Treatment of highly virulent mammarenavirus infections-status quo and future directions

INTRODUCTION

Mammarenaviruses are negative-sense bisegmented enveloped RNA viruses that are endemic in Africa, the Americas, and Europe. Several are highly virulent, causing acute human diseases associated with high case fatality rates, and are considered to be significant with respect to public health impact or bioterrorism threat.

AREAS COVERED

This review summarizes the status quo of treatment development, starting with drugs that are in advanced stages of evaluation in early clinical trials, followed by promising candidate medical countermeasures emerging from bench analyses and investigational animal research.

EXPERT OPINION

Specific therapeutic treatments for diseases caused by mammarenaviruses remain limited to the off-label use of ribavirin and transfusion of convalescent sera. Progress in identifying novel candidate medical countermeasures against mammarenavirus infection has been slow in part because of the biosafety and biosecurity requirements. However, novel methodologies and tools have enabled increasingly efficient high-throughput molecular screens of regulatory-agency-approved small-molecule drugs and led to the identification of several compounds that could be repurposed for the treatment of infection with several mammarenaviruses. Unfortunately, most of them have not yet been evaluated in vivo. The most promising treatment under development is a monoclonal antibody cocktail that is protective against multiple lineages of the Lassa virus in nonhuman primate disease models.

Potent inhibition of arenavirus infection by a novel fusion inhibitor

Several arenaviruses, including Lassa and Lujo viruses in Africa and five New World arenavirus (NWA) species in the Americas, cause life-threatening viral hemorrhagic fevers. In the absence of licensed antiviral therapies, these viruses pose a significant public health risk. The envelope glycoprotein complex (GPC) mediates arenavirus entry through a pH-dependent fusion of the viral and host endosomal membranes. It thus is recognized as a viable target for small-molecule fusion inhibitors. Here, we report on the antiviral activity and pre-clinical development of the novel broad-spectrum arenavirus fusion inhibitors, ARN-75039 and ARN-75041. In Tacaribe virus (TCRV) pseudotyped and native virus assays, the ARN compounds were active in the low to sub-nanomolar range with selectivity indices exceeding 1000. Pharmacokinetic analysis of the orally administered compounds revealed an extended half-life in mice supporting once-daily dosing, and the compounds were well tolerated at the highest tested dose of 100 mg/kg. In a proof-of-concept prophylactic efficacy study, doses of 10 and 35 mg/kg of either compound dramatically improved survival outcome and potently inhibited TCRV replication in serum and various tissues. Additionally, in contrast to surviving mice that received ribavirin or placebo, animals treated with ARN-75039 or ARN-75041 were cured of TCRV infection. In a follow-up study with ARN-75039, impressive therapeutic efficacy was demonstrated under conditions where treatment was withheld until after the onset of disease. Taken together, the data strongly support the continued development of ARN-75039 as a candidate therapeutic for the treatment of severe arenaviral diseases.

Prevention of CD8 T Cell Deletion during Chronic Viral Infection

During chronic viral infections, CD8 T cells rapidly lose antiviral and immune-stimulatory functions in a sustained program termed exhaustion. In addition to this loss of function, CD8 T cells with the highest affinity for viral antigen can be physically deleted. Consequently, treatments designed to restore function to exhausted cells and control chronic viral replication are limited from the onset by the decreased breadth of the antiviral T cell response. Yet, it remains unclear why certain populations of CD8 T cells are deleted while others are preserved in an exhausted state. We report that CD8 T cell deletion during chronic viral infection can be prevented by therapeutically lowering viral replication early after infection. The initial resistance to deletion enabled long-term maintenance of antiviral cytolytic activity of the otherwise deleted high-affinity CD8 T cells. In combination with decreased virus titers, CD4 T cell help and prolonged interactions with costimulatory molecules B7-1/B7-2 were required to prevent CD8 T cell deletion. Thus, therapeutic strategies to decrease early virus replication could enhance virus-specific CD8 T cell diversity and function during chronic infection.

