The kinetics of immature murine thymocyte development in vivo

The dynamics of cell generation and turnover in the young adult murine thymus has been studied by in vivo administration of [6-3H]deoxythymidine, isolation of thymocyte subpopulations by negative depletion and cell sorting procedures, and assessment of dividing cells and their products by autoradiography. The flow of label through subpopulations of CD4-CD8- thymocytes defined by the markers heat stable antigen (HSA), phagocyte glycoprotein 1 (Pgp-1), interleukin 2 receptor (p55) (IL-2R), and CD3 was determined, to check the developmental sequence deduced from intrathymic transfer and molecular approaches. In addition, the flow of label 'downstream' into the CD4+CD8+ cortical populations was followed to check if cells expressing CD8 alone were obligatory intermediates. The main findings were: (i) support for the following sequence within the CD4-CD8- group: HSA++Pgp-1+IL-2R(-)----HSA++Pgp-1-IL-2R(+)----HSA++Pgp-1-IL- 2R-; (ii) the majority of cell generation and cell turnover within the CD4-CD8- population was due to the HSA++IL-2R-Pgp-1- subpopulation; (iii) the rate of cell output from the proposed intermediate CD3-CD4-CD8+ subpopulation was equivalent to only 55% of the cell output from its proposed precursor, the most mature CD4-CD8- subpopulation, suggesting that many double negatives differentiate directly (or via CD3-CD4+CD8- intermediates) into double positives; and (iv) the CD4-CD8-HSA- (and CD3+) thymic subpopulation contained very few cycling cells and turned over extremely slowly, indicating that these slowly accumulating product cells are off the mainstream of T cell development.