The effect of conditioned medium derived from human placental multipotent mesenchymal stromal cells on neutrophils: possible implications for placental infection

The role of human placental multipotent mesenchymal stromal cells (hPMSCs) in placental inflammation is unknown. We hypothesize that hPMSCs are involved in the early phases of placental infection. hPMSCs were isolated from term placentas and neutrophils from peripheral blood. The expression of toll-like receptors (TLRs) and cytokines by hPMSCs was determined by RT-PCR, flow cytometry and enzyme-linked immunosorbent assay. The effect of conditioned medium of hPMSCs with or without lipopolysaccharide (LPS) pretreatment on neutrophil functions: migration, apoptosis and production of reactive oxygen species (ROS) was assessed by flow cytometry and western blot. hPMSCs expressed TLR1, TLR3, TLR4, TLR6, TLR7 and TLR9. LPS stimulation increased the expression of TLR4 and the production of IL-6 and IL-8 by hPMSCs. Neutrophils exhibited chemotaxis to hPMSC-conditioned medium, which was inhibited by IL-8 depletion. Neutrophil CD11b activation was promoted by hPMSC-conditioned medium, which was further enhanced in media from hPMSCs pretreated with LPS. hPMSC-conditioned medium reduced neutrophil ROS production. Neutrophil phagocytosis was increased by LPS alone but not by hPMSC-conditioned medium with or without LPS stimulation. hPMSC-conditioned medium induced STAT3 activation in neutrophils, which was inhibited by neutralizing antibody to IL-6. hPMSC-conditioned medium rescued neutrophils from apoptosis, but this effect was significantly reduced in conditioned medium of hPMSCs with LPS pretreatment. Depletion of IL-6 from the conditioned medium further inhibited the anti-apoptotic effect on neutrophils. Our results demonstrate that hPMSCs can interact with peripheral blood neutrophils in response to inflammatory signals of the placenta. Cytokines produced by hPMSCs can induce neutrophil chemotaxis and reduce neutrophil apoptosis.