TLR2 Regulates Mast Cell IL-6 and IL-13 Production During Listeria monocytogenes Infection

Listeria monocytogenes (L.m) is efficiently controlled by several cells of the innate immunity, including the Mast Cell (MC). MC is activated by L.m inducing its degranulation, cytokine production and microbicidal mechanisms. TLR2 is required for the optimal control of L.m infection by different cells of the immune system. However, little is known about the MC receptors involved in recognizing this bacterium and whether these interactions mediate MC activation. In this study, we analyzed whether TLR2 is involved in mediating different MC activation responses during L.m infection. We found that despite MC were infected with L.m, they were able to clear the bacterial load. In addition, MC degranulated and produced ROS, TNF-α, IL-1β, IL-6, IL-13 and MCP-1 in response to bacterial infection. Interestingly, L.m induced the activation of signaling proteins: ERK, p38 and NF-κB. When TLR2 was blocked, L.m endocytosis, bactericidal activity, ROS production and mast cell degranulation were not affected. Interestingly, only IL-6 and IL-13 production were affected when TLR2 was inhibited in response to L.m infection. Furthermore, p38 activation depended on TLR2, but not ERK or NF-κB activation. These results indicate that TLR2 mediates only some MC activation pathways during L.m infection, mainly those related to IL-6 and IL-13 production.

The Emerging Role of Mast Cells in Response to Fungal Infection

Mast cells (MCs) have been considered as the core effector cells of allergic diseases. However, there are evidence suggesting that MCs are involved in the mechanisms of fungal infection. MCs are mostly located in the border between host and environment and thus may have easy contact with the external environmental pathogens. These cells express receptors which can recognize pathogen-associated molecular patterns such as Toll-like receptors (TLR2/4) and C-type Lectins receptors (Dectin-1/2). Currently, more and more data indicate that MCs can be interacted with some fungi (Candida albicans, Aspergillus fumigatus and Sporothrix schenckii). It is demonstrated that MCs can enhance immunity through triggered degranulation, secretion of cytokines and chemokines, neutrophil recruitment, or provision of extracellular DNA traps in response to the stimulation by fungi. In contrast, the involvement of MCs in some immune responses may lead to more severe symptoms, such as intestinal barrier function loss, development of allergic bronchial pulmonary aspergillosis and increased area of inflammatory in S. schenckii infection. This suggests that MCs and their relevant signaling pathways are potential treatment regimens to prevent the clinically unwanted consequences. However, it is not yet possible to make definitive statements about the role of MCs during fungal infection and/or pathomechanisms of fungal diseases. In our article, we aim to review the function of MCs in fungal infections from molecular mechanism to signaling pathways, and illustrate the role of MCs in some common host-fungi interactions.

Group A Streptococcus Prevents Mast Cell Degranulation to Promote Extracellular Trap Formation

The resurgence of Group A Streptococcus (GAS) infections in the past two decades has been a rising major public health concern. Due to a large number of GAS infections occurring in the skin, mast cells (MCs), innate immune cells known to localize to the dermis, could play an important role in controlling infection. MCs can exert their antimicrobial activities either early during infection, by degranulation and release of antimicrobial proteases and the cathelicidin-derived antimicrobial peptide LL-37, or by forming antibacterial MC extracellular traps (MCETs) in later stages of infection. We demonstrate that MCs do not directly degranulate in response to GAS, reducing their ability to control bacterial growth in early stages of infection. However, MC granule components are highly cytotoxic to GAS due to the pore-forming activity of LL-37, while MC granule proteases do not significantly affect GAS viability. We therefore confirmed the importance of MCETs by demonstrating their capacity to reduce GAS survival. The data therefore suggests that LL-37 from MC granules become embedded in MCETs, and are the primary effector molecule by which MCs control GAS infection. Our work underscores the importance of a non-traditional immune effector cell, utilizing a non-conventional mechanism, in the defense against an important human pathogen.

Vasoactive Intestinal Polypeptide and Mast Cells Regulate Increased Passage of Colonic Bacteria in Patients With Irritable Bowel Syndrome

BACKGROUND & AIMS

Irritable bowel syndrome (IBS) is associated with intestinal dysbiosis and symptoms of IBS develop following gastroenteritis. We aimed to study the passage of live bacteria through the colonic epithelium, and determine the role of mast cells (MCs) and vasoactive intestinal polypeptide (VIP) in barrier regulation in IBS and healthy individuals.

METHODS

Colon biopsies from 32 women with IBS and 15 age-matched healthy women (controls) were mounted in Ussing chambers; we measured numbers of fluorescently labeled Escherichia coli HS and Salmonella typhimurium that passed through from the mucosal side to the serosal side of the tissue. Some biopsies were exposed to agents that block the VIP receptors (VPAC1 and VPAC2) or MCs. Levels of VIP and tryptase were measured in plasma and biopsy lysates. Number of MCs and MCs that express VIP or VIP receptors were quantified by immunofluorescence. Biopsies from an additional 5 patients with IBS and 4 controls were mounted in chambers and Salmonella were added; we studied passage routes through the epithelium by transmission electron microscopy and expression of tight junctions by confocal microscopy.

RESULTS

In colon biopsies from patients with IBS, larger numbers of E coli HS and S typhimurium passed through the epithelium than in biopsies from controls (P < .0005). In transmission electron microscopy analyses, bacteria were found to cross the epithelium via only the transcellular route. Bacterial passage was reduced in biopsies from patients with IBS and controls after addition of antibodies against VPACs or ketotifen, which inhibits MCs. Plasma samples from patients with IBS had higher levels of VIP than plasma samples from controls. Biopsies from patients with IBS had higher levels of tryptase, larger numbers of MCs, and a higher percentage of MCs that express VPAC1 than biopsies from controls. In biopsies from patients with IBS, addition of Salmonella significantly reduced levels of occludin; subsequent addition of ketotifen significantly reversed this effect.

CONCLUSIONS

We found that colonic epithelium tissues from patients with IBS have increased translocation of commensal and pathogenic live bacteria compared with controls. The mechanisms of increased translocation include MCs and VIP.

Proliferation of Prostate Stromal Cell Induced by Benign Prostatic Hyperplasia Epithelial Cell Stimulated With Trichomonas vaginalis via Crosstalk With Mast Cell

BACKGROUND

Chronic inflammation has a role in the pathogenesis of benign prostatic hyperplasia (BPH) and prostate cancer. Mast cells have been detected in chronic inflammatory infiltrate of the prostate, and it is possible that the interaction between prostate epithelial cells and Trichomonas vaginalis influences the activity of mast cells in the prostate stroma. Activated mast cells might influence the biological functions of nearby tissues and cells. In this study, we investigated whether mast cells reacted with the culture supernatant of BPH epithelial cells infected with T. vaginalis may induce the proliferation of prostate stromal cells.

METHODS

To measure the proliferation of prostate stromal cells in response to chronic inflammation caused by the infection of BPH-1 cells with T. vaginalis, the CCK-8 assay and wound healing assay were used. ELISAs, quantitative real-time PCR, western blotting and immunofluorescence were used to measure the production and expression of inflammatory cytokine and cytokine receptor.

