BackgroundThe salmon louse (Lepeophtheirus salmonis Krøyer), an ectoparasitic copepod with a complex life cycle causes significant losses in salmon aquaculture. Pesticide treatments against the parasite raise environmental concerns and their efficacy is gradually decreasing. Improvement of fish resistance to lice, through biological control methods, needs better understanding of the protective mechanisms. We used a 21 k oligonucleotide microarray and RT-qPCR to examine the time-course of immune gene expression changes in salmon skin, spleen, and head kidney during the first 15 days after challenge, which encompassed the copepod and chalimus stages of lice development.ResultsLarge scale and highly complex transcriptome responses were found already one day after infection (dpi). Many genes showed bi-phasic expression profiles with abrupt changes between 5 and 10 dpi (the copepod-chalimus transitions); the greatest fluctuations (up- and down-regulation) were seen in a large group of secretory splenic proteases with unknown roles. Rapid sensing was witnessed with induction of genes involved in innate immunity including lectins and enzymes of eicosanoid metabolism in skin and acute phase proteins in spleen. Transient (1-5 dpi) increase of T-cell receptor alpha, CD4-1, and possible regulators of lymphocyte differentiation suggested recruitment of T-cells of unidentified lineage to the skin. After 5 dpi the magnitude of transcriptomic responses decreased markedly in skin. Up-regulation of matrix metalloproteinases in all studied organs suggested establishment of a chronic inflammatory status. Up-regulation of putative lymphocyte G0/G1 switch proteins in spleen at 5 dpi, immunoglobulins at 15 dpi; and increase of IgM and IgT transcripts in skin indicated an onset of adaptive humoral immune responses, whereas MHCI appeared to be down-regulated.ConclusionsAtlantic salmon develops rapid local and systemic reactions to L. salmonis, which, however, do not result in substantial level of protection. The dramatic changes observed after 5 dpi can be associated with metamorphosis of copepod, immune modulation by the parasite, or transition from innate to adaptive immune responses.

In the present study, IgT genes of Atlantic salmon were cloned and characterised. Analysis of our sequence data as well as ESTs reported to the databases revealed three distinct IgT heavy chain sub-variants in salmon, as opposed to two of IgM and IgD. The IgT sub-variants in salmon are 76-80% identical to each other, and 75-82% identical to the reported rainbow trout sequences, whereas the similarity to the orthologous molecules in zebrafish, grass carp, mandarin fish, and grouper is 25-41%. The heavy chains of both secreted and membrane anchored forms of salmon IgT include four constant Ig domains, τ1-τ4. This parallels the IgM heavy chains in elasmobranch fish and higher vertebrates, but differs from IgM in teleost fish where the membrane anchored form include only three constant Ig domains, μ1-μ3. The similarity between τ1 and μ1 in salmon is relatively high (52%) when compared to the remaining part of the molecules (τ2-τ4 and μ2-μ4 are 13-24% similar). To compare τ, μ and δ expressions in different tissues (head kidney, thymus, spleen, gill, skin, hind gut, brain and muscle) of Atlantic salmon, RT-qPCR assays were designed and evaluated. The analyses revealed that IgM transcripts are most abundant (up to 200 times more than IgD) followed by IgT (up to 20 times more than IgD) in most tissues. Highest expression of IgM, IgT, and IgD was in head kidney and spleen.

The polymeric immunoglobulin receptor (pIgR) plays a pivotal role in mucosal immune protection by transporting secretory immunoglobulins to mucosal epithelia, and protecting them from proteolytic degradation. It has been reported that a homolog of the pIgR has a similar role in teleost fish. Considering the role pIgR has in mucosal defenses, this study was initiated to characterize a possible pIgR homolog in Atlantic salmon (Salmo salar) and its relatedness to pIgR of other vertebrates and similar molecules. Two pIgR-like cDNAs and genes of Atlantic salmon (Salsal pIgR and Salsal pIgRL) were cloned and analyzed. In addition, we gathered sequence information of CMRF35-like molecules (CLM) 1, 7, and 8 (designated as CD300 in humans) and made a comparative evaluation to that of the Salsal pIgR and Salsal pIgRL polypeptides. Salsal pIgR and Salsal pIgRL, like pIgR in other teleosts, are composed of two IG V domains, a connecting, a transmembrane, and a cytoplasmic region. The same holds true for Atlantic salmon CLM1 and CLM7, except that they possess putative immunoreceptor tyrosine-based inhibitory motifs (ITIM) in their cytoplasmic tails. The abundance of Salsal pIgR transcript is significantly higher than Salsal pIgRL and CLM in the skin, while Salsal pIgRL transcripts were abundant in the gills, depicting their possible tissue-specific role in mucosal immunity. To further highlight the roles of these molecules in cutaneous mucosal defence, we compared their transcriptional changes in salmon skin and spleen infected with the ectoparasite Lepeophtheirus salmonis which targets skin and mucus of salmonid fish (sampled 3, 14 and 28 days post infection (dpi)). Salsal pIgR and Salsal pIgRL transcripts significantly increased after 14dpi in skin and spleen. CLM1 was up-regulated in skin and down-regulated in spleen, possibly indicating that CLM1 expressing cells had migrated to the target site. Homology modeling using human pIgR domain 1 (PDB 1xed) identified structurally equivalent residues on both Salsal pIgR and Salsal pIgRL, and the same domain disulphide bridge topology. Cysteines 42 and 50 (IMGT numbering) are 7 residues apart in all V domains of Salsal pIgR, Salsal pIgRL, and mammalian [D1].

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