Contrasting expression of immune genes in scaled and scaleless skin of Atlantic salmon infected with young stages of Lepeophtheirus salmonis

Atlantic salmon skin tissues with and without scales were taken from two preferred sites of salmon louse (Lepeophtheirus salmonis) attachment, behind the dorsal fin (scaled) and from the top of the head (scaleless), respectively. Tissues were profiled by qPCR of 32 genes to study responses to copepodids, 4 days post infection (dpi), and during the moult of copepodids to the chalimus stage, at 8 dpi. Basal/constitutive differences were found for many immune-related genes between the two skin sites; e.g., mannose binding protein C was over 100 fold higher expressed in the scaled skin from the back in comparison to the skin without scales from the head. With lice-infection, at 4 dpi most genes in both tissues showed lower values than in the non-infected control. By 8 dpi, the majority of responses increased towards the control levels, including cytokines of Th1, Th17 and Th2 pathways. Immunohistochemistry of three immune factors revealed an even distribution of MHC class II positive cells throughout epidermis, including the top layer of keratinocytes, marked compartmentalization of Mx⁺ and CD8α⁺ cells close to stratum basale, and an increase in numbers of CD8α⁺ cells in response to infection. In conclusion, suppression of immune genes during the copepodid stage likely sets off a beneficial situation for the parasite. At the moult to chalimus stage 8 dpi, only few genes surpassed the non-infected control levels, including CD8α. The gene expression pattern was reflected in the increased number of CD8α expressing cells, thus revealing a relatively minor activation of skin T-cell defenses in Atlantic salmon in response to L. salmonis infection.