Human αβ and γδ thymocyte development:: TCR gene rearrangements, intracellular TCRβ expression, and γδ developmental potential-differences between men and mice

To evaluate the role of the TCR in the alpha beta/gamma delta lineage choice during human thymocyte development, molecular analyses of the TCR locus in gamma delta cells and the TCR beta and S loci in alpha beta cells were undertaken. TCRP variable gene segments remained largely in germline configuration in gamma delta cells, indicating that commitment to the gamma delta lineage occurred before complete TCR beta rearrangements in most cases. The few TCRP rearrangements detected were primarily out-of-frame, suggesting that productive TCRP rearrangements diverted cells away from the gamma delta lineage. In contrast, in alpha beta cells, the TCR gamma locus was almost completely rearranged with a random productivity profile; the TCR delta locus contained primarily nonproductive rearrangements. Productive gamma rearrangements were, however, depleted compared with preselected cells. Productive TCR gamma and 5 rearrangements rarely occurred in the same cell, suggesting that alpha beta cells developed from cells unable to produce a functional gamma delta TCR. Intracellular TCRP expression correlated with the up-regulation of CD4 and concomitant down-regulation of CD34, and plateaued at the early double positive stage. Surprisingly, however, some early double positive thymocytes retained gamma delta potential in culture. We present a model for human thymopoiesis which includes gamma delta development as a default pathway, an instructional role for the TCR in the alpha beta/gamma delta lineage choice, and a prolonged developmental window for beta selection and gamma delta lineage commitment. Aspects that differ from the mouse are the status of TCR gene rearrangements at the nonexpressed loci, the timing of A selection, and maintenance of gamma delta potential through the early double positive stage of development.