RESULTS

BPH-1 cells incubated with live trichomonads produced increased levels of CCL2, IL-1β, IL-6, and CXCL8, and induced the migration of mast cells and monocytes. When the culture supernatant of BPH-1 cells stimulated with trichomonads (TCM) was added to mast cells, they became activated, as confirmed by release of β-hexosaminidase and CXCL8. Prostate stromal cells incubated with the culture supernatant of mast cells activated with TCM (M-TCM) proliferated and expressed increased levels of CXCL8, CCL2, and the cytokine receptors CXCR1 and CCR2. Blocking the chemokine receptors reduced the proliferation of stromal cells and also decreased the production of CXCL8 and CCL2. Moreover, the expression of FGF2, cyclin D1, and Bcl-2 was increased in the proliferated stromal cells stimulated with M-TCM. Additionally, the M-TCM-treated stromal cells were more invasive than control cells.

CONCLUSIONS

The inflammatory mediators released by BPH epithelial cells in response to infection by trichomonads induce the migration and activation of mast cells. The activated mast cells induce the proliferation of prostate stromal cells via CXCL8-CXCR1 and CCL2-CCR2 signaling. Our results therefore show that the inflammatory response by BPH epithelial cells stimulated with T. vaginalis induce the proliferation of prostate stromal cells via crosstalk with mast cells. Prostate 76:1431-1444, 2016. © 2016 Wiley Periodicals, Inc.

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.

Human mast cell activation with viruses and pathogen products

Mast cells have been demonstrated to have critical roles in host defense against a number of types of pathogens. In order to better understand how mast cells participate in effective immune responses, it is important to evaluate their ability to respond directly to pathogens and their products. In the current chapter we provide a methodology to evaluate human mast cell responses to a number of bacterial and fungal pathogen products and to mammalian reovirus as a model of acute viral infection. These methods should provide key information necessary to aid in the effective design of experiments to evaluate human mast cell responses to a number of other organisms. However, it is important to carefully consider the biology of the mast cell subsets and pathogens involved and the optimal experimental conditions necessary to evaluate mediators of interest.

Brain-gut axis in the pathogenesis of Helicobacter pylori infection

Helicobacter pylori (H. pylori) infection is the main pathogenic factor for upper digestive tract organic diseases. In addition to direct cytotoxic and proinflammatory effects, H. pylori infection may also induce abnormalities indirectly by affecting the brain-gut axis, similar to other microorganisms present in the alimentary tract. The brain-gut axis integrates the central, peripheral, enteric and autonomic nervous systems, as well as the endocrine and immunological systems, with gastrointestinal functions and environmental stimuli, including gastric and intestinal microbiota. The bidirectional relationship between H. pylori infection and the brain-gut axis influences both the contagion process and the host’s neuroendocrine-immunological reaction to it, resulting in alterations in cognitive functions, food intake and appetite, immunological response, and modification of symptom sensitivity thresholds. Furthermore, disturbances in the upper and lower digestive tract permeability, motility and secretion can occur, mainly as a form of irritable bowel syndrome. Many of these abnormalities disappear following H. pylori eradication. H. pylori may have direct neurotoxic effects that lead to alteration of the brain-gut axis through the activation of neurogenic inflammatory processes, or by microelement deficiency secondary to functional and morphological changes in the digestive tract. In digestive tissue, H. pylori can alter signaling in the brain-gut axis by mast cells, the main brain-gut axis effector, as H. pylori infection is associated with decreased mast cell infiltration in the digestive tract. Nevertheless, unequivocal data concerning the direct and immediate effect of H. pylori infection on the brain-gut axis are still lacking. Therefore, further studies evaluating the clinical importance of these host-bacteria interactions will improve our understanding of H. pylori infection pathophysiology and suggest new therapeutic approaches.

[Pattern recognition and activation effect of mast cells in vitro infected by Brucella suis S2]

OBJECTIVE

To study the pattern recognition and activation effect of mast cells infected by Brucella (B.) suis S2.

METHODS

Mast cells derived from bone marrow in vitro were infected by B.suis S2. The change in the cell morphology was observed with Wrights-Giemsa's staining, and cell degranulation was tested with toluidine blue staining. The extracellular levels of histamine, IFN-γ and IL-12 of mast cells at 1 and 12 h after infection were detected by indirect ELISA. The uptake pattern of mast cells to B.suis S2 was determined by laser-scanning confocal microscopy. The expressions of TLR4 and TLR8 mRNA were detected by RT-PCR at 12 and 24 h after infection by B.suis S2, and the TLR4 and TLR8 protein expressions were detected by flow cytometry at 24 h.

RESULTS

The form of mast cells infected by B.suis S2 was obviously changed. Significant degranulation was observed at 1 h, and at 1, 12 h post-infection by B.suis S2, the content of histamine secreted by mast cells was significantly higher than that of normal control group (P<0.05), and IFN-γ and IL-12 were not found. At 30 min and 1 h, B.suis S2 bound to the mast cell surface and were not uptaken into the mast cells. Compared with the control group, the expression of TLR4 mRNA increased after 12 h infection by B.suis S2, and was reduced at 24 h. The expression of TLR4 protein rose at 24 h, but the expression of TLR8 mRNA and protein did not alter at 12 and 24 h after infection by B.suis S2.

CONCLUSION

B.suis S2 can bind to the cell surface and activate mast cells, cause their degranulation, induce the release of histamine, but IFN-γ and IL-12 were not found during the observing time. The mechanism may be that B.suis S2 can be recognized by mast cells through TLR4 but can not be phagocytosed by mast cells.

Autophagy enhances bacterial clearance during P. aeruginosa lung infection

Pseudomonas aeruginosa is an opportunistic bacterial pathogen which is the leading cause of morbidity and mortality among cystic fibrosis patients. Although P. aeruginosa is primarily considered an extacellular pathogen, recent reports have demonstrated that throughout the course of infection the bacterium acquires the ability to enter and reside within host cells. Normally intracellular pathogens are cleared through a process called autophagy which sequesters and degrades portions of the cytosol, including invading bacteria. However the role of autophagy in host defense against P. aeruginosa in vivo remains unknown. Understanding the role of autophagy during P. aeruginosa infection is of particular importance as mutations leading to cystic fibrosis have recently been shown to cause a blockade in the autophagy pathway, which could increase susceptibility to infection. Here we demonstrate that P. aeruginosa induces autophagy in mast cells, which have been recognized as sentinels in the host defense against bacterial infection. We further demonstrate that inhibition of autophagy through pharmacological means or protein knockdown inhibits clearance of intracellular P. aeruginosa in vitro, while pharmacologic induction of autophagy significantly increased bacterial clearance. Finally we find that pharmacological manipulation of autophagy in vivo effectively regulates bacterial clearance of P. aeruginosa from the lung. Together our results demonstrate that autophagy is required for an effective immune response against P. aeruginosa infection in vivo, and suggest that pharmacological interventions targeting the autophagy pathway could have considerable therapeutic potential in the treatment of P. aeruginosa lung infection.

