PS1233 HES1 AND HES4 HAVE SIMILAR AND DIVERSE CAPACITIES TO MEDIATE HUMAN NOTCH‐DEPENDENT HEMATOPOIETIC LINEAGE DECISIONS

Abstract

Background:Multipotent hematopoietic stem cells (HSCs) originate in the bone marrow and give rise to all the different cell types in the blood. This process is orchestrated by interactions between cell‐intrinsic mechanisms and the environmental niche that results in the activation of particular molecular networks that drive specific hematopoietic lineage differentiation. One important example is the development of T cells in the thymus. Following colonization by thymus seeding progenitors, Delta‐like‐ligand‐4 (DLL4) that is expressed by thymic epithelial cells triggers the Notch1 receptor that is expressed on these multipotent hematopoietic progenitor cells (HPCs). Activation of the Notch signaling pathway results in the induction of T‐lineage differentiation through the upregulation of T cell‐specific genes that also prevent differentiation towards other hematopoietic lineages.Aims:In both mouse and human, Notch1 activation is the main initial driver to induce T cell development in hematopoietic stem cells. However, although well described in mice, the role of Notch1 target genes during human hematopoietic developmental choices is still rather unclear. Previous studies in human have shown that TCF1, GATA3 and BCL11B are necessary for the initial stages of T cell differentiation by repressing other hematopoietic lineages, but other transcription factors may also function during these developmental choices. Here, we focused on the function of the Notch1 target genes HES1 and HES4.Methods:We cocultured CB CD34+ Lin− hematopoietic precursor cells with enforced HES1 and HES4 expression on OP9‐DLL4 and MS‐5 stromal cells in conditions that support T‐, B‐, NK‐ and myeloid‐lineage differentiation. In addition, we performed RNA sequencing to identify HES1‐ and HES4‐induced molecular changes during human early hematopoiesis.Results:Our data illustrate that HES1 mainly acts as a repressor of differentiation by maintaining a quiescent stem cell signature in CD34+ HPCs. In contrast, while HES4 can inhibit the NK and myeloid potential of HPCs, it does not affect B cell development. Furthermore, even though HES4 does not alter the Notch ligand requirements to induce T cell development, it promotes early T cell differentiation when Notch1 signaling is present.Summary/Conclusion:Overall, we show that even though HES1 and HES4 are both induced through Notch1 activation during normal human T cell development, they display both unique and similar features during human hematopoiesis.