Metal-induced autoimmunity

Naturally occurring autoimmune disease in (NZB X NZW) F1 mice is correlated with suppression of MZ B cell development due to aberrant B Cell Receptor (BCR) signaling, which is exacerbated by exposure to inorganic mercury

Autoimmune diseases are multifactorial and include environmental as well as genetic drivers. Although much progress has been made in understanding the nature of genetic underpinnings of autoimmune disease, by comparison much less is understood regarding how environmental factors interact with genetics in the development of autoimmunity and autoimmune disease. In this report, we utilize the (NZB X NZW) F1 mouse model of Systemic Lupus Erythematosus (SLE). Mercury is a xenobiotic that is environmentally ubiquitous and is epidemiologically linked with the development of autoimmunity. Among other attributes of human SLE, (NZB X NZW) F1 mice spontaneously develop autoimmune-mediated kidney disease. It has been previously shown that if (NZB X NZW) F1 mice are exposed to inorganic mercury (Hg2+), the development of autoimmunity, including autoimmune kidney pathology, is accelerated. We now show that in these mice the development of kidney disease is correlated with a decreased percentage of marginal zone (MZ) B cells in the spleen. In Hg2+-intoxicated mice, kidney disease is significantly augmented, and matched by a greater decrease in MZ B cell splenic percentages than found in control mice. In Hg2+- intoxicated mice, the decrease in MZ B cells appears to be linked to aberrant B Cell Receptor (BCR) signal strength in transitory 2 (T2) B cells, developmental precursors of MZ B cells.

Alopecia Areata Different View; Heavy Metals

Background:

Alopecia areata (AA) is a noncicatricial alopecia affecting any hair-bearing area. Although AA is considered to be an autoimmune disease, oxidative stress has been shown to be an important factor in the etiology of AA. Trace elements are highly essential for humans since they form the building blocks of large molecules, function as the cofactors of enzymes, and have some key biological functions. The aim of this study was to investigate serum levels of heavy metals like iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), magnesium (Mg), cobalt (Co), cadmium (Cd), and lead (Pb) in patients with AA.

Materials and Methods:

The study included a patient group of 30 with AA and a control group of 31 healthy individuals. Serum levels of heavy metals were measured using atomic absorption spectrophotometry.

Results:

The 30 patients included 17 (56.7%) men and 13 (43.3%) women with a mean age of 33.8 (range, 19–48) years. Serum Zn and Mn levels were significantly lower and serum Cd, Fe, Mg, Pb, Co, and Cu levels were significantly higher in the patient group compared to that of the control group (P<0.05 for all).

Conclusion:

The results indicated that low levels of Zn and Mn are associated with AA while other metals were normal. So Zn supplementation may have some beneficial effect in AA while Fe prescription is fruitless.

Low level Hg2+ exposure modulates the B-cell cytoskeletal phosphoproteome

Exposure of Wehi-231 B-cells to Hg²⁺ for 5min resulted in concentration dependent changes in protein phosphorylations. Phosphorylation was quantified using mass spectrometry to analyze TiO₂ and anti-pTyr antibody selected phosphopeptides from Wehi-231 digests. The most frequent and largest amplitude responses to Hg²⁺ exposure were increased phosphorylation although a decrease was observed for 1% of phosphoproteins detected in the untreated cells. A subset of proteins responded with an increase in phosphorylation to Hg²⁺ exposure at low micromolar concentrations. The majority of proteins required Hg²⁺ over 20μM in order to increase phosphorylation. Ser/Thr phosphorylations are prominent in the cytoskeletal organization and the GTPase signaling systems and these systems are notable as the primary ones responding to the lowest concentrations of Hg²⁺. Systems that required higher concentrations of Hg²⁺ to increase phosphorylation included immune receptor signaling. The proteins for which an increase in phosphorylation occurred at Hg²⁺ above 20μM have a higher proportion of pTyr sites. Anti Ig stimulation of Wehi-231 cells confirmed that cytoskeletal protein phosphorylation and GTPase signaling are modulated in physiologically relevant B-cell receptor activation. Candidate kinases that respond to Hg²⁺ exposure at the low μM concentrations include MAP Kinase 1, CaM Kinase II delta and PAK2.

