Serological relationships among viruses in the Hantavirus genus, family Bunyaviridae

Rodent host specificity of European hantaviruses: evidence of Puumala virus interspecific spillover

In order to investigate rodent host specificity of European hantaviruses, experimental infection of colonized and wild-trapped rodents was performed. In addition to the natural rodent reservoir, Clethrionomys glareolus, Puumala hantavirus (PUUV) could infect colonized Microtus agrestis and Lemmus sibiricus, but not Syrian hamsters or Balb/C mice. Neither C. glareolus, nor M. agrestis, could be readily infected by Tula hantavirus (TULV). Wild-trapped Apodemus flavicollis and A. agrarius, the natural reservoirs of Dobrava (DOBV) and Saaremaa (SAAV) hantaviruses, respectively, could both be infected by SAAV. NMRI mice could also be infected by SAAV, but with lower efficiency as compared to Apodemus mice. Balb/C and NMRI laboratory mice, but not C. glareolus, could be infected by DOBV. To our knowledge, this is the first time DOBV and SAAV have been shown to infect adult laboratory mice. Moreover, potential hantavirus spillover infections were investigated in wild-trapped rodents. In addition to the natural host C. glareolus, we also found M. arvalis and A. sylvaticus with a history of PUUV infection. We did not find any C. glareolus or A. sylvaticus infected with TULV, a hantavirus which is known to circulate in the same geographical regions of Belgium.

Hantaan virus antibody prevalence in rodent populations of several provinces of northeastern Thailand

We conducted a serological survey of 632 rodents from the northeast region of Thailand in order to assess the presence of Hantaan-like viruses that may be a risk to the human population. Rodents were collected from rice fields, houses and domestic gardens in five northeastern provinces and tested for IgG reacting sera to Hantaan antigen using enzyme-linked immunoassays. The overall prevalence of Hantavirus infection in rodents was 2.1% (13/632). Species that tested positive included Bandicota indica (4.3% positive within species), Rattus exulans (2.1%), R. losea (1.6%) and R. rattus (0.9%). Species such as R. exulans and R. losea are candidate hosts of unidentified Hantaan-like viruses in Thailand.

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.

Highly sensitive Taqman PCR detection of Puumala hantavirus

An increasing number of clinical cases of Hantavirus infections have been reported from various regions in Asia, Europe and North America. Hantaviruses (family Bunyaviridae, genus Hantavirus) are enveloped and possess a single-stranded trisegmented RNA genome of negative polarity. Rodents or insectivores are natural hosts of hantaviruses and transmit the virus to humans chiefly by aerosolisation. These viruses are the causative agents of haemorrhagic fever with renal and pulmonary syndromes. In the northeast of France, Puumala hantavirus causes, every year, more than 150 mild forms of haemorrhagic fever with a renal syndrome known as nephropathia epidemica. Serological tests may lack sensitivity for diagnosing early stages of infection and virus isolation is limited because it grows poorly in cell culture. Since reverse transcription (RT)-PCR amplification is an efficient method for detecting viral genomes in patient specimens, we developed an assay using a Taqman probe and compared it with the classical RT-PCR amplification. To achieve this goal, a Puumala strain was grown in Vero E6 cells and RNA extracted from the culture supernatant. We found that the semi-nested RT-PCR detected a minimal amount of 300 TCID(50) mL(-1), while the Taqman PCR allowed detection of less than 10 TCID(50) mL(-1 )and provided a quantitative analysis.

Truncated hantavirus nucleocapsid proteins for serotyping Hantaan, Seoul, and Dobrava hantavirus infections

Truncated recombinant nucleocapsid proteins (rNPs) of Hantaan virus (HTNV), Seoul virus (SEOV), and Dobrava virus (DOBV) were expressed by a baculovirus system. The truncated rNPs, which lacked 49 (rNP50) or 154 (rNP155) N-terminal amino acids of the NPs of HTNV, SEOV, and DOBV, were able to differentiate HTNV-, SEOV-, and DOBV-specific immune sera. Recombinant NP50s retained higher reactivities than rNP155s and were proven useful for enzyme-linked immunosorbent assay (ELISA). The ELISAs based on the rNP50s of HTNV, SEOV, and DOBV successfully differentiated three groups of patient sera, previously defined by neutralization tests: 17 with HTNV infection, 12 with SEOV infection, and 20 with DOBV infection. The entire rNP of Puumala virus (PUUV) distinguished PUUV infection from the other types of hantavirus infection. Serotyping with these rNP50s can be recommended as a rapid and efficient system for hantavirus diagnosis.

Cellular receptors and hantavirus pathogenesis

Hantaviruses cause two potentially lethal diseases, HPS and HFRS, and both diseases result in defects in vascular permeability and platelet function. Human beta 3 integrins confer cellular susceptibility to HPS- and HFRS-causing hantaviruses, a fact directly linking platelets, endothelial cells, and hantavirus diseases to the use of [figure: see text] cellular receptors that maintain capillary integrity and regulate platelet function. The role of vitronectin, PAI-1, uPAR, and complement cascades in hantavirus pathogenesis are unstudied but may contribute to specific disease syndromes effected by hantaviruses. The divergence of hantavirus surface glycoproteins and common beta 3-integrin usage provides further insight into the interaction of hantaviruses with cells. G1 and G2 glycoprotein variation is likely to contribute to additional interactions that determine pathogenic responses to individual viruses. beta 3-integrin usage also suggests that common elements exist on G1 or the more highly conserved G2 surface glycoprotein, which mediate viral attachment to integrins. Although there is currently no data defining the virion attachment protein, the development of antibodies that recognize the hantavirus attachment protein and block integrin interactions is of interest since it is likely to provide an additional point for therapeutic intervention and vaccine development. There are a plethora of effects that could be elicited by hantavirus regulation of cellular beta 3 integrins and their ligands that are consistent with hantavirus diseases. Since beta 3 integrins are critical adhesive receptors on platelets and endothelial cells and regulate both vascular permeability and platelet activation and adhesion, the use of these receptors by hantaviruses is likely to be fundamental to hantavirus pathogenesis. The lack of an animal model for hantavirus pathogenesis has prevented a systematic analysis of immune and cellular responses to hantavirus infections, and it impedes our ability to study protective or therapeutic approaches to hantavirus diseases. However, recent findings suggest that human beta 3 integrins within transgenic mice may provide animal models of hantavirus pathogenesis and have the potential to radically alter the ability to investigate hantavirus disease.

