Murine rotavirus genes encoding outer capsid proteins VP4 and VP7 are not major determinants of host range restriction and virulence

Reverse Genetics of Murine Rotavirus: A Comparative Analysis of the Wild-Type and Cell-Culture-Adapted Murine Rotavirus VP4 in Replication and Virulence in Neonatal Mice

Small-animal models and reverse genetics systems are powerful tools for investigating the molecular mechanisms underlying viral replication, virulence, and interaction with the host immune response in vivo. Rotavirus (RV) causes acute gastroenteritis in many young animals and infants worldwide. Murine RV replicates efficiently in the intestines of inoculated suckling pups, causing diarrhea, and spreads efficiently to uninoculated littermates. Because RVs derived from human and other non-mouse animal species do not replicate efficiently in mice, murine RVs are uniquely useful in probing the viral and host determinants of efficient replication and pathogenesis in a species-matched mouse model. Previously, we established an optimized reverse genetics protocol for RV and successfully generated a murine-like RV rD6/2-2g strain that replicates well in both cultured cell lines and in the intestines of inoculated pups. However, rD6/2-2g possesses three out of eleven gene segments derived from simian RV strains, and these three heterologous segments may attenuate viral pathogenicity in vivo. Here, we rescued the first recombinant RV with all 11 gene segments of murine RV origin. Using this virus as a genetic background, we generated a panel of recombinant murine RVs with either N-terminal VP8* or C-terminal VP5* regions chimerized between a cell-culture-adapted murine ETD strain and a non-tissue-culture-adapted murine EW strain and compared the diarrhea rate and fecal RV shedding in pups. The recombinant viruses with VP5* domains derived from the murine EW strain showed slightly more fecal shedding than those with VP5* domains from the ETD strain. The newly characterized full-genome murine RV will be a useful tool for dissecting virus-host interactions and for studying the mechanism of pathogenesis in neonatal mice.

Corrected and republished from: "VP4 Is a Determinant of Alpha-Defensin Modulation of Rotaviral Infection"

IMPORTANCE

Rotavirus is a leading cause of severe diarrhea in young children. Like other fecal-oral pathogens, rotaviruses encounter abundant, constitutively expressed defensins in the small intestine. These peptides are a vital part of the vertebrate innate immune system. By investigating the impact that defensins from multiple species have on the infectivity of different strains of rotavirus, we show that some rotaviral infections can be inhibited by defensins. We also found that rotaviruses may have evolved resistance to defensins in the intestine of their host species, and some even appropriate defensins to increase their infectivity. Because rotaviruses infect a broad range of animals and rotaviral infections are highly prevalent in children, identifying immune defenses against infection and how they vary across species and among viral genotypes is important for our understanding of the evolution, transmission, and zoonotic potential of these viruses as well as the improvement of vaccines.

The Role of the VP4 Attachment Protein in Rotavirus Host Range Restriction in an In Vivo Suckling Mouse Model

The basis for rotavirus (RV) host range restriction (HRR) is not fully understood but is likely multigenic. RV genes encoding VP3, VP4, NSP1, NSP2, NSP3, and NSP4 have been associated with HRR in various studies. With the exception of NSP1, little is known about the relative contribution of the other RV genes to HRR. VP4 has been linked to HRR because it functions as the RV cell attachment protein, but its actual role in HRR has not been fully assessed. We generated a collection of recombinant RVs (rRVs) in an isogenic murine-like RV genetic background, harboring either heterologous or homologous VP4 genes from simian, bovine, porcine, human, and murine RV strains, and characterized these rRVs in vitro and in vivo. We found that a murine-like rRV encoding a simian VP4 was shed, spread to uninoculated littermates, and induced diarrhea comparably to rRV harboring a murine VP4. However, rRVs carrying VP4s from both bovine and porcine RVs had reduced diarrhea, but no change in fecal shedding was observed. Both diarrhea and shedding were reduced when VP4 originated from a human RV strain. rRVs harboring VP4s from human or bovine RVs did not transmit to uninoculated littermates. We also generated two rRVs harboring reciprocal chimeric murine or bovine VP4. Both chimeras replicated and caused disease as efficiently as the parental strain with a fully murine VP4. These data suggest that the genetic origin of VP4 partially modulates HRR in the suckling mouse and that both the VP8* and VP5* domains independently contribute to pathogenesis and transmission. IMPORTANCE Human group A rotaviruses (RVs) remain the most important cause of severe acute gastroenteritis among infants and young children worldwide despite the introduction of several safe and effective live attenuated vaccines. The lack of knowledge regarding fundamental aspects of RV biology, such as the genetic basis of host range restriction (HRR), has made it difficult to predictively and efficiently design improved, next-generation live attenuated rotavirus vaccines. Here, we engineered a collection of VP4 monoreassortant RVs to systematically explore the role of VP4 in replication, pathogenicity, and spread, as measures of HRR, in a suckling mouse model. The genetic and mechanistic bases of HRR have substantial clinical relevance given that this restriction forms the basis of attenuation for several replication-competent human RV vaccines. In addition, a better understanding of RV pathogenesis and the determinants of RV spread is likely to enhance our ability to improve antiviral drug and therapy development.

Rotavirus vaccine-derived cases in Belgium: Evidence for reversion of attenuating mutations and alternative causes of gastroenteritis

Since the introduction of live-attenuated rotavirus vaccines in Belgium in 2006, surveillance has routinely detected rotavirus vaccine-derived strains. However, their genomic landscape and potential role in gastroenteritis have not been thoroughly investigated. We compared VP7 and VP4 nucleotide sequences obtained from rotavirus surveillance with the Rotarix vaccine sequence. As a result, we identified 80 vaccine-derived strains in 5125 rotavirus-positive infants with gastroenteritis from 2007 to 2018. Using both viral metagenomics and reverse transcription qPCR, we evaluated the vaccine strains and screened for co-infecting enteropathogens. Among the 45 patients with known vaccination status, 39 were vaccinated and 87% received the vaccine less than a month before the gastroenteritis episode. Reconstruction of 30 near complete vaccine-derived genomes revealed 0-11 mutations per genome, with 88% of them being non-synonymous. This, in combination with several shared amino acid changes among strains, pointed at selection of minor variant(s) present in the vaccine. We also found that some of these substitutions were true revertants (e.g., F167L on VP4, and I45T on NSP4). Finally, co-infections with known (e.g., Clostridioides difficile and norovirus) and divergent or emerging (e.g., human parechovirus A1, salivirus A2) pathogens were detected, and we estimated that 35% of the infants likely had gastroenteritis due to a 'non-rotavirus' cause. Conversely, we could not rule out the vaccine-derived gastroenteritis in over half of the cases. Continued studies inspecting reversion to pathogenicity should monitor the long-time safety of live-attenuated rotavirus vaccines. All in all, the complementary approach with NGS and qPCR provided a better understanding of rotavirus vaccine strain evolution in the Belgian population and epidemiology of co-infecting enteropathogens in suspected rotavirus vaccine-derived gastroenteritis cases.

VP4 Is a Determinant of Alpha-Defensin Modulation of Rotaviral Infection

Fecal-oral pathogens encounter constitutively expressed enteric alpha-defensins in the intestine during replication and transmission. Alpha-defensins can be potently antiviral and antibacterial; however, their primary sequences, the number of isoforms, and their activity against specific microorganisms often vary greatly between species, reflecting adaptation to species-specific pathogens. Therefore, alpha-defensins might influence not only microbial evolution and tissue tropism within a host but also species tropism and zoonotic potential. To investigate these concepts, we generated a panel of enteric and myeloid alpha-defensins from humans, rhesus macaques, and mice and tested their activity against group A rotaviruses, an important enteric viral pathogen of humans and animals. Rotaviral adaptation to the rhesus macaque correlated with resistance to rhesus enteric, but not myeloid, alpha-defensins and sensitivity to human alpha-defensins. While mouse rotaviral infection was increased in the presence of mouse enteric alpha-defensins, two prominent genotypes of human rotaviruses were differentially sensitive to human enteric alpha-defensins. Furthermore, the effects of cross-species alpha-defensins on human and mouse rotaviruses did not follow an obvious pattern. Thus, exposure to alpha-defensins may have shaped the evolution of some, but not all, rotaviruses. We then used a genetic approach to identify the viral attachment and penetration protein, VP4, as a determinant of alpha-defensin sensitivity. Our results provide a foundation for future studies of the VP4-dependent mechanism of defensin neutralization, highlight the species-specific activities of alpha-defensins, and focus future efforts on a broader range of rotaviruses that differ in VP4 to uncover the potential for enteric alpha-defensins to influence species tropism. IMPORTANCE Rotavirus is a leading cause of severe diarrhea in young children. Like other fecal-oral pathogens, rotaviruses encounter abundant, constitutively expressed defensins in the small intestine. These peptides are a vital part of the vertebrate innate immune system. By investigating the impact that defensins from multiple species have on the infectivity of different strains of rotavirus, we show that some rotaviral infections can be inhibited by defensins. We also found that some, but not all, rotaviruses may have evolved resistance to defensins in the intestine of their host species, and some even appropriate defensins to increase their infectivity. Because rotaviruses infect a broad range of animals and rotaviral infections are highly prevalent in children, identifying immune defenses against infection and how they vary across species and among viral genotypes is important for our understanding of the evolution, transmission, and zoonotic potential of these viruses as well as the improvement of vaccines.

