POSTER 25Specific changes in myeloid cells during labor in human choriodecidua

The timely onset of labor and birth is a critical determinant of perinatal outcomes. Both preterm and post-term birth are associated with an increased risk of adverse pregnancy events. A better understanding of the mechanisms involved in the onset of labor may enable to predict or even control delivery. Chorion, the outermost leaf of fetal membranes, is fused with decidua, providing a large surface of maternal-fetal interaction. Choriodecidua presents the largest gene expression variability in the context of labor. Gene expression profiles showed activation of multiple pathways of inflammation with labor, which relies a lot on myeloid immune cells. However, these cells are difficult to dissociate from tissues and previous studies until now were mostly done on lymphoid cells. To examine cell state changes, single-nucleus RNA-sequencing approach was performed as it achieves comparable gene detection to single-cell RNA-seq, with reduction of cell dissociation and stress response biases. To provide additional information at the protein level, a multiplex immunofluorescence analysis was also performed. Our aim is to gain knowledge in the myeloid cell populations present in the choriodecidua at term pregnancy before and after the onset of labor. Choriodecidua samples were obtained at term pregnancy from non-laboring women (Term not in labor, TNL) delivered by caesarean section and from women who delivered spontaneously after labor (Term in labor, TIL). Briefly, nuclei were isolated from frozen tissues using dounce homogenization in lysis buffer. Further encapsulation and sequencing were performed using the 10X Genomics technology. Data computational analyses were processed using 10X Cell Ranger software and Seurat v.4 pipelines. The multiplex immunofluorescence analysis was performed with the UltiMapper™ I/O APC kit, which contains antibody-conjugates against CD11c, CD20, CD68/CD163, and MHC Class II. DAPI was used for nuclear counterstain. Multiplex image acquisition was achieved using a Leica SP8X STED FLIM microscope. The images were analysed using ImageJ software. Transcriptomic profiles of 57,204 nuclei from 7 TNL and 7 TIL samples were obtained. We identified the expected cell populations, ranked from the most to the less abundant: trophoblasts, stromal cells, endothelial cells-lymphatic and vascular-, myeloid cells and epithelial endometrial cells. While myeloid cell numbers did not appear to differ between the TNL and TIL groups, differential gene expression analysis showed in these immune cells a decrease of the expression of genes involved in antigen presentation and an over-representation of inflammatory genes after labor. Plasticity being a characteristic of myeloid cells, we analysed them in more details. We found up to 12 different myeloid subclusters with 2 of fetal origin. In accordance with these results, multiplex immunofluorescence analysis showed a global decrease in the proportion of antigen presenting cells after labor. Our preliminary results suggest i) a broad degree of cellular heterogeneity in myeloid cells and ii) a switch with labor toward inflammation and loss of antigen presenting cells. Our unbiased snRNA-seq and multiplex immunofluorescence approaches hold the promise of a detailed molecular and spatial characterization of the distinct myeloid cell types and their changes with labor