Noradrenergic neurons regulate monocyte trafficking and mortality during gram-negative peritonitis in mice

Effective host defense requires a robust, yet self-limited response to pathogens. A poorly calibrated response can lead to either bacterial dissemination due to insufficient inflammation or organ injury due to excessive inflammation. Recent evidence suggests that the cholinergic anti-inflammatory reflex helps calibrate the immune response. However, the influence of peripheral noradrenergic neurons, which are primarily sympathetic neurons, in regulating immunity remains incompletely characterized. Using a model of 6-hydroxydopamine-mediated noradrenergic nerve ablation, we show that elimination of noradrenergic neurons improves survival during Klebsiella pneumoniae peritonitis (67 versus 23%, p < 0.005) in mice. The survival benefit results from enhanced MCP-1-dependent monocyte recruitment and a subsequent decrease in bacterial loads. Splenectomy eliminated both the survival benefit of 6-hydroxydopamine and monocyte recruitment, suggesting that monocytes recruited to the peritoneum originate in the spleen. These results suggest that noradrenergic neurons regulate the immune response through two pathways. First, sympathetic nerve-derived norepinephrine directly restrains MCP-1 production by peritoneal macrophages during infection. Second, norepinephrine derived from the vagally innervated splenic nerve regulates splenic monocyte egress. Removal of these two modulators of the immune response enhances antibacterial immunity and improves survival. These results may have implications for how states of catecholamine excess influence the host response to bacterial infections.

Listeria monocytogenes alters mast cell phenotype, mediator and osteopontin secretion in a listeriolysin-dependent manner

Whilst mast cells participate in the immune defence against the intracellular bacterium Listeria monocytogenes, there is conflicting evidence regarding the ability of L. monocytogenes to infect mast cells. It is known that the pore-forming toxin listeriolysin (LLO) is important for mast cell activation, degranulation and the release of pro-inflammatory cytokines. Mast cells, however, are a potential source of a wide range of cytokines, chemokines and other mediators including osteopontin, which contributes to the clearing of L. monocytogenes infections in vivo, although its source is unknown. We therefore aimed to resolve the controversy of mast cell infection by L. monocytogenes and investigated the extent of mediator release in response to the bacterium. In this paper we show that the infection of bone marrow-derived mast cells by L. monocytogenes is inefficient and LLO-independent. LLO, however, is required for calcium-independent mast cell degranulation as well as for the transient and selective downregulation of cell surface CD117 (c-kit) on mast cells. We demonstrate that in addition to the key pro-inflammatory cytokines TNF-α and IL-6, mast cells release a wide range of other mediators in response to L. monocytogenes. Osteopontin, IL-2, IL-4, IL-13 and granulocyte macrophage colony-stimulating factor (GM-CSF), and chemokines including CCL2, CCL3, CCL4 and CCL5 are released in a MyD88-dependent manner. The wide range of mediators released by mast cells in response to L. monocytogenes may play an important role in the recruitment and activation of a variety of immune cells in vivo. The cocktail of mediators, however, is unlikely to skew the immune response to a particular effector response. We propose that mast cells provide a hitherto unreported source of osteopontin, and may provide an important role in co-ordinating the immune response during Listeria infection.

Systemic effects of ingested Lactobacillus rhamnosus: inhibition of mast cell membrane potassium (IKCa) current and degranulation

Exposure of the intestine to certain strains lactobacillus can have systemic immune effects that include the attenuation of allergic responses. Despite the central role of mast cells in allergic disease little is known about the effect of lactobacilli on the function of these cells. To address this we assessed changes in rat mast cell activation following oral treatment with a strain of Lactobacillus known to attenuate allergic responses in animal models. Sprague Dawley rats were fed with L. rhamnosus JB-1 (1×10(9)) or vehicle control for 9 days. Mediator release from peritoneal mast cells (RPMC) was determined in response to a range of stimuli. Passive cutaneous anaphylaxis (PCA) was used to assess mast cell responses in vivo. The Ca(2+) activated K(+) channel (KCa3.1) current, identified as critical to mast cell degranulation, was monitored by whole cell patch-clamp. L. rhamnosus JB-1 treatment lead to significant inhibition of mast cell mediator release in response to a range of stimuli including IgE mediated activation. Furthermore, the PCA response was significantly reduced in treated rats. Patch-clamp studies revealed that RPMC from treated animals were much less responsive to the KCa3.1 opener, DCEBIO. These studies demonstrate that Ingestion of L. rhamnosus JB-1 leads to mast cell stabilization in rats and identify KCa3.1 as an immunomodulatory target for certain lactobacilli. Thus the systemic effects of certain candidate probiotics may include mast cell stabilization and such actions could contribute to the beneficial effect of these organisms in allergic and other inflammatory disorders.

Parasitic infection improves survival from septic peritonitis by enhancing mast cell responses to bacteria in mice

Mammals are serially infected with a variety of microorganisms, including bacteria and parasites. Each infection reprograms the immune system's responses to re-exposure and potentially alters responses to first-time infection by different microorganisms. To examine whether infection with a metazoan parasite modulates host responses to subsequent bacterial infection, mice were infected with the hookworm-like intestinal nematode Nippostrongylus brasiliensis, followed in 2–4 weeks by peritoneal injection of the pathogenic bacterium Klebsiella pneumoniae. Survival from Klebsiella peritonitis two weeks after parasite infection was better in Nippostrongylus-infected animals than in unparasitized mice, with Nippostrongylus-infected mice having fewer peritoneal bacteria, more neutrophils, and higher levels of protective interleukin 6. The improved survival of Nippostrongylus-infected mice depends on IL-4 because the survival benefit is lost in mice lacking IL-4. Because mast cells protect mice from Klebsiella peritonitis, we examined responses in mast cell-deficient Kit^W-sh/Kit^W-sh mice, in which parasitosis failed to improve survival from Klebsiella peritonitis. However, adoptive transfer of cultured mast cells to Kit^W-sh/Kit^W-sh mice restored survival benefits of parasitosis. These results show that recent infection with Nippostrongylus brasiliensis protects mice from Klebsiella peritonitis by modulating mast cell contributions to host defense, and suggest more generally that parasitosis can yield survival advantages to a bacterially infected host.

The antimicrobial peptides piscidins are stored in the granules of professional phagocytic granulocytes of fish and are delivered to the bacteria-containing phagosome upon phagocytosis

Antimicrobial peptides (AMPs) are increasingly recognized as a critical first line of defence against many pathogens. The genes encoding these peptides are expressed in numerous tissue and cell types from a wide variety of different species including mammals, amphibians, fish, and insects. In this study, we report that the AMPs called piscidins were primarily present in the mast cells (MCs) of fish and were only identified in fish belonging to the Order Perciformes. It is striking that histamine was seen to have a similar evolutionary history, since the only piscine MCs endowed with this molecule are in the Perciformes. We also show that both MCs and professional phagocytic granulocytes were armed with different piscidin molecules. In contrast, macrophages were devoid of these AMPs. More importantly, we found by immunoelectron microscopy that piscidins were delivered to the bacteria-containing phagosome of granulocytes upon phagocytosis, suggesting a role for these AMPs in the killing of both extracellular and intracellular pathogenic bacteria.

