Characteristics of submucosal lymphoid tissue located in the proximal colon of the rat.

In this study we have examined the morphology and steroid sensitivity of proximal colonic lymphoid tissue in the Fisher 344 rat. A time course study was conducted in which groups of animals were injected subcutaneously with hydrocortisone sodium succinate (125 mg/kg body weight) and killed on Days 0-4. Thymus, jejunal and ileal Peyer's patches and proximal colonic lymphoid tissue were excised, weighed and processed for histological analysis. The results showed that the maximum cytoreductive effects of the hydrocortisone were evident on Day 2. Thymus and proximal colonic lymphoid tissue weight decreased to 5 and 18% of the control values respectively, before returning towards control values over the next two days. In contrast, jejunal and ileal Peyer's patch weights were unaltered. A dose response experiment was conducted using the same endpoints. Rats were injected subcutaneously with hydrocortisone at 60, 120, 200 and 300 mg/kg body weight and killed on Day 2. The results of this experiment showed that the proximal colonic lymphoid tissue, like thymus, responded with a dose-dependent loss of tissue weight. The spleen and Peyer's patches showed only a slight weight decrease compared to the control. These data showed that the response of proximal colonic lymphoid tissue to steroids was more similar to that of thymus, a primary lymphoid tissue, than to other secondary lymphoid tissues. Finally, grafts of fetal proximal colon under the kidney capsule of syngeneic adults supported the development of this lymphoid aggregate in the absence of luminal antigenic stimulation. These results suggest that the development and functional contribution of proximal colonic lymphoid tissue to the immune system warrants a more detailed examination.