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-tetrachlorodibe nzo-p-dioxin (TCDD) in experimental animals has been documented. In co ntrast, the impact of dioxin on the human immune system remains contro versial, 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 bloo d lymphocyte (PBL) subpopulations in humans and nonhuman primates, inc luding helper-inducer/memory cells (CD4(+)CD29(+)) and B cells (CD20()) occurred in pokeweed mitogen (PWM) stimulated cultures at concentra tions as low as 10(-2)-10(-14) M TCDD. Therefore, the direct effects o f dioxin on human PBL subpopulations have been studied, in order to de termine their usefulness as sensitive biomarkers for human dioxin expo sure. Lymphocyte cultures from healthy individuals were treated with 1 0(-7) M-10(-14) M TCDD in the absence and presence of stimulation with pokeweed mitogen (PWM) or anti-CDS monoclonal antibody (moAb; OKT3) f or 3 days. Cytochrome P450 (CYP1A1) enzyme induction, one of the best studied direct biological study, all stimulated lymphocyte cultures sh owed a dose-dependent significant increase of CYP1A1 activity at dioxi n concentrations of 10(-7) and 10(-9) M. No enzyme activity could be d etected at lower concentrations of TCDD. On the other hand, neither al teration in surface marker distribution nor suppression of lymphocyte proliferation could be demonstrated in mitogen-activated cells followi ng any concentration of TCDD treatment. These data suggest that the in ducibility of CYP1A1 enzyme activity is not correlated with direct imm unotoxic effects in vitro in human PBL. In contrast to a previous repo rt by Neubert et al. (1991), lymphoproliferation and phenotypes of hum an PBL are resistant to dioxin exposure in vitro and therefore appeare d not to be useful as sensitive biomarkers in human exposure studies.e ffects of TCDD on numerous cell types, was assayed in parallel by etho xyresorufin-O-deethylase (EROD) activity. Percentages of the different lymphocytes subsets, including CD2 (T cells); CD4; CD45 RA (suppresso r-inducer/virgin T cells); CD4 CD29; CD8; CD19 (B cells) as well as in terleukin 2 (IL-2) receptor (CD25) and class II antigen (HLA-DR) expre ssion, were analyzed by flow cytometry. DNA synthesis was determined b y 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 o f TCDD. On the other hand, neither alteration in surface marker distri bution nor suppression of lymphocyte proliferation could be demonstrat ed in mitogen-activated cells following any concentration of TCDD trea tment. These data suggest that the inducibility of CYP1A1 enzyme activ ity is not correlated with direct immunotoxic effects in vitro in huma n PBL. In contrast to a previous report by Neubert et al. (1991), lymp hoproliferation and phenotypes of human PBL are resistant to dioxin ex posure in vitro and therefore appeared not to be useful as sensitive b iomarkers in human exposure studies.