Differential In Vitro Infection of Neural Cells by Astroviruses

Novel murine model of human astrovirus infection reveals cardiovascular tropism

Astroviruses are a common cause of gastrointestinal disease in humans and have been linked to fatal cases of encephalitis. A major barrier to the study of human-infecting astroviruses is the lack of an in vivo model as previous attempts failed to identify a host that supports viral replication. We describe a novel murine model of infection using astrovirus VA1/HMO-C (VA1), an astrovirus with high seroprevalence in humans. VA1 is cardiotropic, and viral RNA levels peak in the heart tissue 7 days post-inoculation in multiple different murine genetic backgrounds. Infectious VA1 particles could be recovered from heart tissue 3 and 5 days post-inoculation. Viral capsid was detected intracellularly in the heart tissue by immunostaining, and viral RNA was detected in cardiac myocytes, endocardium, and endothelial cells based on fluorescent in situ hybridization and confocal microscopy. Histologically, we identified inflammatory infiltrates consistent with myocarditis in some mice, with viral RNA colocalizing with the infiltrates. These foci contained CD3 +T cells and CD68 +macrophages. Viral RNA levels increased by >10 fold in the heart tissue or serum samples from Rag1 or Stat1 knockout mice, demonstrating the role of both adaptive and innate immunity in the response to VA1 infection. Based on the in vivo tropisms, we tested cardiac-derived primary cells and determined that VA1 can replicate in primary human cardiac endothelial cells, suggesting a novel cardiovascular tropism in human cells. This novel in vivo model of a human-infecting astrovirus enables further characterization of the host immune response and reveals a new cardiovascular tropism of astroviruses.

IMPORTANCE

Astroviruses routinely cause infections in humans; however, few methods were available to study these viruses. Here, we describe the first animal system to study human-infecting astroviruses by using mice. We demonstrate that mice are susceptible to astrovirus VA1, a strain that commonly infects humans and has been linked to fatal brain infections. The virus infects the heart tissue and is associated with inflammation. When mice with impaired immune systems were infected with VA1, they were found to have higher amounts of the virus in their hearts and blood. We found that VA1 can infect cells from human blood vessels of the heart, which is associated with human health. This model will enable us to better understand how astroviruses cause disease and how the immune system responds to infection. Our findings also suggest that astroviruses could be linked to cardiovascular diseases, including in humans.

The neonatal Fc receptor is a cellular receptor for human astrovirus

Human astroviruses (HAstV) are major causes of gastroenteritis, especially in children, and there are no vaccines or antivirals currently available. Little is known about host factors required for their cellular entry. Here we utilized complementary CRISPR-Cas9-based knockout and activation screens to identify neonatal Fc receptor (FcRn) and dipeptidyl-peptidase IV (DPP4) as entry factors for HAstV infection in vitro. 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 of FcRn, but not DPP4, with HAstV virions and the purified spike protein. This suggests that FcRn is a receptor for HAstVs while DPP4 is a cofactor for entry. Inhibitors for DPP4 and FcRn currently in clinical use prevented HAstV infection in cell lines and human enteroids. Our results reveal mechanisms of HAstV entry as well as druggable targets to limit HAstV infection.

Neonatal Fc receptor is a functional receptor for classical human astrovirus

Human astrovirus (HAstV) is a global cause of gastroenteritis in infants, the elderly, and the immunocompromised. However, the molecular mechanisms that control its susceptibility are not fully understood, as the functional receptor used by the virus has yet to be identified. Here, a genome-wide CRISPR-Cas9 library screen in Caco2 cells revealed that the neonatal Fc receptor (FcRn) can function as a receptor for classical HAstV (Mamastrovirus genotype 1). Deletion of FCGRT or B2M, which encode subunits of FcRn, rendered Caco2 cells and intestinal organoid cells resistant to HAstV infection. We also showed that human FcRn expression renders non-susceptible cells permissive to viral infection and that FcRn binds directly to the HAstV spike protein. Therefore, our findings provide insight into the entry mechanism of HAstV into susceptible cells. We anticipate that this information can be used to develop new therapies targeting human astroviruses, providing new strategies to treat this global health issue.

