Development and Characterization of a Guinea Pig-Adapted Sudan Virus

Emerging and reemerging infectious diseases: global trends and new strategies for their prevention and control

To adequately prepare for potential hazards caused by emerging and reemerging infectious diseases, the WHO has issued a list of high-priority pathogens that are likely to cause future outbreaks and for which research and development (R&D) efforts are dedicated, known as paramount R&D blueprints. Within R&D efforts, the goal is to obtain effective prophylactic and therapeutic approaches, which depends on a comprehensive knowledge of the etiology, epidemiology, and pathogenesis of these diseases. In this process, the accessibility of animal models is a priority bottleneck because it plays a key role in bridging the gap between in-depth understanding and control efforts for infectious diseases. Here, we reviewed preclinical animal models for high priority disease in terms of their ability to simulate human infections, including both natural susceptibility models, artificially engineered models, and surrogate models. In addition, we have thoroughly reviewed the current landscape of vaccines, antibodies, and small molecule drugs, particularly hopeful candidates in the advanced stages of these infectious diseases. More importantly, focusing on global trends and novel technologies, several aspects of the prevention and control of infectious disease were discussed in detail, including but not limited to gaps in currently available animal models and medical responses, better immune correlates of protection established in animal models and humans, further understanding of disease mechanisms, and the role of artificial intelligence in guiding or supplementing the development of animal models, vaccines, and drugs. Overall, this review described pioneering approaches and sophisticated techniques involved in the study of the epidemiology, pathogenesis, prevention, and clinical theatment of WHO high-priority pathogens and proposed potential directions. Technological advances in these aspects would consolidate the line of defense, thus ensuring a timely response to WHO high priority pathogens.

A Single-Dose Intranasal Combination Panebolavirus Vaccine

BACKGROUND

Ebolaviruses Ebola (EBOV), Sudan (SUDV), and Bundibugyo (BDBV) cause severe human disease, which may be accompanied by hemorrhagic syndrome, with high case fatality rates. Monovalent vaccines do not offer cross-protection against these viruses whose endemic areas overlap. Therefore, development of a panebolavirus vaccine is a priority. As a vaccine vector, human parainfluenza virus type 3 (HPIV3) has the advantages of needle-free administration and induction of both systemic and local mucosal antibody responses in the respiratory tract.

METHODS

To minimize the antivector immunity, genes encoding the HPIV3 envelope proteins F and HN were removed from the vaccine constructs, resulting in expression of only the ebolavirus envelope protein-glycoprotein. These second-generation vaccine constructs were used to develop a combination vaccine against EBOV, SUDV, and BDBV.

RESULTS

A single intranasal vaccination of guinea pigs or ferrets with the trivalent combination vaccine elicited humoral responses to each of the targeted ebolaviruses, including binding and neutralizing antibodies, as well as Fc-mediated effector functions. This vaccine protected animals from death and disease caused by lethal challenges with EBOV, SUDV, or BDBV.

CONCLUSIONS

The combination vaccine elicited protection that was comparable to that induced by the monovalent vaccines, thus demonstrating the value of this combination trivalent vaccine.

Filoviruses: Scientific Gaps and Prototype Pathogen Recommendation

Viruses in the family Filoviridae, including the commonly known Ebola (EBOV) and Marburg (MARV) viruses, can cause severe hemorrhagic fever in humans and nonhuman primates. Sporadic outbreaks of filovirus disease occur in sub-Saharan Africa with reported case fatality rates ranging from 25% to 90%. The high mortality and increasing frequency and magnitude of recent outbreaks along with the increased potential for spread from rural to urban areas highlight the importance of pandemic preparedness for these viruses. Despite their designation as high-priority pathogens, numerous scientific gaps exist in critical areas. In this review, these gaps and an assessment of potential prototype pathogen candidates are presented for this important virus family.

The rVSV-EBOV vaccine provides limited cross-protection against Sudan virus in guinea pigs

Recombinant vesicular stomatitis viruses (rVSVs) engineered to express heterologous viral glycoproteins have proven to be remarkably effective vaccines. Indeed, rVSV-EBOV, which expresses the Ebola virus (EBOV) glycoprotein, recently received clinical approval in the United States and Europe for its ability to prevent EBOV disease. Analogous rVSV vaccines expressing glycoproteins of different human-pathogenic filoviruses have also demonstrated efficacy in pre-clinical evaluations, yet these vaccines have not progressed far beyond research laboratories. In the wake of the most recent outbreak of Sudan virus (SUDV) in Uganda, the need for proven countermeasures was made even more acute. Here we demonstrate that an rVSV-based vaccine expressing the SUDV glycoprotein (rVSV-SUDV) generates a potent humoral immune response that protects guinea pigs from SUDV disease and death. Although the cross-protection generated by rVSV vaccines for different filoviruses is thought to be limited, we wondered whether rVSV-EBOV might also provide protection against SUDV, which is closely related to EBOV. Surprisingly, nearly 60% of guinea pigs that were vaccinated with rVSV-EBOV and challenged with SUDV survived, suggesting that rVSV-EBOV offers limited protection against SUDV, at least in the guinea pig model. These results were confirmed by a back-challenge experiment in which animals that had been vaccinated with rVSV-EBOV and survived EBOV challenge were inoculated with SUDV and survived. Whether these data are applicable to efficacy in humans is unknown, and they should therefore be interpreted cautiously. Nevertheless, this study confirms the potency of the rVSV-SUDV vaccine and highlights the potential for rVSV-EBOV to elicit a cross-protective immune response.

