| Summary: | In recent years, lumpfish and different species of wrasse are used as cleaner fishes for removal of sea lice from farmed Atlantic salmon in Europe and Canada. The production is successful, but there are challenges with high mortality due to bacterial infections. In-depth knowledge of the immune system in lumpfish, particularly immune responses upon bacterial infections and identification of immune genes, will make an important basis for development of immune-prophylactic measures. Prior to this study, there were no available sequences of immune genes in lumpfish in public databases. To obtain sequence information from lumpfish, RNA sequencing of head kidney leukocytes (HKLs) exposed to Vibrio anguillarum O1, followed by de novo transcriptome assembly, was performed. The transcriptome encompassed 221659 trinity genes, 9033 differentially expressed genes (DEG) at 6 hours post exposure (hpe) and 15225 DEG at 24 hpe. The DEG analysis of the lumpfish transcriptome clearly showed that the alternative pathway of the complement cascade is one of the chief mechanisms in the Vibrio anguillarum response of lumpfish HKLs. Furthermore, the DEG analysis also clearly showed that the TLR signaling through the canonical NFκB pathway was the major pathway in the adaptation of the innate immune response. Further, a wider MAPK pathway regulation was observed, involving all of its sub pathways, however, at much less potent regulation levels compared with NFκB. Globally, the DEG analysis displayed a picture of a pro-inflammatory reaction, initiated by toll-like receptor TLR5S, and the alternative complement pathway, and resulting in high up-regulation of the cytokines interleukin (IL)-1β, IL-6, CXCL8 (also known as IL-8) and tumor necrosis factor (TNF)-α. Further characterization of the lumpfish IL-1 family, both ligands and receptors (IL-1R), were performed. Downstream signaling pathways were also characterized. Full-length sequences of the ligands IL-1β, IL-18, and the fish-specific IL-1 family members nIL-1F and IL-1Fm2, the ...
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