Also, the strong homeostatic proliferation that rapidly replenishes the Treg-cell compartment after depletion

of FOXP3+ cells was found to depend on the presence of DCs, in addition to interleukin-2 (IL-2) signaling [25]. Further to their role in Treg-cell homeostasis, steady-state DCs can induce the de novo differentiation of naïve CD4+ T cells into Treg cells in the periphery. These peripherally induced Treg (pTreg) cells [26] are thought to have a nonredundant role in maintaining T-cell tolerance, particularly at environmental interfaces such as skin and mucosal tissues [27]. The induction of pTreg cells by DCs, in vivo as well as in vitro, requires the presence of transforming growth factor β (TGF-β) [28], is greatly enhanced by the vitamin Ponatinib clinical trial A metabolite retinoic acid [29], and is inhibited by the proinflammatory complement fragments C3a and C5a [30]. The capacity to induce pTreg cells seems to be restricted to certain DC subtypes that can produce retinoic acid and reside in peripheral tissues,

such as mucosal CD103+ DCs [29], dermal CD207+DCs [31], and are thus migratory but not lymph node resident DCs [32, 33]. The immature phenotype of steady-state DCs is a prerequisite for tolerance induction via T-cell-intrinsic mechanisms. Upon activation, DCs lose the capacity to delete or anergize autoreactive naïve T cells [14-16]. Similarly, the induction of dominant peripheral tolerance depends Hedgehog antagonist on the DC activation state, although some DC-activating

stimuli might still allow for the DC-dependent induction of pTreg cells. For example, when activated with the TLR3 ligand poly-IC, DCs lose the ability to induce pTreg cells in vitro [34], and DC activation through CD40 ligation prevents pTreg-cell induction by cognate Ag-presenting DCs in vivo [28]. By contrast, DC activation via certain PRRs such as TLR2 has been shown to induce retinoic acid production in DCs, subsequently leading to DC-dependent Exoribonuclease pTreg cell differentiation [35]. However, in general, an immature DC state is essential for induction and maintenance of peripheral tolerance. Facilitated by the development of DC-specific gene targeting, several DC-intrinsic mechanisms have been found to maintain the immature and tolerogenic phenotype of steady-state DCs by downmodulating the signaling pathways that are induced by proinflammatory stimuli. DC-specific deletion of the ubiquitin-editing enzyme A20, which negatively regulates nuclear factor-κβ (NF-κB) signaling, resulted in spontaneous DC activation, expansion of the activated T-cell population and multiorgan autoimmune disease [36]. Mice overexpressing a short splice variant of the ubiquitin-editing enzyme CYLD, which also downregulates the NF-κB pathway, have impaired peripheral tolerance induction, and DCs from these mice display an activated phenotype [37].