Mast cell degranulation is induced by A549 airway epithelial cell infected with respiratory syncytial virus

Rhinovirus infection of the airway epithelium enhances mast cell immune responses via epithelial-derived interferons

BACKGROUND

Mast cells (MCs) within the airway epithelium in asthma are closely related to airway dysfunction, but cross talk between airway epithelial cells (AECs) and MCs in asthma remains incompletely understood. Human rhinovirus (RV) infections are key triggers for asthma progression, and AECs from individuals with asthma may have dysregulated antiviral responses.

OBJECTIVE

We utilized primary AECs in an ex vivo coculture model system to examine cross talk between AECs and MCs after epithelial rhinovirus infection.

METHODS

Primary AECs were obtained from 11 children with asthma and 10 healthy children, differentiated at air-liquid interface, and cultured in the presence of laboratory of allergic diseases 2 (LAD2) MCs. AECs were infected with rhinovirus serogroup A 16 (RV16) for 48 hours. RNA isolated from both AECs and MCs underwent RNA sequencing. Direct effects of epithelial-derived interferons on LAD2 MCs were examined by real-time quantitative PCR.

RESULTS

MCs increased expression of proinflammatory and antiviral genes in AECs. AECs demonstrated a robust antiviral response after RV16 infection that resulted in significant changes in MC gene expression, including upregulation of genes involved in antiviral responses, leukocyte activation, and type 2 inflammation. Subsequent ex vivo modeling demonstrated that IFN-β induces MC type 2 gene expression. The effects of AEC donor phenotype were small relative to the effects of viral infection and the presence of MCs.

CONCLUSIONS

There is significant cross talk between AECs and MCs, which are present in the epithelium in asthma. Epithelial-derived interferons not only play a role in viral suppression but also further alter MC immune responses including specific type 2 genes.

IL-33 Induces an Antiviral Signature in Mast Cells but Enhances Their Permissiveness for Human Rhinovirus Infection

Mast cells (MCs) are classically associated with allergic asthma but their role in antiviral immunity is unclear. Human rhinoviruses (HRVs) are a major cause of asthma exacerbations and can infect and replicate within MCs. The primary site of HRV infection is the airway epithelium and MCs localise to this site with increasing asthma severity. The asthma susceptibility gene, IL-33, encodes an epithelial-derived cytokine released following HRV infection but its impact on MC antiviral responses has yet to be determined. In this study we investigated the global response of LAD2 MCs to IL-33 stimulation using RNA sequencing and identified genes involved in antiviral immunity. In spite of this, IL-33 treatment increased permissiveness of MCs to HRV16 infection which, from the RNA-Seq data, we attributed to upregulation of ICAM1. Flow cytometric analysis confirmed an IL-33-dependent increase in ICAM1 surface expression as well as LDLR, the receptors used by major and minor group HRVs for cellular entry. Neutralisation of ICAM1 reduced the IL-33-dependent enhancement in HRV16 replication and release in both LAD2 MCs and cord blood derived MCs. These findings demonstrate that although IL-33 induces an antiviral signature in MCs, it also upregulates the receptors for HRV entry to enhance infection. This highlights the potential for a gene-environment interaction involving IL33 and HRV in MCs to contribute to virus-induced asthma exacerbations.

Responses of Mast Cells to Pathogens: Beneficial and Detrimental Roles

Mast cells (MCs) are strategically located in tissues close to the external environment, being one of the first immune cells to interact with invading pathogens. They are long living effector cells equipped with different receptors that allow microbial recognition. Once activated, MCs release numerous biologically active mediators in the site of pathogen contact, which induce vascular endothelium modification, inflammation development and extracellular matrix remodeling. Efficient and direct antimicrobial mechanisms of MCs involve phagocytosis with oxidative and non-oxidative microbial destruction, extracellular trap formation, and the release of antimicrobial substances. MCs also contribute to host defense through the attraction and activation of phagocytic and inflammatory cells, shaping the innate and adaptive immune responses. However, as part of their response to pathogens and under an impaired, sustained, or systemic activation, MCs may contribute to tissue damage. This review will focus on the current knowledge about direct and indirect contribution of MCs to pathogen clearance. Antimicrobial mechanisms of MCs are addressed with special attention to signaling pathways involved and molecular weapons implicated. The role of MCs in a dysregulated host response that can increase morbidity and mortality is also reviewed and discussed, highlighting the complexity of MCs biology in the context of host-pathogen interactions.

