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.