Exposure Cessation During Adulthood Did Not Prevent Immunotoxicity Caused by Developmental Exposure to Low-Level Trichloroethylene in Drinking Water

Trichloroethylene metabolite modulates DNA methylation-dependent gene expression in Th1-polarized CD4+ T cells from autoimmune-prone mice

Trichloroethylene (TCE) is an industrial solvent and widespread environmental contaminant associated with CD4+ T-cell activation and autoimmune disease. Prior studies showed that exposure to TCE in the drinking water of autoimmune-prone mice expanded effector/memory CD4+ T cells with an interferon-γ (IFN-γ)-secreting Th1-like phenotype. However, very little is known how TCE exposure skews CD4+ T cells towards this pro-inflammatory Th1 subset. As observed previously, TCE exposure was associated with hypermethylation of regions of the genome related to transcriptional repression in purified effector/memory CD4 T cells. We hypothesized that TCE modulates transcriptional and/or epigenetic programming of CD4+ T cells as they differentiate from a naive to effector phenotype. In the current study, purified naive CD4 T cells from both male and female autoimmune-prone MRL/MpJ mice were activated ex vivo and polarized towards a Th1 subset for 4 days in the presence or absence of the oxidative metabolite of TCE, trichloroacetaldehyde hydrate (TCAH) in vitro. An RNA-seq assessment and reduced representation bisulfite sequencing for DNA methylation were conducted on Th1 cells or activated, non-polarized cells. The results demonstrated TCAH's ability to regulate key genes involved in the immune response and autoimmunity, including Ifng, by altering the level of DNA methylation at the gene promoter. Intriguing sex differences were observed and for the most part, the effects were more robust in females compared to males. In conclusion, TCE via TCAH epigenetically regulates gene expression in CD4+ T cells. These results may have implications for mechanistic understanding or future therapeutics for autoimmunity.

SET-mediated epigenetic dysregulation of p53 impairs trichloroethylene-induced DNA damage response

Trichloroethylene (TCE) was a widely used industrial solvent, and now has become a major environmental pollutant. Exposure to TCE has been found to result in significant damage to the liver, leading to hepatic toxicity. In our previous study, we discovered that a histone chaperon called SET plays a crucial role in mediating the DNA damage and apoptosis caused by TCE in hepatic cells. However, the precise function of SET in the response to DNA damage is still not fully understood. In this study, we evaluated TCE-induced DNA damage of hepatic L-02 cells with SET-knockdown, then analyzed alterations of H3K79me3 and p53 in hepatic cells and carcinogenic mice livers. Results suggested that SET interferes with DNA response via mediating down-regulation of p53 and partially suppressing H3K79me3 under treatment of TCE. To further verify the regulatory cascade, H3K79me3 was reduced and p53 was knocked down in L-02 cells respectively, and extent of DNA damage was evaluated. Reduced H3K79me3 was found leading to down-regulation of p53 which further exacerbated TCE-induced DNA injury. These findings demonstrated that SET-H3K79me3-p53 served as an epigenetic regulatory axis involved in TCE-induced DNA damage response.

Developmental trichloroethylene exposure enhances predictive markers of autoimmunity in a sex-specific manner in disease-resistant female mice

Trichloroethylene (TCE) is a widely used industrial chemical and common environmental pollutant. Exposure to TCE promotes CD4⁺ T cell-driven autoimmunity including autoimmune hepatitis (AIH) in both humans and female autoimmune-prone mice. Because the developing immune system is more sensitive during development, we predicted that non- autoimmune-prone, C57/Bl6 (B6) mice would exhibit some autoimmune-related changes using the Developmental Origins of Health and Disease (DOHaD) model of exposure. Both male and female mice were exposed to vehicle or an environmentally relevant dose of 5 μg/ml TCE (0.9 mg/kg/day) beginning at 2 weeks pre-conception and ending at weaning. CD4+ T cells were assessed for phenotypic markers by flow cytometry. An assessment of cytokines elicited ex vivo after 4d polarization from naïve to CD4⁺ T helper subsets (i.e., Th1, Th17, and T reg) was conducted. mRNA expression of liver genes associated with inflammation, regeneration/repair associated with AIH disease progression in autoimmune-prone mice were evaluated by qRT-PCR. The results demonstrated TCE's ability to induce autoimmune- related biomarkers in B6 mice to an even greater degree in females compared to males when exposed during development.

