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

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

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 adaptive immune response does not influence hantavirus disease or persistence in the Syrian hamster.

Pathogenic New World hantaviruses cause severe disease in humans characterized by a vascular leak syndrome, leading to pulmonary oedema and respiratory distress with case fatality rates approaching 40%. Hantaviruses infect microvascular endothelial cells without conspicuous cytopathic effects, indicating that destruction of the endothelium is not a mechanism of disease. In humans, high levels of inflammatory cytokines are present in the lungs of patients that succumb to infection. This, along with other observations, suggests that disease has an immunopathogenic component. Currently the only animal model available to study hantavirus disease is the Syrian hamster, where infection with Andes virus (ANDV), the primary agent of disease in South America, results in disease that closely mimics that seen in humans. Conversely, inoculation of hamsters with a passaged Sin Nombre virus (SNV), the virus responsible for most cases of disease in North America, results in persistent infection with high levels of viral replication. We found that ANDV elicited a stronger innate immune response, whereas SNV elicited a more robust adaptive response in the lung. Additionally, ANDV infection resulted in significant changes in the blood lymphocyte populations. To determine whether the adaptive immune response influences infection outcome, we depleted hamsters of CD4(+) and CD8(+) T cells before infection with hantaviruses. Depletion resulted in inhibition of virus-specific antibody responses, although the pathogenesis and replication of these viruses were unaltered. These data show that neither hantavirus replication, nor pathogenesis caused by these viruses, is influenced by the adaptive immune response in the Syrian hamster.

Kinetics of immune responses in deer mice experimentally infected with Sin Nombre virus.

Deer mice are the principal reservoir hosts of Sin Nombre virus, the etiologic agent of most hantavirus cardiopulmonary syndrome cases in North America. Infection of deer mice results in persistence without conspicuous pathology, and most, if not all, infected mice remain infected for life, with periods of viral shedding. The kinetics of viral load, histopathology, virus distribution, and immune gene expression in deer mice were examined. Viral antigen was detected as early as 5 days postinfection and peaked on day 15 in the lungs, hearts, kidneys, and livers. Viral RNA levels varied substantially but peaked on day 15 in the lungs and heart, and antinucleocapsid IgG antibodies appeared in some animals on day 10, but a strong neutralizing antibody response failed to develop during the 20-day experiment. No clinical signs of disease were observed in any of the infected deer mice. Most genes were repressed on day 2, suggesting a typical early downregulation of gene expression often observed in viral infections. Several chemokine and cytokine genes were elevated, and markers of a T cell response occurred but then declined days later. Splenic transforming growth factor beta (TGF-ß) expression was elevated early in infection, declined, and then was elevated again late in infection. Together, these data suggest that a subtle immune response that fails to clear the virus occurs in deer mice.

Sin Nombre hantavirus decreases survival of male deer mice.

How pathogens affect their hosts is a key question in infectious disease ecology, and it can have important influences on the spread and persistence of the pathogen. Sin Nombre virus (SNV) is the etiological agent of hantavirus pulmonary syndrome (HPS) in humans. A better understanding of SNV in its reservoir host, the deer mouse, could lead to improved predictions of the circulation and persistence of the virus in the mouse reservoir, and could help identify the factors that lead to increased human risk of HPS. Using mark-recapture statistical modeling on longitudinal data collected over 15 years, we found a 13.4% decrease in the survival of male deer mice with antibodies to SNV compared to uninfected mice (both male and female). There was also an additive effect of breeding condition, with a 21.3% decrease in survival for infected mice in breeding condition compared to uninfected, non-breeding mice. The data identified that transmission was consistent with density-dependent transmission, implying that there may be a critical host density below which SNV cannot persist. The notion of a critical host density coupled with the previously overlooked disease-induced mortality reported here contribute to a better understanding of why SNV often goes extinct locally and only seems to persist at the metapopulation scale, and why human spillover is episodic and hard to predict.

Sin Nombre virus-specific immunoglobulin M and G kinetics in hantavirus pulmonary syndrome and the role played by serologic responses in predicting disease outcome.

