Antiserum prepared against the M37 strain of rotavirus, recovered from an asymptomatic newborn infant in Venezuela, neutralized two prototype human rotaviruses that define two separate serotypes: serotype 1 (Wa) and serotype 4 (ST3). Thus, the M37 strain is a naturally occurring intertypic rotavirus. Analysis of reassortant viruses produced during coinfection in vitro indicated that the observed dual serotype specificity of M37 resulted from sharing a related outer (1-3).The genome of rotaviruses consists of 11 discrete segments (genes) of double-stranded (ds) RNA. These genes reassort with high efficiency during coinfection, and this property has facilitated the selection of reassortant viruses with a mixed constellation of genes derived from two biologically and antigenically distinct rotaviruses. Analysis of reassortant viruses has provided much of our current understanding of gene-product relationships. For example, the fourth gene segment has been shown to code for the outer capsid hemagglutinin protein VP3 (4), which is also the site for protease activation of infectivity (4) and for host-range restriction of rotavirus infectivity (5, 6). The major subgroup antigen(s) was shown to be coded for by the sixth RNA genome segment (7,8). The eighth or ninth genome segment, depending on the virus strain, was shown to code for a major neutralization antigen, VP7 (6,7,9).Hybridoma technology has also aided the functional and structural analysis of the relatively complex rotaviruses. For example, some monoclonal antibodies directed against the fourth rotaviral gene product, VP3, exhibit both hemagglutination-inhibiting and neutralizing activity (8-10). Also, subgroup-specific monoclonal antibodies react with the sixth gene product, VP6 (11), whereas certain monoclonal antibodies directed against the eighth or ninth rotaviral gene product, VP7, exhibit a high level of neutralizing activity (9, 10, 12, 13). Until now the outer capsid VP7 protein has been considered the major neutralization antigen. The recent observation that some monoclonal antibodies directed against VP3 exhibit a moderate to high level of neutralizing activity was not surprising, however, because this antigen is also located on the outer capsid (9, 10, 13).During the course of analyzing rotavirus isolates by the plaque-reduction neutralization (PRN) technique, we observed that hyperimmune guinea pig antiserum raised against the Venezuelan neonatal rotavirus isolate M37 neutralized both serotype 1 (strain Wa) and serotype 4 (strain St. Thomas no. 3) rotaviruses to the same degree (14). A combined genetic and serologic analysis of this "intertypic bridging" phenomenon indicated that the VP3 and VP7 outer capsid proteins of the M37 rotavirus each played a role in the observed dual serotype of the neonatal rotavirus isolate. This indicated that these neutralization specificities present on VP3 and VP7 segregate independently in nature.MATERIALS AND METHODS Viruses. The following cultivatable rotaviruses were used in this study: human rotaviru...

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Rotaviruses are the leading cause of severe diarrhea in infants and young children worldwide. Thus, the development of an effective rotavirus vaccine is a major public health goal. This study was performed to identify the gene or genes responsible for rotavirus virulence or host range restriction and attenuation in a natural host. Such knowledge could have an important bearing on the selection of candidate live vaccine strains. We addressed this issue by analyzing the response of gnotobiotic piglets to orally administered porcine x human rotavirus reassortants. It was possible to determine (i) which porcine rotavirus genes were required for the induction of diarrhea, and (ii) which human rotavirus genes are associated with the host range restriction because the parental porcine rotavirus (SB-1A strain) caused diarrhea in piglets, whereas the parental human rotavirus (DS-1 strain) was attenuated in piglets. Substitution of the 3rd (VP3) or 4th (VP4) or 9th (VP7) or 10th (NS28 (NSP4)) gene of the avirulent human strain for the corresponding gene of porcine rotavirus that was virulent for gnotobiotic piglets yielded viral reassortants that failed to induce diarrhea. Further analysis indicated that reassortants which possessed only one, two, or three of these porcine rotavirus genes on a background of human rotavirus genes also failed to induce diarrhea. However, diarrhea was induced when all four of these porcine rotavirus genes were included in a reassortant in which the remaining seven genes were derived from the human rotavirus. These observations suggest that it may be possible to attenuate wild-type human rotavirus strains that are virulent for humans by selective genetic reassortment with an animal rotavirus strain that is attenuated for humans.