The capacity of DNA vaccines to prime CD8+ T cells makes them excellent candidates for vaccines that are designed to emphasize cellular immunity. However, the long-term stability of CD8+ T cell memory induced by DNA vaccination is poorly characterized. Here, the quality of CD8+ T cell recall responses in mice was investigated more than 1 year after DNA vaccination with the Sendai virus nucleoprotein gene. Cytotoxic T lymphocyte (CTL) activity specific for both dominant and subdominant epitopes could be recalled readily 1 year after vaccination and the frequencies of CTL precursors specific for both of these epitopes were relatively high. These CTL responded strongly to subsequent Sendai virus infection in terms of their ability to migrate to the lung and to differentiate into effector cells. In addition, the recall response to virus infection, as determined by CTL activity in the lungs and IFN-gamma responses in the spleen, was both faster and greater in magnitude than that in control-immunized mice. Significantly, virus titres were reduced at least 100-fold in the lungs of mice that were immunized more than 1 year before infection, as compared with control mice. These data demonstrate that CD8+ T cell memory elicited by DNA vaccination is functionally relevant and persists for at least 1 year.


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