Monoclonal antibody therapy demonstrates increased virulence of a lineage VII strain of Lassa virus in nonhuman primates

Lassa virus (LASV) is a World Health Organization (WHO) priority pathogen that causes high morbidity and mortality. Recently, we showed that a combination of three broadly neutralizing human monoclonal antibodies known as Arevirumab-3 (8.9F, 12.1F, 37.2D) based on the lineage IV Josiah strain protected 100% of cynomolgus macaques against heterologous challenge with lineage II and III strains of LASV when therapy was initiated beginning at day 8 after challenge. LASV strains from Benin and Togo represent a new lineage VII that are more genetically diverse from lineage IV than strains from lineages II and III. Here, we tested the ability of Arevirumab-3 to protect macaques against a LASV lineage VII Togo isolate when treatment was administered beginning 8 days after exposure. Unexpectedly, only 40% of treated animals survived challenge. In a subsequent study we showed that Arevirumab-3 protected 100% of macaques from lethal challenge when treatment was initiated 7 days after LASV Togo exposure. Based on our transcriptomics data, successful Arevirumab-3 treatment correlated with diminished neutrophil signatures and the predicted development of T cell responses. As the in vitro antiviral activity of Arevirumab-3 against LASV Togo was equivalent to lineage II and III strains, the reduced protection in macaques against Togo likely reflects the faster disease course of LASV Togo in macaques than other strains. This data causes concern regarding the ability of heterologous vaccines and treatments to provide cross protection against lineage VII LASV isolates.

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.

Alternating Arenavirus Vector Immunization Generates Robust Polyfunctional Genotype Cross-Reactive Hepatitis B Virus-Specific CD8 T-Cell Responses and High Anti-Hepatitis B Surface Antigen Titers

Hepatitis B Virus (HBV) is a major driver of infectious disease mortality. Curative therapies are needed and ideally should induce CD8 T cell-mediated clearance of infected hepatocytes plus anti-hepatitis B surface antigen (HBsAg) antibodies (anti-HBs) to neutralize residual virus. We developed a novel therapeutic vaccine using non-replicating arenavirus vectors. Antigens were screened for genotype conservation and magnitude and genotype reactivity of T cell response, then cloned into Pichinde virus (PICV) vectors (recombinant PICV, GS-2829) and lymphocytic choriomeningitis virus (LCMV) vectors (replication-incompetent, GS-6779). Alternating immunizations with GS-2829 and GS-6779 induced high-magnitude HBV T cell responses, and high anti-HBs titers. Dose schedule optimization in macaques achieved strong polyfunctional CD8 T cell responses against core, HBsAg, and polymerase and high titer anti-HBs. In AAV-HBV mice, GS-2829 and GS-6779 were efficacious in animals with low pre-treatment serum HBsAg. Based on these results, GS-2829 and GS-6779 could become a central component of cure regimens.

Entry inhibitors as arenavirus antivirals

Arenaviruses belonging to the Arenaviridae family, genus mammarenavirus, are enveloped, single-stranded RNA viruses primarily found in rodent species, that cause severe hemorrhagic fever in humans. With high mortality rates and limited treatment options, the search for effective antivirals is imperative. Current treatments, notably ribavirin and other nucleoside inhibitors, are only partially effective and have significant side effects. The high lethality and lack of treatment, coupled with the absence of vaccines for all but Junín virus, has led to the classification of these viruses as Category A pathogens by the Centers for Disease Control (CDC). This review focuses on entry inhibitors as potential therapeutics against mammarenaviruses, which include both New World and Old World arenaviruses. Various entry inhibition strategies, including small molecule inhibitors and neutralizing antibodies, have been explored through high throughput screening, genome-wide studies, and drug repurposing. Notable progress has been made in identifying molecules that target receptor binding, internalization, or fusion steps. Despite promising preclinical results, the translation of entry inhibitors to approved human therapeutics has faced challenges. Many have only been tested in in vitro or animal models, and a number of candidates showed efficacy only against specific arenaviruses, limiting their broader applicability. The widespread existence of arenaviruses in various rodent species and their potential for their zoonotic transmission also underscores the need for rapid development and deployment of successful pan-arenavirus therapeutics. The diverse pool of candidate molecules in the pipeline provides hope for the eventual discovery of a broadly effective arenavirus antiviral.

Lymphocytic choriomeningitis arenavirus requires cellular COPI and AP-4 complexes for efficient virion production

Lymphocytic choriomeningitis virus (LCMV) is a bisegmented negative-sense RNA virus classified within the Arenaviridae family of the Bunyavirales order. LCMV is associated with fatal disease in immunocompromized populations, and as the prototypical arenavirus, acts as a model for the many serious human pathogens within this group. Here, we examined the dependence of LCMV multiplication on cellular trafficking components using a recombinant LCMV expressing enhanced green fluorescent protein in conjunction with a curated siRNA library. The screen revealed a requirement for subunits of both the coat protein 1 (COPI) coatomer and adapter protein 4 (AP-4) complexes. By rescuing a recombinant LCMV harboring a FLAG-tagged glycoprotein (GP-1) envelope spike (rLCMV-GP1-FLAG), we showed infection resulted in marked co-localization of individual COPI and AP-4 components with both LCMV nucleoprotein (NP) and GP-1, consistent with their involvement in viral processes. To further investigate the role of both COPI and AP-4 complexes during LCMV infection, we utilized the ARF-I inhibitor brefeldin A (BFA) that prevents complex formation. Within a single 12-h cycle of virus multiplication, BFA pre-treatment caused no significant change in LCMV-specific RNA synthesis, alongside no significant change in LCMV NP expression, as measured by BFA time-of-addition experiments. In contrast, BFA addition resulted in a significant drop in released virus titers, approaching 50-fold over the same 12-h period, rising to over 600-fold over 24 h. Taken together, these findings suggest COPI and AP-4 complexes are important host cell factors required for the formation and release of infectious LCMV.

IMPORTANCE

Arenaviruses are rodent-borne, segmented, negative-sense RNA viruses, with several members responsible for fatal human disease, with the prototypic member lymphocytic choriomeningitis virus (LCMV) being under-recognised as a pathogen capable of inflicting neurological infections with fatal outcome. A detailed understanding of how arenaviruses subvert host cell processes to complete their multiplication cycle is incomplete. Here, using a combination of gene ablation and pharmacological inhibition techniques, we showed that host cellular COPI and AP-4 complexes, with native roles in cellular vesicular transport, were required for efficient LCMV growth. We further showed these complexes acted on late stages of the multiplication cycle, post-gene expression, with a significant impact on infectious virus egress. Collectively, our findings improve the understanding of arenaviruses host-pathogen interactions and reveal critical cellular trafficking pathways required during infection.

Alternative translation contributes to the generation of a cytoplasmic subpopulation of the Junín virus nucleoprotein that inhibits caspase activation and innate immunity

The highly pathogenic arenavirus, Junín virus (JUNV), expresses three truncated alternative isoforms of its nucleoprotein (NP), i.e., NP53kD, NP47kD, and NP40kD. While both NP47kD and NP40kD have been previously shown to be products of caspase cleavage, here, we show that expression of the third isoform NP53kD is due to alternative in-frame translation from M80. Based on this information, we were able to generate recombinant JUNVs lacking each of these isoforms. Infection with these mutants revealed that, while all three isoforms contribute to the efficient control of caspase activation, NP40kD plays the predominant role. In contrast to full-length NP (i.e., NP65kD), which is localized to inclusion bodies, where viral RNA synthesis takes place, the loss of portions of the N-terminal coiled-coil region in these isoforms leads to a diffuse cytoplasmic distribution and a loss of function in viral RNA synthesis. Nonetheless, NP53kD, NP47kD, and NP40kD all retain robust interferon antagonistic and 3'-5' exonuclease activities. We suggest that the altered localization of these NP isoforms allows them to be more efficiently targeted by activated caspases for cleavage as decoy substrates, and to be better positioned to degrade viral double-stranded (ds)RNA species that accumulate in the cytoplasm during virus infection and/or interact with cytosolic RNA sensors, thereby limiting dsRNA-mediated innate immune responses. Taken together, this work provides insight into the mechanism by which JUNV leverages apoptosis during infection to generate biologically distinct pools of NP and contributes to our understanding of the expression and biological relevance of alternative protein isoforms during virus infection.IMPORTANCEA limited coding capacity means that RNA viruses need strategies to diversify their proteome. The nucleoprotein (NP) of the highly pathogenic arenavirus Junín virus (JUNV) produces three N-terminally truncated isoforms: two (NP47kD and NP40kD) are known to be produced by caspase cleavage, while, here, we show that NP53kD is produced by alternative translation initiation. Recombinant JUNVs lacking individual NP isoforms revealed that all three isoforms contribute to inhibiting caspase activation during infection, but cleavage to generate NP40kD makes the biggest contribution. Importantly, all three isoforms retain their ability to digest double-stranded (ds)RNA and inhibit interferon promoter activation but have a diffuse cytoplasmic distribution. Given the cytoplasmic localization of both aberrant viral dsRNAs, as well as dsRNA sensors and many other cellular components of innate immune activation pathways, we suggest that the generation of NP isoforms not only contributes to evasion of apoptosis but also robust control of the antiviral response.

