Identification and isolation of a T4+T8+ cell with high T3 expression in human thymus: a possible late intermediate in thymocyte differentiation.

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Abstract
By using sensitive three-color fluorescence flow cytometric techniques, we were able to identify a T4+T8+ thymocyte with high T3 surface density (T3H) representing 4 to 9% of thymocytes. To characterize the T3HT4+T8+ cell, thymic subpopulations with high T3 surface density (T3H) and lower T3 density (T3L/T3-) were compared with regard to T6 expression. The T3H subpopulation was characterized by lower numbers of T6+ cells and reduced levels of T6 antigen density, whereas the T3L/T3- population was greater than 90% T6+ and expressed this antigen at high cell surface density. In addition, T3H fractions appeared to possess higher levels of nuclear activation with respect to the T3L/T3- population as indicated by increased log 90 degrees scatter profiles. These results suggest that thymocytes with high T3 surface expression are not only more differentiated, but also more activated than the majority of the thymic population. The T3HT4+T8+ fraction could be distinguished from T4+T8+ thymocytes with lower T3 density not only by an increased log 90 degrees scatter profile, but also by the presence of T4+T8+ cells with reduced levels of T8 surface antigen. Our results indicate that T4+T8+ thymocytes with high T3 surface density are a distinct subpopulation and may represent the immediate precursors of the phenotypically more mature T3HT4+T8- and T3HT8+T4- subpopulations found in human thymus.