Isolation and antigenic characterization of Lassa virus

Hexestrol, an estrogen receptor agonist, inhibits Lassa virus entry

Lassa virus (LASV) is the causative agent of human Lassa fever which in severe cases manifests as hemorrhagic fever leading to thousands of deaths annually. However, no approved vaccines or antiviral drugs are currently available. Recently, we screened approximately 2,500 compounds using a recombinant vesicular stomatitis virus (VSV) expressing LASV glycoprotein GP (VSV-LASVGP) and identified a P-glycoprotein inhibitor as a potential LASV entry inhibitor. Here, we show that another identified candidate, hexestrol (HES), an estrogen receptor agonist, is also a LASV entry inhibitor. HES inhibited VSV-LASVGP replication with a 50% inhibitory concentration (IC50) of 0.63 µM. Importantly, HES also inhibited authentic LASV replication with IC50 values of 0.31 µM-0.61 µM. Time-of-addition and cell-based membrane fusion assays suggested that HES inhibits the membrane fusion step during virus entry. Alternative estrogen receptor agonists did not inhibit VSV-LASVGP replication, suggesting that the estrogen receptor itself is unlikely to be involved in the antiviral activity of HES. Generation of a HES-resistant mutant revealed that the phenylalanine at amino acid position 446 (F446) of LASVGP, which is located in the transmembrane region, conferred resistance to HES. Although mutation of F446 enhanced the membrane fusion activity of LASVGP, it exhibited reduced VSV-LASVGP replication, most likely due to the instability of the pre-fusion state of LASVGP. Collectively, our results demonstrated that HES is a promising anti-LASV drug that acts by inhibiting the membrane fusion step of LASV entry. This study also highlights the importance of the LASVGP transmembrane region as a target for anti-LASV drugs.IMPORTANCELassa virus (LASV), the causative agent of Lassa fever, is the most devastating mammarenavirus with respect to its impact on public health in West Africa. However, no approved antiviral drugs or vaccines are currently available. Here, we identified hexestrol (HES), an estrogen receptor agonist, as the potential antiviral candidate drug. We showed that the estrogen receptor itself is not involved in the antiviral activity. HES directly bound to LASVGP and blocked membrane fusion, thereby inhibiting LASV infection. Through the generation of a HES-resistant virus, we found that phenylalanine at position 446 (F446) within the LASVGP transmembrane region plays a crucial role in the antiviral activity of HES. The mutation at F446 caused reduced virus replication, likely due to the instability of the pre-fusion state of LASVGP. These findings highlight the potential of HES as a promising candidate for the development of antiviral compounds targeting LASV.

Novel insights for a nonlinear deterministic-stochastic class of fractional-order Lassa fever model with varying kernels

Lassa fever is a hemorrhagic virus infection that is usually spread by rodents. It is a fatal infection that is prevalent in certain West African countries. We created an analytical deterministic-stochastic framework for the epidemics of Lassa fever employing a collection of ordinary differential equations with nonlinear solutions to identify the influence of propagation processes on infected development in individuals and rodents, which include channels that are commonly overlooked, such as ecological emergent and aerosol pathways. The findings shed light on the role of both immediate and subsequent infectiousness via the power law, exponential decay and generalized Mittag-Leffler kernels. The scenario involves the presence of a steady state and an endemic equilibrium regardless of the fundamental reproduction number, [Formula: see text], making Lassa fever influence challenging and dependent on the severity of the initial sub-populations. Meanwhile, we demonstrate that the stochastic structure has an exclusive global positive solution via a positive starting point. The stochastic Lyapunov candidate approach is subsequently employed to determine sufficient requirements for the existence and uniqueness of an ergodic stationary distribution of non-negative stochastic simulation approaches. We acquire the particular configuration of the random perturbation associated with the model's equilibrium [Formula: see text] according to identical environments as the presence of a stationary distribution. Ultimately, modeling techniques are used to verify the mathematical conclusions. Our fractional and stochastic findings exhibit that when all modes of transmission are included, the impact of Lassa fever disease increases. The majority of single dissemination pathways are less detrimental with fractional findings; however, when combined with additional spread pathways, they boost the Lassa fever stress.

Lassa fever - the road ahead

Lassa virus (LASV) is endemic in the rodent populations of Sierra Leone, Nigeria and other countries in West Africa. Spillover to humans occurs frequently and results in Lassa fever, a viral haemorrhagic fever (VHF) associated with a high case fatality rate. Despite advances, fundamental gaps in knowledge of the immunology, epidemiology, ecology and pathogenesis of Lassa fever persist. More frequent outbreaks, the potential for further geographic expansion of Mastomys natalensis and other rodent reservoirs, the ease of procurement and possible use and weaponization of LASV, the frequent importation of LASV to North America and Europe, and the emergence of novel LASV strains in densely populated West Africa have driven new initiatives to develop countermeasures for LASV. Although promising candidates are being evaluated, as yet there are no approved vaccines or therapeutics for human use. This Review discusses the virology of LASV, the clinical course of Lassa fever and the progress towards developing medical countermeasures.

