Type I interferon response is delayed in human astrovirus infections

The neonatal Fc receptor and DPP4 are human astrovirus receptors

Human astroviruses (HAstV) are major global causes of gastroenteritis, but little is known about host factors required for their cellular entry. Here, we utilized complementary CRISPR-Cas9-based knockout and activation screening approaches and identified neonatal Fc receptor (FcRn) and dipeptidyl-peptidase IV (DPP4) as entry factors for HAstV infection of human intestinal epithelial cells. Disruption of FcRn or DPP4 reduced HAstV infection in permissive cells and, reciprocally, overexpression of these factors in non-permissive cells was sufficient to promote infection. We observed direct binding between FcRn and HAstV virions as well as purified spike protein. Finally, inhibitors for DPP4 and FcRn currently in clinical use prevent HAstV infection in cell lines and primary human enteroids. Thus, our results reveal mechanisms of HAstV entry as well as druggable targets.

One-Sentence Summary

Targeting FcRn or DPP4 using available therapies effectively prevents human astrovirus infection in human enteroid cultures.

Indoleamine 2,3-dioxygenase 1 regulates cell permissivity to astrovirus infection

Astroviruses cause a spectrum of diseases spanning asymptomatic infections to severe diarrhea, but little is understood about their pathogenesis. We previously determined that small intestinal goblet cells were the main cell type infected by murine astrovirus-1. Here, we focused on the host immune response to infection and inadvertently discovered a role for indoleamine 2,3-dioxygenase 1 (Ido1), a host tryptophan catabolizing enzyme, in the cellular tropism of murine and human astroviruses. We identified that Ido1 expression was highly enriched among infected goblet cells, and spatially corresponded to the zonation of infection. Because Ido1 can act as a negative regulator of inflammation, we hypothesized it could dampen host antiviral responses. Despite robust interferon signaling in goblet cells, as well as tuft cell and enterocyte bystanders, we observed delayed cytokine induction and suppressed levels of fecal lipocalin-2. Although we found Ido-/- animals were more resistant to infection, this was not associated with fewer goblet cells nor could it be rescued by knocking out interferon responses, suggesting that IDO1 instead regulates cell permissivity. We characterized IDO1-/- Caco-2 cells and observed significantly reduced human astrovirus-1 infection. Together this study highlights a role for Ido1 in astrovirus infection and epithelial cell maturation.

Replication of Porcine Astrovirus Type 1-Infected PK-15 Cells In Vitro Affected by RIG-I and MDA5 Signaling Pathways

The interferon (IFN) system is an extremely powerful antiviral response in animal cells. The subsequent effects caused by porcine astrovirus type 1 (PAstV1) IFN activation are important for the host's response to viral infections. Here, we show that this virus, which causes mild diarrhea, growth retardation, and damage of the villi of the small intestinal mucosa in piglets, induces an IFN response upon infection of PK-15 cells. Although IFN-β mRNA was detected within infected cells, this response usually occurs during the middle stages of infection, after genome replication has taken place. Treatment of PAstV1-infected cells with the interferon regulatory factor 3 (IRF3) inhibitor BX795 decreased IFN-β expression, whereas the nuclear factor kappa light chain enhancer of activated B cells (NF-κB) inhibitor BAY11-7082 did not. These findings indicate that PAstV induced the production of IFN-β via IRF3-mediated rather than NF-κB-mediated signaling pathways in PK-15 cells. Moreover, PAstV1 increased the protein expression levels of retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) in PK-15 cells. The knockdown of RIG-I and MDA5 decreased the expression levels of IFN-β and the viral loads and increased the infectivity of PAstV1. In conclusion, PAstV1 induced the production of IFN-β via the RIG-I and MDA5 signaling pathways, and the IFN-β produced during PAstV1 infection inhibited viral replication. These results will help provide new evidence that PAstV1-induced IFNs may protect against PAstV replication and pathogenesis. IMPORTANCE Astroviruses (AstVs) are widespread and can infect multiple species. Porcine astroviruses produce mainly gastroenteritis and neurological diseases in pigs. However, astrovirus-host interactions are less well studied, particularly with respect to their antagonism of IFN. Here, we report that PAstV1 acts via IRF3 transcription pathway activation of IFN-β. In addition, the knockdown of RIG-I and MDA5 attenuated the production of IFN-β induced by PAstV1 in PK-15 cells and increased efficient viral replication in vitro. We believe that these findings will help us to better understand the mechanism of how AstVs affect the host IFN response.

