Evolution-Driven Attenuation of Alphaviruses Highlights Key Glycoprotein Determinants Regulating Viral Infectivity and Dissemination

Differential restriction of chikungunya virus in primary human cardiac endothelial cells occurs at multiple steps in the viral life cycle

Arthropod-borne viruses (arboviruses) constitute a significant ongoing public health threat, as the mechanisms of pathogenesis remain incompletely understood. Cardiovascular symptomatology is emerging as an important manifestation of arboviral infection. We have recently studied the cardiac tropism implicated in cardiac infection in mice for the alphavirus chikungunya virus (CHIKV), and we therefore sought to evaluate the cardiac tropism of other emerging alphaviruses and arboviruses. Using human primary cardiac cells, we found that arboviruses from diverse viral families were able to replicate within these cells. Interestingly, we noted that while the closely related alphavirus Mayaro virus (MAYV) could replicate to high titers in primary human cardiac microvascular endothelial cells, pulmonary, and brain endothelial cells, the Indian Ocean Lineage of CHIKV (CHIKV-IOL) was restricted in all endothelial cells tested. Upon further investigation, we discovered that this restriction occurs at both entry and egress stages. Additionally, we observed that compared to CHIKV, MAYV may antagonize or evade the innate immune response more efficiently in human cardiac endothelial cells to increase infection. Overall, this study explores the tropism of arboviruses in human primary cardiac cells and characterizes the strain-specific restriction of CHIKV-IOL in human endothelial cells. Further work is needed to understand how the differential restriction of alphaviruses in human endothelial cells impacts pathogenesis in a living model, as well as the specific host factors responsible.

Host-adaptive mutations in Chikungunya virus genome

Chikungunya virus (CHIKV) is the causative agent of chikungunya fever (CHIKF), and its primary vectors are the mosquitoes Aedes aegypti and Aedes albopictus. CHIKV was initially endemic to Africa but has spread globally in recent years and affected millions of people. According to a risk assessment by the World Health Organization, CHIKV has the potential seriously impact public health. A growing body of research suggests that mutations in the CHIKV gene that enhance viral fitness in the host are contributing to the expansion of the global CHIKF epidemic. In this article, we review the host-adapted gene mutations in CHIKV under natural evolution and laboratory transmission conditions, which can help improve our understanding of the adaptive evolution of CHIKV and provide a basis for monitoring and early warning of future CHIKV outbreaks.

A vaccine candidate based on baculovirus displaying chikungunya virus E1-E2 envelope confers protection against challenge in mice

Chikungunya fever is a re-emerging mosquito-borne disease caused by the chikungunya virus (CHIKV) and produces acute arthritis that can progress to chronic disease with arthralgia. The first approved live-attenuated chikungunya vaccine has only recently become available for use in humans in the USA, but the access in endemic regions remains unmet. Here, we exploited the baculovirus display technology to develop a vectored vaccine candidate that exposes the CHIKV membrane proteins E1 and E2 on the baculovirus surface. Using recombinant baculovirus as vector vaccines has both productive and regulatory advantages: they are safe for handling and easy to produce in high titers and are non-pathogenic and non-replicative in mammals but have strong adjuvant properties by inducing humoral and cellular immune responses. CHIKV E1 and E2 envelope proteins with their own signal and transmembrane sequences were expressed on the surface of budded baculovirus virions. Immunization of C57BL/6 mice with non-adjuvanted recombinant baculovirus induced IgG antibodies against E2 with a predominant IgG2c subtype, neutralizing antibodies and a specific IFN-γ CD8+ T-cell response. Immunization with a second dose significantly boosted the antibody response, and mice immunized with two doses of the vaccine candidate were completely protected against challenge with CHIKV showing no detectable viremia or signs of disease. Altogether, baculovirus display of CHIKV envelope proteins served as an efficient vaccine platform against CHIKV.IMPORTANCEThe global spread of chikungunya virus (CHIKV) has disproportionately impacted the Americas that experienced a fourfold increase in 2023 in cases and deaths compared with the same period in 2022. The disease is characterized by acute fever and debilitating joint pain that can become chronic. Despite the socioeconomic burden related to the high morbidity rates of CHIKV infection, a vaccine for CHIKV is currently approved only in the USA. Vaccines are the most effective preventive measure against viral diseases, and advances in the development of different vaccine platforms such as nucleic acids and viral vectors have prompted the rapid deployment of vaccines to contain the COVID-19 pandemic. Here, we report the use of baculovirus display as a strategy for the design of a novel vaccine that provides sterilizing immunity in a mouse model of chikungunya disease. Our results encourage further research regarding the potential of baculovirus as platforms for human vaccine design.

