Lassa virus infection of human dendritic cells and macrophages is productive but fails to activate cells

Emerging and reemerging infectious diseases: global trends and new strategies for their prevention and control

To adequately prepare for potential hazards caused by emerging and reemerging infectious diseases, the WHO has issued a list of high-priority pathogens that are likely to cause future outbreaks and for which research and development (R&D) efforts are dedicated, known as paramount R&D blueprints. Within R&D efforts, the goal is to obtain effective prophylactic and therapeutic approaches, which depends on a comprehensive knowledge of the etiology, epidemiology, and pathogenesis of these diseases. In this process, the accessibility of animal models is a priority bottleneck because it plays a key role in bridging the gap between in-depth understanding and control efforts for infectious diseases. Here, we reviewed preclinical animal models for high priority disease in terms of their ability to simulate human infections, including both natural susceptibility models, artificially engineered models, and surrogate models. In addition, we have thoroughly reviewed the current landscape of vaccines, antibodies, and small molecule drugs, particularly hopeful candidates in the advanced stages of these infectious diseases. More importantly, focusing on global trends and novel technologies, several aspects of the prevention and control of infectious disease were discussed in detail, including but not limited to gaps in currently available animal models and medical responses, better immune correlates of protection established in animal models and humans, further understanding of disease mechanisms, and the role of artificial intelligence in guiding or supplementing the development of animal models, vaccines, and drugs. Overall, this review described pioneering approaches and sophisticated techniques involved in the study of the epidemiology, pathogenesis, prevention, and clinical theatment of WHO high-priority pathogens and proposed potential directions. Technological advances in these aspects would consolidate the line of defense, thus ensuring a timely response to WHO high priority pathogens.

Lassa Fever: Critical Review and Prospects for Control

Lassa Fever is a deadly viral haemorrhagic disease, causing annually several hundreds of deaths in West Africa. This zoonotic disease is primarily transmitted to humans by rodents of the genus Mastomys, even though other rodents reportedly carry the Lassa virus, while secondary interhuman transmission accounts for approximately 20% of cases. Although this disease has been endemic in rural zones of Nigeria, Sierra Leone, Liberfia, and Guinea for hundreds of years, it is also characterised by epidemic outbreaks in the dry season, responsible for heavy death tolls. No licensed vaccine or satisfying treatment is currently available. Disease management is hindered by the incomplete knowledge of the epidemiology and distribution of the disease, resulting from an inadequate health and surveillance system. Additional scientific constraints such as the genetic diversity of the virus and the lack of understanding of the mechanisms of immune protection complexify the development of a vaccine. The intricate socio-economic context in the affected regions, and the lack of monetary incentive for drug development, allow the disease to persist in some of West Africa's poorest communities. The increase in the number of reported cases and in the fatality rate, the expansion of the endemic area, as well as the threat Lassa Fever represents internationally should urge the global community to work on the disease control and prevention. The disease control requires collaborative research for medical countermeasures and tailored public health policies. Lassa Fever, created by the interconnection between animals, humans, and ecosystems, and embedded in an intricate social context, should be addressed with a 'One Health' approach. This article provides an overview of Lassa Fever, focusing on Nigeria, and discusses the perspectives for the control of disease.

Vascular dysfunction in hemorrhagic viral fevers: opportunities for organotypic modeling

The hemorrhagic fever viruses (HFVs) cause severe or fatal infections in humans. Named after their common symptom hemorrhage, these viruses induce significant vascular dysfunction by affecting endothelial cells, altering immunity, and disrupting the clotting system. Despite advances in treatments, such as cytokine blocking therapies, disease modifying treatment for this class of pathogen remains elusive. Improved understanding of the pathogenesis of these infections could provide new avenues to treatment. While animal models and traditional 2D cell cultures have contributed insight into the mechanisms by which these pathogens affect the vasculature, these models fall short in replicatingin vivohuman vascular dynamics. The emergence of microphysiological systems (MPSs) offers promising avenues for modeling these complex interactions. These MPS or 'organ-on-chip' models present opportunities to better mimic human vascular responses and thus aid in treatment development. In this review, we explore the impact of HFV on the vasculature by causing endothelial dysfunction, blood clotting irregularities, and immune dysregulation. We highlight how existing MPS have elucidated features of HFV pathogenesis as well as discuss existing knowledge gaps and the challenges in modeling these interactions using MPS. Understanding the intricate mechanisms of vascular dysfunction caused by HFV is crucial in developing therapies not only for these infections, but also for other vasculotropic conditions like sepsis.

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.

The underlying mechanisms of arenaviral entry through matriglycan

Matriglycan, a recently characterized linear polysaccharide, is composed of alternating xylose and glucuronic acid subunits bound to the ubiquitously expressed protein α-dystroglycan (α-DG). Pathogenic arenaviruses, like the Lassa virus (LASV), hijack this long linear polysaccharide to gain cellular entry. Until recently, it was unclear through what mechanisms LASV engages its matriglycan receptor to initiate infection. Additionally, how matriglycan is synthesized onto α-DG by the Golgi-resident glycosyltransferase LARGE1 remained enigmatic. Recent structural data for LARGE1 and for the LASV spike complex informs us about the synthesis of matriglycan as well as its usage as an entry receptor by arenaviruses. In this review, we discuss structural insights into the system of matriglycan generation and eventual recognition by pathogenic viruses. We also highlight the unique usage of matriglycan as a high-affinity host receptor compared with other polysaccharides that decorate cells.

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.

Ebola Virus Uses Tunneling Nanotubes as an Alternate Route of Dissemination

Ebola virus (EBOV) disease is marked by rapid virus replication and spread. EBOV enters the cell by macropinocytosis and replicates in the cytoplasm, and nascent virions egress from the cell surface to infect neighboring cells. Here, we show that EBOV uses an alternate route to disseminate: tunneling nanotubes (TNTs). TNTs, an actin-based long-range intercellular communication system, allows for direct exchange of cytosolic constituents between cells. Using live, scanning electron, and high-resolution quantitative 3-dimensional microscopy, we show that EBOV infection of primary human cells results in the enhanced formation of TNTs containing viral nucleocapsids. TNTs promote the intercellular transfer of nucleocapsids in the absence of live virus, and virus could replicate in cells devoid of entry factors after initial stall. Our studies suggest an alternate model of EBOV dissemination within the host, laying the groundwork for further investigations into the pathogenesis of filoviruses and, importantly, stimulating new areas of antiviral design.

