Experimental utility of rabies virus-neutralizing human monoclonal antibodies in post-exposure prophylaxis

Fatal Human Rabies Infection With Suspected Host-Mediated Failure of Post-Exposure Prophylaxis Following a Recognized Zoonotic Exposure-Minnesota, 2021

BACKGROUND

No human rabies post-exposure prophylaxis (PEP) failure has been documented in the United States using modern cell culture-based vaccines. In January 2021, an 84-year-old male died from rabies 6 months after being bitten by a rabid bat despite receiving timely rabies PEP. We investigated the cause of breakthrough infection.

METHODS

We reviewed medical records, laboratory results, and autopsy findings and performed whole-genome sequencing (WGS) to compare patient and bat virus sequences. Storage, administration, and integrity of PEP biologics administered to the patient were assessed; samples from leftover rabies immunoglobulin were evaluated for potency. We conducted risk assessments for persons potentially exposed to the bat and for close patient contacts.

RESULTS

Rabies virus antibodies present in serum and cerebrospinal fluid were nonneutralizing. Antemortem blood testing revealed that the patient had unrecognized monoclonal gammopathy of unknown significance. Autopsy findings showed rabies meningoencephalitis and metastatic prostatic adenocarcinoma. Rabies virus sequences from the patient and the offending bat were identical by WGS. No deviations were identified in potency, quality control, administration, or storage of administered PEP. Of 332 persons assessed for potential rabies exposure to the case patient, 3 (0.9%) warranted PEP.

CONCLUSIONS

This is the first reported failure of rabies PEP in the Western Hemisphere using a cell culture-based vaccine. Host-mediated primary vaccine failure attributed to previously unrecognized impaired immunity is the most likely explanation for this breakthrough infection. Clinicians should consider measuring rabies neutralizing antibody titers after completion of PEP if there is any suspicion for immunocompromise.

Efficiency Comparative Approach of Plant-Produced Monoclonal Antibodies against Rabies Virus Infection

Rabies encephalitis is a fatal zoonotic viral disease caused by the neurotropic rabies virus. It remains a major public health concern as it causes almost 100% fatality and has no effective medication after the onset of the disease. However, this illness is preventable with the timely administration of effective post-exposure prophylaxis (PEP) consisting of the rabies vaccine and passive immune globulins (HRIG and ERIG). Recently, conventional PEP has been shown to have many limitations, resulting in little support for these expensive and heterologous globulins. Monoclonal antibody (mAb) production via recombinant technology in animal and human cell cultures, as well as a plant-based platform, was introduced to overcome the costly and high-tech constraints of former preparations. We used transient expression technology to produce two mAbs against the rabies virus in Nicotiana benthamiana and compared their viral neutralizing activity in vitro and in vivo. The expression levels of selective mAbs E559 and 62-71-3 in plants were estimated to be 17.3 mg/kg and 28.6 mg/kg in fresh weight, respectively. The plant-produced mAbs effectively neutralized the challenge virus CVS-11 strain in a cell-based RFFIT. In addition, the combination of these two mAbs in a cocktail protected hamsters from rabies virus infection more effectively than standard HRIG and ERIG. This study suggests that the plant-produced rabies antibody cocktail has promising potential as an alternative biological to polyclonal RIG in rabies PEP.

Comparative pathogenesis of different phylogroup I bat lyssaviruses in a standardized mouse model

A plethora of bat-associated lyssaviruses potentially capable of causing the fatal disease rabies are known today. Transmitted via infectious saliva, occasionally-reported spillover infections from bats to other mammals demonstrate the permeability of the species-barrier and highlight the zoonotic potential of bat-related lyssaviruses. However, it is still unknown whether and, if so, to what extent, viruses from different lyssavirus species vary in their pathogenic potential. In order to characterize and systematically compare a broader group of lyssavirus isolates for their viral replication kinetics, pathogenicity, and virus release through saliva-associated virus shedding, we used a mouse infection model comprising a low (10² TCID₅₀) and a high (10⁵ TCID₅₀) inoculation dose as well as three different inoculation routes (intramuscular, intranasal, intracranial). Clinical signs, incubation periods, and survival were investigated. Based on the latter two parameters, a novel pathogenicity matrix was introduced to classify lyssavirus isolates. Using a total of 13 isolates from ten different virus species, this pathogenicity index varied within and between virus species. Interestingly, Irkut virus (IRKV) and Bokeloh bat lyssavirus (BBLV) obtained higher pathogenicity scores (1.14 for IRKV and 1.06 for BBLV) compared to rabies virus (RABV) isolates ranging between 0.19 and 0.85. Also, clinical signs differed significantly between RABV and other bat lyssaviruses. Altogether, our findings suggest a high diversity among lyssavirus isolates concerning survival, incubation period, and clinical signs. Virus shedding significantly differed between RABVs and other lyssaviruses. Our results demonstrated that active shedding of infectious virus was exclusively associated with two RABV isolates (92% for RABV-DogA and 67% for RABV-Insectbat), thus providing a potential explanation as to why sustained spillovers are solely attributed to RABVs. Interestingly, 3D imaging of a selected panel of brain samples from bat-associated lyssaviruses demonstrated a significantly increased percentage of infected astrocytes in mice inoculated with IRKV (10.03%; SD±7.39) compared to RABV-Vampbat (2.23%; SD±2.4), and BBLV (0.78%; SD±1.51), while only individual infected cells were identified in mice infected with Duvenhage virus (DUVV). These results corroborate previous studies on RABV that suggest a role of astrocyte infection in the pathogenicity of lyssaviruses.

