Distribution and behavior of the Ah receptor in murine T lymphocytes

Exposure of C57Bl/6 mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes suppression of both cell-mediated and humoral immunity. Studies with mice congenic at the Ah locus have demonstrated that this suppression is mediated by the aryl hydrocarbon receptor (AhR). The Ah receptor is a ligand-dependent transcription factor that has been well-studied in hepatocytes; however, the mechanism of TCDD-mediated immunotoxicity remains unknown. Lack of mechanistic understanding is due, in part, to difficulty demonstrating a direct effect of TCDD on immune function in vitro. In this study, we have investigated the behavior of the murine AhR in T cells using isolated spleen T lymphocytes and T cell clones (10.5.17, F4, and F1.A.2) derived from Ah-responsive mouse strains. The presence of the AhR in whole cell extracts of resting and activated splenic lymphocytes and T cell clones was examined by Western blotting. Increased 7-ethoxyresorufin-o-deethylase (EROD) activity was observed in T cell clones and spleen cells; however, the level of EROD induction by TCDD was approximately 100-fold less than the level induced in hepa cells. The behavior of the murine T cell AhR was examined by assessing TCDD-induced nuclear translocation and DNA binding. The intracellular distribution of the AhR was studied by subcellular fractionation of both resting and activated T cells in the presence and absence of TCDD. DNA binding was measured using an electrophoretic mobility shift assay. The AhR was detected in all cell types examined, but the AhR translocated to the nucleus only in activated, TCDD-treated T cells. While AhR derived from TCDD-treated wild-type hepa cells bound specifically to a DRE, no binding was detected when an identical amount of AhR obtained from activated T cells was used. Our inability to detect binding of the T cell nuclear AhR complex to a consensus response element, combined with the observation that it is difficult to reproduce the in vivo immunotoxic effects of TCDD in vitro, suggests that T cells may lack a factor(s) required for AhR binding to a DRE, or may contain a suppressor factor which inhibits AhR binding to DNA. Based on these data, TCDD appears to affect T cell function via an indirect mechanism.