Combating Lassa Fever in West African Sub-Region: Progress, Challenges, and Future Perspectives

Lassa fever (LF) is a rodent-borne disease that threatens human health in the sub-region of West Africa where the zoonotic host of Lassa virus (LASV) is predominant. Currently, treatment options for LF are limited and since no preventive vaccine is approved for its infectivity, there is a high mortality rate in endemic areas. This narrative review explores the transmission, pathogenicity of LASV, advances, and challenges of different treatment options. Our findings indicate that genetic diversity among the different strains of LASV and their ability to circumvent the immune system poses a critical challenge to the development of LASV vaccines/therapeutics. Thus, understanding the biochemistry, physiology and genetic polymorphism of LASV, mechanism of evading host immunity are essential for development of effective LASV vaccines/therapeutics to combat this lethal viral disease. The LASV nucleoprotein (NP) is a novel target for therapeutics as it functions significantly in several aspects of the viral life cycle. Consequently, LASV NP inhibitors could be employed as effective therapeutics as they will potentially inhibit LASV replication. Effective preventive control measures, vaccine development, target validation, and repurposing of existing drugs, such as ribavirin, using activity or in silico-based and computational bioinformatics, would aid in the development of novel drugs for LF management.

Hemorrhagic fever viruses: Pathogenesis, therapeutics, and emerging and re-emerging potential

Hemorrhagic fever viruses (HFVs) pose a threat to global public health owing to the emergence and re-emergence of highly fatal diseases. Viral hemorrhagic fevers (VHFs) caused by these viruses are mostly characterized by an acute febrile syndrome with coagulation abnormalities and generalized hemorrhage that may lead to life-threatening organ dysfunction. Currently, the events underlying the viral pathogenicity associated with multiple organ dysfunction syndrome still underexplored. In this minireview, we address the current knowledge of the mechanisms underlying VHFs pathogenesis and discuss the available development of preventive and therapeutic options to treat these infections. Furthermore, we discuss the potential of HFVs to cause worldwide emergencies along with factors that favor their spread beyond their original niches.

Lassa Virus Countermeasures

Lassa Fever (LF) is a viral hemorrhagic fever endemic in West Africa. LF begins with flu-like symptoms that are difficult to distinguish from other common endemic diseases such as malaria, dengue, and yellow fever making it hard to diagnose clinically. Availability of a rapid diagnostic test and other serological and molecular assays facilitates accurate diagnosis of LF. Lassa virus therapeutics are currently in different stages of preclinical development. Arevirumab, a cocktail of monoclonal antibodies, demonstrates a great safety and efficacy profile in non-human primates. Major efforts have been made in the development of a Lassa virus vaccine. Two vaccine candidates, MeV-NP and pLASV-GPC are undergoing evaluation in phase I clinical trials.

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.

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.

Understanding Host–Virus Interactions: Assessment of Innate Immune Responses in Mastomys natalensis Cells after Arenavirus Infection

Mastomys natalensis is the natural host of various arenaviruses, including the human-pathogenic Lassa virus. Homologous arenaviruses, defined here as those having M. natalensis as a natural host, can establish long-lasting infection in M. natalensis, while these animals rapidly clear arenaviruses having another rodent species as a natural host (heterologous viruses). Little is known about the mechanisms behind the underlying arenavirus–host barriers. The innate immune system, particularly the type I interferon (IFN) response, might play a role. In this study, we developed and validated RT-PCR assays to analyse the expression of M. natalensis interferon-stimulated genes (ISGs). We then used these assays to study if homologous and heterologous viruses induce different IFN responses in M. natalensis cells. Infection experiments were performed with the homologous Lassa and Morogoro viruses and the related but heterologous Mobala virus. Compared to the direct induction with IFN or Poly(I:C), arenaviruses generally induced a weak IFN response. However, the ISG-expression profiles of homologous and heterologous viruses were similar. Our data indicate that, at least in M. natalensis cells, the IFN system is not a major factor in the virus–host barrier for arenaviruses. Our system provides a valuable tool for future in vivo investigation of arenavirus host restrictions at the level of the innate immune response.

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.

CD4 T-cell depletion prevents Lassa fever associated hearing loss in the mouse model

Lassa virus (LASV) is the causative agent of Lassa fever (LF), which presents as a lethal hemorrhagic disease in severe cases. LASV-induced hearing loss in survivors is a huge socioeconomic burden, however, the mechanism(s) leading to hearing loss is unknown. In this study, we evaluate in a mouse LF model the auditory function using auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) to determine the mechanisms underlying LASV-induced hearing loss. In the process, we pioneered measures of ABR and DPOAE tests in rodents in biosafety level 4 (BSL-4) facilities. Our T cell depletion studies demonstrated that CD4 T-cells play an important role in LASV-induced hearing loss, while CD8 T-cells are critical for the pathogenicity in the acute phase of LASV infection. Results presented in this study may help to develop future countermeasures against acute disease and LASV-induced hearing loss.

Infection of Human Endothelial Cells with Lassa Virus Induces Early but Transient Activation and Low Type I IFN Response Compared to the Closely-Related Nonpathogenic Mopeia Virus

Lassa virus (LASV), an Old World arenavirus, is responsible for hemorrhagic fevers in western Africa. The privileged tropism of LASV for endothelial cells combined with a dysregulated inflammatory response are the main cause of the increase in vascular permeability observed during the disease. Mopeia virus (MOPV) is another arenavirus closely related to LASV but nonpathogenic for non-human primates (NHPs) and has never been described in humans. MOPV is more immunogenic than LASV in NHPs and in vitro in human immune cell models, with more intense type I IFN and adaptive cellular responses. Here, we compared the transcriptomic and proteomic responses of human umbilical vein endothelial cells (HUVECs) to infection with the two viruses to further decipher the mechanisms involved in their differences in immunogenicity and pathogenicity. Both viruses replicated durably and efficiently in HUVECs, but the responses they induced were strikingly different. Modest activation was observed at an early stage of LASV infection and then rapidly shut down. By contrast, MOPV induced a late but more intense response, characterized by the expression of genes and proteins mainly associated with the type I IFN response and antigen processing/presentation. Such a response is consistent with the higher immunogenicity of MOPV relative to LASV, whereas the lack of an innate response induced in HUVECs by LASV is consistent with its uncontrolled systemic dissemination through the vascular endothelium.