SIGNIFICANCE

Mercury (Hg) is a wide spread environmental toxicant. Epidemiological and laboratory studies suggest that exposure to environmental Hg at current levels, which have been perceived to be non-toxic, may contribute to immune system dysfunction and autoimmune disease in humans and animals respectively. While we have previously shown that exposure of B lymphocytes to low levels of mercury interferes with B-cell receptor signaling mediated by post transcriptional phosphorylation events, overall the mechanism that is responsible for increased autoimmunity in mercury exposed human or animal populations is not well understood. The current study evaluated the dose dependent actions of mercury to change phosphorylation in the Wehi-231 cell line, an immature B-cell model in which actions of mercury on development of cell function can be evaluated. The study identified the cytoskeletal proteins as the most sensitive to modulation by mercury with changes in Ser/Thr phosphorylation being observed at the lowest concentrations of mercury. These findings indicate that the actions of mercury on B-cell immune function and development are at least in part likely mediated through changes in cytoskeletal protein phosphorylation.

Low level exposure to inorganic mercury interferes with B cell receptor signaling in transitional type 1 B cells

Mercury (Hg) has been implicated as a factor contributing to autoimmune disease in animal models and humans. However the mechanism by which this occurs has remained elusive. Since the discovery of B cells it has been appreciated by immunologists that during the normal course of B cell development, some immature B cells must be generated that produce immunoglobulin reactive to self-antigens (auto-antibodies). However in the course of normal development, the vast majority of immature auto-reactive B cells are prevented from maturing by processes collectively known as tolerance. Autoimmune disease arises when these mechanisms of tolerance are disrupted. In the B cell compartment, it is firmly established that tolerance depends in part upon negative selection of self-reactive immature (transitional type 1) B cells. In these cells negative selection depends upon signals generated by the B Cell Receptor (BCR), in the sense that those T1 B cells who's BCRs most strongly bind to, and so generate the strongest signals to self-antigens are neutralized. In this report we have utilized multicolor phosphoflow cytometry to show that in immature T1 B cells Hg attenuates signal generation by the BCR through mechanisms that may involve Lyn, a key tyrosine kinase in the BCR signal transduction pathway. We suggest that exposure to low, environmentally relevant levels of Hg, disrupts tolerance by interfering with BCR signaling in immature B cells, potentially leading to the appearance of mature auto-reactive B cells which have the ability to contribute to auto-immune disease.

Dietary n-3 PUFAs augment caspase 8 activation in Staphylococcal aureus enterotoxin B stimulated T-cells

Epidemiological studies have linked consumption of n-3 PUFAs with a variety of beneficial health benefits, particularly with respect to putative anti-inflammatory effects. Unfortunately, many of these results remain somewhat controversial because in most instances there has not been a linkage to specific molecular mechanisms. For instance, dietary exposure to low levels of mercury has been shown to be damaging to neural development, but concomitant ingestion of n-3 PUFAs as occurs during consumption of fish, has been shown to counteract the detrimental effects. As the mechanisms mediating the neurotoxicity of environmental mercury are not fully delineated, it is difficult to conceptualize a testable molecular mechanism explaining how n-3 PUFAs negate its neurotoxic effects. However, environmental exposure to mercury also has been linked to increased autoimmunity. By way of a molecular understanding of this immuno-toxic association, disruption of CD95 signaling is well established as a triggering factor for autoimmunity, and we have previously shown that environmentally relevant in vitro and dietary exposures to mercury interfere with CD95 signaling. In particular we have shown that activation of caspase 8, as well as downstream activation of caspase 3, in response to CD95 agonist stimulation is depressed by mercury. More recently we have shown in vitro that the n-3 PUFA docosahexaenoic acid counteracts the negative effect of mercury on CD95 signaling by restoring caspase activity. We hypothesized that concomitant ingestion of n-3 PUFAs with mercury might be protective from the immuno-toxic effects of mercury, as it is with mercury's neuro-toxic effects, and in the case of immuno-toxicity this would be related to restoration of CD95 signal strength. We now show that dietary ingestion of n-3 PUFAs generally promotes CD95 signaling by upregulating caspase 8 activation. Apart from accounting for the ability of n-3 PUFAs to specifically counteract autoimmune sequelae of mercury exposure, this novel finding for the first time suggests a testable molecular mechanism explaining the overall anti-inflammatory properties of n-3 PUFAs.

