Mast Cell Stabilizers in the Treatment of Rosacea: A Review of Existing and Emerging Therapies

Rosacea is a chronic inflammatory skin disease characterized by centrofacial erythema, papules, pustules, and telangiectasias. The onset of rosacea typically occurs after 30 years of age. It is estimated that approximately 2–5% of adults worldwide are affected. While the exact etiology of rosacea remains unknown, its pathogenesis is thought to be multifactorial with both environmental and genetic factors implicated. Ultraviolet radiation, heat, steam, ingested agents, including spicy foods and alcohol, host vasculature, dermal matrix degeneration, genetic susceptibility, and microbial organisms, including Demodex mites and Heliobacter pylori, have been implicated in the development of rosacea. Recently, mast cells (MCs) have emerged as key players in the pathogenesis of rosacea through the release of pro-inflammatory cytokines, chemokines, proteases, and antimicrobial peptides leading to cutaneous vasodilation, angiogenesis, and tissue fibrosis. Several existing and emerging topical, oral, and injectable therapeutics have been associated with improvement of rosacea symptoms based on their ability to stabilize and downregulate activated MCs. Herein, we review the data implicating MCs in the pathogenesis of rosacea and discuss interventions that may stabilize this pathway.

The Classification, Pathogenesis, Diagnostic Workup, and Management of Urticaria: An Update

Wheals and angioedema are the signature signs of urticaria, and itch is the key symptom. Urticaria, in most patients, is acute and resolves within days (acute urticaria, AU). Chronic urticaria (CU) can be of long duration and results not only in severely impaired quality of life but also has a socioeconomic impact due to work productivity impairment. In some patients with CU, the wheals and angioedema are induced exclusively by defined and definite triggers (chronic inducible urticaria, CIndU). In most patients with CU, wheals and angioedema develop unprompted, spontaneously (chronic spontaneous urticaria, CSU). The management of CU aims for the complete control and absence of its signs and symptoms. This is achieved, in most patients, by prophylactic treatment until spontaneous remission occurs. Modern, second-generation H1-antihistamines are the first-line therapy, with the option of updosing to fourfold, and omalizumab is used when this fails.

Infiltration of Mast Cells in Scalp Biopsies of Patients with Alopcia Areata or Androgenic Alopecia Versus Healthy Individuals: A Case Control Study

BACKGROUND

Alopecia areata (AA) and androgenic alopecia (AGA) are of the most common types of alopecias. Recently, the role of mastcells in inflammatory diseases has become the focus of many studies. However, few studies have been conducted on their role in AA and AGA. Therefore, our study aimed to quantitatively evaluate the presence of mastcells in the AA and AGA specimens.

MATERIALS AND METHODS

Three groups of AA, AGA, and healthy control were studied (each group with 20 subjects). Patients were randomly selected from those referred to the dermatology clinics of Shahid Beheshti University. Specimens were obtained from the scalp, and perifollicular and perivascular areas were investigated.

RESULTS

Significantly higher perifollicular and perivascular mastcell counts were seen in both AGA and AA groups as compared to healthy control (P<0.001 for both). Moreover, AA patients had more frequent perivascular mastcells than the AGA group (P=0.042). Among patients aged <40 years, perifollicular and perivascular mastcell counts were not significantly different among three groups; however, subjects over 40 years of age in both groups had significantly more perifollicular and perivascular mastcells than healthy participants. There was a significant positive correlation between disease severity and mast cell counts in both perifollicular and perivascular areas in AA patients (P=0.001 for both).

CONCLUSION

There is a significantly increased infiltration of mastcells in AA and AGA patients, and this increase is age and severity dependent. Moreover, the increase in mastcell proliferation is more dominant in AA patients.

COVID-19: Famotidine, Histamine, Mast Cells, and Mechanisms

SARS-CoV-2 infection is required for COVID-19, but many signs and symptoms of COVID-19 differ from common acute viral diseases. Currently, there are no pre- or post-exposure prophylactic COVID-19 medical countermeasures. Clinical data suggest that famotidine may mitigate COVID-19 disease, but both mechanism of action and rationale for dose selection remain obscure. We explore several plausible avenues of activity including antiviral and host-mediated actions. We propose that the principal famotidine mechanism of action for COVID-19 involves on-target histamine receptor H₂ activity, and that development of clinical COVID-19 involves dysfunctional mast cell activation and histamine release.

