Alanine scanning of the rabies virus glycoprotein antigenic site III using recombinant rabies virus: implication for post-exposure treatment

Immune Evasion Mechanism of Neurotropic Viruses

The persistent challenge posed by viruses that infect the central nervous system lies in their sophisticated ability to evade the host immune system. This review explores into the complex mechanisms of immune evasion employed by these neurotropic viruses, focussing on their modulation of host immune responses, evasion of adaptive immunity, and the cellular and molecular strategies that enable their persistence. Key areas explored include viral latency and reactivation, the inhibition of apoptosis, and antigenic variation, with a detailed examination of viral proteins and their interactions with host cellular processes.

Development of novel canine phage display-derived neutralizing monoclonal antibody fragments against rabies virus from immunized dogs

Animal rabies is a potentially fatal infectious disease in mammals, especially dogs. Currently, the number of rabies cases in pet dogs is increasing in several regions of Thailand. However, no passive postexposure prophylaxis (PEP) has been developed to combat rabies infection in animals. As monoclonal antibodies (MAbs) are promising biological therapies for postinfection, we developed a canine-neutralizing MAb against rabies virus (RABV) via the single-chain variable fragment (scFv) platform. Immunized phage-displaying scFv libraries were constructed from PBMCs via the pComb3XSS system. Diverse canine VHVLκ and VHVLλ libraries containing 2.4 × 108 and 1.3 × 106 clones, respectively, were constructed. Five unique clones that show binding affinity with the RABV glycoprotein were then selected, of which K9RABVscFv1 and K9RABVscFv16 showed rapid fluorescent foci inhibition test (RFFIT) neutralizing titers above the human protective level of 0.5 IU/ml. Finally, in silico docking predictions revealed that the residues on the CDRs of these neutralizing clones interact mainly with similar antigenic sites II and III on the RABV glycoprotein. These candidates may be used to develop complete anti-RABV MAbs as a novel PEP protocol in pet dogs and other animals.

Lyssavirus Vaccine with a Chimeric Glycoprotein Protects across Phylogroups

Rabies is nearly 100% lethal in the absence of treatment, killing an estimated 59,000 people annually. Vaccines and biologics are highly efficacious when administered properly. Sixteen rabies-related viruses (lyssaviruses) are similarly lethal, but some are divergent enough to evade protection from current vaccines and biologics, which are based only on the classical rabies virus (RABV). Here we present the development and characterization of LyssaVax, a vaccine featuring a structurally designed, functional chimeric glycoprotein (G) containing immunologically important domains from both RABV G and the highly divergent Mokola virus (MOKV) G. LyssaVax elicits high titers of antibodies specific to both RABV and MOKV Gs in mice. Immune sera also neutralize a range of wild-type lyssaviruses across the major phylogroups. LyssaVax-immunized mice are protected against challenge with recombinant RABV and MOKV. Altogether, LyssaVax demonstrates the utility of structural modeling in vaccine design and constitutes a broadened lyssavirus vaccine candidate.

Inactivated rabies virus vectored SARS-CoV-2 vaccine prevents disease in a Syrian hamster model

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emergent coronavirus that has caused a worldwide pandemic. Although human disease is often asymptomatic, some develop severe illnesses such as pneumonia, respiratory failure, and death. There is an urgent need for a vaccine to prevent its rapid spread as asymptomatic infections accounting for up to 40% of transmission events. Here we further evaluated an inactivated rabies vectored SARS-CoV-2 S1 vaccine CORAVAX in a Syrian hamster model. CORAVAX adjuvanted with MPLA-AddaVax, a TRL4 agonist, induced high levels of neutralizing antibodies and generated a strong Th1-biased immune response. Vaccinated hamsters were protected from weight loss and viral replication in the lungs and nasal turbinates three days after challenge with SARS-CoV-2. CORAVAX also prevented lung disease, as indicated by the significant reduction in lung pathology. This study highlights CORAVAX as a safe, immunogenic, and efficacious vaccine that warrants further assessment in human trials.

