Cell-cycle arrest prevents induction of P-lig in naive but not in memory cells. Expression of P-lig was measured on naive and memory/effector cells before and 3 days after activation by 0.1 μM OVA_323-339 peptide and cytokines (IL-12, IFN-γ, and anti-IL-4) in the presence or absence of l-mimosine. Naive (CD45RB^high) and memory T cells (CD45RB^low) were identified on day 3 after activation by combining CFSE staining and CD45RB expression (A). (B) The frequency of P-lig–positive cells within KJ1.26-1⁺ naive and memory/effector cells (mean + SD from 3 separate experiments).