Epitope mapping studies with neutralizing and non-neutralizing monoclonal antibodies to the G1 and G2 envelope glycoproteins of Hantaan virus

Extensive genetic diversity of severe fever with thrombocytopenia syndrome virus circulating in Hubei Province, China, 2018-2022

Severe fever with thrombocytopenia syndrome virus (SFTSV), an etiological agent causing febrile human disease was identified as an emerging tick-borne bunyavirus. The clinical disease characteristics and case fatality rates of SFTSV may vary across distinct regions and among different variant genotypes. From 2018 to 2022, we surveyed and recruited 202 severe fever with thrombocytopenia syndrome (SFTS) patients in Hubei Province, a high-incidence area of the epidemic, and conducted timely and systematic research on the disease characteristics, SFTSV diversity, and the correlation between virus genome variation and clinical diseases. Our study identified at least 6 genotypes of SFTSV prevalent in Hubei Province based on the analysis of the S, M, and L genome sequences of 88 virus strains. Strikingly, the dominant genotype of SFTSV was found to change during the years, indicating a dynamic shift in viral genetic diversity in the region. Phylogenetic analysis revealed the genetic exchange of Hubei SFTSV strains was relatively frequent, including 3 reassortment strains and 8 recombination strains. Despite the limited sample size, SFTSV C1 genotype may be associated with higher mortality compared to the other four genotypes, and the serum amyloid A (SAA) level, an inflammatory biomarker, was significantly elevated in these patients. Overall, our data summarize the disease characteristics of SFTSV in Hubei Province, highlight the profound changes in viral genetic diversity, and indicate the need for in-depth monitoring and exploration of the relationship between viral mutations and disease severity.

Mechanistic basis for potent neutralization of Sin Nombre hantavirus by a human monoclonal antibody

Rodent-borne hantaviruses are prevalent worldwide and upon spillover to human populations, cause severe disease for which no specific treatment is available. A potent antibody response is key for recovery from hantavirus infection. Here we study a highly neutralizing human monoclonal antibody, termed SNV-42, which was derived from a memory B cell isolated from an individual with previous Sin Nombre virus (SNV) infection. Crystallographic analysis demonstrates that SNV-42 targets the Gn subcomponent of the tetrameric (Gn-Gc)₄ glycoprotein assembly that is relevant for viral entry. Integration of our 1.8 Å structure with the (Gn-Gc)₄ ultrastructure arrangement indicates that SNV-42 targets the membrane-distal region of the virus envelope. Comparison of the SNV-42 paratope encoding variable genes with inferred germline gene segments reveals high sequence conservation, suggesting that germline-encoded antibodies inhibit SNV. Furthermore, mechanistic assays reveal that SNV-42 interferes with both receptor recognition and fusion during host-cell entry. This work provides a molecular-level blueprint for understanding the human neutralizing antibody response to hantavirus infection.

Human antibody recognizing a quaternary epitope in the Puumala virus glycoprotein provides broad protection against orthohantaviruses

The rodent-borne hantavirus Puumala virus (PUUV) and related agents cause hemorrhagic fever with renal syndrome (HFRS) in humans. Other hantaviruses, including Andes virus (ANDV) and Sin Nombre virus, cause a distinct zoonotic disease, hantavirus cardiopulmonary syndrome (HCPS). Although these infections are severe and have substantial case fatality rates, no FDA-approved hantavirus countermeasures are available. Recent work suggests that monoclonal antibodies may have therapeutic utility. We describe here the isolation of human neutralizing antibodies (nAbs) against tetrameric Gn/Gc glycoprotein spikes from PUUV-experienced donors. We define a dominant class of nAbs recognizing the "capping loop" of Gn that masks the hydrophobic fusion loops in Gc. A subset of nAbs in this class, including ADI-42898, bound Gn/Gc complexes but not Gn alone, strongly suggesting that they recognize a quaternary epitope encompassing both Gn and Gc. ADI-42898 blocked the cell entry of seven HCPS- and HFRS-associated hantaviruses, and single doses of this nAb could protect Syrian hamsters and bank voles challenged with the highly virulent HCPS-causing ANDV and HFRS-causing PUUV, respectively. ADI-42898 is a promising candidate for clinical development as a countermeasure for both HCPS and HFRS, and its mode of Gn/Gc recognition informs the development of broadly protective hantavirus vaccines.

Structural Basis for a Neutralizing Antibody Response Elicited by a Recombinant Hantaan Virus Gn Immunogen

Hantaviruses are a group of emerging pathogens capable of causing severe disease upon zoonotic transmission to humans. The mature hantavirus surface presents higher-order tetrameric assemblies of two glycoproteins, Gn and Gc, which are responsible for negotiating host cell entry and constitute key therapeutic targets. Here, we demonstrate that recombinantly derived Gn from Hantaan virus (HTNV) elicits a neutralizing antibody response (serum dilution that inhibits 50% infection [ID₅₀], 1:200 to 1:850) in an animal model. Using antigen-specific B cell sorting, we isolated monoclonal antibodies (mAbs) exhibiting neutralizing and non-neutralizing activity, termed mAb HTN-Gn1 and mAb nnHTN-Gn2, respectively. Crystallographic analysis reveals that these mAbs target spatially distinct epitopes at disparate sites of the N-terminal region of the HTNV Gn ectodomain. Epitope mapping onto a model of the higher order (Gn-Gc)₄ spike supports the immune accessibility of the mAb HTN-Gn1 epitope, a hypothesis confirmed by electron cryo-tomography of the antibody with virus-like particles. These data define natively exposed regions of the hantaviral Gn that can be targeted in immunogen design. IMPORTANCE The spillover of pathogenic hantaviruses from rodent reservoirs into the human population poses a continued threat to human health. Here, we show that a recombinant form of the Hantaan virus (HTNV) surface-displayed glycoprotein, Gn, elicits a neutralizing antibody response in rabbits. We isolated a neutralizing (HTN-Gn1) and a non-neutralizing (nnHTN-Gn2) monoclonal antibody and provide the first molecular-level insights into how the Gn glycoprotein may be targeted by the antibody-mediated immune response. These findings may guide rational vaccine design approaches focused on targeting the hantavirus glycoprotein envelope.