Effects of Gamma Radiation on FcεRI and TLR-Mediated Mast Cell Activation

Ionizing gamma radiation has several therapeutic indications including bone marrow transplantation and tumor ablation. Among immune cells, susceptibility of lymphocytes to gamma radiation is well known. However, there is little information on the effects of gamma radiation on mast cells, which are important in both innate and acquired immunity. Previous studies have suggested that mast cells may release histamine in response to high doses of gamma radiation, whereas other reports suggest that mast cells are relatively radioresistant. No strong link has been established between gamma radiation and its effect on mast cell survival and activation. We examined both human and murine mast cell survival and activation, including mechanisms related to innate and acquired immune responses following gamma radiation. Data revealed that human and murine mast cells were resistant to gamma radiation-induced cytotoxicity and, importantly, that irradiation did not directly induce beta-hexosaminidase release. Instead, a transient attenuation of IgE-mediated beta-hexosaminidase release and cytokine production was observed which appeared to be the result of reactive oxygen species formation after irradiation. Mast cells retained the ability to phagocytose Escherichia coli particles and respond to TLR ligands as measured by cytokine production after irradiation. In vivo, there was no decrease in mast cell numbers in skin of irradiated mice. Additionally, mast cells retained the ability to respond to Ag in vivo as measured by passive cutaneous anaphylaxis in mice after irradiation. Mast cells are thus resistant to the cytotoxic effects and alterations in function after irradiation and, despite a transient inhibition, ultimately respond to innate and acquired immune activation signals.

Human mast cells, bacteria, and intestinal immunity

Mast cells are versatile tissue regulator cells controlling major intestinal functions such as epithelial secretion, epithelial permeability, blood flow, neuroimmune interactions, and peristalsis. Most importantly, mast cells are key regulators of the integrity and function of the gastrointestinal barrier. At the same time, they can act as immunomodulatory cells by reacting to various exogenous signals from bacteria, viruses, and parasites through innate recognition receptors, such as Toll-like receptors (TLRs) or through receptors of the specific immune system, such as immunoglobulins (Igs) bound to their cell surface. This mast cell function is enhanced by an intensive cross talk of mast cells with other cells of the innate or adaptive immune systems. Finally, mast cells act as inflammatory cells mediating diseases such as allergy, once they become dysregulated because of excess of allergen, allergen-specific IgE and cytokines, or invading microbes. The present article focuses on the human mast cell functions in the intestine and compares the data with those derived from animal experiments. In particular, the role of bacteria and TLRs expression by mast cells for allergic reactions are discussed.

Clostridium difficile toxins A and B directly stimulate human mast cells

Clostridium difficile toxins A and B (TcdA and TcdB) are the causative agents of antibiotic-associated pseudomembranous colitis. Mucosal mast cells play a crucial role in the inflammatory processes underlying this disease. We studied the direct effects of TcdA and TcdB on the human mast cell line HMC-1 with respect to degranulation, cytokine release, and the activation of proinflammatory signal pathways. TcdA and TcdB inactivate Rho GTPases, the master regulators of the actin cytoskeleton. The inactivation of Rho GTPases induced a reorganization of the actin cytoskeleton accompanied by morphological changes of cells. The TcdB-induced reorganization of the actin cytoskeleton in HMC-1 cells reduced the number of electron-dense mast cell-specific granules. Accordingly, TcdB induced the release of hexosaminidase, a marker for degranulation, in HMC-1 cells. The actin rearrangement was found to be responsible for degranulation since latrunculin B induced a comparable hexosaminidase release. In addition, TcdB as well as latrunculin B induced the activation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase 1/2 and also resulted in a p38 MAPK-dependent increased formation of prostaglandins D(2) and E(2). The autocrine stimulation of HMC-1 cells by prostaglandins partially contributed to the degranulation. Interestingly, TcdB-treated HMC-1 cells, but not latrunculin B-treated HMC-1 cells, showed a strong p38 MAPK-dependent increase in interleukin-8 release. Differences in the mast cell responses to TcdB and latrunculin B are probably due to the presence of functionally inactive Rho GTPases in toxin-treated cells. Thus, the HMC-1 cell line is a promising model for studying the direct effects of C. difficile toxins on mast cells independently of the tissue context.

The multiple mechanisms of Ca2+ signalling by listeriolysin O, the cholesterol-dependent cytolysin of Listeria monocytogenes

Cholesterol-dependent cytolysins (CDCs) represent a large family of conserved pore-forming toxins produced by several Gram-positive bacteria such as Listeria monocytogenes, Streptococcus pyrogenes and Bacillus anthracis. These toxins trigger a broad range of cellular responses that greatly influence pathogenesis. Using mast cells, we demonstrate that listeriolysin O (LLO), a prototype of CDCs produced by L. monocytogenes, triggers cellular responses such as degranulation and cytokine synthesis in a Ca(2+)-dependent manner. Ca(2+) signalling by LLO is due to Ca(2+) influx from extracellular milieu and release of from intracellular stores. We show that LLO-induced release of Ca(2+) from intracellular stores occurs via at least two mechanisms: (i) activation of intracellular Ca(2+) channels and (ii) a Ca(2+) channels independent mechanism. The former involves PLC-IP(3)R operated Ca(2+) channels activated via G-proteins and protein tyrosine kinases. For the latter, we propose a novel mechanism of intracellular Ca(2+) release involving injury of intracellular Ca(2+) stores such as the endoplasmic reticulum. In addition to Ca(2+) signalling, the discovery that LLO causes damage to an intracellular organelle provides a new perspective in our understanding of how CDCs affect target cells during infection by the respective bacterial pathogens.

Toll-like receptor-MyD88 and Fc receptor pathways of mast cells mediate the thyroid dysfunctions observed during nonthyroidal illness

Bacterial infections and other pathologic conditions induce complex dysfunctions of the hypothalamic-pituitary-thyroid axis, collectively known as nonthyroidal illness (NTI). To explore the pathogenesis of bacterial NTI, we injected Mycobacterium tuberculosis extracts or Escherichia coli LPS in mice lacking key components of the Toll-like receptor or crystallizable fragment (Fc) receptor pathways. In wild-type mice, the bacterial components induced a hypothyroidism characterized by elements of both hypothalamic and thyroidal dysfunction. This NTI hypothyroidism did not develop in mice lacking the MyD88 adaptor or in those with a reduced number of mast cells. The hypothyroid responsiveness to LPS, however, was restored upon reconstitution with mast cells derived from the bone marrow of wild-type donors. In addition to bacterial components, whole immunoglobulins induced NTI hypothyroidism in wild-type mice, but not in those lacking activating Fc receptors or mast cells. The study demonstrates a link between Toll-like and Fc receptor signaling and thyroid gland function, uncovering a role of mast cells in murine NTI.

New and emerging roles for mast cells in host defence

Mast cells are highly effective sentinel cells, found close to blood vessels and especially common sites of potential infection, such as the skin, airways and gastrointestinal tract. Mast cells participate actively in the innate immune responses to many pathogens through a broad spectrum of mediators that can be selectively generated. They also have a role as innate effector cells in enhancing the earliest processes in the development of acquired immune responses. Studies of bacterial and parasitic models have revealed mast cell dependent regulation of effector cell recruitment, mucosal barrier function and lymph node hypertrophy. An important role for mast cells in viral infection is also implied by several in vivo and in vitro studies. There are multiple direct and indirect pathways by which mast cells can be selectively activated by pathogens including Toll-like receptors, co-receptors and complement component receptors. Understanding the mechanisms and scope of the contribution of mast cells to host defence will be crucial to regulating their activity therapeutically.

