Foundational characterization of a new bat-borne ebolavirus and its potential as a human pathogen

There are many documented cases of pathogens from animals jumping species resulting in a disease that affects humans. One of the primary animals associated with zoonosis is bats due to their global prevalence, diverse genetics, and host a wide variety of viruses that can also infect humans. The importance of studying these viruses has been highlighted by the current coronavirus disease 2019 (COVID-19) pandemic, as the causative agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is believed to have originated from bats.

The viruses of the filovirus genus are enveloped with a single-strand, negative-sense RNA genome and are classified into several viral family groups, including Ebolavirus. There are six known Ebolavirus species: Ebola virus (EBOV), Sudan (SUDV), Bundibugyo (BDBV), Reston (RESTV), Tai Forest (TAFV), and Bombali (BOMV). Due to the highly occurring zoonosis events involving Ebolaviruses, researchers sought to evaluate the zoonotic potential of BOMV to assess the level of risk to public health.

A preprint version of the study is available on the Cell Press* preprint server while the article undergoes peer review.

The inhibition effects of remdesivir and monoclonal antibodies on BOMV

In response to recent outbreaks of EBOV, clinical trials have taken place to test small molecule inhibitors and monoclonal antibodies (mAbs) as potential therapeutic agents. In this study, the authors tested potential EBOV therapies to see whether they would be effective against BOMV.

Remdesivir is a broad-spectrum antiviral therapy utilized to treat many viral infections, such as EBOV and SARS-CoV-2. The researchers discovered that remdesivir suppressed BOMV and EBOV minigenome replication at equal 50% effective concentrations (EC50) of 500nM. BOMV-ZsG and EBOV-ZsG infection was also prevented by Remdesivir, with EC50 values of 17nM and 6nM, respectively.

The authors tested the ability of mAbs produced to target the EBOV glycoprotein to inhibit infection from BOMV. During the West African Ebola virus disease (EBV) pandemic, patients were administered ZMapp, a mAb cocktail. The individual components of ZMapp (2G4 and 4G7) that have known neutralization effects against EBOV glycoprotein and the well-characterized EBOV mAb KZ52 were tested against EBOV and BOMV.

All the above-mentioned mAbs were shown to block EBOV infection, while they all failed to neutralize BOMV. The targets for these mAbs are the glycan cap, receptor-binding domain, and internal fusion loop (IFL). However, mAbs MBP431, MBP047, and MBP087 which target the heptad repeats, glycoprotein base, or the membrane-proximal external region (MPER) domains, blocked both EBOV and BOMV.

Gene expression in host cells when infected with BOMV

Macrophages and dendritic cells were found to be the principal targets of filovirus replication and essential players in the establishment of EVD in clinical studies of EBOV-infected individuals and animal models. Given the importance of macrophages in developing an adaptive immune response, they are also suspects in the weakening of innate and adaptive immune responses seen in severe and deadly filovirus infections.

As a result, the authors intended to see if BOMV could infect and multiply in human macrophages, as well as examine the macrophages' reaction to BOMV infection for virulence potential. In primary human macrophages, they examined the replication capability and transcriptional response of BOMV and EBOV infections. To evaluate macrophage response to virus infection, researchers infected macrophages with recombinant Rift Valley fever dd virus (RVFVdd), a bunyavirus modified to lack an interferon antagonist.

By comparing host gene responses in macrophages pre-treated with the remdesivir, transcriptional alterations were linked to viral multiplication. The intracellular viral RNA of both EBOV and BOMV was reduced following remdesivir treatment, whereas the RNA of RVFVdd remained unaffected due to remdesivir being incapable of inhibiting RVFVdd polymerase.

Host cell transcription was altered by EBOV, BOMV, and RVFVdd infection, and genes with a significant fold change were selected. The authors discovered that most differentially expressed genes (DEGs) were caused by viral replication, shown by a reduction in correlation to genes from remdesivir treated virus-infected cells. Principal component analysis was used to perform unsupervised clustering of the DEGs, revealing clustering of EBOV and BMOV separate from RVFVdd with a 64% variance. The genes causing the variances have been shown to encode inflammatory cytokines, chemokines, and interferon-stimulating genes.

Both BOMV- and EBOV-infected cells altered a similar number of cellular genes; however, the magnitude of counts above or below the mock control for maximal expression was 3.4x higher in EBOV-infected cells. Although virus replication impacted the majority of cellular genes, only 41% of the altered genes were shared by both EBOV and BOMV. Filovirus replication alters the host cell's transcriptional profile, although EBOV and BOMV modify the cell in different ways to establish sustained replication.

Implications

Recent global events concerning the emergence of novel influenza viruses, coronaviruses, and filoviruses from potential zoonotic sources have highlighted the importance of surveillance. Recent efforts to identify new viruses have resulted in the discovery of a large number of viral sequences. Still, only a portion of these has been evaluated for their zoonotic potential.

BOMV is a novel filovirus related to Ebolaviruses such as EBOV, SUDV, and BDBV, which have one of the highest case-fatality rates of any viral infection. The authors of this study successfully produced the first functional characterization of BMOV and assessed its capability to infect human cells and also its potential pathogenicity.

*Important notice

Cell Press publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Colin Lightfoot

Colin graduated from the University of Chester with a B.Sc. in Biomedical Science in 2020. Since completing his undergraduate degree, he worked for NHS England as an Associate Practitioner, responsible for testing inpatients for COVID-19 on admission.