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Autoimmunity Induced by Xenobiotics

K.M. Pollard, D.H. Kono,* D.K. Lee, B. Hildebrandt, P. Hultman,** E.M. Tan

* Department of Immunology, TSRI
** Linkoping University, Linkoping, Sweden

Our work focuses on murine systemic autoimmunity elicited by exposure to mercury and the similarities between this murine model of autoimmunity and systemic autoimmune disease in humans.

XENOBIOTIC-INDUCED ACCELERATION OF SYSTEMIC AUTOIMMUNITY

Systemic lupus erythematosus develops spontaneously in BXSB mice, and these animals express the H-2^b haplotype that confers resistance to mercury-induced autoimmune disease in C57BL/6 mice. These 2 strains were used to examine xenobiotic-induced acceleration of the appearance of immunologic features characteristic of the natural history of systemic autoimmunity. For 1 month, 4-week-old BXSB and C57BL/6 mice were given twice weekly injections of 40 µg of either HgCl₂ or NiCl₂ in phosphate-buffered saline (PBS) or PBS alone.

After the 1 month of treatment, BXSB and C57BL/6 mice given mercury but not those given nickel had elevations in total IgG, IgG1, and IgG2a and an increase in the prevalence of antinuclear antibodies. Although treatment with mercury increased the autoantibody response to chromatin in male BXSB mice, acceleration of the features of autoimmunity was not dramatic and was limited to male mice; no C57BL/6 mice had evidence of an autoimmune response. However, 8-week-old female BXSB mice treated with HgCl₂ or PBS for 1 month had significant increases in antibodies to chromatin (Fig. 1). In the mice treated with mercury, the antibodies were predominantly of the IgG2a and IgG2b isotypes (Fig. 2, left panel). With the exception of an IgG1 component in the response of 1 mouse, the amount of IgG1 or IgG3 antibody to chromatin was small. At 32 weeks after the start of treatment, some of the mice given PBS had antibodies to chromatin. The IgG subclasses of the antichromatin antibodies in both PBS- and HgCl₂-treated mice were similar, mainly IgG2b and IgG2a (Fig. 2, right panel). At the time of sacrifice, HgCl₂-treated female BXSB mice had increases in renal deposits of immune complexes and proteinuria.

This study suggests that transient exposure to HgCl₂ results in the accelerated appearance of IgG2a and IgG2b antibodies to chromatin and immune-complex disease that characterize the idiopathic response in BXSB mice. These findings suggest that HgCl₂ acts as a trigger to complement genetic susceptibility to systemic autoimmunity.

XENOBIOTIC-INDUCED CELL DEATH AND ALTERED PROTEOLYSIS OF A SELF-ANTIGEN

Administration of mercury to H-2^s mice results in a genetically restricted autoantibody response against the nucleolar protein fibrillarin. The mechanisms by which a xenobiotic such as mercury elicits an autoantibody response that predominantly targets a single intracellular protein autoantigen remain uncertain but may require access of mercury to the intracellular environment.

Mercury-induced cell death of the murine macrophage cell line J774A.1 was associated with loss of fibrillarin antigenicity and modification of the molecular properties of fibrillarin. The addition of radiolabeled fibrillarin to cell lysates from J774A.1 cells killed by exposure to HgCl₂ revealed that cell death was associated with protease activity capable of cleaving fibrillarin. Proteolysis of radiolabeled fibrillarin depended on the duration of incubation in cell lysate and on concentration of the cell lysate and was inhibited by the serine protease inhibitors aprotinin and 4-(2-aminoethyl)-benzene-sulfonyl fluoride hydrochloride. Unlike cell death induced by etoposide, ethanol, or heat, death induced by HgCl₂ resulted in the generation of proteolytic fragments that depended on the ability of fibrillarin to interact with mercury.

We propose that the binding of mercury to fibrillarin alters the cleavage of the protein-metal complex by means of proteolytic activity associated with cell death. This aberrant proteolysis may lead to the generation of "cryptic" epitopes that can be recognized by autoreactive T cells in murine strains genetically susceptible to mercury-induced autoimmunity, resulting in autoantibodies against fibrillarin.

PUBLICATIONS

Kono, D.H., Balomenos, D., Pearson, D.L., Park, M.S., Hildebrandt, B., Hultman, P., Pollard, K.M. The prototypic Th2 autoimmunity induced by mercury is dependent on IFN-/gammapub/ and not Th1/Th2 imbalance. J. Immunol., in press.