Mast cell-derived TNF-alpha primes sensory nerve endings in a pulmonary hypersensitivity reaction

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Biomarkers in Interstitial Cystitis/Bladder Pain Syndrome with and without Hunner Lesion: A Review and Future Perspectives

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a debilitating urinary bladder condition that presents with a wide variety of clinical phenotypes. It is commonly characterized by persistent pelvic pain and lower urinary tract symptoms, such as urinary frequency and urgency. Current clinicopathological and genomic evidence has indicated that IC/BPS with Hunner lesions is a clinically relevant distinct subtype with proven bladder pathology of subepithelial chronic inflammatory changes that are characterized by enhanced local immune responses and epithelial denudation. However, other forms of IC/BPS lacking Hunner lesions are a symptom syndrome complex of non-inflammatory conditions with little evidence of bladder etiology, characterized by aberrant neural activity in neurotransmission systems which leads to central nervous sensitization with potential involvement of urothelial malfunction, or clinical presentation of somatic and/or psychological symptoms beyond the bladder. Given such distinct potential pathophysiology between IC/BPS subtypes, disease biomarkers of IC/BPS should be provided separately for subtypes with and without Hunner lesions. Tailored approaches that target characteristic immunological inflammatory processes and epithelial denudation for IC/BPS with Hunner lesions, or the sensitized/altered nervous system, urothelial malfunction, association with other functional somatic syndromes, and psychosocial problems for IC/BPS without Hunner lesions, are essential to identify optimal and reliable disease-specific IC/BPS biomarkers.

Vagus nerve stimulation enhances the cholinergic anti-inflammatory pathway to reduce lung injury in acute respiratory distress syndrome via STAT3

The cholinergic anti-inflammatory pathway (CAIP) is important for antagonizing inflammation and treating several diseases, including acute respiratory distress syndrome (ARDS), and is related to vagus nerve integrity. However, its underlying pathophysiological mechanism is still unclear. We hypothesized that CAIP regulates lung injury repair after ARDS through the STAT3 signaling pathway, which is an important downstream effector of α7nAchR. We enhanced CAIP activity by subjecting rats to vagus nerve stimulation (VNS), and administered the α-7 acetylcholine receptor (α7nAchR) agonist and antagonist to determine whether VNS can reduce lung injury by regulating the pulmonary inflammatory response through CAIP. After being subjected to VNS, the secretion of TNF-α and IL-1β was decreased, while the level of IL-10 was increased in the rat model of ARDS. Moreover, VNS treatment reduced lung mRNA levels of M1 macrophage markers, while increased those of M2 macrophage markers. The expression of Caspase-1 decreased, while that of STAT3 increased in lung tissue after VNS treatment. The aforementioned effects of VNS were reversed by cutting the cervical vagus efferent branch and blocking α7nAchR. These findings suggest that VNS inhibits the ARDS inflammatory response by promoting CAIP activity. Next, we used lentivirus knockdown of STAT3 expression to explore the mechanism of VNS through CAIP on lung inflammation in ARDS model rats. VNS activates α7nAchR, increases STAT3 expression, reduces Caspase-1 expression, suppresses inflammation by inhibiting inflammatory pyroptosis and M1 to M2 macrophage transformation, which may constitute the main mechanism of VNS action in ARDS.

Interstitial cystitis/bladder pain syndrome: The evolving landscape, animal models and future perspectives

Interstitial cystitis/bladder pain syndrome is a debilitating condition of unknown etiology characterized by persistent pelvic pain with lower urinary tract symptoms and comprises a wide variety of potentially clinically useful phenotypes with different possible etiologies. Current clinicopathological and genomic evidence suggests that interstitial cystitis/bladder pain syndrome should be categorized by the presence or absence of Hunner lesions, rather than by clinical phenotyping based on symptomatology. The Hunner lesion subtype is a distinct inflammatory disease with proven bladder etiology characterized by epithelial denudation and enhanced immune responses frequently accompanied by clonal expansion of infiltrating B cells, with potential engagement of infection. Meanwhile, the non-Hunner lesion subtype is a non-inflammatory disorder with little evidence of bladder etiology. It is potentially associated with urothelial malfunction and neurophysiological dysfunction, and frequently presents with somatic and/or psychological symptoms, that commonly result in central nervous sensitization. Animal models of autoimmune cystitis and neurogenic sensitization might serve as disease models for the Hunner lesion and non-Hunner lesion subtypes, respectively. Here, we revisit the taxonomy of interstitial cystitis/bladder pain syndrome according to current research, and discuss its potential pathophysiology and representative animal models. Categorization of interstitial cystitis/bladder pain syndrome based on cystoscopy is mandatory to design optimized treatment and research strategies for each subtype. A tailored approach that specifically targets the characteristic inflammation and epithelial denudation for the Hunner lesion subtype, or the urothelial malfunction, sensitized/altered nervous system and psychosocial problems for the non-Hunner lesion subtype, is essential for better clinical management and research progress in this complex condition.

Postural orthostatic tachycardia syndrome and the potential role of mast cell activation

Though a sizeable amount of data connects mast cell activity to the neurologic system, less is known about the true clinical implications of this relationship. Even less is understood about treatment strategies in those with both allergic and neurologic complaints. This is particularly true in postural orthostatic tachycardia syndrome (POTS), a common type of dysautonomia, where patients are burdened by symptoms of orthostatic cerebral hypoperfusion and several other comorbidities that are likely influenced by autonomic tone. Some patients describe characteristic allergic symptoms, in the absence of typical IgE mediated triggers, and also improvement with traditional mast cell directed medications. Further work is necessary to determine whether these anecdotal observations are valid. The answer to this question will likely be addressed as the mechanisms of POTS are better characterized, which may include a phenotype with distinct mast cell involvement.

The cholinergic anti-inflammatory pathway: an innovative treatment strategy for respiratory diseases and their comorbidities

Over the past few decades, it has been clarified that the nervous system and immune system have overlapping distributions and their interactions are critical in the regulation of immunological and inflammatory responses. The cholinergic anti-inflammatory pathway, including the parasympathetic nerve systems and humoral factors orchestrate the immune responses to protect the body during infection and tissue injury. Recent investigations have attempted to clarify how the parasympathetic nerve systems attenuate the systemic inflammatory responses and identified the α7 nicotinic acetylcholine receptor (α7nAChR) as a crucial target for attenuating the release of inflammatory cytokines from inflammatory cells including macrophages and dendritic cells. This modulatory circuit pathway possibly exists in the lungs and might be involved in regulating inflammation and immunity during infection and other inflammatory lung diseases including asthma and COPD, which means that modulation of the cholinergic anti-inflammatory pathway is a possible therapeutic target for lung diseases.

