Mucosal immunology in Atlantic salmon: with focus on gills and the newly discovered interbrachial lymphatic tissue (ILT).

Fundamental knowledge on salmon and fish immunology is an essential weapon in the battle against infectious disease, posing a serious threat to the productivity of the aquaculture industry. Recently, our group discovered a lymphoid tissue in the gills of Atlantic salmon not previously described in fish. It is localized in the groove between the primary lamellas and is together with the gill filaments a major part of the gill associated lymphoid tissue. A central focus of this project will therefore be dedicated to the immunological function of this interbranchial lymphoid tissue (ILT) in gills, addressing its morphological structure, immunological features and its response towards pathogens and infection. The studies will provide important information for understanding the initial mucosal phase of infections and may help improve future vaccine strategies, and importantly, give us more information on the ILTs significance in the immune system. Following the specific aims and plan of the project, four scientific papers have so far been published in international journals. The papers all aim at characterizing the development and morphology of the ILT together with presence and stimulation of important immune markers. The work has shown that the ILT is much larger than originally described by stretching up along the primary lamellas, increasing its surface area considerably. Consequently, the ILT is now defined into proximal and distal ILT and new size estimations has concluded that it can constitute as much as 3-7% of the total gill volume. It was shown to consist of a heterogenous population of T-cells with a proposed regulatory role based on its high expression of FoxP3. However, the absence of transcripts corresponding to recombination-activating gene 1(RAG1) (essential for recombination in the thymic cortex) and autoimmune regulator (Aire) (involved in negative selection in the thymic medulla) have limited the possibility of the ILT being a primary lymphoid tissue and a functional thymus-analogue as previously proposed. Analysis of the full transcriptome fully supported and strengthened previous described characteristics of the ILT, the results also displayed differential expression of lymphoid tissue related chemokines and their receptors, shedding new light on the stromal network and its impact on orchestrating the functional role of this tissue. Based on the new data, we can confirm that the ILT consists of a tissue with no analogy to previously known lymphatic structures. The ILT in teleost fish may have evolved as a unique mucosa-associated lymphoid tissue missioned to protect the vulnerable gills. It seems to constitute a delicate lymphoid network responsible for the local immune regulation, orchestrating lymphocyte movement, tolerance and possibly antigen presentation. With characteristics reminiscent of lymphoid T cell zones, it may represent an early form of a secondary lymphoid organ.

The project will address the lymphatic structures in the gills of Atlantic salmon and their functions related to interactions between host and pathogens at site of the infection. The aim is to reveal where and how infection of some dreaded salmonid pathog ens occurs and most importantly how the lymphatic tissues with emphasis on the interbrachial lymphoid tissue (ILT) respond. Such mechanisms in the ILT have to date not been addressed, as this structure only recently was discovered by our group and its fun ction is still unknown. This ILT is localized in the gills, in the base of the filaments and stretches the whole length of the arch. It consists largely of T-cells and MHC class II+ cells in a network of epithelial cells and is cowered by a more solid lay er of flattened epithelial cells. It largely resembles an organized lymphoid tissue, and due to its strategic location, it is expected to be important for the immunological function of the gill. In this project, lymphatic tissues and host responses will b e characterised using histology and morphology combined with gene transcriptional approaches. As a part of the project we will make use of whole transcriptome sequencing to examine tissue specific expression and differential expression related to infectio n. Different developmental stages, such as fingerlings, pre-smolts and adult fish, will be sampled to describe maturation of the ILT and other lymphatic structures in the gills. Apart from characterization of the immunological function of the ILT and gill s, the overlaying goal is to strengthen the knowledge regarding the fundamental structures of lymphatic tissues of salmonides and fish in general. Viewed in a broader perspective, this information will also gain general and interesting results with refere nce to the evolvement and evolution of the immune system. Results will provide important information for understanding the mucosal phase of infections in gills and may help improve future vaccine strategies.