Effects of PCB 126 on primary immune organ development in chicken embryos

This experiment evaluated the immunotoxic effects of developmental exposure to a planar polychlorinated biphenyl (3, 3', 4,4',5-pentachlorobiphenyl; P CB 126) in chicken embryos. Previous investigations on the immunotoxic effe cts of PCBs and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in developing av ian embryos were undertaken with embryos exposed only during the latter sta ger of incubation. To simulate exposure in embryos in the wild, chicken egg s were injected with PCB 126 (sunflower oil carrier) into the air cell befo re initiation of incubation. It was hypothesized that exposure to PCB 126 d uring the complete incubation period would decrease immune organ masses and lymphocyte numbers. Doses of PCB 126 ranged from 0.051 to 0.80 ng/g egg. C ontrol groups consisted of carrier-injected and noninjected eggs. The thymu s and bursa of Fabricius were removed and weighed on d 20 of incubation (1 d before hatch). The immune organs were homogenized, and viable lymphoid ce lls were counted using the trypan blue exclusion method. Probit analysis es timated the LD20 to be 0.21 ng/g and the LD50 to be 1.01 ng/g. Thymus mass dropped sharply between 0.13 and 0.32 ng/g, and lymphoid cell numbers in th e thymus fell sharply between 0.051 and 0.13 ng/g. Bursa mass began to decr ease at the lowest dose of 0.051 ng/g and reached a minimum at 0.32 ng/g. T he number of viable cells decreased slightly at 0.051 ng/g and reached a mi nimum at the 0.13- and 0.32-ng/g doses, in general, lymphoid cell numbers w ere more sensitive to PCB 126 than organ masses, and the bursa tended to be more sensitive than the thymus. Doses necessary to reduce the number of vi able lymphoid cells in the thymus and bursa were at least one order of magn itude lower with full-term incubation as compared to exposure only during l ater stages of incubation.