Andes virus M genome segment is not sufficient to confer the virulence associated with Andes virus in Syrian hamsters

Differential pathogenesis between Andes virus strains CHI-7913 and Chile-9717869in Syrian Hamsters

Hantavirus cardiopulmonary syndrome (HCPS) is a severe respiratory disease caused by orthohantaviruses in the Americas with a fatality rate as high as 35%. In South America, Andes orthohantavirus (Hantaviridae, Orthohantavirus, ANDV) is a major cause of HCPS, particularly in Chile and Argentina, where thousands of cases have been reported since the virus was discovered. Two strains of ANDV that are classically used for experimental studies of the virus are Chile-9717869, isolated from the natural reservoir, the long-tailed pygmy rice rat, and CHI-7913, an isolate from a lethal human case of HCPS. An important animal model for studying pathogenesis of HCPS is the lethal Syrian golden hamster model of ANDV infection. In this model, ANDV strain Chile-9717869 is uniformly lethal and has been used extensively for pathogenesis, vaccination, and therapeutic studies. Here we show that the CHI-7913 strain, despite having high sequence similarity with Chile-9717869, does not cause lethal disease in Syrian hamsters. CHI-7913, while being able to infect hamsters and replicate to moderate levels, showed a reduced ability to replicate within the tissues compared with Chile-9717869. Hamsters infected with CHI-7913 had reduced expression of cytokines IL-4, IL-6, and IFN-γ compared with Chile-9717869 infected animals, suggesting potentially limited immune-mediated pathology. These results demonstrate that certain ANDV strains may not be lethal in the classical Syrian hamster model of infection, and further exploration into the differences between lethal and non-lethal strains provide important insights into molecular determinants of pathogenic hantavirus infection.Importance:Andes orthohantavirus (ANDV) is a New World hantavirus that is a major cause of hantavirus cardiopulmonary syndrome (HCPS, also referred to as hantavirus pulmonary syndrome) in South America, particularly in Chile and Argentina. ANDV is one of the few hantaviruses for which there is a reliable animal model, the Syrian hamster model, which recapitulates important aspects of human disease. Here we infected hamsters with a human isolate of ANDV, CHI-7913, to assess its pathogenicity compared with the classical lethal Chile-9717869 strain. CHI-7913 had 22 amino acid differences compared with Chile-9717869, did not cause lethal disease in hamsters, and showed reduced ability to replicate in vivo Our data indicate potentially important molecular signatures for pathogenesis of ANDV infection in hamsters and may lead to insights into what drives pathogenesis of certain hantaviruses in humans.

Phylogeographic diversity and hybrid zone of Hantaan orthohantavirus collected in Gangwon Province, Republic of Korea

Background

Hantaan orthohantavirus (Hantaan virus, HTNV), harbored by Apodemus agrarius (the striped field mouse), causes hemorrhagic fever with renal syndrome (HFRS) in humans. Viral genome-based surveillance at new expansion sites to identify HFRS risks plays a critical role in tracking the infection source of orthohantavirus outbreak. In the Republic of Korea (ROK), most studies demonstrated the serological prevalence and genetic diversity of orthohantaviruses collected from HFRS patients or rodents in Gyeonggi Province. Gangwon Province is a HFRS-endemic area with a high incidence of patients and prevalence of infected rodents, ROK. However, the continued epidemiology and surveillance of orthohantavirus remain to be investigated.

Methodology/Principal findings

Whole-genome sequencing of HTNV was accomplished in small mammals collected in Gangwon Province during 2015–2018 by multiplex PCR-based next-generation sequencing. To elucidate the geographic distribution and molecular diversity of viruses, we conducted phylogenetic analyses of HTNV tripartite genomes. We inferred the hybrid zone using cline analysis to estimate the geographic contact between two different HTNV lineages in the ROK. The graph incompatibility based reassortment finder performed reassortment analysis. A total of 12 HTNV genome sequences were completely obtained from A. agrarius newly collected in Gangwon Province. The phylogenetic and cline analyses demonstrated the genetic diversity and hybrid zone of HTNV in the ROK. Genetic exchange analysis suggested the possibility of reassortments in Cheorwon-gun, a highly HFRS-endemic area.

Conclusions/Significance

The prevalence and distribution of HTNV in HFRS-endemic areas of Gangwon Province enhanced the phylogeographic map for orthohantavirus outbreak monitoring in ROK. This study revealed the hybrid zone reflecting the genetic diversity and evolutionary dynamics of HTNV circulating in Gangwon Province. The results arise awareness of rodent-borne orthohantavirus diseases for physicians in the endemic area of ROK.