Identification and Characterization of a Small-Molecule Rabies Virus Entry Inhibitor

Rabies virus (RABV) causes a severe and fatal neurological disease, but morbidity is vaccine preventable and treatable prior to the onset of clinical symptoms. However, immunoglobulin (IgG)-based rabies postexposure prophylaxis (PEP) is expensive, restricting access to life-saving treatment, especially for patients in low-income countries where the clinical need is greatest, and does not confer cross-protection against newly emerging phylogroup II lyssaviruses. Toward identifying a cost-effective replacement for the IgG component of rabies PEP, we developed and implemented a high-throughput screening protocol utilizing a single-cycle RABV reporter strain. A large-scale screen and subsequent direct and orthogonal counterscreens identified a first-in-class direct-acting RABV inhibitor, GRP-60367, with a specificity index (SI) of >100,000. Mechanistic characterization through time-of-addition studies, transient cell-to-cell fusion assays, and chimeric vesicular stomatitis virus (VSV) recombinants expressing the RABV glycoprotein (G) demonstrated that GRP-60367 inhibits entry of a subset of RABV strains. Resistance profiling of the chemotype revealed hot spots in conserved hydrophobic positions of the RABV G protein fusion loop that were confirmed in transient cell-to-cell fusion assays. Transfer of RABV G genes with signature resistance mutations into a recombinant VSV backbone resulted in the recovery of replication-competent virions with low susceptibility to the inhibitor. This work outlines a tangible strategy for mechanistic characterization and resistance profiling of RABV drug candidates and identified a novel, well-behaved molecular probe chemotype that specifically targets the RABV G protein and prevents G-mediated viral entry.IMPORTANCE Rabies PEP depends on anti-RABV IgG, which is expensive and in limited supply in geographical areas with the highest disease burden. Replacing the IgG component with a cost-effective and shelf-stable small-molecule antiviral could address this unmet clinical need by expanding access to life-saving medication. This study has established a robust protocol for high-throughput anti-RABV drug screens and identified a chemically well-behaved, first-in-class hit with nanomolar anti-RABV potency that blocks RABV G protein-mediated viral entry. Resistance mapping revealed a druggable site formed by the G protein fusion loops that has not previously emerged as a target for neutralizing antibodies. Discovery of this RABV entry inhibitor establishes a new molecular probe to advance further mechanistic and structural characterization of RABV G that may aid in the design of a next-generation clinical candidate against RABV.

Status of antiviral therapeutics against rabies virus and related emerging lyssaviruses

Rabies virus (RABV) constitutes a major social and economic burden associated with 60 000 deaths annually worldwide. Although pre-exposure and post-exposure treatment options are available, they are efficacious only when initiated before the onset of clinical symptoms. Aggravating the problem, the current RABV vaccine does not cross-protect against the emerging zoonotic phylogroup II lyssaviruses. A requirement for an uninterrupted cold chain and high cost of the immunoglobulin component of rabies prophylaxis generate an unmet need for the development of RABV-specific antivirals. We discuss desirable anti-RABV drug profiles, past efforts to address the problem and inhibitor candidates identified, and examine how the rapidly expanding structural insight into RABV protein organization has illuminated novel druggable target candidates and paved the way to structure-aided drug optimization. Special emphasis is given to the viral RNA-dependent RNA polymerase complex as a promising target for direct-acting broad-spectrum RABV inhibitors.

Non-neutralizing antibodies elicited by recombinant Lassa-Rabies vaccine are critical for protection against Lassa fever

Lassa fever (LF), caused by Lassa virus (LASV), is a viral hemorrhagic fever for which no approved vaccine or potent antiviral treatment is available. LF is a WHO priority disease and, together with rabies, a major health burden in West Africa. Here we present the development and characterization of an inactivated recombinant LASV and rabies vaccine candidate (LASSARAB) that expresses a codon-optimized LASV glycoprotein (coGPC) and is adjuvanted by a TLR-4 agonist (GLA-SE). LASSARAB elicits lasting humoral response against LASV and RABV in both mouse and guinea pig models, and it protects both guinea pigs and mice against LF. We also demonstrate a previously unexplored role for non-neutralizing LASV GPC-specific antibodies as a major mechanism of protection by LASSARAB against LF through antibody-dependent cellular functions. Overall, these findings demonstrate an effective inactivated LF vaccine and elucidate a novel humoral correlate of protection for LF.

The Final (Oral Ebola) Vaccine Trial on Captive Chimpanzees?