The surface glycoproteins of hantaviruses

Hantaviruses are rodent-borne viruses distributed worldwide, transmitted through the air and with the ability to spread from person to person. They maintain a non-symptomatic persistent infection in their rodent hosts, but their spillover to humans produces a renal or pulmonary syndrome associated with high fatality rates. Hantavirus particles are lipid-enveloped and display a characteristic surface lattice built up of tetragonal spikes composed of two glycoproteins, Gn and Gc. The pleomorphism of these particles has hindered cryo-EM efforts to obtain detailed structural information and only by using a combination of X-ray crystallography and cryo-electron tomography it was possible to build an atomic model of the surface lattice. Here we review these structural efforts and the unanticipated evolutionary relations between hantaviruses and alphaviruses highlighted by these studies.

Broad and potently neutralizing monoclonal antibodies isolated from human survivors of New World hantavirus infection

New World hantaviruses (NWHs) are endemic in North and South America and cause hantavirus cardiopulmonary syndrome (HCPS), with a case fatality rate of up to 40%. Knowledge of the natural humoral immune response to NWH infection is limited. Here, we describe human monoclonal antibodies (mAbs) isolated from individuals previously infected with Sin Nombre virus (SNV) or Andes virus (ANDV). Most SNV-reactive antibodies show broad recognition and cross-neutralization of both New and Old World hantaviruses, while many ANDV-reactive antibodies show activity for ANDV only. mAbs ANDV-44 and SNV-53 compete for binding to a distinct site on the ANDV surface glycoprotein and show potently neutralizing activity to New and Old World hantaviruses. Four mAbs show therapeutic efficacy at clinically relevant doses in hamsters. These studies reveal a convergent and potently neutralizing human antibody response to NWHs and suggest therapeutic potential for human mAbs against HCPS.

Engdahl et al. show that monoclonal antibodies isolated from human survivors of New World hantavirus infection display broad and potent neutralization across hantavirus species and recognize distinct sites on the glycoprotein spike. Multiple antibodies demonstrate potential therapeutic candidates for New World hantavirus infection. Some antibodies also neutralized Old World hantaviruses.

Molecular rationale for antibody-mediated targeting of the hantavirus fusion glycoprotein

The intricate lattice of Gn and Gc glycoprotein spike complexes on the hantavirus envelope facilitates host-cell entry and is the primary target of the neutralizing antibody-mediated immune response. Through study of a neutralizing monoclonal antibody termed mAb P-4G2, which neutralizes the zoonotic pathogen Puumala virus (PUUV), we provide a molecular-level basis for antibody-mediated targeting of the hantaviral glycoprotein lattice. Crystallographic analysis demonstrates that P-4G2 binds to a multi-domain site on PUUV Gc and may preclude fusogenic rearrangements of the glycoprotein that are required for host-cell entry. Furthermore, cryo-electron microscopy of PUUV-like particles in the presence of P-4G2 reveals a lattice-independent configuration of the Gc, demonstrating that P-4G2 perturbs the (Gn-Gc)4 lattice. This work provides a structure-based blueprint for rationalizing antibody-mediated targeting of hantaviruses.

Humoral Immunity to Hantavirus Infection

Hantaviruses are pathogens that sometimes pass from animals to humans, and they are found in parts of Europe, Asia, and North and South America. When human infection occurs, these viruses can cause kidney or lung failure, and as many as 40% of infected people die. Currently, there are no vaccines or therapeutics for hantavirus-related diseases available. A first step in developing prevention measures is determining what type of immune response is protective. Increasingly it has become clear that the induction of a type of response called a neutralizing antibody response is critical for protection from severe disease. Although virologists first described this family of viruses in the 1950s, there is limited information on what features on the surface of hantaviruses are recognized by the immune system. Here, we review the current state of knowledge of this information, which is critical for the design of effective therapeutics and vaccines.

Hantaviruses are zoonotic pathogens found in parts of Europe, Asia, South America, and North America, which can cause renal and respiratory failure with fatality rates up to 40%. There are currently no FDA-approved vaccines or therapeutics for hantavirus-related diseases; however, it is evident that a robust neutralizing antibody response is critical for protection from severe disease. Although virologists first described this family of viruses in the 1950s, there is limited information on the neutralizing epitopes that exist on the hantavirus antigenic glycoproteins, Gn and Gc, and sites important for the design of effective therapeutics and vaccines. We provide a thorough summary of the hantavirus field from an immunological perspective. In particular, we discuss our current structural knowledge of antigenic proteins Gn and Gc, identification of B cell neutralizing epitopes, previously isolated monoclonal antibodies and their cross-reactivity between different hantavirus strains, and current developments toward vaccines and therapeutics. We conclude with some outstanding questions in the field and emphasize the need for additional studies of the human antibody response to hantavirus infection.

IMPORTANCE Hantaviruses are pathogens that sometimes pass from animals to humans, and they are found in parts of Europe, Asia, and North and South America. When human infection occurs, these viruses can cause kidney or lung failure, and as many as 40% of infected people die. Currently, there are no vaccines or therapeutics for hantavirus-related diseases available. A first step in developing prevention measures is determining what type of immune response is protective. Increasingly it has become clear that the induction of a type of response called a neutralizing antibody response is critical for protection from severe disease. Although virologists first described this family of viruses in the 1950s, there is limited information on what features on the surface of hantaviruses are recognized by the immune system. Here, we review the current state of knowledge of this information, which is critical for the design of effective therapeutics and vaccines.

