LFA-1 binding to ICAM-I provides a costimulatory signal for CD8+ T cell activation that results in increased IL-2 mRNA levels and protein production to support proliferation. CD28 binding to its B7 ligands has the same effect, and the two costimulatory receptors activate some of the game intracellular signaling events, including up-regulation of phosphatidylinositol (PI) 3-kinase activity. However, costimulation by LFA-1 depends upon the activity of this enzyme, whereas costimulation by CD28 does not, as evidenced by differential effects of specific inhibitors of PI 3-kinase: When cells are costimulated with ICAM-1 in the presence of the inhibitors wortmannin or LY294002, proliferation is blocked, but increases in IL-2 mRNA levels and protein production are not. Costimulation also results in increased surface expression of CD25, which is essential for formation of an active IL.2R. This is blocked by the PI 3-kinase inhibitors when costimulation is via LFA-1 but not when it is via CD28. Finally, IL-2-driven proliferation is not blocked by the inhibitors once CD25 surface expression has increased. Thus, the PI 3-kinase-dependent step in CD8 T cell costimulation by LFA-1 is up-regulation of IL-2R expression. In contrast, CD28 engagement also increases IL-2R surface expression, but the up-regulation does not depend upon PI 3-kinase activity.