Report Date: August 2012
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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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 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). in vitro studies have indicated that the potency of 2,3,7,8-tetrabromodibenzofuran relative to that of TCDD may be as high as 0.97, suggesting that this compound may be as potent as TCDD (Behnisch et al., 2003). Furthermore, in vitro evidence has suggested that TBDF may be more potent than 2,3,7,8-tetrachlorodibenzofuran, its chlorinated analogue (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 evaluated 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., 1995; Johnson et al., 2000).
The NTP requested that an evaluation of two HAHs, TBDF and TCDF, be conducted to assess their immunosuppressive effects following a single oral administration. These studies were conducted in female B6C3F1/N mice. Five TBDF dose levels were utilized (3, 9, 15, 30, and 90 µg/kg), given in a single oral administration. Five TCDF 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 TBDF and TCDF administration.
A single exposure to TBDF resulted in increases in both terminal body weight and body weight gain on day 11 at the 9, 15, and 30 µg/kg dose levels but no effects were observed at either the low (3 µg/kg) or high (90 µg/kg) dose levels. Relative liver weights were increased at day 3 following exposure to TBDF at all dose levels, although by day 11, relative liver weights were increased only for the 90 µg/kg dose. TBDF exposure did not affect thymus weights, while relative spleen weights were decreased at dose levels ≥ 9 µg/kg. Hematological parameters were unaffected by TBDF exposure. No effects were observed on spleen cell number, however, the T-Dependent Antibody Response, as measured by the IgM AFC response and serum anti-sRBC IgM antibody titers, was significantly decreased following a single TBDF exposure. Specifically, both the spleen IgM AFC response (when evaluated as both Specific Activity and Total Spleen Activity) and the serum anti-sRBC IgM antibody levels were significantly decreased at all dose levels.
TCDF exposure had no effect on either body weight or body weight gain. Absolute liver weights were increased at day 3 following exposure to 30 µg/kg TCDF, while relative liver weights were increased at both the 30 and 90 µg/kg dose levels. By day 11, no effects on liver weights remained. Furthermore, TCDF exposure did not affect spleen weight, thymus weight, hematological parameters, or spleen cell number. Humoral immune responses were also decreased following TCDF exposure. Both Specific Activity and Total Spleen Activity in the AFC assay were significantly decreased at the 90 µg/kg TCDF dose, while Total Spleen Activity was also decreased at 9 and 30 µg/kg TCDF. No effects were observed on serum anti-sRBC IgM antibody titers.
Pairwise multiple comparisons between the various dose groups of these compounds indicated that TBDF produced greater suppression of the TDAR parameters (AFC response, anti-sRBC IgM antibody titers) than did TCDF at equal doses. Furthermore, TBDF exposure at 9, 15, and 30 µg/kg produced greater suppression of the AFC response than TCDF exposure at the higher dose levels of 15, 30, and 90 µg/kg, respectively.
In conclusion, when administered in a single oral exposure at doses up to 90 µg/kg, TBDF and TCDF produced significant decreases in the humoral immune response as measured by serum IgM anti-sRBC antibody titers and by the AFC assay. The suppression observed following TBDF exposure was greater than that following TCDF exposure, suggesting that TBDF is a more potent immunosuppressive agent than TCDF.
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