Abstract
A serial study of rotavirus infections in 5-day-old gnotobiotic piglets is described in terms of pathogenesis and immune response. An immunofluorescent microtitre technique for titrating rotavirus in LLCMK2 cells was developed and used with immunofluorescent and histological examinations of the small intestine to study for 21 days the course of infections by two porcine rotavirus isolates. The appearance of rotavirus-specific antibody, as measured by a virus neutralization test and an indirect immuno-fluorescent test, was determined in serum, saliva and faeces, and the numbers and distribution of immunoglobulin-containing cells throughout the small intestine examined. The two porcine rotaviruses examined in this study were shown by cross neutralisation and double immunodiffusion to be antigenically dissimilar. Using body weight loss as a measure of virulence, the two isolates could also be clearly distinguished. However, both isolates induced, in 5-day-old gnotobiotic piglets, an acute enteric disease because of. destruction by virus of the villus epithelium of the small intestine. Recovery from intestinal damage appeared to be biphasic. The first phase comprised a balance between virus production and cell susceptibility and the second involved an immune response. Antibody appeared in serum, saliva and faeces, of which that appearing in the faeces correlated with virus elimination. Since large numbers of immunoglobulin-containing cells (presumptively antibody secreting) were detectable in the lamina propria of the small intestine, following infection, it is reasonable to assume that the faecal antibody is locally produced. The most important class of immunoglobulin produced appeared to be IgM. A non-immunoglobulin rotavirus inhibitor could also be detected at titres equivalent to induced antibody levels, in older piglets, both infected and uninfected. The reductions in body weight, and hence differences in virulence, were correlated with changes in the severity of the gut lesion (as measured by the ratio villus length : crypt length). Whilst both isolates could produce similarly severe lesions in a particular area, virulence was found to be associated with the severity of the lesion throughout the small intestine and also the length of time for which the severe lesion persisted. Persistence of the lesion was thought to be connected with the induction of relatively ineffective antibody by the virulent rotavirus, possibly of a different immunoglobulin class to that induced by the other isolate. Viral replication, and hence gut lesion, throughout the small intestine was thought to be governed by differences in cell differentiation along the length of the intestine and also possibly the production of viral inhibitors in the anterior region. Some evidence exists for the role of cell-mediated immunity and macrophages in control of rotavirus infections, and may help to further explain differences in virulence, but more work is necessary before firm conclusions can be drawn.