Attenuation hotspots in neurotropic human astroviruses

During the last decade, the detection of neurotropic astroviruses has increased dramatically. The MLB genogroup of astroviruses represents a genetically distinct group of zoonotic astroviruses associated with gastroenteritis and severe neurological complications in young children, the immunocompromised, and the elderly. Using different virus evolution approaches, we identified dispensable regions in the 3' end of the capsid-coding region responsible for attenuation of MLB astroviruses in susceptible cell lines. To create recombinant viruses with identified deletions, MLB reverse genetics (RG) and replicon systems were developed. Recombinant truncated MLB viruses resulted in imbalanced RNA synthesis and strong attenuation in iPSC-derived neuronal cultures confirming the location of neurotropism determinants. This approach can be used for the development of vaccine candidates using attenuated astroviruses that infect humans, livestock animals, and poultry.

Zinc Finger Protein BCL11A Contributes to the Abortive Infection of Hirame novirhabdovirus (HIRRV) in B Lymphocytes of Flounder (Paralichthys olivaceus)

Hirame novirhabdovirus (HIRRV) infection is characterized by a pronounced viremia, and the high viral load is typically detected in immune-related organs and the circulatory system. In the present study, we demonstrated that HIRRV has the capacity to invade part of flounder membrane-bound IgM (mIgM⁺) B lymphocyte. Eight quantitative real-time PCR (qRT-PCR) standard curves involving HIRRV genomic RNA (gRNA), cRNA, and six mRNAs were established based on the strand-specific reverse transcription performed with tagged primers. It was revealed that viral RNA synthesis, especially the replication of gRNA, was inhibited in B cells, and the intracellular HIRRV even failed to produce infectious viral particles. Moreover, a range of genes with nucleic acid binding activity or related to viral infection were screened out based on the transcriptome analysis of HIRRV-infected B cells, and five molecules were further selected because of their different expression patterns in HIRRV-infected B cells and hirame natural embryo (HINAE) cells. The overexpression of these genes followed by HIRRV infection and RNA binding protein immunoprecipitation (RIP) assay revealed that the flounder B cell lymphoma/leukemia 11A (BCL11A), a highly conserved zinc finger transcription factor, is able to inhibit the proliferation of HIRRV by binding with full-length viral RNA mainly via its zinc finger domains at the C terminus. In conclusion, these data indicated that the high transcriptional activity of BCL11A in flounder mIgM⁺ B lymphocytes is a crucial factor for the abortive infection of HIRRV, and our findings provide new insights into the interaction between HIRRV and teleost B cells. IMPORTANCE HIRRV is a fish rhabdovirus that is considered as an important pathogen threatening the fish farming industry represented by flounder because of its high infectivity and fatality rate. To date, research toward understanding the complex pathogenic mechanism of HIRRV is still in its infancy and faces many challenges. Exploration of the relationship between HIRRV and its target cells is interesting and necessary. Here, we revealed that flounder mIgM⁺ B cells are capable of suppressing viral RNA synthesis and result in an unproductive infection of HIRRV. In addition, our results demonstrated that zinc finger protein BCL11A, a transcription factor in B cells, is able to suppress the replication of HIRRV. These findings increased our understanding of the underlying characteristics of HIRRV infection and revealed a novel antiviral mechanism against HIRRV based on the host restriction factor in teleost B cells, which sheds new light on the research into HIRRV control.

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.

The highly conserved stem-loop II motif is important for the lifecycle of astroviruses but dispensable for SARS-CoV-2

The stem-loop II motif (s2m) is an RNA element present in viruses from divergent viral families, including astroviruses and coronaviruses, but its functional significance is unknown. We created deletions or substitutions of the s2m in astrovirus VA1 (VA1), classic human astrovirus 1 (HAstV1) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). For VA1, recombinant virus could not be rescued upon partial deletion of the s2m or substitutions of G-C base pairs. Compensatory substitutions that restored the G-C base-pair enabled recovery of VA1. For HAstV1, a partial deletion of the s2m resulted in decreased viral titers compared to wild-type virus, and reduced activity in a replicon system. In contrast, deletion or mutation of the SARS-CoV-2 s2m had no effect on the ability to rescue the virus, growth in vitro , or growth in Syrian hamsters. Our study demonstrates the importance of the s2m is virus-dependent.