Anti-HFRS Human IgG Produced in Transchromosomic Bovines Has Potent Hantavirus Neutralizing Activity and Is Protective in Animal Models

We explored an emerging technology to produce anti-Hantaan virus (HTNV) and anti-Puumala virus (PUUV) neutralizing antibodies for use as pre- or post-exposure prophylactics. The technology involves hyperimmunization of transchomosomic bovines (TcB) engineered to express human polyclonal IgG antibodies with HTNV and PUUV DNA vaccines encoding G_nG_c glycoproteins. For the anti-HTNV product, TcB was hyperimmunized with HTNV DNA plus adjuvant or HTNV DNA formulated using lipid nanoparticles (LNP). The LNP-formulated vaccine yielded fivefold higher neutralizing antibody titers using 10-fold less DNA. Human IgG purified from the LNP-formulated animal (SAB-159), had anti-HTNV neutralizing antibody titers >100,000. SAB-159 was capable of neutralizing pseudovirions with monoclonal antibody escape mutations in G_n and G_c demonstrating neutralization escape resistance. SAB-159 protected hamsters from HTNV infection when administered pre- or post-exposure, and limited HTNV infection in a marmoset model. An LNP-formulated PUUV DNA vaccine generated purified anti-PUUV IgG, SAB-159P, with a neutralizing antibody titer >600,000. As little as 0.33 mg/kg of SAB-159P protected hamsters against PUUV infection for pre-exposure and 10 mg/kg SAB-159P protected PUUV-infected hamsters post-exposure. These data demonstrate that DNA vaccines combined with the TcB-based manufacturing platform can be used to rapidly produce potent, human, polyclonal, escape-resistant anti-HTNV, and anti-PUUV neutralizing antibodies that are protective in animal models.

Neutralizing Monoclonal Antibodies against the Gn and the Gc of the Andes Virus Glycoprotein Spike Complex Protect from Virus Challenge in a Preclinical Hamster Model

Hantaviruses are the etiological agent of hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). The latter is associated with case fatality rates ranging from 30% to 50%. HCPS cases are rare, with approximately 300 recorded annually in the Americas. Recently, an HCPS outbreak of unprecedented size has been occurring in and around Epuyén, in the southwestern Argentinian state of Chubut. Since November of 2018, at least 29 cases have been laboratory confirmed, and human-to-human transmission is suspected. Despite posing a significant threat to public health, no treatment or vaccine is available for hantaviral disease. Here, we describe an effort to identify, characterize, and develop neutralizing and protective antibodies against the glycoprotein complex (Gn and Gc) of Andes virus (ANDV), the causative agent of the Epuyén outbreak. Using murine hybridoma technology, we generated 19 distinct monoclonal antibodies (MAbs) against ANDV GnGc. When tested for neutralization against a recombinant vesicular stomatitis virus expressing the Andes glycoprotein (GP) (VSV-ANDV), 12 MAbs showed potent neutralization and 8 showed activity in an antibody-dependent cellular cytotoxicity reporter assay. Escape mutant analysis revealed that neutralizing MAbs targeted both the Gn and the Gc. Four MAbs that bound different epitopes were selected for preclinical studies and were found to be 100% protective against lethality in a Syrian hamster model of ANDV infection. These data suggest the existence of a wide array of neutralizing antibody epitopes on hantavirus GnGc with unique properties and mechanisms of action.IMPORTANCE Infections with New World hantaviruses are associated with high case fatality rates, and no specific vaccine or treatment options exist. Furthermore, the biology of the hantaviral GnGc complex, its antigenicity, and its fusion machinery are poorly understood. Protective monoclonal antibodies against GnGc have the potential to be developed into therapeutics against hantaviral disease and are also great tools to elucidate the biology of the glycoprotein complex.

Analysis of the glycoproteins of Seoul orthohantavirus strain B1 associated with fusion activity

We analyzed two virus variants (S1 and L1) from Seoul orthohantavirus strain B1. Strain B1 produces large opaque plaques when plated on Vero E6 cell monolayers. However, although the L1 variant produced the large opaque plaques common to the strain, the variant S1 produced small clear ones on Vero E6 cells. Five days after Vero E6 cells were infected with the S1 variant, polykaryons formed spontaneously. However, the cells infected with the L1 variant did not show the formation of syncytia. An analysis of the pH dependency of the cell fusion demonstrated that the L1 variant could induce cell fusion, but only at a pH that was 0.2 units lower than the pH at which the S1 variant induced it. Sequencing of the M genome segment of the two virus variants revealed amino acid substitutions at 4 positions in the Gn and Gc gene products of the S1 variant. Two of these substitutions occurred in the extracellular domain of Gn and changed the charge of the protein. Our findings suggest that these amino acid substitutions caused the S1 variant Gn protein to induce fusion at an elevated pH.

Current status of severe fever with thrombocytopenia syndrome in China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by SFTS virus (SFTSV). SFTSV is associated with a high mortality rate and has been reported in China, South Korea and Japan. SFTSV undergoes rapid changes owing to evolution, gene mutations, and reassortment between different strains of SFTSV. In this review, we summarize the recent cases and general properties of SFTS, focusing on the epidemiology, genetic diversity, clinical features, and diagnostics of SFTSV in China. From 2010 to October 2016, SFTS cases were reported in 23 provinces of China, with increased numbers yearly. Infection and death cases are mainly found in central China, where the Haemaphysalis longicornis ticks are spread. The national average mortality rate of SFTS infection was 5.3%, with higher risk to elder people. The main epidemic period was from May to July, with a peak in May. Thus, SFTS reminds a significant public health problem, and development of prophylactic vaccines and effective antiviral drugs will be highly needed.

