Activation of the gut-associated lymphoid tissue with expression of interleukin-2 receptors that peaks during weaning in the rat

Background: Weaning exposes the intestinal mucosa to food and bacterial ant igens at an age when the immune system is believed to be immature and funct ionally defective. The purpose of this study was to investigate changes in activation and phenotype of immune cells of the gut-associated lymphoid tis sue during weaning. Methods: Litters of infant rats were studied from pre- to postweaned life. The activation status, assessed by interleukin-2 receptor (IL-2R) expressio n, and phenotype of cells in the gut-associated lymphoid tissue were examin ed by immunostaining. Results: Interleukin-2 receptor expression peaked two to fourfold at midwea ning (day 21) in mesenteric lymph nodes, jejunal lamina propria, Peyer's pa tches, and intraepithelial lymphocytes, compared with adult animals (day 70 ). CD45+ cells expanded in the lamina propria, epithelium, and lymphocyte-f illed villi. With CD45 as the denominator, 10% to 50% of lymphocytes in the lamina propria and epithelium were alpha beta T-cell receptor (TCR)+, but the remaining cells had a null phe notype, because then were low numbers of gamma delta TCR+ T cells, B cells, and macrophages. Natural killer cells p eaked at midweaning in the lamina propria (9%) and epitheliuln (20%) but we re less than 5% of CD45+ cells after weaning. Conclusions: Rather than being immature or functionally inactive, the gut-a ssociated lymphoid tissue reacts appropriately during weaning with expressi on of IL-2R and expansion of alpha beta TCR+ T-cells.