High Seropositivity Rate of Neutralizing Antibodies to Astrovirus VA1 in Human Populations

Astroviruses are common pathogens of the human gastrointestinal tract, but they have been recently identified from cases of fatal meningoencephalitis. Astrovirus VA1 is the most frequently detected astrovirus genotype from cases of human encephalitis, but the prevalence of neutralizing antibodies to VA1 in human sera is unknown. We developed a focus reduction neutralization assay (FRNT) for VA1 and measured the seroprevalence of neutralizing antibodies from two cohorts of adult and pediatric serum samples: (i) an age-stratified cohort from St. Louis, MO, collected from 2007 to 2008 and (ii) a cohort from the Peruvian Amazonian River Basin collected in the late 1990s. In the St. Louis cohort, the lowest seropositivity rate was in children 1 year of age (6.9%), rising to 63.3% by ages 9 to 12, and 76.3% of adults ≥20 years were positive. The Peruvian Amazon cohort showed similar seropositivity rates across all ages, with individuals under age 20 having a rate of 75%, while 78.2% of adults ≥20 years were seropositive. In addition, we also identified the presence neutralizing antibodies to VA1 from commercial lots of intravenous immunoglobulin (IVIG). Our results demonstrate that a majority of humans are exposed to VA1 by adulthood, with the majority of infections occurring between 2 and 9 years of age. In addition, our results indicate that VA1 has been circulating in two geographically and socioeconomically divergent study cohorts over the past 20 years. Nonetheless, a significant proportion of the human population lacks neutralizing immunity and remains at risk for acute infection. IMPORTANCE Astroviruses are human pathogens with emerging disease associations, including the recent recognition of their capacity to cause meningoencephalitis. Astrovirus VA1 is the most commonly identified astrovirus genotype from cases of human encephalitis, but it is unknown what percentage of the human population has neutralizing antibodies to VA1. We found that 76.3 to 78.2% of adult humans ≥20 years of age in two geographically and socioeconomically distinct cohorts are seropositive for VA1, with the majority of infections occurring between 2 and 9 years of age. These results demonstrate that VA1 has been circulating in human populations over the past 2 decades and that most humans develop neutralizing antibodies against this virus by adulthood. However, a subset of humans lack evidence of neutralizing antibodies and are at risk for diseases caused by VA1, including encephalitis.

Comparative Analysis of Novel Strains of Porcine Astrovirus Type 3 in the USA

Porcine astrovirus type 3 (PoAstV3) has been previously identified as a cause of polioencephalomyelitis in swine and continues to cause disease in the US swine industry. Herein, we describe the characterization of both untranslated regions, frameshifting signal, putative genome-linked virus protein (VPg) and conserved antigenic epitopes of several novel PoAstV3 genomes. Twenty complete coding sequences (CDS) were obtained from 32 diagnostic cases originating from 11 individual farms/systems sharing a nucleotide (amino acid) percent identity of 89.74-100% (94.79-100%), 91.9-100% (96.3-100%) and 90.71-100% (93.51-100%) for ORF1a, ORF1ab and ORF2, respectively. Our results indicate that the 5'UTR of PoAstV3 is highly conserved highlighting the importance of this region in translation initiation while their 3'UTR is moderately conserved among strains, presenting alternative configurations including multiple putative protein binding sites and pseudoknots. Moreover, two predicted conserved antigenic epitopes were identified matching the 3' termini of VP27 of PoAstV3 USA strains. These epitopes may aid in the design and development of vaccine components and diagnostic assays useful to control outbreaks of PoAstV3-associated CNS disease. In conclusion, this is the first analysis predicting the structure of important regulatory motifs of neurotropic mamastroviruses, which differ from those previously described in human astroviruses.

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.

Human Astroviruses: A Tale of Two Strains

Since the 1970s, eight closely related serotypes of classical human astroviruses (HAstV) have been associated with gastrointestinal illness worldwide. In the late 2000s, three genetically unique human astrovirus clades, VA1-VA3, VA2-VA4, and MLB, were described. While the exact disease associated with these clades remains to be defined, VA1 has been associated with central nervous system infections. The discovery that VA1 could be grown in cell culture, supports exciting new studies aimed at understanding viral pathogenesis. Given the association of VA1 with often lethal CNS infections, we tested its susceptibility to the antimicrobial drug, nitazoxanide (NTZ), which we showed could inhibit classical HAstV infections. Our studies demonstrate that NTZ inhibited VA1 replication in Caco2 cells even when added at 12 h post-infection, which is later than in HAstV-1 infection. These data led us to further probe VA1 replication kinetics and cellular responses to infection in Caco-2 cells in comparison to the well-studied HAstV-1 strain. Overall, our studies highlight that VA1 replicates more slowly than HAstV-1 and elicits significantly different cellular responses, including the inability to disrupt cellular junctions and barrier permeability.