Globally, there are at present 17 different officially recognized lyssavirus species posing a potential threat for human and animal health. Bats have been identified as carriers for the vast majority of those zoonotic viruses, which cause the fatal disease rabies and are transmitted through infectious saliva. The occurrence of sporadic spillover events where lyssaviruses are spread from bats to other mammalian species highlights the importance of studying pathogenicity and virus shedding in regard to a potentially sustained onward cross-species transmission. Therefore, as part of this study, we compared 13 different isolates from ten lyssavirus species in a standardized mouse infection model, focusing on clinical signs, incubation periods, and survival. Based on the latter two, a novel pathogenicity index to classify different lyssavirus species was established. This pathogenicity index varied within and between different lyssavirus species and revealed a higher ranking of other bat-related lyssaviruses in comparison to the tested Rabies virus (RABV) isolates. Altogether, our results demonstrate a high diversity among the investigated isolates concerning pathogenicity and clinical picture. Furthermore, we comparatively analyzed virus shedding via saliva and while there was no indication towards a reduced pathogenicity of bat-associated lyssaviruses as opposed to RABV, shedding was increased in RABV isolates. Additionally, we investigated neuronal cell tropism and revealed that bat lyssaviruses are not only capable of infecting neurons but also astrocytes.

Bat and Lyssavirus Exposure among Humans in Area that Celebrates Bat Festival, Nigeria, 2010 and 2013

Using questionnaires and serologic testing, we evaluated bat and lyssavirus exposure among persons in an area of Nigeria that celebrates a bat festival. Bats from festival caves underwent serologic testing for phylogroup II lyssaviruses (Lagos bat virus, Shimoni bat virus, Mokola virus). The enrolled households consisted of 2,112 persons, among whom 213 (10%) were reported to have ever had bat contact (having touched a bat, having been bitten by a bat, or having been scratched by a bat) and 52 (2%) to have ever been bitten by a bat. Of 203 participants with bat contact, 3 (1%) had received rabies vaccination. No participant had neutralizing antibodies to phylogroup II lyssaviruses, but >50% of bats had neutralizing antibodies to these lyssaviruses. Even though we found no evidence of phylogroup II lyssavirus exposure among humans, persons interacting with bats in the area could benefit from practicing bat-related health precautions.

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.

Comparison of a Novel Human Rabies Monoclonal Antibody to Human Rabies Immunoglobulin for Postexposure Prophylaxis: A Phase 2/3, Randomized, Single-Blind, Noninferiority, Controlled Study

Background

Lack of access to rabies immunoglobulin (RIG) contributes to high rabies mortality. A recombinant human monoclonal antibody (SII RMAb) was tested in a postexposure prophylaxis (PEP) regimen in comparison with a human RIG (HRIG)-containing PEP regimen.

Methods

This was a phase 2/3, randomized, single-blind, noninferiority study conducted in 200 participants with World Health Organization category III suspected rabies exposures. Participants received either SII RMAb or HRIG (1:1 ratio) in wounds and, if required, intramuscularly on day 0, along with 5 doses of rabies vaccine intramuscualarly on days 0, 3, 7, 14 and 28. The primary endpoint was the ratio of the day 14 geometric mean concentration (GMC) of rabies virus neutralizing activity (RVNA) as measured by rapid fluorescent focus inhibition test for SII RMAb recipients relative to HRIG recipients.

Results

One hundred ninety-nine participants received SII RMAb (n = 101) or HRIG (n = 98) and at least 1 dose of vaccine. The day 14 GMC ratio of RVNA for the SII RMAb group relative to the HRIG group was 4.23 (96.9018% confidence interval [CI], 2.59-6.94) with a GMC of of 24.90 IU/mL (95% CI, 18.94-32.74) for SII RMAb recipients and 5.88 IU/mL (95% CI, 4.11-8.41) for HRIG recipients. The majority of local injection site and systemic adverse reactions reported from both groups were mild to moderate in severity.

Conclusions

A PEP regimen containing SII RMAb was safe and demonstrated noninferiority to HRIG PEP in RVNA production. The novel monoclonal potentially offers a safe and potent alternative for the passive component of PEP and could significantly improve the management of bites from suspected rabid animals.

Clincical Trials Registration

CTRI/2012/05/002709.

Recent advances in the development of monoclonal antibodies for rabies post exposure prophylaxis: A review of the current status of the clinical development pipeline

Despite successful control in many parts of the world, rabies virus continues to result in tens of thousands of deaths each year. Death from rabies can be prevented by timely and appropriate post exposure prophylaxis including wound cleaning and administration of vaccine and rabies immunoglobulin. Currently, rabies immunoglobulin is derived from the blood plasma of horses or humans and has several limitations relating to supply, cost and quality. Monoclonal antibodies produced through recombinant DNA technologies could potentially overcome these limitations. The first anti-rabies monoclonal antibody has recently gained regulatory approval in India and there are several other candidates being evaluated in clinical trials. Given the advances in the field, rabies monoclonal antibodies have been recently considered by the World Health Organization's Strategic Advisory Group of Experts on Immunization and included in updated WHO immunization policy recommendations for rabies published in April 2018. This article reviews the current landscape of the clinical trial development of anti-rabies monoclonal antibodies and the historical clinical trial pathways followed for blood-derived rabies immunoglobulin before discussing challenges in the clinical evaluation, regulatory approval, uptake and monitoring of these products.

SYN023, a novel humanized monoclonal antibody cocktail, for post-exposure prophylaxis of rabies

Rabies is a neglected zoonotic disease that is preventable in humans by appropriate post-exposure prophylaxis (PEP). However, current PEP relies on polyclonal immune globulin products purified from pooled human (HRIG) or equine (ERIG) plasma that are either in chronic shortage or in association with safety concerns. Here, we present the development of an antibody cocktail, SYN023, made of two novel monoclonal antibodies (MAb) CTB011 and CTB012 that could serve as safer and more cost-effective alternatives to the current RIG products. Both CTB011 and CTB012 are humanized MAbs that bind to non-overlapping epitopes on the rabies virus (RABV) glycoprotein (G) with sub-nanomolar affinities. Sequence analysis revealed that many of the critical residues in binding are highly conserved across different species of lyssaviruses. When combined at a 1:1 ratio, CTB011/CTB012 exhibited neutralization capabilities equivalent or superior to HRIG against 10 North American street RABV isolates in vitro and 15 prevalent Chinese RABV strains in animal models. Finally, SYN023, at a dosage of 0.03 mg/kg, was able to offer the same degree of protection as standard HRIG administration (20 IU/kg) in Syrian hamsters challenged with a highly virulent bat (Tadarida brasiliensis) RABV variant. Taken together, the high-potency and broad-spectrum neutralization demonstrated by SYN023 make it an effective candidate for human rabies PEP consideration.

