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The development of efficient and cheap vaccines against several aquatic viruses is necessary for a sustainable fish farming industry. Toll-like receptor (TLR) ligands have already been used as good adjuvants in human vaccines. With more understanding of TLR expression, function, and ligand specificity in fish, more efficient adjuvants for fish viral vaccines can be developed. In this paper, we examine all known TLRs in Atlantic salmon (Salmo salar) and demonstrate that head kidney and spleen are the main organs expressing TLRs in salmon. We also show that adherent head kidney leucocytes from salmon are able to respond to many of the known agonists for human TLRs, and that viral infection can induce up-regulation of several TLRs. These findings substantiate these receptors' role in immune responses to pathogens in salmonids making their ligands attractive as vaccine adjuvant candidates.

There is an urgent need for more efficient viral vaccines in finfish aquaculture worldwide. Here, we report the use of poly(I:C) stabilized with chitosan as an adjuvant for development of better finfish vaccines. The adjuvant was co-injected with inactivated viral hemorrhagic septicemia virus (VHSV) (CSpIC+iV vaccine) in adult zebrafish and its efficiency in protection against VHSV infection was compared to a live, attenuated VHS virus vaccine (aV). Both free and stabilized poly(I:C) were strong inducers of an antiviral state, measured by transcriptional activation of the genes of viral sensors: toll-like receptors, interferons, and interferon-stimulated genes, such as MXa within 48 h after injection. Both the CSpIC+iV and the aV formulations provided a significant protection against VHSV-induced mortality. However, when plasma from survivors was tested for neutralizing antibodies in an in vitro protection assay, we could not demonstrate any protective effect. On the contrary, plasma from aV vaccinated fish enhanced cytopathic effects, indicating that antibody-dependent entry may play a role in this system. Our results show that poly(I:C) is a promising candidate as an adjuvant for fish vaccination against viral pathogens, and that the zebrafish is a promising model for aquaculture-relevant vaccination studies.

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The live attenuated vesicular stomatitis virus-vectored Ebola vaccine rVSV-ZEBOV is currently undergoing clinical trials in West Africa. The vaccine is to be stored at -70°C or less. Since maintaining the cold chain is challenging in rural areas, the rVSV-ZEBOV vaccine's short-term and long-term stability at different temperatures was examined. Different dilutions were tested since the optimal vaccine dosage had not yet been determined at the start of this experiment. The results demonstrate that the original vaccine formulation was stable for 1 week at 4°C and for 24 hours at 25°C. The stability of the vaccine was compromised by both high temperatures and dilution.