Detection and genetic diversity of Mopeia virus in Mastomys natalensis from different habitats in the Limpopo National Park, Mozambique

Mammarenaviruses have been a growing concern for public health in Africa since the 1970s when Lassa virus cases in humans were first described in west Africa. In southern Africa, a single outbreak of Lujo virus was reported to date in South Africa in 2008 with a case fatality rate of 80%. The natural reservoir of Lassa virus is Mastomys natalensis while for the Lujo virus the natural host has yet to be identified. Mopeia virus was described for the first time in M. natalensis in the central Mozambique in 1977 but few studies have been conducted in the region. In this study, rodents were trapped between March and November 2019in villages, croplands fields and mopane woodland forest. The aim was to assess the potential circulation and to evaluate the genetic diversity of mammarenaviruses in M. natalensis trapped in the Limpopo National Park and its buffer zone in Massingir district, Mozambique. A total of 534 M. natalensis were screened by RT-PCR and the overall proportion of positive individuals was 16.9%. No significant differences were detected between the sampled habitats (χ2 = 0.018; DF = 1; p = 0.893). The Mopeia virus (bootstrap value 91%) was the Mammarenavirus circulating in the study area sites, forming a specific sub-clade with eight different sub-clusters. We concluded that Mopeia virus circulates in all habitats investigated and it forms a different sub-clade to the one reported in central Mozambique in 1977.

Inoculation route-dependent Lassa virus dissemination and shedding dynamics in the natural reservoir - Mastomys natalensis

Lassa virus (LASV), a Risk Group-4 zoonotic haemorrhagic fever virus, affects sub-Saharan African countries. Lassa fever, caused by LASV, results in thousands of annual deaths. Although decades have elapsed since the identification of the Natal multimammate mouse (Mastomys natalensis) as a natural reservoir of LASV, little effort has been made to characterize LASV infection in its reservoir. The natural route of infection and transmission of LASV within M. natalensis remains unknown, and the clinical impact of LASV in M. natalensis is mostly undescribed. Herein, using an outbred colony of M. natalensis, we investigate the replication and dissemination dynamics of LASV in this reservoir following various inoculation routes. Inoculation with LASV, regardless of route, resulted in a systemic infection and accumulation of abundant LASV-RNA in many tissues. LASV infection in the Natal multimammate mice was subclinical, however, clinical chemistry values were transiently altered and immune infiltrates were observed histologically in lungs, spleens and livers, indicating a minor disease with coordinated immune responses are elicited, controlling infection. Intranasal infection resulted in unique virus tissue dissemination dynamics and heightened LASV shedding, compared to subcutaneous inoculation. Our study provides important insights into LASV infection in its natural reservoir using a contemporary infection system, demonstrating that specific inoculation routes result in disparate dissemination outcomes, suggesting intranasal inoculation is important in the maintenance of LASV in the natural reservoir, and emphasizes that selection of the appropriate inoculation route is necessary to examine aspects of viral replication, transmission and responses to zoonotic viruses in their natural reservoirs.

A seasonal model to assess intervention strategies for preventing periodic recurrence of Lassa fever

Lassa haemorrhagic fever is listed in WHO's Blueprint priority list of diseases and pathogens prioritized for research and development, affecting several hundreds of thousands of people each year. Lassa fever is spread via infected Natal multimammate mice and also through human-to-human contacts and it is a particular threat to pregnant women. Despite its importance, relatively few mathematical models have been established for modelling Lassa fever transmission up to now. We establish and study a new compartmental model for Lassa fever transmission including asymptomatic carriers, quarantine and periodic coefficients to model annual weather changes. We determine parameter values providing the best fit to data from Nigerian states Edo and Ondo from 2018–20. We perform uncertainty analysis and PRCC analysis to assess the importance of different parameters and numerical simulations to estimate the possible effects of control measures in eradicating the disease. The results suggest that the most important parameter which might be subject of control measures is death rate of mice, while mouse-to-human and human-to-human transmission rates also significantly influence the number of infected. However, decreasing the latter two parameters seems insufficient to eradicate the disease, while a parallel application of decreasing transmission rates and increasing mouse death rate might be able to stop the epidemic.

Lassa fever in Benin: description of the 2014 and 2016 epidemics and genetic characterization of a new Lassa virus

We report two outbreaks of Lassa fever that occurred in Benin in 2014 and 2016 with 20 confirmed cases and 50% (10/20) mortality. Benin was not previously considered to be an endemic country for Lassa fever, resulting in a delay to diagnose the disease and its human transmission. Molecular investigations showed the viral genomes to be similar to that of the Togo strain, which is genetically very different from other known strains and confirms the existence of a new lineage. Endemic circulation of Lassa virus in a new territory and the genetic diversity thus confirm that this virus represents a growing threat for West African people. Given the divergence of the Benin strain from the prototypic Josiah Sierra Leone strain frequently used to generate vaccine candidates, the efficacy of vaccine candidates should also be demonstrated with this strain.

