Effects of the TCDD congeners 3,3',4,4'-tetrachlorobiphenyl and 3,3',4,4'-tetrachloroazoxybenzene on lymphoid development in the bursa of Fabricius of the chick embryo.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and its congeners, such as 3,3',4,4'-tetrachlorobiphenyl (TCB) and 3,3',4,4'-tetrachloroazoxybenzene (TCAOB), act on targets in the immune system, probably by interacting with the Ah-receptor, causing a characteristic pattern of effects typified by inhibition of lymphoid development in the thymus. There are, however, also reports of effects on B cells. Birds have a well-defined site of B-lymphocyte development, the bursa of Fabricius, analogous to the thymus for T cells. Thus, we wanted to determine whether this organ is suitable for studying effects on B-cell development. TCB and TCAOB were administered by injection into the air sacs of White Leghorn eggs on Day 13 of incubation. Effects observed on Day 19 were as follows: (1) There was a reduction in bursal dry weight but not in body weight in the treated groups. (2) The number of lymphoid cells in the bursae decreased in a dose-dependent manner, and bursae from high-dose groups (300 and 30 micrograms/kg egg of TCB and TCAOB, respectively) were almost completely devoid of lymphoid cells. For the reduction of lymphoid cell number, the ED50 for TCB and TCAOB was approximately 45 and 1.4 micrograms/kg egg, respectively. (3) Histological sections showed that embryos from treated egges contained fewer bursal follicles and that follicles contained fewer lymphoid cells compared with controls. (4) Aryl hydrocarbon hydroxylase activity was 30 and 50 times that of the control at high doses of TCB and TCAOB (estimated ED50: 200-300 and 4 micrograms/kg egg, respectively). These findings suggest that lymphoid development in the bursa of Fabricius of the chick embryo is inhibited by TCDD congeners.