Induction and maintenance of peripheral tolerance is an important phenomenon for the control of homeostasis in the immune system. There is now compelling evidence for CD4(+) T cells that prevent immune pathology, both in autoimmunity and in transplantation. However, the mechanisms involved in the specific differentiation of these T cells are unknown. We had previously shown that repetitive stimulations of naive T cells in the presence of IL-10 induce the differentiation of T regulatory cells 1. We further dissected the mechanism of IL-10 function and demonstrated that IL-10 acts by the down-regulation of most costimulatory molecules without modifying the expression of CD58. Using artificial APCs expressing various costimulatory molecules, we demonstrated that, in contrast to other costimulation patterns, costimulation via CD2 alone, in the absence of costimulations through CD28- or LFA-1, induced T cell anergy in an IL-10-independent pathway along with the differentiation of Ag-specific regulatory T cells. T regulatory cell-1 differentiation via CD2 was very efficient as both high IL-10 secretion and regulatory function were observed after the first stimulation of naive T cells with CD32-CD58 L cells. The possibility to rapidly induce the differentiation of Ag-specific regulatory T cells will certainly accelerate their characterization and their potential use as regulators of T cell-mediated diseases.