The induction of antigen-specific regulatory T cells. The following 3 strategies can be distinguished. (A) Antigen presentation by dendritic cells (DCs) especially dedicated to the induction of regulatory T cells (Trs; mucosal or cutaneous DCs or plasmacytoid DCs). The molecular mechanisms underlying the tolerogenic properties of these DCs remain to be elucidated. (B) Antigen presentation by antigen-presenting cells (APCs) with suboptimal costimulation, achieved through use of CD3, CD4, CD8, or CD40-ligand monoclonal blocking antibodies or immature APCs. (C) Antigen presentation by mature APCs with an additional regulatory signal provided by either cell-surface molecules such as LFA-3, ICOS-ligand, Notch-ligand, or soluble factors, such as IL10 with or without TGFβ, α-MSH (melanocyte-stimulating hormone), or high doses of antithrombin III.