Protective neutralizing antibodies from human survivors of Crimean-Congo hemorrhagic fever.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a World Health Organization priority pathogen. CCHFV infections cause a highly lethal hemorrhagic fever for which specific treatments and vaccines are urgently needed. Here, we characterize the human immune response to natural CCHFV infection to identify potent neutralizing monoclonal antibodies (nAbs) targeting the viral glycoprotein. Competition experiments showed that these nAbs bind six distinct antigenic sites in the Gc subunit. These sites were further delineated through mutagenesis and mapped onto a prefusion model of Gc. Pairwise screening identified combinations of non-competing nAbs that afford synergistic neutralization. Further enhancements in neutralization breadth and potency were attained by physically linking variable domains of synergistic nAb pairs through bispecific antibody (bsAb) engineering. Although multiple nAbs protected mice from lethal CCHFV challenge in pre- or post-exposure prophylactic settings, only a single bsAb, DVD-121-801, afforded therapeutic protection. DVD-121-801 is a promising candidate suitable for clinical development as a CCHFV therapeutic.

Immunization-Elicited Broadly Protective Antibody Reveals Ebolavirus Fusion Loop as a Site of Vulnerability.

While neutralizing antibodies are highly effective against ebolavirus infections, current experimental ebolavirus vaccines primarily elicit species-specific antibody responses. Here, we describe an immunization-elicited macaque antibody (CA45) that clamps the internal fusion loop with the N terminus of the ebolavirus glycoproteins (GPs) and potently neutralizes Ebola, Sudan, Bundibugyo, and Reston viruses. CA45, alone or in combination with an antibody that blocks receptor binding, provided full protection against all pathogenic ebolaviruses in mice, guinea pigs, and ferrets. Analysis of memory B cells from the immunized macaque suggests that elicitation of broadly neutralizing antibodies (bNAbs) for ebolaviruses is possible but difficult, potentially due to the rarity of bNAb clones and their precursors. Unexpectedly, germline-reverted CA45, while exhibiting negligible binding to full-length GP, bound a proteolytically remodeled GP with picomolar affinity, suggesting that engineered ebolavirus vaccines could trigger rare bNAb precursors more robustly. These findings have important implications for developing pan-ebolavirus vaccine and immunotherapeutic cocktails.

Design and characterization of protective pan-ebolavirus and pan-filovirus bispecific antibodies.

Monoclonal antibodies (mAbs) are an important class of antiviral therapeutics. MAbs are highly selective, well tolerated, and have long in vivo half-life as well as the capacity to induce immune-mediated virus clearance. Their activities can be further enhanced by integration of their variable fragments (Fvs) into bispecific antibodies (bsAbs), affording simultaneous targeting of multiple epitopes to improve potency and breadth and/or to mitigate against viral escape by a single mutation. Here, we explore a bsAb strategy for generation of pan-ebolavirus and pan-filovirus immunotherapeutics. Filoviruses, including Ebola virus (EBOV), Sudan virus (SUDV), and Marburg virus (MARV), cause severe hemorrhagic fever. Although there are two FDA-approved mAb therapies for EBOV infection, these do not extend to other filoviruses. Here, we combine Fvs from broad ebolavirus mAbs to generate novel pan-ebolavirus bsAbs that are potently neutralizing, confer protection in mice, and are resistant to viral escape. Moreover, we combine Fvs from pan-ebolavirus mAbs with those of protective MARV mAbs to generate pan-filovirus protective bsAbs. These results provide guidelines for broad antiviral bsAb design and generate new immunotherapeutic candidates.

Protocadherin-1 is essential for cell entry by New World hantaviruses.

The zoonotic transmission of hantaviruses from their rodent hosts to humans in North and South America is associated with a severe and frequently fatal respiratory disease, hantavirus pulmonary syndrome (HPS)1,2. No specific antiviral treatments for HPS are available, and no molecular determinants of in vivo susceptibility to hantavirus infection and HPS are known. Here we identify the human asthma-associated gene protocadherin-1 (PCDH1)3-6 as an essential determinant of entry and infection in pulmonary endothelial cells by two hantaviruses that cause HPS, Andes virus (ANDV) and Sin Nombre virus (SNV). In vitro, we show that the surface glycoproteins of ANDV and SNV directly recognize the outermost extracellular repeat domain of PCDH1-a member of the cadherin superfamily7,8-to exploit PCDH1 for entry. In vivo, genetic ablation of PCDH1 renders Syrian golden hamsters highly resistant to a usually lethal ANDV challenge. Targeting PCDH1 could provide strategies to reduce infection and disease caused by New World hantaviruses.

