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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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 (PBDD/Fs) 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 analogues (Ma et al., 2009). However, there are few in vivo studies that have evaluated the relative potencies of the PBDD/Fs. The brominated dibenzofurans have been reported to be more abundant than brominated dioxins in shellfish (Fernandes et al, 2009) and also in incinerated wastes containing brominated flame retardants (Ma et al., 2009). It has been reported that the biologic and toxic activity of dibenzo-p-dioxins is reduced following the addition of a non-lateral halogen group to those compounds having either Cl or Br in each of the lateral (2, 3, 7, and 8) positions (Mason, et al., 1987), however this was not studied for the dibenzofurans. The WHO has assigned a dioxin toxic equivalency factor of 0.05 to 1,2,3,7,8-pentachlorodibenzofuran; Van den Berg et al., 1998). However, in vitro studies have suggested the relative potency of this chlorinated dibenzofuran may be as high as 0.40 (Behnisch et al, 2003). Furthermore, the relative potency of its brominated analogue, 1,2,3,7,8-pentabromodibenzofuran may be as high as 0.52 (Behnisch et al, 2003).
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 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, 1,2,3,7,8-PeBDF and 1,2,3,7,8-PeCDF, be conducted to assess their immunosuppressive effects following a single oral administration. These studies were conducted in female B6C3F1/N mice. Five 1,2,3,7,8-PeBDF dose levels were utilized (3, 9, 15, 30, and 90 µg/kg), given in a single oral administration. Five 1,2,3,7,8-PeCDF dose levels were utilized (3, 9, 15, 30, and 90 µg/kg), given in a single oral administration. Corn oil was used as the vehicle for 1,2,3,7,8-PeBDF and 1,2,3,7,8-PeCDF administration.
A single exposure to 1,2,3,7,8-PeBDF had no effect on body weight or body weight gain. Absolute liver weights were increased as compared to vehicle control mice at day 3 following exposure to 1,2,3,7,8-PeBDF in a dose-related trend, although by day 11, no differences remained. 1,2,3,7,8-PeBDF exposure did not affect absolute weights of the major immune organs, the spleen and the thymus. In addition, hematological parameters were unaffected by 1,2,3,7,8-PeBDF exposure, with the exception of a statistically significant increase in absolute basophil numbers at the high dose (90 µg/kg). Furthermore, no effects were observed on spleen cell number, however, the T-Dependent Antibody Response, as measured by the spleen IgM AFC response and serum anti-sRBC IgM antibody titers, was significantly decreased following a single 1,2,3,7,8-PeBDF exposure in a dose-related manner. Specifically, the AFC response (when evaluated as both Specific Activity and Total Spleen Activity) was significantly decreased at doses ≥ 30 µg/kg, while Specific Activity was also decreased at the 3 µg/kg dose. Serum anti-sRBC IgM antibody levels were significantly decreased at the 90 µg/kg dose only.
1,2,3,7,8-PeCDF exposure had no effect on either body weight or body weight gain. Absolute liver weights were significantly increased over the liver weights of vehicle control mice at day 3 following exposure to 90 µg/kg 1,2,3,7,8-PeCDF only. By day 11, no effects on liver weights remained. 1,2,3,7,8-PeCDF exposure did not affect spleen weight, thymus weight, or spleen cell number. Hematological parameters were also unaffected, with the exception of increases in the absolute numbers of neutrophils and basophils at the 90 µg/kg dose of 1,2,3,7,8-PeCDF. In contrast, humoral immune responses were significantly decreased following exposure to 1,2,3,7,8-PeCDF. Both Specific Activity and Total Spleen Activity in the AFC assay were significantly decreased in mice treated with 90 µg/kg 1,2,3,7,8-PeCDF. No effects were observed on serum anti-sRBC IgM antibody titers.
Pairwise multiple comparisons between the various dose groups of these compounds suggested that 1,2,3,7,8-PeBDF produced greater suppression of humoral immune responses (AFC response, sRBC ELISA) than did 1,2,3,7,8-PeCDF.
In conclusion, when administered in a single oral exposure at doses up to 90 µg/kg, both 1,2,3,7,8-PeBDF and 1,2,3,7,8-PeCDF produced significant decreases in the AFC response to sRBC, while only the brominated analogue decreased serum IgM anti-sRBC antibody titers. The suppression observed following 1,2,3,7,8-PeBDF exposure appears to be greater than that following 1,2,3,7,8-PeCDF exposure, suggesting that 1,2,3,7,8-PeBDF is a more potent immunosuppressive agent than 1,2,3,7,8-PeCDF.
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