Identification of arboviruses and certain rodent-borne viruses: reevaluation of the paradigm

Diagnostic and epidemiologic virology laboratories have in large part traded conventional techniques of virus detection and identification for more rapid, novel, and sensitive molecular methods. By doing so, useful phenotypic characteristics are not being determined. We feel that the impact of this shift in emphasis has impaired studies of the biology of viruses. This position paper is a plea to the scientific and administrative communities to reconsider the importance of such information. We also suggest a revised paradigm for virus isolation and characterization and provide a rationale for accumulating biologic (phenotypic) information.

Expression of a human, neutralizing monoclonal antibody specific to puumala virus G2-protein in stably-transformed insect cells

We cloned the heavy- and light-chain antibody genes of a human X (humanxmouse) trioma secreting a neutralizing, IgG monoclonal antibody to the G2-protein of Puumala virus. The antibody genes were inserted separately into plasmid transfer vector pIEI-4 such that the genes were under control of the baculovirus immediate early gene promoter, IEI. Trichoplusia ni (TN) cells were co-transfected with these constructs and a selection plasmid containing a neomycin-resistance gene. Cloned transformants expressing the IgG monoclonal antibody were identified by ELISA of transfected TN cell culture supernatants. TN cell lines were established from four selected clones, of which one was chosen for detailed analysis. Specificity of the insect cell-expressed human antibody was determined by ELISA with Puumala virus-infected cell lysates and by immune-precipitation of radiolabeled Puumala virus proteins. The expressed IgG retained the ability to neutralize Puumala virus in plaque-reduction neutralization assays. Using competitive polymerase chain reaction methods, multiple copies of integrated heavy- and light-chain antibody genes were detected in the insect cell genome. The transformed insect cells were stable and continuously expressed biologically active IgG. We conclude that this methodology provides an alternative eukaryotic source for the generation of human antibodies.

Genetic diversity of hantaviruses associated with hemorrhagic fever with renal syndrome in the far east of Russia

To identify the hantaviruses causing hemorrhagic fever with renal syndrome (HFRS) in the Far East of Russia, blood samples collected from HFRS patients in 1994-1998, were examined by reverse transcription-polymerase chain reaction. In addition, 36 sera were tested by an immunofluorescence assay for antibodies against Hantaan, Seoul, Puumala, and Khabarovsk viruses, and 54 samples were tested by plaque reduction neutralization test. With both serological assays, the highest antibody titers were to Hantaan and/or Seoul viruses. Of 110 blood samples 36 were found RT-PCR positive. Phylogenetic analysis the sequences of a 256-nucleotide (nt) fragment of the hantavirus M genome segment revealed at least 3 genetically distinct hantavirus lineages. Nucleotide sequence comparison showed that two of the lineages, designated as FE and Amur (AMR), differed from one another by 15.9-21.2% and from Hantaan virus by 9.8-17.5%. The third lineage, VDV, differed from Seoul virus by 2.6-5.1%. All S segment sequences were from FE lineage, and differed from Hantaan virus by 10.7-12.6%. Thirty of the 36 (83%) analyzed sequences were found to be the FE genotype, which is very similar to that of Hantaan virus, strain 76-118. Of the remaining hantaviruses, 11% were the AMR genotype, and 6% the VDV genotype, which are genetically novel genotypes of Hantaan or Seoul viruses, respectively.

Evaluation of immunoglobulin M (IgM) and IgG enzyme immunoassays in serologic diagnosis of West Nile Virus infection

A unique urban encephalitis epidemic in Romania signaled the emergence of neurological infection due to West Nile (WN) virus as a novel public health threat in Eastern Europe and provided an opportunity to evaluate patterns of immunoglobulin G (IgG) and IgM reactivity in IgM capture and IgG enzyme-linked immunosorbent assays (ELISAs). WN virus infection was diagnosed serologically in 236 of 290 patients from whom acute serum or cerebrospinal fluid (CSF) samples were available. In 37% of serum samples and in 25% of CSF samples collected in the first week of illness, anti-WN virus IgM antibody was detected in the absence of virus-specific IgG. The switch to an IgG antibody response occurred after 4 to 5 days of illness and earlier in CSF than in serum. A specific humoral immune response was detected in the CSF before the serum in some patients for whom paired CSF and serum samples from the same day were available. IgM antibody in convalescent serum samples persisted beyond 2 months after the onset of illness in more than 50% of patients. ELISA optical density values and antibody concentrations were well correlated for both IgM and IgG immunoassays. Anti-WN virus IgM antibody in acute-phase samples did not cross-react significantly with flaviviruses in other antigenic groups.

