Targeted removal of macrophage-secreted interleukin-1 receptor antagonist protects against lethal Candida albicans sepsis.

Invasive fungal infections are associated with high mortality rates, and the lack of efficient treatment options emphasizes an urgency to identify underlying disease mechanisms. We report that disseminated Candida albicans infection is facilitated by interleukin-1 receptor antagonist (IL-1Ra) secreted from macrophages in two temporally and spatially distinct waves. Splenic CD169+ macrophages release IL-1Ra into the bloodstream, impeding early neutrophil recruitment. IL-1Ra secreted by monocyte-derived tissue macrophages further impairs pathogen containment. Therapeutic IL-1Ra neutralization restored the functional competence of neutrophils, corrected maladapted hyper-inflammation, and eradicated the otherwise lethal infection. Conversely, augmentation of macrophage-secreted IL-1Ra by type I interferon severely aggravated disease mortality. Our study uncovers how a fundamental immunoregulatory mechanism mediates the high disease susceptibility to invasive candidiasis. Furthermore, interferon-stimulated IL-1Ra secretion may exacerbate fungal dissemination in human patients with secondary candidemia. Macrophage-secreted IL-1Ra should be considered as an additional biomarker and potential therapeutic target in severe systemic candidiasis.

Fatty acid-induced mitochondrial uncoupling elicits inflammasome-independent IL-1α and sterile vascular inflammation in atherosclerosis.

Chronic inflammation is a fundamental aspect of metabolic disorders such as obesity, diabetes and cardiovascular disease. Cholesterol crystals are metabolic signals that trigger sterile inflammation in atherosclerosis, presumably by activating inflammasomes for IL-1ß production. We found here that atherogenesis was mediated by IL-1α and we identified fatty acids as potent inducers of IL-1α-driven vascular inflammation. Fatty acids selectively stimulated the release of IL-1α but not of IL-1ß by uncoupling mitochondrial respiration. Fatty acid-induced mitochondrial uncoupling abrogated IL-1ß secretion, which deviated the cholesterol crystal-elicited response toward selective production of IL-1α. Our findings delineate a previously unknown pathway for vascular immunopathology that links the cellular response to metabolic stress with innate inflammation, and suggest that IL-1α, not IL-1ß, should be targeted in patients with cardiovascular disease.

Differential sensitivity of inflammatory macrophages and alternatively activated macrophages to ferroptosis.

Acumulation of oxidized membrane lipids ultimately results in ferroptotic cell death, which can be prevented by the selenoenzyme glutathione peroxidase 4 (Gpx4). In vivo conditions promoting ferroptosis and susceptible cell types are still poorly defined. In this study, we analyzed the conditional deletion of Gpx4 in mice specifically in the myeloid cell lineages. Surprisingly, development and maintenance of LysM+ macrophages and neutrophils, as well as CD11c+ monocyte-derived macrophages and dendritic cells were unaffected in the absence of Gpx4. Gpx4-deficient macrophages mounted an unaltered proinflammatory cytokine response including IL-1ß production following stimulation with TLR ligands and activation of several inflammasomes. Accordingly, Gpx4fl/fl LysM-cre mice were protected from bacterial and protozoan infections. Despite having the capacity to differentiate to alternatively activated macrophages (AAM), these cells lacking Gpx4 triggered ferroptosis both in vitro and in vivo following IL-4 overexpression and nematode infection. Exposure to nitric oxide restored viability of Gpx4-deficient AAM, while inhibition of iNOS in proinflammatory macrophages had no effect. These data together suggest that activation cues of tissue macrophages determine sensitivity to lipid peroxidation and ferroptotic cell death.

The regulation of inflammation by oxidized phospholipids.

During inflammation or under conditions of oxidative stress, the polyunsaturated fatty acid side chains of phospholipids in cellular membranes or lipoproteins can be oxidatively modified. This process generates a complex mixture of structurally diverse oxidized phospholipid (OxPL) species, each of which may exert distinct biological effects. The presence of OxPLs has been documented in acute and chronic microbial infections, metabolic disorders, and degenerative diseases. It is now well recognized that OxPLs actively influence biological processes and contribute to the induction and resolution of inflammation. While many pro- and anti-inflammatory effects have been documented for bulk OxPL preparations, we are only beginning to understand the exact molecular mechanisms and signaling events that mediate the individual proinflammatory or anti-inflammatory bioactivities of discrete isolated OxPL species. Here, we review the current knowledge on the regulation of inflammation by OxPLs and summarize recent studies that establish cyclopentenone-containing OxPLs as a category of potent anti-inflammatory lipid mediators.

