Crystal structure of the human astrovirus capsid spike

Mouse and human immune responses share neutralization epitopes of HAstV-VA1

Astroviruses are highly divergent and infect a wide variety of animal hosts. In 2009, a genetically divergent human astrovirus (HAstV) strain VA1 was first identified in an outbreak of acute gastroenteritis. This strain has also been associated with fatal central nervous system disease. In this work, we report the isolation of three high-affinity neutralizing monoclonal antibodies (Nt-MAbs) targeting the capsid spike domain of HAstV-VA1. These antibodies (7C8, 2A2, 3D8) were used to select individual HAstV-VA1 mutants resistant to their neutralizing activity and a HAstV-VA1 triple mutant that escapes neutralization from all three Nt-MAbs. Sequencing of the virus genome capsid region revealed escape mutations that map to the surface of the capsid spike domain, define three potentially independent neutralization epitopes, and help delineate four antigenic sites in human astroviruses. Notably, two of the escape mutations were found to be present in the spike sequence of the HAstV-VA1-PS strain isolated from an immunodeficient patient with encephalitis, suggesting that those mutations arose as a result of the immune pressure generated by the patient's immunotherapy. In agreement with this observation, human serum samples exhibiting strong neutralization activity against wild-type HAstV-VA1 had a 2.6-fold reduction in neutralization titer when evaluated against the triple-escape HAstV-VA1 mutant, suggesting that both mouse and human antibody responses target shared neutralization epitopes. The isolated Nt-MAbs reported in this work will help to characterize the functional domains of the virus during cell entry and have the potential for developing a specific antibody therapy for the neurological disease associated with HAstV-VA1.

IMPORTANCE

Human astroviruses (HAstVs) have been historically associated with acute gastroenteritis. However, the genetically divergent HAstV-VA1 strain has been associated with central nervous system disease. In this work high-affinity neutralizing monoclonal antibodies directed to HAstV-VA1 were isolated and characterized. The proposed binding sites for these antibodies and for neutralizing antibodies against classical HAstVs suggest that there are at least four neutralization sites on the capsid spike of astroviruses. Our data show that natural infection with human astrovirus VA1 elicits a robust humoral immune response that targets the same antigenic sites recognized by the mouse monoclonal antibodies and strongly suggests the emergence of a variant HAstV-VA1 virus in an immunodeficient patient with prolonged astrovirus infection. The isolated Nt-MAb reported in this work will help to define the functional sites of the virus involved in cell entry and hold promise for developing a specific antibody therapy for the neurological disease associated with HAstV-VA1.

Antigenic determinants of HAstV-VA1 neutralization and their relevance in the human immune response

Astroviruses are highly divergent and infect a wide variety of animal hosts. In 2009, a genetically divergent human astrovirus (HAstV) strain VA1 was first identified in an outbreak of acute gastroenteritis. This strain has also been associated with fatal central nervous system disease. In this work, we report the isolation of three high-affinity neutralizing monoclonal antibodies (Nt-MAbs) targeting the capsid spike domain of HAstV-VA1. These antibodies (7C8, 2A2, 3D8) were used to select individual HAstV-VA1 mutants resistant to their neutralizing activity and also select a HAstV-VA1 triple mutant that escapes neutralization from all three Nt-MAbs. Sequencing of the virus genome capsid region revealed escape mutations that map to the surface of the capsid spike domain, define three potentially independent neutralization epitopes, and help delineate four antigenic sites in rotaviruses. Notably, two of the escape mutations were found to be present in the spike sequence of the HAstV-VA1-PS strain isolated from an immunodeficient patient with encephalitis, suggesting that those mutations arose as a result of the immune pressure generated by the patient's immunotherapy. In accordance with this observation, human serum samples exhibiting strong neutralization activity against wild-type HAstV-VA1 had a 2.6-fold reduction in neutralization titer when evaluated against the triple-escape HAstV-VA1 mutant, indicating shared neutralization epitopes between the mouse and human antibody response. The isolated Nt-MAbs reported in this work will help characterize the functional sites of the virus during cell entry and have the potential for developing a specific antibody therapy for the neurological disease associated with HAstV-VA1.

Importance

Human astroviruses (HAstVs) have been historically associated with acute gastroenteritis. However, the genetically divergent HAstV-VA1 strain has been associated with central nervous system disease. This work isolated high-affinity neutralizing monoclonal antibodies directed to HAstV-VA1. The proposed binding sites for these antibodies, together with previously reported sites for neutralizing antibodies against classical HAstVs, suggest the existence of at least four neutralization sites on the capsid spike of astroviruses. Our data show that natural infection with human astrovirus VA1 elicits a robust humoral immune response that targets the same antigenic sites recognized by the mouse monoclonal antibodies and strongly suggests the emergence of a variant HAstV-VA1 virus in an immunodeficient patient with prolonged astrovirus infection. The isolated Nt-MAb reported in this work will be helpful in defining the functional sites of the virus involved in cell entry and hold promise for developing a specific antibody therapy for the neurological disease associated with HAstV-VA1.

Structure and antigenicity of the divergent human astrovirus VA1 capsid spike

Human astrovirus (HAstV) is a known cause of viral gastroenteritis in children worldwide, but HAstV can cause also severe and systemic infections in immunocompromised patients. There are three clades of HAstV: classical, MLB, and VA/HMO. While all three clades are found in gastrointestinal samples, HAstV-VA/HMO is the main clade associated with meningitis and encephalitis in immunocompromised patients. To understand how the HAstV-VA/HMO can infect the central nervous system, we investigated its sequence-divergent capsid spike, which functions in cell attachment and may influence viral tropism. Here we report the high-resolution crystal structures of the HAstV-VA1 capsid spike from strains isolated from patients with gastrointestinal and neuronal disease. The HAstV-VA1 spike forms a dimer and shares a core beta-barrel structure with other astrovirus capsid spikes but is otherwise strikingly different, suggesting that HAstV-VA1 may utilize a different cell receptor, and an infection competition assay supports this hypothesis. Furthermore, by mapping the capsid protease cleavage site onto the structure, the maturation and assembly of the HAstV-VA1 capsid is revealed. Finally, comparison of gastrointestinal and neuronal HAstV-VA1 sequences, structures, and antigenicity suggests that neuronal HAstV-VA1 strains may have acquired immune escape mutations. Overall, our studies on the HAstV-VA1 capsid spike lay a foundation to further investigate the biology of HAstV-VA/HMO and to develop vaccines and therapeutics targeting it.

Molecular characteristics and pathogenicity of a novel chicken astrovirus variant

It is well-established that the genetic diversity, regional prevalence, and broad host range of astroviruses significantly impact the poultry industry. In July 2022, a small-scale commercial broiler farm in China reported cases of growth retardation and a 3% mortality rate. From chickens displaying proventriculitis and pancreatitis, three chicken astroviruses (CAstV) isolates were obtained and named SDAU2022-1-3. Complete genomic sequencing and analysis revealed the unique characteristics of these isolates from known CAstV strains in ORF1a, ORF1b, and ORF2 genes, characterized by an unusually high variability. Analysis of amino acid mutations in ORF1a, ORF1b, and ORF2 indicated that the accumulation of these mutations played a pivotal role in the emergence of the variant strain. Inoculation experiments demonstrated that affected chickens exhibited liver and kidney enlargement, localized proventricular hemorrhage, and a dark reddish-brown appearance in about two-thirds of the pancreas. Histopathological examination unveiled hepatic lymphocytic infiltration, renal tubular epithelial cell swelling, along with lymphocytic proventriculitis and pancreatitis. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis indicated viremia and viral shedding at 3 days post-infection (dpi). The proventriculus displayed the highest viral loads, followed by the liver, kidney, duodenum, and pancreas. Liver parameters (AST and ALT) and kidney parameters (UA and UN) demonstrated mild damage consistent with earlier findings. While the possibility of new mutations in the ORF2 gene of CAstV causing proventriculitis and pancreatitis warrants further investigation, these findings deepen our comprehension of CAstV's pathogenicity in chickens. Additionally, they serve as valuable references for subsequent research endeavors.

