Methyl mercury exposure via placenta and milk impairs natural killer (NK) cell function in newborn rats

Biochemical Parameters of Female Wistar Rats and Their Offspring Exposed to Inorganic Mercury in Drinking Water during the Gestational and Lactational Periods

The aim of this study was to investigate the effects of inorganic mercury (Hg²⁺) exposure on biochemical parameters of dams and their offspring exposed to metal in drinking water. Female Wistar rats were exposed to 0, 10, and 50 µg Hg²⁺/mL (as HgCl₂) for 42 days corresponding to gestational (21 days) and lactational (21 days) periods. The offspring were sacrificed on postnatal days 10, 20, 30, and 40. Dams exposed to Hg²⁺ presented a decrease in water intake in gestation [total: F(2,19) = 15.84; p ≤ 0.0001; daily: F(2,21) = 12.71; p = 0.0002] and lactation [total: F(2,19) = 4.619; p = 0.024; daily: F(2,21) = 5.309; p = 0.0136] without alteration in food intake. Dams exposed to 50 µg Hg²⁺/mL had an increase in kidney total [F(2,21) = 8.081; p = 0.0025] and relative [F(2,21) = 14.11; p = 0.0001] weight without changes in biochemical markers of nephrotoxicity. Moreover, dams had an increase in hepatic [F(2,10) = 3.847; p = 0.0577] and renal [F(2,11) = 6.267; p = 0.0152] metallothionein content concomitantly with an increase in renal Hg levels after Hg²⁺ exposure. Regarding offspring, the exposure to Hg²⁺in utero and breast milk increased the relative liver [F(2,18) = 5.33; p = 0.0152] and kidney [F(2,18) = 3.819; p = 0.0415] weight only on the postnatal day 40. In conclusion, dams were able to handle the Hg²⁺ avoiding the classic Hg²⁺ toxic effects as well as protecting the offspring. We suggest that this protection is related to the hepatic and renal metallothionein content increase.

Placental Epigenetics in Children's Environmental Health

There is a growing interest in understanding the mechanisms that drive the developmental origins of health and disease, and the role of epigenetic regulation has risen to the forefront of these studies. In particular, the placenta may be a model organ to consider as a mediator of the impact of the environment on developmental programming of children's health, as this organ plays a critical role in directing development and regulating the fetal environment. Several recent studies have begun to examine how environmental toxicant exposures can impact the placental epigenome, focusing on studies of DNA methylation and microRNA expression. This review highlights several of these studies and emphasizes the potential the placenta may hold on the broader understanding of the impact of the intrauterine environment on long-term health.

Placental DNA Methylation Related to Both Infant Toenail Mercury and Adverse Neurobehavioral Outcomes

BACKGROUND

Prenatal mercury (Hg) exposure is associated with adverse child neurobehavioral outcomes. Because Hg can interfere with placental functioning and cross the placenta to target the fetal brain, prenatal Hg exposure can inhibit fetal growth and development directly and indirectly.

OBJECTIVES

We examined potential associations between prenatal Hg exposure assessed through infant toenail Hg, placental DNA methylation changes, and newborn neurobehavioral outcomes.

METHODS

The methylation status of > 485,000 CpG loci was interrogated in 192 placental samples using Illumina's Infinium HumanMethylation450 BeadArray. Hg concentrations were analyzed in toenail clippings from a subset of 41 infants; neurobehavior was assessed using the NICU Network Neurobehavioral Scales (NNNS) in an independent subset of 151 infants.

RESULTS

We identified 339 loci with an average methylation difference > 0.125 between any two toenail Hg tertiles. Variation among these loci was subsequently found to be associated with a high-risk neurodevelopmental profile (omnibus p-value = 0.007) characterized by the NNNS. Ten loci had p < 0.01 for the association between methylation and the high-risk NNNS profile. Six of 10 loci reside in the EMID2 gene and were hypomethylated in the 16 high-risk profile infants' placentas. Methylation at these loci was moderately correlated (correlation coefficients range, -0.33 to -0.45) with EMID2 expression.

CONCLUSIONS

EMID2 hypomethylation may represent a novel mechanism linking in utero Hg exposure and adverse infant neurobehavioral outcomes.

