These PRRs can detect a broad range of molecular patterns that are associated with either infection (pathogen-associated molecular pattern; PAMPs) [2, 3] or cell death and trauma (damage-associated molecular patterns; DAMPs) [2, 4]. Following activation through PRRs, DCs undergo a maturation process that is characterized by upregulation of MHC class II and costimulatory molecules on their cell surface, proinflammatory

cytokine production, and DC migration to draining lymph nodes. In the lymph nodes, mature DCs function as the prototype of professional APCs to prime naïve T cells and control T-cell activation [5]. In addition to detecting pathogens or tissue damage directly through PRRs, DCs can be indirectly activated by factors that signal the presence of pathogens. For example, type I interferons (IFNs), which are produced rapidly in the course of viral and bacterial infections, AZD2281 in vivo have been reported to enhance the Ag-presentation R788 cell line efficiency of DCs, as well as DC migration to lymphoid tissues [6]. Moreover, type I IFN receptor signaling in DCs has been found to be essential for

T-cell priming in response to various PAMPs [7], as well as for the induction of virus-specific [8] and tumor-specific T-cell responses [9]. Notably, the interaction of DCs with CD4+ T cells provides additional important stimuli for DC maturation [10]. For example, ligation of CD40 on DCs by CD154 on T cells promotes DC activation, leading to priming of cytotoxic T lymphocytes (CTLs) [11] and CD4+ T-cell differentiation. Over the past decade, it has become clear that, in addition to their role in priming effector T-cell responses against invading pathogens, DCs have a crucial role in self-tolerance. These opposing DC functions are controlled through the regulation of DC maturation in the steady state, and this checkpoint is crucial for the maintenance

of immune homeostasis. In this article, Cell press we review the signals that can induce DC maturation in the steady state and discuss the suppressive mechanisms that counterbalance DC-activating signals to preserve peripheral tolerance. The contribution of steady-state DCs to the maintenance of peripheral tolerance was first shown in animal models, in which Ag could be targeted to immature DCs. Immature steady-state DCs had previously been notoriously difficult to study, as their isolation and manipulation rapidly induce DC maturation [12, 13]. To overcome this problem, the group of Ralph Steinman used mAbs against DC surface receptors to target Ags to DCs in vivo. Antigen delivery to steady-state DCs in the absence of inflammatory signals resulted in a transient activation and proliferation of Ag-specific CD4+ and CD8+ T cells, which was followed by deletion of these T cells and the establishment of Ag-specific T-cell tolerance [14, 15].