Structure and immunogenicity of the murine astrovirus capsid spike

Human astroviruses (HAstVs) are small, non-enveloped icosahedral RNA viruses that are a significant cause of diarrhoea in young children. Despite their worldwide prevalence, HAstV pathogenesis studies and vaccine development remain challenging due to the lack of an animal model for HAstV infection. The recent development of a murine astrovirus (MuAstV) infection model in mice provides the opportunity to test proof-of-concept vaccines based on MuAstV antigens. To help establish a system in which an astrovirus capsid spike-based vaccine could be tested in vivo, we designed and produced a recombinant MuAstV capsid spike protein based on predicted secondary structure homology to HAstV spike proteins. The recombinant MuAstV spike can be expressed with high efficiency in Escherichia coli and retains antigenicity to polyclonal antibodies elicited by MuAstV infection. We determined the crystal structure of the MuAstV spike to 1.75 Å and assessed its structural conservation with HAstV capsid spike. Despite low sequence identity between the MuAstV and HAstV spikes and differences in their overall shapes, they share related structural folds. Additionally, we found that vaccination with MuAstV spike induced anti-MuAstV-spike antibodies, highlighting that the recombinant spike is immunogenic. These studies lay a foundation for future in vivo MuAstV challenge studies to test whether MuAstV spike can be the basis of an effective vaccine.

Entry and egress of human astroviruses

Astroviruses encapsidate a positive-sense, single-stranded RNA genome into ∼30nm icosahedral particles that infect a wide range of mammalian and avian species, but their biology is not well understood. Human astroviruses (HAstV) are divided into three clades: classical HAstV serotypes 1-8, and novel or non-classical HAstV of the MLB and VA clades. These viruses are part of two genogroups and phylogenetically cluster with other mammalian astroviruses, highlighting their zoonotic potential. HAstV are a highly prevalent cause of nonbacterial gastroenteritis, primarily in children, the elderly and immunocompromised. Additionally, asymptomatic infections and extraintestinal disease (e.g., encephalitis), are also observed, mostly in immunocompetent or immunocompromised individuals, respectively. While these viruses are highly prevalent, no approved vaccines or antivirals are available to prevent or treat infections. This is in large part due to their understudied nature and the limited understanding of even very basic features of their life cycle and pathogenesis at the cellular and organismal level. This review will summarize molecular features of human astrovirus biology, pathogenesis, and tropism, and then focus on two stages of the viral life cycle, namely entry and egress, since these are proven targets for therapeutic interventions. We will further highlight gaps in knowledge in hopes of stimulating future research into these understudied viruses.

Human astrovirus capsid protein releases a membrane lytic peptide upon trypsin maturation

The human astrovirus (HAstV) is a non-enveloped, single-stranded RNA virus that is a common cause of gastroenteritis. Most non-enveloped viruses use membrane disruption to deliver the viral genome into a host cell after virus uptake. The virus-host factors that allow for HAstV cell entry are currently unknown but thought to be associated with the host-protease-mediated viral maturation. Using in vitro liposome disruption analysis, we identified a trypsin-dependent lipid disruption activity in the capsid protein of HAstV serotype 8. This function was further localized to the P1 domain of the viral capsid core, which was both necessary and sufficient for membrane disruption. Site-directed mutagenesis identified a cluster of four trypsin cleavage sites necessary to retain the lipid disruption activity, which is likely attributed to a short stretch of sequence ending at arginine 313 based on mass spectrometry of liposome-associated peptides. The membrane disruption activity was conserved across several other HAstVs, including the emerging VA2 strain, and effective against a wide range of lipid identities. This work provides key functional insight into the protease maturation process essential to HAstV infectivity and presents a method to investigate membrane penetration by non-enveloped viruses in vitro. IMPORTANCE Human astroviruses (HAstVs) are an understudied family of viruses that cause mild gastroenteritis but have recent cases associated with a more severe neural pathogenesis. Many important elements of the HAstV life cycle are not well understood, and further elucidating them can help understand the various forms of HAstV pathogenesis. In this study, we utilized an in vitro liposome-based assay to describe and characterize a previously unreported lipid disruption activity. This activity is dependent on the protease cleavage of key sites in HAstV capsid core and can be controlled by site-directed mutagenesis. Our group observed this activity in multiple strains of HAstV and in multiple lipid conditions, indicating this may be a conserved activity across the AstV family. The discovery of this function provides insight into HAstV cellular entry, pathogenesis, and a possible target for future therapeutics.

Structure of the divergent human astrovirus MLB capsid spike

Despite their worldwide prevalence and association with human disease, the molecular bases of human astrovirus (HAstV) infection and evolution remain poorly characterized. Here, we report the structure of the capsid protein spike of the divergent HAstV MLB clade (HAstV MLB). While the structure shares a similar folding topology with that of classical-clade HAstV spikes, it is otherwise strikingly different. We find no evidence of a conserved receptor-binding site between the MLB and classical HAstV spikes, suggesting that MLB and classical HAstVs utilize different receptors for host-cell attachment. We provide evidence for this hypothesis using a novel HAstV infection competition assay. Comparisons of the HAstV MLB spike structure with structures predicted from its sequence reveal poor matches, but template-based predictions were surprisingly accurate relative to machine-learning-based predictions. Our data provide a foundation for understanding the mechanisms of infection by diverse HAstVs and can support structure determination in similarly unstudied systems.

Complete genome sequence and phylogenetic analysis of a goose astrovirus isolate in China

Astroviruses are considered the cause of gastroenteritis in humans and animals. Studies in recent years show avian astroviruses are also associated with duckling hepatitis, gosling gout, and chicken nephritis. In this study, a GAstV strain, designated as JS2019/China, was detected in dead goslings from a commercial goose farm in Jiangsu province of China. Viral strain was proliferated in goose embryos and sequence analysis showed the isolated strain had a classical structure arrangement and a series of conserved regions compared with other GAstVs. Sequence comparison and phylogenetic analysis of whole genome and ORF2 revealed that JS2019/China belongs to the GAstV-1 group, which consists of most of the GAstV strains. Amino acid analysis indicated that some mutants might have an impact on viral protease capacity, such as V505I and K736E of ORF1a and T107I, F342S, and S606P of ORF2. Taken together, a novel GAstV strain was isolated and genomic analysis and protein polymorphism analysis indicated that some amino acid mutants might affect the viral virulence.

A Review of Emerging Goose Astrovirus Causing Gout

In recent years, an infection in geese caused by goose astrovirus (GAstV) has repeatedly occurred in coastal areas of China and rapidly spread to inland provinces. The infection is characterized by joint and visceral gout and is fatal. The disease has caused huge economic losses to China's goose industry. GAstV is a nonenveloped, single-stranded, positive-sense RNA virus. As it is a novel virus, there is no specific classification. Here, we review the current understanding of GAstV. The virus structure, isolation, diagnosis and detection, innate immune regulation, and transmission route are discussed. In addition, since GAstV can cause gout in goslings, the possible role of GAstV in gout formation and uric acid metabolism is discussed. We hope that this review will inform researchers to rapidly develop effective methods to prevent and treat this disease.

Heterotropic roles of divalent cations in the establishment of allostery and affinity maturation of integrin αXβ2

Allosteric activation and silencing of leukocyte β2-integrins transpire through cation-dependent structural changes, which mediate integrin biosynthesis and recycling, and are essential to designing leukocyte-specific drugs. Stepwise addition of Mg²⁺ reveals two mutually coupled events for the αXβ2 ligand-binding domain-the αX I-domain-corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the Mg²⁺-binding site establish long-range couplings, leading to both pH- and Mg²⁺-occupancy-dependent biphasic stability change in the αX I-domain fold. The ligand-binding sensorgrams show composite affinity events for the αX I-domain accounting for the multiplicity of the αX I-domain conformational states existing in the solution. On cell surfaces, increasing Mg²⁺ concentration enhanced adhesiveness of αXβ2. This work highlights how intrinsically flexible pH- and cation-sensitive architecture endows a unique dynamic continuum to the αI-domain structure on the intact integrin, thereby revealing the importance of allostery establishment and affinity maturation in both extracellular and intracellular integrin events.

