T helper cell type 2 cytokines coordinately regulate immunoglobulin E-dependent cysteinyl leukotriene production by human cord blood-derived mast cells: profound induction of leukotriene C(4) synthase expression by interleukin 4

Functional characterization of human cysteinyl leukotriene 1 receptor gene structure

The 5-lipoxygenase pathway has been strongly implicated in the pathogenesis of chronic inflammatory disorders, such as bronchial asthma and atherosclerosis. Cysteinyl leukotrienes (cysLTs), 5-lipoxygenase pathway products, are recognized now not only as important factors in asthmatic inflammation, but also as mediators of cell trafficking and innate immune responses. To study a role of cysLTs in inflammatory reactions we have characterized the gene structure of human cysteinyl leukotriene receptor type I (cysLT(1)R). The cysLT(1)R gene consists of 5 exons that are variably spliced and a single promoter region with multiple transcription start sites. Four different cysLT(1)R transcripts were identified. RT-PCR showed dominant and wide expression of the transcript I, containing exons 1, 4, and 5, with the strongest presence in blood leukocytes, spleen, thymus, lung, and heart. The expression of cysLT(1)R is functionally regulated at the transcriptional level by IL-4 through a STAT6 response element localized to the proximal cysLT(1)R promoter region. IL-4 stimulation increased cysLT(1)R mRNA (real-time PCR) and surface protein expression (flow cytometry) in a time-dependent fashion. CysLTs (LTD(4) and LTC(4)) induced an increased production of a potent monocyte chemoattractant CCL2 (MCP-1) in IL-4-primed THP-1 cells in a dose-dependent manner. This effect was effectively inhibited by the cysLT(1)R-selective antagonist MK571 in a dose-dependent manner and only partially by a nonselective cysLT(1)R/cysLT(2)R inhibitor BAY-u9773, implying a cysLT(1)R-mediated mechanism. Thus, cysLTs signaling through cysLT(1)R might contribute to inflammatory reactions by cooperating with IL-4 in enhanced CCL2 production in human monocytic cells.

Phosphorylation by protein kinase a inhibits nuclear import of 5-lipoxygenase

The enzyme 5-lipoxygenase initiates the synthesis of leukotrienes from arachidonic acid. Protein kinase A phosphorylates 5-lipoxygenase on Ser(523), and this reduces its activity. We report here that phosphorylation of Ser(523) also shifts the subcellular distribution of 5-lipoxygenase from the nucleus to the cytoplasm. Phosphorylation and redistribution of 5-lipoxygenase could be produced by overexpression of the protein kinase A catalytic subunit alpha, by pharmacological activators of protein kinase A, and by prostaglandin E(2). Mimicking phosphorylation by replacing Ser(523) with glutamic acid caused cytoplasmic localization; replacement of Ser(523) with alanine prevented phosphorylation and redistribution in response to protein kinase A activation. Because Ser(523) is positioned within the nuclear localization sequence-518 of 5-lipoxygenase, the ability of protein kinase A to phosphorylate and alter the localization of green fluorescent protein fused to the nuclear localization sequence-518 peptide was also tested. Site-directed replacement of Ser(523) with glutamic acid within the peptide impaired nuclear accumulation; overexpression of the protein kinase A catalytic subunit alpha and pharmacological activation of protein kinase caused phosphorylation of the fusion protein at Ser(523), and the phosphorylated protein was found chiefly in the cytoplasm. Taken together, these results indicate that phosphorylation of Ser(523) inhibits the nuclear import function of a nuclear localization sequence, resulting in the accumulation of 5-lipoxygenase enzyme in the cytoplasm. As cytoplasmic localization can be associated with reduced leukotriene synthetic capacity, phosphorylation of Ser(523) serves to inhibit leukotriene production by both impairing catalytic activity and by placing the enzyme in a site that is unfavorable for action.

Human airway epithelial cell determinants of survival and functional phenotype for primary human mast cells

Mast cells (MCs) are found in increased numbers at airway mucosal surfaces in asthmatic patients. Because human airway epithelial cells (HAECs) actively participate in airway inflammatory responses and are in direct contact with MCs in the mucosa, we hypothesized that HAEC-MC interactions may contribute to the differentiation and survival of MCs in the airway mucosa. Here, we show that HAECs express mRNA and protein for soluble and membrane-bound stem cell factor, releasing soluble stem cell factor into the cell culture supernatant at a concentration of 5.9 +/- 0.1 ng per 10(6) HAEC. HAECs were able to support MC survival in coculture in the absence of any exogenous cytokines for at least 4 d. Before the initiation of coculture, MCs were uniformly tryptase and chymase (MC(TC)) double positive, but by 2 d of coculture the majority of MCs expressed tryptase (MC(T)) alone. MCs supported in coculture generated low amounts of cysteinyl-leukotrienes (cys-LT) after FcepsilonRI-dependent activation (0.2 +/- 0.1 ng of cys-LT per 10(6) cells) and required priming with IL-4 and IL-3 during coculture to achieve a quantity of cys-LT generation within the range expected for human lung mucosal MC (26.5 +/- 16 ng of cys-LT per 10(6) cells). In these culture conditions, HAECs were able to direct mucosal MC protease phenotype, but T cell-derived Th2 cytokines were required for the expression of a functional airway MC eicosanoid phenotype. Thus, distinct cell types may direct unique aspects of reactive mucosal MC phenotype in the airways.

Eicosanoids: mediators and therapeutic targets in fibrotic lung disease

Fibrosis is a common end-stage sequella of a number of acute and chronic lung diseases. Current concepts of pathogenesis implicate dysregulated interactions between epithelial cells and mesenchymal cells. Although investigative efforts have documented important roles for cytokines and growth factors in the pathogenesis of fibrotic lung diseases, these observations have not as yet been translated into efficacious therapies, and there is a pressing need for new pathogenetic insights and therapeutic approaches for these devastating disorders. Eicosanoids are lipid mediators derived from arachidonic acid, the most studied of which are the prostaglandins and leukotrienes. Although they are primarily known for their roles in asthma, pain, fever and vascular responses, present evidence indicates that eicosanoids exert relevant effects on immune/inflammatory, as well as structural, cells pertinent to fibrogenesis. In general, leukotrienes promote, whereas prostaglandin E(2) opposes, fibrogenic responses. An imbalance of eicosanoids also exists in pulmonary fibrosis, which favours the production of leukotrienes over prostaglandin E(2). This review highlights the role of this imbalance in the evolution of fibrotic lung disease, discusses the mechanisms by which it may arise and considers approaches for therapeutic targeting of eicosanoids in these conditions.

IL-4-induced priming of human intestinal mast cells for enhanced survival and Th2 cytokine generation is reversible and associated with increased activity of ERK1/2 and c-Fos

In synergy with stem cell factor (SCF), IL-4 strongly enhances mast cell proliferation and shifts IgE-dependent cytokine production in mature human mast cells toward an increased release of Th2 cytokines such as IL-3, IL-5, and IL-13 and a decreased IL-6 expression. In this study we analyzed the kinetics and the mechanisms of these IL-4 effects on mast cells purified from intestinal tissue. If the cells were first cultured with IL-4 for 14 days and then without IL-4 for another 14 days, mast cells lost the capacity of producing higher amounts of Th2 cytokines and regained the capacity of producing IL-6. The IL-4-induced up-regulation of mast cell proliferation and FcepsilonRI expression was also reversible if IL-4 was withdrawn for 14 days. Interestingly, in contrast to IL-4, proliferation and phenotype of human intestinal mast cells were not affected by IL-13 although both cytokines were capable of inducing STAT6 activation. Instead, IL-4 treatment (but not IL-13 treatment) was associated with an increased activity of ERK1/2 and c-Fos, the downstream target of ERK1/2 and component of the transcription factor AP-1. Consistently, mast cell proliferation and cytokine expression in response to IL-4 was blocked by the MEK inhibitor PD98059. In summary, our data show that the IL-4 effects on human intestinal mast cell functions are reversible and accompanied by an increased activity of ERK1/2 and c-Fos.

Surface CD88 functionally distinguishes the MCTC from the MCT type of human lung mast cell

BACKGROUND

MC(T) and MC(TC) types of human mast cells (MCs) are distinguished from one another on the basis of the protease compositions of their secretory granules, but their functional and developmental relationships have been uncertain.

