Towards a human Lassa fever vaccine

The Adaptive Immune Response against Bunyavirales

The Bunyavirales order includes at least fourteen families with diverse but related viruses, which are transmitted to vertebrate hosts by arthropod or rodent vectors. These viruses are responsible for an increasing number of outbreaks worldwide and represent a threat to public health. Infection in humans can be asymptomatic, or it may present with a range of conditions from a mild, febrile illness to severe hemorrhagic syndromes and/or neurological complications. There is a need to develop safe and effective vaccines, a process requiring better understanding of the adaptive immune responses involved during infection. This review highlights the most recent findings regarding T cell and antibody responses to the five Bunyavirales families with known human pathogens (Peribunyaviridae, Phenuiviridae, Hantaviridae, Nairoviridae, and Arenaviridae). Future studies that define and characterize mechanistic correlates of protection against Bunyavirales infections or disease will help inform the development of effective vaccines.

Molecular Engineering of a Mammarenavirus with Unbreachable Attenuation

Several mammarenaviruses cause severe hemorrhagic fever (HF) disease in humans and pose important public health problems in their regions of endemicity. There are no United States (US) Food and Drug Administration (FDA)-approved mammarenavirus vaccines, and current anti-mammarenavirus therapy is limited to an off-label use of ribavirin that has limited efficacy. Mammarenaviruses are enveloped viruses with a bi-segmented negative-strand RNA genome. Each genome segment contains two open reading frames (ORF) separated by a noncoding intergenic region (IGR). The large (L) segment encodes the RNA dependent RNA polymerase, L protein, and the Z matrix protein, whereas the small (S) segment encodes the surface glycoprotein precursor (GPC) and nucleoprotein (NP). In the present study, we document the generation of a recombinant form of the prototypic mammarenavirus lymphocytic choriomeningitis virus (LCMV) expressing a codon deoptimized (CD) GPC and containing the IGR of the S segment in both the S and L segments (rLCMV/IGR-CD). We show that rLCMV/IGR-CD is fully attenuated in C57BL/6 (B6) mice but able to provide complete protection upon a single administration against a lethal challenge with LCMV. Importantly, rLCMV/IGR-CD exhibited an unbreachable attenuation for its safe implementation as a live-attenuated vaccine (LAV). IMPORTANCE Several mammarenaviruses cause severe disease in humans and pose important public health problems in their regions of endemicity. Currently, no FDA-licensed mammarenavirus vaccines are available, and anti-mammarenaviral therapy is limited to an off-label use of ribavirin whose efficacy is controversial. Here, we describe the generation of recombinant version of the prototypic mammarenavirus lymphocytic choriomeningitis virus (rLCMV) combining the features of a codon deoptimized (CD) GPC and the noncoding intergenic region (IGR) of the S segment in both S and L genome segments, called rLCMV/IGR-CD. We present evidence that rLCMV/IGR-CD has excellent safety and protective efficacy features as live-attenuated vaccine (LAV). Importantly, rLCMV/IGR-CD prevents, in coinfected mice, the generation of LCMV reassortants with increased virulence. Our findings document a well-defined molecular strategy for the generation of mammarenavirus LAV candidates able to trigger long-term protective immunity, upon a single immunization, while exhibiting unique enhanced safety features, including unbreachable attenuation.

Third-generation smallpox vaccine strain-based recombinant vaccines for viral hemorrhagic fevers

Vaccinia virus has been used as a smallpox vaccine. Now that smallpox has been eradicated, the vaccinia virus is expected to be used as a bioterrorism countermeasure and a recombinant vaccine vector for other infectious diseases, such as viral hemorrhagic fevers. Many vaccinia virus strains were used as smallpox vaccines in the smallpox eradication campaign coordinated by the World Health Organization. These strains can be classified into generations, according to the history of improving production methods and efforts to reduce the adverse reactions. Significantly, the third-generation of smallpox vaccine strains, which include modified vaccinia Ankara (MVA) and LC16m8, are currently popular as recombinant vaccine vectors due to their well-balanced safety and immunogenicity profiles. The present review firstly focuses on the characteristics of the smallpox vaccine generations. The historical background of the development of the third-generation smallpox vaccine strains is detailed, along with the history of the transition of the vaccinia virus generation used as vectors for hemorrhagic fever vaccines to the third generation. Among the vaccinia viruses, MVA is currently the most commonly used vector for developing hemorrhagic fever vaccines, including dengue fever, yellow fever, Ebola viral disease, Lassa fever, Rift Valley fever, and Crimean-Congo hemorrhagic fever. LC16m8 is a vaccine candidate for severe fever with thrombocytopenia syndrome. The current status and recent advances in the development of these hemorrhagic fever vaccines using third-generation vaccinia strains are discussed.

Routine and pulse vaccination for Lassa virus could reduce high levels of endemic disease: A mathematical modelling study

Lassa fever is an acute viral illness caused by Lassa virus (LASV), a rodent-borne pathogen. LASV is endemic to much of Sub-Saharan West Africa, where seasonal outbreaks cause significant morbidity and mortality. Increased global awareness of LASV has led to development of improved diagnostic tests, treatments and vaccines. As vaccine candidates are trialled, it is essential to assess the potential outcomes of introducing a LASV vaccination program in endemic regions. This study investigates the potential outcomes of routine and pulse vaccination strategies using a deterministic mathematical model that captures seasonal LASV transmission between rodents and humans. For plausible parameter values, we find that immunization of 40% of infants at 70% vaccine effectiveness achieves a population-level reduction in infectious case numbers of 30%, while coverage of 60% at 90% vaccine effectiveness achieves a 56% reduction. Similar reductions can be achieved more rapidly via population-wide pulse vaccination at 11% coverage (30% reduction at 70% effectiveness) or 23% coverage (56% reduction at 90% effectiveness) repeated every 10 years. Similar pulse vaccine doses delivered at reduced frequency, but increased coverage achieves a greater reduction in infectious cases. Findings around infant vaccination are sensitive to our assumption that immunity is life-long, while pulse-vaccination has only slightly reduced effect if immunity lasts 10-30 years. An effective LASV vaccination program would incorporate pulse vaccination in addition to routine childhood immunization to limit disease. Estimates of feasible vaccine coverage and effectiveness are needed to fully quantify the likely benefits of a vaccination program in LASV endemic regions.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

