Absence of an N-Linked Glycosylation Motif in the Glycoprotein of the Live-Attenuated Argentine Hemorrhagic Fever Vaccine, Candid #1, Results in Its Improper Processing, and Reduced Surface Expression

Restoration of virulence in the attenuated Candid#1 vaccine virus requires reversion at both positions 168 and 427 in the envelope glycoprotein GPC

Live-attenuated virus vaccines provide long-lived protection against viral disease but carry inherent risks of residual pathogenicity and genetic reversion. The live-attenuated Candid#1 vaccine was developed to protect Argentines against lethal infection by the Argentine hemorrhagic fever arenavirus, Junín virus. Despite its safety and efficacy in Phase III clinical study, the vaccine is not licensed in the US, in part due to concerns regarding the genetic stability of attenuation. Previous studies had identified a single F427I mutation in the transmembrane domain of the Candid#1 envelope glycoprotein GPC as the key determinant of attenuation, as well as the propensity of this mutation to revert upon passage in cell culture and neonatal mice. To ascertain the consequences of this reversion event, we introduced the I427F mutation into recombinant Candid#1 (I427F rCan) and investigated the effects in two validated small-animal models: in mice expressing the essential virus receptor (human transferrin receptor 1; huTfR1) and in the conventional guinea pig model. We report that I427F rCan displays only modest virulence in huTfR1 mice and appears attenuated in guinea pigs. Reversion at another attenuating locus in Candid#1 GPC (T168A) was also examined, and a similar pattern was observed. By contrast, virus bearing both revertant mutations (A168T+I427F rCan) approached the lethal virulence of the pathogenic Romero strain in huTfR1 mice. Virulence was less extreme in guinea pigs. Our findings suggest that genetic stabilization at both positions is required to minimize the likelihood of reversion to virulence in a second-generation Candid#1 vaccine.IMPORTANCELive-attenuated virus vaccines, such as measles/mumps/rubella and oral poliovirus, provide robust protection against disease but carry with them the risk of genetic reversion to the virulent form. Here, we analyze the genetics of reversion in the live-attenuated Candid#1 vaccine that is used to protect against Argentine hemorrhagic fever, an often-lethal disease caused by the Junín arenavirus. In two validated small-animal models, we find that restoration of virulence in recombinant Candid#1 viruses requires back-mutation at two positions specific to the Candid#1 envelope glycoprotein GPC, at positions 168 and 427. Viruses bearing only a single change showed only modest virulence. We discuss strategies to genetically harden Candid#1 GPC against these two reversion events in order to develop a safer second-generation Candid#1 vaccine virus.

The role of glycosylation patterns of viral glycoproteins and cell entry receptors in arenavirus infection

Mammarenaviruses are enveloped RNA viruses that can be associated with rodent-transmitted diseases in humans. Their virions are composed of a nucleocapsid surrounded by a lipid bilayer with glycoprotein (GP) spikes interacting with receptors on target cells. Both the GP and receptors are highly glycosylated, with glycosylation patterns being crucial for virus binding and cell entry, viral tropism, immune responses, or therapy strategies. These effects have been previously described for several different viruses. In case of arenaviruses, they remain insufficiently understood. Thus, it is important to determine the mechanisms of glycosylation of viral proteins and receptors responsible for infection, in order to fully understand the biology of arenaviruses. In this article, we have summarized and critically evaluated the available literature data on the glycosylation of mammarenavirus-associated proteins to facilitate further research in this field.

The roles of XJ13 and XJ44-specific mutations within the Candid #1 GPC in Junin virus attenuation

Junin virus (JUNV) is a member of the Arenaviridae family of viruses and is the pathogen responsible for causing Argentine hemorrhagic fever, a potentially lethal disease endemic to Argentina. A live attenuated vaccine for human use, called Candid#1, is approved only in Argentina. Candid#1 vaccine strain of Junin virus was obtained through serial passage in mouse brain tissues followed by passage in Fetal Rhesus macaque lung fibroblast (FRhL) cells. Previously, the mutations responsible for attenuation of this virus in Guinea pigs were mapped in the gene encoding for glycoprotein precursor (GPC) protein. The resulting Candid#1 glycoprotein complex has been shown to cause endoplasmic reticulum (ER) stress in vitro resulting in the degradation of the GPC. To evaluate the attenuating properties of specific mutations within GPC, we created recombinant viruses expressing GPC mutations specific to key Candid#1 passages and evaluated their pathogenicity in our outbred Hartley guinea pig model of Argentine hemorrhagic fever. Here, we provide evidence that early mutations in GPC obtained through serial passaging attenuate the visceral disease and increase immunogenicity in guinea pigs. Specific mutations acquired prior to the 13th mouse brain passage (XJ13) are responsible for attenuation of the visceral disease while having no impact on the neurovirulence of Junin virus. Additionally, our findings demonstrate that the mutation within an N-linked glycosylation motif, acquired prior to the 44th mouse brain passage (XJ44), is unstable but necessary for complete attenuation and enhanced immunogenicity of Candid#1 vaccine strain. The highly conserved N-linked glycosylation profiles of arenavirus glycoproteins could therefore be viable targets for designing attenuating viruses for vaccine development against other arenavirus-associated illnesses.

