Effects of PCB 126 on thymocyte surface marker expression and immune organ development in chicken embryos

Previous studies have shown that chicken (Gallus domesticus) embryos are sensitive to the immunotoxic effects of Ah receptor agonists. These chemicals cause atrophy of the thymus gland and bursa of Fabricius, the sites of T- and B-lymphocyte maturation, respectively. The objectives of this study were (1) to evaluate the effects of 3,3,4,4',5-pentachlorobiphenyl (PCB 126) on thymocyte phenotypes (CD4-CD8-, CD4+CD8+, CD4+CD8-, CD4-CD8+, TCRalphabeta+, and TCRgammadelta) in chicken embryos, and (2) to compare phenotype alterations with masses and cellularity of lymphoid organs. To simulate exposure in wild avian embryos, chicken eggs were injected with PCB 126 (sunflower oil carrier) into the air cell before incubation. Doses ranged from 0.051 to 0.8 ng/g egg with carrier-injected and noninjected control groups. The thymus and bursa were removed, weighed, and homogenized on d 20 of egg incubation (1 d before hatch). Thymocyte phenotypes were quantified by flow cytometry using monoclonal antibodies for CD4, CD8, TCRalphabeta (Vbeta1), and TCRgammadelta. Right thymus mass declined with dose, decreasing significantly between 0.32 and 0.8 ng/g to a size 28% lower than controls. Live lymphoid cell numbers in the right thymus dropped markedly (21% lower than controls) between 0.051 and 0.13 ng/g, with a further decrease (35% lower than controls) at higher doses. There was no significant change in the percentage of thymocytes expressing TCRalphabeta. The total number of TCRalphabeta+ thymocytes decreased with dose as a function of the declines in TCRalphabeta+ percentages and total thymocyte numbers. The percentages of all other measured phenotypes were unaltered by PCB 126. The total number of CD4+CD8+ cells, and to a lesser degree CD4-CD8+ cells, decreased in a dose-dependent manner following the pattern of total live thymocytes. The number of viable lymphoid cells in the bursa decreased to 45% lower than controls at 0.13 ng/g and fell to 76% lower than controls at 0.8 ng/g. Lymphoid atrophy occurred at doses that were 8- to 12-fold lower with full-term incubation as compared to exposure only during later stages of incubation, and the lymphoid atrophy was associated with decreased TCRalphabeta+ thymocytes at higher doses. These immunological effects were observed at concentrations of PCB 126 comparable to those found in Great Lakes herring gull eggs, after correcting for interspecies differences in sensitivity to PCB 126.