Evolution of antiviral resistance captures a transient interdomain functional interaction between chikungunya virus envelope glycoproteins

Envelope proteins drive virus and host-cell membrane fusion to achieve virus entry. Fusogenic proteins are classified into structural classes that function with remarkable mechanistic similarities. Fusion proceeds through coordinated movements of protein domains in a sequence of orchestrated steps. Structures for the initial and final conformations are available for several fusogens, but folding intermediates have largely remained unresolved and interdependency between regions that drive conformational rearrangements is not well understood. Chikungunya virus (CHIKV) particles display heterodimers of envelope proteins E1 and E2 associated as trimeric spikes that respond to acidic pH to trigger fusion. We have followed experimental evolution of CHIKV under the selective pressure of a novel small-molecule entry inhibitor. Mutations arising from selection mapped to two residues located in distal domains of E2 and E1 heterodimer and spikes. Here, we pinpointed the antiviral mode of action to inhibition of fusion. Phenotypic characterization of recombinant viruses indicated that the selected mutations confer a fitness advantage under antiviral pressure, and that the double-mutant virus overcame antiviral inhibition of fusion while single-mutants were sensitive. Further supporting a functional connection between residues, the double-mutant virus displayed a higher pH-threshold for fusion than single-mutant viruses. Finally, mutations implied distinct outcomes of replication and spreading in mice, and infection rates in mosquitoes underscoring the fine-tuning of envelope protein function as a determinant for establishment of infection. Together with molecular dynamics simulations that indicate a link between these two residues in the modulation of the heterodimer conformational rearrangement, our approach captured an otherwise unresolved interaction.

Differential restriction of chikungunya virus in primary human cardiac endothelial cells occurs at multiple steps in the viral life cycle

Arthropod-borne viruses (arboviruses) constitute a significant ongoing public health threat, as the mechanisms of pathogenesis remain incompletely understood. Cardiovascular symptomatology is emerging as an important manifestation of arboviral infection. We have recently studied the cardiac tropism and mechanisms implicated in cardiac damage in mice for the alphavirus chikungunya virus (CHIKV), and we therefore sought to evaluate the cardiac tropism of other emerging alphaviruses and arboviruses. Using human primary cardiac cells, we found that arboviruses from diverse viral families were able to replicate within these cells. Interestingly, we noted that while the closely related alphavirus Mayaro virus (MAYV) could replicate to high titers in primary human cardiac microvascular endothelial cells, pulmonary, and brain endothelial cells, the Indian Ocean Lineage of CHIKV (CHIKV-IOL) was completely restricted in all endothelial cells tested. Upon further investigation, we discovered that this restriction occurs at both entry and egress stages. Additionally, we observed that compared to CHIKV, MAYV may antagonize or evade the innate immune response more efficiently in human cardiac endothelial cells to increase infection. Overall, this study explores the tropism of arboviruses in human primary cardiac cells and characterizes the strain-specific restriction of CHIKV-IOL in human endothelial cells. Further work is needed to understand how the differential restriction of alphaviruses in human endothelial cells impacts pathogenesis in a living model, as well as the specific host factors responsible.

New Highly Selective Antivirals for Chikungunya Virus identified from the Screening of a Drug-Like Compound Library

Chikungunya fever is a mosquito-borne disease caused by Chikungunya virus (CHIKV). Treatment of CHIKV infections is currently supportive and does not limit viral replication or symptoms of persistent chronic arthritis. Although there are multiple compounds reported as antivirals active against CHIKV in vitro, there are still no effective and safe antivirals. Thus, active research aims at the identification of new chemical structures with antiviral activity. Here, we report the screen of the Pandemic Response Box library of small molecules against a fully infectious CHIKV reporter virus. Our screening approach successfully identified previously reported CHIKV antiviral compounds within this library and further expanded potentially active hits, supporting the use of reporter-virus-based assays in high-throughput screening format as a reliable tool for antiviral drug discovery. Four molecules were identified as potential drug candidates against CHIKV: MMV1634402 (Brilacidin) and MMV102270 (Diphyllin), which were previously shown to present broad-spectrum antiviral activities, in addition to MMV1578574 (Eravacycline), and the antifungal MMV689401 (Fluopicolide), for which their antiviral potential is uncovered here.