A Sporadic and Lethal Lassa Fever Case in Forest Guinea, 2019

Lassa fever is a rodent-borne disease caused by Lassa virus (LASV). It causes fever, dizziness, vertigo, fatigue, coughing, diarrhea, internal bleeding and facial edema. The disease has been known in Guinea since 1960 but only anectodical acute cases have been reported to date. In January 2019, a 35-year-old man, a wood merchant from Kissidougou, Forest Guinea, presented himself at several health centers with persistent fever, frequent vomiting and joint pain. He was repeatedly treated for severe malaria, and died three weeks later in Mamou regional hospital. Differential diagnosis identified LASV as the cause of death. No secondary cases were reported. The complete LASV genome was obtained using next-generation sequencing. Phylogenetic analysis showed that this strain, namely the Kissidougou strain, belongs to the clade IV circulating in Guinea and Sierra Leone, and is thought to have emerged some 150 years ago. Due to the similarity of symptoms with malaria, Lassa fever is still a disease that is difficult to recognize and that may remain undiagnosed in health centers in Guinea.

Bayesian estimation of Lassa virus epidemiological parameters: Implications for spillover prevention using wildlife vaccination

Lassa virus is a significant burden on human health throughout its endemic region in West Africa, with most human infections the result of spillover from the primary rodent reservoir of the virus, the natal multimammate mouse, M. natalensis. Here we develop a Bayesian methodology for estimating epidemiological parameters of Lassa virus within its rodent reservoir and for generating probabilistic predictions for the efficacy of rodent vaccination programs. Our approach uses Approximate Bayesian Computation (ABC) to integrate mechanistic mathematical models, remotely-sensed precipitation data, and Lassa virus surveillance data from rodent populations. Using simulated data, we show that our method accurately estimates key model parameters, even when surveillance data are available from only a relatively small number of points in space and time. Applying our method to previously published data from two villages in Guinea estimates the time-averaged R₀ of Lassa virus to be 1.74 and 1.54 for rodent populations in the villages of Bantou and Tanganya, respectively. Using the posterior distribution for model parameters derived from these Guinean populations, we evaluate the likely efficacy of vaccination programs relying on distribution of vaccine-laced baits. Our results demonstrate that effective and durable reductions in the risk of Lassa virus spillover into the human population will require repeated distribution of large quantities of vaccine.

Lassa virus is a chronic source of illness throughout West Africa, and is considered to be a threat for widespread emergence. Because most human infections result from contact with infected rodents, interventions that reduce the number of rodents infected with Lassa virus represent promising opportunities for reducing the public health burden of this disease. Evaluating how well alternative interventions are likely to perform is complicated by our relatively poor understanding of viral epidemiology within the reservoir population. Here we develop a novel statistical approach that couples mathematical models and viral surveillance data from rodent populations to robustly estimate key epidemiological parameters. Applying our method to existing data from Guinea yields well-resolved parameter estimates and allows us to simulate a variety of rodent vaccination programs. Together, our results demonstrate that rodent vaccination alone is unlikely to be an effective tool for reducing the public health burden of Lassa fever within West Africa.

50 Years of Lassa Fever Research

Lassa fever was first described as a clinical entity fifty years ago. The causative agent Lassa virus was isolated from these first known cases. This chapter reviews the key publications on Lassa fever research that appeared in the scientific literature at that time and over the ensuing decades.

Lassa fever diagnostics: past, present, and future

Lassa fever is a unique viral hemorrhagic fever that is endemic in parts of West Africa, primarily Sierra Leone, Guinea, Liberia, and Nigeria. The disease is caused by the Lassa virus, an Old World arenavirus that has as primary reservoir host the multimammate rodent Mastomys nataliensis, which lives in association with humans. Recent estimates suggest LF causes two million cases and 5000-10000 deaths annually, mainly in West Africa. Clinical diagnosis and laboratory confirmation have always been major challenges for effective management and control of the disease in afflicted areas of West Africa. Recent advancements in molecular biology, recombinant DNA technology, and genomics sequencing has facilitated major advancement in development of better diagnostic and surveillance tools for Lassa fever virus. These include, the multiplex, magnetic bead-based immunodiagnostics for both Lassa virus antigens and antibodies; molecular probe-based quantitative real-time PCR for genomic signatures; rapid diagnostics tests that detects the most prevalent West African lineages; and the successful utilization of next-generation sequencing technology to diagnose and characterize Lassa virus in West Africa. These advances will continue to improve disease treatment, control, and prevention. In this review we will discuss progression of Lassa virus diagnostics from the past and into the future.

Antibody therapy for Lassa fever

Serum from convalescent Lassa fever patients was previously shown to be ineffective as a source of protective antibodies in some early studies. Subsequently, monoclonal antibodies (MAbs) to the Lassa virus (LASV) glycoprotein produced by memory B cells of West African patients who survived Lassa fever were identified. Development of MAbs as potential Lassa immunotherapeutics was facilitated by structural studies and mutational analyses that identified protective epitopes on the prefusion form of the LASV glycoprotein. Human mAbs were screened for reactivity to different neutralizing epitopes, potency, and broad reactivity against multiple lineages of LASV. MAbs were downselected in a guinea pig model of Lassa fever. A cocktail of three human MAbs designated Arevirumab-3 rescued 100% of Cynomolgus macaques at advanced stages of disease more than a week post-infection. Antibody therapeutics may be further developed in clinical trials in endemic areas potentially offering a key treatment option for Lassa fever.

