Engagement of the Aryl Hydrocarbon Receptor in Mycobacterium tuberculosis-Infected Macrophages Has Pleiotropic Effects on Innate Immune Signaling

Mechanisms underlying aryl hydrocarbon receptor-driven divergent macrophage function

Macrophages play an essential role in the innate immune system by differentiating into functionally diverse subsets in order to fight infection, repair damaged tissues, and regulate inappropriate immune responses. This functional diversity stems from their ability to adapt and respond to signals in the environment, which is in part mediated through aryl hydrocarbon receptor (AHR)-signaling. AHR, an environmental sensor, can be activated by various ligands, ranging from environmental contaminants to microbially derived tryptophan metabolites. This review discusses what is currently known about how AHR-signaling influences macrophage differentiation, polarization, and function. By discussing studies that are both consistent and divergent, our goal is to highlight the need for future research on the mechanisms by which AHR acts as an immunological switch in macrophages. Ultimately, understanding the contexts in which AHR-signaling promotes and/or inhibits differentiation, proinflammatory functions, and immunoregulatory functions, will help uncover functional predictions of immunotoxicity following exposure to environmental chemicals as well as better design AHR-targeted immunotherapies.

Computer-aided discovery of novel aryl hydrocarbon receptor ligands to regulate CYP1A1 expression in inflammatory macrophages

The environmental factor aryl hydrocarbon receptor (AhR), a key protein connecting the external environmental signals (e.g., environmental endocrine disruptor TCDD) to internal cellular processes, is involved in the activation of peripheral macrophages and inflammatory response in human body. Thus, there is widespread interest in finding compounds to anti-inflammatory response in macrophages by targeting human AhR. Here, ensemble docking based-virtual screening was first used to screen a library (~200,000 compounds) against human AhR ligand binding domain (LBD) and 25 compounds were identified as potential inhibitors. Then, 9 out of the 25 ligands were found to down-regulate the mRNA expression of CYP1A1 (a downstream gene of AhR signaling) in AhR overexpressing macrophages. The most potent compound AE-411/41415610 was selected for further study and found to reduce both mRNA and protein expressions level of CYP1A1 in mouse peritoneal macrophage. Moreover, protein chip signal pathway analysis indicated that AE-411/41415610 play a role in regulating JAK-STAT and AKT-mTOR pathways. In sum, the discovered hits with novel scaffolds provided a starting point for future design of more effective AhR-targeted lead compounds to regulate CYP1A1 expression of inflammatory peritoneal macrophages.

Role of aryl hydrocarbon receptors in infection and inflammation

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by various ligands, including pollutants, microorganisms, and metabolic substances. It is expressed extensively in pulmonary and intestinal epithelial cells, where it contributes to barrier defense. The expression of AhR is pivotal in regulating the inflammatory response to microorganisms. However, dysregulated AhR expression can result in endocrine disorders, leading to immunotoxicity and potentially promoting the development of carcinoma. This review focuses on the crucial role of the AhR in facilitating and limiting the proliferation of pathogens, specifically in relation to the host cell type and the species of etiological agents involved in microbial pathogen infections. The activation of AhR is enhanced through the IDO1-AhR-IDO1 positive feedback loop, which is manipulated by viruses. AhR primarily promotes the infection of SARS-CoV-2 by inducing the expression of angiotensin-converting enzyme 2 (ACE2) and the secretion of pro-inflammatory cytokines. AhR also plays a significant role in regulating various types of T-cells, including CD4+ T cells and CD8+ T cells, in the context of pulmonary infections. The AhR pathway plays a crucial role in regulating immune responses within the respiratory and intestinal barriers when they are invaded by viruses, bacteria, parasites, and fungi. Additionally, we propose that targeting the agonist and antagonist of AhR signaling pathways could serve as a promising therapeutic approach for combating pathogen infections, especially in light of the growing prevalence of drug resistance to multiple antibiotics.

New insight into arginine and tryptophan metabolism in macrophage activation during tuberculosis

Arginine and tryptophan are pivotal in orchestrating cytokine-driven macrophage polarization and immune activation. Specifically, interferon-gamma (IFN-γ) stimulates inducible nitric oxide synthase (iNOS) expression), leading to the conversion of arginine into citrulline and nitric oxide (NO), while Interleukin-4 (IL4) promotes arginase activation, shifting arginine metabolism toward ornithine. Concomitantly, IFN-γ triggers indoleamine 2,3-dioxygenase 1 (IDO1) and Interleukin-4 induced 1 (IL4i1), resulting in the conversion of tryptophan into kynurenine and indole-3-pyruvic acid. These metabolic pathways are tightly regulated by NAD+-dependent sirtuin proteins, with Sirt2 and Sirt5 playing integral roles. In this review, we present novel insights that augment our understanding of the metabolic pathways of arginine and tryptophan following Mycobacterium tuberculosis infection, particularly their relevance in macrophage responses. Additionally, we discuss arginine methylation and demethylation and the role of Sirt2 and Sirt5 in regulating tryptophan metabolism and arginine metabolism, potentially driving macrophage polarization.

Transforming growth factor-β, Interleukin-23 and interleukin-1β modulate TH22 response during active multidrug-resistant tuberculosis

We previously reported that patients with multidrug-resistant tuberculosis (MDR-TB) showed low systemic and Mtb-induced Th22 responses associated to high sputum bacillary load and severe lung lesions suggesting that Th22 response could influence the ability of these patients to control bacillary growth and tissue damage. In MDR-TB patients, the percentage of IL-22+ cells inversely correlates with the proportion of senescent PD-1+ T cells. Herein, we aimed to evaluate the pathways involved on the regulation of systemic and Mtb-induced Th22 response in MDR-TB and fully drug-susceptible TB patients (S-TB) and healthy donors. Our results show that while IL-1β and IL-23 promote Mtb-induced IL-22 secretion and expansion of IL-22+ cells, TGF-β inhibits this response. Systemic and in vitro Mtb-induced Th22 response inversely correlates with TGF-β amounts in plasma and in PBMC cultures respectively. The number of circulating PD-1+ T cells directly correlates with plasmatic TGF-β levels and blockade of PD-1/PD-L1 signalling enhances in vitro Mtb-induced expansion of IL-22+ cells. Thus, TGF-β could also inhibit Th22 response through upregulation of PD-1 expression in T cells. Higher percentage of IL-23+ monocytes was observed in TB patients. In contrast, the proportion of IL-1β+ monocytes was lower in TB patients with bilateral lung cavities (BCC) compared to those patients with unilateral cavities (UCC). Interestingly, TB patients with BCC showed higher plasmatic and Mtb-induced TGF-β secretion than patients with UCC. Thus, high TGF-β secretion and subtle differences in IL-23 and IL-1β expression could diminish systemic and in vitro Mtb-induced Th22 response along disease progression in TB patients.