Changes in the chikungunya virus E1 glycoprotein domain II and hinge influence E2 conformation, infectivity, and virus-receptor interactions

In a previous study to understand how the chikungunya virus (CHIKV) E1 glycoprotein β-strand c functions, we identified several attenuating variants at E1 residue V80 and the emergence of second-site mutations in the fusion loop (E1-M88L) and hinge region (E1-N20Y) with the V80 variants in vivo. The emergence of these mutations led us to question how changes in E1 may contribute to CHIKV infection at the molecular level. Here, we use molecular dynamics to understand how changes in the E1 glycoprotein may influence the CHIKV glycoprotein E1-E2 complex. We found that E1 domain II variants lead to E2 conformational changes, allowing us to hypothesize that emerging variants E1-M88L and E1-N20Y could also change E2 conformation and function. We characterized CHIKV E1-M88L and E1-N20Y in vitro and in vivo to understand how these regions of the E1 glycoprotein contribute to host-specific infection. We found that CHIKV E1-N20Y enhanced infectivity in mosquito cells, while the CHIKV E1-M88L variant enhanced infectivity in both BHK-21 and C6/36 cells and led to changes in viral cholesterol-dependence. Moreover, we found that E1-M88L and E1-N20Y changed E2 conformation, heparin binding, and interactions with the receptor Mxra8. Interestingly, the CHIKV E1-M88L variant increased replication in Mxra8-deficient mice compared to WT CHIKV, yet was attenuated in mouse fibroblasts, suggesting that residue E1-M88 may function in a cell-type-dependent entry. Taken together, these studies show that key residues in the CHIKV E1 domain II and hinge region function through changes in E1-E2 dynamics to facilitate cell- and host-dependent entry.IMPORTANCEArboviruses are significant global public health threats, and their continued emergence around the world highlights the need to understand how these viruses replicate at the molecular level. The alphavirus glycoproteins are critical for virus entry in mosquitoes and mammals, yet how these proteins function is not completely understood. Therefore, it is critical to dissect how distinct glycoprotein domains function in vitro and in vivo to address these gaps in our knowledge. Here, we show that changes in the CHIKV E1 domain II and hinge alter E2 conformations leading to changes in virus-receptor and -glycosaminoglycan interactions and cell-specific infection. These results highlight that adaptive changes in E1 can have a major effect on virus attachment and entry, furthering our knowledge of how alphaviruses infect mammals and insects.

Analysis of highly frequent point mutations in glycoprotein C, glycoprotein N, and nucleoprotein of CCHFV

Crimean-Congo hemorrhagic fever virus (CCHFV) is classified among top 10 priority pathogens by World Health Organization. CCHFV belongs to Bunyaviridae family and negative sense ssRNA genome composed of three RNA segments: L, M, and S. RNA viruses show higher mutation rate as compared to DNA viruses. To gain deeper understanding of impact of point mutations in CCHFV M and S segment, mutation profiling, homology modeling, and molecular dynamic (MD) simulation were performed. Structural glycoproteins (glycoprotein C [Gc] and glycoprotein N [Gn]) of CCHFV are important for host-virus interaction and genome packaging, whereas CCHFV nucleoprotein (NP) is crucial for viral replication. Hence, current study is focused on evaluation of eight mutations in structural glycoproteins (Gc: 7 and Gn: 1) of M segment and seven mutations in NP of S segment. All these mutations were highly frequent, with mutation frequency between 0.81 and 1.0 and found to be persistent in the recent strains of CCHFV. Solubility analysis predicted that selected point mutations reduce solubility of Gc protein and increase solubility of Gn and NP proteins. MD simulation study deciphered that A1046V and G1158E in Gc protein, I778T in Gn protein, and H195R in NP protein displayed large deviation and fluctuation, and affected intramolecular interactions. In conclusion, we observed that point mutations could impact structure, stability, and host-virus interaction of protein, and might lead to evolution of new strains for better survival and drug resistance.

