Characterization of homotypic and heterotypic VP7 neutralization sites of rhesus rotavirus

A Novel Rotavirus Reverse Genetics Platform Supports Flexible Insertion of Exogenous Genes and Enables Rapid Development of a High-Throughput Neutralization Assay

Despite the success of rotavirus vaccines, rotaviruses remain one of the leading causes of diarrheal diseases, resulting in significant childhood morbidity and mortality, especially in low- and middle-income countries. The reverse genetics system enables the manipulation of the rotavirus genome and opens the possibility of using rotavirus as an expression vector for heterologous proteins, such as vaccine antigens and therapeutic payloads. Here, we demonstrate that three positions in rotavirus genome-the C terminus of NSP1, NSP3 and NSP5-can tolerate the insertion of reporter genes. By using rotavirus expressing GFP, we develop a high-throughput neutralization assay and reveal the pre-existing immunity against rotavirus in humans and other animal species. Our work shows the plasticity of the rotavirus genome and establishes a high-throughput assay for interrogating humoral immune responses, benefiting the design of next-generation rotavirus vaccines and the development of rotavirus-based expression platforms.

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.

VP4- and VP7-specific antibodies mediate heterotypic immunity to rotavirus in humans

Human rotaviruses (RVs) are the leading cause of severe diarrhea in young children worldwide. The molecular mechanisms underlying the rapid induction of heterotypic protective immunity to RV, which provides the basis for the efficacy of licensed monovalent RV vaccines, have remained unknown for more than 30 years. We used RV-specific single cell-sorted intestinal B cells from human adults, barcode-based deep sequencing of antibody repertoires, monoclonal antibody expression, and serologic and functional characterization to demonstrate that infection-induced heterotypic immunoglobulins (Igs) primarily directed to VP5*, the stalk region of the RV attachment protein, VP4, are able to mediate heterotypic protective immunity. Heterotypic protective Igs against VP7, the capsid glycoprotein, and VP8*, the cell-binding region of VP4, are also generated after infection; however, our data suggest that homotypic anti-VP7 and non-neutralizing VP8* responses occur more commonly in people. These results indicate that humans can circumvent the extensive serotypic diversity of circulating RV strains by generating frequent heterotypic neutralizing antibody responses to VP7, VP8*, and most often, to VP5* after natural infection. These findings further suggest that recombinant VP5* may represent a useful target for the development of an improved, third-generation, broadly effective RV vaccine and warrants more direct examination.

A G3P[13] porcine group A rotavirus emerging in China is a reassortant and a natural recombinant in the VP4 gene

Group A rotaviruses (RVAs) are a major cause of serious intestinal disease in piglets. In this study, a novel pig strain was identified in a stool sample from China. The strain was designated RVA/Pig/China/LNCY/2016/G3P[13] and had a G3-P[13]-I5-R1-C1-M1-A8-N1-T1-E1-H1 genome. The viral protein 7 (VP7) and non-structural protein 4 (NSP4) genes of RVA/Pig/China/LNCY/2016/G3P[13] were closely related to cogent genes of human RVAs, suggesting that a reassortment between pig and human strains had occurred. Recombination analysis showed that RVA/Pig/China/LNCY/2016/G3P[13] is a natural recombinant strain between the P[23] and P[7] RVA strains, and crossover points for recombination were found at nucleotides (nt) 456 and 804 of the VP4 gene. Elucidating the biological characteristics of porcine rotavirus (PoRV) will be helpful for further analyses of the epidemic characteristics of this virus. The results of this study provide valuable information for RVA recombination and evolution and will facilitate future investigations into the molecular pathogenesis of RVAs.

Modeling of the rotavirus group C capsid predicts a surface topology distinct from other rotavirus species

Rotavirus C (RVC) causes sporadic gastroenteritis in adults and is an established enteric pathogen of swine. Because RVC strains grow poorly in cell culture, which hinders generation of virion-derived RVC triple-layered-particle (TLP) structures, we used the known Rotavirus A (RVA) capsid structure to model the human RVC (Bristol) capsid. Comparative analysis of RVA and RVC capsid proteins showed major differences at the VP7 layer, an important target region for vaccine development due to its antigenic properties. Our model predicted the presence of a surface extended loop in RVC, which could form a major antigenic site on the capsid. We analyzed variations in the glycosylation patterns among RV capsids and identified group specific conserved sites. In addition, our results showed a smaller RVC VP4 foot, which protrudes toward the intermediate VP6 layer, in comparison to that of RVA. Finally, our results showed major structural differences at the VP8* glycan recognition sites.

Increased detection of G3P[9] and G6P[9] rotavirus strains in hospitalized children with acute diarrhea in Bulgaria

Rotavirus severe disease in children is now vaccine-preventable and the roll-out of vaccination programs globally is expected to make a significant impact in the reduction of morbidity and mortality in children <5 years of age. Rotavirus is also a pathogen of other mammals and birds, and its segmented RNA genome can lead to the emergence of new or unusual strains in human population via interspecies transmission and reassortment events. Despite the efficacy and impact of rotavirus vaccine in preventing severe diarrhea, the correlates of protection remain largely unknown. Therefore, rotavirus strain surveillance before, during and after the introduction of immunization programs remains a crucial for monitoring rotavirus vaccine efficacy and impact. In this context, molecular characterization of 1323 Bulgarian rotavirus strains collected between June 2010 and May 2013 was performed. A total of 17 strains of interest were analyzed by partial sequence analysis. Twelve strains were characterized as G3P[9] and G6P[9] of potential animal origin. Phylogenetic analysis and comparisons with the same specificity strains detected sporadically between 2006 and 2010 revealed the constant circulation of these unusual human strains in Bulgaria, although in low prevalence, and their increased potential for person-to-person spread.

