Environmental Health Perspectives Volume 107, Supplement 5, October 1999
Lupus-Prone Mice as Models to Study Xenobiotic-Induced Acceleration of Systemic Autoimmunity
K. Michael Pollard,1 Deborah L. Pearson,1 Per Hultman,3 Bernhard Hildebrandt,1 and Dwight H. Kono2
1Department of Molecular and Experimental Medicine, 2Department of Immunology, The Scripps Research Institute, La Jolla, California USA; 3Division of Molecular and Immunological Pathology, Department of Health and Environment, Linköping University, Linköping, Sweden
Abstract
The linkage between xenobiotic exposures and autoimmune diseases remains to be clearly defined. However, recent studies have raised the possibility that both genetic and environmental factors act synergistically at several stages or checkpoints to influence disease pathogenesis in susceptible populations. These observations predict that individuals susceptible to spontaneous autoimmunity should be more susceptible following xenobiotic exposure by virtue of the presence of predisposing background genes. To test this possibility, mouse strains with differing genetic susceptibility to murine lupus were examined for acceleration of autoimmune features characteristic of spontaneous systemic autoimmune disease following exposure to the immunostimulatory metals nickel and mercury. Although NiCl
2 exposure did not exacerbate autoimmunity, HgCl
2 significantly accelerated systemic disease in a strain-dependent manner. Mercury-exposed (NZB
NZW)F
1 mice had accelerated lymphoid hyperplasia, hypergammaglobulinemia, autoantibodies, and immune complex deposits. Mercury also exacerbated immunopathologic manifestations in MRL+/+ and MR
-lpr mice. However, there was less disease acceleration in
lpr mice compared with MRL+/+ mice, likely due to the fact that environmental factors are less critical for disease induction when there is strong genetic susceptibility. Non-major histocompatability complex genes also contributed to mercury-exacerbated disease, as the nonautoimmune AKR mice, which are H-2 identical with the MRL, showed less immunopathology than either the MRL/
lpr or MRL+/+ strains. This study demonstrates that genetic susceptibility to spontaneous systemic autoimmunity can be a predisposing factor for HgCl
2-induced exacerbation of autoimmunity. Such genetic predisposition may have to be considered when assessing the immunotoxicity of xenobiotics. Additional comparative studies using autoimmune-prone and nonautoimmune mice strains with different genetic backgrounds will help determine the contribution that xenobiotic exposure makes in rendering sensitive populations susceptible to autoimmune diseases.
Key words: animal model, autoimmunity, lupus, mercury, nickel, xenobiotic.
-- Environ Health Perspect 107(suppl 5):729-735 (1999).
http://ehpnet1.niehs.nih.gov/docs/1999/suppl-5/729-735pollard/abstract.html
This article is based on a presentation at the Workshop on Linking Environmental Agents and Autoimmune Diseases held 1-3 September 1998 in Research Triangle Park, North Carolina.
Address correspondence to K.M. Pollard, Dept. of Molecular and Experimental Medicine, SBR6, 10550 North Torrey Pines Rd., La Jolla, CA 92037. Telephone: (619) 784-9214. Fax: (619) 784-2131. E-mail: mpollard@scripps.edu
We thank R.L. Rubin, The Scripps Research Institute, La Jolla, CA, for providing chromatin preparations. This publication was made possible by National Institute of Environmental Health Sciences grants ES07511, ES08080, and ES08666; Swedish Medical Research Council grant 09453; and Deutsche Forschungsgemeinschaft Fellowship Hi 618-1/1. This is publication number 12149 MEM from The Scripps Research Institute, La Jolla, CA.
Received 15 January 1999; accepted 28 April 1999.
Analysis of the role of xenobiotics in autoimmunity, particularly in humans, is hampered by identification of potentially sensitive populations. Epidemiologic studies have revealed associations between xenobiotic exposure and certain systemic autoimmune diseases (
1,2) but do not identify susceptible individuals. Conversely, although it is appreciated that systemic autoimmune diseases are under multigenic control (
3), identification of the genetic elements involved and whether they respond to environmental exposures is in its infancy. However, recent studies have raised the possibility that both genetic and environmental factors act synergistically at several stages or checkpoints of disease pathogenesis (
3). This exacerbating role for environmental agents has been suggested by a number of studies that have shown that exogenous agents can accelerate the onset of autoimmunity in animal models (
4-7). These studies predict that individuals susceptible to spontaneous autoimmunity should be more susceptible following xenobiotic exposure by virtue of the presence of predisposing background genes. Such accelerated disease would be expected to more closely resemble the immunologic features of idiopathic autoimmunity rather than that induced by xenobiotic exposure in a non-autoimmune-prone host.
