Abstract

Fish consumption has dramatically increased while wild fish stocks are in decline. Therefore, the commercial aquaculture industry is expanding worldwide in order to provide sufficient high quality fish protein. Unfortunately, serious losses from disease outbreaks destroy approximately 10% of all cultured fish. Due to a lack of effective therapies, vaccination and quarantine are the only options for controlling infectious fish disease. DNA vaccination is a novel method to produce immunizing protein, in vivo that elicits strong and long-lasting humoral and cellular immunity. Nonspecific immunity is the first line of host defense, and it is also the inducer of specific immunity. The objective of this study was to determine if DNA vaccines could induce a nonspecific cytotoxic cell response in channel catfish. Channel catfish fingerlings were immunized with either a plasmid encoding a reporter gene (pCMV-β), a plasmid without a gene insert [pcDNA 3.1 (+)], or saline as control. Nonspecific cytotoxicity in the two plasmid-injected groups was increased compared with saline-injected fish, but there was no difference in cytotoxicity between the plasmid-injected groups. These results suggest that unmethylated CpG-like motifs in the plasmid backbone of DNA vaccines are able to induce nonspecific cytotoxic activity. To test this hypothesis, catfish were injected with either unmethylated plasmid or DNase treated plasmid. Catfish injected with DNase treated plasmid had decreased nonspecific cytoxicity when compared with catfish injected with intact plasmid. These findings suggest that the plasmid backbone of DNA vaccines can enhance nonspecific cytotoxicity in channel catfish. Further studies are underway to evaluate the role of plasmid CpG motifs in channel catfish DNA vaccines. Additionally, the cooperation between the nonspecific immune enhancement from the plasmid vector and the specific immune induction from the expressed antigen gene insert will be elucidated.