Recent developments on Junin virus, a causative agent for Argentine haemorrhagic fever

Junin virus consists of ribonucleic acid as the genome and is responsible for a rapidly changing tendency of the virus. The virus is accountable for ailments in the human body and causes Argentine Haemorrhagic Fever (AHF). The infection is may be transmitted through contact between an infected animal/host and a person, and later between person to person. Prevention of outbreaks of AHF in humans can be a tough practice, as their occurrence is infrequent and unpredictable. In this review, recent information from the past 5 years available on the Junin virus including the risk of its emergence, infectious agents, its pathogenesis in humans, available diagnostic and therapeutic approaches, and disease management has been summarised. Altogether, this article would be highly significant in understanding the mechanistic basis behind virus interaction and other processes during the life cycle. Currently, no specific therapeutic options are available to treat the Junin virus infection. The information covered in this review could be important for finding possible treatment options for Junin virus infections.

Regulation of Stress-Activated Kinases in Response to Tacaribe Virus Infection and Its Implications for Viral Replication

Arenaviruses include important zoonotic pathogens that cause hemorrhagic fever (e.g., Junín virus; JUNV) as well as other viruses that are closely related but apathogenic (e.g., Tacaribe virus; TCRV). We have found that, while TCRV and JUNV differ in their ability to induce apoptosis in infected cells, due to active inhibition of caspase activation by the JUNV nucleoprotein, both viruses trigger similar upstream pro-apoptotic signaling events, including the activation/phosphorylation of p53. In the case of TCRV, the pro-apoptotic factor Bad is also phosphorylated (leading to its inactivation). These events clearly implicate upstream kinases in regulating the induction of apoptosis. Consistent with this, here we show activation in TCRV-infected cells of the stress-activated protein kinases p38 and JNK, which are known to regulate p53 activation, as well as the downstream kinase MK2 and transcription factor c-Jun. We also observed the early transient activation of Akt, but not Erk. Importantly, the chemical inhibition of Akt, p38, JNK and c-Jun all dramatically reduced viral growth, even though we have shown that inhibition of apoptosis itself does not. This indicates that kinase activation is crucial for viral infection, independent of its downstream role in apoptosis regulation, a finding that has the potential to shed further light on the determinants of arenavirus pathogenesis, as well as to inform future therapeutic approaches.

Potential and action mechanism of favipiravir as an antiviral against Junin virus

Favipiravir is a nucleoside analogue that inhibits the replication and transcription of a broad spectrum of RNA viruses, including pathogenic arenaviruses. In this study, we isolated a favipiravir-resistant mutant of Junin virus (JUNV), which is the causative agent of Argentine hemorrhagic fever, and analyzed the antiviral mechanism of favipiravir against JUNV. Two amino acid substitutions, N462D in the RNA-dependent RNA polymerase (RdRp) and A168T in the glycoprotein precursor GPC, were identified in the mutant. GPC-A168T substitution enhanced the efficiency of JUNV internalization, which explains the robust replication kinetics of the mutant in the virus growth analysis. Although RdRp-N462D substitution did not affect polymerase activity levels in a minigenome system, comparisons of RdRp error frequencies showed that the virus with RdRp-D462 possessed a significantly higher fidelity. Our next generation sequence (NGS) analysis showed a gradual accumulation of both mutations as we passaged the virus in presence of favipiravir. We also provided experimental evidence for the first time that favipiravir inhibited JUNV through the accumulation of transition mutations, confirming its role as a purine analogue against arenaviruses. Moreover, we showed that treatment with a combination of favipiravir and either ribavirin or remdesivir inhibited JUNV replication in a synergistic manner, blocking the generation of the drug-resistant mutant. Our findings provide new insights for the clinical management and treatment of Argentine hemorrhagic fever.

Development of antivirals requires cautious and extensive assessment of action mechanism as well as potential for emergence of resistant phenotype of the virus. In recent years, favipiravir has been put forward as a promising candidate for the treatment of Argentine hemorrhagic fever (AHF) caused by Junin virus (JUNV). We, therefore, aimed to provide experimental evidence on action mechanism of favipiravir to help guide its clinical use. Here we show that favipiravir causes lethal mutation that impairs virus infectivity. More importantly, we demonstrate that the virus has the capability to escape favipiravir selective pressure by acquiring two amino acid substitutions on glycoprotein precursor and polymerase proteins. This observation raises concern over the use of only favipiravir in therapeutic regimens. To overcome this risk, we show that combination of favipiravir with other nucleoside analogues demonstrates a synergistic effect and suppresses the ability of JUNV to escape drug pressure. Favipiravir, ribavirin, and remdesivir have a broad spectrum of antiviral activity. Therefore, combination therapies of these drugs would be expected to have potential therapeutic effects for not only AHF but also the diseases caused by a variety of viruses, including emerging RNA viruses.