A neutralizing recombinant human antibody Fab fragment against Puumala hantavirus

A combinatorial human antibody Fab pComb3H library, generated from splenic lymphocytes of a Puumala hantavirus (PUUV) immune individual, was selected against PUUV using the phage display technique. Panning was carried out with antigens immobilized by MAbs directed to the two PUUV envelope glycoproteins G1 and G2. Thirteen Fabs, with reactivity directed to PUUV and specifically the G2 protein, as assessed by immunofluorescence and ELISA respectively, were isolated in crude preparations. By a focus reduction neutralization test (FRNT), four of the 13 crude Fab preparations exhibited type-specific neutralization of PUUV (strain Sotkamo) with 44-54% reduction in the number of foci. After affinity purification, the four Fab clones exhibited 50% focus reduction of PUUV at concentrations below 2 microg/ml. Sequencing of the heavy and light chain complementarity determining regions (CDR) 1-3 showed that the four selected clones were identical within the antibody binding regions. In inhibition tests with the PUUV G2-specific MAbs, 4G2 and 1C9, a new epitope important for neutralization, designated as G2-a3, was defined. This epitope, overlapping partially the neutralizing epitope recognized by the human MAb 1C9, seems to be unique for the PUUV serotype since none of the Fab clones neutralized any of the other hantaviruses tested.

Murine leukemia virus pseudotypes of La Crosse and Hantaan Bunyaviruses: a system for analysis of cell tropism

We have developed a system for the preparation of La Crosse virus (LAC) and Hantaan virus (HTN) pseudotypes using a murine leukemia virus vector. After concentration, the pseudotypes were present in quantities sufficient to analyze cell tropism and neutralization. Cells resistant to LAC could not be infected with the MLV (LAC) pseudotypes, and the pseudotypes were sensitive to neutralizing monoclonal antibodies prepared against LAC glycoproteins, as well as to inhibition by a soluble form of the virus cell-attachment protein, G1. Perhaps because of lower expression of the HTN glycoproteins at the cell surface, MLV (HTN) pseudotypes were present at lower titers. However, they were also sensitive to appropriate neutralizing antibodies. This pseudotype system will be useful for analysis of the entry process of the Bunyaviridae, and for neutralization studies with some Bunyaviruses whose high virulence normally requires specialized containment facilities.

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.

Dobrava virus infection: serological diagnosis and cross-reactions to other hantaviruses

Recent data have shown that Dobrava (DOB) hantavirus is the cause of severe haemorrhagic fever with renal syndrome (HFRS) in central and eastern Europe. To determine whether serological assays need to be based on the homologous viral antigen rather than on closely related hantavirus antigens, acute and convalescent sera from patients with HFRS collected in former Yugoslavia were examined for IgM and IgG to three hantavirus antigens; DOB, Hantaan (HTN) and Puumala (PUU). Focus reduction neutralization test was included for comparison and confirmation of the enzyme-linked immunosorbent assay (ELISA) results. Although the results showed that the cross-reactivity was high between these three antigens during the acute phase of the disease, one of 155 patients serum samples reacted only in the DOB antigen-based IgM assay. The evaluation of IgG reactivities revealed that a DOB antigen-based IgG ELISA has to be used in sero-epidemiological studies; 7.1% (11/155) of the acute phase/early convalescent sera and 12.5% (2/16) of the late convalescent sera, respectively, reacted only with the homologous DOB antigen.

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.

The muskrat (Ondatra zibethicus) as a new reservoir for puumala-like hantavirus strains in Europe

We have used an indirect immunofluorescent assay (IFA) and reverse transcription-PCR (RT-PCR) to screen the sera and tissues of muskrats (Ondatra zibethica) caught in the northwest of Brandenburg and in the northeast of Saxony-Anhalt, Germany, for hantavirus infection. Kidney and/or lung tissue from 6 (3.1%, CI = 1.1-6.5%) out of 197 muskrats were found to be positive for genomic sequences of hantavirus by RT-PCR. We could also demonstrate that 14 (5%, CI = 2.9-8.7%) out of 266 muskrat's sera available for testing contained hantavirus-specific antibodies in IFA. Thus, a total of 8% of the investigated muskrat population was found to be positive for hantavirus infection by RT-PCR and IFA. None of the animals was found positive in both tests. Further analysis of the RT-PCR amplified fragments by genomic sequencing revealed sequences mostly related to the puumala (PUU) S segment sequence of the Hällnäs B1 hantavirus strain (97-99% similarity). Our data therefore demonstrate that Ondatra zibethicus serves as an additional reservoir for puumala-like hantavirus strains in Europe. The epidemiological implications of this finding for hantavirus infection in Europe and elsewhere are discussed.

Antigenic characterization of Hantaan and Seoul virus nucleocapsid proteins expressed by recombinant baculovirus: application of a truncated protein, lacking an antigenic region common to the two viruses, as a serotyping antigen

Hantaan virus (HTN) and Seoul virus (SEO) are members of the genus Hantavirus in the family Bunyaviridae and are causative agents of hemorrhagic fever with renal syndrome. The complete and truncated nucleocapsid proteins (NP) of HTN and SEO were expressed by a recombinant baculovirus system. Antigenic characterization of the NP using monoclonal antibodies (MAbs) indicated that the binding sites for the serotype-specific MAbs were located between amino acids (aa) 155 and 429. A Western blot assay indicated that the serotype-specific epitopes were conformation dependent. An indirect immunofluorescence antibody (IFA) assay with the truncated NP (aa 155 to 429) was able to distinguish convalescent-phase sera from HTN and SEO patients. However, the antibody titers with the truncated NP were lower than those with the whole NP. The truncated NP of SEO (aa 155 to 429) could be used as an enzyme-linked immunosorbent assay (ELISA) antigen, but the truncated NP from HTN lost its reactivity when used for ELISA. The IFA assay using baculovirus-expressed truncated NP as an antigen is a rapid, simple, and safe test for distinguishing between HTN and SEO infections by serotype.