Evidence on the human health effects of low-level methylmercury exposure

BACKGROUND

Methylmercury (MeHg) is a known neuro-toxicant. Emerging evidence indicates it may have adverse effects on the neuro-logic and other body systems at common low levels of exposure. Impacts of MeHg exposure could vary by individual susceptibility or be confounded by beneficial nutrients in fish containing MeHg. Despite its global relevance, synthesis of the available literature on low-level MeHg exposure has been limited.

OBJECTIVES

We undertook a synthesis of the current knowledge on the human health effects of low-level MeHg exposure to provide a basis for future research efforts, risk assessment, and exposure remediation policies worldwide.

DATA SOURCES AND EXTRACTION

We reviewed the published literature for original human epidemiologic research articles that reported a direct biomarker of mercury exposure. To focus on high-quality studies and those specifically on low mercury exposure, we excluded case series, as well as studies of populations with unusually high fish consumption (e.g., the Seychelles), marine mammal consumption (e.g., the Faroe Islands, circumpolar, and other indigenous populations), or consumption of highly contaminated fish (e.g., gold-mining regions in the Amazon).

DATA SYNTHESIS

Recent evidence raises the possibility of effects of low-level MeHg exposure on fetal growth among susceptible subgroups and on infant growth in the first 2 years of life. Low-level effects of MeHg on neuro-logic outcomes may differ by age, sex, and timing of exposure. No clear pattern has been observed for cardio-vascular disease (CVD) risk across populations or for specific CVD end points. For the few studies evaluating immunologic effects associated with MeHg, results have been inconsistent.

CONCLUSIONS

Studies targeted at identifying potential mechanisms of low-level MeHg effects and characterizing individual susceptibility, sexual dimorphism, and non-linearity in dose response would help guide future prevention, policy, and regulatory efforts surrounding MeHg exposure.

Mercury toxicity and treatment: a review of the literature

Mercury is a toxic heavy metal which is widely dispersed in nature. Most human exposure results from fish consumption or dental amalgam. Mercury occurs in several chemical forms, with complex pharmacokinetics. Mercury is capable of inducing a wide range of clinical presentations. Diagnosis of mercury toxicity can be challenging but can be obtained with reasonable reliability. Effective therapies for clinical toxicity have been described.

Induction of autoimmunity to brain antigens by developmental mercury exposure

A.SW mice, which are known to be prone to mercury (Hg)-induced immune nephritis, were assessed for their ability to develop autoimmunity to brain antigens after developmental exposure to Hg. Maternal drinking water containing subclinical doses of 1.25μM methyl Hg (MeHg) or 50μM Hg chloride (HgCl(2)) were used to evaluate developmental (exposure from gestational day 8 to postnatal day 21) induction of immune responses to brain antigens. Only HgCl(2) induced autoantibody production; the HgCl(2)-exposed offspring showed an increased number of CD4(+) splenic T cells expressing CD25 and V(β) 8.3 chains, and the brain-reactive immunoglobulin G (IgG) antibodies were predominantly against nuclear proteins (30 and 34 kD). The antibodies were deposited in all brain regions. Although male and female A.SW mice exposed to HgCl(2) showed deposition of IgG in multiple brain regions, inflammation responses were observed only in the cerebellum (CB) of female A.SW mice; these responses were associated with increased levels of exploratory behavior. The developmental exposure to MeHg also induced inflammation in the CB and increased exploratory behavior of the female A.SW mice, but the change did not correlate with increased IgG in the brain. Interestingly, the non-Hg-exposed female A.SW mice habituated (adapted to the information and/or stimuli of a new environment) more than the male A.SW mice during exploratory behavior assessment, and the Hg exposure eliminated the habituation (i.e., no changes in behavior with subsequent trials), making the female behaviors more like those of the male A.SW mice. Additionally, gender differences in A.SW brain cytokine expressions prior to Hg exposure were eliminated by the Hg exposure.

Developmental immunotoxicity of methylmercury: the relative sensitivity of developmental and immune parameters