Machupo Virus with Mutations in the Transmembrane Domain and Glycosylation Sites of the Glycoprotein Is Attenuated and Immunogenic in Animal Models of Bolivian Hemorrhagic Fever

Several highly pathogenic mammarenaviruses cause severe hemorrhagic and neurologic disease in humans for which vaccines and antivirals are limited or unavailable. New World (NW) mammarenavirus Machupo virus (MACV) infection causes Bolivian hemorrhagic fever in humans. We previously reported that the disruption of specific N-linked glycan sites on the glycoprotein (GPC) partially attenuates MACV in an interferon alpha/beta and gamma (IFN-α/β and -γ) receptor knockout (R-/-) mouse model. However, some capability to induce neurological pathology still remained. The highly pathogenic Junin virus (JUNV) is another NW arenavirus closely related to MACV. An F427I substitution in the GPC transmembrane domain (TMD) rendered JUNV attenuated in a lethal mouse model after intracranial inoculation. In this study, we rationally designed and rescued a MACV containing mutations at two glycosylation sites and the corresponding F438I substitution in the GPC TMD. The MACV mutant is fully attenuated in IFN-α/β and -γ R-/- mice and outbred guinea pigs. Furthermore, inoculation with this mutant MACV completely protected guinea pigs from wild-type MACV lethal challenge. Last, we found the GPC TMD F438I substitution greatly impaired MACV growth in neuronal cell lines of mouse and human origins. Our results highlight the critical roles of the glycans and the TMD on the GPC in arenavirus virulence, which provide insight into the rational design of potential vaccine candidates for highly pathogenic arenaviruses. IMPORTANCE For arenaviruses, the only vaccine available is the live attenuated Candid#1 vaccine, a JUNV vaccine approved in Argentina. We and others have found that the glycans on GPC and the F427 residue in the GPC TMD are important for virulence of JUNV. Nevertheless, mutating either of them is not sufficient for full and stable attenuation of JUNV. Using reverse genetics, we disrupted specific glycosylation sites on MACV GPC and also introduced the corresponding F438I substitution in the GPC TMD. This MACV mutant is fully attenuated in two animal models and protects animals from lethal infection. Thus, our studies highlight the feasibility of rational attenuation of highly pathogenic arenaviruses for vaccine development. Another important finding from this study is that the F438I substitution in GPC TMD could substantially affect MACV replication in neurons. Future studies are warranted to elucidate the underlying mechanism and the implication of this mutation in arenavirus neural tropism.

Junin Virus Activates p38 MAPK and HSP27 Upon Entry

Junín virus (JUNV), a New World arenavirus, is a rodent-borne virus and the causative agent of Argentine hemorrhagic fever. Humans become infected through exposure to rodent host secreta and excreta and the resulting infection can lead to an acute inflammatory disease with significant morbidity and mortality. Little is understood about the molecular pathogenesis of arenavirus hemorrhagic fever infections. We utilized Reverse Phase Protein Microarrays (RPPA) to compare global alterations in the host proteome following infection with an attenuated vaccine strain, Candid#1 (CD1), and the most parental virulent strain, XJ13, of JUNV in a human cell culture line. Human small airway epithelial cells were infected with CD1 or XJ13 at an MOI of 10, or mock infected. To determine proteomic changes at early timepoints (T = 1, 3, 8 and 24 h), the JUNV infected or mock infected cells were lysed in compatible buffers for RPPA. Out of 113 proteins that were examined by RPPA, 14 proteins were significantly altered following JUNV infection. Several proteins were commonly phosphorylated between the two strains and these correspond to entry and early replication events, to include p38 mitogen-activated protein kinase (MAPK), heat shock protein 27 (HSP27), and nuclear factor kappa B (NFκB). We qualitatively confirmed the alterations of these three proteins following infection by western blot analysis. We also determined that the inhibition of either p38 MAPK, with the small molecule inhibitor SB 203580 or siRNA knockdown, or HSP27, by siRNA knockdown, significantly decreases JUNV replication. Our data suggests that HSP27 phosphorylation at S82 upon virus infection is dependent on p38 MAPK activity. This work sheds light on the nuances of arenavirus replication.

Immunization with GP1 but Not Core-like Particles Displaying Isolated Receptor-Binding Epitopes Elicits Virus-Neutralizing Antibodies against Junín Virus