A systems toxicology approach identifies Lyn as a key signaling phosphoprotein modulated by mercury in a B lymphocyte cell model

Network and protein-protein interaction analyses of proteins undergoing Hg²⁺-induced phosphorylation and dephosphorylation in Hg²⁺-intoxicated mouse WEHI-231 B cells identified Lyn as the most interconnected node. Lyn is a Src family protein tyrosine kinase known to be intimately involved in the B cell receptor (BCR) signaling pathway. Under normal signaling conditions the tyrosine kinase activity of Lyn is controlled by phosphorylation, primarily of two well known canonical regulatory tyrosine sites, Y-397 and Y-508. However, Lyn has several tyrosine residues that have not yet been determined to play a major role under normal signaling conditions, but are potentially important sites for phosphorylation following mercury exposure. In order to determine how Hg²⁺ exposure modulates the phosphorylation of additional residues in Lyn, a targeted MS assay was developed. Initial mass spectrometric surveys of purified Lyn identified 7 phosphorylated tyrosine residues. A quantitative assay was developed from these results using the multiple reaction monitoring (MRM) strategy. WEHI-231 cells were treated with Hg²⁺, pervanadate (a phosphatase inhibitor), or anti-Ig antibody (to stimulate the BCR). Results from these studies showed that the phosphoproteomic profile of Lyn after exposure of the WEHI-231 cells to a low concentration of Hg²⁺ closely resembled that of anti-Ig antibody stimulation, whereas exposure to higher concentrations of Hg²⁺ led to increases in the phosphorylation of Y-193/Y-194, Y-501 and Y-508 residues. These data indicate that mercury can disrupt a key regulatory signal transduction pathway in B cells and point to phospho-Lyn as a potential biomarker for mercury exposure.

Toxic effects of dietary methylmercury on immune function and hematology in American kestrels (Falco sparverius)

Fifty-nine adult male American kestrels (Falco sparverius) were assigned to one of three diet formulations including 0 (control), 0.6, and 3.9 µg/g (dry wt) methylmercury (MeHg). Kestrels received their diets daily for 13 weeks to assess the effects of dietary MeHg on immunocompetence. Immunotoxic endpoints included assessment of cell-mediated immunity (CMI) using the phytohemagglutinin (PHA) skin-swelling assay and primary and secondary antibody-mediated immune responses (IR) via the sheep red blood cell (SRBC) hemagglutination assay. Select hematology and histology parameters were evaluated to corroborate the results of functional assays and to assess immunosuppression of T and B cell-dependent components in spleen tissue. Kestrels in the 0.6 and 3.9 µg/g MeHg groups exhibited suppression of CMI, including lower PHA stimulation indexes (p = 0.019) and a 42 to 45% depletion of T cell-dependent splenic lymphoid tissue (p = 0.006). Kestrels in the 0.6 µg/g group exhibited suppression of the primary IR to SRBCs (p = 0.014). MeHg did not have a noticeable effect on the secondary IR (p = 0.166). Elevation of absolute heterophil counts (p < 0.001), the heterophil to lymphocyte ratio (p < 0.001), and total white blood cell counts (p = 0.003) was apparent in the 3.9 µg/g group at week 12. Heterophilia, or the excess of heterophils in peripheral blood above normal ranges, was apparent in seven of 17 (41%) kestrels in the 3.9 µg/g group and was indicative of an acute inflammatory response or physiological stress. This study revealed that adult kestrels were more sensitive to immunotoxic effects of MeHg at environmentally relevant dietary concentrations than they were to reproductive effects as previously reported.

Toxic effects of dietary methylmercury on immune system development in nestling American kestrels (Falco sparverius)

This study evaluated the effects of dietary methylmercury (MeHg) on immune system development in captive-reared nestling American kestrels (Falco sparverius) to determine whether T cell-mediated and antibody-mediated adaptive immunity are targets for MeHg toxicity at environmentally relevant concentrations. Nestlings received various diets, including 0 (control), 0.6, and 3.9 µg/g (dry wt) MeHg for up to 18 d posthatch. Immunotoxicity endpoints included cell-mediated immunity (CMI) using the phytohemagglutinin (PHA) skin-swelling assay and antibody-mediated immune response via the sheep red blood cell (SRBC) hemagglutination assay. T cell- and B cell-dependent histological parameters in the spleen, thymus, and bursa of Fabricius were correlated with the functional assays. For nestlings in the 0.6 and 3.9 µg/g MeHg groups, CMI was suppressed by 73 and 62%, respectively, at 11 d of age. Results of this functional assay were correlated with T cell-dependent components of the spleen and thymus. Dose-dependent lymphoid depletion in spleen tissue directly affected the proliferation of T-lymphocyte populations, insofar as lower stimulation indexes from the PHA assay occurred in nestlings with lower proportions of splenic white pulp and higher THg concentrations. Nestlings in the 3.9 µg/g group also exhibited lymphoid depletion and a lack of macrophage activity in the thymus. Methylmercury did not have a noticeable effect on antibody-mediated immune function or B cell-dependent histological correlates. We conclude that T cell-mediated immunosuppression is the primary target of MeHg toward adaptive immunity in developing kestrels. This study provides evidence that environmentally relevant concentrations of MeHg may compromise immunocompetence in a developing terrestrial predator and raises concern regarding the long-term health effects of kestrels that were exposed to dietary MeHg during early avian development.