The genetic and molecular epidemiological studies on small mammals derived from hemorrhagic fever with renal syndrome (HFRS)-endemic areas have consistently conducted for the public health surveillance and mitigation of orthohantavirus outbreak in the Republic of Korea (ROK). Implementing viral genome-based surveillance at new expansion sites that may identify HFRS risks is critical for tracking the location of orthohantavirus infections and diagnosing HFRS. In the present study, whole-genome sequences of Hantaan virus (HTNV) from small mammals in Gangwon Province were recovered using multiplex PCR-based next-generation sequencing during 2015–2018. In HFRS-endemic regions including Cheorwon-gun, Chuncheon-si, and Hwacheon-gun, additional HTNV genome sequences contribute to establish a high-resolution phylogeographic map for tracking emerging orthohantavirus-induced diseases. The cline analysis revealed a remarkable hybrid zone by showing spatial contact regions of HTNV at two sites (Cheorwon-gun and Hwacheon-gun) and the spatial separation and sequence divergence across genome segments of HTNV in Gangwon Province. These results demonstrate the genetic diversity and hybrid zone of HTNV circulating in Gangwon Province. These findings increase an awareness raising about HFRS in the endemic area of ROK.

Thrips as the Transmission Bottleneck for Mixed Infection of Two Orthotospoviruses

Mixed infections provide opportunities for viruses to increase genetic diversity by facilitating genomic reassortment or recombination, and they may lead to the emergence of new virus species. Mixed infections of two economically important orthotospoviruses, Tomato spotted wilt orthotospovirus (TSWV) and Impatiens necrotic spot orthotospovirus (INSV), were found in recent years, but no natural reassortants between INSV and TSWV were ever reported. The goal of this study was to establish how vector preferences and the ability to transmit INSV and TSWV influence transmission and establishment of mixed infections. Our results demonstrate that thrips prefer to oviposit on TSWV and INSV mixed-infected plants over singly infected or healthy plants, providing young nymphs with the opportunity to acquire both viruses. Conversely, we observed that thrips served as a bottleneck during transmission and favored transmission of one of the two viruses over the second one, or over transmission of both viruses simultaneously. This constraint was relaxed in plants, when transmission of TSWV and INSV occurred sequentially, demonstrating that plants serve as orthotospovirus permissive hosts, while thrips serve as a bottleneck. Viral fitness, as measured by virus replication, transmission, and competition with other viral strains, is not well studied in mixed infection. Our study looks at the success of transmission during mixed infection of orthotopoviruses, enhancing the understanding of orthotospovirus epidemiology and evolution.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

Tula orthohantavirus nucleocapsid protein is cleaved in infected cells and may sequester activated caspase-3 during persistent infection to suppress apoptosis

The family Hantaviridae mostly comprises rodent-borne segmented negative-sense RNA viruses, many of which are capable of causing devastating disease in humans. In contrast, hantavirus infection of rodent hosts results in a persistent and inapparent infection through their ability to evade immune detection and inhibit apoptosis. In this study, we used Tula hantavirus (TULV) to investigate the interplay between viral and host apoptotic responses during early, peak and persistent phases of virus infection in cell culture. Examination of early-phase TULV infection revealed that infected cells were refractory to apoptosis, as evidenced by the complete lack of cleaved caspase-3 (casp-3C) staining, whereas in non-infected bystander cells casp-3C was highly abundant. Interestingly, at later time points, casp-3C was abundant in infected cells, but the cells remained viable and able to continue shedding infectious virus, and together these observations were suggestive of a TULV-associated apoptotic block. To investigate this block, we viewed TULV-infected cells using laser scanning confocal and wide-field deconvolution microscopy, which revealed that TULV nucleocapsid protein (NP) colocalized with, and sequestered, casp-3C within cytoplasmic ultrastructures. Consistent with casp-3C colocalization, we showed for the first time that TULV NP was cleaved in cells and that TULV NP and casp-3C could be co-immunoprecipitated, suggesting that this interaction was stable and thus unlikely to be solely confined to NP binding as a substrate to the casp-3C active site. To account for these findings, we propose a novel mechanism by which TULV NP inhibits apoptosis by spatially sequestering casp-3C from its downstream apoptotic targets within the cytosol.

