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

mAb therapy controls CNS-resident lyssavirus infection via a CD4 T cell-dependent mechanism

Infections with rabies virus (RABV) and related lyssaviruses are uniformly fatal once virus accesses the central nervous system (CNS) and causes disease signs. Current immunotherapies are thus focused on the early, pre-symptomatic stage of disease, with the goal of peripheral neutralization of virus to prevent CNS infection. Here, we evaluated the therapeutic efficacy of F11, an anti-lyssavirus human monoclonal antibody (mAb), on established lyssavirus infections. We show that a single dose of F11 limits viral load in the brain and reverses disease signs following infection with a lethal dose of lyssavirus, even when administered after initiation of robust virus replication in the CNS. Importantly, we found that F11-dependent neutralization is not sufficient to protect animals from mortality, and a CD4 T cell-dependent adaptive immune response is required for successful control of infection. F11 significantly changes the spectrum of leukocyte populations in the brain, and the FcRγ-binding function of F11 contributes to therapeutic efficacy. Thus, mAb therapy can drive potent neutralization-independent T cell-mediated effects, even against an established CNS infection by a lethal neurotropic virus.

Advances in the progress of monoclonal antibodies for rabies

Rabies is a highly fatal zoonotic disease caused by the rabies virus invading the central nervous system. When suspected of exposure to the rabies virus, post-exposure prophylaxis should be administered as soon as possible. Monoclonal antibodies (mAbs) neutralizing the rabies virus could be better in human rabies post-exposure prophylaxis than equine or human rabies immune globulin in terms of supply, cost, and efficacy. This article reviews anti-rabies mAbs produced by multiple techniques, and the results of clinical trials for anti-rabies mAbs cocktails recognizing non-overlapping epitopes are also discussed.

A broad-spectrum and highly potent human monoclonal antibody cocktail for rabies prophylaxis

Post-exposure prophylaxis (PEP) is highly effective in preventing disease progression of rabies when used in timely and appropriate manner. The key treatment for PEP is infiltration of rabies immune globulin (RIG) into lesion site after bite exposure, besides wound care and vaccination. Unfortunately, however, RIG is expensive and its supply is limited. Currently, several anti-rabies virus monoclonal antibody (mAb) products are under development as alternatives to RIG, and two recently received regulatory approval in India. In this study, fully human mAbs that recognize different rabies virus glycoprotein conformational antigenic site (II and III) were created from peripheral blood mononuclear cells of heathy vaccinated subjects. These mAbs neutralized a diverse range of lyssavirus types. As at least two anti-rabies virus mAbs are recommended for use in human PEP to ensure broad coverage against diverse lyssaviruses and to minimize possible escape variants, two most potent mAbs, NP-19-9 and 11B6, were selected to be used as cocktail treatment. These two mAbs were broadly reactive to different types of lyssaviruses isolates, and were shown to have no interference with each other. These results suggest that NP-19-9 and 11B6 are potent candidates to be used for PEP, suggesting further studies involving clinical studies in human.

Isolation and Characterization of Cross-Reactive Human Monoclonal Antibodies That Potently Neutralize Australian Bat Lyssavirus Variants and Other Phylogroup 1 Lyssaviruses

Australian bat lyssavirus (ABLV) is a rhabdovirus that circulates in four species of pteropid bats (ABLVp) and the yellow-bellied sheath-tailed bat (ABLVs) in mainland Australia. In the three confirmed human cases of ABLV, rabies illness preceded fatality. As with rabies virus (RABV), post-exposure prophylaxis (PEP) for potential ABLV infections consists of wound cleansing, administration of the rabies vaccine and injection of rabies immunoglobulin (RIG) proximal to the wound. Despite the efficacy of PEP, the inaccessibility of human RIG (HRIG) in the developing world and the high immunogenicity of equine RIG (ERIG) has led to consideration of human monoclonal antibodies (hmAbs) as a passive immunization option that offers enhanced safety and specificity. Using a recombinant vesicular stomatitis virus (rVSV) expressing the glycoprotein (G) protein of ABLVs and phage display, we identified two hmAbs, A6 and F11, which completely neutralize ABLVs/ABLVp, and RABV at concentrations ranging from 0.39 and 6.25 µg/mL and 0.19 and 0.39 µg/mL respectively. A6 and F11 recognize overlapping epitopes in the lyssavirus G protein, effectively neutralizing phylogroup 1 lyssaviruses, while having little effect on phylogroup 2 and non-grouped diverse lyssaviruses. These results suggest that A6 and F11 could be effective therapeutic and diagnostic tools for phylogroup 1 lyssavirus infections.

The Generation of the Human mAb RabD4 Specific to the Rabies Virus Glycoprotein and Characterization Thereof

We generated a novel human neutralizing human mAb RabD4 against rabies virus glycoprotein using in vitro stimulation of human peripheral B cells produced by immunized donor. The human mAb RabD4 showed a high antigen-binding activity and virus-neutralizing activity in the FAVN test with the CVS-11 rabies virus.