Long-term Sudan virus Ebola survivors maintain multiple antiviral defense mechanisms.

BACKGROUND: The critical issues of sustained memory immunity following ebolavirus disease among long-term survivors (EVD) are still unclear. METHODS: Here, we examine virus-specific immune and inflammatory responses in 12 Sudan virus (SUDV) long-term survivors from Uganda's 2000-1 Gulu outbreak, 15 years after recovery following in vitro challenge. Total RNA from isolated SUDV-stimulated and unstimulated PBMCs was extracted and analyzed. Matched serum samples were also collected to determine SUDV IgG levels and functionality. RESULTS: We detected persistent humoral (58%, 7 of 12) and cellular (33%, 4 of 12) immune responses in SUDV long-term survivors and identified critical molecular mechanisms of innate and adaptive immunity. Gene expression in immune pathways, the IFN signaling system, antiviral defense response, and activation and regulation of T- and B-cell responses were observed. SUDV long-term survivors also maintained robust virus-specific IgG antibodies capable of polyfunctional responses, including neutralizing and innate Fc effector functions. CONCLUSIONS: Data integration identified significant correlations among humoral and cellular immune responses and pinpointed a specific innate and adaptive gene expression signature associated with long-lasting immunity. This could help identify natural and vaccine correlates of protection against ebolavirus disease.

Prophylactic Administration of the Monoclonal Antibody Adintrevimab Protects against SARS-CoV-2 in Hamster and Non-Human Primate Models of COVID-19.

Adintrevimab is a human immunoglobulin G1 monoclonal antibody engineered to have broad neutralization against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants and other SARS-like coronaviruses with pandemic potential. In both Syrian golden hamster and rhesus macaque models, prophylactic administration of a single dose of adintrevimab provided protection against SARS-CoV-2/WA1/2020 infection in a dose-dependent manner, as measured by significant reductions in lung viral load and virus-induced lung pathology, and by inhibition of viral replication in the upper and lower respiratory tract.

Development of an antibody cocktail for treatment of Sudan virus infection.

Antibody-based therapies are a promising treatment option for managing ebolavirus infections. Several Ebola virus (EBOV)-specific and, more recently, pan-ebolavirus antibody cocktails have been described. Here, we report the development and assessment of a Sudan virus (SUDV)-specific antibody cocktail. We produced a panel of SUDV glycoprotein (GP)-specific human chimeric monoclonal antibodies (mAbs) using both plant and mammalian expression systems and completed head-to-head in vitro and in vivo evaluations. Neutralizing activity, competitive binding groups, and epitope specificity of SUDV mAbs were defined before assessing protective efficacy of individual mAbs using a mouse model of SUDV infection. Of the mAbs tested, GP base-binding mAbs were more potent neutralizers and more protective than glycan cap- or mucin-like domain-binding mAbs. No significant difference was observed between plant and mammalian mAbs in any of our in vitro or in vivo evaluations. Based on in vitro and rodent testing, a combination of two SUDV-specific mAbs, one base binding (16F6) and one glycan cap binding (X10H2), was down-selected for assessment in a macaque model of SUDV infection. This cocktail, RIID F6-H2, provided protection from SUDV infection in rhesus macaques when administered at 50 mg/kg on days 4 and 6 postinfection. RIID F6-H2 is an effective postexposure SUDV therapy and provides a potential treatment option for managing human SUDV infection.

Monoclonal antibody therapy for Junin virus infection.

Countermeasures against potential biothreat agents remain important to US Homeland Security, and many of these pharmaceuticals could have dual use in the improvement of global public health. Junin virus, the causative agent of Argentine hemorrhagic fever (AHF), is an arenavirus identified as a category A high-priority agent. There are no Food and Drug Administration (FDA) approved drugs available for preventing or treating AHF, and the current treatment option is limited to administration of immune plasma. Whereas immune plasma demonstrates the feasibility of passive immunotherapy, it is limited in quantity, variable in quality, and poses safety risks such as transmission of transfusion-borne diseases. In an effort to develop a monoclonal antibody (mAb)-based alternative to plasma, three previously described neutralizing murine mAbs were expressed as mouse-human chimeric antibodies and evaluated in the guinea pig model of AHF. These mAbs provided 100% protection against lethal challenge when administered 2 d after infection (dpi), and one of them (J199) was capable of providing 100% protection when treatment was initiated 6 dpi and 92% protection when initiated 7 dpi. The efficacy of J199 is superior to that previously described for all other evaluated drugs, and its high potency suggests that mAbs like J199 offer an economical alternative to immune plasma and an effective dual use (bioterrorism/public health) therapeutic.