Cyclopentenone-containing oxidized phospholipids and their isoprostanes as pro-resolving mediators of inflammation.

Inflammation represents a powerful innate immune response that defends tissue homeostasis. However, the appropriate termination of inflammatory processes is essential to prevent the development of chronic inflammatory disorders. The resolution of inflammation is actively induced by specialized pro-resolving lipid mediators, which include eicosanoids, resolvins, protectins and maresins. The responsible pro-resolution pathways have emerged as promising targets for anti-inflammatory therapies since they mitigate excessive inflammation without compromising the anti-microbial defenses of the host. We have recently shown that the lipid peroxidation of membrane phospholipids, which is associated with inflammatory conditions, generates oxidized phospholipid (OxPL) species with potent pro-resolving activities. These pro-resolving OxPLs contain a cyclopentenone as their common determinant, and are structurally and functionally related to endogenous pro-resolving prostaglandins. Here, we review the regulation of inflammatory responses by OxPLs with particular focus on the bioactivities and structural characteristics of cyclopentenone-OxPLs, and discuss the impact of the responsible signaling pathways on inflammatory diseases. This article is part of a Special Issue entitled: Lipid modification and lipid peroxidation products in innate immunity and inflammation edited by Christoph J. Binder.

IL-21 restricts virus-driven Treg cell expansion in chronic LCMV infection.

Foxp3+ regulatory T (Treg) cells are essential for the maintenance of immune homeostasis and tolerance. During viral infections, Treg cells can limit the immunopathology resulting from excessive inflammation, yet potentially inhibit effective antiviral T cell responses and promote virus persistence. We report here that the fast-replicating LCMV strain Docile triggers a massive expansion of the Treg population that directly correlates with the size of the virus inoculum and its tendency to establish a chronic, persistent infection. This Treg cell proliferation was greatly enhanced in IL-21R-/- mice and depletion of Treg cells partially rescued defective CD8+ T cell cytokine responses and improved viral clearance in some but not all organs. Notably, IL-21 inhibited Treg cell expansion in a cell intrinsic manner. Moreover, experimental augmentation of Treg cells driven by injection of IL-2/anti-IL-2 immune complexes drastically impaired the functionality of the antiviral T cell response and impeded virus clearance. As a consequence, mice became highly susceptible to chronic infection following exposure to low virus doses. These findings reveal virus-driven Treg cell proliferation as potential evasion strategy that facilitates T cell exhaustion and virus persistence. Furthermore, they suggest that besides its primary function as a direct survival signal for antiviral CD8+ T cells during chronic infections, IL-21 may also indirectly promote CD8+ T cell poly-functionality by restricting the suppressive activity of infection-induced Treg cells.

Discovery of a highly potent anti-inflammatory epoxyisoprostane-derived lactone.

Epoxyisoprostanes EI (1) and EC (2) are effective inhibitors of the secretion of proinflammatory cytokines IL-6 and IL-12. In detailed studies toward the investigation of the molecular mode of action of these structures, a highly potent lactone (3) derived from 1 was identified. The known isoprostanoids 1 and 2 are most likely precursors of 3, the product of facile intramolecular reaction between the epoxide with the carboxylic acid in 2.

Extralymphatic virus sanctuaries as a consequence of potent T-cell activation.

T helper cells can support the functions of CD8(+) T cells against persistently infecting viruses such as murine lymphocytic choriomeningitis virus (LCMV), cytomegalovirus, hepatitis C virus and HIV. These viruses often resist complete elimination and remain detectable at sanctuary sites, such as the kidneys and other extralymphatic organs. The mechanisms underlying this persistence are not well understood. Here we show that mice with potent virus-specific T-cell responses have reduced levels and delayed formation of neutralizing antibodies, and these mice fail to clear LCMV from extralymphatic epithelia. Transfer of virus-specific B cells but not virus-specific T cells augmented virus clearance from persistent sites. Virus elimination from the kidneys was associated with the formation of IgG deposits in the interstitial space, presumably from kidney-infiltrating B cells. CD8(+) T cells in the kidneys of mice that did not clear virus from this site were activated but showed evidence of exhaustion. Thus, we conclude that in this model of infection, site-specific virus persistence develops as a consequence of potent immune activation coupled with reductions in virus-specific neutralizing antibodies. Our results suggest that sanctuary-site formation depends both on organ anatomy and on the induction of different adaptive immune effector mechanisms. Boosting T-cell responses alone may not reduce virus persistence.

