Exocytosed Mast Cell Granules Research Articles

Engulfment of mast cell secretory granules on skin inflammation boosts dendritic cell migration and priming efficiency

Mast cells (MCs) are best known as key effector cells of allergic reactions, but they also play an important role in host defense against pathogens. Despite increasing evidence for a critical effect of MCs on adaptive immunity, the underlying mechanisms are poorly understood. Here we monitored MC intercellular communication with dendritic cells (DCs), MC activation, and degranulation and tracked the fate of exocytosed mast cell granules (MCGs) during skin inflammation. Using a strategy to stain intracellular MCGs invivo, we tracked the MCG fate after skin inflammation-induced MC degranulation. Furthermore, exogenous MCGs were applied to MC-deficient mice by means of intradermal injection. MCG effects on DC functionality and adaptive immune responses invivo were assessed by combining intravital multiphoton microscopy with flow cytometry and functional assays. We demonstrate that dermal DCs engulf the intact granules exocytosed by MCs on skin inflammation. Subsequently, the engulfed MCGs are actively shuttled to skin-draining lymph nodes and finally degraded inside DCs within the lymphoid tissue. Most importantly, MCG uptake promotes DC maturation and migration to skin-draining lymph nodes, partially through MC-derived TNF, and boosts their T-cell priming efficiency. Surprisingly, exogenous MCGs alone are sufficient to induce a prominent DC activation and T-cell response. Our study highlights a unique feature of peripheral MCs to affect lymphoid tissue-borne adaptive immunity over distance by modifying DC functionality through delivery of granule-stored mediators.

Dermal dendrocytes FXIIIA+ phagocytizing extruded mast cell granules in drug‐induced acute urticaria

Few authors have been attempting between mast cells and dermal dendrocytes interactions on urticaria. To describe the extruded mast cell granules and dermal dendrocytes in drug-induced acute urticaria. Seven patients with drug-induced acute urticaria were enrolled in the study. We token skin biopsies of urticaria lesion and perilesional skin. The 14 fragments collected were processed to immunogold electron microscopy using single stains to tryptase and FXIIIa, besides double immunogold labeling with both. Some sections demonstrated mast cells in degranulation process, both in anaphylactic and piecemeal degranulation types. After double immunogold staining, 10 nm (FXIIIa) and 15 nm (Tryptase) gold particles were present together over the granules in mast cells indicating that tryptase and FXIIIa are each localized within the granules of these cells. Interestingly, we found a strong evidence of than the exocytosed mast cell granules contents both FXIIIa and tryptase immunolabeled are phagocytized by dermal dendrocytes. The current observations provide morphological evidence that the exocytosis-phagocytosis mechanisms of mast cell granules represents one pathophysiological example of mast cells-dermal dendrocytes interactions in urticaria.

Activated mast cells increase the level of endothelin-1 mRNA in cocultured endothelial cells and degrade the secreted Peptide.

Subendothelial mast cells have been implicated in the pathogenesis of allergic inflammation, in atherosclerosis, and in the regulation of vascular tone. Because endothelin-1 (ET-1) is an important regulator of vascular tone and has also been implicated in the pathogenesis of atherosclerosis, we studied the role of mast cells in the metabolism of endothelial cell-derived ET-1. In mast cell-endothelial cell cocultures, activation of the mast cells with ensuing degranulation was accompanied by the increased expression of ET-1 mRNA in the endothelial cells, yet the immunoreactive ET-1 protein in the coculture medium disappeared almost completely during the 24-hour coculture. Activation of the mast cells with the ensuing degranulation resulted in proteolytic degradation of ET-1 by the 2 neutral proteases, chymase and carboxypeptidase A, of the exocytosed mast cell granules. With synthetic ET-1 and purified mast cell granule enzymes, efficient degradation of ET-1 by chymase and carboxypeptidase A was verified. These in vitro results imply a novel role for mast cell-derived neutral proteases in ET-1 metabolism and suggest that activated subendothelial mast cells are important local regulators of ET-1 metabolism.

Chymase bound to heparin is resistant to its natural inhibitors and capable of proteolyzing high density lipoproteins in aortic intimal fluid

Degranulated mast cells are present in the human arterial intima. After degranulation of rat serosal mast cells, the secreted neutral serine protease chymase remains bound to the heparin proteoglycan matrix of the exocytosed granules, forming granule remnants. Addition of granule remnants to human aortic intimal fluid results in proteolysis of the apoAI present in the intimal fluid, which contains physiological inhibitors of chymase. To study the physiological mechanism of this protection of granule remnant-bound chymase against its inhibitors, we performed experiments using HDL 3 as substrate. Chymase, when bound to the heparin proteoglycans of granule remnants, but not when released from them, resisted inhibition by the mammalian protease inhibitors α 1-antitrypsin, α 2-antichymotrypsin, α 2-macroglobulin, and eglin C. Importantly, the heparin proteoglycan-bound chymase, but not unbound chymase, degraded its inhibitor (α 1-antitrypsin) in the presence of its substrate (HDL 3). Finally, binding to heparin proteoglycans of a physiological inhibitor of chymase (mucus protease inhibitor (MPI)) or of another substrate of chymase (LDL) did not inhibit the degradation of HDL 3 by granule remnant-bound chymase. This study demonstrates that binding of chymase to the heparin proteoglycan chains of the exocytosed mast cell granules allows the protease to remain active and degrade HDL 3 in the presence of its physiological inhibitors and in the presence of high concentrations of LDL, such as are found in the interstitial fluid of the arterial intima.