Ebola virus entry requires the cholesterol transporter Niemann-Pick C1.

Infections by the Ebola and Marburg filoviruses cause a rapidly fatal haemorrhagic fever in humans for which no approved antivirals are available. Filovirus entry is mediated by the viral spike glycoprotein (GP), which attaches viral particles to the cell surface, delivers them to endosomes and catalyses fusion between viral and endosomal membranes. Additional host factors in the endosomal compartment are probably required for viral membrane fusion; however, despite considerable efforts, these critical host factors have defied molecular identification. Here we describe a genome-wide haploid genetic screen in human cells to identify host factors required for Ebola virus entry. Our screen uncovered 67 mutations disrupting all six members of the homotypic fusion and vacuole protein-sorting (HOPS) multisubunit tethering complex, which is involved in the fusion of endosomes to lysosomes, and 39 independent mutations that disrupt the endo/lysosomal cholesterol transporter protein Niemann-Pick C1 (NPC1). Cells defective for the HOPS complex or NPC1 function, including primary fibroblasts derived from human Niemann-Pick type C1 disease patients, are resistant to infection by Ebola virus and Marburg virus, but remain fully susceptible to a suite of unrelated viruses. We show that membrane fusion mediated by filovirus glycoproteins and viral escape from the vesicular compartment require the NPC1 protein, independent of its known function in cholesterol transport. Our findings uncover unique features of the entry pathway used by filoviruses and indicate potential antiviral strategies to combat these deadly agents.

Ebola virus entry requires the host-programmed recognition of an intracellular receptor.

Ebola and Marburg filoviruses cause deadly outbreaks of haemorrhagic fever. Despite considerable efforts, no essential cellular receptors for filovirus entry have been identified. We showed previously that Niemann-Pick C1 (NPC1), a lysosomal cholesterol transporter, is required for filovirus entry. Here, we demonstrate that NPC1 is a critical filovirus receptor. Human NPC1 fulfills a cardinal property of viral receptors: it confers susceptibility to filovirus infection when expressed in non-permissive reptilian cells. The second luminal domain of NPC1 binds directly and specifically to the viral glycoprotein, GP, and a synthetic single-pass membrane protein containing this domain has viral receptor activity. Purified NPC1 binds only to a cleaved form of GP that is generated within cells during entry, and only viruses containing cleaved GP can utilize a receptor retargeted to the cell surface. Our findings support a model in which GP cleavage by endosomal cysteine proteases unmasks the binding site for NPC1, and GP-NPC1 engagement within lysosomes promotes a late step in entry proximal to viral escape into the host cytoplasm. NPC1 is the first known viral receptor that recognizes its ligand within an intracellular compartment and not at the plasma membrane.

Postexposure antibody prophylaxis protects nonhuman primates from filovirus disease.

Antibody therapies to prevent or limit filovirus infections have received modest interest in recent years, in part because of early negative experimental evidence. We have overcome the limitations of this approach, leveraging the use of antibody from nonhuman primates (NHPs) that survived challenge to filoviruses under controlled conditions. By using concentrated, polyclonal IgG antibody from these survivors, we treated filovirus-infected NHPs with multiple doses administered over the clinical phase of disease. In the first study, Marburg virus (MARV)-infected NHPs were treated 15 to 30 min postexposure with virus-specific IgG, with additional treatments on days 4 and 8 postexposure. The postexposure IgG treatment was completely protective, with no signs of disease or detectable viremia. MARV-specific IgM antibody responses were generated, and all macaques survived rechallenge with MARV, suggesting that they generated an immune response to virus replication. In the next set of studies, NHPs were infected with MARV or Ebola virus (EBOV), and treatments were delayed 48 h, with additional treatments on days 4 and 8 postexposure. The delayed treatments protected both MARV- and EBOV-challenged NHPs. In both studies, two of the three IgG-treated NHPs had no clinical signs of illness, with the third NHP developing mild and delayed signs of disease followed by full recovery. These studies clearly demonstrate that postexposure antibody treatments can protect NHPs and open avenues for filovirus therapies for human use using established Food and Drug Administration-approved polyclonal or monoclonal antibody technologies.

