Favipiravir inhibits in vitro Usutu virus replication and delays disease progression in an infection model in mice

Development of a live-attenuated chimeric vaccine against the emerging Usutu virus

Usutu virus (USUV) is an emerging arthropod-borne flavivirus that has expanded into multiple European countries during the past several decades. USUV infection in human has been linked to severe neurological complications, and no vaccine is now available against USUV. In this work, we develop a live-attenuated chimeric USUV vaccine (termed ChinUSUV) based on the full-length infectious cDNA clone of the licensed Japanese encephalitis virus (JEV) vaccine strain SA14-14-2. In vitro studies demonstrate that ChinUSUV replicates efficiently and maintains its genetic stability. Remarkably, ChinUSUV exhibits a significant attenuation phenotype in multiple mouse models even compared with the licensed JEV vaccine. A single immunization with ChinUSUV elicits potent IgG and neutralizing antibody responses as well as T cell response. Passive transfer of sera from ChinUSUV-immunized mice confers significant protection against lethal homologous challenge in suckling mice. Taken together, our results suggest that ChinUSUV represents a potential USUV vaccine candidate that merits further development.

Competing interests during the key N-glycosylation of 6-chloro-7-deaza-7-iodopurine for the synthesis of 7-deaza-2'-methyladenosine using Vorbrüggen conditions

A short 3-step synthesis of the antiviral agent 7DMA is described herein. The nature of a major by-product formed during the key N-glycosylation of 6-chloro-7-deaza-7-iodopurine with perbenzoylated 2-methyl-ribose under Vorbrüggen conditions was also investigated. Spectroscopic analyses support that the solvent itself is converted into a nucleophilic species competing with the nucleobase and further reacting with the activated riboside in an unanticipated fashion. These findings call for a revision of reaction conditions when working with weakly reactive nucleobases in the presence of Lewis acids. 7DMA thus obtained was evaluated for its efficacy against an emerging flavivirus in vitro.

Usutu virus: An emerging flavivirus with potential threat to public health in Africa: Nigeria as a case study

Usutu virus (USUV) is an arthropod-borne virus (arbovirus) of the flaviviridae family (genus Flavivirus) which belong to the Japanese encephalitis virus complex. Culex mosquitoes have been implicated in the transmission of this pathogen. The major susceptible hosts of USUV are migratory birds, thereby potentiating its ability to spread from one region to another globally. Nigeria has the largest economy in Africa with a significant percentage of the gross domestic product relying on the agricultural and animal production industry. This review explores the zoonotic potentials of the virus in Africa, especially Nigeria, with special focus on the devastating sequelae this might lead to in the future if necessary precautionary policies are not enacted and adopted to bolster the surveillance system for mosquito-borne viruses.

Human Usutu Virus Infections in Europe: A New Risk on Horizon?

The Usutu virus (USUV), a neurotropic mosquito-borne flavivirus discovered in 1959 in South Africa, has spread over the last twenty years across the European continent. This virus follows an enzootic cycle involving mosquitoes and birds. This caused epizootics with significant bird mortality in Europe in 2016 and 2018. It can also occasionally infect humans and other mammals, including horses and bats, which act as incidental or dead-end hosts. The zoonotic risk associated with this succession of avian epizootics in Europe deserves attention, even if, to date, human cases remain exceptional. Human infection is most often asymptomatic or responsible for mild clinical symptoms. However, human Usutu infections have also been associated with neurological disorders, such as encephalitis and meningoencephalitis. One of the major complexities of the study of USUV pathogenesis is the presence of a great diversity of lineages which could co-circulate spatiotemporally. In this review we discuss several aspects of the circulation of Usutu virus in humans in Europe, the neurological disorders associated, involved viral lineages, and the issues and questions raised by their circulation.

