Immune players in acquired protection to enteromyxum leei (Myxozoa) in gilthead sea bream, Sparus aurata (Teleostei: Perciformes)

The intestinal myxosporean parasite Enteromyxum leei causes chronic catarrhal enteritis in gilthead sea bream (GSB, Sparus aurata) leading to intestinal dysfunction, poor growth, and higher susceptibility to handling and stress. It entails severe economic losses to the aquaculture sector. Previous observations in our lab showed that fish that recovered from an E. leei infection did not get infected upon re-exposure, hinting towards the possibility of protective immunization of GSB against E. leei. To study this in more detail, we re-challenged putative resistant (R) GSB that recovered from this myxozoan infection, by exposure to E. leei effluent infected water. Another group of naïve (N) GSB (never exposed to the parasite) was also challenged. Both fish groups were sampled at 0, 61 and 86 days post-exposure (p.e.) and different specific and non-specific humoral factors were measured in serum, such as the total peroxidase activity, total IgM and IgT (by ELISA), and the presence and quantity of specific antibodies against the parasite by immunohistochemistry (IHC). The expression of a panel of immune-related genes was analysed in head kidney, anterior and posterior intestines of N and R fish after 86 days p.e. At this final sampling point, 83.3% of N fish vs 0% of R fish tested positive for E. leei by histology. The total peroxidase activity decreased with the progression of the infection in both groups, but only significantly in R fish, probably due to a higher consumption of this enzyme to fight the parasite. Total IgT and IgM levels did not significantly differ between groups. However, IHC evidenced that R fish had a higher initial level of specific anti-E. leei IgM than N fish. The PCR array showed a differential response between N and R fish and among tissues. R fish had significant up-regulation of IgM, IgT, il10 and gzmA and down-regulation of il1ß in anterior intestine. Complement (c3 and fcl) and the anti-protease alfa2m were significantly higher expressed in