Dose dependence of CTL precursor frequency induced by a DNA vaccine and correlation with protective immunity against influenza virus challenge

Intramuscular injection of BALB/c mice with a DNA plasmid encoding nucleopr otein (NP) from influenza virus A/PR/8/34 (H1N1) provides cross-strain prot ection against lethal challenge with influenza virus A/HK/68 (H3N2). CTL sp ecific for the H-2K(d)-restricted epitope NP147-155 are present in these mi ce and are thought to play a role in the protection. To assess the effectiv eness of NP DNA immunization in comparison with influenza virus infection i n the induction of CTL responses, we monitored the frequency of CTL precurs ors (CTLp) in mice following i.m. injection with NP DNA or intranasal infec tion with influenza virus and showed that the CTLp frequency in NP DNA-immu nized mice can reach levels found in mice that had been infected with influ enza virus. We also measured the CTLp frequency, anti-NP Ab titers, and T c ell proliferative responses in mice that were injected with titrated dosage s of NP DNA and documented a correlation of the CTLp frequency and the Ab t iters, but not proliferative responses, with the injection dose. Furthermor e, we observed a positive correlation between the frequency of NP147-155 ep itope-specific CTLp and the extent of protective immunity against cross-str ain influenza challenge induced by NP DNA injection. Collectively, these re sults and our early observations from adoptive transfer experiments of in v itro activated lymphocytes from NP DNA-immunized mice suggest a protective function of NP-specific CTLp in mice against cross-strain influenza virus c hallenge.