Polyhalogenated dibenzo-p-dioxins and dibenzofurans and the immune system. 4. Effects of multiple-dose treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on peripheral lymphocyte subpopulations of a non-human primate (Callithrix jacchus).

Non-human primates (Callithrix jacchus) were treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) over a period of 30 weeks, and lymphocyte subpopulations of venous blood were monitored using monoclonal antibodies and flow cytometry (FACScan). There was no clear-cut change in the total lymphocyte population during this study. In the first part of the study the new-world monkeys (marmosets) were treated for 24 weeks with a weekly dose of 0.3 ng TCDD/kg body wt. At the end of this treatment period a level corresponding to an actual cumulative dose of about 2.5-2.7 ng TCDD/kg body wt was expected. The percentage and the absolute number of the CD4+CDw29+ cells ("helper inducer" or "memory" cells) surmounted the physiologically occurring increase. Concomitantly the percentage of the CD4+CD45RA+ cells ("suppressor-inducer" or "naive" cells) decreased. There was, at the same period, no change in the total T cell population (CD2+ cells) or in the cells carrying the CD8 or the CD4 epitope. When increasing the weekly dose to 1.5 ng TCDD/kg body wt, a transient increase in the percentage and the absolute number of the CD8+CD56+ cell population ("cytotoxic T cells") was observed 3 weeks after the increase in dosing. At this time the expected decrease in the percentage or the absolute number of CD4+CDw29+ cells was just detectable and this decline was at its maximum 6 weeks after switching to the higher weekly doses. The reduction in the percentage and the absolute number of CD4+CDw29+ cells persisted 5 weeks after discontinuation of the dosing, but this cell population was again within normal limits 7 weeks later. Because the two subpopulations are changed in opposite directions, the ratio CD4+CDw29+/CD4+CD45RA+ is a very sensitive measure of the effect induced by TCDD. There was a pronounced decrease in the percentage of the CD20+ cells (B1 cells), but their percentage and number rapidly normalized, in contrast to the CD4+CDw29+ cells, when the dosing was discontinued. At the end of the treatment period the apparent body burden was calculated to correspond to an actual dose of about 9-10 ng TCDD/kg body wt. Such an actual dose level might be assumed to be reached under steady-state conditions in chronic experiments with daily doses of about 135 pg TCDD/kg body wt (assuming a half-life for TCDD in the marmoset of 6-8 weeks). Extrapolations of the results obtained at higher doses to very low exposures is not justified with respect to the effects induced by TCDD on the immune system of marmosets. At lower doses the effect is clearly reversed.