OBJECTIVE

These studies better define the functional properties and developmental relationship of MC(T) and MC(TC) cells.

METHODS

Mast cells were dispersed from human skin and lung, purified with anti-Kit antibody, and separated into CD88+ and CD88- populations by cell sorting. These cells were evaluated by immunocytochemistry with antitryptase and antichymase mAbs; for chymase and tryptase mRNA by real-time RT-PCR; for conversion of MC(T) to MC(TC) cells during cell culture with recombinant human stem cell factor and recombinant human IL-6; and for degranulation and leukotriene C 4 (LTC 4 ) secretion when stimulated with anti-FcepsilonRI, substance P, C5a, and compound 48/80.

RESULTS

Mature MC(T) and MC(TC) cells were separated from one another on the basis of selective expression of CD88, the C5aR, on MC(TC) cells. Lung MC(T) cells had negligible levels of chymase mRNA and retained their MC(T) phenotype in culture. Mature MC(TC) cells from skin and lung degranulated in response to FcepsilonRI cross-linking, C5a, compound 48/80, and substance P. Lung MC(TC) cells released LTC 4 on activation, but no LTC 4 was detected when skin-derived MC(TC) cells were activated. MC(T) cells from lung degranulated and released LTC 4 in response to anti-FcepsilonRI and substance P, but not to C5a and compound 48/80.

CONCLUSION

These observations functionally distinguish MC(T) from MC(TC) types of human mast cells and suggest important differences that may affect their participation in diseases such as asthma and urticaria.

Interleukin-4 (IL-4) pathway

Interleukin-4 (IL-4) is a cytokine produced by T(H)2 type helper T cells and by mast cells, basophils, and eosinophils. This cytokine can elicit many responses, some of which are associated with allergy and asthma. Studies with long-term cell lines and primary cells have revealed differences in the signaling between these two experimental systems. Understanding these differences is important because therapeutic strategies targeting IL-4 and its signaling pathways are currently being tested to treat allergy and asthma.

Long-term exacerbation by interleukin 13 of IgE-mediated eosinophilia in rats

Recent work shows that at least two cycles of antigen challenge applied in a 7-day interval are required to yield tissue eosinophil accumulation in IgE-passively sensitized rats. Since interleukin (IL)-13 is widely regarded as a key mediator in eosinophilic responses associated with mast cells and IgE, we investigated whether this cytokine could replace the first cycle of sensitization and challenge in its proeosinophilic role. We found that IL-13 (25 and 50 ng/cavity) injected into the rat pleural space led to eotaxin generation and a dose-dependent accumulation of eosinophils following IgE-passive sensitization and challenge 7 days later. IL-13 failed to cause eosinophil chemotaxis in vitro but induced eosinophil accumulation into the pleural cavity of naïve rats, which peaked 1 day and faded 72 h post-challenge. No changes were found 1 week after intrapleural injection of IL-13, except an approximately 40-50% increase in the number of adhered and non-adhered pleural mast cells. As recovered from the pleural effluent 1 week after IL-13, mast cells expressed the same amount of IgE bound on their surface as compared to controls. However, they generated 3-fold more LTC(4) following IgE-sensitization and challenge in vitro, keeping intact the amount of histamine released. Finally, pretreatment with zileuton (50 microg/cavity) 1 h before allergen challenge prevented eosinophil accumulation in those animals injected with IL-13 1 week before. In conclusion, our findings show that IL-13 causes a long-term exacerbation of the IgE-mediated eosinophilic response in a mechanism associated with heightened cysteinyl-leukotriene (cys-LT) production by resident mast cells.

Changes of 5-lipoxygenase pathway and proinflammatory mediators in cerebral cortex and lung tissue of sensitized rats

AIM

To explore the change of 5-lipoxygenase (5-LO) pathway expression and proinflammatory mediators level of lung tissue and cerebral cortex, and the possible regulatory mechanism through central nervous 5-LO pathways to pulmonary inflammatory status in antigen repeated challenged rats.

METHODS

Four groups of rats were treated as control, asthma model, asthma model treatment with dexamethasone (DXM, 0.5 mg/kg, i.p.) and ketotifen (5 mg/kg, i.g.). Tumor necrosis factor (TNF)-alpha, interleukin (IL)-4, interferon (IFN)-gamma, and nitric oxide (NO) were detected by ELISA kits. The mRNA expression of 5-LO and LTA4-hydrolase (LTA4-H) was analyzed by reverse transcription-polymerase chain reaction (RT-PCR), and the protein content of 5-LO was measured by Western blot.

RESULTS

Increase of TNF-alpha, IL-4, NO level, and decrease of IFN-gamma level in bronchoalveolar lavage fluid (BALF) and cerebral cortex in sensitized rats were shown after repeated antigen challenge. The expression of 5-LO and LTA4-H mRNA, and 5-LO protein levels were increased in lung tissue and cerebral cortex in asthma rats. In comparison with the asthma model, DXM significantly inhibited the increase of cytokine levels and the expression of 5-LO pathway enzyme (P<0.05). Ketotifen also inhibited the increase of TNF-alpha level and 5-LO pathway enzyme expression in lung and cerebral cortex, but had no effect on the level of NO, IL-4, and IFN-gamma.

CONCLUSION

The correlative increase of 5-LO pathway enzyme expression and proinflammatory mediators of brain may have a regulatory effect on pulmonary inflammation in asthma.

Adenine nucleotides inhibit cytokine generation by human mast cells through a Gs-coupled receptor

ATP and ADP activate functionally distinct G protein-coupled purinergic (P2Y) receptors. We determined the expression and function of adenine nucleotide-specific P2Y receptors on cord blood-derived human mast cells (hMCs). Human MCs expressed mRNA encoding the ADP-specific P2Y1, P2Y12, and P2Y13 receptors; the ATP/UTP-specific P2Y2 receptor; and the ATP-selective P2Y11 receptor. ADP (0.05-50 muM) induced calcium flux that was completely blocked by a P2Y1 receptor-selective antagonist and was not cross-desensitized by ATP. Low doses of ADP induced strong phosphorylation of ERK and p38 MAPKs; higher doses stimulated eicosanoid production and exocytosis. Although MAPK phosphorylation was blocked by a combination of P2Y1- and P2Y12-selective antagonists, neither interfered with secretion responses. Unexpectedly, both ADP and ATP inhibited the generation of TNF-alpha in response to the TLR2 ligand, peptidoglycan, and blocked the production of TNF-alpha, IL-8, and MIP-1beta in response to leukotriene D(4). These effects were mimicked by two ATP analogues, adenosine 5'-O-(3-thiotriphosphate) and 2',3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate (BzATP), but not by adenosine. ADP, ATP, adenosine 5'-O-(3-thiotriphosphate), and 2',3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate each induced cAMP accumulation, stimulated the phosphorylation of CREB, and up-regulated the expression of inducible cAMP early repressor, a CREB-dependent inhibitor of cytokine transcription. Human MCs thus express several ADP-selective P2Y receptors and at least one G(s)-coupled ADP/ATP receptor. Nucleotides could therefore contribute to MC-dependent microvascular leakage in atherosclerosis, tissue injury, and innate immunity while simultaneously limiting the extent of subsequent inflammation by attenuating the generation of inducible cytokines by MCs.

