A surrogate BSL2-compliant infection model recapitulating key aspects of human Marburg virus disease

Marburg virus disease (MVD) is a severe infectious disease caused by the Marburg virus (MARV), posing a significant threat to humans. MARV needs to be operated under strict biosafety Level 4 (BSL-4) laboratory conditions. Therefore, accessible and practical animal models are urgently needed to advance prophylactic and therapeutic strategies for MARV. In this study, we constructed a recombinant vesicular stomatitis virus (VSV) expressing the Marburg virus glycoprotein (VSV-MARV/GP). Syrian hamsters infected with VSV-MARV/GP presented symptoms such as thrombocytopenia, lymphopenia, haemophilia, and multiorgan failure, developing a severe systemic disease akin to that observed in human MARV patients. Notably, the pathogenicity was found to be species-specific, age-related, sex-associated, and challenge route-dependent. Subsequently, the therapeutic efficacy of the MR191 monoclonal antibody was validated in this model. In summary, this alternative model is an effective tool for rapidly screening medical countermeasures against MARV GP in vivo under BSL-2 conditions.

Evaluation of Vaccines and Therapeutics Against Marburg Virus in Nonhuman Primate Models

Marburg virus (MARV) has caused sporadic outbreaks of severe hemorrhagic fever in Africa in humans and nonhuman primates (NHPs) and has the potential to be used as a biological weapon. Currently, there are no licensed vaccines or therapeutics to respond to outbreaks or deliberate misuse. Vaccine and therapeutic efficacy testing against MARV requires animal models that accurately mimic human disease. In vitro testing in cell culture cannot appropriately model the complex immunological host responses required to accurately predict efficacy in humans, which will ultimately be required for licensure of a medical countermeasure (MCM). While small animal models for MARV have been valuable for dissecting disease processes and the screening of vaccine and drug candidates, there are several caveats to their use including required adaptation of the virus, lack of host-specific reagents, or the need of an immunocompromised host. Conversely, the NHP MARV disease model addresses all shortcomings of small animal models and closely recapitulates all hallmark features of human disease. As such, NHPs have served as the "gold standard" for testing filovirus MCMs and will most likely be required for regulatory approval. Here, we describe the use of NHPs for vaccine and therapeutic evaluation against MARV.

Isolation and Propagation of Marburgviruses

Marburgviruses (Marburg virus and Ravn virus) have caused ~20 outbreaks of human Marburg virus disease since their discovery in 1967. These zoonotic viruses are categorized as priority pathogens with no licensed vaccines or treatment options. The foundation for all experimental work including deciphering disease parameters and countermeasure development is a defined virus stock. This chapter provides a guide for the isolation and growth of marburgviruses in tissue culture.

Monoclonal antibody therapy demonstrates increased virulence of a lineage VII strain of Lassa virus in nonhuman primates

Lassa virus (LASV) is a World Health Organization (WHO) priority pathogen that causes high morbidity and mortality. Recently, we showed that a combination of three broadly neutralizing human monoclonal antibodies known as Arevirumab-3 (8.9F, 12.1F, 37.2D) based on the lineage IV Josiah strain protected 100% of cynomolgus macaques against heterologous challenge with lineage II and III strains of LASV when therapy was initiated beginning at day 8 after challenge. LASV strains from Benin and Togo represent a new lineage VII that are more genetically diverse from lineage IV than strains from lineages II and III. Here, we tested the ability of Arevirumab-3 to protect macaques against a LASV lineage VII Togo isolate when treatment was administered beginning 8 days after exposure. Unexpectedly, only 40% of treated animals survived challenge. In a subsequent study we showed that Arevirumab-3 protected 100% of macaques from lethal challenge when treatment was initiated 7 days after LASV Togo exposure. Based on our transcriptomics data, successful Arevirumab-3 treatment correlated with diminished neutrophil signatures and the predicted development of T cell responses. As the in vitro antiviral activity of Arevirumab-3 against LASV Togo was equivalent to lineage II and III strains, the reduced protection in macaques against Togo likely reflects the faster disease course of LASV Togo in macaques than other strains. This data causes concern regarding the ability of heterologous vaccines and treatments to provide cross protection against lineage VII LASV isolates.