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 and characterization of novel chimeric monoclonal antibodies for broad spectrum neutralization of rabies virus

Current post-exposure prophylaxis for rabies virus infection has several limitations in terms of supply, cost, safety, and efficacy. Attempts to replace human or equine rabies immune globulins (HRIG or ERIG) have been made by several companies and institutes. We developed potent monoclonal antibodies to neutralize a broad spectrum of rabies viruses by screening hybridomas received from the U.S. Centers for Disease Control and Prevention (CDC). Two kinds of chimeric human antibodies (chimeric #7 and #17) were constructed by cloning the variable regions from selected hybridomas and the constant region of a human antibody. Two antibodies were bound to antigenic site III and I/IV, respectively, and were able to neutralize 51 field isolates of rabies virus that were isolated at different times and places such as Asia, Africa, North America, South America, and Australia. These two antibodies neutralize rabies viruses with high efficacy in an in vivo test using Syrian hamster and mouse models and show low risk for adverse immunogenicity.

In Vivo Efficacy of a Cocktail of Human Monoclonal Antibodies (CL184) Against Diverse North American Bat Rabies Virus Variants

Following rabies virus (RABV) exposure, a combination of thorough wound washing, multiple-dose vaccine administration and the local infiltration of rabies immune globulin (RIG) are essential components of modern post-exposure prophylaxis (PEP). Although modern cell-culture-based rabies vaccines are increasingly used in many countries, RIG is much less available. The prohibitive cost of polyclonal serum RIG products has prompted a search for alternatives and design of anti-RABV monoclonal antibodies (MAbs) that can be manufactured on a large scale with a consistent potency and lower production costs. Robust in vitro neutralization activity has been demonstrated for the CL184 MAb cocktail, a 1:1 protein mixture of two human anti-RABV MAbs (CR57/CR4098), against a large panel of RABV isolates. In this study, we used a hamster model to evaluate the efficacy of experimental PEP against a lethal challenge. Various doses of CL184 and commercial rabies vaccine were assessed for the ability to protect against lethal infection with representatives of four distinct bat RABV lineages of public health relevance: silver-haired bat (Ln RABV); western canyon bat (Ph RABV); big brown bat (Ef-w1 RABV) and Mexican free-tailed bat RABV (Tb RABV). 42⁻100% of animals survived bat RABV infection when CL184 (in combination with the vaccine) was administered. A dose-response relationship was observed with decreasing doses of CL184 resulting in increasing mortality. Importantly, CL184 was highly effective in neutralizing and clearing Ph RABV in vivo, even though CR4098 does not neutralize this virus in vitro. By comparison, 19⁻95% survivorship was observed if human RIG (20 IU/kg) and vaccine were used following challenge with different bat viruses. Based on our results, CL184 represents an efficacious alternative for RIG. Both large-scale and lower cost production could ensure better availability and affordability of this critical life-saving biologic in rabies enzootic countries and as such, significantly contribute to the reduction of human rabies deaths globally.

Four Thousand Years of Concepts Relating to Rabies in Animals and Humans, Its Prevention and Its Cure

The epitome of the One Health paradigm-and of its shortcomings-rabies has been known to humankind for at least 4000 years. We review the evolution through history of concepts leading to our current understanding of rabies in dogs and humans and its prevention, as transmitted by accessible and surviving written texts. The tools and concepts currently available to control rabies were developed at the end of the 19th Century, including the first live, attenuated vaccine ever developed for humans and the first post-exposure prophylaxis (PEP) regimen. No progress, however, has been made in etiological treatment, leaving clinicians who provide care to animals or patients with symptomatic rabies as powerless today as their colleagues in Mesopotamia, 40 centuries ago. Rabies remains to date the most lethal infectious disease known to humans. Widespread access to timely, effective, and affordable PEP in rural areas of developing countries is urgently needed.

Safety and General Considerations for the Use of Antibodies in Infectious Diseases

Monocolonal antibodies are valuable potential new tools for meeting unmet needs in treating infectious dieseases and to provide alternatives and supplements to antibiotics in these times of growing resistance. Especially when considering the ability to screen for antibodies reacting to very diverse target antigens and the ability to design and engineer them to work specifically to hit and overcome their strategies, like toxins and their hiding in specific cells to evade the immuneresponse and their special features enabling killing of the infectious agents and or the cells harbouring them. Antibodies are generally very safe and adverse effects of treatments with therapeutic antibodies are usually related to exaggeration of the intended pharmacology. In this chapter general safety considerations for the use of antibodies is reviewed and the general procedures for nonclinical testing to support their clinical development. Special considerations for anti-infective mAb treatments are provided including the special features that makes nonclinical safety programs for anti-infective mAbs much more simple and restricted. However at a cost since only limited information for clinical safety and modeling can be derived from such programs. Then strategies for optimally designing antibodies are discussed including the use of combination of antibodies. Finally ways to facilitate development of more than the currently only three approved mAb based treatments are discussed with a special focus on high costs and high price and how collaboration and new strategies for development in emerging markets can be a driver for this.