High mast cell density associated with granulomatous formation in tuberculous lymphadenitis

Mast cells are one of the main inflammatory cells involved in the pathogenesis of tuberculosis. Previous reports revealed that mast cells participated in both acute and chronic states of infection with Mycobacterium tuberculosis through direct contact or indirect enhancement by releasing mediators. The authors evaluated mast cell density on tissue sections of tuberculous lymphadenitis stained with 0.1% toluidine blue from 45 cases, all of which were retrieved from the surgical pathology files of King Chulalongkorn Memorial Hospital from 1999 to 2006. A number of mast cells were correlated semiquantitatively with granulomas which were formed by aggregation of epithelioid histiocytes, multinucleated giant cells, and caseous necrosis. We found that mast cell density was significantly increased in lymph nodes with greater granuloma involvement (p = 0.030) and multinucleated giant cell formation (p = 0.010). These findings indicate a significant correlation between mast cell density and the granulomatous formation responsible for M. tuberculosis.

Neutrophil histamine contributes to inflammation in mycoplasma pneumonia

Mycoplasmas cause chronic inflammation and are implicated in asthma. Mast cells defend against mycoplasma infection and worsen allergic inflammation, which is mediated partly by histamine. To address the hypothesis that mycoplasma provokes histamine release, we exposed mice to Mycoplasma pulmonis, comparing responses in wild-type and mast cell–deficient Kit^W-sh/Kit^W-sh (W-sh) mice. Low histamine levels in uninfected W-sh mice confirmed the conventional wisdom that mast cells are principal sources of airway and serum histamine. Although mycoplasma did not release histamine acutely in wild-type airways, levels rose up to 50-fold above baseline 1 week after infection in mice heavily burdened with neutrophils. Surprisingly, histamine levels also rose profoundly in infected W-sh lungs, increasing in parallel with neutrophils and declining with neutrophil depletion. Furthermore, neutrophils from infected airway were highly enriched in histamine compared with naive neutrophils. In vitro, mycoplasma directly stimulated histamine production by naive neutrophils and strongly upregulated mRNA encoding histidine decarboxylase, the rate-limiting enzyme in histamine synthesis. In vivo, treatment with antihistamines pyrilamine or cimetidine decreased lung weight and severity of pneumonia and tracheobronchitis in infected W-sh mice. These findings suggest that neutrophils, provoked by mycoplasma, greatly expand their capacity to synthesize histamine, thereby contributing to lung and airway inflammation.

Pseudomonas aeruginosa-Induced Human Mast Cell Apoptosis Is Associated with Up-Regulation of Endogenous Bcl-xSand Down-Regulation of Bcl-xL

Mast cells play a critical role in the host defense against bacterial infection. Recently, apoptosis has been demonstrated to be essential in the regulation of host response to Pseudomonas aeruginosa. In this study we show that human mast cell line HMC-1 and human cord blood-derived mast cells undergo apoptosis as determined by the ssDNA formation after infection with P. aeruginosa. P. aeruginosa induced activation of caspase-3 in mast cells as evidenced by the cleavage of D4-GDI, an endogenous caspase-3 substrate and the generation of an active form of caspase-3. Interestingly, P. aeruginosa treatment induced up-regulation of Bcl-x(S) and down-regulation of Bcl-x(L). Bcl-x(S), and Bcl-x(L) are alternative variants produced from the same Bcl-x pre-mRNA. The former is proapoptotic and the latter is antiapoptotic likely through regulating mitochondrial membrane integrity. Treatment of mast cells with P. aeruginosa induced release of cytochrome c from mitochondria and loss of mitochondrial membrane potentials. Moreover, P. aeruginosa treatment reduced levels of Fas-associated death domain protein-like IL-1beta-converting enzyme-inhibitory proteins (FLIPs) that are endogenous apoptosis inhibitors through counteraction with caspase-8. Thus, human mast cells undergo apoptosis after encountering P. aeruginosa through a mechanism that likely involves both the Bcl family protein mitochondrial-dependent and the FLIP-associated caspase-8 pathways.

Streptococcus pneumoniae induces mast cell degranulation

Streptococcus pneumoniae colonizes the nasopharynx of healthy human carriers, but occasionally can spread in the body causing severe diseases. The mucosa of the respiratory tract is enriched in mast cells, key players of the innate immune response. Here, we report on the interaction of various strains of S. pneumoniae with the mast cell line RBL-2H3. Live, but not heat-killed, bacteria were found to induce mast cell degranulation in a dose- and time-dependent manner, only partially controlled by cytosolic calcium, with no production of TNF-alpha and IL-6. Non-encapsulated pneumococcal strains exhibited different potencies in triggering mast cells. We propose here that the induction of mast cell degranulation by pneumococcal factors not accompanied by the production of pro-inflammatory cytokines may be a specific strategy elaborated by this bacterium to promote its own spreading from the respiratory mucosa into the environment.

Mast Cells Have a Pivotal Role in TNF-Independent Lymph Node Hypertrophy and the Mobilization of Langerhans Cells in Response to Bacterial Peptidoglycan

Peptidoglycan (PGN) from Gram-positive bacteria, activates multiple immune effector cells. PGN-induced lymph node (LN) hypertrophy and dendritic cell mobilization in vivo were investigated following PGN injection into the skin. Both LN activation and the migration of Langerhans cells (LCs) to draining LNs were dependent on the presence of mast cells as demonstrated using mast cell deficient W/W(v) mice. However, these responses did not require TLR2, TLR4, or MYD88. TNF-deficient mice exhibited normal increases in LN cellularity but significantly reduced LC migration. In contrast, responses to IgE-mediated mast cell activation were highly TNF dependent. Complement component C3-deficient mice showed decreased LN hypertrophy and abrogated LC migration in response to PGN. These data demonstrate a critical role for mast cells and complement in LN responses to PGN and illustrate a novel TNF-independent mechanism whereby mast cells participate in the initiation of immunity.

Mast cells protect mice from Mycoplasma pneumonia

RATIONALE

As the smallest free-living bacteria and a frequent cause of respiratory infections, mycoplasmas are unique pathogens. Mice infected with Mycoplasma pulmonis can develop localized, life-long airway infection accompanied by persistent inflammation and remodeling.

OBJECTIVE

Because mast cells protect mice from acute septic peritonitis and gram-negative pneumonia, we hypothesized that they defend against mycoplasma infection. This study tests this hypothesis using mast cell-deficient mice.

METHODS

Responses to airway infection with M. pulmonis were compared in wild-type and mast cell-deficient Kit(W-sh)/Kit(W-sh) mice and sham-infected control mice.

