Differential induction of apoptosis in activated and resting T cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its repercussion on T cell responsiveness.

TCDD is well known for its immunotoxic effects on T cells, although the exact mechanism of toxicity remains unknown. In the current study, we investigated the effect of TCDD administration on resting and activated T cells within the same animal. To this end, C57BL/6 mice were injected intraperitoneally with either TCDD (50 microg/kg body weight) or the vehicle and were injected with anti-CD3 mAbs into the rear footpads to polyclonally activate T cells in the popliteal lymph nodes (LN). Axillary LN cells harvested from the same groups of mice served as a source of resting T cells. When the LN cells were tested for their proliferative responsiveness to stimulation with anti-CD3 mAbs in vitro, the activated popliteal LN, but not the resting axillary LN cells from TCDD-treated mice exhibited a significant decrease in responsiveness when compared to the vehicle controls. Inasmuch as TCDD has been shown to induce apoptosis in thymocytes, we addressed whether TCDD triggered apoptosis in LN cells, using the terminal deoxynucleotidyl transferase (TdT)-mediated FITC-dUTP nick end labeling (TUNEL) method. The axillary and popliteal LN cells from TCDD-treated mice failed to exhibit significant levels of apoptosis when freshly harvested. However, upon in vitro culture for 24 h with either tissue culture medium alone or with anti-CD3 mAbs, activated popliteal LN cells from TCDD-treated mice showed a significant increase in apoptosis when compared to similar cells from vehicle-treated mice. In contrast, resting axillary LN cells from TCDD-treated mice, similarly cultured in vitro, exhibited decreased levels of apoptosis when compared to the controls. Using a double-staining technique, the activated popliteal LN cells undergoing increased apoptosis in TCDD-treated animals were confirmed to be CD3+ T cells. Together, these data demonstrate that TCDD exerts differential effects on activated and resting T cells, even within the same animal, by inhibiting the proliferative responsiveness of activated, but not resting, T cells. Furthermore, this effect may be mediated by the ability of TCDD to induce increased apoptosis in activated, but not resting, T cells.