Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides

Multifaceted activation of STING axis upon Nipah and measles virus-induced syncytia formation

Activation of the DNA-sensing STING axis by RNA viruses plays a role in antiviral response through mechanisms that remain poorly understood. Here, we show that the STING pathway regulates Nipah virus (NiV) replication in vivo in mice. Moreover, we demonstrate that following both NiV and measles virus (MeV) infection, IFNγ-inducible protein 16 (IFI16), an alternative DNA sensor in addition to cGAS, induces the activation of STING, leading to the phosphorylation of NF-κB p65 and the production of IFNβ and interleukin 6. Finally, we found that paramyxovirus-induced syncytia formation is responsible for loss of mitochondrial membrane potential and leakage of mitochondrial DNA in the cytoplasm, the latter of which is further detected by both cGAS and IFI16. These results contribute to improve our understanding about NiV and MeV immunopathogenesis and provide potential paths for alternative therapeutic strategies.

A neutralizing antibody prevents postfusion transition of measles virus fusion protein

Measles virus (MeV) presents a public health threat that is escalating as vaccine coverage in the general population declines and as populations of immunocompromised individuals, who cannot be vaccinated, increase. There are no approved therapeutics for MeV. Neutralizing antibodies targeting viral fusion are one potential therapeutic approach but have not yet been structurally characterized or advanced to clinical use. We present cryo-electron microscopy (cryo-EM) structures of prefusion F alone [2.1-angstrom (Å) resolution], F complexed with a fusion-inhibitory peptide (2.3-Å resolution), F complexed with the neutralizing and protective monoclonal antibody (mAb) 77 (2.6-Å resolution), and an additional structure of postfusion F (2.7-Å resolution). In vitro assays and examination of additional EM classes show that mAb 77 binds prefusion F, arrests F in an intermediate state, and prevents transition to the postfusion conformation. These structures shed light on antibody-mediated neutralization that involves arrest of fusion proteins in an intermediate state.

Setup of Aerosol Delivery System to Prevent Measles Virus Infection in Nonhuman Primate Model

Measles virus is one of the most contagious airborne human viruses which keeps causing outbreaks in numerous countries over the world despite the existence of an efficient vaccine. Fusion inhibitory lipopeptides were shown to inhibit viral entry into target cells, and their adequate administration into the respiratory tract may provide a novel preventive approach against airborne infections. Aerosol delivery presents the best administration route to deliver such preventive compounds to the upper and lower respiratory tract. This approach offers a conceptually new strategy to protect the population at risk against infection by respiratory viruses, including measles. It is a noninvasive needle-free approach, which may be used when antiviral protection is required, without any medical assistance. In this chapter, we describe the nebulization approach of lipopeptide compounds in nonhuman primates and the subsequent measles virus challenge.

Molecular Modeling Identification of Key Secondary Metabolites from Xylopia aethiopica as Promising Therapeutics Targeting Essential Measles Viral Proteins

This study computationally screened three key compounds (vanillin (VAN), oxophoebine (OPB), and dihydrochalcone (DHC)) derived from Xylopia aethiopica (Guinea pepper), a medicinal plant with known antiviral activity, against key druggable measles virus (MV) proteins (fusion protein (FUP), haemagglutinin protein (HMG), and phosphoprotein (PSP)). Each molecular species was subjected to a 100 ns molecular dynamics (MD) simulation following docking, and a range of postdynamic parameters including free binding energy and pharmacokinetic properties were determined. The docking scores of the resulting OPB-FUP (-5.4 kcal/mol), OPB-HMG (-8.1 kcal/mol), and OPB-PSP (-8.0 kcal/mol) complexes were consistent with their respective binding energy values (-25.37, -28.74, and -40.68 kcal/mol), and higher than that of the reference standard, ribavirin (RBV) in each case. Furthermore, all the investigated compounds were thermodynamically compact and stable, especially HMG of MV, and this observation could be attributed to the resulting intermolecular interactions in each system. Overall, OPB may possess inhibitory properties against MV glycoproteins (FUP and HMG) and PSP that play important roles in the replication of MV and measles pathogenesis. While OPB could serve as a scaffold for the development of novel MV fusion and entry inhibitors, further in vitro and in vivo evaluation is highly recommended.

Early Permissiveness of Central Nervous System Cells to Measles Virus Infection Is Determined by Hyperfusogenicity and Interferon Pressure

The cessation of measles virus (MeV) vaccination in more than 40 countries as a consequence of the COVID-19 pandemic is expected to significantly increase deaths due to measles. MeV can infect the central nervous system (CNS) and lead to lethal encephalitis. Substantial part of virus sequences recovered from patients' brain were mutated in the matrix and/or the fusion protein (F). Mutations of the heptad repeat domain located in the C terminal (HRC) part of the F protein were often observed and were associated to hyperfusogenicity. These mutations promote brain invasion as a hallmark of neuroadaptation. Wild-type F allows entry into the brain, followed by limited spreading compared with the massive invasion observed for hyperfusogenic MeV. Taking advantage of our ex vivo models of hamster organotypic brain cultures, we investigated how the hyperfusogenic mutations in the F HRC domain modulate virus distribution in CNS cells. In this study, we also identified the dependence of neural cells susceptibility on both their activation state and destabilization of the virus F protein. Type I interferon (IFN-I) impaired mainly astrocytes and microglial cells permissiveness contrarily to neurons, opening a new way of consideration on the development of treatments against viral encephalitis.

Nebulized fusion inhibitory peptide protects cynomolgus macaques from measles virus infection

Measles is the most contagious airborne viral infection and the leading cause of child death among vaccine-preventable diseases. We show here that aerosolized lipopeptide fusion inhibitor, derived from heptad-repeat regions of the measles virus (MeV) fusion protein, blocks respiratory MeV infection in a non-human primate model, the cynomolgus macaque. We use a custom-designed mesh nebulizer to ensure efficient aerosol delivery of peptide to the respiratory tract and demonstrate the absence of adverse effects and lung pathology in macaques. The nebulized peptide efficiently prevents MeV infection, resulting in the complete absence of MeV RNA, MeV-infected cells, and MeV-specific humoral responses in treated animals. This strategy provides an additional means to fight against respiratory infection in non-vaccinated people, that can be readily translated to human trials. It presents a proof-of-concept for the aerosol delivery of fusion inhibitory peptides to protect against measles and other airborne viruses, including SARS-CoV-2, in case of high-risk exposure.