Allergic airway inflammation induces migration of mast cell populations into the mouse airway

Mast cells (MCs) and airway nerves play an important role in allergic asthma. However, little is known about the MCs and their interaction with airway nerves during allergic airway inflammation. This study aims to investigate the distribution and proliferation of MC populations in different lung compartments, along with the association of mast cells with nerve endings, using a house dust mite (HDM) model for allergic airway inflammation. BALB/c mice were exposed to HDM extract intranasally (25 μg/50 μl) for 5 consecutive days a week over 7 weeks. Immunofluorescence and Edu stains were used to examine the colocalisation of MCs and nerves and the proliferation of MCs, respectively. HDM treatment caused an increased migration of MCs into bronchi, alveolar parenchyma and airway vessels. The proportions of tryptase-chymase expressing MC (MC_TC) increased significantly in the bronchi and the alveolar parenchyma but not in the vascular tissues, by allergic airway inflammation. The association of MCs with nerves was found only in the bronchi and there were no changes in comparison of controls to HDM-treated animals. The present study shows a strong migration of tryptase expressing MC (MC_T) and MC_TC into the bronchi and the alveolar parenchyma, as well as of MC_T in the vascular compartment under HDM treatment. This supports the hypothesis that these mast cell populations may contribute to allergic airway inflammation.

The effect of cannabinoids on dinitrofluorobenzene-induced experimental asthma in mice

Cannabinoids have anti-inflammatory effects and can produce bronchodilation in the airways. We have investigated the effects of cannabinoids on tracheal hyperreactivity and airway inflammation in dinitrofluorobenzene (DNFB)-induced experimental non-atopic asthma in mice. 5-hydroxytryptamine (5-HT)-induced contraction response was enhanced while carbachol- and electrical field stimulation-induced contractions, and isoprenaline-induced relaxation responses were remained unchanged in DNFB group. The increased 5-HT-induced contractions were inhibited by incubation with either atropine or tetrodotoxin. DNFB application resulted in increased macrophage number in the bronchoalveolar lavage fluid (BALF). In vivo ACEA (CB1 agonist) treatment prevented the increase in 5-HT contractions, while JWH133 (CB2 agonist) had no effect. However, neither ACEA nor JWH133 prevented the increase in macrophage number in BALF. In vitro ACEA incubation also inhibited the increase in 5-HT contraction in DNFB group. These results show that cannabinoid CB1 receptor agonist can prevent tracheal hyperreactivity to 5-HT in DNFB-induced non-atopic asthma in mice.

Microbes taming mast cells: Implications for allergic inflammation and beyond

There is increasing awareness of a relationship between our microbiota and the pathogenesis of allergy and other inflammatory diseases. In investigating the mechanisms underlying microbiota modulation of allergy the focus has been on the induction phase; alterations in the phenotype and function of antigen presenting cells, induction of regulatory T cells and shifts in Th1/Th2 balance. However there is evidence that microbes can influence the effector phase of disease, specifically that certain potentially beneficial bacteria can attenuate mast cell activation and degranulation. Furthermore, it appears that different non-pathogenic bacteria can utilize distinct mechanisms to stabilize mast cells, acting locally though direct interaction with the mast cell at mucosal sites or attenuating systemic mast cell dependent responses, likely through indirect signaling mechanisms. The position of mast cells on the frontline of defense against pathogens also suggests they may play an important role in fostering the host-microbiota relationship. Mast cells are also conduits of neuro-immuo-endocrine communication, suggesting the ability of microbes to modulate cell responses may have implications for host physiology beyond immunology. Further investigation of mast cell regulation by non-pathogenic or symbiotic bacteria will likely lead to a greater understanding of host microbiota interaction and the role of the microbiome in health and disease.

The parasympathetic nervous system as a regulator of mast cell function

Often considered as the archetype of neuroimmune communication, much of our understanding of the bidirectional relationship between the nervous and immune systems has come from the study of mast cell-nerve interaction. Mast cells play a role in resistance to infection and are extensively involved in inflammation and subsequent tissue repair. Thus, the relationship between mast cells and neurons enables the involvement of peripheral and central nervous systems in the regulation of host defense mechanisms and inflammation. Recently, with the identification of the cholinergic anti-inflammatory pathway, there has been increased interest in the role of the parasympathetic nervous system in regulating immune responses. Classical neurotransmitters and neuropeptides released from cholinergic and inhibitory NANC neurons can modulate mast cell activity, and there is good evidence for the existence of parasympathetic nerve-mast cell functional units in the skin, lung, and intestine that have the potential to regulate a range of physiological processes.

Neural control of the endocrine pancreas

The autonomic nervous system affects glucose metabolism partly through its connection to the pancreatic islet. Since its discovery by Paul Langerhans, the precise innervation patterns of the islet has remained elusive, mainly because of technical limitations. Using 3-dimensional reconstructions of axonal terminal fields, recent studies have determined the innervation patterns of mouse and human islets. In contrast to the mouse islet, endocrine cells within the human islet are sparsely contacted by autonomic axons. Instead, the invading sympathetic axons preferentially innervate smooth muscle cells of blood vessels. This innervation pattern suggests that, rather than acting directly on endocrine cells, sympathetic nerves may control hormone secretion by modulating blood flow in human islets. In addition to autonomic efferent axons, islets also receive sensory innervation. These axons transmit sensory information to the brain but also have the ability to locally release neuroactive substances that have been suggested to promote diabetes pathogenesis. We discuss recent findings on islet innervation, the connections of the islet with the brain, and the role islet innervation plays during the progression of diabetes.

A novel regulator of lung inflammation and immunity: pulmonary parasympathetic inflammatory reflex

In this review, we first analyzed the current status of cholinergic anti-inflammatory pathway and then put forward a novel regulatory machinery-pulmonary parasympathetic inflammatory reflex, which is composed by lung vagal sensors at afferent arm, α7 nAChR (α7 nicotinic acetylcholine receptors)-expressing cells at efferent arm and the brain information integrating center. This modulatory circuit might loop the lungs, immune and nervous systems and play a very important role in regulating lung infection, inflammation and immunity through the neural innervations and signals when the lungs encounter pathogenic challenges.

