title: Unraveling a revealing paradox: Why major histocompatibility complex I-signaled thymocytes "paradoxically" appear as CD4+8lo transitional cells during positive selection of CD8+ T cells.
abstract: The mechanism by which T cell receptor specificity determines the outcome of the CD4/CD8 lineage decision in the thymus is not known. An important clue is the fact that major histocompatibility complex (MHC)-I-signaled thymocytes paradoxically appear as CD4+8lo transitional cells during their differentiation into CD8+ T cells. Lineage commitment is generally thought to occur at the CD4+8+ (double positive) stage of differentiation and to result in silencing of the opposite coreceptor gene. From this perspective, the appearance of MHC-I-signaled thymocytes as CD4+8lo cells would be due to effects on CD8 surface protein expression, not CD8 gene expression. But contrary to this perspective, this study demonstrates that MHC-I-signaled thymocytes appear as CD4+8lo cells because of transient down-regulation of CD8 gene expression, not because of changes in CD8 surface protein expression or distribution. This study also demonstrates that initial cessation of CD8 gene expression in MHC-I-signaled thymocytes is not necessarily indicative of commitment to the CD4+ T cell lineage, as such thymocytes retain the potential to differentiate into CD8+ T cells. These results challenge classical concepts of lineage commitment but fulfill predictions of the kinetic signaling model.
authors.affiliation: Laboratory of Immune Cell Biology, National Cancer Institute, Bethesda, MD 20892, USA.
authors.forename: Remy
authors.initials: R
authors.lastname: Bosselut
authors.forename: Terry I
authors.initials: TI
authors.lastname: Guinter
authors.forename: Susan O
authors.initials: SO
authors.lastname: Sharrow
authors.forename: Alfred
authors.initials: A
authors.lastname: Singer
mesh_terms: D000818:Animals; D016827:CD8 Antigens / Q000235:genetics* / Q000378:metabolism; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology / Q000502:physiology*; D002454:Cell Differentiation / Q000502:physiology; D019070:Cell Lineage; D015536:Down-Regulation; D005434:Flow Cytometry; D015395:Histocompatibility Antigens Class I / Q000276:immunology / Q000378:metabolism*; D000949:Histocompatibility Antigens Class II / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008285:Major Histocompatibility Complex / Q000276:immunology*; D051379:Mice; D008822:Mice, Transgenic; D015398:Signal Transduction / Q000502:physiology*; D016176:T-Lymphocyte Subsets / Q000276:immunology / Q000502:physiology*; D013950:Thymus Gland / Q000166:cytology / Q000276:immunology* / Q000378:metabolism
chemical_list: D016827:CD8 Antigens; D015395:Histocompatibility Antigens Class I; D000949:Histocompatibility Antigens Class II