ACTIVATION OF MURINE LYMPHOCYTES BY THE HEAVY METAL MITOGENS, ZINC AND MERCURY DIVALENT CATIONS.

Abstract

Splenic and lymph node lymphocytes from Balb/C mice were activated in vitro by the heavy metal cations, Zn⁺⁺ and Hg⁺⁺, as noted by the several-fold increases in ³H-thymidine incorporation at 144 hours of culture. Optimal mitogenic concentrations of Zn⁺⁺ and Hg⁺⁺ were 200 μM and 10 μM, respectively. Data from experiments in which three different methods were used to enrich for either T or B splenic lymphocytes, i.e. cell passage over nylon wool columns, use of athymic Nu/Nu mouse spleen cells, or cell lysis with monoclonal anti-Thy-1 antibody plus complement, suggested that Zn⁺⁺ and Hg⁺⁺ were mitogens for T cells. Removal of macrophages from spleen cells by treatment with carbonyl iron followed by cell passage through nylon wool eliminated the lymphocyte responses to Zn⁺⁺ and to Hg⁺⁺. Moreover, addition of these macrophage-depleted lymphocytes to monolayers of resident peritoneal macrophages restored the lymphocyte responses to these mitogens. Both Zn⁺⁺ and Hg⁺⁺ activated splenic lymphocytes to display lectin-dependent cytotoxicity and to produce gamma interferon. Furthermore, Zn⁺⁺ induced low levels of natural killer activity in spleen cells. In contrast to spleen and lymph node cells, thymocytes and bone marrow lymphocytes did not respond to either cation under standard culture conditions. However, when cultured in the presence of E. coli-derived lipopolysaccharide (LPS) and 2-mercaptoethanol for 144 hours, thymocytes were activated by Zn⁺⁺ (200 μM) but not by Hg⁺⁺. Quantities of LPS as low as 1.0 ng/ml satisfied this culture requirement. Purified interleukin 1 could not replace the helping activity mediated by LPS. Thymocyte subpopulation studies showed that Zn⁺⁺ activated enriched peanut lectin receptor-negative mature thymocytes, but LPS was required for the response. Spleen cells from mice, intraperitoneally injected with ZnCl₂ for 7 to 14 days, were not activated in vivo as assessed by ³H-thymidine incorporation in vitro, nor did they display enhanced responses to T-cell or B-cell mitogens. However, zinc administration had negative effects by decreasing spleen cell numbers by 31% and thymic weight by 59%. A theoretical model is presented in which Zn⁺⁺ and Hg⁺⁺ may mediate their stimulating effects in vitro by altering histocompatibility "self" structures on the surface of lymphocytes and macrophages via interactions with sulfhydryl groups on these structures to which T lymphocytes with receptors for "altered self" structures respond with proliferation or cytotoxicity.