Induction and persistence of Mx protein in tissues, blood and plasma of Atlantic salmon parr, Salmo salar, injected with poly I:C.

The expression of Mx transcripts and Mx protein was monitored at weekly intervals for 7 weeks, by qRT-PCR and immunohistochemistry, in the kidney, liver, gill and blood of Atlantic salmon parr following injection of poly I:C. Elevated levels of Mx transcripts compared to PBS injected control fish were found in the tissues at week 1. Background levels were then found up to week 7, with the exception of week 4 when high levels were again found in poly I:C injected fish as well as control fish. Immunostaining for Mx protein in the kidney, liver and gill of poly I:C injected fish was higher than in control fish from weeks 1-4, but little staining was found in the tissues of both poly I:C treated and control fish thereafter. Blood monocytes stained consistently in all fish, suggesting that this leucocyte type constitutively expressed Mx protein. From weeks 2-4, lymphocytes of both groups consistently stained for Mx protein but the consistency decreased at weeks 5-7. Staining of neutrophils was also inconsistent. Western blots of plasma showed an immunoreactive band of 76 kDa typical of salmon Mx protein. Semi-quantitative measurements of dot blots showed poly I:C injected fish to have higher levels of plasma Mx protein than controls on weeks 1-4 with very low levels on weeks 5-7. The results indicate that following induction of an interferon response with poly I:C, Atlantic salmon parr maintain elevated levels of Mx protein in tissues, leucocytes and blood plasma for about 4 weeks. Production of Mx protein by blood monocytes appears to be constitutive, though production by lymphocytes and neutrophils was less consistent.