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OBJECTIVE: Mast cells (MCs) are a heterogeneous population of tissue-resident bone marrow-derived cells; distinct MC subpopulations are situated at specific microanatomic locations. The phenotype of the murine synovial MC remains undefined. Since MCs have been implicated in the pathogenesis of inflammatory arthritis, we sought to define the phenotype of the murine synovial MC population in normal and arthritic joints. We also examined the contribution of lymphocytes to synovial MC physiology. METHODS: The MC phenotype in healthy and K/BxN serum transfer-induced arthritic synovial tissue was defined using immunohistochemical staining of prototypic MC-specific proteases (murine MC proteases [mMCP] 1, 2, 4, 5, 6, and 7) (chymases and tryptases). MC numbers and density were determined by histomorphometry in healthy and arthritic synovia. The lymphocyte contribution to MC populations was assessed using RAG-null mice. RESULTS: We found that synovial MCs display a connective tissue mast cell (CTMC) phenotype in both normal and arthritic synovial tissue, which expresses mMCP-4, -5, -6, and -7, but not mMCP-1 or mMCP-2. In addition, MC hyperplasia was seen in the arthritic synovium. In RAG-null mice, the phenotype and degree of MC hyperplasia were identical to those observed in normal mice with and without arthritis. Furthermore, in contrast to skin CTMCs, all synovial MCs expressed mMCP-6, demonstrating discrete differences between synovial CTMCs and other anatomic CTMC populations. CONCLUSION: Our findings demonstrate that the murine synovial MC population is composed of lymphocyte-independent CTMCs and identify arthritic synovium as a model system by which to gain insight into the poorly understood physiology of CTMCs in chronic inflammation. [ABSTRACT FROM AUTHOR]

In contrast to resident constitutive mast cells (CMCs), mucosal MCs (MMCs) appear in lung and trachea of sensitized mice only following inhalation challenge. We monitored the influx and maturation of MCs by their expression of Kit, FcεRI, β7 integrin and side scatter (SSC) by flow cytometry. Influx of MC progenitors (MCps) (FcεRIlo, Kitint, β7hi, SSClo) peaks 1 day after challenges and subsides to baseline by day 7 post-challenge. The mature MMCs appear as a distinct population on day 7 and peak at day 14 with higher SSC and FcεRI expression, but lower β7 and Kit expression. A distinct transitional population is present between 1 and 7 days post-challenge. Maturation occurs more rapidly in the trachea. The resident tracheal CMC had higher SSC, FcεRI, and Kit and lower β7 integrin expression than the MMC. By histology, the MMCs follow similar kinetics to the flow cytometry-identified mature MMCs and are notably persistent for >42 days. Steroid treatment reduced inflammation and MCp influx but had no effect on established MMC. Thus changes in SSC, FcεRI, and Kit together with expression of αE/α4:β7 integrins characterizes the development of induced MMCs from MCps and distinguishes them from resident CMC in the trachea and large airways.

The role of stromal fibroblasts in chronic inflammation is unfolding. In rheumatoid arthritis, leukocyte-derived cytokines TNF and IL-17A work together, activating fibroblasts to become a dominant source of the hallmark cytokine IL-6. However, IL-17A alone has minimal effect on fibroblasts. To identify key mediators of the synergistic response to TNF and IL-17A in human synovial fibroblasts, we performed time series, dose-response, and gene-silencing transcriptomics experiments. Here we show that in combination with TNF, IL-17A selectively induces a specific set of genes mediated by factors including cut-like homeobox 1 (CUX1) and IκBζ (NFKBIZ). In the promoters of CXCL1 , CXCL2 , and CXCL3 , we found a putative CUX1-NF-κB binding motif not found elsewhere in the genome. CUX1 and NF-κB p65 mediate transcription of these genes independent of LIFR, STAT3, STAT4, and ELF3. Transcription of NFKBIZ , encoding the atypical IκB factor IκBζ, is IL-17A dose-dependent, and IκBζ only mediates the transcriptional response to TNF and IL-17A, but not to TNF alone. In fibroblasts, IL-17A response depends on CUX1 and IκBζ to engage the NF-κB complex to produce chemoattractants for neutrophil and monocyte recruitment., Competing Interests: The authors declare no competing interest.

Mast cells constitutively express ß-catenin and expand in solid tumors such as colon and skin cancer. However, the role of ß-catenin signaling in mast cells and the cause or effect of mast cell expansion and tumor growth has yet to be established. In earlier studies we used mast cell depletion and protease staining approaches, to provide evidence for a causative role of mast cells in small bowel polyposis, and related specific phenotypes and distributions of tumor infiltrating mast cells to stages of tumor growth. Here we report that, stabilization of ß-catenin expands mast cells to promote high incidence of colon polyposis and infrequent small bowel polyps and skin cancer. Expression of a dominant acting ß-catenin in mast cells (5CreCAT) stimulated maturation and expression of granule stored proteases. Both mucosal and connective tissue type mast cells accumulated in colonic small bowel polyps independent of gender, and mice developed chronic systemic inflammation with splenomegaly. Reconstitution of polyposis-prone mice with bone marrow from 5CreCAT mice resulted in focal expansion of connective tissue like mast cells, which are normally rare in benign polyps and characteristically expand during adenoma-to-carcinoma transition. Our findings highlight a hitherto unknown contribution of ß-catenin signaling in mast cells to their maturation and to increased risk of colon cancer., (Copyright © 2019 Saadalla, Lima, Tsai, Osman, Singh, Linden, Dennis, Haeryfar, Gurish, Gounari and Khazaie.)

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by pathologic T cell-B cell interactions and autoantibody production. Defining the T cell populations that drive B cell responses in SLE may enable design of therapies that specifically target pathologic cell subsets. Here, we evaluated the phenotypes of CD4+ T cells in the circulation of 52 SLE patients drawn from multiple cohorts and identified a highly expanded PD-1hiCXCR5-CD4+ T cell population. Cytometric, transcriptomic, and functional assays demonstrated that PD-1hiCXCR5-CD4+ T cells from SLE patients are T peripheral helper (Tph) cells, a CXCR5- T cell population that stimulates B cell responses via IL-21. The frequency of Tph cells, but not T follicular helper (Tfh) cells, correlated with both clinical disease activity and the frequency of CD11c+ B cells in SLE patients. PD-1hiCD4+ T cells were found within lupus nephritis kidneys and correlated with B cell numbers in the kidney. Both IL-21 neutralization and CRISPR-mediated deletion of MAF abrogated the ability of Tph cells to induce memory B cell differentiation into plasmablasts in vitro. These findings identify Tph cells as a highly expanded T cell population in SLE and suggest a key role for Tph cells in stimulating pathologic B cell responses.