Leveraging the therapeutic, biological, and self-assembling potential of peptides for the treatment of viral infections

Peptides and peptide-based materials have an increasing role in the treatment of viral infections through their use as active pharmaceutical ingredients, targeting moieties, excipients, carriers, or structural components in drug delivery systems. The discovery of peptide-based therapeutic compounds, coupled with the development of new stabilization and formulation strategies, has led to a resurgence of antiviral peptide therapeutics over the past two decades. The ability of peptides to bind cell receptors and to facilitate membrane penetration and subsequent intracellular trafficking enables their use in various antiviral systems for improved targeting efficiency and treatment efficacy. Importantly, the self-assembly of peptides into well-defined nanostructures provides a vast library of discrete constructs and supramolecular biomaterials for systemic and local delivery of antiviral agents. We review here the recent progress in exploiting the therapeutic, biological, and self-assembling potential of peptides, peptide conjugates, and their supramolecular assemblies in treating human viral infections, with an emphasis on the treatment strategies for Human Immunodeficiency Virus (HIV).

Potency of Fusion-Inhibitory Lipopeptides against SARS-CoV-2 Variants of Concern

The ability of SARS-CoV-2 to evolve in response to selective pressures poses a challenge to vaccine and antiviral efficacy. The S1 subunit of the spike (S) protein contains the receptor-binding domain and is therefore under selective pressure to evade neutralizing antibodies elicited by vaccination or infection. In contrast, the S2 subunit of S is only transiently exposed after receptor binding, which makes it a less efficient target for antibodies. As a result, S2 has a lower mutational frequency than S1. We recently described monomeric and dimeric SARS-CoV-2 fusion-inhibitory lipopeptides that block viral infection by interfering with S2 conformational rearrangements during viral entry. Importantly, a dimeric lipopeptide was shown to block SARS-CoV-2 transmission between ferrets in vivo. Because the S2 subunit is relatively conserved in newly emerging SARS-CoV-2 variants of concern (VOCs), we hypothesize that fusion-inhibitory lipopeptides are cross-protective against infection with VOCs. Here, we directly compared the in vitro efficacies of two fusion-inhibitory lipopeptides against VOC, in comparison with a set of seven postvaccination sera (two doses) and a commercial monoclonal antibody preparation. For the beta, delta, and omicron VOCs, it has been reported that convalescent and postvaccination sera are less potent in virus neutralization assays. Both fusion-inhibitory lipopeptides were equally effective against all five VOCs compared to ancestral virus, whereas postvaccination sera and therapeutic monoclonal antibody lost potency to newer VOCs, in particular to omicron BA.1 and BA.2. The neutralizing activity of the lipopeptides is consistent, and they can be expected to neutralize future VOCs based on their mechanism of action.

Nebulized fusion inhibitory peptide protects cynomolgus macaques from measles virus infection

Measles is the most contagious airborne viral infection and the leading cause of child death among vaccine-preventable diseases. We show here that aerosolized lipopeptide fusion inhibitors, derived from heptad-repeat regions of the measles virus (MeV) fusion protein, block respiratory MeV infection in a non-human primate model, the cynomolgus macaque. We used a custom-designed mesh nebulizer to ensure efficient aerosol delivery of peptides to the respiratory tract and demonstrated the absence of adverse effects and lung pathology in macaques. The nebulized peptide efficiently prevented MeV infection, resulting in the complete absence of MeV RNA, MeV-infected cells, and MeV-specific humoral responses in treated animals. This strategy provides an additional shield which complements vaccination to fight against respiratory infection, presenting a proof-of-concept for the aerosol delivery of fusion inhibitory peptides to protect against measles and other airborne viruses, including SARS-CoV-2, in case of high-risk exposure, that can be readily translated to human trials.

Repurposing an In Vitro Measles Virus Dissemination Assay for Screening of Antiviral Compounds

Measles virus (MV) is a highly contagious respiratory virus responsible for outbreaks associated with significant morbidity and mortality among children and young adults. Although safe and effective measles vaccines are available, the COVID-19 pandemic has resulted in vaccination coverage gaps that may lead to the resurgence of measles when restrictions are lifted. This puts individuals who cannot be vaccinated, such as young infants and immunocompromised individuals, at risk. Therapeutic interventions are complicated by the long incubation time of measles, resulting in a narrow treatment window. At present, the only available WHO-advised option is treatment with intravenous immunoglobulins, although this is not approved as standard of care. Antivirals against measles may contribute to intervention strategies to limit the impact of future outbreaks. Here, we review previously described antivirals and antiviral assays, evaluate the antiviral efficacy of a number of compounds to inhibit MV dissemination in vitro, and discuss potential application in specific target populations. We conclude that broadly reactive antivirals could strengthen existing intervention strategies to limit the impact of measles outbreaks.

Antiviral peptide engineering for targeting membrane-enveloped viruses: Recent progress and future directions

Membrane-enveloped viruses are a major cause of global health challenges, including recent epidemics and pandemics. This mini-review covers the latest efforts to develop membrane-targeting antiviral peptides that inhibit enveloped viruses by 1) preventing virus-cell fusion or 2) disrupting the viral membrane envelope. The corresponding mechanisms of antiviral activity are discussed along with peptide engineering strategies to modulate membrane-peptide interactions in terms of potency and selectivity. Application examples are presented demonstrating how membrane-targeting antiviral peptides are useful therapeutics and prophylactics in animal models, while a stronger emphasis on biophysical concepts is proposed to refine mechanistic understanding and support potential clinical translation.

Antivirals targeting paramyxovirus membrane fusion

The Paramyxoviridae family includes enveloped single-stranded negative-sense RNA viruses such as measles, mumps, human parainfluenza, canine distemper, Hendra, and Nipah viruses, which cause a tremendous global health burden. The ability of paramyxoviral glycoproteins to merge viral and host membranes allows entry of the viral genome into host cells, as well as cell-cell fusion, an important contributor to disease progression. Recent molecular and structural advances in our understanding of the paramyxovirus membrane fusion machinery gave rise to various therapeutic approaches aiming at inhibiting viral infection, spread, and cytopathic effects. These therapeutic approaches include peptide mimics, antibodies, and small molecule inhibitors with various levels of success at inhibiting viral entry, increasing the potential of effective antiviral therapeutic development.

