Inflammatory changes and melanization in the Atlantic salmon (Salmo salar) targeted by In situ hybridization

Abstract

Research on the immune system of salmonid fish is attracting increased attention, however, progress within this field has been hampered by the general lack of cell markers targeting different immune molecules. Here, we circumvent these challenges by applying in situ hybridization (ISH) to reveal basic cellular responses in salmon immune reactions. Using RNAscope® (ACD, Newark, CA), we have established ISH targeting immunoglobulin (Ig) M and T RNA. Further, we also target other immune genes where we previously had no markers, and tyrosinase, thought to be important for the function of the melano-macrophages which are special, melanin-producing immune cells found in ectothermic vertebrates. We provided target gene sequences to the manufacturer, which supplied us with probes, and we performed ISH according to the manufacturer’s instructions. Different inflammatory conditions in farmed salmon, including cancerous changes from the gut and muscle tissue with chronic myositis and melanization due to the presence of melano-macrophages, were screened by standard histology and subsequently subjected to ISH. For the IgM results, we compared IHC and ISH results, as we have antisera targeting IgM. The results were highly similar. ISH thus seems as a good method that may be used in salmon when applicable and especially to target transcripts where no antibody recognizing the resulting protein is available. Further, we reveal hitherto non-described cellular compositions of inflammatory changes in salmon using probes detecting several different immune cell transcripts. Our results show that ISH using RNAscope® works in Atlantic salmon and is highly reproducible.

The Distribution of IgT mRNA+ Cells in the Gut of the Atlantic Salmon (Salmo salar L.)

“…Even though IgT has been considered equivalent to IgA in mammals and birds, this view could be nuanced, as IgT presents unique traits depending on the species in which it is found. Progress within this field has been hampered by the general lack of cell markers targeting different immune molecules [15]. Research on different teleost Ig isotypes has mainly relied on gene expression studies.…”