SAV Ag CpG/polyI:C formulated vaccination potentiate protective immune responses in Atlantic salmon - no additive or synergistic effect present when co-injecting a Rhabdovirus G-DNA vaccine

Abstract

Vaccines of today rely on adjuvant efficacy to provide a link between innate and adaptive immunity. CpG oligodeoxynucleotides (ODN) and polyinosinic:polycytidylic acid (polyI:C) are both toll-like receptor (TLR) ligands signaling through a combination of pathogen recognition receptors (PRR) and in mammals CpG/polyI:C are known to induce T-helper 1 (Th1) and cytotoxic T-lymphocyte (CTL) responses. Currently, the commercial vaccine against salmonid alpha virus (SAV) is based on inactivated whole-virus antigen combined with oil (Montanide ISA), mainly stimulating humoral responses. Here, the purpose was to differentiate between adjuvant and antigen induced protection for both water and oil formulated SAV Ag combined with CpG/polyI:C. Moreover, i.p. injected SAV Ag was also co-injected (i.m.) with Novirhabdovirus G DNA vaccine (vhsG), a potent inducer of innate antiviral responses in fish. This to observe if vhsG could enhance the immunity induced by the SAV Ag vaccine and also combined with SAV Ag CpG/polyI:C, to test whether vhsG could maximize the protection further. Expression of early antiviral genes, protection and elicited humoral responses were used to differentiate between the various vaccine formulations, pre- and post- cohabitant SAV challenge. SAV Ag alone and SAV Ag CpG/polyI:C provided full protection against SAV masking the objective to differentiate between Ag and adjuvant induced protection. Nonetheless, CpG/polyI:C again induced high innate immune gene expression (IFNa1, Vig-1, Mx and IFNγ) post vaccination and a potent induction of heat stable (neutralizing antibodies) and heat volatile humoral responses (complement) pre- and post- challenge. The stimulatory effect of vhsG co-injected treatments on early innate immune gene expression was modest. Co-injection of vhsG with SAV Ag CpG/polyI:C did not provide any beneficial adjuvant effects, rather indicating an inhibitory interplay between CpG/polyI:C and vhsG. Results presented here, again demonstrate the immunostimulatory potency of CpG/polyI:C to be used as an adjuvant in viral vaccines for fish.