Development of T cell receptor-gamma delta cells. Phenotypic and functional correlations of T cell receptor-gamma delta thymocyte maturation.

The development of TCR-gamma delta cells during thymic ontogeny has been studied using fetal thymic organ cultures of normal and transgenic (Tg) mice. The expression of the cell-surface markers--heat stable Ag (HSA), MEL-14, CD5, CD25 (IL-2R), and CD44 (Pgp-1)--correlated with TCR-gamma delta maturation. As the fetal thymus developed, there was an increase in HSA-, CD5dull, and CD44+ cells for each TCR-gamma delta cell subset. Moreover, the expression of recombination activating genes-1 and -2 (RAG-1 and RAG-2) also correlated with TCR-gamma delta maturation as only HSA+ TCR-gamma delta cells transcribed these genes. Cyclosporin A inhibited the development of the TCR-gamma delta thymocytes if it was introduced early during thymic ontogeny by arresting the differentiation of TCR-gamma delta thymocytes at the HSA+ stage. Immature HSA+ TCR-gamma delta thymocytes isolated from both TCR-gamma delta Tg and normal mice did not respond to nominal Ag or anti-TCR mAb unless exogenous IL-2 was added to the cultures. In contrast, HSA- TCR-gamma delta cells from Tg and normal mice responded to TCR/ligand interactions in the absence of additional IL-2. Finally, the development of functionally mature TCR-gamma delta cells could be induced in vitro. Interaction of the HSA+ Tg+ TCR-gamma delta cells with anti-TCR-gamma delta mAb or Ag-bearing thymic stromal cells resulted in RAG-1 and RAG-2 down-regulation. These data strongly suggest that TCR-gamma delta HSA+, RAG+ thymocytes differentiate into a more mature stage under the pressure of positive selection and that TCR-gamma delta cell development is regulated in a manner similar to TCR-alpha beta cells. In addition, the ability of Cyclosporin A to inhibit TCR-gamma delta cell development combined with the findings that Ag-bearing stromal cells can induce Tg TCR-gamma delta cell development suggests that maturation and selection of TCR-gamma delta cells depends on receptor-mediated physiologic stimuli delivered during thymic development.