Immune Modulation by Cadmium and Lead in the Acute Reporter Antigen–Popliteal Lymph Node Assay

Immune modulation by heavy metals may cause serious adverse health effects in humans, although the mechanisms involved are not well understood. Both cadmium and lead are important environmental and occupational toxins. Therefore, in the current study, the costimulatory/adjuvant effects and the T-cell-activating potential of these metals (i.e., CdCl2 and PbCl2), are examined. These immune-modulating properties are critical in the development of conditions such as allergy, hypersensitivity, and autoimmunity. Using the direct popliteal lymph node assay (PLNA) and reporter antigen-popliteal lymph node assay (RA-PLNA) both metals were examined individually for immunotoxicity. Mercury (i.e., HgCl2) was included for comparative purposes as its effects in the RA-PLNA are well documented. Seven days following a single footpad injection containing metal and/or RA (trinitrophenyl-ovalbumin [TNP-OVA] or TNP-Ficoll), BALB/c mice were sacrificed and the popliteal lymph nodes (PLNs) removed. PLN cellularity, TNP-specific antibody-secreting cells (ASCs), and lymphocyte subsets were assessed. All three metals strongly stimulated T- and B-cell proliferation and ASC production following coinjection with the RA TNP-OVA. In each case, ASC production was skewed towards the IgG1 isotype. In addition, all three metals induced IgG production to TNP-Ficoll (although relatively weakly in the case of Cd). These results show that each of these metals can provide adjuvant signals to promote lymphocyte proliferation and enhance adaptive immune responses to unrelated antigens. Skewing of immune responses towards T helper type 2 responses suggests that each of these metals can enhance allergic and hypersensitivity reactions to environmental antigens. Furthermore, the induction of IgG responses to TNP-Ficoll, a T-cell-independent antigen, indicates that each of these metals can activate neoantigen-specific T cells. T-cell activation by metals can lead to metal hypersensitivity and has been implicated in the development of autoimmunity. This is the first report of immune modulation by CdCl2 and PbCl2 in the RA-PLNA.

Susceptibility of lupus-prone NZM mouse strains to lead exacerbation of systemic lupus erythematosus symptoms

It has been repeatedly shown that the heavy metal mercury can induce or exacerbate lupus like autoimmunity in susceptible strains of rats and mice. A hallmark of such autoimmune induction is the accompaniment of an immune shift, in which there is usually an initial skewing toward a Th2-like immune environment. Another heavy metal, lead (Pb), has also been found to induce a Th2 shift in mice. However, exposure of normal mouse strains to Pb does not appear to induce autoimmunity. In order to investigate whether mice genetically predisposed to murine systemic lupus erythematosus (SLE) are susceptible to a Pb-induced exacerbation of lupus, males and females of four New Zealand mixed (NZM) mouse strains, along with BALB/c and C57Bl/6 controls, were administered three 100-microliter intraperitoneal injections of either 1.31 mM lead or sodium acetate per week for 3 wk. The four NZM strains chosen, NZM391, NZM2328, NZM88, and NZM2758, have differential genetic penetrance for SLE with variances in certain manifestations of the disease, but all of these strains naturally develop glomerulonephritis and produce high titers of anti-nuclear autoantibodies. The mice were prebled for baseline values and were bled directly after the injection period (d 1) and monthly thereafter for 5 mo. Sera were assessed for anti-double-stranded DNA titers, urea nitrogen levels, and creatine kinase activity, as well as four total immunoglobulin (Ig) G2a and IgG1 levels. Mortality and morbidity of the mice were also recorded. All NZM strains showed an acute, non-gender-based, susceptibility to Pb at d 1, but the control strains were unaffected. Over time, it became apparent that the strains diverged: The NZM391 strain showed gender-independent susceptibility to Pb enhancement of lupus manifestations and mortality; the NZM2328 strain exhibited gender-independent Pb susceptibility to manifestations, although only females had increased mortality; the NZM2758 strain exhibited non-gender-based elevations in urea nitrogen and creatine kinase activity levels; and the NZM88 strain displayed male susceptibility to anti-DNA and life span. Surprisingly, Pb increased the longevity of NZM88 and NZM2758 females. These results indicate that Pb indeed can exacerbate SLE in lupus-prone mice; however, even among lupus-prone strains, genetic differences determine the degree of exacerbation. Using the known phenotype and genetic differences, one can identify and characterize possible traits and loci associated with Pb susceptibility.