Mouse models of Ebola virus tolerance and lethality: characterization of CD-1 mice infected with wild-type, guinea pig-adapted, or mouse-adapted virus

Development of lethal models of Ebola virus disease has been achieved by the serial passage of virus isolates from human cases in mice and guinea pigs. Use of mice infected with non-adapted virus has been limited due to the absence of overt clinical disease. In recent years, newly recognized sequelae identified in human cases has highlighted the importance of continued investigations of non-lethal infection both in humans and animal models. Here, we revisit the use of rodent-adapted and non-adapted Ebola virus (EBOV) in mice to investigate infection tolerance and future utility of these models in pathogenesis and therapeutic intervention studies. We found that like non-adapted wild-type EBOV, guinea pig-adapted EBOV resulted in widespread tissue infection, variably associated with tissue pathology, and alterations in clinical and immunological analytes in the absence of overt disease. Notably, infection with either non-lethal variant did not greatly differ from lethal mouse-adapted EBOV until near the time end-point criteria are reached in these mice. These data support future investigations of pathogenesis, convalescence, and sequelae in mouse models of virus tolerance.

Animal Model Alternatives in Filovirus and Bornavirus Research

The order Mononegavirales contains a variety of highly pathogenic viruses that may infect humans, including the families Filoviridae, Bornaviridae, Paramyxoviridae, and Rhabodoviridae. Animal models have historically been important to study virus pathogenicity and to develop medical countermeasures. As these have inherent shortcomings, the rise of microphysiological systems and organoids able to recapitulate hallmarks of the diseases caused by these viruses may have enormous potential to add to or partially replace animal modeling in the future. Indeed, microphysiological systems and organoids are already used in the pharmaceutical R&D pipeline because they are prefigured to overcome the translational gap between model systems and clinical studies. Moreover, they may serve to alleviate ethical concerns related to animal research. In this review, we discuss the value of animal model alternatives in human pathogenic filovirus and bornavirus research. The current animal models and their limitations are presented followed by an overview of existing alternatives, such as organoids and microphysiological systems, which might help answering open research questions.

Natural history of Sudan ebolavirus infection in rhesus and cynomolgus macaques

Due to its high mortality rate and continued re-emergence, Ebolavirus disease (EVD) continues to pose a serious threat to global health. A group of viruses within the genus Ebolavirus causes this severe hemorrhagic disease in humans: Ebola virus (EBOV; species Zaire ebolavirus), Sudan virus (SUDV; species Sudan ebolavirus), Bundibugyo virus, and Taï Forest virus. EBOV and SUDV are associated with the highest case fatality rates. While the host response to EBOV has been comprehensively examined, limited data exists for SUDV infection. For medical countermeasure testing, well-characterized SUDV nonhuman primate (NHP) models are thus needed. Here, we describe a natural history study in which rhesus (N = 11) and cynomolgus macaques (N = 14) were intramuscularly exposed to a 1000 plaque-forming unit dose of SUDV (Gulu variant). Time-course analyses of various hematological, pathological, serological, coagulation, and transcriptomic findings are reported. SUDV infection was uniformly lethal in cynomolgus macaques (100% mortality), whereas a single rhesus macaque subject (91% mortality) survived to the study endpoint (median time-to-death of ∼8.0 and ∼8.5 days in cynomolgus and rhesus macaques, respectively). Infected macaques exhibited hallmark features of human EVD. The early stage was typified by viremia, granulocytosis, lymphopenia, albuminemia, thrombocytopenia, and decreased expression of HLA-class transcripts. At mid-to-late disease, animals developed fever and petechial rashes, and expressed high levels of pro-inflammatory mediators, pro-thrombotic factors, and markers indicative of liver and kidney injury. End-stage disease was characterized by shock and multi-organ failure. In summary, macaques recapitulate human SUDV disease, supporting these models for use in the development of vaccines and therapeutics.

Development of a Well-Characterized Cynomolgus Macaque Model of Sudan Virus Disease for Support of Product Development

The primary objective of this study was to characterize the disease course in cynomolgus macaques exposed to Sudan virus (SUDV), to determine if infection in this species is an appropriate model for the evaluation of filovirus countermeasures under the FDA Animal Rule. Sudan virus causes Sudan virus disease (SVD), with an average case fatality rate of approximately 50%, and while research is ongoing, presently there are no approved SUDV vaccines or therapies. Well characterized animal models are crucial for further developing and evaluating countermeasures for SUDV. Twenty (20) cynomolgus macaques were exposed intramuscularly to either SUDV or sterile phosphate-buffered saline; 10 SUDV-exposed animals were euthanized on schedule to characterize pathology at defined durations post-exposure and 8 SUDV-exposed animals were not part of the scheduled euthanasia cohort. Survival was assessed, along with clinical observations, body weights, body temperatures, hematology, clinical chemistry, coagulation, viral load (serum and tissues), macroscopic observations, and histopathology. There were statistically significant differences between SUDV-exposed animals and mock-exposed animals for 26 parameters, including telemetry body temperature, clinical chemistry parameters, hematology parameters, activated partial thromboplastin time, serum viremia, and biomarkers that characterize the disease course of SUDV in cynomolgus macaques.