Unique Interferon Pathway Regulation by the Andes Virus Nucleocapsid Protein Is Conferred by Phosphorylation of Serine 386

Andes virus (ANDV) causes hantavirus pulmonary syndrome (HPS) and is the only hantavirus shown to spread person to person and cause a highly lethal HPS-like disease in Syrian hamsters. The unique ability of ANDV N protein to inhibit beta interferon (IFNβ) induction may contribute to its virulence and spread. Here we analyzed IFNβ regulation by ANDV N protein substituted with divergent residues from the nearly identical Maporal virus (MAPV) N protein. We found that MAPV N fails to inhibit IFNβ signaling and that replacing ANDV residues 252 to 296 with a hypervariable domain (HVD) from MAPV N prevents IFNβ regulation. In addition, changing ANDV residue S386 to the histidine present in MAPV N or the alanine present in other hantaviruses prevented ANDV N from regulating IFNβ induction. In contrast, replacing serine with phosphoserine-mimetic aspartic acid (S386D) in ANDV N robustly inhibited interferon regulatory factor 3 (IRF3) phosphorylation and IFNβ induction. Additionally, the MAPV N protein gained the ability to inhibit IRF3 phosphorylation and IFNβ induction when ANDV HVD and H386D replaced MAPV residues. Mass spectroscopy analysis of N protein from ANDV-infected cells revealed that S386 is phosphorylated, newly classifying ANDV N as a phosphoprotein and phosphorylated S386 as a unique determinant of IFN regulation. In this context, the finding that the ANDV HVD is required for IFN regulation by S386 but dispensable for IFN regulation by D386 suggests a role for HVD in kinase recruitment and S386 phosphorylation. These findings delineate elements within the ANDV N protein that can be targeted to attenuate ANDV and suggest targeting cellular kinases as potential ANDV therapeutics.IMPORTANCE ANDV contains virulence determinants that uniquely permit it to spread person to person and cause highly lethal HPS in immunocompetent hamsters. We discovered that ANDV S386 and an ANDV-specific hypervariable domain permit ANDV N to inhibit IFN induction and that IFN regulation is directed by phosphomimetic S386D substitutions in ANDV N. In addition, MAPV N proteins containing D386 and ANDV HVD gained the ability to inhibit IFN induction. Validating these findings, mass spectroscopy analysis revealed that S386 of ANDV N protein is uniquely phosphorylated during ANDV infection. Collectively, these findings reveal new paradigms for ANDV N protein as a phosphoprotein and IFN pathway regulator and suggest new mechanisms for hantavirus regulation of cellular kinases and signaling pathways. Our findings define novel IFN-regulating virulence determinants of ANDV, identify residues that can be modified to attenuate ANDV for vaccine development, and suggest the potential for kinase inhibitors to therapeutically restrict ANDV replication.

Innate immune responses elicited by Sin Nombre virus or type I IFN agonists protect hamsters from lethal Andes virus infections

Sin Nombre virus (SNV) and Andes virus (ANDV) cause hantavirus pulmonary syndrome (HPS) in humans. Both SNV and ANDV infect Syrian hamsters, but only ANDV causes lethal disease. A co-infection study was performed to determine which virus, SNV or ANDV, would dominate the survival outcome in hamsters. Infection of hamsters with SNV 1 day before ANDV challenge did not result in disease characteristic of the latter virus, and all animals survived challenge. Control animals infected solely with ANDV all succumbed by day 14. In contrast, when viruses were injected at the same site concurrently, all hamsters succumbed to HPS disease. Hantaviruses are segmented viruses; therefore we investigated which segment might be responsible for the protective phenotype of SNV by using two SNV/ANDV reassortant viruses, both with reciprocal M-segments from the other virus (denoted ASA and SAS). Both reassortants asymptomatically infect hamsters, similar to SNV. However, unlike SNV, 1 day prior preinfection with the reassortant virus did not prevent ANDV lethality. The ASA reassortant virus, but not SAS, protected hamsters from lethal ANDV infection when administered 3 days prior to ANDV challenge. Similar to SNV preinfection, the potent innate immune stimulator poly I:C administered to hamsters 1 day before ANDV challenge prevented lethal ANDV disease. Combined, these results suggest that the difference in pathogenicity of SNV and ANDV in hamsters involves differences in early host-pathogen interactions and resultant anti-viral immune responses of both the innate and adaptive immune system.

Reassortment events in the evolution of hantaviruses

Hantaviruses (order Bunyavirales, family Hantaviridae), known as important zoonotic human pathogens, possess the capacity to exchange genome segments via genetic reassortment due to their tri-segmented genome. Although not as frequent as in the arthropod-borne bunyaviruses, reports indicating reassortment events in the evolution of hantaviruses have been recently accumulating. The intra- and inter-lineage reassortment between closely related variants has been repeatedly reported for several hantaviruses including the rodent-borne human pathogens such as Sin Nombre virus, Puumala virus, Dobrava-Belgrade virus, or Hantaan virus as well as for the more recently recognized shrew-borne hantaviruses, Imjin and Seewis. Reassortment between more distantly related viruses was rarely found but seems to play a beneficial role in the process of crossing the host species barriers. Besides the findings based on phylogenetic studies of naturally occurring strains, hantavirus reassortants were generated also in in vitro studies. Interestingly, only reassortants with exchanged M segments could be generated suggesting that a high degree of genetic compatibility is required for the S and L segments while the exchange of M segment is better tolerated or is particularly beneficial. Altogether, the numerous reports on hantavirus reassortment, summarized in this review, clearly demonstrate that reassortment events play a significant role in hantavirus evolution and contributed to the currently recognized hantavirus diversity.