Evaluation of rodent control to fight Lassa fever based on field data and mathematical modelling

The Natal multimammate mouse (Mastomys natalensis) is the reservoir host of Lassa virus, an arenavirus that causes Lassa haemorrhagic fever in humans in West Africa. Because no vaccine exists and therapeutic options are limited, preventing infection through rodent control and human behavioural measures is currently considered to be the only option. In order to assess the efficacy of rodent control, we performed a 4-year field experiment in rural Upper Guinea and developed a mathematical model to simulate different control strategies (annual density control, continuous density control, and rodent vaccination). For the field study, rodenticide baits were placed each year in three rural villages, while three other villages were used as controls. Rodents were trapped before and after every treatment and their antibody status and age were determined. Data from the field study were used to parameterize the mathematical model. In the field study, we found a significant negative effect of rodent control on seroprevalence, but this effect was small especially given the effort. Furthermore, the rodent populations recovered rapidly after rodenticide application, leading us to conclude that an annual control strategy is unlikely to significantly reduce Lassa virus spillover to humans. In agreement with this finding, the mathematical model suggests that the use of continuous control or rodent vaccination is the only strategy that could lead to Lassa virus elimination. These field and model results can serve as a guide for determining how long and frequent rodent control should be done in order to eliminate Lassa virus in rural villages.

Rodent control to fight Lassa fever: Evaluation and lessons learned from a 4-year study in Upper Guinea

Lassa fever is a viral haemorrhagic fever caused by an arenavirus. The disease is endemic in West African countries, including Guinea. The rodents Mastomys natalensis and Mastomys erythroleucus have been identified as Lassa virus reservoirs in Guinea. In the absence of a vaccine, rodent control and human behavioural changes are the only options to prevent Lassa fever in highly endemic areas. We performed a 4 year intervention based on chemical rodent control, utilizing anticoagulant rodenticides in 3 villages and evaluating the rodent abundance before and after treatment. Three additional villages were investigated as controls. Analyses to assess the effectiveness of the intervention, bait consumption and rodent dynamics were performed. Anthropological investigations accompanied the intervention to integrate local understandings of human–rodent cohabitation and rodent control intervention. Patterns of bait consumption showed a peak at days 5–7 and no consumption at days 28–30. There was no difference between Bromadiolone and Difenacoum bait consumption. The main rodent species found in the houses was M. natalensis. The abundance of M. natalensis, as measured by the trapping success, varied between 3.6 and 16.7% before treatment and decreased significantly to 1–2% after treatment. Individuals in treated villages welcomed the intervention and trapping because mice are generally regarded as a nuisance. Immediate benefits from controlling rodents included protection of food and belongings. Before the intervention, local awareness of Lassa fever was non-existent. Despite their appreciation for the intervention, local individuals noted its limits and the need for complementary actions. Our results demonstrate that chemical treatment provides an effective tool to control local rodent populations and can serve as part of an effective, holistic approach combining rodent trapping, use of local rodenticides, environmental hygiene, house repairs and rodent-proof storage. These actions should be developed in collaboration with local stakeholders and communities.

In the absence of a Lassa fever vaccine, rodent control is the primary prevention option. An effective rodent control intervention must understand human behaviour towards the rodent such as: human–rodent interactions, cohabitation, and local rodent control measures. We conducted a rodent control intervention at community level in a Lassa Virus endemic area in Upper Guinea (Guinea) accompanied by an anthropological study on people’s perceptions and recommendations on the intervention. Based on our results we seek to broaden the rodent control intervention by including environmental hygiene, house repairs and rodent-proof storage. Chemical treatment has proven effective for rodent control but other factors involved in human-rodent interactions should also be addressed. Our findings highlight the need for Lassa fever prevention and rodent control initiatives to work in collaboration with communities and undertake a holistic approach towards rodent control.

Development of real-time reverse transcriptase qPCR assays for the detection of Punta Toro virus and Pichinde virus

Background

Research with high biocontainment pathogens such as Rift Valley fever virus (RVFV) and Lassa virus (LASV) is expensive, potentially hazardous, and limited to select institutions. Surrogate pathogens such as Punta Toro virus (PTV) for RVFV infection and Pichinde virus (PICV) for LASV infection allow research to be performed under more permissive BSL-2 conditions. Although used as infection models, PTV and PICV have no standard real-time RT-qPCR assays to detect and quantify pathogenesis. PTV is also a human pathogen, making a standardized detection assay essential for biosurveillance. Here, we developed and characterized two real-time RT-qPCR assays for PICV and PTV by optimizing assay conditions and measuring the limit of detection (LOD) and performance in multiple clinical matrices.