Rotavirus Interactions With Host Intestinal Epithelial Cells

Rotavirus (RV) is the foremost enteric pathogen associated with severe diarrheal illness in young children (<5years) and animals worldwide. RV primarily infects mature enterocytes in the intestinal epithelium causing villus atrophy, enhanced epithelial cell turnover and apoptosis. Intestinal epithelial cells (IECs) being the first physical barrier against RV infection employs a range of innate immune strategies to counteract RVs invasion, including mucus production, toll-like receptor signaling and cytokine/chemokine production. Conversely, RVs have evolved numerous mechanisms to escape/subvert host immunity, seizing translation machinery of the host for effective replication and transmission. RV cell entry process involve penetration through the outer mucus layer, interaction with cell surface molecules and intestinal microbiota before reaching the IECs. For successful cell attachment and entry, RVs use sialic acid, histo-blood group antigens, heat shock cognate protein 70 and cell-surface integrins as attachment factors and/or (co)-receptors. In this review, a comprehensive summary of the existing knowledge of mechanisms underlying RV-IECs interactions, including the role of gut microbiota, during RV infection is presented. Understanding these mechanisms is imperative for developing efficacious strategies to control RV infections, including development of antiviral therapies and vaccines that target specific immune system antagonists within IECs.

Enteric Viral Co-Infections: Pathogenesis and Perspective

Enteric viral co-infections, infections involving more than one virus, have been reported for a diverse group of etiological agents, including rotavirus, norovirus, astrovirus, adenovirus, and enteroviruses. These pathogens are causative agents for acute gastroenteritis and diarrheal disease in immunocompetent and immunocompromised individuals of all ages globally. Despite virus–virus co-infection events in the intestine being increasingly detected, little is known about their impact on disease outcomes or human health. Here, we review what is currently known about the clinical prevalence of virus–virus co-infections and how co-infections may influence vaccine responses. While experimental investigations into enteric virus co-infections have been limited, we highlight in vivo and in vitro models with exciting potential to investigate viral co-infections. Many features of virus–virus co-infection mechanisms in the intestine remain unclear, and further research will be critical.

Serial Passaging of the Human Rotavirus CDC-9 Strain in Cell Culture Leads to Attenuation: Characterization from In Vitro and In Vivo Studies

Live oral rotavirus vaccines have been developed by serial passaging in cell culture and found to be safe in infants. However, mechanisms for the adaptation and attenuation of rotavirus vaccines are not fully understood. We prepared a human rotavirus vaccine strain, CDC-9 (G1P[8]), which when grown in MA104 cells to passage 11 or 12 (P11/P12) had no nucleotide or amino acid sequence changes from the original virus in stool. Upon adaptation and passages in Vero cells, the strain underwent five amino acid changes at P28 and one additional change at P44/P45 in the VP4 gene. We performed virologic, immunological, and pathogenic characterization of wild-type CDC-9 virus at P11/P12 and its two mutants at P28 or P44/P45 using in vitro and in vivo model systems. We found that mutants CDC-9 P28 and P44 induced upregulated expression of immunomodulatory cytokines. On the other hand, the two mutant viruses induced lower STAT1 phosphorylation and grew to 2-log-higher titers than wild-type virus in human Caco-2 cells and simian Vero cells. In neonatal rats, CDC-9 P45 showed reduced rotavirus shedding in fecal specimens and did not induce diarrhea compared to wild-type virus and modulated cytokine responses comparably to Rotarix infection. These findings indicate that mutant CDC-9 is attenuated and safe. Our study is the first to provide insight into the possible mechanisms of human rotavirus adaptation and attenuation and supports ongoing efforts to develop CDC-9 as a new generation of rotavirus vaccine for live oral or parenteral administration.IMPORTANCE Mechanisms for in vitro adaptation and in vivo attenuation of human rotavirus vaccines are not known. The present study is the first to comprehensively compare the in vitro growth characteristics, virulence, and host response of a wild-type and an attenuated human rotavirus strain, CDC-9, in Caco-2 cells and neonatal rats. Our study identifies critical sequence changes in the genome that render human rotavirus adapted to growth to high levels in Vero cells and attenuated and safe in neonatal rats; thus, the study supports clinical development of CDC-9 for oral or parenteral vaccination in children.

Rotavirus Reprograms Multiple Interferon Receptors and Restricts Their Intestinal Antiviral and Inflammatory Functions

Rotaviruses (RV) cause acute severe diarrhea in the absence of substantial intestinal inflammation. They are also highly infectious in their homologous host species. The replication capacity of RV in the small bowel is substantially due to its ability to inhibit different types of interferons (IFNs). Here, we found that during RV infection in vitro, both virus-infected and uninfected bystander cells resist STAT1 phosphorylation and interferon regulatory factor 7 (IRF7) induction in response to exogenous interferon (IFN). Functionally, cellular transcription in response to stimulation with IFN, but not intracellular double-stranded RNA (dsRNA), was inhibited by RV. Further, IFNAR1 stimulation during RV infection significantly repressed a set of virus-induced transcripts. Regulation of IFN signaling in vivo was studied in suckling mice using the highly infectious murine EW RV strain. Kinetic studies indicated that sustained EW RV replication and IFN induction in the small intestine are accompanied by significant decreases in IFN-stimulated transcripts. Lipopolysaccharide (LPS)-mediated intestinal damage, driven by STAT1-induced inflammation, was also prevented in EW RV-infected mice. Remarkably, by ectopically stimulating either IFNAR1 or IFNGR1 in EW RV-infected mice, we could eliminate several intestinal antiviral and inflammatory transcriptional responses to RV. In contrast to infection with homologous RV, infection with a STAT1-sensitive heterologous RV strain induced IFN-stimulated transcripts, inflammatory cytokines, and intestinal expression of STAT1-pY701. Finally, RV strain-specific STAT1 regulation also likely determines the intestinal activation of multiple caspases. The simian RRV strain, but not murine EW RV, uniquely triggers the cleavage of both extrinsic and intrinsic caspases (caspases 8, 9, and 3) in a STAT1-mediated manner. Collectively, our findings reveal efficient reprograming of multiple IFN receptors toward a negative-feedback mode of signaling, accompanied by suppression of IFN-mediated antiviral, apoptotic, and inflammatory functions, during natural RV intestinal infection.IMPORTANCE Rotavirus is a highly infectious pathogen that causes severe diarrhea. Replication of RV in the small intestine is restricted to homologous host species, and host range restriction is substantially determined by the interferon response. In this study, we demonstrate that during infection, RV bystander cells resist exogenous IFN-mediated STAT1 signaling and transcription. In a suckling mouse model, ectopically stimulating different intestinal interferon receptors during RV infection eliminates several innate and inflammatory antiviral responses. Different intestinal inflammatory cytokines were also suppressed by homologous RV, as was intestinal damage in response to endotoxin. The ability of RV to suppress IFN-mediated receptors likely impacts intestinal cell homeostasis, as the cleavage of multiple intestinal caspases during RV infection is mediated by the IFN-STAT1 signaling pathway. Together, our results provide a mechanism underlying both the remarkable interferon resistance of homologous RV and its ability to prevent substantial inflammatory damage to the small bowel.