Single-cell transcriptomics reveals immune response of intestinal cell types to viral infection

Human intestinal epithelial cells form a primary barrier protecting us from pathogens, yet only limited knowledge is available about individual contribution of each cell type to mounting an immune response against infection. Here, we developed a framework combining single-cell RNA-Seq and highly multiplex RNA FISH and applied it to human intestinal organoids infected with human astrovirus, a model human enteric virus. We found that interferon controls the infection and that astrovirus infects all major cell types and lineages and induces expression of the cell proliferation marker MKI67. Intriguingly, each intestinal epithelial cell lineage exhibits a unique basal expression of interferon-stimulated genes and, upon astrovirus infection, undergoes an antiviral transcriptional reprogramming by upregulating distinct sets of interferon-stimulated genes. These findings suggest that in the human intestinal epithelium, each cell lineage plays a unique role in resolving virus infection. Our framework is applicable to other organoids and viruses, opening new avenues to unravel roles of individual cell types in viral pathogenesis.

Neurotropic Astroviruses in Animals

Astrovirus infections are among the main causes of diarrhea in children, but their significance for animal health has remained underestimated and largely unknown. This is changing due to the increasing amount of newly identified neurotropic astroviruses in cases of nonsuppurative encephalitis and neurological disease in humans, pigs, ruminant species and minks. Neurological cases in ruminants and humans usually occur sporadically and as isolated cases. This contrasts with the situation in pigs and minks, in which diseases associated with neurotropic astroviruses are endemic and occur on the herd level. Affected animals show neurological signs such as mild ataxia to tetraplegia, loss of orientation or trembling, and the outcome is often fatal. Non-suppurative inflammation with perivascular cuffing, gliosis and neuronal necrosis are typical histological lesions of astrovirus encephalitis. Since astroviruses primarily target the gastrointestinal tract, it is assumed that they infect the brain through the circulatory system or retrograde following the nerves. The phylogenetic analysis of neurotropic astroviruses has revealed that they are genetically closely related, suggesting the presence of viral determinants for tissue tropism and neuroinvasion. In this review, we summarize the current knowledge on neurotropic astrovirus infections in animals and propose future research activities.

Experimental Methods to Study the Pathogenesis of Human Enteric RNA Viruses

Every year, millions of children are infected with viruses that target the gastrointestinal tract, causing acute gastroenteritis and diarrheal illness. Indeed, approximately 700 million episodes of diarrhea occur in children under five annually, with RNA viruses norovirus, rotavirus, and astrovirus serving as major causative pathogens. Numerous methodological advancements in recent years, including the establishment of novel cultivation systems using enteroids as well as the development of murine and other animal models of infection, have helped provide insight into many features of viral pathogenesis. However, many aspects of enteric viral infections remain elusive, demanding further study. Here, we describe the different in vitro and in vivo tools available to explore different pathophysiological attributes of human enteric RNA viruses, highlighting their advantages and limitations depending upon the question being explored. In addition, we discuss key areas and opportunities that would benefit from further methodological progress.

Beyond the Gastrointestinal Tract: The Emerging and Diverse Tissue Tropisms of Astroviruses

Astroviruses are single stranded, positive-sense RNA viruses that have been historically associated with diseases of the gastrointestinal tract of vertebrates, including humans. However, there is now a multitude of evidence demonstrating the capacity of these viruses to cause extraintestinal diseases. The most striking causal relationship is neurological diseases in humans, cattle, pigs, and other mammals, caused by astrovirus infection. Astroviruses have also been associated with disseminated infections, localized disease of the liver or kidneys, and there is increasing evidence suggesting a potential tropism to the respiratory tract. This review will discuss the current understanding of the tissue tropisms for astroviruses and their emerging capacity to cause disease in multiple organ systems.