Migration, recombination, and reassortment are involved in the evolution of severe fever with thrombocytopenia syndrome bunyavirus

Severe fever with thrombocytopenia syndrome bunyavirus (SFTSV) has been identified as the etiological agent causing severe fever with thrombocytopenia syndrome (SFTS). SFTSV was reported in recent years as a newly emerging tick-borne virus in China, Japan and South Korea and is a novel member of the genus Phlebovirus, family Bunyaviridae, which is suspected to be transmitted by the tick Haemaphysalis longicornis. The genetic diversity and evolutionary relationships between geographically distributed SFTSV strains are currently unclear. In this study we used extensive bioinformatics analyses to provide deep insight into the mechanisms of evolution and relationships among SFTSV strains. The genetic diversity of SFTSV was characterized and found to be generated through recombination and reassortment events. Further, potential correlations between the geographic distribution and migration pathways of SFTSV were subject to in-depth analysis. The potential of birds migration related to SFTSV migration were also discussed. The results of this study will facilitate better understanding of the mechanisms of evolution of SFTSV, which will be important in developing public-health interventions and strategies for SFTS disease control and prevention in endemic areas.

Crystal Structure of Glycoprotein C from a Hantavirus in the Post-fusion Conformation

Hantaviruses are important emerging human pathogens and are the causative agents of serious diseases in humans with high mortality rates. Like other members in the Bunyaviridae family their M segment encodes two glycoproteins, G_N and G_C, which are responsible for the early events of infection. Hantaviruses deliver their tripartite genome into the cytoplasm by fusion of the viral and endosomal membranes in response to the reduced pH of the endosome. Unlike phleboviruses (e.g. Rift valley fever virus), that have an icosahedral glycoprotein envelope, hantaviruses display a pleomorphic virion morphology as G_N and G_C assemble into spikes with apparent four-fold symmetry organized in a grid-like pattern on the viral membrane. Here we present the crystal structure of glycoprotein C (G_C) from Puumala virus (PUUV), a representative member of the Hantavirus genus. The crystal structure shows G_C as the membrane fusion effector of PUUV and it presents a class II membrane fusion protein fold. Furthermore, G_C was crystallized in its post-fusion trimeric conformation that until now had been observed only in Flavi- and Togaviridae family members. The PUUV G_C structure together with our functional data provides intriguing evolutionary and mechanistic insights into class II membrane fusion proteins and reveals new targets for membrane fusion inhibitors against these important pathogens.

Hantaviruses (family: Bunyaviridae) encompass pathogens responsible to serious human diseases and economic burden worldwide. Following endocytosis, these enveloped RNA viruses are directed to an endosomal compartment where a sequence of pH-dependent conformational changes of the viral envelope glycoproteins mediates the fusion between the viral and endosomal membranes. The lack of high-resolution structural information for the entry of hantaviruses impair our ability to rationalize new treatments and prevention strategies. We determined the three-dimensional structure of a glycoprotein C from Puumala virus (PUUV) using X-ray crystallography. The two structures (at pH 6.0 and 8.0) were determined to 1.8 Å and 2.3 Å resolutions, respectively. Both structures reveal a class II membrane fusion protein in its post-fusion trimeric conformation with novel structural features in the trimer assembly and stabilization. Our structures suggest that neutralizing antibodies against G_C target its conformational changes as inhibition mechanism and highlight new molecular targets for hantavirus-specific membrane fusion inhibitors. Furthermore, combined with the available structures of other class II proteins, we remodeled the evolutionary relationships between virus families encompassing these proteins.

Induction of specific humoral and cellular immune responses in a mouse model following gene fusion of HSP70C and Hantaan virus Gn and S0.7 in an adenoviral vector

Heat shock proteins (HSPs) display adjuvant functions when given as fusion proteins to enhance vaccination efficiency. To evaluate enhanced potency of Hantaan virus (HTNV) glycoprotein (GP) and nucleocapsid protein (NP) immunogenicity by heat shock protein 70 (HSP70), a recombinant adenovirus rAd-GnS0.7-pCAG-HSP70C expression vector was developed by genetically linking the HSP70 C-terminal gene (HSP70 359-610 aa, HSP70C) to the Gn and 0.7 kb fragment of the NP (aa1-274-S0.7). C57BL/6 mice were immunized with these recombinant adenoviral vectors. A series of immunological assays determined the immunogenicity of the recombinant adenoviral vectors. The results showed that rAd-GnS0.7-pCAG-HSP70C induced a stronger humoral and cellular immune response than other recombinant adenoviruses (rAd-GnS0.7-pCAG and rAd-GnS0.7) and the HFRS vaccine control. Animal protection experiments showed that rAd-GnS0.7-pCAG-HSP70C was effective at protecting C57BL/6 mice from HTNV infection. The results of the immunological experiments showed that HSP70C lead to enhanced vaccine potency, and suggested significant potential in the development of genetically engineered vaccines against HTNV.

A recombinant pseudotyped lentivirus expressing the envelope glycoprotein of hantaan virus induced protective immunity in mice

Background

Hantaviruses cause acute hemorrhagic fever with renal syndrome (HFRS). Currently, several types of inactivated HFRS vaccines are widely used, however the limited ability of these immunogen to elicit neutralizing antibodies restricts vaccine efficacy. Development of an effective vaccine to overcome this weakness is must.