Pathogenesis of Chicken Astrovirus Related Illnesses

Of the several known viruses, chicken astrovirus (CAstV) has been associated with diarrhea, runting-stunting syndrome, severe kidney disease, and gout, and white chick syndrome (WCS) in young broiler chicks. Discovered in 2004, CAstV consists of two genogroups with an expanding subgroup because of the diversity exhibited in its viral capsid sequence. Despite these findings, there exists a dearth of knowledge on its pathogenesis. This review highlights the pathogenesis and development of in vivo and in vitro models.

Development of a potential diagnostic monoclonal antibody against capsid spike protein VP27 of the novel goose astrovirus

Goose astrovirus (GAstVs) is an emerging pathogen of goslings that causes fatal gout, kidney hemorrhages, renomegaly, and high mortality. The GAstVs VP27 protein is an important capsid protein and a candidate for the development of diagnostic reagents. The aim of this study was to clone and express the VP27 gene for preparation of a specific monoclonal antibody (mAb). The VP27 protein was expressed and purified in the supernatant of Escherichia coli BL21. Then, the mAb was obtained with the hybridoma technique and named 2AF11. It was differentiated as IgG1 with the help of immunoglobulin subclass tests. This mAb can specifically recognize the VP27 protein in GAstVs-infected cells, as evidenced by western blot analysis and immunofluorescent assay. Furthermore, this mAb could also detect the VP27 protein in GAstVs-infected tissues, as demonstrated by immunohistochemistry. These findings indicate that this mAb has high diagnostic potential. Therefore, the newly produced anti-VP27 mAb, 2AF11, could be a useful tool as a specific diagnostic marker for GAstVs.

Identification of novel B-cell epitopes on the capsid protein of type 1 porcine astrovirus, using monoclonal antibodies

Porcine astrovirus (PAstV) is prevalent in pigs worldwide and could cause clinical symptoms such as diarrhea and encephalitis. The capsid protein (Cap) of PAstV plays a determinant role for virus immunological characteristics. In this study, the major antigenic regions of PAstV1 Cap were expressed through prokaryotic expression systems and immunized to BALB/c mice. Finally, two anti-Cap monoclonal antibodies (named mAb F4-4 and D3F10) were screened by indirect immune-fluorescence assay (IFA). A series of truncated GST-fused or artificially synthesized peptides were used to detect their reactivity with the mAbs and PAstV positive serum. Two novel B cell epitopes (120-GNNTFG-125, 485-RISDPTWFSA-494) were identified by using these two mAbs. Moreover, sequence alignment result showed that epitope 120-GNNTFG-125 was highly conserved in type 1 PAstV capsid protein. Cross-reactivity analysis further confirmed the genotype-specificity of mAb F4-4. The results of this study demonstrated to be the first description of monoclonal antibody preparation and B-cell epitope mapping on PAstV capsid protein, which may provide new information on the biological significance of PAstV capsid protein and lay a foundation for the development of PAstV immunological tests and genotype diagnostic methods.

Structures of Two Human Astrovirus Capsid/Neutralizing Antibody Complexes Reveal Distinct Epitopes and Inhibition of Virus Attachment to Cells

Human astrovirus is an important cause of viral gastroenteritis worldwide. Young children, the elderly, and the immunocompromised are especially at risk for contracting severe disease. However, no vaccines exist to combat human astrovirus infection. Evidence points to the importance of antibodies in protecting healthy adults from reinfection. To develop an effective subunit vaccine that broadly protects against diverse astrovirus serotypes, we must understand how neutralizing antibodies target the capsid surface at the molecular level. Here, we report the structures of the human astrovirus capsid spike domain bound to two neutralizing monoclonal antibodies. These antibodies bind two distinct conformational epitopes on the spike surface. We add to existing evidence that the human astrovirus capsid spike contains a receptor-binding domain and demonstrate that both antibodies neutralize human astrovirus by blocking virus attachment to host cells. We identify patches of conserved amino acids which overlap or border the antibody epitopes and may constitute a receptor-binding site. Our findings provide a basis for developing therapies to prevent and treat human astrovirus gastroenteritis.

IMPORTANCE Human astroviruses infect nearly every person in the world during childhood and cause diarrhea, vomiting, and fever. Despite the prevalence of this virus, little is known about how antibodies block astrovirus infection. Here, we determined the crystal structures of the astrovirus capsid protein in complex with two virus-neutralizing antibodies. We show that the antibodies bind to two distinct sites on the capsid spike domain, however, both antibodies block virus attachment to human cells. Importantly, our findings support the use of the human astrovirus capsid spike as an antigen in a subunit-based vaccine to prevent astrovirus disease.

Genome characterization, prevalence and tissue distribution of astrovirus, hepevirus and norovirus among wild and laboratory rats (Rattus norvegicus) and mice (Mus musculus) in Hungary

Rodents including rats are reservoir of several pathogens capable of affecting human health. In this study, faecal and different organ specimens from free-living Norway rats (Rattus norvegicus) (N = 18) and faecal samples from laboratory rodents (rats N = 21 and mice N = 20) collected from different geographic areas in Hungary between 2017 and 2020 were investigated by viral metagenomics and conventional RT-PCR methods. The complete genome of three different RNA viruses, rat astrovirus, rat norovirus and rat hepevirus were characterized and analysed in detail. Rat norovirus was detected in faecal (17.6%, 3/17) and kidney (7.1%, 1/14) samples; rat astrovirus in faecal (23.5%, 4/17) and spleen (13.3%, 2/15) samples, and rat hepevirus in 43% to 67% the faecal, liver, kidney, lung, heart, muscle, brain and blood samples from Norway rats, respectively. Rat norovirus was also identifiable in 5% (1/21) of laboratory rats and rat astrovirus in 40% (8/20) of faecal samples from laboratory mice. Co-infections were found in 28% (5/18) wild Norway rats. The highest RNA viral load of astrovirus (1.81 × 108 copy/g) and norovirus (3.49 × 107 copy/g) were measured in faecal samples; while the highest RNA viral load of hepevirus (1.16 × 109 copy/g) was found in liver samples of Norway rats, respectively. This study confirms the wide geographic distribution and high prevalence of astrovirus, norovirus and hepevirus among wild rats in Hungary with confirmation of different organ involvement of as well as the detection of norovirus and astrovirus in laboratory rats and mice, respectively. This finding further strengthens the role of rodents in the spread of viral pathogens especially infecting human.

Exploring the Cause of Diarrhoea and Poor Growth in 8-11-Week-Old Pigs from an Australian Pig Herd Using Metagenomic Sequencing

Diarrhoea and poor growth among growing pigs is responsible for significant economic losses in pig herds globally and can have a wide range of possible aetiologies. Next generation sequencing (NGS) technologies are useful for the detection and characterisation of diverse groups of viruses and bacteria and can thereby provide a better understanding of complex interactions among microorganisms potentially causing clinical disease. Here, we used a metagenomics approach to identify and characterise the possible pathogens in colon and lung samples from pigs with diarrhoea and poor growth in an Australian pig herd. We identified and characterized a wide diversity of porcine viruses including RNA viruses, in particular several picornaviruses—porcine sapelovirus (PSV), enterovirus G (EV-G), and porcine teschovirus (PTV), and a porcine astrovirus (PAstV). Single stranded DNA viruses were also detected and included parvoviruses like porcine bocavirus (PBoV) and porcine parvovirus 2 (PPV2), porcine parvovirus 7 (PPV7), porcine bufa virus (PBuV), and porcine adeno-associated virus (AAV). We also detected single stranded circular DNA viruses such as porcine circovirus type 2 (PCV2) at very low abundance and torque teno sus viruses (TTSuVk2a and TTSuVk2b). Some of the viruses detected here may have had an evolutionary past including recombination events, which may be of importance and potential involvement in clinical disease in the pigs. In addition, our metagenomics data found evidence of the presence of the bacteria Lawsonia intracellularis, Brachyspira spp., and Campylobacter spp. that may, together with these viruses, have contributed to the development of clinical disease and poor growth.

