Chromatin accessibility shapes development-specific lineage plasticity in hematopoiesis.

abstractCell differentiation is governed by dynamic changes in chromatin accessibility, and its dysregulation underlies multiple disease states. Before birth, the development of the hematopoietic system constitutes a period of broad differentiation potential, with certain immune cells arising exclusively during ontogeny. Although age is known to affect lineage bias, the underlying molecular differences driving lineage preference in fetal and adult human hematopoietic stem and progenitor cells (HSPCs) remain unclear. Through single-cell cultures of hematopoietic stem cells (HSCs), we observed that fetal cells frequently generate mixed-lineage colonies, whereas adult HSCs are biased toward myeloid output. To investigate how these lineage preferences were encoded at the chromatin level, we performed single-cell assay for transposase-accessible chromatin using sequencing on first-trimester HSPCs. Although adult HSCs showed enrichment of lineage-specific transcription factor motifs, fetal cells lacked such enrichment, consistent with their broader differentiation potential. We additionally uncovered a development-specific plasticity in fetal lymphoid progenitors, manifested as a hybrid lymphomyeloid chromatin program not present in adult progenitors. Additionally, the motif and putative regulatory elements for PAX5, a master regulator of B-cell development, showed markedly reduced accessibility in fetal cells, supporting a more plastic and less restricted lymphoid state. This enhanced embryonic lineage plasticity may underlie the prenatal susceptibility to mutational drivers of acute lymphoblastic leukemia.