The in vitro proliferation of murine lymphocytes to mercuric chloride is restricted to mature T cells and is interleukin 1 dependent

Modulatory Effects of Mercury (II) Chloride (HgCl2 ) on Chicken Macrophage and B-Lymphocyte Cell Lines with Viral-Like Challenges In Vitro

Mercury (Hg) is a toxic trace metal ubiquitously distributed in the environment. Inorganic mercury (as HgCl₂ ) can cause immunotoxicity in birds, but the mechanisms of action are still not fully resolved, especially with respect to responses to viral infections. To investigate the potential immunomodulatory effects of Hg²⁺ on specific cell types of the avian immune system, chicken macrophage (HD-11) and B-lymphocyte (DT40) cell lines were applied as in vitro models for the innate and adaptive immune systems, respectively. The cells were stimulated with synthetic double-stranded RNA, which can be recognized by toll-like receptor-3 to mimic a viral infection. The Hg²⁺ showed concentration-dependent cytotoxicity in both cell lines, with similar median effect concentrations at 30 µM. The cytotoxicity of Hg²⁺ was closely related to glutathione (GSH) depletion and reactive oxygen species induction, whereas the de novo synthesis of GSH acted as a primary protective strategy. Nitric oxide produced by activated macrophages was strongly inhibited by Hg²⁺ , and was also influenced by cellular GSH levels. Cell proliferation, gene expression of microRNA-155, and cellular IgM levels in B cells were decreased at noncytotoxic Hg²⁺ concentrations. The secretion of antiviral interferon-α was induced by Hg²⁺ in both cell lines. Overall, our results suggest that Hg²⁺ exposure can cause immunomodulatory effects in birds by disrupting immune cell proliferation and cytokine production, and might result in disorders of the avian immune system. Environ Toxicol Chem 2021;40:2813-2824. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The Association Between Mercury Exposure and Atopic Dermatitis in Early Childhood: A Mothers and Children's Environmental Health Study

BACKGROUND

Atopic dermatitis is a chronic and relapsing inflammatory skin disease. Although mercury has been suggested as a risk factor, the underlying mechanism and the relationship between mercury and atopic dermatitis remains unclear. The objective of the present study was to investigate the relationship between mercury exposure and the presence of atopic dermatitis in early childhood.

METHODS

This study is part of the prospective Mothers and Children's Environmental Health cohort study. A total of 1,751 pregnant women were enrolled in Mothers and Children's Environmental Health. After delivery, children were followed up. Blood samples were collected and mothers were asked about the presence of atopic dermatitis in their children via a questionnaire at 6, 12, 24, 36, and 60 months of age.

RESULTS

After excluding participants who did not meet the inclusion criteria, a total of 1,061 mother-children pairs were included in the analysis. The geometric mean of mercury concentrations in cord blood was 5.1 µg/L. In adjusted models, cord blood mercury exposure (odds ratio [OR] = 1.1; 95% confidence interval [CI] = 1.0, 1.2 at 12-24 months) and postnatal mercury exposure (OR = 1.2; 95% CI = 1.0, 1.5 at 24-36 months, OR = 1.4; 95% CI = 1.1, 1.8 at 48-60 months) were associated with the presence of atopic dermatitis in children.

CONCLUSIONS

Postnatal mercury exposure at 24 months of age increases the risk of atopic dermatitis in children.

Mercury-induced inflammation and autoimmunity

BACKGROUND

Human exposure to mercury leads to a variety of pathologies involving numerous organ systems including the immune system. A paucity of epidemiological studies and suitable diagnostic criteria, however, has hampered collection of sufficient data to support a causative role for mercury in autoimmune diseases. Nevertheless, there is evidence that mercury exposure in humans is linked to markers of inflammation and autoimmunity. This is supported by experimental animal model studies, which convincingly demonstrate the biological plausibility of mercury as a factor in the pathogenesis of autoimmune disease.

SCOPE OF THE REVIEW

In this review, we focus on ability of mercury to elicit inflammatory and autoimmune responses in both humans and experimental animal models.

