In this proposal the investigator seeks to delineate mechanisms by which neutrophils can damage tissues in a variety of clinical syndromes such as vasculitis and the Adult Respiratory Distress Syndrome (ARDS). During the past two years the focus on the laboratory has been upon neutrophil-mediators of this injury including reactive oxygen species and lysosomal proteases. Neutrophil oxidants and proteases can work hand in hand to potentiate neutrophil mediated tissue damage by the ability of oxidants to inactivate potent anti-proteases and thereby allow extensive proteolytic injury to go unchecked. The toxicity of neutrophil derived oxidants can be enhanced in the presence of iron which can catalyze the production of hydroxyl radical through the Haber Weiss reaction. It has been demonstrated in this laboratory that lactoferrin can act as an important catalyst of granulocyte- induced target cell damage through enhancing effector..target cell adhesion and potentiating oxidative damage to targets. The effects of lactoferrin on neutrophil-mediated endothelial injury in vitro and in vivo will be examined. Lactoferrin releases its iron from protein as free ionic Fe2+ through a reduction of lactoferrin's iron by neutrophil-derived superoxide. This free Fe2+ can then act as a hydroxyl radical-catalyst, enhance cellular lipid peroxidation and, in vivo, increase vascular damage. The second specific aim focuses upon platelet activating factor (PAF) to "prime" neutrophils and provoke enhanced granulocyte mediated tissue injury. Platelet activating factor in miniscule quantities greatly enhances granulocyte oxidative responses to subsequent stimulation. Since PAF is derived from both neutrophils and endothelial cells, it is speculated that PAF may elicit a powerful paracrine and autocrine amplification in neutrophil-related microvascular injury states such as ARDS. Miniscule quantities of PAF derived from both cell types can potentiate otherwise trivial activators of PMNs to make them lethal to endothelium. These hypotheses will be tested by "priming" neutrophils with PAF and examining in vitro and in vivo endothelial damage.