Macrophage infection, activation, and histopathological findings in ebolavirus infection

Macrophages contribute to Ebola virus disease through their susceptibility to direct infection, their multi-faceted response to ebolaviruses, and their association with pathological findings in tissues throughout the body. Viral attachment and entry factors, as well as the more recently described influence of cell polarization, shape macrophage susceptibility to direct infection. Moreover, the study of Toll-like receptor 4 and the RIG-I-like receptor pathway in the macrophage response to ebolaviruses highlight important immune signaling pathways contributing to the breadth of macrophage responses. Lastly, the deep histopathological catalogue of macrophage involvement across numerous tissues during infection has been enriched by descriptions of tissues involved in sequelae following acute infection, including: the eye, joints, and the nervous system. Building upon this knowledge base, future opportunities include characterization of macrophage phenotypes beneficial or deleterious to survival, delineation of the specific roles macrophages play in pathological lesion development in affected tissues, and the creation of macrophage-specific therapeutics enhancing the beneficial activities and reducing the deleterious contributions of macrophages to the outcome of Ebola virus disease.

A single immunization with a modified vaccinia Ankara vectored vaccine producing Sudan virus-like particles protects from lethal infection

A new vectored vaccine MVA-VLP-SUDV was generated against Sudan ebolavirus (SUDV) combining the advantages of the immunogenicity of a live attenuated vaccine vector (Modified Vaccinia Ankara, MVA) with the authentic conformation of virus-like particles (VLPs). The vaccine expresses minimal components to generate self-assembling VLPs in the vaccinee: the envelope glycoprotein GP and the matrix protein VP40. Guinea pigs vaccinated with one dose of MVA-VLP-SUDV generated SUDV-specific binding and neutralizing antibody responses as well as Fc-mediated protective effects. These responses were boosted by a second vaccine dose. All vaccinated animals which received either one or two vaccine doses were protected from death and disease symptoms following challenge with a lethal dose of SUDV. These data demonstrate single dose protection and potency of the MVA-VLP platform for use in emergency situations to contain outbreaks.

Asymmetric and non-stoichiometric glycoprotein recognition by two distinct antibodies results in broad protection against ebolaviruses

Several ebolaviruses cause outbreaks of severe disease. Vaccines and monoclonal antibody cocktails are available to treat Ebola virus (EBOV) infections, but not Sudan virus (SUDV) or other ebolaviruses. Current cocktails contain antibodies that cross-react with the secreted soluble glycoprotein (sGP) that absorbs virus-neutralizing antibodies. By sorting memory B cells from EBOV infection survivors, we isolated two broadly reactive anti-GP monoclonal antibodies, 1C3 and 1C11, that potently neutralize, protect rodents from disease, and lack sGP cross-reactivity. Both antibodies recognize quaternary epitopes in trimeric ebolavirus GP. 1C11 bridges adjacent protomers via the fusion loop. 1C3 has a tripartite epitope in the center of the trimer apex. One 1C3 antigen-binding fragment anchors simultaneously to the three receptor-binding sites in the GP trimer, and separate 1C3 paratope regions interact differently with identical residues on the three protomers. A cocktail of both antibodies completely protected nonhuman primates from EBOV and SUDV infections, indicating their potential clinical value.

Pan-ebolavirus serology study of healthcare workers in the Mbandaka Health Region, Democratic Republic of the Congo

Although multiple antigenically distinct ebolavirus species can cause human disease, previous serosurveys focused on only Zaire ebolavirus (EBOV). Thus, the extent of reactivity or exposure to other ebolaviruses, and which sociodemographic factors are linked to this seroreactivity, are unclear. We conducted a serosurvey of 539 healthcare workers (HCW) in Mbandaka, Democratic Republic of the Congo, using ELISA-based analysis of serum IgG against EBOV, Sudan ebolavirus (SUDV) and Bundibugyo ebolavirus (BDBV) glycoproteins (GP). We compared seroreactivity to risk factors for viral exposure using univariate and multivariable logistic regression. Seroreactivity against different GPs ranged from 2.2-4.6%. Samples from six individuals reacted to all three species of ebolavirus and 27 samples showed a species-specific IgG response. We find that community health volunteers are more likely to be seroreactive against each antigen than nurses, and in general, that HCWs with indirect patient contact have higher anti-EBOV GP IgG levels than those with direct contact. Seroreactivity against ebolavirus GP may be associated with positions that offer less occupational training and access to PPE. Those individuals with broadly reactive responses may have had multiple ebolavirus exposures or developed cross-reactive antibodies. In contrast, those individuals with species-specific BDBV or SUDV GP seroreactivity may have been exposed to an ebolavirus not previously known to circulate in the region.

STAT-1 Knockout Mice as a Model for Wild-Type Sudan Virus (SUDV)

Currently there is no FDA-licensed vaccine or therapeutic against Sudan ebolavirus (SUDV) infections. The largest ever reported 2014–2016 West Africa outbreak, as well as the 2021 outbreak in the Democratic Republic of Congo, highlight the critical need for countermeasures against filovirus infections. A well-characterized small animal model that is susceptible to wild-type filoviruses would greatly add to the screening of antivirals and vaccines. Here, we infected signal transducer and activator of transcription-1 knock out (STAT-1 KO) mice with five different wildtype filoviruses to determine susceptibility. SUDV and Marburg virus (MARV) were the most virulent, and caused 100% or 80% lethality, respectively. Zaire ebolavirus (EBOV), Bundibugyo ebolavirus (BDBV), and Taï Forest ebolavirus (TAFV) caused 40%, 20%, and no mortality, respectively. Further characterization of SUDV in STAT-1 KO mice demonstrated lethality down to 3.1 × 10¹ pfu. Viral genomic material was detectable in serum as early as 1 to 2 days post-challenge. The onset of viremia was closely followed by significant changes in total white blood cells and proportion of neutrophils and lymphocytes, as well as by an influx of neutrophils in the liver and spleen. Concomitant significant fluctuations in blood glucose, albumin, globulin, and alanine aminotransferase were also noted, altogether consistent with other models of filovirus infection. Finally, favipiravir treatment fully protected STAT-1 KO mice from lethal SUDV challenge, suggesting that this may be an appropriate small animal model to screen anti-SUDV countermeasures.