Mast cells in human fatty streaks and atheromas: implications for intimal lipid accumulation.

Recent immunohistochemical observations on atherosclerotic lesions in human aortas and coronary arteries have revealed that these lesions contain mast cells. Mast cells, better known for their actions in allergic reactions of skin and mucosal surfaces, are filled with cytoplasmic secretory granules that contain histamine, heparin and neutral proteases, notably chymase. When activated these cells degranulate, and in this way may influence lipoprotein metabolism in their immediate environment. Indeed, animal studies have suggested that exocytosed mast cell granules may carry LDL into peritoneal macrophages, and so induce their conversion into foam cells. With immunoelectron microscopic techniques, evidence was obtained suggesting that such a 'granule carrier pathway' may operate in vivo in the human arterial intima.

Chymase in exocytosed rat mast cell granules effectively proteolyzes apolipoprotein AI-containing lipoproteins, so reducing the cholesterol efflux-inducing ability of serum and aortic intimal fluid.

Degranulated mast cells are present in human fatty streaks. Chymase in granules released from degranulated rat serosal mast cells, i.e., in granule remnants, proteolyzes human high density lipoprotein3 (HDL3), and so reduces its ability to induce cholesterol efflux from macrophage foam cells in vitro. In this study we found that remnant chymase, by proteolyzing human serum and human aortic intimal fluid, prevents these two physiologic fluids from effectively inducing cholesterol efflux from cultured macrophage foam cells. Inhibition was strongest when remnants were added to apolipoprotein AI (apoAI)-containing lipoproteins; the remnants had no effect on the weaker efflux produced by apoAI-deficient serum. Western blot analysis showed that granule remnants degrade apoAI in serum and in internal fluid. When released from remnants, chymase lost its ability to proteolyze HDL3 in the presence of serum. Thus, remnant chymase (but not isolated chymase) was able to resist the natural protease inhibitors present in serum and in intimal fluid. The results imply participation of exocytosed mast cell granules in foam cell formation in atherogenesis.

Fusion of proteolyzed low-density lipoprotein in the fluid phase: a novel mechanism generating atherogenic lipoprotein particles.

During atherogenesis, lipid droplets appear in the extracellular space of the arterial intima. We previously observed generation of lipid droplets on the surface of exocytosed mast cell granules when granule neutral proteases degraded the granule-bound LDL particles and the particles became unstable and fused [Kovanen, P.T., & Kokkonen, J.O. (1991) J. Biol. Chem. 266, 4430-4436]. We have now extended our studies to the fluid phase and examined the effects of several proteases (trypsin, alpha-chymotrypsin, Pronase, plasmin, kallikrein, and thrombin) all known for their ability to cleave the apolipoprotein B-100 component (apoB-100) of LDL. The fused LDL particles were separated from unfused particles by gel filtration or by density gradient ultracentrifugation. Proteolytic degradation of LDL with trypsin, alpha-chymotrypsin, or Pronase led to fragmentation of apoB-100 and release of the fragments from the LDL particles and triggered particle fusion. In contrast, proteolytic degradation of LDL with plasmin, kallikrein, or thrombin, which also led to fragmentation of apoB-100 but not to release of fragments, did not trigger particle fusion. With advancing degradation of apoB-100, particles having progressively lower densities and larger sizes were generated. Thus, after incubation for 24 h with alpha-chymotrypsin (apoB-100:alpha-chymotrypsin mass ratio 10:1) 40% of the apoB-100 was degraded and about 30% of the LDL particles had fused and reached diameters of up to 70 nm and densities ranging from 1.020 to < 1.005 g/mL. When the proteolyzed LDL particles, both unfused and fused, were incubated with macrophages, only those particles that had undergone fusion were ingested and converted into intracellular cholesteryl ester droplets. Thus proteolysis of LDL with release of apoB-100 fragments renders the particles sufficiently unstable to fuse and thus to become liable to ingestion by macrophages. Since the fused LDL particles resemble the extracellular lipid droplets in the atherosclerotic arterial intima and generate foam cells in vitro, these findings support the idea that proteolytic fusion of LDL is an atherogenic process.