SARS-CoV-2 Causes a Different Cytokine Response Compared to Other Cytokine Storm-Causing Respiratory Viruses in Severely Ill Patients

Hyper-induction of pro-inflammatory cytokines, also known as a cytokine storm or cytokine release syndrome (CRS), is one of the key aspects of the currently ongoing SARS-CoV-2 pandemic. This process occurs when a large number of innate and adaptive immune cells activate and start producing pro-inflammatory cytokines, establishing an exacerbated feedback loop of inflammation. It is one of the factors contributing to the mortality observed with coronavirus 2019 (COVID-19) for a subgroup of patients. CRS is not unique to the SARS-CoV-2 infection; it was prevalent in most of the major human coronavirus and influenza A subtype outbreaks of the past two decades (H5N1, SARS-CoV, MERS-CoV, and H7N9). With a comprehensive literature search, we collected changing the cytokine levels from patients upon infection with the viral pathogens mentioned above. We analyzed published patient data to highlight the conserved and unique cytokine responses caused by these viruses. Our curation indicates that the cytokine response induced by SARS-CoV-2 is different compared to other CRS-causing respiratory viruses, as SARS-CoV-2 does not always induce specific cytokines like other coronaviruses or influenza do, such as IL-2, IL-10, IL-4, or IL-5. Comparing the collated cytokine responses caused by the analyzed viruses highlights a SARS-CoV-2-specific dysregulation of the type-I interferon (IFN) response and its downstream cytokine signatures. The map of responses gathered in this study could help specialists identify interventions that alleviate CRS in different diseases and evaluate whether they could be used in the COVID-19 cases.

Host innate immune responses of geese infected with goose origin nephrotic astrovirus

A novel goose astrovirus (GoAstV) outbreak in goslings, characterized by severe articular and visceral gout with high mortality, occurred in China. Although the pathogenesis of GoAstV-infected goslings has been explored in several studies, the host-immune response remains unclear. In this study, a goose astrovirus was isolated from goslings in Xiaogan, and designated as the HBXG strain. The full-length genome of HBXG was 7170 nt. A sequence analysis and phylogenetic trees revealed HBXG belonged to the novel GoAstV. We evaluated the viral distribution systematically and estimated immune related gene expression in HBXG-infected goslings. Results showed that GoAstV replicated quickly in many tissues and the highest titer was observed in the kidney, which reached 10^9.6 copies. TLR3, RIG-I and MDA5 were involved in the host-immune response to GoAstV, and the expression of IFN types I (IFN-α, IFN-β), inflammatory cytokines (IL-8, IL-10, TNF-α), antiviral proteins (Mx, OASL, PKR) and MHC-I were also upregulated during the infection. In contrast, the expression of proinflammatory cytokines (IL-1β, IL-6) and MHC-II were inhibited at 3 dpi. This study suggests that GoAstV is highly pathogenic to goslings, causing multiple systemic infections in tissues and the host-immune response is activated early in infection. However, rapid viral replication, suppression of inflammatory cytokines (IL-1β, IL-6) and MHC-II expressions were the possible reasons why the host-immune response cannot provide enough protection against GoAstV infection. This study is the first report to illuminate the immune response in goslings infected with GoAstV and offers insight into the pathogenesis of GoAstV.