Pathogenesis and shedding of Usutu virus in juvenile chickens

Usutu virus (USUV; family: Flaviviridae, genus: Flavivirus), is an emerging zoonotic arbovirus that causes severe neuroinvasive disease in humans and has been implicated in the loss of breeding bird populations in Europe. USUV is maintained in an enzootic cycle between ornithophilic mosquitos and wild birds. As a member of the Japanese encephalitis serocomplex, USUV is closely related to West Nile virus (WNV) and St. Louis encephalitis virus (SLEV), both neuroinvasive arboviruses endemic in wild bird populations in the United States. An avian model for USUV is essential to understanding zoonotic transmission. Here we describe the first avian models of USUV infection with the development of viremia. Juvenile commercial ISA Brown chickens were susceptible to infection by multiple USUV strains with evidence of cardiac lesions. Juvenile chickens from two chicken lines selected for high (HAS) or low (LAS) antibody production against sheep red blood cells showed markedly different responses to USUV infection. Morbidity and mortality were observed in the LAS chickens, but not HAS chickens. LAS chickens had significantly higher viral titers in blood and other tissues, as well as oral secretions, and significantly lower development of neutralizing antibody responses compared to HAS chickens. Mathematical modelling of virus-host interactions showed that the viral clearance rate is a stronger mitigating factor for USUV viremia than neutralizing antibody response in this avian model. These chicken models provide a tool for further understanding USUV pathogenesis in birds and evaluating transmission dynamics between avian hosts and mosquito vectors.

Favipiravir antiviral efficacy against SARS-CoV-2 in a hamster model

Despite no or limited pre-clinical evidence, repurposed drugs are massively evaluated in clinical trials to palliate the lack of antiviral molecules against SARS-CoV-2. Here we use a Syrian hamster model to assess the antiviral efficacy of favipiravir, understand its mechanism of action and determine its pharmacokinetics. When treatment is initiated before or simultaneously to infection, favipiravir has a strong dose effect, leading to reduction of infectious titers in lungs and clinical alleviation of the disease. Antiviral effect of favipiravir correlates with incorporation of a large number of mutations into viral genomes and decrease of viral infectivity. Antiviral efficacy is achieved with plasma drug exposure comparable with those previously found during human clinical trials. Notably, the highest dose of favipiravir tested is associated with signs of toxicity in animals. Thereby, pharmacokinetic and tolerance studies are required to determine whether similar effects can be safely achieved in humans.

A Bacterially-Expressed Recombinant Envelope Protein from Usutu Virus Induces Neutralizing Antibodies in Rabbits

Background: Recently, an emerging flavivirus, Usutu virus (USUV), has caused an epidemic among birds in Europe, resulting in a massive die-off in Eurasian blackbirds. Currently found only in Europe and Africa, it can be envisioned that Usutu virus will follow the path of other flaviviruses, like West Nile virus and Zika virus, and will spread via its mosquito vectors and bird hosts to other parts of the world. Several cases of human infections by Usutu virus have already been published. Anticipating this spread, development of an efficacious vaccine would be highly desirable. Method: This study describes the production in E. coli, purification, and refolding of a partial USUV envelope protein. Prior to immunization, the protein was characterized using size exclusion chromatography, transmission electron microscopy and dynamic light scattering, showing the limited presence of virus-like structures, indicating that the protein solution is probably a mixture of mono and multimeric envelope proteins. Results: Immunizations of two rabbits with the refolded E-protein fraction, mixed with a strong adjuvant, resulted in the generation of neutralizing antibodies, as evidenced in an in vitro assay. Discussion: The way forward towards a subunit vaccine against Usutu virus infection is discussed.

