Interaction of neutrophils with the endothelium is important both in the recruitment of neutrophils to the inflamed site and in the modulation of several neutrophil functions. The localized accumulation of functional neutrophils is pivotal in the host's defense against infection and the orderly elimination of these neutrophils is pivotal in the host's defense against infection and the orderly elimination of these neutrophils is important in down-regulation of the inflammatory response. Blood neutrophils are unique among leukocytes in that their circulating half- life is only between 4-10 hours after which they undergo spontaneous apoptosis (""""""""passive cell death"""""""") which is associated with the loss of several neutrophil functions and their clearance in the spleen. However, at sites of tissue inflammation, neutrophils have an increased life span and enhanced functional responses compared to circulating neutrophils. Apoptosis of these neutrophils and their engulfment by tissue macrophages is associated with the resolution of inflammation. We have identified two components which regulate the fate of neutrophils at sites of inflammation: cytokine activated endothelium secrete factors which significantly delay the """"""""passive cell death"""""""" of neutrophils in vitro and in vivo, while phagocytosis of complement opsonized particles, mediated by the beta2 integrin, Mac-1, triggers rapid apoptosis, a process which we refer to as """"""""phagocytosis induced cell death"""""""" (PICD). Our working hypothesis is that the endothelium, which constitutes the interface between blood neutrophils and the tissue actively delays the passive cell death of neutrophils, thereby contributing to the increased pool of functional neutrophils at inflamed sites, and PICD represents an effective mechanism for eliminating neutrophils that have phagocytosed and therefore reached the end of their useful life span. We propose that defects in either component could lead to tissue damage and/or compromised host defense. Dysfunctional up-regulation of the endothelial anti-apoptotic factor(s) could result in increased accumulation of PMNs in the extravascular tissue and prolong the inflammatory response. On the other hand, a reduction in the endothelial derived anti-apoptotic factor(s) could lead to premature apoptosis of neutrophils in the tissue, thereby resulting in compromised host defense. We also postulate that defects in the effector signaling pathways leading to """"""""phagocytosis induced cell death"""""""", may induce disease by abrogating apoptosis of neutrophils that have phagocytosed bacteria and cellular debris. Our studies are closely related to the theme of this Program Project. Our overall objective is to investigate the molecular mechanisms involved in communication between endothelial cells and neutrophils to modulate the survival of neutrophils at sites of inflammation.
The specific aims of this proposal are to: i) molecularly define the endothelial-derived """"""""anti- apoptotic"""""""" factor(s) and determine how the expression of these factor(s) are regulated by endothelial cells; (ii) investigate the intracellular signaling cascades in neutrophils that may transmit the endothelial mediated delay of apoptosis, or Mac-1 triggered apoptosis, and determine how these two mechanisms are related; iii) critically test the biological relevance of the regulation of neutrophil apoptosis in in vivo models of inflammation.
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