The role of human mast cell-derived cytokines in eosinophil biology

Eosinophil-mediated diseases, such as allergic asthma, eosinophilic fasciitis, and certain hypersensitivity pulmonary disorders, are characterized by eosinophil infiltration and tissue injury. Mast cells and T cells often colocalize to these areas. Recent data suggest that mast cells can contribute to eosinophil-mediated inflammatory responses. Activation of mast cells can occur by antigen and immunoglobulin E (IgE) via the high-affinity receptor (FcepsilonRI) for IgE. The liberation of proteases, leukotrienes, lipid mediators, and histamine can contribute to tissue inflammation and allow recruitment of eosinophils to tissue. In addition, the synthesis and expression of a plethora of cytokines and chemokines (such as granulocyte-macrophage colony-stimulating factor [GM-CSF], interleukin-1 [IL-1], IL-3, IL-5, tumor necrosis factor-alpha [TNF-alpha], and the chemokines IL-8, regulated upon activation normal T cell expressed and secreted [RANTES], monocyte chemotactic protein-1 [MCP-1], and eotaxin) by mast cells can influence eosinophil biology. Stem cell factor (SCF)-c-kit, cytokine-cytokine receptor, and chemokine-chemokine receptor (CCR3) interactions leading to nuclear factor kappaB (NF-kappaB), mitogen-activated protein kinase (MAPK) expression, and other signaling pathways can modulate eosinophil function. Eosinophil hematopoiesis, activation, survival, and elaboration of mediators can all be regulated thus by mast cells in tissue. Moreover, because eosinophils can secrete SCF, eosinophils can regulate mast cell function in a paracrine manner. This two-way interaction between eosinophils and mast cells can pave the way for chronic inflammatory responses in a variety of human diseases. This review summarizes this pivotal interaction between human mast cells and eosinophils.

Role of human mast cells and basophils in bronchial asthma

Mast cells and basophils are the only cells expressing the tetrameric (alphabetagamma2) structure of the high affinity receptor for IgE (FcepsilonRI) and synthesizing histamine in humans. Human FcepsilonRI+ cells are conventionally considered primary effector cells of bronchial asthma. There is now compelling evidence that these cells differ immunologically, biochemically, and pharmacologically, which suggests that they might play distinct roles in the appearance and fluctuation of the asthma phenotype. Recent data have revealed the complexity of the involvement of human mast cells and basophils in asthma and have shed light on the control of recruitment and activation of these cells in different lung compartments. Preliminary evidence suggests that these cells might not always be detrimental in asthma but, under some circumstances, they might exert a protective effect by modulating certain aspects of innate and acquired immunity and allergic inflammation.

Anti-interleukin-4 therapy

Interleukin 4 (IL-4) mediates important pro-inflammatory functions in asthma, including T helper cell type 2 lymphocyte differentiation, induction of IgE production, up-regulation of IgE receptors, expression of vascular cell-adhesion molecule 1, promotion of eosinophil transmigration into the lungs, inhibition of T-lymphocyte apoptosis, and mucus hypersecretion. The role of IL-4 in the pathogenesis of asthma is supported by identification of polymorphisms linked to asthma in the IL-4 gene promoter and proteins involved in IL-4 signaling. Several approaches to IL-4 antagonism are or have been in clinical development. This article examines IL-4 and the antagonists that have been developed. Early trial results and the future of anti-IL-4 therapy are discussed.

Mast-cell responses to pathogens

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

A Novel Role of Cysteinyl Leukotrienes to Promote Dendritic Cell Activation in the Antigen-Induced Immune Responses in the Lung

Although the critical role of cysteinyl leukotrienes (cysLTs) in the inflammation, especially eosinophilic lung inflammation, in asthma has been well documented, their role in the early stage of Ag-specific immune response has not been completely clarified. In the present study, with a mouse model of asthma and in vitro studies we demonstrated that cysLTs potentiated dendritic cell (DC) functions such as Ag-presenting capacity and cytokine production. The cysLT-1 receptor antagonist (LTRA) strongly suppressed the activation of these DC functions and led to inhibition of subsequent not only Th2, but also Th1, responses in the early stage of immune response. Moreover, treatment with LTRA during the early stage of the immune response potently suppressed the development of Ag inhalation-induced eosinophilic airway inflammation, mucus production, and airway hyper-reactivity in vivo. Treatment with LTRA significantly increased PGE(2) production in the lung, and treatment with the cyclooxygenase inhibitor indomethacin abolished LTRA's suppressive effect on DCs and deteriorated the Th2 and Th1 responses and airway inflammation. With in vitro studies, we also confirmed that cysLTs production by DCs increased with LPS stimulation, and that LTRA directly suppressed the alloantigen-presenting capacity of DCs. These results suggested that cysLTs potentiate DC functions both in vivo and in vitro, and that LTRA could be beneficial to suppress the initial immune response in many immune-mediated disorders beyond asthma.

Cysteinyl leukotrienes and their receptors: cellular distribution and function in immune and inflammatory responses

The cysteinyl leukotrienes (cys-LTs) are a family of potent bioactive lipids that act through two structurally divergent G protein-coupled receptors, termed the CysLT(1) and CysLT(2) receptors. The cloning and characterization of these two receptors has not only reconciled findings of previous pharmacologic profiling studies of contractile tissues, but also has uncovered their expression on a wide array of circulating and tissue-dwelling leukocytes. With the development of receptor-selective reagents, as well as mice lacking critical biosynthetic enzymes, transporter proteins, and the CysLT(1) receptor, diverse functions of cys-LTs and their receptors in immune and inflammatory responses have been identified. We review cys-LT biosynthesis; the molecular biology and distribution of the CysLT(1) and CysLT(2) receptors; the functions of cys-LTs and their receptors in the recruitment and activation of effector leukocytes and induction of adaptive immunity; and the development of fibrosis and airway remodeling in animal models of lung injury and allergic inflammation.

Lysophosphatidic acid accelerates the development of human mast cells

Mast cells (MCs) initiate immune responses from mucosal surfaces and perivascular spaces. Stem cell factor (SCF) regulates MC development and viability, but the role of innate serum factors in MC development is unexplored. Cultured cord blood-derived human MCs (hMCs) express mRNA transcripts for all 4 known receptors for lysophosphatidic acid (LPA), an abundant serum-associated lipid growth factor. In an SCF-dependent serum-free culture system, LPA (2.5-10 microM) increased the total number of hMCs by approximately 10-fold compared with cultures maintained in the absence of LPA under otherwise identical conditions. LPA was comitogenic with SCF but did not prolong MC survival. LPA-mediated proliferation was blocked by VPC-32179, a competitive antagonist of LPA(1) and LPA(3) receptors, and by pertussis toxin, and it was also attenuated by GW9662, a selective antagonist of peroxisome proliferator-activated receptor (PPAR)-gamma. LPA accelerated the acquisition of hMC granules and increased Kit expression. hMCs derived in the presence of LPA were functional, as evidenced by their immunoglobulin E (IgE)-dependent histamine release and by their characteristic proliferative responses to interleukin-3 (IL-3), IL-4, and IL-9 in combination with SCF. Thus, LPA acts through LPA receptor and PPAR-gamma-dependent pathways to accelerate hMC proliferation and differentiation, and it modulates their phenotype without providing cytoprotection. LPA could facilitate MC hyperplasia in inflammation associated with either innate or adaptive immunity.

Protection from fluorescein isothiocyanate-induced fibrosis in IL-13-deficient, but not IL-4-deficient, mice results from impaired collagen synthesis by fibroblasts

Intratracheal injection of FITC results in acute lung injury and progresses to fibrosis by day 21 postchallenge. In response to FITC, BALB/c mice produce IL-4 and IL-13 in the lung. To investigate whether IL-4 and/or IL-13 were important profibrotic mediators in this model, we examined the fibrotic response to FITC in mice that were genetically deficient in IL-4 (IL-4(-/-)), IL-13 (IL-13(-/-)), or IL-4 and IL-13 combined (IL-4/13(-/-)). Baseline levels of collagen were similar in all mice. In response to FITC, both BALB/c and IL-4(-/-) mice developed fibrosis, whereas the IL-13(-/-) and IL-4/13(-/-) mice were significantly protected, as measured by total lung collagen levels and histology. Total leukocyte recruitment to the lung was similar in all four strains of mice when measured on days 7, 14, and 21 post-FITC. BALB/c mice showed prominent eosinophilia on day 7 that was absent in IL-4(-/-), IL-13(-/-), and IL-4/13(-/-) mice, suggesting that eosinophilia is not necessary for development of a fibrotic response. There were no significant differences in the percentages of any other leukocytes analyzed between the genotypes. Similarly, protection in IL-13(-/-) mice was not associated with alterations in cytokine or eicosanoid profiles. Interestingly, TGF-beta1 production was not reduced in IL-13(-/-) mice. Analyses of fibroblasts isolated from the four genotypes demonstrated that although there were similar numbers of fibroblasts present in cultures of lung minces, fibroblasts from IL-13-deficient strains have reduced basal and stimulated levels of collagen production. IL-13Ralpha1 expression increases on fibroblasts during fibrotic responses in vivo, and IL-13 increases collagen synthesis in fibroblasts. Thus, IL-13 mediates its profibrotic actions through direct effects on fibroblast production of extracellular matrix.

