Downregulation of CD1 marks acquisition of functional maturation of human thymocytes and defines a control point in late stages of human T cell development.
We have investigated whether in the human thymus transition of CD4+CD8+ double positive (DP) to CD4+ or CD8+ single positive (SP) cells is sufficient for generation of functional immunocompetent T cells. Using the capacity of thymocytes to expand in vitro in response to PHA and IL-2 as a criterion for functional maturity, we found that functional maturity of both SP and DP thymocytes correlates with downregulation of CD1a. CD1a- cells with a persistent DP phenotype were also found in neonatal cord blood, suggesting that at least a proportion of mature DP cells can emigrate from the thymus. The requirements for generating functional T cells were investigated in a hybrid human/mouse fetal thymic organ culture. MHC class II-positive, but not MHC class II-negative, mouse thymic microenvironments support differentiation of human progenitors into TCR alpha beta+CD4+ SP cells, indicating that mouse MHC class II can positively select TCR alpha beta +CD4+ SP human cells. Strikingly, these SP are arrested in the CD1a+ stage and could not be expanded in vitro with PHA and IL-2. CD1a+CD4+ SP thymocytes do not represent an end stage population because purified CD1a+CD4+ SP thymocytes differentiate to expandable CD1a- cells upon cocultivation with human thymic stromal cells. Taken together these data indicate that when CD1a+ DP TCR alpha beta low cells mature, these cells interact with MHC, but that an additional, apparently species-specific, signal is required for downregulation of CD1a to generate functional mature TCR alpha beta + cells.
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Netherlands Cancer Institute, Amsterdam, Netherlands.
Blom
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Hori
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Weijer
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Spits
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D000375:Aging / Q000276:immunology; D000818:Animals; D015703:Antigens, CD / Q000096:biosynthesis*; D018949:Antigens, CD1 / Q000096:biosynthesis*; D015704:CD4 Antigens / Q000096:biosynthesis; D002454:Cell Differentiation; D002648:Child; D002675:Child, Preschool; D017931:DNA Primers; D005434:Flow Cytometry; D018507:Gene Expression Regulation, Developmental; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000276:immunology; D018398:Homeodomain Proteins; D006801:Humans; D007223:Infant; D008213:Lymphocyte Activation; D051379:Mice; D009924:Organ Culture Techniques; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D016693:Receptors, Antigen, T-Cell, alpha-beta / Q000096:biosynthesis; D013601:T-Lymphocytes / Q000276:immunology*; D013950:Thymus Gland / Q000254:growth & development / Q000276:immunology*
D015703:Antigens, CD; D018949:Antigens, CD1; D015704:CD4 Antigens; D017931:DNA Primers; D018398:Homeodomain Proteins; D016693:Receptors, Antigen, T-Cell, alpha-beta; C064658:RAG-1 protein