title: Peptide specificity of thymic selection of CD4+CD25+ T cells.
abstract: The CD4(+)CD25(+) regulatory T cells can be found in the thymus, but their need to undergo positive and negative selection has been questioned. Instead, it has been hypothesized that CD4(+)CD25(+) cells mature following TCR binding to MHC backbone, to low abundant MHC/peptide complexes, or to class II MHC loaded with peripheral autoantigens. In all these circumstances, processes that are distinct from positive and negative selection would govern the provenance of CD4(+)CD25(+) cells in the thymus. By comparing the development of CD4(+)CD25(-) and CD4(+)CD25(+) cells in mice expressing class II MHC molecules bound with one or many peptide(s), we show that the CD4(+)CD25(+) cells appear during natural selection of CD4(+) T cells. The proportion of CD4(+)CD25(+) cells in the population of CD4(+) thymocytes remains constant, and their total number reflects the complexity of selecting class II MHC/peptide complexes. Hence, thymic development of CD4(+)CD25(+) cells does not exclusively depend on the low-density, high-affinity MHC/peptide complexes or thymic presentation of peripheral self-Ags, but, rather, these cells are selected as a portion of the natural repertoire of CD4(+) T cells. Furthermore, while resistant to deletion mediated by endogenous superantigen(s), these cells were negatively selected on class II MHC/peptide complexes. We postulate that while the CD4(+)CD25(+) thymocytes are first detectable in the thymic medulla, their functional commitment occurs in the thymic cortex.
authors.affiliation: Institute of Molecular Medicine and Genetics, Medical College of Georgia, Augusta, GA 30912, USA.
authors.forename: Rafal
authors.initials: R
authors.lastname: Pacholczyk
authors.forename: Piotr
authors.initials: P
authors.lastname: Kraj
authors.forename: Leszek
authors.initials: L
authors.lastname: Ignatowicz
mesh_terms: D000818:Animals; D001324:Autoantigens / Q000096:biosynthesis / Q000378:metabolism; D015704:CD4 Antigens / Q000096:biosynthesis*; D002454:Cell Differentiation / Q000276:immunology; D002455:Cell Division / Q000276:immunology; D002478:Cells, Cultured; D017637:Clonal Deletion; D018920:Coculture Techniques; D018984:Epitopes, T-Lymphocyte / Q000276:immunology*; D000949:Histocompatibility Antigens Class II / Q000096:biosynthesis / Q000378:metabolism; D006706:Homeostasis / Q000276:immunology; D008213:Lymphocyte Activation; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008822:Mice, Transgenic; D010455:Peptides / Q000276:immunology* / Q000378:metabolism; D011485:Protein Binding / Q000276:immunology; D011948:Receptors, Antigen, T-Cell / Q000096:biosynthesis; D015375:Receptors, Interleukin-2 / Q000096:biosynthesis*; D016176:T-Lymphocyte Subsets / Q000166:cytology* / Q000276:immunology* / Q000378:metabolism; D013950:Thymus Gland / Q000166:cytology* / Q000276:immunology* / Q000378:metabolism
chemical_list: D001324:Autoantigens; D015704:CD4 Antigens; D018984:Epitopes, T-Lymphocyte; D000949:Histocompatibility Antigens Class II; D010455:Peptides; D011948:Receptors, Antigen, T-Cell; D015375:Receptors, Interleukin-2