New World arenaviruses are rodent-transmitted viruses and include a number of pathogens that are responsible for causing severe human disease. This includes Junín virus (JUNV), which is the causative agent of Argentine hemorrhagic fever. The wild nature and mobility of the rodent reservoir host makes it difficult to control the disease, and currently passive immunization with high-titer neutralizing antibody-containing plasma from convalescent patients is the only specific therapy. However, dwindling supplies of naturally available convalescent plasma, and challenges in developing similar resources for other closely related viruses, have made the development of alternative antibody-based therapeutic approaches of critical importance. In this study, we sought to induce a neutralizing antibody response in rabbits against the receptor-binding subunit of the viral glycoprotein, GP1, and the specific peptide sequences in GP1 involved in cellular receptor contacts. While these specific receptor-interacting peptides did not efficiently induce the production of neutralizing antibodies when delivered as a particulate antigen (as part of hepatitis B virus core-like particles), we showed that recombinant JUNV GP1 purified from transfected mammalian cells induced virus-neutralizing antibodies at high titers in rabbits. Further, neutralization was observed across a range of unrelated JUNV strains, a feature that is critical for effectiveness in the field. These results underscore the potential of GP1 alone to induce a potent neutralizing antibody response and highlight the importance of epitope presentation. In addition, effective virus neutralization by rabbit antibodies supports the potential applicability of this species for the future development of immunotherapeutics (e.g., based on humanized monoclonal antibodies). Such information can be applied in the design of vaccines and immunogens for both prevention and specific therapies against this and likely also other closely related pathogenic New World arenaviruses.

Glycoprotein N-linked glycans play a critical role in arenavirus pathogenicity

Several arenaviruses cause hemorrhagic fevers in humans with high case fatality rates. A vaccine named Candid#1 is available only against Junin virus (JUNV) in Argentina. Specific N-linked glycans on the arenavirus surface glycoprotein (GP) mask important epitopes and help the virus evade antibody responses. However the role of GPC glycans in arenavirus pathogenicity is largely unclear. In a lethal animal model of hemorrhagic fever-causing Machupo virus (MACV) infection, we found that a chimeric MACV with the ectodomain of GPC from Candid#1 vaccine was partially attenuated. Interestingly, mutations resulting in acquisition of N-linked glycans at GPC N83 and N166 frequently occurred in late stages of the infection. These glycosylation sites are conserved in the GPC of wild-type MACV, indicating that this is a phenotypic reversion for the chimeric MACV to gain those glycans crucial for infection in vivo. Further studies indicated that the GPC mutant viruses with additional glycans became more resistant to neutralizing antibodies and more virulent in animals. On the other hand, disruption of these glycosylation sites on wild-type MACV GPC rendered the virus substantially attenuated in vivo and also more susceptible to antibody neutralization, while loss of these glycans did not affect virus growth in cultured cells. We also found that MACV lacking specific GPC glycans elicited higher levels of neutralizing antibodies against wild-type MACV. Our findings revealed the critical role of specific glycans on GPC in arenavirus pathogenicity and have important implications for rational design of vaccines against this group of hemorrhagic fever-causing viruses.

Several arenaviruses cause severe hemorrhagic fevers in humans. The only vaccine against arenavirus infections is Candid#1, a live attenuated vaccine against Argentine hemorrhagic fever. So far, we have successfully attenuated additional one of the arenaviruses, Machupo virus, the causative agent of Bolivian hemorrhagic fever. Unraveling this attenuation mechanism might help the development of live-attenuated vaccines for other arenaviruses. In this study, we revealed that the specific glycans of the viral glycoproteins play an important role in pathogenicity in vivo. The glycans facilitate the virus to evade neutralizing antibodies. This study would contribute to the development of arenavirus vaccine candidates.

A novel circulating tamiami mammarenavirus shows potential for zoonotic spillover

A detailed understanding of the mechanisms underlying the capacity of a virus to break the species barrier is crucial for pathogen surveillance and control. New World (NW) mammarenaviruses constitute a diverse group of rodent-borne pathogens that includes several causative agents of severe viral hemorrhagic fever in humans. The ability of the NW mammarenaviral attachment glycoprotein (GP) to utilize human transferrin receptor 1 (hTfR1) as a primary entry receptor plays a key role in dictating zoonotic potential. The recent isolation of Tacaribe and lymphocytic choriominingitis mammarenaviruses from host-seeking ticks provided evidence for the presence of mammarenaviruses in arthropods, which are established vectors for numerous other viral pathogens. Here, using next generation sequencing to search for other mammarenaviruses in ticks, we identified a novel replication-competent strain of the NW mammarenavirus Tamiami (TAMV-FL), which we found capable of utilizing hTfR1 to enter mammalian cells. During isolation through serial passaging in mammalian immunocompetent cells, the quasispecies of TAMV-FL acquired and enriched mutations leading to the amino acid changes N151K and D156N, within GP. Cell entry studies revealed that both substitutions, N151K and D156N, increased dependence of the virus on hTfR1 and binding to heparan sulfate proteoglycans. Moreover, we show that the substituted residues likely map to the sterically constrained trimeric axis of GP, and facilitate viral fusion at a lower pH, resulting in viral egress from later endosomal compartments. In summary, we identify and characterize a naturally occurring TAMV strain (TAMV-FL) within ticks that is able to utilize hTfR1. The TAMV-FL significantly diverged from previous TAMV isolates, demonstrating that TAMV quasispecies exhibit striking genetic plasticity that may facilitate zoonotic spillover and rapid adaptation to new hosts.