Promises and pitfalls for recombinant oligoclonal antibodies-based therapeutics in cancer and infectious disease

Monoclonal antibodies (mAbs) have revolutionized the diagnosis and treatment of many human diseases and the application of combinations of mAbs has demonstrated improved therapeutic activity in both preclinical and clinical testing. Combinations of antibodies have several advantages such as the capacities to target multiple and mutating antigens in complex pathogens and to engage varied epitopes on multiple disease-related antigens (e.g. receptors) to overcome heterogeneity and plasticity. Oligoclonal antibodies are an emerging therapeutic format in which a novel antibody combination is developed as a single drug product. Here, we will provide historical context on the use of oligoclonal antibodies in oncology and infectious diseases and will highlight practical considerations related to their preclinical and clinical development programs.

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.

History and Practice: Antibodies in Infectious Diseases

Antibodies and passive antibody therapy in the treatment of infectious diseases is the story of a treatment concept which dates back more than 120 years, to the 1890s, when the use of serum from immunized animals provided the first effective treatment options against infections with Clostridium tetani and Corynebacterium diphtheriae. However, after the discovery of penicillin by Fleming in 1928, and the subsequent introduction of the much cheaper and safer antibiotics in the 1930s, serum therapy was largely abandoned. However, the broad and general use of antibiotics in human and veterinary medicine has resulted in the development of multi-resistant strains of bacteria with limited to no response to existing treatments and the need for alternative treatment options. The combined specificity and flexibility of antibody-based treatments makes them very valuable tools for designing specific antibody treatments to infectious agents. These attributes have already caused a revolution in new antibody-based treatments in oncology and inflammatory diseases, with many approved products. However, only one monoclonal antibody, palivizumab, for the prevention and treatment of respiratory syncytial virus, is approved for infectious diseases. The high cost of monoclonal antibody therapies, the need for parallel development of diagnostics, and the relatively small markets are major barriers for their development in the presence of cheap antibiotics. It is time to take a new and revised look into the future to find appropriate niches in infectious diseases where new antibody-based treatments or combinations with existing antibiotics, could prove their value and serve as stepping stones for broader acceptance of the potential for and value of these treatments.

Current status of rabies and prospects for elimination

Rabies is one of the most deadly infectious diseases, with a case-fatality rate approaching 100%. The disease is established on all continents apart from Antarctica; most cases are reported in Africa and Asia, with thousands of deaths recorded annually. However, the estimated annual figure of almost 60,000 human rabies fatalities is probably an underestimate. Almost all cases of human rabies result from bites from infected dogs. Therefore, the most cost-effective approach to elimination of the global burden of human rabies is to control canine rabies rather than expansion of the availability of human prophylaxis. Mass vaccination campaigns with parenteral vaccines, and advances in oral vaccines for wildlife, have allowed the elimination of rabies in terrestrial carnivores in several countries worldwide. The subsequent reduction in cases of human rabies in such regions advocates the multidisciplinary One Health approach to rabies control through the mass vaccination of dogs and control of canine populations.

Met-CCL5 represents an immunotherapy strategy to ameliorate rabies virus infection

Background

Infection of rabies virus (RABV) causes central nervous system (CNS) dysfunction and results in high mortality in human and animals. However, it is still unclear whether and how CNS inflammation and immune response contribute to RABV infection.

Methods

Suckling mice were intracerebrally infected with attenuated RABV aG and CTN strains, followed by examination of chemokine or cytokine production, inflammatory cell infiltration and neuron apoptosis in the brain. Furthermore, the suckling mice and adult mice that were intracerebrally infected with aG and the adult mice that were intramuscularly infected with street RABV HN10 were treated with CCL5 antagonist (Met-CCL5) daily beginning on day 2 postinfection. The survival rates and inflammation responses in the CNS of these mice were analyzed.

Results

Excessive CCL5 in the CNS was associated with CNS dysfunction, inflammation, and macrophage or lymphocyte infiltration after attenuated or street RABV infection. Administration of exogenous CCL5 induced excessive infiltration of immune cells into the CNS and enhanced inflammatory chemokine and cytokine production. Met-CCL5 treatment significantly prolonged survival time of the suckling mice inoculated with aG and adult mice infected with aG and HN10.

Conclusions

These results suggest that CCL5 in the CNS is a key regulator involved in inducing rabies encephalomyelitis. Furthermore, treatment with the CCL5 antagonist Met-CCL5 prolongs survival time of the mice infected with attenuated or street RABVs, which might represent a novel therapeutic strategy to ameliorate RABV infection.

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.

