T-CELL DEVELOPMENT WITHIN THE INTESTINAL-MUCOSA - CLUES TO A NOVEL IMMUNE-ENDOCRINE NETWORK

Small intestine intraepithelial lymphocytes (IELs) comprise a heteroge neous and phenotypically complex population of T cells that are part o f the gut-associated lymphoid tissues (GALTs). Recent studies from a n umber of laboratories indicate that murine IELs are greatly enriched f ar extrathymic T cells, although many aspects of the IEL extrathymic d evelopmental pathway remain controversial, and there is currently no c onsensus of opinion as to which IELs are extrathymic and which are thy mus-derived. Those differences reflect variations in the IEL repertoir e in athymic animals depending upon the specific model used to study I ELs, and they correlate with the age at which mice became or were rend ered athymic, implying that the thymus participates either directly or indirectly in the local extrathymic IEL developmental process. In thi s article, the basic findings regarding intestinal T cell development are discussed, and a hypothesis is provided which links neuroendocrine interactions targeted to the intestine epithelium to the striking rel ationship between animal developmental age and the thymopoietic potent ial of the intestine. (C) 1997 Elsevier Science Ltd.