Mammarenaviruses include emergent pathogens responsible of severe disease in humans in zoonotic events. The ability to use the human Transferrin receptor 1 (hTfR1) strongly correlates with their pathogenicity in humans. We isolated a new infectious Tamiami virus strain (TAMV-FL) from host-seeking ticks, which, contrary to the previous rodent-derived reference strain, can use hTfR1 to enter human cells. Moreover, serial passaging of TAMV-FL in human immunocompetent cells selected for two substitutions in the viral envelope glycoprotein: N151K and D156N. These substitutions increase the ability to highjack hTfR1 and the binding capacity to heparan sulfate proteoglycans and cause delayed endosomal escape. Our findings provide insight into the acquisition of novel traits by currently circulating TAMV that increase its potential to trespass the inter-species barrier.

BH3-only sensors Bad, Noxa and Puma are Key Regulators of Tacaribe virus-induced Apoptosis

Pathogenicity often differs dramatically among even closely related arenavirus species. For instance, Junín virus (JUNV), the causative agent of Argentine hemorrhagic fever (AHF), is closely related to Tacaribe virus (TCRV), which is normally avirulent in humans. While little is known about how host cell pathways are regulated in response to arenavirus infection, or how this contributes to virulence, these two viruses have been found to differ markedly in their ability to induce apoptosis. However, details of the mechanism(s) governing the apoptotic response to arenavirus infections are unknown. Here we confirm that TCRV-induced apoptosis is mitochondria-regulated, with associated canonical hallmarks of the intrinsic apoptotic pathway, and go on to identify the pro- and anti-apoptotic Bcl-2 factors responsible for regulating this process. In particular, levels of the pro-apoptotic BH3-only proteins Noxa and Puma, as well as their canonical transcription factor p53, were strongly increased. Interestingly, TCRV infection also led to the accumulation of the inactive phosphorylated form of another pro-apoptotic BH3-only protein, Bad (i.e. as phospho-Bad). Knockout of Noxa or Puma suppressed apoptosis in response to TCRV infection, whereas silencing of Bad increased apoptosis, confirming that these factors are key regulators of apoptosis induction in response to TCRV infection. Further, we found that while the highly pathogenic JUNV does not induce caspase activation, it still activated upstream pro-apoptotic factors, consistent with current models suggesting that JUNV evades apoptosis by interfering with caspase activation through a nucleoprotein-mediated decoy function. This new mechanistic insight into the role that individual BH3-only proteins and their regulation play in controlling apoptotic fate in arenavirus-infected cells provides an important experimental framework for future studies aimed at dissecting differences in the apoptotic responses between arenaviruses, their connection to other cell signaling events and ultimately the relationship of these processes to pathogenesis.

Arenaviruses are important zoonotic pathogens that present a serious threat to human health. While some virus species cause severe disease, resulting in hemorrhagic fever and/or neurological symptoms, other closely related species exhibit little or no pathogenicity. The basis for these dramatically different outcomes is insufficiently understood, but investigations of host cell responses have suggested that apoptosis, i.e. non-inflammatory programmed cell death, is regulated differently between pathogenic and apathogenic arenaviruses. However, many questions remain regarding how these viruses interact with cell death pathways upon infection. Here we demonstrate that apoptosis induced by the avirulent Tacaribe virus (TCRV), proceeds via the mitochondria (i.e. the intrinsic apoptotic signaling pathway), and is regulated by a combination of factors that appear to balance activation (i.e. Noxa and Puma) and inactivation (i.e. Bad-P) of this cascade. During TCRV infection, the balance of these pro- and anti-apoptotic signals shifts the equilibrium late in the infection towards cell death. Importantly, we also found that the highly pathogenic Junín virus (JUNV), which does not trigger caspase activation or apoptotic cell death, nonetheless induces pro-apoptotic factors, thus supporting the existence of a specific mechanism by which this virus is able to evade apoptosis at late stages in this process.

A single mutation (V64G) within the RING Domain of Z attenuates Junin virus

Junin virus (JUNV) is a New World arenavirus that is the causative agent of Argentine hemorrhagic fever (AHF). Candid#1 (Can) is a live-attenuated vaccine strain of JUNV that since its introduction has resulted in a marked decrease in AHF incidence within the endemic regions of the Pampas in Argentina. Originally, the viral determinants and mechanisms of Can attenuation were not well understood. Recent work has identified the glycoprotein as the major attenuating factor for Can. The establishment of attenuating strategies based on any of the other viral proteins, however, has not been pursued. Here, we document the role of Can Z resulting in incompatibilities with wild type JUNV that results in decreased growth in vitro. In addition, this incompatibility results in attenuation of the virus in the guinea pig model. Further, we identify a single mutation (V64G) in the Z protein that is able to confer this demonstrated attenuation. By establishing and characterizing a novel attenuation strategy for New World mammarenaviruses, we hope to aid future vaccine development for related emerging pathogens including Machupo virus (MACV), Guanarito virus (GTOV), and Sabia virus (SABV).

