A truncated C-terminal fragment of Mycobacterium tuberculosis HSP70 gene enhanced potency of HBV DNA vaccine

DNA vaccine represents an attractive approach to therapy of chronic hepatitis B virus (HBV) infection because of its ability to generate antigen-specific immunity; nevertheless, there is still a need to increase the potency of DNA vaccine. Mycobacterium tuberculosis heat shock protein70 (HSP70) has both chaperon and cytokine functions, and has been shown to act as an adjuvant when co-administered with peptide antigens or given as fusion proteins. Here we evaluated the effects of two truncated HSP70 molecules, N-terminal domain (HSP701–360, amino acids 1–360) and C-terminal domain (HSP70359–610, amino acids 359–610) of mycobacterial HSP70, on the potency of antigen-specific immunity generated by a HBV DNA vaccination. We found that only the HSP70359–610-fused HBV DNA vaccination resulted in a significant increase in hepatitis B surface antigen (HBsAg)-specific humoral response, while the HSP701–360- or the complete HSP70 molecule-fused vaccine did not. Moreover, HSP70359–610-fused DNA vaccine did not induce anti-HSP70 antibody. Interestingly, HSP70359–610 not only enhanced HBsAg-specific cytotoxic lymphocytes (CTL) responses but also overcame the epitope suppression caused by Ld-restricted epitope. Meanwhile, HSP70369–610 mediated T helper (Th) cell balance towards Th1 pathway. In a HBV transgenic mouse model, the HSP70359–610 fusion vaccine facilitated clearance of circulating HBsAg and down-regulation of HBV replication. These results suggested that the truncated mycobacterial HSP70 molecule, HSP70359–610, might be a superior candidate to deliver the adjuvant function in HBV DNA vaccination instead of the complete HSP70 molecule.