The availability of animal models of systemic autoimmune disease provides a valuable resource with which to study the responses of a population sensitive to xenobiotic-induced acceleration of autoimmunity. Among the best-known animal models of human systemic autoimmune disease are those mouse strains that spontaneously develop features of systemic lupus erythematosus (SLE) (3,8,9). Each of the lupus-prone strains has been derived from different genetic backgrounds, which results in both quantitative and qualitative differences in disease expression. In some cases specific features of autoimmune disease have been ascribed to individual genetic defects in individual strains (9). Lupus-prone strains thus provide unique genetic backgrounds for the study of xenobiotic influences on the constellation of immunologic features that comprise systemic autoimmunity. The strains used in this study include (NZB NZW)F1 (NZBWF1) (H-2d/z) female mice, which due to the influence of sex hormones develop systemic autoimmunity earlier in life than their male counterparts (3,8). Also examined were MRL (H-2k) mice, which consist of two substrains, MRL/lpr and MRL+/+ (3,8). In the MRL/lpr, autoimmunity occurs very early in life and is associated with massive lymphoproliferation due to abnormal Fas (CD95) expression. In contrast, the MRL+/+ substrain, which has normal Fas expression, develops less severe systemic autoimmune disease, with onset considerably later in life than the MRL/lpr. In order to compare the influence of non-major hisocompatability complex (MHC) genes on xenobiotic acceleration of autoimmunity, the nonautoimmune strain AKR (H-2k) from which the MRL substrains derive their H-2 was also tested.
Xenobiotics selected for study were two metals, mercury and nickel, known to produce distinctly different immunologic responses. Mercury induces autoimmunity in mice (10) and rats (11) and an immune-mediated kidney disease in humans (12). Mercury-induced autoimmunity (HgIA) in rodents is associated with three major pathologic sequelae--lymphoproliferation, hypergammaglobulinemia, and the development of autoimmunity--manifested as autoantibody production and immune complex disease (10,11). Elicitation of these pathologic features is dependent on genetic background, with lymphadenopathy occurring in most strains; autoimmunity is more restricted, being controlled in large part by the MHC (10). HgIA was initially thought to be a CD4+ T-helper (Th)2 response because of increased levels of predominantly interleukin (IL)-4, and IgG1 autoantibodies after HgCl2 exposure (13). However all the features of systemic autoimmunity (hypergammaglobulinemia, autoantibodies, immune complex deposits) are now known to be dependent upon the Th1 cytokine interferon (IFN)-
(14).
In contrast, exposure to nickel is associated with allergic rather than autoimmune reactions. Nickel elicits increases in nitric oxide production (15), activation of nuclear factor-
B (16), and delayed-type hypersensitivity (DTH) reactions in humans (17) and rodents (18). Nickel-induced DTH is characterized by the presence of IFN--producing Th1 and Th0 CD4+ T cells (19).
In this study, particular emphasis was placed on the potential of these xenobiotics to exacerbate the natural course of systemic autoimmune disease in lupus-prone mice as revealed by lymphadenopathy, elevations of serum immunoglobulin, appearance of autoantibodies, and immune complex deposits. Although NiCl2 exposure had little effect on autoimmunity, mercury significantly accelerated systemic disease in a strain-dependent manner. Thus, mercury exposure exacerbated almost all features of autoimmunity in NZBWF1 mice. Mercury also exacerbated immunopathologic features in both MRL substrains, although less effect on the humoral response was observed in mice with the lpr mutation. Non-MHC and non-lpr genes contributed to the mercury-exacerbated disease in the MRL, as H-2 identical nonautoimmune AKR mice showed less immunopathology. These studies suggest that autoimmune-prone mice are useful models for nonhuman studies on the effects of xenobiotics on sensitive populations.
Mice
Female NZBWF
1 (H-2
d/z), MRL-Fas
lpr (MRL/
lpr) (H-2
k), MRL/Fas
+/+ (MRL+/+) (H-2
k), and AKR/J (H-2
k) mice were obtained from The Scripps Research Institute Animal Colony (La Jolla, CA) and maintained under specific pathogen-free conditions. All experimental procedures using animals followed the guidelines set down in the National Institutes of Health
Guide for the Care and Use of Laboratory Animals (
20)
.
Treatment of Mice
Groups of up to eight mice (4 weeks of age) were injected subcutaneously twice per week for 4 weeks with 100 µL phosphate-buffered saline (PBS) containing 40 µg HgCl2, 40 µg NiCl2, or PBS alone. Mice were bled before the first injection and sacrificed on day 30; serum was stored at -70°C until use. Autopsies were performed as previously described (21). Samples of kidney and spleen were harvested for analysis of immune complex deposition and histology (see "Tissue Complex Deposits" and "Light Microscopy").