Objective: Mouse mast cell protease-4 (mMCP-4, also known as chymase) has both pro- and anti-inflammatory roles depending on the disease model. However, its effects have not been studied in surgically wounded skin. Given the significant clinical applications of modulating the inflammatory response in wound healing, we examined the role of mMCP-4 and the effect of its inhibitor chymostatin on leukocyte and polymorphonuclear cell (PMN) recruitment in our skin model. Approach: Recruitment was assessed on day-1 postwounding of three groups of mice ( n  = 10 each): mMCP-4 null mice, wild-type (WT) mice treated with the mMCP-4 inhibitor chymostatin, and WT with no other intervention. Leukocytes were stained with CD-45 cell marker, and PMN cells were stained with chloroacetate esterase. Results: The WT mice had 27 ± 9 leukocytes per field compared with 11 ± 6 for the mMCP-4 nulls, a decrease of 60% ( p  = 0.03), whereas the chymostatin-injected group had a count comparable with the uninjected WT controls at 24 ± 9. The WT group had a PMN count of 96 ± 12 cells, compared with just 24 ± 8 in the mMCP-4 null group, a decrease of 75% ( p  = 0.001), whereas the chymostatin-treated group had 60 ± 18 cells, a decrease of 38% compared with the WT group ( p  = 0.03). Innovation: We showed that the inflammatory process can be influenced by impeding the arrival of PMNs into the surgically injured site using the mMCP-4 inhibitor chymostatin. Conclusion: Chymase contributes to the recruitment of white blood cells in surgically wounded skin.

Mouse mast cell protease-4 (mMCP4) is a chymase that has been implicated in cardiovascular diseases, including myocardial infarction (MI). This study tested a direct role of mMCP4 in mouse post-MI cardiac dysfunction and myocardial remodeling. Immunoblot and immunofluorescent double staining demonstrated mMCP4 expression in cardiomyocytes from the infarct zone from mouse heart at 28 day post-MI. At this time point, mMCP4-deficient Mcpt4 -/- mice showed no difference in survival from wild-type (WT) control mice, yet demonstrated smaller infarct size, improved cardiac functions, reduced macrophage content but increased T-cell accumulation in the infarct region compared with those of WT littermates. mMCP4-deficiency also reduced cardiomyocyte apoptosis and expression of TGF-β1, p-Smad2, and p-Smad3 in the infarct region, but did not affect collagen deposition or α-smooth muscle actin expression in the same area. Gelatin gel zymography and immunoblot analysis revealed reduced activities of matrix metalloproteinases and expression of cysteinyl cathepsins in the myocardium, macrophages, and T cells from Mcpt4 -/- mice. Immunoblot analysis also found reduced p-Smad2 and p-Smad3 in the myocardium from Mcpt4 -/- mice, yet fibroblasts from Mcpt4 -/- mice showed comparable levels of p-Smad2 and p-Smad3 to those of WT fibroblasts. Flow cytometry, immunoblot analysis, and immunofluorescent staining demonstrated that mMCP4-deficiency reduced the expression of proapoptotic cathepsins in cardiomyocytes and protected cardiomyocytes from H 2 O 2 -induced apoptosis. This study established a role of mMCP4 in mouse post-MI dysfunction by regulating myocardial protease expression and cardiomyocyte death without significant impact on myocardial fibrosis or survival post-MI in mice., (Copyright © 2019 Elsevier B.V. All rights reserved.)

High-dimensional single-cell analyses have improved the ability to resolve complex mixtures of cells from human disease samples; however, identifying disease-associated cell types or cell states in patient samples remains challenging because of technical and interindividual variation. Here, we present mixed-effects modeling of associations of single cells (MASC), a reverse single-cell association strategy for testing whether case-control status influences the membership of single cells in any of multiple cellular subsets while accounting for technical confounders and biological variation. Applying MASC to mass cytometry analyses of CD4 + T cells from the blood of rheumatoid arthritis (RA) patients and controls revealed a significantly expanded population of CD4 + T cells, identified as CD27 - HLA-DR + effector memory cells, in RA patients (odds ratio, 1.7; P = 1.1 × 10 -3 ). The frequency of CD27 - HLA-DR + cells was similarly elevated in blood samples from a second RA patient cohort, and CD27 - HLA-DR + cell frequency decreased in RA patients who responded to immunosuppressive therapy. Mass cytometry and flow cytometry analyses indicated that CD27 - HLA-DR + cells were associated with RA (meta-analysis P = 2.3 × 10 -4 ). Compared to peripheral blood, synovial fluid and synovial tissue samples from RA patients contained about fivefold higher frequencies of CD27 - HLA-DR + cells, which comprised ~10% of synovial CD4 + T cells. CD27 - HLA-DR + cells expressed a distinctive effector memory transcriptomic program with T helper 1 (T H 1)- and cytotoxicity-associated features and produced abundant interferon-γ (IFN-γ) and granzyme A protein upon stimulation. We propose that MASC is a broadly applicable method to identify disease-associated cell populations in high-dimensional single-cell data., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Mast cells (MCs) are tissue resident sentinels that mature and orchestrate inflammation in response to infection and allergy. While they are also frequently observed in tumors, the contribution of MCs to carcinogenesis remains unclear. Here, we show that sequential oncogenic events in gut epithelia expand different types of MCs in a temporal-, spatial-, and cytokine-dependent manner. The first wave of MCs expands focally in benign adenomatous polyps, which have elevated levels of IL-10, IL-13, and IL-33, and are rich in type-2 innate lymphoid cells (ILC2s). These vanguard MCs adhere to the transformed epithelial cells and express murine mast cell protease 2 (mMCP2; a typical mucosal MC protease) and, to a lesser extent, the connective tissue mast cell (CTMC) protease mMCP6. Persistence of MCs is strictly dependent on T cell-derived IL-10, and their loss in the absence of IL-10-expressing T cells markedly delays small bowel (SB) polyposis. MCs expand profusely in polyposis-prone mice when T cells overexpress IL-10. The frequency of polyp-associated MCs is unaltered in response to broad-spectrum antibiotics, arguing against a microbial component driving their recruitment. Intriguingly, when polyps become invasive, a second wave of mMCP5 + /mMCP6 + CTMCs expands in the tumor stroma and at invasive tumor borders. Ablation of mMCP6 expression attenuates polyposis, but invasive properties of the remaining lesions remain intact. Our findings argue for a multistep process in SB carcinogenesis in which distinct MC subsets, and their elaborated proteases, guide disease progression., Competing Interests: The authors declare no conflict of interest., (Copyright © 2018 the Author(s). Published by PNAS.)

Competing Interests: Conflict-of-interest disclosure: The authors declare no competing financial interests.

The growth factor receptor Kit is involved in hematopoietic and nonhematopoietic development. Mice bearing Kit defects lack mast cells; however, strains bearing different Kit alleles exhibit diverse phenotypes. Herein, we investigated factors underlying differential sensitivity to IgG-mediated arthritis in 2 mast cell-deficient murine lines: KitWsh/Wsh, which develops robust arthritis, and KitW/Wv, which does not. Reciprocal bone marrow transplantation between KitW/Wv and KitWsh/Wsh mice revealed that arthritis resistance reflects a hematopoietic defect in addition to mast cell deficiency. In KitW/Wv mice, restoration of susceptibility to IgG-mediated arthritis was neutrophil independent but required IL-1 and the platelet/megakaryocyte markers NF-E2 and glycoprotein VI. In KitW/Wv mice, platelets were present in numbers similar to those in WT animals and functionally intact, and transfer of WT platelets did not restore arthritis susceptibility. These data implicated a platelet-independent role for the megakaryocyte, a Kit-dependent lineage that is selectively deficient in KitW/Wv mice. Megakaryocytes secreted IL-1 directly and as a component of circulating microparticles, which activated synovial fibroblasts in an IL-1-dependent manner. Transfer of WT but not IL-1-deficient megakaryocytes restored arthritis susceptibility to KitW/Wv mice. These findings identify functional redundancy among Kit-dependent hematopoietic lineages and establish an unanticipated capacity of megakaryocytes to mediate IL-1-driven systemic inflammatory disease.

