SOME EFFECTS OF 2,3,7,8-TETRACHLORODIBENZO-PARA-DIOXIN (T4CDD) AND OF 2,3,4,7,8-PENTACHLORODIBENZOFURAN (P5CDF) ON PERIPHERAL LYMPHOCYTES OF PRIMATES INVIVO AND INVITRO

The evaluation of possible cancerogenic and immunotoxic effects, with respect to a risk assessment in man, of comparatively small doses of polyhalogenated dibenzo-p-dioxins and dibenzofurans (PHDDs/PHDFs) may be of special significance. Information on possible actions of PHDDs/PHDFs on the immune system at low dose levels is scarce and contradictory. Furthermore, reliable and useful data on primates are lacking. We studied the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (T4CDD) and of 2,3,4,7,8-pentachlorodibenzofuran (P5CDF) on peripheral blood lymphocytes of a non-human primate (Callithrix jacchus) in vivo and in vitro, and additionally on human lymphocytes in vitro. Subsequent to a single s.c.-injection of T4CDD, lymphocytes were isolated from venous blood. Lymphocyte subpopulations were characterized with monoclonal antibodies using flow cytometry (FACScan). A decline in the percentage of the population with the CD4+CDw29+ high-density marker (>>helperinducer cells<<, or >>memory<< T cells) as well as with the CD20 epitope (>>B1<< cells) was consistently observed. The effect was less pronounced on the absolute cell basis (cells/mu-l blood). The smallest effect level was a single dose of 10 ng T4CDD/kg body wt, while 3 ng T4CDD/kg body wt did not consistently exhibit a clear-cut effect, and may be close to a NOEL (no-observed-effect-level). The effect occurred with a lag phase. When using the same experimental conditions 2, 3, 4, 7, 8-pentachlorodibenzofuran, a main congener found in breast milk in Germany, seemed to be clearly less potent (less than 1/2) when compared with T4CDD. Under conditions of a repeated dose study a transient increase in the percentage and absolute number of CD56+CD8+ cells (cytotoxic T cells) was observed, preceding the long lasting effect on the CD4+CDw29+ and the CD20+ cells. A causal relationship between these two events is feasible. It is noteworthy that an opposite effect, i.e. an increase in the percentage of CD4+CDw29+ cells instead of a decrease was induced in a multiple-dose study with even smaller doses (0.3 ng TCDD/kg bw weekly), indicating that extrapolations of the effects observed at the higher doses to very low exposures is not justified. Additionally, the effect of T4CDD was studied in vitro on cultures stimulated with pokeweed mitogen, using lymphocytes from marmosets or human volunteers. A decline in the percentage of defined lymphocyte subpopulations could be demonstrated following a 3-day proliferation in the presence of T4CDD in the culture medium. The CD4+CD29w+ as well as CD20+ cells again proved to be especially susceptible. The smallest concentrations leading reproducibly to an effect were in the range of 10(-13) to 10(-14) M T4CDD. In single studies an effect with even smaller concentrations seemed to occur. These are the lowest concentrations ever found to be effective in an in vitro assay with T4CDD. At the present stage of knowledge it cannot be decided whether the effects seen in vivo and in vitro are caused by the same mechanism. The data of the in vitro studies provide the first evidence observed for an effect induced directly by T4CDD on peripheral lymphocytes. While the mechanism is presently unknown several explanations are possible (interference with maturation of CD4+CDw29+ --> CD4+CD45R+ cells, enhancement of apoptosis, binding to specific receptors, change in distribution of lymphocytes in vivo, etc.) and must be verified. Our studies have shown "biological" (not necessarily toxic) effects induced by very small doses of TCDD. However, the significance of these findings for a risk assessment in man is presently unknown.