Mast cells: versatile gatekeepers of pain

Mast cells are important first responders in protective pain responses that provoke withdrawal from intense, noxious environmental stimuli, in part because of their sentinel location in tissue-environment interfaces. In chronic pain disorders, the proximity of mast cells to nerves potentiates critical molecular cross-talk between these two cell types that results in their synergistic contribution to the initiation and propagation of long-term changes in pain responses via intricate signal networks of neurotransmitters, cytokines and adhesion molecules. Both in rodent models of inflammatory pain and chronic pain disorders, as well as in increasing evidence from the clinic, it is abundantly clear that understanding the mast cell-mediated mechanisms underlying protective and maladaptive pain cascades will lead to improved understanding of mast cell biology as well as the development of novel, targeted therapies for the treatment and management of debilitating pain conditions.

Contact hypersensitivity to oxazolone provokes vulvar mechanical hyperalgesia in mice

The interplay among pain, allergy and dysregulated inflammation promises to yield significant conceptual advances in immunology and chronic pain. Hapten-mediated contact hypersensitivity reactions are used to model skin allergies in rodents but have not been utilized to study associated changes in pain perception in the affected skin. Here we characterized changes in mechanical hyperalgesia in oxazolone-sensitized female mice challenged with single and repeated labiar skin exposure to oxazolone. Female mice were sensitized with topical oxazolone on their flanks and challenged 1-3 times on the labia. We then measured mechanical sensitivity of the vulvar region with an electronic pressure meter and evaluated expression of inflammatory genes, leukocyte influx and levels of innervation in the labiar tissue. Oxazolone-sensitized mice developed vulvar mechanical hyperalgesia after a single labiar oxazolone challenge. Hyperalgesia lasted up to 24 hours along with local influx of neutrophils, upregulation of inflammatory cytokine gene expression, and increased density of cutaneous labiar nerve fibers. Three daily oxazolone challenges produced vulvar mechanical hyperalgesic responses and increases in nerve density that were detectable up to 5 days post-challenge even after overt inflammation resolved. This persistent vulvar hyperalgesia is resonant with vulvodynia, an understudied chronic pain condition that is remarkably prevalent in 18-60 year-old women. An elevated risk for vulvodynia has been associated with a history of environmental allergies. Our pre-clinical model can be readily adapted to regimens of chronic exposures and long-term assessment of vulvar pain with and without concurrent inflammation to improve our understanding of mechanisms underlying subsets of vulvodynia and to develop new therapeutics for this condition.

TRPA1-dependent pruritus in IL-13-induced chronic atopic dermatitis

Chronic debilitating pruritus is a cardinal feature of atopic dermatitis (AD). Little is known about the underlying mechanisms. Antihistamines lack efficacy in treating itch in AD, suggesting the existence of histamine-independent itch pathways in AD. Transient receptor potential ankyrin 1 (TRPA1) is essential in the signaling pathways that promote histamine-independent itch. In this study, we tested the hypothesis that TRPA1-dependent neural pathways play a key role in chronic itch in AD using an IL-13-transgenic mouse model of AD. In these mice, IL-13 causes chronic AD characterized by intensive chronic itch associated with markedly enhanced growth of dermal neuropeptide-secreting afferent nerve fibers and enhanced expression of TRPA1 in dermal sensory nerve fibers, their dorsal root ganglia, and mast cells. Inhibition of TRPA1 with a specific antagonist in these mice selectively attenuated itch-evoked scratching. Genetic deletion of mast cells in these mice led to significantly diminished itch-scratching behaviors and reduced TRPA1 expression in dermal neuropeptide containing afferents in the AD skin. Interestingly, IL-13 strongly stimulates TRPA1 expression, which is functional in calcium mobilization in mast cells. In accordance with these observations in the AD mice, TRPA1 expression was highly enhanced in the dermal afferent nerves, mast cells, and the epidermis in the lesional skin biopsies from patients with AD, but not in the skin from healthy subjects. These studies demonstrate a novel neural mechanism underlying chronic itch in AD and highlight the complex interactions among TRPA1(+) dermal afferent nerves and TRPA1(+) mast cells in a Th2-dominated inflammatory environment.

TNF-alpha neutralizing antibody blocks thermal sensitivity induced by compound 48/80-provoked mast cell degranulation

Background: Neuro-inflammatory circuits in the tissue regulate the complex pathophysiology of pain. Protective nociceptive pain serves as an early warning system against noxious environmental stimuli. Tissue-resident mast cells orchestrate the increased thermal sensitivity following injection of basic secretagogue compound 48/80 in the hind paw tissues of ND4 mice. Here we investigated the effects of pre-treatment with TNF-α neutralizing antibody on compound 48/80-provoked thermal hyperalgesia.

Methods: We treated ND4 Swiss male mice with intravenous anti-TNF-α antibody or vehicle 30 minutes prior to bilateral, intra-plantar compound 48/80 administration and measured changes in the timing of hind paw withdrawal observed subsequent to mice being placed on a 51oC hotplate. We also assessed changes in tissue swelling, TNF-α gene expression and protein abundance, mast cell degranulation, and neutrophil influx in the hind paw tissue.

Findings: We found that TNF-α neutralization significantly blocked thermal hyperalgesia, and reduced early tissue swelling. TNF-α neutralization had no significant effect on mast cell degranulation or neutrophil influx into the tissue, however. Moreover, no changes in TNF-α protein or mRNA levels were detected within 3 hours of administration of compound 48/80.

Interpretation:  The neutralizing antibodies likely target pre-formed TNF-α including that stored in the granules of tissue-resident mast cells. Pre-formed TNF-α, released upon degranulation, has immediate effects on nociceptive signaling prior to the induction of neutrophil influx. These early effects on nociceptors are abrogated by TNF-α blockade, resulting in compromised nociceptive withdrawal responses to acute, harmful environmental stimuli.