MAJOR CONCLUSIONS

Although subtle differences exist, the inflammatory and autoimmune responses elicited by mercury exposure in humans and experimental animal models show many similarities. Proinflammatory cytokine expression, lymphoproliferation, autoantibody production, and nephropathy are common outcomes. Animal studies have revealed significant strain dependent differences in inflammation and autoimmunity suggesting genetic regulation. This has been confirmed by the requirement for individual genes as well as genome wide association studies. Importantly, many of the genes required for mercury-induced inflammation and autoimmunity are also required for idiopathic systemic autoimmunity. A notable difference is that mercury-induced autoimmunity does not require type I IFN. This observation suggests that mercury-induced autoimmunity may arise by both common and specific pathways, thereby raising the possibility of devising criteria for environmentally associated autoimmunity.

GENERAL SIGNIFICANCE

Mercury exposure likely contributes to the pathogenesis of autoimmunity.

Protective Effect of Leaf Ethanolic Extract Etlingera hemisphaerica Blume Against Mercuric Chloride Toxicity in Blood of Mice

This research was intended to investigate the protective effect of leaf ethanolic extract Etlingera hemisphaerica Blume (LE3H) against mercuric chloride (HgCl₂) toxicity in blood of mice (Mus musculus). The experimental animals, 95 male M. musculus, received drink and food ad libitum. Three materials were tested: LE3H (0.13, 0.26, 0.39 mg/g body weight [bw]) was administered by gavage; HgCl₂ (5 mg/kg bw) was administrated by gavage or intraperitoneal injection; and Imunos (the nutritional supplement to stimulate the immune system; 0.2 mg/g bw), as a positive control for LE3H treatment, was given by gavage. Blood samples were taken from the tails for determining number of blood cells. The animals were killed by cervical dislocation (CD), and then blood samples were collected from the hearts for protein electrophoresis. Results revealed the same number of leukocytes with LE3H (0.39 mg/g bw) treatment as with the Imunos treatment. HgCl₂ administration increased leukocytes and decreased erythrocytes; HgCl₂ administration followed by LE3H (0.39 mg/g bw) treatment protected the amount of blood cells as well as the control. HgCl₂ administration showed a new 125 kDa protein and caused overexpression of 48 kDa protein; this protein profile could be protected by LE3H (0.39 mg/g bw) treatment as in the control condition. We conclude that LE3H provides a protective effect against HgCl₂ toxicity in blood of M. musculus.

Induction of Systemic Autoimmunity by a Xenobiotic Requires Endosomal TLR Trafficking and Signaling from the Late Endosome and Endolysosome but Not Type I IFN

Type I IFN and nucleic acid-sensing TLRs are both strongly implicated in the pathogenesis of lupus, with most patients expressing IFN-induced genes in peripheral blood cells and with TLRs promoting type I IFNs and autoreactive B cells. About a third of systemic lupus erythematosus patients, however, lack the IFN signature, suggesting the possibility of type I IFN-independent mechanisms. In this study, we examined the role of type I IFN and TLR trafficking and signaling in xenobiotic systemic mercury-induced autoimmunity (HgIA). Strikingly, autoantibody production in HgIA was not dependent on the type I IFN receptor even in NZB mice that require type I IFN signaling for spontaneous disease, but was dependent on the endosomal TLR transporter UNC93B1 and the endosomal proton transporter, solute carrier family 15, member 4. HgIA also required the adaptor protein-3 complex, which transports TLRs from the early endosome to the late endolysosomal compartments. Examination of TLR signaling pathways implicated the canonical NF-κB pathway and the proinflammatory cytokine IL-6 in autoantibody production, but not IFN regulatory factor 7. These findings identify HgIA as a novel type I IFN-independent model of systemic autoimmunity and implicate TLR-mediated NF-κB proinflammatory signaling from the late endocytic pathway compartments in autoantibody generation.

