The dose-response effects of 2,2′,3,3′,4,5,5′,6,6′-,2,2′,3,3′,4,4′,5,6,6′-and 2,2′,3,3′,4,4′,5,5′,6-nonachlorodiphenyl ether (non-aCDE) and decachlorodiphenyl ether (decaCDE) on the splenic plaque-forming cell (PFC) response to sheep red blood cells (SRBCs) and the induction of hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity was determined in aryl hydrocarbon (Ah)-responsive C57BL/6 and less Ah-responsive DBA/2 mice. All the congeners exhibited immunotoxicity at doses between 2.5 and 10 μmol/kg in C57BL/6 mice whereas in DBA/2 mice doses≥25 μmol/kg were required to cause inhibition of the PFC response to SRBCs. The results also showed that the nonaCDE isomers and decaCDE were more active as inducers of hepatic EROD activity in C57BL/6 than DBA/2 mice; however, there was not a correlation between the induced EROD activity and the CYP1A1 and CYP1A2 mRNA levels in the C57BL/6 mice. These data suggested that the immunotoxicity of these compounds was mediated through the Ah receptor. However, the results showed that the immunotoxicity of the nonaCDE isomers and decaCDE was unexpectedly high compared to that of lower chlorinated diphenyl ethers and there were no apparent structure-activity relationships among the higher chlorinated congeners. This suggests that some of the immunosuppressive effects observed for the nonaCDE isomers and decaCDE may be Ah receptor-independent.
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