Development of a live attenuated trivalent porcine rotavirus A vaccine against disease caused by recent strains most prevalent in South Korea

Porcine rotaviruses cause severe economic losses in the Korean swine industry due to G- and P-genotype mismatches between the predominant field and vaccine strains. Here, we developed a live attenuated trivalent porcine group A rotavirus vaccine using 80 cell culture passages of the representative Korean predominant strains G8P[7] 174-1, G9P[23] PRG942, and G5P[7] K71. Vaccination with the trivalent vaccine or its individual components induced no diarrhea during the first 2 weeks post-vaccination, i.e., the vaccines were attenuated. Challenge of trivalent-vaccinated or component-vaccinated piglets with homologous virulent strain(s) did not induce diarrhea for 2 weeks post-challenge. Immunization with the trivalent vaccine or its individual components also alleviated the histopathological lesions in the small intestines caused by challenge with the corresponding original virulent strain(s). Fecal secretory IgAs specific for each of vaccine strains were detected starting at 14 days post-vaccination (dpv), and IgA levels gradually increased up to 28 dpv. Oral immunization with the trivalent vaccine or its individual components induced high levels of serum virus-neutralizing antibody by 7 dpv. No diarrhea was observed in any experimental piglets during five consecutive passages of each vaccine strain. Our data indicated that the live attenuated trivalent vaccine was safe and effective at protecting piglets from diarrhea induced by challenge exposure of homologous virulent strains. This trivalent vaccine will potentially contribute toward controlling porcine rotavirus disease in South Korea and other countries where rotavirus infections with similar G and P genotypes are problematic.

Rotavirus Degrades Multiple Interferon (IFN) Type Receptors To Inhibit IFN Signaling and Protects against Mortality from Endotoxin in Suckling Mice

STAT1 phosphorylation in response to exogenous interferon (IFN) administration can be inhibited by rotaviral replication both in vitro and in vivo In addition many rotavirus strains are resistant to the actions of different IFN types. The regulation by rotaviruses (RVs) of antiviral pathways mediated by multiple IFN types is not well understood. In this study, we find that during infection in vitro and in vivo, RVs significantly deplete IFN type I, II, and III receptors (IFNRs). Regulation of IFNRs occurred exclusively within RV-infected cells and could be abrogated by inhibiting the lysosomal-endosomal degradation pathway. In vitro, IFNR degradation was conserved across multiple RV strains that differ in their modes of regulating IFN induction. In suckling mice, exogenously administered type I, II, or III IFN induced phosphorylation of STAT1-Y701 within intestinal epithelial cells (IECs) of suckling mice. Murine EW strain RV infection transiently activated intestinal STAT1 at 1 day postinfection (dpi) but not subsequently at 2 to 3 dpi. In response to injection of purified IFN-α/β or -λ, IECs in EW-infected mice exhibited impaired STAT1-Y701 phosphorylation, correlating with depletion of different intestinal IFNRs and impaired IFN-mediated transcription. The ability of EW murine RV to inhibit multiple IFN types led us to test protection of suckling mice from endotoxin-mediated shock, an outcome that is dependent on the host IFN response. Compared to mortality in controls, mice infected with EW murine RV were substantially protected against mortality following parenteral endotoxin administration. These studies identify a novel mechanism of IFN subversion by RV and reveal an unexpected protective effect of RV infection on endotoxin-mediated shock in suckling mice.IMPORTANCE Antiviral functions of types I, II, and III IFNs are mediated by receptor-dependent activation of STAT1. Here, we find that RV degrades the types I, II, and III IFN receptors (IFNRs) in vitro In a suckling mouse model, RV effectively blocked STAT1 activation and transcription following injection of different purified IFNs. This correlated with significantly decreased protein expression of intestinal types I and II IFNRs. Recent studies demonstrate that in mice lipopolysaccharide (LPS)-induced lethality is prevented by genetic ablation of IFN signaling genes such as IFNAR1 and STAT1. When suckling mice were infected with RV, they were substantially protected from lethal exposure to endotoxin. These findings provide novel insights into the mechanisms underlying rotavirus regulation of different interferons and are likely to stimulate new research into both rotavirus pathogenesis and endotoxemia.

Rotavirus Strategies Against the Innate Antiviral System

"Rotaviruses represent the most important etiological agents of acute, severe gastroenteritis in the young of many animal species, including humans." This statement, variations of which are a common beginning in articles about rotaviruses, reflects the fact that these viruses have evolved efficient strategies for evading the innate immune response of the host and for successfully replicating in the population. In this review, we summarize what is known about the defense mechanisms that host cells employ to prevent rotavirus invasion and the countermeasures that these viruses have successfully developed to surpass cellular defenses. Rotaviruses use at least two viral multifunctional proteins to directly interact with, and prevent the activation of, the interferon system, and they use at least one other protein to halt the protein synthesis machinery and prevent the expression of most of the transcriptional antiviral program of the cell. Characterization of the confrontation between rotaviruses and their host cells has allowed us to learn about the virus-host coevolution that prevents the damaging effects of the innate immune response.

Experimental Adaptation of Rotaviruses to Tumor Cell Lines

A number of viruses show a naturally extended tropism for tumor cells whereas other viruses have been genetically modified or adapted to infect tumor cells. Oncolytic viruses have become a promising tool for treating some cancers by inducing cell lysis or immune response to tumor cells. In the present work, rotavirus strains TRF-41 (G5) (porcine), RRV (G3) (simian), UK (G6-P5) (bovine), Ym (G11-P9) (porcine), ECwt (murine), Wa (G1-P8), Wi61 (G9) and M69 (G8) (human), and five wild-type human rotavirus isolates were passaged multiple times in different human tumor cell lines and then combined in five different ways before additional multiple passages in tumor cell lines. Cell death caused by the tumor cell-adapted isolates was characterized using Hoechst, propidium iodide, 7-AAD, Annexin V, TUNEL, and anti-poly-(ADP ribose) polymerase (PARP) and -phospho-histone H2A.X antibodies. Multiple passages of the combined rotaviruses in tumor cell lines led to a successful infection of these cells, suggesting a gain-of-function by the acquisition of greater infectious capacity as compared with that of the parental rotaviruses. The electropherotype profiles suggest that unique tumor cell-adapted isolates were derived from reassortment of parental rotaviruses. Infection produced by such rotavirus isolates induced chromatin modifications compatible with apoptotic cell death.

The role of type I interferons in intestinal infection, homeostasis, and inflammation

Type I interferons are a widely expressed family of effector cytokines that promote innate antiviral and antibacterial immunity. Paradoxically, they can also suppress immune responses by driving production of anti-inflammatory cytokines, and dysregulation of these cytokines can contribute to host-mediated immunopathology and disease progression. Recent studies describe their anti-inflammatory role in intestinal inflammation and the locus containing IFNAR, a heterodimeric receptor for the type I interferons has been identified as a susceptibility region for human inflammatory bowel disease. This review focuses on the role of type I IFNs in the intestine in health and disease and their emerging role as immune modulators. Clear understanding of type I IFN-mediated immune responses may provide avenues for fine-tuning existing IFN treatment for infection and intestinal inflammation.

Complete genome characterization of recent and ancient Belgian pig group A rotaviruses and assessment of their evolutionary relationship with human rotaviruses

Group A rotaviruses (RVAs) are an important cause of diarrhea in young pigs and children. An evolutionary relationship has been suggested to exist between pig and human RVAs. This hypothesis was further investigated by phylogenetic analysis of the complete genomes of six recent (G2P[27], G3P[6], G4P[7], G5P[7], G9P[13], and G9P[23]) and one historic (G1P[7]) Belgian pig RVA strains and of all completely characterized pig RVAs from around the globe. In contrast to the large diversity of genotypes found for the outer capsid proteins VP4 and VP7, a relatively conserved genotype constellation (I5-R1-C1-M1-A8-N1-T7-E1-H1) was found for the other 9 genes in most pig RVA strains. VP1, VP2, VP3, NSP2, NSP4, and NSP5 genes of porcine RVAs belonged to genotype 1, which is shared with human Wa-like RVAs. However, for most of these gene segments, pig strains clustered distantly from human Wa-like RVAs, indicating that viruses from both species have entered different evolutionary paths. However, VP1, VP2, and NSP3 genes of some archival human strains were moderately related to pig strains. Phylogenetic analysis of the VP6, NSP1, and NSP3 genes, as well as amino acid analysis of the antigenic regions of VP7, further confirmed this evolutionary segregation. The present results also indicate that the species barrier is less strict for pig P[6] strains but that chances for successful spread of these strains in the human population are hampered by the better adaptation of pig RVAs to pig enterocytes. However, future surveillance of pig and human RVA strains is warranted.