Inhibition of Measles Viral Fusion Is Enhanced by Targeting Multiple Domains of the Fusion Protein

Measles virus (MeV) infection remains a significant public health threat despite ongoing global efforts to increase vaccine coverage. As eradication of MeV stalls, and vulnerable populations expand, effective antivirals against MeV are in high demand. Here, we describe the development of an antiviral peptide that targets the MeV fusion (F) protein. This antiviral peptide construct is composed of a carbobenzoxy-d-Phe-l-Phe-Gly (fusion inhibitor peptide; FIP) conjugated to a lipidated MeV F C-terminal heptad repeat (HRC) domain derivative. Initial in vitro testing showed high antiviral potency and specific targeting of MeV F-associated cell plasma membranes, with minimal cytotoxicity. The FIP and HRC-derived peptide conjugates showed synergistic antiviral activities when administered individually. However, their chemical conjugation resulted in markedly increased antiviral potency. In vitro mechanistic experiments revealed that the FIP-HRC lipid conjugate exerted its antiviral activity predominantly through stabilization of the prefusion F, while HRC-derived peptides alone act predominantly on the F protein after its activation. Coupled with in vivo experiments showing effective prevention of MeV infection in cotton rats, FIP-HRC lipid conjugates show promise as potential MeV antivirals via specific targeting and stabilization of the prefusion MeV F structure.

Molecular Features of the Measles Virus Viral Fusion Complex That Favor Infection and Spread in the Brain

Measles virus (MeV) bearing a single amino acid change in the fusion protein (F)—L454W—was isolated from two patients who died of MeV central nervous system (CNS) infection. This mutation in F confers an advantage over wild-type virus in the CNS, contributing to disease in these patients. Using murine ex vivo organotypic brain cultures and human induced pluripotent stem cell-derived brain organoids, we show that CNS adaptive mutations in F enhance the spread of virus ex vivo. The spread of virus in human brain organoids is blocked by an inhibitory peptide that targets F, confirming that dissemination in the brain tissue is attributable to F. A single mutation in MeV F thus alters the fusion complex to render MeV more neuropathogenic.

Engineering Protease-Resistant Peptides to Inhibit Human Parainfluenza Viral Respiratory Infection

The lower respiratory tract infections affecting children worldwide are in large part caused by the parainfluenza viruses (HPIVs), particularly HPIV3, along with human metapneumovirus and respiratory syncytial virus, enveloped negative-strand RNA viruses. There are no vaccines for these important human pathogens, and existing treatments have limited or no efficacy. Infection by HPIV is initiated by viral glycoprotein-mediated fusion between viral and host cell membranes. A viral fusion protein (F), once activated in proximity to a target cell, undergoes a series of conformational changes that first extend the trimer subunits to allow insertion of the hydrophobic domains into the target cell membrane and then refold the trimer into a stable postfusion state, driving the merger of the viral and host cell membranes. Lipopeptides derived from the C-terminal heptad repeat (HRC) domain of HPIV3 F inhibit infection by interfering with the structural transitions of the trimeric F assembly. Clinical application of this strategy, however, requires improving the in vivo stability of antiviral peptides. We show that the HRC peptide backbone can be modified via partial replacement of α-amino acid residues with β-amino acid residues to generate α/β-peptides that retain antiviral activity but are poor protease substrates. Relative to a conventional α-lipopeptide, our best α/β-lipopeptide exhibits improved persistence in vivo and improved anti-HPIV3 antiviral activity in animals.

Structure of SARS-CoV-2 Spike Glycoprotein for Therapeutic and Preventive Target

The global crisis caused by the coronavirus disease 2019 (COVID-19) led to the most significant economic loss and human deaths after World War II. The pathogen causing this disease is a novel virus called the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As of December 2020, there have been 80.2 million confirmed patients, and the mortality rate is known as 2.16% globally. A strategy to protect a host from SARS-CoV-2 is by suppressing intracellular viral replication or preventing viral entry. We focused on the spike glycoprotein that is responsible for the entry of SARS-CoV-2 into the host cell. Recently, the US Food and Drug Administration/EU Medicines Agency authorized a vaccine and antibody to treat COVID-19 patients by emergency use approval in the absence of long-term clinical trials. Both commercial and academic efforts to develop preventive and therapeutic agents continue all over the world. In this review, we present a perspective on current reports about the spike glycoprotein of SARS-CoV-2 as a therapeutic target.

Current and Future Direct-Acting Antivirals Against COVID-19

The coronavirus disease of 2019 (COVID-19) has caused an unprecedented global crisis. The etiological agent is a new virus called the severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). As of October, 2020 there have been 45.4 million confirmed cases with a mortality rate of 2.6% globally. With the lack of a vaccine and effective treatments, the race is on to find a cure for the virus infection using specific antivirals. The viral RNA-dependent RNA polymerase, proteases, spike protein-host angiotensin-converting enzyme 2 binding and fusion have presented as attractive targets for pan-coronavirus and broad spectrum direct-acting antivirals (DAAs). This review presents a perspective on current re-purposing treatments and future DAAs.

Inhibition of Coronavirus Entry In Vitro and Ex Vivo by a Lipid-Conjugated Peptide Derived from the SARS-CoV-2 Spike Glycoprotein HRC Domain

The emergence of severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), the etiological agent of the 2019 coronavirus disease (COVID-19), has erupted into a global pandemic that has led to tens of millions of infections and hundreds of thousands of deaths worldwide. The development of therapeutics to treat infection or as prophylactics to halt viral transmission and spread is urgently needed. SARS-CoV-2 relies on structural rearrangements within a spike (S) glycoprotein to mediate fusion of the viral and host cell membranes. Here, we describe the development of a lipopeptide that is derived from the C-terminal heptad repeat (HRC) domain of SARS-CoV-2 S that potently inhibits infection by SARS-CoV-2. The lipopeptide inhibits cell-cell fusion mediated by SARS-CoV-2 S and blocks infection by live SARS-CoV-2 in Vero E6 cell monolayers more effectively than previously described lipopeptides. The SARS-CoV-2 lipopeptide exhibits broad-spectrum activity by inhibiting cell-cell fusion mediated by SARS-CoV-1 and Middle East respiratory syndrome coronavirus (MERS-CoV) and blocking infection by live MERS-CoV in cell monolayers. We also show that the SARS-CoV-2 HRC-derived lipopeptide potently blocks the spread of SARS-CoV-2 in human airway epithelial (HAE) cultures, an ex vivo model designed to mimic respiratory viral propagation in humans. While viral spread of SARS-CoV-2 infection was widespread in untreated airways, those treated with SARS-CoV-2 HRC lipopeptide showed no detectable evidence of viral spread. These data provide a framework for the development of peptide therapeutics for the treatment of or prophylaxis against SARS-CoV-2 as well as other coronaviruses.IMPORTANCE SARS-CoV-2, the causative agent of COVID-19, continues to spread globally, placing strain on health care systems and resulting in rapidly increasing numbers of cases and mortalities. Despite the growing need for medical intervention, no FDA-approved vaccines are yet available, and treatment has been limited to supportive therapy for the alleviation of symptoms. Entry inhibitors could fill the important role of preventing initial infection and preventing spread. Here, we describe the design, synthesis, and evaluation of a lipopeptide that is derived from the HRC domain of the SARS-CoV-2 S glycoprotein that potently inhibits fusion mediated by SARS-CoV-2 S glycoprotein and blocks infection by live SARS-CoV-2 in both cell monolayers (in vitro) and human airway tissues (ex vivo). Our results highlight the SARS-CoV-2 HRC-derived lipopeptide as a promising therapeutic candidate for SARS-CoV-2 infections.

