Damping of luciferase expression from a plasmid DNA vaccine construct requires MHC class II molecules and CD4⁺ T cells. (A) Luciferase expression in wild-type and MHC class II knockout (MHC class II KO) mice. Wild-type and MHC class II KO mice were inoculated with a plasmid DNA-Luc vaccine construct (n = 4), and luciferase expression was measured in vivo by IVIS imaging. (B) Luciferase expression in wild-type and CD4⁺ T cell–depleted mice. CD4⁺ T cells were depleted by injecting anti-CD4 antibody 3 days before and every third day after plasmid DNA-Luc inoculation for 28 days, and in vivo luciferase expression was measured. (C,D) SIV-gag AL11 epitope-specific CD8⁺ T-cell responses in vaccinated wild-type and MHC class II KO or CD4⁺ T-cell antibody–depleted mice. Epitope-specific CD8⁺ T-cell responses were measured by D^d/AL11 tetramer staining of CD8⁺ T cells at the indicated times after plasmid DNA vaccine inoculation. The line graphs represent mean values; error bars represent SEM. Statistically significant differences at specific times were determined by the Mann-Whitney test. *Significant difference (P < .05).