Abstract

Contaminant induced immunosuppression by organochlorines, particularly polychlorinated biphenyls (PCBs), has been suspected as a co-factor in the deaths of thousands of cetaceans and pinnipeds over the last three decades. Phagocytosis, part of the innate immune system, is the principal effector mechanism for the ultimate disposal of invading, foreign, or otherwise unwanted cells or particles. The aim of the present study is to characterize the immunomodulatory potential of mixtures of organochlorines on phagocytosis compared to that of individual compounds in beluga whales (Delphinapterus leucas) and Atlantic bottlenose dolphins (Tursiops truncatus). The ability of peripheral blood neutrophils and monocytes to engulf fluorescent microspheres was evaluated using flow cytometry. The ability of organochlorines to directly effect calcium mobilization, a cell-signaling event prior to phagocytosis, also was measured by flow cytometry. The immunomodulatory effects of three non-coplanar PCB congeners, PCB 138,153,180, one coplanar PCB, PCB 169, as well as 2,3,7,8-TCDD and all possible mixtures (26) were tested upon in vitro exposure. In both species, those mixtures containing at least two non-coplanar PCBs reduced both neutrophil and monocyte phagocytosis. Coplanar organochlorines, individually or when added to non-coplanar congeners, did not further suppress phagocytosis, suggesting that modulation of phagocytosis was mediated by an aryl hydrocarbon (Ah) receptor-independent mechanism. Dose-response experiments with individual congeners further demonstrated a non-congener, non-coplanar PCB-induced suppression of phagocytosis, while coplanar congeners had no effects. The non-coplanar PCBs also significantly increased calcium mobilization in dolphin neutrophils in an apparent dose-response relationship, suggesting that calcium mobilization could be involved in the mechanisms of organochlorine immunomodulation on dolphin neutrophil phagocytosis. Our results suggest simple additive interactions of chemicals in a mixture. The calculation of TEQs, however, failed to predict the experimentally induced immunomodulatory effects of organochlorines on beluga whale and bottlenose dolphin phagocytosis, which questions the reliability of TEQs in risk assessment for organochlorine exposure in these species. This is the first report demonstrating the immunosuppressive effects of organochlorines on dolphin and beluga phagocytosis, specifically by non-coplanar PCBs and through an Ah receptor-independent mechanism. A reduction in phagocytosis, the immune system's crucial first line of defense, may increase an animal's susceptibility to a variety of pathogens. A better understanding of the immunomodulatory effects of environmental contaminants will be useful for future assessment of the possible impact of those pollutants on the health of marine mammals and the risks associated with exposure to the ubiquitous organochlorines.