Current developmental and reproductive toxicity protocols include only a limited set of parameters for effects on the developing immune system. In this study, a wide range of immunological parameters were included in a pre- and postnatal developmental toxicity study. Dose-response data were compared to determine the relative sensitivity of different immune and developmental parameters. Mated female Wistar rats were dosed daily by gavage with methylmercury (0, 0.1, 0.4, 0.7, 1.0, 1.5, and 2.0 mg/kg BW/day) from gestational day 6 to postnatal day (PND) 10. In addition to general, reproductive, and developmental parameters, a wide range of immunological parameters were assessed in male offspring at PNDs 21, 42, and 70. The T cell-dependent antibody response to keyhole limpet hemocyanin (KLH) was assessed following sc immunizations on PNDs 21 and 35. Dose-response data were analyzed using the benchmark dose (BMD) approach by fitting dose-response models to the various endpoints. Methylmercury induced effects on developmental parameters, such as growth parameters and pup mortality. Effects on the immune system were found at doses without observed developmental toxicity. Immune effects differed at the three time points and consisted mainly of effects on functional parameters. The parameter with the lowest 5% lower confidence bound of the BMD (BMDL) was the primary KLH-specific IgG antibody response, which showed a dose-dependent decrease with a BMD of 0.039 mg/kg BW/day (CI 0.010-0.12). These data show the relatively high sensitivity of the developing immune system and thereby illustrate the relevance of testing immune parameters in reproductive and developmental toxicity testing protocols.

Methylmercury exposure assessment using dietary and biomarker data among frequent seafood consumers in France CALIPSO study

BACKGROUND

Seafood is considered by toxicologists as the main source of methylmercury (MeHg), but little data is available concerning contamination of seafood and MeHg status of French frequent consumers.

OBJECTIVES

The objectives were to assess food exposure and biomarker of exposure of MeHg from a group of frequent consumers of seafood.

METHODS

Two approaches to exposure assessment were used: the currently used food intake and the biomarker of exposure. A validated food frequency questionnaire (FFQ) was used to assess seafood consumptions for 80 products of 385 frequent consumers aged 18 and over in four French coastal areas. Seafood samples were collected in each region considering preservation methods and supply habits according to a total diet study sampling. Food samples were analyzed for MeHg. Exposure was assessed by combining consumptions with contamination data. Whole blood samples were collected from the volunteers and analyzed for MeHg.

RESULTS

The mean dietary exposure to MeHg or weekly intake (WI FFQ) was 1.51+/-1.17 microg/kg bw/wk. Thirty-five percent of the subjects exceed the Joint Expert Committee on Food Additives (JECFA) provisional tolerable weekly intake (PTWI), whereas the use of the biological results with the JECFA/Environmental Protection Agency (EPA) one-compartment pharmacokinetic model to calculate weekly intake (WI PKM) shows that only 2% of subjects exceed the PTWI. The mean of the individual ratios WI FFQ/WI PKM is 4.3 and the higher the WI FFQ and the blood MeHg level, the lower is the ratio, close to 1-2.

CONCLUSIONS

These analyses support the assumption that the calculated dose of methylmercury is overestimated with the FFQ-based method used in this study. Since FFQ are commonly used in risk assessments, the overestimate of dose is public health protective and this finding is somewhat reassuring from a public health point of view, especially since the JECFA or EPA have applied uncertainty factors of 3.2 or 3, respectively, to take into account the inter-individual pharmacokinetic variability.

Interactions among infections, nutrients and xenobiotics

During recent years there have been several incidents in which symptoms of disease have been linked to consumption of food contaminated by chemical substances (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD). Furthermore, outbreaks of infections in food-producing animals have attracted major attention regarding the safety of consumers, e.g., Bovine Spongiform Encephalitis (BSE) and influenza in chicken. As shown for several xenobiotics in an increasing number of experimental studies, even low-dose xenobiotic exposure may impair immune function over time, as well as microorganism virulence, resulting in more severe infectious diseases and associated complications. Moreover, during ongoing infection, xenobiotic uptake and distribution are often changed resulting in increased toxic insult to the host. The interactions among infectious agents, nutrients, and xenobiotics have thus become a developing concern and new avenue of research in food toxicology as well as in food-borne diseases. From a health perspective, in the risk assessment of xenobiotics in our food and environment, synergistic effects among microorganisms, nutrients, and xenobiotics will have to be considered. Otherwise, such effects may gradually change the disease panorama in society.