Inhibition of Arenaviruses by Combinations of Orally Available Approved Drugs

Neglected diseases caused by arenaviruses such as Lassa virus (LASV) and filoviruses like Ebola virus (EBOV) primarily afflict resource-limited countries, where antiviral drug development is often minimal. Previous studies have shown that many approved drugs developed for other clinical indications inhibit EBOV and LASV and that combinations of these drugs provide synergistic suppression of EBOV, often by blocking discrete steps in virus entry. We hypothesize that repurposing of combinations of orally administered approved drugs provides effective suppression of arenaviruses. In this report, we demonstrate that arbidol, an approved influenza antiviral previously shown to inhibit EBOV, LASV, and many other viruses, inhibits murine leukemia virus (MLV) reporter viruses pseudotyped with the fusion glycoproteins (GPs) of other arenaviruses (Junin virus [JUNV], lymphocytic choriomeningitis virus [LCMV], and Pichinde virus [PICV]). Arbidol and other approved drugs, including aripiprazole, amodiaquine, sertraline, and niclosamide, also inhibit infection of cells by infectious PICV, and arbidol, sertraline, and niclosamide inhibit infectious LASV. Combining arbidol with aripiprazole or sertraline results in the synergistic suppression of LASV and JUNV GP-bearing pseudoviruses. This proof-of-concept study shows that arenavirus infection in vitro can be synergistically inhibited by combinations of approved drugs. This approach may lead to a proactive strategy with which to prepare for and control known and new arenavirus outbreaks.

Identification of a clinical compound losmapimod that blocks Lassa virus entry

Lassa virus (LASV) causes Lassa hemorrhagic fever in humans and poses a significant threat to public health in West Africa. Current therapeutic treatments for Lassa fever are limited, making the development of novel countermeasures an urgent priority. In this study, we identified losmapimod, a p38 mitogen-activated protein kinase (MAPK) inhibitor, from 102 screened compounds as an inhibitor of LASV infection. Losmapimod exerted its inhibitory effect against LASV after p38 MAPK down-regulation, and, interestingly, had no effect on other arenaviruses capable of causing viral hemorrhagic fever. Mechanistic studies showed that losmapimod inhibited LASV entry by affecting the stable signal peptide (SSP)-GP2 subunit interface of the LASV glycoprotein, thereby blocking pH-dependent viral fusion. As an aryl heteroaryl bis-carboxyamide derivative, losmapimod represents a novel chemical scaffold with anti-LASV activity, and it provides a new lead structure for the future development of LASV fusion inhibitors.

Type I interferon signaling attenuates regulatory T cell function in viral infection and in the tumor microenvironment

Regulatory T cells (Tregs) play a cardinal role in the immune system by suppressing detrimental autoimmune responses, but their role in acute, chronic infectious diseases and tumor microenvironment remains unclear. We recently demonstrated that IFN-α/β receptor (IFNAR) signaling promotes Treg function in autoimmunity. Here we dissected the functional role of IFNAR-signaling in Tregs using Treg-specific IFNAR deficient (IFNAR^fl/flxFoxp3^YFP-Cre) mice in acute LCMV Armstrong, chronic Clone-13 viral infection, and in tumor models. In both viral infection and tumor models, IFNAR^fl/flxFoxp3^YFP-Cre mice Tregs expressed enhanced Treg associated activation antigens. LCMV-specific CD8⁺ T cells and tumor infiltrating lymphocytes from IFNAR^fl/flxFoxp3^YFP-Cre mice produced less antiviral and antitumor IFN-γ and TNF-α. In chronic viral model, the numbers of antiviral effector and memory CD8⁺ T cells were decreased in IFNAR^fl/flxFoxp3^YFP-Cre mice and the effector CD4⁺ and CD8⁺ T cells exhibited a phenotype compatible with enhanced exhaustion. IFNAR^fl/flxFoxp3^YFP-Cre mice cleared Armstrong infection normally, but had higher viral titers in sera, kidneys and lungs during chronic infection, and higher tumor burden than the WT controls. The enhanced activated phenotype was evident through transcriptome analysis of IFNAR^fl/flxFoxp3^YFP-Cre mice Tregs during infection demonstrated differential expression of a unique gene signature characterized by elevated levels of genes involved in suppression and decreased levels of genes mediating apoptosis. Thus, IFN signaling in Tregs is beneficial to host resulting in a more effective antiviral response and augmented antitumor immunity.

Type I interferons (IFNs) play a predominant role in the immune response to infectious pathogens. The cellular targets of IFNs have been difficult to dissect because of the ubiquitous expression of the type I interferon receptor (IFNAR). The immune response of mice to lymphocytic choriomeningitis virus (LCMV) is one of the major models for analyzing the action of IFNs. Regulatory T cells (Tregs) have been implicated in the control of LCMV and it has been proposed that IFN may influence their function. The major goal of this study was to define the contribution of IFN signaling on Treg function during different stages LCMV infection. Tregs from mice with selective deletion of IFNAR manifested enhanced suppressive activity during acute/chronic LCMV infection resulting in increased CD8 T cell anergy, defective generation of memory T cells and persistence of virus. Similar effects of IFNAR signaling in Tregs were seen in a tumor model. We identified a unique set of genes in Tregs modulated by IFN signaling that may contribute to the enhanced suppressive function of IFNAR deficient Tregs. IFNs play a beneficial role during acute/chronic viral infections not only by contributing to viral clearance but also by attenuating the function of Tregs.

