Examination of the inflammatory nature of different placental syncytiotrophoblast microparticles (STBM) preparations on human monocytes

Background: Healthy human pregnancies are accompanied by a mild systemic maternal inflammatory response, which includes activation of peripheral blood monocytes. This generalized inflammation is exaggerated in preeclampsia, a placenta-dependent disorder specific to human pregnancies. It has been proposed that placental syncytiotrophoblast membrane microparticles (STBM), which are released into the peripheral blood, might contribute to the maternal response in normal pregnancy and preeclampsia. Aim: The aim of this work was to assess the inflammatory properties of STBM generated in vitro from human term placentas by four different approaches which should mimic physiologic or patho-physiologic conditions, and their mode of action on human monocytes in vitro. Methods: STBM were prepared by: (1) placental explant cultures of villous tissue incubated at 20% O2/5% CO2 (air) at 37°C for 72 hours (eS20); (2) perfusion of the maternal side of a placental cotyledon (pS); (3) placental explant cultures of villous tissue incubated at 3% O2/5% CO2 (hypoxia) at 37°C for 72 hours (eS3); and (4) mechanical dissection of villous tissue (mS). In all approaches, STBM were isolated by serial high-speed centrifugation. STBM were co-incubated with either the human monocytic cell line Mono Mac 6 or human peripheral blood monocytes. In some cases, agents which inhibit cellular functions or signalling pathways were used. Analysis of viability, phenotype and function were performed by real-time PCR, flow cytometry, ELISA and fluorescence microscopy. Results: Viability of Mono Mac 6 cells was not impaired following treatment with STBM. However, STBM only induced a marginal response in Mono Mac 6. None of the STBM population affected the viability of primary monocytes. eS3 and mS decreased the expression of CD54 on peripheral blood monocytes, but did not induce the secretion of IL-1β, IL-6 and IL-8. However, pS and eS20 up-regulated the cell surface expression of CD54 on primary monocytes and stimulated the secretion of IL-1β, IL-6 and IL-8 in a dose- and time-dependent manner. Interestingly, eS20 derived from normal and preeclamptic placentas stimulated monocyte activation to similar degrees. eS20 induced the transcription of several NF-κB responsive genes, including IL-6 and IL-8, and the secretion of IL-6 and IL-8 was reduced upon treatment with NF-κB inhibitors. eS20 was located at the monocytic cell surface and a phagocytosis inhibitor did not reverse the eS20 induced production of IL-6 and IL-8. Primary monocytes and the non-responding Mono Mac 6 cells expressed toll like receptors (TLRs) differently. Pre-incubation of primary monocytes with an inhibitor of intracellular TLR signalling reduced the inflammatory response triggered by eS20. Conclusions: STBM populations evoked neither a proinflammatory nor an anti-inflammatory phenotype in Mono Mac 6 cells. However, STBM prepared at conditions which are believed to mimic the physiologic situation in human pregnancy (eS20 and pS) triggered the secretion of IL-1β, IL-6 and IL-8 and up-regulated the expression of the adhesion molecule CD54 on peripheral blood monocytes. These findings indicate that Mono Mac 6 cells are not the appropriate cells to study the interaction between monocytes and STBM. STBM prepared at non-physiologic (mS) and hypoxic (eS3) conditions, which are thought to mimic the patho-physiologic situation in preeclampsia, did not induce an inflammatory response in peripheral blood monocytes. In addition, the observation that eS20 derived from normal as well as from preeclamptic placentas triggered an equally strong and dose-dependent inflammatory response in primary monocytes, suggests that there are no or only minor qualitative differences between the microparticles. These findings presume that the overt maternal inflammation associated with preeclampsia may be due to the higher concentration of circulating STBM, rather than to a qualitative difference between microparticles released from normal and patho-physiologic placentas. The results also suggest that the inflammatory reaction in monocytes may be initiated by the attachment of STBM to the cell surface and the activation of TLRs. In turn, NF-κB mediates transcription of proinflammatory genes, including IL-1β, IL-6, IL-8 and CD54. The altered expression of CD54 may modulate the adhesion properties of monocytes, whereas the secretion of IL-1β, IL-6 and IL-8 could recruit further immune cells, leading to generalized inflammation.