Serum concentrations of mast cell tryptase are reduced in heavy drinkers

BACKGROUND

Baseline serum tryptase concentrations are commonly used in clinical practice as a marker of the body's mast cell burden. This study aimed to investigate serum tryptase concentrations in heavy drinkers.

METHODS

Serum tryptase concentrations were determined in 126 heavy drinkers (75% males, median age 47 years) who were admitted to the hospital because of alcohol withdrawal syndrome (n = 60), general symptoms with abnormalities on biochemical tests that indicated acute liver disease (n = 19), complications of advanced liver disease (n = 33), and miscellaneous reasons (n = 14). Results were compared with those of 70 healthy controls (66% males, median age 40 years).

RESULTS

Serum tryptase concentrations were lower in heavy drinkers than in healthy controls (median 2.23 μg/l vs. median 3.25 μg/l, p < 0.001). Ten heavy drinkers (7.9%) had undetectable (<1 μg/l) serum tryptase levels versus none of the healthy controls (p = 0.01). The association of low tryptase levels with heavy drinking was independent of age, gender, and smoking status. Among heavy drinkers, the lowest tryptase concentrations were observed in patients with alcohol withdrawal syndrome and patients with general symptoms with abnormalities on biochemical tests that indicated acute liver disease. Furthermore, serum tryptase concentrations were negatively correlated with markers of acute liver damage or alcohol consumption (serum aspartate aminotransferase and gamma-glutamyl transferase). Atopy (skin prick test positivity) was not associated with serum tryptase concentrations in heavy drinkers.

CONCLUSIONS

Serum concentrations of mast cell tryptase are lower in heavy drinkers than in healthy controls.

The effect of hydroxychloroquine on lupus erythematosus-like skin lesions in MRL/lpr mice

OBJECTIVES

To evaluate the effect and safety of hydroxychloroquine (HCQ) on lupus erythematosus (LE)-like skin lesions in the MRL/lpr mouse, a model for systemic LE (SLE).

METHODS

We divided the MRL/lpr mice into three groups that were given: (1) drinking water, (2) HCQ at a dose of 4 mg/kg/d, or (3) HCQ at a dose of 40 mg/kg/d. The HCQ was administered to examine the effect and safety of HCQ on skin lesions and the number of infiltrating cells including mast cells in the dermis.

RESULTS

Six of 13 mice in the group given drinking water, 3 of 11 mice in the group administered low-dose HCQ (4 mg/kg/d), and 1 of 10 mice in the group administered high-dose HCQ (40 mg/kg/d) presented the skin lesions. The average number of mast cells was 81, 50, and 12 (magnification, ×100), the mortality rate was 24%, 8%, and 9% and the mean body weight gain was 4.6 g, 8.0 g and 5.1 g, respectively.

CONCLUSIONS

HCQ was demonstrated to decrease the appearance of LE-like lesions and the number of mast cells in the dermis. Furthermore, there were no obvious systemic adverse effects. This study provides evidence that suggests benefits in human patients.

Implications of scavenger receptors in the safe development of nanotherapeutics

Nanomaterials (NMs) are being utilized in a variety of biomedical applications including drug delivery, diagnostics, and therapeutic targeting. These applications are made possible due to the unique physicochemical properties that are exhibited at the nanoscale. To ensure safe development of NMs for clinical use, it is necessary to understand their interactions with cells and specifically cell surface receptors, which will facilitate either their toxicity and/or clinical function. Recently our research and others have investigated the role of scavenger receptors in mediating NM-cell interactions and responses. Scavenger receptors are expressed by a variety of cell types that are first to encounter NMs during clinical use such as macrophages and endothelial cells. Scavenger receptors are recognized to facilitate uptake of a wide variety of ligands ranging from foreign substances to endogenous lipids/proteins. While interaction of NMs with scavenger receptors may allow therapeutic targeting in some instances, it also presents a challenge for the stealth delivery of NMs and avoidance of the scavenging capability of this class of receptors. Due to their role in facilitating immune responses, scavenger receptor-mediated inflammation is also of concern following NM delivery. The research highlighted in this brief review intends to summarize our current understanding regarding the consequences of NM-scavenger receptor interactions.

Immunohistochemical Expression of Mast Cell in Oral Reactive Lesions

STATEMENT OF THE PROBLEM

Soft tissue reactive lesions are the most common lesions of the oral cavity. Although many studies have shown the interaction of mast cells with fibroblasts and their participation in fibrosis, the role of mast cells in these lesions is not well understood.

