Regulatory T (T(R)) cells manifest constitutive expression of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), but the function of CTLA-4 in mediating the regulatory function of T(R) cells is unclear. We show here that mouse CD4+CD25+ cells, either resting or induced to overexpress CTLA-4 by treatment with antibody to CD3, initiated tryptophan catabolism in dendritic cells through a CTLA-4-dependent mechanism. This process required B7 expression and cytokine production by the dendritic cells. In contrast, T(R) cells cultured in the presence of bacterial lipopolysaccharide induced tryptophan catabolism by dendritic cells in a CTLA-4-independent but cytokine-dependent way. Thus, regulation of immunosuppressive tryptophan catabolism in dendritic cells might represent a major mechanism of action of T(R) cells.

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) plays a critical role in peripheral tolerance. However, regulatory pathways initiated by the interactions of CTLA-4 with B7 counterligands expressed on antigen-presenting cells are not completely understood. We show here that long-term survival of pancreatic islet allografts induced by the soluble fusion protein CTLA-4-immunoglobulin (CTLA-4-Ig) is contingent upon effective tryptophan catabolism in the host. In vitro, we show that CTLA-4-Ig regulates cytokine-dependent tryptophan catabolism in B7-expressing dendritic cells. These data suggest that modulation of tryptophan catabolism is a means by which CTLA-4 functions in vivo and that CTLA-4 acts as a ligand for B7 receptor molecules that transduce intracellular signals.

Tryptophan catabolism is a tolerogenic effector system in regulatory T cell function, yet the general mechanisms whereby tryptophan catabolism affects T cell responses remain unclear. We provide evidence that the short-term, combined effects of tryptophan deprivation and tryptophan catabolites result in GCN2 kinase-dependent down-regulation of the TCR ζ-chain in murine CD8+ T cells. TCR ζ down-regulation can be demonstrated in vivo and is associated with an impaired cytotoxic effector function in vitro. The longer-term effects of tryptophan catabolism include the emergence of a regulatory phenotype in naive CD4+CD25− T cells via TGF-β induction of the forkhead transcription factor Foxp3. Such converted cells appear to be CD25+, CD69−, CD45RBlow, CD62L+, CTLA-4+, BTLAlow and GITR+, and are capable of effective control of diabetogenic T cells when transferred in vivo. Thus, both tryptophan starvation and tryptophan catabolites contribute to establishing a regulatory environment affecting CD8+ as well as CD4+ T cell function, and not only is tryptophan catabolism an effector mechanism of tolerance, but it also results in GCN2-dependent generation of autoimmune-preventive regulatory T cells.

Regulation of tryptophan metabolism by indoleamine 2,3-dioxygenase (IDO) in dendritic cells (DCs) is a highly versatile modulator of immunity. In inflammation, interferon γ (IFN-γ) is the primary IDO inducer to preventhyperinflammatory responses, yet the enzyme is also responsible for longer-term self-tolerance effects. Here we show that treatment of mouse plasmacytoid DCs (pDCs) with transforming growth factor β (TGF-β) conferred regulatory effects on IDO that were mechanistically separable from its enzymic activity. We reveal that IDO is involved in intracellular signaling events responsible for selfamplification and maintenance of a stably regulatory phenotype in pDCs. Thus IDO has a tonic, non-enzymic function that contributes to TGF-β-driven tolerance in non-inflammatory contexts. _____________________________________________________________________Immune regulation is a highly evolved biologic response capable of fine-tuning inflammation and innate immunity, but also of modulating adaptive immunity and establishing tolerance to self 1,2 . Amino acid catabolism is an ancestral survival strategy that can additionally control immune responses in mammals 3 . Several metabolic enzymes are known to possess a second function, which allows them to meet additional functional challenges and needs inside the cell 4 . Three distinct enzymes, namely tryptophan 2,3-dioxygenase (TDO; confined to the liver), IDO (also referred to as IDO1), and the IDO paralogue indoleamine 2,3-dioxygenase-2 (IDO2) catalyze the same rate-limiting step of tryptophan (Trp) catabolism along a common pathway, which leads to Trp starvation and the production of Trp metabolites collectively known as kynurenines [5][6][7] . However, IDO alone is recognized as an authentic regulator of immunity in a variety of physiopathologic conditions, including pregnancy, infection, allergy, autoimmunity, chronic inflammation, transplantation, and immuno-escaping tumoral mechanisms 8,9 .Normally expressed at low basal levels, IDO is rapidly induced by IFN-γ in DCs [8][9][10] . The IFN-γ-IDO axis is considered to be a phylogenetically conserved mechanism of restricting microbial growth and avoiding potentially harmful (hyper) inflammatory responses in the host 11 . However, its regulatory function in pregnancy and long-term prevention of immunopathology have been unclear [8][9][10] . Autocrine or paracrine signaling through transforming growth factor β (TGF-β) can also induce long-term, IDO-dependent effects 12 . The TGF-β-IDO axis was shown to mediate 3 durable regulatory functions, with a primary role in the generation and maintenance of regulatory T (T reg ) cells 13 .Functional flexibility and fostering of T reg responses are features typical of CD11c low B220 high plasmacytoid DCs (pDCs), which are capable of activating but also suppressing both inflammatory or innate responses and adaptive immunity [14][15][16][17] .Although different forms of immunosuppressive mechanisms are exploited by pDCs in distinct environmental conditions 15 , IDO is one of the...

Summary Disease tolerance is the ability of the host to reduce the impact of infection on host fitness. Analysis of disease tolerance pathways could provide new approaches for treating infections and other inflammatory diseases. Typically, an initial exposure to bacterial lipopolysaccharide (LPS) induces a state of refractoriness to further LPS challenge (“endotoxin tolerance”). We found that a first exposure to LPS activated the ligand-operated transcription factor aryl hydrocarbon receptor (AhR) and the hepatic enzyme tryptophan 2,3-dioxygenase 2, which provided an activating ligand to the former, to downregulate early inflammatory gene expression. However, on LPS rechallenge, AhR engaged in long-term regulation of systemic inflammation only in the presence of indoleamine 2,3-dioxygenase 1 (IDO1). AhR complex-associated Src kinase activity promoted IDO1 phosphorylation and signaling ability. The resulting endotoxin-tolerant state was found to protect mice against immunopathology in gram-negative and gram-positive infections, pointing to a role for AhR in contributing to host fitness.