Sustained B7/CD28 interactions and resultant phosphatidylinositol 3-kinase activity maintain G1-->S phase transitions at an optimal rate.

Twenty-four hours of TCR engagement and CD28 costimulation was found sufficient to elicit an optimal rate of cell division over a 72-h period only when a high concentration of IL-2 was produced in the culture and remained readily available to the CD4(+) T cells. The cell division response could be aborted following 24 h of stimulation by the simultaneous abrogation of IL-2R signaling and the blockade of CD28 or TCR ligands. Biochemical and pharmacologic studies indicated that a phosphatidylinositol 3-kinase-Akt signaling cascade costimulated by the TCR and CD28 maintained the blasting cell division rate at a maximal level beyond 24 h even when IL-2 was withdrawn, neutralized, or exhausted. These data show that CD4(+) T cells remain sensitive to antigens (Ag) and costimulatory signals throughout the clonal expansion response. Furthermore, only those T cells that perceive the presence of a continued threat in the form of Ag/MHC complexes and B7 costimulatory ligands or a high concentration of a growth factor are directed to remain in cell cycle.