Development of immunochromatographic strip assay for rapid detection of novel goose astrovirus

The novel goose astrovirus (GoAstV) is an emerging pathogenic virus that has resulted in large economic losses to the goose-rearing industry in China since 2016. The novel goose astrovirus cause gout in goslings with a mortality rate of around 50 %. Therefore, an effective diagnostic approach to monitor the spread of GoAstV is necessary. Here, a novel diagnostic immunochromatographic strip (ICS) assay was developed to detect GoAstV. A rapid immunochromatographic assay based on antibody colloidal gold nanoparticles specific to GoAstV was developed for the detection of GoAstV in goose allantoic fluid and supernatant of tissue homogenate. Monoclonal antibodies (Mabs) were prepared using the hybridoma technology, and the polyclonal antibodies (Pabs) were generated by immunizing the rabbits with recombinant ORF2 protein. In addition, the colloidal gold was prepared by reducing gold salt with sodium citrate coupled with Mabs against GoAstV. The optimal concentrations of the coating antibody and the capture antibody were examined as 1.6 mg/mL and 6 μg/mL. The optimal pH of the colloidal gold labeling was pH 8.0. With the visual observation, the lower limit of the ICS was reported to be approximately 1.2 μg/mL. Common diseases of goose were examined to assess the specificity of the ICS, and no cross-reaction was identified. 40 clinical positive samples were simultaneously detected by using the ICS and the PCR with a 92.5% coincidence rate between them. Furthermore, the mentioned samples could be stored at 25 °C and 4 °C for 4 and 6 months, respectively. It was proved that the ICS in this study was highly specific, sensitive, repeatable and more convenient to rapidly detect GoAstV in clinical samples.

Human Astrovirus 1-8 Seroprevalence Evaluation in a United States Adult Population

Human astroviruses are an important cause of viral gastroenteritis globally, yet few studies have investigated the serostatus of adults to establish rates of previous infection. Here, we applied biolayer interferometry immunosorbent assay (BLI-ISA), a recently developed serosurveillance technique, to measure the presence of blood plasma IgG antibodies directed towards the human astrovirus capsid spikes from serotypes 1-8 in a cross-sectional sample of a United States adult population. The seroprevalence rates of IgG antibodies were 73% for human astrovirus serotype 1, 62% for serotype 3, 52% for serotype 4, 29% for serotype 5, 27% for serotype 8, 22% for serotype 2, 8% for serotype 6, and 8% for serotype 7. Notably, seroprevalence rates for capsid spike antigens correlate with neutralizing antibody rates determined previously. This work is the first seroprevalence study evaluating all eight classical human astrovirus serotypes.

Structural Insights into the Human Astrovirus Capsid

Astroviruses (AstVs) are non-enveloped, positive single-stranded RNA viruses that cause a wide range of inflammatory diseases in mammalian and avian hosts. The T = 3 viral capsid is unique in its ability to infect host cells in a process driven by host proteases. Intercellular protease cleavages allow for viral egress from a cell, while extracellular cleavages allow for the virus to enter a new host cell to initiate infection. High-resolution models of the capsid core indicate a large, exposed region enriched with protease cleavage sites. The virus spike protein allows for binding to target cells and is the major target for naturally occurring and engineered neutralizing antibodies. During maturation, the capsid goes through significant structural changes including the loss of many surface spikes. The capsid interacts with host membranes during the virus life cycle at multiple stages such as assembly, host cell entry and exit. This review will cover recent findings and insights related to the structure of the capsid and its function. Further understanding of the viral capsid structure and maturation process can contribute to new vaccines, gastric therapeutics, and viral engineering applications.

Protein Disulfide Isomerase A4 Is Involved in Genome Uncoating during Human Astrovirus Cell Entry

Although human astroviruses (HAstVs) are important agents of gastroenteritis in young children, the studies aimed at characterizing their biology have been limited, in particular regarding their cell entry process. It has been shown that HAstV serotype 8 enters human cells by a classical clathrin-mediated endocytosis pathway; however, the cell receptor or other cell entry factors that may be relevant for an efficient viral infection are unknown. In this work we used a far-Western blotting approach to identify cellular proteins that interact with the recombinant capsid spike proteins of HAstV serotypes 1, 2, and 8, synthesized in Escherichia coli. We identified the 72 kDa protein disulfide isomerase A4 (PDIA4) as a binding partner for HAstV-1 and -8 spikes, but not for the HAstV-2 spike. In agreement with this observation, the PDI inhibitor 16F16 strongly blocked infection by HAstV serotypes 1 and 8, but not serotype 2. RNA interference of PDIA4 expression selectively blocked HAstV-8 infectivity. We also showed that the PDI activity does not affect virus binding or internalization but is required for uncoating of the viral genome.

Epidemiology, genetic diversity and evolution of canine astrovirus

Astroviruses (AstVs) are major causative agents of gastroenteritis in children and have been reported in many species. Canine astrovirus (CaAstV), as an enteric pathogen, has been widely detected worldwide, but little is known about their genetic diversity and evolution, partly owing to a lack of genomic data. Here, we sequenced 12 nearly full-length CaAstV genomes to address the gap in knowledge. We found 14 (13.2%) and 7 (3.35%) CaAstV-positive samples from pet dogs with and without diarrhoea, respectively. Co-infections were with co-infection with Torque teno canis virus (TTCaV) reported for the first time. Phylogenetic analysis of the ORF2 gene revealed four major lineages. In particular, lineage 4 might have evolved from a recombinant virus from lineage 2 and lineage 3. The strains sequenced here clustered with lineages 2, 3 and 4 in contrast with other Chinese strains identified previously that clustered with lineages 2 and 4. Amino acid sequence alignment within lineage revealed intralineage amino acid diversity and that the type of epidemic strains within lineages changes over time. Three amino acids substitutions located in predicted B-cell epitopes, which might be involved escape of host immunity. Moreover, frequent inter-clade ORF2 gene recombinants were identified. The identification of individual recombination events and a recombinant lineage indicated that recombination plays a crucial role in CaAstV genetic evolution and diversity by generating divergent viruses. Moreover, phylogenetic analysis of ORF1b, the most conserved gene of astrovirus, revealed a close relationship between CaAstV and California sea lion astroviruses. Overall, we report detailed information on the genetic evolution and diversity of CaAstV, which indicates that CaAstV may pose challenges for diagnostics and future control strategies.

Generation and evaluation of a recombinant goose origin Newcastle disease virus expressing Cap protein of goose origin avastrovirus as a bivalent vaccine in goslings

Currently, both goose astrovirus (GoAstV) and goose-origin Newcastle disease virus (NDV) are widely infectious agents for goslings. There is no vaccine for GoAstV. Capsid protein can elicit a neutralizing antibody in human astroviruses (HAstV). Molecular analysis of the genomic region encoding the capsid protein(ORF2) of goose astrovirus has revealed that it contains neutralizing epitopes. Goose-origin NDV is also an infectious agent. A wide range of NDV strains exist that can be commonly used as vaccine vectors. In the present study, the fusion protein cleavage site RRQKR↓F in a backbone of the virulent goose-origin NDV SH-12 was changed into an avirulent motif GRQGR↓L. The modified goose-origin NDV recombinant vaccine virus expressing the Capsid protein (Cap) of GoAstV was generated as a bivalent vaccine using a reverse-genetics approach. The recombinant virus, rNDV/GoAstV-Cap, was attenuated and similar growth dynamics, cytopathic effects, and virus titers in vitro were maintained when compared to the LaSota strain. Expression of the GoAstV-Cap protein in rNDV/GoAstV-Cap infected cells was detected by an immunofluorescence assay and Western blotting. Goslings inoculated with rNDV/GoAstV-Cap showed no apparent signs of disease and induced GoAstV-Cap-specific immune responses and NDV-specific serum antibody responses to a LaSota vaccination control. Complete protection against a pathogenic GoAstV challenge and avelogenic NDV challenge was conferred. The results of the study suggested that rNDV/GoAstV-Cap viruses have the potential to be the safe, stable, and effective bivalent vaccines.