Cross-linking of rotavirus outer capsid protein VP7 by antibodies or disulfides inhibits viral entry

Antibodies that neutralize rotavirus infection target outer coat proteins VP4 and VP7 and inhibit viral entry. The structure of a VP7-Fab complex (S. T. Aoki, et al., Science 324:1444-1447, 2009) led us to reclassify epitopes into two binding regions at inter- and intrasubunit boundaries of the calcium-dependent trimer. It further led us to show that antibodies binding at the intersubunit boundary inhibit uncoating of the virion outer layer. We have now tested representative antibodies for each of the defined structural epitope regions and find that antibodies recognizing epitopes in either binding region neutralize by cross-linking VP7 trimers. Antibodies that bind at the intersubunit junction neutralize as monovalent Fabs, while those that bind at the intrasubunit region require divalency. The VP7 structure has also allowed us to design a disulfide cross-linked VP7 mutant which recoats double-layered particles (DLPs) as efficiently as does wild-type VP7 but which yields particles defective in cell entry as determined both by lack of infectivity and by loss of α-sarcin toxicity in the presence of recoated particles. We conclude that dissociation of the VP7 trimer is an essential step in viral penetration into cells.

Structure of rotavirus outer-layer protein VP7 bound with a neutralizing Fab

Rotavirus outer-layer protein VP7 is a principal target of protective antibodies. Removal of free calcium ions (Ca2+) dissociates VP7 trimers into monomers, releasing VP7 from the virion, and initiates penetration-inducing conformational changes in the other outer-layer protein, VP4. We report the crystal structure at 3.4 angstrom resolution of VP7 bound with the Fab fragment of a neutralizing monoclonal antibody. The Fab binds across the outer surface of the intersubunit contact, which contains two Ca2+ sites. Mutations that escape neutralization by other antibodies suggest that the same region bears the epitopes of most neutralizing antibodies. The monovalent Fab is sufficient to neutralize infectivity. We propose that neutralizing antibodies against VP7 act by stabilizing the trimer, thereby inhibiting the uncoating trigger for VP4 rearrangement. A disulfide-linked trimer is a potential subunit immunogen.

Sequence analysis of the VP7 gene of human rotavirus G1 isolated in Japan, China, Thailand, and Vietnam in the context of changing distribution of rotavirus G-types

Over the last decade, rotavirus G1 has represented the most common genotype worldwide. Since 2000, the prevalence of rotavirus G1 has decreased in some countries such as Japan and China. To monitor the trend of the VP7 encoding gene of rotavirus G1, we performed a sequence analysis of 74 G1 rotavirus strains isolated in Japan, China, Thailand, and Vietnam during the period from 2002 to 2005. The phylogenetic tree showed that all of the studied G1 strains from the four countries clustered into lineage III, the same as the majority of the G1 strains isolated in China and Japan in 1990 and 1991. Examination of the deduced amino acid sequences of the G1 strains from China and Japan revealed an amino acid substitution at position 91 (Asn instead of Thr) in antigenic region A when compared to the G1 strains isolated in China and Japan in 1990, 1991, and global reference strains. For the G1 strains from Thailand and Vietnam, there were three amino acid substitutions, not belonging to any antigenic regions. The study showed that there have been no considerable changes of human rotavirus G1 isolated in Japan, China, Thailand, and Vietnam. Further studies need to be carried out for a better understanding of why such changes in the prevalence of rotavirus G1 occur in these countries.

Amino acid substitutions in the VP7 protein of human rotavirus G3 isolated in China, Russia, Thailand, and Vietnam during 2001–2004

The distribution of rotavirus G-types in the world appears to be changing, especially with the emergence of G3 and G9 in many countries. Sequence analysis of the VP7 gene was performed on the 27 human G3 rotavirus strains isolated in China, Russia, Thailand, and Vietnam during 2001-2004. All the strains studied were clustered into the same branch of the phylogenetic tree. The comparison of the G3 deduced amino acid sequences between the studied Chinese strains and the strains circulating in China during 1986-1992 showed a wide range of amino acid substitutions (up to 13 amino acids in the VP7 antigenic regions). The two considerable changes both from aspartic acid to asparagine were located at positions 96 in antigenic region A and 213 in antigenic region C. Those amino acid substitutions of the Chinese G3 strains might involve in the emergence of G3 rotavirus in China during 2001-2003.

Diversity in Indian equine rotaviruses: identification of genotype G10,P6[1] and G1 strains and a new VP7 genotype (G16) strain in specimens from diarrheic foals in India

Rotaviruses causing severe diarrhea in foals in two organized farms in northern India, during the period from 2003 to 2005, were characterized by electropherotyping, serotyping, and sequence analysis of the genes encoding the outer capsid proteins. Of 137 specimens, 47 (34.31%) were positive for rotavirus and exhibited at least five different electropherotypes (E), E1 to E5. Strains belonging to different electropherotypes exhibited either a different serotype/genotype specificity or a lack of reactivity to typing monoclonal antibodies (MAbs) used in this study. Strains belonging to E1, E2, and E5 exhibited genotype G10,P6[1], G3, and G1 specificities and accounted for 19.0, 42.9, and 9.5% of the isolates, respectively. Though they possessed G10-type VP7, the E1 strains exhibited high reactivity with the G6-specific MAb, suggesting that the uncommon combination of the outer capsid proteins altered the specificity of the conformation-dependent antigenic epitopes on VP7. E3 and E4 strains accounted for 28.6% of the isolates and were untypeable. Sequence analysis of VP7 from E4 strains (Erv92 and Erv99) revealed that they represent a new VP7 genotype, G16. The detection of unexpected bovine rotavirus-derived G10,P6[1] reassortants, G1 serotype strains, and a new genotype (G16) strain in two distant farms reveals an interesting epidemiological situation and diversity of equine rotaviruses in India.