Structure-Guided Improvement of a Dual HPIV3/RSV Fusion Inhibitor

Human parainfluenza virus 3 (HPIV3) and respiratory syncytial virus (RSV) are leading causes of lower respiratory tract infections. There are currently no vaccines or antiviral therapeutics to treat HPIV3 or RSV infections. We recently reported a peptide (VIQKI), derived from the C-terminal heptad repeat (HRC) domain of the HPIV3 fusion (F) glycoprotein that inhibits infection by both HPIV3 and RSV. The dual inhibitory activity of VIQKI is due to its unique ability to bind to the N-terminal heptad repeat (HRN) domains of both HPIV3 and RSV F, thereby preventing the native HRN-HRC interactions required for viral entry. Here we describe the structure-guided design of dual inhibitors of HPIV3 and RSV fusion with improved efficacy. We show that VIQKI derivatives possessing one (I456F) or two (I454F/I456F) phenylalanine substitutions near the N-terminus exhibit more stable assemblies with the RSV-HRN domain and enhanced antiviral efficacy against both HPIV3 and RSV infection. Cocrystal structures of the new Phe-substituted inhibitors coassembled with HPIV3 or RSV-HRN domains reveal that the I456F substitution makes intimate hydrophobic contact with the core trimers of both HPIV3 and RSV F.

Receptor-mediated cell entry of paramyxoviruses: Mechanisms, and consequences for tropism and pathogenesis

Research in the last decade has uncovered many new paramyxoviruses, airborne agents that cause epidemic diseases in animals including humans. Most paramyxoviruses enter epithelial cells of the airway using sialic acid as a receptor and cause only mild disease. However, others cross the epithelial barrier and cause more severe disease. For some of these viruses, the host receptors have been identified, and the mechanisms of cell entry have been elucidated. The tetrameric attachment proteins of paramyxoviruses have vastly different binding affinities for their cognate receptors, which they contact through different binding surfaces. Nevertheless, all input signals are converted to the same output: conformational changes that trigger refolding of trimeric fusion proteins and membrane fusion. Experiments with selectively receptor-blinded viruses inoculated into their natural hosts have provided insights into tropism, identifying the cells and tissues that support growth and revealing the mechanisms of pathogenesis. These analyses also shed light on diabolically elegant mechanisms used by morbilliviruses, including the measles virus, to promote massive amplification within the host, followed by efficient aerosolization and rapid spread through host populations. In another paradigm of receptor-facilitated severe disease, henipaviruses, including Nipah and Hendra viruses, use different members of one protein family to cause zoonoses. Specific properties of different paramyxoviruses, like neurotoxicity and immunosuppression, are now understood in the light of receptor specificity. We propose that research on the specific receptors for several newly identified members of the Paramyxoviridae family that may not bind sialic acid is needed to anticipate their zoonotic potential and to generate effective vaccines and antiviral compounds.

Measles Encephalitis: Towards New Therapeutics

Measles remains a major cause of morbidity and mortality worldwide among vaccine preventable diseases. Recent decline in vaccination coverage resulted in re-emergence of measles outbreaks. Measles virus (MeV) infection causes an acute systemic disease, associated in certain cases with central nervous system (CNS) infection leading to lethal neurological disease. Early following MeV infection some patients develop acute post-infectious measles encephalitis (APME), which is not associated with direct infection of the brain. MeV can also infect the CNS and cause sub-acute sclerosing panencephalitis (SSPE) in immunocompetent people or measles inclusion-body encephalitis (MIBE) in immunocompromised patients. To date, cellular and molecular mechanisms governing CNS invasion are still poorly understood. Moreover, the known MeV entry receptors are not expressed in the CNS and how MeV enters and spreads in the brain is not fully understood. Different antiviral treatments have been tested and validated in vitro, ex vivo and in vivo, mainly in small animal models. Most treatments have high efficacy at preventing infection but their effectiveness after CNS manifestations remains to be evaluated. This review describes MeV neural infection and current most advanced therapeutic approaches potentially applicable to treat MeV CNS infection.

Fusion Inhibitory Lipopeptides Engineered for Prophylaxis of Nipah Virus in Primates

Background

The emerging zoonotic paramyxovirus Nipah virus (NiV) causes severe respiratory and neurological disease in humans, with high fatality rates. Nipah virus can be transmitted via person-to-person contact, posing a high risk for epidemic outbreaks. However, a broadly applicable approach for human NiV outbreaks in field settings is lacking.

Methods

We engineered new antiviral lipopeptides and analyzed in vitro fusion inhibition to identify an optimal candidate for prophylaxis of NiV infection in the lower respiratory tract, and we assessed antiviral efficiency in 2 different animal models.

Results

We show that lethal NiV infection can be prevented with lipopeptides delivered via the respiratory route in both hamsters and nonhuman primates. By targeting retention of peptides for NiV prophylaxis in the respiratory tract, we avoid its systemic delivery in individuals who need only prevention, and thus we increase the safety of treatment and enhance utility of the intervention.

Conclusions

The experiments provide a proof of concept for the use of antifusion lipopeptides for prophylaxis of lethal NiV. These results advance the goal of rational development of potent lipopeptide inhibitors with desirable pharmacokinetic and biodistribution properties and a safe effective delivery method to target NiV and other pathogenic viruses.

Targeting viral entry as a strategy for broad-spectrum antivirals

The process of entry into a host cell is a key step in the life cycle of most viruses. In recent years, there has been a significant increase in our understanding of the routes and mechanisms of entry for a number of these viruses. This has led to the development of novel broad-spectrum antiviral approaches that target host cell proteins and pathways, in addition to strategies focused on individual viruses or virus families. Here we consider a number of these approaches and their broad-spectrum potential.

