EVIDENCE FOR THE INDUCTION OF APOPTOSIS IN THYMOCYTES BY 2,3,7,8-TETVACHLORODIBENZO-P-DIOXIN IN-VIVO

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is well known for its immun otoxic effects particularly on the thymus. The exact mechanism by whic h TCDD induces thymic atrophy is not clear. In the current study, we i nvestigated whether TCDD triggers apoptosis in thymocytes, when admini stered in vivo, by using the TdT-mediated FITC - dUTP nick end labelin g method and analyzing the cell flow cytometrically. Significant apopt osis was detected at 8-12 hr after the TCDD injection but not at 24 hr or beyond, up to 120 hr of study. Furthermore, the induction of apopt osis was confirmed using the JAM test in which thymocytes from TCDD-tr eated mice, labeled with [H-3]thymidine, exhibited increased DNA fragm entation when compared to the controls. Similar to TCDD treatment, adm inistration of dexamethasone (5 or 100 mg/kg) into C57BL/6 mice trigge red apoptosis that was only detected at 12 hr after administration of the drug and not thereafter. When thymocytes from TCDD- or dexamethaso ne-treated mice were cultured in vitro for 24 hr, they exhibited marke d increase in apoptosis when compared to the vehicle-treated controls. However, TCDD, when added to in vitro cultures of thymocytes, failed to trigger apoptosis. Together, our studies demonstrate that TCDD can induce apoptosis in thymocytes in vivo. This can be detected only at a n early stage following TCDD administration, possibly because of rapid clearance of apoptotic cells by the phagocytic cells in vivo. (C) 199 7 Academic Press.