Inhibition of lymphoid cell DNA synthesis by metal allergens at various concentrations. Effect on short-time cultured nonadherent cells compared to nonseparated cells

Immune function effects of dental amalgam in children: a randomized clinical trial

BACKGROUND

Dental amalgam is a widely used restorative material containing 50 percent elemental mercury that emits mercury vapor. No randomized clinical trials have determined whether there are adverse immunological effects associated with this low-level mercury exposure in children. The objective of this study was to evaluate a subpopulation of the participants in the New England Children's Amalgam Trial for in vitro manifestations of immunotoxic effects of dental amalgam.

METHODS

The authors conducted a randomized clinical trial in which children requiring dental restorative treatment were randomly assigned to receive either amalgam for posterior restorations or resin-based composite restorations. They assessed 66 children, aged 6 to 10 years, for total white blood cell counts, specific lymphocyte (T-cell and B-cell) counts and lymphocyte, neutrophil and monocyte responsiveness across a five-year period. Because of the small number of participants, the authors acknowledge that the study is exploratory in nature and has limited statistical power.

RESULTS

The mean number of tooth surfaces restored during the five-year period was 7.8 for the amalgam group and 10.1 for the composite group. In the amalgam group, there was a slight, but not statistically significant, decline in responsiveness of T cells and monocytes at five to seven days after treatment; the authors consistently observed no differences at six, 12 or 60 months.

CONCLUSIONS

The findings of this study confirm that treatment of children with amalgam restorations leads to increased, albeit low-level, exposure to mercury. In this exploratory analysis of immune function, amalgam exposure did not cause overt immune deficits, although small transient effects were observed five to seven days after restoration placement.

CLINICAL IMPLICATIONS

These findings suggest that immunotoxic effects of amalgam restorations are minimal and transient in children and most likely do not need to be of concern to practitioners considering the use of this restorative dental material.

The Children's Amalgam Trial: design and methods

The safety of silver amalgam as a dental restorative material has been controversial since its introduction 150 years ago, but until recently it has been assumed that the exposure to mercury from dental amalgam is limited to the acute placement phase. However, some recent studies have raised safety concerns by demonstrating chronic release of mercury vapor from amalgam fillings during chewing and brushing. The Children's Amalgam Trial is a two-arm randomized trial of safety, comparing amalgam with a mercury-free restorative material. A single masking procedure is used to ensure that all investigators and staff measuring outcomes are unaware of assigned trial arm. The study follows 534 New England children, aged 6-10 years at enrollment, for 5 years. The children were recruited from two northeastern U.S. communities, one in rural Maine and one in urban Massachusetts. No trial subjects received prior amalgam restorations, and all were in need of at least two posterior occlusal fillings. Participants were randomized to receive either amalgam or composite material for all posterior restorations at baseline and at subsequent visits. The primary endpoint will be 5-year change in IQ scores. Secondary endpoints will include measures of other neuropsychological assessments and renal functioning. This paper describes the design of the Children's Amalgam Trial and includes data on baseline characteristics of the subjects.

Spontaneous downregulation of antibody/autoantibody synthesis in susceptible mice upon chronic exposure to mercuric chloride is not owing to a general immunosuppression

Administration of mercuric chloride into susceptible rats and mice induces a systemic autoimmune disease, which is characterized by a T-cell-dependent polyclonal B-cell activation, an increase in serum levels of immunoglobulin (Ig)G1 and IgE, production of antibodies of different specificities and development of renal IgG deposits. A peculiar feature of mercury-induced autoimmunity is that the polyclonal B-cell activation spontaneously disappears in spite of continuous injection of mercury. The exact mechanism(s) for autoregulation of mercury-induced autoimmunity is not well understood. In the present study, we analysed the regulation of mercury-induced immune/autoimmune responses in mice and tested whether spontaneous downregulation of these responses is owing to a general immunosuppression. Mercury-susceptible [SJL (H-2s)] and -resistant [DBA/2 (H-2d)] mice were injected with mercury for 4, 10, 15 and 17 weeks. Immune/autoimmune responses were monitored in these mice. Thereafter, mercury-injected mice for 17 weeks were further immunized with horse red blood cells (HRBC) to study whether the subsequent humoral immune response to a foreign antigen is suppressed. We found that except for IgG1 anti-nucleolar antibody production and renal IgG1 deposition, other characteristics of mercury-induced autoimmunity were downregulated in SJL (H-2s) mice after chronic treatment with mercury. However, these mice did not show any reduction in the number of splenic antibody-secreting cells and/or in serum titres of specific IgM, IgG1 and IgG2a anti-HRBC antibodies in response to HRBC as compared with naïve mice. Similarly, in mercury-resistant DBA/2 (H-2d) mice, chronic treatment with mercury did not either suppress specific antibody responses against HRBC. Our findings show that the autoregulation of mercury-induced immune/autoimmune responses observed after chronic treatment with mercury is not owing to a general immunosuppression.