Improving the Breadth of the Host's Immune Response to Lassa Virus

In 2017, the global Coalition for Epidemic Preparedness (CEPI) declared Lassa virus disease to be one of the world's foremost biothreats. In January 2018, World Health Organization experts met to address the Lassa biothreat. It was commonly recognized that the diversity of Lassa virus (LASV) isolated from West African patient samples was far greater than that of the Ebola isolates from the West African epidemic of 2013⁻2016. Thus, vaccines produced against Lassa virus disease face the added challenge that they must be broadly-protective against a wide variety of LASV. In this review, we discuss what is known about the immune response to Lassa infection. We also discuss the approaches used to make broadly-protective influenza vaccines and how they could be applied to developing broad vaccine coverage against LASV disease. Recent advances in AIDS research are also potentially applicable to the design of broadly-protective medical countermeasures against LASV disease.

A DNA vaccine delivered by dermal electroporation fully protects cynomolgus macaques against Lassa fever

Lassa virus (LASV) is an ambisense RNA virus in the Arenaviridae family and is the etiological agent of Lassa fever, a severe hemorrhagic disease endemic to West and Central Africa.^1,2 There are no US Food and Drug Administration (FDA)-licensed vaccines available to prevent Lassa fever.^1,2 in our previous studies, we developed a gene-optimized DNA vaccine that encodes the glycoprotein precursor gene of LASV (Josiah strain) and demonstrated that 3 vaccinations accompanied by dermal electroporation protected guinea pigs from LASV-associated illness and death. Here, we describe an initial efficacy experiment in cynomolgus macaque nonhuman primates (NHPs) in which we followed an identical 3-dose vaccine schedule that was successful in guinea pigs, and a follow-on experiment in which we used an accelerated vaccination strategy consisting of 2 administrations, spaced 4 weeks apart. In both studies, all of the LASV DNA-vaccinated NHPs survived challenge and none of them had measureable, sustained viremia or displayed weight loss or other disease signs post-exposure. Three of 10 mock-vaccinates survived exposure to LASV, but all of them became acutely ill post-exposure and remained chronically ill to the study end point (45 d post-exposure). Two of the 3 survivors experienced sensorineural hearing loss (described elsewhere). These results clearly demonstrate that the LASV DNA vaccine combined with dermal electroporation is a highly effective candidate for eventual use in humans.

An LASV GPC pseudotyped virus based reporter system enables evaluation of vaccines in mice under non-BSL-4 conditions

Lassa virus (LASV) causes a severe hemorrhagic fever endemic throughout western Africa. Because of the ability to cause lethal disease in humans, limited treatment options, and potential as a bioweapon, the need for vaccines to prevent LASV epidemic is urgent. However, LASV vaccine development has been hindered by the lack of appropriate small animal models for efficacy evaluation independent of biosafety level four (BSL-4) facilities. Here we generated an LASV-glycoprotein precursor (GPC)-pseudotyped Human immunodeficiency virus containing firefly luciferase (Fluc) reporter gene as surrogate to develop a bioluminescent-imaging-based BALB/c mouse model for one-round infection under non-BSL-4 conditions, in which the bioluminescent intensity of Fluc was utilized as endpoint when evaluating vaccine efficacy. Electron microscopy analysis demonstrated that LASV GPC pseudotyped virus appeared structurally similar to native virion. Meanwhile, we constructed DNA vaccine (pSV1.0-LASVGPC) and pseudoparticle-based vaccine (LASVpp) that displayed conformational GPC protein of LASV strain Josiah to vaccinate BALB/c mice using intramuscular electroporation and by intraperitoneal routes, respectively. Vaccinated mice in LASVpp alone and DNA prime+LASVpp boost schedules were protected against 100 AID₅₀ of LASV pseudovirus challenge, and it was found that in vivo efficiencies correlated with their anti-LASV neutralizing activities and MCP-1 cytokine levels in serum sampled before infection. The bioluminescence pseudovirus infection model can be useful tool for the preliminary evaluation of immunogenicity and efficacy of vaccine candidates against LASV outside of BSL-4 containments, and the results with pseudoparticle-based vaccine provided very helpful information for LASV vaccine design.

Structure-function relationship of the mammarenavirus envelope glycoprotein

Mammarenaviruses, including lethal pathogens such as Lassa virus and Junín virus, can cause severe hemorrhagic fever in humans. Entry is a key step for virus infection, which starts with binding of the envelope glycoprotein (GP) to receptors on target cells and subsequent fusion of the virus with target cell membranes. The GP precursor is synthesized as a polypeptide, and maturation occurs by two cleavage events, yielding a tripartite GP complex (GPC) formed by a stable signal peptide (SSP), GP1 and GP2. The unique retained SSP interacts with GP2 and plays essential roles in virion maturation and infectivity. GP1 is responsible for binding to the cell receptor, and GP2 is a class I fusion protein. The native structure of the tripartite GPC is unknown. GPC is critical for the receptor binding, membrane fusion and neutralization antibody recognition. Elucidating the molecular mechanisms underlining the structure-function relationship of the three subunits is the key for understanding their function and can facilitate novel avenues for combating virus infections. This review summarizes the basic aspects and recent research of the structure-function relationship of the three subunits. We discuss the structural basis of the receptor-binding domain in GP1, the interaction between SSP and GP2 and its role in virion maturation and membrane fusion, as well as the mechanism by which glycosylation stabilizes the GPC structure and facilitates immune evasion. Understanding the molecular mechanisms involved in these aspects will contribute to the development of novel vaccines and treatment strategies against mammarenaviruses infection.