Leukotriene receptors in rhinitis and sinusitis

Cysteinyl leukotrienes (CysLTs) mediate their biologic activities through interactions with the CysLT1 and CysLT2 receptors. CysLT1 receptors are prominently expressed on smooth muscle cells and lung fibroblasts, whereas CysLT2 receptors are expressed on heart Purkinje fiber cells, adrenal chromaffin cells, and endothelial cells. Both receptors are expressed on eosinophils and mast cells, but CysLT1 receptors alone are on neutrophils. Antigen-presenting cells more prominently express the type 2 receptor. CysLT1 receptors are uniquely important for bronchospasm, whereas CysLT2 receptors can stimulate endothelial cell adherence, myofibroblast proliferation, and chemokine production by mast cells. Comprehensive inhibition of the proinflammatory activities of CysLTs might require either combination CysLT1 and CysLT2 receptor antagonists or inhibitors of the CysLT synthesis pathway.

Expression, localization, and regulation of NOS in human mast cell lines: effects on leukotriene production

Nitric oxide (NO) is a potent radical produced by nitric oxide synthase (NOS) and has pleiotrophic activities in health and disease. As mast cells (MCs) play a central role in both homeostasis and pathology, we investigated NOS expression and NO production in human MC populations. Endothelial NOS (eNOS) was ubiquitously expressed in both human MC lines and skin-derived MCs, while neuronal NOS (nNOS) was variably expressed in the MC populations studied. The inducible (iNOS) isoform was not detected in human MCs. Both growth factor-independent (HMC-1) and -dependent (LAD 2) MC lines showed predominant nuclear eNOS protein localization, with weaker cytoplasmic expression. nNOS showed exclusive cytoplasmic localization in HMC-1. Activation with Ca(2+) ionophore (A23187) or IgE-anti-IgE induced eNOS phosphorylation and translocation to the nucleus and nuclear and cytoplasmic NO formation. eNOS colocalizes with the leukotriene (LT)-initiating enzyme 5-lipoxygenase (5-LO) in the MC nucleus. The NO donor, S-nitrosoglutathione (SNOG), inhibited, whereas the NOS inhibitor, N(G)-nitro-l-arginine methyl ester (L-NAME), potentiated LT release in a dose-dependent manner. Thus, human MC lines produce NO in both cytoplasmic and nuclear compartments, and endogenously produced NO can regulate LT production by MCs.

Characterization of interleukin-4-stimulated nasal polyp fibroblasts

Chronic hyperplastic eosinophilic sinusitis is an inflammatory disease that results in the accumulation of eosinophils, fibroblasts, mast cells, and goblet cells at the site of injury. A common feature of this disease is the presence of nasal polyposis (NP). The current studies were designed to assess the contribution of interleukin (IL)-4 to fibroblast-mediated inflammation in chronic hyperplastic eosinophilic sinusitis/NP. In addition, we hypothesized that cysteinyl leukotrienes (CysLT) may directly influence fibroblast-mediated fibrotic and remodeling pathways in this disorder. Fibroblasts were isolated from NP tissue. All fibroblast lines expressed the IL-4 receptor. IL-4 induced changes in mRNA and protein expression of fibrotic (transforming growth factor-beta1 and -beta2) and inflammatory cytokines and chemokines (IL-6 and CCL11) by fibroblasts as measured by semiquantitative and quantitative polymerase chain reaction, RNase protection assay, and enzyme-linked immunosorbent assay. The expression of CysLT and other proinflammatory lipid receptors on fibroblasts was evaluated. CysLT1 and CysLT2 receptors were not expressed on fibroblasts; however, LPA(1) receptor was constitutively expressed and LPA(2) receptor expression was upregulated by IL-4. The metabolic cascade involved in CysLT synthesis was not expressed in fibroblasts and could not be induced by IL-4 treatment.

5-Lipoxygenase Pathway, Dendritic Cells, and Adaptive Immunity

5-lipoxygenase (5-LO) pathway is the major source of potent proinflammatory leukotrienes (LTs) issued from the metabolism of arachidonic acid (AA), and best known for their roles in the pathogenesis of asthma. These lipid mediators are mainly released from myeloid cells and may act as physiological autocrine and paracrine signalling molecules, and play a central role in regulating the interaction between innate and adaptive immunity. The biological actions of LTs including their immunoregulatory and proinflammatory effects are mediated through extracellular specific G-protein-coupled receptors. Despite their role in inflammatory cells, such as neutrophils and macrophages, LTs may have important effects on dendritic cells (DC)-mediated adaptive immunity. Several lines of evidence show that DC not only are important source of LTs, but also become targets of their actions by producing other lipid mediators and proinflammatory molecules. This review focuses on advances in 5-LO pathway biology, the production of LTs from DC and their role on various cells of immune system and in adaptive immunity.

FcεRI-dependent gene expression in human mast cells is differentially controlled by T helper type 2 cytokines

BACKGROUND

Mast cells (MCs) proliferate in response to T(H)2 cytokines and express genes de novo after activation. Limited information is available concerning the interplay between these events.

OBJECTIVE

We explored the potential for T(H)2 cytokines to alter activation-dependent gene expression by MCs.

METHODS

Cord blood-derived human (h)MCs maintained in stem cell factor (SCF) alone were compared with replicates treated with IL-4, IL-5, or IL-9, respectively, for their patterns of FcepsilonRI-dependent gene induction using microarray technology.

RESULTS

Activation of SCF-treated hMCs upregulated their expression of roughly 140 transcripts at 2 hours, including genes involved in cell cycle progression and arrest. Each cytokine substantially modified this profile; approximately 800 inducible genes apiece were controlled by IL-5 or IL-9, whereas 169 inducible genes were controlled by IL-4. IL-4 favored the induction of cytokines and of genes associated with cell growth arrest (GADD34, GAS-1, CIDE-A, INK4D, and BAX) and completely abolished the enhanced proliferation observed in the other 3 groups after activation. Conversely, IL-5 priming induced preferential upregulation of genes involved in cell proliferation and did not abolish thymidine incorporation.

CONCLUSIONS

T(H)2 cytokines differentially modulate gene induction in hMCs after FcepsilonRI cross-linkage. IL-4 uniquely controls cytokine gene expression by hMCs and might also limit their activation-driven proliferation.

Bronchial mast cells are the dominating LTC4S-expressing cells in aspirin-tolerant asthma

The increased bronchial production of leukotriene C4 (LTC4) in asthma is assumed to derive from infiltrating eosinophils expressing LTC4-synthase (LTC4S). Multicolor immunohistofluorescence examination of bronchial cryosections from 30 treated, untreated, or bronchial antigen-provoked aspirin-tolerant individuals with asthma and nine control subjects revealed that the dominating LTC4S-expressing cells were mast cells (> 80%), and not eosinophils. Whereas 95% of the mast cells expressed high levels of LTC4S, only 8-27% of the eosinophils expressed low levels. Image analysis revealed a significantly higher LTC4S expression levels in mast cells than in eosinophils. The bronchial mRNA levels for LTC4S did not correlate with the densities of LTC4S-positive eosinophils or mast cells. Treated individuals with asthma with more than 12% reversibility had significantly higher density of LTC4S-positive mast cells than those with less reversibility, and it correlated significantly with reduction in lung function (FEV1-predicted), both before and after salbutamol inhalation. Thus, mucosal mast cells, and not eosinophils, were the dominating LTC4S-containing cells in both untreated and treated aspirin-tolerant asthma. The density of LTC4S-positive mast cells correlated, moreover, with both the reduction in lung function and the degree of reversibility in treated asthma.