The Activated Macrophage - A Tough Fortress for Virus Invasion: How Viruses Strike Back

Macrophages (Mφ) are innate immune cells with a variety of functional phenotypes depending on the cytokine microenvironment they reside in. Mφ exhibit distinct activation patterns that are found within a wide array of activation states ranging from the originally discovered classical pro-inflammatory (M1) to the anti-inflammatory (M2) with their multi-facades. M1 cells are induced by IFNγ + LPS, while M2 are further subdivided into M2a (IL-4), M2b (Immune Complex) and M2c (IL-10) based on their inducing stimuli. Not surprisingly, Mφ activation influences the outcome of viral infections as they produce cytokines that in turn activate cells of the adaptive immune system. Generally, activated M1 cells tend to restrict viral replication, however, influenza and HIV exploit inflammation to support their replication. Moreover, M2a polarization inhibits HIV replication at the post-integration level, while HCMV encoded hrIL-10 suppresses inflammatory reactions by facilitating M2c formation. Additionally, viruses such as LCMV and Lassa Virus directly suppress Mφ activation leading to viral chronicity. Here we review how Mφ activation affects viral infection and the strategies by which viruses manipulate Mφ polarization to benefit their own fitness. An understanding of these mechanisms is important for the development of novel immunotherapies that can sway Mφ phenotype to inhibit viral replication.

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.

Lassa Fever Virus Binds Matriglycan-A Polymer of Alternating Xylose and Glucuronate-On α-Dystroglycan

Lassa fever virus (LASV) can cause life-threatening hemorrhagic fevers for which there are currently no vaccines or targeted treatments. The late Prof. Stefan Kunz, along with others, showed that the high-affinity host receptor for LASV, and other Old World and clade-C New World mammarenaviruses, is matriglycan-a linear repeating disaccharide of alternating xylose and glucuronic acid that is polymerized uniquely on α-dystroglycan by like-acetylglucosaminyltransferase-1 (LARGE1). Although α-dystroglycan is ubiquitously expressed, LASV preferentially infects vascular endothelia and professional phagocytic cells, which suggests that viral entry requires additional cell-specific factors. In this review, we highlight the work of Stefan Kunz detailing the molecular mechanism of LASV binding and discuss the requirements of receptors, such as tyrosine kinases, for internalization through apoptotic mimicry.

A single-shot Lassa vaccine induces long-term immunity and protects cynomolgus monkeys against heterologous strains

A safe and protective Lassa virus vaccine is crucially needed in Western Africa to stem the recurrent outbreaks of Lassa virus infections in Nigeria and the emergence of Lassa virus in previously unaffected countries, such as Benin and Togo. Major challenges in developing a Lassa virus vaccine include the high diversity of circulating strains and their reemergence from 1 year to another. To address each of these challenges, we immunized cynomolgus monkeys with a measles virus vector expressing the Lassa virus glycoprotein and nucleoprotein of the prototypic Lassa virus strain Josiah (MeV-NP). To evaluate vaccine efficacy against heterologous strains of Lassa virus, we challenged the monkeys a month later with heterologous strains from lineage II or lineage VII, finding that the vaccine was protective against these strains. A second cohort of monkeys was challenged 1 year later with the homologous Josiah strain, finding that a single dose of MeV-NP was sufficient to protect all vaccinated monkeys. These studies demonstrate that MeV-NP can generate both long-lasting immune responses and responses that are able to protect against diverse strains of Lassa virus.

The Protein Kinase Receptor Modulates the Innate Immune Response against Tacaribe Virus

The New World (NW) mammarenavirus group includes several zoonotic highly pathogenic viruses, such as Junin (JUNV) or Machupo (MACV). Contrary to the Old World mammarenavirus group, these viruses are not able to completely suppress the innate immune response and trigger a robust interferon (IFN)-I response via retinoic acid-inducible gene I (RIG-I). Nevertheless, pathogenic NW mammarenaviruses trigger a weaker IFN response than their nonpathogenic relatives do. RIG-I activation leads to upregulation of a plethora of IFN-stimulated genes (ISGs), which exert a characteristic antiviral effect either as lone effectors, or resulting from the combination with other ISGs or cellular factors. The dsRNA sensor protein kinase receptor (PKR) is an ISG that plays a pivotal role in the control of the mammarenavirus infection. In addition to its well-known protein synthesis inhibition, PKR further modulates the overall IFN-I response against different viruses, including mammarenaviruses. For this study, we employed Tacaribe virus (TCRV), the closest relative of the human pathogenic JUNV. Our findings indicate that PKR does not only increase IFN-I expression against TCRV infection, but also affects the kinetic expression and the extent of induction of Mx1 and ISG15 at both levels, mRNA and protein expression. Moreover, TCRV fails to suppress the effect of activated PKR, resulting in the inhibition of a viral titer. Here, we provide original evidence of the specific immunomodulatory role of PKR over selected ISGs, altering the dynamic of the innate immune response course against TCRV. The mechanisms for innate immune evasion are key for the emergence and adaptation of human pathogenic arenaviruses, and highly pathogenic mammarenaviruses, such as JUNV or MACV, trigger a weaker IFN response than nonpathogenic mammarenaviruses. Within the innate immune response context, PKR plays an important role in sensing and restricting the infection of TCRV virus. Although the mechanism of PKR for protein synthesis inhibition is well described, its immunomodulatory role is less understood. Our present findings further characterize the innate immune response in the absence of PKR, unveiling the role of PKR in defining the ISG profile after viral infection. Moreover, TCRV fails to suppress activated PKR, resulting in viral progeny production inhibition.

[Review of candidate vaccines for the prevention of Lassa fever]

The Lassa virus one of the main etiological agent of hemorrhagic fevers in the world: according to WHO estimates, it affects 100,000 to 300,000 people annually, which results in up to 10,000 deaths [1]. Although expansion of Lassa fever caused by this pathogen is mostly limited to the West African countries: Sierra Leone, Liberia, Guinea and Nigeria, imported cases have been historically documented in Europe, the United States of America (USA), Canada, Japan, and Israel [2]. In 2017, WHO included the Lassa virus in the list of priority pathogens in need of accelerated research, development of vaccines, therapeutic agents and diagnostic tools regarding infections they cause [3]. This review describes main technological platforms used for the development of vaccines for the prevention of Lassa fever.