Cx43 overexpression is involved in the hyper-proliferation effect of trichloroethylene on human embryonic stem cells

Trichloroethylene (TCE) is a major environmental contaminant. Maternal exposure of TCE is linked to developmental defects, but the mechanisms remain to be elucidated. Along with a strategy of 3Rs principle, human embryonic stem cells (hESCs) are regarded as most promising in vitro models for developmental toxicity studies. TCE interfered with hESCs differentiation, but no report was available for TCE effects on hESCs proliferation. Here, we aimed to explore the toxic effects and mechanisms of TCE on hESCs proliferation. Treatment with TCE, did not affect the pluripotency genes expression. However, TCE enhanced hESCs proliferation, manifested by increased cell number, PCNA expression and EdU incorporation. Moreover, TCE exposure upregulated the protein expression levels of Cx43 and cyclin-dependent kinases. Knockdown of Cx43 attenuated the TCE-induced cell hyper-proliferation and CDK2 upregulation. Furthermore, TCE increased Akt phosphorylation, and the inhibition of Akt blocked the TCE-induced Cx43 overexpression and cell proliferation. In conclusion, TCE exposure resulted in upregulation of Cx43 via Akt phosphorylation, consequently stimulated CDK2 expression, contributing to hyper-proliferation in hESCs. Our study brings to light that TCE stimulated the proliferation of hESCs via Cx43, providing a new research avenue for the causes of TCE-induced developmental toxicity.

Serum levels of miR-21-5p and miR-339-5p associate with occupational trichloroethylene hypersensitivity syndrome

Background

Trichloroethylene (TCE) hypersensitivity syndrome (THS) is a dose-independent and potentially life-threatening disease. In this study, we sought to identify THS-related miRNAs and evaluate its potential clinical value.

Methods

Serum samples of five patients and five matched TCE contacts were used for screening differential miRNAs. Another 34 patients and 34 matched TCE contacts were used for verifying significantly differential miRNAs with SYBR™ Green PCR and MGB PCR. The diagnostic model based on these miRNAs was established via the support vector machine (SVM) algorithm. Correlation between differential miRNAs and liver function was analyzed via the Spearman correlation test.

Results

A total of 69 miRNAs was found to be differentially expressed. MiR-21-5p and miR-339-5p were verified to have significant higher expressions in patients. The sensitivity, specificity and accuracy of disease model were 100, 75 and 86%, respectively. The two miRNAs showed significant correlations with liver function.

Conclusion

These findings suggested that miRNAs profiles in serum of THS patients had changed significantly, and miR-21-5p and miR-339-5p were associated with THS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12995-021-00308-0.

Investigating Molecular Mechanisms of Immunotoxicity and the Utility of ToxCast for Immunotoxicity Screening of Chemicals Added to Food

The development of high-throughput screening methodologies may decrease the need for laboratory animals for toxicity testing. Here, we investigate the potential of assessing immunotoxicity with high-throughput screening data from the U.S. Environmental Protection Agency ToxCast program. As case studies, we analyzed the most common chemicals added to food as well as per- and polyfluoroalkyl substances (PFAS) shown to migrate to food from packaging materials or processing equipment. The antioxidant preservative tert-butylhydroquinone (TBHQ) showed activity both in ToxCast assays and in classical immunological assays, suggesting that it may affect the immune response in people. From the PFAS group, we identified eight substances that can migrate from food contact materials and have ToxCast data. In epidemiological and toxicological studies, PFAS suppress the immune system and decrease the response to vaccination. However, most PFAS show weak or no activity in immune-related ToxCast assays. This lack of concordance between toxicological and high-throughput data for common PFAS indicates the current limitations of in vitro screening for analyzing immunotoxicity. High-throughput in vitro assays show promise for providing mechanistic data relevant for immune risk assessment. In contrast, the lack of immune-specific activity in the existing high-throughput assays cannot validate the safety of a chemical for the immune system.

Sex-Dependent Effects on Liver Inflammation and Gut Microbial Dysbiosis After Continuous Developmental Exposure to Trichloroethylene in Autoimmune-Prone Mice

Trichloroethylene (TCE) is a common environmental toxicant linked with hypersensitivity and autoimmune responses in humans and animal models. While autoimmune diseases are more common in females, mechanisms behind this disparity are not clear. Recent evidence suggests that autoimmunity may be increasing in males, and occupational studies have shown that TCE-mediated hypersensitivity responses occur just as often in males. Previous experimental studies in autoimmune-prone MRL^+/+ mice have focused on responses in females. However, it is important to include both males and females in order to better understand sex-disparity in autoimmune disease. In addition, because of an alarming increase in autoimmunity in adolescents, developmental and/or early life exposures to immune-enhancing environmental pollutants should also be considered. Using MRL^+/+ mice, we hypothesized that TCE would alter markers related to autoimmunity to a greater degree in female mice relative to male mice, and that TCE would enhance these effects. Mice were continuously exposed to either TCE or vehicle beginning at gestation, continuing during lactation, and directly in the drinking water. Both male and female offspring were evaluated at 7 weeks of age. Sex-specific effects were evident. Female mice were more likely than males to show enhanced CD4⁺ T cell cytokine responses (e.g., IL-4 and IFN-γ). Although none of the animals developed pathological or serological signs of autoimmune hepatitis-like disease, TCE-exposed female mice were more likely than males in either group to express higher levels of biomarkers in the liver related to regeneration/repair and proliferation. Levels of bacterial populations in the intestinal ileum were also altered by TCE exposure and were more prominent in females as compared to males. Thus, our expectations were correct in that young adult female mice developmentally exposed to TCE were more likely to exhibit alterations in immunological and gut/liver endpoints compared to male mice.