Targeting the aryl hydrocarbon receptor by gut phenolic metabolites: A strategy towards gut inflammation

The Aryl Hydrocarbon Receptor (AHR) is a ligand-dependent transcription factor able to control complex transcriptional processes in several cell types, which has been correlated with various diseases, including inflammatory bowel diseases (IBD). Numerous studies have described different compounds as ligands of this receptor, like xenobiotics, natural compounds, and several host-derived metabolites. Dietary (poly)phenols have been studied regarding their pleiotropic activities (e.g., neuroprotective and anti-inflammatory), but their AHR modulatory capabilities have also been considered. However, dietary (poly)phenols are submitted to extensive metabolism in the gut (e.g., gut microbiota). Thus, the resulting gut phenolic metabolites could be key players modulating AHR since they are the ones that reach the cells and may exert effects on the AHR throughout the gut and other organs. This review aims at a comprehensive search for the most abundant gut phenolic metabolites detected and quantified in humans to understand how many have been described as AHR modulators and what could be their impact on inflammatory gut processes. Even though several phenolic compounds have been studied regarding their anti-inflammatory capacities, only 1 gut phenolic metabolite, described as AHR modulator, has been evaluated on intestinal inflammatory models. Searching for AHR ligands could be a novel strategy against IBD.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

High-fat diet and alcohol induced-mice could cause colonic injury through molecular mechanisms of endogenous toxins

Due to the complexity and diverse causes, the pathological mechanism of diet-induced colonic injury and colitis remains unclear. In this study, we studied the effects of the combination of a high-fat diet (HFD) plus alcohol on colonic injury in mice. We found HFD plus alcohol treatment induced disturbance of the gut microbiota; increased the production of intestinal toxins lipopolysaccharide (LPS), indole, and skatole; destroyed the stability of the intestinal mucosa; and caused the colonic epithelial cells damage through the activation of nuclear factor (NF)-κB and aromatic hydrocarbon receptors (AhR) signaling pathways. To mimic the effect of HFD plus alcohol in vivo, NCM460 cells were stimulated with alcohol and oleic acid with/without intestinal toxins (LPS, indole, and skatole) in vitro. Combinative treatment of alcohol and oleic acid caused moderate damage on NCM460 cells, while combination with intestinal toxins induced serious cell apoptosis. Western blot data indicated that the activation of NF-κB and AhR pathways further augmented after intestinal toxins treatment in alcohol- and oleic acid-treated colonic cells. This study provided new evidence for the relationship between diet pattern and colonic inflammation, which might partly reveal the pathological development of diet-induced colon disease and the involvement of intestinal toxins.

Stemness and immune evasion conferred by TDO2-AHR pathway are associated with liver metastasis of colon cancer

The aryl hydrocarbon receptor (AHR) pathway modulates the immune system in response to kynurenine, an endogenous tryptophan metabolite. IDO1 and TDO2 catalyze kynurenine production, which promotes cancer progression by compromising host immunosurveillance. However, it is unclear whether the AHR activation regulates the malignant traits of cancer such as metastatic capability or cancer stemness. Here, we carried out systematic analyses of metabolites in patient-derived colorectal cancer spheroids, and identified high levels of kynurenine and TDO2 that were positively associated with liver metastasis. In a mouse colon cancer model, TDO2 expression substantially enhanced liver metastasis, induced AHR-mediated PD-L1 transactivation, and dampened immune responses; these changes were all abolished by PD-L1 knockout. In patient-derived cancer spheroids, TDO2 or AHR activity was required for not only the expression of PD-L1, but also for cancer stem cell (CSC)-related characteristics and Wnt signaling. TDO2 was coexpressed with both PD-L1 and nuclear β-catenin in colon xenograft tumors, and the coexpression of TDO2 and PD-L1 was observed in clinical colon cancer specimens. Thus, our data indicate that the activation of the TDO2-kynurenine-AHR pathway facilitates liver metastasis of colon cancer via PD-L1-mediated immune evasion and maintenance of stemness.

Immunotoxicity of Xenobiotics in Fish: A Role for the Aryl Hydrocarbon Receptor (AhR)?

The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.

Characterization of an Ex Vivo Equine Endometrial Tissue Culture Model Using Next-Generation RNA-Sequencing Technology

Simple Summary

Notwithstanding extensive research into fertility problems in mares, pregnancy rates have remained low mainly because of endometrial inflammation (endometritis). In the field of equine research, endometrial explants have been used to carry out in vitro studies of the mare’s endometrium. However, there has been no wide-ranging assessment of relative stability of this model over time. The aim of this study was to perform an in-depth transcriptomic assessment of endometrial explants over a culture period of 72 h and assess if they are representative of the whole mare. Explants at 24 h demonstrated significant changes when compared to biopsies at 0 h as expected. Even though gene expression changes were seen between 24 and 48 h of culture, prior to this window changes were dominated by the effects of explanting and culture and subsequently, transcription was generally compromised. Our results, therefore have defined the optimal period when explants can be used to study equine endometritis and how the endometrium is modulated during inflammation. It highlights the use of abattoir derived samples to understand the physiology and pathophysiology of the equine endometrium, negating the need to collect repeated uterine biopsies from living mares.

Abstract

Persistent mating-induced endometritis is a major cause of poor fertility rates in the mare. Endometritis can be investigated using an ex vivo equine endometrial explant system which measures uterine inflammation using prostaglandin F_2α as a biomarker. However, this model has yet to undergo a wide-ranging assessment through transcriptomics. In this study, we assessed the transcriptomes of cultured endometrial explants and the optimal temporal window for their use. Endometrium harvested immediately post-mortem from native pony mares (n = 8) were sampled (0 h) and tissue explants were cultured for 24, 48 and 72 h. Tissues were stored in RNALater, total RNA was extracted and sequenced. Differentially expressed genes (DEGs) were defined using DESeq2 (R/Bioconductor). Principal component analysis indicated that the greatest changes in expression occurred in the first 24 h of culture when compared to autologous biopsies at 0 h. Fewer DEGs were seen between 24 and 48 h of culture suggesting the system was more stable than during the first 24 h. No genes were differentially expressed between 48 and 72 h but the low number of background gene expression suggested that explant viability was compromised after 48 h. ESR1, MMP9, PTGS2, PMAIP1, TNF, GADD45B and SELE genes were used as biomarkers of endometrial function, cell death and inflammation across tissue culture timepoints. STRING assessments of gene ontology suggested that DEGs between 24 and 48 h were linked to inflammation, immune system, cellular processes, environmental information processing and signal transduction, with an upregulation of most biomarker genes at 24 h. Taken together our observations indicated that 24–48 h is the optimal temporal window when the explant model can be used, as explants restore microcirculation, perform wound healing and tackle inflammation during this period. This key observation will facilitate the appropriate use of this as a model for further research into the equine endometrium and potentially the progression of mating-induced endometritis to persistent inflammation between 24 and 48 h.

