Current progress towards vaccines for arenavirus-caused diseases

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.

The role of oligomerization for the biological functions of the arenavirus nucleoprotein

The Lassa virus nucleoprotein (NP) is a multifunctional protein that plays an essential role in many aspects of the viral life cycle, including RNA encapsidation, viral transcription and replication, recruitment of ribonucleoprotein complexes to viral budding sites, and inhibition of the host cell interferon response. While it is known that NP is capable of forming oligomers, both the oligomeric state of NP in mammalian cells and the significance of NP oligomerization for its various functions remain unclear. Here, we demonstrate that Lassa virus NP solely forms trimers upon expression in mammalian cells. Using a minigenome assay we show that mutants that are not able to form stable trimers are no longer functional during transcription and/or replication of the minigenome, indicating that NP trimerization is essential for transcription and/or replication of the viral genome. However, mutations leading to destabilization of the NP trimer did not impact the incorporation of NP into virus-like particles or its ability to suppress interferon-induced gene expression, two important functions of arenavirus NP.

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.

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.

Homologous and heterologous glycoproteins induce protection against Junin virus challenge in guinea pigs

Tacaribe virus (TACV) is an arenavirus that is genetically and antigenically closely related to Junin virus (JUNV), the aetiological agent of Argentine haemorrhagic fever (AHF). It is well established that TACV protects experimental animals fully against an otherwise lethal challenge with JUNV. To gain information on the nature of the antigens involved in cross-protection, recombinant vaccinia viruses were constructed that express the glycoprotein precursor (VV-GTac) or the nucleocapsid protein (VV-N) of TACV. TACV proteins expressed by vaccinia virus were indistinguishable from authentic virus proteins by gel electrophoresis. Guinea pigs inoculated with VV-GTac or VV-N elicited antibodies that immunoprecipitated authentic TACV proteins. Antibodies generated by VV-GTac neutralized TACV infectivity. Levels of antibodies after priming and boosting with recombinant vaccinia virus were comparable to those elicited in TACV infection. To evaluate the ability of recombinant vaccinia virus to protect against experimental AHF, guinea pigs were challenged with lethal doses of JUNV. Fifty per cent of the animals immunized with VV-GTac survived, whereas all animals inoculated with VV-N or vaccinia virus died. Having established that the heterologous glycoprotein protects against JUNV challenge, a recombinant vaccinia virus was constructed that expresses JUNV glycoprotein precursor (VV-GJun). The size and reactivity to monoclonal antibodies of the vaccinia virus-expressed and authentic JUNV glycoproteins were indistinguishable. Seventy-two per cent of the animals inoculated with two doses of VV-GJun survived lethal JUNV challenge. Protection with either VV-GJun or VV-GTac occurred in the presence of low or undetectable levels of neutralizing antibodies to JUNV.

Characterization of human CD4(+) T-cell clones recognizing conserved and variable epitopes of the Lassa virus nucleoprotein

T cells must play the major role in controlling acute human Lassa virus infection, because patients recover from acute Lassa fever in the absence of a measurable neutralizing antibody response. T cells alone seem to protect animals from a lethal Lassa virus challenge, because after experimental vaccination no neutralizing antibodies are detectable. In order to study human T-cell reactivity to single Lassa virus proteins, the nucleoprotein (NP) of Lassa virus, strain Josiah, was cloned, expressed in Escherichia coli, and affinity purified. Peripheral blood mononuclear cells (PBMC) obtained from 8 of 13 healthy, Lassa virus antibody-positive individuals living in the Republic of Guinea, western Africa, were found to proliferate in response to the recombinant protein (proliferation index >/=10). PBMC obtained from one individual with a particularly high proliferative response were used to generate 50 NP-specific T-cell clones (TCC). For six of these the epitopes were mapped with overlapping synthetic peptides derived from the sequence of the NP. These CD4(+) TCC displayed high specific proliferation and produced mainly gamma interferon upon stimulation with NP. Because variation of up to 15% in the amino acid sequences of the structural proteins of naturally occurring Lassa virus variants has been observed, the reactivity of the TCC with peptides derived from the homologous epitopes of the Nigeria strain of Lassa virus and of the eastern Africa arenavirus Mopeia was tested. With the Nigeria strain of Lassa virus the levels of homology were 100% for two of these epitopes and 85% for three of them, whereas homology with the respective Mopeia epitopes ranged from 92 to 69%. Reactivity of the TCC with peptides derived from the variable epitopes of the Nigeria strain and of Mopeia was reduced or completely abolished. This report shows for the first time that seropositive individuals from areas of endemicity have very strong memory CD4(+) T-cell responses against the NP of Lassa virus, which are partly strain specific and partly cross-reactive with other Lassa virus strains. Our findings may have important implications for the strategy of designing recombinant vaccines against this mainly T-cell-controlled human arenavirus infection.

Lassa fever: implications of T-cell immunity for vaccine development

Lassa fever is a re-emerging viral hemorrhagic fever, which causes significant human morbidity in endemic regions of West Africa. Attempts to vaccinate against this virus in animal models including non-human primates have revealed that eliciting a strong cellular immune response protects from clinical disease, but not infection, in the absence of measurable neutralizing antibodies. As there is renewed interest in developing a vaccine against Lassa fever for use in humans, several questions should be addressed in view of the scarce knowledge of the mechanisms of natural immunity against this disease. MHC-dependency of a vaccine relying mainly on the induction of T-cell immunity and its ability to cross-protect against different Lassa virus strains will be important issues. Furthermore, the question whether the vaccine can prevent human-to-human transmission of the virus should be discussed and the possibility that vaccination could predispose to immunopathology should be excluded. We are addressing some of the above mentioned problems concerning natural immunity through field studies in the Republic of Guinea, West Africa, and are presently studying the CD4 cell responses of Lassa antibody positive subjects on the basis of T-cell proliferation assays using recombinant Lassa virus proteins.

Molecular approaches for the treatment of hemorrhagic fever virus infections

Viruses causing hemorrhagic fevers in man belong to the following virus groups: togavirus (Chikungunya), flavivirus (dengue, yellow fever, Kyasanur Forest disease, Omsk hemorrhagic fever), arenavirus (Argentinian hemorrhagic fever, Bolivian hemorrhagic fever, Lassa fever), filovirus (Ebola, Marburg), phlebovirus (Rift Valley fever), nairovirus (Crimian-Congo hemorrhagic fever) and hantavirus (hemorrhagic fever with renal syndrome, nephropathic epidemia). Hemorrhagic fever virus infections can be approached by different therapeutic strategies: (i) vaccination; (ii) administration of high-titered antibodies; and (iii) treatment with antiviral drugs. Depending on the molecular target of their interaction, antiviral agents could be classified as follows: IMP dehydrogenase inhibitors (i.e., ribavirin and its derivatives); OMP decarboxylase inhibitors (i.e., pyrazofurin); CTP synthetase inhibitors (i.e., cyclopentylcytosine and cyclopentenylcytosine); SAH hydrolase inhibitors (i.e., neplanocin A); polyanionic substances (i.e., sulfated polymers); interferon and immunomodulators.