Non-coplanar PCB-mediated modulation of human leukocyte phagocytosis: a new mechanism for immunotoxicity

Organochlorine (OC) contaminants, notably polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), are ubiquitous in all ecosystems and found in the tissues of humans and wildlife. Although the immunotoxicity of coplanar, dioxinlike PCBs is well documented, the adverse effects exerted by non-coplanar, non-dioxinlike PCBs have received little attention. Direct causal relationship between PCB and dioxin exposure and the observed detrimental effects on the immune system has yet to be fully established in humans. The immunomodulatory potential of toxic coplanar PCB 169 and TCDD and abundant non-coplanar PCBs 138, 153, and 180 on human leukocyte phagocytosis, an important innate immune function that initiates the clearance of pathogens, was tested upon in vitro exposure. Mixture and concentration-response experiments demonstrated a suppression of phagocytosis by non-coplanar PCBs suggesting a previously unrecognized aryl hydrocarbon receptor (AhR)-independent pathway. Regression analysis revealed that reduction of phagocytosis was mostly explained by the non-coplanar congeners. The effects on phagocytosis could not be accurately predicted by either the currently used toxic equivalence (TEQ) approach or the mouse model, thus undermining the use of the traditional models in the risk assessment for OC mixtures containing non-coplanar congeners. Our results are cause for concern as they suggest an AhR-independent pathway through which non-coplanar PCBs modulate phagocytosis, the immune system's first line of defense, possibly increasing the risk to developing infectious disease.