Biomarkers for understanding Ebola virus disease

RNA Viruses, Toll-Like Receptors, and Cytokines: The Perfect Storm?

BACKGROUND

The interactions between viruses and the host immune response are nuanced and intricate. The cytokine response arguably plays a central role in dictating the outcome of virus infection, balancing inflammation, and healing, which is crucial to resolving infection without destructive immunopathologies.

SUMMARY

Early innate immune responses are key to the generation of a beneficial or detrimental immune response. These initial responses are regulated by a plethora of surface bound, endosomal, and cytoplasmic innate immune receptors known as pattern recognition receptors. Of these, the Toll-like receptors (TLRs) play an important role in the induction of cytokines during virus infection. Recognizing pathogen-associated molecular patterns (PAMPs) such as viral proteins and/or nucleotide sequences, the TLRs act as sentinels for the initiation and propagation of immune responses.

KEY MESSAGES

TLRs are important receptors for initiating the innate response to single-stranded RNA (ssRNA) viruses like influenza A virus (IAV), severe acute respiratory syndrome coronavirus-1 (SARS-CoV-1), SARS-CoV-2, Middle East respiratory syndrome coronavirus, dengue virus, and Ebola virus. Infection with these viruses is also associated with aberrant expression of proinflammatory cytokines that contribute to a harmful cytokine storm response. Herein we discuss the connections between these ssRNA viruses, cytokine storm, and the roles of TLRs.

BACKGROUND

The interactions between viruses and the host immune response are nuanced and intricate. The cytokine response arguably plays a central role in dictating the outcome of virus infection, balancing inflammation, and healing, which is crucial to resolving infection without destructive immunopathologies.

SUMMARY

Early innate immune responses are key to the generation of a beneficial or detrimental immune response. These initial responses are regulated by a plethora of surface bound, endosomal, and cytoplasmic innate immune receptors known as pattern recognition receptors. Of these, the Toll-like receptors (TLRs) play an important role in the induction of cytokines during virus infection. Recognizing pathogen-associated molecular patterns (PAMPs) such as viral proteins and/or nucleotide sequences, the TLRs act as sentinels for the initiation and propagation of immune responses.

KEY MESSAGES

TLRs are important receptors for initiating the innate response to single-stranded RNA (ssRNA) viruses like influenza A virus (IAV), severe acute respiratory syndrome coronavirus-1 (SARS-CoV-1), SARS-CoV-2, Middle East respiratory syndrome coronavirus, dengue virus, and Ebola virus. Infection with these viruses is also associated with aberrant expression of proinflammatory cytokines that contribute to a harmful cytokine storm response. Herein we discuss the connections between these ssRNA viruses, cytokine storm, and the roles of TLRs.

Mapping of susceptibility loci for Ebola virus pathogenesis in mice

Ebola virus (EBOV), a major global health concern, causes severe, often fatal EBOV disease (EVD) in humans. Host genetic variation plays a critical role, yet the identity of host susceptibility loci in mammals remains unknown. Using genetic reference populations, we generate an F2 mapping cohort to identify host susceptibility loci that regulate EVD. While disease-resistant mice display minimal pathogenesis, susceptible mice display severe liver pathology consistent with EVD-like disease and transcriptional signatures associated with inflammatory and liver metabolic processes. A significant quantitative trait locus (QTL) for virus RNA load in blood is identified in chromosome (chr)8, and a severe clinical disease and mortality QTL is mapped to chr7, which includes the Trim5 locus. Using knockout mice, we validate the Trim5 locus as one potential driver of liver failure and mortality after infection. The identification of susceptibility loci provides insight into molecular genetic mechanisms regulating EVD progression and severity, potentially informing therapeutics and vaccination strategies.

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.

Development of a Well-Characterized Rhesus Macaque Model of Ebola Virus Disease for Support of Product Development

Ebola virus (EBOV) is a negative-sense RNA virus that can infect humans and nonhuman primates with severe health consequences. Development of countermeasures requires a thorough understanding of the interaction between host and pathogen, and the course of disease. The goal of this study was to further characterize EBOV disease in a uniformly lethal rhesus macaque model, in order to support development of a well-characterized model following rigorous quality standards. Rhesus macaques were intramuscularly exposed to EBOV and one group was euthanized at predetermined time points to characterize progression of disease. A second group was not scheduled for euthanasia in order to analyze survival, changes in physiology, clinical pathology, terminal pathology, and telemetry kinetics. On day 3, sporadic viremia was observed and pathological evidence was noted in lymph nodes. By day 5, viremia was detected in all EBOV exposed animals and pathological evidence was noted in the liver, spleen, and gastrointestinal tissues. These data support the notion that EBOV infection in rhesus macaques is a rapid systemic disease similar to infection in humans, under a compressed time scale. Biomarkers that correlated with disease progression at the earliest stages of infection were observed thereby identifying potential "trigger-to-treat" for use in therapeutic studies.