Pathogen Dose in Animal Models of Hemorrhagic Fever Virus Infections and the Potential Impact on Studies of the Immune Response

Viral hemorrhagic fever viruses come from a wide range of virus families and are a significant cause of morbidity and mortality worldwide each year. Animal models of infection with a number of these viruses have contributed to our knowledge of their pathogenesis and have been crucial for the development of therapeutics and vaccines that have been approved for human use. Most of these models use artificially high doses of virus, ensuring lethality in pre-clinical drug development studies. However, this can have a significant effect on the immune response generated. Here I discuss how the dose of antigen or pathogen is a critical determinant of immune responses and suggest that the current study of viruses in animal models should take this into account when developing and studying animal models of disease. This can have implications for determination of immune correlates of protection against disease as well as informing relevant vaccination and therapeutic strategies.

Prothrombin Time, Activated Partial Thromboplastin Time, and Fibrinogen Reference Intervals for Inbred Strain 13/N Guinea Pigs (Cavia porcellus) and Validation of Low Volume Sample Analysis

Inbred strain 13/N guinea pigs are used as small animal models for the study of hemorrhagic fever viruses. Coagulation abnormalities, including prolonged clotting times and bleeding, are characteristic of hemorrhagic fever in humans; patients often meet criteria for disseminated intravascular coagulation (DIC). Comprehensively evaluating coagulation function is critical in model development and studies of viral pathogenesis and therapeutic efficacy. Here, using the VetScan VSpro veterinary point-of-care platform, we developed reference intervals in both juvenile and adult strain 13/N guinea pigs for three coagulation parameters: prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen. In addition, for situations or species with limited availability of blood for clinical analysis, we investigated the validity of a modified collection approach for low-volume (0.1 mL) blood sample analysis of PT and aPTT.

Generation and Characterization of a Mouse-Adapted Makona Variant of Ebola Virus

Ebola virus (EBOV) is a zoonotic pathogen that poses a significant threat to public health, causing sporadic yet devastating outbreaks that have the potential to spread worldwide, as demonstrated during the 2013-2016 West African outbreak. Mouse models of infection are important tools for the development of therapeutics and vaccines. Exposure of immunocompetent mice to clinical isolates of EBOV is nonlethal; consequently, EBOV requires prior adaptation in mice to cause lethal disease. Until now, the only immunocompetent EBOV mouse model was based on the Mayinga variant, which was isolated in 1976. Here, we generated a novel mouse-adapted (MA)-EBOV based on the 2014 Makona isolate by inserting EBOV/Mayinga-MA mutations into the EBOV/Makona genome, followed by serial passaging of the rescued virus in suckling mice. The resulting EBOV/Makona-MA causes lethal disease in adult immunocompetent mice within 6 to 9 days and has a lethal dose (LD50) of 0.004 plaque forming units (PFU). Two additional mutations emerged after mouse-adaptation in the viral nucleoprotein (NP) and membrane-associated protein VP24. Using reverse genetics, we found the VP24 mutation to be critical for EBOV/Makona-MA virulence. EBOV/Makona-MA infected mice that presented with viremia, high viral burden in organs, increased release of pro-inflammatory cytokines/chemokines, and lymphopenia. Our mouse model will help advance pre-clinical development of countermeasures against contemporary EBOV variants.

Marburg and Ravn Virus Infections Do Not Cause Observable Disease in Ferrets

Ferrets are used for studying infections with wild-type Ebola virus isolates. Here, we investigated whether these animals are also susceptible to wild-type isolates of Marburg virus (MARV). Ferrets were challenged intramuscularly or intranasally with MARV strain Angola and monitored for 3 weeks. Unexpectedly, the animals neither showed observable signs of disease nor died of infection, and viremia was not detected after challenge. All animals were seropositive for MARV-specific immunoglobulin antibodies. Confirmatory studies with MARV strain Musoke and Ravn virus yielded the same outcomes. Therefore, ferrets may be of limited usefulness for studying the pathogenesis of MARV and Ravn virus infections.

Small Animal Models for Studying Filovirus Pathogenesis

Filovirus small animal disease models have so far been developed in laboratory mice, guinea pigs, and hamsters. Since immunocompetent rodents do not exhibit overt signs of disease following infection with wild-type filoviruses isolated from humans, rodent models have been established using adapted viruses produced through sequential passage in rodents. Rodent-adapted viruses target the same cells/tissues as the wild-type viruses, making rodents invaluable basic research tools for studying filovirus pathogenesis. Moreover, comparative analyses using wild-type and rodent-adapted viruses have provided beneficial insights into the molecular mechanisms of pathogenicity and acquisition of species-specific virulence. Additionally, wild-type filovirus infections in immunodeficient rodents have provided a better understanding of the host factors required for resistance to filovirus infection and of the immune response against the infection. This chapter provides comprehensive information on the filovirus rodent models and rodent-adapted filoviruses. Specifically, we summarize the clinical and pathological features of filovirus infections in all rodent models described to date, including the recently developed humanized and collaborative cross (CC) resource recombinant inbred (RI) intercrossed (CC-RIX) mouse models. We also cover the molecular determinants responsible for adaptation and virulence acquisition in a number of rodent-adapted filoviruses. This chapter clearly defines the characteristic and advantages/disadvantages of rodent models, helping to evaluate the practical use of rodent models in future filovirus studies.

