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.

• Type: Other
• Archiver: European Genome-Phenome Archive (EGA)