TNF-alpha neutralizing antibody blocks thermal sensitivity induced by compound 48/80-provoked mast cell degranulation

BACKGROUND

Neuro-inflammatory circuits in the tissue regulate the complex pathophysiology of pain. Protective nociceptive pain serves as an early warning system against noxious environmental stimuli. Tissue-resident mast cells orchestrate the increased thermal sensitivity following injection of basic secretagogue compound 48/80 in the hind paw tissues of ND4 mice. Here we investigated the effects of pre-treatment with TNF-α neutralizing antibody on compound 48/80-provoked thermal hyperalgesia.

METHODS

We treated ND4 Swiss male mice with intravenous anti-TNF-α antibody or vehicle 30 minutes prior to bilateral, intra-plantar compound 48/80 administration and measured changes in the timing of hind paw withdrawal observed subsequent to mice being placed on a 51oC hotplate. We also assessed changes in tissue swelling, TNF-α gene expression and protein abundance, mast cell degranulation, and neutrophil influx in the hind paw tissue.

FINDINGS

We found that TNF-α neutralization significantly blocked thermal hyperalgesia, and reduced early tissue swelling. TNF-α neutralization had no significant effect on mast cell degranulation or neutrophil influx into the tissue, however. Moreover, no changes in TNF-α protein or mRNA levels were detected within 3 hours of administration of compound 48/80.

INTERPRETATION

  The neutralizing antibodies likely target pre-formed TNF-α including that stored in the granules of tissue-resident mast cells. Pre-formed TNF-α, released upon degranulation, has immediate effects on nociceptive signaling prior to the induction of neutrophil influx. These early effects on nociceptors are abrogated by TNF-α blockade, resulting in compromised nociceptive withdrawal responses to acute, harmful environmental stimuli.

Mast cells on the mind: new insights and opportunities

Mast cells (MCs) are both sensors and effectors in communication among nervous, vascular, and immune systems. In the brain, they reside on the brain side of the blood-brain barrier (BBB), and interact with neurons, glia, blood vessels, and other hematopoietic cells via their neuroactive prestored and newly synthesized chemicals. They are first responders, acting as catalysts and recruiters to initiate, amplify, and prolong other immune and nervous responses upon activation. MCs both promote deleterious outcomes in brain function and contribute to normative behavioral functioning, particularly cognition and emotionality. New experimental tools enabling isolation of brain MCs, manipulation of MCs or their products, and measurement of MC products in very small brain volumes present unprecedented opportunities for examining these enigmatic cells.

Mast cells in lung inflammation

Mast cells play an important role in the lung in both health and disease. Their primary role is to initiate an appropriate program of inflammation and repair in response to tissue damage initiated by a variety of diverse stimuli. They are important for host immunity against bacterial infection and potentially in the host immune response to non small cell lung cancer. In situations of ongoing tissue damage, the sustained release of numerous pro-inflammatory mediators, proteases and cytokines, contributes to the pathophysiology of lung diseases such as asthma and interstitial lung disease. A key goal is the development of treatments which attenuate adverse mast cell function when administered chronically to humans in vivo. Such therapies may offer a novel approach to the treatment of many life-threatening diseases.

Mast cells and the development of allergic airway disease

Murine models have highlighted the importance of T-cells and T_H2 cytokines in development of allergen-induced airway disease. In contrast, the role of mast cells for the development of allergic airway disease has been controversial. Recent studies in murine models demonstrate a significant contribution of mast cells during the development of airway hyperresponsiveness and airway inflammation. Furthermore these models have allowed identifying certain mast cell-produced mediators (e.g. histamine and leukotriene B4) to be involved in the recruitment of effector T-cells into the lung. Additionally, mast cell-produced TNF can directly activate T_H2 cells and contribute to the development of allergic airway disease. These new findings demonstrate a complex role of mast cells and their mediators, not only as effector cells, but also during sensitization and development of allergic airway disease. Therefore mast cells and certain mast cell-produced mediators might be an interesting target for the prevention and treatment of allergic asthma.

Enhanced nerve-mast cell interaction by a neuronal short isoform of cell adhesion molecule-1

Close apposition of nerve and mast cells is viewed as a functional unit of neuro-immune mechanisms, and it is sustained by trans-homophilic binding of cell adhesion molecule-1 (CADM1), an Ig superfamily member. Cerebral nerve-mast cell interaction might be developmentally modulated, because the alternative splicing pattern of four (a-d) types of CADM1 transcripts drastically changed during development of the mouse cerebrum: developing cerebrums expressed CADM1b and CADM1c exclusively, while mature cerebrums expressed CADM1d additionally and predominantly. To probe how individual isoforms are involved in nerve-mast cell interaction, Neuro2a neuroblastoma cells that express CADM1c endogenously were modified to express additionally either CADM1b (Neuro2a-CADM1b) or CADM1d (Neuro2a-CADM1d), and they were cocultured with mouse bone marrow-derived mast cells (BMMCs) and BMMC-derived cell line IC-2 cells, both of which expressed CADM1c. BMMCs were found to adhere to Neuro2a-CADM1d neurites more firmly than to Neuro2a-CADM1b neurites when the adhesive strengths were estimated from the femtosecond laser-induced impulsive forces minimally required for detaching BMMCs. GFP-tagging and crosslinking experiments revealed that the firmer adhesion site consisted of an assembly of CADM1d cis-homodimers. When Neuro2a cells were specifically activated by histamine, intracellular Ca(2+) concentration was increased in 63 and 38% of CADM1c-expressing IC-2 cells that attached to the CADM1d assembly site and elsewhere, respectively. These results indicate that CADM1d is a specific neuronal isoform that enhances nerve-mast cell interaction, and they suggest that nerve-mast cell interaction may be reinforced as the brain grows mature because CADM1d becomes predominant.

Relevance of mast cell-nerve interactions in intestinal nociception

Cross-talk between the immune- and nervous-system is considered an important biological process in health and disease. Because mast cells are often strategically placed between nerves and surrounding (immune)-cells they may function as important intermediate cells. This review summarizes the current knowledge on bidirectional interaction between mast cells and nerves and its possible relevance in (inflammation-induced) increased nociception. Our main focus is on mast cell mediators involved in sensitization of TRP channels, thereby contributing to nociception, as well as neuron-released neuropeptides and their effects on mast cell activation. Furthermore we discuss mechanisms involved in physical mast cell-nerve interactions. This article is part of a Special Issue entitled: Mast cells in inflammation.

