New advances in fish immunology: Characterization of a newly discovered lymphoid organ and its involvement in immune responses.

Aquaculture plays a significant role in our food supply, providing almost half of the fish for human consumption. Unfortunately, the sustainability of this industry is constantly threatened by infectious diseases, raising concerns about animal welfare and food safety. Despite the importance of finding new therapies and vaccines, our limited understanding of the fish immune system is hindering progress. This particularly applies to the branchial cavity region, even though this area serves as a major portal of entry for pathogens and is involved in many diseases affecting farmed fish. It is in this region that we recently discovered a tonsil-like lymphoid organ, named "Nemausean Lymphoid organ" (NELO), in zebrafish and salmon. Located in the lower region of the branchial cavity and connected to the gill immune system, NELO contains numerous immune cells and shows involvement in immune responses. This discovery suggests NELO plays an important role in the fish defence against infectious diseases. The main objective of this project is to investigate the role of NELO in immune responses and its significance for fish diseases and vaccines in aquaculture. To achieve our goal, we will take advantage of the zebrafish model to facilitate our research before further opening our investigations to salmon. We will first study key characteristics such as its cell composition, how it develops, and its functions. For this we will use advanced imaging techniques and different technologies to reveal markers of immune functions at the protein and RNA levels. Additionally, collaboration with international partners will allow for the study of NELO's involvement in immune surveillance during infections and vaccines. The outcomes of this project will enhance our understanding of the fish immune system, contribute to addressing the challenge of infectious disease in aquaculture, and may on the long term provide new insights for human health.

Aquaculture faces major challenges posed by infectious diseases. Despite the importance of these issues, our knowledge of fish immunology, which is a crucial defense against infections and the foundation of vaccination strategies, is surprisingly limited. This is especially true concerning the branchial cavity region, even though it represents a major portal of entry for pathogens. We recently used the well-developed zebrafish model to explore the lymphoid organization of the gills and their surrounding tissues. We recently discovered a new mucosal lymphoid organ interconnected with the gill immune network, which we named the Nemausean Lymphoid Organ (NELO). We found that NELO is composed of numerous immune cells (T cells, B cells, and antigen-presenting cells) embedded in a complex network of supporting cells. Whereas it lacks key markers of primary lymphoid organs, it displays several other features of secondary lymphoid organs. Interestingly, the NELO of Salmon shows the same 3D organization, interconnections with gills, and abundance of T cells as the zebrafish NELO. The aim of this project is to determine the function of NELO and its relevance for fish diseases and vaccines. We will use the zebrafish and salmon models in a comparative and cost-effective approach. First, we will precisely define key features such as its entire cell composition, and functions; for this we will combine high-resolution 3D multiplex immuno-histology (immunofluorescence microscopy / in situ hybridization), single-cell RNA sequencing, and functional assays. Major findings in zebrafish will be compared to the investigations in salmon. Then, with collaborators from a European consortium (NucNanoFish), we will inspect antigen-sampling and immune reactions from NELO upon exposure to different diseases and vaccine treatments. Collectively, our data will greatly enhance our understanding of the fish immune system and be important for the aquaculture industry.