Methods

In the present study, a recombinant pseudotyped lentivirus bearing the hantaan virus (HTNV) envelope glycoproteins (GP), rLV-M, was constructed. C57BL/6 mice were immunized with the rLV-M and a series of immunological assays were conducted to determine the immunogenicity of the recombinant pseudotyped lentivirus. The humoral and cell-mediated immune responses induced by rLV-M were compared with those of the inactivated HFRS vaccine.

Results

Indirect immunofluorescence assay (IFA) showed the rLV-M expressed target proteins in HEK-293cells. In mice, the rLV-M efficiently induced GP-specific humoral responses and protection against HTNV infection. Furthermore, the rLV-M induced higher neutralizing antibody titers than the inactivated HFRS vaccine control.

Conclusions

The results indicated the potential of using a pseudotyped lentivirus as a delivery vector for a hantavirus vaccine immunogen.

Discovery of severe fever with thrombocytopenia syndrome bunyavirus strains originating from intragenic recombination

This study analyzes available severe fever with thrombocytopenia syndrome virus (SFTSV) genomes and reports that a sublineage of lineage I bears a unique M segment recombined from two of three prevailing SFTSV lineages. Through recombination, the sublineage has acquired nearly complete G1 associated with protective epitopes from lineage III, suggesting that this recombination has the capacity to induce antigenic shift of the virus. Therefore, this study provides some valuable implications for the vaccine design of SFTSV.

Interactions and oligomerization of hantavirus glycoproteins

In this report the basis for the structural architecture of the envelope of hantaviruses, family Bunyaviridae, is systematically studied by the interactions of two glycoproteins N and C (Gn and Gc, respectively) and their respective disulfide bridge-mediated homo- and heteromeric oligomerizations. In virion extracts Gn and Gc associated in both homo- and hetero-oligomers which were, at least partially, thiol bridge mediated. Due to strong homo-oligomerization, the hetero-oligomers of Gn and Gc are likely to be mediated by homo-oligomeric subunits. A reversible pH-induced disappearance of a neutralizing epitope in Gc and dissociation of the Gn-Gc complex at pH values below 6.2 provide proteochemical evidence for the fusogenicity of Gc. Incomplete inactivation of virions at acidic pH indicates that additional factors are required for hantavirus fusion, as in the case of pestiviruses of the Flaviviridae. Based on similarities to class II fusion proteins, a structure model was created of hantavirus Gc using the Semliki Forest virus E1 protein as a template. In total, 10 binding regions for Gn were found by peptide scanning, of which five represent homotypic (Gn(I) to Gn(V)) and five represent heterotypic (Gc(I) to Gc(V)) interaction sites that we assign as intra- and interspike connections, respectively. In conclusion, the glycoprotein associations were compiled to a model wherein the surface of hantaviruses is formed of homotetrameric Gn complexes interconnected with Gc homodimers. This organization would create the grid-like surface pattern described earlier for hantaviruses in negatively stained electron microscopy specimens.

Hantaan/Andes virus DNA vaccine elicits a broadly cross-reactive neutralizing antibody response in nonhuman primates

At least four hantavirus species cause disease with prominent renal involvement-hemorrhagic fever with renal syndrome (HFRS); and several hantavirus strains cause disease with significant pulmonary involvement-hantavirus pulmonary syndrome (HPS). The most prevalent and lethal hantaviruses associated with HFRS and HPS are Hantaan virus (HTNV) and Andes virus (ANDV), respectively. Here, we constructed a DNA vaccine plasmid (pWRG/HA-M) that contains both the HTNV and ANDV M gene segments. Rhesus macaques vaccinated with pWRG/HA-M produced antibodies that bound the M gene products (i.e., G1 and G2 glycoproteins), and neutralized both HTNV and ANDV. Neutralizing antibody titers elicited by the dual-immunogen pWRG/HA-M, or single-immunogen plasmids expressing only the HTNV or ANDV glycoproteins, increased rapidly to high levels after a booster vaccination administered 1-2 years after the initial vaccination series. Memory responses elicited by this long-range boost exhibited an increased breadth of cross-neutralizing activity relative to the primary response. This is the first time that hantavirus M gene-based DNA vaccines have been shown to elicit a potent memory response, and to elicit antibody responses that neutralize viruses that cause both HFRS and HPS.

A pseudotype vesicular stomatitis virus containing Hantaan virus envelope glycoproteins G1 and G2 as an alternative to hantavirus vaccine in mice

We examined whether a vesicular stomatitis virus (VSV) pseudotype bearing the hantavirus envelope glycoproteins (GPs) G1 and G2 (VSVdeltaG*HTN) could be used as a safe and effective alternative to native hantavirus. Mice were immunized with purified particles of VSVdeltaG*HTN. After the second immunization, all mice produced anti-GP antibody as detected in ELISA and a neutralization test. After the third immunization, the mice were challenged with Hantaan virus. Neither anti-NP antibody production nor Hantaan virus-specific CD8 T-cell reactions were detected in these mice. The present study demonstrated the potential of using a pseudotype VSV system as a tool for developing a hantavirus vaccine.

Human and rodent humoral immune responses to Andes virus structural proteins

In the present work we identified B-cell epitopes recognized by sera of humans and rodents naturally infected with Andes virus, a hantavirus present in Chile and Argentina. Analysis of patient and rodent sera with overlapping peptides revealed 21 human and rodent epitopes on the three structural proteins. Whereas in the nucleoprotein the region comprising aa 248-260 was shown to be the key determinant of human sera, the major antigenic site of rodent antibody reactivity is located at aa 326-338. In G1, the main epitope recognized by human sera was mapped to aa 14-26, while rodent antibodies bound predominantly to aa 599-611. In contrast, humans and mice had strong responses to three regions in G2 (aa 691-703, aa 918-930, aa 955-967), of which the last two are associated with neutralization of Hantaan virus. This insight affords important information for the development of immunotherapies for the acute phase of hantavirus cardiopulmonary syndrome.