IMPORTANCE

Rotaviruses are an important cause of diarrhea in many species, including pigs and humans. Our understanding of the evolutionary relationship between rotaviruses from both species is limited by the lack of genomic data on pig strains. In this study, recent and ancient Belgian pig rotavirus isolates were sequenced, and their evolutionary relationship with human Wa-like strains was investigated. Our data show that Wa-like human and pig strains have entered different evolutionary paths. Our data indicate that pig P[6] strains form the most considerable risk for interspecies transmission to humans. However, efficient spread of pig strains in the human population is most likely hampered by the adaptation of some crucial viral proteins to the cellular machinery of pig enterocytes. These data allow a better understanding of the risk for direct interspecies transmission events and the emergence of pig rotaviruses or pig-human reassortants in the human population.

Absence of genetic differences among G10P[11] rotaviruses associated with asymptomatic and symptomatic neonatal infections in Vellore, India

Rotaviruses (RVs) are leading causes of severe diarrhea and vomiting in infants and young children. RVs with G10P[11] genotype specificity have been associated with symptomatic and asymptomatic neonatal infections in Vellore, India. To identify possible viral genetic determinants responsible for differences in symptomology, the genome sequences of G10P[11] RVs in stool samples of 19 neonates with symptomatic infections and 20 neonates with asymptomatic infections were determined by Sanger and next-generation sequencing. The data showed that all 39 viruses had identical genotype constellations (G10-P[11]-I2-R2-C2-M2-A1-N1-T1-E2-H3), the same as those of the previously characterized symptomatic N155 Vellore isolate. The data also showed that the RNA and deduced protein sequences of all the Vellore G10P[11] viruses were nearly identical; no nucleotide or amino acid differences were found that correlated with symptomatic versus asymptomatic infection. Next-generation sequencing data revealed that some stool samples, both from neonates with symptomatic infections and from neonates with asymptomatic infections, also contained one or more positive-strand RNA viruses (Aichi virus, astrovirus, or salivirus/klassevirus) suspected of being potential causes of pediatric gastroenteritis. However, none of the positive-strand RNA viruses could be causally associated with the development of symptoms. These results indicate that the diversity of clinical symptoms in Vellore neonates does not result from genetic differences among G10P[11] RVs; instead, other undefined factors appear to influence whether neonates develop gastrointestinal disease symptoms.

IMPORTANCE

Rotavirus (RV) strains have been identified that preferentially replicate in neonates, in some cases, without causing gastrointestinal disease. Surveillance studies have established that G10P[11] RVs are a major cause of neonatal infection in Vellore, India, with half of infected neonates exhibiting symptoms. We used Sanger and next-generation sequencing technologies to contrast G10P[11] RVs recovered from symptomatic and asymptomatic neonates. Remarkably, the data showed that the RNA genomes of the viruses were virtually indistinguishable and lacked any differences that could explain the diversity of clinical outcomes among infected Vellore neonates. The sequencing results also indicated that some symptomatic and some asymptomatic Vellore neonates were infected with other enteric viruses (Aichi virus, astrovirus, salvirus/klassevirus); however, none could be correlated with the presence of symptoms in neonates. Together, our findings suggest that other poorly defined factors, not connected to the genetic makeup of the Vellore G10P[11] viruses, influence whether neonates develop gastrointestinal disease symptoms.

Rotavirus NSP1 protein inhibits interferon-mediated STAT1 activation

Rotavirus (RV) replicates efficiently in intestinal epithelial cells (IECs) in vivo despite the activation of a local host interferon (IFN) response. Previously, we demonstrated that homologous RV efficiently inhibits IFN induction in single infected and bystander villous IECs in vivo. Paradoxically, RV also induces significant type I IFN expression in the intestinal hematopoietic cell compartment in a relatively replication-independent manner. This suggests that RV replication and spread in IECs must occur despite exogenous stimulation of the STAT1-mediated IFN signaling pathway. Here we report that RV inhibits IFN-mediated STAT1 tyrosine 701 phosphorylation in human IECs in vitro and identify RV NSP1 as a direct inhibitor of the pathway. Infection of human HT29 IECs with simian (RRV) or porcine (SB1A or OSU) RV strains, which inhibit IFN induction by targeting either IFN regulatory factor 3 (IRF3) or NF-κB, respectively, resulted in similar regulation of IFN secretion. By flow cytometric analysis at early times during infection, neither RRV nor SB1A effectively inhibited the activation of Y701-STAT1 in response to exogenously added IFN. However, at later times during infection, both RV strains efficiently inhibited IFN-mediated STAT1 activation within virus-infected cells, indicating that RV encodes inhibitors of IFN signaling targeting STAT1 phosphorylation. Expression of RV NSP1 in the absence of other viral proteins resulted in blockage of exogenous IFN-mediated STAT1 phosphorylation, and this function was conserved in NSP1 from simian, bovine, and murine RV strains. Analysis of NSP1 determinants responsible for the inhibition of IFN induction and signaling pathways revealed that these determinants are encoded on discrete domains of NSP1. Finally, we observed that at later times during infection with SB1A, there was almost complete inhibition of IFN-mediated Y701-STAT1 in bystander cells staining negative for viral antigen. This property segregated with the NSP1 gene and was observed in a simian SA11 monoreassortant that encoded porcine OSU NSP1 but not in wild-type SA11 or a reassortant encoding simian RRV NSP1.

Different virulence of porcine and porcine-like bovine rotavirus strains with genetically nearly identical genomes in piglets and calves

Direct interspecies transmissions of group A rotaviruses (RVA) have been reported under natural conditions. However, the pathogenicity of RVA has never been directly compared in homologous and heterologous hosts. The bovine RVA/Cow-tc/KOR/K5/2004/G5P[7] strain, which was shown to possess a typical porcine-like genotype constellation similar to that of the G5P[7] prototype RVA/Pig-tc/USA/OSU/1977/G5P9[7] strain, was examined for its pathogenicity and compared with the porcine G5P[7] RVA/Pig-tc/KOR/K71/2006/G5P[7] strain possessing the same genotype constellation. The bovine K5 strain induced diarrhea and histopathological changes in the small intestine of piglets and calves, whereas the porcine K71 strain caused diarrhea and histopathological changes in the small intestine of piglets, but not in calves. Furthermore, the bovine K5 strain showed extra-intestinal tropisms in both piglets and calves, whereas the porcine K71 strain had extra-intestinal tropisms in piglets, but not in calves. Therefore, we performed comparative genomic analysis of the K71 and K5 RVA strains to determine whether specific mutations could be associated with these distinct clinical and pathological phenotypes. Full-length sequencing analyses for the 11 genomic segments for K71 and K5 revealed that these strains were genetically nearly identical to each other. Two nucleotide mutations were found in the 5′ untranslated region (UTR) of NSP5 and the 3′ UTR of NSP3, and eight amino acid mutations in VP1-VP4 and NSP2. Some of these mutations may be critical molecular determinants for RVA virulence and/or pathogenicity.

Characterization of rotavirus RNAs that activate innate immune signaling through the RIG-I-like receptors

In mammalian cells, the first line of defense against viral pathogens is the innate immune response, which is characterized by induction of type I interferons (IFN) and other pro-inflammatory cytokines that establish an antiviral milieu both in infected cells and in neighboring uninfected cells. Rotavirus, a double-stranded RNA virus of the Reoviridae family, is the primary etiological agent of severe diarrhea in young children worldwide. Previous studies demonstrated that rotavirus replication induces a MAVS-dependent type I IFN response that involves both RIG-I and MDA5, two cytoplasmic viral RNA sensors. This study reports the isolation and characterization of rotavirus RNAs that activate IFN signaling. Using an in vitro approach with purified rotavirus double-layer particles, nascent single-stranded RNA (ssRNA) transcripts (termed in vitro ssRNA) were found to be potent IFN inducers. In addition, large RNAs isolated from rotavirus-infected cells six hours post-infection (termed in vivo 6 hr large RNAs), also activated IFN signaling, whereas a comparable large RNA fraction isolated from cells infected for only one hour lacked this stimulatory activity. Experiments using knockout murine embryonic fibroblasts showed that RIG-I is required for and MDA5 partly contributes to innate immune signaling by both in vitro ssRNA and in vivo 6 hr large RNAs. Enzymatic studies demonstrated that in vitro ssRNA and in vivo 6 hr large RNA samples contain uncapped RNAs with exposed 5’ phosphate groups. RNAs lacking 2’-O-methylated 5’ cap structures were also detected in the in vivo 6 hr large RNA sample. Taken together, our data provide strong evidence that the rotavirus VP3 enzyme, which encodes both guanylyltransferase and methyltransferase activities, is not completely efficient at either 5’ capping or 2’-O-methylation of the 5’ cap structures of viral transcripts, and in this way produces RNA patterns that activate innate immune signaling through the RIG-I-like receptors.

