Inflammatory reactions and melanogenesis in salmon during virus infection

Abstract

Abstract Melanised skeletal muscle changes (black spots) are a serious problem for the salmon industry world-wide, causing extensive production losses. Piscine orthoreovirus (PRV) is associated with the condition. The initial cause for the focal changes remains obscure. Early changes start as muscle bleedings and extensive muscle necrosis, and so far, these changes cannot be explained, However, from this acute phase, the changes may progress into melanised focal changes appearing as chronic granulomatous reactions. Granulomatous melanised changes have been consistently observed in PRV infected fish, and in serious changes, the tissue persistence of virus dominate the changes despite the inflammatory changes. Here, we address the pathogenesis of PRV infection and phases of melanised focal changes development. Fluorescent in situ hybridisation (FISH) assay confirmed the colocalisation of iNOS2 transcripts (M1 activated macrophages) with PRV in red spot whereas both M1 and M2 (Arg2 transcripts) macrophages localized at PRV infected clusters in black spots. Melano-macrophages were found positive with Arg2 transcripts demonstrating their role in tissue repair mechanism. In PRV negative fish low expression of M1 and M2 specific transcripts suggest PRV act as an accelerator for the melanisation in these focal changes. Transcripts specific for melanin production were observed in non-melanised cells in early stages of the condition and in melano-macrophages in late stages. The nature of the melano-macrophages is disputed and the rationale for their melanin production is not understood. Our investigations reveal new characteristics both of the pathogenesis of focal melanised changes as well as of the melano-macrophages involved.