Sammendrag
B cells are critical in the generation of protective immunity. Therefore, understanding the factors that would elicit effective B cell responses is important in vaccine design. This is particularly applicable to salmon wherein most of the currently available vaccines against viral pathogens only provide suboptimal protection. In this study, we focused on the two TNF superfamily ligands BAFF (B cell Activating Factor) and APRIL (A Proliferation Inducing Ligand), whose homologs in mammals are known to be important in B cell homeostasis. Gene expression analyses in different salmon tissues showed that the immune organs had the highest basal level of BAFF and APRIL transcripts. Significant up-regulation of BAFF and APRIL genes was also found in fish infected with salmonid alphavirus 3 (SAV3), with the highest induction observed in the target organs of the virus, implying a potential involvement of these cytokines in the immune response of salmon to pathogens. Apoptosis assay revealed that head kidney leukocytes pre-treated with recombinant BAFF (rBAFF) and APRIL (rAPRIL) had higher IgM+ B cell survival. However, no significant difference in their proliferation compared to the control group were detected. Additionally, higher number of antibody-secreting cells (ASCs) was observed via ELISpot assay in both unstimulated and CpG-activated leukocyte cultures in the presence of rBAFF or rAPRIL. Data from our functional studies suggest that BAFF and APRIL play key roles in maintaining viability of fish IgM+ B cells and ASCs. Thus supporting the idea that salmon BAFF and APRIL, similar to their mammalian counterparts, are important for B cell survival. The recognition of APRIL and BAFF as important piscine B cell survival factors may help devise future vaccination strategies.
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