Immunogenicity and Efficacy Evaluation of Subunit Astrovirus Vaccines

A full understanding of the immune response to astrovirus (AstV) infection is required to treat and control AstV-induced gastroenteritis. Relative contributions of each arm of the immune system in restricting AstV infection remain unknown. In this study, two novel subunit AstV vaccines derived from capsid protein (CP) of mink AstV (MAstV) such as CPΔN (spanning amino acids 161–775) and CPΔC (spanning amino acids 1–621) were evaluated. Their immunogenicity and cytokine production in mice, as well as protective efficacy in mink litters via maternal immunization, were studied. Truncated CPs induced higher levels of serum anti-CP antibodies than CP, with the highest level for CPΔN. No seronegativity was detected after booster immunization with either AstV CP truncates in both mice and mink. All mink moms stayed seropositive during the entire 104-day study. Furthermore, lymphoproliferation responses and Th1/Th2 cytokine induction of mice splenocytes ex vivo re-stimulated by truncated CPs were significantly higher than those by CP, with the highest level for CPΔN. Immunization of mink moms with truncated CPs could suppress virus shedding and clinical signs in their litters during a 51-day study after challenge with a heterogeneous MAstV strain. Collectively, AstV truncated CPs exhibit better parameters for protection than full-length CP.

Targeting the Viral Polymerase of Diarrhea-Causing Viruses as a Strategy to Develop a Single Broad-Spectrum Antiviral Therapy

Viral gastroenteritis is an important cause of morbidity and mortality worldwide, being particularly severe for children under the age of five. The most common viral agents of gastroenteritis are noroviruses, rotaviruses, sapoviruses, astroviruses and adenoviruses, however, no specific antiviral treatment exists today against any of these pathogens. We here discuss the feasibility of developing a broad-spectrum antiviral treatment against these diarrhea-causing viruses. This review focuses on the viral polymerase as an antiviral target, as this is the most conserved viral protein among the diverse viral families to which these viruses belong to. We describe the functional and structural similarities of the different viral polymerases, the antiviral effect of reported polymerase inhibitors and highlight common features that might be exploited in an attempt of designing such pan-polymerase inhibitor.

Isolation of Neutralizing Monoclonal Antibodies to Human Astrovirus and Characterization of Virus Variants That Escape Neutralization

Human astroviruses (HAstVs) cause severe diarrhea and represent an important health problem in children under two years of age. Despite their medical importance, the study of these pathogens has been neglected. To better understand the astrovirus antigenic structure and the basis of protective immunity, in this work we produced a panel of neutralizing monoclonal antibodies (Nt-MAbs) to HAstV serotypes 1, 2, and 8 and identified the mutations that allow the viruses to escape neutralization. We first tested the capacity of the recombinant HAstV capsid core and spike domains to elicit Nt-Abs. Hyperimmunization of animals with the two domains showed that although both induced a potent immune response, only the spike was able to elicit antibodies with neutralizing activity. Based on this finding, we used a mixture of the recombinant spike domains belonging to the three HAstV serotypes to immunize mice. Five Nt-MAbs were isolated and characterized; all of them were serotype specific, two were directed to HAstV-1, one was directed to HAstV-2, and two were directed to HAstV-8. These antibodies were used to select single and double neutralization escape variant viruses, and determination of the amino acid changes that allow the viruses to escape neutralization permitted us to define the existence of four potentially independent neutralization epitopes on the HAstV capsid. These studies provide the basis for development of subunit vaccines that induce neutralizing antibodies and tools to explore the possibility of developing a specific antibody therapy for astrovirus disease. Our results also establish a platform to advance our knowledge on HAstV cell binding and entry.IMPORTANCE Human astroviruses (HAstVs) are common etiological agents of acute gastroenteritis in children, the elderly, and immunocompromised patients; some virus strains have also been associated with neurological disease. Despite their medical importance, the study of these pathogens has advanced at a slow pace. In this work, we produced neutralizing antibodies to the virus and mapped the epitopes they recognize on the virus capsid. These studies provide the basis for development of subunit vaccines that induce neutralizing antibodies, as well as tools to explore the development of a specific antibody therapy for astrovirus disease. Our results also establish a platform to advance our knowledge on HAstV cell binding and entry.

Outbreaks of Neuroinvasive Astrovirus Associated with Encephalomyelitis, Weakness, and Paralysis among Weaned Pigs, Hungary

A large, highly prolific swine farm in Hungary had a 2-year history of neurologic disease among newly weaned (25- to 35-day-old) pigs, with clinical signs of posterior paraplegia and a high mortality rate. Affected pigs that were necropsied had encephalomyelitis and neural necrosis. Porcine astrovirus type 3 was identified by reverse transcription PCR and in situ hybridization in brain and spinal cord samples in 6 animals from this farm. Among tissues tested by quantitative RT-PCR, the highest viral loads were detected in brain stem and spinal cord. Similar porcine astrovirus type 3 was also detected in archived brain and spinal cord samples from another 2 geographically distant farms. Viral RNA was predominantly restricted to neurons, particularly in the brain stem, cerebellum (Purkinje cells), and cervical spinal cord. Astrovirus was generally undetectable in feces but present in respiratory samples, indicating a possible respiratory infection. Astrovirus could cause common, neuroinvasive epidemic disease.

Minimal capsid composition of infectious human astrovirus

Human astrovirus is an important etiological agent of acute gastroenteritis in young children. Despite advances in the characterization of the structure of the virion by cryo-electron microscopy and of capsid proteins by x-ray crystallography, the definition of the minimal polypeptide composition of infectious virus particles has been elusive. In this work we show that mature infectious particles are composed by only two proteins; VP34 that forms the core domain of the virus, and VP27 that constitutes the 30 dimeric spikes present on the virus surface. Our results also indicate that during the transition of immature (90 spikes) to mature (30 spikes) virus particles, that occur during trypsin activation, the viral protein VP25, that most likely forms the 60 spikes that are lost during maturation, detaches from the virus particle. This information is relevant to better understand the biology of virus entry and also for the efficient development of subunit vaccines.

Molecular evolutionary analysis of type-1 human astroviruses identifies putative sites under selection pressure on the capsid protein

Human astroviruses (HAstV) are important enteric pathogens that can be classified into eight sero/genotypes (HAstV-1 to -8). Although the various HAstV types show global spread, type-1 strains tend to be predominant. Molecular analysis of the genomic region encoding the capsid protein (ORF2) has revealed discrete sequence variation, with different lineages within each HAstV type and at least three major lineages have been identified within HAstV-1. Longitudinal epidemiological surveillance has revealed temporal shift of the various HAstV-1 lineages. Metadata analysis of HAstV-1 sequences available in the databases also revealed temporal shifts of the circulation of HAstV-1 lineages, suggesting possible antigenic-related mechanisms of selection at the sub-genotype level. By comparison of HAstV-1 capsid sequences, lineage-defining residues under positive selection were identified. Structural analysis of HAstV-1 capsid allowed identifying at least six residues exposed on the virion surface. Two residues were located in the VP34 (shell region) whilst four residues were mapped in the VP25/27 (protruding region) of HAstV capsid protein, in proximity of the putative receptor binding S site. These findings suggest that mechanisms similar to those observed and/or hypothesized for other enteric viruses are also shaping the evolution of HAstVs, with intra-typic diversification being a possible mechanism to decrease the antigenic pressure to which these viruses are exposed.

Structural Basis for Escape of Human Astrovirus from Antibody Neutralization: Broad Implications for Rational Vaccine Design

Human astroviruses are recognized as a leading cause of viral diarrhea worldwide in children, immunocompromised patients, and the elderly. There are currently no vaccines available to prevent astrovirus infection; however, antibodies developed by healthy individuals during previous infection correlate with protection from reinfection, suggesting that an effective vaccine could be developed. In this study, we investigated the molecular mechanism by which several strains of human astrovirus serotype 2 (HAstV-2) are resistant to the potent HAstV-2-neutralizing monoclonal antibody PL-2 (MAb PL-2). Sequencing of the HAstV-2 capsid genes reveals mutations in the PL-2 epitope within the capsid's spike domain. To understand the molecular basis for resistance from MAb PL-2 neutralization, we determined the 1.35-Å-resolution crystal structure of the capsid spike from one of these HAstV-2 strains. Our structure reveals a dramatic conformational change in a loop within the PL-2 epitope due to a serine-to-proline mutation, locking the loop in a conformation that sterically blocks binding and neutralization by MAb PL-2. We show that mutation to serine permits loop flexibility and recovers MAb PL-2 binding. Importantly, we find that HAstV-2 capsid spike containing a serine in this loop is immunogenic and elicits antibodies that neutralize all HAstV-2 strains. Taken together, our results have broad implications for rational selection of vaccine strains that do not contain prolines in antigenic loops, so as to elicit antibodies against diverse loop conformations.IMPORTANCE Human astroviruses (HAstVs) infect nearly every person in the world during childhood and cause diarrhea, vomiting, and fever. In this study, we investigated how several strains of HAstV are resistant to a virus-neutralizing monoclonal antibody. We determined the crystal structure of the capsid protein spike domain from one of these HAstV strains and found that a single amino acid mutation induces a structural change in a loop that is responsible for antibody binding. Our findings reveal how viruses can escape antibody neutralization and provide insight for the rational design of vaccines to elicit diverse antibodies that provide broader protection from infection.