Vaccine Platform Comparison: Protective Efficacy against Lethal Marburg Virus Challenge in the Hamster Model

Marburg virus (MARV), a filovirus, was first identified in 1967 in Marburg, Germany, and Belgrade, former Yugoslavia. Since then, MARV has caused sporadic outbreaks of human disease with high case fatality rates in parts of Africa, with the largest outbreak occurring in 2004/05 in Angola. From 2021 to 2023, MARV outbreaks occurred in Guinea, Ghana, New Guinea, and Tanzania, emphasizing the expansion of its endemic area into new geographical regions. There are currently no approved vaccines or therapeutics targeting MARV, but several vaccine candidates have shown promise in preclinical studies. We compared three vaccine platforms simultaneously by vaccinating hamsters with either a single dose of an adenovirus-based (ChAdOx-1 MARV) vaccine, an alphavirus replicon-based RNA (LION-MARV) vaccine, or a recombinant vesicular stomatitis virus-based (VSV-MARV) vaccine, all expressing the MARV glycoprotein as the antigen. Lethal challenge with hamster-adapted MARV 4 weeks after vaccination resulted in uniform protection of the VSV-MARV and LION-MARV groups and 83% of the ChAdOx-1 MARV group. Assessment of the antigen-specific humoral response and its functionality revealed vaccine-platform-dependent differences, particularly in the Fc effector functions.

Transcriptomics of Marburg virus-infected primary proximal tubular cells reveals negative correlation of immune response and energy metabolism

Marburg virus, a member of the Filoviridae, is the causative agent of Marburg virus disease (MVD), a hemorrhagic fever with a case fatality rate of up to 90 %. Acute kidney injury is common in MVD and is associated with increased mortality, but its pathogenesis in MVD remains poorly understood. Interestingly, autopsies show the presence of viral proteins in different parts of the nephron, particularly in proximal tubular cells (PTC). These findings suggest a potential role for the virus in the development of MVD-related kidney injury. To shed light on this effect, we infected primary human PTC with Lake Victoria Marburg virus and conducted transcriptomic analysis at multiple time points. Unexpectedly, infection did not induce marked cytopathic effects in primary tubular cells at 20 and 40 h post infection. However, gene expression analysis revealed robust renal viral replication and dysregulation of genes essential for different cellular functions. The gene sets mainly downregulated in PTC were associated with the targets of the transcription factors MYC and E2F, DNA repair, the G2M checkpoint, as well as oxidative phosphorylation. Importantly, the downregulated factors comprise PGC-1α, a well-known factor in acute and chronic kidney injury. By contrast, the most highly upregulated gene sets were those related to the inflammatory response and cholesterol homeostasis. In conclusion, Marburg virus infects and replicates in human primary PTC and induces downregulation of processes known to be relevant for acute kidney injury as well as a strong inflammatory response.

Bioinformatic, Biochemical, and Immunological Mining of MHC Class I Restricted T Cell Epitopes for a Marburg Nucleoprotein Microparticle Vaccine

The Marburg virus (MARV), the virus responsible for Marburg hemorrhagic fever (MHF), is considered a top-priority pathogen for vaccine development. Recent outbreaks in Equatorial Africa have highlighted the urgency of MARV because of its high fatality rate and historical concerns about potential weaponization. Currently, there are no licensed vaccines for MARV. Existing vaccine candidates rely on attenuated recombinant vesicular stomatitis virus carrying MARV glycoprotein (VSVΔG) or the chimpanzee replication-defective adenovirus 3 vector ChAd3-MARV. Although these platforms provide significant protection in animal models, they face challenges because of their limited thermal stability and the need for cold storage during deployment in resource-poor areas. An alternative approach involves using adjuvanted poly (lactic-co-glycolic acid) (PLGA) microparticles loaded with synthetic peptides representing MHC class I-restricted T cell epitopes. This vaccine platform has demonstrated effectiveness in protecting against SARS-CoV-2 and EBoV disease in animal models and has the advantage of not requiring cold storage and remaining stable at room temperature for over six months. This report outlines the design, manufacturing, and in vivo immunogenicity testing of PLGA microparticle human vaccines designed to prevent Marburg hemorrhagic fever.

