Infectious pancreatic necrosis virus (IPNV) infects salmonid fish with high mortality and causes serious economic losses to salmonid aquaculture. Lactobacillus strains have a number of properties that make them attractive candidates as delivery vehicles for the presentation to the mucosa of compounds with pharmaceutical interest, in particular vaccines. Here, Lactobacilli/Escherichia coli shuttle vector pPG1 (surface-displayed) or pPG2 (secretory) with the capsid VP2 gene inserted was transformed into Lactobacillus casei to yield two recombinant strains: Lc:PG1-VP2 and Lc:PG2-VP2, respectively. Rainbow trout immunized respectively with Lc:PG1-VP2, Lc:PG2-VP2, Lc:PG1-VP3 and Lc:PG2-VP3 elicited anti-IPNV immune responses (serum IgM) via oral route. Statistical results of serum IgM titer with neutralizing activity showed that immunogenicity of Lc:PG2-VP2 was more powerful than that of Lc:PG1-VP2 (P < 0.001), Lc:PG1-VP3 (P < 0.001) and Lc:PG2-VP3 (P < 0.001), which was confirmed by viral loads reduction analyzed by real-time RT-PCR in orally immunized rainbow trout after virus challenge. Comparing with negative control, rainbow trout orally dosed with Lc:PG2-VP2 resulted in ∼46-fold reduction in virus load on days 10 post viral challenge as well as Lc:PG1-VP2(∼20-fold), Lc:PG2-VP3(∼6-fold) and Lc:PG1-VP3(∼3-fold). Taken together, Lc:PG2-VP2 exhibited a more appropriate candidate as live bacteria vaccine against IPNV infection in rainbow trout.
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