A review of medications used to control and improve the signs and symptoms of COVID-19 patients

In December 2019, an unprecedented outbreak of pneumonia associated with a novel coronavirus disease 2019 (COVID-19) emerged in Wuhan City, Hubei province, China. The virus that caused the disease was officially named by the World Health Organization (WHO) as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). According to the high transmission rate of SARS-CoV-2, it became a global pandemic and public health emergency within few months. Since SARS-CoV-2 is genetically 80% homologous with the SARS-CoVs family, it is hypothesized that medications developed for the treatment of SARS-CoVs may be useful in the control and management of SARS-CoV-2. In this regard, some medication being tested in clinical trials and in vitro studies include anti-viral RNA polymerase inhibitors, HIV-protease inhibitors, anti-inflammatory agents, angiotensin converting enzyme type 2 (ACE 2) blockers, and some other novel medications. In this communication, we reviewed the general characteristics of medications, medical usage, mechanism of action, as well as SARS-CoV-2 related trials.

Differential pathogenesis of Usutu virus isolates in mice

Usutu virus (USUV; Flavivirus), a close phylogenetic and ecological relative of West Nile virus, is a zoonotic virus that can cause neuroinvasive disease in humans. USUV is maintained in an enzootic cycle between Culex mosquitoes and birds. Since the first isolation in 1959 in South Africa, USUV has spread throughout Africa and Europe. Reported human cases have increased over the last few decades, primarily in Europe, with symptoms ranging from mild febrile illness to severe neurological effects. In this study, we investigated whether USUV has become more pathogenic during emergence in Europe. Interferon α/β receptor knockout (Ifnar1-/-) mice were inoculated with recent USUV isolates from Africa and Europe, as well as the historic 1959 South African strain. The three tested African strains and one European strain from Spain caused 100% mortality in inoculated mice, with similar survival times and histopathology in tissues. Unexpectedly, a European strain from the Netherlands caused only 12% mortality and significantly less histopathology in tissues from mice compared to mice inoculated with the other strains. Viremia was highest in mice inoculated with the recent African strains and lowest in mice inoculated with the Netherlands strain. Based on phylogenetics, the USUV isolates from Spain and the Netherlands were derived from separate introductions into Europe, suggesting that disease outcomes may differ for USUV strains circulating in Europe. These results also suggest that while more human USUV disease cases have been reported in Europe recently, circulating African USUV strains are still a potential major health concern.

In Vitro and In Vivo Models to Study the Zoonotic Mosquito-Borne Usutu Virus

Usutu virus (USUV), a mosquito-borne zoonotic flavivirus discovered in South Africa in 1959, has spread to many European countries over the last 20 years. The virus is currently a major concern for animal health due to its expanding host range and the growing number of avian mass mortality events. Although human infections with USUV are often asymptomatic, they are occasionally accompanied by neurological complications reminiscent of those due to West Nile virus (another flavivirus closely related to USUV). Whilst USUV actually appears less threatening than some other emergent arboviruses, the lessons learned from Chikungunya, Dengue, and Zika viruses during the past few years should not be ignored. Further, it would not be surprising if, with time, USUV disperses further eastwards towards Asia and possibly westwards to the Americas, which may result in more pathogenic USUV strains to humans and/or animals. These observations, inviting the scientific community to be more vigilant about the spread and genetic evolution of USUV, have prompted the use of experimental systems to understand USUV pathogenesis and to boost the development of vaccines and antivirals. This review is the first to provide comprehensive coverage of existing in vitro and in vivo models for USUV infection and to discuss their contribution in advancing data concerning this neurotropic virus. We believe that this paper is a helpful tool for scientists to identify gaps in the knowledge about USUV and to design their future experiments to study the virus.

Pharmacological treatments of COVID-19

The viral infection due to the new coronavirus or coronavirus disease 2019 (COVID-19), which was reported for the first time in December 2019, was named by the World Health Organization (WHO) as Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV2), because of the very similar genome and also its related symptoms to SARS-CoV1. The ongoing COVID-19 pandemic with significant mortality, morbidity, and socioeconomic impact is considered by the WHO as a global public health emergency. Since there is no specific treatment available for SARS-CoV2 infection, and or COVID-19, several clinical and sub-clinical studies are currently undertaken to find a gold-standard therapeutic regimen with high efficacy and low side effect. Based on the published scientific evidence published to date, we summarized herein the effects of different potential therapies and up-to-date clinical trials. The review is intended to help readers aware of potentially effective COVID-19 treatment and provide useful references for future studies.

