Nonspreading Rift Valley fever virus: A potent and flexible vaccine platform

Abstract

Rift Valley fever virus (RVFV) is a serious pathogen for both ruminants and humans. RVF outbreaks have a major impact on the agricultural and related sectors in affected areas with severe socio-economic consequences. Although the virus is confined to the African continent and the Arabian Peninsula, globalization, climate change and global prevalence of potential mosquito vectors constitute a significant risk for further spread to new areas. All these features explain the need for safe, effective and affordable vaccines for both endemic and yet unaffected areas. The aim of the work described in this thesis was to develop a RVF vaccine that optimally combines safety and efficacy. We report the first successful creation of RVFV replicon particles, referred to as nonspreading RVFV (NSR). NSR particles are able to infect new cells, resulting in autonomous genome replication. However, these particles lack the genes encoding surface glycoproteins, therefore no progeny particles can be produced. We demonstrate that a single vaccination with NSR provides full protection in mice and prevents clinical sings and significantly reduces viremia in lambs. Further improvement of NSR resulted in the development of NSR-Gn, containing the gene of one of the viral glycoproteins, Gn. Gn is the major target for neutralizing antibodies and its expression in NSR-Gn infected cells significantly improved immune responses, eliciting sterile protection in lambs. In addition to RVFV vaccine, we aimed to develop a vaccine vector platform suitable for mucosal administration. Many pathogens invade the body via mucosal surfaces. Therefore, vaccines that elicit protective immunity on mucosal surfaces are particularly valuable. As RVFV is very infectious via the respiratory route, we were especially interested in using NSR for respiratory application. As a model for respiratory infection, influenza was used. We developed NSR-HA, which contains the gene of influenza A virus hemagglutinin (HA), one of the major influenza A virus immunogens. A single intranasal vaccination of mice with NSR-HA completely prevented death and clinical signs induced by an influenza virus infection, revealing the potency of NSR as a vector platform suitable for administration via the respiratory mucosa. The efficacy of NSR as a viral vector vaccine for cancer immunotherapy was also studies. Our results in a mouse model show that NSR can reduce tumor growth and even eradicate tumors, suggesting that it is a promising vector for cancer immunotherapy. In our vaccination experiments we consistently observed a high efficaciousness of NSR-based vaccines. We were interested to unravel the molecular basis of the NSR-induced immune response. To that end we investigated the interaction between NSR and dendritic cells (DC). We observed that NSR-infected DCs undergo incomplete maturation, associated with a decrease in CD83, the most prominent marker for DC maturation. However, bystander DCs undergo complete maturation and upregulate CD83. Our results suggest that not only infected, but also bystander DCs play an important role in NSR-mediated immunity. Although we were unable to unravel the exact mechanism of the observed decrease of CD83 in NSR-infected cells, we established that it occurs at the level of protein translation.