Inflammatory responses in lumpfish (Cyclopterus lumpus L.) vaccinated at different water temperatures with intraperitoneal injection

Abstract

The use of lumpfish (Cyclopterus lumpus L.) as cleaner fish in salmonid aquaculture for the biological control of sea lice infestation levels has increased in recent years. The species is relatively new to aquaculture and susceptible to various infectious agents. The health situation for lumpfish is currently considered poor, indicating the necessity for developing more efficient vaccines. Studying inflammatory responses induced by vaccination can contribute to a more detailed understanding of underlying immune mechanisms in lumpfish, which is important in the context of improving vaccine designs and vaccination procedures. Tissue samples from lumpfish intraperitoneally immunized with a divalent oil-adjuvanted injection vaccine at different water temperatures of 5 °C, 10 °C, and 15 °C, collected at 630 day degrees and 18 weeks post injection, were used for histological and immunohistochemical analyses. The relative amount of secretory and membrane-bound immunoglobulin M (IgM) gene transcripts in the head kidney of vaccinated fish was determined by quantitative PCR. Vaccine-induced inflammatory lesions were observed on histological sections of abdominal pancreatic/intestinal tissue from vaccinated fish in all three temperature groups. Immunohistochemical staining of Aeromonas salmonicida vaccine antigen was pronounced along the rim of oil droplets in lesions. Inflammatory cells forming dense aggregations in lesions showed proliferative activity, many of which were identified as eosinophilic-granulocyte-like cells. IgM+ cells were scattered in more loosely arranged inflammatory tissue dominated by connective tissue, showing no apparent difference in numbers between lesions from fish vaccinated at 5 °C, 10 °C, and 15 °C. The observed staining patterns of IgM+ cells on kidney and spleen sections from vaccinated fish were no different from control fish. Relative gene expression analysis of secretory and membrane-bound IgM revealed low overall expression in the head kidney of vaccinated fish at both 630 day degrees and 18 weeks post injection. The results of this study indicate that the vaccine stimulated prolonged local inflammatory responses at the injection site, which was not greatly influenced by temperature. In addition, the results suggest local mechanisms in IgM responses with no corresponding immunoreactivity in the kidney, but further analyses, especially with regard to how antigens are presented and handled by the lumpfish immune system at the local level, are needed for confirmation.