B-cell distribution and composition in the gills and the interbranchial lymphoid tissue (ILT) of the Atlantic salmon (<i>Salmo salar</i> L.)

Fish lack organized mucosa-associated lymphoid tissues as found in birds and mammals. However, a distinct aggregation of lymphoid cells, the interbranchial lymphoid tissue (ILT), was identified in the gills of the Atlantic salmon (Salmo salar) in 2008. Since then, additional structures of similar character have been discovered. The function of the ILT remains elusive, and research has focused primarily on its T cell content. Here, we address the B cell composition of the ILT. Gills of Atlantic salmon from two developmental stages (avg 153 g and avg 1,717 g) were examined from both vaccinated and unvaccinated fish for immunoglobulins IgM, IgT, and IgD, as well as for Activation-Induced Cytidine Deaminase (AID) and Paired Box 5 (Pax5) mRNA by morphological methods. There was an abundance of Ig-transcript+ cells throughout the gill epithelium, with IgD mRNA+ cells being the most numerous. Larger fish had a significantly higher number of IgM mRNA+ cells than the smaller fish, whereas vaccination had no significant impact on any of the isotypes. The immunofluorescence results of Igs paired with Pax5 ISH showed B cells in different stages of maturation within the ILT. Additionally, scattered AID+ cells were present within the pILT. In contrast to what was previously assumed, we show that B cells are abundant in the ILT.