Adjuvant effects of plasmid-generated hairpin RNA molecules on DNA vaccination

Cellular recognition of double-stranded RNA and subsequent antiviral molecular events are important components of host defense, which are responsible for initiating innate immune responses to infection. Here we showed that hairpin RNA molecules with various stem lengths, which were transcribed from antigen-encoding plasmids had profound effects on the host cell apoptosis, foreign gene expression as well as the antigen-specific immune responses elicited by DNA vaccination. The plasmid generating the short-stem (40 bp) RNA molecule showed slight effects on cell apoptosis and reporter gene expression level but stimulated significantly enhanced antigen-specific cellular immune responses. Although the DNA construct encoding the long-stem (750 bp) RNA induced vigrous cell apoptosis, no significant improvement in cell-medicated immune responses was observed when mice were immunized with DNA vaccines encoding the long-stem RNA. In addition, our data also showed that none of DNA vaccine constructs carrying hairpin RNA cassettes enhanced antigen-specific humoral immune responses. This study introduced a novel approach for improving plasmid vector design and showed that the DNA vectors generating hairpin RNA in vivo have a great potential in vaccination and immunotherapy against infectious and malignant diseases..