The virus-induced signaling adaptor molecule enhances DNA-raised immune protection against H5N1 influenza virus infection in mice

As an adaptor molecule in the retinoic acid-inducible gene-I (RIG-I) signaling pathway, the virus-induced signaling adaptor (VISA) molecule activates NF-κB and IRF3 and thereby leads to the production of type I interferons (IFNs). To explore the potential of VISA as a genetic adjuvant for DNA vaccines, a eukaryotic expression plasmid, pVISA, was generated by cloning the VISA gene into the pVAX1vector. For comparison, the pTRIF plasmid was similarly constructed, encoding the known genetic adjuvant TRIF (TIR-domain-containing adapter-inducing interferon-β), an adapter in the Toll-like receptor (TLR) signaling pathway. Mice were immunized with the chimeric DNA vaccine pHA/NP147–155, which encodes the HA (hemagglutinin) fused with NP (nucleoprotein) CTL epitope (NP147–155) of H5N1 influenza virus, either alone or in combination with pVISA or pTRIF. Antigen-specific immune responses were examined in immunized mice. Our results demonstrate that co-immunization of the pHA/NP147–155 plasmid with the VISA adjuvant augmented DNA-raised cellular immune responses and provided protection against H5N1 influenza virus challenge in mice. In addition, our data suggest that VISA acts as a stronger adjuvant for DNA immunization than TRIF. We conclude that co-inoculation with a vector expressing the adaptor molecule VISA enhanced the protective immunity against H5N1 infection induced by pHA/NP147–155 and that VISA could be developed as a novel genetic adjuvant for DNA vaccines.