A microbially produced AhR ligand promotes a Tph1-driven tolerogenic program in multiple sclerosis.

Multiple sclerosis is a debilitating autoimmune disease, characterized by chronic inflammation of the central nervous system. While the significance of the gut microbiome on multiple sclerosis pathogenesis is established, the underlining mechanisms are unknown. We found that serum levels of the microbial postbiotic tryptophan metabolite indole-3-carboxaldehyde (3-IAld) inversely correlated with disease duration in multiple sclerosis patients. Much like the host-derived tryptophan derivative L-Kynurenine, 3-IAld would bind and activate the Aryl hydrocarbon Receptor (AhR), which, in turn, controls endogenous tryptophan catabolic pathways. As a result, in peripheral lymph nodes, microbial 3-IAld, affected mast-cell tryptophan metabolism, forcing mast cells to produce serotonin via Tph1. We thus propose a protective role for AhR-mast-cell activation driven by the microbiome, whereby natural metabolites or postbiotics will have a physiological role in immune homeostasis and may act as therapeutic targets in autoimmune diseases.

Mechanism and clinical evidence of immunotherapy in allergic rhinitis.

Allergic rhinitis is a common upper airway disease caused by hypersensitivity to various aeroallergens. It causes increased inflammation throughout the body and may be complicated by other otolaryngological pathologies such as chronic hyperplastic eosinophilic sinusitis, nasal polyposis, and serous otitis media. Allergic rhinitis is an IgE-mediated disease and immunotherapy can be a possible approach for patients to limit the use of antihistamines and corticosteroids. There is evidence that allergen immunotherapy can prevent the development of new sensitizations and reduce the risk of later development of asthma in patients with allergic rhinitis. However, some patients do not benefit from this approach and the efficacy of immunotherapy in reducing the severity and relapse of symptoms is still a matter of debate. This review highlights new aspects of allergic rhinitis with a particular focus on the impact of sexual dimorphism on the disease manifestation and efficacy to the allergen specific immunotherapy.

LPS Guides Distinct Patterns of Training and Tolerance in Mast Cells.

Mast cells (MCs) are tissue-resident, long lived innate immune cells with important effector and immunomodulatory functions. They are equipped with an eclectic variety of receptors that enable them to sense multiple stimuli and to generate specific responses according on the type, strength and duration of the stimulation. Several studies demonstrated that myeloid cells can retain immunological memory of their encounters - a process termed 'trained immunity' or 'innate immune memory'. As MCs are among the one of first cells to come into contact with the external environment, it is possible that such mechanisms of innate immune memory might help shaping their phenotype and effector functions; however, studies on this aspect of MC biology are still scarce. In this manuscript, we investigated the ability of MCs primed with different stimuli to respond to a second stimulation with the same or different ligands, and determined the molecular and epigenetic drivers of these responses. Our results showed that, while the stimulation with IgE and ß-glucan failed to induce either tolerant or trained phenotypes, LPS conditioning was able to induce a profound and long-lasting remodeling of the signaling pathways involved in the response against LPS or fungal pathogens. On one side, LPS induced a strong state of unresponsiveness to secondary LPS stimulation due to the impairment of the PI3K-AKT signaling pathway, which resulted in the reduced activation of NF-κB and the decreased release of TNF-α and IL-6, compared to naïve MCs. On the other side, LPS primed MCs showed an increased release of TNF-α upon fungal infection with live Candida albicans, thus suggesting a dual role of LPS in inducing both tolerance and training phenotypes depending on the secondary challenge. Interestingly, the inhibition of HDAC during LPS stimulation partially restored the response of LPS-primed MCs to a secondary challenge with LPS, but failed to revert the increased cytokine production of these cells in response to C. albicans. These data indicate that MCs, as other innate immune cells, can develop innate immune memory, and that different stimulatory environments can shape and direct MC specific responses towards the dampening or the propagation of the local inflammatory response.

Repurposing of the Antiepileptic Drug Levetiracetam to Restrain Neuroendocrine Prostate Cancer and Inhibit Mast Cell Support to Adenocarcinoma.

