Increased progenitor cell mobilization in SOCS3^ΔSB/ΔSB mice in response to G-CSF stimulation in vivo. SOCS3^+/+ and SOCS3^ΔSB/ΔSB mice (4 per group) were injected intraperitoneally twice daily with either 2.5 μg/kg G-CSF or endotoxin-free saline/BSA vehicle for 5 days. Mice were killed on day 6. Results are means (± SD). (A) Peripheral blood neutrophil count. (B) Spleen weight. (C) Peripheral blood colony-forming cells. Peripheral blood (10 μL for G-CSF–treated, 30 μL for vehicle-treated) from all mice was cultured in triplicate in the presence of SCF/IL-3 for 7 days, fixed and stained, and the number of colonies was counted at × 40 magnification. Histologic examination showed increased neutrophil infiltration into the following: (D,E) Parasternal muscles. The arrow indicates neutrophils between the muscle fibers. (E,F) Lung. Note the thickening of the alveolar walls, due to increased numbers of neutrophils, in SOCS3^ΔSB/ΔSB mice. (H,I) Intrathecal space. A transverse section through the spinal column at the level of the thoracic vertebrae reveals increased numbers of intrathecal granulocytes in the SOCS3^ΔSB/ΔSB mice. BM indicates bone marrow; V, vertebral column; and SC, spinal cord.