Mast Cells in the Skin: Defenders of Integrity or Offenders in Inflammation?

Mast cells (MCs) are best-known as key effector cells of immediate-type allergic reactions that may even culminate in life-threatening anaphylactic shock syndromes. However, strategically positioned at the host–environment interfaces and equipped with a plethora of receptors, MCs also play an important role in the first-line defense against pathogens. Their main characteristic, the huge amount of preformed proinflammatory mediators embedded in secretory granules, allows for a rapid response and initiation of further immune effector cell recruitment. The same mechanism, however, may account for detrimental overshooting responses. MCs are not only detrimental in MC-driven diseases but also responsible for disease exacerbation in other inflammatory disorders. Focusing on the skin as the largest immune organ, we herein review both beneficial and detrimental functions of skin MCs, from skin barrier integrity via host defense mechanisms to MC-driven inflammatory skin disorders. Moreover, we emphasize the importance of IgE-independent pathways of MC activation and their role in sustained chronic skin inflammation and disease exacerbation.

The impact of TLR7 agonist R848 treatment on mast cell phenotype and activity

Bearing in mind that mast cell contribution to viral clearance is still not fully understood, in this study, we evaluated the effect of Toll-like receptor (TLR)7 viral single-stranded ribonucleic acid (ssRNA) mimic ligand, namely resiquimod (R)848, on mast cell phenotype and activity. We demonstrated that rat peritoneal mast cells exhibit surface and intracellular expression of ssRNA-specific TLR7 molecule, and that mimic ligand switches the self-expression of this receptor. We also detected other proteins associated with the cellular antiviral response: interferon-alpha receptor 1 (IFNAR1), interferon-gamma receptor 1 (IFNGR1), and major histocompatibility complex I (MHC I). Moreover, we showed that R848 caused the decrease of all molecule's expression after prolonged incubation. Interestingly, we found that R848 induced the increase of high-affinity IgE receptor (FcεRI) expression. Finally, we documented that TLR7 ligand-stimulated mast cells synthesize/release interferon (IFN)-α and -β, tumor necrosis factor (TNF), and chemokines CCL3, CXCL8, as well as pro-inflammatory lipid mediators. Our findings confirm that mast cells may respond to TLR7 ligand by altering their phenotype and synthesizing mediators and could serve as active participants in the antiviral immune response.

Protective and pathogenic roles for mast cells during viral infections

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At the host-environment interfaces such as skin or mucosae mast cells act as immune sentinels for variety of pathogens including viruses.

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Mast cells initiate cellular immune responses at the infection site by recruitment of various subsets of T cells.

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Systemic activation of mast cells is associated with vascular pathologies during viral infections.

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Targeting of mast cell specific products has a promising therapeutic potential in treating virus induced immune pathologies.

Mast cells (MCs) are long-lived immune cells. They are armed with preformed mediators within granules that can be instantaneously released in response to an invading pathogen, including certain viruses. At the skin and mucosae, they initiate innate immune responses and promote the development of adaptive immune responses, through cellular recruitment or antigen presentation. However, systemic MC activation may promote immune pathologies through their vasoactive proteases and biogenic amines. Recently, MC products were identified to contribute to pathologies associated with viral hemorrhagic fever, such vascular leakage and thrombocytopenia. Similar associations of MCs with disease severity have been noted for certain respiratory viral pathogens. Here we discuss the specific MC responses to viruses and their influences on functional immune outcomes during infection.

Mast Cell Responses to Viruses and Pathogen Products

Mast cells are well accepted as important sentinel cells for host defence against selected pathogens. Their location at mucosal surfaces and ability to mobilize multiple aspects of early immune responses makes them critical contributors to effective immunity in several experimental settings. However, the interactions of mast cells with viruses and pathogen products are complex and can have both detrimental and positive impacts. There is substantial evidence for mast cell mobilization and activation of effector cells and mobilization of dendritic cells following viral challenge. These cells are a major and under-appreciated local source of type I and III interferons following viral challenge. However, mast cells have also been implicated in inappropriate inflammatory responses, long term fibrosis, and vascular leakage associated with viral infections. Progress in combating infection and boosting effective immunity requires a better understanding of mast cell responses to viral infection and the pathogen products and receptors we can employ to modify such responses. In this review, we outline some of the key known responses of mast cells to viral infection and their major responses to pathogen products. We have placed an emphasis on data obtained from human mast cells and aim to provide a framework for considering the complex interactions between mast cells and pathogens with a view to exploiting this knowledge therapeutically. Long-lived resident mast cells and their responses to viruses and pathogen products provide excellent opportunities to modify local immune responses that remain to be fully exploited in cancer immunotherapy, vaccination, and treatment of infectious diseases.