Small-molecule inhibitors of TBK1 serve as an adjuvant for a plasmid-launched live-attenuated yellow fever vaccine

Plasmid-launched live-attenuated vaccines (PLLAV), also called infectious DNA (iDNA) vaccines, combine the assets of genetic immunization with the potency of replication-competent live viral vaccines. However, due to their origin as bacterial plasmid DNA, efficient delivery of PLLAV may be hampered by innate signaling pathways such as the cGAS-STING-mediated sensing of cytosolic DNA, resulting in an unfavorable proinflammatory and antiviral response locally at the site of immunization. Employing several complementary cell-based systems and using the yellow fever vaccine (YF17D) and the respective PLLAV-YF17D, we screened a panel of small molecules known to interfere with antiviral signaling for their proviral activity and identified two potent inhibitors of the TANK-binding kinase 1 (TBK1), BX795 and CYT387, to enhance YF17D replication and hence efficacy of PLLAV-YF17D transfection. In tissue culture, BX795 could fully revert the block that plasmid transfection poses on YF17D infection in a type I interferon dependent manner, as confirmed by (i) a marked change in gene expression signatures, (ii) a rescue of full YF17D replication, and (iii) a massively increased virus yield. Inhibitors of TBK1 may hence be considered an adjuvant to potentiate novel PLLAV vaccines, which might boost PLLAV delivery toward their use in vivo.

Usutu Virus Infection of Embryonated Chicken Eggs and a Chicken Embryo-Derived Primary Cell Line

Usutu virus (USUV) is a mosquito-borne flavivirus, closely related to the West Nile virus (WNV). Similar to WNV, USUV may cause infections in humans, with occasional, but sometimes severe, neurological complications. Further, USUV can be highly pathogenic in wild and captive birds and its circulation in Europe has given rise to substantial avian death. Adequate study models of this virus are still lacking but are critically needed to understand its pathogenesis and virulence spectrum. The chicken embryo is a low-cost, easy-to-manipulate and ethically acceptable model that closely reflects mammalian fetal development and allows immune response investigations, drug screening, and high-throughput virus production for vaccine development. While former studies suggested that this model was refractory to USUV infection, we unexpectedly found that high doses of four phylogenetically distinct USUV strains caused embryonic lethality. By employing immunohistochemistry and quantitative reverse transcriptase-polymerase chain reaction, we demonstrated that USUV was widely distributed in embryonic tissues, including the brain, retina, and feather follicles. We then successfully developed a primary cell line from the chorioallantoic membrane that was permissive to the virus without the need for viral adaptation. We believe the future use of these models would foster a significant understanding of USUV-induced neuropathogenesis and immune response and allow the future development of drugs and vaccines against USUV.

Comprehensive response to Usutu virus following first isolation in blood donors in the Friuli Venezia Giulia region of Italy: Development of recombinant NS1-based serology and sensitivity to antiviral drugs

Surveillance of Usutu virus is crucial to prevent future outbreaks both in Europe and in other countries currently naïve to the infection, such as the Americas. This goal remains difficult to achieve, notably because of the lack of large-scale cohort studies and the absence of commercially available diagnostic reagents for USUV.

This work started with the first identification of USUV in a blood donor in the Friuli Venezia Giulia (FVG) Region in Northern-Eastern Italy, which is endemic for West Nile virus. Considering that only one IgG ELISA is commercially available, but none for IgM, a novel NS1 antigen based IgG/M ELISA has been developed. This assay tested successfully for the detection of Usutu virus in blood donors with the identification of a second case of transmission and high levels of exposure. Furthermore, two pan-flavivirus antiviral drugs, that we previously characterized to be inhibitors of other flavivirus infectivity, were successfully tested for inhibition of Usutu virus with inhibitory concentrations in the low micromolar range.

