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

Authors: Xu, Dehai; Zhang, Dunhua; Shoemaker, Craig; Beck, Benjamin

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2020
Publication Date: 8/14/2020
Citation: Xu, D., Zhang, D., Shoemaker, C.A., Beck, B.H. 2020. Dose effects of a DNA vaccine encoding the immobilization antigen on the immune response of channel catfish against Ichthyophthirius multifiliis. Fish and Shellfish Immunology. 106: 1031-1041. https://doi.org/10.1016/j.fsi.2020.07.063.
DOI: https://doi.org/10.1016/j.fsi.2020.07.063

Interpretive Summary: A virulent ciliated parasite Ichthyophthirius multifiliis (Ich) results in severe economic loss to aquaculture worldwide. Vaccination against the parasite is an alternative to chemotherapy as currently no effective chemical treatment can be used to control this parasite. This study evaluated whether one single high dose or two low doses of DNA vaccine pcDNA3.1-IAg52b could provide protection for catfish against Ich when delivered with an adjuvant. Results of this study demonstrated that fish vaccinated with the adjuvanted DNA vaccine at a single dose of 20 µg fish -1 or two doses of 10 µg fish -1 had higher anti-Ich antibody levels than fish receiving a single dose of 10 µg fish -1. Survival was higher in fish received the DNA vaccine at 20 µg fish -1 (35.6%) or 2 doses of 10 µg fish -1 (48.9%) than fish that received a single dose of 10 µg fish -1 (15.6%) and mock-vaccinated control (0%). Fish vaccinated with the DNA vaccine showed higher up-regulation on the expression of immune response genes than mock-vaccinated control, including immunoglobulin M (IgM), cluster of differentiation 4 (CD4), major histocompatibility complex I (MHC I), and T cell receptor a (TcR-a). This study demonstrated that DNA vaccines can enhance catfish immune responses and provide fish protection against the parasite infection. Results of this study may allow for development of alternative control strategies for parasite Ich, which can reduce fish loss and increase profitability.

Technical Abstract: Ichthyophthirius multifiliis (Ich) is a virulent ciliated parasite that results in severe economic loss to aquaculture worldwide. Vaccination against the parasite is an alternative to chemotherapy as currently no effective chemical treatment can be used to control this parasite. Channel catfish (Ictalurus punctatus) vaccinated with pcDNA3.1-IAg52b plasmid DNA vaccine encoding immobilization antigen genes of Ich produced anti-Ich antibodies and were partially protected (20% survival) in a previous study. This study evaluated whether a higher dose or two doses of pcDNA3.1-IAg52b vaccine could provide better protection for catfish against Ich when delivered with an adjuvant QCDC (an adjuvant complex composed of Quil A, cholesterol, dimethyl dioctadecyl ammonium bromide and Carbopol). Fish were distributed into 6 groups and vaccinated using following schemes: 1. pcDNA3.1-IAg52b at 10 µg fish-1, 2. pcDNA3.1-IAg52b at 20 µg fish-1, 3. two doses of pcDNA3.1-IAg52b at 10 µg fish-1 with 7 days between doses, 4. pcDNA3.1 at 20 µg fish-1 (mock-vaccinated control), 5. live theronts at 15,000 theronts fish-1 (positive control), and 6. non-vaccinated and non-challenge control. The DNA vaccines (pcDNA3.1-IAg52b and pcDNA3.1 ) in groups 1-4 were adjusted to desired concentration and adjuvanted with QCDC for fish vaccination. Parasite infection levels, serum anti-Ich antibody levels, fish mortality and immune-related gene expression were determined during the trial. Fish vaccinated with QCDC adjuvanted pcDNA3.1-IAg52b at a single dose of 20 µg fish -1 or two doses of 10 µg fish -1 had higher anti-Ich antibody levels than fish receiving a single dose of 10 µg fish -1. Survival was significantly higher in fish received the vaccine at 20 µg fish -1 (35.6%) or 2 doses of 10 µg fish -1 (48.9%) than fish that received a single dose of 10 µg fish -1 (15.6%) and mock-vaccinated control (0%). Fish vaccinated at the dose 20 µg fish-1 had higher expression of vaccine DNA in muscle than fish vaccinated with 10 µg fish-1. Fish vaccinated with the DNA vaccine showed higher up-regulation than mock-vaccinated control in the expression of IgM, CD4, MHC I and TcR-a 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.