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 th e exact mechanism of toxicity remains unknown. In the current study, w e investigated the effect of TCDD administration on resting and activa ted T cells within the same animal. To this end, C57BL/6 mice were inj ected intraperitoneally with either TCDD (50 mu g/kg body weight) or t he vehicle and were injected with anti-CD3 mAbs into the rear footpads to polyclonally activate T cells in the popliteal lymph nodes (LN). A xillary LN cells harvested from the same groups of mice served as a so urce of resting T cells. When the LN cells were tested for their proli ferative responsiveness to stimulation with anti-CD3 mAbs in vitro, th e activated popliteal LN, but not the resting axillary LN cells from T CDD-treated mice exhibited a significant decrease in responsiveness wh en compared to the vehicle controls. Inasmuch as TCDD has been shown t o 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 axilla ry and popliteal LN cells from TCDD-treated mice failed to exhibit sig nificant 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 sh owed a significant increase in apoptosis when compared to similar cell s from vehicle-treated mice. In contrast, resting axillary LN cells fr om TCDD-treated mice, similarly cultured in vitro, exhibited decreased levels of apoptosis when compared to the controls. Using a double-sta ining 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 effec ts on activated and resting T cells, even within the same animal, by i nhibiting the proliferative responsiveness of activated, bur not resti ng, T cells. Furthermore, this effect may be mediated by the ability o f TCDD to induce increased apoptosis in activated. but not resting, T cells. (C) 1998 Elsevier Science Ireland Ltd. A;I rights reserved.