A relevant fraction of castration-resistant prostate cancers (CRPC) evolve into fatal neuroendocrine (NEPC) tumors in resistance to androgen deprivation and/or inhibitors of androgen receptor pathway. Therefore, effective drugs against both CRPC and NEPC are needed. We have previously described a dual role of mast cells (MCs) in prostate cancer, being capable to promote adenocarcinoma but also to restrain NEPC. This finding suggests that a molecule targeting both MCs and NEPC cells could be effective against prostate cancer. Using an in silico drug repurposing approach, here we identify the antiepileptic drug levetiracetam as a potential candidate for this purpose. We found that the protein target of levetiracetam, SV2A, is highly expressed by both NEPC cells and MCs infiltrating prostate adenocarcinoma, while it is low or negligible in adenocarcinoma cells. In vitro, levetiracetam inhibited the proliferation of NEPC cells and the degranulation of MCs. In mice bearing subcutaneous tumors levetiracetam was partially active on both NEPC and adenocarcinoma, the latter effect due to the inhibition of MMP9 release by MCs. Notably, in TRansgenic Adenocarcinoma of the Mouse Prostate (TRAMP) mice subjected to surgical castration to mimic androgen deprivation therapy, levetiracetam reduced onset and frequency of both high grade prostatic intraepithelial neoplasia, adenocarcinoma and NEPC, thus increasing the number of cured mice showing only signs of tumor regression. Our results demonstrate that levetiracetam can directly restrain NEPC development after androgen deprivation, and that it can also block adenocarcinoma progression through the inhibition of some MCs functions. These findings open the possibility of further testing levetiracetam for the therapy of prostate cancer or of MC-mediated diseases.

Coeliac Disease and Mast Cells.

Over the last decades, there has been an impressive progress in our understanding of coeliac disease pathogenesis and it has become clear that the disorder is the final result of complex interactions of environmental, genetic, and immunological factors. Coeliac disease is now considered a prototype of T-cell-mediated disease characterized by loss of tolerance to dietary gluten and the targeted killing of enterocytes by T-cell receptor αß intraepithelial lymphocytes. Accumulating evidence, however, indicates that the induction of a gluten-specific T helper-1 response must be preceded by the activation of the innate immune system. Mast cells are key players of the innate immune response and contribute to the pathogenesis of a multitude of diseases. Here, we review the results of studies aimed at investigating the role of mast cells in the pathogenesis of coeliac disease, showing that these cells increase in number during the progression of the disease and contribute to define a pro-inflammatory microenvironment.

Frontline Science: Mast cells regulate neutrophil homeostasis by influencing macrophage clearance activity.

The receptor tyrosine kinase cKit and its ligand stem cell factor are essential for mast cells (MC) development and survival. Strains with mutations affecting the Kit gene display a profound MC deficiency in all tissues and have been extensively used to investigate the role of MC in both physiologic and pathologic conditions. However, these mice present a variety of abnormalities in other immune cell populations that can affect the interpretation of MC-related responses. C57BL/6 KitW-sh are characterized by an aberrant extramedullary myelopoiesis and systemic neutrophilia. MC deficiency in KitW-sh mice can be selectively repaired by engraftment with in vitro-differentiated MC to validate MC-specific functions. Nevertheless, the impact of MC reconstitution on other immune populations has never been evaluated in detail. Here, we specifically investigated the neutrophil compartment in primary and secondary lymphoid organs of C57BL/6 KitW-sh mice before and after MC reconstitution. We found that, albeit not apparently affecting neutrophils phenotype or maturation, MC reconstitution of KitW-sh mice restored the number of neutrophils at a level similar to that of wild-type C57BL/6 mice. In vitro and ex vivo experiments indicated that MC can influence neutrophil clearance by increasing macrophages' phagocytic activity. Furthermore, the G-CSF/IL-17 axis was also influenced by the presence or absence of MC in KitW-sh mice. These data suggest that MC play a role in the control of neutrophil homeostasis and that this aspect should be taken into account in the interpretation of results obtained using KitW-sh mice.

Endonuclease and redox activities of human apurinic/apyrimidinic endonuclease 1 have distinctive and essential functions in IgA class switch recombination.

The base excision repair (BER) pathway is an important DNA repair pathway and is essential for immune responses. In fact, it regulates both the antigen-stimulated somatic hypermutation (SHM) process and plays a central function in the process of class switch recombination (CSR). For both processes, a central role for apurinic/apyrimidinic endonuclease 1 (APE1) has been demonstrated. APE1 acts also as a master regulator of gene expression through its redox activity. APE1's redox activity stimulates the DNA-binding activity of several transcription factors, including NF-κB and a few others involved in inflammation and in immune responses. Therefore, it is possible that APE1 has a role in regulating the CSR through its function as a redox coactivator. The present study was undertaken to address this question. Using the CSR-competent mouse B-cell line CH12F3 and a combination of specific inhibitors of APE1's redox (APX3330) and repair (compound 3) activities, APE1-deficient or -reconstituted cell lines expressing redox-deficient or endonuclease-deficient proteins, and APX3330-treated mice, we determined the contributions of both endonuclease and redox functions of APE1 in CSR. We found that APE1's endonuclease activity is essential for IgA-class switch recombination. We provide evidence that the redox function of APE1 appears to play a role in regulating CSR through the interleukin-6 signaling pathway and in proper IgA expression. Our results shed light on APE1's redox function in the control of cancer growth through modulation of the IgA CSR process.