Molecular characterization of three ferret badger (Melogale moschata) rabies virus isolates from Jiangxi province, China

Ferret badger (FB) rabies viruses JX09-17(fb), JX09-18 and JX10-37 were isolated from three different regions in Jiangxi province, China, in 2009 and 2010. The complete nucleotide sequence identity between these three isolates was 87-93 %. Compared with the other Chinese rabies virus isolates and vaccine strains, 101 substitutions (53 in JX10-37, 23 in JX09-17(fb) and 25 in JX09-18) in the five structural proteins were observed, and 47 of these substitutions (27 in JX10-37, 14 in JX09-17(fb) and 6 in JX09-18) were unique among lyssaviruses. Amino acid substitutions of S231 and Q333 were noted respectively in the G protein antigenic site I of JX10-37 and site III in JX09-17(fb). Phylogenetic analysis showed that JX09-17(fb) is rooted within the China I lineage, JX09-18 is in China II, and JX10-37 is independent. Evolutionary analysis and comparative sequence data indicate that isolate JX10-37 is a variant virus that diverged from canine rabies viruses around 1933 (range 1886-1963).

Negative effects of a disulfide bond mismatch in anti-rabies G protein single-chain antibody variable fragment FV57

Rabies virus (RV) causes a fatal infectious disease requiring efficient post-exposure prophylaxis (PEP), which includes a rabies vaccine and rabies immunoglobulin (RIG). The single-chain antibody variable fragment (scFv), a small engineered antibody fragment derived from an antibody variable heavy chain and light chain, has the potential to replace the current application of RIG. In previous studies, we constructed and evaluated an anti-rabies virus G protein scFv (FV57) based on the monoclonal antibody CR57. Of the five cysteines in FV57, four are linked in intra-chain disulfide bonds (Cys-VH28/Cys-VH98 and Cys-VL16/Cys-VL84), and one is free (Cys-VL85). However, the thiol in Cys-VL85 neighboring Cys-VL84 in the CDR3 of the light chain is likely to mismatch with the thiol in Cys-VL16 during the renaturing process. In order to study effects of the mismatched disulfide bond, Cys-VL85 and Cys-VL84 of FV57 were mutated to serine to construct mutants FV57(VL85S) and FV57(VL84S). Furthermore, the disulfide bonds in the light chain of FV57, FV57(VL85S) and FV57(VL84S) were deleted by mutating Cys-VL16 to serine. All mutants were prepared and evaluated along with the original FV57. The results indicated that the mismatched disulfide bond of FV57 linking the light chain FR1 and CDR3 would confer deleterious negative effects on its activity against RV, likely due to spatial hindrance in the light chain CDR3. Moreover, avoidance of the disulfide bond mismatch provided an additional 30% protective efficacy against RV infection in the mouse RV challenge model. Thus, modifications of FV57 to eliminate the disulfide bond mismatch may provide a candidate therapeutic agent for effective PEP against rabies.

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

The safety and availability of the human polyclonal sera that is currently utilized for post-exposure treatment (PET) of rabies virus (RABV) infection remain a concern. Recombinant monoclonal antibodies have been postulated as suitable alternatives by WHO. To this extent, CL184, the RABV human antibody combination comprising monoclonal antibodies (mAbs) CR57 and CR4098, has been developed and has delivered promising clinical data to support its use for RABV PET. For this fully human IgG1 cocktail, mAbs CR57 and CR4098 are produced in the PER.C6 human cell line and combined in equal amounts in the final product. During preclinical evaluation, CR57 was shown to bind to antigenic site I whereas CR4098 neutralization was influenced by a mutation of position 336 (N336) located within antigenic site III. Here, alanine scanning was used to analyze the influence of mutations within the potential binding site for CR4098, antigenic site III, in order to evaluate the possibility of mutated rabies viruses escaping neutralization. For this approach, twenty flanking amino acids (10 upstream and 10 downstream) of the RABV glycoprotein (G) asparagine (N336) were exchanged to alanine (or serine, if already alanine) by site-directed mutagenesis. Analysis of G expression revealed four of the twenty mutant Gs to be non-functional, as shown by their lack of cell surface expression, which is a requirement for the production of infectious RABV. Therefore, these mutants were excluded from further study. The remaining sixteen mutants were introduced in an infectious clone of RABV, and recombinant RABVs (rRABVs) were recovered and utilized for in vitro neutralization assays. All of the viruses were effectively neutralized by CR4098 as well as by CR57, indicating that single amino acid exchanges in this region does not affect the broad neutralizing capability of the CL184 mAb combination.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