IgE-Dependent Generation of Foam Cells: An Immune Mechanism Involving Degranulation of Sensitized Mast Cells With Resultant Uptake of LDL by Macrophages

Because a role has been suggested for IgE in cardiovascular diseases and for mast cells in cholesterol accumulation within the macrophages of atherosclerotic lesions, we examined mast cell-macrophage interactions in vitro by using rats with high serum levels of IgE antibodies. The rats were immunized with an antigen (ovalbumin) and adjuvant (Bordetella pertussis vaccine) to provoke synthesis of IgE and to sensitize their mast cells, ie, to allow the IgE to bind to the high-affinity IgE receptors on the mast cell surfaces. On addition of the ovalbumin to suspensions of mast cells isolated from the peritoneal cavity of the immunized rats, the mast cells responded by exocytosing their heparin-proteoglycan--containing granules. When IgE-bearing peritoneal mast cells were cocultured with peritoneal macrophages (also from the immunized rats) in a medium enriched in LDL, addition of ovalbumin to the incubation medium triggered a dose-dependent release of granules and a dose-dependent increase in the rate of LDL uptake by the macrophages. In contrast, ovalbumin had no effect on LDL uptake if the cultures contained only macrophages or if the mast cells and macrophages were from nonimmunized rats. Thus, the sequence of events leading to enhanced uptake of LDL by macrophages depended wholly on IgE-dependent degranulation of the sensitized mast cells. With the aid of gold-labeled LDL we demonstrated that the exocytosed mast cell granules had bound LDL particles and carried them into the macrophages, with subsequent formation of foam cells. The current series of experiments delineates a novel immunologic mechanism for the formation of macrophage foam cells and assigns a potentially atherogenic role to mast cell-bound IgE antibodies.

Chymase of exocytosed mast cell granules resists natural protease inhibitors and degrades HDL in the presence of serum and aortic intimal fluid

Chymase of exocytosed mast cell granules resists natural protease inhibitors and degrades HDL in the presence of serum and aortic intimal fluid

Proteolysis and fusion of low density lipoprotein particles independently strengthen their binding to exocytosed mast cell granules.

Contact between low density lipoproteins (LDL) and exocytosed mast cell granules, the "granule remnants," leads to binding of LDL to the granule remnants via ionic interactions between the apolipoprotein B-100 (apoB-100) component of LDL and the heparin proteoglycan component of the granule remnants. Upon incubation at 37 degrees C, the heparin proteoglycan-bound apoB-100 is progressively proteolyzed by remnant chymase and carboxypeptidase A, which are also bound to the heparin proteoglycans. Thereupon, the LDL particles fuse, and their binding to the granule remnants strengthens, as defined by the decreased ability of NaCl to release LDL from the remnants. We now have examined separately the effects of proteolysis and fusion on LDL binding. Proteolysis without fusion was induced by lowering the incubation temperature to 15 degrees C, and proteolysis-independent fusion was induced by treating granule remnant-bound LDL with sphingomyelinase in the presence of protease inhibitors. It was found that degradation of the heparin proteoglycan-bound apoB-100, even without accompanying particle fusion, increased the strength of LDL binding to the granule remnants, suggesting exposure of buried heparin binding regions of apoB-100. When such proteolyzed LDL particles were allowed to fuse, the strength of their binding to the granule remnants increased still further, probably because of an increase in the number of apoB-100 fragments in the enlarged particles. Proteolysis-independent fusion, induced by sphingomyelinase treatment of granule remnant-bound LDL, also increased the strength of binding. The results show that proteolytic degradation and fusion, the two modifications of granule remnant-bound LDL subsequent to action by chymase and carboxypeptidase A of the granule remnants, represent two separate mechanisms by which LDL particles become tightly bound to the heparin proteoglycans of exocytosed mast cell granules. Since the formation of an atheroma, the hallmark of atherosclerosis, is characterized by accumulation in the proteoglycan matrix of the arterial intima of extracellular lipid droplets resembling the fused LDL particles on the granule remnant surfaces, the modifications of LDL described in this study may provide a clue to the actual processes by which the lipid droplets are anchored to the arterial intima.

Stimulation of rat peritoneal mast cells enhances uptake of low density lipoproteins by rat peritoneal macrophages in vivo

The effect of mast cell stimulation on the uptake of low density lipoproteins (LDL) by macrophages was tested in vivo in the peritoneal cavity of the rat, a site known to contain both macrophages and mast cells. The concentration of LDL in the peritoneal cavity was raised by injecting [14C]sucrose-labeled LDL ([14C]sucrose-LDL). In the treated rats, in the absence of mast cell stimulation, the uptake of LDL by the peritoneal macrophages was low. But when the peritoneal mast cells were concomitantly stimulated by intraperitoneal administration of compound 48/80, an agent known to induce mast cell degranulation, the rate of uptake of labeled LDL by macrophages rose by 7-24-fold. The reason for this rise was that exocytosed mast cell granules bound LDL and carried it into macrophages when phagocytosed. Thus, cyclohexanedione treatment of LDL, or injection of avidin along with LDL, 2 measures known to inhibit binding of LDL to mast cell granules, totally prevented the mast cell-dependent uptake of LDL. Furthermore transmission electron microscopic studies with gold-labeled LDL disclosed phagocytosis of LDL-bearing granules by the peritoneal macrophages. This is the first demonstration of a natural proteoglycan being able to enhance the rate of LDL uptake by macrophages in vivo. These observations on the relation between stimulation of mast cells and uptake of LDL by macrophages in vivo may have relevance in other sites where mast cells and macrophages coexist, such as the arterial intima.