Normal sequence of phenotypic transitions in one cohort of 5-bromo-2'-deoxyuridine-pulse-labeled thymocytes....

"In vivo" kinetics of T cell differentiation and TCR expression in the normal murine thymus were re-evaluated using a new technique for simultaneous detection of bromodeoxyuridine and two surface markers. The transition from CD4-8- precursors to CD4+8+ immature cells was directly observed during cell proliferation, and shown to proceed through transitory intermediates expressing no or low amounts of CD4. CD3-TCR expression also started during this transition and resulted in the production of a majority of TCRlo cells but also of a significant number (1 to 2 x 10(6) of TCRhi immature (heat-stable Ag+) thymocytes. After cessation of proliferation, the maturational transition from CD4+8+ to CD4+8- and CD4-8+ (in this order) was restricted to TCRhi cells produced during CD4+8+ cell generation. The acquisition of the single positive phenotype preceded HSA down-regulation, suggesting that maturation of TCRhi thymocytes proceeds in two separate steps. The major TCRloCD4+8+ subset appeared a dead end subset and showed no up-regulation of TCR expression at any time.