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

• The aim of the present thesis was to study functional interactions between histamine and chemotactic mediators, and the role of histamine for leukocyte recruitment in mast cell-dependent inflammation. Leukocyte rolling, adhesion and emigration. and vascular permeability were studied in vivo by the use of intravital microscopy of the rat mesentery and hamster cheek pouch, and of a complementary histological approach in the undisturbed rat mesentery. Topical challenge with the mast cell secretagogue compound 48/80 increased leukocyte rolling fraction and decreased rolling velocity in post capillary venules of the exposed rat mesentery prepared for intravital microscopy. 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 once spontaneous leukocyte rolling had developed. In the undisturbed rat mesentery lacking spontaneous leukocyte rolling, it was found by the use of a histological approach that the accumulation of polymorphonuclear leukocytes (PMNL) induced by compound 48/80 was markedly inhibited by combined H1- and H2-receptor blockade. These findings indicate that mast cell activation induces P-selectin-dependent leukocyte rolling via the release of histamine in conjunction with other mediator(s). Moreover, compound 48/80 stimulation caused an 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) were inactive in this respect. However, when cimetidine was added to the diphenhydramine treatment, the histamine response 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 the contribution of the H1-receptor appears to the be most important. The duration of the histamine-induced PMNL rolling was approximately 2 h. Furthermore, inhibition of NO synthesis did not affect the PMNL response to histamine stimulation. Pretreatment with the glucocorticoid dexamethasone almost abolished the histamine-induced leukocyte rolling in the rat mesentery, possibly via inhibition of the expression and/or function of PMNL P-selection ligand(s). Topical administration of histamine caused a four fold potentiation of chemoattractant-induced leukocyte adhesion in the exposed rat mesentery. On the other hand, the histamine challenge (which did not increase the fraction of rolling leukocytes) enhanced rolling leukocyte flux in a strictly blood flow-dependent way, caused a clear-cut increase in venular permeability and prolonged the adhesion of leukocytes provoked by chemoattractants. These effects (in addition to induction of P-selectin-dependent rolling) of histamine may contribute 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 by interacting with chemotactic factors through a combination of distinct actions in the microcirculation, and that complete inhibition of histamine-induced microvascular actions may be necessary to reduce leukocyte accumulation in mast cell-dependent inflammation.

Nyckelord

Chemoattractants, dexamethasone, histamine, inflammation, intravital microscopy, mast cells,leukocyte adhesion, leukocyte rolling, microcirculation, mast cells, nitric oxide, leukotrienes, P-selectin.

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vet (ämneskategori)

dok (ämneskategori)