Abstract

Caligus rogercresseyi commonly called sea louse is an endemic copepod ectoparasite, which parasites farmed Salmo salar. This parasitism generates great economic losses, predisposition to secondary infections and welfare issues. Understanding the biology behind the mechanisms involved in the immune response of the host is important for selecting individuals bearing different resistance genetic backgrounds. Fish skin acts as a mechanical barrier and represents a metabolically active tissue; the immune response of the skin is due to the skin's complex structure, mucus and cell composition. In early stages of sea lice infestation the skin response is expected to explain to what extent the fish is susceptible or resistant to the parasite. Early innate immune response of the skin of susceptible and resistant salmonids to L. salmonis had been reported previously, however, this is not the case for the interaction between Caligus rogercresseyi and Salmo salar. In order to investigate the gene expression levels for key genes, such as Matrix metalloproteinase-13, Beta-2-microglobulin, Metalloreductase STEAP, CD4, MHCII, Transferrin, Arachidonate 5-lipoxygenase, Galectin-9 and Prostaglandin D synthase involved in L. salmonis response. We perform an in vivo challenge test of Salmo salar with the pathogen Caligus rogercressy comprising different families from the AQUAGEN breeding program. We characterize the skin immune response in naïve fish (2 days before the challenge start); 9 days post-challenge T (1) and 21 days post-challenge T (21), both in naïve tissue as well as in attachments sites where the host-parasite interaction occurred. Our results show a comparable response of Salmo salar when interacting with Caligus rogercressyi. For example, in louse attachment sites an early pro-inflammatory response was observed in genes; Arachidonate 5-lipoxygenase (LOX-5) and CD4–1, which significantly increased at 9 days post-challenge. The metalloproteinase-13 gene was significantly up regulated at 21 days post-challenge; B2M and MHCII expression significantly decrease in naïve skin at 21 dpi. Galectin-9 and Prostaglandin D synthase were significantly down regulated in the louse attachment sites at 21 dpi. Transferrin showed a bi-phasic transcriptomic response, first the gene was down regulated and its expression increased significantly at 21 days post-challenge at attachment sites. The results show that the transcriptomic response in skin of Salmo salar is complex, evolving slowly as the parasite became adults, when most of the skin damage is observed. Moreover, it appears that the response of the host is quite modest in the sessile stages, possibly due to the fact that Caligus fed mainly on mucus. Some of the results following the full transcriptomic assessment will be presented.