The Importance of Lassa Fever and Its Disease Management in West Africa

Lassa virus (LASV) is a zoonotic pathogen endemic throughout western Africa and is responsible for a human disease known as Lassa fever (LF). Historically, LASV has been emphasized as one of the greatest public health threats in West Africa, with up to 300,000 cases and 5000 associated deaths per year. This, and the fact that the disease has been reported in travelers, has driven a rapid production of various vaccine candidates. Several of these vaccines are currently in clinical development, despite limitations in understanding the immune response to infection. Alarmingly, the host immune response has been implicated in the induction of sensorineural hearing loss in LF survivors, legitimately raising safety questions about any future vaccines as well as efficacy in preventing potential hearing loss. The objective of this article is to revisit the importance and prevalence of LF in West Africa, with focus on Nigeria, and discuss current therapeutic approaches and ongoing vaccine development. In addition, we aim to emphasize the need for more scientific studies relating to LF-associated hearing loss, and to promote critical discussion about potential risks and benefits of vaccinating the population in endemic regions of West Africa.

Recent Discoveries of Novel Mammarenaviruses Infecting Humans and Other Mammals in Asia and Southeast Asia

Mammarenaviruses, a genus of the family Arenaviridae, are capable of infecting mammals and are primarily found in rodent reservoirs worldwide. Mammarenaviruses can be transmitted to humans through contact with infected rodents, and though infection is often asymptomatic, some members of this genus can cause viral haemorrhagic fever which has mortality rates ranging from 1% to 50%. These viruses are typically restricted geographically, based on the geographical range of their host reservoirs. Lymphocytic choriomeningitis virus (LCMV) was previously thought to be the only mammarenavirus found across the globe. However, recent discoveries of two novel human mammarenaviruses, Wenzhou Virus (WENV) and Plateau Pika Virus (PPV), in Asia and Southeast Asia show that mammarenaviruses are more widespread than previously thought. This editorial article aims to raise awareness about these emerging viruses, their genetic and ecological diversities, and clinical significance, and to encourage further study of these emerging viruses.

PDIA4 Is a Host Factor Important for Lymphocytic Choriomeningitis Virus Infection

Mammalian arenaviruses are rodent-borne zoonotic viruses, some of which can cause fatal hemorrhagic diseases in humans. The first discovered arenavirus, lymphocytic choriomeningitis virus (LCMV), has a worldwide distribution and can be fatal for transplant recipients. However, no FDA-approved drugs or vaccines are currently available. In this study, using a quantitative proteomic analysis, we identified a variety of host factors that could be needed for LCMV infection, among which we found that protein disulfide isomerase A4 (PDIA4), a downstream factor of endoplasmic reticulum stress (ERS), is important for LCMV infection. Biochemical analysis revealed that LCMV glycoprotein was the main viral component accounting for PDIA4 upregulation. The inhibition of ATF6-mediated ERS could prevent the upregulation of PDIA4 that was stimulated by LCMV infection. We further found that PDIA4 can affect the LCMV viral RNA synthesis processes and release. In summary, we conclude that PDIA4 could be a new target for antiviral drugs against LCMV.

Luminescent reporter cells enable the identification of broad-spectrum antivirals against emerging viruses

The emerging viruses SARS-CoV-2 and arenaviruses cause severe respiratory and hemorrhagic diseases, respectively. The production of infectious particles of both viruses and virus spread in tissues requires cleavage of surface glycoproteins (GPs) by host proprotein convertases (PCs). SARS-CoV-2 and arenaviruses rely on GP cleavage by PCs furin and subtilisin kexin isozyme-1/site-1 protease (SKI-1/S1P), respectively. We report improved luciferase-based reporter cell lines, named luminescent inducible proprotein convertase reporter cells that we employ to monitor PC activity in its authentic subcellular compartment. Using these sensor lines we screened a small compound library in high-throughput manner. We identified 23 FDA-approved small molecules, among them monensin which displayed broad activity against furin and SKI-1/S1P. Monensin inhibited arenaviruses and SARS-CoV-2 in a dose-dependent manner. We observed a strong reduction in infectious particle release upon monensin treatment with little effect on released genome copies. This was reflected by inhibition of SARS-CoV-2 spike processing suggesting the release of immature particles. In a proof of concept experiment using human precision cut lung slices, monensin potently inhibited SARS-CoV-2 infection, evidenced by reduced infectious particle release. We propose that our PC sensor pipeline is a suitable tool to identify broad-spectrum antivirals with therapeutic potential to combat current and future emerging viruses.

ICTV Virus Taxonomy Profile: Arenaviridae 2023

Arenaviridae is a family for ambisense RNA viruses with genomes of about 10.5 kb that infect mammals, snakes, and fish. The arenavirid genome consists of two or three single-stranded RNA segments and encodes a nucleoprotein (NP), a glycoprotein (GP) and a large (L) protein containing RNA-directed RNA polymerase (RdRP) domains; some arenavirids encode a zinc-binding protein (Z). This is a summary of the International Committee on Taxonomy of Viruses (ICTV) report on the family Arenaviridae, which is available at www.ictv.global/report/arenaviridae.

Lymphocytic Choriomeningitis Virus in Person Living with HIV, Connecticut, USA, 2021

Lymphocytic choriomeningitis virus is an underreported cause of miscarriage and neurologic disease. Surveillance remains challenging because of nonspecific symptomatology, inconsistent case reporting, and difficulties with diagnostic testing. We describe a case of acute lymphocytic choriomeningitis virus disease in a person living with HIV in Connecticut, USA, identified by using quantitative reverse transcription PCR.

Etiologies of Zoonotic Tropical Febrile Illnesses That Are Not Part of the Notifiable Diseases in Colombia

In Colombia, tropical febrile illnesses represent one of the most important causes of clinical attention. Febrile illnesses in the tropics are mainly zoonotic and have a broad etiology. The Colombian surveillance system monitors some notifiable diseases. However, several etiologies are not monitored by this system. In the present review, we describe eleven different etiologies of zoonotic tropical febrile illnesses that are not monitored by the Colombian surveillance system but have scientific, historical, and contemporary data that confirm or suggest their presence in different regions of the country: Anaplasma, Arenavirus, Bartonella, relapsing fever group Borrelia, Coxiella burnetii, Ehrlichia, Hantavirus, Mayaro virus, Orientia, Oropouche virus, and Rickettsia. These could generate a risk for the local population, travelers, and immigrants, due to which they should be included in the mandatory notification system, considering their importance for Colombian public health.

A human monoclonal antibody combination rescues nonhuman primates from advanced disease caused by the major lineages of Lassa virus

There are no approved treatments for Lassa fever (LF), which is responsible for thousands of deaths each year in West Africa. A major challenge in developing effective medical countermeasures against LF is the high diversity of circulating Lassa virus (LASV) strains with four recognized lineages and four proposed lineages. The recent resurgence of LASV in Nigeria caused by genetically distinct strains underscores this concern. Two LASV lineages (II and III) are dominant in Nigeria. Here, we show that combinations of two or three pan-lineage neutralizing human monoclonal antibodies (8.9F, 12.1F, 37.D) known as Arevirumab-2 or Arevirumab-3 can protect up to 100% of cynomolgus macaques against challenge with both lineage II and III LASV isolates when treatment is initiated at advanced stages of disease on day 8 after LASV exposure. This work demonstrates that it may be possible to develop postexposure interventions that can broadly protect against most strains of LASV.