Pathogenesis of puumala and other hantavirus infections

Hantaviruses are rodent/insectivore-borne negative-stranded RNA viruses which belong to the Bunyaviridae family. They do not cause any symptomatic disease in their adult carrier rodents, but in humans they are aetiologic agents of haemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS), both associated with a significant mortality. In cell culture hantaviruses do not cause cytopathic effects and the mechanisms of disease in man are not well understood. Increased capillary permeability is a central phenomenon in the pathogenesis of hantavirus infections. Although the viruses have in vivo a predilection for endothelial cells, it is presumed that inflammatory mediators of the host immune response play a significant role in the capillary leak that may produce abrupt hypotension and shock in severely ill patients. Mediators released by activated macrophages including NO and TNF-alpha are considered important. The pathogenesis of renal failure in HFRS also awaits to be resolved. This review summarises what is known about these phenomena and discusses also the molecular basis of the putative virulence factors of hantaviruses. Finally, the genetic predisposition and HLA association with severe Puumala virus infection will be discussed. Copyright 1998 John Wiley & Sons, Ltd.

Chimaeric HBV core particles carrying a defined segment of Puumala hantavirus nucleocapsid protein evoke protective immunity in an animal model

Hantaviruses are rodent-born agents which are pathogenic in humans causing haemorrhagic fever with renal syndrome or hantavirus pulmonary syndrome. To induce a protective immunity against a European hantavirus (Puumala) we constructed chimaeric hepatitis B virus (HBV) core particles carrying defined fragments of the Puumala virus nucleocapsid protein. After immunisation of bank voles, the natural host of Puumala virus, with core particles possessing an insertion of the N-terminal part of Puumala virus nucleocapsid protein, four of five animals were protected against subsequent virus challenge. The results show that the major protective region of the nucleocapsid protein is located between amino acids 1 and 45 and that chimaeric HBV core-like particles are useful carriers of foreign protective epitopes.

Naked DNA vaccines expressing the prM and E genes of Russian spring summer encephalitis virus and Central European encephalitis virus protect mice from homologous and heterologous challenge

Naked DNA vaccines expressing the prM and E genes of two tick-borne flaviviruses, Russian spring summer encephalitis (RSSE) virus and Central European encephalitis (CEE) virus were evaluated in mice. The vaccines were administered by particle bombardment of DNA-coated gold beads by Accell gene gun inoculation. Two immunizations of 0.5 to 1 microg of RSSE or CEE constructs/dose, delivered at 4-week intervals, elicited cross-reactive antibodies detectable by enzyme-linked immunosorbent assay and high-titer neutralizing antibodies to CEE virus. Cross-challenge experiments demonstrated that either vaccine induced protective immunity to homologous or heterologous RSSE or CEE virus challenge. The absence of antibody titer increases after challenge and the presence of antibodies to E and prM, but not NS1, both before and after challenge suggest that the vaccines prevented productive replication of the challenge virus. One vaccination with 0.5 microg of CEE virus DNA provided protective immunity for at least 2 months, and two vaccinations protected mice from challenge with CEE virus for at least 6 months.

Cell culture adaptation of Puumala hantavirus changes the infectivity for its natural reservoir, Clethrionomys glareolus, and leads to accumulation of mutants with altered genomic RNA S segment

This paper reports the establishment of a model for hantavirus host adaptation. Wild-type (wt) (bank vole-passaged) and Vero E6 cell-cultured variants of Puumala virus strain Kazan were analyzed for their virologic and genetic properties. The wt variant was well adapted for reproduction in bank voles but not in cell culture, while the Vero E6 strains replicated to much higher efficiency in cell culture but did not reproducibly infect bank voles. Comparison of the consensus sequences of the respective viral genomes revealed no differences in the coding region of the S gene. However, the noncoding regions of the S gene were found to be different at positions 26 and 1577. In one additional and independent adaptation experiment, all analyzed cDNA clones from the Vero E6-adapted variant were found to carry substitutions at position 1580 of the S segment, just 3 nucleotides downstream of the mutation observed in the first adaptation. No differences were found in the consensus sequences of the entire M segments from the wt and the Vero E6-adapted variants. The results indicated different impacts of the S and the M genomic segments for the adaptation process and selective advantages for the variants that carried altered noncoding sequences of the S segment. We conclude that the isolation in cell culture resulted in a phenotypically and genotypically altered hantavirus.

A colorimetric PCR-enzyme immunoassay to identify hantaviruses

BACKGROUND

Hantaviruses cause two serious human diseases: hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome. At least nine hantaviruses are known to be pathogenic for humans and numerous others, with unknown disease potential, have been detected in rodents. Assays to quickly identify specific hantaviruses would be useful both for clinical diagnosis and in risk assessment studies.

OBJECTIVES

The goal of our study was to develop and test a specific and sensitive PCR-based assay for identification and differentiation of hantaviruses.

STUDY DESIGN

We developed an assay that combined RNA-PCR amplification and colorimetric enzymatic detection to identify representative European, Asian, and north American hantaviruses. RNAs from 18 hantavirus strains of nine species were amplified in the presence of digoxigenin-dUTP by using a single pair of oligonucleotide primers and polymerase chain reaction (PCR) performed by using rTth DNA polymerase. Digoxigenin-labeled PCR products were hybridized in solution to virus type-specific biotinilated probes, captured onto streptavidin-coated microtiter plates and detected by horseradish peroxidase-labeled anti-digoxigenin antibodies and a chromogenic substrate.

RESULTS AND CONCLUSIONS

The assay correctly identified each homologous virus type tested. The detection limit of the assay was approximately 15 PFU or at least 50 copies of the viral genome. The assay is simple and strain-specific and is adaptable for automation, making it more practical than other available techniques for accurate and reliable diagnosis and typing of hantaviruses.

