Uptake of lymphocytes fed to suckling rats. An autoradiographic study of the transit of labeled cells through the neonatal gastric mucosa

To determine the potential for transfer of maternal lymphocytes to the newborn during suckling, radiolabeled cells were fed to rat neonates of different ages and autoradiographs prepared from tissue sections obtained from the stomach and small intestine. The entire stomach (including contents) and intestinal walls were observed using semiserial 3 μm methacrylate plastic sections. For these studies 100 × 10⁶ [³H]uridine-labeled lymph node cells were fed to 1-4-day-old neonates using a smooth-tipped intubation needle. Some of the lactating female rats had been given radioisotope prior to delivery and "cold" babies were fostered to them to enhance the number of labeled cells in the neonatal digestive tract. Babies were killed at 1, 2, 4, 8, 24 and 48 h after feeding. The stomach contents showed a considerable number of labeled cells (32% at 1 h after feeding) and many of these cells were in close proximity to the epithelium. The gastric epithelium often exhibited gaps between adjacent epithelial cells similar to those we have previously reported in the lactating mammary epithelium, and labeled cells were observed in the immediate area of the gaps. Labeled cells were seen in the neonatal gastric epithelium, lamina propria, mesenteric attachments, in the wall and lumen of small gastric blood vessels and adjacent lymph nodes. Labeled cells were most abundant in the gastric wall at 1-2 h following feeding, indicating that transit through the epithelium occurred rapidly. Although labeled cells were seen in the lumen of the small intestine, none were observed in the tissues of the intestinal wall. When heat-killed labeled cells were fed, no labeled cells were seen in the tissues of the neonatal gastrointestinal tract. It is apparent from these studies that a portion of the leukocytes that are delivered to the neonate during suckling are able to transit the gastric epithelium and establish themselves in the neonate's tissues, presumably to aid in the protection of the immunologically naive infant.