Could new oral vaccine technologies protect endangered wildlife against a rising tide of infectious disease? We used captive chimpanzees to test oral delivery of a rabies virus (RABV) vectored vaccine against Ebola virus (EBOV), a major threat to wild chimpanzees and gorillas. EBOV GP and RABV GP-specific antibody titers increased exponentially during the trial, with rates of increase for six orally vaccinated chimpanzees very similar to four intramuscularly vaccinated controls. Chimpanzee sera also showed robust neutralizing activity against RABV and pseudo-typed EBOV. Vaccination did not induce serious health complications. Blood chemistry, hematologic, and body mass correlates of psychological stress suggested that, although sedation induced acute stress, experimental housing conditions did not induce traumatic levels of chronic stress. Acute behavioral and physiological responses to sedation were strongly correlated with immune responses to vaccination. These results suggest that oral vaccination holds great promise as a tool for the conservation of apes and other endangered tropical wildlife. They also imply that vaccine and drug trials on other captive species need to better account for the effects of stress on immune response.

One-Health: a Safe, Efficient, Dual-Use Vaccine for Humans and Animals against Middle East Respiratory Syndrome Coronavirus and Rabies Virus

Middle East respiratory syndrome coronavirus (MERS-CoV) emerged in 2012 and is a highly pathogenic respiratory virus. There are no treatment options against MERS-CoV for humans or animals, and there are no large-scale clinical trials for therapies against MERS-CoV. To address this need, we developed an inactivated rabies virus (RABV) that contains the MERS-CoV spike (S) protein expressed on its surface. Our initial recombinant vaccine, BNSP333-S, expresses a full-length wild-type MERS-CoV S protein; however, it showed significantly reduced viral titers compared to those of the parental RABV strain and only low-level incorporation of full-length MERS-CoV S into RABV particles. Therefore, we developed a RABV-MERS vector that contained the MERS-CoV S1 domain of the MERS-CoV S protein fused to the RABV G protein C terminus (BNSP333-S1). BNSP333-S1 grew to titers similar to those of the parental vaccine vector BNSP333, and the RABV G-MERS-CoV S1 fusion protein was efficiently expressed and incorporated into RABV particles. When we vaccinated mice, chemically inactivated BNSP333-S1 induced high-titer neutralizing antibodies. Next, we challenged both vaccinated mice and control mice with MERS-CoV after adenovirus transduction of the human dipeptidyl peptidase 4 (hDPP4) receptor and then analyzed the ability of mice to control MERS-CoV infection. Our results demonstrated that vaccinated mice were fully protected from the MERS-CoV challenge, as indicated by the significantly lower MERS-CoV titers and MERS-CoV and mRNA levels in challenged mice than those in unvaccinated controls. These data establish that an inactivated RABV-MERS S-based vaccine may be effective for use in animals and humans in areas where MERS-CoV is endemic.

IMPORTANCE

Rabies virus-based vectors have been proven to be efficient dual vaccines against rabies and emergent infectious diseases such as Ebola virus. Here we show that inactivated rabies virus particles containing the MERS-CoV S1 protein induce potent immune responses against MERS-CoV and RABV. This novel vaccine is easy to produce and may be useful to protect target animals, such as camels, as well as humans from deadly MERS-CoV and RABV infections. Our results indicate that this vaccine approach can prevent disease, and the RABV-based vaccine platform may be a valuable tool for timely vaccine development against emerging infectious diseases.

An Inactivated Rabies Virus-Based Ebola Vaccine, FILORAB1, Adjuvanted With Glucopyranosyl Lipid A in Stable Emulsion Confers Complete Protection in Nonhuman Primate Challenge Models

The 2013-2016 West African Ebola virus (EBOV) disease outbreak was the largest filovirus outbreak to date. Over 28 000 suspected, probable, or confirmed cases have been reported, with a 53% case-fatality rate. The magnitude and international impact of this EBOV outbreak has highlighted the urgent need for a safe and efficient EBOV vaccine. To this end, we demonstrate the immunogenicity and protective efficacy of FILORAB1, a recombinant, bivalent, inactivated rabies virus-based EBOV vaccine, in rhesus and cynomolgus monkeys. Our results demonstrate that the use of the synthetic Toll-like receptor 4 agonist glucopyranosyl lipid A in stable emulsion (GLA-SE) as an adjuvant increased the efficacy of FILORAB1 to 100% protection against lethal EBOV challenge, with no to mild clinical signs of disease. Furthermore, all vaccinated subjects developed protective anti-rabies virus antibody titers. Taken together, these results support further development of FILORAB1/GLA-SE as an effective preexposure EBOV vaccine.