Arenavirus Quasispecies and Their Biological Implications

The family Arenaviridae currently comprises over 20 viral species, each of them associated with a main rodent species as the natural reservoir and in one case possibly phyllostomid bats. Moreover, recent findings have documented a divergent group of arenaviruses in captive alethinophidian snakes. Human infections occur through mucosal exposure to aerosols or by direct contact of abraded skin with infectious materials. Arenaviruses merit interest both as highly tractable experimental model systems to study acute and persistent infections and as clinically important human pathogens including Lassa (LASV) and Junin (JUNV) viruses, the causative agents of Lassa and Argentine hemorrhagic fevers (AHFs), respectively, for which there are no FDA-licensed vaccines, and current therapy is limited to an off-label use of ribavirin (Rib) that has significant limitations. Arenaviruses are enveloped viruses with a bi-segmented negative strand (NS) RNA genome. Each genome segment, L (ca 7.3 kb) and S (ca 3.5 kb), uses an ambisense coding strategy to direct the synthesis of two polypeptides in opposite orientation, separated by a noncoding intergenic region (IGR). The S genomic RNA encodes the virus nucleoprotein (NP) and the precursor (GPC) of the virus surface glycoprotein that mediates virus receptor recognition and cell entry via endocytosis. The L genome RNA encodes the viral RNA-dependent RNA polymerase (RdRp, or L polymerase) and the small (ca 11 kDa) RING finger protein Z that has functions of a bona fide matrix protein including directing virus budding. Arenaviruses were thought to be relatively stable genetically with intra- and interspecies amino acid sequence identities of 90-95 % and 44-63 %, respectively. However, recent evidence has documented extensive arenavirus genetic variability in the field. Moreover, dramatic phenotypic differences have been documented among closely related LCMV isolates. These data provide strong evidence of viral quasispecies involvement in arenavirus adaptability and pathogenesis. Here, we will review several aspects of the molecular biology of arenaviruses, phylogeny and evolution, and quasispecies dynamics of arenavirus populations for a better understanding of arenavirus pathogenesis, as well as for the development of novel antiviral strategies to combat arenavirus infections.

Clinical features and patient management of Lujo hemorrhagic fever

BACKGROUND

In 2008 a nosocomial outbreak of five cases of viral hemorrhagic fever due to a novel arenavirus, Lujo virus, occurred in Johannesburg, South Africa. Lujo virus is only the second pathogenic arenavirus, after Lassa virus, to be recognized in Africa and the first in over 40 years. Because of the remote, resource-poor, and often politically unstable regions where Lassa fever and other viral hemorrhagic fevers typically occur, there have been few opportunities to undertake in-depth study of their clinical manifestations, transmission dynamics, pathogenesis, or response to treatment options typically available in industrialized countries.

METHODS AND FINDINGS

We describe the clinical features of five cases of Lujo hemorrhagic fever and summarize their clinical management, as well as providing additional epidemiologic detail regarding the 2008 outbreak. Illness typically began with the abrupt onset of fever, malaise, headache, and myalgias followed successively by sore throat, chest pain, gastrointestinal symptoms, rash, minor hemorrhage, subconjunctival injection, and neck and facial swelling over the first week of illness. No major hemorrhage was noted. Neurological signs were sometimes seen in the late stages. Shock and multi-organ system failure, often with evidence of disseminated intravascular coagulopathy, ensued in the second week, with death in four of the five cases. Distinctive treatment components of the one surviving patient included rapid commencement of the antiviral drug ribavirin and administration of HMG-CoA reductase inhibitors (statins), N-acetylcysteine, and recombinant factor VIIa.

CONCLUSIONS

Lujo virus causes a clinical syndrome remarkably similar to Lassa fever. Considering the high case-fatality and significant logistical impediments to controlled treatment efficacy trials for viral hemorrhagic fever, it is both logical and ethical to explore the use of the various compounds used in the treatment of the surviving case reported here in future outbreaks. Clinical observations should be systematically recorded to facilitate objective evaluation of treatment efficacy. Due to the risk of secondary transmission, viral hemorrhagic fever precautions should be implemented for all cases of Lujo virus infection, with specialized precautions to protect against aerosols when performing enhanced-risk procedures such as endotracheal intubation.

Inhibition of multiplication of the prototypic arenavirus LCMV by valproic acid

Valproic acid (VPA), a short chain fatty acid commonly used for treatment of neurological disorders, has been shown to inhibit production of infectious progeny of different enveloped viruses including the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV). In this study we have investigated the mechanisms by which VPA inhibits LCMV multiplication in cultured cells. VPA reduced production of infectious LCMV progeny and virus propagation without exerting a major blockage on either viral RNA or protein synthesis, but rather affecting the cell release and specific infectivity of LCMV progeny from infected cells. Our results would support the repurposing of VPA as a candidate antiviral drug to combat arenavirus infections.