A Two-Antibody Pan-Ebolavirus Cocktail Confers Broad Therapeutic Protection in Ferrets and Nonhuman Primates

Recent and ongoing outbreaks of Ebola virus disease (EVD) underscore the unpredictable nature of ebolavirus reemergence and the urgent need for antiviral treatments. Unfortunately, available experimental vaccines and immunotherapeutics are specific for a single member of the Ebolavirus genus, Ebola virus (EBOV), and ineffective against other ebolaviruses associated with EVD, including Sudan virus (SUDV) and Bundibugyo virus (BDBV). Here we show that MBP134^AF, a pan-ebolavirus therapeutic comprising two broadly neutralizing human antibodies (bNAbs), affords unprecedented effectiveness and potency as a therapeutic countermeasure to antigenically diverse ebolaviruses. MBP134^AF could fully protect ferrets against lethal EBOV, SUDV, and BDBV infection, and a single 25-mg/kg dose was sufficient to protect NHPs against all three viruses. The development of MBP134^AF provides a successful model for the rapid discovery and translational advancement of immunotherapeutics targeting emerging infectious diseases.

Virus inoculation and treatment regimens for evaluating anti-filovirus monoclonal antibody efficacy in vivo

The development of monoclonal antibodies to treat disease caused by filoviruses, particularly Ebola virus, has risen steeply in recent years thanks to several key studies demonstrating their remarkable therapeutic potential. The increased drive to develop new and better monoclonal antibodies has necessarily seen an increase in animal model efficacy testing, which is critical to the pre-clinical development of any novel countermeasure. Primary and secondary efficacy testing against filoviruses typically makes use of one or more rodent models (mice, guinea pigs, and occasionally hamsters) or the more recently described ferret model, although the exact choice of model depends on the specific filovirus being evaluated. Indeed, no single small animal model exists for all filoviruses, and the use of any given model must consider the nature of that model as well as the nature of the therapeutic and the experimental objectives. Confirmatory evaluation, on the other hand, is performed in nonhuman primates (rhesus or cynomolgus macaques) regardless of the filovirus. In light of the number of different animal models that are currently used in monoclonal antibody efficacy testing, we sought to better understand how these efficacy tests are being performed by numerous different laboratories around the world. To this end, we review the animal models that are being used for antibody efficacy testing against filoviruses, and we highlight the challenge doses and routes of infection that are used. We also describe the various antibody treatment regimens, including antibody dose, route, and schedule of administration, that are used in these model systems. We do not identify any single best model or treatment regimen, and we do not advocate for field-wide protocol standardization. Instead, we hope to provide a comprehensive resource that will facilitate and enhance the continued pre-clinical development of novel monoclonal antibody therapeutics.

Antibody-Mediated Protective Mechanisms Induced by a Trivalent Parainfluenza Virus-Vectored Ebolavirus Vaccine

Ebolaviruses Zaire (EBOV), Bundibugyo (BDBV), and Sudan (SUDV) cause human disease with high case fatality rates. Experimental monovalent vaccines, which all utilize the sole envelope glycoprotein (GP), do not protect against heterologous ebolaviruses. Human parainfluenza virus type 3-vectored vaccines offer benefits, including needle-free administration and induction of mucosal responses in the respiratory tract. Multiple approaches were taken to induce broad protection against the three ebolaviruses. While GP consensus-based antigens failed to elicit neutralizing antibodies, polyvalent vaccine immunization induced neutralizing responses to all three ebolaviruses and protected animals from death and disease caused by EBOV, SUDV, and BDBV. As immunization with a cocktail of antigenically related antigens can skew the responses and change the epitope hierarchy, we performed comparative analysis of antibody repertoire and Fc-mediated protective mechanisms in animals immunized with monovalent versus polyvalent vaccines. Compared to sera from guinea pigs receiving the monovalent vaccines, sera from guinea pigs receiving the trivalent vaccine bound and neutralized EBOV and SUDV at equivalent levels and BDBV at only a slightly reduced level. Peptide microarrays revealed a preponderance of binding to amino acids 389 to 403, 397 to 415, and 477 to 493, representing three linear epitopes in the mucin-like domain known to induce a protective antibody response. Competition binding assays with monoclonal antibodies isolated from human ebolavirus infection survivors demonstrated that the immune sera block the binding of antibodies specific for the GP glycan cap, the GP1-GP2 interface, the mucin-like domain, and the membrane-proximal external region. Thus, administration of a cocktail of three ebolavirus vaccines induces a desirable broad antibody response, without skewing of the response toward preferential recognition of a single virus.IMPORTANCE The symptoms of the disease caused by the ebolaviruses Ebola, Bundibugyo, and Sudan are similar, and their areas of endemicity overlap. However, because of the limited antigenic relatedness of the ebolavirus glycoprotein (GP) used in all candidate vaccines against these viruses, they protect only against homologous and not against heterologous ebolaviruses. Therefore, a broadly specific pan-ebolavirus vaccine is required, and this might be achieved by administration of a cocktail of vaccines. The effects of cocktail administration of ebolavirus vaccines on the antibody repertoire remain unknown. Here, an in-depth analysis of the antibody responses to administration of a cocktail of human parainfluenza virus type 3-vectored vaccines against individual ebolaviruses was performed, which included analysis of binding to GP, neutralization of individual ebolaviruses, epitope specificity, Fc-mediated functions, and protection against the three ebolaviruses. The results demonstrated potent and balanced responses against individual ebolaviruses and no significant reduction of the responses compared to that induced by individual vaccines.