Mapping the Golgi targeting and retention signal of Bunyamwera virus glycoproteins

The membrane glycoproteins (Gn and Gc) of Bunyamwera virus (BUN; family Bunyaviridae) accumulate in the Golgi complex, where virion maturation occurs. The Golgi targeting and retention signal has previously been shown to reside within the Gn protein. A series of truncated Gn and glycoprotein precursor cDNAs were constructed by progressively deleting the coding region of the transmembrane domain (TMD) and the cytoplasmic tail. We also constructed chimeric proteins of BUN Gc, enhanced green fluorescent protein (EGFP), and human respiratory syncytial virus (HRSV) fusion (F) protein that contain the Gn TMD with various lengths of its adjacent cytoplasmic tails. The subcellular localization of mutated BUN glycoproteins and chimeric proteins was investigated by double-staining immunofluorescence with antibodies against BUN glycoproteins or the HRSV F protein and with antibodies specific for the Golgi complex. The results revealed that Gn and all truncated Gn proteins that contained the intact TMD (residues 206 to 224) were able to translocate to the Golgi complex and also rescued the Gc protein, which is retained in the endoplasmic reticulum when expressed alone, to this organelle. The rescued Gc proteins acquired endo-beta-N-acetylglucosaminidase H resistance. The Gn TMD could also target chimeric EGFP to the Golgi and retain the F protein, which is characteristically expressed on the surface of HRSV-infected cells, in the Golgi. However, chimeric BUN Gc did not translocate to the Golgi, suggesting that an interaction with Gn is involved in Golgi retention of the Gc protein. Collectively, these data demonstrate that the Golgi targeting and retention signal of BUN glycoproteins resides in the TMD of the Gn protein.

Cell fusion activities of Hantaan virus envelope glycoproteins

Hantaan virus (HTNV)-infected Vero E6 cells undergo cell fusion with both infected and uninfected cells under low-pH conditions. Flow cytometry and fluorescence microscopy of HTNV-infected Vero E6 cells showed that envelope glycoproteins (GPs) were located both on the cell surface and in the cytoplasm. Neutralizing monoclonal antibodies (MAbs) against the G1 and G2 envelope GPs inhibited cell fusion, whereas nonneutralizing MAbs against G1 or G2 and MAbs against the nucleocapsid protein (NP) did not. Transfected Vero E6 cells that expressed GPs but not those that expressed NP fused and formed syncytia. These results indicate that HTNV GPs act as fusogens at the cell surface. No fusion activity was observed either in infected Vero cells that were passaged more than 150 times or in BHK-21 cells, although GPs appeared to localize to the cell surface. This variability in fusion induction suggests the involvement of host cell factors in the process of cell membrane fusion.

Analysis of N-linked glycosylation of hantaan virus glycoproteins and the role of oligosaccharide side chains in protein folding and intracellular trafficking

The membrane glycoproteins Gn and Gc of Hantaan virus (HTNV) (family Bunyaviridae) are modified by N-linked glycosylation. The glycoproteins contain six potential sites for the attachment of N-linked oligosaccharides, five sites on Gn and one on Gc. The properties of the N-linked oligosaccharide chains were analyzed by treatment with endoglycosidase H, peptide:N-glycosidase F, tunicamycin, and deoxynojirimycin and were confirmed to be completely of the high-mannose type. Ten glycoprotein gene mutants were constructed by site-directed mutagenesis, including six single N glycosylation site mutants and four double-site mutants. We determined that four sites (N134, -235, -347, and -399) on Gn and the only site (N928) on Gc in their ectodomains are utilized, whereas the fifth site on Gn (N609), which faces the cytoplasm, is not glycosylated. The importance of individual N-oligosaccharide chains varied with respect to folding and intracellular transport. The oligosaccharide chain on residue N134 was found to be crucial for protein folding, whereas single mutations at the other glycosylation sites were better tolerated. Mutation at glycosylation sites N235 and N399 together resulted in Gn misfolding. The endoplasmic reticulum chaperones calnexin and calreticulin were found to be involved in HTNV glycoprotein folding. Our data demonstrate that N-linked glycosylation of HTNV glycoproteins plays important and differential roles in protein folding and intracellular trafficking.

Generation of an HFRS patient-derived neutralizing recombinant antibody to Hantaan virus G1 protein and definition of the neutralizing domain

Hantaan virus (HTNV) in the Hantavirus genus, family Bunyaviridae, is the major cause of severe hemorrhagic fever with renal syndrome (HFRS). We prepared a combinatorial phage display library of human Fabs to HTNV from RNA extracted from the blood lymphocytes of a convalescent HFRS patient. We selected two G1 glycoprotein-specific clones and one nucleocapsid protein (N)-specific clone from the Fab library for further studies. The human Fab antibodies were converted to IgG form in baculovirus/insect cells system by using cassette vectors that we developed earlier. Characterization of the recombinant antibodies revealed that the two G1-specific IgGs, could bind to and neutralize HTNV but not Seoul virus (SEOV). The N-specific IgG did not neutralize either HTNV or SEOV. Sequence analysis revealed that the two G1-specific clones differed by only one predicted amino acid in their complementarity determining regions, CDR3. Epitope mapping studies were carried out with one of the two G1-specific clones and synthetic peptides representing portions of HTNV G1. Results indicated that the recombinant antibody recognizes the core amino acid sequence LTKTLVIGQ, which is found near the C-terminus of HTNV G1. These results are the first to define a neutralizing epitope on the G1 protein of HTNV using an antibody derived from an HFRS patient.