Multifunctional nature of the arenavirus RING finger protein Z

Arenaviruses are a family of enveloped negative-stranded RNA viruses that can cause severe human disease ranging from encephalitis symptoms to fulminant hemorrhagic fever. The bi‑segmented RNA genome encodes four polypeptides: the nucleoprotein NP, the surface glycoprotein GP, the polymerase L, and the RING finger protein Z. Although it is the smallest arenavirus protein with a length of 90 to 99 amino acids and a molecular weight of approx. 11 kDa, the Z protein has multiple functions in the viral life cycle including (i) regulation of viral RNA synthesis, (ii) orchestration of viral assembly and budding, (iii) interaction with host cell proteins, and (iv) interferon antagonism. In this review, we summarize our current understanding of the structural and functional role of the Z protein in the arenavirus replication cycle.

Arenaviruses and lethal mutagenesis. Prospects for new ribavirin-based interventions

Lymphocytic choriomeningitis virus (LCMV) has contributed to unveil some of the molecular mechanisms of lethal mutagenesis, or loss of virus infectivity due to increased mutation rates. Here we review these developments, and provide additional evidence that ribavirin displays a dual mutagenic and inhibitory activity on LCMV that can be relevant to treatment designs. Using 5-fluorouracil as mutagenic agent and ribavirin either as inhibitor or mutagen, we document an advantage of a sequential inhibitor-mutagen administration over the corresponding combination treatment to achieve a low LCMV load in cell culture. This advantage is accentuated in the concentration range in which ribavirin acts mainly as an inhibitor, rather than as mutagen. This observation reinforces previous theoretical and experimental studies in supporting a sequential inhibitor-mutagen administration as a possible antiviral design. Given recent progress in the development of new inhibitors of arenavirus replication, our results suggest new options of ribavirin-based anti-arenavirus treatments.

Arenaviruses and hantaviruses: from epidemiology and genomics to antivirals

The arenaviruses and hantaviruses are segmented genome RNA viruses that are hosted by rodents. Due to their association with rodents, they are globally widespread and can infect humans via direct or indirect routes of transmission, causing considerable human morbidity and mortality. Nevertheless, despite their obvious and emerging importance as pathogens, there are currently no effective antiviral drugs (except ribavirin which proved effective against Lassa virus) with which to treat humans infected by any of these viruses. The EU-funded VIZIER project (Comparative Structural Genomics of Viral Enzymes Involved in Replication) was instigated with an ultimate view of contributing to the development of antiviral therapies for RNA viruses, including the arenaviruses and bunyaviruses. This review highlights some of the major features of the arenaviruses and hantaviruses that have been investigated during recent years. After describing their classification and epidemiology, we review progress in understanding the genomics as well as the structure and function of replicative enzymes achieved under the VIZIER program and the development of new disease control strategies.

Characterization of Junín virus particles inactivated by a zinc finger-reactive compound

Our previous studies reported the inhibitory action against arenaviruses of antiretroviral zinc finger-reactive compounds provided by the National Cancer Institute (USA). These compounds were able to inactivate virions as well as to reduce virus yields from infected cells. Here, the inactivation of the arenavirus Junín (JUNV), agent of Argentine hemorrhagic fever, by the aromatic disulfide NSC20625 was analyzed. The treatment of purified JUNV with this compound eliminated infectivity apparently through irreversible modifications in the matrix Z protein detected by: (a) alterations in the electrophoretic migration profile of Z under non-reducing conditions; (b) an electrodense labeling in the internal layer beneath the envelope and around the matrix Z protein, in negatively stained preparations; (c) changes in the subcellular localization of Z in cells transfected with a recombinant fusion protein JUNVZ-eGFP. The infection of Vero cells with JUNV inactivated particles was blocked at the uncoating of viral nucleocapsid from endosomes, providing new evidence for a functional role of Z in this stage of arenavirus cycle. Furthermore, the inactivated JUNV particles retained the immunoreactivity of the surface glycoprotein GP1 suggesting that this disulfide may be useful in the pursuit of an inactivating agent to obtain a vaccine antigen or diagnostic tool.

