Nitric Oxide Suppresses IFN-γ Production in the Spleen of Mercuric Chloride-Exposed Brown Norway Rats

Recently we have shown that the generation of IFN-γ-producing cells (IFN-γ pc) were severely suppressed in cultures of Con A-stimulated splenocytes obtained from HgCl2-exposed BN but not Lewis rats. Since BN rats develop a TH2-biased polyclonal autoimmune syndrome upon exposure to HgCl2, whereas Lewis rats exhibit a resistant phenotype, possibly by generating a protective TH1 response, downregulation of IFN-γ production may be a critical denominator in the disease process. In the present study it was found that this suppression of IFN-γ production was almost completely antagonized by NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of nitric oxide (NO) synthesis, and potentiated by the depletion of splenic erythrocytes, a cell type known to effectively scavenge NO. Determination of nitrite (NO-2), the stable metabolite of NO, revealed that splenocytes obtained from HgCl2-exposed BN rats produced two- to threefold higher levels of NO-2 than those from HgCl2-exposed Lewis rats and approximately two times more than saline-injected controls. Upon depletion of erythrocytes, splenocytes from normal BN and Lewis rats produced similar amounts of nitrite which was enhanced approximately twofold after in vivo exposure to HgCl2 up to 5 days after initiation of HgCl2 exposure. At Day 7, NO release by Lewis splenocytes returned to baseline levels, whereas NO release by BN splenocytes remained high up to the effector phase of the autoimmune disease. Flow cytometric analysis and spleen cell counts revealed marked differences in the splenic subset composition between Lewis and BN rats with an anomalous low frequency of erythrocytes and CD8+ T cells and a strikingly high number of B cells in the BN spleen. Our findings are in confirmation with a model in which HgCl2 triggers the upregulation of NO synthesis in the spleen. The enhanced production of NO is insufficiently scavenged by erythrocytes in the BN spleen resulting in a suppressive effect on the production of IFN-γ. The sustained inhibitory effect of NO on IFN-γ production may promote the development of a TH2-biased autoimmune syndrome in BN rats.