Adult haematopoietic stem cells (HSCs) are responsible for the lifelong production of blood and immune cells and must respond to extracellular cues including cytokines.

Adult haematopoietic stem cells (HSCs) are responsible for the lifelong production of blood and immune cells and must respond to extracellular cues including cytokines. The JAK/STAT pathway is a highly conserved pathway, activated by many cytokines, in which tyrosine-phosphorylated STATs (pSTATs) function as transcription factors. STAT5 is a pivotal downstream mediator of several cytokines known to regulate haematopoiesis but its function in the HSC compartment remains poorly understood. Here we show that STAT5-deficient HSCs exhibited an unusual phenotype - reduced multi-lineage repopulation and self-renewal combined with reduced cell cycle entry and increased differentiation. This reflected not only loss of a canonical pSTAT5 transcriptional program but also loss of distinct functions mediated by tyrosine-unphosphorylated STAT5 (uSTAT5). Consistent with this concept expression of an unphosphorylatable STAT5B mutant constrained HSC differentiation, promoted HSC maintenance and upregulated transcriptional programs associated with quiescence and stemness. Moreover, treatment with a JAK1/2 inhibitor (ruxolitinib) increased the uSTAT5:pSTAT5 ratio, constrained HSC differentiation and proliferation and promoted HSC maintenance, thus phenocopying uSTAT5B overexpression. Our results therefore reveal a previously unrecognized interplay between pSTAT5 and uSTAT5 in the control of HSC function. In addition the demonstration that JAK inhibition promotes HSC maintenance has implications for gene therapy using HSCs and may contribute to the failure of JAK inhibitors to eradicate myeloproliferative neoplasms. To evaluate the long-term functional capacity of uSTAT5B overexpressing HSCs, WT CD45.2 ESLAM HSCs were sorted and transduced with lentivirus containing STAT5B-YF, STAT5B-WT and EV in SCF and IL-11 cultures. Five days after infection, cells were sorted for viability (DAPI-, Thermo Fisher) and green fluorescent protein (GFP) expression and processed according to the manufacturer’s protocol for 10x Chromium (10x Genomics, Pleasanton, CA) experiments .