PROTECTIVE EFFICACY OF DNA VACCINES AGAINST DUCK HEPATITIS-B VIRUS-INFECTION

The efficacy of DNA vaccines encoding the duck hepatitis B virus (DHBV ) pre-S/S and S proteins were tested in Pekin ducks. Plasmid pcDNA I/A mp DNA containing the DHBV pre-S/S or S genes was injected intramuscul arly three times, at 3-week intervals. All pre-S/S and S-vaccinated du cks developed total anti-DHBs and specific anti-S antibodies with simi lar titers reaching 1/10,000 to 1/50,000 and 1/2,500 to 1/4,000, respe ctively, after the third vaccination. However, following virus challen ge, significant differences in the rate of virus removal from the bloo dstream and the presence of virus replication in the liver were found between the groups. In three of four S-vaccinated ducks, 90% of the in oculum was removed between <5 and 15 min postchallenge (p.c.) and no v irus replication was detected in the liver at 4 days p.c. In contrast, in all four pre-S/S-vaccinated ducks, 90% of the inoculum was removed between 60 and 90 min p.c. and DHBsAg was detected in 10 to 40% of he patocytes. Anti-S serum abolished virus infectivity when preincubated with DHBV before inoculation into 1-day-old ducklings and primary duck hepatocyte cultures, while anti-pre-S/S serum showed very limited cap acity to neutralize virus infectivity in these two systems. Thus, alth ough both DNA vaccines induced high titers of anti-DHBs antibodies, an ti-S antibodies induced by the S-DNA construct were highly effective i n neutralizing virus infectivity,while similar levels of anti-S induce d by the pre-S/S-DNA construct conferred only very limited protection. This phenomenon requires further clarification, particularly in light of the development of newer HBV vaccines containing pre-S proteins an d a possible discrepancy between anti-HBs titers and protective effica cy.