An oral probiotic vaccine loaded by Lactobacillus casei effectively increases defense against GCRV infection in grass carp

Abstract

For a long time, grass carp culture in China has been severely affected by Grass Carp hemorrhagic disease caused by Grass Carp Reovirus (GCRV). At present, vaccines have been widely used for protecting aquatic organisms against infectious diseases, among which oral immunization with Lactobacillus casei is safe and highly effective. This vaccination route has the advantages of easy administration and noninvasive delivery. In this study, the recombinant LC-pVE5523-VP5 was constructed by using the outer capsid protein VP5 of GCRV as the immunogen and pVE5523 as the secretory expression vector. The bacterial powder was prepared from fermented broth by using the vacuum freeze-drying technology. The dried bacterial powder was subsequently mixed with feed and then pressed to pellets. After oral administration of the feed mixed with the recombinant L.casei powder, the expression of the immune-related genes (IFN I, IgM et al.) in grass carp was upregulated significantly. With the increased duration of oral immunization period, serum IgM level was also increased in grass carp. The survival analysis was carried out on the basis of grass carp in response to GCRV challenge. The result showed that the survival rate in the immunized group (74 %) was significantly higher than that in the control group (35 %). The amount of virus replication was also investigated in vaccine-treated fish. The result suggested that the virus content in fish tissue was also significantly less than that of the non-immunized group. The LC-pVE5523-VP5 was still present in the fish intestines 15 days after vaccination. These results indicated that the oral LC-pVE5523-VP5 can effectively protect grass carps from GCRV infection.

Keywords: Grass carp; IFN I; IgM; Lactobacillus casei; Oral vaccine.