Molecular Mechanism for LAMP1 Recognition by Lassa Virus

Lassa virus is a notorious human pathogen that infects many thousands of people each year in West Africa, causing severe viral hemorrhagic fevers and significant mortality. The surface glycoprotein of Lassa virus mediates receptor recognition through its GP1 subunit. Here we report the crystal structure of GP1 from Lassa virus, which is the first representative GP1 structure for Old World arenaviruses. We identify a unique triad of histidines that forms a binding site for LAMP1, a known lysosomal protein recently discovered to be a critical receptor for internalized Lassa virus at acidic pH. We demonstrate that mutation of this histidine triad, which is highly conserved among Old World arenaviruses, impairs LAMP1 recognition. Our biochemical and structural data further suggest that GP1 from Lassa virus may undergo irreversible conformational changes that could serve as an immunological decoy mechanism. Together with a variable region that we identify on the surface of GP1, those could be two distinct mechanisms that Lassa virus utilizes to avoid antibody-based immune response.

IMPORTANCE

Structural data at atomic resolution for viral proteins is key for understanding their function at the molecular level and can facilitate novel avenues for combating viral infections. Here we used X-ray protein crystallography to decipher the crystal structure of the receptor-binding domain (GP1) from Lassa virus. This is a pathogenic virus that causes significant illness and mortality in West Africa. This structure reveals the overall architecture of GP1 domains from the group of viruses known as the Old World arenaviruses. Using this structural information, we elucidated the mechanisms for pH switch and binding of Lassa virus to LAMP1, a recently identified host receptor that is critical for successful infection. Lastly, our structural analysis suggests two novel immune evasion mechanisms that Lassa virus may utilize to escape antibody-based immune response.

Current and emerging strategies for the diagnosis, prevention and treatment of Lassa fever

ABSTRACT 

Lassa fever (LF) is a potentially fatal disease that affects an estimated 300,000–500,000 people in endemic areas of west Africa each year. Though past studies have identified fatality rates of 5–20% in patients suspected to have contracted Lassa virus (LASV), new studies using more precise clinical diagnoses and modern diagnostic assays show fatalities rates above 60% in acutely ill patients from endemic regions. Currently, there are no approved vaccines or therapeutics, and only one Comformité Européenne (CE) marked rapid immunodiagnostic for acute LASV infection. Therefore, preventing LASV transmission is the primary goal in endemic regions. Development of rapid immunodiagnostics and research into the efficacy of current treatment options continues toward saving lives in west Africa as well as creating a line of defense against the nefarious use of LASV in bioterrorism settings.

Genomic profiling of host responses to Lassa virus: therapeutic potential from primate to man

Lassa virus infection elicits distinctive changes in host gene expression and metabolism. We focus on changes in host gene expression that may be biomarkers that discriminate individual pathogens or may help to provide a prognosis for disease. In addition to assessing mRNA changes, functional studies are also needed to discriminate causes of disease from mechanisms of host resistance. Host responses that drive pathogenesis are likely to be targets for prevention or therapy. Host responses to Lassa or its related arenaviruses have been monitored in cell culture, in animal models of hemorrhagic fever, in Lassa-infected nonhuman primates and, to a limited extent, in infected human beings. Here, we describe results from those studies and discuss potential targets for reducing virus replication and mitigating disease.

A bioinformatics tool for epitope-based vaccine design that accounts for human ethnic diversity: application to emerging infectious diseases

BACKGROUND

Peptide vaccination based on multiple T-cell epitopes can be used to target well-defined ethnic populations. Because the response to T-cell epitopes is restricted by HLA proteins, the HLA specificity of T-cell epitopes becomes a major consideration for epitope-based vaccine design. We have previously shown that CD4+ T-cell epitopes restricted by 95% of human MHC class II proteins can be predicted with high-specificity.

METHODS

We describe here the integration of epitope prediction with population coverage and epitope selection algorithms. The population coverage assessment makes use of the Allele Frequency Net Database. We present the computational platform Predivac-2.0 for HLA class II-restricted epitope-based vaccine design, which accounts comprehensively for human genetic diversity.

RESULTS

We validated the performance of the tool on the identification of promiscuous and immunodominant CD4+ T-cell epitopes from the human immunodeficiency virus (HIV) protein Gag. We further describe an application for epitope-based vaccine design in the context of emerging infectious diseases associated with Lassa, Nipah and Hendra viruses. Putative CD4+ T-cell epitopes were mapped on the surface glycoproteins of these pathogens and are good candidates to be experimentally tested, as they hold potential to provide cognate help in vaccination settings in their respective target populations.

CONCLUSION

Predivac-2.0 is a novel approach in epitope-based vaccine design, particularly suited to be applied to virus-related emerging infectious diseases, because the geographic distributions of the viruses are well defined and ethnic populations in need of vaccination can be determined ("ethnicity-oriented approach"). Predivac-2.0 is accessible through the website http://predivac.biosci.uq.edu.au/.

