Abstract

Despite the existence for some time of effective prophylactic vaccines, hepatitis B virus (HBV) infection remains an important global concern. Improvements on existing vaccines could be beneficial, especially in situations where it is desirable or necessary to induce protective immunity more rapidly or with fewer doses. We have compared, in chimpanzees, a current HBV vaccine that contains recombinant hepatitis B surface antigen HBsAg) adsorbed to alum, with two novel vaccine strategies that have proven superior to the current vaccine in mice. The first approach was the use of oligodeoxynucleotides containing CpG motifs (CpG ODN) as an adjuvant to Engerix-B, a commercial HBV vaccine. The addition of CpG ODN to Engerix-B greatly improved the kinetics and magnitude of the humoral response, suggesting that CpG ODN might allow induction of protective immunity in humans more quickly and with fewer vaccine doses. All animals receiving either control or CpG-containing subunit vaccines at 0 and 4 weeks attained titers of HBsAg-specific antibody (anti-HBs) considered protective (> or =10 mIU/ml) and were indeed protected from challenge at 8 weeks with 10(3.5) 50% chimp infectious doses (CID(50)) of intravenous HBV. The second approach was a DNA vaccine with a plasmid vector optimized for content of immunostimulatory CpG motifs. Despite the fact that earlier studies had shown four doses of a similar DNA vaccine (except not optimized for CpG content) to induce strong humoral responses in 1 of 2 chimpanzees, in this study two doses of DNA vaccine (at 0 and 4 weeks) did not generate any detectable anti-HBs in either of 2 chimpanzees, although it did protect 1 that rapidly developed anti-HBs during the incubation period, suggesting priming of an antibody response. The poor results may be due to an inadequate number of doses or amount of plasmid DNA in these larger animals, but nevertheless point to the need to improve delivery methods for DNA vaccines for use in larger animals such as primates.