MEASUREMENTS AND MAIN RESULTS

Endpoints include mortality, body and lymph node weight, mycoplasma antibody titer, and lung mycoplasma burden and histopathology at intervals after infection. The results reveal that infected Kit(W-sh)/Kit(W-sh) mice, compared with other groups, lose more weight and are more likely to die. Live mycoplasma burden is greater in Kit(W-sh)/Kit(W-sh) than in wild-type mice at early time points. Four days after infection, the difference is 162-fold. Titers of mycoplasma-specific IgM and IgA appear earlier and rise higher in Kit(W-sh)/Kit(W-sh) mice, but antibody responses to heat-killed mycoplasma are not different compared with wild-type mice. Infected Kit(W-sh)/Kit(W-sh) mice develop larger bronchial lymph nodes and progressive pneumonia and airway occlusion with neutrophil-rich exudates, accompanied by angiogenesis and lymphangiogenesis. In wild-type mice, pneumonia and exudates are less severe, quicker to resolve, and are not associated with increased angiogenesis.

CONCLUSIONS

These findings suggest that mast cells are important for innate immune containment of and recovery from respiratory mycoplasma infection.

A role for the Mycoplasma pneumoniae adhesin P1 in interleukin (IL)-4 synthesis and release from rodent mast cells

Mycoplasma pneumoniae is a respiratory tract pathogen associated with exacerbations in patients with chronic asthma, yet relatively little is known about the potential role of this organism in asthma pathogenesis. Our previous studies demonstrated that RBL-2H3 mast cells co-cultured with M. pneumoniae released preformed inflammatory mediators, synthesized multiple cytokine mRNA species, and released IL-4 protein. In this study, we sought to determine the mechanism by which M. pneumoniae activates mast cell cytokine production. Cytokine mRNA upregulation and IL-4 protein production in RBL cells were induced almost exclusively by plastic-adherent M. pneumoniae cultures (MpA). Organisms grown under non-adherent conditions (MpN) were unable to induce cytokine responses efficiently. Western blots demonstrated that MpA was enriched for P1, the major M. pneumoniae adhesin, compared to MpN. M. pneumoniae-induced IL-4 release from RBL cells was inhibited >85% by anti-P1 monoclonal antibodies. Additionally, a P1-deficient strain of the bacteria was unable to efficiently induce IL-4 release. Desialation of RBL cell surface glycoproteins by neuraminidase treatment eliminated IL-4 release. We conclude that P1 plays an important role in M. pneumoniae-induced cytokine responses in RBL mast cells and that direct contact between the organism and sialated residues on the RBL surface mediates this activation.

Mast cells are critical mediators of vaccine-induced Helicobacter clearance in the mouse model

BACKGROUND & AIMS

Despite the proven ability of immunization to prevent Helicobacter infection in mouse models, the precise mechanism of protection has remained elusive.

METHODS

We explored the cellular events associated with Helicobacter clearance from the stomach following vaccination by flow cytometry analysis and histological and molecular studies.

RESULTS

Kinetic studies showed that the infection is undetectable in vaccinated mice at day 5 postbacterial challenge. Flow cytometry analysis showed that the percentages of mast cells (CD3 - CD117 + ) increased in the lymphoid cells isolated from the stomach at day 4 postchallenge in urease + cholera toxin (CT)-vaccinated mice in comparison with mice administered with CT alone (9.4% +/- 4.4% and 3.1% +/- 1%, respectively, for vaccinated and CT administered, n = 5; P < .01). Quantitative PCR analysis showed an increased messenger RNA (mRNA) expression of the mast cell proteases 1 and 2 at day 5 postchallenge in the stomach of vaccinated mice. In contrast to wild-type mice, mast cell-deficient mice (W/W v mice) were not protected from H felis colonization after vaccination. Indeed only 1 out of 12 vaccinated W/W v mice showed a negative urease test. Remarkably, vaccinated W/W v mice reconstituted with cultured bone marrow-derived mast cells recovered the ability to clear the infection after vaccination (8 out of 10 mast cell-reconstituted mice showed negative urease tests [ P < .006 as compared with wild-type mice]).

CONCLUSIONS

These experiments show that mast cells are, unexpectedly, critical mediators of anti- Helicobacter vaccination.

Mast-cell responses to pathogens

Mast cells have mainly been studied in the setting of allergic disease, but the importance of mast cells for host defence against several pathogens has now been well established. The location of mast cells, which are found closely associated with blood vessels, allows them to have a crucial sentinel role in host defence. The mast cell has a unique 'armamentarium' of receptor systems and mediators for responding to pathogen-associated signals. Studies of this intriguing immune-effector cell provide important insights into the complex mechanisms by which appropriate innate and acquired immune responses are initiated.

Histological aspects and role of mast cells in Helicobacter pylori-infected gastritis

BACKGROUND

Mast cells are one of the main pro-inflammatory cells, while their knowledge in Helicobacter pylori infection has not been summarized.

METHODS

We reviewed studies on mast cells in H. pylori infection, and summarized the histological aspects and roles.

RESULTS

The density of mast cells is greater in H. pylori-infected than in non-infected subjects. Increased mast cell density in infected gastritis significantly decreases after eradication. On electron microscopy, mast cells in infected gastric mucosa show degranulation. Some experimental studies demonstrate that mast cells are degranulated with H. pylori-derived products.

CONCLUSIONS

Mast cells are actively involved in the pathogenesis of H. pylori-infected gastritis. The possible roles are to initiate and promote the formation of oedema through degranulated and secreted mediators, and to release multiple chemotactic factors, which induce inflammatory cells to infiltrate to the site of oedema, showing acute inflammatory changes. Mast cells also stimulate the degradation of pericellular matrices and the growth of cells in their vicinity, and thereby promote tissue turnover. In chronic H. pylori infection, these reactions continue until the bacteria are eradicated. Mast cells may act both to maintain gastritis and to repair tissue damage in H. pylori-infected chronic gastritis.

Mast cell–mediated inflammatory responses require the α2β1 integrin

Although the α2β1 integrin is widely expressed and has been extensively studied, it has not been previously implicated in mast cell biology. We observed that α2 integrin subunit-deficient mice exhibited markedly diminished neutrophil and interleukin-6 responses during Listeria monocytogenes– and zymosan-induced peritonitis. Since exudative neutrophils of wild-type mice expressed little α2β1 integrin, it seemed unlikely that this integrin mediated neutrophil migration directly. Here, we demonstrate constitutive α2β1 integrin expression on peritoneal mast cells. Although α2-null mice contain normal numbers of peritoneal mast cells, these α2-null cells do not support in vivo mast cell–dependent inflammatory responses. We conclude that α2β1 integrin provides a costimulatory function required for mast cell activation and cytokine production in response to infection.

Mast cell activation by Mycobacterium tuberculosis: mediator release and role of CD48

Mast cells (MC) are abundant in the lung and other peripheral tissue, where they participate in inflammatory processes against bacterial infections. Like other effector cells of the innate immune system, MC interact directly with a wide variety of infectious agents. This interaction results in MC activation and inflammatory mediator release. We demonstrated that MC interact with Mycobacterium tuberculosis, triggering the release of several prestored reagents, such as histamine and beta-hexosaminidase, and de novo synthesized cytokines, such as TNF-alpha and IL-6. A number of M. tuberculosis Ags, ESAT-6, MTSA-10, and MPT-63, have been implicated in MC activation and mediator release. A MC plasmalemmal protein, CD48, was implicated in interactions with mycobacteria because CD48 appeared to aggregate in the MC membrane at sites of bacterial binding and because Abs to CD48 inhibited the MC histamine response to mycobacteria. Cumulatively, these findings suggest that MC, even in the absence of opsonins, can directly recognize M. tuberculosis and its Ags and have the potential to play an active role in mediating the host's innate response to M. tuberculosis infection.

