Figure 9.

T cells that have entered the tryptophan-sensitive arrested state retain their position in mid-G1. (A) T cells were activated in tryptophan-free medium using immobilized anti-CD3/CD28 (•). After a period of preactivation (24–72 h with similar results; 48 h in the experiments shown), tryptophan was added and the time to entry into S phase determined (defined as the initiation of thymidine incorporation). Replicate aliquots of cells were activated in tryptophan-containing medium without the 48-h preincubation period (▪). Lag time in each case was defined as the time to initiation of S phase from the point at which cells saw both tryptophan and anti-CD3. The arrow shows that the lag time to S phase was shortened by 12–16 h due to preactivation in the absence of tryptophan, suggesting that this portion of G1 had been accomplished before the point at which cells arrested. Representative of  seven experiments at 36, 48, and 72 h, all showing the same lag time to S phase. (B) T cells were activated with anti-CD3/CD28 in the presence (▪) or absence (•) of tryptophan. After 14 h (the time of the putative arrest point estimated from A), tryptophan was added to the tryptophan-deficient cultures and entry into S phase determined. T cells rescued at hour 14 showed no delay compared with controls.