Nuclear localization of 5-lipoxygenase as a determinant of leukotriene B4 synthetic capacity

The enzyme 5-lipoxygenase (5-LO) initiates the synthesis of leukotrienes from arachidonic acid. In resting cells, 5-LO can accumulate in either the cytoplasm or the nucleoplasm and, upon cell stimulation, translocates to membranes to initiate leukotriene synthesis. Here, we used mutants of 5-LO with altered subcellular localization to assess the role that nuclear positioning plays in determining leukotriene B4 (LTB4) synthesis. Mutation of either a nuclear localization sequence or a phosphorylation site reduced LTB4 synthesis by 60%, in parallel with reduced nuclear localization of 5-LO. Mutation of both sites together or mutation of all three nuclear localization sequences on 5-LO inhibited LTB4 synthesis by 90% and abolished nuclear localization. Reduced LTB4 generation in mutants could not be attributed to differences in 5-LO amount, enzymatic activity, or membrane association. Instead, 5-LO within the nucleus acts at a different site, the nuclear envelope, than does cytosolic 5-LO, which acts at cytoplasmic and perinuclear membranes. The significance of this difference was suggested by evidence that exogenously derived arachidonic acid colocalized with activated nuclear 5-LO. These results unequivocally demonstrate that the positioning of 5-LO within the nucleus of resting cells is a powerful determinant of the capacity to generate LTB4 upon subsequent activation.

Expression of the type 2 receptor for cysteinyl leukotrienes (CysLT2R) by human mast cells: Functional distinction from CysLT1R

Cysteinyl leukotrienes (cysLTs) mediate vascular leakage and bronchoconstriction through the smooth muscle-associated CysLT type 1 receptor (CysLT1R), one of at least two loosely homologous cysLT-binding G protein-coupled receptors. We previously reported that CysLT1R is expressed by cultured human mast cells (hMCs), and that priming these cells with IL-4 enhances their sensitivity to calcium flux and cytokine generation in response to cys-LTs and the nucleotide ligand, uridine diphosphate (UDP), without increasing their surface expression of CysLT1R. We now report that hMCs express the type 2 receptor for cysLTs (CysLT2R) as well, and that the amount of surface CysLT2R protein increases in response to priming with IL-4. The selective function of CysLT2R was evident based on uninhibited IL-8 secretion by IL-4-primed hMCs stimulated with cys-LTs or UDP in the presence of the selective CysLT1R antagonist MK571. MK571 did inhibit IL-5 generation, calcium flux, and phosphorylation of extracellular signal-regulated kinase. IL-8 secretion was inhibited by pertussis toxin and a selective p38 kinase inhibitor, SB203580. The CysLT2 response may permit the cys-LTs and nucleotides generated in infection and tissue injury to elicit IL-8 generation by hMCs, potentially leading to neutrophilic infiltration, a characteristic of aerosol challenge-induced late-phase responses and of sudden death associated with asthma.

The role of mast cells in asthma

While the role of mast cells in allergic reactions is unequivocal, their precise functions in asthma remain controversial. Mast cells uniquely populate all vascularized organs and tissues, including the upper and lower respiratory tree, even in healthy individuals. Histologic evidence suggests that asthma is accompanied by a mast cell hyperplasia in the inflamed mucosal epithelium and the adjacent smooth muscle. The mechanisms responsible for constitutive mast cell development have been partly elucidated. Moreover, both in vitro studies and in vivo disease models indicate that mast cells have a remarkably flexible program of gene expression, and this program can be drastically altered by the T-cell-derived Th2 cytokines relevant to asthma. Moreover, the role of mast cells in innate immunity is now firmly established, and the capacity for numerous microbial pathogens to initiate their activation in vitro and in vivo suggest mechanisms by which microbes could initiate disease exacerbations.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

Leukotriene C(4) synthase

LTC(4) synthase conjugates LTA(4) with glutathione (GSH) to form LTC(4), the parent compound of the cysteinyl leukotrienes. LTC(4) synthase is a membrane protein that functions as a non-covalent homodimer of two 18-kDa polypeptides. The enzymatic activity of LTC(4) synthase is augmented by Mg(2+) and inhibited by Co(2+) and the FLAP inhibitor MK-886. The K(m) and V(max) values of human LTC(4) synthase are 3.6 microM and 1.3 micromol/mg/min for LTA(4) and 1.6 mM and 2.7 micromol/mg/min for GSH, respectively. The deduced amino acid sequence and the predicted secondary structure of LTC(4) synthase share significant homology to FLAP, mGST-2, and mGST-3. Site-directed mutagenesis of LTC(4) synthase suggests that Arg-51 is involved in opening the epoxide ring of LTA(4) and Tyr-93 in GSH thiolate anion formation during catalytic conjugation. LTC(4) synthase is a TATA-less gene whose transcription involved both cell- and non-specific regulatory elements. LTC(4) synthase gene disrupted mice grow normally, and are attenuated for innate and adaptive immune inflammatory permeability responses.

Minimal persistent inflammation, an emerging concept in the nature and treatment of allergic rhinitis: the possible role of leukotrienes

OBJECTIVE

To review the emerging concept of minimal persistent inflammation in allergic rhinitis and its implications for therapy.

DATA SOURCES

Relevant clinical studies in the English language were reviewed.

STUDY SELECTION

Material was taken from academic/scholarly journals.

RESULTS

Accumulating evidence suggests that allergic rhinitis is a chronic inflammatory disease instead of a disease of acute symptoms. An approach to the therapy for allergic rhinitis should consider that even when symptoms are absent, a minimal level of persistent inflammation may persist. To prevent unexpected exacerbations, the treatment strategy may need to include managing subclinical persistent inflammation. Therapeutic options addressing the major inflammatory elements in allergic rhinitis, including eosinophils, the cysteinyl leukotrienes, and histamine, must be evaluated as management strategies that can achieve effective control. Traditional medications include intranasal corticosteroids, antihistamines, and immunotherapy. Recently, a leukotriene receptor antagonist has been approved for major rhinitis symptoms (congestion, rhinorrhea, sneezing, and pruritus), suggesting a new option for the treatment of allergic rhinitis.

CONCLUSIONS

Because of the possible presence of a minimal persistent inflammation during rhinitis patients' asymptomatic periods, it is important to consider a prophylactic approach to treating allergic rhinitis to prevent or reduce exacerbations during an acute increase in allergen. In light of the advances in the understanding of the pathogenesis of allergic rhinitis, agents must be considered based on their safety, efficacy, and ability to deal with underlying inflammation as well as symptom relief.

Induction of leukotriene C4 synthase after the differentiation of rat basophilic leukemia cells with retinoic acid and a low dose of actinomycin D and its suppression with methylprednisolone

Leukotriene C(4) synthase (LTC(4) S) is a pivotal enzyme for generation of cysteinyl-leukotrienes (cysLTs). LTC(4) S activity in rat basophilic leukemia-1 (RBL-1) cells increased after culture in the presence of retinoic acid (RA) analogues, which was inhibited by cycloheximide or actinomycin D (ACD). Unexpectedly, the co-addition of a low dose of ACD with RA further potentiated the upregulation of the LTC(4) S activity. Daunorubicin and mitomycin C also had a similar effect. When stimulated with calcium ionophore A23187, control cells did not produce cysLTs, but RA-treated cells generated cysLTs and the co-addition of ACD further increased. While LTC(4) S mRNA and protein increased in the cells treated with RA, the co-addition of ACD further potentiated both in proportion to the LTC(4) S activity. The effect of ACD was considered to enhance the transcription rate of LTC(4) S gene, but not the mRNA-stability. The addition of methylprednisolone (MP) inhibited generation of cysLTs from the cells with A23187-stimulation and also did LTC(4) S activity, but did not inhibit 5-lipoxygenase (5-LOX). The suppression of LTC(4) S with MP showed a dependent manner on the time-point and duration of MP-treatment after RA-addition which was correlated with reduction in LTC(4) S mRNA and protein. The cells cultured with RA plus ACD contained more histamine, chymase activity, and granules in the cytoplasm than the control cells, suggesting differentiation to mature mast cells. In consideration of RA-differentiation therapy, it may be of pathophysiological relevance that the antineoplastic agents potentiate RA-induced, steroid-sensitive, induction of LTC(4) S in RBL-1 cells.