CD4 + T cells are central mediators of autoimmune pathology; however, defining their key effector functions in specific autoimmune diseases remains challenging. Pathogenic CD4 + T cells within affected tissues may be identified by expression of markers of recent activation. Here we use mass cytometry to analyse activated T cells in joint tissue from patients with rheumatoid arthritis, a chronic immune-mediated arthritis that affects up to 1% of the population. This approach revealed a markedly expanded population of PD-1 hi CXCR5 - CD4 + T cells in synovium of patients with rheumatoid arthritis. However, these cells are not exhausted, despite high PD-1 expression. Rather, using multidimensional cytometry, transcriptomics, and functional assays, we define a population of PD-1 hi CXCR5 - 'peripheral helper' T (T PH ) cells that express factors enabling B-cell help, including IL-21, CXCL13, ICOS, and MAF. Like PD-1 hi CXCR5 + T follicular helper cells, T PH cells induce plasma cell differentiation in vitro through IL-21 secretion and SLAMF5 interaction (refs 3, 4). However, global transcriptomics highlight differences between T PH cells and T follicular helper cells, including altered expression of BCL6 and BLIMP1 and unique expression of chemokine receptors that direct migration to inflamed sites, such as CCR2, CX3CR1, and CCR5, in T PH cells. T PH cells appear to be uniquely poised to promote B-cell responses and antibody production within pathologically inflamed non-lymphoid tissues.

In contrast to resident constitutive mast cells (CMCs), mucosal MCs (MMCs) appear in the lung and trachea of sensitized mice only following inhalation challenge. We monitored the influx and maturation of MCs by their expression of Kit, FcɛRI, β7-integrin and side scatter (SSC) by flow cytometry. Influx of MC progenitors (MCps) (FcɛRI(lo), Kit(int), β7(hi), and SSC(lo)) peaks 1 day after challenges and subsides to baseline by day 7 after challenge. The mature MMCs appear as a distinct population on day 7 and peak at day 14 with higher SSC and FcɛRI expression, but lower β7 and Kit expression. A distinct transitional population is present between 1 and 7 days after challenge. Maturation occurs more rapidly in the trachea. The resident tracheal CMCs had higher SSC, FcɛRI, and Kit and lower β7-integrin expression than the MMCs. By histology, the MMCs follow similar kinetics to the flow cytometry-identified mature MMCs and are notably persistent for >42 days. Steroid treatment reduced inflammation and MCp influx but had no effect on established MMCs. Thus, changes in SSC, FcɛRI, and Kit together with the expression of αE/α4:β7-integrins characterizes the development of induced MMCs from MCps and distinguishes them from resident CMCs in the trachea and large airways.

Mast cells (MC) are important effector cells involved in a wide range of inflammatory diseases. The lineage-committed, tissue-localized progenitor (MCp) is not easily identified histochemically like the mature MC because they lack the distinctive cytoplasmic granules. However, they can be identified by their unique cell surface phenotype and by their ability to be expanded in culture using selective growth factors. Here we describe the methods that allow evaluation of MCp and mature MC in peripheral tissues under basal and inflammatory conditions. Thus, one can enumerate mature MC as well as immature committed progenitors in order to study basal homing, inflammatory recruitment, maturation, and life span. We also provide an analysis of difficulties that could emerge during these procedures.

Allergic asthma is a complex disease with a strong genetic component where mast cells play a major role by the release of proinflammatory mediators. In the mouse, mast cell protease-6 (mMCP-6) closely resembles the human version of mast cell tryptase, β-tryptase. The gene that encodes mMCP-6, Tpsb2, resides close by the H-2 complex (MHC gene) on chromosome 17. Thus, when the original mMCP-6 knockout mice were backcrossed to the BALB/c strain, these mice were carrying the 129/Sv haplotype of MHC (mMCP-6(-/-)/H-2bc). Further backcrossing yielded mMCP-6(-/-) mice with the BALB/c MHC locus. BALB/c mice were compared with mMCP-6(-/-) and mMCP-6(-/-)/H-2bc mice in a mouse model of experimental asthma. Although OVA-sensitized and challenged wild type mice displayed a striking airway hyperresponsiveness (AHR), mMCP-6(-/-) mice had less AHR that was comparable with that of mMCP-6(-/-)/H-2bc mice, suggesting that mMCP-6 is required for a full-blown AHR. The mMCP-6(-/-)/H-2bc mice had strikingly reduced lung inflammation, IgE responses, and Th2 cell responses upon sensitization and challenge, whereas the mMCP-6(-/-) mice responded similarly to the wild type mice but with a minor decrease in bronchoalveolar lavage eosinophils. These findings suggest that inflammatory Th2 responses are highly dependent on the MHC-haplotype and that they can develop essentially independently of mMCP-6, whereas mMCP-6 plays a key role in the development of AHR., (Copyright © 2014 by The American Association of Immunologists, Inc.)

Papain, a cysteine protease allergen with inherent adjuvant activity, induces potent IL-4 expression by T cells in the popliteal lymph nodes of mice following footpad immunization. In this study, we identify a novel, non-BCR-mediated capacity for B cells to rapidly bind and internalize papain. B cells subsequently regulate the adaptive immune response by enhancing ICOS expression on CD4(+) T cells and amplifying Th2 and follicular helper T cell induction. Ab blockade of ICOS ligand, expressed by popliteal lymph node B cells, but not dendritic cells, at the peak of the response inhibits IL-4 responses in wild-type mice but not B cell-deficient mice. Thus, B cells play a critical role in amplifying adjuvant-dependent Th2 polarization following noncanonical acquisition and internalization of the cysteine protease papain., (Copyright © 2014 by The American Association of Immunologists, Inc.)

Mast cells (MC) are potent innate immune cells that accumulate in chronically inflamed tissues. MC express the IL-33 receptor IL-1 receptor-related protein ST2 at high level, and this IL-1 family cytokine both activates MC directly and primes them to respond to other proinflammatory signals. Whether IL-33 and ST2 play a role in MC survival remains to be defined. In skin-derived human MC, we found that IL-33 attenuated MC apoptosis without altering proliferation, an effect mediated principally through the antiapoptotic molecule B-cell lymphoma-X large (BCLXL). Murine MC demonstrated a similar mechanism, dependent entirely on ST2. In line with these observations, St2(-/-) mice exhibited reduced numbers of tissue MC in inflamed arthritic joints, in helminth-infected intestine, and in normal peritoneum. To confirm an MC-intrinsic role for ST2 in vivo, we performed peritoneal transfer of WT and St2(-/-) MC. In St2(-/-) hosts treated with IL-33 and in WT hosts subjected to thioglycollate peritonitis, WT MC displayed a clear survival advantage over coengrafted St2(-/-) MC. IL-33 blockade specifically attenuated this survival advantage, confirming IL-33 as the relevant ST2 ligand mediating MC survival in vivo. Together, these data reveal a cell-intrinsic role for the IL-33/ST2 axis in the regulation of apoptosis in MC, identifying thereby a previously unappreciated pathway supporting expansion of the MC population with inflammation.