Methods

Total nucleic acid from virus-infected Vero E6 cells was used to optimize TaqMan-minor groove binder (MGB) real-time RT-qPCR assays. A 10-fold dilution series of nucleic acid was used to perform analytical experiments with 60 replicates used to confirm assay LODs. Serum and whole blood spiked with 10-fold dilutions of PTV and PICV virus were assessed as matrices in a mock clinical context. The Cq, or cycle at which the fluoresce of each sample first crosses a threshold line, was determined using the second derivative method using Roche LightCycler 480 software version 1.5.1. Digital droplet PCR (ddPCR) was utilized to quantitatively determine RNA target counts/μl for PTV and PICV.

Results

Optimized PTV and PICV assays had LODs of 1000 PFU/ml and 100 PFU/ml, respectively, and this LOD was confirmed in 60/60 (PTV) and 58/60 (PICV) positive replicates. Preliminary mock clinical LODs remained consistent in serum and whole blood for PTV and PICV at 1000 PFU/ml and 100 PFU/ml. An exclusivity panel showed no cross reaction with near neighbors.

Conclusions

PTV and PICV Taq-man MGB based real-time RT-qPCR assays developed here showed relevant sensitivity and reproducibility in samples extracted from a variety of clinical matrices. These assays will be useful as a standard by researchers for future experiments utilizing PTV and PICV as infection models, offering the ability to track infection and viral replication kinetics during research studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0509-3) contains supplementary material, which is available to authorized users.

Lassa serology in natural populations of rodents and horizontal transmission

Lassa virus causes hemorrhagic fever in West Africa. Previously, we demonstrated by PCR screening that only the multimammate mouse, Mastomys natalensis, hosts Lassa virus in Guinea. In the present study, we used the same specimen collection from 17 villages in Coastal, Upper, and Forest Guinea to investigate the Lassa virus serology in the rodent population. The aim was to determine the dynamics of antibody development in M. natalensis and to detect potential spillover infections in other rodent species. Immunoglobulin G (IgG) antibody screening was performed using the indirect immunofluorescence assay with the Guinean Lassa virus strain Bantou 289 as antigen. The overall seroprevalence was 8% (129/1551) with the following rodents testing positive: 109 M. natalensis, seven Mastomys erythroleucus, four Lemniscomys striatus, four Praomys daltoni, three Mus minutoides, and two Praomys rostratus. Nearly all of them (122/129) originated from Bantou, Tanganya, and Gbetaya, where Lassa virus is highly endemic in M. natalensis. The antibody seroprevalence in M. natalensis from this high-endemic area (27%; 108/396) depended on the village, habitat, host age, and host abundance. A main positive factor was age; the maximum seroprevalence reached 50% in older animals. Our data fit with a model implicating that most M. natalensis rodents become horizontally infected, clear the virus within a period significantly shorter than their life span, and develop antibodies. In addition, the detection of antibodies in other species trapped in the habitats of M. natalensis suggests spillover infections.

Lassa fever vaccine

Lassa fever remains a serious challenge to public health in West Africa threatening both local residents in rural areas and those who serve them, particularly medical care providers. Given the ecology of the rodent host and conditions in the endemic area, a vaccine is mandatory for control. The challenge is to overcome the scientific, political and economic obstacles to producing a human use vaccine candidate. There are some scientific issues to resolve. It is known that the G-protein confers protection but we do not know its duration. If the N-protein is also included there may be a better duration of protection but it is unclear whether the N-protein as a vaccine may possibly enhance the infection. The original vaccinia vector must be replaced by new vectors, chimeras or by delivering DNA in some format. A live vaccine is attractive because it can confer protection in a single shot. A killed vaccine is more stable, particularly for distribution in the tropics but usually requires repeated shots. For practical reasons a live vaccine format should probably be pursued, which could then be combined with a yellow fever vaccine, using the same cold chains, since this disease occupies the same endemic areas in West Africa. Lassa vaccine initiatives have suffered from a lack of funding in the past but bioterrorism has brought new resources to Lassa virus science. Adequate funding and applications of new vaccine technologies give hope that we may soon see a vaccine in clinical trials. However, the difficulty of conducting trials in endemic areas and lack of political stability remain serious problems.

LCMV glycosylation modulates viral fitness and cell tropism

The glycoprotein (GP) of arenaviruses is glycosylated at 11 conserved N-glycosylation sites. We constructed recombinant lymphocytic choriomeningitis virus (rLCMV) featuring either additions or deletions of these N-glycans to investigate their role in the viral life cycle. N-glycosylation at two sites, T87 and S97, were found to be necessary to rescue rLCMV. Three of nine successfully rescued mutants, S116A, T234A, and S373A, under selective pressures in either epithelial, neuronal, or macrophage cells reverted to WT sequence. Of the seven stable N-glycan deletion mutants, five of these led to altered viral fitness and cell tropism, assessed as growth in either mouse primary cortical neurons or bone marrow derived macrophages. These results demonstrate that the deletion of N-glycans in LCMV GP may confer an advantage to the virus for infection of neurons but a disadvantage in macrophages.

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.

