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.

SUMMARY Induction of trained immunity (innate immune memory) is mediated by activation of immune and metabolic pathways that result in epigenetic rewiring of cellular functional programs. Through network-level integration of transcriptomics and metabolomics data, we identify glycolysis, glutaminolysis, and the cholesterol synthesis pathway as indispensable for the induction of trained immunity by β-glucan in monocytes. Accumulation of fumarate, due to glutamine replenishment of the TCA cycle, integrates immune and metabolic circuits to induce monocyte epigenetic reprogramming by inhibiting KDM5 histone demethylases. Furthermore, fumarate itself induced an epigenetic program similar to β-glucan-induced trained immunity. In line with this, inhibition of glutaminolysis and cholesterol synthesis in mice reduced the induction of trained immunity by β-glucan. Identification of the metabolic pathways leading to induction of trained immunity contributes to our understanding of innate immune memory and opens new therapeutic avenues.

SummaryThe protective effects of the tuberculosis vaccine Bacillus Calmette-Guerin (BCG) on unrelated infections are thought to be mediated by long-term metabolic changes and chromatin remodeling through histone modifications in innate immune cells such as monocytes, a process termed trained immunity. Here, we show that BCG induction of trained immunity in monocytes is accompanied by a strong increase in glycolysis and, to a lesser extent, glutamine metabolism, both in an in-vitro model and after vaccination of mice and humans. Pharmacological and genetic modulation of rate-limiting glycolysis enzymes inhibits trained immunity, changes that are reflected by the effects on the histone marks (H3K4me3 and H3K9me3) underlying BCG-induced trained immunity. These data demonstrate that a shift of the glucose metabolism toward glycolysis is crucial for the induction of the histone modifications and functional changes underlying BCG-induced trained immunity. The identification of these pathways may be a first step toward vaccines that combine immunological and metabolic stimulation.

Genetic deficiency of PTX3 affects the antifungal capacity of neutrophils and may contribute to the risk of invasive aspergillosis in patients treated with HSCT. (Funded by the European Society of Clinical Microbiology and Infectious Diseases and others.).

The C-type lectin receptor Dectin-1 plays a pivotal role in antifungal immunity. In this study, the recently characterized human DECTIN1 Y238X early stop codon polymorphism leading to diminished Dectin-1 receptor activity was studied in relation to invasive aspergillosis susceptibility and severity in patients receiving hematopoietic stem cell transplantation. We found that the presence of the DECTIN1 Y238X polymorphism in either donors or recipients of hematopoietic stem cell transplantation increased susceptibility to aspergillosis, with the risk being highest when the polymorphism was present simultaneously in both donors and recipients (adjusted hazard ratio ‫؍‬ 3.9; P ‫؍‬ .005). Functionally, the Y238X polymorphism impaired the production of interferon-␥ and interleukin-10 (IL-10), in addition to IL-1␤, IL-6, and IL-17A, by human peripheral mononuclear cells and Dectin-1 on human epithelial cells contributed to fungal recognition. Mechanistically, studies on preclinical models of infection in intact or bone marrow-transplanted Dectin-1 knockout mice revealed that protection from infection requires a distinct, yet complementary, role of both donor and recipient Dectin-1. This study discloses Dectin-1 deficiency as a novel susceptibility factor for aspergillosis in high-risk patients and identifies a previously unsuspected role for Dectin-1 in antifungal immunity that is the ability to control both resistance and tolerance to the fungus contingent on hematopoietic/ nonhematopoietic compartmentalization. (Blood. 2010;116(24):5394-5402) IntroductionAspergillus spp are ubiquitous in nature, and the spectrum of diseases they cause is myriad, including saprophytic colonization of preexisting cavities (aspergilloma), allergic asthma, hypersensitivity pneumonitis, allergic bronchopulmonary aspergillosis occurring as a complication of bronchial asthma or cystic fibrosis, and disseminated disease associated with high mortality rates in patients with hematologic malignancies and recipients of solid organs and stem cell transplantations. 1 Immunocompetent and nonatopic subjects are relatively resistant to infections, and disease occurs in the setting of host damage. 2 The association of persistent inflammation with intractable infection is common in nonneutropenic patients after hematopoietic stem cell transplantation (HSCT) as well as in allergic fungal diseases. 2 The current understanding of the pathophysiology underlying Aspergillus infection and disease highlights a truly bipolar nature of the inflammatory process in infection. Early inflammation prevents or limits infection, but an uncontrolled response may eventually oppose disease eradication. This condition is crucially exemplified in mice with chronic granulomatous disease, in which an intrinsic, genetically determined failure to control inflammation to sterile fungal components determines the animals' inability to resolve an actual infection with A fumigatus. 3 A main implication of these findings is that, at least in specific clinical settings, it is an exag...

Our resistance to infection is critically dependent upon the ability of pattern recognition receptors to recognise microbial invasion and induce protective immune responses. One such family of receptors are the C-type lectins, which play central roles in antifungal immunity1. These receptors activate key effector mechanisms upon recognition of conserved fungal cell wall carbohydrates. However, several other immunologically active fungal ligands have been described, including melanin2,3, whose mechanisms of recognition remain largely undefined. Here we identify a C-type lectin receptor, Melanin sensing C-type Lectin receptor (MelLec), that plays an essential role in antifungal immunity through recognition of the naphthalene-diol unit of 1,8-dihydroxynaphthalene (DHN)-melanin. MelLec recognises melanin in conidial spores of Aspergillus fumigatus, as well as other DHN-melanised fungi and is ubiquitously expressed by CD31+ endothelial cells in mice. MelLec is also expressed by a sub-population of these cells in mice that co-express EpCAM and which were detected only in the lung and liver. In mouse models, MelLec was required for protection against disseminated infection with A. fumigatus. In humans, MelLec is also expressed by myeloid cells, and we identified a single nucleotide polymorphism of this receptor that negatively affected myeloid inflammatory responses and significantly increased susceptibility of stem-cell transplant recipients to disseminated Aspergillus infections. Thus MelLec is a receptor recognising an immunologically active component commonly found on fungi and plays an essential role in protective antifungal immunity in both mice and humans.

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...

Invasive fungal disease (IFD) shows distinct regional incidence patterns and epidemiological features depending on the geographic region. We conducted a prospective survey in eight centres in Brazil from May 2007 to July 2009. All haematopoietic cell transplant (HCT) recipients and patients with acute myeloid leukaemia (AML) or myelodysplasia (MDS) were followed from admission until 1 year (HCT) or end of consolidation therapy (AML/MDS). The 12-month cumulative incidence (CI) of proven or probable IFD was calculated, and curves were compared using the Grey test. Among 237 AML/MDS patients and 700 HCT recipients (378 allogeneic, 322 autologous), the 1-year CI of IFD in AML/MDS, allogeneic HCT and autologous HCT was 18.7%, 11.3% and 1.9% (p <0.001), respectively. Fusariosis (23 episodes), aspergillosis (20 episodes) and candidiasis (11 episodes) were the most frequent IFD. The 1-year CI of aspergillosis and fusariosis in AML/MDS, allogeneic HCT and autologous HCT were 13.4%, 2.3% and 0% (p <0.001), and 5.2%, 3.8% and 0.6% (p 0.01), respectively. The 6-week probability of survival was 53%, and was lower in cases of fusariosis (41%). We observed a high burden of IFD and a high incidence and mortality for fusariosis in this first multicentre epidemiological study of IFD in haematological patients in Brazil.