However, it is selleck products now recognized that DC are also important for the induction and maintenance of peripheral T cell tolerance . For
instance, mice in which both conventional and plasmacytoid DC subsets have been ablated develop severe, fatal autoimmunity . Notably, patients with the recently identified combined mononuclear cell deficiency DCML [DC, monocyte, B and natural killer (NK) lymphoid-deficient], virtually lacking DC in the blood and interstitial tissues, have a reduced number of Tregs, and a quarter of these patients develop autoimmune disorders . The dual function of DC in initiating immunity, on one hand, and maintaining T cell tolerance on the other hand, can be explained, in part, by the different maturation stages Omipalisib ic50 of DC [18, 19]. In the absence of danger signals provided by infection or inflammation (also referred to as ‘steady state’), DC are largely in an immature differentiation state. They can capture and present antigens to T cells, but in so doing will induce tolerance rather than immunity [20-22]. Maturation of DC can be induced by pathogen-associated molecular patterns (PAMP), e.g.
bacterial lipopolysaccharide (LPS) or viral double-stranded RNA . The process of DC maturation enhances their immunogenicity by up-regulation of major histocompatibility complex (MHC)–peptide complexes and T cell co-stimulatory molecules (e.g. CD80, CD86) on the plasma membrane, and by inducing the production of proinflammatory cytokines (e.g. IL-12) that help and polarize T cell differentiation [24, 25]. However, the notion that immature DC induce tolerance and mature
DC induce immunity has been revised in recent years, as it has become clear that mature DC can also exert pro-tolerogenic effects. For example, DC matured in response to certain PAMP display Bumetanide a typical mature DC surface phenotype but produce anti-inflammatory IL-10 and promote the development of IL-10-producing Tregs [26, 27]. It is now generally accepted that the tolerogenic function of DC is determined by the signals that they receive during maturation; these signals can be derived either from the microenvironment in which DC maturation takes place or from invading pathogens. For instance, anti-inflammatory cytokines [IL-10, transforming growth factor (TGF)-β], immunosuppressive substances (e.g. corticosteroids) or certain PAMP (e.g. schistosomal lysophosphatidylserine) have all been shown to promote the tolerogenic function of DC [27-31]. Several mechanisms by which tolDC induce immune peripheral tolerance have been described, including blocking of T cell clonal expansion and induction of T cell anergy, deletion of T cells and the induction of Tregs. Two major groups of Tregs have been defined: naturally occurring Tregs (nTregs) that arise in the thymus, and adaptive Tregs, that are induced in the periphery (iTregs) [32, 33].