The organotin tributyltin (TBT) is an antifouling biocide used in marine paints and is a common pollutant in harbor estuaries. We previously demonstrated that the immune system of channel catfish, Ictalurus punctatus, is a sensitive target organ of TBT. Exposure strongly suppresses humoral immune responses. Harbor estuaries often contain polychlorinated biphenyls (PCBs) due to their ubuquitous distribution. The coplanar congener 3,3',4,4'5'-polychlorinated biphenyl (PCB-126) is also immunotoxic to channel catfish, but it suppresses only the innate immune responses and only at high doses. In this study we exposed channel catfish to TBT, PCB-126, or both in mixtures, with canola oil (CO) serving as the carrier control. Antibody responses to Vibrio anguillarum and phagocyte oxidative burst activity were measured after (1) a single dose of 0.01 or 1 mg/kg of each or both in combination, and (2) six injections of 1.7 or 170 microg/kg of each (or in combination) given every 3 days over a 16-day period to yield a cumulative dose of 0.01 or 1 mg/kg, respectively. We measured antibody responses to V. anguillarum 21 days after immunization and oxidative burst activities 14 and 21 days after the final treatment. The highest dose of TBT suppressed antibody responses after a single exposure. The high dose of PCB-126 also suppressed antibody responses. The addition of PCB-126 to TBT doses did not alter the antibody responses beyond the effects of TBT alone. In the repeated exposure group, only the high dose of TBT suppressed antibody responses. In animals exposed to mixtures, high levels of PCB-126 enhanced suppression associated with low levels of TBT, whereas PCB-126 protected against suppression associated with high levels of TBT. Single exposures to TBT or PCB-126 suppressed phagocyte oxidative burst activity. In animals exposed to mixtures, as a single exposure, the addition of a low dose PCB-126 protected against low dose TBT-related oxidative burst activity suppression. In the repeated exposure groups TBT suppressed oxidative burst activity, but only at the highest dose on day 21, while high doses of PCB-126 suppressed activity on day 14. Furthermore, low levels of PCB-126 reversed the suppressed oxidative burst activity associated with high levels of TBT on day 21. Overall, this study demonstrates moderate additivity in terms of the immunotoxicity of TBT and PCB-126 mixtures using these two endpoints of immune function in the channel catfish model.

Exposure to planar congeners of halogenated aromatic hydrocarbons (HAHs) leads to a myriad of toxicities, including developmental, reproductive, and immunotoxic effects. Halogenated aromatic hydrocarbons possessing structural similarity to TCDD, such as 3,3',4,4',5-pentachlorobiphenyl (PCB-126), are more prevalent in the environment than TCDD, and they elicit similar toxic effects, primarily through the cytosolic aromatic hydrocarbon receptor (AhR). While polychlorinated biphenyls (PCBs) are ubiquitous pollutants, they rarely exist alone in the environment. The aquatic biocide tributyltin (TBT) is also a widespread environmental contaminant, and numerous studies indicate that it has reproductive, developmental, and immunotoxic effects in a variety of organisms. Unlike planar HAHs, TBT is not associated with any known cellular receptor. The induction of cytochrome P-4501A (CYP1A) activity in most vertebrates is a classical physiological response to planar HAH exposure. TBT has been shown to inhibit the induction of cytochrome P-4.50s at high doses. Recent studies demonstrate that low levels of TBT potentiate PCB-126-associated CYP1A induction in channel catfish, Ictalurus punctatus, and in rodents following intraperitoneal injections. In this study, the effects of dietary exposures to TBT, PCB-126, and mixtures of the two on channel catfish hepatic CYP1A activity, as well as plasma lysozyme activity, were examined. Circulating lysozyme, a marker of proinflammatory responses, was monitored to determine the relative specificity of treatments for CYP1A induction. Plasma lysozyme levels were examined along with hepatic CYP1A protein induction and EROD activity following exposure to nominal doses of 1 and 100 ppb dietary TBT, PCB-126, or mixtures of the two. As expected, the highest level of PCB-126 induced CYP1A, and TBT did not. In mixtures, the low level of TBTpotentiated the ability of the high PCB-126 dose to induce CYP1A. Plasma lysozyme levels were suppressed by both concentrations of TBT and by the low concentrations of PCB- 126 during the initial phase of the response to Vibrio anguillarum. The normal and expected pattern of initial increases in circulating lysozyme levels following immunization, with subsequent return to baseline level, was disrupted by TBT. High levels of PCB-126 potentiated the lysozyme response. As seen with CYP1A activities, exposures to mixtures of TBT and PCB-126 resulted in a potentiation of plasma lysozyme levels. The data show that dietary TBT modulates PCB- 126-induced CYP1A activities and that these mixtures may have potent proinflammatory properties as well.