Construction and Immunological Evaluation of an Adenoviral Vector-Based Vaccine Candidate for Lassa Fever

Lassa virus (LASV) is a rodent-borne arenavirus circulating in West African regions that causes Lassa fever (LF). LF is normally asymptomatic at the initial infection stage, but can progress to severe disease with multiorgan collapse and hemorrhagic fever. To date, the therapeutic choices are limited, and there is no approved vaccine for avoiding LASV infection. Adenoviral vector-based vaccines represent an effective countermeasure against LASV because of their safety and adequate immunogenicity, as demonstrated in use against other emerging viral infections. Here, we constructed and characterized a novel Ad5 (E1-, E3-) vectored vaccine containing the glycoprotein precursor (GPC) of LASV. Ad5-GPC_LASV elicited both humoral and cellular immune responses in BALB/c mice. Moreover, a bioluminescent imaging-based BALB/c mouse model infected with GPC-bearing and luciferase-expressing replication-incompetent LASV pseudovirus was utilized to evaluate the vaccine efficacy. The bioluminescence intensity of immunized mice was significantly lower than that of control mice after being inoculated with LASV pseudovirus. This study suggests that Ad5-GPC_LASV represents a potential vaccine candidate against LF.

Systemic viral spreading and defective host responses are associated with fatal Lassa fever in macaques

Lassa virus (LASV) is endemic in West Africa and induces a viral hemorrhagic fever (VHF) with up to 30% lethality among clinical cases. The mechanisms involved in control of Lassa fever or, in contrast, the ensuing catastrophic illness and death are poorly understood. We used the cynomolgus monkey model to reproduce the human disease with asymptomatic to mild or fatal disease. After initial replication at the inoculation site, LASV reached the secondary lymphoid organs. LASV did not spread further in nonfatal disease and was rapidly controlled by balanced innate and T-cell responses. Systemic viral dissemination occurred during severe disease. Massive replication, a cytokine/chemokine storm, defective T-cell responses, and multiorgan failure were observed. Clinical, biological, immunological, and transcriptomic parameters resembled those observed during septic-shock syndrome, suggesting that similar pathogenesis is induced during Lassa fever. The outcome appears to be determined early, as differentially expressed genes in PBMCs were associated with fatal and non-fatal Lassa fever outcome very early after infection. These results provide a full characterization and important insights into Lassa fever pathogenesis and could help to develop early diagnostic tools.

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.

Natural killer cell responses to emerging viruses of zoonotic origin

Emerging viral diseases pose a major threat to public health worldwide. Nearly all emerging viruses, including Ebola, Dengue, Nipah, West Nile, Zika, and coronaviruses (including SARS-Cov2, the causative agent of the current COVID-19 pandemic), have zoonotic origins, indicating that animal-to-human transmission constitutes a primary mode of acquisition of novel infectious diseases. Why these viruses can cause profound pathologies in humans, while natural reservoir hosts often show little evidence of disease is not completely understood. Differences in the host immune response, especially within the innate compartment, have been suggested to be involved in this divergence. Natural killer (NK) cells are innate lymphocytes that play a critical role in the early antiviral response, secreting effector cytokines and clearing infected cells. In this review, we will discuss the mechanisms through which NK cells interact with viruses, their contribution towards maintaining equilibrium between the virus and its natural host, and their role in disease progression in humans and other non-natural hosts.

Innate Immune Responses to Acute Viral Infection During Pregnancy

Immunological adaptations in pregnancy allow maternal tolerance of the semi-allogeneic fetus but also increase maternal susceptibility to infection. At implantation, the endometrial stroma, glands, arteries and immune cells undergo anatomical and functional transformation to create the decidua, the specialized secretory endometrium of pregnancy. The maternal decidua and the invading fetal trophoblast constitute a dynamic junction that facilitates a complex immunological dialogue between the two. The decidual and peripheral immune systems together assume a pivotal role in regulating the critical balance between tolerance and defense against infection. Throughout pregnancy, this equilibrium is repeatedly subjected to microbial challenge. Acute viral infection in pregnancy is associated with a wide spectrum of adverse consequences for both mother and fetus. Vertical transmission from mother to fetus can cause developmental anomalies, growth restriction, preterm birth and stillbirth, while the mother is predisposed to heightened morbidity and maternal death. A rapid, effective response to invasive pathogens is therefore essential in order to avoid overwhelming maternal infection and consequent fetal compromise. This sentinel response is mediated by the innate immune system: a heritable, highly evolutionarily conserved system comprising physical barriers, antimicrobial peptides (AMP) and a variety of immune cells—principally neutrophils, macrophages, dendritic cells, and natural killer cells—which express pattern-receptors that detect invariant molecular signatures unique to pathogenic micro-organisms. Recognition of these signatures during acute infection triggers signaling cascades that enhance antimicrobial properties such as phagocytosis, secretion of pro-inflammatory cytokines and activation of the complement system. As well as coordinating the initial immune response, macrophages and dendritic cells present microbial antigens to lymphocytes, initiating and influencing the development of specific, long-lasting adaptive immunity. Despite extensive progress in unraveling the immunological adaptations of pregnancy, pregnant women remain particularly susceptible to certain acute viral infections and continue to experience mortality rates equivalent to those observed in pandemics several decades ago. Here, we focus specifically on the pregnancy-induced vulnerabilities in innate immunity that contribute to the disproportionately high maternal mortality observed in the following acute viral infections: Lassa fever, Ebola virus disease (EVD), dengue fever, hepatitis E, influenza, and novel coronavirus infections.

