Caliciviruses

Comparative pathogenesis of tissue culture-adapted and wild-type Cowden porcine enteric calicivirus (PEC) in gnotobiotic pigs and induction of diarrhea by intravenous inoculation of wild-type PEC

Porcine enteric calicivirus (PEC/Cowden) causes diarrhea in pigs, grows in cell culture, and is morphologically and genetically similar to the Sapporo-like human caliciviruses. Genetic analysis revealed that the tissue culture-adapted (TC) Cowden PEC has one distant and three clustered amino acid substitutions in the capsid region and 2 amino acid changes in the RNA polymerase region compared to wild-type (WT) PEC (M. Guo, K.-O. Chang, M. E. Hardy, Q. Zhang, A. V. Parwani, and L. J. Saif, J. Virol. 73:9625-9631, 1999). In this study, the TC PEC, passaged in a porcine kidney cell line, and the WT PEC, passaged in gnotobiotic (Gn) pigs, were used to orally inoculate 13 4- to 6-day-old Gn pigs. No diarrhea developed in the TC-PEC-exposed pigs, whereas moderate diarrhea developed in the WT-PEC orally inoculated pigs, persisting for 2 to 5 days. Fecal virus shedding persisting for at least 7 days was detected by both reverse transcription (RT)-PCR and antigen-enzyme-linked immunosorbent assay (antigen-ELISA) in both TC-PEC and WT-PEC orally inoculated pigs but not in mock-inoculated pigs. The PEC particles were detected by immunoelectron microscopy (IEM) in intestinal contents from all the WT-PEC-inoculated pigs, but not from the TC-PEC-inoculated pigs. Mild (duodenum and jejunum) or no (ileum) villous atrophy was observed in histologic sections of the small intestines of TC-PEC-inoculated pigs, whereas WT PEC caused mild to severe (duodenum and jejunum) villous atrophy and fusion. Scanning electron microscopy confirmed mild shortening and blunting of villi in the duodenum and jejunum of the TC-PEC-inoculated pigs, in contrast to moderate to severe villous shortening and blunting in the duodenum and jejunum of WT-PEC-inoculated pigs. Higher numbers of PEC antigen-positive villous enterocytes were detected by immunofluorescent (IF) staining in the proximal small intestine of the WT-PEC-inoculated pigs, in contrast to low numbers of PEC antigen-positive enterocytes in only one of four TC-PEC-inoculated pigs. No PEC antigen-positive cells were observed in the colon or extraintestinal tissues of all inoculated pigs or in the small intestine of one mock-inoculated pig. Thus, the TC PEC was at least partially attenuated (no diarrhea, mild lesions) after serial passage in cell culture. In further experiments, three 4- to 6-day-old Gn pigs were intravenously (i.v.) inoculated with WT PEC, and all pigs developed diarrhea and villous atrophy in the small intestines resembling that observed in the orally inoculated pigs. Fecal viral shedding persisting for 8 days was detected by both RT-PCR and antigen-ELISA, and PEC was detected by IEM in feces or intestinal contents. The PEC RNA and antigens (at low titers) were detected in acute-phase sera from all the WT-PEC i.v.-inoculated pigs and also from seven of nine of the WT-PEC orally inoculated pigs. Oral or i.v. inoculation of four additional pigs with the PEC-positive acute-phase sera induced diarrhea, small intestinal lesions, PEC shedding in feces, and seroconversion to PEC, confirming the occurrence of viremia during PEC infection, with infectious PEC present in acute-phase sera. No diarrhea, histopathologic changes, or IF staining in the small intestine or fecal or serum detection of PEC was evident in two pigs i.v. mock-inoculated or a pig inoculated i.v. with inactivated WT PEC. To our knowledge, this is the first report of an attenuated enteric calicivirus, the induction of diarrhea, and intestinal lesions in Gn pigs caused by i.v. inoculation of WT PEC and the presence of viremia following PEC infection.

Diagnosis of noncultivatable gastroenteritis viruses, the human caliciviruses

Gastroenteritis is one of the most common illnesses of humans, and many different viruses have been causally associated with this disease. Of those enteric viruses that have been established as etiologic agents of gastroenteritis, only the human caliciviruses cannot be cultivated in vitro. The cloning of Norwalk virus and subsequently of other human caliciviruses has led to the development of several new diagnostic assays. Antigen detection enzyme immunoassays (EIAs) using polyclonal hyperimmune animal sera and antibody detection EIAs using recombinant virus-like particles have supplanted the use of human-derived reagents, but the use of these assays has been restricted to research laboratories. Reverse transcription-PCR assays for the detection of human caliciviruses are more widely available, and these assays have been used to identify virus in clinical specimens as well as in food, water, and other environmental samples. The application of these newer assays has significantly increased the recognition of the importance of human caliciviruses as causes of sporadic and outbreak-associated gastroenteritis.

