Interferon-β and Interferon-γ Are Weak Inhibitors of Ebola Virus in Cell-Based Assays

Therapeutic Strategies against Ebola Virus Infection

Since the 2014–2016 epidemic, Ebola virus (EBOV) has spread to several countries and has become a major threat to global health. EBOV is a risk group 4 pathogen, which imposes significant obstacles for the development of countermeasures against the virus. Efforts have been made to develop anti-EBOV immunization and therapeutics, with three vaccines and two antibody-based therapeutics approved in recent years. Nonetheless, the high fatality of Ebola virus disease highlights the need to continuously develop antiviral strategies for the future management of EBOV outbreaks in conjunction with vaccination programs. This review aims to highlight potential EBOV therapeutics and their target(s) of inhibition, serving as a summary of the literature to inform readers of the novel candidates available in the continued search for EBOV antivirals.

Interferon-alpha or -beta facilitates SARS-CoV-2 pulmonary vascular infection by inducing ACE2

Severe viral pneumonia caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is characterized by a hyperinflammatory state typified by elevated circulating pro-inflammatory cytokines, frequently leading to potentially lethal vascular complications including thromboembolism, disseminated intracellular coagulopathy and vasculitis. Though endothelial infection and subsequent endothelial damage have been described in patients with fatal COVID-19, the mechanism by which this occurs remains elusive, particularly given that, under naïve conditions, pulmonary endothelial cells demonstrate minimal cell surface expression of the SARS-CoV-2 binding receptor ACE2. Herein we describe SARS-CoV-2 infection of the pulmonary endothelium in postmortem lung samples from individuals who died of COVID-19, demonstrating both heterogeneous ACE2 expression and endothelial damage. In primary endothelial cell cultures, we show that SARS-CoV-2 infection is dependent on the induction of ACE2 protein expression and that this process is facilitated by type 1 interferon-alpha (IFNα) or -beta(β)—two of the main anti-viral cytokines induced in severe SARS-CoV-2 infection—but not significantly by other cytokines (including interleukin 6 and interferon γ/λ). Our findings suggest that the stereotypical anti-viral interferon response may paradoxically facilitate the propagation of COVID-19 from the respiratory epithelium to the vasculature, raising concerns regarding the use of exogenous IFNα/β in the treatment of patients with COVID-19.

Filovirus Neutralising Antibodies: Mechanisms of Action and Therapeutic Application

Filoviruses, especially Ebola virus, cause sporadic outbreaks of viral haemorrhagic fever with very high case fatality rates in Africa. The 2013–2016 Ebola epidemic in West Africa provided large survivor cohorts spurring a large number of human studies which showed that specific neutralising antibodies played a key role in protection following a natural Ebola virus infection, as part of the overall humoral response and in conjunction with the cellular adaptive response. This review will discuss the studies in survivors and animal models which described protective neutralising antibody response. Their mechanisms of action will be detailed. Furthermore, the importance of neutralising antibodies in antibody-based therapeutics and in vaccine-induced responses will be explained, as well as the strategies to avoid immune escape from neutralising antibodies. Understanding the neutralising antibody response in the context of filoviruses is crucial to furthering our understanding of virus structure and function, in addition to improving current vaccines & antibody-based therapeutics.

Development of a New Reverse Genetics System for Ebola Virus

Ebola virus is among the most dangerous viral pathogens, with a case fatality rate of up to 90%. Since 2013, the two largest and most complex Ebola outbreaks in West Africa have revealed the lack of investigation on this notorious virus.

Ebola virus (EBOV) is a highly pathogenic negative-stranded RNA virus that has caused several deadly endemics in the past decades. EBOV reverse genetics systems are available for studying live viruses under biosafety level 4 (BSL-4) or subviral particles under BSL-2 conditions. However, these systems all require cotransfection of multiple plasmids expressing viral genome and viral proteins essential for EBOV replication, which is technically challenging and unable to naturally mimic virus propagation using the subviral particle. Here, we established a new EBOV reverse genetics system only requiring transfection of a single viral RNA genome into an engineered cell line that stably expresses viral nucleoprotein (NP), viral protein 35 (VP35), VP30, and large (L) proteins and has been fine-tuned for its superior permissiveness for EBOV replication. Using this system, subviral particles expressing viral VP40, glycoprotein (GP), and VP24 could be produced and continuously propagated and eventually infect the entire cell population. We demonstrated the authentic response of the subviral system to antivirals and uncovered that the VP35 amount is critical for optimal virus replication. Furthermore, we showed that fully infectious virions can be efficiently rescued by delivering the full-length EBOV genome into the same supporting cell, and the efficiency is not affected by genome polarity or virus variant specificity. In summary, our work provides a new tool for studying EBOV under different biosafety levels.

IMPORTANCE Ebola virus is among the most dangerous viral pathogens, with a case fatality rate of up to 90%. Since 2013, the two largest and most complex Ebola outbreaks in Africa have revealed the lack of investigation on this notorious virus. A reverse genetics system is an important tool for studying viruses by producing mutant viruses or generating safer and convenient model systems. Here, we developed an EBOV life cycle modeling system in which subviral particles can spontaneously propagate in cell culture. In addition, this system can be employed to rescue infectious virions of homologous or heterologous EBOV isolates using either sense or antisense viral RNA genomes. In summary, we developed a new tool for EBOV research.