Detection of Serum Antibody
Antibodies nuclear antigens (ANA) were detected using HEp-2 cell slides (Bion Enterprises, Park Ridge, IL) as previously described (22). Prior to assay, sera were diluted 100-fold in PBS containing 0.1% bovine gamma globulin (BGG), 0.5% bovine serum albumin (BSA), 0.001% gelatin, and 0.05% Tween 20. Goat antimouse IgG-fluorescein isothiocyanate (FITC) (Caltag Laboratories, South San Francisco, CA) diluted 100-fold in PBS containing 0.5% BGG, 0.1% BSA, and 0.05% Tween 20 was used as detecting reagent. Antifibrillarin (nucleolar) monoclonal antibody 72B9 (23) was used as control. Intensity of fluorescence was graded on a scale of 0-4+, with 1+ considered positive.
Antichromatin antibodies were detected by enzyme-linked immunosorbent assay (ELISA) as previously described (24). Sera were diluted 100-fold prior to assay, and chromatin-bound antibodies detected with horseradish peroxidase (HRP)-conjugated goat antimouse IgG (Caltag Laboratories) diluted 2,000-fold. Antichromatin monoclonal antibody 1D12 was used as positive control (25).
Serum Immunoglobulin Quantitation
Serum IgG, IgG1, and IgG2a levels were quantified by ELISA, essentially as previously described (26). ELISA plates were coated with 200 µL of 2 µg/mL goat antimouse kappa light chain antibody (Caltag Laboratories) in PBS, and incubated overnight at 4°C. Plates were postcoated for 1 hr with 0.1% gelatin in PBS, followed by three washes with PBS-0.05% Tween 20. Sera were diluted in serum diluent (26), and incubated at room temperature while shaking for 2.5 hr, followed by three washes with PBS-0.05% Tween 20. HRP-conjugated goat antimouse IgG, IgG1, or IgG2a antibodies (Caltag Laboratories) diluted in anti-immunoglobulin diluent (26) were added before incubation with shaking for 90 min. After three washes with PBS-0.05% Tween 20 and four washes with PBS, 2,2´-azinobis(3-ethylbenzthiazolinesulfonic acid) (ABTS) substrate solution was added and optical density read at 405 nm (OD405). Standard curves were generated by serial dilutions of polyclonal mouse reference serum containing predetermined levels of immunoglobulin isotypes and subclasses (The Binding Site, Birmingham, UK). Serum IgG, IgG1, and IgG2a concentrations were quantified by converting the average OD405 of duplicate wells to immunoglobulin concentrations by use of standard curves. Only serum dilutions that gave OD405 values falling within the linear portion of the standard curve were used to calculate immunoglobulin levels.
Tissue Immune Complex Deposits
Sections 2- to 3-mm thick of the kidney and spleen were snap frozen in isopentane-CO2 and prepared for direct immunofluorescence as previously described (27). Cryostat sections 4- to 5-µm thick were fixed in ethanol and incubated with serial dilutions of FITC-conjugated goat antibodies to IgG (-chain specific) or C3 (Southern Biotechnology Associates, Birmingham, AL). The end-point titer of the immune complex deposits was defined as the highest dilution of antibody at which specific fluorescence could be detected and was expressed as the reciprocal titer. A score of 0 was recorded when no specific fluorescence was detected at a dilution of 1/40. The presence of granular deposits in small and medium-size arteries was also examined. All histology slides were examined without knowledge of treatment given or other results.
Light Microscopy
A 2- to 3-mm thick section of kidney was immersed in Histochoice fixative (Amresco, Solon, IL), embedded in paraplast, and cut into 1- to 2-µm sections that were stained with periodic acid Schiff's reagent and with periodic acid silver methenamine. Slides were then examined by light microscopy without knowledge of treatment given or other results. The types of glomerular alteration were determined and the degree of endocapillary cell hyperplasia was scored for each animal as follows: 0 = normal, 0.5 = just detectable alteration, 1 = slight, 2 = moderate, 3 = strong, and 4 = maximal.
Statistical Analysis
Groups were compared by single-factor ANOVA, Mann-Whitney U test, or Fisher's exact test, as appropriate. Comparisons are of HgCl2-treated mice with PBS- and NiCl2- treated animals unless described otherwise. p 
0.05 was considered significant.