Development of broad-spectrum human monoclonal antibodies for rabies post-exposure prophylaxis

Currently available rabies post-exposure prophylaxis (PEP) for use in humans includes equine or human rabies immunoglobulins (RIG). The replacement of RIG with an equally or more potent and safer product is strongly encouraged due to the high costs and limited availability of existing RIG. In this study, we identified two broadly neutralizing human monoclonal antibodies that represent a valid and affordable alternative to RIG in rabies PEP. Memory B cells from four selected vaccinated donors were immortalized and monoclonal antibodies were tested for neutralizing activity and epitope specificity. Two antibodies, identified as RVC20 and RVC58 (binding to antigenic site I and III, respectively), were selected for their potency and broad-spectrum reactivity. In vitro, RVC20 and RVC58 were able to neutralize all 35 rabies virus (RABV) and 25 non-RABV lyssaviruses. They showed higher potency and breath compared to antibodies under clinical development (namely CR57, CR4098, and RAB1) and commercially available human RIG. In vivo, the RVC20-RVC58 cocktail protected Syrian hamsters from a lethal RABV challenge and did not affect the endogenous hamster post-vaccination antibody response.

Co-culture: A quick approach for isolation of street rabies virus in murine neuroblastoma cells

BACKGROUND

Laboratory detection of rabies in most cases is based on detection of the antigen by fluorescent antibody test, however, in weak positive cases confirmative laboratory diagnosis depends on widely accepted mouse inoculation test. Cell lines like neuroblastoma have been used to isolate the virus with greater success not only to target for diagnosis, but also for molecular studies that determine the epidemiology of the circulating street rabies strains and in studies that look at the efficiency of the developed monoclonal antibodies to neutralize the different rabies strains. Due to the recent issues in obtaining ethical permission for mouse experimentation, and also the passages required in the cell lines to isolate the virus, we report herewith a co-culture protocol using the murine neuroblastoma (MNA) cells, which enable quicker isolation of street rabies virus with minimum passages.

OBJECTIVE

This study is not to have an alternative diagnostic assay, but an approach to produce sufficient amount of rabies virus in minimum passages by a co-culture approach in MNA cells.

MATERIALS AND METHODS

The MNA cells are co-cultured by topping the normal cells with infected cells every 48 h and the infectivity was followed up by performing direct fluorescent-antibody test.

RESULTS

The co-culture approach results in 100% infectivity and hence the use of live mouse for experimentation could be avoided.

CONCLUSION

Co-culture method provides an alternative for the situations with limited sample volume and for the quicker isolation of virus which warrants the wild type strains without much modification.

Engineering, expression in transgenic plants and characterisation of E559, a rabies virus-neutralising monoclonal antibody

Rabies post-exposure prophylaxis (PEP) currently comprises administration of rabies vaccine together with rabies immunoglobulin (RIG) of either equine or human origin. In the developing world, RIG preparations are expensive, often in short supply, and of variable efficacy. Therefore, we are seeking to develop a monoclonal antibody cocktail to replace RIG. Here, we describe the cloning, engineering and production in plants of a candidate monoclonal antibody (E559) for inclusion in such a cocktail. The murine constant domains of E559 were replaced with human IgG1κ constant domains and the resulting chimeric mouse-human genes were cloned into plant expression vectors for stable nuclear transformation of Nicotiana tabacum. The plant-expressed, chimeric antibody was purified and biochemically characterized, was demonstrated to neutralize rabies virus in a fluorescent antibody virus neutralization assay, and conferred protection in a hamster challenge model.

Vaccines for lyssaviruses other than rabies

Several new lyssaviruses have emerged in the past decade and it is likely that more remain to be discovered. There are six recognized genotypes of lyssavirus other than the rabies virus (genotype 1). All but one of these has been associated with human cases, with the resulting disease clinically similar to rabies. Rabies vaccines provide a means of pre- and postexposure prophylaxis against rabies and some of the other genotypes, but not all. Those that are crossprotected fall into phylogroup 1 of the genus, and those not protected in phylogroup 2. The crossprotection of phylogroup 1 viruses by rabies vaccines and the development of new, broader range or specific vaccines for phylogroup 2 viruses are reviewed.

Further Studies on the Mechanism of Rabies Virus Neutralization by a Viral Glycoprotein-Specific Monoclonal Antibody, #1-46-12

We previously reported that a conformational epitope-specific monoclonal antibody (mAb; #1-46-12) neutralized the rabies virus by binding only a small number (less than 20) of the antibody molecules per virion, while a linear epitope-specific mAb (#7-1-9) required more than 250 IgG molecules for the neutralization. We also isolated both the epitope-negative (R-31) and-positive (R-61) escape mutants that resisted mAb #1-46-12. Co-infection studies with wild type (wt) and R-61 mutant have shown that although the infectivity of R-61 mutant was not affected by the binding of about 300 IgG molecules per virion, incorporation of a small number of wt G protein into the R-61 virion resulted in dramatic loss of the resistance. In this study, we further investigated properties of the mutant G proteins. The R-61 G protein lost reactivity to the mAb when solubilized, even keeping a trimer form, suggesting that membrane-anchorage is essential for the maintenance of its epitope-positive conformation. On the other hand, incorporation of wt G proteins into the R-31 virions did not affect their resistance to the mAb very much. Although we have not so far found the presumed conformational changes induced by the mAb-binding, we think that these results are not inconsistent with our previously proposed novel model (referred to as a domino effect model) for the virus neutralization by mAb #1-46-12 other than a classical spike-blocking model, which implicates successive spreading of the postulated antibody-induced conformational changes of G protein to the neighboring spikes until abolishing the host cell-binding ability of the virion.