Mercury as an environmental stimulus in the development of autoimmunity - A systematic review

Autoimmune diseases result from an interplay of genetic predisposition and factors which stimulate the onset of disease. Mercury (Hg), a well-established toxicant, is an environmental factor reported to be linked with autoimmunity. Hg exists in several chemical forms and is encountered by humans in dental amalgams, certain vaccines, occupational exposure, atmospheric pollution and seafood. Several studies have investigated the effect of the various forms of Hg, including elemental (Hg⁰), inorganic (iHg) and organic mercury (oHg) and their association with autoimmunity. In vitro studies using peripheral blood mononuclear cells (PBMC) from healthy participants have shown that methylmercury (MeHg) causes cell death at lower concentrations than iHg albeit exposure to iHg results in a more enhanced pro-inflammatory profile in comparison to MeHg. In vivo research utilising murine models susceptible to the development of metal-induced autoimmunity report that exposure to iHg results in a lupus-like syndrome, whilst mice exposed to MeHg develop autoimmunity without the formation of immune complexes. Furthermore, lower concentrations of IgE are detected in MeHg-treated animals in comparison with those treated with iHg. It appears that, oHg has a negative impact on animal models with existing autoimmunity. The research conducted on humans in this area is diverse in study design and the results are conflicting. There is currently no evidence to implicate a role for Hg⁰ exposure from dental amalgams in the development or perpetuation of autoimmune disease, apart from some suggestion of individual sensitivity. Several studies have consistently shown a positive correlation between iHg exposure and serum autoantibody concentrations in gold miners, although the clinical impact of iHg remains unknown. Furthermore, a limited number of studies have reported individuals with autoimmune disease have higher concentrations of blood Hg compared to healthy controls. In summary, it appears that iHg perpetuates markers of autoimmunity to a greater extent than oHg, albeit the impact on clinical outcomes in humans is yet to be elucidated.

An Animal Model Using Metallic Ions to Produce Autoimmune Nephritis

Autoimmune nephritis triggered by metallic ions was assessed in a Long-Evans rat model. The parameters evaluated included antinuclear autoantibody production, kidney damage mediated by immune complexes detected by immunofluorescence, and renal function tested by retention of nitrogen waste products and proteinuria. To accomplish our goal, the animals were treated with the following ionic metals: HgCl₂, CuSO₄, AgNO₃, and Pb(NO₃)₂. A group without ionic metals was used as the control. The results of the present investigation demonstrated that metallic ions triggered antinuclear antibody production in 60% of animals, some of them with anti-DNA specificity. Furthermore, all animals treated with heavy metals developed toxic glomerulonephritis with immune complex deposition along the mesangium and membranes. These phenomena were accompanied by proteinuria and increased concentrations of urea. Based on these results, we conclude that metallic ions may induce experimental autoimmune nephritis.

Association between serum mercury concentration and leukocyte differential count in children

There have been a number of animal studies on the immunological effects of mercury. However, there is a lack of studies investigating the effects of mercury in children. We investigated the association between serum mercury and leukocyte differential count in Korean children. The relationship between mercury and leukocyte differential count (segment, lymphocyte, monocyte, basophil, and eosinophil counts) was analyzed by multivariate linear analysis adjusted for sex, BMI, parental smoking, lead, cadmium, and allergic sensitization in 311 children. Mercury showed a positive correlation with lymphocyte count (coefficient 113.8, 95% confidence interval 26.7-200.9). However, mercury was not associated with total leukocyte, segment, monocyte, basophil, or eosinophil count. Mercury was associated with the increased of lymphocyte count in Korean children. Further studies will be required to ascertain the clinical significance of this association.

Lymphocyte Activation Gene-3 (LAG-3) negatively regulates environmentally-induced autoimmunity

Environmental factors including drugs, mineral oils and heavy metals such as lead, gold and mercury are triggers of autoimmune diseases in animal models or even in occupationally exposed humans. After exposure to subtoxic levels of mercury (Hg), genetically susceptible strains of mice develop an autoimmune disease characterized by the production of highly specific anti-nucleolar autoantibodies, hyperglobulinemia and nephritis. However, mice can be tolerized to the disease by a single low dose administration of Hg. Lymphocyte Activation Gene-3 (LAG-3) is a CD4-related, MHC-class II binding molecule expressed on activated T cells and NK cells which maintains lymphocyte homeostatic balance via various inhibitory mechanisms. In our model, administration of anti-LAG-3 monoclonal antibody broke tolerance to Hg resulting in autoantibody production and an increase in serum IgE level. In addition, LAG-3-deficient B6.SJL mice not only had increased susceptibility to Hg-induced autoimmunity but were also unresponsive to tolerance induction. Conversely, adoptive transfer of wild-type CD4⁺ T cells was able to partially rescue LAG-3-deficient mice from the autoimmune disease. Further, in LAG-3-deficient mice, mercury elicited higher amounts of IL-6, IL-4 and IFN-γ, cytokines known to play a critical role in mercury-induced autoimmunity. Therefore, we conclude that LAG-3 exerts an important regulatory effect on autoimmunity elicited by a common environmental pollutant.