PPAR-γ regulates the effector function of human T helper 9 cells by promoting glycolysis.

T helper 9 (TH9) cells promote allergic tissue inflammation and express the type 2 cytokines, IL-9 and IL-13, as well as the transcription factor, PPAR-γ. However, the functional role of PPAR-γ in human TH9 cells remains unknown. Here, we demonstrate that PPAR-γ drives activation-induced glycolysis, which, in turn, promotes the expression of IL-9, but not IL-13, in an mTORC1-dependent manner. In vitro and ex vivo experiments show that the PPAR-γ-mTORC1-IL-9 pathway is active in TH9 cells in human skin inflammation. Additionally, we find dynamic regulation of tissue glucose levels in acute allergic skin inflammation, suggesting that in situ glucose availability is linked to distinct immunological functions in vivo. Furthermore, paracrine IL-9 induces expression of the lactate transporter, MCT1, in TH cells and promotes their aerobic glycolysis and proliferative capacity. Altogether, our findings uncover a hitherto unknown relationship between PPAR-γ-dependent glucose metabolism and pathogenic effector functions in human TH9 cells.

Nrf2 is essential for cholesterol crystal-induced inflammasome activation and exacerbation of atherosclerosis.

Oxidative stress and inflammation--two components of the natural host response to injury--constitute important etiologic factors in atherogenesis. The pro-inflammatory cytokine interleukin (IL)-1 significantly enhances atherosclerosis, however, the molecular mechanisms of IL-1 induction within the artery wall remain poorly understood. Here we have identified the oxidative stress-responsive transcription factor NF-E2-related 2 (Nrf2) as an essential positive regulator of inflammasome activation and IL-1-mediated vascular inflammation. We show that cholesterol crystals, which accumulate in atherosclerotic plaques, represent an endogenous danger signal that activates Nrf2 and the NLRP3 inflammasome. The resulting vigorous IL-1 response critically depended on expression of Nrf2, and Nrf2-deficient apolipoprotein E (Apoe)-/- mice were highly protected against diet-induced atherogenesis. Importantly, therapeutic neutralization of IL-1α and IL-1ß reduced atherosclerosis in Nrf2+/- Apoe-/- but not in Nrf2-/- Apoe-/- mice, suggesting that the pro-atherogenic effect of Nrf2-signaling was primarily mediated by its permissive role in IL-1 production. Our studies demonstrate a role for Nrf2 in inflammasome activation, and identify cholesterol crystals as disease-relevant triggers of the NLRP3 inflammasome and potent pro-atherogenic cytokine responses. These findings suggest a common pathway through which oxidative stress and metabolic danger signals converge and mutually perpetuate the chronic vascular inflammation that drives atherosclerosis.

Toll-like receptor engagement converts T-cell autoreactivity into overt autoimmune disease.

Autoimmune diabetes mellitus in humans is characterized by immunological destruction of pancreatic beta islet cells. We investigated the circumstances under which CD8(+) T cells specific for pancreatic beta-islet antigens induce disease in mice expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) as a transgene under the control of the rat insulin promoter. In contrast to infection with LCMV, immunization with LCMV-GP derived peptide did not induce autoimmune diabetes despite large numbers of autoreactive cytotoxic T cells. Only subsequent treatment with Toll-like receptor ligands elicited overt autoimmune disease. This difference was critically regulated by the peripheral target organ itself, which upregulated class I major histocompatibility complex (MHC) in response to systemic Toll-like receptor-triggered interferon-alpha production. These data identify the 'inflammatory status' of the target organ as a separate and limiting factor determining the development of autoimmune disease.

Lysosomal recycling terminates CD1d-mediated presentation of short and polyunsaturated variants of the NKT cell lipid antigen alphaGalCer.

Short or polyunsaturated lipid variants of the NKT cell antigen alpha-galactosylceramide (alphaGC) exhibit decreased potency and a Th2 bias in vivo despite conserved TCR contact residues and stable binding to CD1d at neutral and acidic pH. Using reagents to directly visualize lipids in their free or CD1d-bound form, we determined that, contrary to predictions, these lipids reached the lysosome better than alphaGC. However, in contrast with alphaGC, they loaded CD1d at the cell surface and underwent immediate pH-dependent dissociation upon recycling to the lysosome. In cell-free assays, ultrafast dissociation of preformed complexes could be induced at acidic pH only when free competitor lipids were added, suggesting active lipid displacement. These findings provide a common cell biological explanation for the decreased stimulatory properties of short and polyunsaturated alphaGC variants. They also suggest that direct lipid displacement is a potent mechanism underlying highly dynamic lipid exchange reactions in the lysosomal compartment that shape the repertoire of lipids associated with CD1d.

