T-lymphopoietic capacity of the mouse intestinal epithelium

Small intestine intraepithelial lymphocytes (i-IELs) comprise a heterogeneous population of T cells that are part of the gut-associated lymphoid tissues. Recent studies indicate that murine i-IELs are greatly enriched for extrathymic T cells. Yet there is disagreement as to which i-IELS are extrathymic and which are not, and many aspects of intestinal T-cell development remain controversial. In this article, the developmental lineages of murine i-IELs are examined, and the thymopoietic potential of the intestine epithelium is discussed. A mechanism is described whereby all intestinal T cells develop extrathymically, and a concept of intestinal T cells is presented which suggests that lineages of i-IELs need to be viewed according to developmental factors exclusive of thymus dependency.