Tumour necrosis factor alpha mediates transient receptor potential vanilloid 1-dependent bilateral thermal hyperalgesia with distinct peripheral roles of interleukin-1beta, protein kinase C and cyclooxygenase-2 signalling

TNFalpha plays a pivotal role in rheumatoid arthritis (RA) but little is known of the mechanisms that link the inflammatory and nociceptive effects of TNFalpha. We have established a murine model of TNFalpha-induced TRPV1-dependent bilateral thermal hyperalgesia that then allowed us to identify distinct peripheral mechanisms involved in mediating TNFalpha-induced ipsilateral and contralateral hyperalgesia. Thermal hyperalgesia and inflammation were assessed in both hindpaws following unilateral intraplantar (i.pl.) TNFalpha. The hyperalgesic mechanisms were analysed through pharmacogenetic approaches involving TRPV1(-/-) mice and TRPV1 antagonists. To study the mediators downstream of TNFalpha, cyclooxygenase (COX) and PKC inhibitors were utilised and cytokine and prostaglandin levels assessed. The role of neutrophils was determined through use of the selectin inhibitor, fucoidan. We show that TNFalpha (10pmol) causes thermal hyperalgesia (1-4h) in the ipsilateral inflamed and contralateral uninjured hindpaws, which is TRPV1-dependent. GF109203X, a PKC inhibitor, suppressed the hyperalgesia indicating that PKC is involved in TRPV1 sensitisation. Ipsilateral COX-2-derived prostaglandins were also crucial to the development of the bilateral hyperalgesia. The prevention of neutrophil accumulation with fucoidan attenuated hyperalgesia at 4 but not at 1h, indicating a role in the maintenance but not in the induction of bilateral hyperalgesia. However, TNFalpha-induced IL-1beta generation in both paws and the presence of local IL-1beta in the contralateral paw were essential for the development of bilateral hyperalgesia. These results identify a series of peripheral events through which TNFalpha triggers and maintains bilateral inflammatory pain. This potentially allows a better understanding of mechanisms involved in TNFalpha-dependent pain pathways in symmetrical diseases such as arthritis.

Nerve-mast cell and smooth muscle-mast cell interaction mediated by cell adhesion molecule-1, CADM1

Mast cells are a native composer of connective tissue of the skin dermis and intestinal and respiratory mucosa. Independent lines of accumulated evidence indicate the existence of an intensive bidirectional crosstalk between mast cells and sensory nerves and suggest that mast cells and sensory nerves may be viewed as a functional unit, which could be of crucial importance in neuroimmunological pathways. Mast cells appear to have a property of influencing smooth muscle function via not only such nerve-mast cell effects, but also direct pathways. In bronchial asthma, mast cells infiltrate the airway smooth muscle layer, and interact directly with smooth muscle cells, suggesting pathogenic roles for mast cells in airway obstruction. Current studies on mast cell biology identified a novel adhesion molecule of mast cells, namely cell adhesion molecule-1, CADM1. This molecule is unique, because it serves as not only simple glue but also appears to promote functional communication between nerve and mast cells and between smooth muscle and mast cells.

Th17 and allergy

The identification of novel helper T (Th) cell subsets, i.e., IL-17-producing Th cells (Th17 cells) and regulatory T cells (Treg cells), provided new insight into our understanding of the molecular mechanisms involved in the development of infectious and autoimmune diseases as well as immune responses, and thus led to revision of the classic Th1/Th2 paradigm. Several current lines of evidence from gene-deficient mice indicate that IL-17 and Th17 cells, but not IFN-gamma and Th1 cells, are responsible for the development of autoimmune diseases such as murine arthritis and encephalomyelitis, which have classically been considered to be Th1-mediated disorders. Th17 cells may also contribute to the pathogenesis of classically recognized Th2-mediated allergic disorders. In this review, we summarize the current knowledge regarding IL-17 and Th17 cells and discuss their potential roles in the pathogenesis of allergic disorders.

ATP plays a role in neurite stimulation with activated mast cells

Previously, we showed that nerve-mast cell cross-talk can occur bidirectionally and that substance P is a mediator to activate mast cells. Here, we have studied the mediators to activate nerves cocultured with mast cells. Addition of antigen to the cocultures of superior cervical ganglia (SCG) and rat basophilic leukemia cells (RBLs) elicited Ca(2+) response in RBLs and after a lag period induced Ca(2+) signal in SCG neurites. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (purinergic receptor antagonist) or apyrase (ATP-hydrolyzing enzyme) reduced the Ca(2+) signals in neurites, indicating that ATP released from activated mast cells was one of important mediators to activate nerves.

‘Sensing’ autoimmunity in type 1 diabetes

Type 1 diabetes (T1D) results from autoimmune-mediated loss of insulin-producing beta-cells. Recent findings suggest that the events controlling T1D development are not only immunological, but also neuronal in nature. In the non-obese diabetic (NOD) mouse model of T1D, a mutant sensory neuron channel, TRPV1, initiates chronic, progressive beta-cell stress, inducing islet cell inflammation. This novel mechanism of organ-specific damage requires a permissive, autoimmune-prone host, but ascribes tissue specificity to the local secretory dysfunction of sensory afferent neurons. In NOD mice, normalizing this neuronal function by administration of the neurotransmitter substance P clears islet cell inflammation, reduces insulin resistance and restores normoglycemia. Here, we discuss this neuro-immuno-endocrine model, its implications and the involvement of sensory neurons in other autoimmune disorders. These developments might provide novel neuronal-based therapeutic interventions, particularly in diabetes.