We previously established a mast cell (MC)-dependent thermal injury model in mice with ulceration and scar formation that depended on nonredundant functions of mouse MC protease (mMCP)4 and mMCP5. We hypothesized that MC activation is an early event and now find by histology that exocytosis of granule contents occurred by 2 min after thermal injury in wild-type (WT) C57BL/6 mice and in the mMCP4- or mMCP5-deficient mice. The degranulation was equivalent for MCs in the dermis and hypodermis of all three strains, but only the WT mice showed an appreciable increase in epidermal thickness. There was no loss of total MCs, partially degranulated plus intact, during the 4 h of observation. By electron microscopy, MCs in all strains showed early zonal degranulation at 30 s with marked progression in magnitude by 120 s and no mitochondrial injury or cellular necrosis. Concomitantly there was an increase in intercellular spaces indicative of tight junction (TJ) disruption in WT mice but not in the mMCP4- or mMCP5-deficient strains. The desmosomes were intact in all strains. Immunodetection of the TJ protein claudin 4 in WT and mMCP5-deficient mice indicated a significant reduction after scald injury whereas mMCP4(-/-) mice showed no significant changes. Taken together, these findings reveal that a second-degree burn injury can initiate an immediate novel zonal degranulation of MCs throughout all skin layers and a disruption of the epidermal TJs dependent on the nonredundant presence of mMCP4 and mMCP5.

Atopic dermatitis (AD) is a chronic inflammatory skin disease. Here, we show that phospholipase C-β3 (PLC-β3)-deficient mice spontaneously develop AD-like skin lesions and more severe allergen-induced dermatitis than wild-type mice. Mast cells were required for both AD models and remarkably increased in the skin of Plcb3(-/-) mice because of the increased Stat5 and reduced SHP-1 activities. Mast cell-specific deletion of Stat5 gene ameliorated allergen-induced dermatitis, whereas that of Shp1 gene encoding Stat5-inactivating SHP-1 exacerbated it. PLC-β3 regulates the expression of periostin in fibroblasts and TSLP in keratinocytes, two proteins critically involved in AD pathogenesis. Furthermore, polymorphisms in PLCB3, SHP1, STAT5A, and STAT5B genes were associated with human AD. Mast cell expression of PLC-β3 was inversely correlated with that of phospho-STAT5, and increased mast cells with high levels of phospho-STAT5 were found in lesional skin of some AD patients. Therefore, STAT5 regulatory mechanisms in mast cells are important for AD pathogenesis., (Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.)

Mast cells (MCs) are active participants in blood coagulation and innate and acquired immunity. This review focuses on the development of mouse and human MCs, as well as the involvement of their granule serine proteases in inflammation and the connective tissue remodeling that occurs during the different phases of the healing process of wounded skin and other organs. The accumulated data suggest that MCs, their tryptases, and their chymases play important roles in tissue repair. While MCs initially promote healing, they can be detrimental if they are chronically stimulated or if too many MCs become activated at the same time. The possibility that MCs and their granule serine proteases contribute to the formation of keloid and hypertrophic scars makes them potential targets for therapeutic intervention in the repair of damaged skin., (© 2014 Elsevier Inc. All rights reserved.)

Mast cells are derived from committed progenitors that originate in the BM. They mature into histochemically distinguishable, metachromatic mast cells containing numerous cytoplasmic secretory granules. Accumulating evidence demonstrates that mast cell granule maturation is very tightly regulated by many factors including different granule components such as proteoglycans. In this issue of the European Journal of Immunology, Nakazawa et al. [Eur. J. Immunol. 2014. 44: 204-214] highlight a role for mast cell derived histamine as another factor critical for mast cell maturation. Using histidine decarboxylase (HDC) deficient mice that are unable to make histamine, they show poorly formed secretory granules and decreased secretory granule protease expression in peritoneal mast cells. Co-culturing BM-derived mast cells with fibroblasts normally drives granule maturation, but HDC-deficient BM-derived mast cells fail to do so. Exogenously provided histamine partly restores granule differentiation as evidenced by increased tryptase and chymase activity, and this is histamine receptor type H4 -dependent. However, H4 -deficient mice have intact granule formation in peritoneal mast cells, suggesting that when HDC is functional, the intrinsic histamine production is sufficient for most granule maturation processes and H4 is dispensable. This study highlights the role of histamine in the regulation of mast cell maturation, although the cytosolic target remains unknown., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Interleukin (IL)-4 has critical roles in allergic disorders, including food hypersensitivity. The direct effects of the cytokine on the survival and function of mast cells, the key effectors of food anaphylaxis, have not been established. In this study, we demonstrate that IL-4 induces a marked intestinal mastocytosis in mice. This phenotype is reproduced in animals expressing Il4rαF709, an activating variant of the IL-4 receptor α-chain (IL-4Rα). Il4rαF709 mice exhibit enhanced anaphylactic reactions but unaltered physiological responses to vasoactive mediators. IL-4 induces Bcl-2 and Bcl-X(L) and enhances survival and stimulates proliferation in cultured bone marrow-derived mast cells (BMMC). These effects are STAT6 (signal transducer and activator of transcription factor 6)-dependent and are amplified in Il4rαF709 BMMC. In competitive bone marrow chimeras, Il4rαF709 mast cells display a substantial competitive advantage over wild-type mast cells, which, in turn, prevail over IL-4Rα⁻/⁻ mast cells in populating the intestine, establishing a cell-intrinsic effect of IL-4 in intestinal mast cell homeostasis. Our results demonstrate that IL-4-signaling is a key determinant of mast cell expansion in food allergy.

Mast cells (MC) and basophils share expression of the high-affinity receptor for IgE (FcεRI) but can be distinguished by their divergent expression of KIT and CD49b. In BALB/c mice, MC lineage cells expressing high levels of FcεRI by flow cytometry were seen only in bone marrow whereas those expressing intermediate levels of FcεRI were present in bone marrow and spleen of naive mice and in mesenteric lymph nodes (mLN) of Trichinella spiralis-infected mice. These FcεRI(+)KIT(+)CD49b(-) cells had a membrane phenotype similar to i.p. connective tissue-type MC, but were smaller and hypogranular by flow cytometry forward and side scatter profiles, respectively. Consistent with this, they lacked the prominent secretory granules identified by histochemistry and immunodetection for the MC-specific granule proteases that are readily seen in mature jejunal mucosal MC that also are induced by the infection and present at the same time. The concentration of these MC lineage cells in mLN determined by flow cytometry was comparable to that of MC progenitors (MCp) measured by limiting dilution and clonal expansion with maturation. We observed upregulation of IL-4 transcription by MCp in mLN and spleens of helminth-infected 4get mice, and we demonstrated by intracellular cytokine staining production of IL-4 and IL-6 by the mLN MCp in helminth-infected mice. Furthermore, treatment of helminth-infected mice with anti-FcεRI mAb, a protocol known to deplete basophils, also depleted mLN MCp. Thus, this study identifies a hypogranular subset of MCp recruited to mLN by helminth infection that may be an important unrecognized source of cytokines.