Vaccines against hantaviruses

Rodent-borne hantaviruses are etiologic agents for hemorrhagic fever with renal syndrome and hantavirus cardiopulmonary syndrome in the Old World and New World, respectively. These often severe diseases are relatively uncommon in most parts of the world and are sufficiently genetically variable that widely cross-protective vaccines will probably need to be polyvalent. The current status of hantavirus vaccines shall be reviewed and both conventional and speculative new vaccine technologies that may evolve within the field shall be considered.

Hantavirus infections and their prevention

Hantaviruses are rodent-borne bunyaviruses which cause haemorrhagic fever with renal syndrome and Hantavirus pulmonary syndrome in humans. This review covers the host interactions of the viruses, including the rodent reservoirs, the clinical outcome of human infections as well as the pathogenesis and laboratory diagnosis of infections. The current stage in prophylaxis and therapy of hantaviral diseases is described and different approaches in vaccine development are discussed.

Pathogenicity of Hantaan virus in newborn mice: genetic reassortant study demonstrating that a single amino acid change in glycoprotein G1 is related to virulence

Two Hantaan virus strains, clone 1 (cl-1), which is virulent in newborn mice, and its attenuated mutant (mu11E10), were used to examine the pathogenesis of Hantaan virus infection in a mouse model and identify virus factors relating to virulence. After subcutaneous inoculation of newborn BALB/c mice, cl-1 caused fatal disease with high viral multiplication in peripheral organs, but mu11E10 produced nonfatal infection with a low level of virus multiplication. Intracerebral inoculation of either strain caused fatal disease. Histopathological changes in the dead animals were prominent in the brain, indicating that the brain is the target organ and produces the fatal outcome. These results indicate that mu11E10 has a generally less virulent phenotype, and because of decreased multiplication in peripheral tissues, neuroinvasiveness is also decreased. An experiment with genetic reassortant viruses showed that in newborn mice the M segment is the most related to virulence and the L segment is partly related. Sequence comparison detected a single deduced amino acid change (cl-1 Ile to mu11E10 Thr) at amino acid number 515 in glycoprotein G1. One nucleotide change, but no amino acid substitution, was observed in the noncoding region of the L segment. In mouse brain microvascular endothelial cells in vitro, viruses possessing a cl-1-derived M segment grew more rapidly than viruses containing a mu11E10-derived M segment. These results suggest that the single amino acid change in the glycoprotein alters peripheral growth, which affects invasion of the central nervous system in mice.

Temporal and spatial analysis of Sin Nombre virus quasispecies in naturally infected rodents

Sin Nombre virus (SNV) is thought to establish a persistent infection in its natural reservoir, the deer mouse (Peromyscus maniculatus), despite a strong host immune response. SNV-specific neutralizing antibodies were routinely detected in deer mice which maintained virus RNA in the blood and lungs. To determine whether viral diversity played a role in SNV persistence and immune escape in deer mice, we measured the prevalence of virus quasispecies in infected rodents over time in a natural setting. Mark-recapture studies provided serial blood samples from naturally infected deer mice, which were sequentially analyzed for SNV diversity. Viral RNA was detected over a period of months in these rodents in the presence of circulating antibodies specific for SNV. Nucleotide and amino acid substitutions were observed in viral clones from all time points analyzed, including changes in the immunodominant domain of glycoprotein 1 and the 3' small segment noncoding region of the genome. Viral RNA was also detected in seven different organs of sacrificed deer mice. Analysis of organ-specific viral clones revealed major disparities in the level of viral diversity between organs, specifically between the spleen (high diversity) and the lung and liver (low diversity). These results demonstrate the ability of SNV to mutate and generate quasispecies in vivo, which may have implications for viral persistence and possible escape from the host immune system.

Phage-displayed peptides mimicking the discontinuous neutralization sites of puumala Hantavirus envelope glycoproteins

We selected peptide ligands mimicking the surface structure of discontinuous binding sites of Puumala hantavirus-neutralizing monoclonal antibodies from a random 18-amino acid peptide library containing a disulfide bridge in a fixed position and displayed on a filamentous phage. The varying of selection conditions, either by shortening of the association time or by competitive elution with antigen, was crucial for the selection of peptide inserts that could be aligned with the primary sequences of the envelope glycoproteins G1 and G2. Correspondingly, when the envelope glycoprotein sequences were synthesized as overlapping peptides as spots on membrane, the same site in primary structure was found as with phage display, which corroborates the use of the two methods in mapping of conformational epitopes. Also, epitopes reactive with early-phase sera from Puumala virus infection were defined with the pepspot assay in the amino-terminal region of G1. Similarities of the selected phage clones to a monoclonal antibody-escape mutant site and to a linear early-phase epitope were found.

New York 1 and Sin Nombre viruses are serotypically distinct viruses associated with hantavirus pulmonary syndrome

New York 1 virus (NY-1) and Sin Nombre virus (SN) are associated with hantavirus pulmonary syndrome (HPS). NY-1 and SN are derived from unique mammalian hosts and geographic locations but have similar G1 and G2 surface proteins (93 and 97% identical, respectively). Focus reduction neutralization assays were used to define the serotypic relationship between NY-1 and SN. Sera from NY-1-positive Peromyscus leucopus neutralized NY-1 and SN at titers of >/=1/3,200 and 16-fold-lower neutralizing titers to NY-1 than to SN. Reference sera to Hantaan, Seoul, and Prospect Hill viruses also failed to neutralize NY-1. These results indicate that SN and NY-1 define unique hantavirus serotypes and implicate the presence of additional HPS-associated hantavirus serotypes in the Americas.

