Different immunization approaches have been tested to induce a protective immune response against HDV superinfection in chronically WHV-infected woodchucks: immunization with peptides, with prokaryortic and eukaryortic expressed protein, or with vaccinia virus or DNA expressing HDAg. None of these protocols was able to prevent HDV superinfection in all animals.
It is difficult to assess and to compare the studies, particularly because the monitoring after immunization and after challenge is described insufficiently in some studies and the sensitivity of the tests to measure HDV viremia is not comparable. Together these results allow us, however, to conclude that immunization against HDV with conventional vaccines is not possible. Neither a good humoral nor a weak Th-cell immune response is sufficient to protect from HDV superinfection. Immunizations with vaccinia virus or DNA expressing HDAg were at least able to modulate the course of infection. In general, these approaches are known to induce a CTL response, which maybe essential for protection. In further studies HDV vaccination schemes should be optimized to enhance the cellular immune response, e.g., by using different combinations of DNA vaccines with woodchuck cytokine (Lohrengel et al. 1998, 2000). This approach has been shown to enhance the cellular immune response to other hepadnavirus antigens (Mauch et al. 2002; Siegel et al. 2001). In future vaccination studies the monitoring of the immune response with respect to B- and T-cell responses has to be improved to see whether vaccination induced a measurable immune response prior to challenge.
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