Immunization of Neonates with DNA Encoding a Bovine Herpesvirus Glycoprotein Is Effective in the Presence of Maternal Antibodies*

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Abstract
Neonates generally display low immune responsiveness to conventional vaccines, which may be due to the immaturity of their immune system and interference by maternal antibodies. Because of the unique capacity of plasmid DNA for the production of low doses of antigen over extended periods of time, we used DNA immunization as an approach to induce immunity in neonates. Previously, we demonstrated that a plasmid encoding a truncated secreted version of bovine herpesvirus-1 gD (tgD) induces protective immunity in adult animals. For the present study, 3-day-old lambs were immunized intradermally with the tgD-expressing plasmid. The lambs developed antibody as well as T-cell responses to the tgD glycoprotein, which clearly demonstrates the ability of the animals to respond to vaccination at this age. Furthermore, lambs born to tgD-hyperimmunized ewes, thus containing high levels of passively acquired serum antibodies, responded to the tgD DNA vaccine in a similar manner, which shows that the maternal antibodies did not inhibit the development of an immune response. These results indicate that DNA immunization might be a useful approach to vaccinate neonates that possess high levels of maternal antibodies.