Mast Cells Respond to Candida albicans Infections and Modulate Macrophages Phagocytosis of the Fungus.

Mast cells (MCs) are long-lived immune cells widely distributed at mucosal surfaces and are among the first immune cell type that can get in contact with the external environment. This study aims to unravel the mechanisms of reciprocal influence between mucosal MCs and Candida albicans as commensal/opportunistic pathogen species in humans. Stimulation of bone marrow-derived mast cells (BMMCs) with live forms of C. albicans induced the release of TNF-α, IL-6, IL-13, and IL-4. Quite interestingly, BMMCs were able to engulf C. albicans hyphae, rearranging their α-tubulin cytoskeleton and accumulating LAMP1+ vesicles at the phagocytic synapse with the fungus. Candida-infected MCs increased macrophage crawling ability and promoted their chemotaxis against the infection. On the other side, resting MCs inhibited macrophage phagocytosis of C. albicans in a contact-dependent manner. Taken together, these results indicate that MCs play a key role in the maintenance of the equilibrium between the host and the commensal fungus C. albicans, limiting pathological fungal growth and modulating the response of resident macrophages during infections.

Mast cells at the crossroads of microbiota and IBD.

The human gut harbors a wide range of microorganisms that play a fundamental role in the well-being of their host. A dysregulation of the microbial composition can lead to the development or exacerbation of gastrointestinal (GI) disorders. Emerging evidence supports the hypothesis that mast cells (MCs) play a role in host-microbiota communication, modulating the mutual influence between the host and its microbiota through changes in their activation state. The ability of some bacteria to specifically affect MC functions and activation has been extensively studied, with different and sometimes conflicting results, while only little is known about MC-fungi interactions. In this review, the most recent advances in the field of MC-bacteria and MC-fungi interactions will be discussed, with a particular focus on the role of these interactions in the onset of GI disorders such as inflammatory bowel diseases (IBD). Moreover, the connection between some MC-targeting drugs and IBD was discussed, suggesting probiotics as reasonable and promising therapy in the management of IBD patients.

Cross-Talk between Myeloid-Derived Suppressor Cells and Mast Cells Mediates Tumor-Specific Immunosuppression in Prostate Cancer.

Immunotherapy, including the use of checkpoint inhibitors, is a potent therapeutic approach for some cancers, but has limited success with prostate tumors, in which immune suppression is instigated by the tumor. The immunosuppressive capacity of mast cells, which promote adenocarcinoma development in the prostate, prompted our investigation on whether mast cells promote tolerance to SV40 Large-T antigen, the transforming oncogene in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice. The incidence of adenocarcinoma was reduced in the offspring of a cross between TRAMP mice and mast cell-deficient KitWsh mice. TRAMP mice are tolerant to the SV40 Large T antigen, which is otherwise immunogenic in normal syngeneic B6 mice. Genetic ablation of mast cells in TRAMP mice restored their ability to mount a tumor-specific cytotoxic T-cell response. In KitWsh-TRAMP mice, the restored T-cell immunity correlated with the reduced activity of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC), along with their reduced expression of Arg1, Nos2, and Stat3 Having found that CD40L-expressing mast cells can interact in vivo with CD40-expressing PMN-MDSC, we then determined that only KitWsh-TRAMP mice reconstituted with mast cells expressing CD40L could restore PMN-MDSCs suppressive functions, T-cell unresponsiveness and adenocarcinoma development. Thus, mast cells have an immunoregulatory effect on PMN-MDSCs activity through CD40L-CD40 interaction, favoring immunosuppression and tumor onset. In prostate cancer patients, in silico analyses correlated poor clinical outcomes with high expression of genes related to mast cells and PMN-MDSCs. Cancer Immunol Res; 6(5); 552-65. ©2018 AACR.

Is it time for a new classification of mast cells? What do we know about mast cell heterogeneity?