Effect of HgCl2 and NiCl2 on Features of Autoimmunity in NZBWF1 Mice
Mercury-exposed NZBWF
1 mice had elevated serum IgG, IgG1, and IgG2a compared to PBS- and NiCl
2-exposed animals (Table 1, Figure 1). Antibody to nuclear antigens (ANA) (Table 1) of a predominantly dense fine to homogenous nuclear speckling of interphase cells and metaphase chromosomes was found in all NZBWF
1 mice exposed to HgCl
2. All pretreatment sera as well as PBS- and NiCl
2-treated mice showed less frequent ANA responses. Levels of antichromatin antibodies (Table 1, Figure 1) in HgCl
2-exposed NZBWF
1 mice were markedly elevated above those found in either NiCl
2- or PBS-exposed animals.
Figure 1. Hypergammaglobulinemia and antichromatin antibodies in NZBWF1 mice after exposure to mercury. OD405, optical density read at 405 nm; PBS, phosphate-buffered saline. Female NZBWF1 mice were injected with HgCl2, NiCl2, or PBS for 4 weeks as described in "Materials and Methods." Serum IgG, IgG1, and IgG2a (left panels) were determined by enzyme-linked immunosorbent assay (ELISA) before (
) and after (
) metal exposure. IgG antichromatin antibodies (right panel) were determined by ELISA before () and after () metal exposure. See Table 1 for statistical analysis.
Exposure of NZBWF1 mice to mercury increased organ wet weight of spleen and cervical lymph nodes but not mesenteric lymph nodes when compared to PBS- and NiCl2-treated animals (Table 2). All HgCl2-treated mice developed very high titers of IgG and C3 deposits in glomeruli (Table 2). Granular IgG deposits in the glomeruli were localized to the capillary loops in 3/8 mice, restricted to the mesangium in one mouse, and present along the capillary loops as well as in the mesangium in the remaining four mice. Granular deposits of C3 were found in the mesangium in 6/8 animals, whereas 2/8 mice showed C3 deposits along the capillary loops as well as in the mesangium. Renal and splenic vessel walls showed granular deposits with high titers of IgG and C3. In contrast, mice treated with NiCl2 or PBS had much lower titers of IgG in the glomerular mesangium in 2/8 and 3/8 mice, respectively, without any deposits along the capillary loops. The titers of C3 in the glomeruli were also significantly lower in these mice. None of the NiCl2- or PBS-treated animals showed deposits of IgG or C3 in the renal or splenic vessel walls. Histologically there was only slight endocapillary cell hyperplasia in the glomeruli, which did not differ among the three treatment groups, probably because of the young age of mice at sacrifice. There were no alterations in the glomerular basement membrane among the three treatment groups.
Effect of HgCl2 and NiCl2 on Features of Autoimmunity in MRL/lpr Mice
Metal exposure did not lead to elevations of immunoglobulin in MRL/lpr mice, as all three treatment groups developed hypergammaglobulinemia during the experimental period (Table 3, Figure 2). Metal- and saline-exposed MRL/lpr mice showed no significant differences in ANA and antichromatin antibody at the end of the treatment period; each of the groups developed antichromatin antibodies during the exposure period (Table 3, Figure 3).
Figure 2. Hypergammaglobulinemia in MRL and AKR mice after exposure to mercury. PBS, phosphate-buffered saline. Female MRL/lpr, MRL+/+ and AKR mice were injected with HgCl2, NiCl2, or PBS for 4 weeks as described in "Materials and Methods." Serum IgG, IgG1, and IgG2a were determined by enzyme-linked immunosorbent assay (ELISA) before () and after () metal exposure. See Tables 3 and 5 for statistical analysis.
Figure 3. Antichromatin antibodies in MRL and AKR mice after metal exposure. OD405, optical density read at 405 nm; PBS, phosphate-buffered saline. Female MRL/lpr, MRL+/+, and AKR mice were injected with HgCl2, NiCl2, or PBS for 4 weeks as described in "Materials and Methods." IgG antichromatin antibodies were determined by enzyme-linked immunosorbent assay (ELISA) before () and after () metal exposure. See Tables 3 and 5 for statistical analysis.
Mercury-exposed MRL/lpr mice had larger mesenteric lymph nodes than PBS- or NiCl2-treated animals but showed no difference in cervical node or spleen weights (Table 4). Mercury treatment increased the incidence of glomerular, mesangial IgG deposits in all MRL/lpr mice compared with 5/8 mice treated with NiCl2 and only 2/8 PBS-treated animals (Table 4). Mercury significantly increased the titer of mesangial IgG and C3 deposits (Table 4). No vessel wall deposits were seen in any of the animals. There was a mild endocapillary cell hyperplasia in the glomeruli, which tended to be more pronounced in the HgCl2-treated group, but this did not reach statistical significance.
