C57BL/6 and lpr mice were euthanized by exposure to CO 2 followed by cervical dislocation. The femur and tibia of both hind legs were isolated and freed of all soft tissue, and then the ends of both bones were removed. The femur and tibia were placed proximal end down in a 0.6 mL Eppendorf tube, which had been punctured at its lower tip with an 18-gauge needle and placed inside a 1.5 mL Eppendorf tube. The tubes were spun at 2000 X g for 30 seconds and neutrophils were isolated as previously described [23] . After isolation, neutrophils were labeled with calcein-AM (5 μg/ml; Molecular Probes, Eugene, OR) for 30 minutes at 37°C, washed two times in PBS and resuspended at a concentration of 1 x 10 6 /mL.