Oral, perioral and systemic pathosis in HgCl2-induced autoimmunity in the BN rat

Lupus-like oral mucosal lesions in mercury-induced autoimmune response in Brown Norway rats

Background

Administration of mercury at nontoxic doses induces systemic autoimmune disease in Brown Norway (BN) rats. The pathogenesis of lupus-like oral mucosal lesion by mercury-induced autoimmunity is still unclear, even though the oral mucosa is observed to be commonly affected in mercury-treated BN rats. In this study, we investigated the immunopathology of lupus-like oral mucosal lesions in a model of mercury-induced systemic autoimmunity.

Methods

Brown Norway male rats were injected subcutaneously with either phosphate-buffered saline (control) or mercury at a dose of 1.0 mg per kilogram of body weight on days 0, 3, 5, and 7. Blood, kidney, and tongue samples were taken at various timepoints for evaluation by immunohistochemistry, RT-PCR, and lupus band test (LBT).

Results

Oral mucosal lesions were classified according to three consecutive temporal phases on the basis of infiltration of immunocompetent cells as follows: (phase I) infiltration of MHC class II⁺ dendritic cells (DC) and macrophages; (phase II) addition of ED1⁺ macrophage infiltrates; and (phase III) focal infiltration of pan T cells following increased infiltration of DC and macrophages. Dense infiltration of DC and macrophages was observed in the basement membrane (BM) zone of the oral epithelium. Tissue expression of IL-4 mRNA was detected in early lesions (phase I), suggesting that locally produced IL-4 may be responsible for Th2-mediated immune response. A linear and continuous smooth pattern of fluorescence was observed in the oral epithelial BM in addition to renal glomeruli, indicating immune complex deposits.

Conclusions

Local autoimmune responses are involved in the pathogenesis of mercury-induced lupus-like lesions of the oral mucosa.

Lack of graft-versus-host-like pathology in mercury-induced autoimmunity of Brown Norway rats

The repeated administration of mercury to Brown Norway (BN) rats induces the production of autoantibodies to laminin 1 and other autoantigens, accompanied by renal deposition of immunoglobulins and a membranous glomerulonephropathy. A graft-versus-host-like (GVHL) syndrome, characterized by widespread necrotizing leukocytoclastic vasculitis of the bowel, skin, and other tissues, has also been observed after mercury treatment of BN rats. These findings have suggested that the autoimmunity caused by the administration of mercury to BN rats may result as a xenobiotic-induced GVHL effect under the control of OX22+ T lymphocytes. However, previous studies of mercury-induced autoimmunity have never reported any evidence of GVHL lesions. Therefore, we have carefully examined various tissues from a large group of BN rats injected with HgCl₂ to identify possible areas of inflammatory reactions that may have been unnoticed in previous investigations. In addition, we have determined by flow cytometry whether exposure to mercury results in percentage and numerical alterations of OX22+ or other lymphocyte subpopulations in lymphoid organs of HgCl₂-treated BN rats. The present article confirms that mercury induces autoimmune responses to laminin 1 but does not corroborate the hypothesis of a GVHL syndrome regulated by OX22+ lymphocytes. First, changes in OX22+ cells during treatment with HgCl₂ were infrequent and had no significant correlation with the kinetics of autoimmune responses to laminin 1. Second, we detected no GVHL lesions in skin and intestine of mercury-treated BN rats.