Mercuric chloride damages cellular DNA by a non-apoptotic mechanism

Mercury is a xenobiotic metal that is well known to adversely affect the immune system, however, little is known as to the molecular mechanism. Recently, it has been suggested that mercury may induce immune dysfunction by triggering apoptosis in immune cells. Here, we studied the effects of Hg(2+) (HgCl(2)) on U-937 cells, a human cell line with monocytic characteristics. We found that these cells continued to proliferate when exposed to low doses of mercury between 1 and 5 microM. Using the single cell gel electrophoresis (SCGE) or 'comet' assay, we found that mercury damaged DNA at these levels. Between 1 and 50 microM Hg(2+), comet formation was concentration-dependent with the greatest number of comets formed at 5 microM mercury. However, the appearance of mercury-induced comets was qualitatively different from those of control cells treated with anti-fas antibody, suggesting that although mercury might damage DNA, apoptosis was not involved. This was confirmed by the finding that cells treated with 5 microM mercury were negative for annexin-V binding, an independent assay for apoptosis. These data support the notion that DNA damage in surviving cells is a more sensitive indicator of environmental insult than is apoptosis, and suggests that low-concentrations of ionic mercury may be mutagenic.

Low-level methylmercury exposure causes human T-cells to undergo apoptosis: evidence of mitochondrial dysfunction

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are immunotoxic to the human immune system. The major focus of our study is to demonstrate that methylmercuric chloride (MeHgCl) kills human lymphocytes by inducing apoptosis. T-cells exposed to 0.6-5 microM MeHgCl for 24 h were analyzed by flow cytometry. Methylmercury-treated cells exhibited increased Hoechst 33258 fluorescence while maintaining their ability to exclude the vital stain 7-aminoactinomycin. Furthermore, T-cells exposed to methylmercury exhibited changes in light scatter patterns that included decreased forward light scatter and increased side light scatter. The light scatter and fluorescent changes were consistent with morphological alterations displayed by cells during apoptosis. Cell death was further evaluated by assessing annexin V binding to the plasma membrane. Methylmercury-treated cells exhibited increased annexin V binding indicative of phosphatidylserine translocation to the outer leaflet of the plasma membrane. Using the fluorescent probe DiOC6(3), we noted that methylmercury exposure resulted in a decrease in mitochondrial transmembrane potential (Psim). Since a low Psim is associated with altered mitochondrial function, we also determined if exposure to methylmercury potentiated reactive oxygen species (ROS) generation. We noted that treated cells generated ROS, as evidenced by oxidation of hydroethidine and the generation of the fluorescent product, ethidium. Finally, we evaluated the effect of methylmercury on T-cell GSH content utilizing the fluorescent probe monochlorobimane; in the presence of MeHgCl, there is a marked loss in reduced cell thiols. The results of the study indicate that a key event in the induction of T-cell apoptosis by mercuric compounds is depletion in the thiol reserve which predisposes cells to ROS damage and at the same time activates death signaling pathways.

