Profiling Atlantic salmon B cell populations: CpG-mediated TLR-ligation enhances IgM secretion and modulates immune gene expression

Abstract

While TLR-activated pathways are key regulators of B cell responses in mammals, their impact on teleost B cells are scarcely addressed. Here, the potential of Atlantic salmon B cells to respond to TLR ligands was shown by demonstrating a constitutive expression of nucleic-acid sensing TLRs in magnetic sorted IgM⁺ cells. Of the two receptors recognizing CpG in teleosts, <i>tlr9</i> was the dominating receptor with over ten-fold higher expression than <i>tlr21</i>. Upon CpG-stimulation, IgM secretion increased for head kidney (HK) and splenic IgM⁺ cells, while blood B cells were marginally affected. The results suggest that CpG directly affects salmon B cells to differentiate into antibody secreting cells (ASCs). IgM secretion was also detected in the non-treated controls, again with the highest levels in the HK derived population, signifying that persisting ASCs are present in this tissue. In all tissues, the IgM⁺ cells expressed high MHCII levels, suggesting antigen-presenting functions. Upon CpG-treatment the co-stimulatory molecules <i>cd83</i> and <i>cd40</i> were upregulated, while <i>cd86</i> was down-regulated under the same conditions. Finally, <i>ifna1</i> was upregulated upon CpG-stimulation in all tissues, while a restricted upregulation was evident for <i>ifnb</i>, proposing that salmon IgM⁺ B cells exhibit a type I IFN-response.