Passive immunity in the prevention of rabies

Prevention of clinical disease in those exposed to viral infection is an important goal of human medicine. Using rabies virus infection as an example, we discuss the advances in passive immunoprophylaxis, most notably the shift from the recommended polyclonal human or equine immunoglobulins to monoclonal antibody therapies. The first rabies-specific monoclonal antibodies are undergoing clinical trials, so passive immunisation might finally become an accessible, affordable, and routinely used part of global health practices for rabies. Coupled with an adequate supply of modern tissue-culture vaccines, replacing the less efficient and unsafe nerve-tissue-derived rabies vaccines, the burden of this disease could be substantially reduced.

A novel disulfide-stabilized single-chain variable antibody fragment against rabies virus G protein with enhanced in vivo neutralizing potency

Rabies is a fatal infectious disease requiring efficient protection provided by post-exposure prophylaxis (PEP) with rabies immunoglobulin (RIG). The single-chain Fv fragment (scFv) is a small engineered antigen binding protein derived from antibody variable heavy (V(H)) and light (V(L)) chains. This novel antibody format may potentially replace the current application of RIG to detect and neutralize rabies virus (RV). However, the broad use of scFvs is confined by their generally low stability. In this study, a scFv (FV57) was constructed based on the monoclonal antibody, MAB57, against RV. To enhance its stability and neutralizing potency, a disulfide-stabilized scFv, ds-FV57, was also derived by introduction of cysteines at V(H)44 and V(L)100. Furthermore, the cysteine at V(L)85 of ds-FV57 was mutated to serine to construct ds-FV57(VL85Ser) in order to avoid potential mis-formed disulfide bonds which would alter the affinity of the scFv. The stability and activity of all three proteins expressed in Escherichia coli were evaluated. All of the constructed scFvs could provide efficient protection against RV infection both in vivo and in vitro. However, the stability of ds-FV57(VL85Ser) was notably improved, and its in vitro neutralizing potency against RV infection was enhanced. Our findings from these stabilization modifications support the feasibility of developing scFvs for PEP treatment of rabies.

Bats and lyssaviruses

Numerous bat species have been identified as important reservoirs of zoonotic viral pathogens. Rabies and rabies-related viruses constitute one of the most important viral zoonoses and pose a significant threat to public health across the globe. Whereas rabies virus (RABV) appears to be restricted to bats of the New World, related lyssavirus species have not been detected in the Americas and have only been detected in bat populations across Africa, Eurasia, and Australia. Currently, 11 distinct species of lyssavirus have been identified, 10 of which have been isolated from bat species and all of which appear to be able to cause encephalitis consistent with that seen with RABV infection of humans. In contrast, whereas lyssaviruses are apparently able to cause clinical disease in bats, it appears that these lyssaviruses may also be able to circulate within bat populations in the absence of clinical disease. This feature of these highly encephalitic viruses, alongside many other aspects of lyssavirus infection in bats, is poorly understood. Here, we review what is known of the complex relationship between bats and lyssaviruses, detailing both natural and experimental infections of these viruses in both chiropteran and nonchiropteran models. We also discuss potential mechanisms of virus excretion, transmission both to conspecifics and spill-over of virus into nonvolant species, and mechanisms of maintenance within bat populations. Importantly, we review the significance of neutralizing antibodies reported within bat populations and discuss the potential mechanisms by which highly neurovirulent viruses such as the lyssaviruses are able to infect bat species in the absence of clinical disease.

A universal real-time assay for the detection of Lyssaviruses

Rabies virus (RABV) is enzootic throughout most of the world. It is now widely accepted that RABV had its origins in bats. Ten of the 11 Lyssavirus species recognised, including RABV, have been isolated from bats. There is, however, a lack of understanding regarding both the ecology and host reservoirs of Lyssaviruses. A real-time PCR assay for the detection of all Lyssaviruses using universal primers would be beneficial for Lyssavirus surveillance. It was shown that using SYBR^® Green, a universal real-time PCR primer pair previously demonstrated to detect European bat Lyssaviruses 1 and 2, and RABV, was able to detect reverse transcribed RNA for each of the seven virus species available to us. Target sequences of bat derived virus species unavailable for analysis were synthesized to produce oligonucleotides. Lagos Bat-, Duvenhage- and Mokola virus full nucleoprotein gene clones enabled a limit of 5–50 plasmid copies to be detected. Five copies of each of the synthetic DNA oligonucleotides of Aravan-, Khujand-, Irkut-, West Caucasian bat- and Shimoni bat virus were detected. The single universal primer pair was therefore able to detect each of the most divergent known Lyssaviruses with great sensitivity.

Isolation and characterization of novel human monoclonal antibodies possessing neutralizing ability against rabies virus

Rabies is a fatal viral encephalitis which is transmitted by exposure to the bite of rabid animals. Human and equine rabies immunoglobulins are indispensable pharmacological agents for severe bite exposure, as is vaccine. However, several disadvantages, including limited supply, adverse reactions, and high cost, hamper their wide application in developing countries. In the present study, two novel huMabs which neutralize rabies virus were established from vaccinated hyperimmune volunteers using the Epstein-Barr virus transformation method. One MAb (No. 254), which was subclass IgG3, effectively neutralized fixed rabies viruses of CVS, ERA, HEP-Flury, and Nishigahara strains and recognized a well-conserved epitope located in antigenic site II of the rabies virus glycoprotein. No. 254 possessed 68 ng/ml of FRNT₅₀ activity against CVS, 3.7 × 10⁻⁷ M of the Kd value, and the enhancing effect of complement-dependent virolysis. In addition, No. 254 showed effective neutralization potency in vivo in the mouse challenge test. The other MAb, 4D4, was subclass IgM and showed neutralizing activity against CVS and Nishigahara strains. 4D4 recognized a novel antigenic site which is associated with the neurovirulence of rabies, a glycoprotein located between antigenic site I and VI. Both human MAbs against rabies are expected to be utilized as a tool for future post-exposure prophylaxis.

