Hofbauer cells are transcriptionally and epigenomically unique programmable tissue macrophages in human placentas [ATAC-seq]

Tissue-resident macrophage Hofbauer cells (HB cells) play a central role in maintaining placental tissue integrity and homeostasis. HB cells are responsible for fetal immune functions and contribute to angiogenesis, regeneration, and metabolism of chorionic villi. All of these roles have a major impact on the course and outcome of pregnancy through the performance of placental functions. In this study, we sought answers to the following questions: what are the regulatory mechanisms that ensure the specific gene expression of these cells? Are there subgroups within the population of HB cells that were previously thought to be homogeneous? Are HB cells programmable? To answer these questions, Hofbauer cells were isolated from placental villous tissue using enzymatic digestion, gradient-based centrifugation, and both negative and positive immunopurification. ATAC-seq, bulk RNA-seq, in vitro stimulation studies, re-analysis of bulk and single-cell RNA sequencing, and CyTOF were used to assess chromatin accessibility, gene expression regulation, programmability, and heterogeneity of human HB cells. Using these approaches, we found that compared to other adult and fetal macrophages, HB cells have a unique transcriptional regulatory profile that is regulated by the transcription factor-coding NR4A2, NR4A3, GR, and RFX genes. In addition, HB cells showed unique gene expression changes upon stimulation with IL-4 and the PPAR agonist Rosiglitazone. Based on the re-analysis of single-cell RNA-seq data from the first-trimester fetal-maternal interface and CyTOF analysis of our isolates, heterogeneous HB cell subpopulations were identified based on either gene or marker protein expression.