The chikungunya virus E1 glycoprotein fusion loop and hinge alter glycoprotein dynamics leading to cell and host specific changes in infectivity

Alphaviruses infect both mammals and insects, yet the distinct mechanisms that alphaviruses use to infect different hosts are not well defined. In this study, we characterize CHIKV E1 variants in the fusion loop (E1-M88L) and hinge region (E1-N20Y) in vitro and in vivo to understand how these regions of the E1 glycoprotein contribute to host-specific infection. Through cell culture assays, we found that CHIKV E1-N20Y enhanced infectivity in mosquito cells while the CHIKV E1-M88L variant enhanced virus binding and infectivity in both BHK-21 and C6/36 cells, and led to changes in the virus cholesterol-dependence in BHK-21 cells. Given these in vitro results and that residue E1-M88L is in a defined Mxra8 interacting domain, we hypothesized that this residue may be important for receptor usage. However, while the CHIKV E1-M88L variant increased replication in Mxra8-deficient mice compared to WT CHIKV, it was attenuated in vitro in mouse fibroblasts, suggesting that residue E1-M88 may function in a cell-type dependent manner to alter entry. Finally, using molecular dynamics to understand how potential changes in the E1 glycoprotein may impact the CHIKV glycoprotein E1-E2 complex, we found that E1-M88L and other E1 domain II variants lead to changes in both E1 and E2 dynamics. Taken together, these studies show that key residues in the CHIKV E1 fusion loop and hinge region function through changes in E1-E2 dynamics to facilitate cell- and host-dependent entry.

Importance

Arthropod-borne viruses (arboviruses) are significant global public health threats, and their continued emergence around the world highlights the need to understand how these viruses replicate at the molecular level. The alphavirus class II glycoproteins are critical for virus entry in mosquitoes and mammals, yet how these proteins function is not completely understood. Therefore, to address these gaps in our knowledge, it is critical to dissect how distinct glycoprotein domains function in vitro and in vivo . Here, we show that changes in the CHIKV E1 fusion loop and hinge contribute to host-specific entry and E1-E2 dynamics, furthering our knowledge of how alphaviruses infect mammals and insects.

The La Crosse virus class II fusion glycoprotein ij loop contributes to infectivity and replication in vitro and in vivo

Arthropod-borne viruses (arboviruses) are an emerging and evolving global public health threat, with limited antiviral treatments or vaccines available. La Crosse virus (LACV) from the Bunyavirales order is responsible for pediatric encephalitis cases in the United States, yet little is known about the infectivity of LACV. Given the structural similarities between class II fusion glycoproteins of LACV and chikungunya virus (CHIKV), an alphavirus from the Togaviridae family, we hypothesized that LACV would share similar entry mechanisms with CHIKV. To test this hypothesis, we performed cholesterol-depletion and repletion assays and used cholesterol-modulating compounds to study LACV entry and replication. We found that LACV entry was cholesterol dependent, while replication was less affected by cholesterol manipulation. In addition, we generated single-point mutants in the LACV Gc ij loop that corresponded to known CHIKV residues important for virus entry. We found that a conserved histidine and alanine residue in the Gc ij loop impaired virus infectivity and attenuated LACV replication in vitro and in vivo. Finally, we took an evolution-based approach to explore how the LACV glycoprotein evolves in mosquitoes and mice. We found multiple variants that cluster in the Gc glycoprotein head domain, providing evidence for the Gc glycoprotein as a contributor to LACV adaptation. Together, these results begin to characterize the mechanisms of LACV infectivity and how the LACV glycoprotein contributes to replication and pathogenesis. IMPORTANCE Vector-borne viruses are significant health threats that lead to devastating disease worldwide. The emergence of arboviruses, in addition to the lack of effective antivirals or vaccines, highlights the need to study how arboviruses replicate at the molecular level. One potential antiviral target is the class II fusion glycoprotein. Alphaviruses, flaviviruses, and bunyaviruses encode a class II fusion glycoprotein that contains strong structural similarities at the tip of domain II. Here, we show that the bunyavirus La Crosse virus uses a cholesterol-dependent entry pathway similar to the alphavirus chikungunya virus, and residues in the ij loop are important for virus infectivity in vitro and replication in mice. These studies show that genetically diverse viruses may use similar pathways through conserved structure domains, suggesting that these viruses may be targets for broad-spectrum antivirals in multiple arboviral families.