The continual development of safe, effective vaccines against emerging diseases is one of the greatest challenges facing the scientific community. The New World group of mammarenaviruses contains multiple human pathogens, each capable of causing severe hemorrhagic disease. Among these, only Junin virus has a distributed vaccine. By utilizing this vaccine, we are able to determine vaccine development strategies for related New World viruses that represent an emerging threat. Here we demonstrate that manipulation of the viral Z protein is able to produce an incompatibility that ultimately attenuates the virus. This provides yet another tool for future vaccine development to further global public health.

The Glycoprotein of the Live-Attenuated Junin Virus Vaccine Strain Induces Endoplasmic Reticulum Stress and Forms Aggregates prior to Degradation in the Lysosome

Argentine hemorrhagic fever is a potentially lethal disease that is caused by Junin virus (JUNV). There are currently around 5 million individuals at risk of infection within regions of endemicity in Argentina. The live attenuated vaccine strain Candid #1 (Can) is approved for use in regions of endemicity and has substantially decreased the number of annual Argentine hemorrhagic fever (AHF) cases. The glycoprotein (GPC) gene is primarily responsible for attenuation of the Can strain, and we have shown that the absence of an N-linked glycosylation motif in the subunit G1 of the glycoprotein complex of Can, which is otherwise present in the wild-type pathogenic JUNV, causes GPC retention in the endoplasmic reticulum (ER). Here, we show that Can GPC aggregates in the ER of infected cells, forming incorrect cross-chain disulfide bonds, which results in impaired GPC processing into G1 and G2. The GPC fails to cleave into its G1 and G2 subunits and is targeted for degradation within lysosomes. Cells infected with the wild-type Romero (Rom) strain do not produce aggregates that are observed in Can infection, and the stress on the ER remains minimal. While the mutation of the N-linked glycosylation motif (T168A) is primarily responsible for the formation of aggregates, other mutations within G1 that occurred earlier in the passage history of the Can strain also contribute to aggregation of the GPC within the ER.IMPORTANCE The development of vaccines and therapeutics to combat viral hemorrhagic fevers remains a top priority within the Implementation Plan of the U.S. Department of Health and Human Services Public Health Emergency Medical Countermeasures Enterprise. The Can strain, derived from the pathogenic XJ strain of JUNV, has been demonstrated to be both safe and protective against AHF. While the vaccine strain is approved for use in regions of endemicity within Argentina, the mechanisms of Can attenuation have not been elucidated. A better understanding of the viral genetic determinants of attenuation will improve our understanding of the mechanisms contributing to disease pathogenesis and provide critical information for the rational design of live attenuated vaccine candidates for other viral hemorrhagic fevers.

Bivalent Junin & Machupo experimental vaccine based on alphavirus RNA replicon vector

Junin (JUNV) and Machupo (MACV), two mammalian arenaviruses placed on the 2018 WHO watch list, are prevalent in South America causing Argentine and Bolivian hemorrhagic fevers (AHF and BHF), respectively. The live attenuated JUNV vaccine, Candid #1, significantly reduced the incidence of AHF. Vaccination induces neutralizing antibody (nAb) responses which effectively target GP1 (the viral attachment glycoprotein) pocket which accepts the tyrosine residue of the cellular receptor, human transferrin receptor 1 (TfR1). In spite of close genetic relationships between JUNV and MACV, variability in the GP1 receptor binding site (e.g., MACV GP1 loop 10) results in poor MACV neutralization by Candid #1-induced nAbs. Candid #1 is not recommended for vaccination of children younger than 15 years old (a growing "at risk" group), pregnant women, or other immunocompromised individuals. Candid #1's primary reliance on limited missense mutations for attenuation, genetic heterogeneity, and potential stability concerns complicate approval of this vaccine in the US. To address these issues, we applied alphavirus RNA replicon vector technology based on the human Venezuelan equine encephalitis vaccine (VEEV) TC-83 to generate replication restricted virus-like-particles vectors (VLPVs) simultaneously expressing cellular glycoprotein precursors (GPC) of both viruses, JUNV and MACV. Resulting JV&MV VLPVs were found safe and immunogenic in guinea pigs. Immunization with VLPVs induced humoral responses which correlated with complete protection against lethal disease after challenge with pathogenic strains of JUNV (Romero) and MACV (Carvallo).

Comprehensive Interactome Analysis Reveals that STT3B Is Required for N-Glycosylation of Lassa Virus Glycoprotein

Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Lassa virus (LASV) is the causative agent of a fatal hemorrhagic fever in humans. The glycoprotein (GP) of LASV mediates viral entry into host cells, and correct processing and modification of GP by host factors is a prerequisite for virus replication. Here, using an affinity purification-coupled mass spectrometry (AP-MS) strategy, 591 host proteins were identified as interactors of LASV GP. Gene ontology analysis was performed to functionally annotate these proteins, and the oligosaccharyltransferase (OST) complex was highly enriched. Functional studies conducted by using CRISPR-Cas9-mediated knockouts showed that STT3A and STT3B, the two catalytically active isoforms of the OST complex, are essential for the propagation of the recombinant arenavirus rLCMV/LASV glycoprotein precursor, mainly via affecting virus infectivity. Knockout of STT3B, but not STT3A, caused hypoglycosylation of LASV GP, indicating a preferential requirement of LASV for the STT3B-OST isoform. Furthermore, double knockout of magnesium transporter 1 (MAGT1) and tumor suppressor candidate 3 (TUSC3), two specific subunits of STT3B-OST, also caused hypoglycosylation of LASV GP and affected virus propagation. Site-directed mutagenesis analysis revealed that the oxidoreductase CXXC active-site motif of MAGT1 or TUSC3 is essential for the glycosylation of LASV GP. NGI-1, a small-molecule OST inhibitor, can effectively reduce virus infectivity without affecting cell viability. The STT3B-dependent N-glycosylation of GP is conserved among other arenaviruses, including both the Old World and New World groups. Our study provided a systematic view of LASV GP-host interactions and revealed the preferential requirement of STT3B for LASV GP N-glycosylation.

IMPORTANCE Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Adenoviral vector-based vaccine is fully protective against lethal Lassa fever challenge in Hartley guinea pigs

Lassa virus (LASV), the causative agent of Lassa fever (LF), was first identified in 1969. Since then, outbreaks in the endemic countries of Nigeria, Liberia, and Sierra Leone occur on an annual basis resulting in a case-fatality rate of 15-70% in hospitalized patients. There is currently no licensed vaccine and there are limited animal models to test vaccine efficacy. An estimated 37.7 million people are at risk of contracting LASV; therefore, there is an urgent need for the development of a safe, effective vaccine against LASV infection. The LF endemic countries are also inflicted with HIV, Ebola, and malaria infections. The safety in immunocompromised populations must be considered in LASV vaccine development. The novel adenovirus vector-based platform, Ad5 (E1-,E2b-) has been used in clinical trial protocols for treatment of immunocompromised individuals, has been shown to exhibit high stability, low safety risk in humans, and induces a strong cell-mediated and pro-inflammatory immune response even in the presence of pre-existing adenovirus immunity. To this nature, our lab has developed an Ad5 (E1-,E2b-) vector-based vaccine expressing the LASV-NP or LASV-GPC. We found that guinea pigs vaccinated with two doses of Ad5 (E1-,E2b-) LASV-NP and Ad5 (E1-,E2b-) LASV-GPC were protected against lethal LASV challenge. The Ad5 (E1-,E2b-) LASV-NP and LASV-GPC vaccine represents a potential vaccine candidate against LF.

Junín Virus Promotes Autophagy To Facilitate the Virus Life Cycle

Junín virus (JUNV), a member of the family Arenaviridae, is the etiological agent of Argentine hemorrhagic fever (AHF), a potentially deadly endemic-epidemic disease affecting the population of the most fertile farming land of Argentina. Autophagy is a degradative process with a crucial antiviral role; however, several viruses subvert the pathway to their benefit. We determined the role of autophagy in JUNV-infected cells by analyzing LC3, a cytoplasmic protein (LC3-I) that becomes vesicle membrane associated (LC3-II) upon induction of autophagy. Cells overexpressing enhanced green fluorescent protein (EGFP)-LC3 and infected with JUNV showed an increased number of LC3 punctate structures, similar to those obtained after starvation or bafilomycin A1 treatment, which leads to autophagosome induction or accumulation, respectively. We also monitored the conversion of LC3-I to LC3-II, observing LC3-II levels in JUNV-infected cells similar to those observed in starved cells. Additionally, we kinetically studied the number of LC3 dots after JUNV infection and found that the virus activated the pathway as early as 2 h postinfection (p.i.), whereas the UV-inactivated virus did not induce the pathway. Cells subjected to starvation or pretreated with rapamycin, a pharmacological autophagy inductor, enhanced virus yield. Also, we assayed the replication capacity of JUNV in Atg5 knockout or Beclin 1 knockdown cells (both critical components of the autophagic pathway) and found a significant decrease in JUNV replication. Taken together, our results constitute the first study indicating that JUNV infection induces an autophagic response, which is functionally required by the virus for efficient propagation.IMPORTANCE Mammalian arenaviruses are zoonotic viruses that cause asymptomatic and persistent infections in their rodent hosts but may produce severe and lethal hemorrhagic fevers in humans. Currently, there are neither effective therapeutic options nor effective vaccines for viral hemorrhagic fevers caused by human-pathogenic arenaviruses, except the vaccine Candid no. 1 against Argentine hemorrhagic fever (AHF), licensed for human use in areas of endemicity in Argentina. Since arenaviruses remain a severe threat to global public health, more in-depth knowledge of their replication mechanisms would improve our ability to fight these viruses. Autophagy is a lysosomal degradative pathway involved in maintaining cellular homeostasis, representing powerful anti-infective machinery. We show, for the first time for a member of the family Arenaviridae, a proviral role of autophagy in JUNV infection, providing new knowledge in the field of host-virus interaction. Therefore, modulation of virus-induced autophagy could be used as a strategy to block arenavirus infections.

