Development of gamma delta T cells in the adult murine thymus.

The development of T cells belonging to the gamma delta lineage is not well understood. We have analyzed the cells in the adult murine thymus which express the gamma delta TcR on the surface in order to learn more about this process. Our data demonstrate a number of clear subpopulations of gamma delta expressing cells in the thymus based on the expression of Thy-1 and HSA (heat-stable antigen). Only one of these subpopulations, the one expressing both Thy-1 and HSA, contains dividing cells or has a significant rate of turnover. Together with the fact that emigrant gamma delta cells are HSA+Thy-1+, this suggests that this thymic subpopulation is the sole, or major, source of exported cells. However, the turnover of cells from this population is 5 x 10(4) - 10 x 10(4) cells per day, while previous estimates of the rate of export of gamma delta cells are in the order of 10(4) cells per day. Furthermore the V gamma profile of recent gamma delta+ emigrants differs from that of the thymic HSA+Thy-1+ cells. This raises the possibility that only a selected subpopulation of the thymic gamma delta+HSA+Thy-1+ population is exported, and that some gamma delta cells may die in situ in the thymus. The function of the other gamma delta thymic subpopulations, which are turning over very slowly or not at all, (i.e. the HSA-Thy-1- and HSA-Thy-1+ subpopulations) remains unclear.