OASL Triggered by Novel Goose Astrovirus via ORF2 Restricts Its Replication

Although astroviruses causes enteric diseases and encephalitis in humans and nephritis and hepatitis in poultry, astrovirus infection is thought to be self-limiting. However, little is known about its molecular mechanism. In this study, we found that a novel goose astrovirus (GAstV), GAstV-GD, and its open reading frame 2 (ORF2) could efficiently activate the innate immune response and induce a high level of OASL in vitro and in vivo The truncation assay for ORF2 further revealed that the P2 domain of ORF2 contributed to stimulating OASL, whereas the acidic C terminus of ORF2 attenuated such activation. Moreover, the overexpression and knockdown of OASL could efficiently restrict and promote the viral replication of GAstV-GD, respectively. Our data not only give novel insights for elucidating self-limiting infection by astrovirus but also provide virus and host targets for fighting against astroviruses.IMPORTANCE Astroviruses cause gastroenteritis and encephalitis in human, and nephritis, hepatitis, and gout disease in poultry. However, the host immune response activated by astrovirus is mostly unknown. Here, we found that a novel goose astrovirus, GAstV-GD, and its ORF2 protein could efficiently induce a high level of OASL in vitro and in vivo, which could feed back to restrict the replication of GAstV-GD, revealing novel innate molecules triggered by astroviruses and highlighting that the ORF2 of GAstV-GD and OASL can be potential antiviral targets for astroviruses.

Antiviral activity of ribavirin and favipiravir against human astroviruses

BACKGROUND

Recent recognition of invasive astrovirus infections, including encephalitis and viremia in humans, have highlighted the need for effective anti-astrovirus therapeutics. However, there is a paucity of data regarding the in vitro activity of broad-spectrum RNA antivirals against astroviruses, including ribavirin and favipiravir.

OBJECTIVES

We quantified the EC50 values for ribavirin and favipiravir against two human astrovirus strains, astrovirus VA1 (VA1) and human astrovirus 4 (HAstV4).

STUDY DESIGN

Caco-2 cells were infected with VA1 or HAstV4 in the presence of ribavirin or favipiravir (dose range 0.1-1000 μM), and the cells were maintained in media containing the drugs for 72 h. Viral RNA was extracted and quantified by qRT-PCR. As a surrogate for cytotoxicity, cellular adenosine triphosphate (ATP) from each drug treatment was also measured.

RESULTS

VA1 replication was inhibited 10-100-fold by both ribavirin (EC50 = 154 μM) and favipiravir (EC50 = 246 μM). In contrast, ribavirin inhibited HAstV4 replication (EC50 = 268 μM) but favipiravir only reduced replication by 44% at the highest dose. Mild reductions in ATP (17-31%) was only observed at the highest concentration of ribavirin (1000 μM) and no significant decrease in ATP was detected for any concentration of favipiravir.

CONCLUSIONS

Ribavirin inhibited both human astrovirus species and favipiravir was only active against VA1. In the future, the in vivo efficacy of these drugs could be tested with development of an animal model of human astrovirus infection.

Pathogenic Characteristics of a Porcine Astrovirus Strain Isolated in China

Astroviral infection is considered to be one of the causes of mammalian diarrheal diseases. It has been shown that astrovirus infections cause varying degrees of diarrhea in turkeys and mice. However, the pathogenesis of porcine astrovirus is unknown. In this study, the virulence of a cytopathic porcine astrovirus (PAstV) strain (PAstV1-GX1) isolated from the PK-15 cell line was tested using seven-day-old nursing piglets. The results showed that PAstV1-GX1 infection could cause mild diarrhea, growth retardation, and damage of the villi of the small intestinal mucosa. However, all the above symptoms could be restored within 7 to 10days post inoculation (dpi). To evaluate the innate immunity response of PAstV in vivo, the alteration of inflammatory cytokine expression in piglets infected with PAstV1-GX1 was determined using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). The mRNA expression levels of the IFNβ and ISG54 were found to be significantly elevated in virus-infected piglets. In contrast, expression of IFNλ was downregulated in piglets infected with PAstV1-GX1. In addition, the mRNA expression of the tight junction protein 1 and 2 and zonula occludin 1, which are associated with the intestinal barrier permeability, were affected after PAstV1 infection. The present study adds to our understanding of the pathogenic mechanism of PAstV and has established an animal model for further study of pig astrovirus infection.