Enhanced Efficacy of a Codon-Optimized DNA Vaccine Encoding the Glycoprotein Precursor Gene of Lassa Virus in a Guinea Pig Disease Model When Delivered by Dermal Electroporation

Lassa virus (LASV) causes a severe, often fatal, hemorrhagic fever endemic to West Africa. Presently, there are no FDA-licensed medical countermeasures for this disease. In a pilot study, we constructed a DNA vaccine (pLASV-GPC) that expressed the LASV glycoprotein precursor gene (GPC). This plasmid was used to vaccinate guinea pigs (GPs) using intramuscular electroporation as the delivery platform. Vaccinated GPs were protected from lethal infection (5/6) with LASV compared to the controls. However, vaccinated GPs experienced transient viremia after challenge, although lower than the mock-vaccinated controls. In a follow-on study, we developed a new device that allowed for both the vaccine and electroporation pulse to be delivered to the dermis. We also codon-optimized the GPC sequence of the vaccine to enhance expression in GPs. Together, these innovations resulted in enhanced efficacy of the vaccine. Unlike the pilot study where neutralizing titers were not detected until after virus challenge, modest neutralizing titers were detected in guinea pigs before challenge, with escalating titers detected after challenge. The vaccinated GPs were never ill and were not viremic at any timepoint. The combination of the codon-optimized vaccine and dermal electroporation delivery is a worthy candidate for further development.

A historical look at the first reported cases of Lassa fever: IgG antibodies 40 years after acute infection

Lassa fever is an acute and sometimes severe viral hemorrhagic illness endemic in West Africa. One important question regarding Lassa fever is the duration of immunoglobulin G (IgG) antibody after infection. We were able to locate three persons who worked in Nigeria dating back to the 1940s, two of whom were integrally involved in the early outbreaks and investigations of Lassa fever in the late 1960s, including the person from whom Lassa virus was first isolated. Two persons had high titers of Lassa virus-specific IgG antibody over 40 years after infection, indicating the potential for long-term duration of these antibodies. One person was likely infected in 1952, 17 years before the first recognized outbreak. We briefly recount the fascinating stories of these three pioneers and their important contribution to our understanding of Lassa fever.

Vaccine Platforms to Control Arenaviral Hemorrhagic Fevers

Arenaviruses are rodent-borne emerging human pathogens. Diseases caused by these viruses, e.g., Lassa fever (LF) in West Africa and South American hemorrhagic fevers (HFs), are serious public health problems in endemic areas. We have employed replication-competent and replication-deficient strategies to design vaccine candidates potentially targeting different groups "at risk". Our leader LF vaccine candidate, the live reassortant vaccine ML29, is safe and efficacious in all tested animal models including non-human primates. In this study we showed that treatment of fatally infected animals with ML29 two days after Lassa virus (LASV) challenge protected 80% of the treated animals. In endemic areas, where most of the target population is poor and many live far from health care facilities, a single-dose vaccination with ML29 would be ideal solution. Once there is an outbreak, a fast-acting vaccine or post-exposure prophylaxis would be best. The 2(nd) vaccine technology is based on Yellow Fever (YF) 17D vaccine. We designed YF17D-based recombinant viruses expressing LASV glycoproteins (GP) and showed protective efficacy of these recombinants. In the current study we developed a novel technology to clone LASV nucleocapsid within YF17D C gene. Low immunogenicity and stability of foreign inserts must be addressed to design successful LASV/YFV bivalent vaccines to control LF and YF in overlapping endemic areas of West Africa. The 3(rd) platform is based on the new generation of alphavirus replicon virus-like-particle vectors (VLPV). Using this technology we designed VLPV expressing LASV GP with enhanced immunogenicity and bivalent VLPV expressing cross-reactive GP of Junin virus (JUNV) and Machupo virus (MACV), causative agents of Argentinian and Bolivian HF, respectively. A prime-boost regimen required for VLPV immunization might be practical for medical providers, military, lab personnel, and visitors in endemic areas.

Immune responses and Lassa virus infection

Lassa fever is a hemorrhagic fever endemic to West Africa and caused by Lassa virus, an Old World arenavirus. It may be fatal, but most patients recover from acute disease and some experience asymptomatic infection. The immune mechanisms associated with these different outcomes have not yet been fully elucidated, but considerable progress has recently been made, through the use of in vitro human models and nonhuman primates, the only relevant animal model that mimics the pathophysiology and immune responses induced in patients. We discuss here the roles of the various components of the innate and adaptive immune systems in Lassa virus infection and in the control of viral replication and pathogenesis.

Advanced vaccine candidates for Lassa fever

Lassa virus (LASV) is the most prominent human pathogen of the Arenaviridae. The virus is transmitted to humans by a rodent reservoir, Mastomys natalensis, and is capable of causing lethal Lassa Fever (LF). LASV has the highest human impact of any of the viral hemorrhagic fevers (with the exception of Dengue Fever) with an estimated several hundred thousand infections annually, resulting in thousands of deaths in Western Africa. The sizeable disease burden, numerous imported cases of LF in non-endemic countries, and the possibility that LASV can be used as an agent of biological warfare make a strong case for vaccine development. Presently there is no licensed vaccine against LF or approved treatment. Recently, several promising vaccine candidates have been developed which can potentially target different groups at risk. The purpose of this manuscript is to review the LASV pathogenesis and immune mechanisms involved in protection. The current status of pre-clinical development of the advanced vaccine candidates that have been tested in non-human primates will be discussed. Major scientific, manufacturing, and regulatory challenges will also be considered.

Arenavirus evasion of host anti-viral responses

The innate response to infection by an Old World arenavirus is initiated and mediated by extracellular and intracellular receptors, and effector molecules. In response, the invading virus has evolved to inhibit these responses and create the best environment possible for replication and spread. Here, we will discuss both the host's response to infection with data from human infection and lessons learned from animal models, as well as the multitude of ways the virus combats the resulting immune response. Finally, we will highlight recent work identifying TLR2 as an innate sensor for arenaviruses and how the TLR2-dependent response differs depending on the pathogenicity of the strain.

