Immunopathological characterization of red focal changes in Atlantic salmon (Salmo salar) white muscle

Håvard Bjørgen,Subramani Kumar,Gjermund Gunnes,Charles Mcl Press,Espen Rimstad,Erling Olaf Koppang

doi:10.1016/j.vetimm.2020.110035

Håvard Bjørgen, Subramani Kumar + Show 4 more

Open Access

https://doi.org/10.1016/j.vetimm.2020.110035

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Abstract

Farmed Atlantic salmon (Salmo salar) are prone to various conditions affecting the quality of the fillet. A well-known but so far poorly understood condition is the focal red changes in muscle, often referred to as haemorrhages. Such changes are characterized by muscle necrosis, haemorrhages and acute inflammation. They can progress into focal melanised changes, a chronic inflammatory condition with melanin-producing leukocytes. The initial cause of intramuscular haemorrhages is unknown. In this study, we aimed to reveal some of their key immunological features. Samples of red focal changes were investigated by immunohistochemistry (IHC), in situ hybridization (ISH) and RT-qPCR for various immune markers. The results were compared with samples of melanised changes and control muscle, subjected to the same analyses. In all red changes, infiltrates with mononuclear cells were detected, consisting mostly of MHC class I/II+ cells, but also of CD3+ and CD8+ cells. ISH studies on IgM showed few to moderate amounts of B-cells in red focal changes. Trends in the RT-qPCR showed upregulation of genes related to innate immunity in the red changes, whereas genes related to adaptive immunity were upregulated in the melanised changes. An important result was the significant downregulation of the anti-inflammatory cytokine IL10 in all red changes. Our findings indicate that we can rule out an auto invasive nature of the changes. The downregulation of IL10 at an early phase is a trait for the condition.

Highlights

Abundant melanin production may occur with chronic inflammatory conditions in ectothermic species
The immune response in melanised focal changes in Atlantic salmon has been investigated previously in both histologic and transcriptomic studies (Krasnov et al, 2016; Larsen et al, 2012). This is the first report on the immune characterization of red focal changes
IHC showed a varied presence of immune cells in red changes while melanised changes were dominated by granulomatous inflammation, including highly organized granulomas

Summary

Introduction

Abundant melanin production may occur with chronic inflammatory conditions in ectothermic species. Chronic inflammation in the muscle of salmon (Salmo salar) may result in melanised focal changes in the fillet, which are commonly termed black spots (Larsen et al, 2012). With an occurrence of approximately 20 % of the fillets at slaughter, such changes are regarded as the most costly quality problem by the salmon industry (Mørkøre et al, 2015). The hallmark of the inflammatory changes associated with melanised focal changes in skeletal muscle is the presence of melano-macrophages, which are melanin-containing macrophage-like cells found in ectothermic vertebrates (Bjørgen et al, 2019; Larsen et al, 2012). Red focal changes occur less frequent than melanised changes and have a prevalence of about 4 % throughout the production period in seawater (Bjørgen et al, 2019)

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