Effect of HgCl2 and NiCl2 on Features of Autoimmunity in MRL+/+ Mice
Mercury-exposed MRL+/+ mice had elevated serum IgG and IgG1 compared to PBS- and NiCl2-exposed animals, whereas IgG2a levels were elevated compared to NiCl2-treated mice (Table 3, Figure 2). A variety of ANA patterns were found in MRL+/+ mice before and after the various treatments (data not shown). Mercury treatment of MRL+/+ mice resulted in elevated levels of antichromatin antibodies, which were significantly different from PBS- and NiCl2-treated animals (Table 3, Figure 3).
Mercury-exposed MRL+/+ mice had larger cervical nodes than their control counterparts, but spleen weights were not increased and mesenteric nodes were smaller (Table 4). All MRL+/+ mice treated with HgCl2 showed glomerular and mesangial IgG deposits, whereas only 5/8 NiCl2-treated mice and 3/8 PBS-treated mice showed such deposits (Table 4). Renal vessel wall deposits of IgG were seen in 7/8 and C3 in 2/8 of the HgCl2-treated MRL+/+ mice. Only one animal had deposits of Ig and C3 in splenic vessel walls. Vessel wall deposits were not found in either the kidney or spleen of PBS- and NiCl2-treated animals. Compared with PBS-treated animals, there was significantly increased focal/segmental hyperplasia of endocapillary cells in the mercury group (0.59 ± 0.23 vs 1.25 ± 0.13; p < 0.001).
Effect of HgCl2 and NiCl2 on Features of Autoimmunity in AKR Mice
AKR mice also developed hypergammaglobulinemia with increases in IgG and IgG2a following HgCl2 exposure (Table 5). Mercury-treated AKR mice developed ANA but only low levels of antichromatin antibodies (Table 5). Compared to PBS controls, mercury-exposed AKR mice had significant increases of IgG (51 ± 55 vs 274 ± 78; p < 0.0001) and C3 (34 ± 28 vs 120 ± 52; p < 0.005) deposits in the mesangium of the kidney glomeruli. There were no deposits in the kidney vessels or spleen of AKR mice and no histologic abnormalities by light microscopy.
The exacerbation by mercury of systemic autoimmunity in lupus-prone mice identifies a potential model for the study of xenobiotic effects on autoimmune-sensitive populations. The autoimmune features and the extent to which they were accelerated differed between the stains tested, suggesting that genetic background influenced the response to xenobiotic exposure. Much of the disease acceleration appeared to be due to non-MHC lupus-predisposing genes, as an MHC-matched nonautoimmune strain had less severe disease manifestations. Strain-dependent responses, as well as the importance of non-MHC genes in disease severity, suggest that xenobiotic exposure led to acceleration of idiopathic disease beyond the induction of HgIA. Acceleration of autoimmunity was dependent on the specific immunostimulant; another metal that can activate the immune system, nickel, had little effect.
The most pronounced response to mercury exposure was observed in female NZBWF1 mice, considered to be the prototypic murine model for human lupus (11). This strain exhibited lymphadenopathy in the spleen and the cervical lymph nodes, which drain the mercury injection site. NZBWF1 mice also had pronounced hypergammaglobulinemia. Most striking was the marked elevation in antichromatin autoantibodies--an autoantibody response characteristic of murine lupus (28) and idiopathic (29) and drug-related human lupus (30) but found less frequently in murine HgIA (31). Extensive tissue deposits of immunoglobulin and complement, particularly in the vessels of the kidney, are consistent with the progressive glomerulonephritis of idiopathic disease in NZWBF1 mice (32). The higher titer of glomerular deposits in the NZBWF1 compared to the MRL substrains is also consistent with the idiopathic picture in these strains (32). These studies with mercury, and those with other exogenous agents (4,7), suggest that the NZBWF1 strain may be particularly suited to studies on xenobiotic-induced exacerbation of systemic autoimmunity.
