Live attenuated influenza virus expressing human interleukin-2 reveals increased immunogenic potential in young and aged hosts.

Despite the reported efficacy of commercially available influenza virus vaccines, a considerable proportion of the human population does not respond well to vaccination. In an attempt to improve the immunogenicity of live influenza vaccines, an attenuated, cold-adapted (ca) influenza A virus expressing human interleukin-2 (IL-2) from the NS gene was generated. Intranasal immunization of young adult and aged mice with the IL-2-expressing virus resulted in markedly enhanced mucosal and cellular immune responses compared to those of mice immunized with the nonrecombinant ca parent strain. Interestingly, the mucosal immunoglobulin A (IgA) and CD8(+) T-cell responses in the respiratory compartment could be restored in aged mice primed with the IL-2-expressing virus to magnitudes similar to those in young adult mice. The immunomodulating effect of locally expressed IL-2 also gave rise to a systemic CD8(+) T-cell and distant urogenital IgA response in young adult mice, but this effect was less distinct in aged mice. Importantly, only mice immunized with the recombinant IL-2 virus were completely protected from a pathogenic wild-type virus challenge and revealed a stronger onset of virus-specific CD8(+) T-cell recall response. Our findings emphasize the potential of reverse genetics to improve the efficacy of live influenza vaccines, thus rendering them more suitable for high-risk age groups.