Adjuvant formulated virus-like particles expressing native-like forms of the Lassa virus envelope surface glycoprotein are immunogenic and induce antibodies with broadly neutralizing activity

Lassa mammarenavirus (LASV) is a rodent-borne arenavirus endemic to several West African countries. It is the causative agent of human Lassa fever, an acute viral hemorrhagic fever disease. To date, no therapeutics or vaccines against LASV have obtained regulatory approval. Polyclonal neutralizing antibodies derived from hyperimmunized animals may offer a useful strategy for prophylactic and therapeutic intervention to combat human LASV infections. The LASV envelope surface glycoprotein complex (GP) is the major target for neutralizing antibodies, and it is the main viral antigen used for the design of an LASV vaccine. Here, we assessed the immunogenic potential of mammalian cell-derived virus-like particles (VLPs) expressing GP from the prototypic LASV strain Josiah in a native-like conformation as the sole viral antigen. We demonstrate that an adjuvanted prime-boost immunization regimen with GP-derived VLPs elicited neutralizing antibody responses in rabbits, suggesting that effective antigenic epitopes of GP were displayed. Notably, these antibodies exhibited broad reactivity across five genetic lineages of LASV. VLP-based immunization strategies may represent a powerful approach for generating polyclonal sera containing cross-reactive neutralizing antibodies against LASV.

Distinct Molecular Mechanisms of Host Immune Response Modulation by Arenavirus NP and Z Proteins

Endemic to West Africa and South America, mammalian arenaviruses can cross the species barrier from their natural rodent hosts to humans, resulting in illnesses ranging from mild flu-like syndromes to severe and fatal haemorrhagic zoonoses. The increased frequency of outbreaks and associated high fatality rates of the most prevalent arenavirus, Lassa, in West African countries, highlights the significant risk to public health and to the socio-economic development of affected countries. The devastating impact of these viruses is further exacerbated by the lack of approved vaccines and effective treatments. Differential immune responses to arenavirus infections that can lead to either clearance or rapid, widespread and uncontrolled viral dissemination are modulated by the arenavirus multifunctional proteins, NP and Z. These two proteins control the antiviral response to infection by targeting multiple cellular pathways; and thus, represent attractive targets for antiviral development to counteract infection. The interplay between the host immune responses and viral replication is a key determinant of virus pathogenicity and disease outcome. In this review, we examine the current understanding of host immune defenses against arenavirus infections and summarise the host protein interactions of NP and Z and the mechanisms that govern immune evasion strategies.

Lassa Virus, but Not Highly Pathogenic New World Arenaviruses, Restricts Immunostimulatory Double-Stranded RNA Accumulation during Infection

The arenaviruses Lassa virus (LASV), Junín virus (JUNV), and Machupo virus (MACV) can cause severe and fatal diseases in humans. Although these pathogens are closely related, the host immune responses to these virus infections differ remarkably, with direct implications for viral pathogenesis. LASV infection is immunosuppressive, with a very low-level interferon response. In contrast, JUNV and MACV infections stimulate a robust interferon (IFN) response in a retinoic acid-inducible gene I (RIG-I)-dependent manner and readily activate protein kinase R (PKR), a known host double-stranded RNA (dsRNA) sensor. In response to infection with RNA viruses, host nonself RNA sensors recognize virus-derived dsRNA as danger signals and initiate innate immune responses. Arenavirus nucleoproteins (NPs) contain a highly conserved exoribonuclease (ExoN) motif, through which LASV NP has been shown to degrade virus-derived immunostimulatory dsRNA in biochemical assays. In this study, we for the first time present evidence that LASV restricts dsRNA accumulation during infection. Although JUNV and MACV NPs also have the ExoN motif, dsRNA readily accumulated in infected cells and often colocalized with dsRNA sensors. Moreover, LASV coinfection diminished the accumulation of dsRNA and the IFN response in JUNV-infected cells. The disruption of LASV NP ExoN with a mutation led to dsRNA accumulation and impaired LASV replication in minigenome systems. Importantly, both LASV NP and RNA polymerase L protein were required to diminish the accumulation of dsRNA and the IFN response in JUNV infection. For the first time, we discovered a collaboration between LASV NP ExoN and L protein in limiting dsRNA accumulation. Our new findings provide mechanistic insights into the differential host innate immune responses to highly pathogenic arenavirus infections.IMPORTANCE Arenavirus NPs contain a highly conserved DEDDh ExoN motif, through which LASV NP degrades virus-derived, immunostimulatory dsRNA in biochemical assays to eliminate the danger signal and inhibit the innate immune response. Nevertheless, the function of NP ExoN in arenavirus infection remains to be defined. In this study, we discovered that LASV potently restricts dsRNA accumulation during infection and minigenome replication. In contrast, although the NPs of JUNV and MACV also harbor the ExoN motif, dsRNA readily formed during JUNV and MACV infections, accompanied by IFN and PKR responses. Interestingly, LASV NP alone was not sufficient to limit dsRNA accumulation. Instead, both LASV NP and L protein were required to restrict immunostimulatory dsRNA accumulation. Our findings provide novel and important insights into the mechanism for the distinct innate immune response to these highly pathogenic arenaviruses and open new directions for future studies.

The Utility of Human Immune System Mice for High-Containment Viral Hemorrhagic Fever Research

Human immune system (HIS) mice are a subset of humanized mice that are generated by xenoengraftment of human immune cells or tissues and/or their progenitors into immunodeficient mice. Viral hemorrhagic fevers (VHFs) cause severe disease in humans, typically with high case fatality rates. HIS mouse studies have been performed to investigate the pathogenesis and immune responses to VHFs that must be handled in high-containment laboratory facilities. Here, we summarize studies on filoviruses, nairoviruses, phenuiviruses, and hantaviruses, and discuss the knowledge gained from using various HIS mouse models. Furthermore, we discuss the complexities of designing and interpreting studies utilizing HIS mice while highlighting additional questions about VHFs that can still be addressed using HIS mouse models.

E3 Ligase ITCH Interacts with the Z Matrix Protein of Lassa and Mopeia Viruses and Is Required for the Release of Infectious Particles

Lassa virus (LASV) and Mopeia virus (MOPV) are two closely related, rodent-born mammarenaviruses. LASV is the causative agent of Lassa fever, a deadly hemorrhagic fever endemic in West Africa, whereas MOPV is non-pathogenic in humans. The Z matrix protein of arenaviruses is essential to virus assembly and budding by recruiting host factors, a mechanism that remains partially defined. To better characterize the interactions involved, a yeast two-hybrid screen was conducted using the Z proteins from LASV and MOPV as a bait. The cellular proteins ITCH and WWP1, two members of the Nedd4 family of HECT E3 ubiquitin ligases, were found to bind the Z proteins of LASV, MOPV and other arenaviruses. The PPxY late-domain motif of the Z proteins is required for the interaction with ITCH, although the E3 ubiquitin-ligase activity of ITCH is not involved in Z ubiquitination. The silencing of ITCH was shown to affect the replication of the old-world mammarenaviruses LASV, MOPV, Lymphocytic choriomeningitis virus (LCMV) and to a lesser extent Lujo virus (LUJV). More precisely, ITCH was involved in the egress of virus-like particles and the release of infectious progeny viruses. Thus, ITCH constitutes a novel interactor of LASV and MOPV Z proteins that is involved in virus assembly and release.