Healthy individuals' immune response to the Bulgarian Crimean-Congo hemorrhagic fever virus vaccine

Crimean-Congo hemorrhagic fever virus (CCHFV) poses a great threat to public health due to its high mortality and transmission rate and wide geographical distribution. There is currently no specific antiviral therapy for CCHF. This study provides the first in-depth analysis of the cellular and humoral immune response in healthy individuals following injection of inactivated Bulgarian vaccine, the only CCHFV vaccine available at present. Vaccinated individuals developed robust, anti-CCHFV-specific T-cell activity as measured by IFN-γ ELISpot assay. The frequency of IFN-γ secreting T-cells was 10-fold higher in individuals after vaccination with four doses than after one single dose. High levels of CCHFV antibodies were observed following the first dose, but repeated doses were required to achieve antibodies with neutralizing activity against CCHFV. However, the neutralizing activity in these groups was low.

[Arenavirus infections]

Arenaviruses are the collective name for viruses, which belong to the family Arenaviridae. They replicate in the cytoplasm of cells, and were named after the sandy (Latin, arenosus) appearance of the ribosomes often seen in thin sections of virions under electron microscope. Several arenaviruses, such as Lassa virus in West Africa, and Junin, Guanarito, Sabia, Machupo, and Chapare viruses in South America, cause sever viral hemorrhagic fevers (VHF) in humans and represent a serious public health problem. These viruses are categorized as category 1 pathogens thus should be handles in a BSL4 laboratory. Recently, Lujo virus was isolated as a newly discovered novel arenavirus associated with a VHF outbreak in southern Africa in 2008. Although, we have no VHF patients caused by arenaviruses in Japan, except for a single imported Lassa fever case in 1987, it is possible that VHF patients occur as imported cases as for other VHF in the future. Therefore, it is necessary to develop the diagnostics and therapeutics in consideration of patient's severe symptoms and high mortality even in the disease-free countries. In this review, we will broadly discuss the current knowledge from the basic researches to diagnostics and vaccine developments for arenavirus diseases.

Progress in recombinant DNA-derived vaccines for Lassa virus and filoviruses

Developing vaccines for highly pathogenic viruses such as those causing Lassa, Ebola, and Marburg hemorrhagic fevers is a daunting task due to both scientific and logistical constraints. Scientific hurdles to overcome include poorly defined relationships between pathogenicity and protective immune responses, genetic diversity of viruses, and safety in a target population that includes a large number of individuals with compromised immune systems. Logistical obstacles include the requirement for biosafety level-4 containment to study the authentic viruses, the poor public health infrastructure of the endemic disease areas, and the cost of developing these vaccines for use in non-lucrative markets. Recombinant DNA-based vaccine approaches offer promise of overcoming some of these issues. In this review, we consider the status of various recombinant DNA candidate vaccines against Lassa virus and filoviruses which have been tested in animals.

Coverage of related pathogenic species by multivalent and cross-protective vaccine design: arenaviruses as a model system

The arenaviruses are a family of negative-sense RNA viruses that cause severe human disease ranging from aseptic meningitis to hemorrhagic fever syndromes. There are currently no FDA-approved vaccines for the prevention of arenavirus disease, and therapeutic treatment is limited to the use of ribavirin and/or immune plasma for a subset of the pathogenic arenaviruses. The considerable genetic variability observed among the seven arenaviruses that are pathogenic for humans illustrates one of the major challenges for vaccine development today, namely, to overcome pathogen heterogeneity. Over the past 5 years, our group has tested several strategies to overcome pathogen heterogeneity, utilizing the pathogenic arenaviruses as a model system. Because T cells play a prominent role in protective immunity following arenavirus infection, we specifically focused on the development of human vaccines that would induce multivalent and cross-protective cell-mediated immune responses. To facilitate our vaccine development and testing, we conducted large-scale major histocompatibility complex (MHC) class I and class II epitope discovery on murine, nonhuman primate, and human backgrounds for each of the pathogenic arenaviruses, including the identification of protective HLA-restricted epitopes. Finally, using the murine model of lymphocytic choriomeningitis virus infection, we studied the phenotypic characteristics associated with immunodominant and protective T cell epitopes. This review summarizes the findings from our studies and discusses their application to future vaccine design.

A multivalent vaccination strategy for the prevention of Old World arenavirus infection in humans

Arenaviruses cause severe human disease ranging from aseptic meningitis following lymphocytic choriomeningitis virus (LCMV) infection to hemorrhagic fever syndromes following infection with Guanarito virus (GTOV), Junin virus (JUNV), Lassa virus (LASV), Machupo virus (MACV), Sabia virus (SABV), or Whitewater Arroyo virus (WWAV). Cellular immunity, chiefly the CD8(+) T-cell response, plays a critical role in providing protective immunity following infection with the Old World arenaviruses LASV and LCMV. In the current study, we evaluated whether HLA class I-restricted epitopes that are cross-reactive among pathogenic arenaviruses could be identified for the purpose of developing an epitope-based vaccination approach that would cross-protect against multiple arenaviruses. We were able to identify a panel of HLA-A*0201-restricted peptides derived from the same region of the glycoprotein precursor (GPC) of LASV (GPC spanning residues 441 to 449 [GPC(441-449)]), LCMV (GPC(447-455)), JUNV (GPC(429-437)), MACV (GPC(444-452)), GTOV (GPC(427-435)), and WWAV (GPC(428-436)) that displayed high-affinity binding to HLA-A*0201 and were recognized by CD8(+) T cells in a cross-reactive manner following LCMV infection or peptide immunization of HLA-A*0201 transgenic mice. Immunization of HLA-A*0201 mice with the Old World peptide LASV GPC(441-449) or LCMV GPC(447-455) induced high-avidity CD8(+) T-cell responses that were able to kill syngeneic target cells pulsed with either LASV GPC(441-449) or LCMV GPC(447-455) in vivo and provided significant protection against viral challenge with LCMV. Through this study, we have demonstrated that HLA class I-restricted, cross-reactive epitopes exist among diverse arenaviruses and that individual epitopes can be utilized as effective vaccine determinants for multiple pathogenic arenaviruses.

