Mediator synergism in mast celldependent inflammation : role of histamine for leukocyte recruitment

Abstract: H. THORLACIUS The aim of the present thesis was to study functional interactions between histamine and chemotacticmediators, and the role of histamine for leukocyte recruitment in mast cell-dependent inflammation. Leukocyterolling, adhesion and emigration. and vascular permeability were studied in vivo by the use of intravitalmicroscopy of the rat mesentery and hamster cheek pouch, and of a complementary histological approach inthe undisturbed rat mesentery. Topical challenge with the mast cell secretagogue compound 48/80 increased leukocyte rolling fractionand decreased rolling velocity in postcapillary venules of the exposed rat mesentery prepared for intravitalmicroscopy. These effects were inhibited by pretreatment with a monoclonal antibody directed against P-selectin, but not by the combined treatment with H1- and H2-receptor antagonists (mepyramine and cimetidine).Topical application of histamine did not increase leukocyte rolling fraction or reduce rolling velocity oncespontaneous leukocyte rolling had developed. In the undisturbed rat mesentery lacking spontaneous leukocyterolling, it was found by the use of a histological approach that the accumulation of polymorphonuclearleukocytes (PMNL) induced by compound 48/80 was markedly inhibited by combined H1- and H2-receptorblockade. These findings indicate that mast cell activation induces P-selectin-dependent leukocyte rolling viathe release of histamine in conjunction with other mediator(s). Moreover, compound 48/80 stimulation causedan increase in leukocyte adhesion which was also inhibited by the monoclonal antibody against P-selectin,illustrating a critical relationship between leukocyte rolling and adhesion. In the undisturbed rat mesentery, the H1-receptor antagonist (diphenhydramine) inhibited histamine-induced leukocyte rolling substantially, whereas two H2-receptor antagonists (cimetidine and ranitidine) wereinactive in this respect. However, when cimetidine was added to the diphenhydramine treatment, the histamineresponse was further reduced. Moreover, in contrast to an H3-receptor agonist, stimulation with either an H1-receptor agonist or two different H2-receptor agonists was sufficient to provoke significant leukocyte rolling.These findings show that histamine-induced leukocyte rolling involves both H1- and H2-receptors, although thecontribution of the H1-receptor appears to the be most important. The duration of the histamine-inducedPMNL rolling was approximately 2 h. Furthermore, inhibition of NO synthesis did not affect the PMNLresponse to histamine stimulation. Pretreatment with the glucocorticoid dexamethasone almost abolished the histamine-induced leukocyterolling in the rat mesentery, possibly via inhibition of the expression and/or function of PMNL P-selectionligand(s). Topical administration of histamine caused a four fold potentiation of chemoattractant-induced leukocyteadhesion in the exposed rat mesentery. On the other hand, the histamine challenge (which did not increase thefraction of rolling leukocvtes) enhanced rolling leukocyte flux in a strictly blood flow-dependent way, caused aclear-cut increase in venular permeability and prolonged tne adhesion of leukocytes provoked bychemoattractants. These effects (in addition to induction of P-selectin-dependent rolling) of histamine maycontribute to the potentiating effect of histamine on chemoattractant-induced leukocyte adhesion.Furthermore, it was found that threshold doses of histamine could markedly potentiate chemoattractant-induced leukocyte adhesion in the hamster cheek pouch. Moreover, in immunized hamsters, treatment with aH1-receptor antagonist (mepyramine) in a concentration high enough to completely reverse the histamine-induced venular plasma leakage greatly reduced allergic leukocyte accumulation. In conclusion, these findings show that histamine plays an important role for leukocyte recruitment byinteracting with chemotactic factors through a combination of distinct actions in the microcirculation, and thatcomplete inhibition of histamine-induced microvascular actions may be necessary to reduce leukocyteaccumulation in mast cell-dependent inflammation.Key Words: Chemoattractants, dexamethasone, histamine, inflammation, intravital microscopy, mast cells,leukocyte adhesion, leukocyte rolling, microcirculation, mast cells, nitric oxide, leukotrienes, P-selectin. ISBN 91-628-2105-9