T cell-dependence of Lassa fever pathogenesis

Lassa virus (LASV), the causative agent of Lassa fever (LF), is endemic in West Africa, accounting for substantial morbidity and mortality. In spite of ongoing research efforts, LF pathogenesis and mechanisms of LASV immune control remain poorly understood. While normal laboratory mice are resistant to LASV, we report that mice expressing humanized instead of murine MHC class I (MHC-I) failed to control LASV infection and develop severe LF. Infection of MHC-I knockout mice confirmed a key role for MHC-I-restricted T cell responses in controlling LASV. Intriguingly we found that T cell depletion in LASV-infected HHD mice prevented disease, irrespective of high-level viremia. Widespread activation of monocyte/macrophage lineage cells, manifest through inducible NO synthase expression, and elevated IL-12p40 serum levels indicated a systemic inflammatory condition. The absence of extensive monocyte/macrophage activation in T cell-depleted mice suggested that T cell responses contribute to deleterious innate inflammatory reactions and LF pathogenesis. Our observations in mice indicate a dual role for T cells, not only protecting from LASV, but also enhancing LF pathogenesis. The possibility of T cell-driven enhancement and immunopathogenesis should be given consideration in future LF vaccine development.

Lassa virus (LASV) is the causative agent of Lassa fever (LF), accounting for substantial morbidity and mortality in West Africa. Yet the mechanisms leading to disease remain poorly understood. Here we propose a concept whereby the body's immune defense either defeats LASV rapidly or, if unsuccessful, becomes an essential facilitator of disease. This latter paradoxical postulate stems from observations in genetically engineered (HHD) mice, which we found to be susceptible to LF. HHD mice differ from resistant wild type mice in that they have a humanized repertoire of T cells, a main component of the mammalian immune system. Counterintuitively, we could protect HHD mice against LF by experimentally removing their T cells. We further found that LF correlated with widespread activation of macrophages, which again depended on T cells. Similar to T cells, macrophages are important players in our body's defense system, but their inflammatory products are also candidate mediators of LF. Taken together, these findings suggest that LF may represent an inappropriate host response to infection. Specifically, our study demonstrates a two-faced role of T cell responses against LASV. Such detrimental aspects of immune defense need to be given consideration in future LF vaccine development, to avoid enhancement of disease in vaccinated individuals.

Lassa virus

Lassa virus is a RNA virus belonging to the family of Arenaviridae. It was discovered as the causative agent of a hemorrhagic fever--Lassa fever--about 30 years ago. Lassa fever is endemic in West Africa and is estimated to affect some 100,000 people annually. Great progress in the understanding of the life cycle of arenaviruses, including Lassa virus, has been made in recent years. New insights have been gained in the pathogenesis and molecular epidemiology of Lassa fever, and state-of the-art technologies for diagnosing this life-threatening disease have been developed. The intention of this review is to summarize in particular the recent literature on Lassa virus and Lassa fever. Several aspects ranging from basic research up to clinical practice and laboratory diagnosis are discussed and linked together.

Infection of dendritic cells by lymphocytic choriomeningitis virus

Dendritic cells (DCs) comprise the major antigen-presenting cells (APCs) of the host, uniquely programmed to stimulate immunologically naïve T lymphocytes. Viruses that can target and disorder the function of these cells enjoy a selective advantage. The cellular receptor for lymphocytic choriomeningitis virus (LCMV), Lassa fever virus (LFV), and several other arenaviruses is alpha-dystroglycan (alpha-DG). Among cells of the immune system, CD11c+ and DEC-205+ DCs primarily and preferentially express alpha-DG. By selection, strains and variants of LCMV generated as quasi-species that bind alpha-DG with high affinity replicate in the majority of CD11c+ and DEC-205+ (>75%) DCs, causing a generalized immunosuppression, and establish a persistent infection. In contrast, viral strains and variants that bind with low affinity to alpha-DG display minimal replication in CD11c+ and DEC-205+ DCs (<10%), rarely replicate in the white pulp, and generate a robust anti-LCMV CTL response that clears the virus infection. Hence, receptor-virus interaction on DCs in vivo is an essential step in the initiation of virus-induced immunosuppression and viral persistence. Investigation into the mechanism of how virus-infected DCs cause immunosuppression reveals loss of MHC class II surface expression and costimulatory molecules on surface of such DCs. As a consequence DCs are unable to act as APCs, initiate immune responses, and have a defect in migration into the T cell area. These data indicate that LCMV infection influences DC maturation and migration, leading to decreased T cell stimulatory capacity of DCs, events essential for the initiation of immune responses. Because several other viruses known to cause immunosuppression (HIV, measles) interact with DCs, the observations noted here are likely a common selective mechanism by which viruses also are able to evade the host's immune system.

