Search for the target genes involved in the suppression of antibody production by TCDD in C57BL/6 mice

EDC-induced mechanisms of immunotoxicity: a systematic review

Endocrine-disrupting chemicals (EDCs) refer to a group of chemicals that cause adverse effects in human health, impairing hormone production and regulation, resulting in alteration of homeostasis, reproductive, and developmental, and immune system impairments. The immunotoxicity of EDCs involves many mechanisms altering gene expression that depend on the activation of nuclear receptors such as the aryl hydrocarbon receptor (AHR), the estrogen receptor (ER), and the peroxisome proliferator-activated receptor (PPAR), which also results in skin and intestinal disorders, microbiota alterations and inflammatory diseases. This systematic review aims to review different mechanisms of immunotoxicity and immunomodulation of T cells, focusing on T regulatory (Treg) and Th17 subsets, B cells, and dendritic cells (DCs) caused by specific EDCs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), bisphenols (BPs) and polyfluoroalkyl substances (PFASs). To achieve this objective, a systematic study was conducted searching various databases including PubMed and Scopus to find in-vitro, in-vivo, and biomonitoring studies that examine EDC-dependent mechanisms of immunotoxicity. While doing the systematic review, we found species- and cell-specific outcomes and a translational gap between in-vitro and in-vivo experiments. Finally, an adverse outcome pathway (AOP) framework is proposed, which explains mechanistically toxicity endpoints emerging from different EDCs having similar key events and can help to improve our understanding of EDCs mechanisms of immunotoxicity. In conclusion, this review provides insights into the mechanisms of immunotoxicity mediated by EDCs and will help to improve human health risk assessment.

Constitutively active aryl hydrocarbon receptor expressed in T cells increases immunization-induced IFN-gamma production in mice but does not suppress T(h)2-cytokine production or antibody production

The ligand-dependent transcription factor aryl hydrocarbon receptor (AhR) has been implicated in various immune functions. Our previous studies have shown that AhR activation by exposure of ovalbumin (OVA)-immunized mice to the potent ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases immunization-induced IFN-gamma production in the spleen and suppresses the production of T(h)2 cytokines and OVA-specific antibodies. In the present study, we used transgenic (Tg) mice that express a constitutively active mutant of aryl hydrocarbon receptor (CA-AhR) specifically in T-lineage cells to clarify the role of AhR activation in T cells in these reactions. The results of this study clearly demonstrated that AhR activation only in the T cells augments IFN-gamma production upon OVA immunization. By contrast, production of T(h)2 cytokines and antibodies were not significantly suppressed by CA-AhR in the T cells. These results suggest that suppression of T(h)2 cytokines and antibodies production require AhR activation not only in T cells but also in other cell types as caused by TCDD exposure. Alternatively, these results may indicate that IFN-gamma augmentation and T(h)2 cytokines and antibodies suppression depend on different ways of functions of AhR in the T cells and that CA-AhR does not replicate the suppressive effect of TCDD-activated AhR on T(h)2 cytokines and antibodies. Expression of CA-AhR in the T cells was also shown to increase the percentage of CD25(+) cells among CD4(+) cells in the thymus and spleen. Thus, studies using T-cell-specific CA-AhR Tg mice provide a way to dissect the role of AhR in individual cell types and how the AhR functions.

Toxicogenomics: transcription profiling for toxicology assessment

Toxicogenomics, the application of transcription profiling to toxicology, has been widely used for elucidating the molecular and cellular actions of chemicals and other environmental stressors on biological systems, predicting toxicity before any functional damages, and classification of known or new toxicants based on signatures of gene expression. The success of a toxicogenomics study depends upon close collaboration among experts in different fields, including a toxicologist or biologist, a bioinformatician, statistician, physician and, sometimes, mathematician. This review is focused on toxicogenomics studies, including transcription profiling technology, experimental design, significant gene extraction, toxicological results interpretation, potential pathway identification, database input and the applications of toxicogenomics in various fields of toxicological study.

The aryl hydrocarbon receptor is a modulator of anti-viral immunity

Although immune modulation by AhR ligands has been studied for many years, the impact of AhR activation on host defenses against viral infection has not, until recently, garnered much attention. The development of novel reagents and model systems, new information regarding anti-viral immunity, and a growing appreciation for the global health threat posed by viruses have invigorated interest in understanding how environmental signals affect susceptibility to and pathological consequences of viral infection. Using influenza A virus as a model of respiratory viral infection, recent studies show that AhR activation cues signaling events in both leukocytes and non-immune cells. Functional alterations include suppressed lymphocyte responses and increased inflammation in the infected lung. AhR-mediated events within and extrinsic to hematopoietic cells has been investigated using bone marrow chimeras, which show that AhR alters different elements of the immune response by affecting different tissue targets. In particular, suppressed CD8(+) T cell responses are due to deregulated events within leukocytes themselves, whereas increased neutrophil recruitment to and IFN-gamma levels in the lung result from AhR-regulated events extrinsic to bone marrow-derived cells. This latter discovery suggests that epithelial and endothelial cells are overlooked targets of AhR-mediated changes in immune function. Further support that AhR influences host cell responses to viral infection are provided by several studies demonstrating that AhR interacts directly with viral proteins and affects viral latency. While AhR clearly modulates host responses to viral infection, we still have much to understand about the complex interactions between immune cells, viruses, and the host environment.