Complex epigenetic patterns in cerebellum generated after developmental exposure to trichloroethylene and/or high fat diet in autoimmune-prone mice

Trichloroethylene (TCE) is an environmental contaminant associated with immune-mediated inflammatory disorders and neurotoxicity. Based on known negative effects of developmental overnutrition on neurodevelopment, we hypothesized that developmental exposure to high fat diet (HFD) consisting of 40% kcal fat would enhance neurotoxicity of low-level (6 μg per kg per day) TCE exposure in offspring over either stressor alone. Male offspring were evaluated at ∼6 weeks of age after exposure beginning 4 weeks preconception in the dams until weaning. TCE, whether used as a single exposure or together with HFD, appeared to be more robust than HFD alone in altering one-carbon metabolites involved in glutathione redox homeostasis and methylation capacity. In contrast, opposing effects of expression of key enzymes related to DNA methylation related to HFD and TCE exposure were observed. The mice generated unique patterns of anti-brain antibodies detected by western blotting attributable to both TCE and HFD. Taken together, developmental exposure to TCE and/or HFD appear to act in complex ways to alter brain biomarkers in offspring.

Continuous Developmental and Early Life Trichloroethylene Exposure Promoted DNA Methylation Alterations in Polycomb Protein Binding Sites in Effector/Memory CD4+ T Cells

Trichloroethylene (TCE) is an industrial solvent and drinking water pollutant associated with CD4⁺ T cell-mediated autoimmunity. In our mouse model, discontinuation of TCE exposure during adulthood after developmental exposure did not prevent immunotoxicity. To determine whether persistent effects were linked to epigenetic changes we conducted whole genome reduced representation bisulfite sequencing (RRBS) to evaluate methylation of CpG sites in autosomal chromosomes in activated effector/memory CD4⁺ T cells. Female MRL+/+ mice were exposed to vehicle control or TCE in the drinking water from gestation until ~37 weeks of age [postnatal day (PND) 259]. In a subset of mice, TCE exposure was discontinued at ~22 weeks of age (PND 154). At PND 259, RRBS assessment revealed more global methylation changes in the continuous exposure group vs. the discontinuous exposure group. A majority of the differentially methylated CpG regions (DMRs) across promoters, islands, and regulatory elements were hypermethylated (~90%). However, continuous developmental TCE exposure altered the methylation of 274 CpG sites in promoters and CpG islands. In contrast, only 4 CpG island regions were differentially methylated (hypermethylated) in the discontinuous group. Interestingly, 2 of these 4 sites were also hypermethylated in the continuous exposure group, and both of these island regions are associated with lysine 27 on histone H3 (H3K27) involved in polycomb complex-dependent transcriptional repression via H3K27 tri-methylation. CpG sites were overlapped with the Open Regulatory Annotation database. Unlike the discontinuous group, continuous TCE treatment resulted in 129 DMRs including 12 unique transcription factors and regulatory elements; 80% of which were enriched for one or more polycomb group (PcG) protein binding regions (i.e., SUZ12, EZH2, JARID2, and MTF2). Pathway analysis of the DMRs indicated that TCE primarily altered the methylation of genes associated with regulation of cellular metabolism and cell signaling. The results demonstrated that continuous developmental exposure to TCE differentially methylated binding sites of PcG proteins in effector/memory CD4⁺ cells. There were minimal yet potentially biologically significant effects that occurred when exposure was discontinued. These results point toward a novel mechanism by which chronic developmental TCE exposure may alter terminally differentiated CD4⁺ T cell function in adulthood.

Environmental cues received during development shape dendritic cell responses later in life

Environmental signals mediated via the aryl hydrocarbon receptor (AHR) shape the developing immune system and influence immune function. Developmental exposure to AHR binding chemicals causes persistent changes in CD4+ and CD8+ T cell responses later in life, including dampened clonal expansion and differentiation during influenza A virus (IAV) infection. Naïve T cells require activation by dendritic cells (DCs), and AHR ligands modulate the function of DCs from adult organisms. Yet, the consequences of developmental AHR activation by exogenous ligands on DCs later in life has not been examined. We report here that early life activation of AHR durably reduces the ability of DC to activate naïve IAV-specific CD8+ T cells; however, activation of naïve CD4+ T cells was not impaired. Also, DCs from developmentally exposed offspring migrated more poorly than DCs from control dams in both in vivo and ex vivo assessments of DC migration. Conditional knockout mice, which lack Ahr in CD11c lineage cells, suggest that dampened DC emigration is intrinsic to DCs. Yet, levels of chemokine receptor 7 (CCR7), a key regulator of DC trafficking, were generally unaffected. Gene expression analyses reveal changes in Lrp1, Itgam, and Fcgr1 expression, and point to alterations in genes that regulate DC migration and antigen processing and presentation as being among pathways disrupted by inappropriate AHR signaling during development. These studies establish that AHR activation during development causes long-lasting changes to DCs, and provide new information regarding how early life environmental cues shape immune function later in life.