Dual Inhibition of Human Parainfluenza Type 3 and Respiratory Syncytial Virus Infectivity with a Single Agent

Human parainfluenza virus 3 (HPIV3) and respiratory syncytial virus (RSV) cause lower respiratory infection in infants and young children. There are no vaccines for these pathogens, and existing treatments have limited or questionable efficacy. Infection by HPIV3 or RSV requires fusion of the viral and cell membranes, a process mediated by a trimeric fusion glycoprotein (F) displayed on the viral envelope. Once triggered, the pre-fusion form of F undergoes a series of conformational changes that first extend the molecule to allow for insertion of the hydrophobic fusion peptide into the target cell membrane and then refold the trimeric assembly into an energetically stable post-fusion state, a process that drives the merger of the viral and host cell membranes. Peptides derived from defined regions of HPIV3 F inhibit infection by HPIV3 by interfering with the structural transitions of the trimeric F assembly. Here we describe lipopeptides derived from the C-terminal heptad repeat (HRC) domain of HPIV3 F that potently inhibit infection by both HPIV3 and RSV. The lead peptide inhibits RSV infection as effectively as does a peptide corresponding to the RSV HRC domain itself. We show that the inhibitors bind to the N-terminal heptad repeat (HRN) domains of both HPIV3 and RSV F with high affinity. Co-crystal structures of inhibitors bound to the HRN domains of HPIV3 or RSV F reveal remarkably different modes of binding in the N-terminal segment of the inhibitor.

Preventing Measles in Immunosuppressed Cancer and Hematopoietic Cell Transplantation Patients: A Position Statement by the American Society for Transplantation and Cellular Therapy

Until recently, measles exposures were relatively rare and so, consequently, were an afterthought for cancer patients and/or blood and marrow transplant recipients and their providers. Declines in measles herd immunity have reached critical levels in many communities throughout the United States due to increasing vaccine hesitancy, so that community-based outbreaks have occurred. The reemergence of measles as a clinical disease has raised serious concerns among immunocompromised patients and those who work within the cancer and hematopoietic cell transplantation (HCT) community. Since live attenuated vaccines, such as measles, mumps, and rubella (MMR), are contraindicated in immunocompromised patients, and with no approved antiviral therapies for measles, community exposures in these patients can lead to life-threatening infection. The lack of data regarding measles prevention in this population poses a number of clinical dilemmas. Herein specialists in Infectious Diseases and HCT/cellular therapy endorsed by the American Society of Transplant and Cellular Therapy address frequently asked questions about measles in these high-risk cancer patients and HCT recipients and provide expert opinions based on the limited available data.

Type I Interferon Receptor Signaling Drives Selective Permissiveness of Astrocytes and Microglia to Measles Virus during Brain Infection

Fatal neurological syndromes can occur after measles virus (MeV) infection of the brain. The mechanisms controlling MeV spread within the central nervous system (CNS) remain poorly understood. We analyzed the role of type I interferon (IFN-I) receptor (IFNAR) signaling in the control of MeV infection in a murine model of brain infection. Using organotypic brain cultures (OBC) from wild-type and IFNAR-knockout (IFNARKO) transgenic mice ubiquitously expressing the human SLAM (CD150) receptor, the heterogeneity of the permissiveness of different CNS cell types to MeV infection was characterized. In the absence of IFNAR signaling, MeV propagated significantly better in explant slices. In OBC from IFNAR-competent mice, while astrocytes and microglia were infected on the day of explant preparation, they became refractory to infection with time, in contrast to neurons and oligodendrocytes, which remained permissive to infection. This selective loss of permissiveness to MeV infection was not observed in IFNARKO mouse OBC. Accordingly, the development of astrogliosis related to the OBC procedure was exacerbated in the presence of IFNAR signaling. In the hippocampus, this astrogliosis was characterized by a change in the astrocyte phenotype and by an increase of IFN-I transcripts. A proteome analysis showed the upregulation of 84 out of 111 secreted proteins. In the absence of IFNAR, only 27 secreted proteins were upregulated, and none of these were associated with antiviral activities. Our results highlight the essential role of the IFN-I response in astrogliosis and in the permissiveness of astrocytes and microglia that could control MeV propagation throughout the CNS.IMPORTANCE Measles virus (MeV) can infect the central nervous system (CNS), with dramatic consequences. The mechanisms controlling MeV invasion of the CNS remain ill-defined since most previous data were obtained from postmortem analysis. Here, we highlight for the first time the crucial role of the type I interferon (IFN-I) response not only in the control of CNS invasion but also in the early permissiveness of glial cells to measles virus infection.

Measles Virus Bearing Measles Inclusion Body Encephalitis-Derived Fusion Protein Is Pathogenic after Infection via the Respiratory Route

A clinical isolate of measles virus (MeV) bearing a single amino acid alteration in the viral fusion protein (F; L454W) was previously identified in two patients with lethal sequelae of MeV central nervous system (CNS) infection. The mutation dysregulated the viral fusion machinery so that the mutated F protein mediated cell fusion in the absence of known MeV cellular receptors. While this virus could feasibly have arisen via intrahost evolution of the wild-type (wt) virus, it was recently shown that the same mutation emerged under the selective pressure of small-molecule antiviral treatment. Under these conditions, a potentially neuropathogenic variant emerged outside the CNS. While CNS adaptation of MeV was thought to generate viruses that are less fit for interhost spread, we show that two animal models can be readily infected with CNS-adapted MeV via the respiratory route. Despite bearing a fusion protein that is less stable at 37°C than the wt MeV F, this virus infects and replicates in cotton rat lung tissue more efficiently than the wt virus and is lethal in a suckling mouse model of MeV encephalitis even with a lower inoculum. Thus, either during lethal MeV CNS infection or during antiviral treatment in vitro, neuropathogenic MeV can emerge, can infect new hosts via the respiratory route, and is more pathogenic (at least in these animal models) than wt MeV.IMPORTANCE Measles virus (MeV) infection can be severe in immunocompromised individuals and lead to complications, including measles inclusion body encephalitis (MIBE). In some cases, MeV persistence and subacute sclerosing panencephalitis (SSPE) occur even in the face of an intact immune response. While they are relatively rare complications of MeV infection, MIBE and SSPE are lethal. This work addresses the hypothesis that despite a dysregulated viral fusion complex, central nervous system (CNS)-adapted measles virus can spread outside the CNS within an infected host.