Development of a multiplex serological assay reveals a worldwide distribution of murine astrovirus infections in laboratory mice

Laboratory mice play a tremendous role in biomedical research in studies on immunology, infection, cancer and therapy. In the course of standardization of mice used in animal experiments, health monitoring constitutes an important instrument towards microbiological standardization. Infections with murine astroviruses (MuAstV) were only recently discovered and are, therefore, still relatively unknown in laboratory animal science. In rodent health monitoring viral infections within a population are commonly assessed in terms of specific antibodies by serological testing, as active infection and excretion of virus is often temporary and can easily be missed. So far only ongoing infections with astroviruses can be detected by PCR. The objective of this work was the development of a sensitive and specific MuAstV multiplex serological assay with a high-throughput capability to be used in routine testing of laboratory mice. Four different MuAstV proteins were recombinantly expressed and used as antigens. The best reacting antigen, the capsid spike protein VP27, was selected and tested with a panel of 400 sera of mice from units with a known MuAstV status. Assay sensitivity and specificity resulted in 98.5% and 100%, respectively, compared to RT-PCR results. Eventually this assay was used to test 5529 serum samples in total, during routine diagnostics at the German Cancer Research Center (DKFZ) in Heidelberg between 2015 and 2017. High sero-prevalence rates of up to 98% were detected in units with open cages indicating that the virus is highly infectious and circulates within these populations virtually infecting all animals regardless of the mouse strain. In addition, data collected from 312 mice purchased from commercial breeders and from 661 mice from 58 research institutes in 15 countries worldwide allowed the conclusion that MuAstV is widespread in contemporary laboratory mouse populations.

The Expanding Field of Mammalian Astroviruses: Opportunities and Challenges in Clinical Virology

Astroviruses are best known as being one of the leading causes of diarrhea in infants and were first described in this context in 1975. In its first years, astrovirus research was mainly restricted to electron microscopy and serology studies. The ability to culture some of these viruses in vitro allowed a first consequent step forward, especially at the molecular level. Since the emergence of more powerful genetic methods, though, the face of this research field has dramatically changed and evolved. From the exponential number of discoveries of new astrovirus strains in the most varied of animal species to their association with atypical diseases, these viruses revealed a lot of surprises, and many more are probably still waiting to be uncovered. This chapter summarizes the most important knowledge about astroviruses and discusses the implication of the latest findings in this area of research.

Viral hijacking of host caspases: an emerging category of pathogen-host interactions

Viruses co-evolve with their hosts, and many viruses have developed mechanisms to suppress or modify the host cell apoptotic response for their own benefit. Recently, evidence has emerged for the opposite strategy. Some viruses have developed the ability to co-opt apoptotic caspase activity to facilitate their own proliferation. In these strategies, viral proteins are cleaved by host caspases to create cleavage products with novel activities which facilitate viral replication. This represents a novel and interesting class of viral-host interactions, and also represents a new group of non-apoptotic roles for caspases. Here we review the evidence for such strategies, and discuss their origins and their implications for our understanding of the relationship between viral pathogenesis and programmed cell death.

Astrovirus Biology and Pathogenesis

Astroviruses are nonenveloped, positive-sense single-stranded RNA viruses that cause gastrointestinal illness. Although a leading cause of pediatric diarrhea, human astroviruses are among the least characterized enteric RNA viruses. However, by using in vitro methods and animal models to characterize virus-host interactions, researchers have discovered several important properties of astroviruses, including the ability of the astrovirus capsid to act as an enterotoxin, disrupting the gut epithelial barrier. Improved animal models are needed to study this phenomenon, along with the pathogenesis of astroviruses, particularly in those strains that can cause extraintestinal disease. Much like for other enteric viruses, the current dogma states that astroviruses infect in a species-specific manner; however, this assumption is being challenged by growing evidence that these viruses have potential to cross species barriers. This review summarizes these remarkable facets of astrovirus biology, highlighting critical steps toward increasing our understanding of this unique enteric pathogen.

The Broad Host Range and Genetic Diversity of Mammalian and Avian Astroviruses

Astroviruses are a diverse family of viruses that infect a wide range of mammalian and avian hosts. Here we describe the phylogenetic diversity and current classification methodology of astroviruses based on the ORF1b and ORF2 genes, highlighting the propensity of astroviruses to undergo interspecies transmission and genetic recombination which greatly increase diversity and complicate attempts at a unified and comprehensive classification strategy.

Whole genome analysis of porcine astroviruses detected in Japanese pigs reveals genetic diversity and possible intra-genotypic recombination

Porcine astroviruses (PoAstVs) are ubiquitous enteric virus of pigs that are distributed in several countries throughout the world. Since PoAstVs are detected in apparent healthy pigs, the clinical significance of infection is unknown. However, AstVs have recently been associated with a severe neurological disorder in animals, including humans, and zoonotic potential has been suggested. To date, little is known about the epidemiology of PoAstVs among the pig population in Japan. In this report, we present an analysis of nearly complete genomes of 36 PoAstVs detected by a metagenomics approach in the feces of Japanese pigs. Based on a phylogenetic analysis and pairwise sequence comparison, 10, 5, 15, and 6 sequences were classified as PoAstV2, PoAstV3, PoAstV4, and PoAstV5, respectively. Co-infection with two or three strains was found in individual fecal samples from eight pigs. The phylogenetic trees of ORF1a, ORF1b, and ORF2 of PoAstV2 and PoAstV4 showed differences in their topologies. The PoAstV3 and PoAstV5 strains shared high sequence identities within each genotype in all ORFs; however, one PoAstV3 strain and one PoAstV5 strain showed considerable sequence divergence from the other PoAstV3 and PoAstV5 strains, respectively, in ORF2. Recombination analysis using whole genomes revealed evidence of multiple possible intra-genotype recombination events in PoAstV2 and PoAstV4, suggesting that recombination might have contributed to the genetic diversity and played an important role in the evolution of Japanese PoAstVs.

Astrovirus Pathogenesis

Astroviruses are a major cause of diarrhea in the young, elderly, and the immunocompromised. Since the discovery of human astrovirus type 1 (HAstV-1) in 1975, the family Astroviridae has expanded to include two more human clades and numerous mammalian and avian-specific genotypes. Despite this, there is still little known about pathogenesis. The following review highlights the current knowledge of astrovirus pathogenesis, and outlines the critical steps needed to further astrovirus research, including the development of animal models of cell culture systems.

The Astrovirus Capsid: A Review

Astroviruses are enterically transmitted viruses that cause infections in mammalian and avian species. Astroviruses are nonenveloped, icosahedral viruses comprised of a capsid protein shell and a positive-sense, single-stranded RNA genome. The capsid protein undergoes dramatic proteolytic processing both inside and outside of the host cell, resulting in a coordinated maturation process that affects cellular localization, virus structure, and infectivity. After maturation, the capsid protein controls the initial phases of virus infection, including virus attachment, endocytosis, and genome release into the host cell. The astrovirus capsid is the target of host antibodies including virus-neutralizing antibodies. The capsid protein also mediates the binding of host complement proteins and inhibits complement activation. Here, we will review our knowledge on the astrovirus capsid protein (CP), with particular attention to the recent structural, biochemical, and virological studies that have advanced our understanding of the astrovirus life cycle.