Immunological Changes Associated with Chronic Ingestion of Depleted Uranium in Rats

The objective of this study was to evaluate the effect of depleted uranium (DU) on the rat immune system. Three-wk-old, specific pathogen-free Sprague-Dawley rats that received dietary DU for 4 mo were divided into four groups (30 rats per group) according to exposure dose: 0 (control group), 1.3 (DU1.3 group), 13 (DU13 group), and 130 mg U kg-1 (DU130 group). After 4 mo of exposure, the splenic tissues of the rats presented elevated uranium content in accordance with increasing dosage. The rats in the DU130 group exhibited immunological damage characterized by pathomorphological changes of immune organs and significantly lower thymic and splenic relative weight, whereas the influences of DU on the immune system were slight in the other groups. In addition, higher uranium exposure also resulted in reduced numbers of peripheral lymphocytes, lower lymphocyte proliferation, and reduced paw swelling. The authors concluded that the influences of DU on the immune system were slight after ingestion of DU at lower doses but severe after exposure to DU at higher doses.

Requirements for innate immune pathways in environmentally induced autoimmunity

There is substantial evidence that environmental triggers in combination with genetic and stochastic factors play an important role in spontaneous autoimmune disease. Although the specific environmental agents and how they promote autoimmunity remain largely unknown, in part because of diverse etiologies, environmentally induced autoimmune models can provide insights into potential mechanisms. Studies of idiopathic and environmentally induced systemic autoimmunity show that they are mediated by common adaptive immune response genes. By contrast, although the innate immune system is indispensable for autoimmunity, there are clear differences in the molecular and cellular innate components that mediate specific systemic autoimmune diseases, suggesting distinct autoimmune-promoting pathways. Some of these differences may be related to the bifurcation of toll-like receptor signaling that distinguishes interferon regulatory factor 7-mediated type I interferon production from nuclear factor-κB-driven proinflammatory cytokine expression. Accordingly, idiopathic and pristane-induced systemic autoimmunity require both type I interferon and proinflammatory cytokines whereas the less aggressive mercury-induced autoimmunity, although dependent on nucleic acid-binding toll-like receptors, does not require type I interferon but needs proinflammatory cytokines. Scavenger receptors and the inflammasome may contribute to silica-induced autoimmunity. Greater understanding of the innate mechanisms responsible for idiopathic and environmentally induced autoimmunity should yield new information into the processes that instigate and drive systemic autoimmunity.

Pyrrolidine dithiocarbamate augments Hg(2+)-mediated induction of macrophage cell death via oxidative stress-induced apoptosis and necrosis signaling pathways

Exposure to mercury can lead to several injuries in mammals, including immune system dysfunction, and pyrrolidine dithiocarbamate (PDTC), as a metal chelator and antioxidant, has been indicated to increase the cytotoxic effects of toxic metals. However, the toxicological effects and possible mechanisms of mercury in combination with PDTC are mostly unclear. In this study, we showed that PDTC dramatically increase the cytotoxic effect of HgCl(2) on cultured murine macrophages (RAW 264.7 cells). PDTC augmented HgCl(2)-induced cytotoxic effects by facilitating the entry of mercury into the cells. The Hg(2+)/PDTC complex significantly and rapidly increased the formation of reactive oxygen species (ROS) and decreased intracellular glutathione (GSH) levels in these cells. Flow cytometry analysis showed that the numbers of sub-G1 hypodiploid cells and annexin V-FITC binding cells increased after Hg(2+)/PDTC complex exposure, and several features of mitochondria-dependent apoptosis were also induced, including mitochondrial membrane depolarization, cytosolic cytochrome c release, poly(ADP-ribose) polymerase (PARP) and caspase 3/7 activation, and DNA fragmentation. Moreover, both apoptotic and necrotic cells were detected using acridine orange/ethidium bromide dual staining. Meanwhile, depleted intracellular ATP levels and increased lactate dehydrogenase (LDH) release were observed, suggesting the induction of necrotic cell death processes. These Hg(2+)/PDTC complex-induced cytotoxicity-related signals could be reversed by pretreatment with the antioxidant N-acetylcysteine. In conclusion, these results suggest that Hg(2+)/PDTC complex-induced oxidative stress causes macrophage cell death via both apoptosis and necrosis. These findings imply for the first time that PDTC dramatically increases the uptake and toxicological effects of Hg(2+) instead of detoxification.

