Detection and localization of 3,3',4,4'-tetrachlorobiphenyl-induced P4501A protein in avian primary immune tissues.

P4501A can be detected in thymic and bursal microsomes from chickens pretreated with 3,3',4,4'-tetrachlorobiphenyl (TCB) using a polyclonal antibody against purified P4501A from 3-methylcholanthrene (3-MC)-induced chicken embryo liver. A dose-response for induction by TCB of P4501A protein was detected by Western blotting in both bursal and thymic microsomes. Ethoxyresorufin-O-deethylase (EROD), a specific catalytic activity of P4501A, was also induced in a dose-response fashion. More TCB-induced P4501A was detected in thymus than bursa by both methods. No EROD was detected in bursal or thymic microsomes from untreated chickens, although P4501A protein was detected at very low levels in thymic microsomes from untreated chickens. P4501A was detected by immunohistochemistry in scattered patches of non-lymphocytic cells residing in medullary regions of the TCB-induced thymus but was not detected in lymphocytes. This result supports previous work demonstrating that TCB-inducible EROD is much higher in the supporting tissue cell fractions than in lymphocyte fractions of the primary immune tissues. Although EROD was induced by TCB in the late stage embryo after 20 h exposure, no effect of TCB on the cell cycle in thymic or bursal lymphocytes was observed over the same period. The same TCB exposure resulted in bursal but not thymic cellular depletion. Thymic and bursal supporting tissue cells may be primary sites of immunosuppression within these organs by P4501A inducers or substrates whether immunosuppression occurs subsequent to metabolism or through interaction with Ah receptors.