As a genetic adjuvant, CTA improves the immunogenicity of DNA vaccines in an ADP-ribosyltransferase activity- and IL-6-dependent manner.

Cholera toxin (CT) and its subunits (A and B) have been intensively investigated as adjuvants for protein-based vaccines. Their underlying mechanisms vary with respect to the inoculation route used. By fusing the CTA gene to either the HIV-1-derived Tat-Rev-Vif-Integrase-Nef fusion gene or the OVA gene, our study showed that the fusion of CTA in these DNA vaccines had no cytotoxic effect in vitro and significantly improved both the quantity and quality of the elicited CD8(+) T cell responses. Further experiments identified that the fusion of CTA in these DNA vaccines augmented the secretion of IL-6 in a manner that was dependent on its ADP-ribosyltransferase activity, and protein kinase A (PKA) was found to be the major mediator of its downstream signaling. By site-directed mutagenesis of the ADP-ribosyltransferase catalytic center and in vivo RNAi, we demonstrated that the ADP-ribosyltransferase activity and the upregulation of IL-6 were required for the CTA gene-mediated adjuvant effect. These findings demonstrate that when fused to an immunogen gene, the CTA gene could serve as a potent genetic adjuvant, providing new insights into the mechanisms of CTA as an adjuvant.