Background: Sensitization to food antigen can occur through cutaneous exposure., Objective: We sought to test the hypothesis that epicutaneous sensitization with food antigen predisposes to IgE-mediated anaphylaxis on oral allergen challenge., Methods: BALB/c mice were epicutaneously sensitized by repeated application of ovalbumin (OVA) to tape-stripped skin over 7 weeks or orally immunized with OVA and cholera toxin (CT) weekly for 8 weeks and then orally challenged with OVA. Body temperature was monitored, and serum mouse mast cell protease 1 levels were determined after challenge. Tissue mast cell (MC) counts were examined by using chloroacetate esterase staining. Levels of serum OVA-specific IgE and IgG(1) antibodies and cytokines in supernatants of OVA-stimulated splenocytes were measured by means of ELISA. Serum IL-4 levels were measured by using an in vivo cytokine capture assay., Results: Epicutaneously sensitized mice exhibited expansion of connective tissue MCs in the jejunum, increased serum IL-4 levels, and systemic anaphylaxis after oral challenge, as evidenced by decreased body temperature and increased serum mouse mast cell protease 1 levels. Intestinal MC expansion and anaphylaxis were IgE dependent because they did not occur in epicutaneously sensitized IgE(-/-) mice. Mice orally immunized with OVA plus CT did not have increased serum IL-4 levels, expanded intestinal MCs, or anaphylaxis after oral challenge, despite OVA-specific IgE levels and splenocyte cytokine production in response to OVA stimulation, which were comparable with those of epicutaneously sensitized mice., Conclusion: Epicutaneously sensitized mice, but not mice orally immunized with antigen plus CT, have expansion of intestinal MCs and IgE-mediated anaphylaxis after single oral antigen challenge. IgE is necessary but not sufficient for food anaphylaxis, and MC expansion in the gut can play an important role in the development of anaphylaxis., (Copyright © 2013 American Academy of Allergy, Asthma & Immunology. Published by Mosby, Inc. All rights reserved.)

Mast cells (MCs) are constitutively present in most tissues and a distinct subset of MCs can also be induced upon host responses to inflammation. The hematopoietic lineage development of tissue MCs is unique compared to other myeloid-derived cells because it is early lineage progenitors, undetectable by histochemistry, that leave the bone marrow to enter the circulation. These immature lineage MCs immediately undergo transendothelial recruitment into peripheral tissues wherein the appearance of secretory granules with a particular protease phenotype is regulated by the peripheral tissue. In this Perspective, we discuss our current understanding of how these unique immunocytes arise, traffic to various sites, and may or may not mature into tissue-directed granulated phenotypes and query whether a granulated end stage is their only intended role., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Background: Mast cells are important in numerous inflammatory processes. They are also mechanosensitive and likely play an important role in wound healing. The authors hypothesized that mechanical alteration of the wound environment with a distributed suction device could link mast cells to the healing cascade., Methods: Controlled uniform full-thickness wound surface microdeformations were induced by suction combined with an open-pore polyurethane foam (microdeformational wound therapy) in mast cell-deficient WWv mice and their mast cell-sufficient littermates. Wound healing parameters were assessed in the inflammatory, proliferative, and remodeling phases of healing., Results: Wound tissue granulation, cell proliferation, blood vessel sprouting, and collagen maturation were found to be mast cell-dependent throughout the proliferating and remodeling stages of healing., Conclusion: Mast cells are critical in the robust granulation tissue response seen in microdeformational wound therapy and in the modulation of the remodeling phase of wound healing.

Mouse mast cells (MCs) express a large number of serine proteases including tryptases, mouse mast cell protease (mMCP)-6 and -7; chymases, mMCP-1, -2, and -4; and an elastase, mMCP-5; along with carboxypeptidase-A3 (CPA3). In helminth-infected mouse intestine, distinct protease phenotypes are observed for connective tissue MCs (CTMCs) (mMCP-4(+)-7(+), and CPA3(+)) and mucosal MCs (MMCs) (mMCP-1(+) and 2(+)). To determine whether the protease phenotype was regulated by the tissue, we compared the phenotype of constitutive CTMCs and induced MMCs in trachea and large airways in antigen-sensitized unchallenged and challenged mice to MCs in skin and helminthic-infected intestine. We found that in the trachea, unlike in skin and intestine, CTMCs and MMCs both express all six serine proteases and CPA3 (mMCP-1(+), -2(+), 4(+)-7(+), CPA3(+)). This phenotype also holds for the lung CTMCs in the proximal bronchi, whereas the induced MMCs express only four proteases, mMCP-1, -2, -6, and -7. Thus, the T-cell-dependent induction of MMCs in trachea, large bronchi, and small intestine provides numbers but does not determine the protease phenotype. Furthermore, the CTMCs, which are constitutive, also show striking differences at these tissue sites, supporting the view that the differences in expression are tissue directed and not dependent on inflammation.

Background: In atopic subjects food ingestion drives the production of IgE antibodies that can trigger hypersensitivity reactions. The IL-4 pathway plays a critical role in this response, and genetic polymorphisms in its components have been linked to allergy., Objective: We sought to test whether an activating mutation in the IL-4 receptor (IL-4R) α chain enhances allergic responses to a food antigen., Methods: F709 mice, in which the IL-4Rα immunoreceptor tyrosine-based inhibitory motif is inactivated, were gavage fed with ovalbumin (OVA). Reactions to OVA challenge and immune responses, including antibody production and T(H)2 responses, were assessed., Results: F709 mice, but not wild-type control animals, sensitized by means of gavage with OVA and either cholera toxin or staphylococcal enterotoxin B, displayed mast cell activation and systemic anaphylaxis on enteral challenge. Anaphylaxis was elicited even in F709 mice enterally sensitized with OVA alone. Bone marrow chimera experiments established that the increased sensitivity conferred by the F709 genotype was mediated mostly by hematopoietic cells but that nonhematopoietic cells also contributed. F709 mice exhibited increased intestinal permeability to macromolecules. The F709 genotype conferred increased OVA-specific IgE but not IgG1 responses, local and systemic T(H)2 responses, and intestinal mast cell hyperplasia compared with wild-type mice. Anaphylaxis was abrogated in F709 mice lacking IgE or the high-affinity receptor for IgE (FcεRI)., Conclusion: Augmented IL-4Rα signaling confers increased intestinal permeability and dramatically enhanced sensitivity to food allergens. Unlike anaphylaxis to injected antigens, which in rodents can be mediated by either IgE or IgG antibodies, the food-induced response in F709 mice is solely IgE dependent., (Copyright © 2010 American Academy of Allergy, Asthma & Immunology. Published by Mosby, Inc. All rights reserved.)

Mast cells are derived from the hematopoietic progenitors found in bone marrow and spleen. Committed mast cell progenitors are rare in bone marrow suggesting they are rapidly released into the blood where they circulate and move out into the peripheral tissues. This migration is controlled in a tissue specific manner. Basal trafficking to the intestine requires expression of α4β7 integrin and the chemokine receptor CXCR2 by the mast cell progenitors and expression of MAdCAM-1 and VCAM-1 in the intestinal endothelium; and is also controlled by dendritic cells expressing the transcriptional regulatory protein T-bet. None of these play a role in basal trafficking to the lung. With the induction of allergic inflammation in the lung, there is marked recruitment of committed mast cell progenitors to lung and these cells must express α4β7 and α4β1 integrins. Within the lung there is a requirement for expression of VCAM-1 on the endothelium that is regulated by CXCR2, also expressed on the endothelium. There is a further requirement for expression of the CCR2/CCL2 pathways for full recruitment of the mast cell progenitors to the antigen-inflamed lung.