Cutting edge: mast cell antimicrobial activity is mediated by expression of cathelicidin antimicrobial peptide

Cathelicidins (caths) are peptides that are expressed at high levels in neutrophils and some epithelia and can act as natural antibiotics by directly killing a wide range of microorganisms. We hypothesized that caths are expressed in mast cells (MCs), because these cells have been previously associated with inherent antimicrobial activity. Cultured murine MCs contained abundant amounts of cathelin-related antimicrobial peptide (AMP), the murine cath, and this expression was inducible by LPS or lipoteichoic acid. Human skin MCs also expressed cath as detected by immunohistochemical analysis for the human cath LL-37. The functional significance of this expression was shown by comparing MCs cultured from normal mice to MCs from littermates deficient in the cathelin-related AMP gene (Cnlp(-)). MCs derived from Cnlp(-/-) animals had a 50% reduction in their ability to kill group A STREPTOCOCCUS: These MCs expressed equivalent amounts of mRNA for murine beta-defensin-4, a beta-defensin AMP. Thus, different antimicrobials can be identified in MCs, and the presence of cath is necessary for efficient bacterial killing. These observations suggest that the presence of cath is vital to the ability of mammalian MCs to participate in antimicrobial defense.

Pseudomonas aeruginosa activates human mast cells to induce neutrophil transendothelial migration via mast cell-derived IL-1 alpha and beta

The mechanisms of neutrophil (PMN) recruitment to Pseudomonas aeruginosa infection remain incompletely defined. Mast cells (MC) involvement in this process has not been studied previously. In this study, we demonstrate that human cord blood-derived MC phagocytose P. aeruginosa and release mediators that activate HUVEC monolayers for supporting PMN transmigration. Pretreatment of supernatants from P. aeruginosa-MC cocultures with neutralizing anti-IL-1alpha plus anti-IL-1beta Abs, or IL-1R antagonist before addition to HUVEC for stimulation completely abrogated MC-induced PMN transmigration, while anti-TNF-alpha treatment had no effect. The expression of E-selectin and ICAM-1 on HUVEC, the latter a ligand for PMN CD11/CD18, was significantly up-regulated by P. aeruginosa-induced MC mediators. Pretreatment of human PMN with anti-CD18 mAb or pretreatment of HUVEC with a combination of three mAbs (against ICAM-1, ICAM-2, and E-selectin) inhibited by 85% the MC-dependent PMN transmigration. Moreover, P. aeruginosa-induced production of IL-1alpha and IL-1beta was down-regulated by IL-10 and dexamethasone. This study demonstrates for the first time that MC may mediate P. aeruginosa-induced PMN recruitment via production of IL-1alpha and beta. These findings have important implications for diseases involving P. aeruginosa infection and suggest novel targets for modulating P. aeruginosa-induced inflammation.

Chronic stress induces mast cell-dependent bacterial adherence and initiates mucosal inflammation in rat intestine

BACKGROUND & AIMS

Chronic psychological stress is an important factor in relapses of intestinal disorders, but it remains unclear if stress can induce primary gut inflammation in a previously healthy host.

METHODS

Mast cell-deficient (Ws/Ws) rats and wild-type control (+/+) rats were submitted to water avoidance stress or sham stress (1 h/day) for 10 consecutive days, as a model of ongoing life stress.

RESULTS

Both rat groups had similar systemic responses to stress, as assessed by changes in weight, corticosterone levels, and defecation. In +/+ rats, chronic stress induced barrier dysfunction in the ileum and colon (increased macromolecular permeability and depletion of mucus) and ultrastructural changes in epithelial cells (enlarged mitochondria and presence of autophagosomes) associated with bacterial adhesion and penetration into enterocytes. Moreover, hyperplasia and activation of mast cells, infiltration of neutrophils and mononuclear cells, and increased myeloperoxidase (MPO) activity were documented in the mucosa. In intestine of Ws/Ws rats, epithelial function and morphology were unchanged by chronic stress, bacterial-epithelial cell interaction was not demonstrated, and there was no evidence of inflammatory cell infiltration.

CONCLUSIONS

These findings suggest that chronic psychological stress can be an initiating factor in intestinal inflammation by impairing mucosal defenses against luminal bacteria and highlight the importance of mast cells in this process.

Role of Janus kinase 3 in mast cell-mediated innate immunity against gram-negative bacteria

Mast cells play a pivotal role in innate host immune response to gram-negative bacteria. We report that Janus kinase 3 plays a role in mast cell-mediated bacterial clearance and neutrophil recruitment by regulating the release of tumor necrosis factor from mast cells. The role of JAK3 in mast cell-facilitated neutrophil recruitment and bacterial clearance was investigated by comparing the neutrophil influxes and bacterial clearance in mast cell-deficient W/W(v) mice reconstituted with JAK3(+/+) or JAK(-/-) mast cells. The neutrophil influx, bacterial clearance, and survival outcome in W/W(v) mice reconstituted with JAK3(+/+) mast cells was better than in W/W(v) mice reconstituted with JAK3(-/-) mast cells. These findings provide evidence that JAK3 is a key regulator of mast cell-mediated innate immunity against gram-negative bacteria.

Studies of the multifaceted mast cell response to bacteria

The concept of mast cells as playing a critical and multifaceted role in immune defense against pathogens is new, and effective ways to study and validate this notion are required. Recently, a number of approaches have been described that can be used to study the molecular aspects of mast cell recognition of pathogens, and of specific mast cell responses, such as mediator release, bacterial endocytosis and mast cell migration, to pathogens.

Interaction of Bordetella pertussis with mast cells, modulation of cytokine secretion by pertussis toxin

Together with macrophages and dendritic cells, mast cells have recently been shown to interact with certain pathogenic bacteria and present microbial antigens to the immune system. We show here that Bordetella pertussis can adhere to and be phagocytosed by mast cells. In addition, mast cells are able to process and present B. pertussis antigens to T lymphocytes. Furthermore, exposure of mast cells to B. pertussis induced the release of the proinflammatory cytokines tumour necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6). The release of IL-6 was strongly reduced by pertussis toxin expressed by B. pertussis. The production of IL-10, but not that of IL-4, by mast cells was also inhibited by pertussis toxin. Depletion of mast cells in vivo resulted in significant reduction of early TNF-alpha production in bronchoalveolar lavage (BAL) fluids of B. pertussis-infected mice. These data suggest that mast cells may play a role in the induction of immune responses against B. pertussis through the release of cytokines, especially TNF-alpha.

Involvement of cellular caveolae in bacterial entry into mast cells

Caveolae are subcellular structures implicated in the import and transcytosis of macromolecules and in transmembrane signaling. To date, evidence for the existence of caveolae in hematopoietic cells has been ambiguous. Caveolae were detected in the microvilli and intracellular vesicles of cultured mouse bone marrow-derived mast cells (BMMCs). CD48, a receptor for FimH-expressing (type 1 fimbriated) Escherichia coli, was specifically localized to plasmalemmal caveolae in BMMCs. The involvement of caveolae in bacterial entry into BMMCs was indicated because caveolae-disrupting and -usurping agents specifically blocked E. coli entry, and markers of caveolae were actively recruited to sites of bacterial entry. The formation of bacteria-encapsulating caveolar chambers in BMMCs represents a distinct mechanism of microbial entry into phagocytes.