Fine mapping and interaction analysis of a linear rabies virus neutralizing epitope

A novel human antibody AR16, targeting the G5 linear epitope of rabies virus glycoprotein (RVG) was shown to have promising antivirus potency. Using AR16, the minimal binding region within G5 was identified as HDFR (residues 261-264), with key residues HDF (residues 261-263) identified by alanine replacement scanning. The key HDF was highly conserved within phylogroup I Lyssaviruses but not those in phylogroup II. Using computer-aided docking and interaction models, not only the key residues (Asp30, Asp31, Tyr32, Trp53, Asn54, Glu99, Ile101, and Trp166) of AR16 that participated in the interaction with G5 were identified, the van der Waals forces that mediated the epitope-antibody interaction were also revealed. Seven out of eight presumed key residues (Asp30, Asp31, Tyr32, Trp53, Asn54, Glu99, and Ile101) were located at the variable regions of AR16 heavy chains. A novel mAb cocktail containing AR16 and CR57, has the potential to recognize non-overlapping, non-competing epitopes, and neutralize a broad range of rabies virus.

Rabies virus pathogenesis in relationship to intervention with inactivated and attenuated rabies vaccines

Despite progress in vaccine development in the past century the mechanisms behind immune responses elicited by rabies biologics or via natural infection remain largely unknown. In this study, we compared protection elicited by standard, early, or delayed prophylaxis with a reduced number of vaccine doses using inactivated and live-attenuated vaccines. Two-month-old Syrian hamsters, 4-week-old ICR mice or adult rhesus macaques were inoculated with canine rabies virus variants. Thereafter, prophylaxis was initiated 6h, 1, 2, 3, 4, 5, 6 or 7 days post-exposure (p.e.). One or several doses of inactivated (HDCV), or reverse genetically attenuated (live), or gamma-irradiated (inactivated)-ERAG333 vaccines were administered intramuscularly. The dynamics of virus spread were measured over time in the rodent models. Rabies virus reached the spinal cord at day 4 and brain at day 6 p.e. All hamsters succumbed in groups in which live ERAG333 was delayed until days 5 and 6 p.e. However, 78%, 44%, 56% and 22% of hamsters survived when one dose of live ERAG333 was administered 6h, 1, 2, 3, and 4 days p.e., respectively. Similarly, 67% survived when inactivated ERAG333 was administered at 24h p.e. All hamsters succumbed when standard prophylaxis (the Essen regimen) was delayed until days 3-6, but 67% and 33% of hamsters survived when PEP began 1 or 2 days p.e., respectively. Macaques were protected by one dose of attenuated ERAG333 at 24h p.e. The highly attenuated (live) and inactivated ERAG333 vaccines elicited potent protective immune responses, even when prophylaxis initiation was delayed. When 2-5 doses of commercial vaccine and HRIG were administered according to the Essen scheme, 89-100% of the animals survived. Reduced vaccine schedules provided efficacious intervention, regardless of the total number of vaccine doses administered.

Development of a mouse monoclonal antibody cocktail for post-exposure rabies prophylaxis in humans

As the demand for rabies post-exposure prophylaxis (PEP) treatments has increased exponentially in recent years, the limited supply of human and equine rabies immunoglobulin (HRIG and ERIG) has failed to provide the required passive immune component in PEP in countries where canine rabies is endemic. Replacement of HRIG and ERIG with a potentially cheaper and efficacious alternative biological for treatment of rabies in humans, therefore, remains a high priority. In this study, we set out to assess a mouse monoclonal antibody (MoMAb) cocktail with the ultimate goal to develop a product at the lowest possible cost that can be used in developing countries as a replacement for RIG in PEP. Five MoMAbs, E559.9.14, 1112-1, 62-71-3, M727-5-1, and M777-16-3, were selected from available panels based on stringent criteria, such as biological activity, neutralizing potency, binding specificity, spectrum of neutralization of lyssaviruses, and history of each hybridoma. Four of these MoMAbs recognize epitopes in antigenic site II and one recognizes an epitope in antigenic site III on the rabies virus (RABV) glycoprotein, as determined by nucleotide sequence analysis of the glycoprotein gene of unique MoMAb neutralization-escape mutants. The MoMAbs were produced under Good Laboratory Practice (GLP) conditions. Unique combinations (cocktails) were prepared, using different concentrations of the MoMAbs that were capable of targeting non-overlapping epitopes of antigenic sites II and III. Blind in vitro efficacy studies showed the MoMab cocktails neutralized a broad spectrum of lyssaviruses except for lyssaviruses belonging to phylogroups II and III. In vivo, MoMAb cocktails resulted in protection as a component of PEP that was comparable to HRIG. In conclusion, all three novel combinations of MoMAbs were shown to have equal efficacy to HRIG and therefore could be considered a potentially less expensive alternative biological agent for use in PEP and prevention of rabies in humans.

Human mortality from endemic canine rabies is estimated to be 55,000 deaths per year in Africa and Asia, yet rabies remains a neglected disease throughout most of these countries. More than 99% of human rabies cases are caused by infections resulting from a dog-bite injury. In the vast majority of human exposures to rabies, patients require post-exposure prophylaxis (PEP), which includes both passive (rabies immunoglobulin, RIG) and active immunization (rabies vaccine). The number of victims requiring PEP has increased exponentially in recent years, and human and equine RIG (HRIG and ERIG) were not sufficiently available in countries where canine rabies is endemic. Rabies virus-neutralizing monoclonal antibodies (MAbs) of mouse (Mo) origin have been identified as promising alternatives to HRIG and ERIG. We have developed and assessed both in vitro and in vivo unique mouse monoclonal antibody (MoMAb) cocktails, which are highly efficacious. Three novel combinations were shown to have an equal or superior efficacy to HRIG and therefore could be considered a potentially less expensive alternative for passive prophylactic use to prevent the development of rabies in humans, particularly where needed most in developing countries.