BACKGROUND: Sin Nombre virus (SNV) is the primary cause of hantavirus pulmonary syndrome (HPS) in the United States. Although other studies have demonstrated a possible association between neutralizing antibody titers and the severity of HPS, the exact nature of serologic responses and their association with outcomes have not been fully characterized. METHODS: We examined immunoglobulin M (IgM) and immunoglobulin G (IgG) serologic responses in 94 clinical samples from 81 patients with confirmed HPS. We further compared a subset of 31 patients with fatal HPS and 20 surviving patients for whom samples were available within a week after the onset of HPS. RESULTS: SNV-specific IgM antibodies displayed a trend suggesting an early peak, whereas IgG antibody values peaked later. Among individuals with samples from the first week after the onset of HPS, all surviving patients had SNV-specific IgG responses, compared with <50% of patients with fatal HPS, and the distribution of IgG responses was significantly higher in surviving patients. CONCLUSIONS: Production of SNV-specific IgM antibodies occurs early during the clinical course of HPS, whereas production of IgG antibodies may be more protracted. The presence and overall distribution of higher IgG antibody titers in surviving patients with HPS suggests that production of SNV-specific IgG may be a strong predictor of favorable outcomes.

Longitudinal Assessment of Cytokine Expression and Plasminogen Activation in Hantavirus Cardiopulmonary Syndrome Reveals Immune Regulatory Dysfunction in End-Stage Disease.

Pathogenic New World orthohantaviruses cause hantavirus cardiopulmonary syndrome (HCPS), a severe immunopathogenic disease in humans manifested by pulmonary edema and respiratory distress, with case fatality rates approaching 40%. High levels of inflammatory mediators are present in the lungs and systemic circulation of HCPS patients. Previous studies have provided insights into the pathophysiology of HCPS. However, the longitudinal correlations of innate and adaptive immune responses and disease outcomes remain unresolved. This study analyzed serial immune responses in 13 HCPS cases due to Sin Nombre orthohantavirus (SNV), with 11 severe cases requiring extracorporeal membrane oxygenation (ECMO) treatment and two mild cases. We measured viral load, levels of various cytokines, urokinase plasminogen activator (uPA), and plasminogen activator inhibitor-1 (PAI-1). We found significantly elevated levels of proinflammatory cytokines and PAI-1 in five end-stage cases. There was no difference between the expression of active uPA in survivors' and decedents' cases. However, total uPA in decedents' cases was significantly higher compared to survivors'. In some end-stage cases, uPA was refractory to PAI-1 inhibition as measured by zymography, where uPA and PAI-1 were strongly correlated to lymphocyte counts and IFN-γ. We also found bacterial co-infection influencing the etiology and outcome of immune response in two cases. Unsupervised Principal Component Analysis and hierarchical cluster analyses resolved separate waves of correlated immune mediators expressed in one case patient due to a sequential co-infection of bacteria and SNV. Overall, a robust proinflammatory immune response, characterized by an imbalance in T helper 17 (Th17) and regulatory T-cells (Treg) subsets, was correlated with dysregulated inflammation and mortality. Our sample size is small; however, the core differences correlated to survivors and end-stage HCPS are instructive.

Multivalent presentation of antihantavirus peptides on nanoparticles enhances infection blockade.

Viral entry into susceptible host cells typically results from multivalent interactions between viral surface proteins and host entry receptors. In the case of Sin Nombre virus (SNV), a New World hantavirus that causes hantavirus cardiopulmonary syndrome, infection involves the interaction between viral membrane surface glycoproteins and the human integrin alpha(v)beta(3). Currently, there are no therapeutic agents available which specifically target SNV. To address this problem, we used phage display selection of cyclic nonapeptides to identify peptides that bound SNV and specifically prevented SNV infection in vitro. We synthesized cyclic nonapeptides based on peptide sequences of phage demonstrating the strongest inhibition of infection, and in all cases, the isolated peptides were less effective at blocking infection (9.0% to 27.6% inhibition) than were the same peptides presented by phage (74.0% to 82.6% inhibition). Since peptides presented by the phage were pentavalent, we determined whether the identified peptides would show greater inhibition if presented in a multivalent format. We used carboxyl linkages to conjugate selected cyclic peptides to multivalent nanoparticles and tested infection inhibition. Two of the peptides, CLVRNLAWC and CQATTARNC, showed inhibition that was improved over that of the free format when presented on nanoparticles at a 4:1 nanoparticle-to-virus ratio (9.0% to 32.5% and 27.6% to 37.6%, respectively), with CQATTARNC inhibition surpassing 50% when nanoparticles were used at a 20:1 ratio versus virus. These data illustrate that multivalent inhibitors may disrupt polyvalent protein-protein interactions, such as those utilized for viral infection of host cells, and may represent a useful therapeutic approach.