Clinical Laboratory Values as Early Indicators of Ebola Virus Infection in Nonhuman Primates

The Ebola virus (EBOV) outbreak in West Africa during 2013-2016 demonstrated the need to improve Ebola virus disease (EVD) diagnostics and standards of care. This retrospective study compared laboratory values and clinical features of 3 nonhuman primate models of lethal EVD to assess associations with improved survival time. In addition, the study identified laboratory values useful as predictors of survival, surrogates for EBOV viral loads, and triggers for initiation of therapeutic interventions in these nonhuman primate models. Furthermore, the data support that, in nonhuman primates, the Makona strain of EBOV may be less virulent than the Kikwit strain of EBOV. The applicability of these findings as potential diagnostic and management tools for EVD in humans warrants further investigation.

Vaccines against Ebola virus

We have just witnessed the largest and most devastating outbreak of Ebola virus disease, which highlighted the urgent need for development of an efficacious vaccine that could be used to curtail future outbreaks. Prior to 2014, there had been limited impetus worldwide to develop a vaccine since the virus was first discovered in 1976. Though too many lives were lost during this outbreak, it resulted in the significantly accelerated clinical development of a number of candidate vaccines through an extraordinary collaborative global effort coordinated by the World Health Organisation (WHO) and involving a number of companies, trial centres, funders, global stakeholders and agencies. We have acquired substantial safety and immunogenicity data on a number of vaccines in Caucasian and African populations. The rapid pace of events led to the initiation of the landmark efficacy trial testing the rVSV-vectored vaccine, which showed high level efficacy in an outbreak setting when deployed using an innovative ring vaccination strategy. Though the Public Health Emergency of International Concern (PHEIC) declared by the WHO has now been lifted, the global scientific community faces numerous challenges ahead to ensure that there is a licensed, deployable vaccine available for use in future outbreaks for at least the Zaire and Sudan strains of Ebola virus. There remain several unanswered questions on the durability of protection, mechanistic immunological correlates and preferred deployment strategies. This review outlines a brief history of the development of Ebola vaccines, the significant progress made since the scale of the outbreak became apparent, some lessons learnt and how they could shape future development of vaccines and the management of similar outbreaks.

Analysis of Ebola virus polymerase domains to find strain-specific differences and to gain insight on their pathogenicity

Ebola virus, a member of the family Filoviridae has caused immense morbidity and mortality in recent times, especially in West Africa. The infection characterized by chills, fever, diarrhea, and myalgia can progress to hemorrhage and death. Hence, it is a high priority area to better understand its biology in order to expedite vaccine development pipelines. In this regard, this study analyzes the domains in RNA polymerase of fifteen publicly-available Ebola isolates belonging to three strains (Zaire, Sudan and Reston). The protein FASTA sequences of the isolates belonging Zaire, Sudan and Reston strains were extracted from UniProt database and submitted to the interactive web tool SMART for the polymerase domain profiles. Subsequent in silico investigation furnished interesting results that sure can contribute to the understanding of Ebola pathogenesis. The key findings and patterns have been presented, and based on them hypotheses have been formulated for further empirical validation.

Kinetic Analysis of Biomarkers in a Cohort of US Patients With Ebola Virus Disease

BACKGROUND

Ebola virus (EBOV) infection causes a severe and often fatal disease. Despite the fact that more than 30 000 individuals have acquired Ebola virus disease (EVD), the medical and scientific community still does not have a clear understanding of the mechanisms by which EBOV causes such severe disease.

METHODS

In this study, 54 biomarkers in plasma samples serially collected from 7 patients with EVD were analyzed in an attempt to define the kinetics of inflammatory modulators. Two clinical disease groups were defined (moderate and severe) based on the need for clinical support. Biomarkers were evaluated for correlation with viremia and clinical disease in an effort to identify pathways that could be useful targets of therapeutic intervention.

RESULTS

Patients with severe disease had higher viremia than those with moderate disease. Several biomarkers of immune activation and control were significantly elevated in patients with moderate disease. A series of pro-inflammatory cytokines and chemokines were significantly elevated in patients with severe disease.

CONCLUSIONS

Biomarkers that were associated with severe EVD were proinflammatory and indicative of endothelial or coagulation cascade dysfunction, as has been seen historically in patients with fatal outcomes. In contrast, biomarkers that were associated with moderate EVD were suggestive of a strong interferon response and control of both innate and adaptive responses. Therefore, clinical interventions that modulate the phenotype and magnitude of immune activation may be beneficial in treating EVD.