Methylmercury interaction with lymphocyte cholinergic muscarinic receptors in developing rats

Cerebral cholinergic muscarinic receptors (MR) have been suggested as one of the sensitive biochemical endpoints of the central nervous system altered by developmental exposure to the widespread seafood contaminant methylmercury (MeHg). In adult rats, MeHg has been shown to alter MR binding both in the brain and lymphocytes, supporting the use of MR in blood cells as a surrogate marker of CNS changes. The effects of MeHg have been evaluated on rat lymphocyte MR binding (using [3H]QNB as specific muscarinic ligand) in vivo (after perinatal exposure) and in vitro. For comparison, in vitro studies were also performed on human lymphocytes. Exposure to 1 mg MeHg/kg/day during pregnancy and lactation (from GD7 to PND7) significantly enhanced lymphocyte MR density in both adult and young rats 21 days after delivery, with a more pronounced effect in the mothers (B(max) increase of 139%) than in the male offspring (+49%) and female offspring (+73%) as compared with their respective controls (33+/-4, 41+/-8, and 37+/-4 fmol/million cells), in accordance with the higher Hg levels detected in the adult blood (11.3+/-2.2 microg/mL) than in pups (1.3+/-0.4 microg/L in both genders). A lower MeHg dose (0.5 mg/kg/day) was without any effect on lymphocyte MRs. In in vitro studies, MeHg was an almost equipotent inhibitor of (3)H-QNB binding to rat and human lymphocyte MRs (IC50 values were 4.1+/-0.29, 5.2+/-0.51, and 5.0+/-0.9 microM for total rat lymphocytes, rat T lymphocytes, and total human lymphocytes, respectively). Notably, the IC50 values for MeHg to lymphocyte MRs were comparable to the Hg levels reached in blood (5-50 microM) of the PND21 rats exposed to MeHg. The finding that the MR binding is a target for the effects of MeHg in peripheral blood cells is in accordance with our previous data in brain [Coccini et al., 2006. Effects of developmental co-exposure to methylmercury and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) on cholinergic muscarinic receptors in rat brain. Neurotoxicology, in press], and supports the use of this peripheral endpoint as a biomarker of MeHg-induced cerebral muscarinic alterations. The similarity of MeHg IC50 binding data between human and rat in peripheral tissues suggests the possible application of such biomarker to humans exposed to environmental chemicals.

In vitro and in vivo effects of mercuric chloride on thymic endocrine activity, NK and NKT cell cytotoxicity, cytokine profiles (IL-2, IFN-gamma, IL-6): role of the nitric oxide-L-arginine pathway

Mercury (Hg2+) affects cell-mediated immunity, including thymulin production. Thymulin, a zinc-dependent thymic hormone synthesized by thymic epithelial cells (TECs), is involved in NK cell cytotoxicity and Th1 cytokine production (IL-2 and IFN-gamma), which in turn affect both NKT and classic NK spleen cell cytotoxicity. High doses of Hg2+ induce an inflammatory status, increased production of IL-6 and consequent Th1/Th2 imbalance as well as cell-mediated immune depression. The mechanisms by which Hg+ affects the cell-mediated immune response are still unclear. The nitric oxide (NO) pathway may be implicated. The aim of this work was to further explore its noxious role in innate and adaptive immunity and to study the possible role played by the NO pathway. Young Balb/c mice treated in vivo for 1 month with 1.0 mg HgCl2/kg b.w. showed low thymulin activity, depressed NO production (as measured by nitrite and nitrate plasma levels), impaired classic NK spleen cell cytotoxicity, decreased Th1 (IL-2 and IFN-gamma) cytokine profiles, and increased IL-6 production. In vitro, 10(-6) M of HgCl2 inhibited active thymulin kinetics, TEC proliferation, NKT cell cytotoxicity and Th1 cytokine production, whereas IL-6 increased. L-arginine restored thymulin activity, TEC proliferation, NKT cytotoxicity, cytokine profiles and nitrite and nitrate plasma levels both in vivo and in vitro. Since L-arginine is the substrate for NO production, it may compensate for the cell-mediated immune defect induced by HgCl2, via the arginine-NO-pathway. L-arginine is also able to reduce glomerular kidney IgG antibodies deposits induced by higher dose of HgCl2 administration.

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.

Identifying environmental contributions to autism: provocative clues and false leads

The potential role of environmental factors in autism spectrum disorders (ASD) is an area of emerging interest within the public and scientific communities. The high degree of heritability of ASD suggests that environmental influences are likely to operate through their interaction with genetic susceptibility during vulnerable periods of development. Evaluation of the plausibility of specific neurotoxicants as etiological agents in ASD should be guided by toxicological principles, including dose-effect dependency and pharmacokinetic parameters. Clinical and epidemiological investigations require the use of sufficiently powered study designs with appropriate control groups and unbiased case ascertainment and exposure assessment. Although much of the existing data that have been used to implicate environmental agents in ASD are limited by methodological shortcomings, a number of efforts are underway that will allow more rigorous evaluation of the role of environmental exposures in the etiology and/or phenotypic expression of the disorder. Surveillance systems are now in place that will provide reliable prevalence estimates going forward in time. Anticipated discoveries in genetics, brain pathology, and the molecular/cellular basis of functional impairment in ASD are likely to provide new opportunities to explore environmental aspects of this disorder.