Abnormal Tryptophan Metabolism in HIV and Mycobacterium tuberculosis Infection

Host metabolism has recently gained more attention for its roles in physiological functions and pathologic conditions. Of these, metabolic tryptophan disorders generate a pattern of abnormal metabolites that are implicated in various diseases. Here, we briefly highlight the recent advances regarding abnormal tryptophan metabolism in HIV and Mycobacterium tuberculosis infection and discuss its potential impact on immune regulation, disease progression, and neurological disorders. Finally, we also discuss the potential for metabolic tryptophan interventions toward these infectious diseases.

Understanding the Reciprocal Interplay Between Antibiotics and Host Immune System: How Can We Improve the Anti-Mycobacterial Activity of Current Drugs to Better Control Tuberculosis?

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) infection, remains a global health threat despite recent advances and insights into host-pathogen interactions and the identification of diverse pathways that may be novel therapeutic targets for TB treatment. In addition, the emergence and spread of multidrug-resistant Mtb strains led to a low success rate of TB treatments. Thus, novel strategies involving the host immune system that boost the effectiveness of existing antibiotics have been recently suggested to better control TB. However, the lack of comprehensive understanding of the immunomodulatory effects of anti-TB drugs, including first-line drugs and newly introduced antibiotics, on bystander and effector immune cells curtailed the development of effective therapeutic strategies to combat Mtb infection. In this review, we focus on the influence of host immune-mediated stresses, such as lysosomal activation, metabolic changes, oxidative stress, mitochondrial damage, and immune mediators, on the activities of anti-TB drugs. In addition, we discuss how anti-TB drugs facilitate the generation of Mtb populations that are resistant to host immune response or disrupt host immunity. Thus, further understanding the interplay between anti-TB drugs and host immune responses may enhance effective host antimicrobial activities and prevent Mtb tolerance to antibiotic and immune attacks. Finally, this review highlights novel adjunctive therapeutic approaches against Mtb infection for better disease outcomes, shorter treatment duration, and improved treatment efficacy based on reciprocal interactions between current TB antibiotics and host immune cells.

Lipopolysaccharide-Enhanced Responses against Aryl Hydrocarbon Receptor in FcgRIIb-Deficient Macrophages, a Profound Impact of an Environmental Toxin on a Lupus-Like Mouse Model

Fc gamma receptor IIb (FcgRIIb) is the only inhibitory-FcgR in the FcgR family, and FcgRIIb-deficient (FcgRIIb^−/−) mice develop a lupus-like condition with hyper-responsiveness against several stimulations. The activation of aryl hydrocarbon receptor (Ahr), a cellular environmental sensor, might aggravate activity of the lupus-like condition. As such, 1,4-chrysenequinone (1,4-CQ), an Ahr-activator, alone did not induce supernatant cytokines from macrophages, while the 24 h pre-treatment by lipopolysaccharide (LPS), a representative inflammatory activator, prior to 1,4-CQ activation (LPS/1,4-CQ) predominantly induced macrophage pro-inflammatory responses. Additionally, the responses from FcgRIIb^−/− macrophages were more prominent than wild-type (WT) cells as determined by (i) supernatant cytokines (TNF-α, IL-6, and IL-10), (ii) expression of the inflammation associated genes (NF-κB, aryl hydrocarbon receptor, iNOS, IL-1β and activating-FcgRIV) and cell-surface CD-86 (a biomarker of M1 macrophage polarization), and (iii) cell apoptosis (Annexin V), with the lower inhibitory-FcgRIIb expression. Moreover, 8-week-administration of 1,4-CQ in 8 week old FcgRIIb^−/− mice, a genetic-prone lupus-like model, enhanced lupus characteristics as indicated by anti-dsDNA, serum creatinine, proteinuria, endotoxemia, gut-leakage (FITC-dextran), and glomerular immunoglobulin deposition. In conclusion, an Ahr activation worsened the disease severity in FcgRIIb^−/− mice possibly through the enhanced inflammatory responses. The deficiency of inhibitory-FcgRIIb in these mice, at least in part, prominently enhanced the pro-inflammatory responses. Our data suggest that patients with lupus might be more vulnerable to environmental pollutants.

Metabolic Regulation of Immune Responses to Mycobacterium tuberculosis: A Spotlight on L-Arginine and L-Tryptophan Metabolism

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a leading cause of death worldwide. Despite decades of research, there is still much to be uncovered regarding the immune response to Mtb infection. Here, we summarize the current knowledge on anti-Mtb immunity, with a spotlight on immune cell amino acid metabolism. Specifically, we discuss L-arginine and L-tryptophan, focusing on their requirements, regulatory roles, and potential use as adjunctive therapy in TB patients. By continuing to uncover the immune cell contribution during Mtb infection and how amino acid utilization regulates their functions, it is anticipated that novel host-directed therapies may be developed and/or refined, helping to eradicate TB.

