Rhabdovirus-induced microribonucleic acids in rainbow trout (Oncorhynchus mykiss Walbaum)

This thesis deals with microribonucleic acid (microRNA; miRNA) expression during rhabdovirus infection and upon immunization with a DNA vaccine expressing the rhabdovirus glycoprotein in teleost fish. MicroRNAs are conserved, small, endogenous, non-coding regulatory RNAs that modulate gene expression at the post-transcriptional level. The genes whose expression they control are involved in numerous aspects of an organism’s biology in which abnormal miRNA expression is associated with pathologies.
In this thesis, the upregulation of two clustered miRNAs was observed in rainbow trout infected with the fish rhabdovirus, viral hemorrhagic septicemia virus (VHSV) and in fish immunized with a DNA vaccine encoding the glycoprotein of VHSV. It was shown that the two miRNAs, known so far only in teleost fishes, are orthologues of an ancient vertebrate miRNA cluster, which in humans are involved in the regulation of the cell cycle and have been associated with various types of cancers. Interferon (IFN)-related regulatory sequences were found in the promoter of the teleost fish miRNA cluster and its expression was induced by IFNs and IFN-related mechanisms. It was further demonstrated that these teleost miRNAs may participate in the IFN-mediated antiviral response.
IFN-induced miRNAs contributing to the antiviral effects of type I IFN have been demonstrated and an increasing number of cellular miRNAs have been shown to be involved in the antiviral response against different viruses in mammalian cells. The findings in this thesis represent the first report to address the expression of particular miRNAs in response to rhabdovirus infection and to anti-rhabdovirus DNA vaccination in teleost fish, as well as the first teleost fish IFN-elicited miRNAs that could potentially be involved in teleost immune responses. Thus, RNA interference mechanism mediated by cellular miRNAs might play an important role in innate antiviral immune responses in teleost fish. An update on the antiviral roles of miRNAs in mammalian cells and potentially in vertebrate cells is included in a review paper as part of the thesis.