Teresa Zelante

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.

IRF4 Transcription Factor-Dependent CD11b+ Dendritic Cells in Human and Mouse Control Mucosal IL-17 Cytokine Responses

Summary Mouse and human dendritic cells (DCs) are composed of functionally specialized subsets, but precise interspecies correlation is currently incomplete. Here, we showed that murine lung and gut lamina propria CD11b+ DC populations were comprised of two subsets: FLT3- and IRF4-dependent CD24+CD64− DCs and contaminating CSF-1R-dependent CD24−CD64+ macrophages. Functionally, loss of CD24+CD11b+ DCs abrogated CD4+ T cell-mediated interleukin-17 (IL-17) production in steady state and after Aspergillus fumigatus challenge. Human CD1c+ DCs, the equivalent of murine CD24+CD11b+ DCs, also expressed IRF4, secreted IL-23, and promoted T helper 17 cell responses. Our data revealed heterogeneity in the mouse CD11b+ DC compartment and identifed mucosal tissues IRF4-expressing DCs specialized in instructing IL-17 responses in both mouse and human. The demonstration of mouse and human DC subsets specialized in driving IL-17 responses highlights the conservation of key immune functions across species and will facilitate the translation of mouse in vivo findings to advance DC-based clinical therapies.

Aryl hydrocarbon receptor control of a disease tolerance defence pathway

Disease tolerance is the ability of the host to reduce the effect 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 of mice to LPS activated the ligand-operated transcription factor aryl hydrocarbon receptor (AhR) and the hepatic enzyme tryptophan 2,3-dioxygenase, 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 signalling 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.

IL-22 defines a novel immune pathway of antifungal resistance

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 hosts 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.

Immunity and Tolerance to Aspergillus Involve Functionally Distinct Regulatory T Cells and Tryptophan Catabolism

The inherent resistance to diseases caused by Aspergillus fumigatus suggests the occurrence of regulatory mechanisms that provide the host with adequate defense without necessarily eliminating the fungus or causing unacceptable levels of host damage. In this study, we show that a division of labor occurs between functionally distinct regulatory T cells (Treg) that are coordinately activated by a CD28/B-7-dependent costimulatory pathway after exposure of mice to Aspergillus conidia. Early in infection, inflammation is controlled by the expansion, activation and local recruitment of CD4+CD25+ Treg capable of suppressing neutrophils through the combined actions of IL-10 and CTLA-4 on indoleamine 2,3-dioxygenase. The levels of IFN-γ produced in this early phase set the subsequent adaptive stage by conditioning the indoleamine 2,3-dioxygenase-dependent tolerogenic program of dendritic cells and the subsequent activation and expansion of tolerogenic Treg, which produce IL-10 and TGF-β, inhibit Th2 cells, and prevent allergy to the fungus. The coordinate activation of Treg may, however, be subverted by the fungus, as germinating conidia are capable of interfering with anti-inflammatory and tolerogenic Treg programs. Thus, regulation is an essential component of the host response in infection and allergy to the fungus, and its manipulation may allow the pathogen to overcome host resistance and promote disease.

Dectin-1 Y238X polymorphism associates with susceptibility to invasive aspergillosis in hematopoietic transplantation through impairment of both recipient- and donor-dependent mechanisms of antifungal immunity

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-γ and interleukin-10 (IL-10), in addition to IL-1β, 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.

Functional yet Balanced Reactivity to Candida albicans Requires TRIF, MyD88, and IDO-Dependent Inhibition of Rorc

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.

Role of complement and Fcγ receptors in the protective activity of the long pentraxin PTX3 against Aspergillus fumigatus

Pentraxin 3 (PTX3) is a soluble pattern recognition molecule playing a nonredundant role in resistance against Aspergillus fumigatus. The present study was designed to investigate the molecular pathways involved in the opsonic activity of PTX3. The PTX3 N-terminal domain was responsible for conidia recognition, but the full-length molecule was necessary for opsonic activity. The PTX3-dependent pathway of enhanced neutrophil phagocytic activity involved complement activation via the alternative pathway; Fcγ receptor (FcγR) IIA/CD32 recognition of PTX3-sensitized conidia and complement receptor 3 (CR3) activation; and CR3 and CD32 localization to the phagocytic cup. Gene targeted mice (ptx3, FcR common γ chain, C3, C1q) validated the in vivo relevance of the pathway. In particular, the protective activity of exogenous PTX3 against A fumigatus was abolished in FcR common γ chain-deficient mice. Thus, the opsonic and antifungal activity of PTX3 is at the crossroad between complement, complement receptor 3-, and FcγR-mediated recognition. Because short pentraxins (eg, C-reactive protein) interact with complement and FcγR, the present results may have general significance for the mode of action of these components of the humoral arm of innate immunity.

A Crucial Role for Tryptophan Catabolism at the Host/Candida albicans Interface

By mediating tryptophan catabolism, the enzyme indoleamine 2,3-dioxygenase (IDO) has a complex role in immunoregulation in infection, pregnancy, autoimmunity, transplantation, and neoplasia. We hypothesized that IDO might affect the outcome of the infection in mice infected with Candida albicans by virtue of its potent regulatory effects on inflammatory and T cell responses. IDO expression was examined in mice challenged with the fungus along with the consequences of its blockade by in vivo treatment with an enzyme inhibitor. We found that IDO activity was induced at sites of infection as well as in dendritic cells and effector neutrophils via IFN-γ- and CTLA-4-dependent mechanisms. IDO inhibition greatly exacerbated infection and associated inflammatory pathology as a result of deregulated innate and adaptive/regulatory immune responses. However, a role for tryptophan catabolism was also demonstrated in a fungus-autonomous fashion; its blockade in vitro promoted yeast-to-hyphal transition. These results provide novel mechanistic insights into complex events that, occurring at the fungus/pathogen interface, relate to the dynamics of host adaptation to the fungus. The production of IFN-γ may be squarely placed at this interface, where IDO activation probably exerts a fine control over fungal morphology as well as inflammatory and adaptive antifungal responses.