Effects of the Natural Flavonoid Quercetin on Arenavirus Junín Infection

There is no specific chemotherapy approved for the treatment of pathogenic arenaviruses that cause severe hemorrhagic fever (HF) in the population of endemic regions in America and Africa. The present study reports the effects of the natural flavonoid quercetin (QUER) on the infection of A549 and Vero cells with Junín virus (JUNV), agent of the Argentine HF. By infectivity assays, a very effective dose-dependent reduction of JUNV multiplication was shown by cell pretreatment at 2-6 h prior to the infection at non-cytotoxic concentrations, with 50% effective concentration values in the range of 6.1-7.5 µg/mL. QUER was also active by post-infection treatment but with minor efficacy. Mechanistic studies indicated that QUER mainly affected the early steps of virus adsorption and internalization in the multiplication cycle of JUNV. Treatment with QUER blocked the phosphorylation of Akt without changes in the total protein expression, detected by Western blot, and the consequent perturbation of the PI3K/Akt pathway was also associated with the fluorescence redistribution from membrane to cytoplasm of TfR1, the cell receptor recognized by JUNV. Then, it appears that the cellular antiviral state, induced by QUER treatment, leads to the prevention of JUNV entry into the cell.

Passive Transfer of Animal-Derived Polyclonal Hyperimmune Antibodies Provides Protection of Mice from Lethal Lassa Virus Infection

BACKGROUND

Lassa virus (LASV) can cause severe acute systemic infection in humans. No approved antiviral drugs or vaccines are currently available. Antibody-based therapeutics are considered a promising treatment strategy in the management of LASV disease.

METHODS

We used chimeric Ifnar^-/- C57BL/6 (Ifnar^{-/- Bl6}) mice, a lethal LASV mouse model, to evaluate the protective efficacy of polyclonal antibodies purified from sera of rabbits hyperimmunized with virus-like particles displaying native-like LASV glycoprotein GP spikes.

RESULTS

Polyclonal anti-LASV GP antibodies provided 100% protection against lethal LASV infection in a pre- and post-exposure treatment setting and prevented LASV disease. Treatment also significantly lowered viremia level and virus load in organs. When treatment was initiated at the onset of symptoms, the hyperimmune antibodies provided partial protection and increased the survival rate by 80%.

CONCLUSIONS

Our findings support the consideration of animal-derived hyperimmune antibodies targeting GP as an effective treatment option for highly pathogenic LASV.

Reptarenavirus S Segment RNA Levels Correlate with the Presence of Inclusion Bodies and the Number of L Segments in Snakes with Reptarenavirus Infection-Lessons Learned from a Large Breeding Colony

Reptarenaviruses cause boid inclusion body disease (BIBD), a fatal disease particularly impacting captive boa constrictor collections. The development of cytoplasmic inclusion bodies (IBs) comprising reptarenavirus nucleoprotein (NP) in many cell types of affected snakes is characteristic of BIBD. However, snakes can harbor reptarenaviruses without showing IBs, hence representing carriers and a potential source of transmission. The RNA genome of reptarenaviruses comprises a small (S) and a large (L) segment, and the snakes with BIBD commonly carry a swarm of reptarenavirus segments. To design sensitive and reliable tools for the diagnosis of reptarenavirus infection in snake colonies, we used metatranscriptomics to determine the reptarenavirus segments present in a large boa constrictor breeding colony. The analysis identified one reptarenavirus S segment and three L segments in the colony. The sequence data served to design real-time reverse transcription-PCR (RT-PCR) targeting the found S segment. This allowed us to identify all infected animals and to quantify the S segment RNA levels, which we found to correlate with the presence of IBs. We further found a positive correlation between the number of L segments and the S segment RNA level, which could suggest that L segment excess also contributes to the IB formation. Information on cohousing of the snakes showed a clear association of reptarenavirus infection with cohousing in general and cohousing with infected animals. Information on breeding and offspring confirmed that vertical transmission occurred. Furthermore, our data suggest that some animals might be able to clear the infection or at least exhibit transient or intermittent viremia. IMPORTANCE Boid inclusion body disease (BIBD) is caused by reptarenavirus infection, and while reptarenavirus nucleoprotein is the main component of the inclusion bodies (IBs) characteristic of BIBD, not all reptarenavirus-infected snakes demonstrate IBs in their cells. Identification of infected individuals is critical for controlling the spread of the disease; however, the genetic divergence of reptarenaviruses complicates reverse transcription-PCR (RT-PCR)-based diagnostics. Here, we tested a next-generation-sequencing-based approach to establish a tailored "colony-specific" set of diagnostic tools for the detection of reptarenavirus small (S) and large (L) genome segments. With this approach, we could demonstrate that an S-segment-specific RT-PCR is highly effective in identifying the infected individuals. We further found the S segment RNA level to positively correlate with the presence of IBs and the number of L segments, which could direct future studies to identify the BIBD pathogenetic mechanisms.

N6-methyladenosine is required for efficient RNA synthesis of Ebola virus and other haemorrhagic fever viruses

N⁶-methyladenosine (m⁶A) is one of the most abundant modifications of cellular RNA, where it serves various functions. m⁶A methylation of many viral RNA species has also been described; however, little is known about the m⁶A epitranscriptome of haemorrhagic fever-causing viruses like Ebola virus (EBOV). Here, we analysed the importance of the methyltransferase METTL3 for the life cycle of this virus. We found that METTL3 interacts with the EBOV nucleoprotein and the transcriptional activator VP30 to support viral RNA synthesis, and that METTL3 is recruited into EBOV inclusions bodies, where viral RNA synthesis occurs. Analysis of the m⁶A methylation pattern of EBOV mRNAs showed that they are methylated by METTL3. Further studies revealed that METTL3 interaction with the viral nucleoprotein, as well as its importance for RNA synthesis and protein expression, is also observed for other haemorrhagic fever viruses such as Junín virus (JUNV) and Crimean-Congo haemorrhagic fever virus (CCHFV). The negative effects on viral RNA synthesis due to loss of m⁶A methylation are independent of innate immune sensing, as METTL3 knockout did not affect type I interferon induction in response to viral RNA synthesis or infection. Our results suggest a novel function for m⁶A that is conserved among diverse haemorrhagic fever-causing viruses (i.e. EBOV, JUNV and CCHFV), making METTL3 a promising target for broadly-acting antivirals.

Structural conservation of Lassa virus glycoproteins and recognition by neutralizing antibodies

Lassa fever is an acute hemorrhagic fever caused by the zoonotic Lassa virus (LASV). The LASV glycoprotein complex (GPC) mediates viral entry and is the sole target for neutralizing antibodies. Immunogen design is complicated by the metastable nature of recombinant GPCs and the antigenic differences among phylogenetically distinct LASV lineages. Despite the sequence diversity of the GPC, structures of most lineages are lacking. We present the development and characterization of prefusion-stabilized, trimeric GPCs of LASV lineages II, V, and VII, revealing structural conservation despite sequence diversity. High-resolution structures and biophysical characterization of the GPC in complex with GP1-A-specific antibodies suggest their neutralization mechanisms. Finally, we present the isolation and characterization of a trimer-preferring neutralizing antibody belonging to the GPC-B competition group with an epitope that spans adjacent protomers and includes the fusion peptide. Our work provides molecular detail information on LASV antigenic diversity and will guide efforts to design pan-LASV vaccines.

Structural basis for antibody-mediated neutralization of lymphocytic choriomeningitis virus

The mammarenavirus lymphocytic choriomeningitis virus (LCMV) is a globally distributed zoonotic pathogen that can be lethal in immunocompromised patients and can cause severe birth defects if acquired during pregnancy. The structure of the trimeric surface glycoprotein, essential for entry, vaccine design, and antibody neutralization, remains unknown. Here, we present the cryoelectron microscopy (cryo-EM) structure of the LCMV surface glycoprotein (GP) in its trimeric pre-fusion assembly both alone and in complex with a rationally engineered monoclonal neutralizing antibody termed 18.5C-M28 (M28). Additionally, we show that passive administration of M28, either as a prophylactic or therapeutic, protects mice from LCMV clone 13 (LCMVcl13) challenge. Our study illuminates not only the overall structural organization of LCMV GP and the mechanism for its inhibition by M28 but also presents a promising therapeutic candidate to prevent severe or fatal disease in individuals who are at risk of infection by a virus that poses a threat worldwide.

