Analysis of polymeric immunoglobulin receptor- and CD300-like molecules from Atlantic salmon

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].