Leukotriene synthesis is increased by transcriptional up-regulation of 5-lipoxygenase, leukotriene A4 hydrolase, and leukotriene C4 synthase in asthmatic children

Leukotrienes (LTs) are recognized to be important mediators in asthma. Recent studies revealed that LT synthesis is controlled by the regulation of LT-synthesizing enzymes. We determined the synthesis of LTB4 and LTC4 by specific radioimmunoassay, and the messenger RNA (mRNA) expression of LT-synthesizing enzymes by reverse transcriptase polymerase chain reaction in peripheral polymorphonuclear leukocytes, which were obtained from controls and asthmatic children. The synthesis of LTB4 and LTC4, and the mRNA expression of 5-lipoxygenase, LTA4 hydrolase, and LTC4 synthase were enhanced in the patients. The mRNA expression of LT-synthesizing enzymes was up-regulated, resulting in increased LT synthesis, which may play an important role in the pathogenesis of childhood asthma.

Identification of two novel nuclear import sequences on the 5-lipoxygenase protein

The nuclear import of 5-lipoxygenase modulates its capacity to produce leukotrienes from arachidonic acid. However, the molecular determinants of its nuclear import are unknown. Recently, we used structural and functional criteria to identify a novel import sequence at Arg(518) on human 5-lipoxygenase (Jones, S. M., Luo, M., Healy, A. M., Peters-Golden, M., and Brock, T. G. (2002) J. Biol. Chem. 277, 38550-38556). However, this analysis also indicated that other import sequences must exist. Here, we identify two additional sites, at Arg(112) and Lys(158), as nuclear import sequences. Both sites were found to be common to 5-lipoxygenases from different species but not found on other lipoxygenases. Both sites also appeared to be a part of structures that were predominantly random loops. Peptide sequences at these sites were sufficient to direct nuclear import of green fluorescent protein. Mutation of basic residues in these sites impaired nuclear import and combinations of mutations at different sites were additive in effect. Mutations in all three sites were required to disable nuclear accumulation of 5-lipoxygenase in all cells. Significantly, mutation in these sites did not inhibit catalytic function. Taken together, these results indicate that nuclear import of 5-lipoxygenase may reflect the combined functional effects of three discrete import sequences. Mutation of individual sites can, by itself, impair nuclear import, which in turn could impact arachidonic acid metabolism.

Non-IgE-dependent activation of human lung- and cord blood-derived mast cells is induced by eosinophil major basic protein and modulated by the membrane form of stem cell factor

The allergic reaction begins with the antigen-induced aggregation of occupied high-affinity IgE receptors expressed on mast cell surface, their activation, and the release of proinflammatory mediators that cause the "early phase" of this process. In addition, mast cell activation induces the onset of a "late phase" reaction characterized by the tissue infiltration of inflammatory cells, mainly eosinophils. We have hypothesized that during the late phase mast cells interact with and are activated by eosinophils. Here we report that highly purified human lung mast cells became responsive to eosinophil major basic protein (MBP) when in coculture with human lung fibroblasts. In addition, cord blood-derived mast cells maintained in coculture with 3T3 fibroblasts released more histamine and prostaglandin D(2) (PGD(2)) compared with cells maintained in suspension. The fibroblast-derived membrane form of stem cell factor (SCF) was found to be involved in the mast cell increased responsiveness to MBP. In fact, cord blood-derived mast cells cocultured with 3T3 in the presence of antisense for SCF or cocultured with fibroblasts that do not express the membrane form of SCF were inhibited in their histamine-releasing activity toward MBP. In addition, this form of SCF induced the expression of a pertussis toxin-sensitive G(i) protein, G(i3) that interacts with MBP to trigger mast cell non-IgE-dependent activation in a manner similar to other cationic compounds such as compound 48/80. Mast cell responsiveness to eosinophil mediators is a potentially novel evidence for an alternative pathway of allergen-independent activation able to contribute to the perpetuation of allergy.

Extending the understanding of leukotrienes in asthma

PURPOSE OF REVIEW

Leukotriene modifiers have recognized utility in the management of asthma. The aim of this review is to put into context recent research findings that extend our understanding of cysteinyl leukotriene synthesis and actions in the pathogenesis of asthma and allergic disease.

RECENT FINDINGS

Previous literature has shown that T helper type 2 cytokines thought to favor asthma and allergic diseases upregulate leukotriene synthesis. Recent findings show that interleukins-4 and -13 also upregulate cysteinyl leukotriene 1 receptor expression. Conversely, the regulation of cytokine expression by leukotrienes has also been explored: cysteinyl leukotrienes upregulate type 2 cytokine expression and decrease type 1 cytokine expression, favoring an allergic phenotype. Genetic determinants of the expression of leukotriene-forming enzymes include polymorphisms of the 5-lipoxygenase and LTC(4) synthase promoters. Novel actions of leukotrienes continue to be recognized, and a role for leukotrienes in the development of airway remodeling accompanying chronic asthma is discussed. Mounting evidence implicates leukotrienes in the pathogenesis of asthma following viral infections. Finally, advances in the measurement of leukotrienes are reviewed.

SUMMARY

Leukotrienes and their receptors play an important role in the pathogenesis of asthma. Advances in our understanding of the synthesis and actions of these lipid mediators provide the scientific rationale for appropriate utilization of leukotriene modifiers and for envisioning novel leukotriene-based therapeutic approaches in the clinical management of asthma.

2. Cytokines and chemokines

Cytokines and chemokines are redundant secreted proteins with growth, differentiation, and activation functions that regulate and determine the nature of immune responses and control immune cell trafficking and the cellular arrangement of immune organs. Which cytokines are produced in response to an immune insult determines initially whether an immune response develops and subsequently whether that response is cytotoxic, humoral, cell-mediated, or allergic. A cascade of responses can be seen in response to cytokines, and often several cytokines are required to synergize to express optimal function. An additional confounding variable in dissecting cytokine function is that each cytokine may have a completely different function, depending on the cellular source, target, and, most important, specific phase of the immune response during which it is presented. Numerous cytokines have both proinflammatory and anti-inflammatory potential; which activity is observed depends on the immune cells present and their state of responsiveness to the cytokine. For this chapter, cytokines are grouped according to those that are mononuclear phagocytic-derived or T-lymphocytic-derived; that mediate cytotoxic (antiviral and anticancer), humoral, cell-mediated, or allergic immunity; and that are immunosuppressive. The biology of chemokines are then reviewed, grouped by family.

Prostaglandins inhibit 5-lipoxygenase-activating protein expression and leukotriene B4 production from dendritic cells via an IL-10-dependent mechanism

PGs produced from arachidonic acid by the action of cyclooxygenase enzymes play a pivotal role in the regulation of both inflammatory and immune responses. Because leukotriene B4 (LTB4), a product of 5-lipoxygenase (5-LO) pathway, can exert numerous immunoregulatory and proinflammatory activities, we examined the effects of PGs on LTB4 release from dendritic cells (DC) and from peritoneal macrophages. In concentration-dependent manner, PGE1 and PGE2 inhibited the production of LTB4 from DC, but not from peritoneal macrophage, with an IC50 of 0.04 microM. The same effect was observed with MK-886, a 5-LO-activating protein (FLAP)-specific inhibitor. The decreased release of LTB4 was associated with an enhanced level of IL-10. Furthermore, the inhibition of LTB4 synthesis by PGs was significantly reversed by anti-IL-10, suggesting the involvement of an IL-10-dependent mechanism. Hence, we examined the effects of exogenous IL-10 on the 5-LO pathway. We demonstrate that IL-10 suppresses the production of LTB4 from DC by inhibiting FLAP protein expression without any effect on 5-LO and cytosolic phospholipase A2. Taken together, our results suggest links between DC cyclooxygenase and 5-LO pathways during the inflammatory response, and FLAP is a key target for the PG-induced IL-10-suppressive effects.

