CXCR4 and acute respiratory distress syndrome: CXCR4 was the most prevalent fibrocyte chemokine receptor in the bone marrow, blood, and lung in the animal model, and in the blood in human subjects with ARDS (Figs. 1d, 2c, and Supplemental Fig. 1), and we noted a time-dependent progressive gradient of the CXCR4 ligand, CXCL12, between the three compartments during the course of lung injury in the animal model (Fig. 1e-f).