Activation requirements of CD8⁺ CD45RA⁺ revertant T cells. CD8⁺ T-cell subsets were sorted using a MoFlo cell sorter on the basis of CD27 and CD45RA expression. (A) All CD8⁺ T-cell subsets were activated with a concentration range of anti-CD3 and irradiated autologous APCs. Proliferation was determined by tritiated thymidine incorporation 72 hours later (left). The cytotoxic potential of the same T-cell subsets was determined by assessing the relative expression of surface CD107 (right). ♦ indicates CD27⁺CD45RA⁺; ▪, CD27⁺CD45RA^–; ▴, CD27^–CD45RA^–; and ○, CD27^–CD45RA⁺. (B) T-cell subsets were also examined for INF-γ expression 18 hours after exposure to a concentration range of anti-CD3 plus autologous APCs. Finally, freshly isolated CD27^–CD45RA⁺ revertants (C, top) were stained for CD27 and CD28 expression (C, bottom). These results are representative of 3 separate experiments. Numbers refer to the percentage of cells in each quadrant.