Cysteinyl leukotriene interactions with other mediators and with glucocorticosteroids during airway inflammation

Unexpected aspects of the antiasthmatic efficacy of leukotriene modifiers and glucocorticosteroids have been observed. For both classes, the observed effects may be partially explainable on the basis of underrecognized interactions involving leukotrienes. This review examines the interactions between leukotrienes and other mediators of asthma. It details the effects of glucocorticosteroids on leukotriene synthesis and on leukocyte populations in asthmatic airways. Unexpected controller effects of the leukotriene modifiers may reflect the fact that leukotrienes and other mediators of asthma, such as T(H)2 cytokines, positively influence each other's generation. The ability of the leukotriene modifiers to disrupt such extensive interactions means that other relevant mediators are targeted indirectly by leukotriene blockade. Among asthma therapies, the glucocorticosteroids have numerous anti-inflammatory activities, but their effects may be unpredictable. Many processes involved in inflammation appear to escape modulation by glucocorticosteroids, including leukotriene synthesis, and leukotriene generation is among them. Understanding whether glucocorticosteroids reduce cysteinyl leukotriene levels in the airway is important in determining the clinical value of combining glucocorticosteroid therapy with leukotriene modifier therapy.

Mast cells: beyond IgE

Mast cells, historically known for their involvement in type I hypersensitivity, also serve critical protective and homeostatic functions. They directly recognize the products of bacterial infection through several surface receptor proteins, releasing proteases, cytokines, and eicosanoid mediators that recruit neutrophils, limit the spread of bacterial infection, and facilitate subsequent tissue repair. In vitro studies suggest that the spectrum of microbes capable of initiating mast cell activation is broad and extends to common respiratory viruses, mycoplasma, and even products of tissue injury, such as nucleotides. TH2-polarized inflammation elicits a reactive hyperplasia of mast cells at the involved mucosal surfaces in both mice and human subject. Several recombinant TH2 cytokines (IL-3, IL-4, IL-5, and IL-9) act synergistically with stem cell factor to facilitate proliferation of nontransformed human mast cells in vitro. IL-4 induces the expression of critical inflammation-associated genes by human mast cells, such as those encoding leukotriene C4 synthase, Fc(epsilon)RI, and several cytokines. Consequently, priming with IL-4 not only amplifies classical Fc(epsilon)RI-dependent mast cell activation but also dramatically alters the product profile of mast cells activated by innate signals and by chemical mediators of inflammation. Strikingly, IL-4 induces an activation response by mast cells to cysteinyl leukotrienes, which act through a receptor shared with uridine diphosphate to induce cytokine generation without exocytosis. It Is possible that alterations in mast cell phenotype by the TH2 milieu of allergy permits otherwise trivial infections or homeostatic chemical signals to initiate harmful inflammatory cascades and sustain tissue pathology. Drug development must take these nonclassical mast cell activation pathways into account without compromising the beneficial and protective functions of mast cells.

cDNA cloning and expression of rat leukotriene C(4) synthase: elevated expression in rat basophilic leukemia-1 cells after treatment with retinoic acid

Leukotriene C(4) synthase (LTC(4) S) is considered a pivotal enzyme for generation of potent proinflammatory mediators, cysteinyl-leukotrienes (cysLTs). LTC(4) S cDNA was cloned in rat basophilic leukemia-1 (RBL-1) cells, and exhibited 84.8% and 94.5% identity with the reported human and mouse LTC(4) S cDNA sequences, respectively. Homology between the rat LTC(4) S amino acid sequence and the corresponding sequences from the other species was 86.5% and 95.3% with human and mouse sequences, respectively. Rat LTC(4) S thus showed extensive homology with both mouse and human cDNA sequences. The active enzyme as assessed by LTC(4) S activity was expressed in COS-7 cells. While RBL-1 cells after the culture for 48 h in the presence of 0.1 microg/ml all trans -retinoic acid (RA) exhibited 27 times higher LTC(4) S activity than control cells, Northern-blot analysis of RA-treated cells showed upregulation of LTC(4) S mRNA. Polyclonal antibody was raised against the synthesized peptide deduced from the nucleotide sequence. Thus, Western-blot analysis of RBL-1 cells treated with RA and COS-7 cells transfected with pcDNA-LTC(4) S commonly showed a band at approximately 18 kDa in each solubilized enzyme solution, but either control cells did not. This cDNA probe and antibody may be useful for investigating the roles of cysLTs in various experimental models of rats.

Role of STAT6 and mast cells in IL-4- and IL-13-induced alterations in murine intestinal epithelial cell function

Gastrointestinal nematode infections generally invoke a type 2 cytokine response, characterized by the production of IL-4, IL-5, IL-9, and IL-13. Among these cytokines, IL-4 and IL-13 exhibit a functional overlap that can be explained by the sharing of a common receptor or receptor component (IL-4Ralpha). Binding of IL-4 by either the type 1 or 2 IL-4R, or of IL-13 by the type 2 IL-4R, initiates Jak-dependent tyrosine phosphorylation of the IL-4Ralpha-chain and the transcription factor, STAT6. In the present study, we investigated: 1) whether IL-13 has effects on intestinal epithelial cells similar to those observed with IL-4, and 2) whether the effects of IL-4 and IL-13 depend on STAT6 signaling and/or mast cells. BALB/c, STAT6(-/-), and mast cell-deficient W/W(v) mice or their +/+ littermates were treated with a long-lasting formulation of recombinant mouse IL-4 (IL-4C) or with IL-13 for seven days. Segments of jejunum were mounted in Ussing chambers to measure mucosal permeability; chloride secretion in response to PGE(2), histamine, 5-hydroxytryptamine, or acetylcholine; and Na(+)-linked glucose absorption. IL-4C and IL-13 increased mucosal permeability, decreased glucose absorption, and decreased chloride secretion in response to 5-hydroxytryptamine. These effects were dependent on STAT6 signaling. Responses to PGE(2) and histamine, which were dependent on mast cells and STAT6, were enhanced by IL-4C, but not by IL-13. The effects of IL-4 and IL-13 on intestinal epithelial cell function may play a critical role in host protection against gastrointestinal nematodes.

Structural and functional criteria reveal a new nuclear import sequence on the 5-lipoxygenase protein

Leukotrienes are lipid mediators with important roles in immunity. The enzyme 5-lipoxygenase initiates leukotriene synthesis; nuclear import of 5-lipoxygenase modulates leukotriene synthetic capacity. In this study, we used structural and functional criteria to identify potential nuclear import sequences. Specifically, we sought basic residues that 1) were common to different 5-lipoxygenases but not shared with other lipoxygenases, 2) were found on random coil/loop structures, and 3) could be replaced without eliminating catalytic activity. Application of these criteria to the putative bipartite nuclear import sequence of 5-lipoxygenase revealed that this region formed an alpha-helix rather than a random coil, that the critical residue arginine 651 serves a structural role, and that mutation of this residue eliminated catalytic activity. A previously unrecognized region corresponding to residues 518-530 on human 5-lipoxygenase was found to be unique to 5-lipoxygenase and on a random coil. This region alone was sufficient to drive import of green fluorescent protein to the same degree as complete 5-lipoxygenase. Replacement of basic residues in this region of the complete protein was capable of eliminating nuclear import without abolishing catalytic activity. Surprisingly, two subpopulations of cells expressing 5-lipoxygenase with this mutated region could be discerned: those with strongly impaired import and those with normal import. Taken together, these results show that the previously identified region with a bipartite motif is not a functional import sequence, whereas the newly identified basic region constitutes a true nuclear import sequence. Moreover, we suggest that another sequence that can mediate nuclear import of 5-lipoxygenase remains to be identified.

Leukotriene C4 synthase: a pivotal enzyme in cellular biosynthesis of the cysteinyl leukotrienes

Leukotriene C4 synthase (LTC4S) conjugates LTA4 with glutathione (GSH) to form LTC4, the parent compound of the cysteinyl LTs. LTC4S is an 18 kDa membrane protein and functions as a noncovalent homodimer. The enzyme activity of LTC4S is augmented by Mg2+ and inhibited by Co2+ and the function of 5-lipoxygenase (LO) activating protein (FLAP) inhibitor MK-886. The Km and Vmax values are 3.6 microM and 1.3 micromol/mg/min for LTA4 and 1.6 mM and 2.7 micromol/mg/min for GSH, respectively. The deduced amino acid sequence and the predicted secondary of LTC4S shares significant homology to FLAP, mGST-2 and mGST-3 which are all members of MAPEG protein superfamily. LTC4S and FLAP exhibited identical genomic organization of five exons and four introns. Site-directed mutagenesis suggests that Arg-51 is involved in opening the epoxide ring of LTA4 and Tyr-93 in GSH thiolate anion formation during catalytic conjugation. LTC4S is a TATA-less gene whose transcription assessed in a reporter construct involved both cell-specific and nonspecific regulatory elements. LTC4S-/- mice grow normally, and are attenuated for innate and adaptive immune inflammatory permeability responses.