Distribution of rotavirus VP7 genotypes among children suffering from watery diarrhea in Kolkata, India

A combined reverse transcriptase-polymerase chain reaction (RT/PCR) was used to produce cDNA of the VP7 gene of rotavirus present in the stool samples. A total of 150 rotavirus positive stool samples were used in this study. Multiplex PCR, using the type specific primers, revealed the presence of G1 (49/150, 32.7%), G2 (27/150, 18%) and G4 (30/150, 20%) genotypes among the samples collected during 1999-2000 from children suffering from acute watery diarrhea. Eighteen samples (12%) were of mixed genotype and the remaining 16 samples (10.6%) could not be typed. Comparative analysis of the full length genes of the representative strains with corresponding genotypes incorporated in the human-rhesus rotavirus tetravalent vaccine (RRV-TV) formulation demonstrates variations of the circulating G1, G2 and G4 strains with the corresponding G genotypes present in the vaccine strain.

Nucleotide sequence variation of the VP7 gene of two G3-type rotaviruses isolated from dogs

The sequence of the VP7 gene of two rotaviruses isolated from dogs in southern Italy was determined and the inferred amino acid sequence was compared with that of other rotavirus strains. There was very high nucleotide and amino acid identity between canine strain RV198/95 and other canine strains, and to the human strain HCR3A. Strain RV52/96, however, was found to have about 95% identity to the G3 serotype canine strains K9, A79-10 and CU-1 and 96% identity to strain RV198/95 and to the simian strain RRV. Therefore both of the canine strains belong to the G3 serotype. Nevertheless, detailed analysis of the VP7 variable regions revealed that RV52/96 possesses amino acid substitutions uncommon to the other canine isolates. In addition, strain RV52/96 exhibited a nucleotide divergence greater than 16% from all the other canine strains studied; however, it revealed the closest identity (90.4%) to the simian strain RRV. With only a few exceptions, phylogenetic analysis allowed clear differentiation of the G3 rotaviruses on the basis of the species of origin. The nucleotide and amino acid variations observed in strain RV52/96 could account for the existence of a canine rotavirus G3 sub-type.

Purified recombinant rotavirus VP7 forms soluble, calcium-dependent trimers

Rotavirus is a major cause of severe, dehydrating childhood diarrhea. VP7, the rotavirus outer capsid glycoprotein, is a target of protective antibodies and is responsible for the calcium-dependent uncoating of the virus during cell entry. We have purified, characterized, and crystallized recombinant rhesus rotavirus VP7, expressed in insect cells. A critical aspect of the purification is the elution of VP7 from a neutralizing monoclonal antibody column by EDTA. Gel filtration chromatography and equilibrium analytical ultracentrifugation demonstrate that, in the presence of calcium, purified VP7 trimerizes. Trimeric VP7 crystallizes into hexagonal plates. Preliminary X-ray analysis suggests that the crystal packing reproduces the hexagonal component of the icosahedral lattice of VP7 on triple-layered rotavirus particles. These data indicate that the rotavirus outer capsid assembles from calcium-dependent VP7 trimers and that dissociation of these trimers is the biochemical basis for EDTA-induced rotavirus uncoating and loss of VP7 neutralizing epitopes.

Sequence analysis of VP4 and VP7 genes of nontypeable strains identifies a new pair of outer capsid proteins representing novel P and G genotypes in bovine rotaviruses

During a limited epidemiological study, the serotype specificities of several isolates of bovine rotavirus, exhibiting identical electropherotypes, from a single cattle farm near Bangalore, India, could not be determined using a panel of serotyping monoclonal antibodies (MAbs) specific for G serotypes 1-6 and 10. To determine the genotypes of these isolates, the nucleotide sequences of the genes encoding the outer capsid proteins VP4 and VP7 of two representative isolates, Hg18 and Hg23, were determined. The corresponding gene sequences from the two isolates were identical, indicating that these isolates represented a single strain of bovine rotavirus. Comparison of the VP4 nucleotide (nt) and the deduced amino acid (aa) sequences with those of several human and animal rotavirus strains representing all of the currently recognized 20 different VP4 (P) genotypes revealed low nt and aa sequence identities of 61.0 to 74.2% and 57.9 to 78.2% for VP4. The percentages of amino acid homology for the VP8* and VP5* regions of VP4 were 37.7 to 67.9 and 68.1 to 84.2%, respectively. The nt and aa sequences of the VP7 gene were also distinct from those of human and animal strains belonging to the previously established 14 VP7(G) serotypes (65.9 to 75.5% nt and 59.5 to 77.6% aa identities). These findings suggest the classification of the VP4 and VP7 genes of the bovine isolates represented by Hg18 as new P and G genotypes and provide further evidence for the vast genetic/antigenic diversity of group A rotaviruses.