Evaluation of cadmium-induced transcriptome alterations by three color cDNA labeling microarray analysis on a T-cell line

Beside heavy metals, cadmium (Cd(2+)) is a ubiquitous toxic metal with a well established apoptotic and genotoxic effect, chronic exposure of which has been involved in a variety of pathological conditions. In the present study, we investigated by 1455 genes cDNA microarrays the toxic and apoptotic effect of Cd(2+), on the T-cell line CCRF-CEM, applying a three laser differential analysis, on the same microarray slide. The cells were cultured for 6 and 24 h in the absence (control) or presence of Cd(2+) (10 or 20 microM), RNAs were extracted and the produced cDNAs were labeled with rhodamine derivatives fluorescent dyes. A microarray slide was simultaneously hybridized by the labeled cDNAs and analyzed. We found that, in relation to control, treatment of the cells for 6 h with 10 and 20 microM Cd(2+), induces up-regulation in 20 and 34 genes, respectively. Treatment for 24 h with 10 and 20 microM Cd(2+) induces up-regulation in 22 and 84 genes, respectively. Twenty-eight genes were found down-regulated only after treatment for 24 h with Cd(2+) 10 microM. These data suggest that Cd(2+) produces a time- and dose-dependent molecular cascade, induces disturbances in different subcellular compartments, influencing thereafter the normal cellular functions, the differentiation process, the malignant transformation and the cell death.

Costimulatory molecule expression following exposure to orthopaedic implants wear debris

Patients with long-term orthopedic implants may develop inflammatory reactions due to the accumulation of biomaterial particles both around the implant and in distant organs. The exact impact of these particles on the normal immune cell function still remain relatively unclear. Activation of T-cells following exposure to biomaterial particles is driven by macrophages and requires synergistic signals primed by both antigen presentation and costimulation. The pattern of costimulatory molecule expression (CD80,CD86) was primarily examined using immunohistochemistry on tissue specimens of bone/implant interface membranes taken from sites of bone erosion. Additionally, costimulatory molecule expression was also assessed in the monocytic leukemia cell line U937 following exposure to clinically relevant titanium aluminum vanadium (TiAlV) and stainless steel particles (FeCrNi) cultured in vitro. This study demonstrates the induction and prominent expression of CD86 on almost all macrophage subsets at the bone/implant interface, including fused forms and large multinucleated giant cells (MNGC). In vitro analysis also indicated phagocytosis of metal particles by differentiated U937 caused significant induction of both CD80 and CD86 (p < 0.01), although the expression of CD86 dominated following prolonged exposure. The data presented highlights that CD86 is the predominant costimulatory molecule ligating to the complementary CD28 molecule at the inflammatory lesion of the interface. We propose that the intracellular presence of indigestible implant material, in addition to elevated costimulatory molecule expression, may promote T-cell inflammatory reactions at sites close to and distant from the orthopedic implant.

Effects on the immune system associated with living near a pesticide dump site

In this paper, we report results of the second phase of a larger study designed to evaluate the effects on the immune system of living near a Superfund site containing organochlorine pesticides, volatile organic compounds, and metals. Phase II was conducted to determine whether living near the site, consisting of six locations in Aberdeen, North Carolina, is associated with higher plasma organochlorine levels, immune suppression, or DNA damage. Each of 302 residents of Aberdeen and neighboring communities provided a blood specimen, underwent a skin test, and answered a questionnaire. Blood specimens were analyzed for organochlorine pesticides, immune markers, and micronuclei. Of 20 organochlorines tested, only DDE was detected in the blood of participants (except for one individual). Age-adjusted mean plasma DDE levels were 4.05 ppb for Aberdeen residents and 2.95 ppb (p = 0.01) for residents of neighboring communities. Residents of 40-59 years of age who lived within a mile of any site, but particularly the Farm Chemicals site, had higher plasma DDE levels than residents who lived farther away. Residents who lived near the Farm Chemicals site before versus after 1985 also had higher plasma DDE levels. Overall, there were few differences in immune markers between residents of Aberdeen and the neighboring communities. However, residents who lived closer to the dump sites had statistically significantly lower mitogen-induced lymphoproliferative activity than residents who lived farther away (p < 0.05). Residential location was not consistently associated with frequency of micronuclei or skin test responses. Although some statistically significant differences in immune markers were noted in association with residential location, the magnitude of effects are of uncertain clinical importance.