Is autism an autoimmune disease?

Autism spectrum disorder (ASD) is a spectrum of behavioral anomalies characterized by impaired social interaction and communication, often accompanied by repetitive and stereotyped behavior. The condition manifests within the first 3 years of life and persists into adulthood. There are numerous hypotheses regarding the etiology and pathology of ASD, including a suggested role for immune dysfunction. However, to date, the evidence for involvement of the immune system in autism has been inconclusive. While immune system abnormalities have been reported in children with autistic disorder, there is little consensus regarding the nature of these differences which include both enhanced autoimmunity and reduced immune function. In this review, we discuss current findings with respect to immune function and the spectrum of autoimmune phenomena described in children with ASD.

Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

Cord blood lymphocyte functions in newborns from a remote maritime population exposed to organochlorines and methylmercury

The consumption of fish and sea mammals can be an important source of exposure to organochlorine compounds (OCs) and heavy metals in populations relying on seafood for subsistence. Exposure to these substances, especially during the prenatal period, has been shown to induce immunotoxic effects in mammals. Immunological status was assessed in 48 newborns from a remote maritime population living on the Lower and Mid North Shore of the St. Lawrence River (subsistence fishing group) and 60 newborns from the coastal urban center of Sept-Iles (reference group). Women were recruited upon arrival at Sept-Iles regional hospital to give birth. Cord blood samples were collected for organochlorine and heavy metal analyses and to isolate lymphocytes for immunological assays (proportions and functional responses of the main cellular subsets T, B, and NK (natural killer) cells. Concentrations of polychlorinated biphenyls (PCBs) and mercury were respectively three- and twofold higher, significantly greater, in the subsistence fishing group than in the reference group. Compared to the reference group, the subsistence fishing group showed significant decreases in the proportion of the naive helper T-cell subset CD4+CD45RA, T-cell proliferation following an in vitro mitogenic stimulation, and plasma immunoglobulin M (IgM) level, while plasma IgC level was increased. NK cytolytic activities were similar in both groups. The proportion of CD4+CD45RA cells was inversely correlated to mercury and PCBs, while T-cell clonal expansion was negatively associated with PCBs and p,p'-DDE. Mercury was inversely correlated to plasma IgM. Data show that subtle functional alterations of the developing human immune system may result from in utero exposure to OCs and mercury. Epidemiological studies are needed to determine the relevance of these alterations in predicting detrimental health effects in the developing child.

Immune function of bovine leukocytes after in vitro exposure to selected heavy metals

OBJECTIVE

To study effects of in vitro exposure of bovine leukocytes to mercury, cadmium, and lead on phagocytosis, natural killer cell activity, and lymphocyte proliferation.

SAMPLE POPULATION

Leukocytes from 6 nonpregnant Holstein heifers.

PROCEDURE

Leukocytes were exposed in vitro to the aforementioned metals, and leukocyte functions were assessed.

RESULTS

Phagocytosis was suppressed by 10(-5) to 10(-7) M CdCl2 and by 10(-5) and 10(-6) M HgCl2, but not 10(-7) M HgCl2 nor 10(-4) to 10(-6) M PbCl2. Spontaneous and concanavalin A- or phytohemagglutinin-stimulated proliferation of metal-treated bovine blood mononuclear cells was not significantly different from that of nontreated control cells, except for enhanced spontaneous proliferation in response to 10(-5) M HgCl2. When proliferation was expressed as a stimulation index, a dose-dependent increase of spontaneous proliferation was observed in response to exposure to HgCl2 and PbCl2. Compared with response to 10(-6) or 10(-7) M CdCl2, reduction of mitogen-induced and spontaneous proliferation was observed on exposure to 10(-5) M CdCl2. Natural killer cell activity against YAC-1 target cells, evaluated by flow cytometry, was decreased only in cells exposed to 10 M HgCl2.