Reverse genetics approaches to combat pathogenic arenaviruses

Several arenaviruses cause hemorrhagic fever (HF) in humans, and evidence indicates that the worldwide-distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected human pathogen of clinical significance. Moreover, arenaviruses pose a biodefense threat. No licensed anti-arenavirus vaccines are available, and current anti-arenavirus therapy is limited to the use of ribavirin, which is only partially effective and is associated with anemia and other side effects. Therefore, it is important to develop effective vaccines and better antiviral drugs to combat the dual threats of naturally occurring and intentionally introduced arenavirus infections. The development of arenavirus reverse genetic systems is allowing investigators to conduct a detailed molecular characterization of the viral cis-acting signals and trans-acting factors that control each of the steps of the arenavirus life cycle, including RNA synthesis, packaging and budding. Knowledge derived from these studies is uncovering potential novel targets for therapeutic intervention, as well as facilitating the establishment of assays to identify and characterize candidate antiviral drugs capable of interfering with specific steps of the virus life cycle. Likewise, the ability to generate predetermined specific mutations within the arenavirus genome and analyze their phenotypic expression would significantly contribute to the elucidation of arenavirus-host interactions, including the basis of their ability to cause severe HF. This, in turn, could lead to the development of novel, potent and safe arenavirus vaccines.

Treatment of late stage disease in a model of arenaviral hemorrhagic fever: T-705 efficacy and reduced toxicity suggests an alternative to ribavirin

A growing number of arenaviruses are known to cause viral hemorrhagic fever (HF), a severe and life-threatening syndrome characterized by fever, malaise, and increased vascular permeability. Ribavirin, the only licensed antiviral indicated for the treatment of certain arenaviral HFs, has had mixed success and significant toxicity. Since severe arenaviral infections initially do not present with distinguishing symptoms and are difficult to clinically diagnose at early stages, it is of utmost importance to identify antiviral therapies effective at later stages of infection. We have previously reported that T-705, a substituted pyrazine derivative currently under development as an anti-influenza drug, is highly active in hamsters infected with Pichinde virus when the drug is administered orally early during the course of infection. Here we demonstrate that T-705 offers significant protection against this lethal arenaviral infection in hamsters when treatment is begun after the animals are ill and the day before the animals begin to succumb to disease. Importantly, this coincides with the time when peak viral loads are present in most organs and considerable tissue damage is evident. We also show that T-705 is as effective as, and less toxic than, ribavirin, as infected T-705-treated hamsters on average maintain their weight better and recover more rapidly than animals treated with ribavirin. Further, there was no added benefit to combination therapy with T-705 and ribavirin. Finally, pharmacokinetic data indicate that plasma T-705 levels following oral administration are markedly reduced during the latter stages of disease, and may contribute to the reduced efficacy seen when treatment is withheld until day 7 of infection. Our findings support further pre-clinical development of T-705 for the treatment of severe arenaviral infections.

Synthesis and evaluation of N-substituted acridones as antiviral agents against haemorrhagic fever viruses

BACKGROUND

In the present study, a series of N-substituted acridone derivatives was synthesized and evaluated against two haemorrhagic fever viruses (HFV).

METHODS

Compounds were tested against Junin virus (JUNV), an arenavirus agent of Argentine haemorrhagic fever, and dengue virus (DENV), a flavivirus agent of the most prevalent arthropod-borne viral disease in humans.

RESULTS

Among tested compounds, two N-allyl acridones (derivatives 3c and 3f) elicited a potent and selective antiviral activity against JUNV (strain 1V4454) and DENV-2 (strain NGC) with 50% effective concentration values between 2.5 and 5.5 microM, as determined by virus yield inhibition. No cytotoxicity was detected at concentrations up to 1,000 microM, resulting in selectivity indices >181.8-400.0. Both acridones were effective against a wide spectrum of arenaviruses and the four serotypes of DENV. Furthermore, 3c and 3f failed to inactivate virus before cell infection as well as to induce a refractory state by cell pretreatment, indicating that the inhibitory effect was exerted through a blockade in virus multiplication during the infectious process.

CONCLUSION

These data are the first demonstration that acridone derivatives have a potent antiviral activity that block in vitro multiplication of HFV belonging to Arenaviridae and Flaviviridae, such as JUNV and DENV.

In vitro and in vivo activities of T-705 against arenavirus and bunyavirus infections

There is a need for the development of effective antivirals for the treatment of severe viral diseases caused by members of the virus families Bunyaviridae and Arenaviridae. The pyrazine derivative T-705 (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) has demonstrated remarkable antiviral activity against influenza virus and, to a lesser degree, against some other RNA viruses (Y. Furuta, K. Takahashi, Y. Fukuda, M. Kuno, T. Kamiyama, K. Kozaki, N. Nomura, H. Egawa, S. Minami, Y. Watanabe, H. Narita, and K. Shiraki, Antimicrob. Agents Chemother., 46:977-981, 2002). Here, we report that T-705 is highly active against a panel of bunyaviruses (La Crosse, Punta Toro, Rift Valley fever, and sandfly fever viruses) and arenaviruses (Junin, Pichinde, and Tacaribe viruses) by cytopathic effect and virus yield reduction cell-based assays. The 50% effective concentrations for T-705 ranged from 5 to 30 microg/ml and 0.7 to 1.2 microg/ml against the bunyaviruses and arenaviruses examined, respectively. We also demonstrate that orally administered T-705 is efficacious in treating Punta Toro virus in the mouse and hamster infection models, as well as Pichinde virus infection in hamsters. When administered twice daily for 5 to 6 days, beginning 4 h pre- or 24 h post-Punta Toro virus challenge, a 30-mg/kg of body weight/day dose provided complete protection from death and limited viral burden and liver disease. A dose of 50 mg/kg/day was found to be optimal for treating Pichinde infection and limiting viral replication and disease severity. In general, T-705 was found to be more active than ribavirin in cell-based assays and in vivo, as reflected by substantially greater therapeutic indexes. Our results suggest that T-705 may be a viable alternative for the treatment of life-threatening bunyaviral and arenaviral infections.