Single domain antibody multimers confer protection against rabies infection

Post-exposure prophylactic (PEP) neutralizing antibodies against Rabies are the most effective way to prevent infection-related fatality. The outer envelope glycoprotein of the Rabies virus (RABV) is the most significant surface antigen for generating virus-neutralizing antibodies. The small size and uncompromised functional specificity of single domain antibodies (sdAbs) can be exploited in the fields of experimental therapeutic applications for infectious diseases through formatting flexibilities to increase their avidity towards target antigens. In this study, we used phage display technique to select and identify sdAbs that were specific for the RABV glycoprotein from a naïve llama-derived antibody library. To increase their neutralizing potencies, the sdAbs were fused with a coiled-coil peptide derived from the human cartilage oligomeric matrix protein (COMP48) to form homogenous pentavalent multimers, known as combodies. Compared to monovalent sdAbs, the combodies, namely 26424 and 26434, exhibited high avidity and were able to neutralize 85-fold higher input of RABV (CVS-11 strain) pseudotypes in vitro, as a result of multimerization, while retaining their specificities for target antigen. 26424 and 26434 were capable of neutralizing CVS-11 pseudotypes in vitro by 90–95% as compared to human rabies immunoglobulin (HRIG), currently used for PEP in Rabies. The multimeric sdAbs were also demonstrated to be partially protective for mice that were infected with lethal doses of rabies virus in vivo. The results demonstrate that the combodies could be valuable tools in understanding viral mechanisms, diagnosis and possible anti-viral candidate for RABV infection.

Current and future tools for global canine rabies elimination

Even though rabies is almost uniformly fatal, it is readily preventable with currently available tools. Vaccination is highly efficacious for the pre-exposure prophylaxis (PrEP) of rabies in humans and animals, and prompt postexposure prophylaxis (PEP) with vaccine and rabies immune globulin (RIG) can reliably prevent disease in humans. However, access to these tools and knowledge of their proper use are often limited, especially in impoverished, rabies-enzootic countries with the highest disease burden. In the absence of reliable diagnostic capacity and risk assessments, vaccines and RIG are often administered inappropriately, leading to chronic supply shortages and otherwise preventable deaths. Rather than focusing solely on human prophylaxis, it is more cost-effective over the long term to eliminate canine rabies in its natural terrestrial reservoirs. Because more than 99% of human rabies deaths result from dog bites, prevention efforts should focus on dogs. A versatile "One Health" strategy for canine rabies elimination should aim to create sustainable herd immunity in dogs, using proven vaccination strategies at the local level, coupled with community education and humane population management. Such strategies have succeeded in both developed and developing countries, and can be adapted to any locality. Numerous examples in Africa, Asia, and Latin America have shown that community-based, locally guided vaccination and education programs, based on a shared vision and long-term commitment, can eliminate canine rabies. Such programs should have specific goals and measurable outcomes, and should be conducted under the guidance of supportive governments, in collaboration with international partners and nongovernmental organizations. In addition to currently available tools, rabies prevention can be augmented by new dose-sparing human vaccine schedules, alternative routes of vaccine administration, monoclonal antibodies as an alternative to RIG, sensitive and specific point-of-care diagnostics and the development of canine immunocontraceptive methods. Accurate risk assessments of potential human exposures and support for decentralized laboratory capacity will be essential to ensure the most effective utilization of vaccines and RIG until canine rabies has been eliminated.

Eliminating canine rabies, the principal source of human infection: what will it take?

More than 50,000 people die of rabies each year; most are children in developing countries, and almost all have been bitten by dogs. Eliminating canine rabies throughout the world would save thousands of lives and would reduce the economic impact of the disease by dramatically reducing the requirement for postexposure prophylaxis (PEP). Lengthy experience in the industrialized countries and ongoing programs in Latin America, Africa, and Asia have shown that the elimination of rabies in dogs is an achievable goal. The presence of canine rabies in developing countries is associated with poverty, and most deaths occur in the lowest socioeconomic sectors. To be successful, national rabies control programs should share responsibility with local communities for prevention and control activities and maintaining disease-free status. Legislation should be adapted to local conditions and the realities of dog ownership. While the provision of PEP to all bite victims is affordable in many countries, it is usually beyond the capacity of impoverished nations, which deal with many other health priorities. Ministries of health should provide PEP, either free or with a charge preferably at a subsidized price, replacing the current system in many countries, in which biologics are sold by government-owned and private clinics at a cost beyond the means of bite victims. The public health sector should assume responsibility when animal control strategies are not effectively implemented or when PEP is not administered correctly or is not available. A global strategy is needed to identify gaps in surveillance and diagnosis, improve access to PEP and enhance canine immunization and population management. Such approaches based on a "One Health" model should be coordinated across regions, and should extend control efforts to other dog-related zoonoses. This article introduces a symposium in Antiviral Research on the elimination of canine rabies.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

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.

Preparation and identification of anti-rabies virus monoclonal antibodies

To provide a foundation for the development of rapid and specific methods for the diagnosis of rabies virus infection, anti-rabies virus monoclonal antibodies were prepared and rabies virus nucleoprotein and human rabies virus vaccine strain (PV strain) were used as immunogens to immunize 6-8 week old female BALB/c mice. Spleen cells and SP2/0 myeloma cells were fused according to conventional methods: the monoclonal cell strains obtained were selected using the indirect immunofluorescence test; this was followed by preparation of monoclonal antibody ascitic fluid; and finally, systematic identification of subclass, specificity and sensitivity was carried out. Two high potency and specific monoclonal antibodies against rabies virus were obtained and named 3B12 and 4A12, with ascitic fluid titers of 1:8000 and 1:10000, respectively. Both belonged to the IgG2a subclass. These strains secrete potent, stable and specific anti-rabies virus monoclonal antibodies, which makes them well suited for the development of rabies diagnosis reagents.

