Endogenous tryptophan (Trp) metabolites have an important role in mammalian gut immune homeostasis, yet the potential contribution of Trp metabolites from resident microbiota has never been addressed experimentally. Here, we describe a metabolic pathway whereby Trp metabolites from the microbiota balance mucosal reactivity in mice. Switching from sugar to Trp as an energy source (e.g., under conditions of unrestricted Trp availability), highly adaptive lactobacilli are expanded and produce an aryl hydrocarbon receptor (AhR) ligand-indole-3-aldehyde-that contributes to AhR-dependent Il22 transcription. The resulting IL-22-dependent balanced mucosal response allows for survival of mixed microbial communities yet provides colonization resistance to the fungus Candida albicans and mucosal protection from inflammation. Thus, the microbiota-AhR axis might represent an important strategy pursued by coevolutive commensalism for fine tuning host mucosal reactivity contingent on Trp catabolism.

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 Vgamma1(+) gammadelta 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 gammadelta 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-gamma (IFN-gamma), restores production of downstream immunoactive metabolites and enables the emergence of regulatory Vgamma4(+) gammadelta and Foxp3(+) alphabeta 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.

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.See accompanying commentary: http://dx.doi.org/10.1002/eji.200737804

Patients with chronic granulomatous disease (CGD) have a mutated NADPH complex resulting in defective production of reactive oxygen species; these patients can develop severe colitis and are highly susceptible to invasive fungal infection. In NADPH oxidasedeficient mice, autophagy is defective but inflammasome activation is present despite lack of reactive oxygen species production. However, whether these processes are mutually regulated in CGD and whether defective autophagy is clinically relevant in patients with CGD is unknown. Here, we demonstrate that macrophages from CGD mice and blood monocytes from CGD patients display minimal recruitment of microtubule-associated protein 1 light chain 3 (LC3) to phagosomes. This defect in autophagy results in increased IL-1β release. Blocking IL-1 with the receptor antagonist (anakinra) decreases neutrophil recruitment and T helper 17 responses and protects CGD mice from colitis and also from invasive aspergillosis. In addition to decreased inflammasome activation, anakinra restored autophagy in CGD mice in vivo, with increased Aspergillus-induced LC3 recruitment and increased expression of autophagy genes. Anakinra also increased Aspergillus-induced LC3 recruitment from 23% to 51% (P < 0.01) in vitro in monocytes from CGD patients. The clinical relevance of these findings was assessed by treating CGD patients who had severe colitis with IL-1 receptor blockade using anakinra. Anakinra treatment resulted in a rapid and sustained improvement in colitis. Thus, inflammation in CGD is due to IL-1-dependent mechanisms, such as decreased autophagy and increased inflammasome activation, which are linked pathological conditions in CGD that can be restored by IL-1 receptor blockade.interleukin-1 | S. aureus | LPS | Crohn disease | autoinflammatory disease C hronic granulomatous disease (CGD) is an immunodeficiency characterized by defective production of reactive oxygen species (ROS) (1) due to mutations in the proteins forming the NADPH complex (2, 3). The most frequent form of CGD is hereditary and X-linked, and is caused by a mutation in the gene CYBB, which encodes the protein gp91 phox , the catalytic subunit of the NADPH oxidase complex. In autosomal and recessive forms of CGD, the mutations affect the genes encoding p22 , which are all part of the NADPH complex, resulting in a defective NADPH oxidase complex. As a result, patients who have CGD have defective microbial killing by phagocytic cells and an increased susceptibility to infections, especially Staphylococcus aureus and Aspergillus spp. (4, 5). Paradoxically, ROS deficiency in patients with CGD results in a hyperinflammatory state (6), and one-third of the patients develop an inflammatory colitis indistinguishable from Crohn disease (7-10).The hyperinflammatory state in CGD is linked to inflammasome activation (11-13). Studies in mice and humans reveal that autophagy is crucial for IL-1β transcription (14) and processing of pro-IL-1β (15, 16); defects in autophagy result in increased secretion of IL-1β. ROS product...

The role of IL-17 and Th17 cells in immunity vs. pathology associated with the human commensal Candida albicans remains controversial. Both positive and negative effects on immune resistance have been attributed to IL-17/Th17 in experimental candidiasis. In this study, we provide evidence that IL-22, which is also produced by Th17 cells, has a critical, first-line defense in candidiasis by controlling the growth of infecting yeasts as well as by contributing to the host's epithelial integrity in the absence of acquired Th1-type immunity. The two pathways are reciprocally regulated, and IL-22 is upregulated under Th1 deficiency conditions and vice versa. Whereas both IL-17A and F are dispensable for antifungal resistance, IL-22 mediates protection in IL-17RA-deficient mice, in which IL-17A contributes to disease susceptibility. Thus, our findings suggest that protective immunity to candidiasis is made up of a staged response involving an early, IL-22-dominated response followed by Th1/Treg reactivity that will prevent fungal dissemination and supply memory.