Fluorescent Crimean-Congo hemorrhagic fever virus illuminates tissue tropism patterns and identifies early mononuclear phagocytic cell targets in Ifnar-/- mice

Crimean-Congo hemorrhagic fever virus (CCHFV, order Bunyavirales, family Nairoviridae, genus Orthonairovirus) is the tick-borne etiological agent of Crimean-Congo hemorrhagic fever (CCHF) in humans. Animals are generally susceptible to CCHFV infection but refractory to disease. Small animal models are limited to interferon-deficient mice, that develop acute fatal disease following infection. Here, using a ZsGreen1- (ZsG) expressing reporter virus (CCHFV/ZsG), we examine tissue tropism and dissemination of virus in interferon-α/β receptor knock-out (Ifnar^-/-) mice. We demonstrate that CCHFV/ZsG retains in vivo pathogenicity comparable to wild-type virus. Interestingly, despite high levels of viral RNA in all organs assessed, 2 distribution patterns of infection were observed by both fluorescence and immunohistochemistry (IHC), corresponding to the permissiveness of organ tissues. To further investigate viral dissemination and to temporally define cellular targets of CCHFV in vivo, mice were serially euthanized at different stages of disease. Flow cytometry was used to characterize CCHFV-associated alterations in hematopoietic cell populations and to classify infected cells in the blood, lymph node, spleen, and liver. ZsG signal indicated that mononuclear phagocytic cells in the lymphatic tissues were early targets of infection; in late-stage infection, overall, the highest levels of signal were detected in the liver, and ZsG was found in both antigen-presenting and lymphocyte cell populations.

Human infection by tick-borne Crimean-Congo hemorrhagic fever virus (CCHFV) can result in severe disease with up to 30% case fatality rates. While CCHFV is known to be hepatotropic, the presence and implications of virus in other tissues are less clear. Furthermore, to date, early cellular targets of infection in a CCHFV disease model have not been investigated in detail. Here, using a recombinant reporter CCHFV expressing the fluorescent protein ZsGreen1 (ZsG; CCHFV/ZsG) in interferon-α/β receptor knock-out (Ifnar^-/-) mice, which develop acute fatal disease following infection, we investigate both cellular and tissue targets of infection. Importantly, we find that CCHFV/ZsG infection demonstrated comparable pathogenicity to wild-type virus in Ifnar^-/- mice. We used in situ visualization of fluorescent signal in tissues to assess viral dissemination throughout the course of infection, and found robust viral signal in reproductive tissues, previously unrecognized as sites of CCHFV infection. We also used flow cytometry to detect intracellular fluorescent signal, and identified initial target cells of CCHFV infection as macrophage and monocyte populations in lymphatic tissues. These findings support a central role of immune cells in early virus dissemination, and a need for further investigations into reproductive tract involvement in human CCHFV infection.

Junin Virus Triggers Macrophage Activation and Modulates Polarization According to Viral Strain Pathogenicity

The New World arenavirus Junin (JUNV) is the etiological agent of Argentine hemorrhagic fever (AHF). Previous studies of human macrophage infection by the Old-World arenaviruses Mopeia and Lassa showed that while the non-pathogenic Mopeia virus replicates and activates human macrophages, the pathogenic Lassa virus replicates but fails to activate human macrophages. Less is known in regard to the impact of New World arenavirus infection on the human macrophage immune response. Macrophage activation is critical for controlling infections but could also be usurped favoring immune evasion. Therefore, it is crucial to understand how the JUNV infection modulates macrophage plasticity to clarify its role in AHF pathogenesis. With this aim in mind, we compared infection with the attenuated Candid 1 (C#1) or the pathogenic P strains of the JUNV virus in human macrophage cultures. The results showed that both JUNV strains similarly replicated and induced morphological changes as early as 1 day post-infection. However, both strains differentially induced the expression of CD71, the receptor for cell entry, the activation and maturation molecules CD80, CD86, and HLA-DR and selectively modulated cytokine production. Higher levels of TNF-α, IL-10, and IL-12 were detected with C#1 strain, while the P strain induced only higher levels of IL-6. We also found that C#1 strain infection skewed macrophage polarization to M1, whereas the P strain shifted the response to an M2 phenotype. Interestingly, the MERTK receptor, that negatively regulates the immune response, was down-regulated by C#1 strain and up-regulated by P strain infection. Similarly, the target genes of MERTK activation, the cytokine suppressors SOCS1 and SOCS3, were also increased after P strain infection, in addition to IRF-1, that regulates type I IFN levels, which were higher with C#1 compared with P strain infection. Together, this differential activation/polarization pattern of macrophages elicited by P strain suggests a more evasive immune response and may have important implications in the pathogenesis of AHF and underpinning the development of new potential therapeutic strategies.

Differential Immune Responses to Hemorrhagic Fever-Causing Arenaviruses

The family Arenaviridae contains several pathogens of major clinical importance. The Old World (OW) arenavirus Lassa virus is endemic in West Africa and is estimated to cause up to 300,000 infections each year. The New World (NW) arenaviruses Junín and Machupo periodically cause hemorrhagic fever outbreaks in South America. While these arenaviruses are highly pathogenic in humans, recent evidence indicates that pathogenic OW and NW arenaviruses interact with the host immune system differently, which may have differential impacts on viral pathogenesis. Severe Lassa fever cases are characterized by profound immunosuppression. In contrast, pathogenic NW arenavirus infections are accompanied by elevated levels of Type I interferon and pro-inflammatory cytokines. This review aims to summarize recent findings about interactions of these pathogenic arenaviruses with the innate immune machinery and the subsequent effects on adaptive immunity, which may inform the development of vaccines and therapeutics against arenavirus infections.

