Macrophage-Induced Apoptosis Limits Endovascular Trophoblast Invasion in the Uterine Wall of Preeclamptic Women

Impaired invasion of uteroplacental arteries by extravillous trophoblast cells is a key pathogenic mechanism of preeclampsia. We previously demonstrated that reduced trophoblast invasion into uteroplacental spiral arteries was associated with an excess of macrophages in and around these arteries. To explore the significance of these observations, we correlated the extent of extravillous trophoblast apoptosis in placental bed biopsy specimens with macrophage distribution and studied the effect of macrophages upon trophoblast apoptosis in vitro. Extravillous trophoblast hybrid cells were cocultured with activated macrophages exposed to exogenous tumor necrosis factor α (TNFα), anti-tumor necrosis factor receptor I (TNF-RI), and tryptophan depletion, and the rates of trophoblast apoptosis were measured. Extravillous trophoblast hybrid cells showed increased rates of apoptosis following exposure to exogenous TNFα, with tryptophan depletion, and when cocultured with activated macrophages. The proapoptotic effects of macrophages in vitro were completely inhibited only by simultaneous addition of tryptophan and anti–TNF-RI. Our data indicate that macrophages, residing in excess in the placental bed of preeclamptic women, are able to limit extravillous trophoblast invasion of spiral arterial segments through apoptosis mediated by the combination of TNFα secretion and tryptophan depletion. The mechanisms by which macrophages are activated and recruited to the placental bed are presently unknown but are likely central to the pathogenesis of preeclampsia.