Costimulation induces CD4 expression on CD8⁺ cells.

(A) Stimulation of purified CD8⁺ T lymphocytes. CD8⁺ T cells (> 99% purity) were left unstimulated or stimulated with anti-CD3 alone, anti-CD28 alone, PHA, or simultaneously with anti-CD3 and anti-CD28 for 3 days and analyzed for expression of CD4 (PE), CD8 (ECD), and CD25 (FITC). The percentage of CD8⁺ cells expressing CD4 is shown in the upper right-hand corner of each dot plot, and the percentage of total cells expressing CD25 is shown below each dot plot. (B) Costimulation of purified CD8⁺ T cells. Purified CD8⁺ T lymphocytes were costimulated with plate-bound anti-CD3 and increasing concentrations of anti-CD28 for 3 days and analyzed for expression of CD4 (PE), CD8 (ECD), and CD25 (FITC). The amount of soluble anti-CD28 added to each culture is shown above each dot plot, and the percentage of CD8⁺cells expressing CD4 is shown in the upper right-hand corner of each dot plot. The percentage of total cells expressing CD25 is shown below each dot plot.