The MRL/lpr has been used extensively in studies on autoimmunity, primarily to understand the significance of the lpr gene but also because its accelerated disease and early mortality facilitate in vivo studies. However, the long life of the MRL+/+ offers the advantage of examining the effects of long-term xenobiotic exposures on the expression of autoimmunity. Previous studies have suggested that autoimmunity in MRL mice is either accelerated (33,34) or unaffected (33) by xenobiotic exposure. Both MRL substrains were examined in this study to determine if the accelerated disease of the MRL/lpr, and the delayed disease of the MRL+/+, would be similarly affected by exposure to metals. The presence of the lpr gene, which is associated with elevations of IgG and ANA as early as 2 months of age (32,35), appeared to be the predominant influence on immunoglobulin and autoantibody levels, as neither metal led to increases above those of the PBS controls. In contrast, mercury-exposed MRL+/+ mice showed evidence of both accelerated hypergammaglobulinemia and elevated antichromatin antibodies compared to nickel- or PBS-treated groups. Interestingly, both MRL substrains had increased glomerular deposits of IgG compared with those in control mice, and MRL+/+ mice showed more severe immunopathology with IgG and C3 deposits in kidney vessels as well as hyperplasia of endocapillary cells. The increased glomerular disease in lpr mice exposed to HgCl2 despite similar levels of immuloglobulins and autoantibodies as PBS and NiCl2 controls suggests that HgCl2 may also promote disease by mechanisms beyond autoantibody production, possibly related to autoantibody specificity or effector pathways involved in end-organ damage.
The MHC is a major contributor to both idiopathic lupus (3) and HgIA (31,36). It could therefore be argued that exacerbation of autoimmunity in lupus-prone mice is primarily due to an effect on H-2 genes and not acceleration of idiopathic disease per se. To examine this possibility, AKR mice, from which the MRL derives its H-2 genes, were exposed to mercury. Although the AKR possesses the H-2k haploytpe, which confers susceptibility to HgIA (31), the degree of autoimmunity elicited differed from that of MRL mice. AKR mice had predominant elevations of IgG2a, whereas MRL+/+ mice showed increased IgG1. HgCl2-exposed MRL+/+ mice had more severe disease, with higher titers of antichromatin antibodies, increased deposits of IgG and C3 in kidney vessels, and histologic glomerular changes. This comparison between autoimmune-prone strains and an H-2 identical nonautoimmune strain demonstrates the influences of both non-MHC and non-lpr genes on mercury-accelerated systemic autoimmunity in mice predisposed to spontaneous disease. Furthermore, the data demonstrate how an environmental agent, mercury, can act synergistically on individuals with intermediate susceptibility (NZBWF1 and MRL+/+) to accelerate disease, whereas there was less effect on MRL/lpr mice with the highest susceptibility. This is consistent with the threshold liability model (9) and the potential importance of environmental factors in influencing disease induction, particularly in individuals with intermediate susceptibility.
Mercury, in various forms, elicits systemic autoimmunity in a number of nonautoimmune murine strains (10). Susceptibility to HgIA is genetically controlled, as this xenobiotic is able to elicit differing degrees of autoimmunity depending upon the genetic background of the host (10,27). The immunologic features of HgIA, which include lymphadenopathy, hypergammaglobulinemia, autoantibodies, and immune complex disease, resemble features of SLE. Even so, there are significant differences between murine models of HgIA and idiopathic SLE. HgIA is transient, with disease features resolving with time once exposure to mercury ceases (27). In idiopathic SLE, disease is progressive, often leading to death (8,9). At the height of disease HgIA is associated with increased IL-4 and IgG1 autoantibodies (13). In contrast, murine models of idiopathic SLE have autoantibodies of IgG2a and IgG2b isotypes (28) and elevations in IL-1 and IFN- (9).
Several mechanisms have been postulated to account for mercury's autoimmune-promoting effects, including generation of cryptic T-cell determinants (37) or production of T cells reacting against self-class II (38). The finding that mercury did not accelerate humoral responses in the presence of the lpr gene raises the possibility that mercury, like the lpr defect, may act through inhibition of apoptosis. This would be consistent with the polyclonal lymphoid hyperplasia and immunoglobulin increases found in the MRL+/+ after mercury treatment. Mercury could possibly prevent apoptosis by interacting with the sulfhydryl moiety of cysteine in the active site of one or more caspases (39) that are involved in the signaling and effector pathways responsible for induction of apoptotic cell death.
Comparison between the MRL and AKR strains revealed the importance of studies with genetically related nonautoimmune strains in dissecting the contribution of xenobiotic exposure to acceleration of idiopathic disease. Additional studies contrasting the genetically related autoimmune-prone BXSB (H-2b) and nonautoimmune C57BL/6 (H-2b) have also shown that mercury exposure exacerbates idiopathic autoimmunity (40). The significance of such studies cannot be overstated. Human studies have shown that xenobiotic-induced disease, although exhibiting features of the idiopathic syndrome, may lack key pathologic sequlea and have clinical features not associated with idiopathic disease (2,41). Studies that seek to link xenobiotics with autoimmunity must therefore attempt to discriminate between xenobiotic-induced autoimmunity and xenobiotic-accelerated idiopathic disease. Comparative studies using genetically related autoimmune-prone and nonautoimmune mice might help determine the contribution that xenobiotic exposure makes in rendering sensitive populations susceptible to autoimmunity.