Responses of human mast cells and epithelial cells following exposure to influenza A virus

As a part of innate immune defense, the role of mast cells during viral replication has been incompletely understood. In this study, we characterized and compared the responses of the human mast cell line, LAD2, and human lung epithelial cell line, Calu-3, against three influenza A virus strains; A/PR/8/34 (H1N1), A/WS/33 (H1N1) and A/HK/8/68 (H3N2). We found that there were strain-dependent mast cell responses, and different profiles of cytokine, chemokine and antiviral gene expression between the two cell types. All three strains did not induce histamine or β-hexosaminidase release in LAD2. A/HK/8/68 induced release of prostaglandin D2 in LAD2, whereas A/PR/8/34 and A/WS/33 did not. We found that, among those examined, only CCL4 (by A/PR/8/34) was statistically significantly released from LAD2 cells. Furthermore, there was increased mRNA expression of viral recognition receptors (RIG-I and MDA5) and antiviral protein, viperin, but levels and kinetics of the expression were different among the cell types, as well as by the strains examined. Our findings highlight the variability in innate response to different strains of influenza A virus in two human cell types, indicating that further investigation is needed to understand better the role of mast cells and epithelial cells in innate immunity against influenza A viruses.

Mast Cells in Cardiac Fibrosis: New Insights Suggest Opportunities for Intervention

Mast cells (MC) are innate immune cells present in virtually all body tissues with key roles in allergic disease and host defense. MCs recognize damage-associated molecular patterns (DAMPs) through expression of multiple receptors including Toll-like receptors and the IL-33 receptor ST2. MCs can be activated to degranulate and release pre-formed mediators, to synthesize and secrete cytokines and chemokines without degranulation, and/or to produce lipid mediators. MC numbers are generally increased at sites of fibrosis. They are potent, resident, effector cells producing mediators that regulate the fibrotic process. The nature of the secretory products produced by MCs depend on micro-environmental signals and can be both pro- and anti-fibrotic. MCs have been repeatedly implicated in the pathogenesis of cardiac fibrosis and in angiogenic responses in hypoxic tissues, but these findings are controversial. Several rodent studies have indicated a protective role for MCs. MC-deficient mice have been reported to have poorer outcomes after coronary artery ligation and increased cardiac function upon MC reconstitution. In contrast, MCs have also been implicated as key drivers of fibrosis. MC stabilization during a hypertensive rat model and an atrial fibrillation mouse model rescued associated fibrosis. Discrepancies in the literature could be related to problems with mouse models of MC deficiency. To further complicate the issue, mice generally have a much lower density of MCs in their cardiac tissue than humans, and as such comparing MC deficient and MC containing mouse models is not necessarily reflective of the role of MCs in human disease. In this review, we will evaluate the literature regarding the role of MCs in cardiac fibrosis with an emphasis on what is known about MC biology, in this context. MCs have been well-studied in allergic disease and multiple pharmacological tools are available to regulate their function. We will identify potential opportunities to manipulate human MC function and the impact of their mediators with a view to preventing or reducing harmful fibrosis. Important therapeutic opportunities could arise from increased understanding of the impact of such potent, resident immune cells, with the ability to profoundly alter long term fibrotic processes.