CONCLUSION AND CLINICAL RELEVANCE

Bovine leukocytes are susceptible to the immunomodulatory effects of in vitro exposure to heavy metals at concentrations equal to or higher than those at which similar effects are seen for leukocytes from most other animal species for which data are available for comparison. Exception is phagocytosis, which is severely affected by low concentrations of CdCl2 and HgCl2 in cattle. Reduction of defense mechanisms on exposure to metals could lead to increased susceptibility to potential pathogens.

Immune system alteration in the rat after indirect exposure to methyl mercury chloride or methyl mercury sulfide

Methyl mercury is a well-recognized health hazard. It is an environmental contaminant that accumulates in the food chain. The primary source of mercury exposure for humans is through the consumption of contaminated fish. We studied the effects of indirect methyl mercury exposure on the immune system of Sprague-Dawley rats. The effects of different forms of methyl mercury on immune system development were studied in Sprague-Dawley rats at 6 and 12 weeks of age. Rats were indirectly exposed to mercury during gestation and during nursing by exposing pregnant rats to either 5 or 500 micrograms/liter of methyl mercury chloride (CH3HgCl) or 5 micrograms/liter of methyl mercury sulfide [(CH3Hg)2S] in their drinking water. Total body, splenic, and thymic weights were measured, and NK cell cytolytic activity and lymphoproliferative response to T and B cell mitogens were evaluated in the offspring. At 6 weeks of age, total body and splenic weights were significantly increased in both high- and low-dose methyl mercury chloride-exposed groups. Rats exposed to methyl mercury sulfide had a significant increase in thymic weight at 6 weeks of age. At 12 weeks, the total body and organ weights were not different from controls. The lymphocyte proliferative response of splenocytes to PWM was enhanced at 6 weeks in both CH3HgCl exposed groups and not affected in the (CH3Hg)2S exposed group. NK cell activity was not affected in either group at 6 weeks of age. At age 12 weeks, NK cell activity was statistically significantly decreased by 56.6% in both CH3HgCl-exposed groups and not affected in the (CH3Hg)2S-exposed rats. The lymphocyte proliferative response of splenocytes to the B cell mitogen pokeweed remained increased in the CH3HgCl groups. Indirect exposure of rats (during gestation and nursing) to different forms of methyl mercury reveals that chloride forms have prolonged predominantly enhancing effects on lymphoproliferative response of splenocytes, followed by significant depression of NK cell activity.

Immunomodulating effects after perinatal exposure to methylmercury in mice

The influence of methylmercury on the developing immune system was studied in offspring from Balb/c mice exposed to 0, 0.5 or 5 mg Hg/kg as methylmercury in the diet. Dams were exposed for 10 weeks prior to mating, during gestation and lactation. Pups were exposed to mercury until day 15 of lactation, thereafter the pups were given control milk and control diet. Samples for mercury analysis were collected from the pups on days 22 and 50, and for immunological studies on days 10, 22 and 50. The exposure resulted in significantly increased total Hg concentrations in whole blood on day 22 and 50 in offspring from the 5 mg Hg/kg group, and in offspring from the 0.5 mg Hg/kg group on day 22. On day 50, blood mercury levels had decreased to background levels in the 0.5 mg Hg/kg group. Increased numbers of splenocytes and thymocytes were found in offspring from the 0.5 mg Hg/kg group. Flow cytometry analysis of thymocytes revealed increased numbers and altered proportions of lymphocyte subpopulations within the thymus in offspring from both of the exposed groups. The proliferative response of splenocytes to the B-cell mitogen LPS was increased in offspring from dams exposed to 5 mg Hg/kg, and the primary antibody response to a viral antigen was stimulated in pups from dams exposed to 0.5 mg Hg/kg. The present results indicate that placental and lactational transfer of mercury affects thymocyte development and stimulates certain mitogen- or antigen-induced lymphocyte activities in mice.

