Objective: To determine the mechanisms that lead to the decrease in bone marrow production of neutrophils during burn sepsis. Summary Background Data: Impaired bone marrow granulopoiesis during burn sepsis often results in neutropenia despite elevated circulating levels of granulocyte colony- stimulating factor (G-CSF). To date, neither the specific stages of neutrophil maturation involved in the bone marrow suppression nor the mechanisms for the impairment have been determined. Methods: Peripheral blood absolute neutrophil count and G-CSF levels were determined in mice 3 days after randomization to control, burn alone, or burn plus a topical inoculation of Pseudomonas aeruginosa (1000 colony-forming units). Bone marrow aspirates were analyzed for their neutrophil differentiation patterns by Gr-1 antigen expression and their G-CSF receptor status. Histologic analysis of liver, lung, spleen, and wound site was performed. Results: In burn sepsis, absolute neutrophil count was reduced whereas plasma G-CSF levels were elevated, and myeloid differentiation was significantly shifted toward the immature mitotic myeloid cells. Bone marrow G-CSF receptor mRNA levels and G-CSF-stimulated proliferation were substantially decreased in burn sepsis. Histologic analysis revealed no significant neutrophil infiltration into the tissues. Conclusions: In thermal injury with superimposed sepsis, neutropenia and myeloid maturation arrest, despite the elevated levels of GCSF, correlate with the reduction in bone marrow G-CSF receptor expression. These observations may provide a potential mechanism for neutropenia in sepsis.
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