Combinatorial ribavirin and interferon alfacon-1 therapy of acute arenaviral disease in hamsters

Several arenaviruses endemic to South America (Junin, Machupo, and Guanarito) and Africa (Lassa) are known to cause frequently fatal haemorrhagic fever. With the exception of ribavirin, which has demonstrated efficacy in cases of Lassa fever, there is no other effective therapeutic for the treatment of arenaviral haemorrhagic fever. We have recently reported that consensus interferon-a (IFN alfacon-1) can protect hamsters from lethal Pichinde virus (PCV) infection, which serves as a model for acute arenaviral disease in humans. Here we demonstrate highly effective therapy through the combined use of ribavirin with IFN alfacon-1 for the treatment of PCV infection in hamsters. Ribavirin was given orally, twice per day for 7 days, and IFN alfacon-1 was administered intraperitoneally once per day for 10 days. Treatments were initiated 1-5 days post-virus challenge using various dose combinations, many of which were less than optimal when the drugs were given independently. Combining suboptimal doses of ribavirin (5-10 mg/kg/day) with IFN alfacon-1 (5-10 microg/kg/day), we were able to demonstrate increased protection from mortality, reduced viral burden and liver disease, and greatly extended survival times as compared to treatments where drugs were administered alone. Our data indicate that combination therapy results in synergistic activity that may slow down the progression of the disease and decrease fatality rates associated with severe arenaviral infections in humans. Further, combination therapy reduces the effective dosage of ribavirin, which would serve to limit its toxicity.

Novel antiviral strategies to combat human Arenavirus infections

Arenaviruses merit significant attention both as tractable model systems to study acute and persistent viral infections, and as clinically important human pathogens. Evidence indicates that LCMV remains present in the USA and Europe and capable of causing significant morbidity in infected individuals, likely being a neglected human pathogen. Moreover, new arenaviruses are being discovered in the Americas on the average of one every three years, with some of them causing severe hemorrhagic fever. In addition, weaponized forms of these viruses pose a real threat as agents of bioterrorism. Therefore, it is important to develop effective vaccines and better antiviral drugs to combat the dual threats of naturally occurring and intentionally introduced Arenavirus infections. The development of arenavirus reverse genetic systems is allowing investigators to conduct a detailed molecular characterization of the viral cis-acting signals and trans-acting factors that control each of the steps of the Arenavirus life cycle, including RNA synthesis, packaging and budding. We will discuss how this new knowledge is facilitating the establishment of novel assays to identify and characterize compounds capable of interfering with specific steps of the virus life cycle. Likewise, the ability to generate predetermined specific mutations within the arenavirus genome, and analyze their phenotypic expression, would significantly contribute to the elucidation of arenavirus-host interactions, including the bases of their ability to persist, as well as to cause severe HF (hemorrhagic fever) disease in humans. These approaches could also lead to the development of novel potent and safe Arenavirus vaccines.

Identification and characterization of potent small molecule inhibitor of hemorrhagic fever New World arenaviruses

Category A arenaviruses as defined by the National Institute of Allergy and Infectious Diseases (NIAID) are human pathogens that could be weaponized by bioterrorists. Many of these deadly viruses require biosafety level-4 (BSL-4) containment for all laboratory work, which limits traditional laboratory high-throughput screening (HTS) for identification of small molecule inhibitors. For those reasons, a related BSL-2 New World arenavirus, Tacaribe virus, 67-78% identical to Junín virus at the amino acid level, was used in a HTS campaign where approximately 400,000 small molecule compounds were screened in a Tacaribe virus-induced cytopathic effect (CPE) assay. Compounds identified in this screen showed antiviral activity and specificity against not only Tacaribe virus, but also the Category A New World arenaviruses (Junín, Machupo, and Guanarito). Drug resistant variants were isolated, suggesting that these compounds act through inhibition of a viral protein, the viral glycoprotein (GP2), and not through cellular toxicity mechanisms. A lead compound, ST-294, has been chosen for drug development. This potent and selective compound, with good bioavailability, demonstrated protective anti-viral efficacy in a Tacaribe mouse challenge model. This series of compounds represent a new class of inhibitors that may warrant further development for potential inclusion in a strategic stockpile.

