RAG1 and RAG2 expression in human intestinal epithelium: evidence of extrathymic T cell differentiation.

We examined the hypothesis that the adult human gastrointestinal tract is a site of extrathymic T cell differentiation. When T lymphocytes undergo gene rearrangement, the products of both RAG1 and RAG2 genes are expressed; RAG mRNA is present only in tissue governing lymphocyte maturation. In this study, reverse transcription polymerase chain reaction (RT-PCR) was used to detect RAG1-and RAG2-specific mRNA. Total RNA was purified from small intestinal samples from five adults. Peripheral blood mRNA from the same patient was used as a negative control in two cases. Bone marrow RNA preparations from two healthy donors were used as positive controls. cDNA synthesis was carried out using random hexamers. Primers for first round and nested PCR of RAG1 and RAG2 were synthesized. RAG1 and RAG2 mRNA was detected in all bone marrow preparations but was absent in all peripheral blood samples. RAG1 and RAG2 mRNA was detected in the small intestine of four of the five patients studied. RAG1 and RAG2 expression was localized in the epithelial layer and absent in the lamina propria. RAG1 and RAG2 expression in the epithelial layer is strong evidence that T cell differentiation occurs in the adult human intestine.