Differential modulation of Ahr and Arid5a: A promising therapeutic strategy for autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease that involves demyelination of axons in the central nervous system (CNS) and affects patients worldwide. It has been demonstrated that ligand-activated aryl hydrocarbon receptor (Ahr) ameliorates experimental autoimmune encephalomyelitis (EAE), a murine model of MS, by increasing CD4⁺FoxP3⁺ T cells. Recent evidence indicates that AT-rich interactive domain-containing protein 5a (Arid5a) is required for EAE pathogenesis by stabilizing Il6 and OX40 mRNAs. However, the differential modulation of Ahr and Arid5a in autoimmunity as a therapeutic strategy is unexplored. Herein, an in silico, in vitro and in vivo approach identified Flavipin (3,4,5-trihydroxy-6-methylphthalaldehyde) as an Ahr agonist that induces the expression of Ahr downstream genes in mouse CD4⁺ T cells and CD11b⁺ macrophages. Interestingly, Flavipin inhibited the stabilizing function of Arid5a and its counteracting effects on Regnase-1 on the 3′ untranslated region (3′UTR) of target mRNAs. Furthermore, it inhibited the stabilizing function of Arid5a on Il23a 3′UTR, a newly identified target mRNA. In EAE, Flavipin ameliorated disease severity, with reduced CD4⁺IL-17⁺ T cells, IL-6 and TNF-α and increased CD4⁺FoxP3⁺ T cells. Moreover, EAE amelioration was concomitant with reduced CD4⁺OX40⁺ and CD4⁺CD45⁺ T cells in the CNS. RNA interference showed that the modulatory effects of Flavipin on pro- and anti-inflammatory mediators in CD4⁺ T cells and macrophages were Ahr- and/or Arid5a-dependent. In conclusion, our findings reveal differential modulation of Ahr and Arid5a as a new therapeutic strategy for MS.

The Lung Microbiome: A Central Mediator of Host Inflammation and Metabolism in Lung Cancer Patients?

Simple Summary

Lung cancer is the major cause of cancer related deaths in the world. New therapies have improved outcomes. Unfortunately, overall 5 year survival is ~20%. Therefore, better understanding of tumor biology and the microenvironment may lead to new therapeutic targets. The lung microbiome has recently emerged as a major mediator of host inflammation and pathogenesis. Understanding how the lung microbiota exerts its effects on lung cancer and the tumor microenvironment will allow for novel development of therapies.

Abstract

Lung cancer is the leading cause of cancer-related death. Over the past 5–10 years lung cancer outcomes have significantly improved in part due to better treatment options including immunotherapy and molecularly targeted agents. Unfortunately, the majority of lung cancer patients do not enjoy durable responses to these new treatments. Seminal research demonstrated the importance of the gut microbiome in dictating responses to immunotherapy in melanoma patients. However, little is known regarding how other sites of microbiota in the human body affect tumorigenesis and treatment responses. The lungs were traditionally thought to be a sterile environment; however, recent research demonstrated that the lung contains its own dynamic microbiota that can influence disease and pathophysiology. Few studies have explored the role of the lung microbiome in lung cancer biology. In this review article, we discuss the links between the lung microbiota and cancer, with particular focus on immune responses, metabolism and strategies to target the lung microbiome for cancer prevention.

Intestinal microbiota-derived tryptophan metabolites are predictive of Ah receptor activity

Commensal microbiota-dependent tryptophan catabolism within the gastrointestinal tract is known to exert profound effects upon host physiology, including the maintenance of epithelial barrier and immune function. A number of abundant microbiota-derived tryptophan metabolites exhibit activation potential for the aryl hydrocarbon receptor (AHR). Gene expression facilitated by AHR activation through the presence of dietary or microbiota-generated metabolites can influence gastrointestinal homeostasis and confer protection from intestinal challenges. Utilizing untargeted mass spectrometry-based metabolomics profiling, combined with AHR activity screening assays, we identify four previously unrecognized tryptophan metabolites, present in mouse cecal contents and human stool, with the capacity to activate AHR. Using GC/MS and LC/MS platforms, quantification of these novel AHR activators, along with previously established AHR-activating tryptophan metabolites, was achieved, providing a relative order of abundance. Using physiologically relevant concentrations and quantitative gene expression analyses, the relative efficacy of these tryptophan metabolites with regard to mouse or human AHR activation potential is examined. These data reveal indole, 2-oxindole, indole-3-acetic acid and kynurenic acid as the dominant AHR activators in mouse cecal contents and human stool from participants on a controlled diet. Here we provide the first documentation of the relative abundance and AHR activation potential of a panel of microbiota-derived tryptophan metabolites. Furthermore, these data reveal the human AHR to be more sensitive, at physiologically relevant concentrations, to tryptophan metabolite activation than mouse AHR. Additionally, correlation analyses indicate a relationship linking major tryptophan metabolite abundance with AHR activity, suggesting these cecal/fecal metabolites represent biomarkers of intestinal AHR activity.

Indole Propionic Acid, an Unusual Antibiotic Produced by the Gut Microbiota, With Anti-inflammatory and Antioxidant Properties

Most antibiotics are produced by soil microbes and typically interfere with macromolecular synthesis processes as their antibacterial mechanism of action. These natural products are often large and suffer from poor chemical tractability. Here, we discuss discovery, mechanism of action, and the therapeutic potentials of an unusual antibiotic, indole propionic acid (IPA). IPA is produced by the human gut microbiota. The molecule is small, chemically tractable, and targets amino acid biosynthesis. IPA is active against a broad spectrum of mycobacteria, including drug resistant Mycobacterium tuberculosis and non-tuberculous mycobacteria (NTM). Interestingly, the microbiota-produced metabolite is detectable in the serum of healthy individuals, tuberculosis (TB) patients, and several animal models. Thus, the microbiota in our gut may influence susceptibility to mycobacterial diseases. If a gut-lung microbiome axis can be demonstrated, IPA may have potential as a biomarker of disease progression, and development of microbiota-based therapies could be explored. In addition to its antimycobacterial activity, the molecule displays anti-inflammatory and antioxidant properties. This raises the possibility that IPA has therapeutic potential as both antibiotic and add-on host-directed drug for the treatment of TB in patient populations where disease morbidity and mortality is driven by excessive inflammation and tissue damage, such as TB-associated immune reconstitution inflammatory syndrome, TB-meningitis, and TB-diabetes.

Aryl hydrocarbon receptor in mesenchymal stromal cells: new frontiers in AhR biology

Mesenchymal stromal cells (MSCs) are nonhematopoietic cells that have been clinically explored as investigational cellular therapeutics for tissue injury regeneration and immune-mediated diseases. Their pharmaceutical properties arise from activation of endogenous receptors and transcription factors leading to a paracrine effect which mirror the biology of progenitors from which they arise. The aryl hydrocarbon receptor (AhR) is a transcription factor that has been extensively studied as an environmental sensor for xenobiotics, but recent findings suggest it can modulate immunological functions. Both genetic and pharmacological investigations revealed that MSCs express AhR and that it plays roles in inflammation, immunomodulation, and mesodermal plasticity of endogenous MSCs. Further, AhR has been shown to interact with key signaling cascades associated with these conditions. Therefore, AhR has potential to be an attractive target in both endogenous and culture-adapted MSCs for novel therapeutics to treat inflammation and other age-related disorders.

