Pigment-producing granulomatous myopathy in Atlantic salmon: a novel inflammatory response.

Melanin comprises a complex group of pigmented polymers whose primary function is ascribed to dermal solar protection, but may also have an interesting role in innate immunity. In ectothermic vertebrates, melanogenesis is reported in leukocyte populations, but it is not known if this occurs in connection with inflammatory reactions. Melanin accumulations in ectopic locations, in particular muscle, represent a serious quality problem in salmon production. Here, we investigated such changes for the expression of dopachrome tautomerase and tyrosinase as well as some important immune genes and pathogens. Furthermore, the nature of the pathological changes was addressed by morphological methods. Gene transcripts encoding key enzymes in melanogenesis, suggesting a de novo melanin synthesis in pigmented muscle, were found. MHC class II transcripts were up-regulated and there was no indication of bacterial or viral infection. The histological examination revealed granulomatous inflammation with distribution of MHC class II positive cells and T cells, analogous to the pattern found in mammals. Importantly, in contrast to mammals pigmented cells were contributing in the inflammation. We demonstrate that melanin production occurs in granulomatous inflammation in salmon, revealing a close and hitherto unreported link between the pigmentary and immune systems.

Transcription of the tyrosinase gene family in an Atlantic salmon leukocyte cell line (SHK-1) is influenced by temperature, but not by virus infection or bacterin stimulation.

The present study was performed to address putative links between the immune and pigmentary systems. A pigment-producing leukocyte-like cell-line (SHK-1 cells) of Atlantic salmon (Salmo salar L.) was exposed to different temperatures, poly I:C, bacterin or infected with virus (infectious pancreatic necrosis virus or infectious salmon anaemia virus). The effect of this stimulation regarding the transcription-pattern of the tyrosinase gene family (melanin genes) and the immune-related genes MHC class II and IFN-1 was analysed using real-time RT-qPCR. At 10°C cultivation, tyrosinase and dopachrome tautomerase remained unregulated. At 15°C, a moderate up-regulation was induced, while at 20°C, these genes were up-regulated in an exponential manner over time. Temperature did not affect the transcription of the immune-related genes. Virus infections, poly I:C or bacterin had no influence on the transcription of the melanogenesis-related genes, but triggered the immune-related genes. Our findings revealed no connections between the pigmentary and immune systems, but demonstrated a hereto undiscovered temperature-effect on the tyrosinase gene family.