Activation of the Gut-Associated Lymphoid Tissue With Expression of Interleukin-2 Receptors That Peaks During Weaning in the Rat

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Abstract
Background: Weaning exposes the intestinal mucosa to food and bacterial antigens at an age when the immune system is believed to be immature and functionally defective. The purpose of this study was to investigate changes in activation and phenotype of immune cells of the gut-associated lymphoid tissue during weaning. Methods: Litters of infant rats were studied from pre- to postweaned life. The activation status, assessed by interleukin-2 receptor (IL-2R) expression, and phenotype of cells in the gut-associated lymphoid tissue were examined by immunostaining. Results: Interleukin-2 receptor expression peaked two to fourfold at midweaning (day 21) in mesenteric lymph nodes, jejunal lamina propria, Peyer's patches, and intraepithelial lymphocytes, compared with adult animals (day 70). CD45+ cells expanded in the lamina propria, epithelium, and lymphocyte-filled villi. With CD45 as the denominator, 10% to 50% of lymphocytes in the lamina propria and epithelium were αβT-cell receptor (TCR)+, but the remaining cells had a null phenotype, because there were low numbers of γδTCR+ T cells, B cells, and macrophages. Natural killer cells peaked at midweaning in the lamina propria (9%) and epithelium (20%) but were less than 5% of CD45+ cells after weaning. Conclusions: Rather than being immature or functionally inactive, the gut-associated lymphoid tissue reacts appropriately during weaning with expression of IL-2R and expansion of αβTCR+ T-cells.