Exposure of single-walled carbon nanotubes impairs the functions of primarily cultured murine peritoneal macrophages

It is increasingly important to understand the single-walled carbon nanotubes' (SWCNTs) immune response as their increasingly biomedical researches and applications. Macrophages and T cells play important roles in scavenging foreign materials and pathogens and regulating immune response. In this work, primarily cultured murine peritoneal macrophages and purified splenic T cells were utilised to determine the toxic effects of SWCNTs and acid-functionalised SWCNTs (AF-SWCNTs) on the immune system, especially on macrophage functions. Macrophages were exposed to 0-50 μg/ml of CNTs for 24 h and no significant cytotoxicity was found by live/dead and annexin-V-FITC/PI analyses. The TEM images revealed that AF-SWCNTs were engulfed mostly through phagocytosis and located in lysosomes of macrophages. Measurement of mitochondrial membrane potential and proteasome subunit gene expression demonstrated that 10 and 50 μg/ml AF-SWCNTs could damage mitochondrial function and proteasome formation in a concentration-dependent manner. Functional analyses revealed that the percentage of phagocytic cells were affected significantly by 20 μg/ml CNTs, and 5 μg/ml AF-SWCNTs inhibited the phagocytic efficiency of latex beads in macrophages. The accessory cell function was affected by both AF-SWCNTs and SWCNTs at concentrations of 10 and 50 μg/ml, respectively. Furthermore, AF-SWCNT biased naïve T-cell differentiation to Th1 type by inducing the production of IFN-γ and TNF, implying the potential risk of Th1-associated diseases (e.g. autoimmune diseases and inflammation) on AF-SWCNT exposure.

Definition of IFN-γ-related pathways critical for chemically-induced systemic autoimmunity

IFN-γ is essential for idiopathic and murine mercury-induced systemic autoimmunity (mHgIA), and heterozygous IFN-γ(+/-) mice also exhibit reduced disease. This suggests that blocking specific IFN-γ-related pathways that may only partially inhibit IFN-γ production or function will also suppress autoimmunity. To test this hypothesis, mice deficient in genes regulating IFN-γ expression (Casp1, Nlrp3, Il12a, Il12b, Stat4) or function (Ifngr1, Irf1) were examined for mHgIA susceptibility. Absence of either Ifngr1 or Irf1 resulted in a striking reduction of disease, while deficiency of genes promoting IFN-γ expression had modest to no effect. Furthermore, both Irf1- and Ifng-deficiency only modestly reduced the expansion of CD44(hi) and CD44(hi)CD55(lo) CD4(+) T cells, indicating that they are not absolutely required for T cell activation. Thus, there is substantial redundancy in genes that regulate IFN-γ expression in contrast to those that mediate later signaling events. These findings have implications for the therapeutic targeting of IFN-γ pathways in systemic autoimmunity.

β2-microglobulin is required for the full expression of xenobiotic-induced systemic autoimmunity

Mercury exposure in both humans and mice is associated with features of systemic autoimmunity. Murine HgCl₂-induced autoimmunity (mHgIA) requires MHC Class II, CD4⁺ T-cells, co-stimulatory molecules, and interferon-γ (IFN-γ), similar to spontaneous models of systemic lupus erythematosus (SLE). β₂-microglobulin (B2m) is required for functional MHC Class I molecules and the neonatal F(c) receptor (F(c)Rn). Deficiency of B2m in lupus-prone strains is consistently associated with reduced IgG levels, but with variable effects on other manifestations. Herein, we examined the role of B2m in mHgIA and show that in the absence of B2m, mercury-exposed mice failed to exhibit hypergammaglobulinemia, had reduced anti-nucleolar autoantibodies (ANoA), and had a lower incidence of immune complex deposits in splenic blood vessels, whereas IgG anti-chromatin autoantibodies and renal immune deposits were largely unaffected. Subclass analysis of the IgG anti-chromatin, however, revealed a significant reduction in the IgG₁ subtype. Examination of IFNγ, IL-4, and IL-2 in exposed skin, draining lymph nodes, and spleen following mercury exposure showed reduced IL-4 in the spleen and skin in B2m-deficient mice, consistent with the lower IgG₁ anti-chromatin levels, and reduced IFNγ expression in the skin. These findings demonstrate how a single genetic alteration can partially but significantly modify the clinical manifestations of systemic autoimmunity induced by exposure to xenobiotics.

