Paolo Puccetti

Modulation of tryptophan catabolism by regulatory T cells

Regulatory T (TR) 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 TR 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, TR 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 TR cells.

CTLA-4-Ig regulates tryptophan catabolism in vivo.

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.

The Combined Effects of Tryptophan Starvation and Tryptophan Catabolites Down-Regulate T Cell Receptor ζ-Chain and Induce a Regulatory Phenotype in Naive T Cells

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.

Tolerance, DCs and tryptophan: much ado about IDO

Abstract Indoleamine 2,3-dioxygenase (IDO) degrades the indole moiety of tryptophan, serotonin and melatonin, and initiates the production of neuroactive and immunoregulatory metabolites, collectively known as kynurenines. By locally depleting tryptophan and increasing proapoptotic kynurenines, IDO expressed by dendritic cells (DCs) greatly affects T-cell proliferation and survival. IDO induction in DCs could be a common mechanism of deletional tolerance driven by regulatory T cells. Because such tolerogenic responses can be expected to operate in a variety of physiopathological conditions, tryptophan metabolism and kynurenine production might represent a crucial interface between the immune and nervous systems. This Opinion is the first in a series on the regulation of the immune system by metabolic pathways .

T cell apoptosis by tryptophan catabolism

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme that, expressed by different cell types, has regulatory effects on T cells resulting from tryptophan depletion in specific local tissue microenvironments. Different mechanisms, however, might contribute to IDO-dependent immune regulation. We show here that tryptophan metabolites in the kynurenine pathway, such as 3-hydroxyanthranilic and quinolinic acids, will induce the selective apoptosis in vitro of murine thymocytes and of Th1 but not Th2 cells. T cell apoptosis was observed at relatively low concentrations of kynurenines, did not require Fas/Fas ligand interactions, and was associated with the activation of caspase-8 and the release of cytochrome c from mitochondria. When administered in vivo, the two kynurenines caused depletion of specific thymocyte subsets in a fashion qualitatively similar to dexamethasone. These data suggest that the selective deletion of T lymphocytes may be a major mechanism whereby tryptophan metabolism affects immunity under physiopathologic conditions.

Natural killer cells. Characteristics and regulation of activity.

Recently there has been increasing recognition of natural cell-mediated cytotoxicity as a potentially important antitumor effector mechanism in addition to that of specifically immune T cells and of activated macrophages. Although natural cellular cytotoxicity was first recognized only a few years ago (Herberman et al. 1973,1974, McCoy et al. 1973b, Oldham et al. 1973, Rosenberg et al. 1972,1974), there has already been extensive research in many laboratories on the nature of the effector cells, the possible mechanisms of cytotoxicity, the factors regulating the levels of reactivity, and the relevance of natural immunity to in vivo resistance against tumor growth and immune surveillance. A principal component of natural cell-mediated cytotoxicity in rodents and man has been found to be a particular subpopulation of lymphocytes which have been termed natural killer (NK) cells. We have recently reviewed in detail much of the available information on NK cells (Herberman & Holden 1978). In this paper we will only summarize our views on the characteristics of NK cells and focus on a few issues of current interest in our laboratory.

IL-23 and the Th17 pathway promote inflammation and impair antifungal immune resistance.

Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases. We found here that the IL‐23/IL‐17 developmental pathway acted as a negative regulator of the Th1‐mediated immune resistance to fungi and played an inflammatory role previously attributed to uncontrolled Th1 cell responses. Both inflammation and infection were exacerbated by a heightened Th17 response against Candida albicans and Aspergillus fumigatus, two major human fungal pathogens. IL‐23 acted as a molecular connection between uncontrolled fungal growth and inflammation, being produced by dendritic cells in response to a high fungal burden and counter‐regulating IL‐12p70 production. Both IL‐23 and IL‐17 subverted the inflammatory program of neutrophils, which resulted in severe tissue inflammatory pathology associated with infection. Our data are the first demonstrating that the IL‐23/IL‐17 pathway promotes inflammation and susceptibility in an infectious disease model. As IL‐23‐driven inflammation promotes infection and impairs antifungal resistance, modulation of the inflammatory response represents a potential strategy to stimulate protective immune responses to fungi.