Antibodies induced by vaccination with purified chick embryo cell culture vaccine (PCECV) cross-neutralize non-classical bat lyssavirus strains

Tissue-culture vaccines like purified chick embryo cell vaccine (PCECV) have been shown to provide protection against classical rabies virus (RABV) via pre-exposure or post-exposure prophylaxis. A cross-neutralization study was conducted using a panel of 100 human sera, to determine, to what extent after vaccination with PCECV protection exists against non-classical bat lyssavirus strains like European bat lyssavirus (EBLV) type 1 and 2 and Australian bat lyssavirus (ABLV). Virus neutralizing antibody (VNA) concentrations against the rabies virus variants CVS-11, ABLV, EBLV-1 and EBLV-2 were determined by using a modified rapid fluorescent focus inhibition test. For ABLV and EBLV-2, the comparison to CVS-11 revealed almost identical results (100% adequate VNA concentrations >or=0.5 IU/mL; correlation coefficient r(2)=0.69 and 0.77, respectively), while for EBLV-1 more scattering was observed (97% adequate VNA concentrations; r(2)=0.50). In conclusion, vaccination with PCECV produces adequate VNA concentrations against classical RABV as well as non-classical lyssavirus strains ABLV, EBLV-1, and EBLV-2.

Plant vaccines: an immunological perspective

The advent of technologies to express heterologous proteins in planta has led to the proposition that plants may be engineered to be safe, inexpensive vehicles for the production of vaccines and possibly even vectors for their delivery. The immunogenicity of a variety of antigens of relevance to vaccination expressed in different plants has been assessed. The purpose of this article is to examine the utility of plant-expression systems in vaccine development from an immunological perspective.

Generation and characterization of human monoclonal antibodies to G5, a linear neutralization epitope on glycoprotein of rabies virus, by phage display technology

The aim of the present study was to discover distinct human MAbs to RV with high neutralizing potency and a broad neutralization spectrum. A phage display technology was used to produce human scFv to G5, a conserved linear neutralization epitope on Gp of RV. A phage display scFv library with 6 x 10(7) members was constructed and the phage-scFv with 'antigen-binding' activities were selected with synthetic peptide G5-24. The obtained scFv genes were cloned into pET22b(+)/BL21(DE3) and from this we prepared purified scFv fragments. The assay of the specificity characteristics and neutralization capacity showed that two distinct clones with new human immunoglobulin V genes can recognize G5 specifically as well as neutralize different RV strains. They have potential for inclusion in an antibodies combination aimed for use in rabies PEP.

A Human Monoclonal Antibody Cocktail as a Novel Component of Rabies Postexposure Prophylaxis*

The currently recommended treatment for individuals exposed to rabies virus is the combined administration of rabies vaccine and rabies immune globulin (RIG). This review sets out the criteria used to guide development of a cocktail of human monoclonal antibodies as a replacement for RIG. Using this process as a model, the general requirements for development of safe and efficacious monoclonal antibody alternatives to currently used polyclonal serum products are discussed.

Emergence of lyssaviruses in the Old World: the case of Africa

Rabies has a long history of occurrence throughout Africa, spanning hundreds of years. At least four distinct Lyssavirus species persist throughout the continent, among carnivores, bats and other mammals. Rabies virus is the most cosmopolitan member, with primary reservoirs within dogs and mongoose, but other wildlife vectors are important in viral maintenance, such as jackals. Besides a prominent toll on humans and domestic animals, the disease has an underappreciated role in conservation biology, especially for such highly endangered fauna as African wild dogs and Ethiopian wolves. Both Duvenhage and Lagos bat viruses are adapted to bats, but their epidemiology, together with Mokola virus, is poorly understood. Significantly, less than ideal cross-reactivity with modern biologicals used for veterinary and public health interventions is a major cause for concern among these emerging viral agents.

Identification and characterization of a human monoclonal antibody that potently neutralizes a broad panel of rabies virus isolates

Rabies is a zoonosis that results in millions of human exposures worldwide each year. Human monoclonal antibodies (HuMAbs) that neutralize rabies virus may represent one viable strategy for post-exposure prophylaxis in humans, and have many advantages over current human or equine rabies immune globulin. Transgenic mice carrying human immunoglobulin genes were used to isolate human monoclonal antibodies that neutralized rabies virus. Several HuMAbs were identified that neutralized rabies virus variants from a broad panel of isolates of public health significance. HuMAb 17C7 was the most promising antibody identified because it neutralized all rabies virus isolates tested. HuMAb 17C7 recognizes a conformational epitope on the rabies virus glycoprotein which includes antigenic site III. HuMAb 17C7 protected hamsters from a lethal dose of rabies virus in a well-established in vivo model of post-exposure prophylaxis.

Australian bat lyssavirus: a recently discovered new rhabdovirus

Australian bat lyssavirus (ABLV), first identified in 1996, has been associated with two human fatalities. ABLV is genetically and serologically distinct from, but is closely related to, classical rabies. It has a bullet-shaped morphology by electron microscopy. There are two strains of ABLV known: one circulates in frugivorous bats, sub-order Megachiroptera, and the other circulates in the smaller, mainly insectivorous bats, sub-order Microchiroptera. Each strain has been associated with one human fatality. Surveillance indicates infected bats are widespread at a low frequency on the Australian mainland. It is unclear how long ABLV has been present in Australia, although molecular clock studies suggest the two strains separated 950 or 1,700 years ago based on synonymous or non-synonymous nucleotide changes, respectively. Recent serological surveys suggest a closely related virus may exist in the Philippines. Due to demonstrated cross-protection in mice, rabies vaccine is used to prevent infection. Rabies post-exposure prophylaxis (PEP) protocols have been adopted for when a human is scratched or bitten by a suspect bat. A long-term commitment to public health programs that test bats that have been involved in scratch or bite incidents, followed by PEP if appropriate, will be necessary to minimise further human infection.