Venezuelan equine encephalitis virus replicon particle vaccine protects nonhuman primates from intramuscular and aerosol challenge with ebolavirus.

There are no vaccines or therapeutics currently approved for the prevention or treatment of ebolavirus infection. Previously, a replicon vaccine based on Venezuelan equine encephalitis virus (VEEV) demonstrated protective efficacy against Marburg virus in nonhuman primates. Here, we report the protective efficacy of Sudan virus (SUDV)- and Ebola virus (EBOV)-specific VEEV replicon particle (VRP) vaccines in nonhuman primates. VRP vaccines were developed to express the glycoprotein (GP) of either SUDV or EBOV. A single intramuscular vaccination of cynomolgus macaques with VRP expressing SUDV GP provided complete protection against intramuscular challenge with SUDV. Vaccination against SUDV and subsequent survival of SUDV challenge did not fully protect cynomolgus macaques against intramuscular EBOV back-challenge. However, a single simultaneous intramuscular vaccination with VRP expressing SUDV GP combined with VRP expressing EBOV GP did provide complete protection against intramuscular challenge with either SUDV or EBOV in cynomolgus macaques. Finally, intramuscular vaccination with VRP expressing SUDV GP completely protected cynomolgus macaques when challenged with aerosolized SUDV, although complete protection against aerosol challenge required two vaccinations with this vaccine.

Profile and persistence of the virus-specific neutralizing humoral immune response in human survivors of Sudan ebolavirus (Gulu).

To better understand humoral immunity following ebolavirus infection, a serological study of the humoral immune response against the individual viral proteins of Sudan ebolavirus (Gulu) in human survivors was performed. An enzyme-linked immunosorbent assay specific for full-length recombinant viral proteins NP, VP30, VP40, and GP1-649 (GP lacking the transmembrane domain) of Sudan ebolavirus (Gulu) was used as well as a plaque reduction neutralization test. Serum samples from human survivors, which were collected up to 10 years following recovery, were screened and analyzed. Results demonstrate that samples obtained 10 years following infection contain virus-specific antibodies that can neutralize virus. Neutralization correlates well with immunoreactivity against the viral proteins NP, VP30, and GP1-649. Sera from individuals who died or those with no documented infection but immunoreactive to ebolavirus did not neutralize. This work provides insight into the duration, profile of immunoreactivity, and neutralization capacity of the humoral immune response in ebolavirus survivors.

Engineering, structure, and immunogenicity of a Crimean-Congo hemorrhagic fever virus pre-fusion heterotrimeric glycoprotein complex.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne virus that can cause severe disease in humans with case fatality rates of 10-40%. Although structures of CCHFV glycoproteins GP38 and Gc have provided insights into viral entry and defined epitopes of neutralizing and protective antibodies, the structure of glycoprotein Gn and its interactions with GP38 and Gc have remained elusive. Here, we used structure-guided protein engineering to produce a stabilized GP38-Gn-Gc heterotrimeric glycoprotein complex (GP38-GnH-DS-Gc). A cryo-EM structure of this complex provides the molecular basis for GP38's association on the viral surface, reveals the structure of Gn, and demonstrates that GP38-Gn restrains the Gc fusion loops in the prefusion conformation, facilitated by an N-linked glycan attached to Gn. Immunization with GP38-GnH-DS-Gc conferred 40% protection against lethal IbAr10200 challenge in mice. These data define the architecture of a GP38-Gn-Gc protomer and provide a template for structure-guided vaccine antigen development.

Antibodies targeting Crimean-Congo hemorrhagic fever virus GP38 limit vascular leak and viral spread.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a priority pathogen transmitted by tick bites, with no vaccines or specific therapeutics approved to date. Severe disease manifestations include hemorrhage, endothelial dysfunction, and multiorgan failure. Infected cells secrete the viral glycoprotein GP38, whose extracellular function is presently unknown. GP38 is considered an important target for vaccine and therapeutic design as GP38-specific antibodies can protect against severe disease in animal models, albeit through a currently unknown mechanism of action. Here, we show that GP38 induces endothelial barrier dysfunction in vitro, and that CCHFV infection, and GP38 alone, can trigger vascular leak in a mouse model. Protective antibodies that recognize specific antigenic sites on GP38, but not a protective neutralizing antibody binding the structural protein Gc, potently inhibit endothelial hyperpermeability in vitro and vascular leak in vivo during CCHFV infection. This work uncovers a function of the secreted viral protein GP38 as a viral toxin in CCHFV pathogenesis and elucidates the mode of action of non-neutralizing GP38-specific antibodies.

