In the early stage of pregnancy, hypoxia in the placenta is of great significance to the migration and invasion of trophoblasts. In addition, changes to the polarity and activity of macrophages can affect embryo implantation, trophoblast migration and invasion, and vascular remodeling by affecting cytokine secretion. However, the mechanism of the effects of hypoxic conditions in the placenta on trophoblasts remains unknown. We used gene knockdown on macrophages, and drug treatment on trophoblasts, and cultured them under hypoxic and normoxic conditions. The cells were then subjected to wound-healing assays, Transwell cell invasion experiments, quantitative real-time reverse transcription Polymerase Chain Reaction (PCR), western blotting, and immunofluorescence. The polarization of macrophages in each group, the migration and invasion ability of trophoblasts, and changes to the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway were detected. Hypoxic conditions induce M2 polarization of macrophages. The conditioned medium from macrophages under hypoxic conditions increased the migration and invasion of trophoblasts and enhanced the levels of phosphorylated (p)-PI3K and p-AKT in trophoblasts. After C-C motif chemokine ligand 5 knockdown in macrophages, the ability of conditioned medium from macrophages cultured under hypoxic conditions to promote the migration and invasion of trophoblasts was weakened significantly. The use of PI3K/AKT signaling pathway agonists could reverse the attenuation effect caused by C-C motif chemokine ligand 5 knockdown.

Summary sentence

Hypoxic conditions induced the polarization of M2 macrophages and their secretion of C-C motif chemokine ligand 5, thereby activating the PI3K/AKT signaling pathway in trophoblasts, ultimately leading to enhanced trophoblast migration and invasion.

Graphical Abstract

The hypoxic environment of the placenta in early pregnancy might play a role in the mechanism of macrophage polarization, in which hypoxic conditions can induce macrophages to polarize to the M2 type and stimulate the secretion of CCL5. This will further promote the migration and invasion of trophoblast cells. At the same time, CCL5 derived from macrophages is involved in the activation of the PI3K/AKT signaling pathway in trophoblasts under hypoxic conditions.