Report Date: November 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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
1,2:5,6-Dibenzanthracene, also known as dibenz[a,h]anthracene, belongs to a class of compounds known as polycyclic aromatic hydrocarbons. PAHs are formed as a result of incomplete combustion of organic material. Human exposure to PAHs occurs primarily via the diet, inhalation of polluted air (forest fires, wood-burning stoves, smoking, car exhaust fumes), or through occupational exposure (roofing, smelting, coke ovens). While other PAHs, such as benzo[a]pyrene (B[a]P), have been well-studied for their immunosuppressive effects, DBZA has not been thoroughly investigated, even though it has been reported to be more potent than B[a]P (White et al., 1985a; Pufulete et al., 2004). Four days following a single pharyngeal aspiration of DBZA, systemic immunosuppression of both humoral and cell-mediated immune functions has been reported, with the more significant effects being observed on humoral immunity (Smith et. al., 2010).
The National Toxicology Program requested that a dose range-finding study be conducted to establish the potential effects of DBZA on the immune system. These studies were conducted in female B6C3F1/N mice. Animals were treated with corn oil vehicle or 25, 79.3, 250, 793, or 2500 µg/kg DBZA daily for 28 days via subcutaneous injection. DBZA dosing solutions were prepared weekly.
No effects were observed on body weights or on the weight gain of B6C3F1/N mice treated with DBZA. In addition, the absolute weights of the liver, thymus, lung, and kidney were unaffected by DBZA exposure, while dose-related decreases in absolute spleen weights were observed, reaching the level of statistical significance at the 2500 µg/kg dose. Significant dose-related decreases in erythrocyte numbers, hemoglobin concentration, and hematocrit were also observed, while other hematological parameters were unaffected.
Spleen cell numbers were decreased in a dose-dependent manner, and the absolute numbers of the various spleen cell phenotypes, including B cells, total T cells, T-helper and cytotoxic T cell subsets, natural killer cells, and macrophages, were significantly decreased at the 2500 µg/kg dose level. The absolute number of NK cells was also significantly decreased in mice exposed to 793 µg/kg DBZA. A dose-dependent suppression of the T-dependent antibody response to sheep erythrocytes was observed at doses ≥ 793 µg/kg. Both the antibody-forming cell (AFC) assay and the sRBC enzyme-linked immunosorbent assay were significantly suppressed. Effects on cell-mediated immunity were mixed. The mixed leukocyte response was significantly decreased at the 2500 µg/kg dose level in two studies, while no effects were observed on anti-CD3 mediated T cell proliferation or the cytolytic activity of TCTL cells. Innate immune function, as assessed using the NK cell assay, was also decreased at two effector:target ratios in mice exposed to 2500 µg/kg DBZA.
Overall, these results indicate that DBZA is immunosuppressive following 28 days of exposure in the female B6C3F1/N mouse.
White K.L., Lysy H.H., & Holsapple M.P. (1985a). Immunosuppression by polycyclic aromatic hydrocarbons: A structure-activity relationship in B6C3F1/N and DBA/2 mice. Immunopharmacology, 9:155-64.
Pufulete M., Battershill J., Boobis A., & Fielder R. (2004). Approaches to carcinogenic risk assessment for polycyclic aromatic hydrocarbons: A UK perspective. Regul Toxicol Pharmacol, 40:54-66.
Smith D.C., Smith M.J., & White K.L. Jr. (2010). Systemic immunosuppression following a single pharyngeal aspiration of 1,2:5,6-dibenzanthracene in female B6C3F1/N mice. J Immunotoxicol, 7(3):219-31.