| Summary: | Viral diseases represent one of the major threats for salmonid aquaculture. Disease prevention by vaccination relies on the induction of protective antibody responses mediated by B-cells. Upon binding to a specific antigen during vaccination or infection, the B-cells become activated and mature into antibody secreting cells (ASC). The antibodies protect against re-infection and prevent spreading of the pathogen. The main goal of this thesis was to understand more of basic B-cell biology in Atlantic salmon (A.salmon) and study B-cell responses after in vitro stimulation and in vivo challenge experiments. Previously, the toll-like receptor (TLR) ligand CpG has demonstrated promising vaccine adjuvant activity in A. salmon. Here, CpG-stimulation of pure B-cells in culture enhanced secretion of IgM, suggesting CpG-mediated differentiation towards an ASC phenotype. A.salmon B-cells expressed nucleic acid sensing TLRs laying the basis for this stimulation. Infection by salmonid alphavirus (SAV3) causes pancreas disease in A.salmon. In this study, we used intraperitoneal (IP) infection with SAV3 as a model to characterize B-cell responses locally in the peritoneal cavity and in systemic immune tissues. IP-administration of vaccines is common in A. salmon and understanding the B-cell response in the peritoneal cavity is central. IP SAV3 infection induced peritoneal cavity B-cell responses as assessed by increased frequency of IgM+ B-cells and total IgM secreting cells (ASC). These responses were prolonged and positively correlated to anti-SAV3 antibody levels in serum. For the systemic immune sites, virus-induced changes in B-cell responses were modest. While live virus increased the levels of SAV3 specific ASCs both in the peritoneum and systemic immune sites, the response to IP-injected inactivated SAV3 in systemic sites was low and barely detectable. Collectively, the data indicate that A. salmon peritoneal cavity could serve as a peripheral immune site by providing a niche for maintenance of ASC responses. The thesis has provided basic knowledge about A.salmon B-cells and new methods for studying their function have been established. In addition, the importance of analyzing local B-cell responses is emphasized which calls for further exploration.
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