Structure of a Human Astrovirus Capsid-Antibody Complex and Mechanistic Insights into Virus Neutralization

Human astroviruses (HAstVs) are a leading cause of viral diarrhea in young children, the immunocompromised, and the elderly. There are no vaccines or antiviral therapies against HAstV disease. Several lines of evidence point to the presence of protective antibodies in healthy adults as a mechanism governing protection against reinfection by HAstV. However, development of anti-HAstV therapies is hampered by the gap in knowledge of protective antibody epitopes on the HAstV capsid surface. Here, we report the structure of the HAstV capsid spike domain bound to the neutralizing monoclonal antibody PL-2. The antibody uses all six complementarity-determining regions to bind to a quaternary epitope on each side of the dimeric capsid spike. We provide evidence that the HAstV capsid spike is a receptor-binding domain and that the antibody neutralizes HAstV by blocking virus attachment to cells. We identify patches of conserved amino acids that overlap the antibody epitope and may comprise a receptor-binding site. Our studies provide a foundation for the development of therapies to prevent and treat HAstV diarrheal disease.

IMPORTANCE

Human astroviruses (HAstVs) infect nearly every person in the world during childhood and cause diarrhea, vomiting, and fever. Despite the prevalence of this virus, little is known about how antibodies in healthy adults protect them against reinfection. Here, we determined the crystal structure of a complex of the HAstV capsid protein and a virus-neutralizing antibody. We show that the antibody binds to the outermost spike domain of the capsid, and we provide evidence that the antibody blocks virus attachment to human cells. Importantly, our findings suggest that a subunit-based vaccine focusing the immune system on the HAstV capsid spike domain could be effective in protecting children against HAstV disease.

The Immune Response to Astrovirus Infection

Astroviruses are one of the leading causes of pediatric gastroenteritis worldwide and are clinically importantly pathogens in the elderly and immunocompromised populations. Although the use of cell culture systems and small animal models have enhanced our understanding of astrovirus infection and pathogenesis, little is known about the immune response to astrovirus infection. Studies from humans and animals suggest that adaptive immunity is important in restricting classic and novel astrovirus infections, while studies from animal models and cell culture systems suggest that an innate immune system plays a role in limiting astrovirus replication. The relative contribution of each arm of the immune system in restricting astrovirus infection remains unknown. This review summarizes our current understanding of the immune response to astrovirus infection and highlights some of the key questions that stem from these studies. A full understanding of the immune response to astrovirus infection is required to be able to treat and control astrovirus-induced gastroenteritis.

Ancient recombination events and the origins of hepatitis E virus

BACKGROUND

Hepatitis E virus (HEV) is an enteric, single-stranded, positive sense RNA virus and a significant etiological agent of hepatitis, causing sporadic infections and outbreaks globally. Tracing the evolutionary ancestry of HEV has proved difficult since its identification in 1992, it has been reclassified several times, and confusion remains surrounding its origins and ancestry.

RESULTS

To reveal close protein relatives of the Hepeviridae family, similarity searching of the GenBank database was carried out using a complete Orthohepevirus A, HEV genotype I (GI) ORF1 protein sequence and individual proteins. The closest non-Hepeviridae homologues to the HEV ORF1 encoded polyprotein were found to be those from the lepidopteran-infecting Alphatetraviridae family members. A consistent relationship to this was found using a phylogenetic approach; the Hepeviridae RdRp clustered with those of the Alphatetraviridae and Benyviridae families. This puts the Hepeviridae ORF1 region within the "Alpha-like" super-group of viruses. In marked contrast, the HEV GI capsid was found to be most closely related to the chicken astrovirus capsid, with phylogenetic trees clustering the Hepeviridae capsid together with those from the Astroviridae family, and surprisingly within the "Picorna-like" supergroup. These results indicate an ancient recombination event has occurred at the junction of the non-structural and structure encoding regions, which led to the emergence of the entire Hepeviridae family. The Astroviridae capsid is also closely related to the Tymoviridae family of monopartite, T = 3 icosahedral plant viruses, whilst its non-structural region is related to viruses of the Potyviridae; a large family of plant-infecting viruses with a flexible filamentous rod-shaped virion. Thus, we identified a separate inter-viral family recombination event, again at the non-structural/structural junction, which likely led to the creation of the Astroviridae.

CONCLUSIONS

In summary, we have shown that new viral families have been created though recombination at the junction of the genome that encodes non-structural and structural proteins, and such recombination events are implicated in the genesis of important human pathogens; HEV, astrovirus and rubella virus.

Crystal Structure of the Human Astrovirus Capsid Protein

Human astrovirus (HAstV) is a leading cause of viral diarrhea in infants and young children worldwide. HAstV is a nonenveloped virus with a T=3 capsid and a positive-sense RNA genome. The capsid protein (CP) of HAstV is synthesized as a 90-kDa precursor (VP90) that can be divided into three linear domains: a conserved N-terminal domain, a hypervariable domain, and an acidic C-terminal domain. Maturation of HAstV requires proteolytic processing of the astrovirus CP both inside and outside the host cell, resulting in the removal of the C-terminal domain and the breakdown of the rest of the CP into three predominant protein species with molecular masses of ∼34, 27/29, and 25/26 kDa, respectively. We have now solved the crystal structure of VP90(71-415) (amino acids [aa] 71 to 415 of VP90) of human astrovirus serotype 8 at a 2.15-Å resolution. VP90(71-415) encompasses the conserved N-terminal domain of VP90 but lacks the hypervariable domain, which forms the capsid surface spikes. The structure of VP90(71-415) is comprised of two domains: an S domain, which adopts the typical jelly-roll β-barrel fold, and a P1 domain, which forms a squashed β-barrel consisting of six antiparallel β-strands similar to what was observed in the hepatitis E virus (HEV) capsid structure. Fitting of the VP90(71-415) structure into the cryo-electron microscopy (EM) maps of HAstV produced an atomic model for a continuous, T=3 icosahedral capsid shell. Our pseudoatomic model of the human HAstV capsid shell provides valuable insights into intermolecular interactions required for capsid assembly and trypsin-mediated proteolytic maturation needed for virus infectivity. Such information has potential applications in the development of a virus-like particle (VLP) vaccine as well as small-molecule drugs targeting astrovirus assembly/maturation.

IMPORTANCE

Human astrovirus (HAstV) is a leading cause of viral diarrhea in infants and young children worldwide. As a nonenveloped virus, HAstV exhibits an intriguing feature in that its maturation requires extensive proteolytic processing of the astrovirus capsid protein (CP) both inside and outside the host cell. Mature HAstV contains three predominant protein species, but the mechanism for acquired infectivity upon maturation is unclear. We have solved the crystal structure of VP90(71-415) of human astrovirus serotype 8. VP90(71-415) encompasses the conserved N-terminal domain of the viral CP. Fitting of the VP90(71-415) structure into the cryo-EM maps of HAstV produced an atomic model for the T=3 icosahedral capsid. Our model of the HAstV capsid provides valuable insights into intermolecular interactions required for capsid assembly and trypsin-mediated proteolytic maturation. Such information has potential applications in the development of a VLP vaccine as well as small-molecule drugs targeting astrovirus assembly/maturation.

A trivalent vaccine candidate against hepatitis E virus, norovirus, and astrovirus

Hepatitis E virus (HEV), norovirus (NoV), and astrovirus (AstV) are enterically-transmitted viral pathogens causing epidemic or endemic hepatitis (HEV) and gastroenteritis (NoV and AstV) respectively in humans, leading to significant morbidity and mortality worldwide. While a recombinant subunit vaccine against HEVs is available in China, there is no commercial vaccine or antiviral against NoV or AstV. We report here our development of a trivalent vaccine against the three viral pathogens through our new polymer vaccine technology. All HEV, NoV, and AstV are non-enveloped RNA viruses covered by a protein capsid, featuring surface protruding (P) proteins that are responsible for virus-host interaction. These dimeric P proteins elicit neutralizing antibody and are good targets for subunit vaccine development. The trivalent subunit vaccine was developed by fusion of the dimeric P domains of the three viruses together that formed tetramers. This trivalent vaccine elicited significantly higher antibody responses in mice against all three P domains than those induced by a mixture of the three free P domains (mixed vaccine). Furthermore, the post-immune antisera of the trivalent vaccine showed significantly higher neutralizing titers against HEV infection in cell culture and higher blocking activity against NoV binding to HBGA ligands than those of the post-immune sera of the mixed vaccine. Thus, the trivalent vaccine is a promising vaccine candidate against HEV, NoV, and AstV.

