Neutrophils are the primary effector cells of the innate immune response. These highly motile cells rapidly move to sites of infection and injury where they eradicate pathogens and/or remove and restructure damaged tissues. The recruitment and activity of these cells must be tightly regulated because they have the potential to induce severe tissue damage with their potent effector functions. In an in vitro model of sterile injury, where no pathogens are present, we have identified a potential non-formylated peptide released from necrotic cells that attracts neutrophils to a site of injury via the chemotaxis-inducing G-protein coupled formyl peptide receptor 1. This as-of-yet unidentified ligand was not sensitive to enzymatic deformylation. In competitive chemotaxis assays, this signal from disrupted cells acts as an “end-target” chemoattractant, causing neutrophils to ignore competing gradients of the endogenous chemokine interleukin-8, and move precisely to a site of injury.
Additionally, we have investigated the role of two lipid phosphatases, SHIP1 and PTEN, in neutrophil behavior. These two phosphatases antagonize phosphatidylinositol-3 kinase signaling, which is one of the main signaling pathways involved in controlling neutrophil chemotaxis. Using a targetable loxP/Cre recombinase gene excision system in the mouse, we have, for the first time, simultaneously depleted both these phosphatases in neutrophils. In contrast to mice in which neutrophils lack either one or the other, and have little outwardly apparent abnormalities, the resulting double deficient animals had a severe phenotype. These animals exhibited hematopoietic disturbances characterized by an increased proliferative capacity in populations of immature neutrophils, with an enhanced recruitment of neutrophils and, in certain cases other cells such macrophages, to solid organs such as the liver, spleen, brain, lungs. They displayed pronounced hepatomegaly and splenomegaly, occasional neurological disturbances, organ dysfunction, and typically expired at less than four weeks of age. Analysis of neutrophil behavior in very young animals revealed decreased chemotactic function in vivo in the liver. In vitro analyses revealed chemotactic impairments towards certain cues, coupled with increased numbers of cells responding to single gradients of chemoattractants. These studies provide critical insight into the role of SHIP1 and PTEN in neutrophil function, development, and homeostasis.