Permissive replication of homologous murine rotavirus in the mouse intestine is primarily regulated by VP4 and NSP1

Homologous rotaviruses (RV) are, in general, more virulent and replicate more efficiently than heterologous RV in the intestine of the homologous host. The genetic basis for RV host range restriction is not fully understood and is likely to be multigenic. In previous studies, RV genes encoding VP3, VP4, VP7, nonstructural protein 1 (NSP1), and NSP4 have all been implicated in strain- and host species-specific infection. These studies used different RV strains, variable measurements of host range, and different animal hosts, and no clear consensus on the host range restriction determinants emerged. We used a murine model to demonstrate that enteric replication of murine RV EW is 1,000- to 10,000-fold greater than that of a simian rotavirus (RRV) in suckling mice. Intestinal replication of a series of EW × RRV reassortants was used to identify several RV genes that influenced RV replication in the intestine. The role of VP4 (encoded by gene 4) in enteric infection was strain specific. RRV VP4 reduced murine RV infectivity only slightly; however, a reassortant expressing VP4 from a bovine RV strain (UK) severely restricted intestinal replication in the suckling mice. The homologous murine EW NSP1 (encoded by gene 5) was necessary but not sufficient for promoting efficient enteric growth. Efficient enteric replication required a constellation of murine genes encoding VP3, NSP2, and NSP3 along with NSP1.

The battle between rotavirus and its host for control of the interferon signaling pathway

Viral pathogens must overcome innate antiviral responses to replicate successfully in the host organism. Some of the mechanisms viruses use to interfere with antiviral responses in the infected cell include preventing detection of viral components, perturbing the function of transcription factors that initiate antiviral responses, and inhibiting downstream signal transduction. RNA viruses with small genomes and limited coding space often express multifunctional proteins that modulate several aspects of the normal host response to infection. One such virus, rotavirus, is an important pediatric pathogen that causes severe gastroenteritis, leading to ∼450,000 deaths globally each year. In this review, we discuss the nature of the innate antiviral responses triggered by rotavirus infection and the viral mechanisms for inhibiting these responses.

Innate immune response to homologous rotavirus infection in the small intestinal villous epithelium at single-cell resolution

"Bulk" measurements of antiviral innate immune responses from pooled cells yield averaged signals and do not reveal underlying signaling heterogeneity in infected and bystander single cells. We examined such heterogeneity in the small intestine during rotavirus (RV) infection. Murine RV EW robustly activated type I IFNs and several antiviral genes (IFN-stimulated genes) in the intestine by bulk analysis, the source of induced IFNs primarily being hematopoietic cells. Flow cytometry and microfluidics-based single-cell multiplex RT-PCR allowed dissection of IFN responses in single RV-infected and bystander intestinal epithelial cells (IECs). EW replicates in IEC subsets differing in their basal type I IFN transcription and induces IRF3-dependent and IRF3-augmented transcription, but not NF-κB-dependent or type I IFN transcripts. Bystander cells did not display enhanced type I IFN transcription but had elevated levels of certain IFN-stimulated genes, presumably in response to exogenous IFNs secreted from immune cells. Comparison of IRF3 and NF-κB induction in STAT1(-/-) mice revealed that murine but not simian RRV mediated accumulation of IkB-α protein and decreased transcription of NF-κB-dependent genes. RRV replication was significantly rescued in IFN types I and II, as well as STAT1 (IFN types I, II, and III) deficient mice in contrast to EW, which was only modestly sensitive to IFNs I and II. Resolution of "averaged" innate immune responses in single IECs thus revealed unexpected heterogeneity in both the induction and subversion of early host antiviral immunity, which modulated host range.

Genotype constellation and evolution of group A rotaviruses infecting humans

Numerous rotavirus group A (RVA) strains with distinct G-genotype and P-genotype combinations have been described infecting humans worldwide. However, the increasing amount of complete RVA genome data which have become available, suggest that only RVA strains with 2 discrete genotype constellations have been successful in sustaining infection of humans worldwide over longer periods of time. Those genotype constellations have been designated I1-R1-C1-M1-A1-N1-T1-E1-H1 and I2-R2-C2-M2-A2-N2-T2-E2-H2 and are also known as Wa-like and DS-1-like, respectively. RVAs of other genotype constellations which were able to spread to a limited extent in the human population are AU-1-related RVA strains (I3-R3-C3-M3-A3/A12-N3-T3-E3-H3/H6) in combination with G3P[9] or G12P[9], and neonatal G10P[11] RVA strains in India (bovine×human Wa-like reassortants). On the basis of the analysis of complete genomes, it is suggested that the overall genetic diversity of epidemiologically widespread human RVA strains is more limited than generally assumed. This conclusion has consequences for how we look at host range restriction and the criteria according to which the effectiveness of RVA universal mass vaccination programs is assessed.

Full-length genomic analysis of porcine G9P[23] and G9P[7] rotavirus strains isolated from pigs with diarrhea in South Korea

Group A rotaviruses (RVAs) are agents causing severe gastroenteritis in infants and young animals. G9 RVA strains are believed to have originated from pigs. However, this genotype has emerged as the fifth major human RVA genotype worldwide. To better understand the relationship between human and porcine RVA strains, complete RVA genome data are needed. For human RVA strains, the number of complete genome data have grown exponentially. However, there is still a lack of complete genome data on porcine RVA strains. Recently, G9 RVA strains have been identified as the third most important genotype in diarrheic pigs in South Korea in combinations with P[7] and P[23]. This study is the first report on complete genome analyses of 1 G9P[7] and 3 G9P[23] porcine RVA strains, resulting in the following genotype constellation: G9-P[7]/P[23]-I5-R1-C1-M1-A8-N1-T1-E1-H1. By comparisons of these genotype constellations, it was revealed that the Korean G9P[7] and G9P[23] RVA strains possessed a typical porcine RVA backbone, similar to other known porcine RVA strains. However, detailed phylogenetic analyses revealed the presence of intra-genotype reassortments among porcine RVA strains in South Korea. Thus, our data provide genetic information of G9 RVA strains increasingly detected in both humans and pigs, and will help to establish the role of pigs as a source or reservoir for novel human RVA strains.

Pathogenicity characterization of a bovine triple reassortant rotavirus in calves and piglets

Rotaviruses are important human and animal pathogens with high impact on public health and livestock industry. There is little evidence about the cross-species pathogenicity and extra-intestinal infections of animal and human reassortant rotaviruses, particularly based on all 11 genotyping data. In this study, the bovine triple reassortant KJ56-1 strain harboring two bovine-like genome segments, eight porcine-like genome segments, and one human-like genome segment was used to evaluate the cross-species pathogenicity in its parent species, calves and piglets, and to determine its abilities of causing viremia and extra-intestinal tropisms in piglets. The KJ56-1 strain isolated from a calf diarrhea fecal sample replicated without causing diarrhea and severe intestinal pathology in calves. However, piglets inoculated with this strain showed persistent severe diarrhea and marked intestinal pathology. By SYBR Green real-time RT-PCR, viral RNA was detected in the sera, mesenteric lymph node, lung, liver, choroid plexus, and cerebrospinal fluid in the experimental piglets. An immunofluorescence assay confirmed viral replication in these extra-intestinal organs and tissues. These results indicated that the bovine triple reassortant KJ56-1 strain was virulent to piglets but not to calves. Our data also demonstrated that the reassortant rotaviruses had the ability to spread to the bloodstream from the gut, enter and amplify in the mesenteric lymph node, and disseminate to the extra-intestinal organs and tissues.

Conformational Differences Unfold a Wide Range of Enterotoxigenic Abilities Exhibited by rNSP4 Peptides from Different Rotavirus Strains

NSP4 has been recognized as the rotavirus-encoded enterotoxin. However, a few studies failed to support its diarrheagenic activity. As recombinant NSP4 (rNSP4) peptides of different lengths were used in the limited number of studies, a comparison of relative diarrheagenic potential of NSP4 from different strains could not be possible. To better understand the diarrheagenic potential of NSP4 from different strains, in this report we have evaluated the enterotoxigenic activity of the deletion mutant ΔN72 that lacks the N-terminal 72 residues and the biologically relevant ΔN112 peptide which when derived from SA11 rotavirus strain were previously shown to be highly diarrheagenic in newborn mice. Detailed comparative analysis of biochemical and biophysical properties and diarrheagenic activity of the recombinant ΔN72 peptides from seventeen different strains under identical conditions revealed wide differences among themselves in their resistance to trypsin cleavage, thioflavin T (ThT) binding, multimerization and conformation without any correlation with their diarrhea inducing abilities. These results support our previously proposed concept for the requirement of a unique conformation for optimal biological functions conferred by cooperation between the N- and C-terminal regions of the cytoplasmic tail.

