HgCl2-induced perturbation of the T cell network in experimental allergic encephalomyelitis. II. In vivo demonstration of the role of T suppressor and contrasuppressor cells

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.

Induction of CD4+ regulatory T cells by TPA in mice: contra-suppression by CD8+ T cells

Among the eight inbred mouse strains employed in our preceding report, 12-O-tetradecanoylphorbol 13-acetate (TPA) painting alone induced CD4+ regulatory T (Tr) cells in four strains (e.g., C3H/He) at 6-8 weeks of age, but not in the remaining strains (e.g., C57BL/6, BALB/c). In the present study, the effect of growth from 4-14 weeks on delayed-type hypersensitivity (DTH) response was investigated in three inbred murine strains, C3H/He (H-2k), C57BL/6 (H-2b) and BALB/c (H-2d) mice. In all strains older than 10 weeks, DTH response was suppressed exclusively by TPA painting. The defect of suppressive activity for DTH in several of the strains at 6-8 weeks of age was dependent on the presence of cells, which blocked regulatory cell activity at 6-8 weeks of age, but not at 10 weeks of age. The age-dependent difference in regulatory activity was caused by the presence of CD8+ contra-regulatory T (Tcr) cells. CD8+ contra-regulatory T cells are required to contact regulatory cells in order to block DTH suppressive activity. Adhesion molecules were of great importance in contra-suppression, as antibody treatment to LFA-1 or ICAM-1 blocked this activity. ICAM-1 expression on CD4 T cells greatly increased following growth in 10 week-BALB/c mice receiving TPA than 6-week-old mice, however, a slight increase in growth occurred in 6-to-10-week-old animals in which TPA was absent. The degree of increment in body weight was very similar in these inbred strains. Thymus involution in C3H/He mice was the earliest signal among these mice. This result may suggest that the period of differentiation and maturation of T cells in a first lymphoid tissue for the growth process differs in these three inbred strains. This study provides an interesting example of genetic control of maturation or proliferation of peripheral T cells.

Differential expression of T-cell adhesion molecules and LFA-1-dependent intercellular adhesion in HgCl2-induced autoimmunity and immune suppression

Exposure of Brown Norway (BN) rats to HgCl2 induces Th2-mediated systemic autoimmunity. In contrast, in Lewis rats, HgCl2 induces immune suppression, mediated by CD8+ T cells. HgCl2 was previously found to enhance expression of LFA-1, ICAM-1 and CD134 (OX40) on T cells in BN rats. In the present study, T cells from Lewis rats were studied at day 4 after injection of HgCl2. CD8+ T lymphoblasts were significantly increased, which were predominantly CD45RC(hi), and which showed enhanced LFA-1 expression. Furthermore, CD4+CD45RC(hi) T cells showed increased numbers of ICAM-1+ cells, whereas expression of CD134 and CD26 was relatively decreased in CD4+ T lymphoblasts. Ex vivo experiments demonstrated that HgCl2-exposure of BN rats, but not of Lewis rats, significantly enhances PMA [phorbol 12-myristate 13-acetate]-induced lymphocyte aggregation, mediated by LFA-1 and ICAM-1. In conclusion, HgCl2-injected Lewis rats show early signs of T-lymphocyte activation, predominantly on CD8+ cells. Strain-dependent effects of HgCl2 on cell adhesion molecules and expression of CD134 may play an important role in development of either autoimmunity or immune suppression.

Effects of HgCl2 on the expression of autoimmune responses and disease in diabetes-prone (DP) BB rats