Toxicology of autoimmune diseases

Susceptibility to most autoimmune diseases is dependent on polygenic inheritance, environmental factors, and poorly defined stochastic events. One of the significant challenges facing autoimmune disease research is in identifying the specific events that trigger loss of tolerance and autoimmunity. Although many intrinsic factors, including age, sex, and genetics, contribute to autoimmunity, extrinsic factors such as drugs, chemicals, microbes, or other environmental factors can also act as important initiators. This review explores how certain extrinsic factors, namely, drugs and chemicals, can promote the development of autoimmunity, focusing on a few better characterized agents that, in most instances, have been shown to produce autoimmune manifestations in human populations. Mechanisms of autoimmune disease induction are discussed in terms of research obtained using specific animal models. Although a number of different pathways have been delineated for drug/chemical-induced autoimmunity, some similarities do exist, and a working model is proposed.

Differential immunotoxic effects of inorganic and organic mercury species in vitro

Despite the fact that humans are exposed to multiple forms of mercury (elemental, inorganic, and organic), most research on mercury toxicity has focused on methylmercury (MeHg) and on neurotoxic outcomes and mechanisms. Recent work has indicated that the immunotoxic effects of mercury compounds may be significant contributors to human disease as well as mechanistically relevant to other target organ toxicities. In this study, we compared the effects of inorganic Hg (iHg) to organic Hg species (MeHg and ethylmercury, EtHg) in human peripheral blood mononuclear cells (PBMCs) in vitro at sub-cytotoxic concentrations, using methods developed to characterize response of human PBMCs to iHg in vitro. PBMCs were isolated from six volunteer blood donors (three males and three females) and cultured in the presence and absence of lipopolysaccharide (LPS) and low levels (up to 200nM of each Hg species, separately) for 24h in culture. Cell culture supernatants were analyzed for cytokine concentrations with a bead-based multiplex assay. We report that iHg and MeHg both increase pro-inflammatory cytokine release in LPS-stimulated PBMCs, while EtHg decreases IFN-gamma release as well pro-inflammatory cytokine release. IL-17 release is significantly increased only in response to iHg treatment. Levels of anti-inflammatory cytokines (IL-1Ra and IL-10) were not significantly altered by any Hg treatment. These results indicate that both organic and inorganic species of Hg can affect the human immune system, but that they may exert different effects on immune function.

How can a chemical element elicit complex immunopathology? Lessons from mercury-induced autoimmunity

Although most autoimmune diseases develop without a manifest cause, epidemiological studies indicate that external factors play an important role in triggering or aggravating autoimmune processes in genetically predisposed individuals. Nevertheless, most autoimmune disease-promoting environmental agents are unknown because their relationships to immune function are not understood. Thus, the study of animal models of chemically-induced autoimmunity should shed light on the pathways involved and allow us to identify these agents. The rodent model of heavy metal-induced autoimmunity is one of the most intriguing experimental systems available to address such questions. Although the ultimate pathophysiology of this model remains mysterious, recent studies have started to elucidate the mechanisms by which heavy metal exposure leads to immune activation and loss of self-tolerance.

Immunology of mercury

The heavy metal mercury is ubiquitously distributed in the environment resulting in permanent low-level exposure in human populations. Mercury can be encountered in three main chemical forms (elemental, inorganic, and organic) which can affect the immune system in different ways. In this review, we describe the effects of these various forms of mercury exposure on immune cells in humans and animals. In genetically susceptible mice or rats, subtoxic doses of mercury induce the production of highly specific autoantibodies as well as a generalized activation of the immune system. We review studies performed in this model and discuss their implications for the role of environmental chemicals in human autoimmunity.