Astrovirus replication in human intestinal enteroids reveals multi-cellular tropism and an intricate host innate immune landscape

Human astroviruses (HAstV) are understudied positive-strand RNA viruses that cause gastroenteritis mostly in children and the elderly. Three clades of astroviruses, classic, MLB-type and VA-type have been reported in humans. One limitation towards a better understanding of these viruses has been the lack of a physiologically relevant cell culture model that supports growth of all clades of HAstV. Herein, we demonstrate infection of HAstV strains belonging to all three clades in epithelium-only human intestinal enteroids (HIE) isolated from biopsy-derived intestinal crypts. A detailed investigation of infection of VA1, a member of the non-canonical HAstV-VA/HMO clade, showed robust replication in HIE derived from different patients and from different intestinal regions independent of the cellular differentiation status. Flow cytometry and immunofluorescence analysis revealed that VA1 infects several cell types, including intestinal progenitor cells and mature enterocytes, in HIE cultures. RNA profiling of VA1-infected HIE uncovered that the host response to infection is dominated by interferon (IFN)-mediated innate immune responses. A comparison of the antiviral host response in non-transformed HIE and transformed human colon carcinoma Caco-2 cells highlighted significant differences between these cells, including an increased magnitude of the response in HIE. Additional studies confirmed the sensitivity of VA1 to exogenous IFNs, and indicated that the endogenous IFN response of HIE to curtail the growth of strains from all three clades. Genotypic variation in the permissiveness of different HIE lines to HAstV could be overcome by pharmacologic inhibition of JAK/STAT signaling. Collectively, our data identify HIE as a universal infection model for HAstV and an improved model of the intestinal epithelium to investigate enteric virus-host interactions.

Human astroviruses (HAstV) are understudied positive-strand RNA viruses that typically cause gastroenteritis mostly in children and the elderly, but more recent studies also implicate them in neurological disease in immunocompromised patients. To better understand these viruses, a physiologically relevant cell culture model that supports growth of all clades of HAstV would be highly beneficial. Herein, we demonstrated robust infection of HAstV strains belonging to all three clades in epithelium-only human intestinal enteroids (HIE) isolated from biopsy-derived intestinal crypts from different patients and intestinal regions, making HIE a valuable model to study HAstV biology. Using this system, we identify for the first time that VA1 infects several cell types, including intestinal progenitor cells and mature enterocytes. Analysis of the antiviral host response to infection demonstrated that HIE respond to infection with a type I and III interferon response. This response reduced HAstV replication and when blocked resulted in increased infection. Establishment of the HIE system for HAstV research lays the foundation for future basic and translational discoveries.

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.

Characterizing a Murine Model for Astrovirus Using Viral Isolates from Persistently Infected Immunocompromised Mice

Human astroviruses are single-stranded RNA enteric viruses that cause a spectrum of disease ranging from asymptomatic infection to systemic extragastrointestinal spread; however, they are among the least-characterized enteric viruses, and there is a lack of a well-characterized small animal model. Finding that immunocompromised mice were resistant to human astrovirus infection via multiple routes of inoculation, our studies aimed to determine whether murine astrovirus (MuAstV) could be used to model human astrovirus disease. We experimentally infected wild-type mice with MuAstV isolated from immunocompromised mice and found that the virus was detected throughout the gastrointestinal tract, including the stomach, but was not associated with diarrhea. The virus was also detected in the lung. Although virus levels were higher in recently weaned mice, the levels were similar in male and female adult mice. Using two distinct viruses isolated from different immunocompromised mouse strains, we observed virus strain-specific differences in the duration of infection (3 versus 10 weeks) in wild-type mice, indicating that the within-host immune pressure from donor mice shaped the virus kinetics in immunocompetent recipient hosts. Both virus strains elicited minimal pathology and a lack of sustained immunity. In summary, MuAstV represents a useful model for studying asymptomatic human infection and gaining insight into the astrovirus pathogenesis and immunity.IMPORTANCE Astroviruses are widespread in both birds and mammals; however, little is known about the pathogenesis and the immune response to the virus due to the lack of a well-characterized small-animal model. Here we describe two distinct strains of murine astrovirus that cause infections in immunocompetent mice that mirror aspects of asymptomatic human infections, including minimal pathology and short-lived immunity. However, we noted that the duration of infection differed greatly between the strains, highlighting an important facet of these viruses that was not previously appreciated. The ubiquitous nature and diversity of murine astroviruses coupled with the continuous likelihood of reinfection raise the possibility of viral interference with other mouse models of disease.

