Vaccine-associated granulomatous inflammation and melanin accumulation in Atlantic salmon, Salmo salar L., white muscle

The purpose of this study was to investigate the nature of variably sized pigmented foci encountered in fillets of farmed Atlantic salmon, Salmo salar L. The material was sampled on the fillet production line and on salmon farms from fish with an average size of 3 kg from various producers. The fish had been routinely vaccinated by injection. Gross pathology, histology, immunohistochemistry using antisera against major histocompatibility complex (MHC) class II beta chain and transmission electron microscopy (TEM) were used to characterize the changes. Macroscopically, melanized foci were seen penetrating from the peritoneum deep into the abdominal wall, sometimes right through to the skin, and also embedded in the caudal musculature. Histological investigation revealed muscle degeneration and necrosis, fibrosis and granulomatous inflammation containing varying numbers of melano-macrophages. Vacuoles, either empty or containing heterogeneous material, were frequently seen. The presence of abundant MHC class II+ cells indicated an active inflammatory condition. TEM showed large extracellular vacuoles and leucocytes containing homogeneous material of lipid-like appearance. The results showed that the melanized foci in Atlantic salmon fillet resulted from an inflammatory condition probably induced by vaccination. The described condition is not known in wild salmon and in farmed salmon where injection vaccination is not applied.

Production of rabbit antisera against recombinant MHC class II beta chain and identification of immunoreactive cells in Atlantic salmon (Salmo salar)

In the present work, rabbit antisera recognising the Atlantic salmon (Salmo salar) MHC class II beta chain polypeptide were produced and used in immunoblotting, immunohistochemistry and immunogold electron microscopy. A construct encoding the beta1 and beta2 domains fused to the E. coli protein thioredoxin was used to express the recombinant MHC class II beta chain. Immunoblotting revealed a band of approximately 30kDa in total protein samples from head kidney, spleen, gills, thymus and blood leukocytes, while being absent in muscle. The distribution of MHC class II positive cells was immunohistochemically demonstrated in Atlantic salmon epithelial and haemopoietic tissues. Ultrastructural demonstration of immunoreactive organelles in mid-kidney cells was performed by immunogold electron microscopy. The results indicate expression in lymphocytes, macrophages, epithelial cells and endritic-like cells. This is the first study to address morphological MHC class II expression in a fish species.