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.

Antibodies in infectious diseases: polyclonals, monoclonals and niche biotechnology

Antibody preparations have a long history of providing protection from infectious diseases. Although antibodies remain the only natural host-derived defense mechanism capable of completely preventing infection, as products, they compete against inexpensive therapeutics such as antibiotics, small molecule inhibitors and active vaccines. The continued discovery in the monoclonal antibody (mAb) field of leads with broadened cross neutralization of viruses and demonstrable synergy of antibody with antibiotics for bacterial diseases, clearly show that innovation remains. The commercial success of mAbs in chronic disease has not been paralleled in infectious diseases for several reasons. Infectious disease immunotherapeutics are limited in scope as endemic diseases necessitate active vaccine development. Also, the complexity of these small markets draws the interest of niche companies rather than big pharmaceutical corporations. Lastly, the cost of goods for mAb therapeutics is inherently high for infectious agents due to the need for antibody cocktails, which better mimic polyclonal immunoglobulin preparations and prevent antigenic escape. In cases where vaccine or convalescent populations are available, current polyclonal hyperimmune immunoglobulin preparations (pIgG), with modern and highly efficient purification technology and standardized assays for potency, can make economic sense. Recent innovations to broaden the potency of mAb therapies, while reducing cost of production, are discussed herein. On the basis of centuries of effective use of Ab treatments, and with growing immunocompromised populations, the question is not whether antibodies have a bright future for infectious agents, but rather what formats are cost effective and generate safe and efficacious treatments to satisfy regulatory approval.

Characterization of a human antibody fragment Fab and its calcium phosphate nanoparticles that inhibit rabies virus infection with vaccine

Recombinant antibody phage display technology has been used to mimic many aspects of the processes that govern the generation and selection of high-affinity natural human antibodies in the human immune system, especially for infectious disease prophylaxis. An anti-rabies virus immunized phage-display Fab library was constructed from peripheral blood lymphocytes from vaccinated volunteers. The immunized antibody library, with a diversity of 6.7×10(8), was used to select and produce antibodies that bound to rabies virus glycoprotein. After five rounds of immobilized fixed rabies virion panning, four unique DNA sequences were found in the higher binding clones, and only one, Fab094, showed neutralization activity. Fab094 components were analyzed by ELISA, immunoprecipitation and immunofluorescent staining. ELISA and immunofluorescence showed that Fab094 bound specifically to rabies virions. Immunoprecipitation and mass spectrometry showed that Fab094 reacted with rabies virus glycoprotein. To improve the penetration power of Fab094 antibodies, we developed Fab094 calcium phosphate nanoparticles (Fab094-CPNPs) and tested their efficacy. The rapid fluorescent focus inhibition test indicated that the neutralizing antibody titers of Fab094 and Fab094-CPNPs were reached at 200.17 IU/Kg and 246.12 IU/Kg, respectively. These findings were confirmed in vivo in a Kunming mouse challenge model. Our results demonstrate that human Fab094 and Fab094-CPNPs are efficacious candidate drugs to replace rabies immunoglobulin in post-exposure prophylaxis (PEP).

Generation and characterization of the human neutralizing antibody fragment Fab091 against rabies virus

AIM

To transform the human anti-rabies virus glycoprotein (anti-RABVG) single-chain variable fragment (scFv) into a Fab fragment and to analyze its immunological activity.

METHODS

The Fab gene was amplified using overlap PCR and inserted into the vector pComb3XSS. The recombinant vector was then transformed into E coli Top10F' for expression and purification. The purified Fab was characterized using SDS-PAGE, Western blotting, indirect ELISA, competitive ELISA, and the fluorescent antibody virus neutralization test (FAVN), respectively, and examined in a Kunming mouse challenge model in vivo.

RESULTS

A recombinant vector was constructed. The Fab was expressed in soluble form in E coli Top10F'. Specific binding of the Fab to rabies virus was confirmed by indirect ELISA and immunoprecipitation (IP). The neutralizing antibody titer of Fab was 10.26 IU/mL. The mouse group treated with both vaccine and human rabies immunoglobulin (HRIG)/Fab091 (32 IU/kg) showed protection against rabies, compared with the control group (P<0.05, Logrank test).

CONCLUSION

The antibody fragment Fab was shown to be a neutralizing antibody against RABVG. It can be used together with other monoclonal antibodies for post-exposure prophylaxis of rabies virus in future studies.