Human Astrovirus MLB Replication In Vitro: Persistence in Extraintestinal Cell Lines

MLB astroviruses were identified 10 years ago in feces from children with gastroenteritis of unknown etiology and have been unexpectedly detected in severe cases of meningitis/encephalitis, febrile illness of unknown etiology, and respiratory syndromes. The aim of this study was to establish a cell culture system supporting MLB astrovirus replication. We used two clinical strains to infect several cell lines, an MLB1 strain from a gastroenteritis case, and an MLB2 strain associated with a neurologic infection. Efforts to propagate the viruses in the Caco-2 cell line were unsuccessful. In contrast, we identified two human nonintestinal cell lines, Huh-7 and A549, permissive for both genotypes. After serial passages in the Huh-7.5 cell line, the adapted strains were able to establish persistent infections in the Huh-7.5, Huh-7AI, and A549 cell lines, with high viral loads (up to 10 log₁₀ genome copies/ml) detected by quantitative reverse transcription-PCR (RT-qPCR) in the culture supernatant. Immunofluorescence assays demonstrated infection in about 10% of cells in persistently infected cultures. Electron microscopy revealed particles of 32 to 33 nm in diameter after negative staining of cell supernatants and capsid arrays in ultrathin sections with a particularly high production in Huh-7.5 cells. Interferon (IFN) expression by infected cells and effect of exogenous IFN varied depending on the type of infection and the cell line. The availability of a cell culture system to propagate MLB astroviruses represents a key step to better understand their replicative cycle, as well as a source of viruses to conduct a wide variety of basic virologic studies.IMPORTANCE MLB astroviruses are emerging viruses infecting humans. More studies are required to determine their exact epidemiology, but several reports have already identified them as the cause of unexpected clinical diseases, including severe neurologic diseases. Our study provides the first description of a cell culture system for the propagation of MLB astroviruses, enabling the study of their replicative cycle. Moreover, we demonstrated the unknown capacity of MLB astrovirus to establish persistent infections in cell culture. Whether these persistent infections are also established in vivo remains unknown, but the clinical consequences would be of high interest if persistence was confirmed in vivo Finally, our analysis of IFN expression provides some trails to understand the mechanism by which MLB astroviruses can cause persistent infections in the assayed cultures.

Upregulation of CHOP participates in caspase activation and virus release in human astrovirus-infected cells

Human astroviruses (HAstVs), non-enveloped RNA viruses with positive-sense RNA genomes, are an important cause of acute gastroenteritis in young children, although the processes that produce infectious virions are not clearly defined. To track the viral replication complex (RC) upon HAstV1 infection, the subcellular distribution of double-stranded (ds) RNA and of ORF1b, a viral RNA polymerase, was examined by immunocytochemistry. Foci that were positive for dsRNA and for ORF1b were co-localized, and both foci were also co-localized with resident proteins of the endoplasmic reticulum (ER). Focusing on the association between the HAstV RC and ER, we examined the expression of unfolded protein response (UPR) markers and found that targets of eukaryotic translation initiation factor 2α (eIF2α)-activating transcription factor 4 (ATF4), including CCAAT/enhancer-binding protein homologous protein (CHOP), a proapoptotic transcription factor, were upregulated at the late phase in HAstV-infected cells. Consistently, eIF2α was phosphorylated at the late phase of HAstV infection. The formation of foci resembling stress granules, another known downstream response to eIF2α phosphorylation, was also observed at the same period. Phosphorylation of eIF2α was attenuated in protein kinase R (PKR)-knockdown cells, suggesting that, unlike the canonical ER stress response, PKR was involved in eIF2α phosphorylation in response to HAstV infection. Studies have indicated that immature HAstV capsid protein is processed by caspases, and caspase cleavage is integral to particle release. Inhibition of CHOP upregulation reduced caspase activation and the release of HAstV RNA from cells during HAstV infection. Our results suggest that the eIF2α-ATF4-CHOP pathway participates in HAstV propagation.