Vaccine Candidates against Arenavirus Infections

The viral family Arenaviridae contains several members that cause severe, and often lethal, diseases in humans. Several highly pathogenic arenaviruses are classified as Risk Group 4 agents and must be handled in the highest biological containment facility, biosafety level-4 (BSL-4). Vaccines and treatments are very limited for these pathogens. The development of vaccines is crucial for the establishment of countermeasures against highly pathogenic arenavirus infections. While several vaccine candidates have been investigated, there are currently no approved vaccines for arenavirus infection except for Candid#1, a live-attenuated Junin virus vaccine only licensed in Argentina. Current platforms under investigation for use include live-attenuated vaccines, recombinant virus-based vaccines, and recombinant proteins. We summarize here the recent updates of vaccine candidates against arenavirus infections.

Modulation of the Aryl Hydrocarbon Receptor Signaling Pathway Impacts on Junín Virus Replication

Junín virus (JUNV), a member of the family Arenaviridae, is the etiological agent of the Argentine hemorrhagic fever, an endemic disease in the rural region of Argentina lacking a specific chemotherapy. Aryl hydrocarbon receptor (AHR) is expressed in several mammalian tissues and has been indicated as a sensor of ligands from variable sources and a modulator of the cell immune response. Interestingly, recent studies have suggested that the activation or depression of the AHR signaling pathway may play a role in the outcome of diverse human viral infections. In the present report, the effect of the pharmacological modulation of AHR on JUNV in vitro infection was analyzed. An initial microarray screening showed that the AHR pathway was overexpressed in JUNV-infected hepatic cells. Concomitantly, the infection of Vero and Huh-7 cells with the JUNV strains IV4454 and Candid#1 was significantly inhibited in a dose-dependent manner by treatment with CH223191, a specific AHR antagonist, as detected by infectivity assays, real-time RT-PCR and immunofluorescence detection of viral proteins. Furthermore, the pro-viral role of AHR in JUNV infection appears to be independent of the IFN-I pathway. Our findings support the promising perspectives of the pharmacological modulation of AHR as a potential target for the control of AHF.

Viral RNA and infectious virus in mucosal specimens from guinea pigs modelling early phases of lethal and non-lethal Lassa fever

Lassa fever (LF) is endemic to broad regions of West Africa. Infection with Lassa virus (LASV), the etiologic agent of LF, results in a spectrum of clinical signs in humans, including severe and lethal hemorrhagic disease. Person-to-person transmission occurs through direct contact with body fluids or contaminated bedding and clothing. To investigate transmission risk in acute LASV infection, we evaluated viral RNA and infectious virus obtained from conjunctival, nasal, oral, genital, and rectal swab specimens from guinea pigs modelling lethal and non-lethal LF. Viral RNA and infectious virus were detected in all specimen types beginning 8 days post infection, prior to onset of fever. In the pre-clinical and clinical period, virus was isolated from a subset of nasal, oral, genital, and rectal swabs, and from all conjunctival swabs. Overall, conjunctival and nasal specimens most frequently yielded infectious virus. These findings indicate mucosal transmission risk based on virus isolation from various sites early in infection and support potential utility of minimally invasive specimen evaluation by RT-qPCR for LASV diagnostics.

Boid Inclusion Body Disease Is Also a Disease of Wild Boa Constrictors

Reptarenaviruses cause boid inclusion body disease (BIBD), a potentially fatal disease, occurring in captive constrictor snakes boas and pythons worldwide. Classical BIBD, characterized by the formation of pathognomonic cytoplasmic inclusion bodies (IBs), occurs mainly in boas, whereas in pythons, for example, reptarenavirus infection most often manifests as central nervous system signs with limited IB formation. The natural hosts of reptarenaviruses are unknown, although free-ranging/wild constrictor snakes are among the suspects. Here, we report BIBD with reptarenavirus infection in indigenous captive and wild boid snakes in Costa Rica using histology, immunohistology, transmission electron microscopy, and next-generation sequencing (NGS). The snakes studied represented diagnostic postmortem cases of captive and wild-caught snakes since 1989. The results from NGS on archival paraffin blocks confirm that reptarenaviruses were already present in wild boa constrictors in Costa Rica in the 1980s. Continuous sequences that were de novo assembled from the low-quality RNA obtained from paraffin-embedded tissue allowed the identification of a distinct pair of reptarenavirus S and L segments in all studied animals; in most cases, reference assembly could recover almost complete segments. Sampling of three prospective cases in 2018 allowed an examination of fresh blood or tissues and resulted in the identification of additional reptarenavirus segments and hartmanivirus coinfection. Our results show that BIBD is not only a disease of captive snakes but also occurs in indigenous wild constrictor snakes in Costa Rica, suggesting boa constrictors to play a role in natural reptarenavirus circulation. IMPORTANCE The literature describes cases of boid inclusion body disease (BIBD) in captive snakes since the 1970s, and in the 2010s, others and ourselves identified reptarenaviruses as the causative agent. BIBD affects captive snakes globally, but the origin and the natural host of reptarenaviruses remain unknown. In this report, we show BIBD and reptarenavirus infections in two native Costa Rican constrictor snake species, and by studying archival samples, we show that both the viruses and the disease have been present in free-ranging/wild snakes in Costa Rica at least since the 1980s. The diagnosis of BIBD in wild boa constrictors suggests that this species plays a role in the circulation of reptarenaviruses. Additional sample collection and analysis would help to clarify this role further and the possibility of, e.g., vector transmission from an arthropod host.

Understanding Immune Responses to Lassa Virus Infection and to Its Candidate Vaccines

Lassa fever (LF) is a deadly viral hemorrhagic fever disease that is endemic in several countries in West Africa. It is caused by Lassa virus (LASV), which has been estimated to be responsible for approximately 300,000 infections and 5000 deaths annually. LASV is a highly pathogenic human pathogen without effective therapeutics or FDA-approved vaccines. Here, we aim to provide a literature review of the current understanding of the basic mechanism of immune responses to LASV infection in animal models and patients, as well as to several of its candidate vaccines.

Depletion of CD4 and CD8 T Cells Reduces Acute Disease and Is Not Associated with Hearing Loss in ML29-Infected STAT1-/- Mice

Lassa virus (LASV) is a zoonotic virus endemic to western Africa that can cause a potentially lethal and hemorrhagic disease, Lassa fever (LF). Survivors suffer a myriad of sequelae, most notably sudden onset sensorineural hearing loss (SNHL), the mechanism of which remains unclear. Unfortunately, studies aiming to identify the mechanism of these sequelae are limited due to the biosafety level 4 (BSL4) requirements of LASV itself. ML29, a reassortant virus proposed as an experimental vaccine candidate against LASV, is potentially an ideal surrogate model of LF in STAT1-/- mice due to similar phenotype in these animals. We intended to better characterize ML29 pathogenesis and potential sequelae in this animal model. Our results indicate that while both CD4 and CD8 T cells are responsible for acute disease in ML29 infection, ML29 induces significant hearing loss in a mechanism independent of either CD4 or CD8 T cells. We believe that this model could provide valuable information for viral-associated hearing loss in general.