Whole-genome analysis of a human rabies virus from Sri Lanka

The complete genome sequence of a human rabies virus, strain H-08-1320, from Sri Lanka was determined and compared with other rabies viruses. The size of the genome was 11,926 nt, and it was composed of a 58-nucleotide 3' leader, five protein genes--N (1353 nt), P (894 nt), M (609 nt), G (1575 nt), and L (6387 nt)--and a 70-nt 5' trailer. The intergenic region G-L contained 515 nt. The sizes of the nucleoprotein, phosphoprotein, matrix-protein, glycoprotein and large-protein was 450, 296, 202, 524 and 2,128 residues, respectively. The phosphoprotein and large protein were one amino acid shorter and longer, respectively, than those of most rabies viruses. The glycoprotein of H-08-1320 had a unique amino acid substitution at antigenic site I. Whole-genome phylogenetic analysis showed that strain H-08-1320 formed an independent lineage and did not cluster with rabies viruses from other countries.

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.

Rupprecht C. Design of future rabies biologics and antiviral drugs

In recent years, no major paradigm shifts have occurred in the utilization of new products for the prevention and control of rabies. Development of new cost-effective rabies biologics and antiviral drugs is critical in continuing to prevent and reduce disease. Current rabies vaccines are highly effective but have developed largely based on technical improvements in the vaccine industry. In the future, alternative approaches for improved vaccines, including novel avirulent rabies virus (RABV) vectors, should be pursued. Any rabies vaccine that is effective without the need for rabies immune globulin (RIG) will contribute fundamentally to disease prevention by reducing the cost and complexity of postexposure prophylaxis (PEP). The lack of high quality, affordable RIG is a continuing problem. Virus-specific monoclonal antibodies (mAbs) will soon fulfill the PEP requirement for passive immunity, currently met with RIG. Several relevant strategies for mAb production, including use of transgenic mice, humanization of mouse mAbs, and generation of human immune libraries, are underway. As a result of successful PEP and pre-exposure prophylaxis in developed countries, until recently, no significant focused efforts have been devoted to RABV-specific antiviral agents. To date, combination therapy including broad spectrum antiviral agents has been successful in only one case, and reports of antiviral activity are often conflicting. Current antiviral strategies target either the nucleoprotein or phosphoprotein, but drugs targeting the viral polymerase should be considered. Considering the lag from creation of new concepts to experimental development and clinical trials, many years will likely elapse between today's ideas and tomorrow's practices.

Generation of stable cell clones expressing mixtures of human antibodies

Therapeutic monoclonal antibodies, a highly successful class of biological drugs, are conventionally manufactured in mammalian cell lines. A recent approach to increase the therapeutic effectiveness of monoclonal antibodies has been to combine two or more of them; however this increases the complexity of development and manufacture. To address this issue a method to efficiently express multiple monoclonal antibodies from a single cell has been developed and we describe here the generation of stable cell clones that express high levels of a human monoclonal antibody mixture. PER.C6 cells were transfected with a combination of plasmids containing genes encoding three different antibodies. Clones that express the three corresponding antibody specificities were identified, subcloned, and passaged in the absence of antibiotic selection pressure. At several time points, batch production runs were analyzed for stable growth and IgG production characteristics. The majority (11/12) of subclones analyzed expressed all three antibody specificities in constant ratios with total IgG productivity ranging between 15 and 20 pg/cell/day under suboptimal culture conditions after up to 67 population doublings. The growth and IgG production characteristics of the stable clones reported here resemble those of single monoclonal antibody cell lines from conventional clone generation programs. We conclude that the methodology described here is applicable to the generation of stable PER.C6(R) clones for industrial scale production of mixtures of antibodies.

[Monoclonal antibodies for treating infectious diseases]

Monoclonal antibodies (mAb) are attractive biologic drugs because of their specificity and well understood mechanisms of action. So far, most mAb have been developed for treating cancers or immunological disorders. However, the antibiotic resistance crisis, emerging viral diseases and bioterrorism have increased the development of anti-infectious mAb, for which more than twenty clinical trials are in progress to evaluate their safety and efficacy. The synergies obtained using combinations of anti-infectious mAb and small molecule drugs will certainly offer new opportunities for the treatment of infectious diseases.

[The use of passive rabies immunotherapy: from the past to the future]

Rabies is a fatal disease transmitted by infected animals by bite, scratch, licking on broken skin or contamination of mucosis by saliva. The regimen of post-exposure prophylaxis for people not previously vaccinated, that is currently recommended by WHO, consists of a combination of wound cleaning, active immunization and passive immunization when the exposure is of category 3. Most of the products available on the market, in particular human rabies immunoglobulins, highly purified equine rabies immunoglobulins and the derived F(ab')(2) fragments, are now characterized by high potency and safety. Although the interest of passive anti-rabies immunization was first demonstrated in the first half of the 20th century, there is still an inadequate supply of these products to the target populations mostly in developing countries. Therefore, it is urgent to set-up training and information actions for healthcare personnel on the need to use passive immunotherapy and the lack of adverse effects of the related products. For the future, we hope that a scale up of production and a lower price will improve the accessibility to these products. The development of new products based on monoclonal antibodies and molecular biology, and which may be cheaper, is promising.

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.