Polyfunctional CD4+ T cell responses to a set of pathogenic arenaviruses provide broad population coverage

Background

Several arenaviruses cause severe hemorrhagic fever and aseptic meningitis in humans for which no licensed vaccines are available. A major obstacle for vaccine development is pathogen heterogeneity within the Arenaviridae family. Evidence in animal models and humans indicate that T cell and antibody-mediated immunity play important roles in controlling arenavirus infection and replication. Because CD4⁺ T cells are needed for optimal CD8⁺ T cell responses and to provide cognate help for B cells, knowledge of epitopes recognized by CD4⁺ T cells is critical to the development of an effective vaccine strategy against arenaviruses. Thus, the goal of the present study was to define and characterize CD4⁺ T cell responses from a broad repertoire of pathogenic arenaviruses (including lymphocytic choriomeningitis, Lassa, Guanarito, Junin, Machupo, Sabia, and Whitewater Arroyo viruses) and to provide determinants with the potential to be incorporated into a multivalent vaccine strategy.

Results

By inoculating HLA-DRB1*0101 transgenic mice with a panel of recombinant vaccinia viruses, each expressing a single arenavirus antigen, we identified 37 human HLA-DRB1*0101-restricted CD4⁺ T cell epitopes from the 7 antigenically distinct arenaviruses. We showed that the arenavirus-specific CD4⁺ T cell epitopes are capable of eliciting T cells with a propensity to provide help and protection through CD40L and polyfunctional cytokine expression. Importantly, we demonstrated that the set of identified CD4⁺ T cell epitopes provides broad, non-ethnically biased population coverage of all 7 arenavirus species targeted by our studies.

Conclusions

The identification of CD4⁺ T cell epitopes, with promiscuous binding properties, derived from 7 different arenavirus species will aid in the development of a T cell-based vaccine strategy with the potential to target a broad range of ethnicities within the general population and to protect against both Old and New World arenavirus infection.

A multivalent and cross-protective vaccine strategy against arenaviruses associated with human disease

Arenaviruses are the causative pathogens of severe hemorrhagic fever and aseptic meningitis in humans, for which no licensed vaccines are currently available. Pathogen heterogeneity within the Arenaviridae family poses a significant challenge for vaccine development. The main hypothesis we tested in the present study was whether it is possible to design a universal vaccine strategy capable of inducing simultaneous HLA-restricted CD8⁺ T cell responses against 7 pathogenic arenaviruses (including the lymphocytic choriomeningitis, Lassa, Guanarito, Junin, Machupo, Sabia, and Whitewater Arroyo viruses), either through the identification of widely conserved epitopes, or by the identification of a collection of epitopes derived from multiple arenavirus species. By inoculating HLA transgenic mice with a panel of recombinant vaccinia viruses (rVACVs) expressing the different arenavirus proteins, we identified 10 HLA-A02 and 10 HLA-A03-restricted epitopes that are naturally processed in human antigen-presenting cells. For some of these epitopes we were able to demonstrate cross-reactive CD8⁺ T cell responses, further increasing the coverage afforded by the epitope set against each different arenavirus species. Importantly, we showed that immunization of HLA transgenic mice with an epitope cocktail generated simultaneous CD8⁺ T cell responses against all 7 arenaviruses, and protected mice against challenge with rVACVs expressing either Old or New World arenavirus glycoproteins. In conclusion, the set of identified epitopes allows broad, non-ethnically biased coverage of all 7 viral species targeted by our studies.

Arenaviruses cause significant morbidity and mortality worldwide and are also regarded as a potential bioterrorist threat. CD8⁺ T cells restricted by class I MHC molecules clearly play a protective role in murine models of arenavirus infection, yet little is known about the epitopes recognized in the context of human class I MHC (HLA). Here, we defined 20 CD8⁺ T cell epitopes restricted by HLA class I molecules, derived from 7 different species of arenaviruses associated with human disease. To accomplish this task, we utilized epitope predictions, in vitro HLA binding assays, and HLA transgenic mice inoculated with recombinant vaccinia viruses (rVACV) expressing arenavirus antigens. Because our analysis targeted two of the most common HLA types worldwide, we project that the CD8⁺ T cell epitope set provides broad coverage against diverse ethnic groups within the human population. Furthermore, we show that immunization with a cocktail of these epitopes protects HLA transgenic mice from challenge with rVACV expressing antigens from different arenavirus species. Our findings suggest that a cell-mediated vaccine strategy might be able to protect against infection mediated by multiple arenavirus species.