Evaluating the Biological Role of Lassa Viral Z Protein-Mediated RIG-I Inhibition Using a Replication-Competent Trisegmented Pichinde Virus System in an Inducible RIG-IN Expression Cell Line

Lassa virus (LASV) is a mammarenavirus that can cause lethal Lassa fever disease with no FDA-approved vaccine and limited treatment options. Fatal LASV infections are associated with innate immune suppression. We have previously shown that the small matrix Z protein of LASV, but not of a nonpathogenic arenavirus Pichinde virus (PICV), can inhibit the cellular RIG-I-like receptors (RLRs), but its biological significance has not been evaluated in an infectious virus due to the multiple essential functions of the Z protein required for the viral life cycle. In this study, we developed a stable HeLa cell line (HeLa-iRIGN) that could be rapidly and robustly induced by doxycycline (Dox) treatment to express RIG-I N-terminal effector, with concomitant production of type I interferons (IFN-Is). We also generated recombinant tri-segmented PICVs, rP18tri-LZ, and rP18tri-PZ, which encode LASV Z and PICV Z, respectively, as an extra mScarlet fusion protein that is nonessential for the viral life cycle. Upon infection, rP18tri-LZ consistently expressed viral genes at a higher level than rP18tri-PZ. rP18tri-LZ also showed a higher level of a viral infection than rP18tri-PZ did in HeLa-iRIGN cells, especially upon Dox induction. The heterologous Z gene did not alter viral growth in Vero and A549 cells by growth curve analysis, while LASV Z strongly increased and prolonged viral gene expression, especially in IFN-competent A549 cells. Our study provides important insights into the biological role of LASV Z-mediated RIG-I inhibition and implicates LASV Z as a potential virulence factor. IMPORTANCE Lassa virus (LASV) can cause lethal hemorrhagic fever disease in humans but other arenaviruses, such as Pichinde virus (PICV), do not cause obvious disease. We have previously shown that the Z protein of LASV but not of PICV can inhibit RIG-I, a cytosolic innate immune receptor. In this study, we developed a stable HeLa cell line that can be induced to express the RIG-I N-terminal effector domain, which allows for timely control of RIG-I activation. We also generated recombinant PICVs encoding LASV Z or PICV Z as an extra gene that is nonessential for the viral life cycle. Compared to PICV Z, LASV Z could increase viral gene expression and viral infection in an infectious arenavirus system, especially when RIG-I signaling is activated. Our study presented a convenient cell system to characterize RIG-I signaling and its antagonists and revealed LASV Z as a possible virulence factor and a potential antiviral target.

A recombinant VSV-vectored vaccine rapidly protects nonhuman primates against heterologous lethal Lassa fever

Lassa virus (LASV) is recognized by the World Health Organization as one of the top five pathogens likely to cause a severe outbreak. A recent unprecedented resurgence of LASV in Nigeria caused by genetically diverse strains underscores the need for licensed medical countermeasures. Single-injection vaccines that can rapidly control outbreaks and confer long-term immunity are needed. Vaccination of cynomolgus monkeys with a recombinant vesicular stomatitis virus vector expressing the glycoprotein precursor of LASV lineage IV strain Josiah (rVSVΔG-LASV-GPC) induces fast-acting protection in monkeys challenged 3 or 7 days later with a genetically heterologous lineage II isolate of LASV from Nigeria, while nonspecifically vaccinated control animals succumb to challenge. The rVSVΔG-LASV-GPC vaccine induces rapid activation of adaptive immunity and the transcription of natural killer (NK) cell-affiliated mRNAs. This study demonstrates that rVSVΔG-LASV-GPC may provide rapid protection in humans against LASV infections in cases where immediate public-health intervention is required.

The Tetraspanin CD81 Is a Host Factor for Chikungunya Virus Replication

Chikungunya virus (CHIKV) is an arthritogenic reemerging virus replicating in plasma membrane-derived compartments termed "spherules." Here, we identify the human transmembrane protein CD81 as host factor required for CHIKV replication. Ablation of CD81 results in decreased CHIKV permissiveness, while overexpression enhances infection. CD81 is dispensable for virus uptake but critically required for viral genome replication. Likewise, murine CD81 is crucial for CHIKV permissiveness and is expressed in target cells such as dermal fibroblasts, muscle and liver cells. Whereas related alphaviruses, including Ross River virus (RRV), Semliki Forest virus (SFV), Sindbis virus (SINV) and Venezuelan equine encephalitis virus (VEEV), also depend on CD81 for infection, RNA viruses from other families, such as coronaviruses, replicate independently of CD81. Strikingly, the replication-enhancing function of CD81 is linked to cholesterol binding. These results define a mechanism exploited by alphaviruses to hijack the membrane microdomain-modeling protein CD81 for virus replication through interaction with cholesterol. IMPORTANCE In this study, we discover the tetraspanin CD81 as a host factor for the globally emerging chikungunya virus and related alphaviruses. We show that CD81 promotes replication of viral genomes in human and mouse cells, while virus entry into cells is independent of CD81. This provides novel insights into how alphaviruses hijack host proteins to complete their life cycle. Alphaviruses replicate at distinct sites of the plasma membrane, which are enriched in cholesterol. We found that the cholesterol-binding ability of CD81 is important for its function as an alphavirus host factor. This discovery thus broadens our understanding of the alphavirus replication process and the use of host factors to reprogram cells into virus replication factories.

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.

Contrasting Modes of New World Arenavirus Neutralization by Immunization-Elicited Monoclonal Antibodies

Transmission of the New World hemorrhagic fever arenaviruses Junín virus (JUNV) and Machupo virus (MACV) to humans is facilitated, in part, by the interaction between the arenavirus GP1 glycoprotein and the human transferrin receptor 1 (hTfR1). We utilize a mouse model of live-attenuated immunization with envelope exchange viruses to isolate neutralizing monoclonal antibodies (NAbs) specific to JUNV GP1 and MACV GP1. Structures of two NAbs, termed JUN1 and MAC1, demonstrate that they neutralize through disruption of hTfR1 recognition. JUN1 utilizes a binding mode common to all characterized infection- and vaccine-elicited JUNV-specific NAbs, which involves mimicking hTfR1 binding through the insertion of a tyrosine into the receptor-binding site. In contrast, MAC1 undergoes a tyrosine-mediated mode of antigen recognition distinct from that used by the reported anti-JUNV NAbs and the only other characterized anti-MACV NAb. These data reveal the varied modes of GP1-specific recognition among New World arenaviruses by the antibody-mediated immune response. IMPORTANCE The GP1 subcomponent of the New World arenavirus GP is a primary target of the neutralizing antibody response, which has been shown to be effective in the prevention and treatment of infection. Here, we characterize the structural basis of the antibody-mediated immune response that arises from immunization of mice against Junín virus and Machupo virus, two rodent-borne zoonotic New World arenaviruses. We isolate a panel of GP1-specific monoclonal antibodies that recognize overlapping epitopes and exhibit neutralizing behavior, in vitro. Structural characterization of two of these antibodies indicates that antibody recognition likely interferes with GP1-mediated recognition of the transferrin receptor 1. These data provide molecular-level detail for a key region of vulnerability on the New World arenavirus surface and a blueprint for therapeutic antibody development.

Genetic characterisation of a novel reptarenavirus detected in a dead pet red-tailed boa (Boa constrictor)

Boid inclusion body disease (BIBD) is a severe and transmissible disease of snakes worldwide. Reptarenaviruses have been identified as the aetiological agents of BIBD. We determined the almost complete genome sequence of an arenavirus detected in a female red-tailed boa that had succumbed in a private collection in Hungary. We used a combination of next generation sequencing and Sanger sequencing methods. Based on the analysis of the obtained sequence data, the virus, tentatively named Coldvalley virus, seemed to belong to the Reptarenavirus genus of the Arenaviridae family. This classification was confirmed by the genome structure (bisegmented single-stranded RNA) characteristic of the genera Mammarenavirus and Reptarenavirus. The pairwise comparison of the nucleotide and amino acid sequences, as well as the topology of the maximum likelihood phylogenetic trees, suggested that the newly-characterised Coldvalley virus can be classified into the species Rotterdam reptarenavirus.

CD164 is a host factor for lymphocytic choriomeningitis virus entry

Lymphocytic choriomeningitis virus (LCMV) is the prototypic arenavirus and has been utilized for decades as a model to understand the host immune response against viral infection. LCMV infection can lead to fatal meningitis in immunocompromised people and can lead to congenital birth defects and spontaneous abortion if acquired during pregnancy. Using a genetic screen, we uncover host factors involved in LCMV entry that were previously unknown and are candidate therapeutic targets to combat LCMV infection. This study expands our understanding of the entry pathway of LCMV, revealing that its glycoprotein switches from utilizing the known receptor α-DG and heparan sulfate at the plasma membrane to binding the lysosomal mucin CD164 at pH levels found in endolysosomal compartments, facilitating membrane fusion.