Utilizing non-human primate models to combat recent COVID-19/SARS-CoV-2 and viral infectious disease outbreaks

In recent times, global viral outbreaks and diseases, such as COVID-19 (SARS-CoV-2), Zika (ZIKV), monkeypox (MPOX), Ebola (EBOV), and Marburg (MARV), have been extensively documented. Swiftly deciphering the mechanisms underlying disease pathogenesis and devising vaccines or therapeutic interventions to curtail these outbreaks stand as paramount imperatives. Amidst these endeavors, animal models emerge as pivotal tools. Among these models, non-human primates (NHPs) hold a position of particular importance. Their proximity in evolutionary lineage and physiological resemblances to humans render them a primary model for comprehending human viral infections. This review encapsulates the pivotal role of various NHP species-such as rhesus macaques (Macaca mulatta), cynomolgus macaques (Macaca fascicularis), african green monkeys (Chlorocebus sabaeus/aethiops), pigtailed macaques (Macaca nemestrina/Macaca leonina), baboons (Papio hamadryas/Papio anubis), and common marmosets (Callithrix jacchus)-in investigations pertaining to the abovementioned viral outbreaks. These NHP models play a pivotal role in illuminating key aspects of disease dynamics, facilitating the development of effective countermeasures, and contributing significantly to our overall understanding of viral pathogenesis.

Peripheral immune responses to filoviruses in a reservoir versus spillover hosts reveal transcriptional correlates of disease

Several filoviruses, including Marburg virus (MARV), cause severe disease in humans and nonhuman primates (NHPs). However, the Egyptian rousette bat (ERB, Rousettus aegyptiacus), the only known MARV reservoir, shows no overt illness upon natural or experimental infection, which, like other bat hosts of zoonoses, is due to well-adapted, likely species-specific immune features. Despite advances in understanding reservoir immune responses to filoviruses, ERB peripheral blood responses to MARV and how they compare to those of diseased filovirus-infected spillover hosts remain ill-defined. We thus conducted a longitudinal analysis of ERB blood gene responses during acute MARV infection. These data were then contrasted with a compilation of published primate blood response studies to elucidate gene correlates of filovirus protection versus disease. Our work expands on previous findings in MARV-infected ERBs by supporting both host resistance and disease tolerance mechanisms, offers insight into the peripheral immunocellular repertoire during infection, and provides the most direct known cross-examination between reservoir and spillover hosts of the most prevalently-regulated response genes, pathways and activities associated with differences in filovirus pathogenesis and pathogenicity.

Transcriptional profiling of immune responses in NHPs after low-dose, VSV-based vaccination against Marburg virus

Infection with Marburg virus (MARV), the causative agent of Marburg virus disease (MVD), results in haemorrhagic disease and high case fatality rates (>40%) in humans. Despite its public health relevance, there are no licensed vaccines or therapeutics to prevent or treat MVD. A vesicular stomatitis virus (VSV)-based vaccine expressing the MARV glycoprotein (VSV-MARV) is currently in clinical development. Previously, a single 10 million PFU dose of VSV-MARV administered 1-5 weeks before lethal MARV challenge conferred uniform protection in nonhuman primates (NHPs), demonstrating fast-acting potential. Additionally, our group recently demonstrated that even a low dose VSV-MARV (1000 PFU) protected NHPs when given 7 days before MARV challenge. In this study, we longitudinally profiled the transcriptional responses of NHPs vaccinated with this low dose of VSV-MARV either 14 or 7 days before lethal MARV challenge. NHPs vaccinated 14 days before challenge presented with transcriptional changes consistent with an antiviral response before challenge. Limited gene expression changes were observed in the group vaccinated 7 days before challenge. After challenge, genes related to lymphocyte-mediated immunity were only observed in the group vaccinated 14 days before challenge, indicating that the length of time between vaccination and challenge influenced gene expression. Our results indicate that a low dose VSV-MARV elicits distinct immune responses that correlate with protection against MVD. A low dose of VSV-MARV should be evaluated in clinical rails as it may be an option to deliver beneficial public health outcomes to more people in the event of future outbreaks.

