Comparative histopathology of mucosa associated lymphoid tissue in the gills of Atlantic salmon Salmo salar and southern bluefin tuna Thunnus maccoyii

The recent identification of an interbranchial lymphoid tissue (ILT) revealed that an organized mucosa-associated lymphoid tissue (MALT) was present in the gills of salmonids. Due to its location in the gills, and lymphoid nature, it was hypothesized that the ILT could be affected by amoebic gill disease (AGD). AGD is caused by a protozoan parasite, Neoparamoeba perurans. This disease is considered a serious problem for the health and welfare of marine farmed salmonids. AGD affects the gills of fish, causing significant pathology, such as hyperplasia of the lamellae leading to lamellar fusion. This research, therefore, aimed to understand the effects of AGD in the ILT of Atlantic salmon (Salmo salar) using histological techniques. Two studies were undertaken to evaluate the effects of AGD in the structure and cellularity of the ILT. In the first study, Atlantic salmon were exposed to N. perurans trophozoites, and sampled prior to exposure and at 3, 7, 14 and 28 days post-exposure. Histology was performed for the first experiment, and the area of the proximal ILT and cell density were quantified. It was found that AGD caused changes in the morphology of the ILT, which was enlarged at 28 days post-exposure to N. perurans when compared to controls. Proliferating cell nuclear antigen (PCNA) immunostaining revealed that epithelial hyperplasia was the most likely factor contributing to the ILT enlargement in the affected fish. Lymphocyte density in the ILT of affected fish was significantly lower 28 days post-exposure. In the second study, fish were tagged with passive integrated transponder (PIT) tags, exposed four times to N. perurans trophozoites and treated following each exposure in freshwater. This experiment intended to mimic the infection pattern in an aquaculture environment. At the end of the experiment, the effects of exposure time (either single infection or repeated infections) and lesion presence on the ILT were assessed. Changes in the T-cell population were primarily associated with lesion presence. Histological assessments suggested observed changes in the ILT could be indicative of different AGD pathology stages with different cell types present in the ILT. The presence of organized MALT in the gills of non-salmonid teleosts has not been confirmed. However, it is likely that a similar structure may be present in the gills of other fish species. To investigate this hypothesis, the gills of a Scombridae, the southern bluefin tuna (SBT) (Thunnus maccoyii), a species with both high economic and recreational value in Australia, was investigated. Histological observations of the gill indicated the filamental mucosa overlaid a tissue composed mainly of lymphocytes and eosinophilic granulocytes embedded in a meshwork of epithelial cells. A basement membrane underlaid the lymphoid cell accumulation, separating it from the gill connective tissue. To the best of our knowledge, this is the first histological description of a gill associated lymphoid tissue (GIALT) in the gills of SBT. Overall, the results of this research have provided important information regarding the ILT during AGD infection in naive and Atlantic salmon repeatedly exposed to N. perurans. It was found that the ILT is a plastic tissue, changing its size and shape during AGD. Additionally, a difference in the cell population of the ILT was related to AGD severity. This research is also the first to identify a GIALT in the gills of SBT. The major difference between the lymphoid tissue in the SBT and the ILT in salmonids was the presence of a population of eosinophilic granulocytes in the lymphoid tissue present in the gills of SBT. These results aid in the understanding of the GIALT in both Atlantic salmon and SBT. The information acquired in this project will be useful for further investigation of the ILT during AGD, mucosal immunology and mucosal vaccine development. Furthermore, the characterization of an GIALT in the gills of SBT provides novel information that could relate to the phylogeny of MALTs in vertebrates.