Lymphocytic choriomeningitis virus (LCMV) is a rodent-borne zoonotic arenavirus that causes congenital abnormalities and can be fatal for transplant recipients. Using a genome-wide loss-of-function screen, we identify host factors required for LCMV entry into cells. We identify the lysosomal mucin CD164, glycosylation factors, the heparan sulfate biosynthesis machinery, and the known receptor alpha-dystroglycan (α-DG). Biochemical analysis revealed that the LCMV glycoprotein binds CD164 at acidic pH and requires a sialylated glycan at residue N104. We demonstrate that LCMV entry proceeds by the virus switching binding from heparan sulfate or α-DG at the plasma membrane to CD164 prior to membrane fusion, thus identifying additional potential targets for therapeutic intervention.

Inhibition of Arenavirus Entry and Replication by the Cell-Intrinsic Restriction Factor ZMPSTE24 Is Enhanced by IFITM Antiviral Activity

In the absence of effective vaccines and treatments, annual outbreaks of severe human haemorrhagic fever caused by arenaviruses, such as Lassa virus, continue to pose a significant human health threat. Understanding the balance of cellular factors that inhibit or promote arenavirus infection may have important implications for the development of effective antiviral strategies. Here, we identified the cell-intrinsic zinc transmembrane metalloprotease, ZMPSTE24, as a restriction factor against arenaviruses. Notably, CRISPR-Cas9-mediated knockout of ZMPSTE24 in human alveolar epithelial A549 cells increased arenavirus glycoprotein-mediated viral entry in pseudoparticle assays and live virus infection models. As a barrier to viral entry and replication, ZMPSTE24 may act as a downstream effector of interferon-induced transmembrane protein (IFITM) antiviral function; though through a yet poorly understood mechanism. Overexpression of IFITM1, IFITM2, and IFITM3 proteins did not restrict the entry of pseudoparticles carrying arenavirus envelope glycoproteins and live virus infection. Furthermore, gain-of-function studies revealed that IFITMs augment the antiviral activity of ZMPSTE24 against arenaviruses, suggesting a cooperative effect of viral restriction. We show that ZMPSTE24 and IFITMs affect the kinetics of cellular endocytosis, suggesting that perturbation of membrane structure and stability is likely the mechanism of ZMPSTE24-mediated restriction and cooperative ZMPSTE24-IFITM antiviral activity. Collectively, our findings define the role of ZMPSTE24 host restriction activity in the early stages of arenavirus infection. Moreover, we provide insight into the importance of cellular membrane integrity for productive fusion of arenaviruses and highlight a novel avenue for therapeutic development.

Lassa virus glycoprotein complex review: insights into its unique fusion machinery

Lassa virus (LASV), an arenavirus endemic to West Africa, causes Lassa fever—a lethal hemorrhagic fever. Entry of LASV into the host cell is mediated by the glycoprotein complex (GPC), which is the only protein located on the viral surface and comprises three subunits: glycoprotein 1 (GP1), glycoprotein 2 (GP2), and a stable signal peptide (SSP). The LASV GPC is a class one viral fusion protein, akin to those found in viruses such as human immunodeficiency virus (HIV), influenza, Ebola virus (EBOV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These viruses are enveloped and utilize membrane fusion to deliver their genetic material to the host cell. Like other class one fusion proteins, LASV-mediated membrane fusion occurs through an orchestrated sequence of conformational changes in its GPC. The receptor-binding subunit, GP1, first engages with a host cell receptor then undergoes a unique receptor switch upon delivery to the late endosome. The acidic pH and change in receptor result in the dissociation of GP1, exposing the fusion subunit, GP2, such that fusion can occur. These events ultimately lead to the formation of a fusion pore so that the LASV genetic material is released into the host cell. Interestingly, the mature GPC retains its SSP as a third subunit—a feature that is unique to arenaviruses. Additionally, the fusion domain contains two separate fusion peptides, instead of a standard singular fusion peptide. Here, we give a comprehensive review of the LASV GPC components and their unusual features.

RIG-I and MDA5 Protect Mice From Pichinde Virus Infection by Controlling Viral Replication and Regulating Immune Responses to the Infection

RIG-I and MDA5 are major cytoplasmic innate-immune sensor proteins that recognize aberrant double-stranded RNAs generated during virus infection to activate type 1 interferon (IFN-I) and IFN-stimulated gene (ISG) expressions to control virus infection. The roles of RIG-I and MDA5 in controlling replication of Pichinde virus (PICV), a mammarenavirus, in mice have not been examined. Here, we showed that MDA5 single knockout (SKO) and RIG-I/MDA5 double knockout (DKO) mice are highly susceptible to PICV infection as evidenced by their significant reduction in body weights during the course of the infection, validating the important roles of these innate-immune sensor proteins in controlling PICV infection. Compared to the wildtype mice, SKO and DKO mice infected with PICV had significantly higher virus titers and lower IFN-I expressions early in the infection but appeared to exhibit a late and heightened level of adaptive immune responses to clear the infection. When a recombinant rPICV mutant virus (rPICV-NPmut) that lacks the ability to suppress IFN-I was used to infect mice, as expected, there were heightened levels of IFN-I and ISG expressions in the wild-type mice, whereas infected SKO and DKO mice showed delayed mouse growth kinetics and relatively low, delayed, and transient levels of innate and adaptive immune responses to this viral infection. Taken together, our data suggest that PICV infection triggers activation of immune sensors that include but might not be necessarily limited to RIG-I and MDA5 to stimulate effective innate and adaptive immune responses to control virus infection in mice.

Pathogenicity and virulence mechanisms of Lassa virus and its animal modeling, diagnostic, prophylactic, and therapeutic developments

Lassa fever (LF) is a deadly viral hemorrhagic disease that is endemic to West Africa. The causative agent of LF is Lassa virus (LASV), which causes approximately 300,000 infections and 5,000 deaths annually. There are currently no approved therapeutics or FDA-approved vaccines against LASV. The high genetic variability between LASV strains and immune evasion mediated by the virus complicate the development of effective therapeutics and vaccines. Here, we aim to provide a comprehensive review of the basic biology of LASV and its mechanisms of disease pathogenesis and virulence in various animal models, as well as an update on prospective vaccines, therapeutics, and diagnostics for LF. Until effective vaccines and/or therapeutics are available for use to prevent or treat LF, a better level of understanding of the basic biology of LASV, its natural genetic variations and immune evasion mechanisms as potential pathogenicity factors, and of the rodent reservoir-vector populations and their geographical distributions, is necessary for the development of accurate diagnostics and effective therapeutics and vaccines against this deadly human viral pathogen.

Characterization of RNA Sensing Pathways in Hepatoma Cell Lines and Primary Human Hepatocytes

The liver is targeted by several human pathogenic RNA viruses for viral replication and dissemination; despite this, the extent of innate immune sensing of RNA viruses by human hepatocytes is insufficiently understood to date. In particular, for highly human tropic viruses such as hepatitis C virus, cell culture models are needed to study immune sensing. However, several human hepatoma cell lines have impaired RNA sensing pathways and fail to mimic innate immune responses in the human liver. Here we compare the RNA sensing properties of six human hepatoma cell lines, namely Huh-6, Huh-7, HepG2, HepG2-HFL, Hep3B, and HepaRG, with primary human hepatocytes. We show that primary liver cells sense RNA through retinoic acid-inducible gene I (RIG-I) like receptor (RLR) and Toll-like receptor 3 (TLR3) pathways. Of the tested cell lines, Hep3B cells most closely mimicked the RLR and TLR3 mediated sensing in primary hepatocytes. This was shown by the expression of RLRs and TLR3 as well as the expression and release of bioactive interferon in primary hepatocytes and Hep3B cells. Our work shows that Hep3B cells partially mimic RNA sensing in primary hepatocytes and thus can serve as in vitro model to study innate immunity to RNA viruses in hepatocytes.