Th2 Cytokines in Skin Draining Lymph Nodes and Serum IgE Do Not Predict Airway Hypersensitivity to Intranasal Isocyanate Exposure in Mice

Isocyanate exposure in the workplace has been linked to asthma and allergic rhinitis. Recently, investigators have proposed that Th2 cytokine responses in lymph nodes draining the site of dermal application of chemicals including isocyanates may be used to identify sensitizers that cause asthma-like responses. The purpose of this study was to determine if the cytokine profile induced after dermal sensitization with isocyanates and serum IgE predict immediate (IHS) and methacholine-induced late (LHS) respiratory hypersensitivity responses after intranasal challenge. Dermal application of hexylmethane diisocyanate (HMDI), toluene diisocyanate (TDI), or methylene diisocyanate (MDI) significantly increased interleukin-4 (IL-4), IL-5, and IL-13 secretion in parotid lymph node cells. Isophorone diisocyanate (IPDI) increased IL-4 and IL-13, but not IL-5. Tolyl(mono)isocyanate (TMI), tetramethylene xylene diisocyanate (TMXDI), or the contact sensitizer dinitrochlorobenzene (DNCB), only induced minor increases in some of the Th2 cytokines. HMDI, TDI, MDI, and IPDI elicited greater increases in total serum IgE than DNCB, TMI, and TMXDI. All chemicals except TMXDI caused IHS after intranasal challenge of sensitized female BALB/c mice. Only HMDI-, TMI-, or TMXDI-sensitized and challenged mice had increases in LHS. All chemicals elicited epithelial cytotoxicity indicative of nasal airway irritation. The discordance between dermal cytokine profiles and respiratory responses suggests that dermal responses do not necessarily predict respiratory responses. Serum IgE also was not predictive of the respiratory responses to the isocyanates, suggesting that other unknown mechanisms may be involved.

Human mast cells produce and release the cytotoxic lymphocyte associated protease granzyme B upon activation

Mast cells are widely distributed throughout the body and express effector functions in allergic reactions, inflammatory diseases, and host defense. Activation of mast cells results in exocytosis of preformed chemical mediators and leads to novel synthesis and secretion of lipid mediators and cytokines. Here, we show that human mast cells also express and release the cytotoxic lymphocyte-associated protease, granzyme B. Granzyme B was active and localized in cytoplasmic granules, morphologically resembling those present in cytotoxic lymphocytes. Expression and release of granzyme B by mast cell-lines HMC-1 and LAD 2 and by cord blood- and mature skin-derived human mast cells depended on the mode of activation of these cells. In mast cell lines and cord blood-derived mast cells, granzyme B expression was mainly induced by non-physiological stimuli (A23187/PMA, Compound 48/80) and substance P. In contrast, mature skin-derived mast cells only produced granzyme B upon IgE-dependent stimulation. We conclude that granzyme B is expressed and released by human mast cells upon physiologic stimulation. This suggests a role for granzyme B as a novel mediator in mast cell biology.

Involvement of the SgIGSF/Necl-2 adhesion molecule in degranulation of mesenteric mast cells

Spermatogenic immunoglobulin superfamily (SgIGSF) expressed on nerve and mast cells, binds homophilically between both in culture. In the steady-state mesentery of mice, the proportion of morphologically degranulating mast cells was approximately 20%, and it increased nearly two-fold when the mesenteric nerve root was stimulated electrically. In contrast, there was no significant increase detectable in the mesentery of MITF-mutants, from which bone marrow-derived cultured mast cells (BMMCs) lack SgIGSF. BMMCs from SgIGSF-knockout mice transplanted to the mesentery of mast cell-deficient W/W(v) mice did not degranulate in response to the mesenteric nerve stimulation, whereas transfection with SgIGSF cDNA restored those responses. SgIGSF appeared to promote communication between nerves and mast cells in the murine mesentery.

Use of electroacupuncture at ST36 to inhibit anaphylactic and inflammatory reaction in mice

OBJECTIVE

Electroacupuncture (EA) has been used to treat myalgia, allergy and gastroenteropathy in Korea. To determine whether EA can treat anaphylactic and inflammatory reactions, the effect of EA was investigated in a murine model.

METHODS

EA stimulation of the ST36 acupoint was performed for 10 min. Using a passive cutaneous anaphylaxis (PCA) model, the antianaphylactic effects of EA were examined. Interleukin-6 and tumor necrosis factor-alpha were measured using the ELISA method. The level of nuclear factor (NF)-kappaB/RelA protein and NF-kappaB DNA-binding activity was determined using the Western blot analysis and the transcription factor enzyme-linked immunoassay method.

RESULTS

EA inhibits PCA and beta-hexosaminidase release, IL-6 secretion on the PCA, and in addition, EA reduces NF-kappaB DNA-binding activity.

CONCLUSION

These results indicate that EA may possess antianaphylactic and antiinflammatory properties.

Mast cell-derived tumor necrosis factor can promote nerve fiber elongation in the skin during contact hypersensitivity in mice

In humans, lesions of contact eczema or atopic dermatitis can exhibit increases in epidermal nerves, but the mechanism resulting in such nerve elongation are not fully understood. We found that contact hypersensitivity reactions to oxazolone in mice were associated with significant increases in the length of nerves in the epidermis and dermis. Using genetically mast cell-deficient c-kit mutant mice selectively repaired of their dermal mast cell deficiency with either wild-type or tumor necrosis factor (TNF)-deficient mast cells, we found that mast cells, and mast cell-derived TNF, significantly contributed to the elongation of epidermal and dermal PGP 9.5+ nerves and dermal CGRP+ nerves, as well as to the inflammation observed at sites of contact hypersensitivity in response to oxazolone. Moreover, the percentage of mast cells in close proximity to dermal PGP 9.5+ nerve fibers was significantly higher in wild-type mice and in c-kit mutant mice repaired of their dermal mast cell deficiency by the adoptive transfer of wild-type mast cells than in TNF-deficient mice or in TNF-/- mast cell-engrafted c-kit mutant mice. These observations show that mast cells, and mast cell-derived TNF, can promote the elongation of cutaneous nerve fibers during contact hypersensitivity in the mouse.

Free immunoglobulin light chains as target in the treatment of chronic inflammatory diseases

Immunoglobulin free light chains were long considered irrelevant bystander products of immunoglobulin synthesis by B lymphocytes. To date, different studies suggest that free light chains may have important functional activities. For instance, it has been shown that immunoglobulin free light chains can elicit mast cell-driven hypersensitivity responses leading to asthma and contact sensitivity. Free light chains also show other biologic actions such as anti-angiogenic and proteolytic activities or can be used as specific targeting vehicles. Levels of free light chain levels in body fluids increase markedly in diseases such as multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus. In this review, we will focus on the unexpected biological activities of immunoglobulin free light chains with special attention to its possible role in the induction of chronic inflammatory diseases.