Serological diagnosis of hantavirus infections by an enzyme-linked immunosorbent assay based on detection of immunoglobulin G and M responses to recombinant nucleocapsid proteins of five viral serotypes

Worldwide, hantaviruses cause more than 100,000 human infections annually. Rapid and accurate methods are important both in monitoring acute infections and for epidemiological studies. We and others have shown that the amino termini of hantavirus nucleocapsid proteins (Ns) are sensitive tools for the detection of specific antibodies in hantavirus disease. Accordingly, we expressed truncated Ns (amino acids 1 to 117) in Escherichia coli from the five hantaviruses known to be pathogenic to man; Hantaan (HTN), Seoul (SEO), Dobrava (DOB), Sin Nombre (SN), and Puumala (PUU) viruses. In order to obtain pure antigens for use in an enzyme-linked immunosorbent assay (ELISA), the recombinant proteins were purified by polyhistidine-metal chelate affinity chromatography. Polyclonal animal antisera and a panel of serum specimens from hantavirus-infected individuals from Scandinavia, Slovenia, Russia, Korea, China, and the United States were used to evaluate the usefulness of the method. With both human and animal sera, it was possible to designate the antibody response into two groups: those with HTN, SEO, and DOB virus reactivity on the one hand and those with SN and PUU virus reactivity on the other. In sera from Scandinavia, European Russia, and the United States, the antibody response was directed mainly to the PUU and SN virus group. The sera from Asia reacted almost exclusively with the HTN, SEO, and DOB types of viruses. This was true for both the immunoglobulin M (IgM) and IgG antibody responses, indicating that this type of discrimination can be done during the acute phase of hantavirus infections. Both the HTN, SEO, and DOB virus and the PUU and SN virus types of antibody response patterns were found in patients from the Balkan region (Solvenia).

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.

Rapid and specific detection of Sin Nombre virus antibodies in patients with hantavirus pulmonary syndrome by a strip immunoblot assay suitable for field diagnosis

To develop a rapid antibody test for Sin Nombre hantavirus (SNV) infection for diagnosis of hantavirus pulmonary syndrome (HPS) in field settings where advanced instrumentation is not available, a strip immunoblot assay bearing four immobilized antigens for SNV and a recombinant nucleocapsid protein antigen of Seoul hantavirus (SEOV) was prepared. The SNV antigens included a full-length recombinant-expressed nucleocapsid (N) protein (rN), a recombinant-expressed G1 protein (residues 35 to 117), and synthetic peptides derived from N (residues 17 to 59) and G1 (residues 55 to 88). On the basis of the observed reactivities of hantavirus-infected patient and control sera, we determined that a positive assay requires reactivity with SNV or SEOV rN antigen and at least one other antigen. Isolated reactivity to either viral rN antigen is indeterminate, and any pattern of reactivity that does not include reactivity to an rN antigen is considered indeterminate but is unlikely to represent hantavirus infection. Fifty-eight of 59 samples from patients with acute SNV-associated HPS were positive according to these criteria, and one was initially indeterminate. Four of four samples from patients with HPS due to other hantaviruses were positive, as were most samples from patients with SEOV and Puumala virus infections. Of 192 control serum samples, 2 (1%) were positive and 2 were indeterminate. Acute SNV infection was distinguishable from remote SNV infection or infection with hantaviruses other than SNV by the presence of G1 peptide antigen reactivities in the former. The strip immunoblot assay shows promise for the detection of SNV antibodies early in the course of HPS.

Hantaviruses: a global disease problem

Hantaviruses are carried by numerous rodent species throughout the world. In 1993, a previously unknown group of hantaviruses emerged in the United States as the cause of an acute respiratory disease now termed hantavirus pulmonary syndrome (HPS). Before than, hantaviruses were known as the etiologic agents of hemorrhagic fever with renal syndrome, a disease that occurs almost entirely in the Eastern Hemisphere. Since the discovery of the HPS-causing hantaviruses, intense investigation of the ecology and epidemiology of hantaviruses has led to the discovery of many other novel hantaviruses. Their ubiquity and potential for causing severe human illness make these viruses an important public health concern; we reviewed the distribution, ecology, disease potential, and genetic spectrum.

A major antigenic domain of hantaviruses is located on the aminoproximal site of the viral nucleocapsid protein

Hantavirus nucleocapsid protein has recently been shown to be an immunodominant antigen in hemorrhagic with renal syndrome (HFRS) inducing an early and long-lasting immune response. Recombinant proteins representing various regions of the nucleocapsid proteins as well as segments of the G1 and the G2 glycoproteins of hantavirus strains CG18-20 (Puumala serotype) and Hantaan 76-118 have been expressed in E. coli. The antigenicity of these proteins was tested in enzyme immunoassays and immunoblots. These studies revealed that human IgG immune response is primarily directed against epitopes located within the amino acid residues 1 to 119 of the amino terminus of viral nucleocapsid proteins. This fragment was recognized by all HFRS patient sera tested (n = 128). The corresponding enzyme immunoassays proved to be more sensitive than the indirect immunofluorescence assays. Furthermore, the majority of bank vole monoclonal antibodies raised against Puumala virus reacted specifically with this site. A recombinant G1 protein (aa 59 to 401) derived from the CG 18-20 strain was recognized by 19 out of 20 sera from HFRS patients.

