In vitro and in vivo differential expression of rainbow trout (Oncorhynchus mykiss) Mx isoforms in response to viral haemorrhagic septicaemia virus (VHSV) G gene, poly I:C and VHSV

Three different Mx isoforms are known to be present in rainbow trout, however, to date, neither their mechanism of action nor their regulation have been established. Because most previous studies have focused only on one Mx isoform of the three present in rainbow trout, the expression of all isoforms was simultaneously studied in this work in response to the viral haemorrhagic septicaemia virus (VHSV) G gene, poly I:C or VHSV. Thus, RT–PCR assays were specifically designed to amplify each of the Mx1, Mx2 and Mx3 transcripts induced both in vitro (RTG-2 cell line and head kidney leucocytes) and in vivo (muscle, head kidney, spleen and liver). Regardless of the inducer used, in vitro results showed that while in RTG-2 cells Mx3 was predominantly induced, all three isoforms were similarly induced in head kidney leukocytes. In vivo, regardless of the inducer used a predominant expression of Mx3 transcripts was also observed in muscle but expression of all three Mx isoforms or predominantly Mx1 and Mx2 was found in head kidney and spleen. Mx expression in the liver was however more dependant on the inducer used. In conclusion, the results obtained demonstrated, for the first time, that both in vitro and in vivo the expression of the different Mx genes is differentially regulated. Moreover, this is also the first report showing Mx induction after cell transfection with a plasmid coding for the VHSV-G protein.