Astrovirus VA1 in patients with acute gastroenteritis

Human astroviruses (AstVs) are usually associated with acute gastroenteritis. In recent years, atypical animal-like AstVs have been identified, but their pathogenic role in humans has not been determined. Starting from 2010, there has been a growing evidence that AstVs may also be associated with encephalitis in human and animal hosts. Some human atypical AstV strains (VA1, MLB1/MLB2) display neurotropic potential, as they have been repeatedly identified in patients with AstV-related encephalitis, chiefly in immunosuppressed individuals. In this study, a VA1-like AstV was identified from a single stool sample from an outbreak of foodborne acute gastroenteritis occurred in Italy in 2018. On genome sequencing, the virus was related to the VA1-like strain UK1 (99.3% at the nucleotide level). Similar viruses were also found to circulate in paediatric patients hospitalized with AGE in the same time span, 2018, but at low prevalence (0.75%, 3/401). Gathering epidemiological data on atypical AstVs will be useful to assess the risks posed by atypical AstV infections, chiefly in medically fragile patients.

Astrovirus-Associated Polioencephalomyelitis in an Alpaca

An 8-year-old alpaca was admitted to the emergency service of the Clinic for Ruminants in Bern due to a reduced general condition and progressive neurological signs. Despite supportive treatment, its condition deteriorated and the animal had to be euthanized. Histopathological analysis revealed a severe non-suppurative polioencephalomyelitis with neuronal necrosis, most likely of viral origin. We detected abundant neuronal labelling with antibodies directed against two different epitopes of Bovine Astrovirus CH13/NeuroS1 (BoAstV-CH13/NeuroS1), which is a common viral agent associated with non-suppurative encephalitis in Swiss cattle. These findings were further verified by detection of viral RNA by use of in-situ hybridization and real-time RT-PCR. Next generation sequencing revealed that the detected virus genome had a pairwise identity of 98.9% to the genome of BoAstV-CH13/NeuroS1. To our knowledge, this is the first report of an astrovirus-associated polioencephalomyelitis in an alpaca. These results point to the possibility of an interspecies transmission of BoAstV-CH13/NeuroS1.

Intestinal Enteroid Culture for Human Astroviruses

Human astroviruses (HAstV) are non-enveloped, positive-sense single stranded RNA viruses that typically cause gastroenteritis in children, the elderly and among immunocompromised individuals. Some HAstV species have also been implicated in neurological diseases. It is important to study these viruses to understand the pathogenesis and develop therapeutics. Here we describe HAstV infection in epithelium-only human intestinal enteroids (HIE) isolated from biopsy-derived intestinal crypts. Although different HAstV clades have been propagated in transformed immortalized cell lines such as A549, Caco-2, HEK293T and Huh7.5, we chose HIE because they better mimic the human intestine and thus are more physiologically relevant. Additionally, HIE support the replication of all HAstV clades including clinical samples, thus making HIE a valuable potential universal model to study HAstV biology.

Identification of astroviruses in bovine and buffalo calves with enteritis

Astroviruses (AstVs) have been identified in the stools of calves with enteritis and in the brain tissues of bovines with encephalitis but their pathogenic role has not been clarified. In this study, we report the detection and characterization of bovine and water buffalo AstV strains identified in young bovine and buffalo calves with enteritis in Italy between 2012 and 2015. By negative staining transmission electron microscopy (TEM) observation, AstV-like particles were identified in the stools of the animals and AstV RNA was confirmed molecularly. The sequence (~3.2-kb) at the 3′ end of the genome was determined for two bovine and two buffalo AstVs. Sequence and phylogenetic analysis on the partial ORF1b and full-length ORF2 revealed a marked genetic diversity although the viruses were distantly related to other AstV identified from ruminants. Gathering sequence information on ruminant AstVs is important to understand the extent of inter-species circulation and for the development of reliable, specific diagnostic tools.

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Astroviruses in ruminants are neglected enteric viruses.

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Recently, neurotropic strains have been identified in ruminants.

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Astrovirus was detected in outbreaks of enteritis in cattle and buffalo in Italy.

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Genome sequencing of the viruses unveiled marked genetic diversity.

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Implementing the diagnostics is pivotal to conceive surveillance studies.

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.

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.