In vitro immune toxicity of depleted uranium: effects on murine macrophages, CD4+ T cells, and gene expression profiles

Depleted uranium (DU) is a by-product of the uranium enrichment process and shares chemical properties with natural and enriched uranium. To investigate the toxic effects of environmental DU exposure on the immune system, we examined the influences of DU (in the form of uranyl nitrate) on viability and immune function as well as cytokine gene expression in murine peritoneal macrophages and splenic CD4+ T cells. Macrophages and CD4+ T cells were exposed to various concentrations of DU, and cell death via apoptosis and necrosis was analyzed using annexin-V/propidium iodide assay. DU cytotoxicity in both cell types was concentration dependent, with macrophage apoptosis and necrosis occurring within 24 hr at 100 microM DU exposure, whereas CD4+ T cells underwent cell death at 500 microM DU exposure. Noncytotoxic concentrations for macrophages and CD4+ T cells were determined as 50 and 100 microM, respectively. Lymphoproliferation analysis indicated that macrophage accessory cell function was altered with 200 microM DU after exposure times as short as 2 hr. Microarray and real-time reverse-transcriptase polymerase chain reaction analyses revealed that DU alters gene expression patterns in both cell types. The most differentially expressed genes were related to signal transduction, such as c-jun, NF- kappa Bp65, neurotrophic factors (e.g., Mdk), chemokine and chemokine receptors (e.g., TECK/CCL25), and interleukins such as IL-10 and IL-5, indicating a possible involvement of DU in cancer development, autoimmune diseases, and T helper 2 polarization of T cells. The results are a first step in identifying molecular targets for the toxicity of DU and the elucidation of the molecular mechanisms for the immune modulation ability of DU.

In vitro HgCl2 exposure of immune cells at different stages of maturation: effects on phenotype and function

This is the first study to investigate the hypothesis that the immunotoxic effects of inorganic mercury may be modulated by inherent differences in the responsiveness of immune cells related to the age of the donor. We exposed cells from lymph nodes, spleen, and thymus, collected from 7- and 10-day-old CD.1 pups, as well as from adult CD.1 mice, in terms of the effects of mercury in vitro on responses to Con-A stimulation with respect to proliferation, cytokine production, and cell phenotype. The effects of mercury on proliferation were age and organ dependent, while effects on cytokine production were only age dependent. Effects of mercury were observed only on splenocyte T-cell subpopulations and only in cells from 10-day-old pups and from adults. Mercury had no effect on IFN-gamma and IL-4 production by splenocytes from 7-day-old pups, but significantly decreased release of these cytokines by splenocytes from 10-day-old pups and adults. Hg did not affect IL-4 production by lymph node cells or thymocytes. In lymph node cells Hg affected IFN-gamma production only at 7 days. These data indicate that inherent properties of immune cells at different stages of development may influence the response to immunotoxicants.

Mechanisms of heavy metal-induced autoimmunity

Chemical exposure can trigger or accelerate the development of autoimmune manifestations. Although heavy metals are elementary chemical structures, they can have profound and complex effects on the immune system. In genetically susceptible mice or rats, administration of subtoxic doses of mercury induces both the production of highly specific autoantibodies and a polyclonal activation of the immune system. We review in this article some of the mechanisms by which heavy metal exposure can lead to autoimmunity.

Organic mercury compounds and autoimmunity

Based on in vitro studies and short-term in vivo studies, all mercurials were for a long time considered as prototypic immunosuppressive substances. Recent studies have confirmed that organic mercurials such as methyl mercury (MeHg) and ethyl mercury (EtHg) are much more potent immunosuppressors than inorganic mercury (Hg). However, Hg interacts with the immune system in the presence of a susceptible genotype to cause immunostimulation, antinucleolar antibodies targeting fibrillarin, and systemic immune-complex (IC) deposits, a syndrome called Hg-induced autoimmunity (HgIA). Recent studies in mice with a susceptible genotype has revealed that the immunosuppressive effect of MeHg and EtHg will within 1-3 weeks be superseded by immunostimulation causing an HgIA-like syndrome. At equimolar doses of Hg, MeHg has the weakest immunostimulating, autoimmunogen, and IC-inducing effect, while the effect of thimerosal is similar to that of inorganic mercury. The immunosuppression is caused by the organic mercurials per se. Since they undergo rapid transformation to inorganic Hg, studies are being undertaken to delineate the importance of the organic substances per se and the newly formed inorganic Hg for induction of autoimmunity.