Serpinb1a Is Dispensable for the Development and Cytokine Response of Invariant Natural Killer T Cell Subsets.

Invariant natural killer T (iNKT) cells are innate-like T lymphocytes. They quickly respond to antigenic stimulation by producing copious amounts of cytokines and chemokines. iNKT precursors differentiate into three subsets iNKT1, iNKT2, and iNKT17 with specific cytokine production signatures. While key transcription factors drive subset differentiation, factors that regulate iNKT subset homeostasis remain incompletely defined. Transcriptomic analyses of thymic iNKT subsets indicate that Serpinb1a is one of the most specific transcripts for iNKT17 cells suggesting that iNKT cell maintenance and function may be regulated by Serpinb1a. Serpinb1a is a major survival factor in neutrophils and prevents cell death in a cell-autonomous manner. It also controls inflammation in models of bacterial and viral infection as well as in LPS-driven inflammation. Here, we examined the iNKT subsets in neutropenic Serpinb1a-/- mice as well as in Serpinb1a-/- mice with normal neutrophil counts due to transgenic re-expression of SERPINB1 in neutrophils. In steady state, we found no significant effect of Serpinb1a-deficiency on the proliferation and numbers of iNKT subsets in thymus, lymph nodes, lung, liver and spleen. Following systemic activation with α-galactosylceramide, the prototypic glycolipid agonist of iNKT cells, we observed similar serum levels of IFN-γ and IL-4 between genotypes. Moreover, splenic dendritic cells showed normal upregulation of maturation markers following iNKT cell activation with α-galactosylceramide. Finally, lung instillation of α-galactosylceramide induced a similar recruitment of neutrophils and production of iNKT-derived cytokines IL-17, IFN-γ, and IL-4 in wild-type and Serpinb1a-/- mice. Taken together, our results indicate that Serpinb1a, while dominantly expressed in iNKT17 cells, is not essential for iNKT cell homeostasis, subset differentiation and cytokine release.

TNIK signaling imprints CD8+ T cell memory formation early after priming.

Co-stimulatory signals, cytokines and transcription factors regulate the balance between effector and memory cell differentiation during T cell activation. Here, we analyse the role of the TRAF2-/NCK-interacting kinase (TNIK), a signaling molecule downstream of the tumor necrosis factor superfamily receptors such as CD27, in the regulation of CD8+ T cell fate during acute infection with lymphocytic choriomeningitis virus. Priming of CD8+ T cells induces a TNIK-dependent nuclear translocation of ß-catenin with consecutive Wnt pathway activation. TNIK-deficiency during T cell activation results in enhanced differentiation towards effector cells, glycolysis and apoptosis. TNIK signaling enriches for memory precursors by favouring symmetric over asymmetric cell division. This enlarges the pool of memory CD8+ T cells and increases their capacity to expand after re-infection in serial re-transplantation experiments. These findings reveal that TNIK is an important regulator of effector and memory T cell differentiation and induces a population of stem cell-like memory T cells.

PLCγ1 suppression promotes the adaptation of KRAS-mutant lung adenocarcinomas to hypoxia.

Mutant KRAS modulates the metabolic plasticity of cancer cells to confer a growth advantage during hypoxia, but the molecular underpinnings are largely unknown. Using a lipidomic screen, we found that PLCγ1 is suppressed during hypoxia in KRAS-mutant human lung adenocarcinoma cancer cell lines. Suppression of PLCγ1 in hypoxia promotes a less oxidative cancer cell metabolism state, reduces the formation of mitochondrial reactive oxygen species and switches tumour bioenergetics towards glycolysis by impairing Ca2+ entry into the mitochondria. This event prevents lipid peroxidation, antagonizes apoptosis and increases cancer cell proliferation. Accordingly, loss of function of Plcg1 in a mouse model of KrasG12D-driven lung adenocarcinoma increased the expression of glycolytic genes, boosted tumour growth and reduced survival. In patients with KRAS-mutant lung adenocarcinomas, low PLCγ1 expression correlates with increased expression of hypoxia markers and predicts poor patient survival. Thus, our work reveals a mechanism of cancer cell adaptation to hypoxia with potential therapeutic value.

Maternal immunization programs postnatal immune responses and reduces atherosclerosis in offspring.