An introduction to the Marburg virus vaccine consortium, MARVAC

The emergence of Marburg virus (MARV) in Guinea and Ghana triggered the assembly of the MARV vaccine "MARVAC" consortium representing leaders in the field of vaccine research and development aiming to facilitate a rapid response to this infectious disease threat. Here, we discuss current progress, challenges, and future directions for MARV vaccines.

Development of a Well-Characterized Cynomolgus Macaque Model of Marburg Virus Disease for Support of Vaccine and Therapy Development

Marburg virus (MARV) is a filovirus that can infect humans and nonhuman primates (NHPs), causing severe disease and death. Of the filoviruses, Ebola virus (EBOV) has been the primary target for vaccine and therapeutic development. However, MARV has an average case fatality rate of approximately 50%, the infectious dose is low, and there are currently no approved vaccines or therapies targeted at infection with MARV. The purpose of this study was to characterize disease course in cynomolgus macaques intramuscularly exposed to MARV Angola variant. There were several biomarkers that reliably correlated with MARV-induced disease, including: viral load; elevated total clinical scores; temperature changes; elevated ALT, ALP, BA, TBIL, CRP and decreased ALB values; decreased lymphocytes and platelets; and prolonged PTT. A scheduled euthanasia component also provided the opportunity to study the earliest stages of the disease. This study provides evidence for the application of this model to evaluate potential vaccines and therapies against MARV and will be valuable in improving existing models.

Single Dose of a VSV-Based Vaccine Rapidly Protects Macaques From Marburg Virus Disease

Marburg virus (MARV) is a member of the filovirus family that causes hemorrhagic disease with high case fatality rates. MARV is on the priority list of the World Health Organization for countermeasure development highlighting its potential impact on global public health. We developed a vesicular stomatitis virus (VSV)-based vaccine expressing the MARV glycoprotein (VSV-MARV) and previously demonstrated uniform protection of nonhuman primates (NHPs) with a single dose. Here, we investigated the fast-acting potential of this vaccine by challenging NHPs with MARV 14, 7 or 3 days after a single dose vaccination with VSV-MARV. We found that 100% of the animals survived when vaccinated 7 or 14 days and 75% of the animal survived when vaccinated 3 days prior to lethal MARV challenge. Transcriptional analysis of whole blood samples indicated activation of B cells and antiviral defense after VSV-MARV vaccination. In the day -14 and -7 groups, limited transcriptional changes after challenge were observed with the exception of day 9 post-challenge in the day -7 group where we detected gene expression profiles indicative of a recall response. In the day -3 group, transcriptional analysis of samples from surviving NHPs revealed strong innate immune activation. In contrast, the animal that succumbed to disease in this group lacked signatures of antiviral immunity. In summary, our data demonstrate that the VSV-MARV is a fast-acting vaccine suitable for the use in emergency situations like disease outbreaks in Africa.

Lymphopenia Caused by Virus Infections and the Mechanisms Beyond

Viral infections can give rise to a systemic decrease in the total number of lymphocytes in the blood, referred to as lymphopenia. Lymphopenia may affect the host adaptive immune responses and impact the clinical course of acute viral infections. Detailed knowledge on how viruses induce lymphopenia would provide valuable information into the pathogenesis of viral infections and potential therapeutic targeting. In this review, the current progress of viruses-induced lymphopenia is summarized and the potential mechanisms and factors involved are discussed.