The molecular chaperone calnexin interacts with the NSP4 enterotoxin of rotavirus in vivo and in vitro

Calnexin is an endoplasmic reticulum (ER)-associated molecular chaperone proposed to promote folding and assembly of glycoproteins that traverse the secretory pathway in eukaryotic cells. In this study we examined if calnexin interacts with the ER-associated luminal (VP7) and transmembrane (NSP4) proteins of rotavirus. Only glycosylated NSP4 interacted with calnexin and did so in a time-dependent manner (half-life, 20 min). In vitro translation experiments programmed with gene 10 of rhesus rotavirus confirmed that calnexin recognizes only glycosylated NSP4. Castanospermine (a glucosidase I and II inhibitor) experiments established that calnexin associates only with partly deglucosylated (di- or monoglucosylated) NSP4. Furthermore, enzymatic removal of the remaining glucose residues on the N-linked glycan units was essential to disengage the NSP4-calnexin complex. Novel experiments with castanospermine revealed that glucose trimming and the calnexin-NSP4 interaction were not critical for the assembly of infectious virus.

Intragenic recombinations in rotaviruses

In this paper, evidence for intragenic recombination in the VP7 gene between rotavirus strains bearing different serotypes is demonstrated for the first time. Intragenic recombination may be one of the escaping mechanisms from the host immune system for rotavirus. This process involves exchanging antigenic regions, thus questioning the use of multivalent vaccines for the prevention of rotavirus infection.

Serotype specificity of the neutralizing-antibody response induced by the individual surface proteins of rotavirus in natural infections of young children

The relative contribution of the rotavirus surface proteins, VP4 and VP7, to the induction of homotypic as well as heterotypic neutralizing antibodies (NtAbs) in natural infections was studied. The NtAb titers of paired sera from 70 infants with serologically defined primary rotavirus infections were determined with a panel of rotavirus reassortants having one surface protein from a human rotavirus (serotypes G1 to G4 for VP7 and P1A and P1B for VP4) and the other surface protein from a heterologous animal rotavirus strain. A subset of 37 children were evaluated for epitope-specific antibodies to the two proteins by an epitope-blocking assay. The infants were found to seroconvert more frequently to VP4 than to VP7 by both methods, although the titers of the seroconverters were higher to VP7 than to VP4. Both proteins induced homotypic as well as heterotypic NtAbs. G1 VP7 frequently induced a response to both G1 and G3 VP7s, while G3 VP7 and P1A VP4 induced mostly homotypic responses.

Single point mutations may affect the serotype reactivity of serotype G11 porcine rotavirus strains: a widening spectrum?

A panel of single and double neutralization-resistant escape mutants of serotype G11 porcine rotavirus strains A253 and YM, selected with G11 monotype- and serotype-specific neutralizing monoclonal antibodies (MAbs) to VP7, was tested in neutralization assays with hyperimmune sera raised against rotavirus strains of different serotypes. Escape mutants with an amino acid substitution in antigenic region A (amino acids [aa] 87 to 101) resulting in a residue identical or chemically similar to those present at the same positions in serotype G3 strains, at positions 87 for strain A253 and 96 for strain YM, were significantly more sensitive than the parental strains to neutralization with sera against some serotype G3 strains. Also, one YM antigenic variant (YM-5E6.1) acquired reactivity by enzyme-linked immunosorbent assay with MAbs 159, 57/8, and YO-1E2, which react with G3 strains, but not with the serotype G11 parental strain YM. Cross-adsorption studies suggested that the observed cross-neutralization by the G3-specific sera was due to the sera containing antibodies reactive with the parental strain plus antibodies reactive with the epitope(s) on the antigenic variant that mimick the serotype G3 specific one(s). Moreover, antibodies reactive with antigenic region F (aa 235 to 242) of VP7 might also be involved since cross-reactivity to serotype G3 was decreased in double mutants carrying an additional mutation, which creates a potential glycosylation site at position 238. Thus, single point mutations can affect the serotype reactivity of G11 porcine rotavirus strains with both monoclonal and polyclonal antibodies and may explain the origin of rotavirus strains with dual serotype specificity based on sequence divergence of VP7.

Quantification of systemic and local immune responses to individual rotavirus proteins during rotavirus infection in mice

The purpose of the present study was to develop a quantitative assay that could be used to measure the local and systemic immune responses to specific rotavirus proteins following rotavirus infection of adult mice. To measure these responses, we used an immunocytochemical staining assay of Spodoptera frugiperda (Sf-9) cells which were infected with recombinant baculovirus expressing selected rotavirus proteins. The specificity of the assay was documented by using a series of monoclonal antibodies to individual rotavirus proteins. We observed that the assay had high levels of sensitivity and specificity for a series of VP7- and VP4-specific neutralizing monoclonal antibodies which recognized conformation-dependent epitopes on their target proteins. We also studied immunoglobulin G (IgG) immune responses in serum and IgA immune responses in the stools of mice infected with wild-type murine rotavirus strain EHPw. In both sera and stools, the most immunogenic proteins were VP6 and VP4. VP2 was less immunogenic than VP6 or VP4, and the immune responses to VP7, NSP2, and NSP4 were very low in serum and undetectable in stools.