Irreversible effects of trichloroethylene on the gut microbial community and gut-associated immune responses in autoimmune-prone mice

The developing immune system is particularly sensitive to immunotoxicants. This study assessed trichloroethylene (TCE)-induced effects on the gut microbiome and cytokine production during the development in mice. Mice were exposed to TCE (0.05 or 500 μg/mL) at the levels that approximate to environmental or occupational exposure, respectively. Mice were subjected to a continuous developmental exposure to these doses encompassing gestation, lactation and continuing directly in the drinking water postnatally for 154 days (PND154) or PND259. To observe persistence of the effect TCE was removed from the drinking water in a subset of mice on PND154 and were provided regular drinking water until the study terminus (PND259). Abundance of total tissue-associated bacteria reduced only in mice exposed to TCE until PND259. The ratio of Firmicutes/Bacteroidetes did not alter during this continuos exposure; however, cessation of high-dose TCE at PND154 resulted in the increased abundance Bacteroidetes at PND259. Furthermore, high-dose TCE exposure until PND259 resulted in a lower abundance of the genera Bacteroides and Lactobaccilus and increased abundance of genus Bifidobactrium and bacterial family Enterobacteriaceae. TCE exposure until PND154 showed significant changes in the production of interleukin-33; that might play a dual role in maintaining the balance and homeostasis between commensal microbiota and mucosal health. At PND259, interleukin-3, granulocyte-macrophage colony-stimulating factor and Eotaxin were altered in both, the continuous exposure and cessation groups, whereas only a cessation group had a higher level of KC that may facilitate infiltration of neutrophils. The irreversible effects of TCE after a period of exposure cessation suggested a unique programming and potential toxicity of TCE even at the environmental level exposure.

Opposing Actions of Developmental Trichloroethylene and High-Fat Diet Coexposure on Markers of Lipogenesis and Inflammation in Autoimmune-Prone Mice

Trichloroethylene (TCE) is a widespread environmental pollutant associated with immunotoxicity and autoimmune disease. Previous studies showed that mice exposed from gestation through early life demonstrated CD4+ T cell alterations and autoimmune hepatitis. Determining the role of one environmental risk factor for any disease is complicated by the presence of other stressors. Based on its known effects, we hypothesized that developmental overnutrition in the form of a moderately high-fat diet (HFD) consisting of 40% kcal fat would exacerbate the immunotoxicity and autoimmune-promoting effects of low-level (<10 μg/kg/day) TCE in autoimmune-prone MRL+/+ mice over either stressor alone. When female offspring were evaluated at 27 weeks of age we found that a continuous exposure beginning at 4 weeks preconception in the dams until 10 weeks of age in offspring that TCE and HFD promoted unique effects that were often antagonistic. For a number of adiposity endpoints, TCE significantly reversed the expected effects of HFD on expression of genes involved in fatty acid synthesis/insulin resistance, as well as mean pathology scores of steatosis. Although none of the animals developed pathological signs of autoimmune hepatitis, the mice generated unique patterns of antiliver antibodies detected by western blotting attributable to TCE exposure. A majority of cytokines in liver, gut, and splenic CD4+ T cells were significantly altered by TCE, but not HFD. Levels of bacterial populations in the intestinal ileum were also altered by TCE exposure rather than HFD. Thus, in contrast to our expectations this coexposure did not promote synergistic effects.

Epigenetic underpinnings of developmental immunotoxicity and autoimmune disease

The concordance rate for developing autoimmune disease in identical twins is around 50% demonstrating that gene and environmental interactions contribute to disease etiology. The environmental contribution to autoimmune disease is a wide-ranging concept including exposure to immunotoxic environmental chemicals. Because the immune system is immature during development suggests that adult-onset autoimmunity may originate when the immune system is particularly sensitive. Among the pollutants most closely associated with inflammation and/or autoimmunity include Bisphenol-A, mercury, TCDD, and trichloroethylene. These toxicants have been shown to impart epigenetic changes (e.g., DNA methylation) that may alter immune function and promote autoreactivity. Here we review these autoimmune-promoting toxicants and their relation to immune cell epigenetics both in terms of adult and developmental exposure.