PURPOSE

The aim of this study was to evaluate the mast cells (MCs) count in oral soft-tissue reactive lesions including peripheral giant cell granuloma (PGCG), peripheral ossifying fibroma (POF), irritation fibroma (IF) and normal oral mucosa.

MATERIALS AND METHOD

In this cross-sectional study, 50 samples including IF, PGCG, POF (14 cases for each group) and 8 cases of normal oral mucosa were stained with tryptase antibody through immunohistochemistry. The number of mast cells was counted in 5HPF containing maximum counts for each section stained with tryptase. Statistical analysis including Chi-square test and Tukey test with a significance level of 0.05 were considered.

RESULTS

The number of MCs was found to have increased in reactive lesions compared with normal oral mucosa. MCs count in the POF group was higher than the others.

CONCLUSION

These findings suggest a possible role of mast cells in the pathogenesis of reactive oral lesions and induction of fibrous tissues. Chemical mediators released from mast cells might influence other cells, especially fibroblasts, to induce fibrosis.

Systemic mastocytosis: a rare cause of single vertebral body uptake on bone scan

Systemic Mastocytosis is a rare condition characterized by the abnormal proliferation of Mast Cells. Presentation as a solitary vertebral body lesion is extremely uncommon and may be confused with more ominous conditions such as metastasis. Familiarity with the condition can heighten clinical suspicion, direct tissue diagnosis, guide management and indicate appropriate follow up. We present a case of a 64-year-old woman undergoing staging for recently diagnosed breast cancer who was found to have Systemic Mastocytosis of a single vertebral body.

Therapeutic effects of s-petasin on disease models of asthma and peritonitis

To explore the anti-allergic and anti-inflammatory effects of extracts of Petasites genus, we studied the effects of s-petasin, a major sesquiterpene from Petasites formosanus (a butterbur species) on asthma and peritonitis models. In an ovalbumin-induced mouse asthma model, s-petasin significantly inhibited the accumulations of eosinophils, macrophages, and lymphocytes in bronchoalveolar fluids. S-petasin inhibited the antigen-induced degranulation of β-hexosamidase but did not inhibit intracellular Ca(2+) increase in RBL-2H3 mast cells. S-petasin inhibited the LPS induction of iNOS at the RNA and protein levels in mouse peritoneal macrophages. Furthermore, s-petasin inhibited the production of NO (the product of iNOS) in a concentration-dependent manner in the macrophages. Furthermore, in an LPS-induced mouse model of peritonitis, s-petasin significantly inhibited the accumulation of polymorpho nuclear and mononuclear leukocytes in peritoneal cavity. This study shows that s-petasin in Petasites genus has therapeutic effects on allergic and inflammatory diseases, such as, asthma and peritonitis through degranulation inhibition in mast cells, suppression of iNOS induction and production of NO in macrophages, and suppression of inflammatory cell accumulation.

Dectin-1-mediated signaling leads to characteristic gene expressions and cytokine secretion via spleen tyrosine kinase (Syk) in rat mast cells

Dectin-1 recognizes β-glucan and plays important roles for the antifungal immunity through the activation of spleen tyrosine kinase (Syk) in dendritic cells or macrophages. Recently, expression of Dectin-1 was also identified in human and mouse mast cells, although its physiological roles were largely unknown. In this report, rat mast cell line RBL-2H3 was analyzed to investigate the molecular mechanism of Dectin-1-mediated activation and responses of mast cells. Treatment of cells with Dectin-1-specific agonist curdlan induced tyrosine phosphorylation of cellular proteins and the interaction of Dectin-1 with the Src homology 2 domain of Syk. These responses depended on tyrosine phosphorylation of the hemi-immunoreceptor tyrosine-based activation motif in the cytoplasmic tail of Dectin-1, whereas they were independent of the γ-subunit of high-affinity IgE receptor. DNA microarray and real-time PCR analyses showed that Dectin-1-mediated signaling stimulated gene expression of transcription factor Nfkbiz and inflammatory cytokines, such as monocyte chemoattractant protein-1, IL-3, IL-4, IL-13, and tumor necrosis factor (TNF)-α. The response was abrogated by pretreatment with Syk inhibitor R406. These results suggest that Syk is critical for Dectin-1-mediated activation of mast cells, although the signaling differs from that triggered by FcϵRI activation. In addition, these gene expressions induced by curdlan stimulation were specifically observed in mast cells, suggesting that Dectin-1-mediated signaling of mast cells offers new insight into the antifungal immunity.