Historical Perspective: What Constitutes Discovery (of a New Virus)?

A historic review of the discovery of new viruses leads to reminders of traditions that have evolved over 118 years. One such tradition gives credit for the discovery of a virus to the investigator(s) who not only carried out the seminal experiments but also correctly interpreted the findings (within the technological context of the day). Early on, ultrafiltration played a unique role in "proving" that an infectious agent was a virus, as did a failure to find any microscopically visible agent, failure to show replication of the agent in the absence of viable cells, thermolability of the agent, and demonstration of a specific immune response to the agent so as to rule out duplicates and close variants. More difficult was "proving" that the new virus was the etiologic agent of the disease ("proof of causation")-for good reasons this matter has been revisited several times over the years as technologies and perspectives have changed. One tradition is that the discoverers get to name their discovery, their new virus (unless some grievous convention has been broken)-the stability of these virus names has been a way to honor the discoverer(s) over the long term. Several vignettes have been chosen to illustrate several difficulties in holding to the traditions (vignettes chosen include vaccinia and variola viruses, yellow fever virus, and influenza viruses. Crimean-Congo hemorrhagic fever virus, Murray Valley encephalitis virus, human immunodeficiency virus 1, Sin Nombre virus, and Ebola virus). Each suggests lessons for the future. One way to assure that discoveries are forever linked with discoverers would be a permanent archive in one of the universal virus databases that have been constructed for other purposes. However, no current database seems ideal-perhaps members of the global community of virologists will have an ideal solution.

Transcriptome markers of viral persistence in naturally-infected andes virus (bunyaviridae) seropositive long-tailed pygmy rice rats.

Long-tailed pygmy rice rats (Oligoryzomys longicaudatus) are principal reservoir hosts of Andes virus (ANDV) (Bunyaviridae), which causes most hantavirus cardiopulmonary syndrome cases in the Americas. To develop tools for the study of the ANDV-host interactions, we used RNA-Seq to generate a de novo transcriptome assembly. Splenic RNA from five rice rats captured in Chile, three of which were ANDV-infected, was used to generate an assembly of 66,173 annotated transcripts, including noncoding RNAs. Phylogenetic analysis of selected predicted proteins showed similarities to those of the North American deer mouse (Peromyscus maniculatus), the principal reservoir of Sin Nombre virus (SNV). One of the infected rice rats had about 50-fold more viral burden than the others, suggesting acute infection, whereas the remaining two had levels consistent with persistence. Differential expression analysis revealed distinct signatures among the infected rodents. The differences could be due to 1) variations in viral load, 2) dimorphic or reproductive differences in splenic homing of immune cells, or 3) factors of unknown etiology. In the two persistently infected rice rats, suppression of the JAK-STAT pathway at Stat5b and Ccnot1, elevation of Casp1, RIG-I pathway factors Ppp1cc and Mff, and increased FC receptor-like transcripts occurred. Caspase-1 and Stat5b activation pathways have been shown to stimulate T helper follicular cell (TFH) development in other species. These data are also consistent with reports suggestive of TFH stimulation in deer mice experimentally infected with hantaviruses. In the remaining acutely infected rice rat, the apoptotic pathway marker Cox6a1 was elevated, and putative anti-viral factors Abcb1a, Fam46c, Spp1, Rxra, Rxrb, Trmp2 and Trim58 were modulated. Transcripts for preproenkephalin (Prenk) were reduced, which may be predictive of an increased T cell activation threshold. Taken together, this transcriptome dataset will permit rigorous examination of rice rat-ANDV interactions and may lead to better understanding of virus ecology.

Hantavirus pulmonary syndrome: the new American hemorrhagic fever.

