Heavy Metal Superantigen : Genetic Regulation of Lymphocyte Activation by HgC12

Sammanfattning: Injections of subtoxic doses of HgCl2 (mercuric chloride) cause a systemic autoimmune disease in genetically predisposed rats and mice. My work was designed to elucidate how HgCl2 induces lymphocyte activation and how the response to HgCl2 is regulated in susceptible and resistant strains.We showed that HgCl2 at a very low dose (10 mM) primarily activated murine T lymphocytes to proliferation in the presence of adherent cells in a primary in vitro culture, reaching a peak by day 5. Consistent with the in vivo findings, lymphocytes from different strains differed in their capability of responding to HgCl2. Anti-CD4 antibody completely inhibited the proliferation induced by HgCl2, which indicates that CD8+ T cells cannot be activated to division without the help from CD4+ T cells. Moreover, CD4+ T cells from susceptible A.SW and BALB/c mice, but not from resistant DBA/2 mice, were preferentially activated by HgCl2, as indicated by expression of the very early activation antigen CD69 and transformation to blast cells. Unlike CD4+ T cells, CD8+ T cells were activated by HgCl2 irrespective of the origin of the cells. Thus, helper T cells play a crucial role in the immunological effects caused by HgCl2 and may determine the ability of different strains to respond to HgCl2.We found that HgCl2 selectively activated T cells expressing certain TCR Vb elements. Depletion of Vb8+ T cells strongly suppressed HgCl2 -induced proliferation and transformation in spleen cells from BALB/c mice, indicating a specific correlation between Vb usage and the response to HgCl2. It seems unlikely that a specific TCR repertoire predisposes to the development of autoimmunity caused by HgCl2. We also verified that the unresponsiveness of CD4+ T cells from DBA/2 mice to HgCl2 was not due to immunosuppression mediated by CD8+ T cells.We further investigated the production of IL-2, IL-4, IFN-g, and IL-10 after activation by HgCl2, but found no evidence for the suggestion that HgCl2 induces a Th1/Th2 imbalance in resistant/ susceptible strains. A high frequency of IL-2-secreting cells was observed in spleen cells from the high responder A.SW mice, followed by cells from the intermediate responder BALB/c mice, whereas the frequency was low in cells from the non-responder DBA/2 mice. We showed that neutralizing anti-IL-2 antibody profoundly inhibited the proliferative response to HgCl2. Supplement of exogenous IL-2 enabled spleen cells from DBA/2 mice to respond to HgCl2 to a level comparable with cells from BALB/c mice. These findings indicate that lymphocytes from resistant mice have the potential of responding to HgCl2, but the response is limited due to an insufficient source of IL-2.HgCl2 activated B cells from susceptible mice, but not from resistant mice, to produce IgM antibody in a primary in vitro culture.We hypothesize that mercuric ions bind to certain molecules and transform them to super-antigens which in turn polyclonally activate T cells in a Vb-specific manner. This is followed by a polyclonal activation of B cells, resulting in autoantibody production and pathological manifestations in susceptible strains. IL-2 may be a limiting factor that precludes lymphocytes from resistant strains from responding to HgCl2.