Asymptomatic Infection of Marburg Virus Reservoir Bats Is Explained by a Strategy of Immunoprotective Disease Tolerance

Marburg virus (MARV) is among the most virulent pathogens of primates, including humans. Contributors to severe MARV disease include immune response suppression and inflammatory gene dysregulation ("cytokine storm"), leading to systemic damage and often death. Conversely, MARV causes little to no clinical disease in its reservoir host, the Egyptian rousette bat (ERB). Previous genomic and in vitro data suggest that a tolerant ERB immune response may underlie MARV avirulence, but no significant examination of this response in vivo yet exists. Here, using colony-bred ERBs inoculated with a bat isolate of MARV, we use species-specific antibodies and an immune gene probe array (NanoString) to temporally characterize the transcriptional host response at sites of MARV replication relevant to primate pathogenesis and immunity, including CD14⁺ monocytes/macrophages, critical immune response mediators, primary MARV targets, and skin at the inoculation site, where highest viral loads and initial engagement of antiviral defenses are expected. Our analysis shows that ERBs upregulate canonical antiviral genes typical of mammalian systems, such as ISG15, IFIT1, and OAS3, yet demonstrate a remarkable lack of significant induction of proinflammatory genes classically implicated in primate filoviral pathogenesis, including CCL8, FAS, and IL6. Together, these findings offer the first in vivo functional evidence for disease tolerance as an immunological mechanism by which the bat reservoir asymptomatically hosts MARV. More broadly, these data highlight factors determining disparate outcomes between reservoir and spillover hosts and defensive strategies likely utilized by bat hosts of other emerging pathogens, knowledge that may guide development of effective antiviral therapies.

Natural killer cell responses to emerging viruses of zoonotic origin

Emerging viral diseases pose a major threat to public health worldwide. Nearly all emerging viruses, including Ebola, Dengue, Nipah, West Nile, Zika, and coronaviruses (including SARS-Cov2, the causative agent of the current COVID-19 pandemic), have zoonotic origins, indicating that animal-to-human transmission constitutes a primary mode of acquisition of novel infectious diseases. Why these viruses can cause profound pathologies in humans, while natural reservoir hosts often show little evidence of disease is not completely understood. Differences in the host immune response, especially within the innate compartment, have been suggested to be involved in this divergence. Natural killer (NK) cells are innate lymphocytes that play a critical role in the early antiviral response, secreting effector cytokines and clearing infected cells. In this review, we will discuss the mechanisms through which NK cells interact with viruses, their contribution towards maintaining equilibrium between the virus and its natural host, and their role in disease progression in humans and other non-natural hosts.

Marburg virus pathogenesis - differences and similarities in humans and animal models

Marburg virus (MARV) is a highly pathogenic virus associated with severe disease and mortality rates as high as 90%. Outbreaks of MARV are sporadic, deadly, and often characterized by a lack of resources and facilities to diagnose and treat patients. There are currently no approved vaccines or treatments, and the chaotic and infrequent nature of outbreaks, among other factors, makes testing new countermeasures during outbreaks ethically and logistically challenging. Without field efficacy studies, researchers must rely on animal models of MARV infection to assess the efficacy of vaccines and treatments, with the limitations being the accuracy of the animal model in recapitulating human pathogenesis. This review will compare various animal models to the available descriptions of human pathogenesis and aims to evaluate their effectiveness in modeling important aspects of Marburg virus disease.

Distinct Biological Phenotypes of Marburg and Ravn Virus Infection in Macaques

Filoviruses are among the most pathogenic infectious agents known to human, with high destructive potential, as evidenced by the recent Ebola virus epidemic in West Africa. As members of the filovirus family, marburgviruses have caused similar devastating outbreaks, albeit with lower case numbers. In this study we compare the pathogenesis of Ravn virus (RAVV) and Marburg virus (MARV) strains Angola, Musoke, and Ozolin in rhesus and cynomolgus macaques, the 2 nonhuman primate species most commonly used in filovirus research. Our results reveal the most pathogenic MARV strain to be Angola, followed by Musoke, whereas Ozolin is the least pathogenic. We also demonstrate that RAVV is highly pathogenic in cynomolgus macaques but less pathogenic in rhesus macaques. Our results demonstrate a preferential infection of endothelial cells by MARVs; in addition, analysis of tissue samples suggests that lymphocyte and hepatocyte apoptosis might play a role in MARV pathogenicity. This information expands our knowledge about pathogenicity and virulence of marburgviruses.

