Characterization of Triple Negative Clones Isolated from Post-Natal Human Thymus
Human triple negative (CD3- CD4- CD8-) thymocytes and double negative (CD4- CD8-) thymocytes purified from post-natal thymus were cloned with a feeder cell mixture of irradiated PBL, irradiated JY cells and PHA and expanded with IL-2. The cloning efficiency of triple negative thymocytes was less than 1% and the majority of the clones were triple negative. One out of 11 clones was TCR αβ+ CD4+. No TCR γδ + clones were isolated. On the other hand, the cloning efficiency of double negative thymocytes was about 10% and most of the clones isolated were TCR γδ +. We could not find any evidence of in vitro differentiation of triple negative thymocytes into TCR γδ + cells. Some of the triple negative clones expressed CD16- brightly and were apparently NK cells. All CD 16-clones isolated from triple negative thymocytes, however, expressed NKH1, which is also an NK cell marker. Cytoplasmic CD3-δ and CD3-ε Ag which have been reported to be expressed in the most immature thymocytes were not detected in any of these clones. Furthermore, the CD 16- triple negative clones exhibited significant cytolytic activity against K562. Pheno-type of the clones seems to be stable under various conditions in vitro including coculture with human thymic epithelial cells. These data indicate that the CD16- triple negative clones isolated from triple negative thymocytes are similar to a minor subset of NK cells which is CD16- NKH-1+. It is not clear whether they originated from a distinct subset of mature or immature NK cells resident in the thymus tissue or from common precursors of both T and NK lineage.
KeywordsCloning Efficiency Thymus Tissue Human Thymus Immature Thymocyte Thymic Stroma
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