Greater than additive suppression of TLR3-induced IL-6 responses by administration of dieldrin and atrazine

Current risk assessment practices do not consider possible synergistic or antagonistic interactions of compounds to which persons may be exposed during the same period of time. This may simply reflect the minimal amount of data available on such interactions, particularly with regard to immunotoxicology. Therefore, this study was conducted to determine if such interactions occur between the most abundantly used conventional pesticide in the United States (atrazine) and a legacy pesticide that is still present in the United States food supply at levels greater than recognized as safe (dieldrin). The results provide evidence that greater than additive effects on signaling and cytokine production occur and suggest that evaluation of common mixtures for such effects may be needed. The compounds both separately and together directly inhibited cytokine production induced by polyinosinic-polycytidylic acid (poly I:C) by macrophages in cell culture. Subcutaneous administration of dieldrin (10-20 mg/kg, daily for 7 d) and atrazine (one dose on Day 7, 100-200 mg/kg) inhibited the production of IL-6 and IL-12 in the peritoneal cavity in a dose-dependent manner, but IL-10 was either increased or not affected. The suppression of IL-6 production and inhibition of NF-kappa B activation was greater than additive when comparing animals given both compounds to those given either compound separately. However, at lower dosages of both compounds (10 mg/kg dieldrin and 50 mg/kg atrazine), the effect was much greater than additive on IL-6 production (adding the individual effects of atrazine and dieldrin on IL-6 production indicates 20% suppression, whereas the combination yields 80% suppression) and essentially additive for inhibition of the activation of c-JUN (a component of the transcription factor, AP-1). Previously published results indicate that atrazine induces a neuroendocrine stress response, and results reported here indicate that dieldrin at 20 mg/kg increases serum corticosterone concentrations, indicating a stress response. This and other possible mechanisms of the greater than additive effects on cytokine production are discussed. Dieldrin and atrazine administered orally (as opposed to subcutaneously as in the other experiments) also effectively suppressed IL-6 production. These results suggest that interactions other than additive effects for compounds with similar mechanisms of action should be considered in risk assessment. Finally, a molecular mechanism for the greater than additive inhibition of IL-6 production is proposed and a mathematical model incorporating that mechanism is presented.