Meisoindigo Protects Against Focal Cerebral Ischemia-Reperfusion Injury by Inhibiting NLRP3 Inflammasome Activation and Regulating Microglia/Macrophage Polarization via TLR4/NF-κB Signaling Pathway

Ischemic stroke is a devastating disease with long-term disability. However, the pathogenesis is unclear and treatments are limited. Meisoindigo, a second-generation derivative of indirubin, has general water solubility and is well-tolerated. Previous studies have shown that meisoindigo reduces inflammation by inhibiting leukocyte chemotaxis and migration. In the present study, we investigated the hypothesis that meisoindigo was also protective against ischemic stroke, then evaluated its underlying mechanisms. In vivo, adult male C57BL/6J wild-type mice were used to produce a middle cerebral artery occlusion (MCAO) stroke model. On day three after reperfusion, obvious improvement in neurological scores, infarct volume reduction and cerebral edema amelioration were observed in meisoindigo treatment. Moreover, immunofluorescence staining and western-blot showed that the expression of NLRP3 inflammasome and its associated proteins in neurons and microglia was inhibited by meisoindigo. The effects of Meisoindigo on NLRP3 inflammasome inactivation and increased the M2 phenotype of microglia/macrophage through shifting from a M1 phenotype, which was possibly mediated by inhibition of TLR4/NF-κB. Furthermore, we verified the inhibitory effect of meisoindigo on TLR4/NF-κB signaling pathway, and found that meisoindigo treatment could significantly suppressed the expression of TLR4/NF-κB pathway-associated proteins in a dose-dependent manner, meanwhile, which resulted in downregulation of HMGB1 and IL-1β. Next, we established an in vitro oxygen glucose deprivation/Reperfusion (OGD/R) model in HT-22 and BV2 cells to simulate ischemic conditions. Cytotoxicity assay showed that meisoindigo substantially improved relative cell vitality and in HT-22 and BV2 cells following OGD/R in vitro. After suffering OGD/R, the TLR4/NF-κB pathway was activated, the expression of NLRP3 inflammasome-associated proteins and M1 microglia/macrophage were increased, but meisoindigo could inhibit above changes in both HT-22 and BV2 cells. Additionally, though lipopolysaccharide stimulated the activation of TLR4 signaling in OGD/R models, meisoindigo co-treatment markedly reversed the upregulation of TLR4 and following activation of NLRP3 inflammasome and polarization of M1 microglia/macrophages mediated by TLR4. Overall, we demonstrate for the first time that meisoindigo post-treatment alleviates brain damage induced by ischemic stroke in vivo and in vitro experiments through blocking activation of the NLRP3 inflammasome and regulating the polarization of microglia/macrophages via inhibition of the TLR4/NF-κB signaling pathway.

Tissue Metabolic Changes Drive Cytokine Responses to Mycobacterium tuberculosis

Cellular metabolism can influence host immune responses to Mycobacterium tuberculosis. Using a systems biology approach, differential expression of 292 metabolic genes involved in glycolysis, glutathione, pyrimidine, and inositol phosphate pathways was evident at the site of a human tuberculin skin test challenge in patients with active tuberculosis infection. For 28 metabolic genes, we identified single nucleotide polymorphisms that were trans-acting for in vitro cytokine responses to M. tuberculosis stimulation, including glutathione and pyrimidine metabolism genes that alter production of Th1 and Th17 cytokines. Our findings identify novel therapeutic targets in host metabolism that may shape protective immunity to tuberculosis.

Transcriptional blood signatures for active and amphotericin B treated visceral leishmaniasis in India

Amphotericin B provides improved therapy for visceral leishmaniasis (VL) caused by Leishmania donovani, with single dose liposomal-encapsulated Ambisome providing the best cure rates. The VL elimination program aims to reduce the incidence rate in the Indian subcontinent to <1/10,000 population/year. Ability to predict which asymptomatic individuals (e.g. anti-leishmanial IgG and/or Leishmania-specific modified Quantiferon positive) will progress to clinical VL would help in monitoring disease outbreaks. Here we examined whole blood transcriptional profiles associated with asymptomatic infection, active disease, and in treated cases. Two independent microarray experiments were performed, with analysis focussed primarily on differentially expressed genes (DEGs) concordant across both experiments. No DEGs were identified for IgG or Quantiferon positive asymptomatic groups compared to negative healthy endemic controls. We therefore concentrated on comparing concordant DEGs from active cases with all healthy controls, and in examining differences in the transcriptome following different regimens of drug treatment. In these comparisons 6 major themes emerged: (i) expression of genes and enrichment of gene sets associated with erythrocyte function in active cases; (ii) strong evidence for enrichment of gene sets involved in cell cycle in comparing active cases with healthy controls; (iii) identification of IFNG encoding interferon-γ as the major hub gene in concordant gene expression patterns across experiments comparing active cases with healthy controls or with treated cases; (iv) enrichment for interleukin signalling (IL-1/3/4/6/7/8) and a prominent role for CXCL10/9/11 and chemokine signalling pathways in comparing active cases with treated cases; (v) the novel identification of Aryl Hydrocarbon Receptor signalling as a significant canonical pathway when comparing active cases with healthy controls or with treated cases; and (vi) global expression profiling support for more effective cure at day 30 post-treatment with a single dose of liposomal encapsulated amphotericin B compared to multi-dose non-liposomal amphotericin B treatment over 30 days. (296 words; 300 words allowed).

Ironing out Macrophage Immunometabolism

Over the last decade, increasing evidence has reinforced the key role of metabolic reprogramming in macrophage activation. In addition to supporting the specific immune response of different subsets of macrophages, intracellular metabolic pathways also directly control the specialized effector functions of immune cells. In this context, iron metabolism has been recognized as an important component of macrophage plasticity. Since macrophages control the availability of this essential metal, changes in the expression of genes coding for the major proteins of iron metabolism may result in different iron availability for the macrophage itself and for other cells in the microenvironment. In this review, we discuss how macrophage iron can also play a role in immunometabolism.