Immunology and genetics of induced systemic autoimmunity

Systemic lupus erythematosus is a multigenic disorder of unknown etiology. To investigate the role of specific genes in lupus, we have examined the effects of single gene deletions on mercury-induced autoimmunity. Deficiency of certain genes abrogated induction of autoimmunity, while absence of others had little effect. The most interesting observations were obtained with genes related to interferon-gamma. Genes involved in upregulation of IFN-gamma expression did not significantly influence autoimmunity whereas absence of IFN-gamma or IFN-gamma receptor led to greatly reduced autoantibody responses and immunopathology. Absence of IRF-1, a gene expressed in response to IFN-gamma, resulted in selective retention of anti-chromatin autoantibodies demonstrating that specific defects in signaling pathways and gene expression subsequent to IFN-gamma/IFN-gamma receptor interaction influence specific disease parameters. These studies show that single gene deletions can have various outcomes ranging from no effect, suppression of one or more features of disease, to suppression of all features of disease, and that all three outcomes can be observed in the IFN-gamma pathway. IFN-gamma influences the expression and function of other lupus relevant genes such as IL-6 and beta2microglobulin, therefore the effects of these gene deletions on disease expression may also reflect responses downstream of IFN-gamma function.

Costimulation Requirements of Induced Murine Systemic Autoimmune Disease

Costimulation between T cells and APC is required for productive immune responses. A number of receptor/ligand pairs have been shown to mediate costimulation, including CD28/B7 molecules (CD80 and CD86), CD40/CD40 ligand (CD40L, CD154), and LFA-1 (CD18)/ICAM-1 (CD54). T-B cell costimulation also plays a significant role in autoimmune diseases such as systemic lupus erythematosus. Murine HgCl2-induced autoimmunity (mHgIA) is a T cell-dependent systemic autoimmune disease that shares a number of common pathogenic mechanisms with idiopathic lupus. In this report, the significance of costimulation in mHgIA is examined by attempting to induce disease in mice deficient in either CD40L, CD28, or ICAM-1. Unlike absence of ICAM-1, homozygous deficiencies in either CD40L or CD28 significantly reduced the development of mHgIA. CD40L displayed a gene dosage effect as heterozygous mice also showed reduction of autoantibody responses and immunopathology. Markers of T cell activation such as CD44 and CTLA-4 were associated with disease expression in wild-type and ICAM-1-deficient mice but not in CD40L- or CD28-deficient mice. Absence of CTLA-4 expression in CD40L-/- mice suggests that signaling via both CD28 and CD40L is important for T cell activation and subsequent autoimmunity in mHgIA. Attempts to circumvent the absence of CD40L by increasing CD28 signaling via agonistic Ab failed to elicit CTLA-4 expression. These findings indicate that breaking of self-tolerance in mHgIA requires signaling via both the CD28/B7 and CD40/CD40L pathways.

Inorganic mercury attenuates CD95-mediated apoptosis by interfering with formation of the death inducing signaling complex

Inorganic mercury (Hg2+) modulates several lymphocyte signaling pathways and has been implicated as an environmental factor linked to autoimmune disease. From the standpoint that autoimmune diseases represent disorders of cell accumulation, in which dysregulated apoptosis may be one mechanism leading to the accumulation of autoreactive lymphocytes, we have been investigating the influences of Hg2+ on CD95-mediated apoptosis. We demonstrate here that low and noncytotoxic concentrations of Hg2+ impair CD95 agonist-induced apoptosis in representative Type-I and Type-II T cell lines. Hg2+ treatment blocks the CD95 agonist-induced activation of initiator and effector caspases as well as the association between CD95 and the signaling adaptor, FADD. CD95 multimerization does not appear to be affected by Hg2+. Thus, the Hg2+ sensitive step within the CD95 death pathway is localized to the level of the death inducing signaling complex (DISC). Disruption of proper DISC formation may be a biochemical mechanism whereby Hg2+ contributes to autoimmune disease.