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 The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009.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.
Disinfection by-products (DBPs) are contaminants found in drinking water. DBPs are formed as by-products as a result of the chlorination/ozonization process used to purify water to acceptable drinking water standards. The potential effects of drinking water contaminants to affect adversely the immune system is a concern of both the Environmental Protection Agency (EPA) and the National Institute of Environmental Health Sciences (NIEHS). Several drinking water DBPs have been identified and selected for evaluation of their potential effects on the immune system in a joint project between the EPA and the NIEHS.
The National Toxicology Program (NTP) requested that a dose range-finding study be performed in order to establish the potential effects of dibromoacetic acid (DBA) on the immune system and to determine doses that could be used in a full immunotoxicology study. These studies were conducted in female B6C3F1 mice. The animals were exposed to DBA based on the concentration of the test article in the drinking water. Five DBA concentrations of 125, 250, 500, 1000 and 2000 mg/L for 28 days were utilized. DBA solutions were prepared fresh every two weeks in tap water and stored refrigerated. The in-life phase of these studies was carried out between 18 November 1997 and 6 February 1998.
The baseline toxicology studies are summarized in Table ES-1. DBA was administered in the drinking water from water bottles for 28 days at 125, 250, 500, 1000 and 2000 mg/L/day. There was no statistical difference in drinking water consumption from animals exposed to DBA as compared to the tap water controls. Exposure to DBA did not produce any signs of overt toxicity. There was no significant difference in body weight between the exposed and control animals during the experimental period; however, a 40% decrease in body weight gain was observed in the DBA high dose group. No gross pathological lesions were observed in DBA-exposed animals; furthermore, there were no differences observed in terminal body weight, brain or lung weight. Animals exposed to the high dose of DBA had a significant decrease (33%) in relative thymus weight and an increase (21%) in relative spleen weight. DBA produced a dose-related increase in liver weight which was significant at all dose levels with the greatest increase (51%) in relative liver weight being observed at the high dose group. In addition, a dose-related increase in kidney weight was also observed with the increase in the highest four dose levels reaching the level of statistical significance. The greatest increase (21%) in the relative kidney weights was also observed at the high dose group. The erythrocyte count, hemoglobin, hematocrit, MCV, MCH, MCHC, platelets, leukocyte counts and leukocyte differential were unaffected by DBA. While a statistically significant increase in MCH was observed at the 500 mg/L dose level, this increase (1%) was not considered to be biologically relevant. A dose-related increase in reticulocytes, significant at the three highest dose levels, was observed following exposure to DBA with the greatest increase (30%) being observed at the high dose level.
The immunological studies are summarized in Table ES-2. As in the toxicological parameters, exposure to DBA produced marked changes in various immunological parameters. A dose-dependent increase in spleen cell number was observed in animals exposed to DBA. The maximum effect was observed in the 500 mg/L dose group where there was a 20% increase in spleen cell number compared to the vehicle (tap water) control animals. In addition, both the 1000 and 2000 mg/L dose groups have statistically significant increases in spleen cell numbers. In the phenotypic enumeration of the spleen cells, there were dose-related increases in the absolute number of B cells, total T cells, T helper cells, T suppresser/cytotoxic cells and natural killer cells. However, for each of these cell types, the increases appeared to be due primarily to an increase in spleen cell number since little effect was observed on the percentage values. In contrast, the dose-related increase (91%) in the splenic macrophage numbers appeared to be the result of an actual increase in the macrophage number. Exposure to DBA produced a dose-dependent decrease in the antibody-forming cell (AFC) response to sheep erythrocytes, which was significant at the three highest dose levels when evaluated either as specific activity or as total spleen activity. At the highest dose level, the decreases were 50% and 47%, respectively, for each of these parameters. No effect was observed on serum IgM antibody titer to sheep erythrocytes which may be the result of DBA having a greater effect on the spleen than on the bone marrow and lymph nodes. No effect was observed on the mixed leukocyte response (MLR) following DBA exposure. Overall, no effect was observed in the ability of thioglycollate-recruited peritoneal macrophages to kill and/or inhibit the growth of B16F10 tumor cells. In addition, exposure to DBA did not inhibit the ability of the macrophages to respond to a stimulus known to enhance macrophage function. In contrast to the suppression observed in the antibody-forming cell response, animals exposed to DBA had an increase in natural killer (NK) activity when the data were evaluated as lytic units. A statistically significant increase was observed in the three highest dose groups when the results were expressed as specific activity and in the four highest dose levels when expressed as total spleen activity. The greatest effects were observed at the high dose level where specific activity was increased 100% and the total spleen activity increased 143%.
In conclusion, DBA, when administered in the drinking water at doses from 125 to 2000 mg/L, produced marked toxicological and immunomodulatory effects. These effects included changes in organ weights and hematological parameters. In addition, DBA exposure resulted in changes in spleen cell numbers and spleen cell populations, as well as alteration in natural killer cell activity and the antibody-forming cell response to sheep erythrocytes. Based on the results of this range-finding study, further investigation of the effect of DBA on the immune system should be undertaken. As a disinfection by-product, human exposure to DBA can occur from the drinking water and the effects of such exposure has on the immune system warrants further study.