Vaccine challenge of PGK.142T-treated mice. PGK- and PGK.142T-treated BALB/c mice were vaccinated at 6 weeks after vector injection by administration of a plasmid encoding GFP into cardiotoxin-injured muscle. Analysis was performed 12 days after vaccination. (A-B) The frequency of IFN-γ–producing GFP-specific CD8⁺ T cells in the spleen and liver of treated mice was determined by ELISPOT. Data are expressed as the mean ± SD number of GFP-specific CD8⁺ T cells per 10⁶ total CD8⁺ T cells. Note that vaccination was able to induce a primary immune response to GFP in PBS-treated mice, and a secondary immune response to GFP in PGK-treated mice, but not in PGK.142T-treated mice. (C) Liver sections were analyzed by confocal microscopy after staining for GFP and nuclei. Scale bar = 100 μm. Images are representative of 2 separate experiments (n = 6 mice/group). Also shown is the mean C/G from the livers of treated mice (n = 6).