DCs in immune tolerance in steady-state conditions.

Dendritic cells (DCs) are antigen-presenting cells (APCs) characterized by a unique capacity to stimulate naïve T cells and initiate primary immune responses. Recent studies suggest that DCs are also involved in the induction of immunological tolerance in peripheral tissues under steady-state conditions by maintaining the homeostasis of self-reactive CD4(+)Foxp3(+)naturally occurring thymic-derived regulatory T cells (nT(regs)) and de novo generation of antigen-specific CD4(+)Foxp3(+)inducible regulatory T cells (iT(regs)). We demonstrate here the impact of CD11(+)DCs on the antigen-specific differentiation of CD4(+)Foxp3(+)iT(regs) from CD4(+)Foxp3(-)T cells under steady-state and inflammatory conditions. CD11c(+)DCs promoted the transforming growth factor (TGF)-β1-mediated conversion of CD4(+)Foxp3(-)T cells into CD4(+)Foxp3(+)iT(regs) in vitro, while stimulation of CD11c(+)DCs with CpG oligodeoxynucleotide (ODN) abrogated this conversion. Furthermore, antigen-specific generation of CD4(+)Foxp3(+)iT(regs) required the function of CD11(+)DCs under steady-state conditions, whereas such conversion was severely abolished under inflammatory conditions. Thus, these results suggest the crucial role of DCs in the antigen-specific de novo conversion of CD4(+)Foxp3(-)T cells into CD4(+)Foxp3(+)iT(regs) under steady-state conditions, thereby leading to the establishment of peripheral immune tolerance.