Stromal STAT5-Mediated Trophic Activity Regulates Hematopoietic Niche Factors

Signal transducer and activator of transcription 5 (STAT5a and STAT5b) are intrinsically critical for normal hematopoiesis but are also expressed in stromal cells. Here, STAT5ab knockout (KO) was generated with a variety of bone marrow hematopoietic and stromal Cre transgenic mouse strains. Vav1-Cre/+STAT5abfl/fl, the positive control for loss of multipotent hematopoietic function, surprisingly dysregulated niche factor mRNA expression, and deleted STAT5ab in CD45neg cells. Single-cell transcriptome analysis of bone marrow from Vav1-Cre/+ wild-type or Vav1-Cre/+STAT5abfl/fl mice showed hematopoietic stem cell (HSC) myeloid commitment priming. Nes+ cells were detected in both CD45neg and CD45+ clusters and deletion of STAT5ab with Nes-Cre caused hematopoietic repopulating defects. To follow up on these promiscuous Cre promoter deletions in CD45neg and CD45+ bone marrow cell populations, more stroma-specific Cre strains were generated and demonstrated a reduction in multipotent hematopoietic progenitors. Functional support for niche-supporting activity was assessed using STAT5-deficient mesenchymal stem cells (MSCs). With Lepr-Cre/+STAT5abfl/fl, niche factor mRNAs were downregulated with validation of reduced IGF-1 and CXCL12 proteins. Furthermore, advanced computational analyses revealed a key role for STAT5ab/Cish balance with Cish strongly co-expressed in MSCs and HSCs primed for differentiation. Therefore, STAT5ab-associated gene regulation supports the bone marrow microenvironment. Graphical Abstract STAT5 function in hematopoiesis-supporting stromal cells. STAT5ab is expressed in both hematopoietic and stromal cells within a heterogeneous bone marrow niche that includes bone, endothelial cells, pericytes, and stromal cells. Previous studies by our lab and others showed important roles in hematopoietic stem cell and multipotent progenitor activity. However, STAT5ab expression in mesenchymal stromal progenitor cells and osteolineage progenitors (OLC) has not been explored. We show here that secreted niche factor production (arrows of different colors) supports hematopoiesis through a STAT5ab-dependent mechanism.