The rhesus rotavirus gene encoding VP4 is a major determinant in the pathogenesis of biliary atresia in newborn mice

Biliary atresia (BA) is a devastating disease of childhood for which increasing evidence supports a viral component in pathogenesis. The murine model of BA is induced by perinatal infection with rhesus rotavirus (RRV) but not with other strains of rotavirus, such as TUCH. To determine which RRV gene segment(s) is responsible for pathogenesis, we used the RRV and TUCH strains to generate a complete set of single-gene reassortants. Eleven single-gene "loss-of-function" reassortants in which a TUCH gene replaced its RRV equivalent and 11 single-gene "gain-of-function" reassortants in which an RRV gene replaced its TUCH equivalent were generated. Newborn BALB/c mice were inoculated with the reassortants and were monitored for biliary obstruction and mortality. In vitro, the ability to bind to and replicate within cholangiocytes was analyzed. Infection of mice with the "loss-of-function" reassortant R(T(VP4)), where gene 4 from TUCH was placed on an RRV background, eliminated the ability of RRV to cause murine BA. In a reciprocal fashion, the "gain-of-function" reassortant T(R(VP4)) resulted in murine BA with 88% mortality. Compared with those for RRV, R(T(VP4)) binding and titers in cholangiocytes were significantly attenuated, while T(R(VP4)) binding and titers were significantly increased over those for TUCH. Reassortants R(T(VP3)) and T(R(VP3)) induced an intermediate phenotype. RRV gene segment 4 plays a significant role in governing tropism for the cholangiocyte and the ability to induce murine BA. Gene segment 3 did not affect RRV infectivity in vitro but altered its in vivo effect.

The early interferon response to rotavirus is regulated by PKR and depends on MAVS/IPS-1, RIG-I, MDA-5, and IRF3

In mouse embryonic fibroblasts (MEFs), the bovine rotavirus (UK strain) but not the simian rhesus rotavirus (RRV) robustly triggers beta interferon (IFN-β) secretion, resulting in an IFN-dependent restriction of replication. We now find that both rotavirus strains trigger antiviral transcriptional responses early during infection and that both transcriptional responses and IFN-β secretion are completely abrogated in MAVS/IPS-1(-/-) MEFs. Replication of UK virus could be rescued in MAVS/IPS-1(-/-) MEFs, and synthesis of viral RNA significantly increased early during virus infection. UK virus induced IFN-β secretion and transcription of IFN-stimulated genes (ISGs) in both RIG-I(-/-) and MDA-5(-/-) MEFs, and neither receptor was essential by itself for the antiviral response to UK rotavirus. However, when receptors RIG-I and MDA-5 were depleted using RNA interference, we found that both contribute to the magnitude of the IFN response. IRF3 was found to be essential for MAVS/IPS-1-directed ISG transcription and IFN-β secretion during rotavirus infection. Interestingly, absence of the double-stranded RNA-dependent protein kinase PKR led to a profound defect in the capacity of host cells to secrete IFN-β in response to virus. Both PKR and IRF3 restricted the early replication of UK as indicated by significant increases in viral RNA in fibroblasts lacking either gene. Despite the loss in IFN-β secretion in PKR(-/-) MEFs, we did not observe decreased IRF3- or NF-κB-dependent early ISG transcription in these cells. Levels of transcripts encoding IFN-α4, IFN-α5, and IFN-β were high in infected PKR(-/-) MEFs, indicating that during rotavirus infection, PKR functions at a stage between IFN gene transcription and subsequent IFN-β secretion. These findings reveal that activation of the antiviral response by rotavirus is dependent on MAVS/IPS-1 and IRF3 and involves both RIG-I and MDA-5 and that IFN-β secretion during rotavirus infection is regulated by PKR.

Roles of VP4 and NSP1 in determining the distinctive replication capacities of simian rotavirus RRV and bovine rotavirus UK in the mouse biliary tract

Rotavirus replication and virulence are strongly influenced by virus strain and host species. The rotavirus proteins VP3, VP4, VP7, NSP1, and NSP4 have all been implicated in strain and species restriction of replication; however, the mechanisms have not been fully determined. Simian (RRV) and bovine (UK) rotaviruses have distinctive replication capacities in mouse extraintestinal organs such as the biliary tract. Using reassortants between UK and RRV, we previously demonstrated that the differential replication of these viruses in mouse embryonic fibroblasts is determined by the respective NSP1 proteins, which differ substantially in their abilities to degrade interferon (IFN) regulatory factor 3 (IRF3) and suppress the type I IFN response. In this study, we used an in vivo model of rotavirus infection of mouse gallbladder with UK × RRV reassortants to study the genetic and mechanistic basis of systemic rotavirus replication. We found that the low-replication phenotype of UK in biliary tissues was conferred by UK VP4 and that the high-replication phenotype of RRV was conferred by RRV VP4 and NSP1. Viruses with RRV VP4 entered cultured mouse cholangiocytes more efficiently than did those with UK VP4. Reassortants with RRV VP4 and UK NSP1 genes induced high levels of expression of IRF3-dependent p54 in biliary tissues, and their replication was increased 3-fold in IFN-α/β and -γ receptor or STAT1 knockout (KO) mice compared to wild-type mice. Our data indicate that systemic rotavirus strain-specific replication in the murine biliary tract is determined by both viral entry mediated by VP4 and viral antagonism of the host innate immune response mediated by NSP1.

Concentration of acrylamide in a polyacrylamide gel affects VP4 gene coding assignment of group A equine rotavirus strains with P[12] specificity

BACKGROUND

It is universally acknowledged that genome segment 4 of group A rotavirus, the major etiologic agent of severe diarrhea in infants and neonatal farm animals, encodes outer capsid neutralization and protective antigen VP4.

RESULTS

To determine which genome segment of three group A equine rotavirus strains (H-2, FI-14 and FI-23) with P[12] specificity encodes the VP4, we analyzed dsRNAs of strains H-2, FI-14 and FI-23 as well as their reassortants by polyacrylamide gel electrophoresis (PAGE) at varying concentrations of acrylamide. The relative position of the VP4 gene of the three equine P[12] strains varied (either genome segment 3 or 4) depending upon the concentration of acrylamide. The VP4 gene bearing P[3], P[4], P[6], P[7], P[8] or P[18] specificity did not exhibit this phenomenon when the PAGE running conditions were varied.

CONCLUSIONS

The concentration of acrylamide in a PAGE gel affected VP4 gene coding assignment of equine rotavirus strains bearing P[12] specificity.

Rotavirus antagonism of the innate immune response

Rotavirus is a primary cause of severe dehydrating gastroenteritis in infants and young children. The virus is sensitive to the antiviral effects triggered by the interferon (IFN)-signaling pathway, an important component of the host cell innate immune response. To counteract these effects, rotavirus encodes a nonstructural protein (NSP1) that induces the degradation of proteins involved in regulating IFN expression, such as members of the IFN regulatory factor (IRF) family. In some instances, NSP1 also subverts IFN expression by causing the degradation of a component of the E3 ubiquitin ligase complex responsible for activating NF-κB. By antagonizing multiple components of the IFN-induction pathway, NSP1 aids viral spread and contributes to rotavirus pathogenesis.