Neutralization of Junín virus by single domain antibodies targeted against the nucleoprotein

The syndrome viral haemorrhagic fever (VHF) designates a broad range of diseases that are caused by different viruses including members of the family Arenaviridae. Prophylaxis for Argentine Haemorrhagic Fever (AHF), caused by the arenavirus Junín (JUNV), has been achieved by the use of a live attenuated vaccine, named Candid#1. The standard treatment of AHF is transfusion of convalescent human plasma. Our aim was to develop an alternative and safer treatment for AHF based on the use of virus-neutralizing single domain antibodies (VHHs). We describe the first reported VHHs directed against an arenavirus. These VHHs could neutralize Candid#1 by altering virion binding/fusion. Surprisingly, the neutralizing VHHs appeared to be specific for the viral nucleoprotein (N) that is not known to be involved in arenavirus entry. Candid#1 VHH-escape viruses had acquired a predicted N-glycosylation site in the surface glycoprotein GP1 that is present in highly pathogenic JUNV strains. Accordingly, the Candid#1-neutralizing VHHs could not neutralize pathogenic JUNV strains, but they could still bind to cells infected with a pathogenic strain or the escape mutant viruses. These results show that the attenuated strains of JUNV can be potently neutralized by nucleoprotein-specific VHHs.

The S Genome Segment Is Sufficient to Maintain Pathogenicity in Intra-Clade Lassa Virus Reassortants in a Guinea Pig Model

Genome reassortment in Lassa virus (LASV) has been reported in nature, but phenotypic consequences of this phenomenon are not well described. Here we characterize, both in vitro and in vivo, reassortment between 2 LASV strains: the prototypic 1976 Josiah strain and a more recently isolated 2015 Liberian strain. In vitro analysis showed that although cis- and trans-acting elements of viral RNA synthesis were compatible between strains, reassortants demonstrated different levels of viral replication. These differences were also apparent in vivo, as reassortants varied in pathogenicity in the guinea pig model of LASV infection. The reassortant variant containing the Josiah S segment retained the virulence of the parental Josiah strain, but the reassortant variant containing the S segment of the Liberian isolate was highly attenuated compared to both parental strains. Contrary to observations in reassortants between LASV and other arenavirus species, which suggest that L segment-encoded factors are responsible for virulence, these studies highlight a role for S segment-encoded virulence factors in disease, and also suggest that inefficient interactions between proteins of heterologous strains may limit the prevalence of reassortant LASV variants in nature.

Epistastic Interactions within the Junín Virus Envelope Glycoprotein Complex Provide an Evolutionary Barrier to Reversion in the Live-Attenuated Candid#1 Vaccine

The Candid#1 strain of Junín virus was developed using a conventional attenuation strategy of serial passage in nonhost animals and cultured cells. The live-attenuated Candid#1 vaccine is used in Argentina to protect at-risk individuals against Argentine hemorrhagic fever, but it has not been licensed in the United States. Recent studies have revealed that Candid#1 attenuation is entirely dependent on a phenylalanine-to-isoleucine substitution at position 427 in the fusion subunit (GP2) of the viral envelope glycoprotein complex (GPC), thereby raising concerns regarding the potential for reversion to virulence. In this study, we report the identification and characterization of an intragenic epistatic interaction between the attenuating F427I mutation in GP2 and a lysine-to-serine mutation at position 33 in the stable signal peptide (SSP) subunit of GPC, and we demonstrate the utility of this interaction in creating an evolutionary barrier against reversion to the pathogenic genotype. In the presence of the wild-type F427 residue, the K33S mutation abrogates the ability of ectopically expressed GPC to mediate membrane fusion at endosomal pH. This defect is rescued by the attenuating F427I mutation. We show that the recombinant Candid#1 (rCan) virus bearing K33S GPC is viable and retains its attenuated genotype under cell culture conditions that readily select for reversion in the parental rCan virus. If back-mutation to F427 offers an accessible pathway to increase fitness in rCan, reversion in K33S-GPC rCan is likely to be lethal. The epistatic interaction between K33S and F427I thus may minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine.IMPORTANCE The live-attenuated Candid#1 vaccine strain of Junín virus is used to protect against Argentine hemorrhagic fever. Recent findings that a single missense mutation in the viral envelope glycoprotein complex (GPC) is responsible for attenuation raise the prospect of facile reversion to pathogenicity. Here, we characterize a genetic interaction between GPC subunits that evolutionarily forces retention of the attenuating mutation. By incorporating this secondary mutation into Candid#1 GPC, we hope to minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine. A similar approach may guide the design of live-attenuated vaccines against Lassa and other arenaviral hemorrhagic fevers.

Apoptosis during arenavirus infection: mechanisms and evasion strategies

In recent years there has been a greatly increased interest in the interactions of arenaviruses with the apoptotic machinery, and particularly the extent to which these interactions may be an important contributor to pathogenesis. Here we summarize the current state of our knowledge on this subject and address the potential for interplay with other immunological mechanisms known to be regulated by these viruses. We also compare and contrast what is known for arenavirus-induced apoptosis with observations from other segmented hemorrhagic fever viruses.