The recognition of hantavirus pulmonary syndrome (HPS) after the investigation of a cluster of unexplained respiratory deaths in the southwestern United States during the spring of 1993 showcased our ability to recognize new and emerging diseases, given the correct juxtaposition of a new clinical entity with circumscribed epidemiologic features that are analyzed with novel diagnostic methods. In less than a decade, HPS has become established as a pan-American zoonosis due to numerous viruses maintained by sigmodontine rodents with rodent- and virus-specific epidemiologic profiles. The classical features of the syndrome-acute febrile illness associated with prominent cardiorespiratory compromise after direct contact or inhalation of aerosolized rodent excreta-has been extended to include clinical variants, including disease with frank hemorrhage, that have confirmed that this syndrome is a viral hemorrhagic fever. Efforts are under way to refine prevention strategies, to understand the pathogenesis of the shock, and to identify therapeutic modalities.

Infection dynamics of Sin Nombre virus after a widespread decline in host populations.

Many researchers have speculated that infection dynamics of Sin Nombre virus are driven by density patterns of its major host, Peromyscus maniculatus. Few, if any, studies have examined this question systematically at a realistically large spatial scale, however. We collected data from 159 independent field sites within a 1 million-hectare study area in Nevada and California, from 1995-1998. In 1997, there was a widespread and substantial reduction in host density. This reduction in host density did not reduce seroprevalence of antibody to Sin Nombre virus within host populations. During this period, however, there was a significant reduction in the likelihood that antibody-positive mice had detectable virus in their blood, as determined by reverse-transcriptase polymerase chain reaction. Our findings suggest 2 possible causal mechanisms for this reduction: an apparent change in the age structure of host populations and landscape-scale patterns of host density. This study indicates that a relationship does exist between host density and infection dynamics and that this relationship concurrently operates at different spatial scales. It also highlights the limitations of antibody seroprevalence as a metric of infections, especially during transient host-density fluctuations.

Human exposure to particulate matter potentially contaminated with sin nombre virus.

The most common mechanism for human exposure to hantaviruses throughout North America is inhalation of virally contaminated particulates. However, risk factors associated with exposure to particulates potentially contaminated with hantaviruses are generally not well understood. In North America, Sin Nombre virus (SNV) is the most common hantavirus that infects humans, causing hantavirus pulmonary syndrome, which has a significant mortality rate (approximately 35%). We investigated human exposure to particulate matter and evaluated the effects of season, location (sylvan and peridomestic environment), and activity (walking and sweeping) on generation of particulates at the breathing zone (1.5 m above the ground). We found greater volumes of small inhalable particulates during the spring and summer compared to the fall and winter seasons and greater volumes of small inhalable particulates produced in peridomestic, compared to sylvan, environments. Also, greater volumes of particulates were generated at the breathing zone while walking compared to sweeping. Results suggest that more aerosolized particles were generated during the spring and summer months. Our findings suggest that simply moving around in buildings is a significant source of human exposure to particulates, potentially contaminated with SNV, during spring and summer seasons. These findings could be advanced by investigation of what particle sizes SNV is most likely to attach to, and where in the respiratory tract humans become infected.

Prediction of Peromyscus maniculatus (deer mouse) population dynamics in Montana, USA, using satellite-driven vegetation productivity and weather data.

Deer mice (Peromyscus maniculatus) are the main reservoir host for Sin Nombre virus, the primary etiologic agent of hantavirus pulmonary syndrome in North America. Sequential changes in weather and plant productivity (trophic cascades) have been noted as likely catalysts of deer mouse population irruptions, and monitoring and modeling of these phenomena may allow for development of early-warning systems for disease risk. Relationships among weather variables, satellite-derived vegetation productivity, and deer mouse populations were examined for a grassland site east of the Continental Divide and a sage-steppe site west of the Continental Divide in Montana, USA. We acquired monthly deer mouse population data for mid-1994 through 2007 from long-term study sites maintained for monitoring changes in hantavirus reservoir populations, and we compared these with monthly bioclimatology data from the same period and gross primary productivity data from the Moderate Resolution Imaging Spectroradiometer sensor for 2000-06. We used the Random Forests statistical learning technique to fit a series of predictive models based on temperature, precipitation, and vegetation productivity variables. Although we attempted several iterations of models, including incorporating lag effects and classifying rodent density by seasonal thresholds, our results showed no ability to predict rodent populations using vegetation productivity or weather data. We concluded that trophic cascade connections to rodent population levels may be weaker than originally supposed, may be specific to only certain climatic regions, or may not be detectable using remotely sensed vegetation productivity measures, although weather patterns and vegetation dynamics were positively correlated.