Biology and Diseases of Hamsters

The hamster species used as research models include the Syrian (golden), Mesocricetus auratus; the Chinese (striped-back), Cricetulus griseus; the Armenian (gray), C. migratorius; the European, Cricetus cricetus; and the Djungarian, Phodopus campbelli (Russian dwarf) and P. sungorus (Siberian dwarf). Hamsters are classified as members of the order Rodentia, suborder Myomorpha, superfamily Muroidea and in family Cricetidae. Animals in this family are characterized by large cheek pouches, thick bodies, short tails, and an excess of loose skin. They have incisors that erupt continuously and cuspidate molars that do not continue to grow ((I 1/1, C 0/0, PM 0/0, M 3/3) × 2 = 16). In 2010, it was reported that approximately 146,000 hamsters were used in research in the United States (United States Department of Agriculture, 2010).

An innate immunity-regulating virulence determinant is uniquely encoded by the Andes virus nucleocapsid protein

Andes virus (ANDV) is the only hantavirus known to spread from person to person and shown to cause highly lethal hantavirus pulmonary syndrome (HPS) in patients and Syrian hamsters. Hantaviruses replicate in human endothelial cells and accomplish this by restricting the early induction of beta interferon (IFN-β)- and IFN-stimulated genes (ISGs). Our studies reveal that the ANDV nucleocapsid (N) protein uniquely inhibits IFN signaling responses directed by cytoplasmic double-stranded RNA (dsRNA) sensors RIG-I and MDA5. In contrast, N proteins from Sin Nombre, New York-1, and Prospect Hill hantaviruses had no effect on RIG-I/MDA5-directed transcriptional responses from IFN-β-, IFN-stimulated response element (ISRE)-, or κB-containing promoters. Ablating a potential S-segment nonstructural open reading frame (ORF) (NSs) within the ANDV plasmid expressing N protein failed to alter IFN regulation by ANDV N protein. Further analysis demonstrated that expressing the ANDV N protein inhibited downstream IFN pathway activation directed by MAVS, TBK1, and IκB kinase ε (IKKε) but failed to inhibit transcriptional responses directed by constitutive expression of active interferon regulatory factor IRF3-5D or after stimulation by alpha interferon (IFN-α) or tumor necrosis factor alpha (TNF-α). Consistent with IFN pathway-specific regulation, the ANDV N protein inhibited TBK1-directed IRF3 phosphorylation (phosphorylation of serine 396 [pS396]) and TBK1 autophosphorylation (pS172). Collectively, these findings indicate that the ANDV N inhibits IFN signaling responses by interfering with TBK1 activation, upstream of IRF3 phosphorylation and NF-κB activation. Moreover, our findings reveal that ANDV uniquely carries a gene encoding a virulence determinant within its N protein that is capable of restricting ISG and IFN-β induction and provide a rationale for the novel pathogenesis and spread of ANDV.

Andes virus (ANDV) is distinguished from other hantaviruses by its unique ability to spread from person to person and cause lethal hantavirus pulmonary syndrome (HPS)-like disease in Syrian hamsters. However, virulence determinants that distinguish ANDV from other pathogenic hantaviruses have yet to be defined. Here we reveal that ANDV uniquely contains a virulence determinant within its nucleocapsid (N) protein that potently inhibits innate cellular signaling pathways. This novel function of the N protein provides a new mechanism for hantaviruses to regulate interferon (IFN) and IFN-stimulated gene (ISG) induction that is likely to contribute to the enhanced ability of ANDV to replicate, spread, and cause disease. These findings differentiate ANDV from other HPS-causing hantaviruses and provide a potential target for viral attenuation that needs to be considered in vaccine development.

Viruses of the family Bunyaviridae: are all available isolates reassortants?

Viruses of the family Bunyaviridae (the bunyaviruses) possess three distinct linear, single-stranded, negative sense or ambisense RNA segments (large, medium, and small). Dual infections of arthropod and perhaps vertebrate and plant hosts provide substantial opportunity for segment reassortment and an increasingly recognized number of the nearly 300 viruses in this family have been shown to be reassortants. Reassortment of RNA segments (genetic shift) complements genetic drift (accumulation of point mutations) as a powerful mechanism underlying bunyavirus evolution. Here we consider the possibility, if not likelihood, that most if not all bunyaviruses currently recognized may represent reassortants, some of which may be reassortants of existing viruses, and some of which may be reassortants of extinct viruses. If this hypothesis is correct, then the roots of the family and genus trees of bunyaviruses as currently described (or ignored) are incomplete or incorrect.

