To investigate the role of TCR signaling in the exit of CD4+ T cells from cell cycle, we took advantage of a low frequency TEa T cell adoptive transfer technique as well as the Y-Ae mAb to interrupt Ag/MHC recognition before the completion of clonal expansion. Termination of TCR signaling after 36 h of Ag exposure caused an immediate reduction in cell size and deceleration of G1—>SG2M phase cell cycle progression. As a consequence, clonal expansion in the absence of durable TCR signaling decreased by two-thirds. Thus, CD4+ T cells scan for the presence Ag throughout their clonal expansion response, and continuously adjust their rate of cell growth and G1—>S phase transition to match their intensity of TCR signaling.

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-kinaseAkt 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by the TCR and CD28 maintained the blasting cell division rate at a maximal level beyond 24 h even when IL-2 was withdrawn, neutralized, or exhausted.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.