Analysis of a Subacute Sclerosing Panencephalitis Genotype B3 Virus from the 2009-2010 South African Measles Epidemic Shows That Hyperfusogenic F Proteins Contribute to Measles Virus Infection in the Brain

During a measles virus (MeV) epidemic in 2009 in South Africa, measles inclusion body encephalitis (MIBE) was identified in several HIV-infected patients. Years later, children are presenting with subacute sclerosing panencephalitis (SSPE). To investigate the features of established MeV neuronal infections, viral sequences were analyzed from brain tissue samples of a single SSPE case and compared with MIBE sequences previously obtained from patients infected during the same epidemic. Both the SSPE and the MIBE viruses had amino acid substitutions in the ectodomain of the F protein that confer enhanced fusion properties. Functional analysis of the fusion complexes confirmed that both MIBE and SSPE F protein mutations promoted fusion with less dependence on interaction by the viral receptor-binding protein with known MeV receptors. While the SSPE F required the presence of a homotypic attachment protein, MeV H, in order to fuse, MIBE F did not. Both F proteins had decreased thermal stability compared to that of the corresponding wild-type F protein. Finally, recombinant viruses expressing MIBE or SSPE fusion complexes spread in the absence of known MeV receptors, with MIBE F-bearing viruses causing large syncytia in these cells. Our results suggest that alterations to the MeV fusion complex that promote fusion and cell-to-cell spread in the absence of known MeV receptors is a key property for infection of the brain.IMPORTANCE Measles virus can invade the central nervous system (CNS) and cause severe neurological complications, such as MIBE and SSPE. However, mechanisms by which MeV enters the CNS and triggers the disease remain unclear. We analyzed viruses from brain tissue of individuals with MIBE or SSPE, infected during the same epidemic, after the onset of neurological disease. Our findings indicate that the emergence of hyperfusogenic MeV F proteins is associated with infection of the brain. We also demonstrate that hyperfusogenic F proteins permit MeV to enter cells and spread without the need to engage nectin-4 or CD150, known receptors for MeV that are not present on neural cells.

Structure-Stability-Function Mechanistic Links in the Anti-Measles Virus Action of Tocopherol-Derivatized Peptide Nanoparticles

Measles remains one of the leading causes of child mortality worldwide and is re-emerging in some countries due to poor vaccine coverage, concomitant with importation of measles virus (MV) from endemic areas. The lack of specific chemotherapy contributes to negative outcomes, especially in infants or immunodeficient individuals. Fusion inhibitor peptides derived from the MV Fusion protein C-terminal Heptad Repeat (HRC) targeting MV envelope fusion glycoproteins block infection at the stage of entry into host cells, thus preventing viral multiplication. To improve efficacy of such entry inhibitors, we have modified a HRC peptide inhibitor by introducing properties of self-assembly into nanoparticles (NP) and higher affinity for both viral and cell membranes. Modification of the peptide consisted of covalent grafting with tocopherol to increase amphipathicity and lipophilicity (HRC5). One additional peptide inhibitor consisting of a peptide dimer grafted to tocopherol was also used (HRC6). Spectroscopic, imaging, and simulation techniques were used to characterize the NP and explore the molecular basis for their antiviral efficacy. HRC5 forms micellar stable NP while HRC6 aggregates into amorphous, loose, unstable NP. Interpeptide cluster bridging governs NP assembly into dynamic metastable states. The results are consistent with the conclusion that the improved efficacy of HRC6 relative to HRC5 can be attributed to NP instability, which leads to more extensive partition to target membranes and binding to viral target proteins.

Primary resistance mechanism of the canine distemper virus fusion protein against a small-molecule membrane fusion inhibitor

Morbilliviruses (e.g. measles virus [MeV] or canine distemper virus [CDV]) employ the attachment (H) and fusion (F) envelope glycoproteins for cell entry. H protein engagement to a cognate receptor eventually leads to F-triggering. Upon activation, F proteins transit from a prefusion to a postfusion conformation; a refolding process that is associated with membrane merging. Small-molecule morbilliviral fusion inhibitors such as the compound 3G (a chemical analog in the AS-48 class) were previously generated and mechanistic studies revealed a stabilizing effect on morbilliviral prefusion F trimers. Here, we aimed at designing 3G-resistant CDV F mutants by introducing single cysteine residues at hydrophobic core positions of the helical stalk region. Covalently-linked F dimers were generated, which highlighted substantial conformational flexibility within the stalk to achieve those irregular F conformations. Our findings demonstrate that "top-stalk" CDV F cysteine mutants (F-V571C and F-L575C) remained functional and gained resistance to 3G. Conversely, although not all "bottom-stalk" F cysteine variants preserved proper bioactivity, those that remained functional exhibited 3G-sensitivity. According to the recently determined prefusion MeV F trimer/AS-48 co-crystal structure, CDV residues F-V571 and F-L575 may directly interact with 3G. A combination of conformation-specific anti-F antibodies and low-resolution electron microscopy structural analyses confirmed that 3G lost its stabilizing effect on "top-stalk" F cysteine mutants thus suggesting a primary resistance mechanism. Overall, our data suggest that the fusion inhibitor 3G stabilizes prefusion CDV F trimers by docking at the top of the stalk domain.

Effective in Vivo Targeting of Influenza Virus through a Cell-Penetrating/Fusion Inhibitor Tandem Peptide Anchored to the Plasma Membrane

The impact of influenza virus infection is felt each year on a global scale when approximately 5-10% of adults and 20-30% of children globally are infected. While vaccination is the primary strategy for influenza prevention, there are a number of likely scenarios for which vaccination is inadequate, making the development of effective antiviral agents of utmost importance. Anti-influenza treatments with innovative mechanisms of action are critical in the face of emerging viral resistance to the existing drugs. These new antiviral agents are urgently needed to address future epidemic (or pandemic) influenza and are critical for the immune-compromised cohort who cannot be vaccinated. We have previously shown that lipid tagged peptides derived from the C-terminal region of influenza hemagglutinin (HA) were effective influenza fusion inhibitors. In this study, we modified the influenza fusion inhibitors by adding a cell penetrating peptide sequence to promote intracellular targeting. These fusion-inhibiting peptides self-assemble into ∼15-30 nm nanoparticles (NPs), target relevant infectious tissues in vivo, and reduce viral infectivity upon interaction with the cell membrane. Overall, our data show that the CPP and the lipid moiety are both required for efficient biodistribution, fusion inhibition, and efficacy in vivo.