Prevalence of G and P serotypes among equine rotaviruses in the faeces of diarrhoeic foals

Variant types of VP4 and VP7 gene segments of faecal rotaviruses from diarrhoeic foals were identified by restriction endonuclease digestion of reverse transcription/polymerase chain reaction (RT/PCR) products. The variants observed were correlated with serotypes by determination of the sequence of representative RT/PCR products (entire coding sequence for VP7 and the VP8 region of VP4) and comparison to published sequences of equine G and P serotype genes. Both G and P serotypes could be predicted for 95/116 (82%) strains, P serotype only for a further 8 (7%) strains and G serotype only for 1 (1%) strain. All characterised strains belonged to the same P serotype, P12, although minor sequence variations were observed. Of those strains able to be assigned to G serotypes, 84/96 (87.5%) belonged to serotype G3A, and 12/96 (12.5%) belonged to serotype G14. Comparison of G serotyping by ELISA to the RT/PCR method showed that serotyping equine rotaviruses by currently available ELISA methods was prone to error. This study establishes the restricted serotypic diversity of equine rotaviruses, and the significance of serotype G14.

Protective effect of rotavirus VP6-specific IgA monoclonal antibodies that lack neutralizing activity

Rotaviruses are the leading cause of severe gastroenteritis and dehydrating diarrhea in young children and animals worldwide. A murine model and "backpack tumor" transplantation were used to determine the protective effect of antibodies against VP4(an outer capsid viral protein) and VP6(a major inner capsid viral protein). Only two non-neutralizing immunoglobulin A (IgA) antibodies to VP6 were capable of preventing primary and resolving chronic murine rotavirus infections. These antibodies were not active, however, when presented directly to the luminal side of the intestinal tract. These findings support the hypothesis that in vivo intracellular viral inactivation by secretory IgA during transcytosis is a mechanism of host defense against rotavirus infection.

Cross-reactive, serotype- and monotype-specific neutralization epitopes on VP7 of serotype G3 and G5 porcine rotavirus strains

VP7 specific monoclonal antibodies raised against serotype G5 porcine rotavirus strains isolated in Venezuela showed either a serotype G5- or monotype-specific pattern of reactivity by neutralization against a panel of 53 group A rotavirus isolates representative of all established G serotypes. Monoclonal antibodies raised against two G3 porcine strains were either specific for a subset of porcine G3 strains or reactive with another subset of porcine G3 strains and with most G5 strains. Neither were reactive with G3 strains from other species. Analysis of neutralization resistant mutants selected with these monoclonal antibodies indicated that epitopes defined by cross-reactive, serotype- and monotype-specific monoclonal antibodies overlap functionally and that binding and neutralization by these antibodies depended on specific amino acid residues in the region A or C of VP7. Results indicate that a high degree of monotypic variation occurs among G5 and G3 porcine rotavirus strains and the existence of at least one common epitope shared by G5 and G3 porcine strains, in the major neutralization domain of these VP7s.

Interactions between the two surface proteins of rotavirus may alter the receptor-binding specificity of the virus

The infection of target cells by most animal rotavirus strains requires the presence of sialic acids (SAs) on the cell surface. We recently isolated variants from simian rotavirus RRV whose infectivity is no longer dependent on SAs and showed that the mutant phenotype segregates with the gene coding for VP4, one of the two surface proteins of rotaviruses (the other one being VP7). The nucleotide sequence of the VP4 gene of four independently isolated variants showed three amino acid changes, at positions 37 (Leu to Pro), 187 (Lys to Arg), and 267 (Tyr to Cys), in all mutant VP4 proteins compared with RRV VP4. The characterization of revertant viruses from two independent mutants showed that the arginine residue at position 187 changed back to lysine, indicating that this amino acid is involved in the determination of the mutant phenotype. Surprisingly, sequence analysis of reassortant virus DS1XRRV, which depends on SAs to infect the cell, showed that its VP4 gene is identical to the VP4 gene of the variants. Since the only difference between DS1XRRV and the RRV variants is the parental origin of the VP7 gene (human rotavirus DS1 in the reassortant), these findings suggest that the receptor-binding specificity of rotaviruses, via VP4, may be influenced by the associated VP7 protein.

Characterization of the neutralizing epitopes of VP7 of the Gottfried strain of porcine rotavirus

The neutralization epitopes of the outer capsid protein VP7 of a porcine group A rotavirus were studied by using neutralizing monoclonal antibodies (N-MAbs). Six N-MAbs which were specific for the VP7 protein of the Gottfried strain of porcine rotavirus (serotype G4) were used for analyzing the antigenic sites of VP7. Three different approaches were used for this analysis: testing the serological reactivity of each N-MAb against different G serotypes of human and animal rotaviruses, analyzing N-MAb-resistant viral antigenic variants, and performing a nucleotide sequence analysis of the VP7 gene of each of the viral antigenic variants generated. From the serological analyses, three different reactivity patterns were recognized by plaque reduction virus neutralization and cell culture immunofluorescence tests. A single MAb (RG36H9) reacted with animal rotavirus serotypes G3 and G4 but not with human serotypes G3 and G4. The MAb 57/8 (D. A. Benfield, E. A. Nelson, and Y. Hoshino, p. 111, in Abstr. VIIth Internat. Congr. Virol., 1987, and E. R. Mackow, R. D. Shaw, S. M. Matsui, P. T. Vo, D. A. Benfield, and H. B. Greenberg, Virology 165:511-517, 1988) reacted with animal and human rotavirus serotypes G3 and G4 and also with human serotype G9 and bovine serotype G6. The other four MAbs reacted only with the porcine rotavirus serotype G4. The epitope defined by MAb 57/8 and the epitope defined by the other five MAbs appeared to be partially overlapping or close to each other, as identified by viral antigenic variant analysis. However, data from nucleotide and deduced amino acid sequence analyses of the VP7 of each of the viral antigenic variants showed that these two epitopes constituted a large, single neutralization domain.