Novel human monoclonal antibody combination effectively neutralizing natural rabies virus variants and individual in vitro escape mutants

The need to replace rabies immune globulin (RIG) as an essential component of rabies postexposure prophylaxis is widely acknowledged. We set out to discover a unique combination of human monoclonal antibodies (MAbs) able to replace RIG. Stringent criteria concerning neutralizing potency, affinity, breadth of neutralization, and coverage of natural rabies virus (RV) isolates and in vitro escape mutants were set for each individual antibody, and the complementarities of the two MAbs were defined at the onset. First, we identified and characterized one human MAb (CR57) with high in vitro and in vivo neutralizing potency and a broad neutralization spectrum. The linear antibody binding site was mapped on the RV glycoprotein as antigenic site I by characterizing CR57 escape mutants. Secondly, we selected using phage display a complementing antibody (CR4098) that recognized a distinct, nonoverlapping epitope (antigenic site III), showed similar neutralizing potency and breadth as CR57, and neutralized CR57 escape mutants. Reciprocally, CR57 neutralized RV variants escaping CR4098. Analysis of glycoprotein sequences of natural RV isolates revealed that the majority of strains contain both intact epitopes, and the few remaining strains contain at least one of the two. In vitro exposure of RV to the combination of CR57 and CR4098 yielded no escape mutants. In conclusion, a novel combination of human MAbs was discovered suitable to replace RIG.

The human antibody repertoire specific for rabies virus glycoprotein as selected from immune libraries

Antibody phage display technology was used to identify human monoclonal antibodies that neutralize rabies virus (RV). A phage repertoire was constructed using antibody genes harvested from the blood of vaccinated donors. Selections using this repertoire and three different antigen formats of the RV glycoprotein (gp) resulted in the identification of 147 unique antibody fragments specific for the RV gp. Analysis of the DNA sequences of these antibodies demonstrated a large variation in the heavy- and light-chain germ-line gene usage, suggesting that a broad antibody repertoire was selected. The single-chain variable fragment (scFv) antibodies were tested in vitro for RV neutralization, resulting in 39 specificities that neutralize the virus. Of the scFv clones, 21 were converted into full-length human IgG(1) format. Analysis of viral escape variants and binding competition experiments indicated that the majority of the neutralizing antibodies are directed against antigenic site III of the RV gp. The obtained specificities expand the set of human anti-RV antibodies eligible for inclusion in an antibody cocktail aimed for use in rabies post-exposure prophylaxis.

Rabies human diploid cell vaccine elicits cross-neutralising and cross-protecting immune responses against European and Australian bat lyssaviruses

The ability of antibodies elicited against the rabies human diploid cell vaccine (HDCV) to neutralise European bat Lyssaviruses (EBLV types-1 and -2), Australian bat Lyssavirus and classical rabies virus (RABV) has been evaluated using modified fluorescent antibody virus neutralisation (mFAVN) assays. Ninety-six percent (48 of 50) of the human post-vaccinated sera tested cross-neutralised these viruses (>or=0.5 IU/ml). Cross-protection experiments using inbred mice (RIII, k/k haplotype) were also assessed. Mice were given HDCV (twice by the intra-peritoneal route) and challenged (intra-cranially or peripherally) with a lethal dose (25 MLD(50)) of the individual viruses. The vaccine conferred statistically significant protection in 80--100% of animals challenged via the peripheral route. Levels of protection were lower following intra-cranial (i.c.) challenge. Our data provides strong evidence for broad spectrum cross-neutralisation and cross-protection of phylogroup I lyssaviruses using rabies HDCV.

Novel rabies virus-neutralizing epitope recognized by human monoclonal antibody: fine mapping and escape mutant analysis

Anti-rabies virus immunoglobulin combined with rabies vaccine protects humans from lethal rabies infections. For cost and safety reasons, replacement of the human or equine polyclonal immunoglobulin is advocated, and the use of rabies virus-specific monoclonal antibodies (MAbs) is recommended. We produced two previously described potent rabies virus-neutralizing human MAbs, CR57 and CRJB, in human PER.C6 cells. The two MAbs competed for binding to rabies virus glycoprotein. Using CR57 and a set of 15-mer overlapping peptides covering the glycoprotein ectodomain, a neutralization domain was identified between amino acids (aa) 218 and 240. The minimal binding region was identified as KLCGVL (aa 226 to 231), with key residues K-CGV- identified by alanine replacement scanning. The critical binding region of this novel nonconformational rabies virus epitope is highly conserved within rabies viruses of genotype 1. Subsequently, we generated six rabies virus variants escaping neutralization by CR57 and six variants escaping CRJB. The CR57 escape mutants were only partially covered by CRJB, and all CRJB-resistant variants completely escaped neutralization by CR57. Without exception, the CR57-resistant variants showed a mutation at key residues within the defined minimal binding region, while the CRJB escape viruses showed a single mutation distant from the CR57 epitope (N182D) combined with mutations in the CR57 epitope. The competition between CR57 and CRJB, the in vitro escape profile, and the apparent overlap between the recognized epitopes argues against including both CR57 and CRJB in a MAb cocktail aimed at replacing classical immunoglobulin preparations.

Antibody processing and engineering in plants, and new strategies for vaccine production

The use of transgenic plants for the production of recombinant proteins is not a universal solution for all proteins. The choice of this expression system depends very much on the type of protein and its applications. Many proteins will best be made by conventional microbial fermentation, similarly, we are already identifying proteins where plants represent the only practical option for one reason or another. It will be important to understand better the cellular mechanisms of protein folding, assembly and processing in plants, in order to maximise the potential of transgenic plants as a protein production system. One of the main advantages that plants offer is that they are higher eukaryotic organisms with an endomembrane system. Therefore, they fold and assemble recombinant proteins using protein chaperones that are homologous to those in mammalian cells, and they perform post-translational modifications. This allows, for example, the expression of monoclonal antibodies, first described in 1989, as well as a range of other types of immunoglobulin molecules and multimeric complexes.