Adaptive immunity to Lymphocytic choriomeningitis virus: new insights into antigenic determinants

Lymphocytic choriomeningitis virus (LCMV) is one of the most studied infectious disease models in mice. Human infection with LCMV can result in severe disease, ranging from aseptic meningitis in immunocompetent individuals, hydrocephalus, chorioretinitis or microcephaly in fetal infection, or to a highly lethal outcome in immunosuppressed individuals. This review examines recent advances in our understanding of the adaptive immune response to LCMV and how the cell-mediated and humoral immune responses contribute to protective immunity. New insights into the antigenicity of the LCMV proteome and the complexity of the cell-mediated immune response are addressed. We also discuss state-of-the-art approaches for T-cell epitope discovery in murine and human backgrounds and their recent application to LCMV. New findings regarding CD4+ T-cell dysregulation during chronic LCMV infection, and potential avenues for the treatment of chronic viral infection through modulation of the programmed cell death-1 receptor and/or IL-10 signaling pathways, are also evaluated.

Arenavirus entry occurs through a cholesterol-dependent, non-caveolar, clathrin-mediated endocytic mechanism

Arenaviruses are important causes of viral hemorrhagic fevers in humans. Arenavirus infection of cells occurs via a pH-dependent endocytic route, but detailed studies of entry pathways have not been done. We investigated the role of cell membrane cholesterol, caveolae, and clathrin coated pits in infection by Lassa virus (LASV), which utilizes alpha-dystroglycan (alpha-DG) as a receptor, and Pichindé virus (PICV), which does not. Depletion of cellular cholesterol by treatment with methyl betacyclodextrin (MbetaCD) or nystatin/progesterone inhibited PICV replication and transfer of packaged marker gene by LASV or PICV pseudotyped retroviral particles. In cells lacking caveolae due to silencing of the caveolin-1 gene, no inhibition of PICV infection or LASV pseudotype transduction was observed. However, PICV infection and LASV and PICV pseudotype transduction was inhibited when an Eps15 dominant negative mutant was used to inhibit clathrin-mediated endocytosis. Altogether, the results indicate that diverse arenaviruses have a common requirement for cell membrane cholesterol and clathrin mediated endocytosis in establishing infection.

A ML29 reassortant virus protects guinea pigs against a distantly related Nigerian strain of Lassa virus and can provide sterilizing immunity

Lassa virus (LASV) is responsible for the deaths of thousands of people in West Africa annually. Genetic diversity among LASV strains is the highest among the Arenaviridae and represents a great challenge for vaccine development. Guinea pigs vaccinated with a ML29 reassortant vaccine experienced sterilizing immunity and complete protection when challenged on day 30 either with homologous virus or with the distantly related Nigerian isolate. Simultaneous vaccination-challenge or challenge on day 2 after vaccination also protected 60-100% of the animals against both strains, but without sterilizing immunity. These results indicate that simultaneous replication of ML29 and LASV attenuates the virulence of LASV infection.

Identification of protective Lassa virus epitopes that are restricted by HLA-A2

Recovery from Lassa virus (LASV) infection usually precedes the appearance of neutralizing antibodies, indicating that cellular immunity plays a primary role in viral clearance. To date, the role of LASV-specific CD8(+) T cells has not been evaluated in humans. To facilitate such studies, we utilized a predictive algorithm to identify candidate HLA-A2 supertype epitopes from the LASV nucleoprotein and glycoprotein precursor (GPC) genes. We identified three peptides (GPC(42-50), GLVGLVTFL; GPC(60-68), SLYKGVYEL; and GPC(441-449), YLISIFLHL) that displayed high-affinity binding (< or =98 nM) to HLA-A*0201, induced CD8(+) T-cell responses of high functional avidity in HLA-A*0201 transgenic mice, and were naturally processed from native LASV GPC in human HLA-A*0201-positive target cells. HLA-A*0201 mice immunized with either GPC(42-50) or GPC(60-68) were protected against challenge with a recombinant vaccinia virus that expressed LASV GPC. The epitopes identified in this study represent potential diagnostic reagents and candidates for inclusion in epitope-based vaccine constructs. Our approach is applicable to any pathogen with existing sequence data, does not require manipulation of the actual pathogen or access to immune human donors, and should therefore be generally applicable to category A through C agents and other emerging pathogens.

Envelope exchange for the generation of live-attenuated arenavirus vaccines

Arenaviruses such as Lassa fever virus cause significant mortality in endemic areas and represent potential bioterrorist weapons. The occurrence of arenaviral hemorrhagic fevers is largely confined to Third World countries with a limited medical infrastructure, and therefore live-attenuated vaccines have long been sought as a method of choice for prevention. Yet their rational design and engineering have been thwarted by technical limitations. In addition, viral genes had not been identified that are needed to cause disease but can be deleted or substituted to generate live-attenuated vaccine strains. Lymphocytic choriomeningitis virus, the prototype arenavirus, induces cell-mediated immunity against Lassa fever virus, but its safety for humans is unclear and untested. Using this virus model, we have developed the necessary methodology to efficiently modify arenavirus genomes and have exploited these techniques to identify an arenaviral Achilles' heel suitable for targeting in vaccine design. Reverse genetic exchange of the viral glycoprotein for foreign glycoproteins created attenuated vaccine strains that remained viable although unable to cause disease in infected mice. This phenotype remained stable even after extensive propagation in immunodeficient hosts. Nevertheless, the engineered viruses induced T cell-mediated immunity protecting against overwhelming systemic infection and severe liver disease upon wild-type virus challenge. Protection was established within 3 to 7 d after immunization and lasted for approximately 300 d. The identification of an arenaviral Achilles' heel demonstrates that the reverse genetic engineering of live-attenuated arenavirus vaccines is feasible. Moreover, our findings offer lymphocytic choriomeningitis virus or other arenaviruses expressing foreign glycoproteins as promising live-attenuated arenavirus vaccine candidates.