Viral zoonoses and food of animal origin: caliciviruses and human disease

Caliciviruses are important veterinary and human pathogens. The viruses gain their name from characteristic cup-shaped structures seen on the virion surface by negative stain electron microscopy. In humans caliciviruses are a major cause of diarrhoeal disease. There are two fundamentally different genome structures amongst human caliciviruses. The Norwalk-like or small round structured viruses (SRSVs) are viruses that have an amorphous structure when viewed by EM, they have a genome composed of 3 major open reading frames (ORFs). These viruses cause epidemic gastroenteritis amongst all age groups. In contrast, the 'classic' human caliciviruses (HuCVs) display the typical calicivirus surface structure and have their capsid ORF fused to and contiguous with the non structural proteins forming one giant polyprotein. HuCVs are predominantly associated with paediatric infections and are only a minor cause of disease in humans. Spread of disease for both SRSVs and HuCVs is usually by faecal oral transmission. SRSVs are a major cause of foodborne gastroenteritis especially linked to the consumption of sewage-contaminated shellfish. However, there is no evidence that these viruses replicate in shellfish or that they originate from an animal source.

Attachment and entry of recombinant Norwalk virus capsids to cultured human and animal cell lines

Norwalk virus (NV) is the prototype strain of a group of noncultivable human caliciviruses responsible for epidemic outbreaks of acute gastroenteritis. While these viruses do not grow in tissue culture cells or animal models, expression of the capsid protein in insect cells results in the self-assembly of recombinant Norwalk virus-like particles (rNV VLPs) that are morphologically and antigenically similar to native NV. We have used these rNV VLPs to examine virus-cell interactions. Binding and internalization of VLPs to cultured human and animal cell lines were studied in an attempt to identify potentially susceptible cell lines for virus propagation in vitro and to determine if early events in the replication cycle were responsible for the narrow host range and restriction of virus growth in cell culture. Radiolabeled VLPs specifically bound to a saturable number of binding molecules on the cell surface of 13 cell lines from different origins, including human intestine (differentiated and undifferentiated Caco-2) and insect (Spodoptera frugiperda 9) ovary. Differentiated Caco-2 cells bound significantly more rNV VLPs than the other cell lines. Variations in the amount of bound VLPs among the different cell lines did not correlate with the tissue or species of origin. VLP binding was specific, as determined by competition experiments with unlabeled rNV VLPs; however, only 1.4 to 6.8% of the specifically prebound radiolabeled VLPs became internalized into cells. Blocking experiments using polygonal and monoclonal anti-rNV sera and specific antipeptide sera were performed to map the domains on rNV VLPs involved in binding to cells. One monoclonal antibody (NV8812) blocked binding of rNV VLPs to human and animal cell lines. The binding site of monoclonal antibody NV8812 was localized to the C-terminal 300 to 384 residues of the capsid protein by immunoprecipitation with truncated and cleaved forms of the capsid protein. These data suggest that the C-terminal region of the capsid protein is involved in specific binding of rNV VLPs to cells.

Genome organization in the caliciviridae

Caliciviruses cause a wide spectrum of important diseases. These viruses have a positive-sense single-stranded RNA genome; recently, the complete genome sequences of several caliciviruses have been determined. This review outlines the genome organization and phylogenetic relationships of the animal and candidate human caliciviruses.

A conserved sequence motif at the 5' terminus of the Southampton virus genome is characteristic of the Caliciviridae

We have determined the 5'terminal cDNA sequence for the genome of Southampton virus, a recently characterized, human, small round-structured virus (SRSV). Genomic RNA was extracted directly from a stool sample and amplified by RT-PCR by homopolymer tailing of the 3' terminus of the cDNA. The additional sequence increases the overall length of the Southampton virus genome by 12 nucleotides, resulting in a significant change to the genome organization by extending the first large open reading frame (ORF) by 51 amino acids. The 5'terminal bases pGpT and the presence of conserved genome and putative subgenomic RNA terminal motifs are now prominent features shared between the human SRSV Southampton virus and the animal caliciviruses rabbit hemorrhagic disease virus and feline calicivirus.

Specific proteolytic cleavage of recombinant Norwalk virus capsid protein

Norwalk virus (NV) causes epidemic outbreaks of acute nonbacterial gastroenteritis in humans. The NV capsid is made up of a single protein, and expression of the capsid protein in baculovirus recombinants results in spontaneous assembly of the protein into virus-like particles (X. Jiang, M. Wang, D. Y. Graham, and M. K. Estes, J. Virol. 66:6527-6532, 1992). We have investigated whether the NV capsid protein undergoes a specific proteolytic cleavage. Recombinant NV (rNV) particles were digested with trypsin to determine if a specific cleavage occurred. A predominant band with a molecular weight of approximately 32,000 (32K protein) was observed when trypsin-treated rNV was electrophoresed on sodium dodecyl sulfate-polyacrylamide gels. Determination of the N-terminal sequence of this band showed that a trypsin-specific cleavage occurred at amino acid residue 227. Early studies identified two proteins with molecular weights of 59,000 and 30,000 (59K and 30K proteins) in the stool of NV-infected volunteers that were reactive with postinfection antiserum. (H. B. Greenberg, J. R. Valdesuso, A. R. Kalica, R. G. Wyatt, V. J. McAuliffe, A. Z. Kapikian, and R. M. Chanock, J. Virol. 37:994-999, 1981). We hypothesized that the 32K rNV cleavage product might be analogous to the 30K soluble protein detected in stools of NV-infected volunteers. Immunoprecipitation of soluble protein from these stool extracts with a rabbit polyclonal antiserum made against rNV, and Western blot detection with a mouse polyclonal antiserum made against rNV, revealed a single band with an apparent molecular weight of 30,000 that migrated similarly to the trypsin cleavage product observed in vitro. The N terminus of this band was identical to that of the 32K cleavage product of rNV capsid protein. These data show that the 30K protein in stool is produced by specific cleavage of the NV capsid protein in vivo. Trypsin cleavage of isolated soluble rNV 58K capsid protein and of assembled particles showed that only soluble 58K capsid protein is susceptible to cleavage. The presence of a large amount of soluble capsid protein may influence the immune response to or pathogenicity of NV infections.