Sequence conservation within neutralization epitope regions of VP7 and VP4 proteins of human serotype G4 rotavirus isolates

Serotype G4 rotavirus isolates causing four separate epidemics of severe diarrhoea in young children in Melbourne, Australia (from 1974-1990) were investigated for sequence variation in genes encoding the outer capsid proteins, VP4 and VP7. Complementary DNA of the gene encoding the major outer capsid neutralization antigen, VP7, of eighteen isolates was synthesized and amplified by coupled reverse transcription and polymerase chain reaction. Direct sequencing methods were used to derive the deduced amino acid sequences of the immunodominant A, B, and C neutralization epitope regions of the protein. Limited variation was observed among all isolates. A threonine to asparagine change in region A, at amino acid 96, was associated with altered binding of serotype G4-specific neutralizing monoclonal antibodies. The VP8* region of the outer capsid protein VP4 (containing the proposed serotype-specific neutralization epitopes) was investigated in eight isolates. This region was found to highly conserved both within Melbourne isolates and in relation to the standard strains Wa, P, and VA70. The characteristic periodicity of occurrence of serotype G4 isolates causing severe diarrhoea in Melbourne children is unlikely to be due to changes in neutralization epitopes located on the outer capsid proteins, VP7 or VP4.

Infectious diarrhoea. Viruses

Increased knowledge has been gained into the aetiology and pathogenesis of viral gastroenteritis during the past two decades. There are now thought to be four major subclassifications of gastroenteritis-causing viruses; these include rotavirus, enteric adenovirus, calicivirus, including Norwalk and Norwalk-like viruses, and astrovirus. The association of these agents with gastroenteritis has been made by their electron microscopic detection in stool and intestinal biopsy specimens from affected patients, the inability to detect the viruses after recovery from disease, and the subsequent development of immunoglobulin responses after infection; in some instances disease transmission was achieved in human volunteers. The association of these viral agents with gastroenteritis has facilitated the study of classification, epidemiology, immunity, diagnostic tests, methods of treatment and, most importantly, disease prevention strategies such as vaccine development for rotavirus. This chapter highlights the major features of these agents, with special attention being given to the pertinent molecular biology as well as current and future prospects for vaccination. Enteric viral infections of the gastrointestinal tract in patients with AIDS are also discussed.

Intra- and inter-season genetic variability in the VP7 gene of serotype 1 (monotype 1 a) rotavirus clinical isolates

Nucleotide sequence variation was detected in the VP7 gene of serotype 1 (monotype 1 a) rotavirus isolates collected from children admitted to hospital in Melbourne with acute diarrhoea in 1990 and 1991. Two co-circulating electropherotypes were detected during the 1991 winter epidemic. Using chemical cleavage of mismatches in heteroduplexes, the genes encoding VP7 were found to be genetically stable within each electropherotype during the study period, although differences between the two types were apparent. Direct nucleotide sequencing confirmed this finding. The two electropherotypes exhibited four nucleotide differences in the VP7 gene, only one of which resulted in a substitution in the deduced amino acid sequence. The degree of variation was more pronounced between the 1991 isolates and those from the previous winter, with approximately 3% nucleotide sequence diversity between isolates from both winters. The regions encoding the neutralization epitopes of VP7 were conserved among all isolates. Comparison to the local prototype monotype 1 a strain, RV 4 (isolated from a child admitted to hospital in Melbourne in 1981), implies that the 1990 and 1991 isolates have diverged independently. This suggests that genetically distinct strains emerge from a pool of related viruses to predominate in any given year.

Two-way cross-neutralization mediated by a shared P (VP4) serotype between bovine rotavirus strains with distinct G (VP7) serotypes

Among bovine rotavirus strains, there are three G serotypes (G6, G8, and G10) and three P (VP4) serotypes (PB1, PB2, and PB3, which are defined on the basis of strains NCDV, UK, and B223, respectively). Plaque reduction neutralization assays with hyperimmune antisera disclosed two-way antigenic relationships of strain KN-4 with strain KK-3 (G10, PB3) as well as with strains NCDV (G6, PB1) and 0510 (G6, PB2). Neutralization assays with monoclonal antibodies specific for G6, G10, and PB3 revealed that KK-3 and KN-4 had the same P serotype (PB3) but that neither G6- nor G10-specific monoclonal antibody neutralized KN-4. Comparison of the VP7 gene sequence of KN-4 with those of other bovine rotavirus strains indicated that KN-4 was more similar to G6 bovine strains than to KK-3 and other G10 strains, suggesting that the G serotype of KN-4 was G6. From these results, we concluded that the two-way cross-neutralization between KN-4 and NCDV or 0510 was mediated by shared G6 serotype specificity, whereas the two-way cross-neutralization between KN-4 and KK-3 was mediated by shared P serotype specificity (PB3). Thus, KN-4 and KK-3 represent the first reported example of a two-way antigenic relationship mediated only by the P serotype. This article emphasizes the need for adopting a binary serotyping system and development of reagents which will enable classification of rotaviruses based on their G and P serotype specificities.

