The lipid mediators of inflammation include platelet-activating factor (PAF) and several classes of metabolites of arachidonic acid derived from the cyclooxygenase (prostaglandins and thrombox-anes) and lipoxygenase pathways, such as leukotrienes (LTs) and lipox-ins. Of these lipid mediators, LTs and PAF are potent agonists of leukocytes and endothelial cells and show important proinflammatory properties. Several elegant studies have recently provided strong support for a role of one of these mediators, LTB4, in animal models of rheumatoid arthritis [1-3]. Interestingly, LTB4, LTC4/LTD4 and PAF are concomitantly generated at the blood–endothelium interface when neutrophils bind to activated endothelial cells at inflammatory sites in vivo and are exposed to a chemoattractant. The hypothesis explored in this project is that the three classes of lipid mediators collectively and synergistically act to promote and facilitate neutrophil extravasation and accumulation at inflammatory sites. The hypothesis also implicates that, given the high level of redundancy in the abilities of the three lipid mediators to activate neutrophils and endothelial cells, the blockade of one of these mediators can only have a modest and incomplete inhibitory effect on neutrophil extravasation.
The objective of these studies is therefore to compare the role of each individual mediator and the collective effect of the three mediators in regulating neutrophil trafficking. Another objective is to assess whether or not the combined use of antagonists to the three classes of mediators could result in a stronger inhibitory effect on neutrophil trafficking.
The studies are performed using potent and selective LTB4, LTD4 and PAF antagonists in both in vitro and in vivo models. The separate and collective role of the three classes of lipid mediators will be investigated in the migration of human neutrophils through a monolayer of human endothelial cells grown on a gel of extracellular matrix proteins. The studies also involve the use of models of dermal inflammation in rats and rabbits and the mouse air pouch model.
The results show that, in the various models tested, neutrophil migration elicited by proinflammatory agents such as IL-1, tumor necrosis factor alpha and lipopolysaccharide is generally decreased by LTB4 or PAF antagonists, and that the combination of both antagonists results in a greater inhibitory effect, supporting the concept that several lipid mediators generated locally (at the blood–endothelium interface and/or at inflammatory sites) regulate neutrophil trafficking. Additional experiments involving antagonists to the three classes of lipid mediators, as well as LTB4 and PAF receptor knockout mice, are in progress.
Griffiths RJ, Pettipher ER, Koch K, Farrell CA, Breslow R, Conklyn MJ, Smith MA, Hackman BC, Wimberly DJ, Milici AJ, et al.: Leukotriene B4 plays a critical role in the progression of colla- gen-induced arthritis.
Griffiths RJ, Smith MA, Roach ML, Stock JL, Stam EJ, Milici AJ, Scampoli DN, Eskra JD, Byrum RS, Koller BH, Mcneish JD: Collagen-induced arthritis is reduced in 5-lipoxygenase-activating protein-deficient mice.