MAVS signaling is required for preventing persistent chikungunya heart infection and chronic vascular tissue inflammation

Chikungunya virus (CHIKV) infection has been associated with severe cardiac manifestations, yet, how CHIKV infection leads to heart disease remains unknown. Here, we leveraged both mouse models and human primary cardiac cells to define the mechanisms of CHIKV heart infection. Using an immunocompetent mouse model of CHIKV infection as well as human primary cardiac cells, we demonstrate that CHIKV directly infects and actively replicates in cardiac fibroblasts. In immunocompetent mice, CHIKV is cleared from cardiac tissue without significant damage through the induction of a local type I interferon response from both infected and non-infected cardiac cells. Using mice deficient in major innate immunity signaling components, we found that signaling through the mitochondrial antiviral-signaling protein (MAVS) is required for viral clearance from the heart. In the absence of MAVS signaling, persistent infection leads to focal myocarditis and vasculitis of the large vessels attached to the base of the heart. Large vessel vasculitis was observed for up to 60 days post infection, suggesting CHIKV can lead to vascular inflammation and potential long-lasting cardiovascular complications. This study provides a model of CHIKV cardiac infection and mechanistic insight into CHIKV-induced heart disease, underscoring the importance of monitoring cardiac function in patients with CHIKV infections.

The La Crosse virus class II fusion glycoprotein ij loop contributes to infectivity and cholesterol-dependent entry

Arthropod-borne viruses (arboviruses) are an emerging and evolving global public health threat with little to no antiviral treatments. La Crosse virus (LACV) from the Bunyavirales order is responsible for pediatric encephalitis cases in the United States, yet little is known about the infectivity of LACV. Given the structural similarities between class II fusion glycoproteins of LACV and chikungunya virus (CHIKV), an alphavirus from the Togaviridae family, we hypothesized that LACV would share similar entry mechanisms to CHIKV. To test this hypothesis, we performed cholesterol-depletion and repletion assays and used cholesterol modulating compounds to study LACV entry and replication. We found that LACV entry was cholesterol-dependent while replication was less affected by cholesterol manipulation. In addition, we generated single point mutants in the LACV ij loop that corresponded to known CHIKV residues important for virus entry. We found that a conserved histidine and alanine residue in the Gc ij loop impaired virus infectivity and attenuate LACV in vitro and in vivo . Finally, we took an evolution-based approach to explore how the LACV glycoprotein evolution in mosquitoes and mice. We found multiple variants that cluster in the Gc glycoprotein head domain, supporting the Gc glycoprotein as a target for LACV adaptation. Together, these results begin to characterize the mechanisms of LACV infectivity and how the LACV glycoprotein contributes to infectivity and pathogenesis.

Importance

Vector-borne arboviruses are significant health threats leading to devastating disease worldwide. This emergence and the fact that there are little to no vaccines or antivirals targeting these viruses highlights the need to study how arboviruses replicate at the molecular level. One potential antiviral target is the class II fusion glycoprotein. Alphaviruses, flaviviruses, and bunyaviruses encode a class II fusion glycoprotein that contain strong structural similarities in the tip of domain II. Here we show that the bunyavirus La Crosse virus uses similar mechanisms to entry as the alphavirus chikungunya virus and residues in the ij loop are important for virus infectivity. These studies show that genetically diverse viruses use similar mechanisms through concerned structure domains, suggesting these may be a target for broad-spectrum antivirals to multiple arbovirus families.

