Tim-3 and Cell Death in Murine Mercury Induced Autoimmunity

Abstract

There is a role for environmental factors in the pathogenesis of autoimmune diseases in humans and animals. Correlations have been made between mercury (Hg) exposure and the prevalence human autoimmune diseases. The rodent model of Hg-induced autoimmunity is useful for the analysis of systemic autoimmunity that is both environmentally and genetically modulated. In susceptible mice, Hg treatment induces both polyclonal activation of B cells, as well as the production of highly specific autoantibodies targeting the nucleolar protein fibrillarin. Tim proteins are a family of immune cell surface molecules that have been implicated in modulation of autoimmune diseases in both humans and animals. Tim-3, originally reported as a Th1 specific molecule, is expressed on T cells of various subsets, as well as antigen presenting cells and granulocytes. Tim-3 and its known ligand galectin-9 play a role in the down-regulation of immune responses, including experimental autoimmune encephalomyelitis and autoimmune conjunctivitis. Tim-3-Ig fusion protein, as well as blocking and stimulating anti-Tim-3 antibodies, were employed in vivo to modulate the Tim-3-galectin-9 pathway during Hg-induced autoimmunity. Results have shown an increase in Th1 manifestations and decrease in Th2 manifestations when the pathway is inhibited by Tim-3-Ig. Blockade of the pathway using a Tim-3 directed antibody resulted in an increase of all disease manifestations, including Th2-driven IgE production; a stimulating antibody had the opposite effect on IgG and no effect on IgE. In addition to studying the regulation of Hg-induced autoimmunity, a goal of immunologists is to understand the genesis of this disease model. Apoptosis, although anti-inflammatory, has been proposed as a source of autoantigen in autoimmune diseases. Interest in events occurring at the site of Hg injection has prompted us to perform in vitro studies of Hg-induced cell death. Treatment of murine lung epithelial cells or Jurkat T cells with doses of Hg similar to those present at the injection site results in a death process that is morphologically and biochemically distinct from apoptosis. We have observed peripheral movement of the nucleolar protein fibrillarin via confocal microscopic study of Hg-induced cell death. As cells die, there is extrusion of fibrillarin from the nucleus and exposure at the cell surface. Such cell death may allow recognition of fibrillarin by the immune system thereby promoting the formation of anti-nucleolar antibodies.