Seroepidemiology of Hantaan virus infection in Taiwan

In order to investigate the infection rate of Hantaan virus in Taiwan, a total of 6,536 human serum samples were collected from residents, selected by stratified random sampling, from 19 townships covering four different ethnic groups: Aborigines, Fukien Taiwanese, Hakka Taiwanese, and Mainland Chinese. Serum samples were screened for Hantaan virus antibodies by indirect immunofluorescence. The prototype Hantaan virus (76/118)-infected Vero E6 cells were used as the viral antigen for the antibody detection. Among 6,536 human serum samples, 403 (6.2%) samples had Hantaan virus antibodies. The seropositive rates for males and females were 6.1% and 6.2%, respectively. A higher seropositive rate was found among Aborigines on the Orchid Islets (11.5%) and Fukien Taiwanese on the Penghu Islets (11.6%), while the lowest rate was observed among Hakka Taiwanese in the south of Taiwan (2.5%). In comparing with different ethnic groups, the highest prevalence was found among Fukien Taiwanese (8.1%) and the lowest among Mainland Chinese (4.9%). Among the different geographical areas, the highest positive rate was found in western Taiwan (7.1%) and the lowest in southern Taiwan (5.4%). Hantaan virus antibodies were also detected in 22 of 548 (4.0%) rat serum samples. The highest seropositive rate was found in rat sera collected from the Orchid Islets (21.4%). None of the rat sera collected from Hsinchu, Miaoli, Changhua, Nantu, Yunlin, Chiayi, Tainan, and Penghu Counties were positive. Hantaan virus antibodies were found in rats: Rattus rattus (20%), Bandicota indica (9.0%), Rattus norvegicus (8.3%), Bandicota nemorivaga (6.3%), Rattus losea (4.2%), and Apodemus agrarius (1.6%). Hantaan virus antibodies were not detected in rat sera collected from species of Rattus coxinga, Rattus culturatus, Mus musculus, Mus caroli, Suncus murinus, and Apodemus semotus. The results show that the Hantaan or Hantaan-related virus exists and is distributed widely in both human and rats in Taiwan.

Characterization of Tula virus antigenic determinants defined by monoclonal antibodies raised against baculovirus-expressed nucleocapsid protein

Tula virus was recently discovered by RT-PCR in lung samples from European common voles (Microtus arvalis and M. rossiaemeridionalis). Since virus isolation attempts had been unsuccessful, no antigen was available for analysis or for use in immunoassays. To circumvent this, complete Tula virus nucleocapsid protein (bac-TUL-N) was expressed in recombinant baculovirus. Rodent antibody end-point titers to bac-TUL-N and to truncated N fragments indicated that the NH2-terminal region is the major antigenic target and revealed a high cross-reactivity to Puumala virus N. Immunizations with crude bac-TUL-N preparations evoked high antibody responses to native hantavirus N in Balb/c mice and six monoclonal antibodies (Mabs) were generated. Epitope mapping of the Mabs, based on a competitive assay, reactivities to truncated recombinant N fragments, and reactivity patterns to different hantavirus strains, identified five recognition sites on Tula virus N. One epitope, which was identified as specific for Tula virus, was located in a region of N which is highly variable among the hantaviruses (aa 226-293), and four epitopes were mapped to the NH2-terminal region of the protein (aa 1-61). One epitope was expressed only in Tula and Prospect Hill viruses, one epitope in Tula, Prospect Hill, Khabarovsk, and Sin Nombre viruses, while two epitopes were conserved in all examined hantaviruses carried by rodents within the subfamily Arvicolinae of the Muridae family.

Characterization of Langat virus antigenic determinants defined by monoclonal antibodies to E, NS1 and preM and identification of a protective, non-neutralizing preM-specific monoclonal antibody

Hybridomas secreting monoclonal antibodies (MAb) to the tick-borne encephalitis (TBE) group virus, Langat virus (LGTV), were prepared. Of more than 200 MAb screened, 19 antibodies, which cross-reacted with the etiologic agent of Central European encephalitis, were selected for further characterization. Of these MAb, 15 were specific for LGTV E glycoprotein, two for the NS1 protein, and three for preM protein. The two NS1-specific MAb and two of the E-specific MAb reacted with all six of the other TBE group viruses tested while the remainder of the E-specific MAb failed to recognize at least one of the viruses. None of the MAb neutralized LGTV in cell culture assays, but one of the preM-specific MAb protected weanling mice against a virulent LGTV challenge. Although protective antibodies to E and NS1 proteins of TBE viruses were reported, our data provided the first evidence for protection by a non-neutralizing antibody to the preM or M protein of any of the tick-borne flaviviruses.

Antigenic properties and diagnostic potential of puumala virus nucleocapsid protein expressed in insect cells

Puumala virus (PUU) is a member of the genus Hantavirus in the family Bunyaviridae and the causative agent of nephropathia epidemica, a European form of hemorrhagic fever with renal syndrome. Sera of nephropathia epidemica patients react specifically with PUU nucleocapsid (N) protein. In order to safely provide large quantities of antigen for diagnostic purposes, PUU Sotkamo strain N protein was expressed by using the baculovirus system in Sf9 insect cells to up to 30 to 50% of the total cellular protein. The recombinant N protein (bac-PUU-N) was solubilized with 6 M urea, dialyzed, and purified by anion-exchange liquid chromatography. In an immunoglobulin M mu-capture assay purified and unpurified bac-PUU-N antigen showed identical results compared with the results of a similar assay based on native PUU antigen grown in Vero E6 cells. An immunoglobulin G monoclonal antibody-capture assay based on unpurified bac-PUU-N also showed results identical to those of an assay with native PUU-N antigen. Moreover, a panel of monoclonal antibodies reactive with eight different epitopes showed identical reactivity patterns with both natural and bac-PUU-N antigen, while two epitopes in PUU-N expressed as a fusion protein in Escherichia coli were not recognized. Puumala hantavirus N protein expressed by the baculovirus system offers a safe and inexpensive source of specific antigen for large-scale diagnostic and seroepidemiological purposes.