Rabies and other lyssavirus diseases

The full scale of the global burden of human rabies is unknown, owing to inadequate surveillance of this fatal disease. However, the terror of hydrophobia, a cardinal symptom of rabies encephalitis, is suffered by tens of thousands of people each year. The recent discovery of enzootic European bat lyssavirus infection in the UK is indicative of our expanding awareness of the Lyssavirus genus. The main mammalian vector species vary geographically, so the health problems created by the lyssaviruses and their management differ throughout the world. The methods by which these neurotropic viruses hijack neurophysiological mechanisms while evading immune surveillance is beginning to be unravelled by, for example, studies of molecular motor transport systems. Meanwhile, enormous challenges remain in the control of animal rabies and the provision of accessible, appropriate human prophylaxis worldwide.

Function and glycosylation of plant-derived antiviral monoclonal antibody

Plant genetic engineering led to the production of plant-derived mAb (mAbP), which provides a safe and economically feasible alternative to the current methods of antibody production in animal systems. In this study, the heavy and light chains of human anti-rabies mAb were expressed and assembled in planta under the control of two strong constitutive promoters. An alfalfa mosaic virus untranslated leader sequence and Lys-Asp-Glu-Leu (KDEL) endoplasmic reticulum retention signal were linked at the N and C terminus of the heavy chain, respectively. mAbP was as effective at neutralizing the activity of the rabies virus as the mammalian-derived antibody (mAbM) or human rabies Ig (HRIG). The mAbP contained mainly oligomannose type N-glycans (90%) and had no potentially antigenic alpha(1,3)-linked fucose residues. mAbP had a shorter half-life than mAbM. The mAbP was as efficient as HRIG for post-exposure prophylaxis against rabies virus in hamsters, indicating that differences in N-glycosylation do not affect the efficacy of the antibody in this model.

Postexposure prophylaxis for prevention of rabies in dogs

OBJECTIVE

To evaluate postexposure prophylaxis (PEP) in dogs experimentally infected with rabies.

ANIMALS

29 Beagles.

PROCEDURE

Dogs were sedated and inoculated in the right masseter muscle with a salivary gland homogenate from a naturally infected rabid dog (day 0). Six hours later, 5 dogs were treated by administration of 2 murine anti-rabies glycoprotein monoclonal antibodies (mAb) and commercial vaccine; 5 received mAb alone; 5 received purified, heat-treated, equine rabies immune globulin (PHT-ERIG) and vaccine; 5 received PHT-ERIG alone; 4 received vaccine alone; and 5 control dogs were not treated. The mAb or PHT-ERIG was administered at the site of rabies virus inoculation. Additional vaccine doses for groups mAb plus vaccine, PHT-ERIG plus vaccine, and vaccine alone were administered IM in the right hind limb on days 3, 7, 14, and 35.

RESULTS

All control dogs and dogs that received only vaccine developed rabies. In the PHT-ERIG and vaccine group, 2 of 5 dogs were protected, whereas none were protected with PHT-ERIG alone. Use of mAb alone resulted in protection in 4 of 5 dogs. Administration of mAb in combination with vaccine provided protection in all 5 dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

Current national guidelines recommend euthanasia or a 6-month quarantine for unvaccinated animals exposed to rabies. Findings from this study document that vaccine alone following severe exposure was unable to provide protection from rabies. However, vaccine combined with mAb resulted in protection in all treated dogs, revealing the potential use of mAb in PEP against rabies in naive dogs.

Update on rabies

Rabies remains an important public health problem worldwide due to endemic dog rabies in developing countries. Rabies was a re-emerging disease in the United States during the 1990s due to bat rabies virus variants. Australian bat lyssavirus also emerged in Australian bat populations and caused two human deaths. There have been important recent advances in our knowledge of the pathogenesis of rabies and in our ability to diagnose and prevent it.

Rabies re-examined

Rabies is an acute, progressive, incurable viral encephalitis. The causative agents are neurotropic RNA viruses in the family Rhabdoviridae, genus Lyssavirus. Mammalian reservoirs include the Carnivora and Chiroptera, but rabid dogs still pose the greatest hazard worldwide. Viral transmission occurs mainly via animal bite, and once the virus is deposited in peripheral wounds, centripetal passage occurs towards the central nervous system. After viral replication, there is centrifugal spread to major exit portals, the salivary glands. The epidemiological significance of any host "carrier" state remains highly speculative. Although incubation periods average 1-3 months, disease occurrence days or years after exposure has been documented. Rabies should be suspected in patients with a concomitant history of animal bite and traditional clinical presentation, but a lack of such clues makes antemortem diagnosis a challenge. Pathogenetic mechanisms remain poorly understood, and current care entails palliative measures only. Current medical emphasis relies heavily on prevention of exposure and intervention before clinical onset. Prophylaxis encompasses thorough wound treatment, vaccine administration, and inoculation of rabies immunoglobulin. Although it is a major zoonosis, canine rabies can be eliminated, and application of new vaccine technologies permits significant disease control among wildlife species. Nevertheless, despite much technical progress in the past century, rabies is a disease of neglect and presents a modern public-health conundrum.

Sequence analysis of an isolate from a fatal human infection of Australian bat lyssavirus

Australian bat lyssavirus (ABLV), which occurs in pteropid and insectivorous bat populations, causes a rabies-like encephalitis in infected humans. We report the first complete sequence of an ABLV isolate obtained from a human who developed symptoms 27 months after being bitten by an infected flying fox. This isolate is the smallest lyssavirus to be sequenced, with a size of 11,918 nucleotides. Analyses of previously unsequenced regions and the complete genome confirm its close relationship with classical rabies viruses. In addition, a leucine zipper-like motif, not present in the other lyssaviruses, was found in the conserved domain I of the polymerase protein. This is the first report of a lyssavirus to vary in an 11-nucleotide, strictly conserved, complementary terminal sequence. This region is thought to encode important cis-acting regulatory signals; ABLV variation indicates a greater degree of flexibility than was thought for lyssaviruses in this region. A comparison of the pteropid and insectivorous isolates of ABLV indicates considerable differences between the two viruses. If the divergence of the two occurred on the Australian mainland, ABLV may have been endemic to Australia well before European colonisation.