A variant serotype G3 rotavirus isolated from an unusually severe outbreak of diarrhoea in piglets

About 80% of faecal samples from severe outbreak of porcine diarrhoea (scours) were positive for rotavirus. Rotavirus positive samples were analyzed for their antigenic properties and amino acid sequences of the glycoprotein genes. These viruses could not be assigned to any serotypes using serotyping monoclonal antibodies (MAbs) developed for porcine rotaviruses [Nagesha and Holmes: Journal of Medical Virology 35:206-211, 1991b]. When two such viruses were isolated in cell culture and analyzed by neutralization tests using hyperimmune sera they showed only one way antigenic relation with both human and porcine viruses belonging to serotype G3. In addition none of the serotyping MAbs neutralized these two virus isolates. There was no base variation between VP7 genes of faecal and cell culture isolates. Predicted amino acid sequences of the VP7 gene showed marked epitope variation from other porcine type G3 isolates with amino acid substitutions and an additional glycosylation site at residue 238. This antigenic variation seen in rotaviruses appears similar to that of influenza viruses undergoing antigenic drift.

Calcium chelation induces a conformational change in recombinant herpes simplex virus-1-expressed rotavirus VP7

Rotavirus, strain SA11, glycoprotein VP7 that was expressed by a recombinant herpes simplex virus-1 or contained in purified rotavirus particles lost reactivity with the neutralizing monoclonal antibody (mAb) 159, but not with nonneutralizing mAbs, upon chelation of calcium by EGTA. Exposing VP7, but not the neutralizing mAbs, to a transient excess of EGTA over calcium eliminated VP7 neutralizing epitopes. Therefore, a calcium chelation-induced conformational change in VP7, not in the neutralizing mAbs, caused the epitope loss. Addition of excess calcium or strontium, but not magnesium or barium, to EGTA-treated VP7 restored its 159 epitope. These results suggest that VP7 binds calcium in the absence of other rotavirus proteins and that the calcium chelation-induced conformational change in VP7 may mediate uncoating of double-shelled rotavirus particles.

Psoralen preparation of antigenically intact noninfectious rotavirus particles

The use of the synthetic psoralen 4'-aminomethyl-4,5',8-trimethylpsoralen hydrochloride (AMT) is described for the inactivation of infectious rotavirus, a member of the viral family Reoviradae with a double-stranded RNA genome. This method not only provides complete inactivation of the virus but leaves antigenically intact particles. The lack of viral replication following inactivation was determined with an immunohistochemical focus assay. The antigenic authenticity of the particles was determined by monoclonal antibody ELISA and a viral hemagglutination assay.

Humoral immune responses to VP4 and its cleavage products VP5* and VP8* in infants vaccinated with rhesus rotavirus

The humoral immune response to rhesus rotavirus (RRV) VP4 and its cleavage products VP5* and VP8* was determined in paired serum samples from 44 infants vaccinated with RRV or human rotavirus-RRV reassortants and 5 placebo recipients. Our aim was to try to measure the response to those regions of VP4 most closely related to protection. An enzyme-linked immunosorbent assay (ELISA) was used to measure the immunoglobulin G immune response to baculovirus-expressed full-length RRV VP4, full-length VP8*, and the amino-terminal polypeptide of VP5* called VP5*(1) (amino acids 248 to 474). The two antigenic regions of VP4 selected for study, VP5*(1) and VP8*, have previously been shown to contain most of the cross-reactive and strain-specific neutralization epitopes, respectively, while the remaining carboxy-terminal half of VP5* (amino acids 475 to 776) has not been clearly associated with neutralization. All three recombinant proteins were antigenically conserved, since they reacted with a library of neutralizing monoclonal antibodies directed at VP4. There was a high percentage of seroresponders to VP4 (61%) or to VP8* (52%), but fewer infants seroresponded to VP5*(1) (11%). In addition, infants responding to VP5*(1) had considerably lower titers than to VP4 or VP8*. Immune response to VP4 correlated strongly with the responses detected by the plaque reduction neutralization assay but did not correlate with the responses detected by the ELISA to whole RRV. These data imply that the VP5*(1) region is less immunogenic than the VP8* region of VP4 in infants immunized with RRV or RRV reassortants. The low immunogenicity of VP5* might adversely affect the efficacy of RRV vaccine candidates.

Characterization of serum antibody responses to natural rotavirus infections in children by VP7-specific epitope-blocking assays

Knowledge of the immune response to rotavirus is crucial for vaccine development. We compared an epitope-blocking assay (EBA) that uses VP7-specific monoclonal antibodies with neutralization assays (NAs) with polyclonal antisera for detecting serum antibody responses after natural rotavirus infection in children. Twenty-six serum pairs from children living in an orphanage with and without symptoms during two rotavirus outbreaks were evaluated for VP7 type 1-, 2-, 3-, and 4-specific antibody responses. In the first outbreak, which was caused by a VP7 type 3 strain, homotypic antibody responses were detected in 11 of 11 symptomatic children by NA and in 10 of 11 symptomatic children by EBA. Heterotypic antibody responses were detected more frequently (12 of 15 children) by NA than by EBA, and the heterotypic epitope-blocking antibody responses occurred in children older than 14 months of age. Antibody responses in asymptomatic children were more commonly detected by EBA than by NA. EBA results from the sera of children in the second outbreak indicated that it was caused by VP7 type 4, whereas NA results suggested it was caused by VP7 type 3. Our results confirm that EBA is a sensitive and specific method for determining VP7 type-specific immune responses after natural rotavirus infections.

