INTRAEPITHELIAL LYMPHOCYTES - EVIDENCE FOR REGIONAL SPECIALIZATION AND EXTRATHYMIC T-CELL MATURATION IN THE HUMAN GUT EPITHELIUM

The human gut epithelium is a unique immunological compartment, contai ning substantial amounts of intra-epithelial lymphocytes (IEL) with un known functions. In this study we show that distinct and unusual subpo pulations of IEL are present at different levels of human intestine. I EL phenotypes in normal jejunum, ileum and colon were compared using i mmunoflow cytometry and immunohistochemistry. The expression of mRNA f or recombination-activating gene-1 (RAG-I) in IEL from all three level s was compared using reverse-transcription polymerase chain reaction, and the morphology of IEL in site was determined using immunoelectron microscopy, Surface marker profiles of isolated intestinal epithelial cells at all three levels were also investigated. On average the propo rtion of TCR gamma delta IEL was comparable in jejunum, ileum and colo n and the majority were CD4(-)CD8(-). TCR alpha beta IEL were more fre quent in jejunum than ileum and colon and varied in phenotype with gut level, CD4(-)CD8(-) TCR alpha beta IEL dominated in colon but were ab sent in jejunum. CD8(+) TCR alpha beta IEL were present at all levels but only in jejunum did they constitute the majority of all IEL. CD4() TCR alpha beta IEL were present in similar frequencies at all levels of the gut, In general, the majority of IEL had an activated phenotyp e (CD45RO(+), alpha(E) beta(7)(+)) Furthermore, IEL exhibited phenotyp es which are rare in peripheral blood, The thymocyte markers CD1a and CD1c as well as the NK cell marker CD56 were expressed on a fraction o f TCR alpha beta and TCR gamma delta IEL. A small population of 'null' cells (CD45(+)TCR/CD3(-)CD20(-)CD14(-)CD15(-) cells) was also present at equal proportions along the gut, Jejunal but not colonic IEL expre ssed RAG-1 mRNA suggesting that extrathymic T cell maturation occurs i n the epithelium of small intestine, RAG-1 was expressed in CD2(+)TCR/ CD3(-) and CD3(+)/TCR(-) IEL. Ultrastructurally, IEL often formed smal l clusters and intimate contacts with epithelial cells, suggesting cel l cooperation within the epithelium. Some IEL had pseudopodium-like ex tensions penetrating the epithelial basement membrane suggesting trans migration. Epithelial cells in small intestine but not colon expressed heat shock protein 60 and HLA-DR. CD1a, CD1b and CD1c were not expres sed on intestinal epithelial cells at any level. The distinct surface marker profiles of IEL and epithelial cells along small and large inte stine suggest functional regional specialization and are compatible wi th the hypothesis that TCR alpha beta IEL participate in immune reacti ons to lumenal antigens while TCR gamma delta IEL perform surveillance of the epithelium.