Cell-to-Cell Measles Virus Spread between Human Neurons Is Dependent on Hemagglutinin and Hyperfusogenic Fusion Protein

Measles virus (MV) usually causes acute infection but in rare cases persists in the brain, resulting in subacute sclerosing panencephalitis (SSPE). Since human neurons, an important target affected in the disease, do not express the known MV receptors (signaling lymphocyte activation molecule [SLAM] and nectin 4), how MV infects neurons and spreads between them is unknown. Recent studies have shown that many virus strains isolated from SSPE patients possess substitutions in the extracellular domain of the fusion (F) protein which confer enhanced fusion activity. Hyperfusogenic viruses with such mutations, unlike the wild-type MV, can induce cell-cell fusion even in SLAM- and nectin 4-negative cells and spread efficiently in human primary neurons and the brains of animal models. We show here that a hyperfusogenic mutant MV, IC323-F(T461I)-EGFP (IC323 with a fusion-enhancing T461I substitution in the F protein and expressing enhanced green fluorescent protein), but not the wild-type MV, spreads in differentiated NT2 cells, a widely used human neuron model. Confocal time-lapse imaging revealed the cell-to-cell spread of IC323-F(T461I)-EGFP between NT2 neurons without syncytium formation. The production of virus particles was strongly suppressed in NT2 neurons, also supporting cell-to-cell viral transmission. The spread of IC323-F(T461I)-EGFP was inhibited by a fusion inhibitor peptide as well as by some but not all of the anti-hemagglutinin antibodies which neutralize SLAM- or nectin-4-dependent MV infection, suggesting the presence of a distinct neuronal receptor. Our results indicate that MV spreads in a cell-to-cell manner between human neurons without causing syncytium formation and that the spread is dependent on the hyperfusogenic F protein, the hemagglutinin, and the putative neuronal receptor for MV.IMPORTANCE Measles virus (MV), in rare cases, persists in the human central nervous system (CNS) and causes subacute sclerosing panencephalitis (SSPE) several years after acute infection. This neurological complication is almost always fatal, and there is currently no effective treatment for it. Mechanisms by which MV invades the CNS and causes the disease remain to be elucidated. We have previously shown that fusion-enhancing substitutions in the fusion protein of MVs isolated from SSPE patients contribute to MV spread in neurons. In this study, we demonstrate that MV bearing the hyperfusogenic mutant fusion protein spreads between human neurons in a cell-to-cell manner. Spread of the virus was inhibited by a fusion inhibitor peptide and antibodies against the MV hemagglutinin, indicating that both the hemagglutinin and hyperfusogenic fusion protein play important roles in MV spread between human neurons. The findings help us better understand the disease process of SSPE.

Biophysical Properties and Antiviral Activities of Measles Fusion Protein Derived Peptide Conjugated with 25-Hydroxycholesterol

Measles virus (MV) infection is re-emerging, despite the availability of an effective vaccine. The mechanism of MV entry into a target cell relies on coordinated action between the MV hemagglutinin (H) receptor binding protein and the fusion envelope glycoprotein (F) which mediates fusion between the viral and cell membranes. Peptides derived from the C-terminal heptad repeat (HRC) of F can interfere with this process, blocking MV infection. As previously described, biophysical properties of HRC-derived peptides modulate their antiviral potency. In this work, we characterized a MV peptide fusion inhibitor conjugated to 25-hydroxycholesterol (25HC), a cholesterol derivative with intrinsic antiviral activity, and evaluated its interaction with membrane model systems and human blood cells. The peptide (MV.

Antiviral Lipopeptide-Cell Membrane Interaction Is Influenced by PEG Linker Length

A set of lipopeptides was recently reported for their broad-spectrum antiviral activity against viruses belonging to the Paramyxoviridae family, including human parainfluenza virus type 3 and Nipah virus. Among them, the peptide with a 24-unit PEG linker connecting it to a cholesterol moiety (VG-PEG24-Chol) was found to be the best membrane fusion inhibitory peptide. Here, we evaluated the interaction of the same set of peptides with biomembrane model systems and isolated human peripheral blood mononuclear cells (PBMC). VG-PEG24-Chol showed the highest insertion rate and it was among the peptides that induced a larger change on the surface pressure of cholesterol rich membranes. This peptide also displayed a high affinity towards PBMC membranes. These data provide new information about the dynamics of peptide-membrane interactions of a specific group of antiviral peptides, known for their potential as multipotent paramyxovirus antivirals.

Broad spectrum antiviral activity for paramyxoviruses is modulated by biophysical properties of fusion inhibitory peptides

Human paramyxoviruses include global causes of lower respiratory disease like the parainfluenza viruses, as well as agents of lethal encephalitis like Nipah virus. Infection is initiated by viral glycoprotein-mediated fusion between viral and host cell membranes. Paramyxovirus viral fusion proteins (F) insert into the target cell membrane, and form a transient intermediate that pulls the viral and cell membranes together as two heptad-repeat regions refold to form a six-helix bundle structure that can be specifically targeted by fusion-inhibitory peptides. Antiviral potency can be improved by sequence modification and lipid conjugation, and by adding linkers between the protein and lipid components. We exploit the uniquely broad spectrum antiviral activity of a parainfluenza F-derived peptide sequence that inhibits both parainfluenza and Nipah viruses, to investigate the influence of peptide orientation and intervening linker length on the peptides' interaction with transitional states of F, solubility, membrane insertion kinetics, and protease sensitivity. We assessed the impact of these features on biodistribution and antiviral efficacy in vitro and in vivo. The engineering approach based on biophysical parameters resulted in a peptide that is a highly effective inhibitor of both paramyxoviruses and a set of criteria to be used for engineering broad spectrum antivirals for emerging paramyxoviruses.

Foldamers in Medicinal Chemistry

Bio-inspired synthetic backbones leading to foldamers can provide effective biopolymer mimics with new and improved properties in a physiological environment, and in turn could serve as useful tools to study biology and lead to practical applications in the areas of diagnostics or therapeutics. Remarkable progress has been accomplished over the past 20 years with the discovery of many potent bioactive foldamers originating from diverse backbones and targeting a whole spectrum of bio(macro)molecules such as membranes, protein surfaces, and nucleic acids. These current achievements, future opportunities, and key challenges that remain are discussed in this article.

In Vivo Efficacy of Measles Virus Fusion Protein-Derived Peptides Is Modulated by the Properties of Self-Assembly and Membrane Residence

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV hemagglutinin (H) and fusion (F) envelope glycoproteins; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad repeat (HR) regions of F can inhibit MV infection at the entry stage. In previous work, we have generated potent MV fusion inhibitors by dimerizing the F-derived peptides and conjugating them to cholesterol. We have shown that prophylactic intranasal administration of our lead fusion inhibitor efficiently protects from MV infection in vivo We show here that peptides tagged with lipophilic moieties self-assemble into nanoparticles until they reach the target cells, where they are integrated into cell membranes. The self-assembly feature enhances biodistribution and the half-life of the peptides, while integration into the target cell membrane increases fusion inhibitor potency. These factors together modulate in vivo efficacy. The results suggest a new framework for developing effective fusion inhibitory peptides.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. We show here that specific biophysical properties regulate the in vivo efficacy of MV F-derived peptides.

