Jurkat T cells E6.1 were purchased from ATCC. CD3 + human T lymphocytes from healthy donors (Etablissement Francais du Sang) were isolated with the MidiMacs system (Miltenyi Biotec). Cells were activated with a mixture of soluble anti-CD3e (HIT3a, BD Biosciences) and anti-CD28 (BD Biosciences), or with 20-40 ng.ml 21 phorbol 12-myristate 13-acetate (PMA, Sigma) and 300 ng.ml 21 ionomycin (Calbiochem). Carboxyfluorescein Succinimydyl Ester (CFSE) and Brefeldin A were purchased from Sigma, and the calcium-sensitive dye Fluo-4 was from Invitrogen.