Adjuvant effect of CD40 on H5N1 DNA vaccine in mice.

H5N1 viruses cause severe and often fatal disease in humans. DNA vaccines have been shown to provide some protection in many animal models against H5N1 influenza virus infection, but this protection is not complete. To enhance the immunogenicity of an H5N1 DNA vaccine, we constructed the eukaryotic expression systems pcD-CD40 and pcD-HA. The expression of pcD-HA or pcD-CD40 in transfected BHK cells was confirmed by reverse transcription polymerase chain reaction (RT-PCR). The constructs were then used to immunize BALB/c mice intramuscularly three times at two-week intervals. The titers of serum HA-specific antibodies were determined by ELISA, and the expression levels of the cytokines IL-2, IL-4, IL-6 and TNF-α were determined by real-time PCR. The results showed that CD40 as a molecular adjuvant significantly enhanced the production of serum anti-HA antibodies and increased the levels of the Th2 cytokines IL-4 and IL-6, suggesting that co-immunization with CD40 upregulated the humoral immune responses to the DNA vaccine in BALB/c mice. This study will provide important information for the selection of adjuvants for DNA vaccines against HPAI H5N1 viruses or other subtypes of human influenza viruses.