Plasmid DNA switches on Th1 response in mice immunized with antigen-antibody complex

Objective Our previous study showed that plasmid DNA could substitute alum as an adjuvant to enhance anti-HBs response to hepatitis B surface antigen (HBsAg)-anti-HBs immunogenic complex (IC) in mice (Viral Immunology, 1998, 11:65). In the present study, we further investigated the mechanisms of plasmid DNA on the immunogenicity of IC. Methods BALB/c mice were immunized separately with (a) HBsAg, (b) IC, (c) pI/AmpHBs DNA, a plasmid encoding HBsAg, (d) IC combined with pI/AmpHBs, and (e) IC combined with the vector pI/Amp by i.m injection. Anti-HBs IgG subclasses in mouse serum were analyzed. The expression of IFN-γ mRNA and IL-4 mRNA in spleen cells from mice immunized with HBsAg in vitro were assayed by semi-quantification RT-PCR. Results It was showed that serum anti-HBs titer from group d was the highest among immunized groups, and its ratio of IgG2a/IgG1 reaching 0.91 was higher than that from group a (0.73) and group b (0.82), but was lower than that from group c (1.12). Higher level of IFN-γ mRNA was found in spleen cells of group d and its ratio T/C (target/competitor) was 6.25 and 9.09, at 24 and 48 hours after HBsAg-stimulation. IFN-γ mRNA level detected in group d was tenfold higher than that in group b (T/C ratio 0.83/0.13). T/C ratio of IFN-γ in group a was 0.57/0.02, group c 1.05/0.24 and group e 3.68/0.06. IL-4 mRNA level detected in group d was also higher than that in other groups. T/C ratio of IL-4 in group d was 0.96 and 1.68 at 24 and 48 hours after HBsAg-stimulation meaning twofold higher than that in group b (T/C ratio 0.43 and 0.79). Conclusion Results indicated that plasmid DNA combined with IC switches on Th1 response with no adverse effects on inducing high titer of anti-HBs.