Repeated exposure of Brown Norway (BN) rats to relatively low doses of HgCl2 induces autoantibodies to renal antigens (e.g., laminin) and a membranous glomerulonephropathy characterized by proteinuria. In contrast, Lewis (LEW) rats are "resistant" to the autoimmune effects of mercury and, when exposed to this metal, are protected against experimental autoimmune encephalomyelitis (EAE) and Heymann's nephritis. To date, there is no information on "suppressive" effects of mercury in naturally occurring (so-called "spontaneous") rat models of autoimmune disease. Therefore, we have administered HgCl2 to diabetes-prone (DP) BB rats, animals that spontaneously develop both insulin-dependent diabetes mellitus (IDDM) and thyroiditis. We found that DP rats treated with mercury or water for a period of 40-125 days developed autoantibodies to thyroglobulin, with a higher incidence in HgCl2-injected animals (92% vs. 56% in H2O-injected controls). A novel finding of our study was the detection of autoantibodies to laminin in the same rats, again with an increased incidence after HgCl2 treatment (83% vs. 44%). IgG2a was the most frequently detected isotype of antibodies to laminin, followed by IgG1, IgG2b and IgG2c. The IgG isotype profile suggests that treatment with HgCl2 may activate both Th1 and Th2 lymphocytes in BB rats. In spite of these stimulatory effects on autoantibody responses, we found that there was no difference in the incidence of IDDM and thyroiditis between HgCl2-treated and control animals. We conclude that the suppressive effects of mercury previously observed in EAE and Heymann's nephritis of LEW rats do not occur in "spontaneous" autoimmune IDDM and thyroiditis of BB rats. Therefore, immune suppression caused by HgCl2 cannot be considered a common phenomenon, but may be a genetically determined characteristic of LEW rats, possibly related to a specific or unique cytokine profile of this particular rat strain. In contrast, while mercury does not seem to recruit, induce or rescue regulatory T cell function in DP rats, it does stimulate autoantibody responses in these animals.

T lymphocytes in mercury-exposed workers

In this work we have investigated the changes in T-helper and T-suppressor cells and T-cell proliferative response to phytohemagglutinin (PHA) in mercury-exposed workers. The study group consisted of 33 workers from a mercury-producing plant with a mean age of 29 years and a mean exposure period of 19 months. At the time of testing, and for the three previous months, the exposed population had urinary mercury levels below the currently accepted limit of 50 micrograms/g creatinine. A reverse CD4+/CD8+ ratio was observed in the mercury-exposed individuals which was characterized by a reduction in the number of CD4+ lymphocytes. No changes were observed in the proliferative response of lymphocytes from exposed individuals to PHA. Similarly, no proliferative response was observed when lymphocytes from normal individuals were cultivated in the presence of serum from the exposed workers. We found no correlations between lymphocytes changes and urinary mercury concentrations, time of exposure or the age of the workers.

Predictive immunotoxicological test systems: suitability of the popliteal lymph node assay in mice and rats

This article reviews results obtained with popliteal lymph node assays (PLNAs) in rodents and discusses their ability to detect and analyze immunotoxic effects of drugs and other low molecular weight (LMW) chemicals. In its basic form, the PLNA measures activation of the draining lymph node of the hind paw (i.e., the PLN) after injection of a test chemical into the hind foot pad. The assay appears to be appropriate to recognize sensitizing, that is, allergenic and autoimmunogenic, chemicals, as well as nonsensitizing immunostimulatory chemicals. With modifications, PLNAs can detect immunosuppressive chemicals and distinguish sensitizing from nonsensitizing chemicals. Furthermore, modified PLNAs enable detection of known as well as unknown sensitizing metabolites, and may assist in the identification of the self-molecules that act as carriers for chemical sensitization or as targets of chemical-induced autoimmune disease. Experience with PLNAs shows that they are rapid, reproducible, and objective tests for recognition of sensitizing or otherwise immunomodulating chemicals. Because current protocols of toxicity testing are insensitive in predicting a chemical's potential to result in immunomodulation, PLNAs, when further validated, may provide welcome supplements to routine toxicity screening of chemicals, thus enhancing chemical safety.

Characterization of an immunoregulatory cell subset involved in the up-regulation of the autoimmune response to rat male accessory glands

PROBLEM

We studied the regulation of the autoimmune response to rat male accessory glands (RAG) using liposomes as adjuvants.

METHOD

Adult male Wistar rats were submitted to three intraperitoneal (i.p.) immunizations with 750 micrograms of saline extract of RAG associated with liposomes. The delayed type hypersensitivity (DTH) response studied approximately 10 days after each immunization developed after the first immunization, having a remission state after the second one and a clear increase after the third injection. In a further study, spleen mononuclear (SpM) cells obtained form immunized rats 10 days after the third immunization (DTH positive) or from normal rats were separated as adherent (VV+) or nonadherent (VV-) to Vicia villosa population. In VV+ SpM cells from immunized or normal animals an enhanced percentage of OX8+ cells (P < .05 and P < .01, respectively) was found, but in VV- SpM cells from the same groups of rats an enhanced percentage of W3/25+ cells (P < .01 and P < .05, respectively) was found when they were studied by immunofluorescence. Later on, we transferred total VV+ or VV- SpM cells from i.p. immunized rats to immunized recipients 10 days after the second immunization (DTH negative). The DTH response was enhanced in recipients of total or VV+ SpM cells (P < .01). It was also observed that the transfer of VV- SpM cells from immunized rats or total or VV+ SpM cells from normal rats did not reduce the suppression state observed after the second injection (P = NS). The total SpM cells obtained 10 days after the third immunization (DTH positive) were able to transfer autoimmune response to RAG to normal animals (P < .01), whereas VV+ SpM cells did not show that capacity (P = NS).