Influenza A Viruses Replicate Productively in Mouse Mastocytoma Cells (P815) and Trigger Pro-inflammatory Cytokine and Chemokine Production through TLR3 Signaling Pathway

The influenza A viruses (IAVs) cause acute respiratory infection in both humans and animals. As a member of the initial lines of host defense system, the role of mast cells during IAV infection has been poorly understood. Here, we characterized for the first time that both avian-like (α-2, 3-linked) and human-like (α-2, 6- linked) sialic acid (SA) receptors were expressed by the mouse mastocytoma cell line (P815). The P815 cells did support the productive replication of H1N1 (A/WSN/33), H5N1 (A/chicken/ Henan/1/04) and H7N2 (A/chicken/Hebei/2/02) in vitro while the in vivo infection of H5N1 in mast cells was confirmed by the specific staining of nasal mucosa and lung tissue from mice. All the three viruses triggered the infected P815 cells to produce pro-inflammatory cytokines and chemokines including IL-6, IFN-γ, TNF-α, CCL-2, CCL-5, and IP-10, but not the antiviral type I interferon. It was further confirmed that TLR3 pathway was involved in P815 cell response to IAV-infection. Our findings highlight the remarkable tropism and infectivity of IAV to P815 cells, indicating that mast cells may be unneglectable player in the development of IAV infection.

Mast cells in asthma--state of the art

Mast cells (MCs) play a central role in tissue homoeostasis, sensing the local environment through numerous innate cell surface receptors. This enables them to respond rapidly to perceived tissue insults with a view to initiating a co-ordinated programme of inflammation and repair. However, when the tissue insult is chronic, the ongoing release of multiple pro-inflammatory mediators, proteases, cytokines and chemokines leads to tissue damage and remodelling. In asthma, there is strong evidence of ongoing MC activation, and their mediators and cell-cell signals are capable of regulating many facets of asthma pathophysiology. This article reviews the evidence behind this.

Mast cells are associated with exacerbations and eosinophilia in children with severe asthma

The role of mast cells in the pathogenesis of childhood asthma is poorly understood. We aimed to estimate the implication of airway mucosal mast cells in severe asthma and their relationship with clinical, functional, inflammatory and remodelling parameters.Bronchial biopsies were performed in 36 children (5-18 years) with severe asthma: 24 had frequent severe exacerbations and/or daily symptoms in the previous year (symptomatic group), and 12 had few symptoms and a persistent obstructive pattern (paucisymptomatic group). Nine children without asthma were included as control subjects. We assessed mast cells in the submucosa and airway smooth muscle using c-kit antibodies and in the entire biopsy area using Giemsa.The number of submucosal mast cells was higher in the symptomatic group than in the paucisymptomatic group (p=0.02). The number of submucosal mast cells correlated with the number of severe exacerbations (p=0.02, r=0.37). There were positive correlations between the number of submucosal mast cells (p<0.01, r=0.44), airway smooth muscle mast cells (p=0.02, r= 0.40), mast cells stained by Giemsa (p<0.01, r=0.44) and submucosal eosinophils.Mast cells are associated with severe exacerbations and submucosal eosinophilic inflammation in children with severe asthma.

Microwave-irradiation-assisted hybrid chemical approach for titanium dioxide nanoparticle synthesis: microbial and cytotoxicological evaluation

Titanium dioxide nanoparticles (TNPs) are widely used in the pharmaceutical and cosmetics industries. It is used for protection against UV exposure due to its light-scattering properties and high refractive index. Though TNPs are increasingly used, the synthesis of TNPs is tedious and time consuming; therefore, in the present study, microwave-assisted hybrid chemical approach was used for TNP synthesis. In the present study, we demonstrated that TNPs can be synthesized only in 2.5 h; however, the commonly used chemical approach using muffle furnace takes 5 h. The activity of TNP depends on the synthetic protocol; therefore, the present study also determined the effect of microwave-assisted hybrid chemical approach synthetic protocol on microbial and cytotoxicity. The results showed that TNP has the best antibacterial activity in decreasing order from Escherichia coli, Bacillus subtilis, and Staphylococcus aureus. The IC50 values of TNP for HCT116 and A549 were found to be 6.43 and 6.04 ppm, respectively. Cell death was also confirmed from trypan blue exclusion assay and membrane integrity loss was observed. Therefore, the study determines that the microwave-assisted hybrid chemical approach is time-saving; hence, this technique can be upgraded from lab scale to industrial scale via pilot plant scale. Moreover, it is necessary to find the mechanism of action at the molecular level to establish the reason for greater bacterial and cytotoxicological toxicity. Graphical abstract A graphical representation of TNP synthesis.