Nucleotide and deduced amino acid sequences of the M and S genome segments of a Swedish Puumala virus isolate

The Swedish Puumala (PUU) virus strain Vindeln 83-L20, isolated from a bank vole trapped in 1983 near Vindeln, Västerbotten county, Sweden, was characterized by nucleotide sequence analysis. The coding region of the M segment was determined by PCR followed by direct sequencing and the entire S segment was characterized by cloning and nucleotide sequence analysis. The genomic organization was found to be very similar to that of other PUU virus strains regarding open reading frames, polypeptide sizes and potential glycosylation sites. According to phylogenetic analysis 83-L20 was found to represent a new lineage within the Puumala virus serotype in the Hantavirus genus. The M segment sequence of 83-L20 was found to be more closely related to the Finnish PUU virus strains than to strains from Central Europe or from Russia. The evolutionary origin of the S segment was not as clearly resolved since the branching points of all PUU virus strains in the phylogenetic tree were nearly the same.

Molecular linkage of hantavirus pulmonary syndrome to the white-footed mouse, Peromyscus leucopus: genetic characterization of the M genome of New York virus

The complete M segment sequences of hantaviruses amplified from tissues of a patient with hantavirus pulmonary syndrome in the northeastern United States and from white-footed mice, Peromyscus leucopus, from New York were 99% identical and differed from those of Four Corners virus by 23%. The serum of this patient failed to recognize a conserved, immunodominant epitope of the Four Corners virus G1 glycoprotein. Collectively, these findings indicate that P. leucopus harbors a genetically and antigenically distinct hantavirus that causes hantavirus pulmonary syndrome.

Complete genetic characterization and analysis of isolation of Sin Nombre virus

This study reports completion of the genetic characterization of the entire genome of Sin Nombre (SN) virus (NMH10) detected in autopsy tissues from a patient who died of hantavirus pulmonary syndrome (HPS). The large (L) genome segment was found to be 6,562 nucleotides in length and encoded a putative L polymerase that was 2,153 amino acids in length. No evidence of segment reassortment with other well-characterized hantaviruses was obtained. The sequence of the entire S, M, and L genome segments of SN virus (strain NMR11) isolated from a mouse (trapped in the residence of the patient infected with SN virus [NMH10]) by passage two times in Peromyscus maniculatus and then by five passages in E6 Vero cells was determined and compared with that of the virus detected in autopsy tissues. Only 16 nucleotide differences were detected between the virus genomes, and none of these resulted in virus protein amino acid substitutions. Determination of the exact 5'- and 3'-terminal sequences of all genome segments of SN virus and representatives of other serologic groups in the Hantavirus genus, family Bunyaviridae, showed the existence of conserved nucleotide domains that may be involved in important regulatory mechanisms, such as RNA encapsidation, polymerase binding, and control of transcription and replication.

Coding strategy of the S and M genomic segments of a hantavirus representing a new subtype of the Puumala serotype

The hantavirus strain Vranica was previously reported to have been isolated from a bank vole in Bosnia-Hercegovina and associated with the occurrence of hemorrhagic fever with renal syndrome (HRFS) in humans. The complete cDNA nucleotide sequences of the small (S) and medium (M) genomic RNA segments of this virus were determined. Major open reading frames were found in the S and M segment between nucleotide positions 43 and 1341 coding for a polypeptide of 433 amino acid residues and between nucleotide positions 41 and 3,484 coding for 1,148 amino acid residues, respectively. The analysis and the alignment of the nucleotide and the derived amino acid sequences with known sequences of other hantavirus strains demonstrate that Vranica resembles Swedish strains and represents a new virus subtype of the Puumala serotype distinct from the subtypes represented by virus strains CG18-20 and Sotkamo.

A vaccinia virus-vectored Hantaan virus vaccine protects hamsters from challenge with Hantaan and Seoul viruses but not Puumala virus

To investigate the ability of a vaccinia virus-vectored vaccine expressing the M and the S segments of Hantaan (HTN) virus (C. S. Schmaljohn, S. E. Hasty, and J. M. Dalrymple, Vaccine 10:10-13, 1992) to elicit a protective immune response against other hantaviruses, we vaccinated hamsters with the recombinant vaccine and challenged them with HTN, Seoul (SEO), or Puumala (PUU) virus. Neutralizing antibodies to HTN virus were found in all vaccinated hamsters both before and after challenge. Neutralizing antibody titers to SEO virus were present at low levels or were undetectable after two immunizations with the vaccine but were positive in all vaccinated hamsters after challenge with SEO virus and were also positive in control animals that were not challenged. Neutralizing antibodies to PUU virus were observed only in hamsters previously challenged with PUU virus. To assay for virus in the blood and tissues of the hamsters, we developed a nested reverse transcriptase (RT)-PCR with cross-reactive outer primers and serotype-specific inner primers. The RT-PCR specifically detected as little as 1 PFU of virus in serum containing high-titer neutralizing antibodies and was more sensitive than immunofluorescent antibody staining for detecting virus in lung and kidney specimens of infected hamsters. By using the RT-PCR, we found that vaccinated hamsters, challenged with HTN or SEO virus, neither were viremic nor had evidence of virus in their lungs or kidneys. In contrast, vaccinated hamsters challenged with PUU virus were viremic and had PUU virus-specific nucleic acid in their organs.

Antigenic relationships of hantavirus strains analysed by monoclonal antibodies

The antigenic relationships among 71 hantavirus strains, isolated from rodent species or humans in several geographic regions, were examined by immunofluorescence assay (IFA) using human patient sera and a panel of 22 monoclonal antibodies prepared against Hantaan, Seoul, and Puumala viruses. The study included virus strains, mainly from the former USSR, for which little or no serological data were available. Fifty-nine of the 71 isolates could be placed into five antigenic groups of hantaviruses, Hantaan (HTN), Puumala (PUU), Seoul (SEO), Prospect Hill (PH), Dobrava/Belgrade (DOB). Twelve isolates were found antigenically closely related to, but distinct from, HTN (2 strains), PUU (4 strains) and PH (6 strains), respectively. The antigenic characteristics of these 12 isolates suggested two supplementary antigenic subgroups of HTN, one of PUU, and two of PH. The two antigenic subgroups of HTN included strains isolated in the Far-East of Russia. The PUU subgroup included strains isolated in European Russia as well as strains isolated in Far-Eastern Russia. The PH group comprised two subgroups, both represented by strains isolated from M. fortis in Far-Eastern Russia. The results showed that the PUU and PH antigenic groups are more complex than previously known and that PH-like virus strains isolated in Russia are antigenically more closely related to PUU virus when compared to prototype PH virus isolated in the USA.

