The generation, release, and disposal of neutrophils in response to inflammation remains poorly understood. A number of neutrophil-dependent lung disorders, including the Adult Respiratory Distress Syndrome (ARDS) are characterized by release of immature neutrophils from marrow into the circulation. Using murine systems in vivo, a novel embryonic stem cell-derived in vitro system, and murine and human cells in vitro, we propose to test interrelated hypotheses that explore fundamental mechanisms of leukocytosis and lung sequestration. 1. During inflammation, systemic expression of G-CSF is induced by TNF and other cytoses. G-CSF not only increases the number of progenitors, but also retards apoptosis of developing neutrophils, amplifying the resultant generation of neutrophils. 2. G-CSF mobilizes immature neutrophils through modification of three systems - signaling of CXCR4 by SDF-1, the interaction between sialoadhesin and its ligand, and homotypic interactions between ALCAM on both stroma and hematopoietic cells. 3. Immature neutrophils mobilized by G-CSF are larger and demonstrate increased resistance to deformation, leading to their localization in the lung. We believe the results of these experiments will provide new information on fundamental aspects of normal homeostasis as well as insights into mechanisms of neutrophil-dependent inflammatory states.