Visualization of conformational changes and membrane remodeling leading to genome delivery by viral class-II fusion machinery

Chikungunya virus (CHIKV) is a human pathogen that delivers its genome to the host cell cytoplasm through endocytic low pH-activated membrane fusion mediated by class-II fusion proteins. Though structures of prefusion, icosahedral CHIKV are available, structural characterization of virion interaction with membranes has been limited. Here, we have used cryo-electron tomography to visualize CHIKV's complete membrane fusion pathway, identifying key intermediary glycoprotein conformations coupled to membrane remodeling events. Using sub-tomogram averaging, we elucidate features of the low pH-exposed virion, nucleocapsid and full-length E1-glycoprotein's post-fusion structure. Contrary to class-I fusion systems, CHIKV achieves membrane apposition by protrusion of extended E1-glycoprotein homotrimers into the target membrane. The fusion process also features a large hemifusion diaphragm that transitions to a wide pore for intact nucleocapsid delivery. Our analyses provide comprehensive ultrastructural insights into the class-II virus fusion system function and direct mechanistic characterization of the fundamental process of protein-mediated membrane fusion.

Emerging chikungunya virus variants at the E1-E1 inter-glycoprotein spike interface impact virus attachment and Inflammation

Chikungunya virus (CHIKV) is a re-emerging arthropod-borne alphavirus and a serious threat to human health. Therefore, efforts toward elucidating how this virus causes disease and the molecular mechanisms underlying steps of the viral replication cycle are crucial. Using an in vivo transmission system that allows intra-host evolution, we identified an emerging CHIKV variant carrying a mutation in the E1 glycoprotein (V156A) in the serum of mice and saliva of mosquitoes. E1 V156A has since emerged in humans during an outbreak in Brazil, co-occurring with a second mutation, E1 K211T, suggesting an important role for these residues in CHIKV biology. Given the emergence of these variants, we hypothesized that they function to promote CHIKV infectivity and subsequent disease. Here, we show that E1 V156A and E1 K211T modulate virus attachment and fusion and impact binding to heparin, a homolog of heparan sulfate, a key entry factor on host cells. These variants also exhibit differential neutralization by anti-glycoprotein monoclonal antibodies, suggesting structural impacts on the particle that may be responsible for altered interactions at the host membrane. Finally, E1 V156A and E1 K211T exhibit increased titers in an adult arthritic mouse model and induce increased foot-swelling at the site of injection. Taken together, this work has revealed new roles for E1 where discrete regions of the glycoprotein are able to modulate cell attachment and swelling within the host. IMPORTANCE Alphaviruses represent a growing threat to human health worldwide. The re-emerging alphavirus chikungunya virus (CHIKV) has rapidly spread to new geographic regions in the last several decades, causing overwhelming outbreaks of disease, yet there are no approved vaccines or therapeutics. The CHIKV glycoproteins are key determinants of CHIKV adaptation and virulence. In this study, we identify and characterize the emerging E1 glycoprotein variants, V156A and K211T, that have since emerged in nature. We demonstrate that E1 V156A and K211T function in virus attachment to cells, a role that until now has been only attributed to specific residues of the CHIKV E2 glycoprotein. We also demonstrate E1 V156A and K211T to increase foot-swelling of the ipsilateral foot in mice infected with these variants. Observing that these variants and other pathogenic variants occur at the E1-E1 inter-spike interface, we highlight this structurally important region as critical for multiple steps during CHIKV infection. Together, these studies further defines the function of E1 in CHIKV infection and can inform the development of therapeutic or preventative strategies.

Structurally conserved domains between flavivirus and alphavirus fusion glycoproteins contribute to replication and infectious virion production