REFERENCES AND NOTES
1. Rubin RL. Drug-induced lupus. In: Dubois' Lupus Erythematosus, 5th ed (Wallace DJ, Hahn BH, eds). Baltimore, MD:Williams and Wilkins, 1997;871-901.
2. Silver RM. Scleroderma and pseudoscleroderma. In: Systemic Sclerosis (Clements PJ, Furst DE, eds). Baltimore, MD:Williams and Wilkins, 1996;81-98.
3. Kono DH, Theofilopoulos AN. Genetic contributions to SLE. J Autoimmunity 9:437-452 (1996).
4. Hang L, Slack JH, Amundson C, Izui S, Theofilopoulos AN, Dixon FJ. Induction of murine autoimmune disease by chronic polyclonal B cell activation. J Exp Med 157:874-883 (1983).
5. Ansel JC, Mountz J, Steinberg AD, DeFabo E, Green I. Effects of UV radiation on autoimmune strains of mice: increased mortality and accelerated autoimmunity in BXSB male mice. J Invest Dermatol 85:181-186 (1985).
6. Lewis RE, Cruse JM, Johnson WW, Mohammad A. Histopathology and cell-mediated immune reactivity in halothane-associated lymphomagenesis and autoimmunity in BXSB/Mp and MRL/Mp mice. Exp Mol Pathol 36:378-395 (1982).
7. Carpenter DF, Steinberg AD, Schur PH, Talal N. The pathogenesis of autoimmunity in New Zealand mice. II: Acceleration of glomerulonephritis by polyinosinic.polycytidylic acid. Lab Invest 23:628-634 (1970).
8. Theofilopoulos AN. Murine models of lupus. In: Systemic Lupus Erythematosus. 2nd ed (Lahita RG, ed). New York:Churchill Livingstone, 1992;121-194.
9. Theofilopoulos AN, Kono DH. Murine Lupus Models: Gene-specific and genome-wide studies. In: Systemic Lupus Erythematosus, 3rd ed (Lahita RG, ed). New York:Churchill Livingstone, 1999;149-181.
10. Pollard KM, Hultman P. Effects of mercury on the immune system. In: Metal Ions in Biological Systems. Vol 34: Mercury and Its Effects on Environment and Biology (Sigel H, Sigel A, eds). Basel:Marcel Dekker, 1997;421-440.
11. Bigazzi PE. Animal models of autoimmunity: spontaneous and induced. In: The Autoimmune Diseases (Rose NR, Mackay IR, eds). New York:Academic Press, 1998;211-244.
12. Barr RD. The mercurial nephrotic syndrome. E Afr Med J 67:381-386 (1990)
13. Ochel M, Vohr HW, Pfeiffer C, Gleichmann E. IL-4 is required for the IgE and IgG1 increase and IgG1 autoantibody formation in mice treated with mercuric chloride. J Immunol 146:3006-3011 (1991).
14. Kono, DH, Balomenos D, Pearson DL, Park MS, Hildebrandt B, Hultman P, Pollard KM. The prototypic autoimmunity induced by mercury is dependent on IFN- and not Th1/Th2 imbalance. J Immunol 161:234-240 (1998).
15. Tian L, Lawrence DA. Metal-induced modulation of nitric oxide production in vitro by murine macrophages: lead, nickel, and cobalt utilize different mechanisms. Toxicol Appl Pharmacol 141:540-547 (1996).
16. Goebeler M, Roth J, Bröcker E-B, Sorg C, Schultze-Osthoff K. Activation of nuclear factor-B and gene expression in human endothelial cells by the common haptens nickel and cobalt. J Immunol 155:2459-2467 (1995).
17. Bernstein DI. Allergic reactions to workplace allergens. JAMA 278:1907-1913 (1997).
18. Van Hoogstraten IM, Boos C, Boden D, Von Blomberg ME, Scleper RJ, Kraal G. Oral induction of tolerance to nickel sensitization in mice. J Invest Dermatol 101:26-31 (1993).
19. Kapsenberg ML, Wieerenga EA, Bos JD, Jansen HM. Functional subsets of allergen-reactive human CD4+ T cells. Immunol Today 12:392-395 (1991).
20. National Research Council. Guide for the Care and Use of Laboratory Animals. Washington, DC:National Academy Press, 1996.
21. Kono D, Burlingame R, Owens DG, Kuramochi A, Balderas RS, Balomenos, D, Theofilopoulos AN. Lupus susceptibility loci in New Zealand mice. Proc Natl Acad Sci USA 91:10168-10172 (1994).
