the first such aggregation in a teleost and may serve as a site for immune surveillance and vaccine development.

Abstract

Abstract In addition to being the respiratory organ in fish, the gills form a barrier against the external milieu. Innate and adaptive immune system components have been detected in the gills, but lymphoid cell accumulations similar to that seen in the mammalian mucosa have not been described. The present investigations revealed cell accumulations on the caudal edge of interbranchial septum at the base of the gill filaments in the Atlantic salmon. Cytokeratin immunohistochemical staining and identification of a basal membrane and desmosome cell junctions by electron microscopy showed that the cell accumulation was located intraepithelially. Major histocompatibility complex (MHC) class II + cells were detected by immunohistochemistry, and laser capture micro‐dissection and subsequent RT‐PCR analysis revealed expression of T‐cell receptor transcripts in the investigated tissue, suggesting the presence of T cells. The intraepithelial tissue reported here may be a suitable location for immune surveillance of gill infections, as well as a target site for new vaccine approaches and investigations of epithelial immunity. This is the first description of a lymphocyte cell aggregation within a teleostian gill epithelium network, illustrating a phylogenetically early form of leukocyte accumulations in a respiratory organ.