The effect of eukaryotic expression vectors and adjuvants on DNA vaccines in chickens using an avian influenza model

Vaccination of poultry with naked plasmid DNA has been successfully demonst rated with several different poultry pathogens, but the technology needs to be further developed before it can be practically implemented. Many differ ent methods can conceivably enhance the efficacy of DNA vaccines, and this report examines the use of different eukaryotic expression vectors with dif ferent promoters and different adjuvants to express the influenza hemagglut inin protein. Four different promoters in five different plasmids were used to express the hemagglutinin protein of an H5 avian influenza virus, inclu ding two different immediate early cytomegaloviruses (CMVs), Rous sarcoma v irus, chicken actin, and simian virus 40 promoters. All five constructs exp ressed detectable hemagglutinin protein in cell culture, but the pCI-neo HA plasmid with the CMV promoter provided the best response in chickens when vaccinated intramuscularly at 1 day of age on the basis of antibody titer a nd survivability after challenge with a highly pathogenic avian influenza v irus at 6 wk postinoculation. A beneficial response was observed in birds b oostered at 3 wk of age, in birds given larger amounts of DNA, and with the use of multiple injection sites to administer the vaccine. With the use of the pCI-neo construct, the effects of different adjuvants designed to incr ease the uptake of plasmid DNA, including 25% sucrose, diethylaminoethyl de xtran, calcium phosphate, polybrene, and two different cationic liposomes, were examined. Both liposomes rested enhanced antibody titers as compared w ith the positive controls, but the other chemical adjuvants decreased the a ntibody response as compared with the control chickens that received just t he plasmid alone. The results observed are promising for continued studies, but continued improvements in vaccine response and reduced costs are neces sary before the technology can be commercially developed.