Extrathymic T-cell development i-sa topic of considerable interest and debate, with important implications for the mechanisms of T-cell maturation and repertoire selection. Recent evidence has suggested that intraepithelial T Iymphocytes (IELs) of the small intestine can mature and undergo selection in the absence of a thymus. Houletler, IEL precursors are present in the thymus and LB. dezlelopment is known to be influenced by the thymus. Here, Leo LefranCois and Lynn Puddington discuss these data and suggest that the diff erentiation pathway1 of IEL precursors is dependent on whether or not a thymus is present.

Classically, the development of T lymphocytes has been defined as occurring in the thymus. T-cell precursors arrive in the thymus from a hematopoietic source, such as fetal liver, spleen or bone marrow, and develop through several phenotypic intermediates vin a selection process based on T-cell receptor (TCR) specificities and affinities. This process comprises negative and positive selection events that generally lead to the pro duction of non-autoreactive CD4’and CDS+ peripheral T cells that potentially react with foreign antigen and