Poly I:C-induced Mx responses in Atlantic salmon parr, post-smolts and growers

Intraperitoneal injection of 500 microg poly I:C/fish into Atlantic salmon parr in freshwater and post-smolts and growers in seawater (all at 11 degrees C) induced enhanced expression of Mx mRNA in liver tissue 24 h post-injection. The level of Mx transcripts peaked at day 3 (Mx:beta-actin ratio of about 0.8) and the response disappeared by day 7. In post-smolts, mortalities occurred up to day 14 post-injection, which was dose-dependent. Histological examination of tissues revealed severe pathological changes in the liver of poly I:C injected post-smolts resulting from apoptosis and necrosis of hepatocytes. All other organs appeared histologically normal. Levels of Mx mRNA expression on day 3 post-injection were similar for fish with normal and pathological livers. In untreated or control fish injected with PBS, low levels of Mx transcripts (Mx:beta-actin ratio about 0.1) were sometimes detectable in parr but not in growers. Constitutive Mx expression was variable in post-smolts. Some populations had no detectable transcripts while in others moderate ratios (about 0.3) were detectable over a 3-week period of sampling. Poly I:C administered to parr by bath or orally did not induce upregulation of Mx expression.