Endocrine aryl hydrocarbon receptor signaling is induced by moderate cutaneous exposure to ultraviolet light

Links between solar UV exposure and immunity date back to the ancient Greeks with the development of heliotherapy. Skin contains several UV-sensitive chromophores and exposure to sunlight can produce molecules, such as vitamin D3, that act in an endocrine manner. We investigated the role of the aryl hydrocarbon receptor (AHR), an environmental sensor and ligand-regulated transcription factor activated by numerous planar compounds of endogenous, dietary or environmental origin. 15- to 30-minute exposure of cells to a minimal erythemal dose of UVB irradiation in vitro induced translocation of the AHR to the nucleus, rapidly inducing site-specific DNA binding and target gene regulation. Importantly, ex vivo studies with Ahr wild-type or null fibroblasts showed that serum from mice whose skin was exposed to a 15 min UVB dose, but not control serum, contained agonist activity within 30 min of UV irradiation, inducing AHR-dependent gene expression. Moreover, a 15-min cutaneous UVB exposure induced AHR site-specific DNA binding and target gene regulation in vivo within 3–6 hr post-irradiation in blood and in peripheral tissues, including intestine. These results show that cutaneous exposure of mice to a single minimal erythemic dose of UVB induces rapid AHR signaling in multiple peripheral organs, providing compelling evidence that moderate sun exposure can exert endocrine control of immunity through the AHR.

Indirubin modulates CD4+ T-cell homeostasis via PD1/PTEN/AKT signalling pathway in immune thrombocytopenia

Immune thrombocytopenia (ITP) is an acquired autoimmune disease characterized by an immune mediated decrease in platelet number. Disturbance of CD4⁺ T-cell homeostasis with simultaneous decrease of CD4⁺ CD25⁺ Foxp3⁺ regulatory T cells (Tregs) as well as unrestricted proliferation and activation of peripheral CD4⁺ effector T cells underpin the pathophysiology of ITP. Indirubin is an active ingredient of a traditional Chinese herb called Indigofera tinctoria L. which is clinically used for the treatment of ITP patients. Whether indirubin targets the Tregs/effector T cell-axis to restore platelet number is unknown. In our in vitro studies, Indirubin could significantly enhance the number and function of Tregs and meanwhile dampen the activation of effector T cells in a dose-dependent manner. Indirubin was observed to restore the expression of programmed cell-death 1 (PD1) and phosphatase and tensin homolog (PTEN) on the CD4⁺ T cells of ITP patients, leading to the subsequent attenuation of the AKT/mTOR pathway. Furthermore, these observations were recapitulated in an active murine model of ITP with a prominent platelet response. Thus, our results identified a potentially novel mechanism of the therapeutic action of indirubin in the treatment of ITP through regulating the homeostasis of CD4⁺ T cells in a PD1/PTEN/AKT signalling pathway.

Staphylococcus epidermidis Activates Aryl Hydrocarbon Receptor Signaling in Human Keratinocytes: Implications for Cutaneous Defense

Bacterial challenge of keratinocytes with the abundant skin commensal Staphylococcus epidermidis induces distinct innate immune responses, but the underlying molecular mechanisms are still emerging. We report that the aryl hydrocarbon receptor (AhR) was activated in human primary keratinocytes infected with S. epidermidis, leading to induction of the AhR-responsive gene cytochrome P450 1A1 (CYP1A1). In addition, functional AhR was required for S. epidermidis-mediated induction of IL-1β expression in keratinocytes. AhR-dependent gene induction of IL-1β and CYP1A1 was mediated by factor(s) < 2 kDa secreted by S. epidermidis. Blockade of the AhR in a 3D organotypic skin equivalent infected with S. epidermidis attenuated the S. epidermidis-induced CYP1A1 and IL-1β expression. Moreover, S. epidermidis also induced expression of IL-1α and of the antimicrobial peptide human β-defensin-3 in an AhR-dependent manner in a 3D skin equivalent. An increased outgrowth of S. epidermidis on the surface of skin explants treated with a specific AhR inhibitor further indicate a pivotal role of the AhR in mediating an epidermal defense response. Taken together, our data expand the role of the AhR in innate immunity and support a previously unappreciated contribution for the AhR in cutaneous defense.

Addressing the tuberculosis-depression syndemic to end the tuberculosis epidemic

Tuberculosis (TB) and depression act synergistically via social, behavioral, and biological mechanisms to magnify the burden of disease. Clinical depression is a common, under-recognized, yet treatable condition that, if comorbid with TB, is associated with increased morbidity, mortality, community TB transmission, and drug resistance. Depression may increase risk of TB reactivation, contribute to disease progression, and/or inhibit the physiological response to anti-tuberculosis treatment because of poverty, undernutrition, immunosuppression, and/or negative coping behaviors, including substance abuse. Tuberculous infection and/or disease reactivation may precipitate depression as a result of the inflammatory response and/or dysregulation of the hypothalamic-pituitary-adrenal axis. Clinical depression may also be triggered by TB-related stigma, exacerbating other underlying social vulnerabilities, and/or may be attributed to the side effects of anti-tuberculosis treatment. Depression may negatively impact health behaviors such as diet, health care seeking, medication adherence, and/or treatment completion, posing a significant challenge for global TB elimination. As several of the core symptoms of TB and depression overlap, depression often goes unrecognized in individuals with active TB, or is dismissed as a normative reaction to situational stress. We used evidence to reframe TB and depression comorbidity as the 'TB-depression syndemic', and identified critical research gaps to further elucidate the underlying mechanisms. The World Health Organization's Global End TB Strategy calls for integrated patient-centered care and prevention linked to social protection and innovative research. It will require multidisciplinary approaches that consider conditions such as TB and depression together, rather than as separate problems and diseases, to end the global TB epidemic.

Tumor-Repopulating Cells Induce PD-1 Expression in CD8+ T Cells by Transferring Kynurenine and AhR Activation

Despite the clinical successes fostered by immune checkpoint inhibitors, mechanisms underlying PD-1 upregulation in tumor-infiltrating T cells remain an enigma. Here, we show that tumor-repopulating cells (TRCs) drive PD-1 upregulation in CD8⁺ T cells through a transcellular kynurenine (Kyn)-aryl hydrocarbon receptor (AhR) pathway. Interferon-γ produced by CD8⁺ T cells stimulates release of high levels of Kyn produced by TRCs, which is transferred into adjacent CD8⁺ T cells via the transporters SLC7A8 and PAT4. Kyn induces and activates AhR and thereby upregulates PD-1 expression. This Kyn-AhR pathway is confirmed in both tumor-bearing mice and cancer patients and its blockade enhances antitumor adoptive T cell therapy efficacy. Thus, we uncovered a mechanism of PD-1 upregulation with potential tumor immunotherapeutic applications.

