Proangiogenic immature myeloid cells populate the human placenta and their presence correlates with placental and birthweight

Objective: To determine whether proangiogenic immature myeloid cells are present in human placentas. Study Design: Biopsies were obtained from 61 placentas of term pregnancies. Percentage of CD45⁺CD33 ⁺LIN2^-HLADR^- immature myeloid cells of total CD45⁺ hematopoietic cells was determined by flow cytometry. Location of immature myeloid cells in the placenta was identified using confocal microscopy. The proangiogenic potential of immature myeloid cells was analyzed by endothelial tube formation. Results: Immature myeloid cells comprise ∼25% of human placental CD45⁺ hematopoietic cells and infiltrate placentas in proximity of blood vessels. The percentage of immature myeloid cells correlated positively with placental weight (r² = 0.108, P =.01) and birthweight (r² = 0.087, P =.02). Endothelial tube formation was increased in the presence of immature myeloid cells as compared with the presence of CD45⁺LIN2⁺ control cells. Conclusion: Human placentas are populated by immature myeloid cells in the proximity of blood vessels. Consistent with their involvement in angiogenesis, immature myeloid cells accelerated endothelial tube formation. The presence of immature myeloid cells in pathologic pregnancies warrants further studies.