Maternal hypercholesterolemia during pregnancy increases offspring susceptibility to atherosclerosis by an oxidation-dependent mechanism. The present studies investigated whether maternal immunization with oxidized LDL (OxLDL) before pregnancy protects the fetus from atherogenic in utero programming by maternal hypercholesterolemia. Maternal immunization of NZW rabbits and LDL receptor-deficient mice indeed reduced atherosclerosis in adult offspring by up to 56%, but the protective effect could not be attributed to a reduction of fetal exposure to hypercholesterolemia alone, and even nonspecific immune stimulation with adjuvant only provided some protection. Unexpectedly, offspring of immunized mothers developed increased IgM antibodies to selective OxLDL epitopes and increased IgM-LDL immune complexes, compared with offspring of nonimmunized controls. Even naïve offspring of OxLDL-immunized mothers never exposed to postnatal hypercholesterolemia responded to a one-time OxLDL and KLH challenge with greater OxLDL-specific IgM responses, increased OxLDL-specific IgM-secreting B cells, and more IgM-LDL immune complexes. In contrast, maternal immunization with KLH, a T cell-dependent nonmammalian antigen, did not influence postnatal immune responses. Effects of maternal OxLDL-immunization on offspring B cells and selective antibodies were independent of transplacental passage of maternal immunoglobulins. Results show that maternal immunization with antigens prevalent in atherosclerotic lesions reduces atherogenesis in their offspring by mechanisms that include, but are not limited to, reduced fetal exposure to maternal hypercholesterolemia and lipid peroxidation. More importantly, they demonstrate in principle that maternal adaptive immunity to selective antigens influences postnatal B cell and antibody responses in offspring, and that modulation of in utero immune programming may influence immune-modulated diseases later in life.

Synthesis of diglycosylceramides and evaluation of their iNKT cell stimulatory properties.

Stimulation of iNKT cells is highly dependent on the structures of the glycolipids presented by CD1d. Furthermore, antigen processing and CD1d loading in lysosomes play central roles in controlling the stimulatory properties of glycolipid antigens. Previously, we determined that substitution at C6'' on alpha-galactosylceramides did not significantly impact stimulatory properties; however, it was not known if substitution at this position influenced lysosomal processing of oligoglycosylceramides. We have prepared a series of mono- and di-galactosylceramides to observe the impact of C6'' substitution on glycosidase truncation of these glycolipids. We found that substitution did not significantly impact glycosidase activity or loading into CD1d.

Corrugoside, a new immunostimulatory alpha-galactoglycosphingolipid from the marine sponge Axinella corrugata.

Corrugoside (1a), a new immunostimulatory triglycosilated alpha-galactoglycosphingolipid, was isolated from the marine sponge Axinella corrugata, and its structure determined by spectral analysis and chemical degradation. Compound 1a activated murine NKT cells in vitro, with a potency of about 2 logs lower than that of alphaGalCer. Four stereoisomeric glycosphingolipids (2a-2d) were also obtained, beta-glucosylceramides bearing unusual endoperoxide and allylic hydroperoxide functionalities on the sphinganine chain. They were shown to be photooxidation artifacts of the known glycosphingolipids 3, also present in the sponge. A possible role of compound 3 as a singlet oxygen scavenger to protect the organism from oxidative damage is proposed.

Developmental programming: maternal hypercholesterolemia and immunity influence susceptibility to atherosclerosis.

It is increasingly recognized that the in utero environment is an important determinant of adult disease, and extensive epidemiological evidence links dysmetabolic conditions during pregnancy with increased hypertension, cardiovascular disease, and diabetes later in life. The original Barker Hypothesis focused on low birth weight as the primary indicator of postnatal risk, but low birth weight may arise from other, non-metabolic conditions. This has impeded the identification of developmental programming mechanisms. More recently, the focus has shifted to the impact of specific maternal risk factors, such as obesity, metabolic syndrome, and diabetes, on cardiovascular risk in offspring. Inflammation plays a central role in these maternal conditions as well as in offspring atherogenesis, and two key factors that influence inflammation, maternal hypercholesterolemia and maternal immune mechanisms, have been shown to affect the developmental programming of atherosclerosis. Maternal hypercholesterolemia in pregnancy, even if only temporary, is associated with increased fatty streak formation in human fetal arteries and accelerated progression of atherosclerosis in normocholesterolemic children. Conversely, immunization of experimental animals with oxidized low-density lipoprotein cholesterol, an antigen prevalent in atherosclerotic lesions, inhibits the progression of atherosclerosis in the offspring of hypercholesterolemic mothers. These findings indicate it is possible, in principle, to program postnatal immune responses and to reduce atherosclerosis, and potentially other immunomodulated diseases, by targeted maternal immunomodulation.