| Summary: | Type I interferons constitute the first line of defence against viral infections in the vertebrate innate immune system. Mx genes are inducible by type I interferon, natural and synthetic double stranded RNA (dsRNA) and unmethylated bacterial CpG motifs both in fish and mammalian hosts. Mx and interferon expression occur in salmonids after viral infection (i.e. IPNV) although the antiviral role of Mx proteins in fish remains unclear. This study aims to identify alternative Mx stimulatory compounds in Atlantic salmon (Salmo salar L.) and to characterise the kinetics and intensity of the response by semi-quantitative RT-PCR. Mx response at 14ºc was measured from Atlantic salmon parr liver previously injected with either LPS (1 µg/fish, 10µg/fish, 50µg/fish and 100µg/fish), the synthetic double stranded polyribonucleotide poly I: C (500µg/fish), Vibrio bacterin, β-glucan (50µg/fish), whole yeast cells (100µg/fish) or yeast RNA (100µg/fish). Newly hatched salmon fry were immersed once, twice or three times in the Vibrio bacterin diluted five or ten times and sampled for three weeks. The effect of temperature on the poly I: C induced Mx response was analysed by injecting parr with poly I: C at 6°C. Histological samples were taken as mortalities followed poly I: C injection during the first week. Paraffin embedded sections were TUNEL stained in order to reveal presence of interferon-induced apoptosis. Neither LPS nor any of the yeast compounds induced Mx expression in Atlantic salmon parr. Vibrio bacterin injection but not immersion resulted in positive Mx induction. The response peaked on day 2 after injection and finished by day 6. Poly I: C induced Mx responses were more intense and longer lasting. Levels of Mx expression peaked on day 3 and were no longer detectable by day 9 at 14ºc. Lower temperature caused a longer lasting response (at least 21 days), which peaked on day 7. Such response was only slightly less intense than that at 14 ºc and had no delayed onset. Liver histology samples were TUNEL positive hence it seems that interferon produced en masse may have disadvantageous effects in salmonid hepatocytes. In conclusion, both the prolongation of Mx responses at lower temperatures and the capacity of Vibrio vaccines to induce Mx responses by injection are encouraging findings for the Atlantic salmon farming industry. The Marine Laboratory, Aberdeen, Scotland
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