Filovirus Virulence in Interferon α/β and γ Double Knockout Mice, and Treatment with Favipiravir

The 2014 Ebolavirus outbreak in West Africa highlighted the need for vaccines and therapeutics to prevent and treat filovirus infections. A well-characterized small animal model that is susceptible to wild-type filoviruses would facilitate the screening of anti-filovirus agents. To that end, we characterized knockout mice lacking α/β and γ interferon receptors (IFNAGR KO) as a model for wild-type filovirus infection. Intraperitoneal challenge of IFNAGR KO mice with several known human pathogenic species from the genus Ebolavirus and Marburgvirus, except Bundibugyo ebolavirus and Taï Forest ebolavirus, caused variable mortality rate. Further characterization of the prototype Ebola virus Kikwit isolate infection in this KO mouse model showed 100% lethality down to a dilution equivalent to 1.0 × 10⁻¹ pfu with all deaths occurring between 7 and 9 days post-challenge. Viral RNA was detectable in serum after challenge with 1.0 × 10² pfu as early as one day after infection. Changes in hematology and serum chemistry became pronounced as the disease progressed and mirrored the histological changes in the spleen and liver that were also consistent with those described for patients with Ebola virus disease. In a proof-of-principle study, treatment of Ebola virus infected IFNAGR KO mice with favipiravir resulted in 83% protection. Taken together, the data suggest that IFNAGR KO mice may be a useful model for early screening of anti-filovirus medical countermeasures.

Development of a Human Antibody Cocktail that Deploys Multiple Functions to Confer Pan-Ebolavirus Protection

Passive administration of monoclonal antibodies (mAbs) is a promising therapeutic approach for Ebola virus disease (EVD). However, all mAbs and mAb cocktails that have entered clinical development are specific for a single member of the Ebolavirus genus, Ebola virus (EBOV), and ineffective against outbreak-causing Bundibugyo virus (BDBV) and Sudan virus (SUDV). Here, we advance MBP134, a cocktail of two broadly neutralizing human mAbs, ADI-15878 from an EVD survivor and ADI-23774 from the same survivor but specificity-matured for SUDV GP binding affinity, as a candidate pan-ebolavirus therapeutic. MBP134 potently neutralized all ebolaviruses and demonstrated greater protective efficacy than ADI-15878 alone in EBOV-challenged guinea pigs. A second-generation cocktail, MBP134AF, engineered to effectively harness natural killer (NK) cells afforded additional improvement relative to its precursor in protective efficacy against EBOV and SUDV in guinea pigs. MBP134AF is an optimized mAb cocktail suitable for evaluation as a pan-ebolavirus therapeutic in nonhuman primates.

Postexposure Protective Efficacy of T-705 (Favipiravir) Against Sudan Virus Infection in Guinea Pigs

Filoviruses such as Ebola virus (EBOV), Marburg virus (MARV), and Sudan virus (SUDV) cause deadly viral hemorrhagic fever in humans, with high case-fatality rates; however, no licensed therapeutic agent or vaccine has been clinically approved to treat or prevent infection. T-705 (favipiravir) is a novel antiviral drug that has been approved for the treatment of influenza in Japan. T-705 exhibits broad-spectrum antiviral activity against different viruses, including MARV and EBOV, and here, we are the first to report the in vitro and in vivo antiviral activity of T-705 against SUDV. T-705 treatment reduced SUDV replication in Vero E6 cells. Subcutaneous administration of T-705, beginning 1-4 days after infection and continuing for 7 days, significantly protected SUDV-infected guinea pigs, with a survival rate of 83%-100%. Viral RNA replication and infectious virus production were also significantly reduced in the blood, spleen, liver, lungs, and kidney. Moreover, early administration of low-dose T-705 and late administration (at 5 days after infection) of higher-dose T-705 also showed partial protection. Overall, our study is the first to demonstrate the antiviral activity of T-705 against SUDV, suggesting that T-705 may be a potential drug candidate for use during outbreaks.

Multifunctional Pan-ebolavirus Antibody Recognizes a Site of Broad Vulnerability on the Ebolavirus Glycoprotein

Ebolaviruses cause severe disease in humans, and identification of monoclonal antibodies (mAbs) that are effective against multiple ebolaviruses are important for therapeutics development. Here we describe a distinct class of broadly neutralizing human mAbs with protective capacity against three ebolaviruses infectious for humans: Ebola (EBOV), Sudan (SUDV), and Bundibugyo (BDBV) viruses. We isolated mAbs from human survivors of ebolavirus disease and identified a potent mAb, EBOV-520, which bound to an epitope in the glycoprotein (GP) base region. EBOV-520 efficiently neutralized EBOV, BDBV, and SUDV and also showed protective capacity in relevant animal models of these infections. EBOV-520 mediated protection principally by direct virus neutralization and exhibited multifunctional properties. This study identified a potent naturally occurring mAb and defined key features of the human antibody response that may contribute to broad protection. This multifunctional mAb and related clones are promising candidates for development as broadly protective pan-ebolavirus therapeutic molecules.

Small Animal Models for Evaluating Filovirus Countermeasures

The development of novel therapeutics and vaccines to treat or prevent disease caused by filoviruses, such as Ebola and Marburg viruses, depends on the availability of animal models that faithfully recapitulate clinical hallmarks of disease as it is observed in humans. In particular, small animal models (such as mice and guinea pigs) are historically and frequently used for the primary evaluation of antiviral countermeasures, prior to testing in nonhuman primates, which represent the gold-standard filovirus animal model. In the past several years, however, the filovirus field has witnessed the continued refinement of the mouse and guinea pig models of disease, as well as the introduction of the hamster and ferret models. We now have small animal models for most human-pathogenic filoviruses, many of which are susceptible to wild type virus and demonstrate key features of disease, including robust virus replication, coagulopathy, and immune system dysfunction. Although none of these small animal model systems perfectly recapitulates Ebola virus disease or Marburg virus disease on its own, collectively they offer a nearly complete set of tools in which to carry out the preclinical development of novel antiviral drugs.