Transglutaminase II/microRNA-218/-181a loop regulates positive feedback relationship between allergic inflammation and tumor metastasis

The molecular mechanism of transglutaminase II (TGaseII)-mediated allergic inflammation remains largely unknown. TGaseII, induced by antigen stimulation, showed an interaction and co-localization with FcϵRI. TGaseII was necessary for in vivo allergic inflammation, such as triphasic cutaneous reaction, passive cutaneous anaphylaxis, and passive systemic anaphylaxis. TGaseII was necessary for the enhanced metastatic potential of B16F1 melanoma cells by passive systemic anaphylaxis. TGaseII was shown to be a secreted protein. Recombinant TGaseII protein increased the histamine release and β-hexosaminidase activity, and enhanced the metastatic potential of B16F1 mouse melanoma cells. Recombinant TGaseII protein induced the activation of EGF receptor and an interaction between EGF receptor and FcϵRI. Recombinant TGaseII protein displayed angiogenic potential accompanied by allergic inflammation. R2 peptide, an inhibitor of TGaseII, exerted negative effects on in vitro and in vivo allergic inflammation by regulating the expression of TGaseII and FcϵRI signaling. MicroRNA (miR)-218 and miR-181a, decreased during allergic inflammation, were predicted as negative regulators of TGaseII by microRNA array and TargetScan analysis. miR-218 and miR-181a formed a negative feedback loop with TGaseII and regulated the in vitro and in vivo allergic inflammation. TGaseII was necessary for the interaction between mast cells and macrophages during allergic inflammation. Mast cells and macrophages, activated during allergic inflammation, were responsible for the enhanced metastatic potential of tumor cells that are accompanied by allergic inflammation. In conclusion, the TGaseII/miR-218/-181a feedback loop can be employed for the development of anti-allergy therapeutics.

Importance of mast cell Prss31/transmembrane tryptase/tryptase-γ in lung function and experimental chronic obstructive pulmonary disease and colitis

Protease serine member S31 (Prss31)/transmembrane tryptase/tryptase-γ is a mast cell (MC)-restricted protease of unknown function that is retained on the outer leaflet of the plasma membrane when MCs are activated. We determined the nucleotide sequences of the Prss31 gene in different mouse strains and then used a Cre/loxP homologous recombination approach to create a novel Prss31(-/-) C57BL/6 mouse line. The resulting animals exhibited no obvious developmental abnormality, contained normal numbers of granulated MCs in their tissues, and did not compensate for their loss of the membrane tryptase by increasing their expression of other granule proteases. When Prss31-null MCs were activated with a calcium ionophore or by their high affinity IgE receptors, they degranulated in a pattern similar to that of WT MCs. Prss31-null mice had increased baseline airway reactivity to methacholine but markedly reduced experimental chronic obstructive pulmonary disease and colitis, thereby indicating both beneficial and adverse functional roles for the tryptase. In a cigarette smoke-induced model of chronic obstructive pulmonary disease, WT mice had more pulmonary macrophages, higher histopathology scores, and more fibrosis in their small airways than similarly treated Prss31-null mice. In a dextran sodium sulfate-induced acute colitis model, WT mice lost more weight, had higher histopathology scores, and contained more Cxcl-2 and IL-6 mRNA in their colons than similarly treated Prss31-null mice. The accumulated data raise the possibility that inhibitors of this membrane tryptase may provide additional therapeutic benefit in the treatment of humans with these MC-dependent inflammatory diseases.

Mast cell-nerve cell interaction at acupoint: modeling mechanotransduction pathway induced by acupuncture

Mast cells are found abundant at sites of acupoints. Nerve cells share perivascular localization with mast cells. Acupuncture (mechanical stimuli) can activate mast cells to release adenosine triphosphate (ATP) which can activate nerve cells and modulates pain-processing pathways in response to acupuncture. In this paper, a mathematical model was constructed for describing intracellular Ca²⁺ signal and ATP release in a coupled mast cell and nerve cell system induced by mechanical stimuli. The results showed mechanical stimuli lead to a intracellular Ca²⁺ rise in the mast cell and ATP release, ATP diffuses in the extracellular space (ECS) and activates the nearby nerve cells, then induces electrical current in the nerve cell which spreads in the neural network. This study may facilitate our understanding of the mechanotransduction process induced by acupuncture and provide a methodology for quantitatively analyzing acupuncture treatment.