The ability of regulatory T (Treg) cells to inhibit aspects of innate and adaptive immunity is central to their protective function in fungal infections. In murine candidiasis, CD4+CD25+ Treg cells prevent excessive inflammation but enable fungal persistence in the gastrointestinal tract, which underlies the onset of durable antifungal protection. In this study, we show that fungal growth, inflammatory immunity, and tolerance to the fungus were all controlled by the coordinate activation of naturally occurring Treg cells, which limited early inflammation at the sites of infection, and pathogen-induced Treg cells (that regulated the expression of adaptive Th immunity in secondary lymphoid organs). Naturally occurring Treg cells required the TRIF pathway for migration to inflamed sites, where the MyD88 pathway would then restrain their suppressive function. Subsequent inflammatory Th1-type immunity was modulated by induced Treg cells, which required the TRIF pathway as well, and acted through activation of IDO in dendritic cells and Th17 cell antagonism. In vitro, using naive CD4+ cells from TRIF-deficient mice, tryptophan metabolites were capable of inducing the Foxp3-encoding gene transcriptionally and suppressing the gene encoding RORγt, Th17 lineage specification factor. This is the first study to show that the same tryptophan catabolites can foster dendritic cell-supported generation of Foxp3+ cells and mediate, at the same time, inhibition of RORγt-expressing T cells.

We analyzed the contribution of intracellular signaling to the functional plasticity of dendritic cells (DCs) presenting Candida albicans, a human commensal associated with severe diseases. Distinct intracellular pathways were activated by recognition of different fungal morphotypes in distinct DC subsets and in Peyer's patches DCs. Inflammatory DCs initiated Th17/Th2 responses to yeasts through the adaptor myeloid differentiation factor-88 (MyD88), whereas tolerogenic DCs activate Th1/T regulatory cell (Treg) differentiation programs to hyphae involving Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF) as an intermediary of signaling. In addition, signal transducer and activator of transcription 3 (STAT3), affecting the balance between canonical and non-canonical activation of nuclear factor-kappaB (NF-kappaB) and 2,3 indoleamine dioxygenase (IDO), pivotally contributed to DC plasticity and functional specialization. As Candida-induced tolerogenic DCs ameliorated experimental colitis, our data qualify Candida as a commensal with immunoregulatory activity, resulting from the orchestrated usage of multiple, yet functionally distinct, receptor-signaling pathways in DCs. Ultimately, affecting the local Th17/Treg balance might likely be exploited by the fungus for either commensalism or pathogenicity.

Aspergillus fumigatus is a model fungal pathogen and a common cause of severe infections and diseases. CD8 ؉ T cells are present in the human and murine T-cell repertoire to the fungus. However, CD8 ؉ T-cell function in infection and the molecular mechanisms that control their priming and differentiation into effector and memory cells in vivo remain elusive. In the present study, we report that both CD4 ؉ and CD8 ؉ T cells mediate protective memory responses to the fungus contingent on the nature of the fungal vaccine. Mechanistically, class I MHCrestricted, CD8 ؉ memory T cells were activated through TLR3 sensing of fungal RNA by cross-presenting dendritic cells. Genetic deficiency of TLR3 was associated with susceptibility to aspergillosis and concomitant failure to activate memory-protective CD8 ؉ T cells both in mice and in patients receiving stem-cell transplantations. Therefore, TLR3 essentially promotes antifungal memory CD8 ؉ T-cell responses and its deficiency is a novel susceptibility factor for aspergillosis in high-risk patients. IntroductionTLR3 plays a key role in modulating inflammation and innate immunity in the airway. Although best known for recognition of viral double-stranded RNA (dsRNA) and its synthetic analog polyinosinic:polycytidylic acid [poly(I:C)], 1 TLR3 also recognizes endogenous ligands, 2 including heterologous RNA released from or associated with necrotic cells or generated by in vitro transcription. 3 Therefore, TLR3, together with other intracellular signaling proteins, 4 induces or otherwise modulates innate immune responses and inflammation in settings that are not associated with viral dsRNA. TLR3 signaling may also modulate adaptive immune responses by providing cross-priming of cytotoxic T lymphocytes through signaling in dendritic cells (DCs) 5,6 via type I IFNs 7 and in the absence of CD4 ϩ T-cell help. 8 Therefore, it is not surprising that people with mutations in key TLR3 signaling components have a selective immunodeficiency manifested by recurrent episodes of herpes simplex virus 1 encephalitis 9,10 or enteroviral myocarditis/cardiomyopathy. 11 Aspergillus fumigatus is a model fungal pathogen and a common cause of severe infections and diseases. Humans inhale hundreds of conidia every day without adverse consequences, 12 except for a minority of persons in whom defense systems fail and a life-threatening form of disease can develop. CD4 ϩ and CD8 ϩ T cells are present in the human T-cell repertoire to the fungus 13-15 and adoptive transfer of A fumigatus-specific CD4 ϩ T cells conferred protection against invasive fungal infection. 15,16 Recent studies indicate a role for TLR3 in murine aspergillosis. By functioning as an endogenous sensor of fungal RNA, 17 TLR3 mediates expression of the enzyme indoleamine 2,3-dioxygenase (IDO) on both epithelial cells 18 and DCs,19 contributing to the local regulation of innate and adaptive inflammation to the fungus. However, the findings that protective memory CD8 ϩ T cells are induced against fungi 20-23 suggest a possible rol...