NITRIC-OXIDE SUPPRESSES IFN-GAMMA PRODUCTION IN THE SPLEEN OF MERCURIC CHLORIDE-EXPOSED BROWN-NORWAY RATS

Recently we have shown that the generation of IFN-gamma-producing cell s (IFN-gamma pc) were severely suppressed in cultures of Con A-stimula ted splenocytes obtained from HgCl2-exposed BN but not Lewis rats, Sin ce BN rats develop a T(H)2-biased polyclonal autoimmune syndrome upon exposure to HgCl2, whereas Lewis rats exhibit a resistant phenotype, p ossibly by generating a protective T(H)1 response, downregulation of I FN-gamma production may be a critical denominator in the disease proce ss, In the present study it was found that this suppression of IFN-gam ma, production was almost completely antagonized by N-G-monomethyl-L-a rginine (NMMA), a competitive inhibitor of nitric oxide (NO) synthesis , and potentiated by the depletion of splenic erythrocytes, a cell typ e known to effectively scavenge NO, Determination of nitrite (NO2-), t he stable metabolite of NO, revealed that splenocytes obtained from Hg Cl2-exposed BN rats produced two- to threefold higher levels of NO2- t han those from HgCl2-exposed Lewis rats and approximately two times mo re than saline-injected controls. Upon depletion of erythrocytes, sple nocytes from normal BN and Lewis rats produced similar amounts of nitr ite which was enhanced approximately twofold after in vivo exposure to HgCl2 up to 5 days after initiation of HgCl2 exposure, At Day 7, NO r elease by Lewis splenocytes returned to baseline levels, whereas NO re lease by BN splenocytes remained high up to the effector phase of the autoimmune disease, Flow cytometric analysis and spleen cell counts re vealed marked differences in the splenic subset composition between Le wis and BN rats with an anomalous low frequency of erythrocytes and CD 8(+) 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 producti on of NO is insufficiently scavenged by erythrocytes in the BN spleen resulting in a suppressive effect on the production of IFN-gamma. The sustained inhibitory effect of NO on IFN-gamma production may promote the development of a T(H)2-biased autoimmune syndrome in BN rats. (C) 1995 Academic Press, Inc.