Identification of CCL2, RARRES2 and EFNB2 as host cell factors that influence the multistep replication of respiratory syncytial virus

Human respiratory syncytial virus (RSV) is a major causative agent of respiratory tract infections in children worldwide. Preterm children or those with underlying cardiopulmonary disorders are at particularly high risk of developing severe and lethal RSV respiratory tract infections; however, there are currently no effective vaccines or anti-viral drugs. To identify targets for the development of drugs to treat RSV infections, we investigated host cell factors involved in the replication of RSV. To this end, MDCK cells with low susceptibility to RSV were transfected with cDNA libraries derived from RSV-susceptible human lung or HeLa cells. A microarray analysis was subsequently performed on parental MDCK cells and MDCK cells that were converted to an RSV-susceptible form. Among the genes identified, chemokine (C-C motif) ligand 2 (CCL2), retinoic acid receptor responder protein 2 (RARRES2) and ephrin-B2 (EFNB2) had a positive effect on RSV replication. Expression of these genes in MDCK cells resulted in a 10- to 100-fold increase in RSV replication. CCL2 expression also disrupted the distribution of claudin-1, a tight junction protein, suggesting that CCL2 plays a role in claudin-based tight junction formation during RSV replication. The knockdown of EFNB2 and RARRES2 by siRNA in RSV-susceptible cell lines (HEp-2 and A549) resulted in reduced RSV replication, suggesting that EFNB2 and RARRES2 participate in RSV replication. Together, our findings suggest that CCL2, RARRES2 and EFNB2 are host cell factors involved in RSV replication.

Limited replication of influenza A virus in human mast cells

Mast cells are important in innate immunity and protective against certain bacterial infections. However, there is limited evidence that mast cells respond to viruses. As mast cells are abundant in mucosal tissues of the lung, they are in a prime location to detect and respond to influenza virus. In this study, we characterized for the first time the replication cycle of influenza A virus in human mast cells by measuring influenza A virus transcription, RNA replication, protein synthesis, and formation of infectious virus as compared to the replication cycle in epithelial cells. We detected the presence of influenza A viral genomic RNA transcription, replication, and protein synthesis in human mast cells and epithelial cells. However, there was no significant release of infectious influenza A virus from mast cells, whereas epithelial cells produce ~100-fold virus compared with the inoculating dose. We confirmed that influenza A virus infects human mast cells, begins to replicate, but the production of new virus is aborted. Thus, mast cells may lack critical factors essential for productive infection or there are intrinsic or inducible anti-influenza A mechanisms in mast cells.

Role and relevance of mast cells in fungal infections

In addition to their detrimental role in allergic diseases, mast cells (MCs) are well known to be important cells of the innate immune system. In the last decade, they have been shown to contribute significantly to optimal host defense against numerous pathogens including parasites, bacteria, and viruses. The contribution of MCs to the immune responses in fungal infections, however, is largely unknown. In this review, we first discuss key features of mast cell responses to pathogens in general and then summarize the current knowledge on the function of MCs in the defense against fungal pathogens. We especially focus on the potential and proven mechanisms by which MCs can detect fungal infections and on possible MC effector mechanisms in protecting from fungal infections.

Increased replication of respiratory syncytial virus in the presence of cytokeratin 8 and 18

Previously, it was reported that productive viral infection, viral protein synthesis, and viral RNA replication of respiratory syncytial virus (RSV) operated efficiently in two human epithelial cell lines (HEp‐2 and A549), but not in a human mast‐cell line, HMC‐1. Based on these observations, it was hypothesized that HMC‐1 cells lack the machinery required for RSV replication. To identify the host factors required for RSV replication, cDNA subtraction using A549, HEp‐2, and HMC‐1 cells was performed, and cytokeratin 18 (C18) was identified as a candidate host factor. Because C18 is generally expressed in simple epithelia with cytokeratin 8 (C8), HMC‐1 cells that constitutively express C18 and C8 (HMC‐1‐C8/18) were established to evaluate the role of C8/18 in RSV replication. In HMC‐1‐C8/18 cells, RSV RNA replication was increased, and the amount of infective virus produced was also increased in the cellular fraction after RSV spinoculation, whereas RSV production was decreased in A549 cells in which C18 expression was knocked down. These data suggest that the replication of RSV increases in the presence of C8/18. J. Med. Virol. 84:365–370, 2012. © 2011 Wiley Periodicals, Inc.