The Ubiquitin-Proteasome System Is Necessary for Efficient Replication of Human Astrovirus

Astroviruses, members of the family Astroviridae, represent an important cause of human gastroenteritis in the world. The cellular factors required for astrovirus replication have been poorly studied. In this work, we evaluated the relevance of the ubiquitin-proteasome system (UPS) in the replication of Yuc8, a human astrovirus serotype 8 strain. We found that proteasome inhibitors decrease the production of infectious viral progeny at a step in the replication cycle subsequent to virus entry. The inhibition of proteasome activity decreases viral RNA levels and viral protein synthesis; similarly, the inhibition of ubiquitination by chemical inhibitors or RNA interference (RNAi) reduces the production of viral progeny as well as viral protein synthesis. The effect on viral progeny production induced by proteasome inhibitors is not explained by a reduction in the pool of monoubiquitin or the induction of early apoptosis or autophagy. Our observations are consistent with the need of the proteolytic activity of the UPS for the efficient replication of the virus and suggest that UPS is necessary for the production of genomic and subgenomic RNA but not for antigenomic RNA.IMPORTANCE Astroviruses are a major cause of gastroenteritis in young humans and animals, and recently, it was associated with fatal encephalitis in humans. The role of the ubiquitin-proteasome system in the replication of these viruses has not been studied previously. In this work, we present evidence that supports that the proteolytic activity of the proteasome is necessary for efficient viral progeny production and that this proteolytic system is required for the accumulation of both genomic and subgenomic viral RNAs.

Propagation of Astrovirus VA1, a Neurotropic Human Astrovirus, in Cell Culture

Astrovirus VA1/HMO-C (VA1; mamastrovirus 9) is a recently discovered astrovirus genotype that is divergent from the classic human astroviruses (mamastrovirus 1). The gastrointestinal tract is presumed to be the primary site of infection and pathogenicity for astroviruses. However, VA1 has been independently detected in brain tissue of five cases of human encephalitis. Studies of the pathogenicity of VA1 are currently impossible because there are no reported cell culture systems or in vivo models that support VA1 infection. Here, we describe successful propagation of VA1 in multiple human cell lines. The initial inoculum, a filtered clinical stool sample from the index gastroenteritis case cluster that led to the discovery of VA1, was first passaged in Vero cells. Serial blind passage in Caco-2 cells yielded increasing copies of VA1 RNA, and multistep growth curves demonstrated a >100-fold increase in VA1 RNA 72 h after inoculation. The full-length genomic and subgenomic RNA strands were detected by Northern blotting, and crystalline lattices of viral particles of ∼26-nm diameter were observed by electron microscopy in infected Caco-2 cells. Unlike other human astrovirus cell culture systems, which require addition of exogenous trypsin for continued propagation, VA1 could be propagated equally well with or without the addition of trypsin. Furthermore, VA1 was sensitive to the type I interferon (IFN-I) response, as VA1 RNA levels were reduced by pretreatment of Caco-2 cells with IFN-β1a. The ability to propagate VA1 in cell culture will facilitate studies of the neurotropism and neuropathogenesis of VA1.IMPORTANCE Astroviruses are an emerging cause of central nervous system infections in mammals, and astrovirus VA1/HMO-C is the most prevalent astrovirus in cases of human encephalitis. This virus has not been previously propagated, preventing elucidation of the biology of this virus. We describe the first cell culture system for VA1, a key step necessary for the study of its ability to cause disease.