To conclude, this work identifies North-Eastern Italy as endemic for Usutu virus with implications for the screening of transfusion blood. A novel NS1-based ELISA test has been implemented for the detection of IgM/G that will be of importance as a tool for the diagnosis and surveillance of Usutu virus infection. Finally, Usutu virus is shown to be sensitive to a class of promising pan-flavivirus drugs.

Tropical viruses transmitted by ticks or mosquitoes are becoming a health threat in areas of the world that were previously naïve to these infections. Usutu virus is a mosquito-borne virus that is circulating in Europe causing massive outbreaks in birds. Transmission to humans is documented, with some reports of severe neurological disease. However, the real size of transmission to humans suffers from lack of data due to insufficient surveillance. The first confirmed case of human USUV infection in an asymptomatic blood donor from North-Eastern Italy is hereby demonstrated by molecular assays and virus isolation. Specific Usutu virus serology has also been developed taking advantage of the NS1 viral antigen, which is tested on a number of blood donors demonstrating a high level of Usutu positivity. These findings confirm the human transmission in the region and offer a novel tool for specific Usutu virus surveillance. Finally, two drugs that were previously shown to have a wide spectrum of activity towards members of this family of viruses are shown to inhibit also Usutu virus, opening the way to a novel class antivirals.

Antivirals in medical biodefense

The viruses historically implicated or currently considered as candidates for misuse in bioterrorist events are poxviruses, filoviruses, bunyaviruses, orthomyxoviruses, paramyxoviruses and a number of arboviruses causing encephalitis, including alpha- and flaviviruses. All these viruses are of concern for public health services when they occur in natural outbreaks or emerge in unvaccinated populations. Recent events and intelligence reports point to a growing risk of dangerous biological agents being used for nefarious purposes. Public health responses effective in natural outbreaks of infectious disease may not be sufficient to deal with the severe consequences of a deliberate release of such agents. One important aspect of countermeasures against viral biothreat agents are the antiviral treatment options available for use in post-exposure prophylaxis. These issues were adressed by the organizers of the 16th Medical Biodefense Conference, held in Munich in 2018, in a special session on the development of drugs to treat infections with viruses currently perceived as a threat to societies or associated with a potential for misuse as biothreat agents. This review will outline the state-of-the-art methods in antivirals research discussed and provide an overview of antiviral compounds in the pipeline that are already approved for use or still under development.

New Insights into the Susceptibility of Immunocompetent Mice to Usutu Virus

Usutu virus (USUV) is a mosquito-borne flavivirus that shares many similarities with the closely related West Nile virus (WNV) in terms of ecology and clinical manifestations. Initially distributed in Africa, USUV emerged in Italy in 1996 and managed to co-circulate with WNV in many European countries in a similar mosquito-bird life cycle. The rapid geographic spread of USUV, the seasonal mass mortalities it causes in the European avifauna, and the increasing number of infections with neurological disease both in healthy and immunocompromised humans has stimulated interest in infection studies to delineate USUV pathogenesis. Here, we assessed the pathogenicity of two USUV isolates from a recent Belgian outbreak in immunocompetent mice. The intradermal injection of USUV gave rise to disorientation and paraplegia and was associated with neuronal death in the brain and spinal cord in a single mouse. Intranasal inoculation of USUV could also establish the infection; viral RNA was detected in the brain 15 days post-infection. Overall, this pilot study probes the suitability of this murine model for the study of USUV neuroinvasiveness and the possibility of direct transmission in mammals.

Usutu virus disease: a potential problem for North America?

Usutu virus is an emerging mosquito-borne flavivirus initially identified in South Africa in 1959 that is now circulating throughout parts of Africa, Europe, and the Middle East. It is closely related to West Nile virus, and has similar vectors, amplifying bird hosts, and epidemiology. Usutu virus infection can occur in humans and may be asymptomatic or cause systemic (e.g., fever, rash, and hepatitis) or neuroinvasive (e.g., meningitis and encephalitis) disease. Given few reported cases, the full clinical spectrum is not known. No anti-viral treatment is available, but it can be largely prevented by avoiding mosquito bites. Because of similar mosquitoes, birds, and climate to Europe, the potential for introduction to North America is possible.

