Live recombinant Lactococcus lactis vaccine expressing immobilization antigen (i-Ag) for protection against Ichthyophthirius multifiliis in goldfish.

The parasite Ichthyophthirius multifiliis (Ich) has been reported in various freshwater fishes worldwide and results in severe losses to both food and aquarium fish production. 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, the present study was conducted to evaluate a live recombinant Lactococcus lactis vaccine expressing immobilization antigen (IAG-52X) in protection against I. multifiliis. A 1266 bp gene fragment containing a potential antigenic epitope of the 48 kDa immobilization antigen of I. multifiliis was assembled from six synthetic ohgonucleotides and cloned into pSIP409 and electrotransformed into Lactobacillus plantarum NC8. The recombinant vaccine candidate was then orally fed into goldfish. The expression of immune-related genes: complement component 3 (C3), MHC I, IgM gene in blood from goldfish at different time points after immunization were evaluated. Immunized fish were than challenged with a lethal dose of infectious I. multifiliis. The cumulative mortality and relative percentage survival (RPS) were also determined. Our results showed that the antibody level in the blood and skin of the immunized fish was statistically significant (P < 0.05) in relation to the control groups. Goldfish orally immunized with NC8-pSIP409- IAG-52X had high serum antibody titers that ranged from 32 to 256 after 28d post immunization, while fish fed with NC8-pSIP409 or PBS had no detectable immobilizing antibody response. Expression of IgM, C3, MHC I genes in the group immunized with IAG-52X were significantly (P < 0.05) up regulated as compared with control group, indicating that different immune cells were actively involved in cellular immune response. The results showed that the average survival rate of fish orally immunized with 10 ⁸ and 10 ⁶ NC8-pSIP409-IAG-52X was 60% and 50% respectively. Therefore, NC8-pSIP409-IAG-52X could become a promising oral vaccine candidate against I. multifiliis.
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