Hamster-adapted Sin Nombre virus causes disseminated infection and efficiently replicates in pulmonary endothelial cells without signs of disease

To date, a laboratory animal model for the study of Sin Nombre virus (SNV) infection or associated disease has not been described. Unlike infection with Andes virus, which causes lethal hantavirus pulmonary syndrome (HPS)-like disease in hamsters, SNV infection is short-lived, with no viremia and little dissemination. Here we investigated the effect of passaging SNV in hamsters. We found that a host-adapted SNV achieves prolonged and disseminated infection in hamsters, including efficient replication in pulmonary endothelial cells, albeit without signs of disease.

The Syrian hamster model of hantavirus pulmonary syndrome

Hantavirus pulmonary syndrome (HPS) is a relatively rare, but frequently fatal disease associated with New World hantaviruses, most commonly Sin Nombre and Andes viruses in North and South America, respectively. It is characterized by fever and the sudden, rapid onset of severe respiratory distress and cardiogenic shock, which can be fatal in up to 50% of cases. Currently there are no approved antiviral therapies or vaccines for the treatment or prevention of HPS. A major obstacle in the development of effective medical countermeasures against highly pathogenic agents like the hantaviruses is recapitulating the human disease as closely as possible in an appropriate and reliable animal model. To date, the only animal model that resembles HPS in humans is the Syrian hamster model. Following infection with Andes virus, hamsters develop HPS-like disease which faithfully mimics the human condition with respect to incubation period and pathophysiology of disease. Perhaps most importantly, the sudden and rapid onset of severe respiratory distress observed in humans also occurs in hamsters. The last several years has seen an increase in studies utilizing the Andes virus hamster model which have provided unique insight into HPS pathogenesis as well as potential therapeutic and vaccine strategies to treat and prevent HPS. The purpose of this article is to review the current understanding of HPS disease progression in Syrian hamsters and discuss the suitability of utilizing this model to evaluate potential medical countermeasures against HPS.

Hantaviruses in rodents and humans, Xi'an, PR China

Xi'an, the capital of Shaanxi province, located in north-west China, is one of the major endemic areas for haemorrhagic fever with renal syndrome (HFRS). In this study, the epidemiological data of HFRS in Xi'an from 1959 to 2010, especially in the past ten years (2001-2010), were surveyed. The features of hantavirus (HV) host carriers, the molecular characteristics of the HV S gene from hosts and patients, and the genome of the viral isolate were also investigated. Data showed that there might be a ten-year cycle of HFRS in Xi'an. Although the main population group infected over the past ten years was still the 16-59-year-old male farmers, the composition of the population and geographical distribution of HFRS cases have changed slowly, accompanied by the development of environmental and socio-economic situations. Apodemus agrarius remains the dominant host of HV. The HV strains from host rodents and patients in Xi'an belonged to the Hantaan virus (HTNV); no Seoul virus strains were found. Phylogenetic analysis of the small segments of strains taken from hosts and patients, and the whole genome of a viral isolate showed that the virus circulating in Xi'an had high similarity to Guizhou strains. The study also indicated that the vaccine candidate strain A16 isolated during the past century in Xi'an might be a recombinant strain of HTNV and the Amur virus, thus it may not be an optimal vaccine strain.

The Role of the Endothelium in HPS Pathogenesis and Potential Therapeutic Approaches

American hantaviruses cause a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). Hantaviruses nonlytically infect endothelial cells and cause dramatic changes in barrier functions of the endothelium without disrupting the endothelium. Instead hantaviruses cause changes in the function of infected endothelial cells that normally regulate fluid barrier functions of capillaries. The endothelium of arteries, veins, and lymphatic vessels is unique and central to the function of vast pulmonary capillary beds, which regulate pulmonary fluid accumulation. The endothelium maintains vascular barrier functions through a complex series of redundant receptors and signaling pathways that serve to both permit fluid and immune cell efflux into tissues and restrict tissue edema. Infection of the endothelium provides several mechanisms for hantaviruses to alter capillary permeability but also defines potential therapeutic targets for regulating acute pulmonary edema and HPS disease. Here we discuss interactions of HPS causing hantaviruses with the endothelium, potential endothelial cell-directed permeability mechanisms, and therapeutic targeting of the endothelium as a means of reducing the severity of HPS disease.