Exploitation of glycosylation in enveloped virus pathobiology

Glycosylation is a ubiquitous post-translational modification responsible for a multitude of crucial biological roles. As obligate parasites, viruses exploit host-cell machinery to glycosylate their own proteins during replication. Viral envelope proteins from a variety of human pathogens including HIV-1, influenza virus, Lassa virus, SARS, Zika virus, dengue virus, and Ebola virus have evolved to be extensively glycosylated. These host-cell derived glycans facilitate diverse structural and functional roles during the viral life-cycle, ranging from immune evasion by glycan shielding to enhancement of immune cell infection. In this review, we highlight the imperative and auxiliary roles glycans play, and how specific oligosaccharide structures facilitate these functions during viral pathogenesis. We discuss the growing efforts to exploit viral glycobiology in the development of anti-viral vaccines and therapies.

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.

Use of a Scalable Replicon-Particle Vaccine to Protect Against Lethal Lassa Virus Infection in the Guinea Pig Model

Lassa fever is a viral zoonosis that can be transmitted from person to person, especially in the hospital setting. The disease is endemic to several countries in West Africa and can be a major contributor to morbidity and mortality in affected areas. There are no approved vaccines to prevent Lassa virus infection. In this work, we present a vaccine candidate that combines the scalability and efficacy benefits of a live vaccine with the safety benefits of single-cycle replication. The system consists of Lassa virus replicon particles devoid of the virus essential glycoprotein gene, and a cell line that expresses the glycoprotein products, enabling efficient vaccine propagation. Guinea pigs vaccinated with these particles showed no clinical reaction to the inoculum and were protected against fever, weight loss, and lethality after infection with Lassa virus.

Vaccine platforms for the prevention of Lassa fever

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The epidemiological significance of Lassa fever in West Africa is discussed.

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Viral ecology, pathology, and immunobiology of Lassa virus infection is described.

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Multiple vaccine candidates have been tested in pre-clinical models.

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Lassa fever vaccine candidates have yet to progress to clinical trials.

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Five platform technologies have been selected for expedited development.

Lassa fever is an acute viral haemorrhagic illness caused by Lassa virus (LASV), which is endemic throughout much of West Africa. The virus primarily circulates in the Mastomys natalensis reservoir and is transmitted to humans through contact with infectious rodents or their secretions; human-to-human transmission is documented as well. With the exception of Dengue fever, LASV has the highest human impact of any haemorrhagic fever virus. On-going outbreaks in Nigeria have resulted in unprecedented mortality. Consequently, the World Health Organization (WHO) has listed LASV as a high priority pathogen for the development of treatments and prophylactics. Currently, there are no licensed vaccines to protect against LASV infection. Although numerous candidates have demonstrated efficacy in animal models, to date, only a single candidate has advanced to clinical trials. Lassa fever vaccine development efforts have been hindered by the high cost of biocontainment requirements, the absence of established correlates of protection, and uncertainty regarding the extent to which animal models are predictive of vaccine efficacy in humans. This review briefly discusses the epidemiology and biology of LASV infection and highlights recent progress in vaccine development.

Hemorrhagic Fever-Causing Arenaviruses: Lethal Pathogens and Potent Immune Suppressors

Hemorrhagic fevers (HF) resulting from pathogenic arenaviral infections have traditionally been neglected as tropical diseases primarily affecting African and South American regions. There are currently no FDA-approved vaccines for arenaviruses, and treatments have been limited to supportive therapy and use of non-specific nucleoside analogs, such as Ribavirin. Outbreaks of arenaviral infections have been limited to certain geographic areas that are endemic but known cases of exportation of arenaviruses from endemic regions and socioeconomic challenges for local control of rodent reservoirs raise serious concerns about the potential for larger outbreaks in the future. This review synthesizes current knowledge about arenaviral evolution, ecology, transmission patterns, life cycle, modulation of host immunity, disease pathogenesis, as well as discusses recent development of preventative and therapeutic pursuits against this group of deadly viral pathogens.

Virus⁻Host Interactions Involved in Lassa Virus Entry and Genome Replication

Lassa virus (LASV) is the causative agent of Lassa fever, a human hemorrhagic disease associated with high mortality and morbidity rates, particularly prevalent in West Africa. Over the past few years, a significant amount of novel information has been provided on cellular factors that are determinant elements playing a role in arenavirus multiplication. In this review, we focus on host proteins that intersect with the initial steps of the LASV replication cycle: virus entry and genome replication. A better understanding of relevant virus⁻host interactions essential for sustaining these critical steps may help to identify possible targets for the rational design of novel therapeutic approaches against LASV and other arenaviruses that cause severe human disease.

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

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

Lassa virus activates myeloid dendritic cells but suppresses their ability to stimulate T cells

Lassa virus (LASV) is responsible for a viral hemorrhagic fever in humans and the death of 3,000 to 5,000 people every year. The immune response to LASV is poorly understood, but type I interferon (IFN-I) and T-cell responses appear to be critical for the host. We studied the response of myeloid dendritic cells (mDC) to LASV, as mDCs are involved in both IFN-I production and T-cell activation. We compared the response of primary human mDCs to LASV and Mopeia virus (MOPV), which is similar to LASV, but non-pathogenic. We showed that mDCs produced substantial amounts of IFN-I in response to both LASV and MOPV. However, only MOPV-infected mDCs were able to activate T cells. More surprisingly, coculture with T cells completely inhibited the activation of LASV-infected mDCs. These differences between LASV and MOPV were mostly due to the LASV nucleoprotein, which has major immunosuppressive properties, but the glycoprotein was also involved. Overall, these results suggest that mDCs may be important for the global response to LASV and play a role in the outcome of Lassa fever.

Lassa fever is a viral hemorrhagic fever and a major public health issue in West Africa. Lassa virus, the causative agent of Lassa fever, is listed by the World Health Organization as one of the emerging pathogens likely to cause severe outbreaks in the near future. Indeed, there is currently no vaccine and no treatment against Lassa virus. Determinants of Lassa virus high pathogenicity are not completely understood. However, it has been shown that rapid type I interferon response and efficient T cell response were critical to survive Lassa fever. Dendritic cells are at the crossroads of innate and adaptive immunity. Their direct response to viral infection includes type I interferon production. They can also present viral antigens, initiating the T cell responses. We decided to investigate how dendritic cells respond to Lassa virus to evaluate their importance in the global immune response. We showed that primary human myeloid dendritic cells are activated by Lassa virus infection, and produce type I interferon. However, Lassa virus-infected dendritic cells were not able to activate T cells. We also elucidated the roles of viral proteins in the modulation of dendritic cell responses.