Proteolytic histone modification by mast cell tryptase, a serglycin proteoglycan-dependent secretory granule protease

A hallmark feature of mast cells is their high content of cytoplasmic secretory granules filled with various preformed compounds, including proteases of tryptase-, chymase-, and carboxypeptidase A3 type that are electrostatically bound to serglycin proteoglycan. Apart from participating in extracellular processes, serglycin proteoglycan and one of its associated proteases, tryptase, are known to regulate cell death by promoting apoptosis over necrosis. Here we sought to outline the underlying mechanism and identify core histones as primary proteolytic targets for the serglycin-tryptase axis. During the cell death process, tryptase was found to relocalize from granules into the cytosol and nucleus, and it was found that the absence of tryptase was associated with a pronounced accumulation of core histones both in the cytosol and in the nucleus. Intriguingly, tryptase deficiency resulted in defective proteolytic modification of core histones even at baseline conditions, i.e. in the absence of cytotoxic agent, suggesting that tryptase has a homeostatic impact on nuclear events. Indeed, tryptase was found in the nucleus of viable cells and was shown to cleave core histones in their N-terminal tail. Moreover, it was shown that the absence of the serglycin-tryptase axis resulted in altered chromatin composition. Together, these findings implicate histone proteolysis through a secretory granule-derived serglycin-tryptase axis as a novel principle for histone modification, during both cell homeostasis and cell death.

Mast cell chymase degrades the alarmins heat shock protein 70, biglycan, HMGB1, and interleukin-33 (IL-33) and limits danger-induced inflammation

During infection and tissue damage, virulence factors and alarmins are pro-inflammatory and induce activation of various immune cells including macrophages and mast cells (MCs). Activated MCs instantly release preformed inflammatory mediators, including several proteases. The chymase mouse mast cell protease (MCPT)-4 is thought to be pro-inflammatory, whereas human chymase also degrades pro-inflammatory cytokines, suggesting that chymase instead limits inflammation. Here we explored the contribution of MCPT4 and human chymase to the control of danger-induced inflammation. We found that protein extracts from wild type (WT), carboxypeptidase A3-, and MCPT6-deficient mice and MCs and recombinant human chymase efficiently degrade the Trichinella spiralis virulence factor heat shock protein 70 (Hsp70) as well as endogenous Hsp70. MC-(W(sash))-, serglycin-, NDST2-, and MCPT4-deficient extracts lacked this capacity, indicating that chymase is responsible for the degradation. Chymase, but not MC tryptase, also degraded other alarmins, i.e. biglycan, HMGB1, and IL-33, a degradation that was efficiently blocked by the chymase inhibitor chymostatin. IL-7, IL-22, GM-CSF, and CCL2 were resistant to chymase degradation. MCPT4-deficient conditions ex vivo and in vivo showed no reduction in added Hsp70 and only minor reduction of IL-33. Peritoneal challenge with Hsp70 resulted in increased neutrophil recruitment and TNF-α levels in the MCPT4-deficient mice, whereas IL-6 and CCL2 levels were similar to the levels found in WT mice. The rapid and MC chymase-specific degradation of virulence factors and alarmins may depend on the presence of accessible extended recognition cleavage sites in target substrates and suggests a protective and regulatory role of MC chymase during danger-induced inflammation.

Allosteric control of βII-tryptase by a redox active disulfide bond

The S1A serine proteases function in many key biological processes such as development, immunity, and blood coagulation. S1A proteases contain a highly conserved disulfide bond (Cys(191)-Cys(220) in chymotrypsin numbering) that links two β-loop structures that define the rim of the active site pocket. Mast cell βII-tryptase is a S1A protease that is associated with pathological inflammation. In this study, we have found that the conserved disulfide bond (Cys(220)-Cys(248) in βII-tryptase) exists in oxidized and reduced states in the enzyme stored and secreted by mast cells. The disulfide bond has a standard redox potential of -301 mV and is stoichiometrically reduced by the inflammatory mediator, thioredoxin, with a rate constant of 350 m(-1) s(-1). The oxidized and reduced enzymes have different substrate specificity and catalytic efficiency for hydrolysis of both small and macromolecular substrates. These observations indicate that βII-tryptase activity is post-translationally regulated by an allosteric disulfide bond. It is likely that other S1A serine proteases are similarly regulated.