Induction of protective immunity in a Syrian hamster model against a cytopathogenic strain of Andes virus

Andes virus (ANDV) is responsible for the Hantavirus Pulmonary Syndrome cases in Argentina and neighboring countries, with moderate to high case-fatality rates. ANDV has some particular features, which make it unique among other members of the Hantavirus genus such as person-to-person transmission and causing a disease similar to Hantavirus Pulmonary Syndrome in the hamster as an animal model. The kinetics of replication in Vero E6 cells of an ANDV strain isolated in Argentina, called Andes/ARG, was studied. Cytopathic effect and the formation of clear plaques were observed and therefore Andes/ARG could be quantified by classic plaque assay. The Andes/ARG strain was found to be highly lethal in Syrian hamsters allowing experiments to demonstrate the protective potential of vaccines. A recombinant nucleocapsid protein of ANDV induced a long lasting antibody response and protective immunity against a homologous challenge, but to a lower extent against heterologous challenge by the Seoul virus.

The Experimental Use of Syrian Hamsters

The Syrian hamster (Mesocricetus auratus) is a widely used experimental animal model. This chapter focuses primarily on the most current research uses of the hamster. More classical uses are covered only as they pertain to these current uses. Hamsters possess unique anatomical and physiological features, which make them desirable research models. Unlike other commonly used laboratory rodents, hamsters possess a cheek pouch, which can be easily everted and examined at both the gross and microscopic level. The hamster's relative size also allows for better visualization of certain biological systems including the respiratory and reproductive systems when compared to the mouse. Further, laboratory hamsters develop a variety of inherited diseases, which display similarities to human conditions. Hamsters possessing some of these inherited traits are commercially available. They are susceptible to a variety of carcinogens and develop tumors that other research animals less commonly develop. Also they are susceptible to the induction of a variety of metabolic disorders through the use of dietary manipulations. The antagonistic nature of hamsters is used to study the effect of treatment on male aggressive and defensive behaviors. Syrian hamsters display several unique characteristics that make them desired models for carcinogenesis studies.

Pathogenesis of the hantavirus pulmonary syndrome

Hantavirus pulmonary syndrome caused by hantaviruses in the Americas presents as a broad clinical spectrum ranging from brief febrile prodrome with only thrombocytopenia to rapidly progressive fulminant pulmonary edema and shock. This vascular leak syndrome confined almost exclusively to the lung is initiated by the noncytolytic infection of capillary endothelial cells. A number of pathogenic mechanisms have been proposed, including immune cell-mediated injury, cytokine-mediated injury and enhanced VEGF responses from intercellular junctions resulting from highly specific virus–integrin interactions. This review examines evidence for each of these potential mechanisms, with relevant references to its sister syndrome, hemorrhagic fever with renal syndrome, in Eurasia. Any mechanism or combination of mechanisms must be able to explain the massive pulmonary capillary leak at the severe extreme of the spectrum, a disease manifestation without parallel in clinical medicine.

Phylogenetic exploration of hantaviruses in Paraguay reveals reassortment and host switching in South America

Background

Longitudinal mark-recapture studies of rodents in two sites in the Mbaracayú Biosphere Reserve in the Interior Atlantic Forest of eastern Paraguay have revealed a complex and intriguing pattern of hantaviruses harbored by rodents in this area. Full-length sequencing and phylogenetic analyses were conducted for several rodents from Akodon montensis and Oligoryzomys fornesi. The phylogenetic relationships of these viruses were analyzed in the context of hantaviruses in South America with published S- and M-segment sequences.

Findings

Phylogenetic analyses of hantaviruses identified in the Mbaracayú Biosphere Reserve in Paraguay revealed Jabora and Juquitiba viruses are harbored by Akodon montensis and Oligoryzomys fornesi, respectively. These analyses revealed that in general the constituents of the major subclade for the S- and M-segments differ for the South American hantaviruses. Further, the two major groups within subclade C for the M-segment reflect in general the lethality associated with the viruses within each group.

Conclusions

Phylogenetic studies of Jabora and Juquitiba viruses and other Paraguayan viruses in the context of American hantaviruses revealed reassortment and host-switching in the evolution of South American hantaviruses.

A natural M RNA reassortant arising from two species of plant- and insect-infecting bunyaviruses and comparison of its sequence and biological properties to parental species

Reassortment allows multicomponent viruses to exchange genome segments, a process well-documented in the vertebrate- and arthropod-infecting members of the family Bunyaviridae but not between distinct species of the plant- and insect-infecting members of the genus Tospovirus. Genome sequence comparisons of a virus causing severe tospovirus-like symptoms in Florida tomato with Groundnut ringspot virus (GRSV) and Tomato chlorotic spot virus (TCSV) demonstrated that reassortment has occurred, with the large (L) and small (S) RNAs coming from GRSV and the medium (M) RNA coming from TCSV (i.e. L(G)M(T)S(G)). Neither parental genotype is known to occur in the U.S. suggesting that L(G)M(T)S(G) was introduced as a reassortant. L(G)M(T)S(G) was transmitted by western flower thrips (Frankliniella occidentalis [Pergande]), and was not able to overcome the Sw5 resistance gene of tomato. Our demonstration of reassortment between GRSV and TCSV suggests caution in defining species within the family Bunyaviridae based on their ability to reassort.