Mast cell-induced lung injury in mice infected with H5N1 influenza virus

Although an important role for mast cells in several viral infections has been demonstrated, its role in the invasion of highly pathogenic H5N1 influenza virus is unknown. In the present study, we demonstrate that mast cells were activated significantly by H5N1 virus (A/chicken/Henan/1/2004) infection both in vivo and in vitro. Mast cells could possibly intensify the lung injury that results from H5N1 infection by releasing proinflammatory mediators, including histamine, tryptase, and gamma interferon (IFN-γ). Lung lesions and apoptosis induced by H5N1 infection were reduced dramatically by treatment with ketotifen, which is a mast cell degranulation inhibitor. A combination of ketotifen and the neuraminidase inhibitor oseltamivir protected 100% of the mice from death postinfection. In conclusion, our data suggest that mast cells play a crucial role in the early stages of H5N1 influenza virus infection and provide a new approach to combat highly pathogenic influenza virus infection.

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.

CD23-dependent transcytosis of IgE and immune complex across the polarized human respiratory epithelial cells

IgE-mediated allergic inflammation occurs when allergens cross-link IgE on the surface of immune cells, thereby triggering the release of inflammatory mediators as well as enhancing Ag presentations. IgE is frequently present in airway secretions, and its level can be enhanced in human patients with allergic rhinitis and bronchial asthma. However, it remains completely unknown how IgE appears in the airway secretions. In this study, we show that CD23 (FcεRII) is constitutively expressed in established or primary human airway epithelial cells, and its expression is significantly upregulated when airway epithelial cells were subjected to IL-4 stimulation. In a transcytosis assay, human IgE or IgE-derived immune complex (IC) was transported across a polarized Calu-3 monolayer. Exposure of the Calu-3 monolayer to IL-4 stimulation also enhanced the transcytosis of either human IgE or the IC. A CD23-specific Ab or soluble CD23 significantly reduced the efficiency of IgE or IC transcytosis, suggesting a specific receptor-mediated transport by CD23. Transcytosis of both IgE and the IC was further verified in primary human airway epithelial cell monolayers. Furthermore, the transcytosed Ag-IgE complexes were competent in inducing degranulation of the cultured human mast cells. Because airway epithelial cells are the first cell layer to come into contact with inhaled allergens, our study implies CD23-mediated IgE transcytosis in human airway epithelial cells may play a critical role in initiating and contributing to the perpetuation of airway allergic inflammation.

Mast cell lines HMC-1 and LAD2 in comparison with mature human skin mast cells--drastically reduced levels of tryptase and chymase in mast cell lines

To circumvent the costly isolation procedure associated with tissue mast cells (MC), two human MC lines, i.e. HMC-1 and LAD2, are frequently employed, but their relation to mature MC is unknown. Here, we quantitatively assessed their expression of MC markers in direct comparison to skin MC (sMC). sMC expressed all lineage markers at highest and HMC-1 cells at lowest levels. LAD2 cells expressed comparable high-affinity IgE receptor alpha (FcepsilonRIalpha) and FcepsilonRIgamma but less FcepsilonRIbeta than sMC and displayed slightly reduced, but robust FcepsilonRI-mediated histamine release. Only minor differences were found for total histamine content and c-Kit expression. Huge, and to this level unexpected, differences were found for MC tryptase and chymase, with sMC >>> LAD2 > HMC-1. Taken together, HMC-1 cells represent very immature malignantly transformed MC, whereas LAD2 cells can be considered intermediately differentiated. Because of the minute levels of MC proteases, MC lines can serve as surrogates of tissue MC to a limited degree only.

Mast cell-orchestrated immunity to pathogens

Although mast cells were discovered more than a century ago, their functions beyond their role in allergic responses remained elusive until recently. However, there is a growing appreciation that an important physiological function of these cells is the recognition of pathogens and modulation of appropriate immune responses. Because of their ability to instantly release several pro-inflammatory mediators from intracellular stores and their location at the host-environment interface, mast cells have been shown to be crucial for optimal immune responses during infection. Mast cells seem to exert these effects by altering the inflammatory environment after detection of a pathogen and by mobilizing various immune cells to the site of infection and to draining lymph nodes. Interestingly, the character and timing of these responses can vary depending on the type of pathogen stimulus, location of pathogen recognition and sensitization state of the responding mast cells. Recent studies using mast cell activators as effective vaccine adjuvants show the potential of harnessing these cells to confer protective immunity against microbial pathogens.