LACK OF DIRECT IMMUNOSUPPRESSIVE EFFECTS OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD) ON HUMAN PERIPHERAL-BLOOD LYMPHOCYTE SUBSETS IN-VITRO

A wide variety of immunosuppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in experimental animals has been documented. In contrast, the impact of dioxin on the human immune system remains controversial, although adverse health effects have been reported in humans after occupational or accidental exposure to dioxin. Recently, Neubert et al. (1991) found that a dose-dependent decrease of peripheral blood lymphocyte (PBL) subpopulations in humans and nonhuman primates, including helper-inducer/memory cells (CD4(+)CD29(+)) and B cells (CD20(+)) occurred in pokeweed mitogen (PWM) stimulated cultures at concentrations as low as 10(-2)-10(-14) M TCDD. Therefore, the direct effects of dioxin on human PBL subpopulations have been studied, in order to determine their usefulness as sensitive biomarkers for human dioxin exposure. Lymphocyte cultures from healthy individuals were treated with 10(-7) M-10(-14) M TCDD in the absence and presence of stimulation with pokeweed mitogen (PWM) or anti-CDS monoclonal antibody (moAb; OKT3) for 3 days. Cytochrome P450 (CYP1A1) enzyme induction, one of the best studied direct biological study, all stimulated lymphocyte cultures showed a dose-dependent significant increase of CYP1A1 activity at dioxin concentrations of 10(-7) and 10(-9) M. No enzyme activity could be detected at lower concentrations of TCDD. On the other hand, neither alteration in surface marker distribution nor suppression of lymphocyte proliferation could be demonstrated in mitogen-activated cells following any concentration of TCDD treatment. These data suggest that the inducibility of CYP1A1 enzyme activity is not correlated with direct immunotoxic effects in vitro in human PBL. In contrast to a previous report by Neubert et al. (1991), lymphoproliferation and phenotypes of human PBL are resistant to dioxin exposure in vitro and therefore appeared not to be useful as sensitive biomarkers in human exposure studies.effects of TCDD on numerous cell types, was assayed in parallel by ethoxyresorufin-O-deethylase (EROD) activity. Percentages of the different lymphocytes subsets, including CD2 (T cells); CD4; CD45 RA (suppressor-inducer/virgin T cells); CD4 CD29; CD8; CD19 (B cells) as well as interleukin 2 (IL-2) receptor (CD25) and class II antigen (HLA-DR) expression, were analyzed by flow cytometry. DNA synthesis was determined by H-3-thymidine uptake after 3 days of culture. In the present study, all stimulated lymphocyte cultures showed a dose-dependent significant increase of CYP1A1 activity at dioxin concentrations of 10(-7) and 10(-9) M. No enzyme activity could be detected at lower concentrations of TCDD. On the other hand, neither alteration in surface marker distribution nor suppression of lymphocyte proliferation could be demonstrated in mitogen-activated cells following any concentration of TCDD treatment. These data suggest that the inducibility of CYP1A1 enzyme activity is not correlated with direct immunotoxic effects in vitro in human PBL. In contrast to a previous report by Neubert et al. (1991), lymphoproliferation and phenotypes of human PBL are resistant to dioxin exposure in vitro and therefore appeared not to be useful as sensitive biomarkers in human exposure studies.