Using cationic liposomes as carriers for long dsRNA to trigger an antiviral response in rainbow trout cell lines

Long dsRNA induces the expression of type I interferons (IFNs) and IFN-stimulated genes (ISGs) to establish an antiviral state. When induced prophylactically, this antiviral state can reduce the severity and mortality of viral infections. One of the limiting factors in delivering dsRNA in animal models is the lack of an effective carrier that protects the dsRNA from degradation in the extracellular space. In this study, commercially available cationic liposomes composed of stearylamine, L-α-phosphatidylcholine, and cholesterol were analyzed for their ability to encapsulate and deliver a 621-bp dsRNA sequence. This encapsulated dsRNA was delivered to two Oncorhynchus mykiss cell lines, RTG-2 and RTgill-W1, to activate the IFN pathway and reduce chum salmon reovirus (CSV) infection. EMSA analysis revealed that the liposomes effectively encapsulated 55 and 800 µg/mL doses of dsRNA, remained stable when stored at 4°C and - 20°C, and protected the encapsulated dsRNA from degradation by RNase III. Cell viability assays determined that liposomes loaded with dsRNA were highly cytotoxic after 24 h of exposure. A shorter exposure of 2 h resulted in reduced cytotoxicity and enhanced expression of the ISG Mx1 in both dsRNA alone and dsRNA-liposome-treated cells; however, the elevated Mx1 induction was not sufficient in the dsRNA-liposome treatment group to provide protection against viral infection. Meanwhile, the unencapsulated dsRNA significantly reduced the CSV titer and amount of syncytia formation. Thus, while dsRNA represents an important immune modulator in fish cells, this liposome formulation is too toxic for antiviral applications.