Mast cells (MCs) are derived from committed precursors that leave the hematopoietic tissue, migrate in the blood, and colonize peripheral tissues where they terminally differentiate under microenvironment stimuli. They are distributed in almost all vascularized tissues where they act both as immune effectors and housekeeping cells, contributing to tissue homeostasis. Historically, MCs were classified into 2 subtypes, according to tryptic enzymes expression. However, MCs display a striking heterogeneity that reflects a complex interplay between different microenvironmental signals delivered by various tissues, and a differentiation program that decides their identity. Moreover, tissue-specific MCs show a trained memory, which contributes to shape their function in a specific microenvironment. In this review, we summarize the current state of our understanding of MC heterogeneity that reflects their different tissue experiences. We describe the discovery of unique cell molecules that can be used to distinguish specific MC subsets in vivo, and discuss how the improved ability to recognize these subsets provided new insights into the biology of MCs. These recent advances will be helpful for the understanding of the specific role of individual MC subsets in the control of tissue homeostasis, and in the regulation of pathological conditions such as infection, autoimmunity, and cancer.

Reciprocal influence of B cells and tumor macro and microenvironments in the ApcMin/+ model of colorectal cancer.

One of the most fascinating aspects of the immune system is its dynamism, meant as the ability to change and readapt according to the organism needs. Following an insult, we assist to the spontaneous organization of different immune cells which cooperate, locally and at distance, to build up an appropriate response. Throughout tumor progression, adaptations within the systemic tumor environment, or macroenvironment, result in the promotion of tumor growth, tumor invasion and metastasis to distal organs, but also to dramatic changes in the activity and composition of the immune system. In this work, we show the changes of the B-cell arm of the immune system following tumor progression in the ApcMin/+ model of colorectal cancer. Tumor macroenvironment leads to an increased proportion of total and IL-10-competent B cells in draining LNs while activates a differentiation route that leads to the expansion of IgA+ lymphocytes in the spleen and peritoneum. Importantly, serum IgA levels were significantly higher in ApcMin/+ than Wt mice. The peculiar involvement of IgA response in the adenomatous transformation had correlates in the gut-mucosal compartment where IgA-positive elements increased from normal mucosa to areas of low grade dysplasia while decreasing upon overt carcinomatous transformation. Altogether, our findings provide a snapshot of the tumor education of B lymphocytes in the ApcMin/+ model of colorectal cancer. Understanding how tumor macroenvironment affects the differentiation, function and distribution of B lymphocytes is pivotal to the generation of specific therapies, targeted to switching B cells to an anti-, rather than pro-, tumoral phenotype.

Rheostatic Functions of Mast Cells in the Control of Innate and Adaptive Immune Responses.

Mast cells are evolutionarily ancient cells, endowed with a unique developmental, phenotypic, and functional plasticity. They are resident cells that participate in tissue homeostasis by constantly sampling the microenvironment. As a result of their large repertoire of receptors, they can respond to multiple stimuli and selectively release different types and amounts of mediator. Here, we present and discuss the recent mast cell literature, focusing on studies that demonstrate that mast cells are more than a switch that is turned 'off' when in the resting state and 'on' when in the degranulating state. We propose a new vision of mast cells in which, by operating in a 'rheostatic' manner, these cells finely modulate not only immune responses, but also the pathogenesis of several inflammatory disorders, including infection, autoimmunity, and cancer.

Mast cells contribute to autoimmune diabetes by releasing interleukin-6 and failing to acquire a tolerogenic IL-10+ phenotype.

Mast cells (MCs) are innate immune cells that exert positive and negative immune modulatory functions capable to enhance or limit the intensity and/or duration of adaptive immune responses. Although MCs are crucial to regulate T cell immunity, their action in the pathogenesis of autoimmune diseases is still debated. Here we demonstrate that MCs play a crucial role in T1D pathogenesis so that their selective depletion in conditional MC knockout NOD mice protects them from the disease. MCs of diabetic NOD mice are overly inflammatory and secrete large amounts of IL-6 that favors differentiation of IL-17-secreting T cells at the site of autoimmunity. Moreover, while MCs of control mice acquire an IL-10+ phenotype upon interaction with FoxP3+ Treg cells, MCs of NOD mice do not undergo this tolerogenic differentiation. Our data indicate that overly inflammatory MCs unable to acquire a tolerogenic IL-10+ phenotype contribute to the pathogenesis of autoimmune T1D.

Mast cells are associated with the onset and progression of celiac disease.

