title: Development and functional properties of thymic and extrathymic T lymphocytes.
abstract: Although vertebrate hematopoiesis is known to occur in a variety of locations, the differentiation of T lymphocytes takes place only in the thymus. Conversely, the only known function of the thymus is to produce T lymphocytes. As thymic function progressively diminishes with age in all individuals, and decreased thymic capacity has been linked to an impaired resistance to infections and cancer, increasing thymic function and/or extrathymic T-cell development could thus play an important role in health management in elderly subjects. However, it is currently unclear why the thymus is essential for T-cell development and precisely what is necessary to make a functional T lymphopoietic organ. In this review, we shall discuss the current literature on the similarities and differences between thymic and extrathymic T cells. A major conclusion is that innate T cells can be generated extrathymically, but only the thymus can support the development of classic adaptive T cells. Functional differences between thymic and extrathymic T cells provide a rationale for conservation of the thymus as the sole primary T lymphoid organ. These differences also have implications for the management of thymus involution.
authors.affiliation: Institute for Research in Immunology and Cancer; Department of Medicine, Universite de Montreal, Montreal, QC, Canada H3C 3J7.
authors.forename: Krista M
authors.initials: KM
authors.lastname: Heinonen
authors.forename: Claude
authors.initials: C
authors.lastname: Perreault
mesh_terms: D000818:Animals; D001854:Bone Marrow Cells / Q000502:physiology; D002454:Cell Differentiation; D019070:Cell Lineage; D006412:Hematopoietic Stem Cells / Q000502:physiology; D006801:Humans; D008213:Lymphocyte Activation; D038041:Lymphopoiesis; D017154:Stromal Cells / Q000502:physiology; D013601:T-Lymphocytes / Q000502:physiology*; D013950:Thymus Gland / Q000166:cytology*