| Background Gastric cancer is the No. 1 killer among all malignant tumors in China. Conventional methods work poorly in treating gastric cancer, and with few tumor-specific antigen identified, there is no effective cancer vaccine developed to combat gastric cancer. MG7-Ag, discovered by our institute, is a marker molecule of gastric cancer. The antigenicity of MG7-Ag was found to locate in a suger chain, which suggested that the MG7-Ag was a kind of carbohydrate antigen. By screening the phage display library, we have identified the mimicry peptide of MG7-Ag, competitive antibody binding assay showed that the mimicry peptide could mimic the primary antigen well, which provided the premise for further using it to develop vaccines. DNA vaccine is a relatively new kind of vaccine. It can induce specific, potent and persisting immunity against given antigen and can be easily constructed. There have been some human experiments of DNA vaccines against tumors such as myeloma, lymphoma, melanoma and prostate cancer, their findings showed that tumor DNA vaccines can induce anti-tumor immunity and the DNA given proved to be safety. The immunizing routes of DNA vaccines include: intramuscular injection, intradermal injection, subcutaneous injection and skin gene gun bombard. It is recently discovered oral administration maybe another way. By using a attenuated Salmonella typhimurium as carrier, studies found that the eukaryotic expression vectors of exogenous antigen harbored by the bacteria could be transferred into dendritic cells (DC) in the spleen then ex- pressed and presented by the DC. As the DCs are professional antigen presenting cells, it seems that oral DNA vaccines using S. typhimurium is more potent in inducing immune responses. Animal experiments have found that oral DNA vaccines using S. typhimurium as carrier provided potent protection against certain bacteria and tumors, and animals immunized survived fatal dose of bacteria or tumor challenges. Several measures have been taken to improve the potency of DNA vaccines. These measures include: co-admistration of cytokines or plasmids of cytokines such as IL-i, 11-2, GM-CSF and JF7N- i; conjugating antigens with carrier protein such as KLH and HBcAg; developing multi-epitope DNA vaccines; including in the vaccine some inimunostimulating motif of certain plasmids, for example CpG et al. As the CD4+ T helper cells (TH cells) play important roles in immune response, immobilizing the CD4+ TH cells may enhance immune response induced by vaccines. Studies have found that when TH epitopes and CTL epitopes (or B cell epitopes) were included into DNA vaccines in tandem, the DNA vaccines induced more potent immune response than those without TH epitopes. In such cases, the TH epitopes and CTL epitopes were presented by the same APC and thus brought the TH cells in proximity to the CTLs, which facilitated the helper effect. TH cells produce TEll or TH2 type cytokines to stimulate CTL and B cells in the cellular and humoral immunity. Interactions between TEl cells and APCs can upregulate the expression of costimulatory molecules on the APCs and improve their ability of antigen presenting. It has been clearly shown that DNA vaccines including both tumor specific CTL (or B cell) epitopes and specific TH epitopes are more potent in inducing anti-tumor immunity. In 1994, Alexander et al invented a universal TH epitope by introducing MI-IC II binding anchoring amino acids into a polyalanine backbone. PADRE (Pan-DR binding epitope),...
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