Pathogenicity of Ebola and Marburg Viruses Is Associated With Differential Activation of the Myeloid Compartment in Humanized Triple Knockout-Bone Marrow, Liver, and Thymus Mice

Ebola virus (EBOV) and Marburg virus (MARV) outbreaks are highly lethal, and infection results in a hemorrhagic fever with complex etiology. These zoonotic viruses dysregulate the immune system to cause disease, in part by replicating within myeloid cells that would normally innately control viral infection and shape the adaptive immune response. We used triple knockout (TKO)-bone marrow, liver, thymus (BLT) humanized mice to recapitulate the early in vivo human immune response to filovirus infection. Disease severity in TKO-BLT mice was dissimilar between EBOV and MARV with greater severity observed during EBOV infection. Disease severity was related to increased Kupffer cell infection in the liver, higher levels of myeloid dysfunction, and skewing of macrophage subtypes in EBOV compared with MARV-infected mice. Overall, the TKO-BLT model provided a practical in vivo platform to study the human immune response to filovirus infection and generated a better understanding of how these viruses modulate specific components of the immune system.

Filovirus Strategies to Escape Antiviral Responses

This chapter describes the various strategies filoviruses use to escape host immune responses with a focus on innate immune and cell death pathways. Since filovirus replication can be efficiently blocked by interferon (IFN), filoviruses have evolved mechanisms to counteract both type I IFN induction and IFN response signaling pathways. Intriguingly, marburg- and ebolaviruses use different strategies to inhibit IFN signaling. This chapter also summarizes what is known about the role of IFN-stimulated genes (ISGs) in filovirus infection. These fall into three categories: those that restrict filovirus replication, those whose activation is inhibited by filoviruses, and those that have no measurable effect on viral replication. In addition to innate immunity, mammalian cells have evolved strategies to counter viral infections, including the induction of cell death and stress response pathways, and we summarize our current knowledge of how filoviruses interact with these pathways. Finally, this chapter delves into the interaction of EBOV with myeloid dendritic cells and macrophages and the associated inflammatory response, which differs dramatically between these cell types when they are infected with EBOV. In summary, we highlight the multifaceted nature of the host-viral interactions during filoviral infections.

Recent advances in marburgvirus research

Marburgviruses are closely related to ebolaviruses and cause a devastating disease in humans. In 2012, we published a comprehensive review of the first 45 years of research on marburgviruses and the disease they cause, ranging from molecular biology to ecology. Spurred in part by the deadly Ebola virus outbreak in West Africa in 2013-2016, research on all filoviruses has intensified. Not meant as an introduction to marburgviruses, this article instead provides a synopsis of recent progress in marburgvirus research with a particular focus on molecular biology, advances in animal modeling, and the use of Egyptian fruit bats in infection experiments.

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.

Protection Against Marburg Virus Using a Recombinant VSV-Vaccine Depends on T and B Cell Activation

Marburg virus (MARV) is the causative agent of hemorrhagic fever outbreaks with high case fatality rates. Closely related to Ebola virus, MARV is a filamentous virus with a negative-sense, single-stranded RNA genome. Although extensive studies on filovirus countermeasures have been conducted, there are no licensed treatments against MARV infections. An experimental vaccine based on the recombinant vesicular stomatitis virus (VSV) expressing the MARV-Musoke glycoprotein demonstrated complete protection when a single dose was administered 28 days and up to 14 months prior to MARV challenge. Here, we analyzed the protective efficacy of an updated vaccine expressing the MARV-Angola glycoprotein (VSV-MARV). A single dose of VSV-MARV given 5 weeks before challenge provided uniform protection with no detectable viremia. The vaccine induced B and T cell proliferation and, importantly, antigen-specific IgG production. Transcriptomic signatures confirm these findings and suggest innate immunity engendered by VSV-MARV may direct the development of protective humoral immunity.

Marburgviruses: An Update

Ebolaviruses have gained much attention recently due to the outbreak from 2014 through 2016. The related marburgviruses also have been responsible for large outbreaks with high case fatality rates. The purpose of this article is to provide the clinical laboratory scientist with a review of the most current developments in marburgvirus research. The PubMed database was reviewed using the keywords "Marburg virus," "Ravn virus," and "marburgviruses," with publication dates from January 1, 2015 through June 20, 2017. The search yielded 345 articles. In total, 52 articles met the inclusion criteria and were reviewed. Advances have been made in the areas of ecology and host reservoir studies, seroprevalence studies, pathology and pathogenesis studies, laboratory assay development, and treatment and vaccine development. Marburgviruses are highly lethal viruses that pose a significant threat to the human population. Although numerous advances have been made, there are still large gaps in knowledge, and it is imperative that scientists gain more information to fully understand virus/host interactions. An approved vaccine and treatment remain elusive.