The switch-associated protein 70 (SWAP-70) bundles actin filaments and contributes to the regulation of F-actin dynamics

Coordinated assembly and disassembly of actin into filaments and higher order structures such as stress fibers and lamellipodia are fundamental for cell migration and adhesion. However, the precise spatiotemporal regulation of F-actin structures is not completely understood. SWAP-70, a phosphatidylinositol 3,4,5-trisphosphate-interacting, F-actin-binding protein, participates in actin rearrangements through yet unknown mechanisms. Here, we show that SWAP-70 is an F-actin-bundling protein that oligomerizes through a Gln/Glu-rich stretch within a coiled-coil region. SWAP-70 bundles filaments in parallel and anti-parallel fashion through its C-terminal F-actin binding domain and delays dilution-induced F-actin depolymerization. We further demonstrate that SWAP-70 co-localizes and directly interacts with cofilin, an F-actin severing and depolymerization factor, and contributes to the regulation of cofilin activity in vivo. In line with these activities, upon stem cell factor stimulation, murine bone marrow-derived mast cells lacking SWAP-70 display aberrant regulation of F-actin and actin free barbed ends dynamics. Moreover, proper stem cell factor-dependent cofilin activation via dephosphorylation and subcellular redistribution into a detergent-resistant cytoskeletal compartment also require SWAP-70. Together, these findings reveal an important role of SWAP-70 in the dynamic spatiotemporal regulation of F-actin networks.

Prevention of F-actin assembly switches the response to SCF from chemotaxis to degranulation in human mast cells

Following antigen/IgE-mediated aggregation of high affinity IgE-receptors (FcεRI), mast cells (MCs) degranulate and release inflammatory mediators leading to the induction of allergic reactions including anaphylaxis. Migration of MCs to resident tissues and sites of inflammation is regulated by tissue chemotactic factors such as stem cell factor (SCF (KIT ligand)). Despite inducing similar early signaling events to antigen, chemotactic factors, including SCF, produce minimal degranulation in the absence of other stimuli. We therefore investigated whether processes regulating MC chemotaxis are rate limiting for MC mediator release. To investigate this issue, we disrupted actin polymerization, a requirement for MC chemotaxis, with latrunculin B and cytochalasin B, then examined chemotaxis and mediator release in human (hu)MCs induced by antigen or SCF. As expected, such disruption minimally affected early signaling pathways, but attenuated SCF-induced human mast cell chemotaxis. In contrast, SCF, in the absence of other stimuli, induced substantial degranulation in a concentration-dependent manner following actin disassembly. It also moderately enhanced antigen-mediated human mast cell degranulation which was further enhanced in the presence of SCF. These observations suggest that processes regulating cell migration limit MC degranulation as a consequence of cytoskeletal reorganization.

Mechanisms of granule membrane recapture following exocytosis in intact mast cells

In secretory cells, several exocytosis-coupled forms of endocytosis have been proposed including clathrin-mediated endocytosis, kiss-and-run endocytosis, cavicapture, and bulk endocytosis. These forms of endocytosis can be induced under different conditions, but their detailed molecular mechanisms and functions are largely unknown. We studied exocytosis and endocytosis in mast cells with both perforated-patch and whole-cell configurations of the patch clamp technique using cell capacitance measurements in combination with amperometric serotonin detection. We found that intact mast cells exhibit an early endocytosis that follows exocytosis induced by compound 48/80. Direct observation of individual exocytic and endocytic events showed a higher percentage of capacitance flickers (27.3%) and off-steps (11.4%) in intact mast cells than in dialyzed cells (5.4% and 2.9%, respectively). Moreover, we observed a type of endocytosis of large pieces of membrane that were likely formed by cumulative fusion of several secretory granules with the cell membrane. We also identified "large-capacitance flickers" that occur after large endocytosis events. Pore conductance analysis indicated that these transient events may represent "compound cavicapture," most likely due to the flickering of a dilated fusion pore. Using fluorescence imaging of individual exocytic and endocytic events we observed that granules can fuse to granules already fused with the plasma membrane, and then the membranes and dense cores of fused granules are internalized. Altogether, our results suggest that stimulated exocytosis in intact mast cells is followed by several forms of compensatory endocytosis, including kiss-and-run endocytosis and a mechanism for efficient retrieval of the compound membrane of several secretory granules through a single membrane fission event.