Thromboxane A2 induces leukotriene B4 synthesis that in turn mediates neutrophil diapedesis via CD 18 activation

Previous studies have indicated that thromboxane (Tx) and leukotriene (LT) B4 act synergistically to induce neutrophil (PMN) adhesion in the microvasculature. This study explores the ability of Tx to induce LTB4 synthesis, which then leads to activation of PMN and endothelial adhesion receptors. Tx-mimic (U46619, 1 microgram/ml) was administered into abraded skin chambers placed on the backs of rabbits (n = 6). After 3 hr LTB4 was synthesized in the blister fluid at 385 pg/ml, a value higher than levels in saline-treated blisters, 10 pg/ml (P less than 0.05). The LTB4 generation following Tx-mimic was correlated (P less than 0.05, r = 0.70) with neutrophil diapedesis. These averaged 645 PMN/mm3, values higher than saline values of 20 PMN/mm3 (P less than 0.05). Intravenous (iv) treatment of other rabbits (n = 4) with the lipoxygenase inhibitor diethylcarbamazine at 60 mg/kg, followed by 40 mg/kg/hr, prevented Tx-mimic-induced LTB4 synthesis (10 pg/ml) and diapedesis (19 PMN/mm3) (both P less than 0.05). Intravenous treatment of yet other rabbits (n = 4) with the anti-CD 18 monoclonal antibody R 15.7 at 1 mg/kg abolished Tx-induced diapedesis (3 PMN/mm3) (P less than 0.05). In contrast, local administration of 3 ng of the protein synthesis inhibitor actinomycin D, to prevent expression of endothelial adhesion proteins, limited TNF- but not Tx-induced diapedesis. The data indicate that Tx-induced diapedesis is mediated by the generation of LTB4 and the activation of neutrophil CD 18 but not endothelial adhesion proteins.