A cytosol derived factor of Group B streptococcus prevent its invasion into human epithelial cells

Group B streptococcus (GBS) or Streptococcus agalactiae, is an opportunistic pathogen causing a wide range of infections like pneumonia, sepsis, and meningitis in newborn, pregnant women and adults. While this bacterium has adapted well to asymptomatic colonization of adult humans, it still remains a potentially devastating pathogen to susceptible infants. Advances in molecular techniques and refinement of in vitro and in vivo model systems have elucidated key elements of the pathogenic process, from initial attachment to the maternal vaginal epithelium to penetration of the newborn blood-brain barrier. Still, the formidable array of GBS virulence factors makes this bacterium at the forefront of neonatal pathogens. The involvement of bacterial components in the host-pathogen interaction of GBS pathogenesis and its related diseases is not clearly understood. In this study we demonstrated the role of a 39 kDa factor from GBS which plays an important role in the process of its invasion. We found a homogeneous 39 kDa factor from the cytosol of GBS after following a combination of sequential purification steps involving molecular sieving and ion exchange chromatography using ACTA-FPLC system. Its N-terminal sequence showed a homology with xenobiotic response element type transcriptional regulator protein, a 40 kDa protein of Streptococcus. This factor leads to inhibition of GBS invasion in HeLa and A549 cells. This protein also showed sensitivity and specific cross reactivity with the antibodies raised against it in New Zealand white rabbits by western immunoblotting. This inhibitory factor was further confirmed tolerant for its cytotoxicity. These results add a novel aspect to bacterial pathogenesis where bacteria's own intracellular protein component can act as a potential therapeutic candidate by decreasing the severity of disease thus promoting its invasion inhibition.

Cerebral tryptophan metabolism and outcome of tuberculous meningitis: an observational cohort study

BACKGROUND

Immunopathology contributes to the high mortality of tuberculous meningitis, but the biological pathways involved are mostly unknown. We aimed to compare cerebrospinal fluid (CSF) and serum metabolomes of patients with tuberculous meningitis with that of controls without tuberculous meningitis, and assess the link between metabolite concentrations and mortality.

METHODS

In this observational cohort study at the Hasan Sadikin Hospital (Bandung, Indonesia) we measured 425 metabolites using liquid chromatography-mass spectrometry in CSF and serum from 33 HIV-negative Indonesian patients with confirmed or probable tuberculous meningitis and 22 control participants with complete clinical data between March 12, 2009, and Oct 27, 2013. Associations of metabolite concentrations with survival were validated in a second cohort of 101 patients from the same centre. Genome-wide single nucleotide polymorphism typing was used to identify tryptophan quantitative trait loci, which were used for survival analysis in a third cohort of 285 patients.

FINDINGS

Concentrations of 250 (70%) of 351 metabolites detected in CSF were higher in patients with tuberculous meningitis than in controls, especially in those who died during follow-up. Only five (1%) of the 390 metobolites detected in serum differed between patients with tuberculous meningitis and controls. CSF tryptophan concentrations showed a pattern different from most other CSF metabolites; concentrations were lower in patients who survived compared with patients who died (9-times) and to controls (31-times). The association of low CSF tryptophan with patient survival was confirmed in the validation cohort (hazard ratio 0·73; 95% CI 0·64-0·83; p<0·0001; per each halving). 11 genetic loci predictive for CSF tryptophan concentrations in tuberculous meningitis were identified (p<0·00001). These quantitative trait loci predicted survival in a third cohort of 285 HIV-negative patients in a prognostic index including age and sex, also after correction for possible confounders (p=0·0083).

INTERPRETATION

Cerebral tryptophan metabolism, which is known to affect Mycobacterium tuberculosis growth and CNS inflammation, is important for the outcome of tuberculous meningitis. CSF tryptophan concentrations in tuberculous meningitis are under strong genetic influence, probably contributing to the variable outcomes of tuberculous meningitis. Interventions targeting tryptophan metabolism could improve outcomes of tuberculous meningitis.

FUNDING

Royal Dutch Academy of Arts and Sciences; Netherlands Foundation for Scientific Research; Radboud University; National Academy of Sciences; Ministry of Research, Technology, and Higher Education, Indonesia; European Research Council; and PEER-Health.

A blood-based biomarker panel indicates IL-10 and IL-12/23p40 are jointly associated as predictors of β-amyloid load in an AD cohort

Alzheimer’s Disease (AD) is the most common form of dementia, characterised by extracellular amyloid deposition as plaques and intracellular neurofibrillary tangles of tau protein. As no current clinical test can diagnose individuals at risk of developing AD, the aim of this project is to evaluate a blood-based biomarker panel to identify individuals who carry this risk. We analysed the levels of 22 biomarkers in clinically classified healthy controls (HC), mild cognitive impairment (MCI) and Alzheimer’s participants from the well characterised Australian Imaging, Biomarker and Lifestyle (AIBL) study of aging. High levels of IL-10 and IL-12/23p40 were significantly associated with amyloid deposition in HC, suggesting that these two biomarkers might be used to detect at risk individuals. Additionally, other biomarkers (Eotaxin-3, Leptin, PYY) exhibited altered levels in AD participants possessing the APOE ε4 allele. This suggests that the physiology of some potential biomarkers may be altered in AD due to the APOE ε4 allele, a major risk factor for AD. Taken together, these data highlight several potential biomarkers that can be used in a blood-based panel to allow earlier identification of individuals at risk of developing AD and/or early stage AD for which current therapies may be more beneficial.

NB 06: From a simple lysosomotropic aSMase inhibitor to tools for elucidating the role of lysosomes in signaling apoptosis and LPS-induced inflammation

Ceramide generation is involved in signal transduction of cellular stress response, in particular during stress-induced apoptosis in response to stimuli such as minimally modified Low-density lipoproteins, TNFalpha and exogenous C₆-ceramide. In this paper we describe 48 diverse synthetic products and evaluate their lysosomotropic and acid sphingomyelinase inhibiting activities in macrophages. A stimuli-induced increase of C₁₆-ceramide in macrophages can be almost completely suppressed by representative compound NB 06 providing an effective protection of macrophages against apoptosis. Compounds like NB 06 thus offer highly interesting fields of application besides prevention of apoptosis of macrophages in atherosclerotic plaques in vessel walls. Most importantly, they can be used for blocking pH-dependent lysosomal processes and enzymes in general as well as for analyzing lysosomal dependent cellular signaling. Modulation of gene expression of several prominent inflammatory messengers IL1B, IL6, IL23A, CCL4 and CCL20 further indicate potentially beneficial effects in the field of (systemic) infections involving bacterial endotoxins like LPS or infections with influenza A virus.

