HLA-DR is absent in primitive macrophages through epigenetic silencing of master regulator CIITA

The earliest macrophages are generated during embryonic development from erythro-myeloid progenitors (EMPs) via primitive haematopoiesis. This process is poorly understood in humans and in the mouse is thought to be spatially restricted to the yolk sac. Human fetal placental macrophages, Hofbauer cells (HBC), arise during the primitive haematopoietic wave at ~18 days post conception. Here we identify a population of placental erythro-myeloid progenitors (PEMPs) in the early human placenta that give rise to HBC. PEMP are fetal CD34+CD43+ progenitors found exclusively at early gestational timepoints. Transcriptomic analyses reveal that PEMP have conserved features of primitive yolk sac EMPs, including the lack of HLF expression. Using in vitro single-cell culture experiments we show that PEMP generate HBC-like cells that completely lack HLA-DR expression  and demonstrate that this is a conserved feature of all macrophages generated by primitive haematopoiesis in humans. Finally, we demonstrate that the class II transactivator, CTIIA, the master regulator HLA-DR expression, is epigenetically silenced during primitive haematopoiesis, leading to HLA-DRneg macrophages. These findings demonstrate that HBC are derived locally from PEMP and establish the human placenta as an additional site of primitive haematopoiesis. In this study we characterise CD34+CD43+ progenitors in human placental villi. We find that these placental CD34+CD43+ progenitors have conserved features with yolk sac erythro-myeloid progenitors (EMP), including a lack of HLF expression, a defining feature of EMP. Hence, we termed placental CD34+CD43+ progenitors, as placental EMP (PEMP). We subsequently show that PEMP differentiate into primitive HLA-DRneg HBC-like macrophages in vitro. Finally, wWe demonstrate that a conserved feature of human primitive macrophages across embryonic tissues, as well as extra-embryonic tissues, is a complete of lack of expression of HLA Class II molecules. Analysis of methylation status of the class II transactivator, CTIIA, reveals that CTIIA promoter is epigenetically silenced in primitive macrophages during the first trimester of pregnancy. With timeAs pregnancy progresses, primitive macrophages begin to express HLA-DR. Our data suggests that there is an erosion of epigenetic silencing throughout gestation, underpinnings the observed transition of HBC from an HLA-DRneg to HLA-DRpos state. >>>Submitter states that raw data for this study are not publicly available due to patient privacy concerns. Raw data will be submitted for restricted access at EGA.<<<