The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program (NTP). The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by the NTP on March 2009 (see http://ntp.niehs.nih.gov/ntp/htdocs/levels/09-3566%20NTP-ITOX-R1.pdf). The findings and conclusions for this study should not be construed to represent the views of the NTP or the US Government.
Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine)has been reported to be the most commonly employed herbicide inthe United States. Atrazine (ATZ) is routinely used in combatingweeds in corn and sorghum crops and is also widely used in combinationwith other herbicides. As a result of its low biodegradability,it has a high potential for contaminating surface waters. Atrazine'spotential environmental hazard for polluting drinking water haslead Italy to band this herbicide. Wide-spread contamination ofground water has been reported in the Midwest of the United States.Prior to these studies, the effect of atrazine on the immune systemhad only been studied to a limited extent.
Atrazine was nominated to the NTPfor toxicological evaluation and was selected for immunotoxicitystudies by the chemical manager. Thus, the purpose of these studieswas to determine the effect of atrazine on the immune system.
The baseline toxicology studies aresummarized in Table ES-1. Mice exposed toatrazine at doses of 500 mg/kg have significant decreases in bodyweight at the end of the first and second week of exposure. Insome studies the decrease in body weight was in excess of 10%of the vehicle control group. Body weight gain when evaluatedover the first week period was significantly decreased at alldose levels, and when evaluated over the two week exposure period,both the middle (45%) and high (105%) dose groups were significantlydecreased. While the liver weights were unaffected by the atrazineexposure, a dose-dependent decrease was observed in spleen (33%)and thymus (52%) weights. Additionally, kidney weights were decreased(15%) in the atrazine exposure animals at the high dose. Nostatistically significant effects were observed on leukocyte numbers,leukocyte differentials, reticulocytes or mean corpuscular hemoglobinconcentrations. A statistically significant decrease was observedin the erythroid elements: erythrocytes (14%), hemoglobin (5%)and hematocrit (5%). An increase was observed in mean corpuscularhemoglobin which was dose related. However, the significant increasein mean corpuscular volume was observed only in the low dose group,and while the other dose levels were elevated compared to thevehicle control, they did not reach the level of statistical significance(p less than 0.05).
Table ES-2 summarizesthe immunology studies. Exposure to atrazine decreased the absolutenumber of B cells (18%) and decreased the absolute number of CD4+CD8+(40%) T cell subset. Total T cells and the other T cell subsetswere not affected. Atrazine had no effect on the humoral immuneresponse to sheep erythrocytes, evaluated either in the plaque-formingcell assay (PFC) or as serum titers. The proliferative responseto mitogens, Con A and LPS, were not affected. However, a dose-dependentincrease was observed in the proliferative response to allogeneiccells as evaluated in the MLR. The animals receiving the highdose of atrazine had a response which was increased 30% comparedto the vehicle controls. Overall, there was no significant effecton the CTL response, natural killer cell activity or the functionalactivity of the reticuloendothelial system.
The results of the host resistancestudies are summarized in Table ES-3. In thetwo host resistance models evaluated, host resistance to Listeriamonocytogenes was not affected, while a decrease in host resistanceto the B16F10 Melanoma tumor model was observed.
In summary, although atrazine wasadministered at dose levels which produced significant decreasesin terminal body weights and weight gain as well as effects onvarious erythroid elements, most of the functional assays usedto evaluate immunocompetence were not adversely affected. However,due to the dose-dependent increase in the mixed leukocyte responseand the dose-dependent decrease in host resistance to the B16F10melanoma tumor model, atrazine was found to adversely affect theimmune system and, thus, is considered to be an immunotoxic compoundwhen administered under the experimental conditions used in thisprotocol.
|Day 8||Decreased||4%||500 mg/kg|
|Day 15||Decreased||6%||500 mg/kg|
|Day 8-1||Decreased||137%||500 mg/kg|
|Day 15-1||Decreased||105%||500 mg/kg|
|Gross Pathology||No Effect|
|MCV||Increased||11%||25 mg/kg||Low Dose Only|
SUMMARY TABLE FOR IMMUNOLOGYSTUDIES
|IgM Humoral Immune Response to Sheep Erythrocytes|
|Spleen IgM AFC to sRBC||Increased||35%||25 mg/kg||Not Biologically Significant|
|Serum Titers to sRBC||No Effect|
|Proliferation Assays, i.e. Mitogens, Mixed Leukocyte Response|
|Con A||No Effect|
|Cytotoxic T Lymphocyte Activity|
|Functional Activity of the Reticuloendothelial System|
|NK Cell Activity|
|1:25||Decreased||40%||250 mg/kg||Not Biologically Significant|
SUMMARY TABLE FOR HOST RESISTANCESTUDIES
|Listeria monocytogenes||No Effect|
|B16F10 Melanoma||Decreased||207%||500 mg/kg|
Report Date: May 1994