Characterization of the arenavirus RING finger Z protein regions required for Z-mediated inhibition of viral RNA synthesis

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is an enveloped virus with a bisegmented negative-strand RNA genome whose proteomic capability is limited to four polypeptides, namely, nucleoprotein; surface glycoprotein (GP), which is proteolytically processed into GP1 and GP2; polymerase (L); and a small (11-kDa) RING finger protein (Z). Using a reverse genetic system based on the ARM strain of LCMV, we have previously shown that Z has a strong inhibitory activity on LCMV minigenome transcription and RNA replication (T. I. Cornu and J. C. de la Torre, J. Virol. 75:9415-9426, 2001). In the present study, we have identified regions and specific amino acid residues within Z which contribute to its inhibitory activity on RNA synthesis mediated by the LCMV polymerase. Z proteins from different LCMV strains had similar inhibitory activities on the expression of the LCMV ARM minigenome, whereas the Z protein of the genetically more distantly related Tacaribe virus had an approximately 10-fold lower inhibitory activity on ARM minigenome expression. Results from the use of chimera proteins between Z and Xenopus Neuralized, a nonviral RING finger protein, indicated that the structural integrity of the Z RING domain (RD) was required but not sufficient for the inhibitory activity of Z. Serial deletion mutants of the N and C termini of Z showed that the N terminus (residues 1 through 16) and C terminus (residues 79 through 90) do not contribute to the Z inhibitory activity. A highly conserved tryptophan (W) residue located at position 36 in ARM-Z, next to the second conserved cysteine (C) residue of the Z RD, also contributed to the Z inhibitory activity.

RING finger Z protein of lymphocytic choriomeningitis virus (LCMV) inhibits transcription and RNA replication of an LCMV S-segment minigenome

Arenaviruses have a bisegmented negative-strand RNA genome whose proteomic capability is limited to only four polypeptides, namely, nucleoprotein (NP), surface glycoprotein (GP) that is proteolytically processed into GP1+GP2, polymerase (L), and a small (11-kDa) RING finger protein (Z). The role of Z during the Lymphocytic choriomeningitis virus (LCMV) life cycle is poorly understood. We investigated the function of Z in virus transcription and replication by using a reverse genetic system for the prototypic arenavirus LCMV. This system involves an LCMV minigenome and the minimal viral trans-acting factors (NP and L), expressed from separated cotransfected plasmids. Cotransfection of the Z cDNA strongly inhibited LCMV minigenome expression. The effect required synthesis of Z protein; its magnitude was dose dependent and occurred with levels of Z protein substantially lower than those observed in LCMV-infected cells. Coexpression of Z did not prevent the encapsidation of plasmid supplied minigenome, but it affected both transcription and RNA replication similarly. Mutations in Z that unfolded its RING finger domain eliminated its inhibitory activity, but RING proteins not related to Z did not affect LCMV minigenome expression. Consistent with the minigenome results, cells transiently expressing Z exhibited decreased susceptibility to infection with LCMV.

Towards a human Lassa fever vaccine

Arenaviruses, such as Lassa fever, establish chronic infections in rodents, leading to incidental transmission to humans. Lassa fever is a clinically severe disease, yet the absence of second attacks implies life-long immunity. The aim of this review is to consider whether such immunity could be provided by vaccines. The South American arenaviruses are controlled by neutralising antibody and a clinical trial of live, attenuated vaccine for Argentinian haemorrhagic fever provided 84% protection. In contrast, there is no evidence for protective humoral immunity against Old World arenaviruses which are controlled by cell-mediated immune responses. Nevertheless, vaccination with Lassa glycoproteins can protect monkeys from disease, implying that protection may be achievable, even though the immunological mechanisms are distinct. Recombinant vaccinia viruses expressing various forms of Lassa glycoproteins can protect both guinea-pigs and primates, while additional protective responses can be mounted against nucleocapsid genes. However, vaccines based upon vaccinia constructs are no longer tenable for African populations with a high seroprevalence of HIV infection. The scientific challenge now remains to find alternative methods of delivering T-cell immunity against glycoproteins from Lassa virus in ways which can overcome the local economic and political hurdles to vaccine development.

Hemorrhagic fever virus-induced changes in hemostasis and vascular biology

Viral hemorrhagic fever (VHF) denotes a virus-induced acute febrile, hemorrhagic disease reported from wide areas of the world. Hemorrhagic fever (HF) viruses are encapsulated, single-stranded RNA viruses that are associated with insect or rodent vectors whose interaction with humans defines the mode of disease transmission. There are 14 HF viruses, which belong to four viral families: Arenaviridae, Bunyaviridae, Filoviridae and Flaviviridae. This review presents, in order, the following aspects of VHF: (1) epidemiology, (2) anomalies of platelets and coagulation factors, (3) vasculopathy, (4) animal models of VHFs, (5) pathogenic mechanisms, and (6) treatment and future studies. HF viruses produce the manifestations of VHFs either by direct effects on cellular functions or by activation of immune and inflammatory pathways. In Lassa fever, Rift Valley fever and Crimean-Congo HF, the main feature of fatal illness appears to be impaired/delayed cellular immunity, which leads to unchecked viremia. However, in HF with renal syndrome and dengue HF, the immune response plays an active role in disease pathogenesis. The interplay of hemostasis, immune response, and inflammation is very complex. Molecular biologic techniques and the use of animal models have helped to unravel some of these interactions.

