CD4+CD25+ regulatory T cells (Tregs) are essential negative regulators of immune 
responses. Here, we examined the signaling properties of human Tregs, using 
CD4+CD25+ Treg and CD4+CD25- control (Tcont) cell lines generated from cord 
blood. Treg cell lines were markedly hyporesponsive to stimulation with 
dendritic cells and with anti-CD3/CD28-coated beads. Hyporesponsiveness was 
reversed by exogenous interleukin-2 (IL-2). T-cell receptor 
(TCR)-CD3/CD28-mediated activation of Rap1 and Akt was retained in Tregs, but 
activation of Ras, mitogenactivated protein kinase 1/2 (MEK1/2), and 
extracellular signal-regulated kinase 1/2 (Erk1/2) was impaired. Tregs were 
blocked from cell cycle progression due to decrease of cyclin E and cyclin A and 
increase of p27kip1 (p27kip cyclin dependent kinase inhibitor). IL-2 induced 
sustained increase of cyclin E and cyclin A and prevented up-regulation of 
p27kip1. Tregs had high susceptibility to apoptosis that was reversed by IL-2, 
which correlated with activation of Erk1/2, up-regulation of Bcl-x(L) (B-cell 
CLL/lymphoma 2-like nuclear gene encoding mitochondrial protein, transcript 
variant 2), and phosphorylation of Bad (Bcl2 antagonist of cell death) at 
Ser112. Thus, Tregs share biochemical characteristics of anergy, including 
abortive activation of Ras-MEK-Erk, increased activation of Rap1, and increased 
expression of p27kip1. In addition, our results indicate that 
TCR-CD3/CD28-mediated and IL-2 receptor-mediated signals converge at the level 
of MEK-Erk kinases to regulate Treg survival and expansion and suggest that 
manipulation of the MEK-Erk axis may represent a novel strategy for Treg 
expansion for immunotherapy.