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; PCB 126) in chicken embryos. Previous investigations on the immunotoxic effects of PCBs and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in developing avian embryos were undertaken with embryos exposed only during the latter stages of incubation. To simulate exposure in embryos in the wild, chicken eggs were injected with PCB 126 (sunflower oil carrier) into the air cell before initiation of incubation. It was hypothesized that exposure to PCB 126 during 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. Control groups consisted of carrier-injected and noninjected eggs. The thymus 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 cells were counted using the trypan blue exclusion method. Probit analysis estimated 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 the thymus fell sharply between 0.051 and 0.13 ng/g. Bursa mass began to decrease at the lowest dose of 0.051 ng/g and reached a minimum at 0.32 ng/g. The number of viable cells decreased slightly at 0.051 ng/g and reached a minimum at the 0.13- and 0.32-ng/g doses. In general, lymphoid cell numbers were 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 viable lymphoid cells in the thymus and bursa were at least one order of magnitude lower with full-term incubation as compared to exposure only during later stages of incubation.