Human bocavirus 1 infection of CACO-2 cell line cultures

Human bocavirus 1 (HBoV1) is a parvovirus associated with pneumonia in infants. It has been detected in different tissues, including colorectal tumors. In this study, we investigated whether Caco-2 cell line, derived from human colon cancer, can be utilized as a model for HBoV1 replication. We demonstrate HBoV1 replication in Caco-2 cultures supplemented with DEAE-dextran after inoculation with respiratory material from infected patients presenting with acute respiratory infection. A viral cycle of rapid development is displayed. However, in spite of HBoV1 DNA 4-fold increment in the supernatants and monolayers by day 1, evidencing that the system allows the virus genome replication after the entry occurred, infectious progeny particles were not produced. These results are consistent with an infection that is limited to a single growth cycle, which can be associated to mutations in the NS1 and VP1/VP2 regions of HBoV1 genome. Further research will contribute to fully elucidate these observations.

•

HBoV1 replicates within 24 h in standard and differentiated Caco-2 cells.

•

DNA is detected in attached cell and supernatant medium.

•

Immunofluorescence tests evidences HBoV1 infection.

•

DNA replication occurred but infectious progeny particles were not produced.

•

The viral genome presents deleterious mutations at NS1 and VP1/VP2 regions.

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.

Epidemiology of Classic and Novel Human Astrovirus: Gastroenteritis and Beyond

Since they were identified in 1975, human astroviruses have been considered one of the most important agents of viral acute gastroenteritis in children. However, highly divergent astroviruses infecting humans have been recently discovered and associated with extra-intestinal infections. The report of cases of fatal meningitis and encephalitis, especially in immunocompromised individuals, has broadened their disease spectrum. Although zoonotic transmission among animal and human astroviruses has not been clearly recognized, the genetic similarity between some human and animal viruses makes it likely to occur. This review provides an update on the epidemiology of both classic and novel human astroviruses, and a comprehensive view on confirmed or potential association between astrovirus and human disease.

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 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.

Type I Interferon Response Limits Astrovirus Replication and Protects against Increased Barrier Permeability In Vitro and In Vivo

Little is known about intrinsic epithelial cell responses against astrovirus infection. Here we show that human astrovirus type 1 (HAstV-1) infection induces type I interferon (beta interferon [IFN-β]) production in differentiated Caco2 cells, which not only inhibits viral replication by blocking positive-strand viral RNA and capsid protein synthesis but also protects against HAstV-1-increased barrier permeability. Excitingly, we found similar results in vivo using a murine astrovirus (MuAstV) model, providing new evidence that virus-induced type I IFNs may protect against astrovirus replication and pathogenesis in vivo.

IMPORTANCE

Human astroviruses are a major cause of pediatric diarrhea, yet little is known about the immune response. Here we show that type I interferon limits astrovirus infection and preserves barrier permeability both in vitro and in vivo. Importantly, we characterized a new mouse model for studying astrovirus replication and pathogenesis.

Examination of a plasmid-based reverse genetics system for human astrovirus

A plasmid-based reverse genetics system for human astrovirus type 1 (HAstV1) is examined. Upon transfection into 293T cells, the plasmid vector, which harbors a HAstV1 expression cassette, expressed astroviral RNA that appeared to be capable of viral RNA replication, as indicated by the production of subgenomic RNA and capsid protein expression irrespective of the heterologous 5' ends of the transcribed RNA. Particles infectious to Caco-2 cells were made in this system; however, their infectivity was much lower than would be expected from the amount of particles apparently produced. Using Huh-7 cells as the transfection host with the aim of improving viral capsid processing for virion maturation partially restored the efficiency of infectious particle formation. Our results support the possibility that the DNA transfection process induces a cellular response that targets late, but not early, stages of HAstV1 infection.