Mucosal immune activation and maturation of the small intestine at weaning in the hypothymic (nude) rat.

Activation of the mucosal immune system peaks at weaning on days 21 and 22 of life in the rat. We have investigated activation in the gut associated lymphoid tissue and maturation of intestinal mucosa in hypothymic (nude) and in phenotypically normal heterozygous CBH rats at 22 days of life. Intestinal maturation, as assessed by villus area, crypt length, crypt cell production rate and disaccharidase activity, was similar in hypothymic and normal rats, and indices of mucosal immune function were elevated in both groups at this time. The proportion of mononuclear cells from the mesenteric lymph node expressing IL-2R was 11% in heterozygous and 14% in hypothymic rats as determined by flow cytometry. Immunoperoxidase staining of MLN sections confirmed the presence of IL-2R+ cells in the T-dependent interfollicular areas. However, the number of T-cells was considerably depleted in hypothymic rats. Intraepithelial lymphocyte counts and serum rat mucosal mast cell protease II concentrations were similar in the two groups, while counts of jejunal mucosal mast cells and eosinophils were paradoxically increased in hypothymic animals. As T lymphocyte function is thought to be impaired in hypothymic rats, the intact mucosal immune activity in hypothymic rats could be due to activation of intrinsic "thymic independent" T lymphocytes in hypothymic rats, or to engraftment with extrinsic maternal milk-derived lymphocytes and their activation in the infant rat gut.