Genetic reassortment between high-virulent and low-virulent Dobrava-Belgrade virus strains

The tri-segmented RNA genome of hantaviruses facilitates genetic reassortment by segment swapping when cells are co-infected with different virus strains. We found efficient in vitro reassortment between members of two different genetic lineages of the Dobrava-Belgrade virus species, the weakly virulent DOBV-Aa and highly virulent DOBV-Af. In all reassortants, S and L segments originated from the same parental strain, and only the M segment was exchanged. To identify functional differences between the parental strains DOBV-Aa and DOBV-Af in cell culture and to compare them with the reassortants, we studied elements of the innate immunity in virus-infected cells. The contrasting phenotypes of the parental viruses were maintained by the reassortants carrying the respective S and L segments of the parental virus and were not influenced by the origin of the M segment.

A global perspective on hantavirus ecology, epidemiology, and disease

Hantaviruses are enzootic viruses that maintain persistent infections in their rodent hosts without apparent disease symptoms. The spillover of these viruses to humans can lead to one of two serious illnesses, hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome. In recent years, there has been an improved understanding of the epidemiology, pathogenesis, and natural history of these viruses following an increase in the number of outbreaks in the Americas. In this review, current concepts regarding the ecology of and disease associated with these serious human pathogens are presented. Priorities for future research suggest an integration of the ecology and evolution of these and other host-virus ecosystems through modeling and hypothesis-driven research with the risk of emergence, host switching/spillover, and disease transmission to humans.

Generation and characterization of genetic reassortants between Puumala and Prospect Hill hantavirus in vitro

Hantaviruses belong to the family Bunyaviridae characterized by tri-segmented RNA genomes. Depending on the hantavirus species, infection can lead to hantavirus cardiopulmonary or haemorrhagic fever with renal syndrome. In vitro studies suggest that pathogenic hantaviruses evade induction of innate antiviral responses, and this ability might determine the virulence in humans. Since reverse genetic systems are not available, in vitro reassortment is currently the only way to culture defined hantavirus variants. Here, we demonstrate for the first time the generation of a reassortant between a pathogenic Old World and a non-pathogenic New World hantavirus in vitro. The reassortant contained the glycoprotein coding M-segment derived from the pathogenic Puumala virus (PUUV) and the other genomic segments coding for the nucleocapsid protein and RNA-dependent RNA-polymerase from Prospect Hill virus (PHV), which is taken as non-pathogenic in humans. Exchange of the M-segment was confirmed by sequencing and virus neutralization test with PUUV-specific sera. Functional analysis of the reassortant and parental viruses revealed characteristic growth kinetics and innate immune responses as determined by expression analyses for lambda interferon and MxA, and by interferon-stimulated response element reporter gene studies. Consistent with previous studies with other pathogenic hantaviruses, PUUV elicited reduced innate responses if compared with PHV. In all these functional assays the reassortant revealed PHV-like phenotypes. Thus, neither the PUUV M-segment nor entry via specific M-segment directed pathways modulated the virus type-specific innate responses. Moreover, the data imply that this approach might be an option for production of attenuated viruses that could be used as vaccines against pathogenic hantaviruses.

Andes virus recognition of human and Syrian hamster beta3 integrins is determined by an L33P substitution in the PSI domain

Andes virus (ANDV) causes a fatal hantavirus pulmonary syndrome (HPS) in humans and Syrian hamsters. Human alpha(v)beta(3) integrins are receptors for several pathogenic hantaviruses, and the function of alpha(v)beta(3) integrins on endothelial cells suggests a role for alpha(v)beta(3) in hantavirus directed vascular permeability. We determined here that ANDV infection of human endothelial cells or Syrian hamster-derived BHK-21 cells was selectively inhibited by the high-affinity alpha(v)beta(3) integrin ligand vitronectin and by antibodies to alpha(v)beta(3) integrins. Further, antibodies to the beta(3) integrin PSI domain, as well as PSI domain polypeptides derived from human and Syrian hamster beta(3) subunits, but not murine or bovine beta(3), inhibited ANDV infection of both BHK-21 and human endothelial cells. These findings suggest that ANDV interacts with beta(3) subunits through PSI domain residues conserved in both Syrian hamster and human beta(3) integrins. Sequencing the Syrian hamster beta(3) integrin PSI domain revealed eight differences between Syrian hamster and human beta(3) integrins. Analysis of residues within the PSI domains of human, Syrian hamster, murine, and bovine beta(3) integrins identified unique proline substitutions at residues 32 and 33 of murine and bovine PSI domains that could determine ANDV recognition. Mutagenizing the human beta(3) PSI domain to contain the L33P substitution present in bovine beta(3) integrin abolished the ability of the PSI domain to inhibit ANDV infectivity. Conversely, mutagenizing either the bovine PSI domain, P33L, or the murine PSI domain, S32P, to the residue present human beta(3) permitted PSI mutants to inhibit ANDV infection. Similarly, CHO cells transfected with the full-length bovine beta(3) integrin containing the P33L mutation permitted infection by ANDV. These findings indicate that human and Syrian hamster alpha(v)beta(3) integrins are key receptors for ANDV and that specific residues within the beta(3) integrin PSI domain are required for ANDV infection. Since L33P is a naturally occurring human beta(3) polymorphism, these findings further suggest the importance of specific beta(3) integrin residues in hantavirus infection. These findings rationalize determining the role of beta(3) integrins in hantavirus pathogenesis in the Syrian hamster model.