Effects of methyl mercury on cytokines, inflammation and virus clearance in a common infection (coxsackie B3 myocarditis)

A myocarditic coxsackievirus B3 (CB3) infection in Balb/c mice was used to investigate the effects of 12 weeks of methyl mercury (MeHg) exposure (3.69 mg/g diet) on inflammatory heart lesions, virus in the heart, the cytokine response, i.e. cachectin/TNF-alpha and gamma-interferon (IFN-gamma) levels in plasma, and on disease complications and mortality. This dose of MeHg did not influence mortality in this infection model. The inflammatory and necrotic lesions in the ventricular myocardium 7 days after the inoculation covered 2.2% of the tissue section area in infected control mice. This damage was increased (n.s.) by 50% (to 3.3% of the tissue section area) in MeHg-treated mice. The response pattern of lymphocyte subsets in situ in myocardial inflammatory lesions was corroborated using an immune histological technique. MeHg treatment tended to increase (2.2-fold, n.s.) the number of Mac 2+ cells (macrophages) in the heart muscle in this infection. Plasma levels of both TNF-alpha and IFN-gamma increased on day 3 of the infection in MeHg-treated as well as in non-MeHg-treated mice, but the mean IFN-gamma response was more pronounced in the MeHg-treated mice. On day 7 of the infection, when most animals still showed clinical signs of disease, cytokine levels were back to normal. MeHg-exposure in non-infected mice did not affect cytokine levels. In situ hybridization of virus RNA in myocardial tissue showed remaining virus in those mice who had the lowest plasma IFN-gamma levels. A 20% increased (P < 0.05) lymphoproliferative response to the T cell mitogen Con A was observed as a result of the MeHg treatment. Even heart tissue lesions and virus persistence tended to be influenced by MeHg in a direction compatible with the development of chronic disease.

High consumption of fatty fish from the Baltic Sea is associated with changes in human lymphocyte subset levels

Fatty fish from the Baltic Sea accumulate immunotoxic persistent organochlorine compounds. In a previous study we found inverse correlations between such fish consumption and natural killer (NK) cell levels in a Swedish population. The present study concerns 68 Latvian subjects with high, low or intermediate fish consumption. High fish consumption correlated positively with B cell levels and CD4+/CD8+ ratios, but negatively with levels of cytotoxic (CD8+) T cells. Furthermore, NK cell levels correlated inversely with plasma selenium, one of several strong correlates with fish intake. A high fish diet includes a set of possible immunomodulating agents. It is presently not possible to pinpoint the cause for the observed subset deviations or to establish their possible biological importance.

Parameters of immunological competence in subjects with high consumption of fish contaminated with persistent organochlorine compounds

Consumption of fatty fish species, like salmon and herring, from the Baltic Sea is an important source of human exposure to persistent organochlorine compounds, e.g. polychlorinated dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs). Many of these compounds show immunotoxic and hepatotoxic effects in animals. We have now studied immunological competence, including lymphocyte subsets, in 23 males with a high consumption of fish from the Baltic Sea and in a control group of 20 males with virtually no fish consumption. The high consumers had lower proportions and numbers of natural killer (NK) cells, identified by the CD 56 marker, in peripheral blood than the non-consumers. Weekly intake of fatty fish correlated negatively with proportions of NK cells (rs = -0.32, P = 0.04). There were also, in a subsample of 11 subjects, significant negative correlations between numbers of NK cells and blood levels of a toxic non-ortho-PCB congener (IUPAC 126; rs = -0.68, P = 0.02) and a mono-ortho congener (IUPAC 118; rs = -0.76, P = 0.01). A similar correlation, in 12 subjects, was seen for p,p'-DDT (rs = -0.76, P = 0.01). The corresponding negative correlation, in 13 subjects, with blood levels of PCDD/Fs was not significant (rs = -0.57, P = 0.07). No significant association was seen between organic mercury in erythrocytes and NK cells. Fish consumption was not associated with levels of any other lymphocyte subset. Neither were there any correlations with plasma immunoglobulins or liver enzyme activities. Our study indicates that accumulation of persistent organochlorine compounds in high consumers of fatty fish may adversely affect NK cell levels.

Effects of methyl mercury exposure on spleen and blood natural killer (NK) cell activity in the mouse

The effect of 12 weeks of exposure to methyl mercury (MeHg) (3.9 micrograms/g diet) on the immune function was studied in female Balb/c mice. This MeHg dose did not affect body, kidney, liver or spleen weight. Thymus weight and cell number decreased by 22% (P less than 0.05) and 50% (P less than 0.001), respectively. The lymphoproliferative response to T and B cell mitogens, however, tended to increase in both lymphoid organs. Natural killer cell activity was reduced by 44% (P less than 0.01) and 75% (P less than 0.05) in the spleen and blood, respectively. The number of red blood cells increased slightly (12%, P less than 0.05), whereas white blood cell counts were unaffected. These results indicate that MeHg evokes immune suppressive effects on protective cytotoxic capacity that is of major importance in the pathogenesis of several diseases.