Single-Dose Immunization with Virus Replicon Particles Confers Rapid Robust Protection against Rift Valley Fever Virus Challenge

Abstract

c Rift Valley fever virus (RVFV) causes outbreaks of severe disease in people and livestock throughout Africa and the Arabian Peninsula. The potential for RVFV introduction outside the area of endemicity highlights the need for fast-acting, safe, and efficacious vaccines. Here, we demonstrate a robust system for the reverse genetics generation of a RVF virus replicon particle (VRP RVF ) vaccine candidate. Using a mouse model, we show that VRP RVF immunization provides the optimal balance of safety and single-dose robust efficacy. VRP RVF can actively synthesize viral RNA and proteins but lacks structural glycoprotein genes, preventing spread within immunized individuals and reducing the risk of vaccine-induced pathogenicity. VRP RVF proved to be completely safe following intracranial inoculation of suckling mice, a stringent test of vaccine safety. Single-dose subcutaneous immunization with VRP RVF , although it is highly attenuated, completely protected mice against a virulent RVFV challenge dose which was 100,000-fold greater than the 50% lethal dose (LD 50 ). Robust protection from lethal challenge was observed by 24 h postvaccination, with 100% protection induced in as little as 96 h. We show that a single subcutaneous VRP RVF immunization initiated a systemic antiviral state followed by an enhanced adaptive response. These data contrast sharply with the much-reduced survivability and immune responses observed among animals immunized with nonreplicating viral particles, indicating that replication, even if confined to the initially infected cells, contributes substantially to protective efficacy at early and late time points postimmunization. These data demonstrate that replicon vaccines successfully bridge the gap between safety and efficacy and provide insights into the kinetics of antiviral protection from RVFV infection.

Rift Valley Fever Virus Clearance and Protection from Neurologic Disease Are Dependent on CD4 + T Cell and Virus-Specific Antibody Responses

“…The dramatic in vivo attenuation of viruses lacking a functional NSs (7,8,18,19) highlights the importance of the early innate response in controlling RVFV replication. Indeed, several of the most promising replication-competent vaccine candidates lack NSs and are associated with an early systemic antiviral response (20) followed by a strong neutralizing antibody response correlating with protection from subsequent challenge with wild-type RVFV (18,(20)(21)(22)(23). These vaccine studies corroborate results from early passive transfer studies with nonhuman primates (24) and mice (25) that indicated protection from RVFV challenge is antibody mediated.…”

“…In this system, wild-type RVFV (Egyptian strain ZH501) infection is uniformly and rapidly lethal, with mice succumbing to fulminant hepatitis 2 to 3 days after infection (20). However, an RVFV strain containing a full-gene deletion of NSs (⌬NSs virus) is highly attenuated in immunocompetent mice and provides an ideal platform for evaluating the role of adaptive immunity in the context of an active innate response.…”

A Glycoprotein Subunit Vaccine Elicits a Strong Rift Valley Fever Virus Neutralizing Antibody Response in Sheep

“…The vaccine induced potentially protective (i.e., 1:40), virus neutralizing titers with single vaccination in five of the six animals within 2 weeks pv (Table 1). These results compared favorably with the outcome of recently reported vaccinations using vaccines based on RVFV glycoproteins, such as GnGc VLPs and Gne , de Boer et al 2010, Mandell et al 2010a, Kortekaas et al 2012, Oreshkova et al 2013, as well as a Newcastle disease virus-vectored vaccine (NDFL-GnGc) (Kortekaas et al 2010a, Kortekaas et al 2010b) and virus replicon particles (Dodd et al 2012, Oreshkova et al 2013, some of which have also been reported to elicit neutralizing antibodies with single vaccination in sheep (Kortekaas et al 2010a, Kortekaas, et al 2012, Oreshkova, et al 2013.…”

“…Strategies to develop RVFV vaccines include subunit (Schmaljohn et al 1989, Mandell et al 2010a, DNA (Spik et al 2006), viruslike particles (VLPs) , de Boer et al 2010, Kortekaas et al 2012, virus replicon particles (Kortekaas et al 2011, Dodd et al 2012, Oreshkova et al 2013), virus-vectored (Wallace et al 2006, Heise et al 2009) modified live vaccines, developed from recombinant viruses engineered using reverse genetics (Ikegami et al 2006, Bird et al 2008, Billecocq et al 2008, Habjan et al 2008, Bird et al 2011, live attenuated (Smithburn 1949, Caplen et al 1985, Muller et al 1995, Dungu et al 2010, Pittman 2012, Morrill et al 2013, and inactivated whole virus vaccines (Pittman et al 2000). Although subunit vaccines for RVFV are generally considered safe, and recently some progress has been made in their development, evaluation of immunogenicity and/or efficacy in a target species, sheep, has been performed for a few candidates (Kortekaas et al 2012, Oreshkova et al 2013).…”

Creation of Rift Valley Fever Viruses with Four-Segmented Genomes Reveals Flexibility in Bunyavirus Genome Packaging

“…It was previously proposed that the M segment plays a pivotal role in the copackaging of the L and S segments (20). However, we and others have demonstrated that the M segment is not required for the packaging of L and S genome segments into RVFV replicon particles (23,24), and a two-segmented RVFV that expresses the Gn and Gc genes from the NSs location of the S segment was shown to be viable without an Mtype genome segment (25). Altogether, these studies have provided important new insights into bunyavirus packaging; however, they also made clear that many questions on this topic remain to be answered.…”