Interferon alfacon-1 protects hamsters from lethal pichinde virus infection

Hemorrhagic fever of arenaviral origin is a frequently fatal infectious disease of considerable priority to the biodefense mission. Historically, the treatment of arenaviral infections with alpha interferons has not yielded favorable results. Here we present evidence that interferon alfacon-1, a nonnaturally occurring bioengineered alpha interferon approved for the treatment of chronic hepatitis C, is active against Pichinde and Tacaribe arenaviruses in cell culture. In the hamster model of Pichinde virus (PCV) infection, interferon alfacon-1 treatment significantly protected animals from death, prolonged the survival of those that eventually died, reduced virus titers, and limited liver damage characteristic of PCV-induced disease. Moreover, interferon alfacon-1 also demonstrated therapeutic activity, to a lesser degree, when the initiation of treatment was delayed up to 2 days post-virus challenge. Despite the observed advantages of interferon alfacon-1 therapy, efforts to stimulate the immune system with the known interferon inducer poly(I:C12U) (Ampligen) offered only limited protection against lethal PCV challenge. Taken together, these data suggest that the increased potency of the bio-optimized interferon alfacon-1 molecule may be critical to the observed antiviral effects. These data are the first report demonstrating efficacious treatment of acute arenaviral disease with alpha interferon therapy, and further study is warranted.

Arenavirus extinction through lethal mutagenesis

Viral hemorrhagic fevers represent serious human public health problems causing devastating and often lethal disease. Several hemorrhagic fevers are caused by arenaviruses including Lassa fever virus (LFV) and the South American viral hemorrhagic fevers (SAHF). In recent years, increased air travel between Africa and other areas has led to the importation of LFV into the US, Europe, Japan, and Canada. This has raised awareness about arenaviruses as potential emerging viruses. Moreover, because of its severe morbidity and high mortality, and transmissibility from human to human, weaponized forms of LFV poses a real threat as agent of bioterrorism. No licensed vaccine is available in the US, and currently there is not efficacious therapy to treat these infections. Therefore, the importance of developing novel effective antiviral drugs to combat HF arenaviruses, for which the prototypic Arenavirus lymphocytic choriomeningitis virus (LCMV) provides us with an excellent model system. Recent findings have shown that LCMV multiplication both in cultured cells and in vivo is highly susceptible to the mutagenic agent 5-fluorouracil (FU). FU-mediated extinction of LCMV was associated with only modest increases in virus mutation frequencies, but did not significantly affect virus replication and transcription, or virus particle formation. These findings indicate that, as with other riboviruses, lethal mutagenesis is effective also against LCMV raising the possibility of using this novel antiviral strategy to combat pathogenic arenaviruses.

Lethal mutagenesis of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV)

Passage of the prototypic arenavirus lymphocytic choriomenigitis virus (LCMV) in cultured cells in the presence of the mutagenic agent 5-fluorouracil (FU) resulted in efficient and systematic virus extinction under conditions that did not significantly affect cell survival. FU-mediated extinction of LCMV was associated with 3.6- to 10-fold increases in the mutation frequencies for the three viral genes examined, but with only very modest effects on virus replication and transcription during a single round of infection. Likewise, FU did not affect expression of a LCMV minigenome. In contrast, the well documented antiviral effect of ribavirin against LCMV was not associated with significant increases in virus mutation frequencies, but rather with a dramatic inhibition of both viral RNA synthesis and LCMV minigenome expression. Mutagen induced viral extinction has been recently reported for positive strand RNA viruses polio and foot-and-mouth disease, and the lentivirus HIV-1. Our findings indicate that lethal mutagenesis can be effective also against LCMV, a negative strand RNA virus. Moreover, FU treatment prevented the establishment of LCMV persistent infection in mice deficient in B and T cells, suggesting the feasibility in vivo of lethal mutagenesis as a novel antiviral strategy.

Arenaviruses other than Lassa virus

The family Arenaviridae includes 23 viral species, of which 5 can cause viral hemorrhagic fevers with a case fatality rate of about 20%. These five viruses are Junin, Machupo, Guanarito, Sabia and Lassa virus, the manipulation of which requires biosafety level 4 facilities. They are included in the Category A Pathogen List established by the Center for Disease Control and Prevention that groups agents with the greatest potential for adverse public health impact and mass casualties whether a situation characterized by a ill-intentioned abuse of natural or engineered arenavirus would be encountered. The aims of this article are to (i) summarize the current situation; (ii) provide information to help anticipating the effects to be expected in such a situation; and to (iii) emphasize the need for fundamental research to allow the development of diagnostic, prevention and therapeutic tools as countermeasures to weaponized arenaviruses.