Mercury-induced autoreactive anti-class II T cell line protects from experimental autoimmune encephalomyelitis by the bias of CD8+ antiergotypic cells in Lewis rats

Brown-Norway (BN) rats injected with HgCl2 develop a systemic autoimmune disease associated with a polyclonal B cell activation, due to autoreactive T cells specific for self-class II molecules, while Lewis (LEW) rats injected with HgCl2 do not exhibit autoimmunity and develop a non-antigen-specific, CD8-mediated immunosuppression assessed by a depression of T cell functions, and a protection against experimental autoimmune encephalomyelitis (EAE). Resistance to HgCl2-induced autoimmunity is not due to these suppressor cells since treatment with an anti-CD8 monoclonal antibody (mAb) did not allow autoimmunity to appear. The absence of autoimmunity in this strain could result from the absence of autoreactive T cells, or from quantitative or qualitative differences of these cells between susceptible and resistant strains. In the present study, we show that CD4+ anti-class II T cells are present in HgCl2-injected LEW rats and are as frequent as in BN rats when assessed by limiting dilution analysis. LEW CD4+ autoreactive T cell lines were derived. They proliferated in the presence of normal class II-bearing cells, secreted interleukin 2, and did not induce B cells to produce immunoglobulins. Transfer of one of these lines, LEW Hg A, into normal LEW rats led to the appearance of CD8+ cells responsible for a non-antigen-specific immunosuppression that induced complete protection from EAE. Immunosuppression was abrogated after treatment with an anti-CD8 mAb. In vitro, CD8+ cells from rats injected with the LEW Hg A T cell line proliferated in the presence of activated T cells whatever their origin. We conclude that HgCl2 induces CD4+ autoreactive T cells that proliferate in the presence of class II+ cells in susceptible BN as well as in resistant LEW rats. But while these cells collaborate with B cells to produce autoantibodies in BN rats, they initiate in LEW rats a suppressor circuit involving antiergotypic CD8+ suppressor cells.

Immunotoxicology of cadmium and mercury on B-lymphocytes--I. Effects on lymphocyte function

Heavy metals have been shown to exert immunotoxic effects on humoral immunity. To ascertain the mechanisms by which these immunotoxic effects are exerted, the effects of CdCl2 and HgCl2 on the biology of murine B-lymphocytes were studied. It was shown that CdCl2 and HgCl2 inhibited B-cell RNA and DNA synthesis. The IC50 (the concentration required to inhibit a specific B-cell function by 50%) for CdCl2 was 30 microM for RNA synthesis and DNA synthesis. The IC50 for HgCl2 was 50 and 120 nM for RNA and DNA synthesis, respectively. Cell cycle analysis revealed that B-cells were arrested throughout the cell cycle with CdCl2 and HgCl2. The inhibitory effects exerted by CdCl2 and HgCl2 were rapid, inhibiting RNA synthesis within 2 h of activation. Differentiation to Ig secretion was inhibited by CdCl2 and HgCl2 in culture and there appeared to be selective effects on specific Ig isotypes. IgG3 production was most sensitive to inhibition by CdCl2 and HgCl2 followed by IgG1 and IgG2b and then IgM and IgG2a. Changes in the expression of B-cell surface antigens induced by LPS were also influenced by CdCl2. LPS-induced increases in class II MHC expression was inhibited by CdCl2, as was the constitutive expression of class I MHC antigen. A summary of the IC50 for CdCl2 and HgCl2 are presented. In summary, both CdCl2 and HgCl2 exert early, inhibitory effects on B-cell activation. This is manifested by the inhibition of RNA, DNA and antibody synthesis. However, selective effects on the production of specific Ig isotypes by these metals may influence the ability of B-cells to mount effective immune responses to pathogens.