An optimized chikungunya virus trans-amplifying RNA vaccine candidate induces potent immune responses with only 1 ng of antigen encoding RNA

Introduction

The human pathogenic chikungunya virus (CHIKV) has caused outbreaks in over 100 countries worldwide and is raising public health concern. Further spread by mosquito vectors due to globalization and climate change is expected. The symptoms of a CHIKV infection include fever, headache, myalgia, arthritis, and joint pain. Although the mortality rate is rather low, infections can result in debilitating arthralgia that can become chronic. Accordingly, we aim to explore potential CHIKV vaccine candidates.

Methods

Our first-generation CHIKV vaccine candidate was based on trans-amplifying RNA (taRNA), consisting of two RNAs: a non-replicating mRNA encoding for the CHIKV nonstructural proteins, forming the replicase complex and a trans-replicon (TR) RNA encoding the CHIKV envelope proteins. The TR-RNA is efficiently amplified by the replicase in trans leading to a high antigen expression and potent immune responses. To optimize our taRNA platform, we now simplified the TR-RNA to the essential elements by removing the subgenomic promoter and redesigning the 5’ untranslated region. Additionally, we formulated our vaccine candidates with lipid nanoparticles and evaluated several modifications. Humoral and cellular immune responses were assessed after prime-boost immunization of mice.

Results

First, we could prove in vitro that the respective antigen can now be directly translated from the STR-RNA as from a normal mRNA. However, as the conserved sequence elements were preserved, the STR-RNA was efficiently amplified by the CHIKV replicase. Here, the usage of only the envelope proteins led to a high protein expression, whereas the addition of the capsid protein allowed the release of virus-like particles. The LNP formulation of our taRNA vaccine candidate led to a 4.3-fold greater antibody titer compared to intradermal vaccination in saline. In comparison to the previous TR-RNA, the STR-RNA induced a further 4-fold increase in antibody titers. Here, in contrast to our previous results, all mice developed potent neutralizing antibody responses. Importantly, we could drastically reduce the taRNA dose. Neutralizing antibodies could be induced with as little as 1 ng STR-RNA and 5 ng STR-RNA were sufficient to protect all mice from a CHIKV challenge infection.

Discussion

The CHIKV is of increasing public health concern after its rapid global spread. In comparison to mRNA vaccines, the platform of taRNA allows to significantly reduce the required RNA doses for protective immunity. Nevertheless, the potential of taRNA as vaccine in humans still needs to be established in clinical trials.

Conclusion

In conclusion, taRNA represents a promising safe and efficient vaccination strategy for emerging infectious diseases.