Isolation and characterization of human neutralizing antibodies to rabies virus derived from a recombinant immune antibody library

A human immune Fab library was constructed using RNAs from peripheral blood lymphocytes obtained from rabies virus hyperimmune volunteers on phagemid vector. The size of the constructed Fab library was 2 x 10(7) Escherichia coli transformants. After four rounds of panning on whole inactivated rabies virus (PV-11), phage clones displaying rabies virus-specific human Fab were selected. The specificity of soluble Fab antibody fragments, derived from positive phage clones was verified by ELISA. Among 20 specific Fab clones, the genetic sequence of 6 of them (FabRV01, FabRV02, FabRV03, FabRV04, FabRV05, and FabRV06) was analyzed. The variable heavy (VH) and variable light (VL) domains were found to share 90% and 93% homology with sequences encoded by the corresponding human germline genes, respectively. The soluble Fab fragments, expressed in Escherichia coli were purified by a single step Nickel-NTA affinity chromatography via a hexa-histidine tag and their binding specificities to rabies virus were confirmed. Three of the Fab antibodies, FabRV01, FabRV02 and FabRV03, showed binding characteristics to rabies virus glycoprotein antigenic site III with affinities in the K(D) range 7 x 10(-9) to 5 x 10(-8)M. The Fab fragments showed dose-dependent neutralization properties for the challenge virus standard (CVS-11).

Combination therapy using chimeric monoclonal antibodies protects mice from lethal H5N1 infection and prevents formation of escape mutants

BACKGROUND

Given that there is a possibility of a human H5N1 pandemic and the fact that the recent H5N1 viruses are resistant to the anti-viral drugs, newer strategies for effective therapy are warranted. Previous studies show that single mAbs in immune prophylaxis can be protective against H5N1 infection. But a single mAb may not be effective in neutralization of a broad range of different strains of H5N1 and control of potential neutralization escape mutants.

METHODS/PRINCIPAL FINDINGS

We selected two mAbs which recognized different epitopes on the hemagglutinin molecule. These two mAbs could each neutralize in vitro escape mutants to the other and in combination could effectively neutralize viruses from clades 0, 1, 2.1, 2.2, 2.3, 4, 7 and 8 of influenza A H5N1 viruses. This combination of chimeric mAbs when administered passively, pre or post challenge with 10 MLD50 (50% mouse lethal dose) HPAI H5N1 influenza A viruses could protect 100% of the mice from two different clades of viruses (clades 1 and 2.1). We also tested the efficacy of a single dose of the combination of mAbs versus two doses. Two doses of the combination therapy not only affected early clearance of the virus from the lung but could completely prevent lung pathology of the H5N1 infected mice. No escape variants were detected after therapy.

CONCLUSIONS/SIGNIFICANCE

Our studies provide proof of concept that the synergistic action of two or more mAbs in combination is required for preventing the generation of escape mutants and also to enhance the therapeutic efficacy of passive therapy against H5N1 infection. Combination therapy may allow for a lower dose of antibody to be administered for passive therapy of influenza infection and hence can be made available at reduced economic costs during an outbreak.

Specificity tuning of antibody fragments to neutralize two human chemokines with a single agent

Chemokines are important mediators of the immune response that are responsible for the trafficking of immune cells between lymphoid organs and migration towards sites of inflammation.Using phage display selection and a functional screening approach, we have isolated a panel of single-chain fragment variable (scFv) capable of neutralizing the activity of the human chemokine CXCL10 (hCXCL10). One of the isolated scFv was weakly cross-reactive against another human chemokine CXCL9,but was unable to block its biological activity. We diversified the complementarity determining region 3 (CDR3) of the light chain variable domain (VL) of this scFv and combined phage display with high throughput antibody array screening to identify variants capable of neutralizing both chemokines. Using this approach it is therefore possible to engineer pan-specific antibodies that could prove very useful to antagonize redundant signaling pathways such as the chemokine signaling network.

Passive protection assay of monoclonal antibodies against dengue virus in suckling mice

Dengue fever and dengue hemorrhagic fever/dengue shock syndrome are highly infectious diseases caused by dengue virus (DV). Specific monoclonal antibodies (mAbs) against DV are vital for diagnosis, pathological studies, and passive immune therapy. In this study, purified DV serotype 2 (DV2) was used as antigen and BALB/c mice were immunized to induce specific antibodies. We established five hybridoma cell lines, called 78#, 1E7, 7F7, 8F12, and 8H1, respectively, and evaluated them by enzyme-linked immunosorbent assay, indirect immunofluorescence assay, Western blot, plaque reduction neutralization test, and suckling mice protection assay. Lines 78#, 1E7, 7F7, and 8F12 showed a neutralizing effect, and lines 78#, 1E7, 8F12, and 8H1 recognized envelope glycoprotein of DV2. Among them, lines 78# and 8F12 had stronger neutralizing ability in vitro and could protect some suckling mice from virus challenge. Our results demonstrate that immunization with purified virion is efficient for the production of specific neutralizing mAbs against DV2, and these mAbs could be useful tools for studying or treating DV infection.