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

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

Current research for a vaccine against Lassa hemorrhagic fever virus

Lassa virus (LASV) is a rodent-borne arenavirus endemic to several West African countries that causes Lassa fever (LF). LF is typically mild but it can cause severe disease characterized by hemorrhagic fever and multi-organ failure. A current outbreak of LASV in Nigeria has seen greater than 300 cases with a case fatality rate of 22%. Currently, there are limited treatment options and no vaccine candidates are approved to prevent LASV infection. The Coalition for Epidemic Preparedness Innovations has identified LASV as an emerging pathogen of high consequence and this has resulted in a push for several preclinical vaccine candidates to be advanced toward clinical trials. Here, we discuss several important aspects of LASV infection including immunobiology, immune evasion, and correlates of protection against LF, which have been identified through animal models and human infections. In addition, we discuss several vaccine candidates that have shown efficacy in animal models that could be advanced toward clinical trials. The increased fatality rate seen in the recent LASV outbreak in Nigeria highlights the importance of developing effective treatment and prevention strategies against LF. The spike in LASV cases seen in West Africa has the potential for increased mortality and human-to-human transmission, making the development and testing of effective vaccines for LASV critical.

NRP2 and CD63 Are Host Factors for Lujo Virus Cell Entry

Arenaviruses cause fatal hemorrhagic disease in humans. Old World arenavirus glycoproteins (GPs) mainly engage α-dystroglycan as a cell-surface receptor, while New World arenaviruses hijack transferrin receptor. However, the Lujo virus (LUJV) GP does not cluster with New or Old World arenaviruses. Using a recombinant vesicular stomatitis virus containing LUJV GP as its sole attachment and fusion protein (VSV-LUJV), we demonstrate that infection is independent of known arenavirus receptor genes. A genome-wide haploid genetic screen identified the transmembrane protein neuropilin 2 (NRP2) and tetraspanin CD63 as factors for LUJV GP-mediated infection. LUJV GP binds the N-terminal domain of NRP2, while CD63 stimulates pH-activated LUJV GP-mediated membrane fusion. Overexpression of NRP2 or its N-terminal domain enhances VSV-LUJV infection, and cells lacking NRP2 are deficient in wild-type LUJV infection. These findings uncover this distinct set of host cell entry factors in LUJV infection and are attractive focus points for therapeutic intervention.

Interactions of Human Dermal Dendritic Cells and Langerhans Cells Treated with Hyalomma Tick Saliva with Crimean-Congo Hemorrhagic Fever Virus

Crimean-Congo hemorrhagic fever virus is one the most important and wide spread tick-borne viruses. Very little is known about the transmission from the tick and the early aspects of pathogenesis. Here, we generate human cutaneous antigen presenting cells-dermal dendritic cells and Langerhans cells-from umbilical cord progenitor cells. In order to mimic the environment created during tick feeding, tick salivary gland extract was generated from semi-engorged Hyalomma marginatum ticks. Our findings indicate that human dermal dendritic cells and Langerhans cells are susceptible and permissive to Crimean-Congo hemorrhagic fever virus infection, however, to different degrees. Infection leads to cell activation and cytokine/chemokine secretion, although these responses vary between the different cell types. Hyalomma marginatum salivary gland extract had minimal effect on cell responses, with some synergy with viral infection with respect to cytokine secretion. However, salivary gland extract appeared to inhibit antigen presenting cells (APCs) migration. Based on the findings here we hypothesize that human dermal dendritic cells and Langerhans cells serve as early target cells. Rather affecting Crimean-Congo hemorrhagic fever virus replication, tick saliva likely immunomodulates and inhibits migration of these APCs from the feeding site.

A Vaccine Platform against Arenaviruses Based on a Recombinant Hyperattenuated Mopeia Virus Expressing Heterologous Glycoproteins

Several Old World and New World arenaviruses are responsible for severe endemic and epidemic hemorrhagic fevers, whereas other members of the Arenaviridae family are nonpathogenic. To date, no approved vaccines, antivirals, or specific treatments are available, except for Junín virus. However, protection of nonhuman primates against Lassa fever virus (LASV) is possible through the inoculation of the closely related but nonpathogenic Mopeia virus (MOPV) before challenge with LASV. We reasoned that this virus, modified by using reverse genetics, would represent the basis for the generation of a vaccine platform against LASV and other pathogenic arenaviruses. After showing evidence of exoribonuclease (ExoN) activity in NP of MOPV, we found that this activity was essential for multiplication in antigen-presenting cells. The introduction of multiple mutations in the ExoN site of MOPV NP generated a hyperattenuated strain (MOPVExoN6b) that is (i) genetically stable over passages, (ii) has increased immunogenic properties compared to those of MOPV, and (iii) still promotes a strong type I interferon (IFN) response. MOPVExoN6b was further modified to harbor the envelope glycoproteins of heterologous pathogenic arenaviruses, such as LASV or Lujo, Machupo, Guanarito, Chapare, or Sabia virus in order to broaden specific antigenicity while preserving the hyperattenuated characteristics of the parental strain. Our MOPV-based vaccine candidate for LASV, MOPEVACLASV, was used in a one-shot immunization assay in nonhuman primates and fully protected them from a lethal challenge with LASV. Thus, our hyperattenuated strain of MOPV constitutes a promising new live-attenuated vaccine platform to immunize against several, if not all, pathogenic arenaviruses.IMPORTANCE Arenaviruses are emerging pathogens transmitted to humans by rodents and responsible for endemic and epidemic hemorrhagic fevers of global concern. Nonspecific symptoms associated with the onset of infection make these viruses difficult to distinguish from other endemic pathogens. Moreover, the unavailability of rapid diagnosis in the field delays the identification of the virus and early care for treatment and favors spreading. The vaccination of exposed populations would be of great help to decrease morbidity and human-to-human transmission. Using reverse genetics, we generated a vaccine platform for pathogenic arenaviruses based on a modified and hyperattenuated strain of the nonpathogenic Mopeia virus and showed that the Lassa virus candidate fully protected nonhuman primates from a lethal challenge. These results showed that a rationally designed recombinant MOPV-based vaccine is safe, immunogenic, and efficacious in nonhuman primates.