A second-degree epidermal scald burn in mice elicits an inflammatory response mediated by natural IgM directed to nonmuscle myosin with complement activation that results in ulceration and scarring. We find that such burn injury is associated with early mast cell (MC) degranulation and is absent in WBB6F1-Kit(W)/Kit(Wv) mice, which lack MCs in a context of other defects due to a mutation of the Kit receptor. To address further an MC role, we used transgenic strains with normal lineage development and a deficiency in a specific secretory granule component. Mouse strains lacking the MC-restricted chymase, mouse MC protease (mMCP)-4, or elastase, mMCP-5, show decreased injury after a second-degree scald burn, whereas mice lacking the MC-restricted tryptases, mMCP-6 and mMCP-7, or MC-specific carboxypeptidase A3 activity are not protected. Histologic sections showed some disruption of the epidermis at the scald site in the protected strains suggesting the possibility of topical reconstitution of full injury. Topical application of recombinant mMCP-5 or human neutrophil elastase to the scalded area increases epidermal injury with subsequent ulceration and scarring, both clinically and morphologically, in mMCP-5-deficient mice. Restoration of injury requires that topical administration of recombinant mMCP-5 occurs within the first hour postburn. Importantly, topical application of human MC chymase restores burn injury to scalded mMCP-4-deficient mice but not to mMCP-5-deficient mice revealing nonredundant actions for these two MC proteases in a model of innate inflammatory injury with remodeling.

In C57BL/6 mice, the recruitment of mast cell progenitors (MCps) to the lung is a feature of Ag-induced pulmonary inflammation that requires sensitization and challenge and is totally inhibited by the administration of anti-CD4 at the time of challenge. When mAb to TGFbeta1 or to IL-10R was administered at the time of challenge, the recruitment of MCp/10(6) mononuclear cells (MNCs) to the lung was inhibited by 56.3 and 69.6%, respectively, whereas mAb to IL-4, IFN-gamma, IL-6, IL-17A, and IL-17F had no effect. In sensitized and challenged C57BL/6 mice lacking TGFbetaRII on CD4(+) cells, the recruitment of MCp/10(6) MNCs was reduced by 67.8%. The requirement for TGFbeta1 and IL-10 suggested a role for CD4(+)CD25(+) T regulatory cells. Mice treated with anti-CD25 at the time of Ag-challenge showed a reduction in the recruitment of MCp/10(6) MNCs by 77.2% without any reduction in MNC influx. These results reveal an unexpected role for T regulatory cells in promoting the recruitment of MCps to the lungs of C57BL/6 mice with Ag-induced pulmonary inflammation.

Tissue-resident mast cells (MCs) are important in allergic diseases. In a mouse model of allergic airways inflammation, an increase in peribronchiolar MCs was associated with increased concentrations of the chemokine CCL2 in lung lavage. MC progenitors (MCps) arising in bone marrow (BM) are recruited to tissues by transendothelial migration, and we found that CCL2 is chemotactic for MCps in freshly isolated BM in vitro. Immature, but not mature, BM-derived MCs migrated in response to CCL2 when cultured in IL-3+stem cell factor (SCF) but not when cultured in IL-3 alone. However, the cells under both culture conditions expressed mRNA for CCR2, the receptor for CCL2, and bound the radiolabeled chemokine with similar affinities, highlighting SCF as a key mediator in coupling CCR2 to downstream events, culminating in chemotaxis. Immature BM-derived MCs from IL-3 +SCF cultures, when administered i.v., accumulated at skin sites injected with CCL2 in vivo. MCp recruitment to the allergen-sensitized/challenged lung was significantly reduced in CCR2(-/-) and CCL2(-/-) mouse strains. However, reconstitution studies of sublethally irradiated and BM-reconstituted mice indicated that BM cells and stromal elements could provide CCL2, whereas the CCR2 function resided with stromal elements rather than BM cells. These experiments revealed a new function of SCF in chemokine receptor coupling, but they suggest a complex role of the CCL2/CCR2 axis in recruiting MCps during pulmonary inflammation.

Nuclear factor of activated T cell (NFAT) transcription factors are key regulators of gene transcription within immune cells. The NFAT-interacting protein, (NIP45), augments NFAT-driven IL-4 expression by a mechanism that relies on arginine methylation. To establish the function of NIP45 in vivo, we generated mice with a targeted deletion of the gene encoding this cofactor. NIP45-deficient T helper cells displayed profound defects in the expression of NFAT-regulated cytokine genes, including IL-4. Whereas NIP45 deficiency does not interfere with T helper cell NFAT activation or lineage-specific transcription-factor expression, NIP45 acts as an enhancer for the assembly of protein arginine methyltransferase 1 and the protein arginine methyltransferase 1-linked histone 4 arginine 3 methylation with the IL-4 promoter. Our study reveals an essential role for NIP45 in promoting robust cytokine expression in vivo, which is required for the efficient handling of parasites. We propose that NIP45 acts as a molecular rheostat serving to amplify the type-2 immune response.

Altered intestinal barrier function is postulated to be a central predisposing factor to intestinal diseases, including inflammatory bowel diseases and food allergies. However, the mechanisms involved in maintaining homeostatic intestinal barrier integrity remain undefined. In this study, we demonstrate that mice deficient in mast cells (Kit(W-sh/W-sh) [Wsh]) or mast cell chymase (Mcpt4(-/-)) have significantly decreased basal small intestinal permeability compared with wild-type (WT) mice. Altered intestinal barrier function was linked to decreased intestinal epithelial cell migration along the villus/crypt axis, altered intestinal morphology, and dysregulated claudin-3 crypt expression. Remarkably, engraftment of Wsh mice with WT but not Mcpt4(-/-) mast cells restored intestinal epithelial cell migration, morphology, and intestinal epithelial barrier function. Collectively, these findings identify a mechanism by which mast cells regulate homeostatic intestinal epithelial migration and barrier function.

Pulmonary mast cell progenitor (MCp) numbers increase dramatically in sensitized and aerosolized Ag-challenged mice. This increase depends on CD4(+) T cells, as no MCp increase occurs in the lungs of sensitized wild-type (WT) mice after mAb depletion of CD4(+) but not CD8(+) cells before aerosol Ag challenge. Neither the genetic absence of IL-4, IL-4Ralpha chain, STAT-6, IFN-gamma, or IL-12p40 nor mAb blockade of IFN-gamma, IL-3, IL-4, IL-5, IL-6, IL-10, IL-13, IL-17A, IL-12p40, or IL-12p40Rbeta1 before Ag challenge in WT mice reduces the pulmonary MCp increase. However, sensitized and Ag-challenged IL-9-deficient mice and sensitized WT mice given mAb to IL-9 just before Ag challenge show significant reductions in elicited lung MCp/10(6) mononuclear cells of 47 and 66%, respectively. CD1d-deficient mice and WT mice receiving anti-CD1d before Ag challenge also show significant reductions of 65 and 59%, respectively, in elicited lung MCp/10(6) mononuclear cells, revealing an additional requirement for MCp recruitment. However, in Jalpha18-deficient mice, which lack only type 1 or invariant NKT cells, the increase in the numbers of lung MCp with Ag challenge was intact, indicating that their recruitment must be mediated by type 2 NKT cells. Furthermore, anti-CD1d treatment of IL-9-deficient mice or anti-IL-9 treatment of CD1d-deficient mice does not further reduce the significant partial impairment of MCp recruitment occurring with a single deficiency. These findings implicate type 2 NKT cells and IL-9 as central regulators that function in the same pathway mediating the Ag-induced increase in numbers of pulmonary MCp.

