The effector functions of specific CD8 T cells are crucial in mediating
influenza heterologous protection. However, new approaches for
influenza vaccines that can trigger effective CD8 T cell responses have not been extensively explored. We report here the generation of single-cycle infectious influenza virus that lacks a functional
hemagglutinin (HA) gene on an X31 genetic background and demonstrate its potential for triggering protective CD8 T cell immunity against heterologous influenza virus challenge. In vitro, X31-sciIV can infect MDCK cells, but infectious virions are not produced unless HA is transcomplemented. In vivo, intranasal immunization with X31-sciIV does not cause any clinical symptoms in mice but generates
influenza-specific CD8 T cells in lymphoid (mediastinal lymph nodes and spleen) and nonlymphoid tissues, including lung and bronchoalveolar lavage fluid, as measured by H2-Db NP366 and PA224 tetramer staining. In addition, a significant proportion of X31-sciIV-induced
antigen-specific respiratory CD8 T cells expressed
VLA-1, a marker that is associated with heterologous
influenza protection. Further, these
influenza-specific CD8 T cells produce
antiviral cytokines when stimulated with NP366 and PA224
peptides, indicating that CD8 T cells triggered by X31-sciIV are functional. When challenged with a lethal dose of heterologous PR8 virus, X31-sciIV-primed mice were fully protected from death. However, when CD8 T cells were depleted after priming or before priming, mice could not effectively control virus replication or survive the lethal challenge, indicating that X31-sciIV-induced memory CD8 T cells mediate the heterologous protection. Thus, our results demonstrate the potential for sciIV as a CD8 T cell-inducing
vaccine. Importance: One of the challenges for
influenza prevention is the existence of multiple influenza virus subtypes and variants and the fact that new strains can emerge yearly. Numerous studies have indicated that the effector functions of specific CD8 T cells are crucial in mediating
influenza heterologous protection. However,
influenza vaccines that can trigger effective CD8 T cell responses for heterologous protection have not been developed. We report here the generation of an X31 (H3N2) virus-derived single-cycle infectious influenza virus, X31-sciIV. A one-dose immunization with X31-sciIV is capable of inducing functional influenza virus-specific CD8 T cells that can be recruited into respiratory tissues and provide protection against lethal heterologous challenge. Without these cells, protection against lethal challenge was essentially lost. Our data indicate that an
influenza vaccine that primarily relies on CD8 T cells for protection could be developed.