Phage-displayed peptide targeting on the Puumala hantavirus neutralization site

We have selected ligands for Puumala hantavirus, the causative agent of nephropathia epidemica, from a seven-amino-acid peptide library flanked by cysteines and displayed on a filamentous phage. To direct the selection to areas on the virus particle which are essential for infection, phages were competitively eluted with neutralizing monoclonal antibodies specific for the viral glycoproteins. The selected phage populations were specific for the same sites as the antibodies and mimicked their functions. The peptide insert, CHWMFSPWC, when displayed on the phages, completely inhibited Puumala virus infection in cell culture at the same effective concentration as the eluting antibody specific for envelope glycoprotein G2. The binding of the phage clones to the virus and inhibition of infection were not necessarily coincident; Pro-6 was critical for virus inhibition, while consensus residues Trp-2 and Phe-4 were essential for binding. The strategy described can be applied to any virus for production of molecules mimicking the effect of neutralizing antibodies.

Single amino acid substitutions in Puumala virus envelope glycoproteins G1 and G2 eliminate important neutralization epitopes

Two monoclonal antibody escape virus mutants (MARs), rescued from a human MAb to glycoprotein 2 (G2) and a bank vole monoclonal antibody (MAb) directed to glycoprotein 1 (G1) of Puumala virus, strain Sotkamo, were produced by using a combination of neutralization tests and antigen detection. The MARs and the original virus were analyzed by nucleotide sequencing and the responsible mutations were defined and characterized. The G1 mutation was found to constitute an A to T nucleotide substitution, giving raise to an aspartic acid to valine mutation at residue 272, potentially increasing the hydrophobicity of this region. The G2 mutation was found to constitute a C to T substitution, altering the residue 944 from serine into the more hydrophobic phenylalanine and resulting in secondary structure alterations. The mutation was found to be in close vicinity to a glycosylation site. Synthetic peptides covering the regions of the native virus, defined by the MARs, were produced and evaluated for reactivity with the corresponding MAb. The peptides were not recognized by the MAbs, and did not inhibit the binding of the MAbs in competition assays. Sera from mice immunized with the peptides were not able to recognize the native protein. This indicates that the epitopes are non-linear and/or glycosylated in the native state, or alternatively, that the G1 and G2 MAbs binds to regions away from the mutations.

The evolution of hantaviruses

Hantaviruses exist in most regions of the world. The many different strains identified thus far have widely divergent roles in human disease and infect a wide range of rodent hosts. The sequence data available for the genomes of these viruses allows us to study indirectly the evolutionary patterns of the hantaviruses. In this paper, we describe relationships among the M genomic segments of hantaviruses, and attempt to relate these to the evolutionary relationships of the virus' rodent hosts.

Hantaviruses: clinical, microbiologic, and epidemiologic aspects

Hantaviruses comprise a genus of the family Bunyaviridae. Bunyaviruses are enveloped viruses with a negative-sense, tripartite RNA genome. Hantaviruses are etiologic agents for two acute and severe illnesses of man, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Each hantavirus is primarily associated with a single rodent host species or genus, and is transmitted to man through accidental inhalation or ingestion of virus-contaminated rodent excreta. The distribution of hantaviruses is worldwide. HFRS is caused by infection with Hantaan, Seoul, Dobrava/Belgrade, and Puumala hantaviruses, all of which are enzootic in murid rodents of Old World origin. HPS is caused by any of several hantavirus species associated with indigenous New World rodents of the subfamily Sigmodontinae, family Muridae. HFRS and HPS have numerous common epidemiologic, clinical, and laboratory characteristics. Common features include fever, myalgia, thrombocytopenia, neutrophilia, and a profound capillary leak syndrome associated with hypotension, decreased cardiac output, and shock. Worldwide, HPS is much less common than HFRS but is associated with a higher mortality rate. Recovery from hantavirus disease is generally complete, although chronic renal insufficiency may be a rare sequel of HFRS.

Characterization of human antibody responses to four corners hantavirus infections among patients with hantavirus pulmonary syndrome

Hantavirus pulmonary syndrome (HPS) is a human disease caused by a newly identified hantavirus, which we will refer to as Four Corners virus (FCV). FCV is related most closely to Puumala virus (PUU) and to Prospect Hill virus (PHV). Twenty-five acute HPS serum samples were tested for immunoglobulin G (IgG) and IgM antibody reactivities to FCV-encoded recombinant proteins in Western blot (immunoblot) assays. All HPS serum samples contained both IgG and IgM antibodies to the FCV nucleocapsid (N) protein. FCV N antibodies cross-reacted with PUU N and PHV N proteins. A dominant FCV N epitope was mapped to the segment between amino acids 17 and 59 (QLVTARQKLKDAERAVELDPDDVNKSTLQSRRAAVSALETKLG). All HPS serum samples contained IgG antibodies to the FCV glycoprotein-1 (G1) protein, and 21 of 25 serum samples contained FCV G1 IgM antibodies. The FCV G1 antibodies did not cross-react with PUU G1 and PHV G1 proteins. The FCV G1 type-specific antibody reactivity mapped to a segment between amino acids 59 and 89 (LKIESSCNFDLHVPATTTQKYNQVDWTKKSS). One hundred twenty-eight control serum samples were tested for IgG reactivities to the FCV N and G1 proteins. Nine (7.0%) contained FCV N reactivities, 3 (2.3%) contained FCV G1 reactivities, and one (0.8%) contained both FCV N and FCV G1 reactivities. The epitopes recognized by antibodies present in control serum samples were different from the epitopes recognized by HPS antibodies, suggesting that the control antibody reactivities were unrelated to FCV infections. These reagents constitute a type-specific assay for FCV antibodies.