Dose effects of a DNA vaccine encoding immobilization antigen on immune response of channel catfish against Ichthyophthirius multifiliis.

Channel catfish (Ictalurus punctatus) vaccinated with pcDNA3.1-IAg52b plasmid DNA vaccine encoding immobilization antigen genes of Ichthyophthirius multifiliis (Ich) produced anti-Ich antibodies and were partially protected (20% survival) in a previous study. Here we evaluated whether a higher dose or two doses of pcDNA3.1-IAg52b vaccine could provide better protection for catfish against Ich. Fish were distributed into 6 groups and vaccinated using following schemes: 1.10 μg pcDNA3.1-IAg52b fish, 2.20 μg pcDNA3.1-IAg52b fish, 3. two doses of 10 μg pcDNA3.1-IAg52b fish with 7 days between doses, 4.20 μg pcDNA3.1 fish (mock-vaccinated control), 5.15,000 live theronts fish (positive control), and 6. non-vaccinated and non-challenge control. Parasite infection levels, serum anti-Ich antibody levels, fish mortality and immune-related gene expression were determined during the trial. Fish vaccinated with a single dose of 20 μg pcDNA3.1-IAg52b fish or two doses of 10 μg fish had higher anti-Ich antibody levels than fish receiving a single dose of 10 μg fish. Survival was significantly higher in fish receiving 20 μg vaccine fish (35.6%) or 2 doses of 10 μg fish (48.9%) than fish injected with a single dose of 10 μg fish (15.6%) or mock-vaccinated control (0%). Fish vaccinated at the dose 20 μg fish had higher expression of vaccine DNA in muscle than fish vaccinated with 10 μg fish. Fish vaccinated with the DNA vaccine showed higher up-regulation than mock-vaccinated control in the expression of IgM, CD4, MHC I and TcR-α genes during most of time points after vaccination. Further studies are needed to improve efficacy of DNA vaccines by using multiple antigens in the DNA vaccines.