title: Difference in skin immune responses to infection with salmon louse (Lepeophtheirus salmonis) in Atlantic salmon (Salmo salar L.) of families selected for resistance and susceptibility.
abstract: Atlantic salmon is susceptible to the salmon louse (Lepeophtheirus salmonis) and the variation in susceptibility within the species can be exploited in selective breeding programs for louse resistant fish. In this study, lice counts were completed on 3000 siblings from 150 families of Atlantic salmon identified as high resistant (HR) and low resistant (LR) families in two independent challenge trials. Skin samples behind the dorsal fin (nearby lice attachment) were collected from ten extreme families (HR or LR) and analyzed by qPCR for the expression of 32 selected genes, including a number of genes involved in T helper cell (Th) mediated immune responses, which have been previously implied to play important roles during salmon louse infections. Most genes showed lower expression patterns in the LR than in HR fish, suggesting an immunosuppressed state in LR families. The average number of lice (chalimi) was 9 in HR and 15 in LR fish. Large variation in lice counts was seen both within resistant and susceptible families, which enabled us to subdivide the groups into HR < 10 and HR > 10, and LR < 10 and LR > 10 to better understand the effect of lice burden per se. As expected, expression patterns were influenced both by genetic background and the number of attached parasites. Higher number of lice (>10) negatively affected gene expression in both HR and LR families. In general, strongest down-regulation was seen in LR > 10 and lesser down-regulation in HR < 10. HR in general and especially HR < 10 fish were better at resisting suppression of expression of both Th1 and Th2 genes. However, the best inverse correlation with infection level was seen for the prototypical Th1 genes, including several members from the interferon pathways. In addition, skin histomorphometry suggests that infected LR salmon had thicker epidermis in the area behind the dorsal fin and larger mucous cell size compared to infected HR fish, however marginally significant (p = 0.08). This histomorphometric finding was in line with the immune response being skewed in LR towards the Th2 rather than a Th1 profile. Our findings suggest that the ability to resist lice infection depends on the ability to avoid immunosuppression and not as much on the physical tissue barrier functions.
authors.affiliation: Norwegian University of Life Sciences, Faculty of Biosciences and Veterinary Medicine, Sea Lice Research Center, PO Box 8146, N-0033 Oslo, Norway.
authors.forename: Helle
authors.initials: H
authors.lastname: Holm
authors.affiliation: Aquagen AS, Havnegata 9, N-7010 Trondheim, Norway.
authors.forename: Nina
authors.initials: N
authors.lastname: Santi
authors.affiliation: Aquagen AS, Havnegata 9, N-7010 Trondheim, Norway.
authors.forename: Sissel
authors.initials: S
authors.lastname: Kjøglum
authors.affiliation: Weifa AS, Stittlidalen 4, Fikkjebakke, 3766 Sannidal, PO Box 98, NO-37911 Kragerø, Norway.
authors.forename: Nebojsa
authors.initials: N
authors.lastname: Perisic
authors.affiliation: Norwegian University of Life Sciences, Faculty of Biosciences and Veterinary Medicine, Sea Lice Research Center, PO Box 8146, N-0033 Oslo, Norway.
authors.forename: Stanko
authors.initials: S
authors.lastname: Skugor
authors.affiliation: Norwegian University of Life Sciences, Faculty of Biosciences and Veterinary Medicine, Sea Lice Research Center, PO Box 8146, N-0033 Oslo, Norway. Electronic address: oystein.evensen@nmbu.no.
authors.forename: Øystein
authors.initials: Ø
authors.lastname: Evensen
mesh_terms: D000818:Animals; D033342:Copepoda / Q000502:physiology*; D004198:Disease Susceptibility / Q000276:immunology / Q000662:veterinary; D004478:Ectoparasitic Infestations / Q000276:immunology / Q000469:parasitology / Q000662:veterinary*; D005260:Female; D005393:Fish Diseases / Q000235:genetics* / Q000276:immunology* / Q000469:parasitology; D029941:Fish Proteins / Q000235:genetics* / Q000378:metabolism; D005786:Gene Expression Regulation; D008297:Male; D015999:Multivariate Analysis; D060888:Real-Time Polymerase Chain Reaction; D020317:Salmo salar; D012867:Skin / Q000276:immunology; D018417:Th1 Cells / Q000276:immunology; D058504:Th17 Cells / Q000276:immunology; D018418:Th2 Cells / Q000276:immunology; D059467:Transcriptome
chemical_list: D029941:Fish Proteins
keywords: Atlantic salmon; Salmon louse; Selective breeding; Skin immune responses; Th1/Th2 immunity