Characterization of Sudan Ebolavirus infection in ferrets

Sudan virus (SUDV) outbreaks in Africa are highly lethal; however, the development and testing of novel antivirals and vaccines for this virus has been limited by a lack of suitable animal models. Non-human primates (NHP) remain the gold standard for modeling filovirus disease, but they are not conducive to screening large numbers of experimental compounds and should only be used to test the most promising candidates. Therefore, other smaller animal models are a valuable asset. We have recently developed a guinea-pig adapted SUDV virus that is lethal in guinea pigs. In our current study, we show that ferrets are susceptible to wild-type SUDV, providing a small animal model to directly study clinical isolates, screen experimental anti-SUDV compounds and potentially study viral transmission.

Type I interferon receptor knockout mice as models for infection of highly pathogenic viruses with outbreak potential

Due to their inability to generate a complete immune response, mice knockout for type I interferon (IFN) receptors (Ifnar^–/–) are more susceptible to viral infections, and are thus commonly used for pathogenesis studies. This mouse model has been used to study many diseases caused by highly pathogenic viruses from many families, including the Flaviviridae, Filoviridae, Arenaviridae, Bunyaviridae, Henipaviridae, and Togaviridae. In this review, we summarize the findings from these animal studies, and discuss the pros and cons of using this model versus other known methods for studying pathogenesis in animals.

Animal models for filovirus infections

The family Filoviridae, which includes the genera Marburgvirus and Ebolavirus, contains some of the most pathogenic viruses in humans and non-human primates (NHPs), causing severe hemorrhagic fevers with high fatality rates. Small animal models against filoviruses using mice, guinea pigs, hamsters, and ferrets have been developed with the goal of screening candidate vaccines and antivirals, before testing in the gold standard NHP models. In this review, we summarize the different animal models used to understand filovirus pathogenesis, and discuss the advantages and disadvantages of each model with respect to filovirus disease research.

Testing Experimental Therapies in a Guinea Pig Model for Hemorrhagic Fever

Hemorrhagic fever viruses are among the deadliest pathogens known to humans, and often, licensed medical countermeasures are unavailable to prevent or treat infections. Guinea pigs are a commonly used animal for the preclinical development of any experimental candidates, typically to confirm data generated in mice and as a way to validate and support further testing in nonhuman primates. In this chapter, we use Sudan virus (SUDV), a lethal filovirus closely related to Ebola virus, as an example of the steps required for generating a guinea pig-adapted isolate that is used to test a monoclonal antibody-based therapy against viral hemorrhagic fevers.

Human immune system mouse models of Ebola virus infection

Human immune system (HIS) mice, immunodeficient mice engrafted with human cells (with or without donor-matched tissue), offer a unique opportunity to study pathogens that cause disease predominantly or exclusively in humans. Several HIS mouse models have recently been used to study Ebola virus (EBOV) infection and disease. The results of these studies are encouraging and support further development and use of these models in Ebola research. HIS mice provide a small animal model to study EBOV isolates, investigate early viral interactions with human immune cells, screen vaccines and therapeutics that modulate the immune system, and investigate sequelae in survivors. Here we review existing models, discuss their use in pathogenesis studies and therapeutic screening, and highlight considerations for study design and analysis. Finally, we point out caveats to current models, and recommend future efforts for modeling EBOV infection in HIS mice.

Detection of Zaire ebolavirus in swine: Assay development and optimization

Ebolaviruses (family Filoviridae, order Mononegavirales) cause often fatal, haemorrhagic fever in primates including humans. Pigs have been identified as a species susceptible to Reston ebolavirus (RESTV) infection, with indicated transmission to humans in the Philippines; however, their role during Ebola outbreaks in Africa needs to be clarified. To perform surveillance studies, detection of ebolavirus requires a prerequisite validation of viral RNA and antibody detection methods in swine samples. These diagnostic tests also need to be suitable for deployment to low-level containment laboratories. In this study, we developed a set of tests for detection of antibodies against Zaire ebolavirus (EBOV) in swine. Recombinant EBOV nucleoprotein was produced using a baculovirus expression system for indirect ELISA development. Evaluation of this assay was performed using laboratory and field samples, achieving a diagnostic specificity of 99%. Importantly, the indirect ELISA was able to detect antibodies to EBOV at 7 dpi, 3 days earlier than virus neutralization tests (VNT). The format of the VNT in this work was modified to a microtitre plaque reduction neutralization assay (miPRNT) complemented with immunostaining to provide a more rapid and highly specific assay. Finally, a confirmatory immunoblot assay was generated to supplement the indirect ELISA results.