Development and Characterization of a Novel Non-Lytic Cancer Immunotherapy Using a Recombinant Arenavirus Vector Platform

Engineered viral vectors represent a promising strategy to trigger antigen-specific antitumor T cell responses. Arenaviruses have been widely studied because of their ability to elicit potent and protective T cell responses. Here, we provide an overview of a novel intravenously administered, replication-competent, non-lytic arenavirus-based vector technology that delivers tumor antigens to induce antigen-specific anti-cancer T cell responses. Preclinical studies in mice and cell culture experiments with human peripheral blood mononuclear cells demonstrate that arenavirus vectors preferentially infect antigen-presenting cells. This, in conjunction with a non-lytic functional activation of the infected antigen-presenting cells, leads to a robust antigen-specific CD8⁺ T cell response. T cell migration to, and infiltration of, the tumor microenvironment has been demonstrated in various preclinical tumor models with vectors encoding self- and non-self-antigens. The available data also suggest that arenavirus-based vector therapy can induce immunological memory protecting from tumor rechallenge. Based on promising preclinical data, a phase 1/2 clinical trial was initiated and is currently ongoing to test the activity and safety of arenavirus vectors, HB-201 and HB-202, created using lymphocytic choriomeningitis virus and Pichinde virus, respectively. Both vectors have been engineered to deliver non-oncogenic versions of the human papilloma virus 16 (HPV16) antigens E7 and E6 and will be injected intravenously with or without an initial intratumoral dose. This dose escalation/expansion study is being conducted in patients with recurrent or metastatic HPV16+ cancers. Promising preliminary data from this ongoing clinical study have been reported. Immunogenicity data from several patients demonstrate that a single injection of HB-201 or HB-202 monotherapy is highly immunogenic, as evidenced by an increase in inflammatory cytokines/chemokines and the expansion of antigen-specific CD8⁺ T cell responses. This response can be further enhanced by alternating injections of HB-202 and HB-201, which has resulted in frequencies of circulating HPV16 E7/E6-specific CD8⁺ T cells of up to 40% of the total CD8⁺ T cell compartment in peripheral blood in analyses to date. Treatment with intravenous administration also resulted in a disease control rate of 73% among 11 evaluable patients with head and neck cancer dosed every three weeks, including 2 patients with a partial response.

Isolation of Reconstructed Functional Ribonucleoprotein Complexes of Machupo Virus

Arenaviruses initiate infection by delivering a transcriptionally competent ribonucleoprotein (RNP) complex into the cytosol of host cells. The arenavirus RNP consists of the large (L) RNA-dependent RNA polymerase (RdRP) bound to a nucleoprotein (NP)-encapsidated genomic RNA (viral RNA [vRNA]) template. During transcription and replication, L must transiently displace RNA-bound NP to allow for template access into the RdRP active site. Concomitant with RNA replication, new subunits of NP must be added to the nascent complementary RNAs (cRNA) as they emerge from the product exit channel of L. Interactions between L and NP thus play a central role in arenavirus gene expression. We developed an approach to purify recombinant functional RNPs from mammalian cells in culture using a synthetic vRNA and affinity-tagged L and NP. Negative-stain electron microscopy of purified RNPs revealed they adopt diverse and flexible structures, like RNPs of other Bunyavirales members. Monodispersed L-NP and trimeric ring-like NP complexes were also obtained in excess of flexible RNPs, suggesting that these heterodimeric structures self-assemble in the absence of suitable RNA templates. This work allows for further biochemical analysis of the interaction between arenavirus L and NP proteins and provides a framework for future high-resolution structural analyses of this replication-associated complex. IMPORTANCE Arenaviruses are rodent-borne pathogens that can cause severe disease in humans. All arenaviruses begin the infection cycle with delivery of the virus replication machinery into the cytoplasm of the host cell. This machinery consists of an RNA-dependent RNA polymerase-which copies the viral genome segments and synthesizes all four viral mRNAs-bound to the two nucleoprotein-encapsidated genomic RNAs. How this complex assembles remains a mystery. Our findings provide direct evidence for the formation of diverse intracellular arenavirus replication complexes using purification strategies for the polymerase, nucleoprotein, and genomic RNA of Machupo virus, which causes Bolivian hemorrhagic fever in humans. We demonstrate that the polymerase and nucleoprotein assemble into higher-order structures within cells, providing a model for the molecular events of arenavirus RNA synthesis. These findings provide a framework for probing the architectures and functions of the arenavirus replication machinery and thus advancing antiviral strategies targeting this essential complex.

CP100356 Hydrochloride, a P-Glycoprotein Inhibitor, Inhibits Lassa Virus Entry: Implication of a Candidate Pan-Mammarenavirus Entry Inhibitor

Lassa virus (LASV)—a member of the family Arenaviridae—causes Lassa fever in humans and is endemic in West Africa. Currently, no approved drugs are available. We screened 2480 small compounds for their potential antiviral activity using pseudotyped vesicular stomatitis virus harboring the LASV glycoprotein (VSV-LASVGP) and a related prototypic arenavirus, lymphocytic choriomeningitis virus (LCMV). Follow-up studies confirmed that CP100356 hydrochloride (CP100356), a specific P-glycoprotein (P-gp) inhibitor, suppressed VSV-LASVGP, LCMV, and LASV infection with half maximal inhibitory concentrations of 0.52, 0.54, and 0.062 μM, respectively, without significant cytotoxicity. Although CP100356 did not block receptor binding at the cell surface, it inhibited low-pH-dependent membrane fusion mediated by arenavirus glycoproteins. P-gp downregulation did not cause a significant reduction in either VSV-LASVGP or LCMV infection, suggesting that P-gp itself is unlikely to be involved in arenavirus entry. Finally, our data also indicate that CP100356 inhibits the infection by other mammarenaviruses. Thus, our findings suggest that CP100356 can be considered as an effective virus entry inhibitor for LASV and other highly pathogenic mammarenaviruses.

Screening and Identification of Lujo Virus Inhibitors Using a Recombinant Reporter Virus Platform

Lujo virus (LUJV), a highly pathogenic arenavirus, was first identified in 2008 in Zambia. To aid the identification of effective therapeutics for LUJV, we developed a recombinant reporter virus system, confirming reporter LUJV comparability with wild-type virus and its utility in high-throughput antiviral screening assays. Using this system, we evaluated compounds with known and unknown efficacy against related arenaviruses, with the aim of identifying LUJV-specific and potential new pan-arenavirus antivirals. We identified six compounds demonstrating robust anti-LUJV activity, including several compounds with previously reported activity against other arenaviruses. These data provide critical evidence for developing broad-spectrum antivirals against high-consequence arenaviruses.

Second-Generation Live-Attenuated Candid#1 Vaccine Virus Resists Reversion and Protects against Lethal Junín Virus Infection in Guinea Pigs

Live-attenuated virus vaccines are highly effective in preventing viral disease but carry intrinsic risks of residual virulence and reversion to pathogenicity. The classically derived Candid#1 virus protects seasonal field workers in Argentina against zoonotic infection by Junín virus (JUNV) but is not approved in the United States, in part due to the potential for reversion at the attenuating locus, a phenylalanine-to-isoleucine substitution at position 427 in the GP2 subunit of the GPC envelope glycoprotein. Previously, we demonstrated facile reversion of recombinant Candid#1 (rCan) in cell culture and identified an epistatic interaction between the attenuating I427 and a secondary K33S mutation in the stable signal peptide (SSP) subunit of GPC that imposes an evolutionary barrier to reversion. The magnitude of this genetic barrier is manifest in our repeated failures to rescue the hypothetical revertant virus. In this study, we show that K33S rCan is safe and attenuated in guinea pigs and capable of eliciting potent virus-neutralizing antibodies. Immunized animals are fully protected against lethal challenge with virulent JUNV. In addition, we employed a more permissive model of infection in neonatal mice to investigate genetic reversion. RNA sequence analysis of the recovered virus identified revertant viruses in pups inoculated with the parental rCan virus and none in mice receiving K33S rCan (P < 0.0001). Taken together, our findings support the further development of K33S rCan as a safe second-generation JUNV vaccine. IMPORTANCE Our most successful vaccines comprise weakened strains of virus that initiate a limited and benign infection in immunized persons. The live-attenuated Candid#1 strain of Junín virus (JUNV) was developed to protect field workers in Argentina from rodent-borne hemorrhagic fever but is not licensed in the United States, in part due to the likelihood of genetic reversion to virulence. A single-amino-acid change in the GPC envelope glycoprotein of the virus is responsible for attenuation, and a single nucleotide change may regenerate the pathogenic virus. Here, we take advantage of a unique genetic interaction between GPC subunits to design a mutant Candid#1 virus that establishes an evolutionary barrier to reversion. The mutant virus (K33S rCan) is fully attenuated and protects immunized guinea pigs against lethal JUNV infection. We find no instances of reversion in mice inoculated with K33S rCan. This work supports the further development of K33S rCan as a second-generation JUNV vaccine.

