Burn Injury-Induced G-CSF Secretion Depletes Spi-C+ Erythroblastic Island Macrophages and Impairs Medullary Erythropoiesis.

The erythroblastic island (EBI) functions as a niche where erythroblastic island macrophages (EBIMφ) are positioned within rings of erythroblasts, providing support and signals that orchestrate efficient erythropoiesis. Herein, we postulated burn injury impacts the EBI, given the nearly universal presence of anemia and inflammation in burn patients, and a divergent myeloid transcriptional signature we observed in murine bone marrow following burn injury, where G-CSF secretion increased neutrophil and monocyte transcripts but reduced EBIMφ transcripts. We utilized flow cytometry to investigate EBIMφ after imaging flow cytometry confirmed the heme induced transcription factor Spi-C as a robust marker of EBIMφ in Spicigfp/igfp mice. Two populations referred to as Spic+ macrophages and Spic+ Gr1 low monocytes possessed cell intrinsic GFP, with Spic+ macrophages distinguished by higher levels of GFP, F4/80, autofluorescence, and negligible CD115 staining compared to Gr1 low monocytes. Further, at least 50% of the Spic+ macrophages stained for CD163 while staining was negligible on Gr1 low monocytes. Since CD163 distinguished the populations and macrophage remnants could convolute flow cytometry analysis, total CD163 protein was quantified in marrow aspirates to corroborate results. Application of Spicigfp/igfp mice in studies revealed a G-CSF dependent reduction of Spic+ macrophages in post burn marrow that coincided with a loss of erythroid cells and that recombinant G-CSF alone was sufficient to reduce Spic+ macrophages in the marrow. These results provide the first evidence that burn injuries impact the EBI niche through G-CSF dependent depletion of EBIMφ and support the use of Spicigfp/igfp mice in investigation of EBIMφ.