Identification of interferon-stimulated genes that attenuate Ebola virus infection

The West Africa Ebola outbreak was the largest outbreak ever recorded, with over 28,000 reported infections; this devastating epidemic emphasized the need to understand the mechanisms to counteract virus infection. Here, we screen a library of nearly 400 interferon-stimulated genes (ISGs) against a biologically contained Ebola virus and identify several ISGs not previously known to affect Ebola virus infection. Overexpression of the top ten ISGs attenuates virus titers by up to 1000-fold. Mechanistic studies demonstrate that three ISGs interfere with virus entry, six affect viral transcription/replication, and two inhibit virion formation and budding. A comprehensive study of one ISG (CCDC92) that shows anti-Ebola activity in our screen reveals that CCDC92 can inhibit viral transcription and the formation of complete virions via an interaction with the viral protein NP. Our findings provide insights into Ebola virus infection that could be exploited for the development of therapeutics against this virus.

Here, Kuroda et al. screen a library of nearly 400 interferon-stimulated genes (ISGs) and identify several ISGs that inhibit Ebola virus entry, viral transcription/replication, or virion formation. The study provides insights into interactions between Ebola and the host cells.

The role of mononuclear phagocytes in Ebola virus infection

The filovirus, Zaire Ebolavirus (EBOV), infects tissue macrophages (Mϕs) and dendritic cells (DCs) early during infection. Viral infection of both cells types is highly productive, leading to increased viral load. However, virus infection of these two cell types results in different consequences for cellular function. Infection of Mϕs stimulates the production of proinflammatory and immunomodulatory cytokines and chemokines, leading to the production of a cytokine storm, while simultaneously increasing tissue factor production and thus facilitating disseminated intravascular coagulation. In contrast, EBOV infection of DCs blocks DC maturation and antigen presentation rendering these cells unable to communicate with adaptive immune response elements. Details of the known interactions of these cells with EBOV are reviewed here. We also identify a number of unanswered questions that remain about interactions of filoviruses with these cells.

Immune barriers of Ebola virus infection

Since its initial emergence in 1976 in northern Democratic Republic of Congo (DRC), Ebola virus (EBOV) has been a global health concern due to its virulence in humans, the mystery surrounding the identity of its host reservoir and the unpredictable nature of Ebola virus disease (EVD) outbreaks. Early after the first clinical descriptions of a disease resembling a 'septic-shock-like syndrome', with coagulation abnormalities and multi-system organ failure, researchers began to evaluate the role of the host immune response in EVD pathophysiology. In this review, we summarize how data gathered during the last 40 years in the laboratory as well as in the field have provided insight into EBOV immunity. From molecular mechanisms involved in EBOV recognition in infected cells, to antigen processing and adaptive immune responses, we discuss current knowledge on the main immune barriers of infection as well as outstanding research questions.

Novel Stable Ebola Virus Minigenome Replicon Reveals Remarkable Stability of the Viral Genome

Ebola virus (EBOV) causes severe hemorrhagic fever in humans and other primates with a high case fatality rate. No approved drug or vaccine of EBOV is available, which necessitates better understanding of the virus life cycle. Studies on EBOV have been hampered because experimentations involving live virus are restricted to biosafety level 4 (BSL4) laboratories. The EBOV minigenome system has provided researchers with the opportunity to study EBOV under BSL2 conditions. Here, we developed a novel EBOV minigenome replicon which, to our knowledge, is the first EBOV cell culture system that can stably replicate and transcribe the EBOV minigenome. The minigenomic RNA harboring a Gaussia luciferase and hygromycin-resistant marker can replicate for months in a helper cell stably expressing viral nucleoprotein (NP), viral protein 35 (VP35), VP30, and L proteins. Quantification of viral RNA (vRNA), cRNA, and mRNA levels of the EBOV minigenome demonstrated that the stable EBOV replicon had much-more-active minigenome replication than previously developed transient-transfection-based EBOV minigenome systems, which recapitulate viral primary transcription more than genome replication. Interestingly, minigenome replication in the stable EBOV replicon cells was insensitive to interferon treatment or RNA interference. Moreover, RNase digestion of the replicon cell lysates revealed the remarkably stable nature of the EBOV minigenomic vRNA ribonucleoprotein complex, which may help improve understanding of EBOV persistence in convalescent patients.IMPORTANCE The scope and severity of the recent Ebola outbreak in Western Africa justified a more comprehensive investigation of the causative risk group 4 agent Ebola virus (EBOV). Study of EBOV replication and antiviral development can be facilitated by developing a cell culture system that allows experimentation under biosafety level 2 conditions. Here, we developed a novel stable EBOV minigenome replicon which, to our knowledge, is the first EBOV cell culture system that can stably replicate and transcribe the EBOV minigenome. The replicon system had more-active genome replication than previously developed transient-transfection-based EBOV minigenome systems, providing a convenient surrogate system to study EBOV replication. Furthermore, self-replicating minigenomic vRNA in the replicon cells displayed strong stability in response to interferon treatment, RNA silencing, and RNase digestion, which may provide an explanation for the persistence of EBOV in survivors.