Cimetidine (CIM), a histamine 2-receptor antagonist, is postulated to enhance immune responses owing to its inhibitory effects on suppressor T cells. In this report, we evaluated effects of cimetidine on the potency of antigen-specific immunity generated by DNA vaccine encoding hepatitis B surface antigen (HBsAg, pcD-S2). Our data demonstrate that CIM as adjuvant significantly increased HBsAg-specific cell-mediated and humoral immunities that were characterized by higher Ig2a/IgG1 ratio. In addition, CIM significantly promotes an elevated level of IL-4 and IFN-gamma in antigen-specific CD4(+) T cells and a robust antigen-specific cytotoxic response in the animals immunized with pcD-S2 plus CIM. Further, CIM induces pro-inflammatory cytokine expression such as the IL-12 and down-regulates anti-inflammatory cytokine expression such as IL-10 and TGF-beta, which may lead to an impairment of CD4(+)CD25(+) Treg cell-mediated suppression. Collectively these findings suggest that CIM enhances the immune responses of HBV DNA vaccine through the stimulation of pro-inflammatory and inhibition of anti-inflammatory cytokine expression patterns. (C) 2008 Elsevier Inc. All rights reserved.