Usutu Virus: An Arbovirus on the Rise

The Usutu virus (USUV) is a flavivirus that is drawing increasing attention because of its potential for emergence. First isolated in Africa, it was introduced into Europe where it caused significant outbreaks in birds, such as in Austria in 2001. Since then, its geographical distribution has rapidly expanded, with increased circulation, especially in the last few years. Similar to West Nile virus (WNV), the USUV enzootic transmission cycle involves Culex mosquitoes as vectors, and birds as amplifying reservoir hosts, with humans and other mammals likely being dead-end hosts. A similarity in the ecology of these two viruses, which co-circulate in several European countries, highlights USUV’s potential to become an important human pathogen. While USUV has had a severe impact on the blackbird population, the number of human cases remains low, with most infections being asymptomatic. However, some rare cases of neurological disease have been described, both in healthy and immuno-compromised patients. Here, we will discuss the transmission dynamics and the current state of USUV circulation in Europe.

Establishment of a Cell Culture Model of Persistent Flaviviral Infection: Usutu Virus Shows Sustained Replication during Passages and Resistance to Extinction by Antiviral Nucleosides

Chronic viral disease constitutes a major global health problem, with several hundred million people affected and an associated elevated number of deaths. An increasing number of disorders caused by human flaviviruses are related to their capacity to establish a persistent infection. Here we show that Usutu virus (USUV), an emerging zoonotic flavivirus linked to sporadic neurologic disease in humans, can establish a persistent infection in cell culture. Two independent lineages of Vero cells surviving USUV lytic infection were cultured over 82 days (41 cell transfers) without any apparent cytopathology crisis associated. We found elevated titers in the supernatant of these cells, with modest fluctuations during passages but no overall tendency towards increased or decreased infectivity. In addition to full-length genomes, viral RNA isolated from these cells at passage 40 revealed the presence of defective genomes, containing different deletions at the 5' end. These truncated transcripts were all predicted to encode shorter polyprotein products lacking membrane and envelope structural proteins, and most of non-structural protein 1. Treatment with different broad-range antiviral nucleosides revealed that USUV is sensitive to these compounds in the context of a persistent infection, in agreement with previous observations during lytic infections. The exposure of infected cells to prolonged treatment (10 days) with favipiravir and/or ribavirin resulted in the complete clearance of infectivity in the cellular supernatants (decrease of ~5 log10 in virus titers and RNA levels), although modest changes in intracellular viral RNA levels were recorded (<2 log10 decrease). Drug withdrawal after treatment day 10 resulted in a relapse in virus titers. These results encourage the use of persistently-infected cultures as a surrogate system in the identification of improved antivirals against flaviviral chronic disease.

Status of antiviral therapeutics against rabies virus and related emerging lyssaviruses

Rabies virus (RABV) constitutes a major social and economic burden associated with 60 000 deaths annually worldwide. Although pre-exposure and post-exposure treatment options are available, they are efficacious only when initiated before the onset of clinical symptoms. Aggravating the problem, the current RABV vaccine does not cross-protect against the emerging zoonotic phylogroup II lyssaviruses. A requirement for an uninterrupted cold chain and high cost of the immunoglobulin component of rabies prophylaxis generate an unmet need for the development of RABV-specific antivirals. We discuss desirable anti-RABV drug profiles, past efforts to address the problem and inhibitor candidates identified, and examine how the rapidly expanding structural insight into RABV protein organization has illuminated novel druggable target candidates and paved the way to structure-aided drug optimization. Special emphasis is given to the viral RNA-dependent RNA polymerase complex as a promising target for direct-acting broad-spectrum RABV inhibitors.