The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of the NTP or the U.S. Government.
The World Health Organization and the United States Environmental Protection Agency regulate polychlorinated dibenzo-p-dioxins and dibenzofurans using the toxic equivalency factor methodology. A number of studies have reported that the polybrominated dibenzo-p-dioxins and dibenzofurans are present in sediment (Takigama et al., 2005), fish (Hayward et al., 2007; Fernandes et al., 2008), and in human serum (Schecter et al., 2005). There are in vitro studies (Behnisch et al., 2003) suggesting that some of the brominated analogs may be more potent than 2,3,7,8-tetrachlorodibenzodioxin. It has been reported that the TEFs for the PBDD/Fs are greater than for the PCDD/F o-p-dioxin is more potent than TCDD (Behnisch et al., 2003; Mason et al., 1987). Accordingly, it was proposed to study DiCDD and compare the results with 2,3,7-tribromodibenzo-p-dioxin. TriBDD has been found in seafood (Fernandes et al., 2008, 2009) and has been reported to have similar affinity as TCDD for binding the aryl hydrocarbon receptor in vitro (Mason et al., 1987).
As part of the National Toxicology Program evaluation of the TEF methodology for use in dioxin risk assessments, the relative potencies of the brominated dioxins and their chlorinated analogs have been tested for equivalence by investigating the effects of these compounds on humoral immunity in mice. Halogenated aromatic hydrocarbons have well-documented effects on the immune system, and a TEF approach to examining the relative potencies of dioxins, polychlorinated biphenyls, and dibenzofurans demonstrated that TCDD is one of the most potent inhibitors of immune function in the mouse (Davis and Safe, 1988). Inhibition of the antibody-forming cell response to sheep red blood cells in mice following an acute single dose of dioxins has been shown to be one of the most sensitive indicators of exposure to these compounds (Harper et al., 1993; Johnson et al, 2000).
The NTP requested that an evaluation of two HAHs, TriBDD and DiCDD, be conducted to assess their immunosuppressive effects following a single oral administration. These studies were conducted in female B6C3F1/N mice. Five TriBDD dose levels were utilized (1, 3, 5, 10, and 30 μg/kg), given in a single oral administration. Five DiCDD dose levels were utilized (0.1, 0.3, 0.5, 1, and 3 μg/kg), given in a single oral administration. Corn oil was used as the vehicle for TriBDD and DiCDD administration.
Exposure to neither TriBDD nor DiCDD produced adverse effects on either body weight or body weight gain. No effects were observed on the weights of the major organs of the immune system, the thymus and the spleen, however, there was a decreasing trend in spleen cell numbers following exposure to DiCDD but not TriBDD. Liver weights were unaffected by TriBDD exposure. In contrast, DiCDD exposure at the high dose (3 μg/kg) produced significant increases in liver weights when expressed as both absolute (wet) weight and percent of body weight (relative weight). Effects on hematological parameters were sporadic and not dose-responsive. The T-Dependent Antibody Response, as measured using the AFC and the sRBC enzyme-linked immunosorbent assay, were unaffected by TriBDD at doses up to 30 μg/kg. For DiCDD, however, both the AFC response and the serum IgM antibody titers to sRBC were significantly decreased following exposure.
Pairwise multiple comparisons between dose groups demonstrated that DiCDD at doses ranging from 0.1 μg/kg to 3 μg/kg produced greater immunosuppression of the humoral immune responses (AFC, sRBC ELISA) than did TriBDD at the high dose of 30 μg/kg.
In conclusion, when administered in a single oral exposure at doses up to 30 μg/kg, TriBDD did not affect the humoral immune response, as measured by serum IgM anti-sRBC antibody titers and by the AFC assay. In contrast, DiCDD suppressed both of these measures of humoral immunity, where all exposure levels (0.1 - 3 μg/kg) produced significant decreases in serum anti-sRBC antibody levels, and all exposure levels except 0.3 μg/kg produced significant decreases in the AFC assay. These results suggest that DiCDD is a more potent immunosuppressive agent than TriBDD.