Genetic variation of wild Puumala viruses within the serotype, local rodent populations and individual animal

Reverse transcriptase polymerase chain reaction cloning and sequencing were used to determine the range of S gene/N protein variability in wild Puumala virus (PUU) strains and to study phylogenetic relationships between two groups of strains which originated from Finland and from European Russia. Analyses of the nucleotide and predicted amino acid sequences showed: (1) all PUU strains shared a common ancient ancestor; and (2) the more recent ancestors were different for the Finnish branch and the Russian branch of PUU strains. A cluster of amino acid substitutions in the N protein of Finnish strains was found; this cluster was located within a highly variable region of the molecule carrying B-cell epitopes (Vapalahti et al., J. Med. Virol., 1995, in press). Different levels of S gene/N protein diversity of PUU were revealed supporting the view of geographical clustering of genetic variants. Puumala virus from individual voles was found to be a complex mixture of closely related variants-quasispecies. The ratio of non-silent to silent nucleotide mutations registered in the S genes/N proteins of PUU quasispecies was 4- to 16-fold higher than that in Puumala virus strains, resulting in a more wide range of quasispecies N protein sequence diversity.

Human B-cell epitopes of Puumala virus nucleocapsid protein, the major antigen in early serological response

Puumala virus (PUU) is a member of the Hantavi rus genus in the family Bunyaviridae and the etiologic agent of nephropathia epidemica (NE), a form of haemorrhagic fever with renal syndrome (HFRS). In this study we compared the immunofluorescence patterns of NE sera and antibodies raised against recombinant PUU proteins and confirm that the nucleocapsid protein is the major target in the early IgG response of NE patients and provides the molecular basis for simple and rapid differentiation between acute illness and old immunity by granular vs. diffuse fluorescence staining in the indirect immunofluorescence test. The differential kinetics of B-cell responses to PUU nucleocapsid vs. envelope proteins was emphasized further by the endpoint titres of IgG antibodies to N, G1 and G2 proteins in NE patients. The granular fluorescence correlated with low IgG avidity in 99.8%, and diffuse fluorescence with high avidity in 100% of 617 NE sera studied. Epitope scanning with overlapping 14-mer peptides covering the whole nucleocapsid protein by a shift of 3 amino acids revealed six major antigenic epitopes recognized by sera from acute-phase NE patients. The epitopes clustered mainly in the hydrophilic regions, and two of them in a highly variable region which could probably serve as an antigen to distinguish serologically between infections of closely related hantaviruses, some apparently apathogenic, some causing lethal infections. The anti-peptide epitope pattern varied between different individuals and a collection of several pin-bound peptides was needed to be recognised by most NE sera studied.

Isolation of black creek canal virus, a new hantavirus from Sigmodon hispidus in Florida

Numerous rodents were trapped for serologic and virologic studies following the identification of a hantavirus pulmonary syndrome (HPS) case in Dade County, Florida. Cotton rats (Sigmodon hispidus) were the most frequently capture rodent and displayed the highest seroprevalence to a variety of hantavirus antigens. Hantavirus genome RNA was detected in all the seropositive cotton rats tested, using a reverse transcriptase-polymerase chain reaction (RT-PCR) assay. A virus was isolated from tissues of two seropositive cotton rats by cultivation of lung and spleen homogenates on Vero E6 cells. Nucleotide sequence information obtained by direct RT-PCR and the serologic relationships of this virus with the other hantaviruses indicate that this virus, Black Creek Canal virus, represents a new hantavirus distinct from the previously known serotypes.

Phylogenetically distinct hantavirus implicated in a case of hantavirus pulmonary syndrome in the northeastern United States

Hantavirus pulmonary syndrome (HPS) is an acute respiratory illness with high mortality. It is caused by a newly described New World hantavirus known as Four Corners virus (FCV). Nearly all cases of HPS have occurred in the western United States. The etiologic agents in those cases have been closely related to each other, based upon comparisons of their genetic sequences. We have molecularly cloned the S genomic segment of a hantavirus (Rl-1) implicated in a case on HPS in the northeastern United States. Nucleotide sequence analysis shows that the Rl-1 virus has many similarities to FCV, but is clearly distinct from the western forms of that virus. These data suggest that HPS can be caused by multiple agents that together form a distinctive evolutionary clade.

Sensitive detection of hantaviruses by biotin-streptavidin enhanced immunoassays based on bank vole monoclonal antibodies

Antigen capture enzyme-linked immunosorbent assays (ELISAs) based on detection of the viral nucleocapsid protein (N) were designed for rapid diagnosis of hantavirus infections. Several combinations of bank vole (Clethrionomys glareolus) monoclonal antibodies with various N-epitope specificities were used for the development of two double-antibody sandwich forms of ELISA; PUU virus AG-ELISA for an exclusive detection of Puumala-related viruses, and Hantavirus AG-ELISA for a more extensive detection of all serotypes of hemorrhagic fever with renal syndrome (HFRS) viruses. The biotin-streptavidin system, in combination with horseradish peroxidase, rendered the assays' sensitivities similar to or even higher than immunoblotting. Calculated detection limits for the PUU virus and the Hantavirus AG-ELISAs were 405 and 50 focus forming units or 80 and 10 infected Vero E6 cells, respectively. The assays were evaluated and found to be suitable for convenient and routine detection of hantaviruses in cell cultures and in infected animal tissue.