Natural History and Pathogenesis of Wild-Type Marburg Virus Infection in STAT2 Knockout Hamsters

Marburg virus (MARV; family Filoviridae) causes sporadic outbreaks of Marburg hemorrhagic fever in sub-Saharan Africa with case fatality rates reaching 90%. Wild-type filoviruses, including MARV and the closely related Ebola virus, are unable to suppress the type I interferon response in rodents, and therefore require adaptation of the viruses to cause disease in immunocompetent animals. In the current study, we demonstrate that STAT2 knockout Syrian hamsters are susceptible to infection with different wild-type MARV variants. MARV Musoke causes a robust and systemic infection resulting in lethal disease. Histopathological findings share features similar to those observed in human patients and other animal models of filovirus infection. Reverse-transcription polymerase chain reaction analysis of host transcripts shows a dysregulation of the innate immune response. Our results demonstrate that the STAT2 knockout hamster represents a novel small animal model of severe MARV infection and disease without the requirement for virus adaptation.

Virulence of Marburg Virus Angola Compared to Mt. Elgon (Musoke) in Macaques: A Pooled Survival Analysis

Angola variant (MARV/Ang) has replaced Mt. Elgon variant Musoke isolate (MARV/MtE-Mus) as the consensus standard variant for Marburg virus research and is regarded as causing a more aggressive phenotype of disease in animal models; however, there is a dearth of published evidence supporting the higher virulence of MARV/Ang. In this retrospective study, we used data pooled from eight separate studies in nonhuman primates experimentally exposed with either 1000 pfu intramuscular (IM) MARV/Ang or MARV/MtE-Mus between 2012 and 2017 at the United States Army Medical Research Institute of Infectious Diseases (USAMRIID). Multivariable Cox proportional hazards regression was used to evaluate the association of variant type with time to death, the development of anorexia, rash, viremia, and 10 select clinical laboratory values. A total of 47 cynomolgus monkeys were included, of which 18 were exposed to MARV/Ang in three separate studies and 29 to MARV/MtE-Mus in five studies. Following universally fatal Marburg virus exposure, compared to MARV/MtE-Mus, MARV/Ang was associated with an increased risk of death (HR = 22.10; 95% CI: 7.08, 68.93), rash (HR = 5.87; 95% CI: 2.76, 12.51) and loss of appetite (HR = 35.10; 95% CI: 7.60, 162.18). Our data demonstrate an increased virulence of MARV/Ang compared to MARV/MtE-Mus variant in the 1000 pfu IM cynomolgus macaque model.

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.

Development and characterization of a guinea pig model for Marburg virus

The Angolan strain of Marburg virus (MARV/Ang) can cause lethal disease in humans with a case fatality rate of up to 90%, but infection of immunocompetent rodents do not result in any observable symptoms. Our previous work includes the development and characterization of a MARV/Ang variant that can cause lethal disease in mice (MARV/Ang-MA), with the aim of using this tool to screen for promising prophylactic and therapeutic candidates. An intermediate animal model is needed to confirm any findings from mice studies before testing in the gold-standard non-human primate (NHP) model. In this study, we serially passaged the clinical isolate of MARV/Ang in the livers and spleens of guinea pigs until a variant emerged that causes 100% lethality in guinea pigs (MARV/Ang-GA). Animals infected with MARV/Ang-GA showed signs of filovirus infection including lymphocytopenia, thrombocytopenia, and high viremia leading to spread to major organs, including the liver, spleen, lungs, and kidneys. The MARV/Ang-GA guinea pigs died between 7–9 days after infection, and the LD₅₀ was calculated to be 1.1×10^–1 TCID₅₀ (median tissue culture infective dose). Mutations in MARV/Ang-GA were identified and compared to sequences of known rodent-adapted MARV/Ang variants, which may benefit future studies characterizing important host adaptation sites in the MARV/Ang viral genome.

