FIG. 4.

In vivo influenza-virus-specific CTL assay. A 1:1 mixture of 10⁷ cells of each target cell population was injected intravenously into mice 7 d after the third injection with the indicated DNA vaccines. After 4 h, the mice were killed and the splenocytes were analyzed for the presence of CFSE^high (pulsed with influenza epitopes), and CFSE^low (unpulsed) target cell populations. (A) Representative histogram plot of splenocytes obtained from a mouse vaccinated with each DNA construct at 4 h post-transfer of CFSE-labeled target cells. (B) The percentage of specific lysis (+standard deviation) in five animals was calculated as described in the materials and methods section (*p < 0.05, **p < 0.01 compared to the vector-immunized group; p < 0.05 for the pHA/NP-immunized group compared to the pHA-immunized group).