The survival and turnover of mature and immature CD8 T cells.

The present investigation has examined the phenotype, survival and fate of immature and mature CD8 T cells. CD4- CD8+ 'single positive' thymocytes (a model for recent thymic emigrants) were Thy-1+ CD45RC- RT6- before transfer to normal euthymic recipients, but changed phenotype within 7-10 days to become Thy-1- CD45RC+ RT6(+)--the phenotype of mature resting CD8 T cells. Following transfer to athymic nude recipients CD8 T cells from thoracic duct lymph of allotype-marked rats increased 12-17-fold during the first 2 months. Proliferation occurred in the complete absence of CD4 T cells and the donor CD8 T cells persisted [at 15-18% of peripheral blood lymphocytes (PBL)] for the life of the recipients. When combined with equal numbers of CD4 T cells, however, CD8 T cells occupied only 3-4% of PBL; in these animals CD4 T cells plateaued at 15-16% of PBL. The results suggested that CD8 T cells competed poorly with rapidly dividing CD4 T cells for limited space in a recirculating pool in which total T-cell numbers are homeostatically regulated. Although able to proliferate and self-renew in athymic nude recipients, when transferred to normal euthymic animals donor-derived mature CD8 T cells declined in number with time; their half-life was estimated to be 34 days. Similar studies with purified CD4- CD8+ 'single positive' thymocytes gave a comparable half-life of 37 days. The results indicated that lifespan was not due to an ageing process among CD8 T cells, but was rather a reflection of cell turnover dependent on thymic output.