Defective tryptophan catabolism underlies inflammation in mouse chronic granulomatous disease.

Half a century ago, chronic granulomatous disease (CGD) was first described as a disease fatally affecting the ability of children to survive infections. Various milestone discoveries have since been made, from an insufficient ability of patients’ leucocytes to kill microbes to the underlying genetic abnormalities. In this inherited disorder, phagocytes lack NADPH oxidase activity and do not generate reactive oxygen species, most notably superoxide anion, causing recurrent bacterial and fungal infections. Patients with CGD also suffer from chronic inflammatory conditions, most prominently granuloma formation in hollow viscera. The precise mechanisms of the increased microbial pathogenicity have been unclear, and more so the reasons for the exaggerated inflammatory response. Here we show that a superoxide-dependent step in tryptophan metabolism along the kynurenine pathway is blocked in CGD mice with lethal pulmonary aspergillosis, leading to unrestrained Vγ1+ γδ T-cell reactivity, dominant production of interleukin (IL)-17, defective regulatory T-cell activity and acute inflammatory lung injury. Although beneficial effects are induced by IL-17 neutralization or γδ T-cell contraction, complete cure and reversal of the hyperinflammatory phenotype are achieved by replacement therapy with a natural kynurenine distal to the blockade in the pathway. Effective therapy, which includes co-administration of recombinant interferon-γ (IFN-γ), restores production of downstream immunoactive metabolites and enables the emergence of regulatory Vγ4+ γδ and Foxp3+ αβ T cells. Therefore, paradoxically, the lack of reactive oxygen species contributes to the hyperinflammatory phenotype associated with NADPH oxidase deficiencies, through a dysfunctional kynurenine pathway of tryptophan catabolism. Yet, this condition can be reverted by reactivating the pathway downstream of the superoxide-dependent step.

Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic 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 is the main inducer of IDO for the prevention of hyperinflammatory responses, yet IDO is also responsible for self-tolerance effects in the longer term. 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 found that IDO was involved in intracellular signaling events responsible for the self-amplification and maintenance of a stably regulatory phenotype in pDCs. Thus, IDO has a tonic, nonenzymic function that contributes to TGF-β-driven tolerance in noninflammatory contexts.

IL-23 and Th17 Cells Enhance Th2-Cell–mediated Eosinophilic Airway Inflammation in Mice

RATIONALE The IL-23-IL-17A-producing CD4(+) T-cell (Th17 cell) axis plays an important role in the development of chronic inflammatory diseases, including autoimmune diseases. However, the role of the IL-23-Th17 cell axis in the regulation of allergic airway inflammation is still largely unknown. OBJECTIVES To determine the role of IL-23 and Th17 cells in allergic airway inflammation. METHODS We examined the effect of anti-IL-23 antibody on antigen-induced airway inflammation. We also investigated the effect of enforced expression of IL-23 on allergic airway inflammation by generating lung-specific IL-23 transgenic mice. Moreover, we examined the effect of adoptive transfer of antigen-specific Th17 cells on allergic airway inflammation. MEASUREMENTS AND MAIN RESULTS IL-23 mRNA was expressed in the lung of sensitized mice upon antigen inhalation, and the neutralization of IL-23 decreased antigen-induced eosinophil recruitment and Th2 cytokine production in the airways. The enforced expression of IL-23 in the airways significantly enhanced antigen-induced eosinophil and neutrophil recruitment into the airways; Th2 cytokine, IL-17A, and tumor necrosis factor (TNF)-alpha production in the airways; goblet cell hyperplasia; and airway hyperresponsiveness. Moreover, IL-23-mediated enhancement of antigen-induced Th2 cytokine production and eosinophil recruitment in the airways was still observed in the mice lacking IL-17A. Furthermore, although adoptive transfer of antigen-specific Th17 cells alone induced neutrophil but not eosinophil recruitment into the airways upon antigen inhalation, cotransfer of Th17 cells with Th2 cells significantly enhanced antigen-induced Th2-cell-mediated eosinophil recruitment into the airways and airway hyperresponsiveness. CONCLUSIONS IL-23 and Th17 cells not only induce Th17-cell-mediated neutrophilic airway inflammation but also up-regulate Th2-cell-mediated eosinophilic airway inflammation.