Screening of Eurasian Tundra Reindeer for Viral Sequences by Next-Generation Sequencing

Reindeer husbandry is essential for the livelihood and culture of indigenous people in the Arctic. Parts of the herding areas are also used as pastures for farm animals, facilitating potential transmission of viruses between species. Following the Covid-19 pandemic, viruses circulating in the wild are receiving increased attention, since they might pose a potential threat to human health. Climate change will influence the prevalence of infectious diseases of both humans and animals. The aim of this study was to detect known and previously unknown viruses in Eurasian tundra reindeer. In total, 623 nasal and 477 rectal swab samples were collected from reindeer herds in Fennoscandia, Iceland, and Eastern Russia during 2016–2019. Next-generation sequencing analysis and BLAST-homology searches indicated the presence of viruses of domesticated and wild animals, such as bovine viral diarrhea virus, bovine papillomavirus, alcephaline herpesvirus 1 and 2, deer mastadenovirus B, bovine rotavirus, and roe deer picobirnavirus. Several viral species previously found in reindeer and some novel species were detected, although the clinical relevance of these viruses in reindeer is largely unknown. These results indicate that it should be possible to find emerging viruses of relevance for both human and animal health using reindeer as a sentinel species.

High-Throughput Screening of an FDA-Approved Drug Library Identifies Inhibitors against Arenaviruses and SARS-CoV-2

Arenaviruses are a large family of enveloped negative-strand RNA viruses that include several causative agents of severe hemorrhagic fevers. Currently, there are no FDA-licensed drugs to treat arenavirus infection except for the off-labeled use of ribavirin. Here, we performed antiviral drug screening against the Old World arenavirus lymphocytic choriomeningitis virus (LCMV) using an FDA-approved drug library. Five drug candidates were identified, including mycophenolic acid, benidipine hydrochloride, clofazimine, dabrafenib, and apatinib, for having strong anti-LCMV effects. Further analysis indicated that benidipine hydrochloride inhibited LCMV membrane fusion, and an adaptive mutation on the LCMV glycoprotein D414 site was found to antagonize the anti-LCMV activity of benidipine hydrochloride. Mycophenolic acid inhibited LCMV replication by depleting GTP production. We also found mycophenolic acid, clofazimine, dabrafenib, and apatinib can inhibit the newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Owing to their FDA-approved status, these drug candidates can potentially be used rapidly in the clinical treatment of arenavirus and SARS-CoV-2 infection.

Activation of the STAT3 Signaling Pathway by the RNA-Dependent RNA Polymerase Protein of Arenavirus

Arenaviruses cause chronic and asymptomatic infections in their natural host, rodents, and several arenaviruses cause severe hemorrhagic fever that has a high mortality in infected humans, seriously threatening public health. There are currently no FDA-licensed drugs available against arenaviruses; therefore, it is important to develop novel antiviral strategies to combat them, which would be facilitated by a detailed understanding of the interactions between the viruses and their hosts. To this end, we performed a transcriptomic analysis on cells infected with arenavirus lymphocytic choriomeningitis virus (LCMV), a neglected human pathogen with clinical significance, and found that the signal transducer and activator of transcription 3 (STAT3) signaling pathway was activated. A further investigation indicated that STAT3 could be activated by the RNA-dependent RNA polymerase L protein (Lp) of LCMV. Our functional analysis found that STAT3 cannot affect LCMV multiplication in A549 cells. We also found that STAT3 was activated by the Lp of Mopeia virus and Junin virus, suggesting that this activation may be conserved across certain arenaviruses. Our study explored the interactions between arenaviruses and STAT3, which may help us to better understand the molecular and cell biology of arenaviruses.

Experimental Morogoro Virus Infection in Its Natural Host, Mastomys natalensis

Natural hosts of most arenaviruses are rodents. The human-pathogenic Lassa virus and several non-pathogenic arenaviruses such as Morogoro virus (MORV) share the same host species, namely Mastomys natalensis (M. natalensis). In this study, we investigated the history of infection and virus transmission within the natural host population. To this end, we infected M. natalensis at different ages with MORV and measured the health status of the animals, virus load in blood and organs, the development of virus-specific antibodies, and the ability of the infected individuals to transmit the virus. To explore the impact of the lack of evolutionary virus-host adaptation, experiments were also conducted with Mobala virus (MOBV), which does not share M. natalensis as a natural host. Animals infected with MORV up to two weeks after birth developed persistent infection, seroconverted and were able to transmit the virus horizontally. Animals older than two weeks at the time of infection rapidly cleared the virus. In contrast, MOBV-infected neonates neither developed persistent infection nor were able to transmit the virus. In conclusion, we demonstrate that MORV is able to develop persistent infection in its natural host, but only after inoculation shortly after birth. A related arenavirus that is not evolutionarily adapted to M. natalensis is not able to establish persistent infection. Persistently infected animals appear to be important to maintain virus transmission within the host population.

Identification of Novel Antiviral Compounds Targeting Entry of Hantaviruses

Hemorrhagic fever viruses, among them orthohantaviruses, arenaviruses and filoviruses, are responsible for some of the most severe human diseases and represent a serious challenge for public health. The current limited therapeutic options and available vaccines make the development of novel efficacious antiviral agents an urgent need. Inhibiting viral attachment and entry is a promising strategy for the development of new treatments and to prevent all subsequent steps in virus infection. Here, we developed a fluorescence-based screening assay for the identification of new antivirals against hemorrhagic fever virus entry. We screened a phytochemical library containing 320 natural compounds using a validated VSV pseudotype platform bearing the glycoprotein of the virus of interest and encoding enhanced green fluorescent protein (EGFP). EGFP expression allows the quantitative detection of infection and the identification of compounds affecting viral entry. We identified several hits against four pseudoviruses for the orthohantaviruses Hantaan (HTNV) and Andes (ANDV), the filovirus Ebola (EBOV) and the arenavirus Lassa (LASV). Two selected inhibitors, emetine dihydrochloride and tetrandrine, were validated with infectious pathogenic HTNV in a BSL-3 laboratory. This study provides potential therapeutics against emerging virus infection, and highlights the importance of drug repurposing.

Reduced Nucleoprotein Availability Impairs Negative-Sense RNA Virus Replication and Promotes Host Recognition

Negative-sense RNA viruses (NSVs) rely on prepackaged viral RNA-dependent RNA polymerases (RdRp) to replicate and transcribe their viral genomes. Their replication machinery consists of an RdRp bound to viral RNA which is wound around a nucleoprotein (NP) scaffold, forming a viral ribonucleoprotein complex. NSV NP is known to regulate transcription and replication of genomic RNA; however, its role in maintaining and protecting the viral genetic material is unknown. Here, we exploited host microRNA expression to target NP of influenza A virus and Sendai virus to ascertain how this would impact genomic levels and the host response to infection. We find that in addition to inducing a drastic decrease in genome replication, the antiviral host response in the absence of NP is dramatically enhanced. Additionally, our data show that insufficient levels of NP prevent the replication machinery of these NSVs to process full-length genomes, resulting in aberrant replication products which form pathogen-associated molecular patterns in the process. These dynamics facilitate immune recognition by cellular pattern recognition receptors leading to a strong host antiviral response. Moreover, we observe that the consequences of limiting NP levels are universal among NSVs, including Ebola virus, Lassa virus, and measles virus. Overall, these results provide new insights into viral genome replication of negative-sense RNA viruses and highlight novel avenues for developing effective antiviral strategies, adjuvants, and/or live-attenuated vaccines.IMPORTANCE Negative-sense RNA viruses comprise some of the most important known human pathogens, including influenza A virus, measles virus, and Ebola virus. These viruses possess RNA genomes that are unreadable to the host, as they require specific viral RNA-dependent RNA polymerases in conjunction with other viral proteins, such as nucleoprotein, to be replicated and transcribed. As this process generates a significant amount of pathogen-associated molecular patterns, this phylum of viruses can result in a robust induction of the intrinsic host cellular response. To circumvent these defenses, these viruses form tightly regulated ribonucleoprotein replication complexes in order to protect their genomes from detection and to prevent excessive aberrant replication. Here, we demonstrate the balance that negative-sense RNA viruses must achieve both to replicate efficiently and to avoid induction of the host defenses.