Cell biology. Bacterial spelunkers

Bacteria that are engulfed by phagocytic cells of the immune system are usually destroyed once inside the host cell but not always. Why is it that sometimes engulfed bacteria survive and thrive quite happily inside the host cell? As Mulvey and Hultgren explain in their Perspective, the answer may lie in small indentations in the host cell plasma membrane called caveolae that direct certain signal transduction pathways inside the host cell (Shin et al.). If bacteria adhere to regions of the host cell surface that is rich in caveolae, they are better able to survive once inside the cell.

Clinical and environmental isolates of Burkholderia cepacia exhibit differential cytotoxicity towards macrophages and mast cells

Burkholderia cepacia is an emerging opportunistic pathogen that causes fatal infections in patients suffering from cystic fibrosis (CF) and chronic granulomatous disease. Various environmental isolates of B. cepacia are, however, capable of degrading environmental pollutants, such as trichloroethylene, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), etc., and are also highly effective in controlling plant diseases caused by nematodes and fungi. Such strains have therefore been proposed for environmental release to clean up toxic dump sites or as biopesticides. Various efforts to distinguish between clinical and environmental isolates of B. cepacia with regard to their virulence characteristics have produced ambiguous results, suggesting that newer methods are needed to test for the presence or absence of pathogenic potential in B. cepacia strains proposed for environmental release. We now report that several clinical strains of B. cepacia secrete cytotoxic factors that allow macrophage and mast cell death in the presence of external ATP. Several environmental strains had reduced activity in this regard. We also demonstrate that, while all the strains secrete enzymes that have nucleoside diphosphate kinase (Ndk), adenylate kinase (Ak) and 5'-nucleotidase activity, the level of secretion of the 5'-nucleotidase (and/or ATPase/phosphatase) appears to be lower in the environmental strains than in the clinical strains. The secretion of these enzymes is specifically activated in the presence of eukaryotic proteins such as alpha2-macroglobulin. As macrophage-or mast cell surface-associated P2Z receptors promote their cell death in the presence of mM concentrations of ATP, and as the secreted ATP-using enzymes generate various phosphorylated or non-phosphorylated adenine nucleotides that may even be better agonists than ATP in activating the P2Z receptors or may act through the activation of additional purinergic receptors, such enzymes may play an important role in allowing B. cepacia to evade host defence.

[Mast cell of upper gastrointestinal tract mucosa and Helicobacter pylori infection in children]

Using histological, immunohistochemical and electron microscopic examinations of gastric and duodenal mucosa the authors showed the increase in the number of mast cells with ultrastructural changes characteristic of their degranulation in H. pylori infected children. The damage of various intensification in glandular and covering epithelial cells and the inflammatory response were also proved. The results suggest the role of mastocytes in the inflammation of gastric and duodenal mucosa in H. pylori infected children.

Role of mast cell leukotrienes in neutrophil recruitment and bacterial clearance in infectious peritonitis

Stimulated mast cells release a variety of chemotactic factors such as tumor necrosis factor alpha (TNF-alpha) and leukotriene B4. Recent studies have shown that mast cell-derived TNF-alpha plays a critical role in host defense against Gram negative bacterial infections by the recruitment of neutrophils to the sites of infection. In the present study, we sought to investigate if mast cells release leukotriene (LT) B4 in response to bacteria and, if so, to establish its in vivo relevance. We show that mast cells release significant amounts of LTB4 and LTC4 in response to exposure to FimH-expressing type 1 fimbriated Escherichia coli in vitro. To test the functional significance of mast cell-derived LTs during an E. coli infection in vivo, we examined the effect of a LT-synthesis inhibitor, A-63162, on bacterial clearance and neutrophil influx in an infectious peritonitis model in mast cell-deficient mice (WBB6F1-W/WV) and their normal congenic control (WBB6F1-+/+) mice. Our results show that a treatment with A-63162 reduced neutrophil influx and bacterial clearance in the peritoneal cavities of mast cell-sufficient but not -deficient mice. Thus, mast cell-derived LTs contribute to host defense by mediating early neutrophil influx and bacterial clearance at sites of infection.

Internalization of FimH+ Escherichia coli by the human mast cell line (HMC-1 5C6) involves protein kinase C

Rodent mast cells (MC) play critical roles in host defense against bacterial infection. However, bacteria-mediated signaling mechanisms in MC have not been studied. In addition, the response of human MC to bacteria is not fully investigated. This study examined the interaction between human MC and type 1 fimbriated Escherichia coli and the mechanisms involved using the human MC line HMC-1 5C6 and human cord blood-derived MC. These MC internalized significant numbers of FimH+ E. coli, but not its isogenic FimH- mutant. In HMC-1 cells, bacterial internalization was stimulated by protein kinase C (PKC) activation [short-term phorbol myristate acetate (PMA) treatment] and dramatically decreased by PKC inhibitors or PKC depletion (long-term PMA treatment). Moreover, bacterial internalization was accompanied by significant expression of PKCbeta1 and delta. Fluorescence microscopy demonstrated accumulation of PKCbeta1 on internalized bacteria. These data indicate that human MC has the capacity to internalize bacteria and PKC may be a critical intracellular mediator of this function.

Bacteria-host cell interaction mediated by cellular cholesterol/glycolipid-enriched microdomains

Gram negative bacterial infection is a leading cause of fatality and is attributed, at least in part, to the bacteria's capacity to persist in the host in spite of appropriate antibiotic therapy. It has been suggested that bacteria evade antibiotics by hiding within host cells. We sought to investigate this important aspect of infections in mast cells, which are inflammatory cells found in close proximity to the host-environment interface and which have recently been reported to play a crucial role in the early innate immune response to bacteria. We examined mast cell interactions with FimH-expressing E. coli, one of the major opportunistic pathogens of humans. We determined that in serum free conditions, these bacteria were able to trigger mast cell uptake without loss of bacterial viability. CD48, a mannose containing GPI (glycosylphosphatidylinositol)-linked molecule was found to be the receptor of FimH-expressing E. coli in mouse mast cells. We found that the internalization via CD48 was blocked by filipin, a cholesterol binding drug known to disrupt cholesterol/glycolipid-enriched microdomains and the bacteria-encasing vacuoles were rich in cholesterol inside cells. Interestingly, we found that mast cells subsequently expelled majority of the intracellular bacteria in 24 hours. This expulsion process was blocked by lovastatin/cyclodextrin treatment, which is known to inhibit cellular trafficking of cholesterol/glycolipid-enriched microdomains. Thus, the bacterial entry into and expulsion from mast cells were critically dependent on cholesterol/glycolipid-enriched microdomains, which represents a novel mode of tussle between the pathogen and the mast cell occurring in opsonin deficient sites in the body or even at other sites in naive or immunocompromised hosts which have low systemic levels of E. coli specific antibody.