Bone Marrow Adipocyte Secreted Resistin Is a Positive Regulator of Long-Term Hematopoietic Stem Cell Engraftment and Myelopoiesis

Myeloablative hematopoietic stem cell transplant (HSCT) can cure cancer, bone marrow failure, hemoglobinopathies or immunodeficiencies. The aim of HSC transplant is to restore the endogenous hematopoiesis by long-term, multi-lineage hematopoiesis derived from a healthy donor or genetically corrected patient’s HSC. In the bone marrow (BM), HSCs are responsive to the BM microenvironment (ME) to engraft and regenerate hematopoiesis. HSC and ME-dependent retinoic acid signaling is crucial to regenerate hematopoiesis. Bone marrow adipocytes (BMA) are a crucial cellular component of the BM ME that can occupy up to 70% of the BM volume. BMA and their precursor leptin-receptor expressing stromal cells express adiponectin. Differentiation and activity of adiponectin-expressing cells are largely dependent on RXR signaling. To understand how RXRs regulate the activity of BM ME adiponectin-expressing (BMA) cells and its effect on hematopoiesis, we generated tamoxifen (TAM)-inducible, BMA-specific RXR α/β deficient (AdipoQCre-ERTi2-RXRα/β^Δ/Δ or AdipoQ-RXRα/β^Δ/Δ) mice. We found ~70% reduction in BMA cells in AdipoQ-RXRα/β^Δ/Δ BM. Competitive hematopoietic repopulation after transplantation of AdipoQ-RXRα/β^Δ/Δ BM generated comparable levels of hematopoietic chimerism. However, the HSC, multipotential progenitor and myelopoietic recovery after 5-FU treatment or after HSC transplantation of myeloablated animals were significantly impaired. Competitive transplantation of purified HSC (Lin^-Sca1^-cKit^-CD48^-CD150⁺) confirmed the results of non-purified BM transplantation indicating that BM HSC from AdipoQ-RXRα/β^Δ/Δ are significantly less efficient in hematopoietic regeneration. To understand the effects of RXR deficient BMA ME on HSC/Ps, we compared the single-cell RNAseq transcriptome of Lin^-Sca1⁺c-Kit⁺ (LSK) BM cells from WT and AdipoQ-RXRα/β^Δ/Δ mice. Pathway enrichment analysis suggested a significant reduction in mitochondrial oxidative phosphorylation and expression of electron transfer chain (ETC) complex genes and reduced expression of TNFa/NF-kB signaling pathway genes. Metabolomic analysis and adipokine profiling from the extracellular BM fluid indicated a significant decrease in the levels of triglycerides and a ~80% reduction in the levels of Resistin, a secreted pro-inflammatory cytokine, in BMA-RXR deficient BM. In vivo Resistin neutralization hampered the normal blood cell production, bone marrow cellularity and myeloid cell production. To test efficacy of Resistin towards HCS maintenance during ex-vivo HSC culture, we treated isolated WT LSK with Resistin. Short-term Resistin treatment induced transient NF-kB activation in HSC, increased the levels of myeloid progenitors and the capacity of treated HSC to support long-term hematopoiesis as assessed in serial, competitive repopulation assays. Together, our data indicate that BMA-RXR dependent Resistin is a positive regulator of long-term hematopoiesis under stress conditions and can be used during ex vivo manipulation of HSCs destined to transplantation and myeloid cell reconstitution.