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Neutrophil maturation and activation determine anatomic site of clearance from circulation

The long term disposition of circulating neutrophils and the site of disappearance from circulation remain unclear. We investigated neutrophil localization in mice using 111In-labeled murine peripheral blood neutrophils, mature bone marrow neutrophils, and peritoneal exudate neutrophils to track in vivo localization of these different cell populations. Infused peripheral neutrophils were found to localize equally between liver and marrow sites by 4 h (31.2 ± 1.9 vs. 31.9 ± 1.8%), whereas exudate neutrophils predominantly localized to liver (42.0 ± 1.1%) and marrow-derived neutrophils to the marrow (65.9 ± 6.6%) where they were found to localize predominantly in the hematopoietic cords. Stimulation of marrow neutrophils before infusion caused a shift in localization from marrow to liver, and subsequent induction of an inflammatory site after infusion and marrow sequestration led to remobilization of infused marrow neutrophils but not of peripheral neutrophils. These results indicate that the marrow participates in removing neutrophils from circulation, with evidence  supporting both storage and perhaps disposal functions. Furthermore, models for circulating  neutrophil homeostasis should consider that the site of retention is governed by the maturation and activation states of the cell.