Alphaviruses and flaviviruses have class II fusion glycoproteins that are essential for virion assembly and infectivity. Importantly, the tip of domain II is structurally conserved between the alphavirus and flavivirus fusion proteins, yet whether these structural similarities between virus families translate to functional similarities is unclear. Using in vivo evolution of Zika virus (ZIKV), we identified several novel emerging variants including an envelope glycoprotein variant in β-strand c (V114M) of domain II. We have previously shown that the analogous β-strand c and the ij loop, located in the tip of domain II of the alphavirus E1 glycoprotein, are important for infectivity. This led us to hypothesize that flavivirus E β-strand c also contributes to flavivirus infection. We generated this ZIKV glycoprotein variant and found that while it had little impact on infection in mosquitoes, it reduced replication in human cells and mice, and increased virus sensitivity to ammonium chloride, as seen for alphaviruses. In light of these results and given our alphavirus ij loop studies, we mutated a conserved alanine at the tip of the flavivirus ij loop to valine to test its effect on ZIKV infectivity. Interestingly, this mutation inhibited infectious virion production of ZIKV and yellow fever virus, but not West Nile virus. Together, these studies show that shared domains of the alphavirus and flavivirus class II fusion glycoproteins harbor structurally analogous residues that are functionally important and contribute to virus infection in vivo. Importance Arboviruses are a significant global public health threat, yet there are no antivirals targeting these viruses. This problem is in part due to our lack of knowledge on the molecular mechanisms involved in the arbovirus life cycle. In particular, virus entry and assembly are essential processes in the virus life cycle and steps that can be targeted for the development of antiviral therapies. Therefore, understanding common, fundamental mechanisms used by different arboviruses for entry and assembly is essential. In this study, we show that flavivirus and alphavirus residues located in structurally conserved and analogous regions of the class II fusion proteins contribute to common mechanisms of entry, dissemination, and infectious virion production. These studies highlight how class II fusion proteins function and provide novel targets for development of antivirals.

Overview on Chikungunya Virus Infection: From Epidemiology to State-of-the-Art Experimental Models

Chikungunya virus (CHIKV) is currently one of the most relevant arboviruses to public health. It is a member of the Togaviridae family and alphavirus genus and causes an arthritogenic disease known as chikungunya fever (CHIKF). It is characterized by a multifaceted disease, which is distinguished from other arbovirus infections by the intense and debilitating arthralgia that can last for months or years in some individuals. Despite the great social and economic burden caused by CHIKV infection, there is no vaccine or specific antiviral drugs currently available. Recent outbreaks have shown a change in the severity profile of the disease in which atypical and severe manifestation lead to hundreds of deaths, reinforcing the necessity to understand the replication and pathogenesis processes. CHIKF is a complex disease resultant from the infection of a plethora of cell types. Although there are several in vivo models for studying CHIKV infection, none of them reproduces integrally the disease signature observed in humans, which is a challenge for vaccine and drug development. Therefore, understanding the potentials and limitations of the state-of-the-art experimental models is imperative to advance in the field. In this context, the present review outlines the present knowledge on CHIKV epidemiology, replication, pathogenesis, and immunity and also brings a critical perspective on the current in vitro and in vivo state-of-the-art experimental models of CHIKF.

Therapeutic alphavirus cross-reactive E1 human antibodies inhibit viral egress

Alphaviruses cause severe arthritogenic or encephalitic disease. The E1 structural glycoprotein is highly conserved in these viruses and mediates viral fusion with host cells. However, the role of antibody responses to the E1 protein in immunity is poorly understood. We isolated E1-specific human monoclonal antibodies (mAbs) with diverse patterns of recognition for alphaviruses (ranging from Eastern equine encephalitis virus [EEEV]-specific to alphavirus cross-reactive) from survivors of natural EEEV infection. Antibody binding patterns and epitope mapping experiments identified differences in E1 reactivity based on exposure of epitopes on the glycoprotein through pH-dependent mechanisms or presentation on the cell surface prior to virus egress. Therapeutic efficacy in vivo of these mAbs corresponded with potency of virus egress inhibition in vitro and did not require Fc-mediated effector functions for treatment against subcutaneous EEEV challenge. These studies reveal the molecular basis for broad and protective antibody responses to alphavirus E1 proteins.

Alphavirus Virulence Determinants

Alphaviruses are important pathogens that continue to cause outbreaks of disease in humans and animals worldwide. Diseases caused by alphavirus infections include acute symptoms of fever, rash, and nausea as well as chronic arthritis and severe-to-fatal conditions including myocarditis and encephalitis. Despite their prevalence and the significant public health threat they pose, there are currently no effective antiviral treatments or vaccines against alphaviruses. Various genetic determinants of alphavirus virulence, including genomic RNA elements and specific protein residues and domains, have been described by researchers to play key roles in the development of disease, the immune response to infection, and virus transmissibility. Here, we focus on the determinants that are currently described in the literature. Understanding how these molecular determinants shape viral infections can lead to new strategies for the development of therapies and vaccines to combat these viruses.