Hantavirus pulmonary syndrome.

Hantavirus pulmonary syndrome (HPS) is a severe disease characterized by a rapid onset of pulmonary edema followed by respiratory failure and cardiogenic shock. The HPS associated viruses are members of the genus Hantavirus, family Bunyaviridae. Hantaviruses have a worldwide distribution and are broadly split into the New World hantaviruses, which includes those causing HPS, and the Old World hantaviruses [including the prototype Hantaan virus (HTNV)], which are associated with a different disease, hemorrhagic fever with renal syndrome (HFRS). Sin Nombre virus (SNV) and Andes virus (ANDV) are the most common causes of HPS in North and South America, respectively. Case fatality of HPS is approximately 40%. Pathogenic New World hantaviruses infect the lung microvascular endothelium without causing any virus induced cytopathic effect. However, virus infection results in microvascular leakage, which is the hallmark of HPS. This article briefly reviews the knowledge on HPS-associated hantaviruses accumulated since their discovery, less than 20 years ago.

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).

Ribavirin, human convalescent plasma and anti-beta3 integrin antibody inhibit infection by Sin Nombre virus in the deer mouse model.

The New World hantavirus Sin Nombre virus (SNV) is an aetiological agent for the often-fatal hantavirus cardiopulmonary syndrome (HCPS). There is no disease model for SNV and specific treatments for HCPS do not exist. By using the deer mouse infectious model, the in vivo inhibitory potential of ribavirin, human anti-SNV immune plasma (HIP), an anti-beta3 antibody (ReoPro) and a polyclonal rabbit anti-recombinant nucleocapsid (N) antibody against SNV was investigated. Concurrent intraperitoneal administration of 100 mg ribavirin kg(-1) prevented seroconversion in all mice at day 15 post-inoculation (p.i.). No evidence of infection was detectable by immunohistochemical staining or by quantitative RT-PCR in two of these six mice. Lower doses of ribavirin, between 5 and 50 mg kg(-1), were much less effective at inhibiting infection. Mice given 200 microl aliquots of dilutions as high as 1 : 20 of HIP (neutralizing-antibody titre 800) failed to seroconvert by day 15 p.i. SNV N antigen staining and viral S genome were undetectable in these mice. A subset of mice given higher dilutions of HIP became infected. Treatment with 6 mg ReoPro kg(-1) did not prevent seroconversion, but was able to reduce viral load. Mice treated with 200 microl anti-N antibody or negative human plasma seroconverted when challenged with SNV, and antigen staining and viral loads were comparable to those seen in untreated controls. These results show that ReoPro can lower viral loads and that ribavirin and HIP, but not anti-N antibody, inhibit seroconversion and reduce viral loads in a dose-dependent manner.

Early innate immune responses to Sin Nombre hantavirus occur independently of IFN regulatory factor 3, characterized pattern recognition receptors, and viral entry.

Sin Nombre virus (SNV) is a highly pathogenic New World virus and etiologic agent of hantavirus cardiopulmonary syndrome. We have previously shown that replication-defective virus particles are able to induce a strong IFN-stimulated gene (ISG) response in human primary cells. RNA viruses often stimulate the innate immune response by interactions between viral nucleic acids, acting as a pathogen-associated molecular pattern, and cellular pattern-recognition receptors (PRRs). Ligand binding to PRRs activates transcription factors which regulate the expression of antiviral genes, and in all systems examined thus far, IFN regulatory factor 3 (IRF3) has been described as an essential intermediate for induction of ISG expression. However, we now describe a model in which IRF3 is dispensable for the induction of ISG transcription in response to viral particles. IRF3-independent ISG transcription in human hepatoma cell lines is initiated early after exposure to SNV virus particles in an entry- and replication-independent fashion. Furthermore, using gene knockdown, we discovered that this activation is independent of the best-characterized RNA- and protein-sensing PRRs including the cytoplasmic caspase recruitment domain-containing RNA helicases and the TLRs. SNV particles engage a heretofore unrecognized PRR, likely located at the cell surface, and engage a novel IRF3-independent pathway that activates the innate immune response.