Haemorrhagic fevers and ecological perturbations

Hemorrhagic fever is a clinical and imprecise definition for several different diseases. Their main common point is to be zoonoses. These diseases are due to several viruses which belong to different families. The Flaviviridae have been known for the longest time. They include the Amaril virus that causes yellow fever and is transported by mosquitoes. Viruses that have come to light more recently belong to three other families: Arenaviridae, Bunyaviridae, and Filoviridae. They are transmitted by rodents (hantaviruses and arenaviruses) or from unknown reservoirs (Ebola Marburg). The primary cause of most outbreaks of hemorrhagic fever viruses is ecological disruption resulting from human activities. The expansion of the world population perturbs ecosystems that were stable a few decades ago and facilitates contacts with animals carrying viruses pathogenic to humans. Another dangerous human activity is the development of hospitals with poor medical hygiene. Lassa, Crimean-Congo or Ebola outbreaks are mainly nosocomial. There are also natural environmental changes: the emergence of Sin Nombre in the U.S. resulted from heavier than usual rain and snow during spring 1993 in the Four Corners. Biological industries also present risks. In 1967, collection of organs from monkeys allowed the discovery in Marburg of a new family of viruses, the Filoviridae. Hemorrhagic fever viruses are cause for worry, and the avenues to reduce their toll are still limited.

Cross-protection against lymphocytic choriomeningitis virus mediated by a CD4+ T-cell clone specific for an envelope glycoprotein epitope of Lassa virus

Recombinant vaccinia virus expressing the Lassa virus (LV) envelope glycoprotein precursor, V-LSGPC, was used to study the basis of LV-induced cross-protective immunity against the closely related arenavirus lymphocytic choriomeningitis virus (LCMV). C3H/HeJ mice primed with V-LSGPC developed neither circulating antibodies nor CD8+ cytotoxic T cells specific for LCMV, yet they resisted a normally lethal LCMV challenge. Spleen cells from such mice gave a proliferative response to LCMV in vitro that was inhibitable by anti-CD4 antibody. Synthetic peptides corresponding to predicted T-cell sites common to the envelope glycoprotein precursor (GP-C) of LV and that of LCMV were used to map the specificity of the proliferative response to an epitope located between amino acids 403 and 417 of LV GP-C. Several CD4+ T-cell clones specific for the 403-417 peptide were isolated and found to produce gamma interferon in response to both the peptide and LCMV. One of these clones, C9, was selected for further study. C9 lysed I-AK-bearing target cells, and when adoptively transferred to C3H/HeJ mice, it was capable of mediating both a peptide-specific delayed hypersensitivity reaction and resistance to lethal LCMV challenge. These collective findings demonstrate, for the first time, that CD4+ T cells can play a major role in arenavirus-specific cross-protective immunity.

Viral diseases involving the liver

Even though HAV, HBV and HNANB viruses are responsible for most of the viral hepatitis cases, many other viruses have been reported to cause hepatic injury. These viruses may involve the liver, either as part of a systemic illness (e.g. EBV, CMV, HSV) or as the primary target organ (e.g. yellow fever virus, Lassa fever virus, Ebola virus). Clinically overt hepatocellular dysfunction is rare in such viral infections. Biochemical disturbance of hepatic functions shown, for example, by rises in AST and ALT, is a frequent event and indicates hepatic damage. Morphological changes of the liver include varying degrees of hepatic necrosis with a paucity of inflammatory activities. Intranuclear or cytoplasmic inclusion bodies may be characteristic findings in these diseases. Laboratory diagnosis depends upon serology and liver histology. Treatment is still largely supportive in most of these diseases, although recent trials of antiviral agents show promise against some viruses. Chronic sequelae, such as cirrhosis or hepatocellular cancer, are not encountered. More work is needed to elucidate the pathogenesis of hepatic injury in these illnesses.

Lassa fever. Effective therapy with ribavirin

In a study of Lassa fever in Sierra Leone, West Africa, we identified two variables associated with a high risk of death, and we evaluated the efficacy of ribavirin and Lassa virus-convalescent plasma for the treatment of Lassa fever. A serum aspartate aminotransferase level greater than or equal to 150 IU per liter at the time of hospital admission was associated with a case-fatality rate of 55 percent (33 of 60). Patients with the same risk factor who were treated for 10 days with intravenous ribavirin, begun within the first 6 days after the onset of fever, had a case-fatality rate of 5 percent (1 of 20) (P = 0.0002 by Fisher's exact test). Patients whose treatment began seven or more days after the onset of fever had a case-fatality rate of 26 percent (11 of 43) (P = 0.01). Viremia with levels greater than or equal to 10(3.6) TCID50 per milliliter on admission was associated with a case-fatality rate of 76 percent (35 of 46). Patients with this risk factor who were treated with intravenous ribavirin within the first six days after onset of fever had a case-fatality rate of 9 percent (1 of 11) (P = 0.006), whereas those treated after seven days or more of illness had a fatality rate of 47 percent (9 of 19) (P = 0.035). Oral ribavirin was also effective in patients at high risk of death. Lassa-convalescent plasma did not significantly reduce mortality in any of the high-risk groups. We conclude that ribavirin is effective in the treatment of Lassa fever and that it should be used at any point in the illness, as well as for postexposure prophylaxis.