Cell cycle progression in response to TCR–CD3/CD28 is blocked in Treg cell lines and can be rescued by exogenous IL-2. Treg and Tcont cell lines were stimulated with anti-CD3/CD28–coated dynal beads for the indicated time intervals; cell lysates were prepared and analyzed by (A-B) SDS-PAGE and immunoblot with indicated antibodies. (C) The same cell lines were stimulated with anti-CD3/CD28–coated dynal beads for 72 hours in total (indicated as D3) or for 56 hours, after which IL-2 (50 U/mL) was added and the culture was continued until 72 hours in total (indicated as D3/IL-2). Cell lysates were analyzed by SDS-PAGE and immunoblot with indicated antibodies. Results are representative of 3 independent experiments. zap70 indicates zeta chain associated protein kinase.