Toxicology 170 (2002) 139 – 151 Mechanism of 2,3,7,8-tetrachlorodibenzo-p -dioxin (TCDD)-induced decrease in anti-CD3-activated CD4 + T cells: the roles of apoptosis, Fas, and TNF E.A. Dearstyne 1 , N.I. Kerkvliet * Department of Enironmental and Molecular Toxicology and Enironmental Health Sciences Center, Room 1007, Agricultural Life Sciences Building, Oregon State Uniersity, Corallis, OR 97331, USA Received 24 April 2001; accepted 8 October 2001 Abstract The environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p -dioxin (TCDD), suppresses T cell functions and reduces T cell numbers in multiple models of immune stimulation. However, the underlying mechanism(s) by which TCDD induces these changes has yet to be elucidated. We hypothesized that TCDD affects T cells through the induction or augmentation of apoptosis. In these studies, we used antibody to CD4, annexin V, and 7-AAD in three-color flow cytometric analyses to examine the relationship between the decrease in CD4 + T cells and cell death in mice treated with anti-CD3 and TCDD. In addition, we examined two signaling pathways, Fas and TNF, in order to elucidate a potential mechanism by which TCDD increases cell death. Our results show that the TCDD-induced decrease in CD4 + T cell number correlated with an increase in the percentage of dead cells, but not with cells expressing an early apoptotic phenotype. The TCDD-induced decrease in CD4 + T cells was attenuated in Fas- and FasL-deficient mice (lpr and gld, respectively), but not by treatment with a neutralizing antibody to TNF. While these results suggest that the Fas pathway may be important in TCDD-induced T cell death, however, the effect of TCDD on the Fas pathway remains unclear. Taken together, our data suggest that TCDD-induced suppression of CD4 + T cells involves, in part, increased cell death that may be mediated by Fas/FasL interaction. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: TCDD; CD4 + T cell; Apoptosis; Fas; TNF www.elsevier.com/locate/toxicol 1. Introduction Exposure to 2,3,7,8-tetrachlorodibenzo-p - dioxin (TCDD), an environmental contaminant and prototypic ligand for the Ah receptor, sup- presses T cell numbers and function in multiple models of immune stimulation (Lundberg et al., 1991, 1992; Prell et al., 1995; Kerkvliet et al., * Corresponding author. Fax: +1-541-737-0497. E-mail address: nancy.kerkvliet@orst.edu (N.I. Kerkvliet). 1 Present address: NeoRx Corporation, 410 West Harrison Street, Seattle, WA 98119, USA. 0300-483X/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved. PII:S0300-483X(01)00542-X