Preclinical Development of Inactivated Rabies Virus-Based Polyvalent Vaccine Against Rabies and Filoviruses

We previously described the generation of a novel Ebola virus (EBOV) vaccine based on inactivated rabies virus (RABV) containing EBOV glycoprotein (GP) incorporated in the RABV virion. Our results demonstrated safety, immunogenicity, and protective efficacy in mice and nonhuman primates (NHPs). Protection against viral challenge depended largely on the quality of the humoral immune response against EBOV GP.Here we present the extension and improvement of this vaccine by increasing the amount of GP incorporation into virions via GP codon-optimization as well as the addition of Sudan virus (SUDV) and Marburg virus (MARV) GP containing virions. Immunogenicity studies in mice indicate similar immune responses for both SUDV GP and MARV GP compared to EBOV GP. Immunizing mice with multiple antigens resulted in immune responses similar to immunization with a single antigen. Moreover, immunization of NHP with the new inactivated RABV EBOV vaccine resulted in high titer neutralizing antibody levels and 100% protection against lethal EBOV challenge when applied with adjuvant.Our results indicate that an inactivated polyvalent vaccine against RABV filoviruses is achievable. Finally, the novel vaccines are produced on approved VERO cells and a clinical grade RABV/EBOV vaccine for human trials has been produced.

Rhabdovirus-based vaccine platforms against henipaviruses

The emerging zoonotic pathogens Hendra virus (HeV) and Nipah virus (NiV) are in the genus Henipavirus in the family Paramyxoviridae. HeV and NiV infections can be highly fatal to humans and livestock. The goal of this study was to develop candidate vaccines against henipaviruses utilizing two well-established rhabdoviral vaccine vector platforms, recombinant rabies virus (RABV) and recombinant vesicular stomatitis virus (VSV), expressing either the codon-optimized or the wild-type (wt) HeV glycoprotein (G) gene. The RABV vector expressing the codon-optimized HeV G showed a 2- to 3-fold increase in incorporation compared to the RABV vector expressing wt HeV G. There was no significant difference in HeV G incorporation in the VSV vectors expressing either wt or codon-optimized HeV G. Mice inoculated intranasally with any of these live recombinant viruses showed no signs of disease, including weight loss, indicating that HeV G expression and incorporation did not increase the neurotropism of the vaccine vectors. To test the immunogenicity of the vaccine candidates, we immunized mice intramuscularly with either one dose of the live vaccines or 3 doses of 10 μg chemically inactivated viral particles. Increased codon-optimized HeV G incorporation into RABV virions resulted in higher antibody titers against HeV G compared to inactivated RABV virions expressing wt HeV G. The live VSV vectors induced more HeV G-specific antibodies as well as higher levels of HeV neutralizing antibodies than the RABV vectors. In the case of killed particles, HeV neutralizing serum titers were very similar between the two platforms. These results indicated that killed RABV with codon-optimized HeV G should be the vector of choice as a dual vaccine in areas where rabies is endemic.

IMPORTANCE

Scientists have been tracking two new viruses carried by the Pteropid fruit bats: Hendra virus (HeV) and Nipah virus (NiV). Both viruses can be fatal to humans and also pose a serious risk to domestic animals. A recent escalation in the frequency of outbreaks has increased the need for a vaccine that prevents HeV and NiV infections. In this study, we performed an extensive comparison of live and killed particles of two recombinant rhabdoviral vectors, rabies virus and vesicular stomatitis virus (VSV), expressing wild-type or codon-optimized HeV glycoprotein, with the goal of developing a candidate vaccine against HeV. Based on our data from the presented mouse immunogenicity studies, we conclude that a killed RABV vaccine would be highly effective against HeV infections and would make an excellent vaccine candidate in areas where both RABV and henipaviruses pose a threat to human health.