Crimean-Congo Hemorrhagic Fever Survivors Elicit Protective Non-Neutralizing Antibodies that Target 11 Overlapping Regions on Viral Glycoprotein GP38.

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, unique to Nairoviridae, is a target of protective antibodies, but extensive mapping of the human antibody response to GP38 has not been previously performed. Here, we isolated 188 GP38-specific antibodies from human survivors of infection. Competition experiments showed that these antibodies bind across five distinct antigenic sites, encompassing eleven overlapping regions. Additionally, we reveal structures of GP38 bound with nine of these antibodies targeting different antigenic sites. Although GP38-specific antibodies were non-neutralizing, several antibodies were found to have protection equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and inform the development of broadly effective CCHFV antibody therapeutics.

Structure of an antibody in complex with its mucin domain linear epitope that is protective against Ebola virus.

Antibody 14G7 is protective against lethal Ebola virus challenge and recognizes a distinct linear epitope in the prominent mucin-like domain of the Ebola virus glycoprotein GP. The structure of 14G7 in complex with its linear peptide epitope has now been determined to 2.8 Å. The structure shows that this GP sequence forms a tandem ß-hairpin structure that binds deeply into a cleft in the antibody-combining site. A key threonine at the apex of one turn is critical for antibody interaction and is conserved among all Ebola viruses. This work provides further insight into the mechanism of protection by antibodies that target the protruding, highly accessible mucin-like domain of Ebola virus and the structural framework for understanding and characterizing candidate immunotherapeutics.

Two point mutations in protocadherin-1 disrupt hantavirus recognition and afford protection against lethal infection.

Andes virus (ANDV) and Sin Nombre virus (SNV) are the etiologic agents of severe hantavirus cardiopulmonary syndrome (HCPS) in the Americas for which no FDA-approved countermeasures are available. Protocadherin-1 (PCDH1), a cadherin-superfamily protein recently identified as a critical host factor for ANDV and SNV, represents a new antiviral target; however, its precise role remains to be elucidated. Here, we use computational and experimental approaches to delineate the binding surface of the hantavirus glycoprotein complex on PCDH1's first extracellular cadherin repeat domain. Strikingly, a single amino acid residue in this PCDH1 surface influences the host species-specificity of SNV glycoprotein-PCDH1 interaction and cell entry. Mutation of this and a neighboring residue substantially protects Syrian hamsters from pulmonary disease and death caused by ANDV. We conclude that PCDH1 is a bona fide entry receptor for ANDV and SNV whose direct interaction with hantavirus glycoproteins could be targeted to develop new interventions against HCPS.

The SARS-CoV-2 monoclonal antibody combination, AZD7442, is protective in nonhuman primates and has an extended half-life in humans.

Despite the success of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines, there remains a need for more prevention and treatment options for individuals remaining at risk of coronavirus disease 2019 (COVID-19). Monoclonal antibodies (mAbs) against the viral spike protein have potential to both prevent and treat COVID-19 and reduce the risk of severe disease and death. Here, we describe AZD7442, a combination of two mAbs, AZD8895 (tixagevimab) and AZD1061 (cilgavimab), that simultaneously bind to distinct, nonoverlapping epitopes on the spike protein receptor binding domain to neutralize SARS-CoV-2. Initially isolated from individuals with prior SARS-CoV-2 infection, the two mAbs were designed to extend their half-lives and reduce effector functions. The AZD7442 mAbs individually prevent the spike protein from binding to angiotensin-converting enzyme 2 receptor, blocking virus cell entry, and neutralize all tested SARS-CoV-2 variants of concern. In a nonhuman primate model of SARS-CoV-2 infection, prophylactic AZD7442 administration prevented infection, whereas therapeutic administration accelerated virus clearance from the lung. In an ongoing phase 1 study in healthy participants (NCT04507256), a 300-mg intramuscular injection of AZD7442 provided SARS-CoV-2 serum geometric mean neutralizing titers greater than 10-fold above those of convalescent serum for at least 3 months, which remained threefold above those of convalescent serum at 9 months after AZD7442 administration. About 1 to 2% of serum AZD7442 was detected in nasal mucosa, a site of SARS-CoV-2 infection. Extrapolation of the time course of serum AZD7442 concentration suggests AZD7442 may provide up to 12 months of protection and benefit individuals at high-risk of COVID-19.