Measles Virus Fusion Protein: Structure, Function and Inhibition

Measles virus (MeV), a highly contagious member of the Paramyxoviridae family, causes measles in humans. The Paramyxoviridae family of negative single-stranded enveloped viruses includes several important human and animal pathogens, with MeV causing approximately 120,000 deaths annually. MeV and canine distemper virus (CDV)-mediated diseases can be prevented by vaccination. However, sub-optimal vaccine delivery continues to foster MeV outbreaks. Post-exposure prophylaxis with antivirals has been proposed as a novel strategy to complement vaccination programs by filling herd immunity gaps. Recent research has shown that membrane fusion induced by the morbillivirus glycoproteins is the first critical step for viral entry and infection, and determines cell pathology and disease outcome. Our molecular understanding of morbillivirus-associated membrane fusion has greatly progressed towards the feasibility to control this process by treating the fusion glycoprotein with inhibitory molecules. Current approaches to develop anti-membrane fusion drugs and our knowledge on drug resistance mechanisms strongly suggest that combined therapies will be a prerequisite. Thus, discovery of additional anti-fusion and/or anti-attachment protein small-molecule compounds may eventually translate into realistic therapeutic options.

Sequential conformational changes in the morbillivirus attachment protein initiate the membrane fusion process

Despite large vaccination campaigns, measles virus (MeV) and canine distemper virus (CDV) cause major morbidity and mortality in humans and animals, respectively. The MeV and CDV cell entry system relies on two interacting envelope glycoproteins: the attachment protein (H), consisting of stalk and head domains, co-operates with the fusion protein (F) to mediate membrane fusion. However, how receptor-binding by the H-protein leads to F-triggering is not fully understood. Here, we report that an anti-CDV-H monoclonal antibody (mAb-1347), which targets the linear H-stalk segment 126-133, potently inhibits membrane fusion without interfering with H receptor-binding or F-interaction. Rather, mAb-1347 blocked the F-triggering function of H-proteins regardless of the presence or absence of the head domains. Remarkably, mAb-1347 binding to headless CDV H, as well as standard and engineered bioactive stalk-elongated CDV H-constructs treated with cells expressing the SLAM receptor, was enhanced. Despite proper cell surface expression, fusion promotion by most H-stalk mutants harboring alanine substitutions in the 126-138 "spacer" section was substantially impaired, consistent with deficient receptor-induced mAb-1347 binding enhancement. However, a previously reported F-triggering defective H-I98A variant still exhibited the receptor-induced "head-stalk" rearrangement. Collectively, our data spotlight a distinct mechanism for morbillivirus membrane fusion activation: prior to receptor contact, at least one of the morbillivirus H-head domains interacts with the membrane-distal "spacer" domain in the H-stalk, leaving the F-binding site located further membrane-proximal in the stalk fully accessible. This "head-to-spacer" interaction conformationally stabilizes H in an auto-repressed state, which enables intracellular H-stalk/F engagement while preventing the inherent H-stalk's bioactivity that may prematurely activate F. Receptor-contact disrupts the "head-to-spacer" interaction, which subsequently "unlocks" the stalk, allowing it to rearrange and trigger F. Overall, our study reveals essential mechanistic requirements governing the activation of the morbillivirus membrane fusion cascade and spotlights the H-stalk "spacer" microdomain as a possible drug target for antiviral therapy.

Morbillivirus infections: an introduction

Research on morbillivirus infections has led to exciting developments in recent years. Global measles vaccination coverage has increased, resulting in a significant reduction in measles mortality. In 2011 rinderpest virus was declared globally eradicated - only the second virus to be eradicated by targeted vaccination. Identification of new cellular receptors and implementation of recombinant viruses expressing fluorescent proteins in a range of model systems have provided fundamental new insights into the pathogenesis of morbilliviruses, and their interactions with the host immune system. Nevertheless, both new and well-studied morbilliviruses are associated with significant disease in wildlife and domestic animals. This illustrates the need for robust surveillance and a strategic focus on barriers that restrict cross-species transmission. Recent and ongoing measles outbreaks also demonstrate that maintenance of high vaccination coverage for these highly infectious agents is critical. This introduction briefly summarizes the most important current research topics in this field.

Measles fusion machinery is dysregulated in neuropathogenic variants

Paramyxoviruses, including the human pathogen measles virus (MV), enter host cells by fusing their viral envelope with the target cell membrane. This fusion process is driven by the concerted actions of the two viral envelope glycoproteins, the receptor binding protein (hemagglutinin [H]) and the fusion (F) protein. H attaches to specific proteinaceous receptors on host cells; once the receptor engages, H activates F to directly mediate lipid bilayer fusion during entry. In a recent MV outbreak in South Africa, several HIV-positive people died of MV central nervous system (CNS) infection. We analyzed the virus sequences from these patients and found that specific intrahost evolution of the F protein had occurred and resulted in viruses that are "CNS adapted." A mutation in F of the CNS-adapted virus (a leucine-to-tryptophan change present at position 454) allows it to promote fusion with less dependence on engagement of H by the two known wild-type (wt) MV cellular receptors. This F protein is activated independently of H or the receptor and has reduced thermal stability and increased fusion activity compared to those of the corresponding wt F. These functional effects are the result of the single L454W mutation in F. We hypothesize that in the absence of effective cellular immunity, such as HIV infection, MV variants bearing altered fusion machinery that enabled efficient spread in the CNS underwent positive selection.

IMPORTANCE

Measles virus has become a concern in the United States and Europe due to recent outbreaks and continues to be a significant global problem. While live immunization is available, there are no effective therapies or prophylactics to combat measles infection in unprotected people. Additionally, vaccination does not adequately protect immunocompromised people, who are vulnerable to the more severe CNS manifestations of disease. We found that strains isolated from patients with measles virus infection of the CNS have fusion properties different from those of strains previously isolated from patients without CNS involvement. Specifically, the viral entry machinery is more active and the virus can spread, even in the absence of H. Our findings are consistent with an intrahost evolution of the fusion machinery that leads to neuropathogenic MV variants.

Henipavirus pathogenesis and antiviral approaches

Hendra virus and Nipah virus are closely related, recently emerged zoonotic paramyxoviruses, belonging to the Henipavirus genus. Both viruses induce generalized vasculitis affecting particularly the respiratory tract and CNS. The exceptionally broad species tropism of Henipavirus, the high case fatality rate and person-to-person transmission associated with Nipah virus outbreaks emphasize the necessity of effective antiviral strategies for these intriguing threatening pathogens. Current therapeutic approaches, validated in animal models, target early steps in viral infection; they include the use of neutralizing virus-specific antibodies and blocking membrane fusion with peptides that bind the viral fusion protein. A better understanding of Henipavirus pathogenesis is critical for the further advancement of antiviral treatment, and we summarize here the recent progress in the field.