22. Takeuchi K, Turley SJ, Tan EM, Pollard KM. Analysis of the autoantibody response to fibrillarin in human disease and murine models of autoimmunity J. Immunol 154:691-971 (1995).
23. Reimer G, Pollard KM, Penning CA, Ochs RL, Lischwe MA, Busch H, Tan EM. Monoclonal autoantibody from a (New Zealand black New Zealand white)F1 mouse and some human scleroderma sera target an Mr 34,000 nucleolar protein of the U3 RNP particle. Arthritis Rheum 30:793-800 (1987).
24. Burlingame RW, Rubin RL. Subnucleosome structures as substrates in enzyme-linked immunosorbent assays. J Immunol Meth 143:187-199 (1990).
25. Kotzin BL, Lafferty JA, Portanova JP, Rubin RL, Tan EM. Monoclonal anti-histone autoantibodies derived from murine models of lupus. J Immunol 133:2554-2559 (1984).
26. Bell S, Hobbs M, Rubin R. Isotype-restricted hyperimmunity in a murine model of the toxic oil syndrome. J Immunol 148:3369-3376 (1992).
27. Hultman P, Turley SJ, Eneström S, Lindh U, Pollard KM. Murine genotype influences the specificity, magnitude and persistence of murine mercury-induced autoimmunity. J Autoimmunity 9:139-149 (1996).
28. Burlingame RW, Rubin RL, Balderas RS, Theofiopoulos AN. Genesis and evolution of antichromatin autoantibodies in murine lupus implicates T-dependent immunization with self antigen. J Clin Invest 91:1687-1695 (1993).
29. Burlingame RW, Boey ML, Starkebaum G, Rubin RL. The central role of chromatin in autoimmune responses to histones and DNA in systemic lupus erythematosus. J Clin Invest 94:184-192 (1994).
30. Rubin RL, Bell SA, Burlingame RW. Autoantibodies associated with lupus induced by diverse drugs target a similar epitope in the (H2A-H2B)-DNA complex. J Clin Invest 90(1):165-173 (1992).
31. Hultman P, Bell LJ, Eneström S, Pollard KM. Murine susceptibility to mercury. 1: Autoantibody profiles and systemic immune deposits in inbred, congenic, and intra-H-2 recombinant strains. Clin Immunol Immunopathol 65:98-108 (1992).
32. Andrews BS, Eisenberg RA, Theofilopoulos AN, Izui S, Wilson CB, McConahey PJ, Murphy ED, Roths JB, Dixon FJ. Spontaneous murine lupus-like syndromes: clinical and immunopathological manifestations in several strains. J Exp Med 148:1198-1215 (1987).
33. Rodgers KE. Effects of oral administration of malathion on the course of disease in MRL-lpr mice. J Autoimmunity 10:367-373 (1997).
34. Kahn MF, Kaphalia BS, Ansari GA. Time-dependent autoimmune response of dichloroacetyl chloride in female MRl+/+ mice. Immunopharmacol Immunotoxicol 19:265-277 (1997).
35. Theofilopoulos AN, Dixon FJ. Experimental murine systemic lupus erythematosus. In: Systemic Lupus Erythematosus (Lahita RG, ed). New York:John Wiley and Sons, 1987;121-202.
36. Hultman P, Bell LJ, Eneström S, Pollard KM. Murine susceptibility to mercury. 2: Autoantibody profiles and renal immune deposits in hybrid, backcross, and H-2d congenic mice. Clin Immunol Immunopathol 68:9-20 (1993).
37. Griem P, Wulferink M, Sachs B, González JB, Gleichmann E. Allergic and autoimmune reactions to xenobiotics: how do they arise? Immunol Today 19:133-141 (1998).
38. Druet P, Pelletier L. Th2 and Th1 autoreactive anti-class II cell lines in the rat suppress or induce autoimmunity. J Autoimmunity 9:221-226 (1996).
39. Alnemri ES, Livingston DJ, Nicholson DW, Salvesen G, Thornberry NA, Wong WW, Yuan J. Human ICE/CED-3 protease nomenclature [Letter]. Cell 87:171 (1996).
40. Pollard KM, Pearson DL, Hultman P, Hildebrandt B, Kono DH. Unpublished data.
41. Fritzler MJ, Rubin RL. Drug-induced lupus. In: Dubois' Lupus Erythematosus. 4th ed (Wallace DJ, Hahn BH, eds). London:Lea and Febiger, 1993;442-453.
Last Updated: September 21, 1999
