Lipopolysaccharide induces apoptosis of cytotrophoblasts by activating an innate immune reaction in vitro

BACKGROUND: Enhanced apoptosis of cytotrophoblasts in early pregnancy is associated with high risk of intrauterine growth retardation and preeclampsia, which are two common pregnant complications. Its etiological factors remain unclear. Cytotrophoblasts share some traits with innate immune cells and may show response to lipopolysaccharide. This study was conducted to demonstrate whether lipopolysaccharide has apoptosis-inducing effects on cytotrophoblast and the role of innate immune reaction in this process. METHODS: Cytotrophoblasts were isolated from early pregnant villous tissues and cultured with serum-free medium. Subsequently, cytotrophoblasts were treated with lipopolysaccharide at the concentrations of 0 (control), 25, 50, 100 and 200 ng/ml for 24 hours. Apoptosis of cytotrophoblasts was determined by light microscopy, Hoechst 33258 DNA staining with a fluorescent microscope, transmission electron microscope and annexin V-fluorescein isothiocyanate-conjugated/propidium iodide (PI) staining with flow cytometry. Then expression of caspase-3 was detected by Western blot. Confocal immunofluorescence technique was used to detect tumor necrosis factor alpha expression in cytotrophoblasts. The levels of tumor necrosis factor alpha in the culture medium were detected by enzyme-linked immunosorbent assay. RESULTS: Under light, fluorescence microscope and transmission electron microscope, characteristic alternations of apoptosis in cytotrophoblasts were observed after lipopolysaccharide treatment. Flow cytometry results showed that lipopolysaccharide significantly increased apoptosis indexes of cytotrophoblasts. Significant statistical differences were found in the above groups (P = 0.01). The mean relative densities of bands corresponding to caspase-3 were significantly increased in groups treated with lipopolysaccharide, as compared with the normal control (P < 0.001). Tumor necrosis factor a expression was found to increase in cytotrophoblasts by confocal immunofluorescence technique and in culture medium by enzyme-linked immunosorbent assay after lipopolysaccharide treatment. A positive correlation was found between tumor necrosis factor a expression and apoptosis indexes of cytotrophoblasts (r = 0.747, P < 0.001). CONCLUSION: Apoptosis of cytotrophoblasts could be induced by lipopolysaccharide, in which innate immune reaction is the important mechanism.