A DNA Vaccine against Yellow Fever Virus: Development and Evaluation

Abstract

Attenuated yellow fever (YF) virus 17D/17DD vaccines are the only available protection from YF infection, which remains a significant source of morbidity and mortality in the tropical areas of the world. The attenuated YF virus vaccine, which is used worldwide, generates both long-lasting neutralizing antibodies and strong T-cell responses. However, on rare occasions, this vaccine has toxic side effects that can be fatal. This study presents the design of two non-viral DNA-based antigen formulations and the characterization of their expression and immunological properties. The two antigen formulations consist of DNA encoding the full-length envelope protein (p/YFE) or the full-length envelope protein fused to the lysosomal-associated membrane protein signal, LAMP-1 (pL/YFE), aimed at diverting antigen processing/presentation through the major histocompatibility complex II precursor compartments. The immune responses triggered by these formulations were evaluated in H2b and H2d backgrounds, corresponding to the C57Bl/6 and BALB/c mice strains, respectively. Both DNA constructs were able to induce very strong T-cell responses of similar magnitude against almost all epitopes that are also generated by the YF 17DD vaccine. The pL/YFE formulation performed best overall. In addition to the T-cell response, it was also able to stimulate high titers of anti-YF neutralizing antibodies comparable to the levels elicited by the 17DD vaccine. More importantly, the pL/YFE vaccine conferred 100% protection against the YF virus in intracerebrally challenged mice. These results indicate that pL/YFE DNA is an excellent vaccine candidate and should be considered for further developmental studies.

Author Summary

DNA and other nucleic acid vaccine technologies are advancing quickly, and new potent delivery methods are demonstrating great potential in human clinical trials. In this manuscript, we report a highly protective DNA vaccine against the yellow fever virus. This vaccine was engineered with a molecular adjuvant technology to enhance the exposure of the vaccine antigens to the immune system, resulting in augmented CD4 ⁺ helper responses. We postulate that the robust CD4 ⁺ responses help the B cells and the CD8 ⁺ cells mature more efficiently and produce better antibodies and cytotoxic cells, respectively. Our results show that vaccination with this yellow fever DNA formulation elicited protective levels of neutralizing antibodies and very strong cellular responses at similar levels to the responses elicited by the live attenuated 17DD vaccine. In addition, these results also suggest a very important role for cellular responses in mediating protection against yellow fever virus. The results reported here are very promising and further studies may lead to a new yellow fever vaccine for human use.