A bursa of Fabricius structural analogue in the teleost fish Atlantic salmon

Abstract

Knowledge of the construction of immune organs of different species is important to understand the evolution of immunity. In birds, the bursa of Fabricius is a cloaca-based lymphoid organ that has been vital for our current understanding of the adaptive immune system of vertebrates. So far, this structure has not been described in other species. Here, we show a substantial lymphoepithelial tissue in a previously undescribed bursa in the Atlantic salmon, situated analogous to the placement of the avian bursa of Fabricius, also developing from the ectoderm. Investigations of three different age groups of Atlantic salmon revealed a dynamic lymphoepithelium that regressed following sexual maturation. This is similar to the situation in birds. Morphological investigations including dissection, radiography, in situ hybridization and immunohistochemistry revealed presence of T and B cells embedded within a CCL19-positive lymphoepithelium. CCL19 expression is indicative for lymphoid organs, and thus we suggest that the salmonid bursa should be regarded as such. We found however no evidence for primary lymphoid organ functions including recombination events in the lymphocytes, this contrasting the situation in birds. Our results demonstrate the presence of a salmonid lymphoepithelial compartment with anatomical and developmental similarities to early stages in the organogenesis of the avian bursa of Fabricius. Such information provides novel and exciting possibilities both for our understanding of the function of the immune system in salmon and for our understanding of the development of lymphoid structures and niches in vertebrates.