The normal alveolus (left side) and polymorphonuclear neutrophil (PMN) migration into alveolar spaces during a murine model of ventilator-induced lung injury (right side). Shown are the two critical barriers, the endothelium and epithelium. PMNs adhere to the capillary endothelium and migrate through the interstitium and the epithelium into the fluid-filled alveolar airspace. Alveolar macrophages secrete cytokines and interleukins that activate local neutrophils. In humans there are two major CXC receptors with overlapping ligands (CXCR1/IL-8; CXCR2/IL-8, GRO peptides, ENA-78). Mice have only CXCR2 (CXCR2/KC, MIP-2). The ligands are produced by alveolar macrophages and other cells in the lungs.