Molecular Mechanisms of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Cardiovascular Embryotoxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons are widespread environmental contaminants and potent developmental toxicants. Hallmarks of embryonic exposure include edema, hemorrhage, and mortality. Recent studies in zebrafish and chicken have revealed direct impairment of cardiac muscle growth that may underlie these overt symptoms. TCDD toxicity is mediated by the aryl hydrocarbon receptor, but downstream targets remain unclear. Oxidative stress and growth factor modulation have been implicated in TCDD cardiovascular toxicity. Gene expression profiling is elucidating additional pathways by which TCDD might act. We review our understanding of the mechanism of TCDD embryotoxicity at morphological and molecular levels.

Environmental toxins as modulators of antiviral immune responses

Exposure to environmental contaminants has a profound effect on immune function, yet mechanistic understanding of how pollutants deregulate immune responses has, for many chemicals, remained elusive. Available data suggest that certain pollutants alter host immune responses and increase susceptibility to viral infection. In particular, data from a combination of epidemiological and animal studies show that exposure to dioxins, cigarette smoke, diesel exhaust and other air pollutants increase pathology associated with infection. Mechanistically, some of these chemicals disrupt the kinetics and efficacy of innate and adaptive responses to infection, whereas others influence viral latency. While there remain considerable gaps in our knowledge of the complex interactions between viruses, immune cells, and the host environment, these observations indicate that pollutants are important but overlooked contributors to susceptibility and pathogenesis of viral infections.

Toxicogenomics in the assessment of immunotoxicity

Microarray analysis is used for simultaneous measurement of expression of thousands of genes in a given sample and as such extends and deepens our understanding of biological processes. Application of the technique in toxicology is referred to as toxicogenomics. The examples of assessment of immunotoxicity by gene expression profiling presented and discussed here, show that microarray analysis is able to detect known and novel effects of a wide range of immunomodulating agents. Besides the elucidation of mechanisms of action, toxicogenomics is also applied to predict consequences of exposing biological systems to toxic agents. Successful attempts to classify compounds using signature gene expression profiles have been reported. These did, however, not specifically focus on immunotoxicity. Databases containing expression profiles can facilitate the applications of toxicogenomics. Platforms and methodologies for gene expression profiling may vary, however, hampering data compiling across different laboratories. Therefore, attention is paid to standardization of the generation, reporting, and management of microarray data. Obtained gene expression profiles should be anchored to pathological and functional endpoints for correct interpretation of results. These issues are also important when using toxicogenomics in risk assessment. The application of toxicogenomics in evaluation of immunotoxicity is thus not yet without challenges. It already contributes to the understanding of immunotoxic processes and the development of in vitro screening assays, though, and is therefore expected to be of value for mechanistic insight into immunotoxicity and hazard identification of existing and novel compounds.

The immune system as a target for environmental chemicals: Xenoestrogens and other compounds

The immune system in higher organisms is under integrated control and has the capacity to rapidly respond to the environment. Recently, there has been a significant increase in the prevalence of allergic diseases. Environmental factors likely play a major role in the explosion of allergy. Although the "hygiene hypothesis" may explain the increase in allergic diseases which are prone to T helper 2 (Th2) immune responses, recent findings highlight the possible involvement of environmental xenobiotic chemicals which can modulate normal immune function. Interestingly, several reports suggest that the prevalence of systemic lupus erythematosus, a Th2-type autoimmune disease, is also increasing, although the development of high-sensitivity immunological tests may be a possible cause. The increased prevalence of autoimmune disease in women, the sexual dimorphism of the immune response, and the immunomodulatory effects of sex steroids, have focused attention on the role of chemicals which influence sex steroids in the development of immune diseases. Moreover, recent reports indicate that some environmental chemicals can work on nuclear hormone receptors, other than sex hormone receptors, and modulate immune reactions. This review focuses on the impact of environmental chemicals on immune system function and pathogenesis of immune diseases, including allergy and autoimmune diseases.

Constitutively Active Aryl Hydrocarbon Receptor Expressed Specifically in T-Lineage Cells Causes Thymus Involution and Suppresses the Immunization-Induced Increase in Splenocytes

The aryl hydrocarbon receptor (AhR) is a transcription factor belonging to the basic helix-loop-helix-PER-ARNT-SIM superfamily. Xenobiotics, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, bind the receptor and trigger diverse biological reactions. Thymocyte development and T cell-dependent immune reactions are sensitive targets of AhR-dependent 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity. However, the exact role of the AhR in T cells in animals exposed to exogenous ligands has not been clarified because indirect effects of activated AhR in other cell types cannot be excluded. In this study, we generated transgenic (Tg) mice expressing a constitutively active mutant of AhR under the regulation of a T cell-specific CD2 promoter to examine AhR function in T cells. The mRNAs of the constitutively active mutant of AhR and an AhR-induced gene, CYP1A1, were expressed in the thymus and spleen of the Tg mice. The transgene expression was clearly detected in the thymocytes, CD4, and CD8 T cells, but not in the B cells or thymus stromal cells. These Tg mice had a decreased number of thymocytes and an increased percentage of CD8 single-positive thymocytes, but their splenocytes were much less affected. By contrast, the increase in number of T cells and B cells taking place in the spleen after immunization was significantly suppressed in the Tg mice. These results clearly show that AhR activation in the T-lineage cells is directly involved in thymocyte loss and skewed differentiation. They also indicate that AhR activation in T cells and not in B cells suppresses the immunization-induced increase in both T cells and B cells.