Structural, Mechanistic, and Antigenic Characterization of the Human Astrovirus Capsid

Human astroviruses (HAstVs) are nonenveloped, positive-sense, single-stranded RNA viruses that are a leading cause of viral gastroenteritis. HAstV particles display T=3 icosahedral symmetry formed by 180 copies of the capsid protein (CP), which undergoes proteolytic maturation to generate infectious HAstV particles. Little is known about the molecular features that govern HAstV particle assembly, maturation, infectivity, and immunogenicity. Here we report the crystal structures of the two main structural domains of the HAstV CP: the core domain at 2.60-Å resolution and the spike domain at 0.95-Å resolution. Fitting of these structures into the previously determined 25-Å-resolution electron cryomicroscopy density maps of HAstV allowed us to characterize the molecular features on the surfaces of immature and mature T=3 HAstV particles. The highly electropositive inner surface of HAstV supports a model in which interaction of the HAstV CP core with viral RNA is a driving force in T=3 HAstV particle formation. Additionally, mapping of conserved residues onto the HAstV CP core and spike domains in the context of the immature and mature HAstV particles revealed dramatic changes to the exposure of conserved residues during virus maturation. Indeed, we show that antibodies raised against mature HAstV have reactivity to both the HAstV CP core and spike domains, revealing for the first time that the CP core domain is antigenic. Together, these data provide new molecular insights into HAstV that have practical applications for the development of vaccines and antiviral therapies.

IMPORTANCE Astroviruses are a leading cause of viral diarrhea in young children, immunocompromised individuals, and the elderly. Despite the prevalence of astroviruses, little is known at the molecular level about how the astrovirus particle assembles and is converted into an infectious, mature virus. In this paper, we describe the high-resolution structures of the two main astrovirus capsid proteins. Fitting these structures into previously determined low-resolution maps of astrovirus allowed us to characterize the molecular surfaces of immature and mature astroviruses. Our studies provide the first evidence that astroviruses undergo viral RNA-dependent assembly. We also provide new insight into the molecular mechanisms that lead to astrovirus maturation and infectivity. Finally, we show that both capsid proteins contribute to the adaptive immune response against astrovirus. Together, these studies will help to guide the development of vaccines and antiviral drugs targeting astrovirus.

A novel astrovirus from dromedaries in the Middle East

The recent emergence of Middle East respiratory syndrome coronavirus from the Middle East and its discovery from dromedary camels has boosted interest in the search for novel viruses in dromedaries. The existence of astroviruses (AstVs) in dromedaries was previously unknown. We describe the discovery of a novel dromedary camel AstV (DcAstV) from dromedaries in Dubai. Among 215 dromedaries, DcAstV was detected in faecal samples of four [three (1.5 %) adult dromedaries and one (8.3 %) dromedary calf] by reverse transcription-PCR. Sequencing of the four DcAstV genomes and phylogenetic analysis showed that the DcAstVs formed a distinct cluster. Although DcAstV was most closely related to a recently characterized porcine AstV 2, their capsid proteins only shared 60-66 % amino acid identity, with a mean amino acid genetic distance of 0.372. Notably, the N-terminal halves of the capsid proteins of DcAstV shared ≤ 85 % amino acid identity, but the C-terminal halves only shared ≤ 49 % amino acid identity compared with the corresponding proteins in other AstVs. A high variation of the genome sequences of DcAstV was also observed, with a mean amino acid genetic distance of 0.214 for ORF2 of the four strains. Recombination analysis revealed a possible recombination event in ORF2 of strain DcAstV-274. The low Ka/Ks ratios (number of non-synonymous substitutions per non-synonymous site to number of synonymous substitutions per synonymous site) of the four ORFs in the DcAstV genomes supported the suggestion that dromedaries are the natural reservoir where AstV is stably evolving. These results suggest that AstV is a novel species of the genus Mamastrovirus in the family Astroviridae. Further studies are important to understand the pathogenic potential of DcAstV.

A Comprehensive Study of Neutralizing Antigenic Sites on the Hepatitis E Virus (HEV) Capsid by Constructing, Clustering, and Characterizing a Tool Box

The hepatitis E virus (HEV) ORF2 encodes a single structural capsid protein. The E2s domain (amino acids 459-606) of the capsid protein has been identified as the major immune target. All identified neutralizing epitopes are located on this domain; however, a comprehensive characterization of antigenic sites on the domain is lacking due to its high degree of conformation dependence. Here, we used the statistical software SPSS to analyze cELISA (competitive ELISA) data to classify monoclonal antibodies (mAbs), which recognized conformational epitopes on E2s domain. Using this novel analysis method, we identified various conformational mAbs that recognized the E2s domain. These mAbs were distributed into 6 independent groups, suggesting the presence of at least 6 epitopes. Twelve representative mAbs covering the six groups were selected as a tool box to further map functional antigenic sites on the E2s domain. By combining functional and location information of the 12 representative mAbs, this study provided a complete picture of potential neutralizing epitope regions and immune-dominant determinants on E2s domain. One epitope region is located on top of the E2s domain close to the monomer interface; the other is located on the monomer side of the E2s dimer around the groove zone. Besides, two non-neutralizing epitopes were also identified on E2s domain that did not stimulate neutralizing antibodies. Our results help further the understanding of protective mechanisms induced by the HEV vaccine. Furthermore, the tool box with 12 representative mAbs will be useful for studying the HEV infection process.

The C-terminal end of the capsid protein of Avian Nephritis Virus is antigenic and induces broadly cross-reactive antibodies

Avian nephritis virus (ANV) has been isolated frequently from commercial broilers in many countries. The prevalence and economic impact of ANV however has been difficult to ascertain due to the lack of convenient serological techniques. In this study the full-length and fragments of the ANV capsid protein were expressed in Baculovirus and affinity purified recombinant proteins used for the detection of ANV antibodies in ELISA. The crystal structure of Human Astrovirus (HAstV) was used as a model to determine potential homologous C-terminal antigenic regions in ANV. The rp37 fragment from three ANV strains NSW_3, ANV-1 and ANV-2, and a shorter NSW_3 fragment (rp33) were compared for their ability to detect ANV antibodies in seven reference chicken sera. The ANV-1 rp37 antigen was the most strain specific whereas the NSW_3 rp37 and rp33 antigens detected antibodies in all heterologous sera, including ANV-1 serum. Irrespective of the strain used, the two NSW_3 protein fragments rp37 and rp33 were found to be superior as antigens for ELISA when compared to the full-length capsid protein rp75. An ELISA designed using the NSW_3 rp33 could reliably differentiate between uninfected and infected commercial broiler flocks, as demonstrated by statistically significant differences between the OD values. This study identified an ANV immunogenic region and successfully used recombinant protein expression of this region to detect cross-reactive ANV antibodies. The results of this study facilitate future studies into the epidemiology and importance of ANV infections in commercial poultry.

Complete genome sequence of canine astrovirus with molecular and epidemiological characterisation of UK strains

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Astroviruses are a common cause of gastroenteritis in many species including man.

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We sought to determine whether canine astrovirus is circulating in the UK.

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Canine astrovirus was identified in four dogs with gastroenteritis.

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Sequencing the capsid of each isolate identified significant genetic heterogeneity.

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The first full genome sequence of canine astrovirus has also been determined.

Astroviruses are a common cause of gastroenteritis in children worldwide. These viruses can also cause infection in a range of domestic and wild animal species. Canine astrovirus (CaAstV) was first identified in the USA, and has since been reported in dogs from Europe, the Far East and South America. We sought to determine whether CaAstV is circulating in the UK dog population, and to characterise any identified strains. Stool samples were collected from pet dogs in the UK with and without gastroenteritis, and samples were screened for CaAstV by qPCR. Four CaAstV positive samples were identified from dogs with gastroenteritis (4/67, 6.0%), whereas no samples from healthy dogs were positive (p < 0.001). Sequencing of the capsid sequences from the four CaAstV strains found significant genetic heterogeneity, with only 80% amino acid identity between strains. The full genome sequence of two UK CaAstV strains was then determined, confirming that CaAstV conforms to the classic genome organisation of other astroviruses with ORF1a and ORF1b separated by a frameshift and ORF2 encoding the capsid protein. This is the first report describing the circulation of CaAstV in UK dogs with clinical signs of gastroenteritis, and the first description of the full-length genomes of two CaAstV strains.