Reduced Expression of Polymeric Immunoglobulin Receptors in the Intestine of Young Rats Fed a Fiber-free Diet

In this study, we investigated the influence of a fiber-free diet on the intestinal secretory immune system in young animals. Four-week-old rats were fed either a purified diet containing sucrose as the only carbohydrate source (fiber(–) diet) or a diet supplemented with 15% natural crude fiber from sugar beets (fiber(+) diet). After 14 days of feeding, we measured total IgA content in 24-hr fecal samples and in intestinal tissues and the expression of intestinal polymeric immunoglobulin receptors (pIgRs), which are essential for IgA secretion. The excretion of total IgA in the feces was significantly lower in rats fed the fiber(–) diet than in those fed the fiber(+) diet (27% vs. 100%; p < 0.05). However, the total IgA content in the intestinal tissue extracts did not differ between the groups. The pIgR signal intensities observed by immunohistochemistry were somewhat lower in the colon of the rats fed the fiber(–) diet. Western blot analysis showed that pIgR protein expression in the distal colon of rats fed the fiber(–) diet was significantly lower than that in rats fed the fiber(+) diet (38% vs. 100%, p < 0.05). Conversely, colonic pIgR mRNA expression did not differ between the groups. Thus, we conclude that a fiber-free diet decreases colonic pIgR protein expression by a posttranscriptional mechanism, resulting in decreased luminal secretory immune system activity and thus, suboptimal protection of the colonic mucosa.