Embryonic resorption and polycyclic aromatic hydrocarbons: putative immune-mediated mechanisms.

Polycyclic aromatic hydrocarbons (PAHs) are released into the environment as a result of incomplete fossil fuel combustion from industrial furnaces, wood-burning stoves, and automobile exhaust fumes; however, the primary source of human exposure to these compounds is cigarette smoke. Embryonic and fetal loss after treatment with high doses of PAHs have been well documented in animal studies; however, few studies have addressed the reproductive consequences of long-term, low-level exposure to these chemicals. We previously reported that low doses of PAHs administered to ICR mice over a period of 9 weeks prior to conception resulted in early embryonic resorptions, whereby treated dams lost approximately 50% of their litter. During the course of these studies, we observed greater numbers of infiltrating uterine natural killer (uNK) cells into the placenta of PAH-exposed conceptuses. While exposure to high levels of PAHs has been shown to be immunosuppressive, increasing evidence suggests that chronic, low-dose exposure to PAHs may stimulate immune cells. Thus, we hypothesized that low-dose, chronic PAH exposure in our mouse model is mediating embryonic resorption by hyperstimulating maternal immune cells. In this review of the literature, we outline the rationale of our argument and present preliminary data, focussing upon PAH-mediated alterations in uNK cell dynamics and how these changes may be linked to early embryonic resorptions.