TCell Immune Responses After Immunization of Mice or Woodchucks

Immunization studies in mice and woodchucks support the hypothesis that Th-cell and CTL responses can be generated, that may contribute to a protective immunity to HDV. Different DNA immunization protocols using either plasmids expressing the small or the large HDAg induced significant CD4+ Th-cell proliferative responses in mice of different strains (Mauch et al. 2002). Huang et al. further characterized the cytokine profile after immunization in mice and found a prominent increase in Th1 cytokine production, especially in IFN-y and interleukin (IL)-2, but not in IL-4 (Huang et al. 2000).

A cytolytic activity of an HDV-specific CTL response was demonstrated by Mauch et al. in mice (Mauch et al. 2002). After immunization with DNA coding for the small or large HDAg specific CD4+ Th and CD8+ CTL responses were detected in different mouse strains. Both CD4+ and CD8+ T cells were required for the antitumor activity in a syngenic tumor model as determined by in vivo T-cell depletion experiments.

In woodchucks a proliferative Th-cell immune response was detected after immunization and after challenge, and was further characterized using a panel of overlapping synthetic peptides spanning the whole HDAg (D'Ugo et al. 2004; Fiedler et al. 2001). In both studies the small HDAg expressed in yeast was used. In our study HDAg/CpG immunization induced a Th-cell response in WHV carrier woodchucks equivalent to that induced in WHV negative ones by immunization with protein and complete Freund's adjuvant (CFA). Both protocols are known to induce a strong Th1 cell immune response

(Chu et al. 1997; Forsthuber et al. 1996; Schirmbeck et al. 1999). A polyspecific, but weak lymphoproliferative response was seen. Overall, 10 peptides were able to induce a Th-cell immune response (Fig. 2). The recognized epitopes grouped into two clusters, one near the N terminus and one closer to the C terminus of HDAg. We did not investigate the proliferative response after challenge. D'Ugo et al. detected a Th-cell immune response against HDAg in two of four woodchucks immunized with HDAg/MF59 and in three of four animals immunized with HDAg/CFA. Before challenge, no significant response to any of the peptides was detected. After challenge, all animals of the HDAg/MF59 group showed a specific response to at least two peptides, one animal of the HDAg/CFA group, and one the control group (three animals) showed also evident responses.

As described in detail later (see Sect. 4.1), the detected immune response in either study, however, was not able to protect from HDV superinfection. The differences in the detection of a Th-cell immune response after immunization could be caused by different immunization protocols and/or the outbred status of the woodchucks.

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