2,3,7,8-Tetrachlorodibenzo-p-dioxin (
TCDD) exposure results in adverse effects on the immune system of experimental animals. The purpose of this study was to compare the effects of a single and repeated dosing of
TCDD on splenic T-cell subpopulations in Long Evans rats 9 days post-exposure to
TCDD. A single dose (25 microg/kg
body weight) of
TCDD resulted in reduced
body weight. The percentage and total number of CD4+ or CD8+ subsets and percentage of CD4+ or CD8+ cell cycling in the S and G2M phases were similar in the single dosed (25 microg/kg
body weight)
TCDD group compared with the vehicle control. A repeated dose (5 microg/kg/day for 5 days) of
TCDD also resulted in a significant reduction in
body weight. However, multiple doses of
TCDD significantly decreased the percentage of the CD4+ subset and the percentage of CD4+ cells cycling in the S and G2M phases. No significant change occurred in the CD8+ cell subpopulation after single or multiple dosing with
TCDD. These results demonstrated that repeated dosing of
TCDD decreased the total percentage of CD4+ cells and the percentage of CD4+ cells cycling 9 days post-exposure, while an analogous single dose of
TCDD failed to affect the CD4+ cell subpopulation. The difference in
biological responses to a single versus 'equivalent' multiple (cumulative) dose of
TCDD is discussed.