Genetic variation in rotavirus serotype 4 subtypes

Serotype 4 rotavirus strains have been classified into two antigenic "subtypes" by a solid phase immune electron microscopy technique in which cross-absorbed animal polyclonal immune sera are used as the source of antibodies. The sequences of the gene encoding the outer capsid glycoprotein VP7 from a single serotype 4 rotavirus field strain identified as subtype A ("ST3-like") and from three field strains identified as subtype B ("VA70-like") were determined. A comparison of the deduced amino acid sequences indicated that 15 amino acid residues were divergent between subtypes but were conserved within a subtype. Three of these 15 amino acid residues (at positions 96, 212, and 217) were located in regions of the VP7 that have been defined as serotype-specific antigenic sites. These data suggest that VP7 subtype differences may result from critical amino acid substitutions within an immunodominant serotype 4-specific antigenic site. Whether these differences are an important mechanism in the epidemiology of rotaviruses requires further investigation.

Neutralizing epitopes on herpes simplex virus-1-expressed rotavirus VP7 are dependent on coexpression of other rotavirus proteins

We constructed a recombinant thymidine kinase-negative herpes simplex virus type 1 (HSV-1) that expressed the rotavirus major outer capsid glycoprotein, VP7. In the recombinant HSV-1, a promoter from the 5' noncoding region of the HSV-1 glycoprotein B locus regulated the expression of VP7 as a HSV-1 gamma 1 gene product. HSV-1-expressed VP7 resembled rotavirus-expressed VP7 in its SDS-PAGE mobility, high mannose-type glycosylation, disulfide bonding, perinuclear to cytoplasmic localization, intracellular retention, and reactivity with polyclonal antisera and nonneutralizing antibodies. Unlike rotavirus-expressed VP7, HSV-1-expressed VP7 lacked several neutralizing epitopes by immuno-histochemical staining and by ELISA. One neutralizing epitope identified on HSV-1-expressed VP7 by ELISA was masked by paraformaldehyde fixation of recombinant HSV-1- but not rotavirus-infected cells. Neutralizing epitopes were restored to HSV-1-expressed VP7 by coinfection of cells with the HSV-1 recombinant and a heterologous rotavirus that lack the neutralizing epitopes. The recovered neutralizing epitopes were detected on double-shelled rotavirus particles produced in the coinfected cells. This study indicates that the formation of several neutralizing epitopes on rotavirus VP7 requires interaction of VP7 with other rotavirus proteins. In addition, HSV-1 was a useful vector for studying the localization, processing, and antigenicity of an RNA virus glycoprotein.

Comparisons of rotavirus VP7-typing monoclonal antibodies by competition binding assay

Three sets of neutralizing monoclonal antibodies (MAbs) used to type the outer capsid protein VP7 of four group A rotavirus serotypes (1 through 4) were compared in competition immunoassays. Reciprocal competition was observed for each of the VP7 type 2-, 3-, and 4-specific MAbs. The VP7 type 1 MAbs exhibited variable competition patterns with other VP7 type 1 MAbs. MAb RV4:3, which has been used to recognize antigenic variants within VP7 type 1 strains, showed reciprocal competition with the four VP7 type 3 MAbs (RV3:1, YO-1E2, 4F8, and 159) using a VP7 type 3 virus (SA11) as antigen. MAb 2C9, also prepared against VP7 type 1, reacted with VP7 type 3 strains and competed with a VP7 type 3 MAb, 159, using RRV as antigen. Use of the different sets of VP7 type-specific MAbs in the enzyme-linked immunosorbent assay permitted the recognition of six antigenic variants within VP7 types 1, 2, and 3 among specimens whose VP7 type could not be determined previously with only one set of typing MAbs. These results demonstrate differences of typing ability among these VP7-specific MAbs and emphasize the need to improve the sensitivity of typing systems by incorporating panels of MAbs reacting with several neutralizing epitopes.

Evidence for two serotype G3 subtypes among equine rotaviruses

Ten cultivable equine rotavirus isolates, two of North American, six of British, and two of Irish origin, were compared with standard rotavirus strains and with each other by cross neutralization, neutralization with a panel of monoclonal antibodies (MAbs), hybridization to a simian rotavirus (SA-11) VP7 gene probe, and reaction with rotavirus subgrouping and serotyping MAbs in enzyme-linked immunosorbent assays. Six isolates, two of which had previously been serotyped as G3 by other workers, were found to be serotype G3; one was confirmed to be G5, and three were not related to serotypes G1 to G10. The serotype G3 strains were divisible into two subtypes, G3A and G3B, on the basis of cross neutralization. This division was also apparent in reactions with neutralizing VP7-specific MAbs and in the liquid hybridization assay. Two of the isolates were not bound by either subgroup MAb, six were bound by both subgroup I and II MAbs, and two were bound by only the subgroup I MAb. The assays used in this characterization provide a range of epidemiological information for use in future field investigations.

Rotavirus serotype G3 predominates in horses

Foal fecal group A rotavirus strains were characterized by electropherotype, serotype, and subgroup and shown to be distinctly different from rotaviruses of other mammals. Of 86 strains that were electropherotyped, 98% had similar profiles, with gene segments 3 and 4 close together and segments 7, 8, and 9 widely spaced. Of 70 strains that had sufficient detectable VP7 antigen to be serotyped by enzyme-linked immunosorbent assays (ELISAs), 63% were serotype G3 (39% were subtype G3A and 24% were subtype G3B), 4% were serotype G13, and 33% were untypeable. Serotypes G1, G2, G4, G5, G6, G9, G10, and G14 were not detected, although G5 and G14 strains have been identified among cultivable equine strains. Of 50 strains that had sufficient detectable VP6 antigen to be subgrouped by ELISAs, only 12% were able to be assigned to either subgroup I or II, with the remaining 88% belonging to neither subgroup.