A live attenuated vaccine for Lassa fever made by reassortment of Lassa and Mopeia viruses

Lassa virus (LASV) and Mopeia virus (MOPV) are closely related Old World arenaviruses that can exchange genomic segments (reassort) during coinfection. Clone ML29, selected from a library of MOPV/LASV (MOP/LAS) reassortants, encodes the major antigens (nucleocapsid and glycoprotein) of LASV and the RNA polymerase and zinc-binding protein of MOPV. Replication of ML29 was attenuated in guinea pigs and nonhuman primates. In murine adoptive-transfer experiments, as little as 150 PFU of ML29 induced protective cell-mediated immunity. All strain 13 guinea pigs vaccinated with clone ML29 survived at least 70 days after LASV challenge without either disease signs or histological lesions. Rhesus macaques inoculated with clone ML29 developed primary virus-specific T cells capable of secreting gamma interferon in response to homologous MOP/LAS and heterologous MOPV and lymphocytic choriomeningitis virus. Detailed examination of two rhesus macaques infected with this MOPV/LAS reassortant revealed no histological lesions or disease signs. Thus, ML29 is a promising attenuated vaccine candidate for Lassa fever.

Viral zoonoses in Europe

A number of new virus infections have emerged or re-emerged during the past 15 years. Some viruses are spreading to new areas along with climate and environmental changes. The majority of these infections are transmitted from animals to humans, and thus called zoonoses. Zoonotic viruses are, as compared to human-only viruses, much more difficult to eradicate. Infections by several of these viruses may lead to high mortality and also attract attention because they are potential bio-weapons. This review will focus on zoonotic virus infections occurring in Europe.

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.

Lessons from nosocomial viral haemorrhagic fever outbreaks

The outbreak of Marburg haemorrhagic fever in Angola in 2004-2005 shows once again the devastating and rapid spread of viral haemorrhagic fevers in medical settings where hygiene practices are poorly applied or ignored. The legacy of years of war and poverty in Angola has resulted in very poor medical education and services. The initial high rate of infection among infants in Angola may have been related to poor hospital practices, possibly administration of vaccines. Though the outbreak in Angola was in a part of Africa not previously known to have filovirus infection, prior ecological modelling had predicted this location and many others. Prevention of future outbreaks will not be easy. The urgent need is dissemination of knowledge and the training, discipline and resources for good clinical practice. Educating the public to demand higher standards could be a powerful tool. Good practices are difficult to establish and maintain on the scale needed.

The signal peptide of the Junín arenavirus envelope glycoprotein is myristoylated and forms an essential subunit of the mature G1-G2 complex

Arenaviruses comprise a diverse family of rodent-borne viruses that are responsible for recurring and emerging outbreaks of viral hemorrhagic fevers worldwide. The Junín virus, a member of the New World arenaviruses, is endemic to the pampas grasslands of Argentina and is the etiologic agent of Argentine hemorrhagic fever. In this study, we have analyzed the assembly and function of the Junín virus envelope glycoproteins. The mature envelope glycoprotein complex is proteolytically processed from the GP-C precursor polypeptide and consists of three noncovalently associated subunits, G1, G2, and a stable 58-amino-acid signal peptide. This tripartite organization is found both on virions of the attenuated Candid 1 strain and in cells expressing the pathogenic MC2 strain GP-C gene. Replacement of the Junín virus GP-C signal peptide with that of human CD4 has little effect on glycoprotein assembly while abolishing the ability of the G1-G2 complex to mediate pH-dependent cell-cell fusion. In addition, we demonstrate that the Junín virus GP-C signal peptide subunit is myristoylated at its N-terminal glycine. Alanine substitution for the modified glycine residue in the GP-C signal peptide does not affect formation of the tripartite envelope glycoprotein complex but markedly reduces its membrane fusion activity. In contrast to the classical view that signal peptides act primarily in targeting nascent polypeptides to the endoplasmic reticulum, we suggest that the signal peptide of the arenavirus GP-C may serve additional functions in envelope glycoprotein structure and trafficking.

Mucosal arenavirus infection of primates can protect them from lethal hemorrhagic fever

Arenaviruses are transmitted from rodents to human beings by blood or mucosal exposure. The most devastating arenavirus in terms of human disease is Lassa fever virus, causing up to 300,000 annual infections in West Africa. We used a model for Lassa fever in which Rhesus macaques were infected with a related virus, lymphocytic choriomeningitis virus (LCMV). Our goals were to determine the outcome of infection after mucosal inoculation and later lethal challenge, to characterize protective immune responses, and to test cross-protection between a virulent (LCMV-WE) and an avirulent (LCMV-ARM) strain of virus. Although intravenous infections in the monkey model were uniformly lethal, intragastric infections recapitulated the spectrum of clinical outcomes seen in human exposure to Lassa fever virus: death, recovery from disease, and most often, subclinical infection. Plaque neutralization, ELISA, lymphocyte proliferation, and chromium-release assays were used to monitor humoral and cellular immune responses. Cross protection between the two strains was observed. The three out of seven monkeys that experienced protection were also the three with the strongest cell-mediated immunity.

Lassa fever: epidemiology, clinical features, and social consequences

Lassa fever is endemic in west Africa, where it probably kills several thousand people each year. With access to the region improving, the opportunity, and the need, to improve our understanding of this disease are increasing

Molecular diagnostics of viral hemorrhagic fevers

This review addresses the diagnostics of viral hemorrhagic fevers (VHFs). In the first part, an overview is given on classical methods of VHF diagnostics as well as novel molecular diagnostic tools. Currently available polymerase chain reaction (PCR) assays for diagnosis of VHF are summarized and discussed. In the second part, VHF diagnostics are described in particular for Lassa fever, yellow fever, and Crimean-Congo hemorrhagic fever, based on cases that were imported into or occurred within Europe. The third part is focussed on important differential diagnoses of VHF.