Discovery of a Potent and Selective Chikungunya Virus Envelope Protein Inhibitor through Computer-Aided Drug Design

The worldwide expansion of chikungunya virus (CHIKV) into tropical and subtropical areas in the last 15 years has posed a currently unmet need for vaccines and therapeutics. The E2-E1 envelope glycoprotein complex binds receptors on the host cell and promotes membrane fusion during CHIKV entry, thus constituting an attractive target for the development of antiviral drugs. In order to identify CHIKV antivirals acting through inhibition of the envelope glycoprotein complex function, our first approach was to search for amenable druggable sites within the E2-E1 heterodimer. We identified a pocket located in the interface between E2 and E1 around the fusion loop. Then, via a structure-based virtual screening approach and in vitro assay of antiviral activity, we identified compound 7 as a specific inhibitor of CHIKV. Through a lead optimization process, we obtained compound 11 that demonstrated increased antiviral activity and low cytotoxicity (EC₅₀ 1.6 μM, CC₅₀ 56.0 μM). Molecular dynamics simulations were carried out and described a possible interaction pattern of compound 11 and the E1-E2 dimer that could be useful for further optimization. As expected from target site selection, compound 11 inhibited virus internalization during CHIKV entry. In addition, virus populations resistant to compound 11 included mutation E2-P173S, which mapped to the proposed binding pocket, and second site mutation E1-Y24H. Construction of recombinant viruses showed that these mutations conferred antiviral resistance in the parental background. Finally, compound 11 presents acceptable solubility values and is chemically and enzymatically stable in different media. Altogether, these findings uncover a suitable pocket for the design of CHIKV entry inhibitors with promising antiviral activity and pharmacological profiles.

Mapping the evolutionary landscape of Zika virus infection in immunocompromised mice

The fundamental basis of how arboviruses evolve in nature and what regulates the adaptive process remain unclear. To address this problem, we established a Zika virus (ZIKV) vector-borne transmission system in immunocompromised mice to study the evolutionary characteristics of ZIKV infection. Using this system, we defined factors that influence the evolutionary landscape of ZIKV infection and show that transmission route and specific organ microenvironments impact viral diversity and defective viral genome production. In addition, we identified in mice the emergence of ZIKV mutants previously seen in natural infections, including variants present in currently circulating Asian and American strains, as well as mutations unique to the mouse infections. With these studies, we have established an insect-to-mouse transmission model to study ZIKV evolution in vivo. We also defined how organ microenvironments and infection route impact the ZIKV evolutionary landscape, providing a deeper understanding of the factors that regulate arbovirus evolution and emergence.

Distinct New York City Aedes albopictus Mosquito Populations Display Differences in Salivary Gland Protein D7 Diversity and Chikungunya Virus Replication

In an increasingly interconnected world, the exposure and subsequent spread of emergent viruses has become inevitable. This is particularly true for Aedes (Ae.) mosquito-vectored viruses, whose range has increased over the past decade from tropical to temperate regions. However, it is unclear if all populations of Ae. mosquitoes in temperate New York City are able to successfully replicate and transmit arboviruses. To answer this question, we reared Ae. albopictus mosquitoes living in a temperate climate from three locations in New York City. We first sequenced the salivary antiviral protein D7 from individual mosquitoes in each population and found single nucleotide variants that are both shared and unique for each Ae. albopictus population. We then fed each population chikungunya virus (CHIKV) via an artificial blood meal. All three mosquito populations could be infected with CHIKV, yet viral titers differed between populations at 7 days post infection. Moreover, we found that these mosquitoes could transmit CHIKV to mice, and that virus RNA reached the saliva as early as two days post infection. Upon sequencing of the saliva CHIKV genomic RNA, we found mutations at sites correlated with increased transmission and virulence. These studies show that NYC Ae. albopictus populations can be infected with and transmit CHIKV, CHIKV is able to evolve in these mosquitoes, and that host salivary factors display population-specific diversity. Taken together, these studies highlight the need to study how distinct mosquito populations control viral infections, both at the virus and host level.