Chromatin Accessibility Shapes Developmental-Specific Lineage Plasticity in Hematopoiesis.

Cell differentiation is governed by dynamic changes in chromatin accessibility, and its dysregulation underlies multiple disease states. Prior to birth, development of the hematopoietic system constitutes a period of broad differentiation potential, with certain immune cells arising exclusively during ontogeny. While 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 towards myeloid output. To investigate how these lineage preferences were encoded at the chromatin level, we performed single-cell ATAC-sequencing on first-trimester HSPCs. While 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 developmental-specific plasticity in fetal lymphoid progenitors, manifested as a hybrid lympho-myeloid 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.