Enhanced attenuation of chikungunya vaccines expressing antiviral cytokines

Abstract Chikungunya virus (CHIKV) is an emerging virus responsible for millions of infections globally within the last 15 years and has the potential to become endemic in the US. CHIK disease is characterized by severe febrile illness, with 30–60% of cases leading to debilitating chronic joint pain. No licensed treatments are available to protect against CHIK disease; thus, there is a tremendous need to generate a safe and effective vaccine. Live-attenuated vaccines (LAVs) are an appealing immunization strategy because they typically generate long-term protection from a single dose. However, LAVs often cause post-vaccination side effects and produce systemic viral replication, which can potentially lead to reversion to a pathogenic phenotype or transmission to mosquitoes; thus, safer LAV platforms are needed. To that end, we sought to improve the traditional LAV platform by combining attenuating strategies; as a vaccine backbone, we used a previously developed chimera of CHIKV and the closely related Semliki Forest virus (SFV) where we replaced the E2 domain C region of CHIKV with the corresponding domain from SFV (CHIKV-SFV/DomC) that was highly attenuated in mice and mosquitoes. To further attenuate the backbone, we inserted IFN-γ or IL-21, important antiviral cytokine genes, into the viral genome. The IFN-γ- and IL-21-expressing candidates were significantly attenuated post-vaccination, generating reduced footpad swelling with minimal systemic replication and dissemination capacity compared to the parental vaccine. Additionally, these candidates provided complete protection to mice challenged with WT CHIKV. This novel dual attenuation strategy combining an attenuated chimeric backbone and an antiviral cytokine has the possibility to be applied for the attenuation of any RNA virus.