Pivotal role of endogenous tachykinins and the NK1 receptor in mediating leukocyte accumulation, in the absence of oedema formation, in response to TNFα in the cutaneous microvasculature

Tachykinins including substance P (SP) are well known to play a role in influencing oedema formation and leukocyte accumulation during tissue insult and inflammation. Cutaneous inflammatory models to characterize a TNFalpha-dependent mechanism where endogenous SP act via the NK1 receptor to promote leukocyte accumulation in the absence of oedema formation were used. We found that TNFalpha induced dose-dependent leukocyte accumulation at 4 h, which returned towards basal levels at 8 h in NK1+/+ mice. This response was absent in both the NK1+/+ mice treated with an NK1 receptor antagonist and NK1-/- mice. At the highest dose IL-6 induced a significant accumulation in NK1+/+ and NK1-/- mice but IL-12 was ineffective. SP induced skin oedema but none of the cytokines did. Either co-injection of SP with low dose of TNFalpha (0.3 pmol/site) or SP previously injected (30 min) to TNFalpha evoked a significant increase in MPO activity when compared with that induced by the cytokine alone. In contrast, SP injected i.d. 3.5 h after TNFalpha failed to produce additive response. Control, but not capsaicin-pretreated rats (to deplete sensory nerves), exhibited a marked increase in MPO activity in response to TNFalpha. Histological analysis showed that TNFalpha caused tissue infiltrate of leukocytes in NK1+/+ mice, whilst leukocytes accumulated at intravascular sites in NK1-/- mice, but did not appear to emigrate, suggesting a defect in trans-endothelial migration. Interestingly, monocytes in addition to neutrophils accumulated 4 h post TNFalpha injection. In conclusion, the NK1 receptor plays a functional role in mediating leukocyte accumulation independently of the historically important NK1 mediated oedema formation. It seems that TNFalpha directly activates sensory nerve in addition to its chemoattractant activity. The NK1 receptor agonist influences the accumulation of monocytes in addition to that of PMN by 4 h, thus revealing an important influence of the NK1 receptor on TNFalpha mediated events in mouse skin.

Direct Interaction Between Nerves and Mast Cells Mediated by the SgIGSF/SynCAM Adhesion Molecule

Accumulating evidence has so far indicated that cross-talk between the nervous and immune systems plays a pivotal role in the pathophysiology of various diseases. As a prototypic demonstration of neuro-immune systems, the interaction between nerves and mast cells has been examined intensively. Anatomically, mast cells are often located in close proximity to nerves. Functionally, both cells communicate with each other in a bi-directional manner. Substance P released from nerves and proteases and cytokines from mast cells have proved to be important mediators in such communication. On the other hand, the molecules involved in membrane-membrane contacts between nerves and mast cells were largely unknown. In 2003, both cells were found to express the identical adhesion molecule, named SynCAM (synaptic cell adhesion molecule) or SgIGSF (spermatogenic immunoglobulin superfamily). Since SgIGSF/SynCAM binds homophilically, its involvement in nerve-mast cell interaction was examined in vitro. Superior cervical ganglia expressed SgIGSF/SynCAM along their neurites. Adhesion to these neurites of mast cells lacking SgIGSF/SynCAM was poor, and this was normalized by ectopic expression of SgIGSF/SynCAM. Moreover, SgIGSF/SynCAM-expressing mast cells were more competent in communicating with the neurites. Further understanding of the adhesion molecule-dependent interaction will be expected to open a new avenue in the field of neuro-immune cross-talk.

Involvement of transient receptor potential vanilloid 1 in the vascular and hyperalgesic components of joint inflammation

OBJECTIVE

To investigate the endogenous involvement of transient receptor potential vanilloid 1 (TRPV1) in a model of knee joint inflammation in the mouse.

METHODS

Following characterization of wild-type (WT) and TRPV1-knockout mice, inflammation was induced via intraarticular (IA) injection of Freund's complete adjuvant (CFA). Knee swelling was assessed as diameter, and inflammatory heat hyperalgesia was determined using the Hargreaves technique, for up to 3 weeks. At 18 hours, acute hyperpermeability was measured with 125I-albumin, and cytokines and myeloperoxidase activity, a marker of neutrophils, were assayed in synovial fluid extracts. The possibility that exogenous tumor necrosis factor alpha (TNFalpha) was involved in influencing TRPV1 activation was investigated in separate experiments.

RESULTS

Increased levels of knee swelling, hyperpermeability, leukocyte accumulation, and TNFalpha were found in WT mice 18 hours after IA CFA treatment compared with saline treatment. Significantly less knee swelling and hyperpermeability were found in TRPV1-/- mice, but leukocyte accumulation and TNFalpha levels were similar in WT and TRPV1-/- mice. Knee swelling in response to CFA remained significantly higher for a longer period in WT mice compared with TRPV1-/- mice, with thermal hyperalgesic sensitivity observed at 24 hours and at 1 week in WT, but not TRPV1-/-, mice. Knee swelling was attenuated (P < 0.05) in TRPV1-/- compared with WT mice 4 hours after IA administration of TNFalpha.

CONCLUSION

Our findings indicate that TRPV1 has a role in acute and chronic inflammation in the mouse knee joint. Thus, selective antagonism of TRPV1 should be considered as a potential target for treatment of acute and chronic joint inflammation.

Significance of Conversation between Mast Cells and Nerves

More and more studies are demonstrating interactions between the nervous system and the immune system. However, the functional relevance of this interaction still remains to be elucidated. Such associations have been found in the intestine between nerves and mast cells as well as between eosinophils and plasma cells. Similar morphologic associations have been demonstrated in the liver, mesentery, urinary bladder, and skin. Unmyelinated axons especially were found to associate with mast cells as well as Langerhans' cells in primate as well as murine skin. Although there are several pathways by which immune cells interact with the nervous system, the focus in this review will be on the interaction between mast cells and nerves.