Study of Andes virus entry and neutralization using a pseudovirion system

Andes virus (ANDV), a member of the Hantavirus genus in the family Bunyaviridae, causes an acute disease characteristic of New-World hantaviruses called hantavirus pulmonary syndrome (HPS). HPS is a highly pathogenic disease with a case-fatality rate of 40%. ANDV is the only hantavirus reported to spread directly from human-to-human. The aim of the present study was to develop a quantitative and high-throughput pseudovirion assay to study ANDV infection and neutralization in biosafety level 2 facilities (BSL-2). This pseudovirion assay is based on incorporation of ANDV glycoproteins onto replication-defective vesicular stomatitis virus (VSV) cores in which the gene for the surface G protein has been replaced by that encoding Renilla luciferase. Infection by the pseudovirions can be quantified by luciferase activity of infected cell lysates. ANDV pseudovirions were neutralized by ANDV-specific antisera, and there was good concordance between specificity and neutralization titers of ANDV hamster sera as determined by our pseudovirion assay and a commonly used plaque reduction neutralization titer (PRNT) assay. In addition, the pseudovirions were used to evaluate the requirements for ANDV entry, like pH dependency and the role of beta3 integrin, the reported receptor for other pathogenic hantaviruses, on entry.

Temporal analysis of Andes virus and Sin Nombre virus infections of Syrian hamsters

Andes virus (ANDV) and Sin Nombre virus (SNV) are rodent-borne hantaviruses that cause a highly lethal hemorrhagic fever in humans known as hantavirus pulmonary syndrome (HPS). There are no vaccines or specific drugs to prevent or treat HPS, and the pathogenesis is not understood. Syrian hamsters infected with ANDV, but not SNV, develop a highly lethal disease that closely resembles HPS in humans. Here, we performed a temporal pathogenesis study comparing ANDV and SNV infections in hamsters. SNV was nonpathogenic and viremia was not detected despite the fact that all animals were infected. ANDV was uniformly lethal with a mean time to death of 11 days. The first pathology detected was lymphocyte apoptosis starting on day 4. Animals were viremic and viral antigen was first observed in multiple organs by days 6 and 8, respectively. Levels of infectious virus in the blood increased 4 to 5 logs between days 6 and 8. Pulmonary edema was first detected ultrastructurally on day 6. Ultrastructural analysis of lung tissues revealed the presence of large inclusion bodies and substantial numbers of vacuoles within infected endothelial cells. Paraendothelial gaps were not observed, suggesting that fluid leakage was transcellular and directly attributable to infecting virus. Taken together, these data imply that HPS treatment strategies aimed at preventing virus replication and dissemination will have the greatest probability of success if administered before the viremic phase; however, because vascular leakage is associated with infected endothelial cells, a therapeutic strategy targeting viral replication might be effective even at later times (e.g., after disease onset).

A variable region in the Crimean-Congo hemorrhagic fever virus L segment distinguishes between strains isolated from different geographic regions

Alignment of Crimean-Congo hemorrhagic fever virus (CCHFV) L genome segment full-length sequences reveals an overall high level of conservation among strains, with greater than 90% of translated amino acid residues strictly conserved. However, a region of marked variability identified previously, corresponding to L polyprotein amino acid positions 760-810, shares only 40% overall identity between strains. The variable regions sequences of 16 laboratory-adapted CCHFV strains were determined, including 11 strains from European Russia, one strain from Bulgaria, and four strains from the Central Asian countries of Tajikistan, Turkmenistan, and Uzbekistan. Phylogenetic analysis demonstrates this L segment variable region sequence divides CCHFV strains into similar geographically-defined groupings observed for S segment-derived trees, but with higher bootstrap support and a much smaller character set required for analysis.