T-regulatory (Treg) cells play a major role in cancer by suppressing protective antitumor immune responses. A series of observations (from a single laboratory) suggest that Treg cells are protective in cancer by virtue of their ability to control cancer-associated inflammation in an interleukin (IL)-10-dependent manner. Here, we report that the ability of Treg cells to produce IL-10 and control inflammation is lost in the course of progressive disease in a mouse model of hereditary colon cancer. Treg cells that expand in adenomatous polyps no longer produce IL-10 and instead switch to production of IL-17. Aberrant Treg cells from polyp-ridden mice promote rather than suppress focal mastocytosis, a critical tumor-promoting inflammatory response. The cells, however, maintain other Treg characteristics, including their inability to produce IL-2 and ability to suppress proliferation of stimulated CD4 T cells. By promoting inflammation and suppressing T-helper functions, these cells act as a double-edged knife propagating tumor growth.

Studies performed using cultured cells indicate that IgE functions not only to trigger degranulation of mast cells following allergen exposure, but also to enhance their survival. Such an influence of IgE on mast cell homeostasis during allergic responses in vivo has not been established. In this study, we show that inhalation of Aspergillus fumigatus extract in mice induced a dramatic rise in IgE accompanied by an increase in airway mast cells. These had an activated phenotype with high levels of FcepsilonRI. Plasma mast cell protease-1 was also increased, indicating an elevated systemic mast cell load. In addition, enhanced levels of IL-5 and eosinophils were observed in the airway. Both mast cell expansion and activation were markedly attenuated in IgE(-/-) animals that are incapable of producing IgE in response to A. fumigatus. The recruitment of eosinophils to the airways was also reduced in IgE(-/-) mice. Analyses of potential cellular targets of IgE revealed that IgE Abs are not required for the induction of mast cell progenitors in response to allergen, but rather act by sustaining the survival of mature mast cells. Our results identify an important role for IgE Abs in promoting mast cell expansion during allergic responses in vivo.

Although mast cells (MCs) often are abundant in the synovial tissues of patients with rheumatoid arthritis, the contribution of MCs to joint inflammation and cartilage loss remains poorly understood. MC-restricted tryptase/heparin complexes have proinflammatory activity, and significant amounts of human tryptase beta (hTryptase-beta) are present in rheumatoid arthritis synovial fluid. Mouse MC protease-6 (mMCP-6) is the ortholog of hTryptase-beta, and this serine protease is abundant in the synovium of arthritic mice. We now report that C57BL/6 (B6) mice lacking their tryptase/heparin complexes have attenuated arthritic responses, with mMCP-6 as the dominant tryptase responsible for augmenting neutrophil infiltration in the K/BxN mouse serum-transfer arthritis model. While inflammation in this experimental arthritis model was not dependent on protease-activated receptor-2, it was dependent on the chemokine receptor CXCR2. In support of the latter data, exposure of synovial fibroblasts to hTryptase-beta/heparin or mMCP-6/heparin complexes resulted in expression of the neutrophil chemotactic factors CXCL1/KC, CXCL5/LIX, and CXCL8/IL-8. Our proteomics, histochemistry, and immunohistochemistry data also revealed substantial loss of cartilage-derived aggrecan proteoglycans in the arthritic joints of wild-type B6 mice but not mMCP-6-null B6 mice. These observations demonstrate the functional contribution of MC-restricted tryptase/heparin complexes in the K/BxN mouse arthritis model and connect our mouse findings with rheumatoid arthritis pathophysiology.

Although the innate immune function of mast cells in the acute phase of parasitic and bacterial infections is well established, their participation in chronic immune responses to indolent infection remains incompletely understood. In parasitic infection with Trichinella spiralis, the immune response incorporates both lymphocyte and mast cell-dependent effector functions for pathogen eradication. Among the mechanistic insights still unresolved in the reaction to T. spiralis are the means by which mast cells respond to parasites and the mast cell effector functions that contribute to the immunologic response to this pathogen. We hypothesized that mast cell elaboration of tryptase may comprise an important effector component in this response. Indeed, we find that mice deficient in the tryptase mouse mast cell protease-6 (mMCP-6) display a significant difference in their response to T. spiralis larvae in chronically infected skeletal muscle tissue. Mechanistically, this is associated with a profound inability to recruit eosinophils to larvae in mMCP-6-deficient mice. Analysis of IgE-deficient mice demonstrates an identical defect in eosinophil recruitment. These findings establish that mast cell secretion of the tryptase mMCP-6, a function directed by the activity of the adaptive immune system, contributes to eosinophil recruitment to the site of larval infection, thereby comprising an integral link in the chronic immune response to parasitic infection.

Chemokine receptors regulate the trafficking of leukocytes by mediating chemotaxis and by their influence on the expression and/or affinity of leukocyte integrins. Using blocking mAb, we showed that antigen-induced recruitment of mast cell progenitors (MCp) to the lung requires interaction of a4 integrins on the MCp with endothelial vascular cell adhesion molecule 1 (VCAM-1). In seeking a chemokine component, we found that CXCR2-deficient but not CCR3- or CCR5-deficient sensitized and antigen-challenged mice have significantly fewer lung MCp 1 day after challenge and fewer tracheal intraepithelial MC 1 week after challenge, implying that recruited MCp provide the source for these mature MC. Unexpectedly, reconstitution of sensitized, sublethally irradiated +/+ and -/- mice with bone marrow cells of either genotype indicated that expression of CXCR2 by the migrating MCp was not required. Instead, receptor function by resident lung cells was required because normal BM did not reconstitute MCp recruitment in irradiated CXCR2(-/-) mice. The reduced MCp influx into the lung of CXCR2(-/-) mice was accompanied by reduced induction of VCAM-1 transcripts and reduced endothelial surface expression. Thus, these studies demonstrate a role for a chemokine receptor in regulating endothelial VCAM-1 expression, MCp migration, and the level of intraepithelial MC in the lung of aerosolized, antigen-challenged mice.

It is generally agreed that most colon cancers develop from adenomatous polyps, and it is this fact on which screening strategies are based. Although there is overwhelming evidence to link intrinsic genetic lesions with the formation of these preneoplastic lesions, recent data suggest that the tumor stromal environment also plays an essential role in this disease. In particular, it has been suggested that CD34(+) immature myeloid precursor cells are required for tumor development and invasion. Here we have used mice conditional for the stabilization of beta-catenin or defective for the adenomatous polyposis coli (APC) gene to reinvestigated the identity and importance of tumor-infiltrating hematopoietic cells in polyposis. We show that, from the onset, polyps are infiltrated with proinflammatory mast cells (MC) and their precursors. Depletion of MC either pharmacologically or through the generation of chimeric mice with genetic lesions in MC development leads to a profound remission of existing polyps. Our data suggest that MC are an essential hematopoietic component for preneoplastic polyp development and are a novel target for therapeutic intervention.

The appreciation of the role of the mast cell (MC) in inflammatory processes has expanded dramatically during the last decade. Many of these processes, especially more prolonged responses, are accompanied by an increase in the number of MCs, and much of this increase is likely because of recruitment of immature progenitors with subsequent maturation under the control of the tissue microenvironment. We have begun to identify many of the cell-surface molecules that control this influx and have traced the development of these cells back to their hematopoietic roots. This development proceeds along the myelomonocytic pathway with distinct intermediates having been identified in both bone marrow and spleen. The expression of alpha4beta7 integrins has played a prominent role in this process, as it helped identify a bipotent basophil MC precursor in the spleens of C57BL/6 mice. This integrin also controls basal influx into the intestine and, along with alpha4beta1 integrins, plays a critical role in recruitment to inflamed lungs. Investigation of chemokines and chemokine receptors in these processes led to the identification of a dual role for the murine interleukin-8 receptor CXCR2. This alpha-chemokine receptor affects MC progenitor trafficking by its expression by MC progenitors and by its expression on stromal cells, likely endothelium, affecting trafficking to both intestine under basal conditions and lung during inflammatory recruitment.

