Oral immunization with a recombinant Lactobacillus expressing CK6 fused with VP2 protein against IPNV in rainbow trout (Oncorhynchus mykiss)

Infectious pancreatic necrosis virus (IPNV) infects wild and cultured salmonid fish causing high mortality with serious economic losses to salmonid aquaculture. Ideally, the method of oral immunization should prevent the infection of rainbow trout juveniles with IPNV. In the present study, genetically engineered Lactobacillus casei 393 pPG-612-VP2/L. casei 393 and pPG-612-CK6-VP2/L. casei 393 constitutively expressing VP2 protein of IPNV were constructed. The recombinant strains pPG-612-CK6-VP2/L. casei 393 and pPG-612-VP2/L. casei 393 were orally administrated to juvenile rainbow trouts, and significant titers of IgM and IgT of pPG-612-CK6-VP2/L. casei 393 were observed. The results demonstrate that the recombinants could elicit both local mucosal and systemic immune responses. The proliferation of spleen lymphocytes in trouts immunized with pPG-612-CK6-VP2/L. casei 393 showed that the recombinant strain could induce a strong cellular immune response. The IL-1β, IL-8, CK6, MHC-II, Mx, β-defensin, and TNF-1α levels in the spleen and gut suggest that the target molecular chemokine has the ability to attract relevant immune cells to participate in the inflammatory response and enhance the function of the innate immune response. Additionally, the pPG-612-CK6-VP2/L. casei 393 induced the expression of cytokines, which have the effect of promoting inflammation to drive the differentiation of macrophages and clear target cells. After challenging with IPNV, the reduction in viral load caused by pPG-612-CK6-VP2/L. casei 393 was significantly higher than that of the other groups. Thus, the recombinant pPG-612-CK6-VP2/L. casei 393 is a promising candidate for the development of an oral vaccine against IPNV.