IRF3 inhibition by rotavirus NSP1 is host cell and virus strain dependent but independent of NSP1 proteasomal degradation

Rotavirus host range restriction forms a basis for strain attenuation although the underlying mechanisms are unclear. In mouse fibroblasts, the inability of rotavirus NSP1 to mediate interferon (IFN) regulatory factor 3 (IRF3) degradation correlates with IFN-dependent restricted replication of the bovine UK strain but not the mouse EW and simian RRV strains. We found that UK NSP1 is unable to degrade IRF3 when expressed in murine NIH 3T3 cells in contrast to the EW and RRV NSP1 proteins. Surprisingly, UK NSP1 expression led to IRF3 degradation in simian COS7 cells, indicating that IRF3 degradation by NSP1 is host cell dependent, a finding further supported using adenovirus-expressed NSP1 from NCDV bovine rotavirus. By expressing heterologous IRF3 proteins in complementary host cells, we found that IRF3 is the minimal host factor constraining NSP1 IRF3-degradative ability. NSP1-mediated IRF3 degradation was enhanced by transfection of double-stranded RNA (dsRNA) in a host cell-specific manner, and in IRF3-dependent positive regulatory domain III reporter assays, NSP1 inhibited IRF3 function in response to pathway activation by dsRNA, TBK-1, IRF3, or constitutively activated IRF3-5D. An interesting observation arising from these experiments is the ability of transiently expressed UK NSP1 to inhibit poly(I:C)-directed IRF3 activity in NIH 3T3 cells in the absence of detectable IRF3 degradation, an unexpected finding since UK virus infection was unable to block IFN secretion, and UK NSP1 expression did not result in suppression of IRF3-directed activation of the pathway. RRV and EW but not UK NSP1 was proteasomally degraded, requiring E1 ligase activity, although NSP1 degradation was not required for IRF3 degradation. Using a chimeric RRV NSP1 protein containing the carboxyl 100 residues derived from UK NSP1, we found that the RRV NSP1 carboxyl 100 residues are critical for its IRF3 inhibition in murine cells but are not essential for NSP1 degradation. Thus, NSP1's ability to degrade IRF3 is host cell dependent and is independent of NSP1 proteasomal degradation.

Variation in antagonism of the interferon response to rotavirus NSP1 results in differential infectivity in mouse embryonic fibroblasts

Rotavirus NSP1 has been shown to function as an E3 ubiquitin ligase that mediates proteasome-dependent degradation of interferon (IFN) regulatory factors (IRF), including IRF3, -5, and -7, and suppresses the cellular type I IFN response. However, the effect of rotavirus NSP1 on viral replication is not well defined. Prior studies used genetic analysis of selected reassortants to link NSP1 with host range restriction in the mouse, suggesting that homologous and heterologous rotaviruses might use their different abilities to antagonize the IFN response as the basis of their host tropisms. Using a mouse embryonic fibroblast (MEF) model, we demonstrate that heterologous bovine (UK and NCDV) and porcine (OSU) rotaviruses fail to effectively degrade cellular IRF3, resulting in IRF3 activation and beta IFN (IFN-beta) secretion. As a consequence of this failure, replication of these viruses is severely restricted in IFN-competent wild-type, but not in IFN-deficient (IFN-alpha/beta/gamma receptor- or STAT1-deficient) MEFs. On the other hand, homologous murine rotaviruses (ETD or EHP) or the heterologous simian rotavirus (rhesus rotavirus [RRV]) efficiently degrade cellular IRF3, diminish IRF3 activation and IFN-beta secretion and are not replication restricted in wild-type MEFs. Genetic reassortant analysis between UK and RRV maps the distinctive phenotypes of IFN antagonism and growth restriction in wild-type MEFs to NSP1. Therefore, there is a direct relationship between the replication efficiencies of different rotavirus strains in MEFs and strain-related variations in NSP1-mediated antagonism of the type I IFN response.

Role of interferon in homologous and heterologous rotavirus infection in the intestines and extraintestinal organs of suckling mice

Recent studies demonstrated that viremia and extraintestinal rotavirus infection are common in acutely infected humans and animals, while systemic diseases appear to be rare. Intraperitoneal infection of newborn mice with rhesus rotavirus (RRV) results in biliary atresia (BA), and this condition is influenced by the host interferon response. We studied orally inoculated 5-day-old suckling mice that were deficient in interferon (IFN) signaling to evaluate the role of interferon on the outcome of local and systemic infection after enteric inoculation. We found that systemic replication of RRV, but not murine rotavirus strain EC, was greatly enhanced in IFN-alpha/beta and IFN-gamma receptor double-knockout (KO) or STAT1 KO mice but not in mice deficient in B- or T-cell immunity. The enhanced replication of RRV was associated with a lethal hepatitis, pancreatitis, and BA, while no systemic disease was observed in strain EC-infected interferon-deficient mice. In IFN-alpha/beta receptor KO mice the extraintestinal infection and systemic disease were only moderately increased, while RRV infection was not augmented and systemic disease was not present in IFN-gamma receptor KO mice. The increase of systemic infection in IFN-deficient mice was also observed during simian strain SA11 infection but not following bovine NCDV, porcine OSU, or murine strain EW infection. Our data indicate that the requirements for the interferon system to inhibit intestinal and extraintestinal viral replication in suckling mice vary among different heterologous and homologous rotavirus strains, and this variation is associated with lethal systemic disease.

Acrylamide concentration affects the relative position of VP7 gene of serotype G2 strains as determined by polyacrylamide gel electrophoresis

BACKGROUND

During the course of development and characterization of various rotavirus reassortants, we found that the relative position of the gene encoding neutralization and protective antigen VP7 of certain rotavirus strains in a PAGE gel was influenced by the concentration of acrylamide.

OBJECTIVES

We investigated systematically various factors that affected the relative position of the rotavirus VP7 gene in a PAGE gel.

STUDY DESIGN

We analyzed dsRNAs of selected rotavirus strains bearing G1, G2, G3 or G9 specificity by PAGE at varying concentrations of acrylamide.

RESULTS

We demonstrated that the relative position of the VP7 gene of three G2 strains varied depending upon the concentration of acrylamide in a PAGE gel, which occurred not only in a homologous G2 virus gene background but also in a heterologous G3 virus gene background; and the VP7 gene bearing G1, G3, G4 or G9 specificity did not display this phenomenon when the PAGE running conditions were varied.

CONCLUSIONS

The concentration of acrylamide in a PAGE gel was the major factor that influenced the relative position of the VP7 gene of G2 rotavirus strains (i.e., VP7 gene coding assignment by PAGE).

Full genomic analysis of human rotavirus strain B4106 and lapine rotavirus strain 30/96 provides evidence for interspecies transmission

The Belgian rotavirus strain B4106, isolated from a child with gastroenteritis, was previously found to have VP7 (G3), VP4 (P[14]), and NSP4 (A genotype) genes closely related to those of lapine rotaviruses, suggesting a possible lapine origin or natural reassortment of strain B4106. To investigate the origin of this unusual strain, the gene sequences encoding VP1, VP2, VP3, VP6, NSP1, NSP2, NSP3, and NSP5/6 were also determined. To allow comparison to a lapine strain, the 11 double-stranded RNA segments of a European G3P[14] rabbit rotavirus strain 30/96 were also determined. The complete genome similarity between strains B4106 and 30/96 was 93.4% at the nucleotide level and 96.9% at the amino acid level. All 11 genome segments of strain B4106 were closely related to those of lapine rotaviruses and clustered with the lapine strains in phylogenetic analyses. In addition, sequence analyses of the NSP5 gene of strain B4106 revealed that the altered electrophoretic mobility of NSP5, resulting in a super-short pattern, was due to a gene rearrangement (head-to-tail partial duplication, combined with two short insertions and a deletion). Altogether, these findings confirm that a rotavirus strain with an entirely lapine genome complement was able to infect and cause severe disease in a human child.

Rotavirus nonstructural protein 1 subverts innate immune response by inducing degradation of IFN regulatory factor 3

IFN regulatory factor 3 (IRF3), a constitutively expressed protein localizing largely to the cytoplasm, is a primary effector of the innate immune response. Infection can trigger the phosphorylation, dimerization, and nuclear translocation of IRF3, where the factor stimulates the expression and release of IFN. In this study, we determined that the rotavirus gene 5 product, nonstructural protein 1 (NSP1), interacts with IRF3 in the infected cell. To understand the importance of the interaction, we compared IRF3 activation by rotaviruses expressing wild-type and C-truncated forms of NSP1. The analysis showed that IRF3 underwent dimerization and nuclear translocation and stimulated IFN promoter activity in infected cells expressing truncated NSP1. In contrast, infected cells expressing wild-type NSP1 were characterized by the rapid degradation of IRF3 during the replication cycle, severe decreases in IRF3 dimerization and nuclear translocation, and lack of IFN promoter activity. The implication of these results, that wild-type NSP1 is an antagonist of the IFN-signaling pathway, was confirmed in transient expression assays, which showed that wild-type NSP1, but not the C-truncated protein, induced the degradation of IRF3 fusion proteins. Related experiments indicated that NSP1 mediates IRF3 degradation through a proteasome-dependent pathway. The critical role of NSP1 in promoting cell-to-cell spread of rotavirus was demonstrated by using gene 5-specific short interfering RNAs in plaque assays. Although several viruses have been described that subvert the innate immune response by preventing IRF3 activation, rotavirus is identified as one that accomplishes this task by inducing the degradation of IRF3.

