Intraperitoneal G-CSF Stimulation Achieves Human-like Neutrophil Levels in NSG Mice Without Inducing Systemic Inflammation.

Neutrophils are central executors of innate immunity. Yet murine models are inherently limited by low baseline neutrophil counts. NSG mice are among the most widely used models for xenotransplantation and studies on the humanized immune system. Although G-CSF is known to stimulate granulopoiesis, the dose- and schedule-dependent effects of intraperitoneal G-CSF administration have not been systematically characterized in this immunodeficient background. Male NSG mice received intraperitoneal G-CSF according to one of five regimens (n = 6 per group): group 0 served as the saline control, group 1 received a single dose of 250 µg/kg G-CSF administered at 48 h; group 2 received a single dose of 250 µg/kg G-CSF administered at 24 h; group 3 received three doses of 250 µg/kg administered G-CSF at 0 h, 24 h, and 48 h and group 4 received a single dose of 500 µg/kg G-CSF administered at 48 h. All animals were sacrificed at 72 h. Circulating neutrophils were then quantified by flow cytometry, bone marrow neutrophil proportions by panoptic smear analysis, and splenic neutrophil abundance by Ly6G immunofluorescence. Systemic neutrophil activation was assessed via plasma neutrophil elastase (NE) activity and cell-free DNA (cfDNA) levels. Repeated G-CSF administration (Group 3) induced an approximately 13-fold expansion of circulating neutrophils, approaching the human physiological range, with significant increases also observed in bone marrow and a trend towards increased neutrophil abundance in the spleen. A single dose of 250 µg/kg administered at 24 h (group 2), produced significant neutrophil expansion in peripheral blood and bone marrow but not in the spleen, while all other single-dose regimens failed to induce significant expansion in any compartment. NE activity and cfDNA concentrations and a selected cytokine panel remained unaltered across all groups. This systematic comparison establishes repeated intraperitoneal G-CSF administration as a reproducible strategy to achieve human-like neutrophil levels in NSG mice without inducing systemic inflammation. This provides a validated protocol with direct utility in translational models of neutrophil-dependent diseases.