Polyhalogenated dibenzo-p-dioxins and dibenzofurans and the immune system. 1. Effects on peripheral lymphocyte subpopulations of a non-human primate (Callithrix jacchus) after treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Abstract

Monoclonal antibodies were used to analyse the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on peripheral lymphocytes from marmosets (Callithrix jacchus) following injections of single low doses of TCDD. A reduction of the percentage and the total number of lymphocytes among the white blood cells was seen 3 weeks after a single administration of 300 ng TCDD/kg body wt, but not following 167 ng TCDD/kg or less. After treatment of marmosets with the single subcutaneous dose of 10 ng TCDD/kg body wt, we observed an average decrease of about 20% in the percentage of CD4⁺ cells in the venous blood of treated animals. This change was not seen at the time of maximum absorption (2 weeks after the injection), but was demonstrable after 4 weeks and reached its maximum 15 weeks after the injection. The time course of the changes suggests an indirect effect (possibly via the thymus). Due to the considerable inter- and intra-individual variability in the number of peripheral lymphocytes the effect was less convincing on the basis of the total CD4⁺ cells per microliter blood. There was a significant concomitant increase in the percentage of cells with the CD8⁺ marker. A closer analysis of the lymphocyte subpopulation involved revealed a predominant effect on the cells with the CD4CDw29 (“leu 3a⁺4B4⁺”) surface marker (“helperinducer cells”). Subsequent to a dose of 10 ng TCDD/kg body wt the percentage of these cells was reduced by 50% or more when compared with the (24) controls. On an absolute basis (number of cells/μl blood) these CD4⁺CDw29⁺ cells were clearly reduced in only two out of four marmosets. There was no significant effect on the percentage of the CD4⁺CD45R⁺ (“leu3a⁺2H4⁺”) subpopulation (“suppressor-inducer cells”). The ratios: CD4⁺CDw29⁺/CD4⁺CD45R⁺, or CD4⁺CDw29⁺/CD8⁺ appear to be convenient measures for monitoring the effect described. Since the difference between the percentage of CD2⁺ cells minus the sum of CD4⁺ plus CD8⁺ cells was found to be increased following rather high doses (167 ng TCDD/kg body wt or more), this suggests that immature cells (CD2⁺ CD4-CD8-) are released into the periphery as a result of the exposure to TCDD. Furthermore, a decrease in the percentage of CD20⁺ (B₁ ⁺) cells (about 50% when compared with controls) was observed in the treated animals following a single dose of 10 ng TCDD/kg body wt or higher. Subsequent to a dose of 10 ng TCDD/kg body wt the percentage of CD56⁺ (NKH1⁺) cells seemed to be slightly increased. The dose-response for the effects observed was rather poor. At present it cannot be decided whether the changes have to be considered as adverse health effects. All of the animals involved were apparently healthy and did not exhibit, e.g. any infections. However, the deviations described certainly represent biologicaleffects induced at very low dose levels. From the data available up till now, a single dose of 10 ng TCDD/kg body wt seems to be the “lowest-observed-effect-level” (LOEL) in the marmoset for all the effects studied. More extensive experiments within the dose range between 1 and 10 ng TCDD/kg body wt are under way in our laboratory. When peripheral lymphocytes of TCDD-treated animals were incubated in vitro with a mitogen (PWM = poke weed mitogen) an exaggeration of thedecrease in the percent of cells with the CD4 surface marker and a concomitant increase in the percentage of CD8⁺ cells was observed. This seems to be an especially sensitive experimental approach for demonstrating biological effects (but not necessarily adverse health effects) of this class of substances. Such an effect was demonstrated and found to be statistically significant already 2 weeks after a single injection of 10 ng TCDD/kg body wt in comparison to vehicle-treated controls.