BACKGROUND: Celiac disease (CD) is an immune-mediated disorder characterized by an accumulation of immune cells in the duodenal mucosa as a consequence of both adaptive and innate immune responses to undigested gliadin peptides. Mast cells (MCs) are innate immune cells that are a major source of costimulatory signals and inflammatory mediators in the intestinal mucosa. Although MCs have previously been associated with CD, functional studies have never been performed. OBJECTIVE: We aimed at evaluating the role of MCs in the pathogenesis of CD. METHODS: Intestinal biopsy specimens of patients with CD were scored according to the Marsh classification and characterized for leukocyte infiltration and MC distribution. Moreover, MC reactivity to gliadin and its peptides was characterized by using in vitro assays. RESULTS: Infiltrating MCs were associated with the severity of mucosal damage, and their numbers were increased in patients with higher Marsh scores. MCs were found to directly respond to nonimmunodominant gliadin fragments by releasing proinflammatory mediators. Immunohistochemical characterization of infiltrating MCs and the effects of gliadin peptides on intestinal MCs indicated an increase in proinflammatory MC function in advanced stages of the disease. This was also associated with increased neutrophil accumulation, the prevalence of M1 macrophages, and the severity of tissue damage. CONCLUSION: We provide a description of the progressive stages of CD, in which MCs are the hallmark of the inflammatory process. Thus the view of CD should be revised, and the contribution of MCs in the onset and progression of CD should be reconsidered in developing new therapeutic approaches.

Deciphering new mechanisms on T-cell costimulation by human mast cells.

It is well established that full activation of T cells to recognize a specific antigen requires additional signals. These secondary signals are generated by the interaction of costimulatory molecules expressed on APCs. Classical APCs include DCs, macrophages, Langerhans cells, and B cells. However, in recent years, several haematopoietic and nonhaematopoietic cells have been described to express MHC class II antigens and, in appropriate conditions, costimulatory molecules. In this issue, Suurmond et al. [Eur. J. Immunol. 2016. 46: 1132-1141] show, for the first time, that human mast cells not only express costimulatory molecules of the TNF-receptor and CD28 families, but can also costimulate T cells through a yet-to-be-defined CD28-independent interaction.

Co-Occurrence of Chronic Spontaneous Urticaria with Immunoglobulin A Deficiency and Autoimmune Diseases.

BACKGROUND: Immunoglobulin (Ig) A deficiency is a primary immunodeficiency in which autoimmunity is frequently observed. Thirty to fifty percent of patients with spontaneous chronic urticaria have autoantibodies that are able to cross-link FcεRI on mast cells and basophils. METHODS: We investigated whether spontaneous chronic urticaria in patients with IgA deficiency meets the criteria for autoimmunity. Four patients were screened for positivity to a skin prick test and an autologous serum skin test and for the presence of other autoimmune diseases. Patient sera were tested for the ability to activate basophils and mast cells in vitro by measuring surface CD63 expression and ß-hexosaminidase release, respectively. RESULTS: The autologous serum test was positive in all patients, and patient sera were found to induce CD63 upregulation on basophils and degranulation of an LAD2 mast cell line. Moreover, all patients were affected by other autoimmune disorders. CONCLUSION: For the first time, these data point out chronic autoimmune urticaria in subjects with an IgA deficiency and confirm that different autoimmune disorders are common among patients with an IgA deficiency. Patients with chronic autoimmune spontaneous urticaria should be screened for IgA deficiency, especially if they are affected by other autoimmune disorders. Thus, spontaneous urticaria could mirror more complex systemic diseases, such as immune deficiency.

Integrating innate and adaptive immune cells: Mast cells as crossroads between regulatory and effector B and T cells.

A diversity of immune mechanisms have evolved to protect normal tissues from infection, but from immune damage too. Innate cells, as well as adaptive cells, are critical contributors to the correct development of the immune response and of tissue homeostasis. There is a dynamic "cross-talk" between the innate and adaptive immunomodulatory mechanisms for an integrated control of immune damage as well as the development of the immune response. Mast cells have shown a great plasticity, modifying their behavior at different stages of immune response through interaction with effector and regulatory populations of adaptive immunity. Understanding the interplays among T effectors, regulatory T cells, B cells and regulatory B cells with mast cells will be critical in the future to assist in the development of therapeutic strategies to enhance and synergize physiological immune-modulator and -suppressor elements in the innate and adaptive immune system.

Mast cell/MDSC a liaison immunosuppressive for tumor microenvironment.

The instauration of an immunosuppressive microenvironment is a key event in cancer development and progression. Here, we discuss increasing evidences of the crosstalk between myeloid-derived suppressor cells (MDSCs) and mast cells (MCs) as a new fuel for the cancer immunosuppressive machinery.