Duration of calcineurin and Erk signals regulates CD4/CD8 lineage commitment of thymocytes

CD4/CD8 lineage commitment of thymocytes is controlled by the T cell receptor-mediated signals and is mimicked in vitro by a long-pulse stimulation of isolated CD4(+)CD8(+) thymocytes with proper combinations of phorbol myristate acetate and the calcium ionophore ionomycin. CD4 lineage commitment required higher intracellular Ca2+ levels than CD8 lineage commitment in this culture system. The calcineurin inhibitor FK506 at 1 nM inhibited the development of thymocytes to either lineage, but 0.3 nM FK506 significantly switched the development from the CD4 cell fate to the CD8 cell fate. The switch in lineage commitment was also observed when 1 nM FK506 was added 8 h after the start of the culture. Delayed addition of 20 muM U0126, an Mek (Erk kinase) inhibitor, also induced the switch. These results suggest that the intensity of calcineurin activity and the duration of both calcineurin and Erk pathway activation are crucial for thymocyte lineage commitment. (C) 2002 Elsevier Science (USA). All rights reserved.