Ferrets Infected with Bundibugyo Virus or Ebola Virus Recapitulate Important Aspects of Human Filovirus Disease

Bundibugyo virus (BDBV) is the etiological agent of a severe hemorrhagic fever in humans with a case-fatality rate ranging from 25 to 36%. Despite having been known to the scientific and medical communities for almost 1 decade, there is a dearth of studies on this pathogen due to the lack of a small animal model. Domestic ferrets are commonly used to study other RNA viruses, including members of the order Mononegavirales To investigate whether ferrets were susceptible to filovirus infections, ferrets were challenged with a clinical isolate of BDBV. Animals became viremic within 4 days and succumbed to infection between 8 and 9 days, and a petechial rash was observed with moribund ferrets. Furthermore, several hallmarks of human filoviral disease were recapitulated in the ferret model, including substantial decreases in lymphocyte and platelet counts and dysregulation of key biochemical markers related to hepatic/renal function, as well as coagulation abnormalities. Virological, histopathological, and immunohistochemical analyses confirmed uncontrolled BDBV replication in the major organs. Ferrets were also infected with Ebola virus (EBOV) to confirm their susceptibility to another filovirus species and to potentially establish a virus transmission model. Similar to what was seen with BDBV, important hallmarks of human filoviral disease were observed in EBOV-infected ferrets. This study demonstrates the potential of this small animal model for studying BDBV and EBOV using wild-type isolates and will accelerate efforts to understand filovirus pathogenesis and transmission as well as the development of specific vaccines and antivirals.

IMPORTANCE

The 2013-2016 outbreak of Ebola virus in West Africa has highlighted the threat posed by filoviruses to global public health. Bundibugyo virus (BDBV) is a member of the genus Ebolavirus and has caused outbreaks in the past but is relatively understudied, likely due to the lack of a suitable small animal model. Such a model for BDBV is crucial to evaluating vaccines and therapies and potentially understanding transmission. To address this, we demonstrated that ferrets are susceptible models to BDBV infection as well as to Ebola virus infection and that no virus adaptation is required. Moreover, these animals develop a disease that is similar to that seen in humans and nonhuman primates. We believe that this will improve the ability to study BDBV and provide a platform to test vaccines and therapeutics.

Rodent-Adapted Filoviruses and the Molecular Basis of Pathogenesis

Ebola, Marburg, and Ravn viruses, all filoviruses, are the causative agents of severe hemorrhagic fever. Much of what we understand about the pathogenesis of filovirus disease is derived from work with animal models, including nonhuman primates, which are considered the "gold standard" filovirus model since they faithfully recapitulate the clinical hallmarks of filovirus disease. However, rodent models, including the mouse, guinea pig, and hamster, also exist for Ebola, Marburg, and Ravn viruses, and although they may not reproduce all the clinical signs of filovirus disease, thanks to their relative ease of use and low cost, they are often the first choice for initial descriptions of virus pathogenesis and evaluation of antiviral prophylactics and therapeutics. Since filoviruses do not cause significant disease in adult, immunocompetent rodents, these models rely on "rodent-adapted" viruses that have been passaged several times through their host until virulence and lethality are achieved. In the process of adaptation, the viruses acquire numerous nucleotide/amino acid mutations that contribute to virulence in their rodent host. Interestingly, virus protein 24 (VP24) and nucleoprotein (NP) appear to be major virulence factors for ebolaviruses in rodents, whereas VP40 appears to be the major virulence factor for marburgviruses. By characterizing these mutations and understanding the molecular mechanisms that lead to the acquisition of virulence, we can gain better insight into the pathogenic processes that underlie filovirus disease in humans. These processes, and the viral and/or cellular proteins that contribute to them, will make attractive targets for the development of novel therapeutics and counter-measures.

The Domestic Ferret (Mustela putorius furo) as a Lethal Infection Model for 3 Species of Ebolavirus

Small-animal models have been developed for several Filoviridae species; however, serial adaptation was required to produce lethal infection. These adapted viruses have sequence changes in several genes, including those that modulate the host immune response. Nonhuman primate models do not require adaptation of filoviruses. Here, we describe lethal models of disease for Bundibugyo, Sudan, and Zaire species of Ebolavirus in the domestic ferret, using wild-type nonadapted viruses. Pathologic features were consistent with disease in primates. Of particular importance, this is the only known small-animal model developed for Bundibugyo and the only uniformly lethal animal model for Bundibugyo.

Antibody Treatment of Ebola and Sudan Virus Infection via a Uniquely Exposed Epitope within the Glycoprotein Receptor-Binding Site

Previous efforts to identify cross-neutralizing antibodies to the receptor-binding site (RBS) of ebolavirus glycoproteins have been unsuccessful, largely because the RBS is occluded on the viral surface. We report a monoclonal antibody (FVM04) that targets a uniquely exposed epitope within the RBS; cross-neutralizes Ebola (EBOV), Sudan (SUDV), and, to a lesser extent, Bundibugyo viruses; and shows protection against EBOV and SUDV in mice and guinea pigs. The antibody cocktail ZMapp™ is remarkably effective against EBOV (Zaire) but does not cross-neutralize other ebolaviruses. By replacing one of the ZMapp™ components with FVM04, we retained the anti-EBOV efficacy while extending the breadth of protection to SUDV, thereby generating a cross-protective antibody cocktail. In addition, we report several mutations at the base of the ebolavirus glycoprotein that enhance the binding of FVM04 and other cross-reactive antibodies. These findings have important implications for pan-ebolavirus vaccine development and defining broadly protective antibody cocktails.

Vaccines against 'the other' Ebolavirus species

The Ebolavirus genus includes five member species, all of which pose a threat to global public health. These viruses cause fatal hemorrhagic fever in humans and nonhuman primates, and are considered category A pathogens due to the risk of their use as a bioweapon. The potential for an outbreak, either as a result of a natural emergence, deliberate release, or imported case underscores the need for protective vaccines. Recent progress in advancing vaccines for use against the strain of Zaire ebolavirus (EBOV) responsible for the West African Ebola outbreak offers reasons for optimism against EBOV, and demonstrates that protection against other Ebolavirus species is achievable.