STUDIES ON THE MECHANISMS INVOLVED IN THE INFLAMMATORY RESPONSE IN A REVERSED PASSIVE ARTHUS REACTION IN GUINEA-PIG SKIN - CONTRIBUTION OF NEUTROPHILS AND ENDOGENOUS MEDIATORS

1 Mediators of inflammation can increase vascular permeability in at l east two different ways: by acting directly on endothelial cells or, i ndirectly, through an incompletely understood mechanism, dependent on circulating neutrophils. Neutrophil-dependent oedema formation has bee n described in the skin of rabbits, rats, hamsters, mice and man. In c ontrast, we presented evidence in a previous study that local oedema f ormation induced by i,d. injection of chemoattractants in guinea-pig s kin was neutrophil-independent. In the present study, we sought eviden ce of neutrophil-dependent oedema formation in immune-complex-mediated vasculitis, the reversed passive Arthus (RPA) reaction, in guinea-pig skin. We also investigated whether haemorrhage in the RPA reaction wa s neutrophil-dependent (as it is in other species) and the role of end ogenous mediators of inflammation (prostaglandins, nitric oxide, hista mine, PAF and leukotrienes) in contributing to the local inflammatory response. 2 In the RPA reaction, most oedema formation occurred over t he first 60 min whereas In-111-neutrophil accumulation was still incre asing from 60 to 240 min. The different kinetics of these two events s uggested that they may be dissociated. 3 Oedema formation was partiall y inhibited by a long-acting PAF antagonist (UK-74,505) and an H-1 his tamine receptor antagonist (mepyramine) but not by a 5-lipoxygenase in hibitor (ZM 230487). A nitric oxide synthesis inhibitor (N-G-nitro-L-a rginine methyl ester, L-NAME) suppressed oedema formation by 68% where as a cyclo-oxygenase inhibitor suppressed oedema by 27%. 4 In-111-neut rophil accumulation in the RPA reaction was partially suppressed by UK -74,505. In contrast, ZM 230487 was without effect at doses which abro gated arachidonic acid-induced In-111-neutrophil accumulation. 5 The a nti-CD18 monoclonal antibody, (mAb) 6.5E F(ab')(2), effectively inhibi ted In-111-neutrophil accumulation induced by PAF, zymosan-activated p lasma (ZAP) and in the RPA reaction. However, oedema formation measure d in the same sites was not altered. In contrast, oedema formation in the RPA reaction was partially suppressed by 6.5E whole mAb which was 2.5 times more potent than 6.5E F(ab')(2) at inhibiting guinea-pig neu trophil adhesion to protein-coated plastic. Haemorrhage induced by PAF and in the RPA reaction was significantly inhibited by 6.5E F(ab')(2) pretreatment. 6 We conclude that in the RPA reaction in guinea-pig sk in, oedema formation is partially neutrophil-dependent as assessed by using an anti-CD18 mAb, whereas ZAP-induced oedema formation is neutro phil-independent. Haemorrhage was also dependent on neutrophil accumul ation. In addition, our studies support a role for PAF in mediating bo th oedema formation and In-111-neutrophil accumulation in the RPA reac tion. Endogenous release of histamine also appears to be important in mediating oedema formation suggesting that mast cells play a critical role in increases of vascular permeability in inflammatory reactions i n guinea-pig skin. Moreover, our results confirm previous findings whi ch suggest a dominant role for nitric oxide in maintaining cutaneous b lood how in the guinea-pig.