Tirzepatide preserves hematopoietic stem and progenitor cycling while remodeling inflammatory monocytes in obese mice

Obesity expands myeloid progenitors, myelopoiesis and increases the production of monocytes. While weight loss (WL) alleviates aspects of this inflammatory dysregulation, it is not known whether GLP-1 receptor agonists or other traditional modalities of WL differentially modify hematopoietic stem/progenitor cells (HSPCs), hematopoiesis, or inflammatory cell production. To test this, we compared the hematopoietic compartment in lean, obese and weight-reduced mice from tirzepatide treatment and caloric restriction (CR) implemented to match the body weight in both groups. At equal WL, we found CR induced multilineage cytopenias, whereas tirzepatide preserved blood lineages while specifically reducing classical Ly6C ^hi CCR2 ⁺ monocytes. To define the mechanisms underlying these changes we performed single-cell mRNA sequencing of bone marrow HSPCs and mature mononuclear blood cells. CR-HSPCs suppressed gene sets associated with nutrient sensing, proliferation and oxidative phosphorylation (OXPHOS) and exhibited lower inferred cell cycle activity, whereas tirzepatide-HSPCs attenuated these changes. Unlike CR, we found that across progressively differentiated cells from HSPCs to mature blood monocytes, tirzepatide increasingly suppressed OXPHOS and simultaneously shifted the maturation spectrum away from classical monocytes. Following six weeks of tirzepatide withdrawal and weight regain, Ly6C ^hi CCR2 ⁺ monocytes rebounded to levels seen in obese mice. These findings suggest that tirzepatide uncouples WL from the broad hematopoietic suppression seen in CR by preserving progenitor activity but selectively remodeling inflammatory/classical monocytes. We demonstrate that WL modality differentially impacts hematopoietic adaptation and provide evidence that classical monocytes are an effector cell through which tirzepatide may dampen obesity-associated inflammation.