The immunomodulatory effect(s) of lead and cadmium on the cells of immune system in vitro

A number of studies documented that the heavy metals are not only toxic for the organisms but they may modulate immune responses. The immunomodulatory activity was proved in several in vivo and in vitro model systems. In the current study, immunomodulatory activities of lead and cadmium are presented. The viability of both lymphocytes and macrophages was affected by heavy metals in a dose- and time-dependent manner. In the case of lead, the depression of N-oxide production closely correlated with increased blast transformation of spleen cells induced by concanavalin A (ConA). On the contrary, cadmium suppressed the production of N-oxides but stimulated significantly the proliferation of spleen cells. The production of cytokines by lymphocytes and macrophages was dependent on the in vitro model used. Generally, the treatment of macrophages with lead results in disregulation of the production of proinflammatory cytokines [tumour necrosis factor alpha (TNF-alpha), interleukin 1alpha (IL-1alpha) and interleukin 6 (IL-6)] and preferential production of Th1 type of cytokines (IFN-gamma and IL-2). Cadmium seemed to trigger the Th2 cytokine regulatory pathway [interleukin 4 (IL-4), interleukin 10 (IL-10)]. The results suggest the metal-induced changes in immunoregulatory mechanism of host with potentially severe clinical consequences.

Environmental chemical exposures and risk of herpes zoster

This study investigated whether residence in Aberdeen, North Carolina, the location of the Aberdeen pesticides dumps site (a national priority list Superfund site containing organochlorine pesticides, volatile organic compounds, and metals), is associated with immune suppression as indicated by a higher incidence of herpes zoster and recent occurrences of other common infectious diseases. Study participants included 1,642 residents, 18-64 years of age, who responded to a telephone survey concerning potential occupational and recreational exposures to pesticides and other chemicals, lifetime history of herpes zoster (shingles), and the recent occurrence of other common infectious diseases. Stratified and logistic regression analyses were used to compare the cumulative incidence of herpes zoster among Aberdeen residents and residents of nearby communities. There was little evidence of an overall increased risk of herpes zoster among Aberdeen residents during the period 1951-1994 [relative risk (RR), 1.3; 95% confidence interval (CI), 0.8-2.1]. However, an elevated risk of herpes zoster was noted consistently among Aberdeen residents of younger ages as compared to residents of the nearby communities. The RR was 2.0 (CI, 1.0-4.0) among those 18-40 years of age and was not affected by controlling for potential confounders. The RR of herpes zoster was also consistently elevated in all age groups for the period before 1985. No differences were noted between residents of Aberdeen and those of the nearby communities with respect to the recent occurrence of other common infectious diseases. These results support the plausibility of an association between exposure to the Aberdeen pesticides dumps site and immune suppression and the potential use of herpes zoster as a marker of immune suppression in studies of environmental chemical exposures.

Major histocompatibility complex class II antigens are required for both cytokine production and proliferation induced by mercuric chloride in vitro

Autoimmune diseases induced by mercuric chloride are genetically determined, at least one gene being major histocompatibility complex (MHC)-linked. Previously, we showed that in vitro mercury stimulation induced a high proliferative response in lymphocytes from susceptible mice (high-responders) and that the proliferative response could be restored in lymphocytes from low-responders by pretreating the cells with mercury. We also found that the continuous presence of mercury induced IL-2 and IFN-gamma production, while pretreatment with mercury induced IL-4 production. In this study, we showed that anti-MHC class II monoclonal antibodies blocked both the mercury-induced proliferative responses in lymphocytes from high-responders and the restored proliferative responses in low-responders. In addition, anti-MHC class II antibodies also inhibited the mercury-induced IL-2, IFN-gamma and IL-4 cytokine production in vitro. The results demonstrate that MHC class II antigens directly participate in mercury-induced cytokine production and cell activation, and are required at the onset of the initiation.