A hamster model for Marburg virus infection accurately recapitulates Marburg hemorrhagic fever

Marburg virus (MARV), a close relative of Ebola virus, is the causative agent of a severe human disease known as Marburg hemorrhagic fever (MHF). No licensed vaccine or therapeutic exists to treat MHF, and MARV is therefore classified as a Tier 1 select agent and a category A bioterrorism agent. In order to develop countermeasures against this severe disease, animal models that accurately recapitulate human disease are required. Here we describe the development of a novel, uniformly lethal Syrian golden hamster model of MHF using a hamster-adapted MARV variant Angola. Remarkably, this model displayed almost all of the clinical features of MHF seen in humans and non-human primates, including coagulation abnormalities, hemorrhagic manifestations, petechial rash, and a severely dysregulated immune response. This MHF hamster model represents a powerful tool for further dissecting MARV pathogenesis and accelerating the development of effective medical countermeasures against human MHF.

The Role of Cytokines and Chemokines in Filovirus Infection

Ebola- and marburgviruses are highly pathogenic filoviruses and causative agents of viral hemorrhagic fever. Filovirus disease is characterized by a dysregulated immune response, severe organ damage, and coagulation abnormalities. This includes modulation of cytokines, signaling mediators that regulate various components of the immune system as well as other biological processes. Here we examine the role of cytokines in filovirus infection, with an emphasis on understanding how these molecules affect development of the antiviral immune response and influence pathology. These proteins may present targets for immune modulation by therapeutic agents and vaccines in an effort to boost the natural immune response to infection and/or reduce immunopathology.

Considerations in the Use of Nonhuman Primate Models of Ebola Virus and Marburg Virus Infection

The filoviruses, Ebola virus and Marburg virus, are zoonotic pathogens that cause severe hemorrhagic fever in humans and nonhuman primates (NHPs), with case-fatality rates ranging from 23% to 90%. The current outbreak of Ebola virus infection in West Africa, with >26 000 cases, demonstrates the long-underestimated public health danger that filoviruses pose as natural human pathogens. Currently, there are no vaccines or treatments licensed for human use. Licensure of any medical countermeasure may require demonstration of efficacy in the gold standard cynomolgus or rhesus macaque models of filovirus infection. Substantial progress has been made over the last decade in characterizing the filovirus NHP models. However, there is considerable debate over a variety of experimental conditions, including differences among filovirus isolates used, routes and doses of exposure, and euthanasia criteria, all of which may contribute to variability of results among different laboratories. As an example of the importance of understanding these differences, recent data with Ebola virus shows that an addition of a single uridine residue in the glycoprotein gene at the editing site attenuates the virus. Here, we draw on decades of experience working with filovirus-infected NHPs to provide a perspective on the importance of various experimental conditions.

Considerations in the Use of Nonhuman Primate Models of Ebola Virus and Marburg Virus Infection

The filoviruses, Ebola virus and Marburg virus, are zoonotic pathogens that cause severe hemorrhagic fever in humans and nonhuman primates (NHPs), with case-fatality rates ranging from 23% to 90%. The current outbreak of Ebola virus infection in West Africa, with >26 000 cases, demonstrates the long-underestimated public health danger that filoviruses pose as natural human pathogens. Currently, there are no vaccines or treatments licensed for human use. Licensure of any medical countermeasure may require demonstration of efficacy in the gold standard cynomolgus or rhesus macaque models of filovirus infection. Substantial progress has been made over the last decade in characterizing the filovirus NHP models. However, there is considerable debate over a variety of experimental conditions, including differences among filovirus isolates used, routes and doses of exposure, and euthanasia criteria, all of which may contribute to variability of results among different laboratories. As an example of the importance of understanding these differences, recent data with Ebola virus shows that an addition of a single uridine residue in the glycoprotein gene at the editing site attenuates the virus. Here, we draw on decades of experience working with filovirus-infected NHPs to provide a perspective on the importance of various experimental conditions.