AIM: Persistent hepatitis B virus (HBV) infection is characterized by a weak CD8+ T cell response to HBV. Immunotherapeutic strategies that overcome tolerance and boost these suboptimal responses may facilitate viral clearance in chronically infected individuals. Therefore, we examined whether CD25+CD4+ regulatory T (Treg) cells might be involved in a inhibition of CD8+T cell priming or in the modulation of the magnitude of the ’peak’ antiviral CD8+ T cell response primed by DNA immunization. METHODS: B10.D2 mice were immunized once with plasmid pCMV-S. Mice received 500 μg of anti-CD25 mAb injected intraperitoneally 3 d before DNA immunization to deplete CD25+ cells. Induction of HBV-specific CD8+ T cells in peripheral blood mononuclear cells (PBMCs) was measured by S28-39 peptide loaded DimerX staining and their function was analyzed by intracellular IFN-γ staining. RESULTS: DNA immunization induced HBV-specific CD8+ T cells. At the peak T cell response (d 10), 7.1±2.0% of CD8+ T cells were HBV-specific after DNA immunization, whereas 12.7±3.2% of CD8+ T cells were HBV-specific in Treg-depleted mice, suggesting that DNA immunization induced more antigen-specific CD8+ T cells in the absence of CD25+ Treg cells (n = 6, P<0.05). Similarly, fewer HBV specific memory T cells were detected in the presence of these cells (1.3±0.4%) in comparison to Treg-depleted mice (2.6±0.9%) on d 30 after DNA immunization (n - 6, P<0.01). Both IFN-γ production and the avidity of the HBV-specific CD8+ T cell response to antigen were higher in HBV-specific CD8+ T cells induced in the absence of Treg cells. CONCLUSION: CD25+ Treg cells suppress priming and/or expansion of antigen-specific CD8+ T cells during DNA immunization and the peak CD8+ T cell response is enhanced by depleting this cell population. Furthermore, Treg cells appear to be involved in the contraction phase of the CD8+ T cell response and may affect the quality of memory T cell pools. The elimination of Treg cells or their inhibition may be important in immunotherapeutic strategies to control HBV infection by inducing virus-specific cytotoxic T lymphocyte responses in chronically infected subjects. Immunotherapeutic strategies that overcome tolerance and boost these suboptimal responses may facilitate viral clearance in chronically infected individuals. Thus, we review whether CD25 + CD4 + regulatory T (Treg) cells might be involved in a inhibition of CD8 + T cell priming or in the modulation of the magnitude of the ’peak’ antiviral CD8 + T cell response primed by DNA immunization. METHODS: B10.D2 mice were immunized once with the plasmid pCMV-S. Mice received 500 μg of anti-CD25 mAb injected intraperitoneally 3 d before DNA immunization to deplete CD25 + cells. Induction of HBV-specific CD8 + T cells in peripheral blood mononuclear cells (PBMCs) was measured by S28-39 peptide loaded DimerX staining and their function was analyzed by intracellular IFN-γ staining. RESULTS: DNA immunization induced HBV-specific CD8 + T cells. At the peak T cell response (d10), 7.1 ± 2.0% of CD8 + T suggesting that DNA immunization induced more antigen-specific CD8 + T cells in the absence of CD25 + Treg cells (n = 12.7 ± 3.2% of CD8 + T cells were HBV- specific in Treg- depleted mice. 6, P <0.05). Similarly, fewer HBV specific memory T cells were detected in the presence of these cells (1.3 ± 0.4%) in comparison to Treg-depleted mice (2.6 ± 0.9%) on d 30 after DNA immunization - 6, P <0.01). Both IFN-γ production and the avidity of the HBV-specific CD8 + T cell response to antigen were higher in HBV-specific CD8 + T cells induced in the absence of Treg cells. CONCLUSION: CD25 + Treg cells suppress priming and / or expansion of antigen-specific CD8 + T cells during DNA immunization and the peak CD8 + T cell response is enhanced by depleting this cell population. Furthermore, Treg cells appear to be involved in the contraction phase of the CD8 + T cell response and may affect the quality of memory T cell pools. The elimination of Treg cells or their inhibition may be important in immunotherapeutic strategies to control HBV infection by inducing virus-specific cytotoxic T lymphocyte responses in chronically infected subjects.