The Spermatogenic Ig Superfamily/Synaptic Cell Adhesion Molecule Mast-Cell Adhesion Molecule Promotes Interaction with Nerves

Nerve-mast cell interaction is involved in both homeostatic and pathologic regulations. The molecules that sustain this association have not been identified. Because synaptic cell adhesion molecule (SynCAM), alternatively named spermatogenic Ig superfamily (SgIGSF), is expressed on both nerves and mast cells and because it binds homophilically, this molecule may be a candidate. To examine this possibility, mast cells with or without SgIGSF/SynCAM were cocultured with superior cervical ganglion neurons that express SgIGSF/SynCAM, and the number of mast cells attached to neurites was counted. The attachment of mast cells with SgIGSF/SynCAM, i.e., bone marrow-derived mast cells (BMMC) from wild-type mice, was inhibited dose-dependently by blocking Ab to SgIGSF/SynCAM. Mast cells without SgIGSF/SynCAM, i.e., BMMC from microphthalmia transcription factor-deficient mice and BMMC-derived cell line IC-2 cells, were defective in attachment to neurite, and transfection with SgIGSF/SynCAM normalized this. When the nerves were specifically activated by scorpion venom, one-quarter of the attached IC-2 cells mobilized Ca(2+) after a few dozen seconds, and ectopic SgIGSF/SynCAM doubled this proportion. At points of contact between neurites and wild-type BMMC, SgIGSF/SynCAM was locally concentrated in both neurites and BMMC. SgIGSF/SynCAM on mast cells appeared to predominantly mediate attachment and promote communication with nerves.

Elicitation of allergic asthma by immunoglobulin free light chains

The observation that only 50% of patients with adult asthma manifest atopy indicates that other inflammatory mechanisms are likely involved in producing the characteristic features of this disorder; namely reversible airway obstruction, hyperresponsiveness, and pulmonary inflammation. Our recent discovery that antigen-specific Ig free light chains (LCs) mediate hypersensitivity-like responses suggests that these molecules may be of import in the pathophysiology of asthma. Using a murine experimental model of nonatopic asthma, we now have shown that an LC antagonist, the 9-mer peptide F991, can abrogate the development of airway obstruction, hyperresponsiveness, and pulmonary inflammation. Further, passive immunization with antigen-specific LCs and subsequent airway challenge can elicit a mast cell-dependent reaction leading to acute bronchoconstriction. These findings, and the demonstration that the concentration of free kappa LCs in the sera of patients with adult asthma were significantly increased (as compared with age-matched nonasthmatic individuals), provide previously undescribed insight into the pathogenesis of asthma. In addition, the ability to inhibit pharmacologically LC-induced mast cell activation provides a therapeutic means to prevent or ameliorate the adverse bronchopulmonary manifestations of this incapacitating disorder.

Live Cell Imaging to Study Signaling Molecules in Allergic Reactions

Mast cells are widely distributed throughout the body, predominantly near blood vessels and nerves, and express effector functions in allergic reactions, inflammatory diseases, and host defense. The activation of mast cells results in secretion of the preformed chemical mediators in their granules by a regulated process of exocytosis and leads to synthesis and secretion of lipid mediators and cytokines. Their soluble factors contribute to allergic inflammation. Mast cells are associated with hypersensitivity reactions, not only in the classical immunoglobulin E (IgE)-dependent mechanism but also in an IgE-independent manner. In particular, investigations of potential anatomical and functional interactions between mast cells and the nervous system have recently attracted great interest. To understand these molecular mechanisms in mast cell activation, the ability to visualize, track, and quantify molecules and events in living mast cells is an essential and powerful tool. Recent dramatic advances in imaging technology and labeling techniques have enabled us to carry out these tasks with high spatiotemporal resolution using confocal laser scanning microscopes, green fluorescent protein and its derivatives, and image analysis systems. Here we review our investigations of the dynamic processes of intracellular signaling molecules, cellular structure, and interactions with neurons in mast cells to provide basic and valuable information for allergy and clinical immunology using these new imaging methods.

Association of tumor necrosis factor-alpha polymorphisms and ozone-induced change in lung function

Ozone is a major air pollutant with adverse health effects which exhibit marked inter-individual variability. In mice, regions of genetic linkage with ozone-induced lung injury include the tumor necrosis factor-alpha (TNF), lymphotoxin-alpha (LTA), Toll-like receptor 4 (TLR4), superoxide dismutase (SOD2), and glutathione peroxidase (GPX1) genes. We genotyped polymorphisms in these genes in 51 individuals who had undergone ozone challenge. Mean change in FEV1 with ozone challenge, as a percentage of baseline, was -3% in TNF -308G/A or A/A individuals, compared with -9% in G/G individuals (p = 0.024). When considering TNF haplotypes, the smallest change in FEV1 with ozone exposure was associated with the TNF haplotype comprising LTA +252G/TNF -1031T/TNF -308A/TNF -238G. This association remained statistically significant after correction for age, sex, disease, and ozone concentration (p = 0.047). SOD2 or GPX1 genotypes were not associated with lung function, and the TLR4 polymorphism was too infrequent to analyze. The results of this study support TNF as a genetic factor for susceptibility to ozone-induced changes in lung function in humans, and has potential implications for stratifying health risks of air pollution.

Mast cells play a pivotal role in ischaemia reperfusion injury to skeletal muscles

Ischaemia reperfusion (IR) injury is a serious complication of cardiovascular disease, transplantation and replantation surgery. Once established there is no effective method of treatment. Although studies using mast cell-depleted (Wf/Wf) mice implicate mast cells in this pathology, they do not exclude a contribution by other deficiencies expressed in Wf/Wf mice. In order to obtain conclusive evidence for the role of mast cells, we engrafted cultured bone marrow-derived mast cells (BMMC) from normal mice into their Wf/Wf littermates. After 12 weeks, the hind limbs of Wf/Wf, engrafted Wf/Wf and normal littermates were subjected to IR injury. Muscle viability was assessed by both morphology and by nitroblue tetrazolium histochemical assay. Here, we present conclusive evidence for a causal role of mast cells in IR injury. Our data show that muscles from Wf/Wf mice subjected to IR have a significantly greater proportion of viable fibres than normal littermates subjected to identical injury (78.9+/-5.2 vs 27.2+/-3.7%, respectively). When Wf/Wf IR-resistant mice were engrafted with BMMC from normal littermates and subjected to IR, the proportion of viable muscle fibres was significantly reduced (78.9+/-5.2 vs 37.0+/-6.5%). Thus, engraftment of BMMC into Wf/Wf mice restores the susceptibility of skeletal muscle to IR injury irrespective of other abnormalities in Wf/Wf mice. In this model, the numerical density of mast cells undergoes a significant decrease within 1 h of reperfusion, indicating extensive mast cell degranulation. We conclude that mast cells are pivotal effector cells in the pathogenesis of IR injury of murine skeletal muscle.