Antigens-specific T cell responses. In test groups, female BalB/C mice were intradermally vaccinated with pGag, pEnv or pPol, in the absence or presence of pTat. Blank plasmid group was used as negative control. pTat alone group was designed to test whether there are cross-reactive epitopes between Tat and HIV antigens Gag, Env and Pol (splenocytes from mice vaccinated with pTat alone were stimulated by Gag, Env and Pol peptides, respectively). Fresh splenocytes were stimulated by the corresponding positive peptides, and antigen-specifc IFN-gamma response was detected by ELISPOT assay. These experiments were repeated once. The data was shown as means ± SD. Part A showed the Gag-specific IFN-gamma response (splenocytes were stimulated by Gag peptide). “pGag(2)”: two immunizations with Gag expression plasmid alone (50 μg pGag and 50 μg empty vector each time); “pGag + pTat(2)”: two immunizations with the mixture of Gag expression plasmid and Tat expression plasmid (50 μg pGag and 50 μg pTat each time); “pGag(3)”: three immunizations with Gag expression plasmid alone (50 μg pGag and 50 μg empty vector each time); “pTat”: two immunizations with pTat (50 μg pTat and 50 μg empty vector each time); “blank”: two immunizations with empty vector (100 μg each time). Part B (Env-specific IFN-gamma response) and Part C (Pol-specific IFN-gamma response) can be done in the same manner. Symbol** means P < 0.01; Symbol* means P < 0.05.