Targeted gene disruption reveals the role of cysteinyl leukotriene 1 receptor in the enhanced vascular permeability of mice undergoing acute inflammatory responses

The cysteinyl leukotrienes (cysLTs), leukotriene (LT) C(4), LTD(4), and LTE(4), are proinflammatory lipid mediators generated in the mouse by hematopoietic cells such as macrophages and mast cells. There are two mouse receptors for the cysLTs, CysLT(1) receptor (CysLT(1)R) and CysLT(2)R, which are 38% homologous and are located on mouse chromosomes X and 14, respectively. To clarify the different roles of the CysLT(1)R and CysLT(2)R in inflammatory responses in vivo, we generated CysLT(1)R-deficient mice by targeted gene disruption. These mice developed normally and were fertile. In an intracellular calcium mobilization assay with fura-2 acetoxymethyl ester, peritoneal macrophages from wild-type littermates, which express both CysLT(1)R and CysLT(2)R, responded substantially to 1 x 10(-6) m LTD(4) and slightly to 1 x 10(-6) m LTC(4), whereas the macrophages from CysLT(1)R-deficient mice did not respond to either LTD(4) or LTC(4). Plasma protein extravasation, but not neutrophil infiltration, was significantly reduced in CysLT(1)R-deficient mice subjected to zymosan A-induced peritoneal inflammation. Plasma protein extravasation was also significantly diminished in CysLT(1)R-deficient mice undergoing IgE-mediated passive cutaneous anaphylaxis as compared with the wild-type mice. Thus, the cysLTs generated in vivo by either monocytes/macrophages or mast cells utilize CysLT(1)R for the response of the microvasculature in acute inflammation.

Mast cell growth, differentiation, and death

Mast cells (MC) begin development in the bone marrow. Following initial lineage commitment, the cells move into the vasculature as a committed progenitor (MCp) that is poorly phenotypically defined, but appears to be an agranular cell lacking the high-affinity IgE receptor characteristic of the mature tissue-localized MC. Full maturation occurs after the cells move into the various tissues. In the mouse, MCp localizing in the connective tissues appear to differentiate into mature MC, whereas those localizing in the lung and mucosal compartment of the small intestine remain largely as committed MCp. Movement of the MCp into the small intestine is controlled by the alpha 4 beta 7 integrin, whereas the factors controlling movement into other tissues remain to be defined. Following an inflammatory stimulus, Th2-derived cytokines drive the maturation process of these MCps, leading to the mature mucosal MC hyperplasia associated with events such as an intestinal helminth infection and possibly human allergy such as asthma and rhinitis. The expanded MC number disappears as the stimulus resolves. Various routes are used in the resolution of the MC hyperplasia including apoptosis, shedding along with the villous epithelium, and recirculation back to the spleen for elimination. Unlike the reactive MC that appears in association with inflammation, the connective tissue-localized MC is a long-lived radiation-resistant cell, which appears to depend principally on the presence of stem cell factor (SCF) for its persistence.

Cysteinyl leukotrienes and uridine diphosphate induce cytokine generation by human mast cells through an interleukin 4-regulated pathway that is inhibited by leukotriene receptor antagonists

We previously reported that interleukin (IL)-4 upregulates the expression of leukotriene C(4) synthase (LTC(4)S) by human cord blood--derived mast cells (hMCs), augments their high-affinity Fc receptor for IgE (Fc(epsilon)RI)-dependent generation of eicosanoids and cytokines, and induces a calcium flux in response to cysteinyl leukotrienes (cys-LTs) and uridine diphosphate (UDP) that is blocked by cys-LT receptor antagonists. We speculated that this IL-4-dependent, receptor-mediated response to the cys-LTs and UDP might induce cytokine generation by hMCs without concomitant exocytosis. Unlike hMCs maintained in cytoprotective stem cell factor (SCF) alone, hMCs primed for 5 d with IL-4 responded to UDP (1microM), LTC(4) (100 nM), and LTD(4) (100 nM) by producing IL-5, tumor necrosis factor (TNF)-alpha, and especially large quantities of macrophage inflammatory protein (MIP)-1beta de novo at 6 h, preceded by the induced expression of the corresponding mRNAs. Cys-LT- and UDP-mediated cytokine production by the primed hMCs occurred without histamine release or PGD(2) generation and was inhibited by the CysLT1 receptor antagonist MK571. Additionally, pretreatment of hMCs with MK571 or with the cys-LT biosynthetic inhibitor MK886 decreased IL-5 and TNF-alpha production in response to IgE receptor cross-linkage, implying a positive feedback by endogenously produced cys-LTs. Cys-LTs and UDP thus orchestrate a novel, IL-4-regulated, non-IgE-dependent hMC activation for cytokine gene induction that could be initiated by microbes, cellular injury, or neurogenic or inflammatory signals; and this pathobiologic event would not be recognized in tissue studies where hMC activation is classically defined by exocytosis.

Pronounced eosinophilic lung inflammation and Th2 cytokine release in human lipocalin-type prostaglandin D synthase transgenic mice

PGD(2) is a major lipid mediator released from mast cells, but little is known about its role in the development of allergic reactions. We used transgenic (TG) mice overexpressing human lipocalin-type PGD synthase to examine the effect of overproduction of PGD(2) in an OVA-induced murine asthma model. The sensitization of wild-type (WT) and TG mice was similar as judged by the content of OVA-specific IgE. After OVA challenge, PGD(2), but not PGE(2), substantially increased in the lungs of WT and TG mice with greater PGD(2) increment in TG mice compared with WT mice. The numbers of eosinophils and lymphocytes in the bronchoalveolar lavage (BAL) fluid were significantly greater in TG mice than in WT mice on days 1 and 3 post-OVA challenge, whereas the numbers of macrophages and neutrophils were the same in both WT and TG mice. The levels of IL-4, IL-5, and eotaxin in BAL fluid were also significantly higher in TG mice than in WT mice, although the level of IFN-gamma in the BAL fluid of TG mice was decreased compared with that in WT mice. Furthermore, lymphocytes isolated from the lungs of TG mice secreted less IFN-gamma than those from WT mice, whereas IL-4 production was unchanged between WT and TG mice. Thus, overproduction of PGD(2) caused an increase in the levels of Th2 cytokines and a chemokine, accompanied by the enhanced accumulation of eosinophils and lymphocytes in the lung. These results indicate that PGD(2) plays an important role in late phase allergic reactions in the pathophysiology of bronchial asthma.

Leukotriene and prostanoid pathway enzymes in bronchial biopsies of seasonal allergic asthmatics

Cysteinyl-leukotrienes and prostaglandin D2 generated by the 5-lipoxygenase (5-LO) and cyclooxygenase (COX) pathways, respectively, cause bronchoconstriction, leukocyte recruitment, and bronchial hyperresponsiveness in asthma. We characterized the cellular expression of 5-LO and COX enzymes using immunohistochemistry on bronchial biopsies from 12 allergic asthmatic patients before and during seasonal exposure to birch pollen. Bronchial responsiveness (p = 0.004) and symptoms (p < 0.005) increased and peak expiratory flow (PEF; p < or = 0.02) decreased in the pollen season. In-season biopsies had 2-fold more cells immunostaining for 5-LO (p = 0.02), 5-LO-activating protein (FLAP; p = 0.04), and leukotriene (LT)A4 hydrolase (p = 0.05), and 4-fold more for the terminal enzyme for cysteinyl-leukotriene synthesis, LTC4 synthase (p = 0.02). Immunostaining for COX-1, COX-2, and PGD2 synthase was unchanged. Increased staining for LTC4 synthase was due to increased eosinophils (p = 0.035) and an increased proportion of eosinophils expressing the enzyme (p = 0.047). Macrophages also increased (p = 0.019), but mast cells and T-lymphocyte subsets were unchanged. Inverse correlations between PEF and 5-LO(+) cell counts link increased expression of 5-LO pathway enzymes in eosinophils and macrophages within the bronchial mucosa to deterioration of lung function during seasonal allergen exposure.