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From the * Netherlands Cancer Institute, 1066 CX Amsterdam, Netherlands; and 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
Laboratory of
AIDS Immunology, Institute for Virus Research, Kyoto University, Kyoto, Japan
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
+CD4+ SP cells, indicating that mouse MHC class II
can positively select TCR
+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
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
+ cells.
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