Loss of STAT1 and Nitric Oxide Signaling Compromises the Development of the Stress Erythropoiesis Niche

Introduction/Rationale: Steady-state erythropoiesis generates erythrocytes in the bone marrow to replace senescent cells cleared by the spleen. Inflammation from infection or tissue damage disrupts this balance by skewing hematopoiesis toward myelopoiesis. To compensate, stress erythropoiesis maintains red cell output until inflammation resolves. Unlike steady-state erythropoiesis, stress erythropoiesis depends on TNFα and IFNγ, which recruit short-term hematopoietic stem cells (ST-HSCs) and monocytes to the spleen, where they generate stress erythroid progenitors (SEPs) and form erythroblastic islands (EBIs). IFNγ–STAT1 signaling induces CCL2 expression and CCR2⁺ monocyte recruitment to initiate this niche, while nitric oxide (NO) signaling later resolves inflammation and supports erythroid maturation.

Methods: Inflammatory anemia was induced using heat-killed Brucella abortus. Spleens from wild-type, Stat1⁻/⁻, and Nos2⁻/⁻ mice were analyzed for splenomegaly, Ly6C⁺ monocyte recruitment, SEP proliferation, and BFU-E formation. Flow cytometry and erythrophagocytosis assays characterized monocyte differentiation and SEP maturation. Sickle-cell mouse spleens were also examined for Ly6C⁺ recruitment and BFU-Es.

Results: Stat1⁻/⁻ mice showed reduced CCL2 expression, impaired monocyte recruitment, and decreased SEP expansion, indicating STAT1 initiates the inflammatory niche for stress erythropoiesis. Nos2⁻/⁻ mice exhibited greater splenomegaly, delayed anemia recovery, fewer BFU-Es, and disrupted erythroid maturation from excess Ly6C⁺ monocytes. Sickle cell mice showed similar defects, including increased splenomegaly and reduced BFU-Es.

Conclusion: STAT1 and NOS2 act sequentially to regulate stress erythropoiesis. STAT1-driven IFNγ signaling initiates monocyte recruitment, while NOS2-derived NO promotes macrophage differentiation, SEP proliferation, and anemia recovery. Low NO in Nos2⁻/⁻ and sickle-cell mice disrupts erythroid regeneration, identifying STAT1–NOS2 signaling as a therapeutic target in inflammatory anemia.