Molecular mechanisms for natural immunity towards infectious salmon anemia virus and infectious pancreatic necrosis virus

Abstract

When cells are infected with virus, a chain of biochemical processes are initiated to protect the host cell and organism. Some of these processes are effectuated immediately (innate immunity) whilst other processes might need several days before becoming effective (adaptive immunity). An essential part of the innate immune system is the receptors which recognize extracellular or endocytosed/phagocytosed foreign organisms (TLR). In addition there are cytoplasmic receptors (the RLHs RIG-I and MDA-5 or the NLRs) that initiate signaling cascades upon interaction with foreign macromolecules. When these receptors bind foreign molecules, chain-reactions lead to the synthesis and secretion of interferons, certain interleukins, and the activation of different caspases mediating regulated cell-death (apoptosis). The NLRs recognize bacterial components like peptidoglycan containing diaminopimelic-acid or muramyl dipeptide, while the RLHs recognize viral components. When RIG-I bind to viral RNA, down-stream signaling leads to the activation of a mitochondrial antiviral signaling protein (MAVS) that result in increased production of interferon. Activation of virus responsive genes is not always sufficient for protection of salmon against acute infections with the highly pathogenic viruses. The diseases causing the highest ecological and economic impacts in farmed fish in Europe are caused by RNA-viruses. In this thesis two pathogenic viruses that are the causative agents of two important fish diseases in aquaculture are studied; Infectious Salmon Anemia Virus (ISAV) and Infectious Pancreatic Necrotic Virus (IPNV). Cell lines from Atlantic salmon were infected with these viruses to study the intracellular biochemical changes that are initiated upon infection. In this thesis the importance of the pattern recognition receptor RIG-I and the signaling protein MAVS were studied by transfecting cells with plasmids containing genes coding for these proteins. The effect of overexpressing these proteins on virus replication and CPE were analyzed. In addition to this, the subcellular localization of these proteins was studied by transfecting cells with plasmids coding for variants of RIG-I and MAVS protein fused to Green Fluorescent Protein (GFP). Fluorescence microscopy was utilized to observe and study the effect of virus infection on intracellular localization. Our findings do not conclusively demonstrate that overexpression of RIG-I or MAVS protect the cells from ISAV or IPNV infection.