Immunotoxic effects of mercuric compounds on human lymphocytes and monocytes. III. Alterations in B-cell function and viability

The major goal of the study was to determine the effects of high and low levels of mercury on human B-cells. Following treatment of B-cells with HgCl2 (0-1000 ng) and MeHgCl2 (0-100 ng), their activation by mitogens was evaluated. Both forms of mercury caused a dose dependent reduction in B-cell proliferation in the presence or absence of monocytes. MeHgCl was approximately 10 times more potent than HgCl2. Mercury also inhibited the ability of these cells to synthesize IgM and IgG. Analysis of the expression of activation markers indicated that CD69, an early marker of cell activation, was not effected by mercury. In comparison, B-cell expression of the low affinity IgE receptor and the transferrin receptor were significantly reduced. Of particular interest, cells activated by mitogen for 48 hr became refractory to the immunotoxic effects of mercury. When exposed to high levels of HgCl2 (0.5-10 micrograms/ml) and MeHgCl (0.05-1 micrograms/ml), there was minimal reduction in B-cell viability at 1-4 hr, however, after exposure to mercury for 24 hr, cell death was apparent. MeHgCl was approximately 5-10 times more potent than HgCl2. Electron microscopic analysis revealed early nuclear alterations characterized by hyperchromaticity, nuclear fragmentation and condensation of nucleoplasm. Both forms of mercury caused a rapid and sustained elevation in the intracellular levels of Ca++. The results of this investigation clearly show that mercury-containing compounds are immunomodulatory; moreover, the decrease in B-cell function indicates that this metal is immunotoxic at very low exposure levels. Furthermore, the cytotoxic events are consistent with the notion that mercury initiates changes associated with programmed cell death.

Immunotoxic effects of mercuric compounds on human lymphocytes and monocytes. II. Alterations in cell viability

The major goal of this investigation was to examine the cytotoxic properties of both HgCl2 and MeHgCl, in terms of their ability to alter human T-cell and monocyte viability. Following treatment with HgCl2 (0-20 micrograms/ml) or MeHgCl (0-2 micrograms/ml), there was minimal reduction in lymphocyte viability at 1-4 hr. However, after exposure to mercury for 24 hr, cell death was apparent. In comparison, monocytes exhibited significant loss of viability during the early exposure periods. MeHgCl was approximately 5-10 times more potent than HgCl2. Other indicators of cell death were also determined. Measurement of the energy charge ratio indicated profound changes in cellular energy conservation. Electron microscopic analysis of cells treated with mercury revealed early nuclear alterations characterized by hyperchromaticity, nuclear fragmentation and condensation of nucleoplasm. In concert with these nuclear changes, there was destruction of cytoplasmic organelles with loss of membrane integrity. Studies of phospholipid synthesis by mercury treated cells confirmed that there were alterations in membrane structure. Thus, there was a decrease in total phosphatide synthesis by treated cells. Moreover, monocyte phospholipid synthesis appeared to be more sensitive to the presence of mercury then lymphocytes. Finally, both forms of mercury caused a rapid and sustained elevation in the intracellular levels of Ca++. These morphological and biochemical changes are consistent with the notion that mercury initiates cytotoxic changes associated with programmed cell death.

Immunotoxic effects of mercuric compounds on human lymphocytes and monocytes. I. Suppression of T-cell activation

Considerable attention has been directed at defining the health deficits associated with exposure to mercurial compounds. While numerous studies have been conducted, the findings have been somewhat contradictory and have led to a confused understanding of the immunotoxicology of mercury. It is becoming clear, however, that the immunotoxic effects of heavy metals in general, and mercury in particular, are dependent upon the assays and source of cells. The major goal of our study was to assess whether low level mercury exposure modulates human T-cell function. Following treatment of T-cells with HgCl2 (0-1000 ng) and MeHgCl (0-100 ng), their activation by mitogens was evaluated. Both forms of mercury caused a dose dependent reduction in T cell proliferation, however, the effect was dependent upon the presence of monocytes. Moreover, in the absence of monocytes, HgCl2 enhance PMA induced T-cell proliferation. MeHgCl was approximately 5-10 times more potent than HgCl2. Mercury also inhibited the ability of these cells to synthesize and secrete IL-1. Analysis of the expression of activation markers on the cell surface indicated that one of the earliest markers of lymphocyte activation, CD69, was not effected by mercury. In comparison, T-cell expression of IL-2R and the transferrin receptor was impaired. Of particular interest, cells activated by mitogen for 24 hr became refractory to the immunotoxic effects of mercury. The results of this investigation clearly show that mercury-containing compounds are immunomodulatory; moreover, the decrease in T-cell function following exposure to mercury indicates that this metal is immunotoxic at very low exposure levels.