Three-dimensional structure of baculovirus-expressed Norwalk virus capsids

The three-dimensional structure of the baculovirus-expressed Norwalk virus capsid has been determined to a resolution of 2.2 nm using electron cryomicroscopy and computer image processing techniques. The empty capsid, 38.0 nm in diameter, exhibits T = 3 icosahedral symmetry and is composed of 90 dimers of the capsid protein. The striking features of the capsid structure are arch-like capsomeres, at the local and strict 2-fold axes, formed by dimers of the capsid protein and large hollows at the icosahedral 5- and 3-fold axes. Despite its distinctive architecture, the Norwalk virus capsid has several similarities with the structures of T = 3 single-stranded RNA (ssRNA) viruses. The structure of the protein subunit appears to be modular with three distinct domains: the distal globular domain (P2) that appears bilobed, a central stem domain (P1), and a lower shell domain (S). The distal domains of the 2-fold related subunits interact with each other to form the top of the arch. The lower domains of the adjacent subunits associate tightly to form a continuous shell between the radii of 11.0 and 15.0 nm. No significant mass density is observed below the radius of 11.0 mm. It is suspected that the hinge peptide in the adjoining region between the central domain and the shell domain may facilitate the subunits adapting to various quasi-equivalent environments. Architectural similarities between the Norwalk virus capsid and the other ssRNA viruses have suggested a possible domain organization along the primary sequence of the Norwalk virus capsid protein. It is suggested that the N-terminal 250 residues constitute the lower shell domain (S) with an eight-strand beta-barrel structure and that the C-terminal residues beyond 250 constitute the protruding (P1+P2) domains. A lack of an N-terminal basic region and the ability of the Norwalk virus capsid protein to form empty T = 3 shells suggest that the assembly pathway and the RNA packing mechanisms may be different from those proposed for tomato bushy stunt virus and southern bean mosaic virus but similar to that in tymoviruses and comoviruses.

Identification and sequence determination of the capsid protein gene of human astrovirus serotype 1

Abstract We present the sequence of an open reading frame (ORF) at the 3′ end of human astrovirus serotype 1. Primer extension experiments showed that the RNA expressing this gene is shorter than the complete ORF, and could form a protein of M_r 85 540. The protein was expressed by recombinant baculovirus and was recognized by anti‐virion serum, indicating a structural role. Sequence comparison indicates that astrovirus serotypes 1 and 2 differ markedly in the C‐terminal half of the protein but are well conserved towards the N‐terminus.

Genomic organization and expression of astroviruses and caliciviruses

Astroviruses and caliciviruses are two families defined initially by their characteristic morphology. Many of these viruses have been difficult to grow in culture. Molecular biology has now provided a valuable insight into the nature of these viruses, and in many respects knowledge of genome structure now outstrips that of more classical virological features. However these advances have allowed a more detailed approach to virus classification and have led to the establishment of the Astroviridae as a distinct virus family.

Detection of Norwalk virus in the UK by the polymerase chain reaction

We have developed a polymerase chain reaction for the detection of Norwalk virus using the published sequence of the virus RNA dependent RNA polymerase gene and have used this to clone and sequence this region of a virus from a UK outbreak. We have applied this method to a panel of UK Norwalk-like viruses using both Tet-z and Taq DNA polymerases and found that amplification produces a multiplicity of bands from stool samples. However, in combination with Southern blotting, Taq polymerase amplification detected virus in 13 of a panel of 30 clinical samples known to contain these viruses and also detected astroviruses in a mixed infection. Amplification using Tet-z DNA polymerase was less efficient (6/30) and detected predominantly viruses typed as UK type 2 by solid phase immune electron microscopy.

The complete nucleotide sequence of a feline calicivirus

We have determined the complete sequence of a feline calicivirus. The virus genome is 7690 bases long and contains two large open reading frames. Proteins specified by these have similarity to those encoded in the corresponding regions of a candidate calicivirus rabbit hemorrhagic disease virus, but are distinctly different from those specified by another such virus, hepatitis E virus. A third, small open reading frame at the 3' end of the genome is present in both feline and rabbit viruses but is absent from hepatitis E. These findings suggest that the calicivirus family, which consists of a single genus, may require subdivision.