Hemorrhagic fever viruses as biological weapons: medical and public health management

OBJECTIVE

To develop consensus-based recommendations for measures to be taken by medical and public health professionals if hemorrhagic fever viruses (HFVs) are used as biological weapons against a civilian population.

PARTICIPANTS

The Working Group on Civilian Biodefense included 26 representatives from academic medical centers, public health, military services, governmental agencies, and other emergency management institutions.

EVIDENCE

MEDLINE was searched from January 1966 to January 2002. Retrieved references, relevant material published prior to 1966, and additional sources identified by participants were reviewed.

CONSENSUS PROCESS

Three formal drafts of the statement that synthesized information obtained in the evidence-gathering process were reviewed by the working group. Each draft incorporated comments and judgments of the members. All members approved the final draft.

CONCLUSIONS

Weapons disseminating a number of HFVs could cause an outbreak of an undifferentiated febrile illness 2 to 21 days later, associated with clinical manifestations that could include rash, hemorrhagic diathesis, and shock. The mode of transmission and clinical course would vary depending on the specific pathogen. Diagnosis may be delayed given clinicians' unfamiliarity with these diseases, heterogeneous clinical presentation within an infected cohort, and lack of widely available diagnostic tests. Initiation of ribavirin therapy in the early phases of illness may be useful in treatment of some of these viruses, although extensive experience is lacking. There are no licensed vaccines to treat the diseases caused by HFVs.

Arenavirus Infection — Connecticut, 1994

On August 20, 1994, the Connecticut Department of Public Health and Addiction Services received a report of a case of acute illness in a virologist suspected to be associated with Sabiá virus, a newly described arenavirus. This report preliminary findings from the case investigation.

Arenavirus infection in the guinea pig model: antiviral therapy with recombinant interferon-alpha, the immunomodulator CL246,738 and ribavirin

Human arenaviral infections have a high mortality, and are dangerous to work with in the laboratory. There is a need for good antiviral agents to treat these infections. Pichinde virus infection of the inbred strain 13 guinea pig is a relatively safe, good animal model for human arenavirus infections. Mortality is consistently 100% between days 12 and 25 (mean 14.8) days after infection. When infected animals were treated with recombinant human interferon alpha A, or with CL246,783, an immunomodulator known to induce interferon, no beneficial effect was noted. When animals received ribavirin, 25 mg/kg/day for the first 14 days of infection, the course of infection was prolonged, with death occurring a mean of 22.5 days after infection. If ribavirin was administered for 28 days, mortality was reduced to 25%, with those animals dying a mean of 21.0 days after infection. These results confirm the studies that indicate that ribavirin is a useful agent for treating arenaviral infections. However, treatment with this agent must be prolonged. They also demonstrate the potential usefulness of Pichinde virus infection in strain 13 guinea pigs as an animal model of human disease.

Generation of memory cell-mediated immune responses after secondary infection of mice with pichinde virus

Pichinde virus (PV), a member of the arenavirus group, was found to elicit strong cell-mediated immune responses in various strains of mice. After primary i.v. inoculation, augmentation of natural killer (NK) cell activity occurred and peaked 3 to 4 days after infection. The NK response was followed by a second peak of cytotoxic activity that was found to be H-2 restricted, virus specific, and mediated by Thy-1.2+, Lyt-2.2+ lymphocytes. This cytotoxic T lymphocyte (CTL) response peaked 7 days post infection. Neutralizing antibodies were not detectable after PV infection of the mice. In light of this, we investigated the generation and kinetics of secondary cell-mediated immune responses after reinjection of homologous virus in vivo. Slight but significant augmentation of NK activity was observed 1 day after secondary virus challenge. As in the primary response, effectors of this NK activity rapidly became sensitive to anti-Thy-1.2 and complement treatment. NK activity rapidly returned to background levels and was followed by an anamnestic CTL response that peaked 4 days after reinjection of the virus. Thus, cell-mediated immune responses appeared more rapidly after secondary challenge in vivo, and the temporal relationship between NK and CTL generation was maintained. Both secondary NK and CTL responses were generated in mice that had been pretreated with cyclophosphamide (CY), suggesting that memory cell-mediated immune responses can be reactivated in vivo without undergoing cell division. In contrast, treatment with CY before primary infection delayed the appearance of virus-induced NK activity and abrogated the generation of H-2-restricted virus-specific CTL. Rechallenge of these CY-treated NK-primed mice resulted in the rapid generation of a secondary NK response that was not followed by either a primary or secondary CTL response. The data suggest that cells mediating a nonspecific effector function may possess specific memory. We discuss our results with respect to possible NK-CTL relationships.