Role of mast cell-derived mediators for leukocyte/endothelium-interactions and microvascular mechanisms in inflammation and in anaphylaxis.

Sammanfattning: The overall objective of this thesis was to study the roles of mast cell-derived mediators for leukocyte/endothelium interactions and microvascular mechanisms in inflammation and in anaphylaxis, using mast cell-deficient Ws/Ws rats and their wild-type +/+ littermates. The efflux of endogenous histamine and edema formation evoked by subplantar injection of compound 48/80 in rat hindpaws was dose-dependent in +/+ rats, and was essentially lacking in Ws/Ws rats. These findings suggest a critical role of histamine as trigger of edema formation in vivo. Spontaneous leukocyte rolling was studied by intravital microscopy of venules in rat mesentery and cremaster muscle. Rolling and rolling velocity in mast cell-deficient animals exactly matched that seen in mild-type animals. Challenge with 48/80 markedly increased leukocyte adhesion and emigration in +/+ animals, whereas in animals lacking mast cells adherent and extravascular leukocytes remained very few and did not increase after challenge with 48/80. The data strongly suggest that the presence or activation of mast cells has no bearing on spontaneous leukocyte rolling. Experimental surgical skin flaps in mast cell-deficient rats accumulated little histamine and leukocytes and exhibited little tissue necrosis, whereas flaps in wild-type rats were associated with marked accumulation of histamine and leukocytes and showed substantial tissue loss. Antihistamine treatment in the wild-type rats halved leukocyte accumulation and increased flap survival. The data suggest a role for histamine in skin wound healing. Ovalbumin-sensitized rats, both Ws/Ws and +/+ but not sham sensitized animals, exhibited hypotensive responses after intravenous antigen challenge. There was no mast cell activation in the mesentery of senzitised +/+ rats after intravenous antigen challenge. Hypotension induced by intravenous injection of dextran or compound 48/80 occurred in +/+ rats, but not in Ws/Ws rats, and was inhibited by pre-treatment with histamine receptor antagonists. The data indicate that the hypotensive response induced by antigen- mediated anaphylaxis is independent of mast cell activation. Plasma exudation and influx of leukocytes in air pouches after immune complex reaction were markedly lower in Ws/Ws rats than in +/+ rats. Leukocyte rolling, adhesion and transmigration in the mesentery were all substantially increased 6 h after immune complex reaction in +/+ rats bin not in Ws/Ws rats. Leukocytes adhered under flow conditions in markedly larger numbers to endothelial cell monolayers stimulated with peritoneal fluid (collected 6 h after immune complex reaction) from +/+ rats than from Ws/Ws rats. The data indicate a dominant role of mast cell-derived mediators in the leukocyte response to an IgG immune complex reaction in rat skin and mesentery. In summary, the data presented in this thesis show distinctive roles of mast cells in inflammation and anaphylaxis. As studied in the rat, mast cells are of critical importance in local tissue response to immune complex activation and surgical trauma, and are likely important in wound healing. Systemic anaphylaxis to allergen is not critically dependent on mast cells. Mast cell-deficient rats should be most useful in further delineating physiological and pathophysiological roles of mast cells.