Normal mouse lungs lack appreciable numbers of mast cells (MCs) or MC progenitors (MCp's), yet the appearance of mature MCs in the tracheobronchial epithelial surface is a characteristic of allergic, T-cell-dependent pulmonary inflammation. We hypothesized that pulmonary inflammation would recruit MCp's to inflamed lungs and that this recruitment would be regulated by distinct adhesion pathways. Ovalbumin-sensitized and challenged mice had a greater than 28-fold increase in the number of MCp's in the lungs. In mice lacking endothelial vascular cell adhesion molecule 1 (VCAM-1) and in wild-type mice administered blocking monoclonal antibody (mAb) to VCAM-1 but not to mucosal addressin CAM-1 (MadCAM-1), recruitment of MCp's to the inflamed lung was reduced by greater than 75%. Analysis of the integrin receptors for VCAM-1 showed that in beta7 integrin-deficient mice, recruitment was reduced 73% relative to wild-type controls, and in either BALB/c or C57BL/6 mice, mAb blocking of alpha4, beta1, or beta7 integrins inhibited the recruitment of MCp's to the inflamed lung. Thus, VCAM-1 interactions with both alpha4beta1 and alpha4beta7 integrins are essential for the recruitment and expansion of the MCp populations in the lung during antigen-induced pulmonary inflammation. Furthermore, the MCp is currently unique among inflammatory cells in its partial dependence on alpha4beta7 integrins for lung recruitment.

Mast cells (MCs) are found principally in peripheral tissues yet are of bone marrow origin. Recent studies in mice trace the MC lineage from the common myeloid progenitor through the granulocyte-macrophage progenitor in the bone marrow to a committed MC progenitor (MCP). Additionally, at least in the mouse, a bipotent basophil-MC progenitor has been identified in the spleen, suggesting a physiologic role for this organ in MC development. MCPs are especially abundant in the mouse intestine, likely ensuring the capacity for a rapid expansion of MCs in the intestinal epithelium during the effector response to helminth infection and perhaps providing a pool of committed cells capable of redistribution to other tissues. Migration of MCPs to the intestine is constitutive and controlled by alpha chemokine receptor 2 and alpha4beta7 integrins expressed on the MCPs, with the latter integrin interacting with endothelial vascular cell adhesion molecule 1 and mucosal addressin cell adhesion molecule 1. In contrast, normal mouse lung tissue contains few MCPs and MCs, and these cellular reservoirs are not affected by the lack of alpha chemokine receptor 2 or alpha4beta7 integrin. Nonetheless, robust recruitment of MCPs to the lung occurs during experimentally induced allergic pulmonary inflammation and requires alpha4beta7 and alpha4beta1 integrins interacting with vascular cell adhesion molecule 1 but not with mucosal addressin cell adhesion molecule 1. Thus although MCs are present in all organs, the pathways responsible for the trafficking of MCPs from the circulation are organ specific and include both constitutive and inducible systems, ensuring both resident MCs and the potential for incremental recruitment in accord with the requirements of the immune response. These findings in mice await confirmation in human subjects.

Basophils and mast cells, which are selectively endowed with the high-affinity IgE receptor and mediate a range of adaptive and innate immune responses, have an unknown developmental relationship. Here, by evaluating the expression of the beta7 integrin, a molecule that is required for selective homing of mast cell progenitors (MCPs) to the periphery, we identified bipotent progenitors that are capable of differentiating into either cell type in the mouse spleen. These basophil/mast cell progenitors (BMCPs) gave rise to basophils and mast cells at the single-cell level and reconstituted both mucosal and connective tissue mast cells. We also identified the basophil progenitor (BaP) and the MCP in the bone marrow and the gastrointestinal mucosa, respectively. We further show that the granulocyte-related transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) plays a primary role in the fate decision of BMCPs, being expressed in BaPs but not in MCPs. Thus, circulating basophils and tissue mast cells share a common developmental stage at which their fate decision might be controlled principally by C/EBPalpha.

Eosinophil lineage-committed progenitors (EoPs) are phenotypically isolatable in the steady-state murine bone marrow. Purified granulocyte/monocyte progenitors (GMPs) gave rise to eosinophils as well as neutrophils and monocytes at the single cell level. Within the short-term culture of GMPs, the eosinophil potential was found exclusively in cells activating the transgenic reporter for GATA-1, a transcription factor capable of instructing eosinophil lineage commitment. These GATA-1-activating cells possessed an IL-5Ralpha(+)CD34(+)c-Kit(lo) phenotype. Normal bone marrow cells also contained IL-5Ralpha(+)CD34(+)c-Kit(lo) EoPs that gave rise exclusively to eosinophils. EoPs significantly increased in number in response to helminth infection, suggesting that the EoP stage is physiologically involved in eosinophil production in vivo. EoPs expressed eosinophil-related genes, such as the eosinophil peroxidase and the major basic protein, but did not express basophil/mast cell-related mast cell proteases. The enforced retroviral expression of IL-5Ralpha in GMPs did not enhance the frequency of eosinophil lineage read-outs, whereas IL-5Ralpha(+) GMPs displayed normal neutrophil/monocyte differentiation in the presence of IL-5 alone. Thus, IL-5Ralpha might be expressed specifically at the EoP stage as a result of commitment into the eosinophil lineage. The newly identified EoPs could be the cellular target in the treatment of a variety of disorders mediated by eosinophils.

Ischemia with subsequent reperfusion (IR) injury is a significant clinical problem that occurs after physical and surgical trauma, myocardial infarction, and organ transplantation. IR injury of mouse skeletal muscle depends on the presence of both natural IgM and an intact C pathway. Disruption of the skeletal muscle architecture and permeability also requires mast cell (MC) participation, as revealed by the fact that IR injury is markedly reduced in c-kit defective, MC-deficient mouse strains. In this study, we sought to identify the pathobiologic MC products expressed in IR injury using transgenic mouse strains with normal MC development, except for the lack of a particular MC-derived mediator. Histologic analysis of skeletal muscle from BALB/c and C57BL/6 mice revealed a strong positive correlation (R(2) = 0.85) between the extent of IR injury and the level of MC degranulation. Linkage between C activation and MC degranulation was demonstrated in mice lacking C4, in which only limited MC degranulation and muscle injury were apparent. No reduction in injury was observed in transgenic mice lacking leukotriene C(4) synthase, hemopoietic PGD(2) synthase, N-deacetylase/N-sulfotransferase-2 (enzyme involved in heparin biosynthesis), or mouse MC protease (mMCP) 1. In contrast, muscle injury was significantly attenuated in mMCP-5-null mice. The MCs that reside in skeletal muscle contain abundant amounts of mMCP-5 which is the serine protease that is most similar in sequence to human MC chymase. We now report a cytotoxic activity associated with a MC-specific protease and demonstrate that mMCP-5 is critical for irreversible IR injury of skeletal muscle.