The respiratory tract microbiome and lung inflammation: a two-way street

The lungs are not sterile or free from bacteria; rather, they harbor a distinct microbiome whose composition is driven by different ecological rules than for the gastrointestinal tract. During disease, there is often a shift in community composition towards Gammaproteobacteria, the bacterial class that contains many common lung-associated gram-negative "pathogens." Numerous byproducts of host inflammation are growth factors for these bacteria. The extracellular nutrient supply for bacteria in the lungs, which is severely limited during health, markedly increases due to the presence of mucus and vascular permeability. While Gammaproteobacteria benefit from airway inflammation, they also encode molecular components that promote inflammation, potentially creating a cyclical inflammatory mechanism. In contrast, Prevotella species that are routinely acquired via microaspiration from the oral cavity may participate in immunologic homeostasis of the airways.vAreas of future research include determining for specific lung diseases (1) whether an altered lung microbiome initiates disease pathogenesis, promotes chronic inflammation, or is merely a marker of injury and inflammation, (2) whether the lung microbiome can be manipulated therapeutically to change disease progression, (3) what molecules (metabolites) generated during an inflammatory response promote cross-kingdom signaling, and (4) how the lung "ecosystem" collapses during pneumonia, to be dominated by a single pathogen.

Ligand activation of the Ah receptor contributes to gastrointestinal homeostasis

The Ah receptor (AHR) is capable of binding a structurally diverse group of compounds that can be found in the diet, produced by bacteria in the gut and through endogenous metabolism. The gastrointestinal tract is a rich source of AHR ligands, which have been shown to protect the gut upon challenge with either pathogenic bacteria or toxic chemicals. The human AHR can be activated by a broader range of ligands compared to the mouse AHR, suggesting that studies in mice may underestimate the impact of AHR ligands in the human gut. The protective effect of AHR activation appears to be due to modulating the immune system within the gut. While several mechanisms have been established, due to the increasingly pleotropic nature of the AHR, other mechanisms of action likely exist that remain to be identified. The major contributors to AHR function in the gut and the most appropriate level of receptor activation that maintains intestinal homeostasis warrants further investigation.

Aryl hydrocarbon receptor signaling modifies Toll-like receptor-regulated responses in human dendritic cells

Currently, it is not well understood how ligands of the aryl hydrocarbon receptor (AhR) modify inflammatory responses triggered by Toll-like receptor (TLR) agonists in human dendritic cells (DCs). Here, we show that AhR ligands 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the tryptophan derivatives 6-formylindolo[3,2-b] carbazole (FICZ), kynurenine (kyn), and the natural dietary compound indole-3-carbinol (I3C) differentially modify cytokine expression in human monocyte-derived DCs (MoDCs). The results show that TLR-activated MoDCs express higher levels of AhR and are more sensitive toward the effects of AhR ligands. Depending on the cytokine, treatment with AhR ligands led to a synergistic or antagonistic effect of the TLR-triggered response in MoDCs. Thus, activation of AhR increased the expression of interleukin (IL)-1β, but decreased the expression of IL-12A in TLR-activated MoDCs. Furthermore, TCDD and FICZ may have opposite effects on the expression of cytochrome P4501A1 (CYP1A1) in TLR8-activated MoDCs indicating that the effect of the specific AhR ligand may depend on the presence of the specific TLR agonist. Gene silencing showed that synergistic effects of AhR ligands on TLR-induced expression of IL-1β require a functional AhR and the expression of NF-κB RelB. On the other hand, repression of IL-12A by TCDD and FICZ involved the induction of the caudal type homeobox 2 (CDX2) transcription factor. Additionally, the levels of DC surface markers were decreased in MoDCs by TCDD, FICZ and I3C, but not by kyn. Overall, these data demonstrate that AhR modulates TLR-induced expression of cytokines and DC-specific surface markers in MoDCs involving NFκB RelB and the immune regulatory factor CDX2.

The Transcriptional Repressor Polycomb Group Factor 6, PCGF6, Negatively Regulates Dendritic Cell Activation and Promotes Quiescence

Pro-inflammatory signals provided by the microenvironment are critical to activate dendritic cells (DCs), components of the innate immune system that shape both innate and adaptive immunity. However, to prevent inappropriate immune activation, mechanisms must be in place to restrain DC activation to ensure DCs are activated only once sufficient stimuli have been received. Here, we report that DC activation and immunogenicity are regulated by the transcriptional repressor Polycomb group factor 6 (PCGF6). Pcgf6 is rapidly downregulated upon stimulation, and this downregulation is necessary to permit full DC activation. Silencing PCGF6 expression enhanced both spontaneous and stimulated DC activation. We show that PCGF6 associates with the H3K4me3 demethylase JARID1c, and together, they negatively regulate H3K4me3 levels in DCs. Our results identify two key regulators, PCGF6 and JARID1c that temper DC activation and implicate active transcriptional silencing via histone demethylation as a previously unappreciated mechanism for regulating DC activation and quiescence.

Cockroach allergen exposure and risk of asthma

Cockroach sensitization is an important risk factor for the development of asthma. However, its underlying immune mechanisms and the genetic etiology for differences in allergic responses remain unclear. Cockroach allergens identification and their expression as biologically active recombinant proteins have provided a basis for studying the mechanisms regarding cockroach allergen-induced allergic sensitization and asthma. Glycans in allergens may play a crucial role in the immunogenicity of allergic diseases. Protease-activated receptor (PAR)-2, Toll-like receptor (TLR), and C-type lectin receptors have been suggested to be important for the penetration of cockroach allergens through epithelial cells to mediate allergen uptake, dendritic cell maturation, antigen-presenting cell (APC) function in T-cell polarization, and cytokine production. Environmental pollutants, which often coexist with the allergen, could synergistically elicit allergic inflammation, and aryl hydrocarbon receptor (AhR) activation and signaling may serve as a link between these two elements. Genetic factors may also play an important role in conferring the susceptibility to cockroach sensitization. Several genes have been associated with cockroach sensitization and asthma-related phenotypes. In this review, we will discuss the epidemiological evidence for cockroach allergen-induced asthma, cockroach allergens, the mechanisms regarding cockroach allergen-induced innate immune responses, and the genetic basis for cockroach sensitization.