Rotavirus serotype G9P[8] and acute gastroenteritis outbreak in children, Northern Australia

Amino acid substitutions on the VP7 and NSP4 proteins were identified in regions known to influence function and may have contributed to the emergence and increased dominance of the outbreak strains.

During 2001, an outbreak of severe acute gastroenteritis swept through Central and northern Australia and caused serious disruption to health services. We tracked and characterized the rotavirus strain implicated in the outbreak. Comparison of the electropherotypes of outbreak samples suggested that one G9P[8] strain was likely responsible for the outbreak. Samples were obtained from geographically distinct regions of Australia where the epidemic had occurred. The outbreak strains showed identical nucleotide sequences in genes encoding three rotavirus proteins, VP7, VP8, and NSP4, but they were distinct from G9P[8] strains isolated in previous years. Several of the amino acid substitutions on the VP7 and NSP4 proteins were identified in regions known to influence function and may have contributed to the emergence and increased dominance of the outbreak strains. Rotavirus serotype surveillance should continue with methods capable of identifying new and emerging types.

Cloning and sequence analysis of dsRNA segments 5, 6 and 7 of a novel non-group A, B, C adult rotavirus that caused an outbreak of gastroenteritis in China

A diarrhoeal outbreak among adults in China was caused by a new rotavirus, termed ADRV-N, that does not react with antisera directed against group A, B or C rotaviruses [Zhonghua Liu Xing Bing Xue Za Zhi (Chin. Epidemiol.) 19 (1998) 336]. ADRV-N can be propagated in cell cultures [Zhonghua Yi Xue Za Zhi (Natl. Med. J. China) 82 (2002) 14]. We present the complete sequences for ADRV-N genome segments 5 and 6, and a full ORF sequence of genome segment 7. The deduced amino acid sequences suggest that these segments encode NSP1, VP6 and NSP3, respectively. These three ADRV-N genome segments have a unique -ACCCC-3' terminal sequence. The 5'-GG- terminus of segments 5 and 6 is the same as that of other rotaviruses. The amino acid similarity between VP6 and NSP3 of ADRV-N and the cognate sequences of their closest counterpart, group B IDIR, was 37 and 35%, respectively. The ADRV-N NSP1 has a double-stranded RNA binding motif (DSRM) and a putative autoproteolytic cleavage motif upstream from the DSRM. The putative ADRV-N NSP3 has a truncated C-terminus compared to the cognate protein of group B rotaviruses. All the available data demonstrate that ADRV-N differs significantly from the known rotaviruses and strongly suggest that ADRV-N is the first recognized member of a new group of rotaviruses infecting humans.

Human rotavirus strains bearing VP4 gene P[6] allele recovered from asymptomatic or symptomatic infections share similar, if not identical, VP4 neutralization specificities

A rotavirus VP4 gene P[6] allele has been documented in a number of countries to be characteristically associated with an endemic predominantly asymptomatic infection in neonates in maternity hospital nurseries. The mechanisms underlying the endemicity and asymptomatic nature of such neonatal infections remain unknown. Rotavirus strains sharing this same P genotype, however, have more recently been recovered from an increasing number of symptomatic diarrheal episodes in infants and young children in various parts of the world. Previously, we have shown that an asymptomatic P[6] rotavirus neonatal infection is not associated with a unique VP7 (G) serotype but may occur in conjunction with various G types. Although amino acid sequence comparisons of the VP4 gene between selected "asymptomatic" and "symptomatic" P[6] rotavirus strains have been reported and yielded information concerning their VP4 genotypes, serotypic comparisons of the outer capsid spike protein VP4 of such viruses have not been studied systematically by two-way cross-neutralizations. We determined the VP4 neutralization specificities of four asymptomatic and four symptomatic P[6] strains: two each of asymptomatic and symptomatic strains by two-way tests, and two each of additional asymptomatic and symptomatic strains by one-way tests. Both asymptomatic and symptomatic P[6] strains were shown to bear similar, if not identical, VP4 neutralization specificities. Thus, P[6] rotavirus strains causing asymptomatic or symptomatic infections did not appear to belong to unique P (VP4) serotypes. In addition, a close VP4 serotypic relationship between human P[6] rotavirus strains and the porcine P[6] rotavirus Gottfried strain was confirmed.

Rotavirus infectious particles use lipid rafts during replication for transport to the cell surface in vitro and in vivo

The pathway by which rotavirus is released from the cell is poorly understood but recent work has shown that, prior to cell lysis, rotavirus is released almost exclusively from the apical surface of the infected cell. By virtue of their unique biochemical and physical properties, viruses have exploited lipid rafts for host cell entry and/or assembly. Here we characterized the association of rhesus rotavirus (RRV) with lipid rafts during the rotavirus replication cycle. We found that newly synthesized infectious virus associates with rafts in vitro and in vivo. RRV proteins cosegregated with rafts on density gradients. Viral infectivity and genomic dsRNA also cosegregated with the raft fractions. Confocal microscopic analysis of raft and RRV virion proteins demonstrated colocalization within the cell. In addition, cholesterol depletion interfered with the association of RRV particles with rafts and reduced the release of infectious particles from the cell. Furthermore, murine rotavirus associates with lipid rafts in intestinal epithelial cells during a natural infection in vivo. Our results confirm the association of rotavirus infectious particles with rafts during replication in vitro and in vivo and strongly support the conclusion that this virus uses these microdomains for transport to the cell surface during replication.

Rotavirus genome segment 7 (NSP3) is a determinant of extraintestinal spread in the neonatal mouse

We used the neonatal mouse model of rotavirus infection to study extraintestinal spread following oral inoculation. Five-day-old pups were inoculated with either SA11-Cl3, SA11-Cl4, SA11-4F, RRV, or B223. By using virus detection in the liver as a proxy determination for extraintestinal spread, rotavirus strains capable of extraintestinal spread at high frequency (rhesus rotavirus [RRV]) and very low frequency (SA11-Cl4) were identified. Both strains productively infected the gastrointestinal tract. Oral inoculation of mice with RRV/ SA11-Cl4 reassortants and determination of virus titers in the gut and liver revealed that the extraintestinal spread phenotype segregated with RRV genome segment 7 to a high level of significance (P = 10(-3)). RRV segment 7 also segregated with the growth of virus in the gut (P = 10(-5)). Although infection of the gut was clearly required for tropism to the liver, there was no correlation between virus titers in the gut and detection of virus in the liver. Five days after intraperitoneal administration to bypass the gut barrier to virus spread, RRV and SA11-Cl4 both were recovered in the liver. However, only RRV was found in the liver following subcutaneous inoculation, suggesting that this peripheral site presented a similar barrier to virus spread as the gut. Sequence analysis of segment 7 from parental RRV and SA11-Cl4 and selected reassortants showed that (i) amino acid differences were distributed throughout the coding sequences and not concentrated in any particular functional motif and (ii) parental sequence was preserved in reassortants. These data support the hypothesis that NSP3, coded for by genome segment 7, plays a significant role in viral growth in the gut and spread to peripheral sites. The mechanism of NSP3-mediated tropism is under investigation.

Pathogenesis of rotavirus diarrhea

Rotavirus diarrhea is a major cause of infantile gastroenteritis worldwide. This review is mainly devoted to the effects of Rotavirus on intestinal epithelial transport and to the pathophysiological mechanisms proposed to underlie the intestinal fluid secretion caused by the virus.

Avian-to-mammal transmission of an avian rotavirus: analysis of its pathogenicity in a heterologous mouse model

It has been suggested that group A avian rotaviruses can be transmitted to mammals, but there is no direct evidence that such viruses induce disease in mammals. Suckling mice were orally inoculated with two avian rotaviruses. A pigeon rotavirus, PO-13, was found to induce diarrhea, but a turkey rotavirus, Ty-3, did not. The diarrhea induced by PO-13 was dependent on the age of the mouse. In histopathological examinations, antigens of PO-13 were sporadically detected in absorptive cells in the ileum, and lesions were observed as ballooning degenerations of absorptive cells in a region from the duodenum to the ileum. However, the rotavirus antigen was not detected in the majority of these degenerative cells. These results indicated that PO-13 could infect and induce diarrhea in suckling mice. This is the first evidence of an avian rotavirus being experimentally transmissible to a mammal.