Leukotriene B4 receptor BLT1 mediates early effector T cell recruitment

Leukotriene A4 hydrolase expression in PEL cells is regulated at the transcriptional level and leads to increased leukotriene B4 production

Primary effusion lymphoma (PEL) is a herpesvirus-8-associated lymphoproliferative disease characterized by migration of tumor cells to serous body cavities. PEL cells originate from postgerminal center B cells and share a remarkable alteration in B cell transcription factor expression and/or activation with classical Hodgkin's disease cells. Comparative analysis of gene expression by cDNA microarray of BCBL-1 cells (PEL), L-428 (classical Hodgkin's disease), and BJAB cells revealed a subset of genes that were differentially expressed in BCBL-1 cells. Among these, four genes involved in cell migration and chemotaxis were strongly up-regulated in PEL cells: leukotriene A4 (LTA4) hydrolase (LTA4H), IL-16, thrombospondin-1 (TSP-1), and selectin-P ligand (PSGL-1). Up-regulation of LTA4H was investigated at the transcriptional level. Full-length LTA4H promoter exhibited 50% higher activity in BCBL-1 cells than in BJAB or L-428 cells. Deletion analysis of the LTA4H promoter revealed a positive cis-regulatory element active only in BCBL-1 cells in the promoter proximal region located between -76 and -40 bp. Formation of a specific DNA-protein complex in this region was confirmed by EMSA. Coculture of ionophore-stimulated primary neutrophils with BCBL-1 cells leads to an increased production of LTB4 compared with coculture with BJAB and L-428 cells as measured by enzyme immunoassay, demonstrating the functional significance of LTA4H up-regulation.

CARMA1 is critical for the development of allergic airway inflammation in a murine model of asthma

CARMA1 has been shown to be important for Ag-stimulated activation of NF-kappaB in lymphocytes in vitro and thus could be a novel therapeutic target in inflammatory diseases such as asthma. In the present study, we demonstrate that mice with deletion in the CARMA1 gene (CARMA1(-/-)) do not develop inflammation in a murine model of asthma. Compared with wild-type controls, CARMA1(-/-) mice did not develop airway eosinophilia, had no significant T cell recruitment into the airways, and had no evidence for T cell activation in the lung or draining lymph nodes. In addition, the CARMA1(-/-) mice had significantly decreased levels of IL-4, IL-5, and IL-13, did not produce IgE, and did not develop airway hyperresponsiveness or mucus cell hypertrophy. However, adoptive transfer of wild-type Th2 cells into CARMA1(-/-) mice restored eosinophilic airway inflammation, cytokine production, airway hyperresponsiveness, and mucus production. This is the first demonstration of an in vivo role for CARMA1 in a disease process. Furthermore, the data clearly show that CARMA1 is essential for the development of allergic airway inflammation through its role in T lymphocytes, and may provide a novel means to inhibit NF-kappaB for therapy in asthma.

Pathogenic T-cell recruitment into the airway in human disease

Effector T cells significantly contribute to inflammatory diseases. These cells are recruited into tissue, where they orchestrate an inflammatory response that can either protect against infection or sometimes stimulate human disease. The recruitment of T cells into tissue from the blood and lymphoid compartments is an active process controlled by chemokines and the chemokine receptors expressed on distinct effector T-cell subsets. Thus, the chemokines secreted in the tissue will determine the specific types of T lymphocyte recruited into that tissue based on the chemokine receptors expressed on these cells. It follows that the chemokine receptor profile on T cells isolated from the lungs of patients with inflammatory pulmonary disease will define the subtype of pathogenic T lymphocytes mediating the disease process and will identify the mechanisms that recruit these cells into the lung. This article reviews data from both human and animal studies that define the chemokine receptors involved in the recruitment of T lymphocytes into the lung in various inflammatory pulmonary diseases, including asthma, obliterative bronchiolitis, sarcoidosis, and chronic eosinophilic pneumonia. We then speculate on the potential role of these chemokine receptors in the pathogenesis of these disorders and potential novel therapeutic approaches suggested by these data.

Intestinal and pulmonary mucosal T cells: local heroes fight to maintain the status quo

Mucosal immunity in the lung and intestine is controlled by complex multifaceted systems. While mucosal T cells are essential for protection against invading pathogens owing to their proximity to the outside world, powerful systems must also be in place to harness ongoing inflammatory processes. In each site, distinct anatomical structures play key roles in mounting and executing both protective and deleterious mucosal T cell responses. Although analogies can be drawn regarding the immune systems of these two organs, there are substantial dissimilarities necessitated by unique physiologic constraints. Here, we discuss how T cell activation and effector function are generated in the mucosae.

CD8+ T lymphocytes and leukotriene B4: novel interactions in the persistence and progression of asthma

The contribution of CD8+ T cells to the development of airway hyperresponsiveness and airway inflammation has received increased attention recently. CD8+ T cells, which are capable of secreting TH2 cytokines, including IL-4, IL-5, and IL-13, have been described in asthmatic subjects and in animals sensitized and challenged with allergen. A subset of these IL-13-producing CD8+ T cells, effector memory CD8+ T cells in the mouse, express a high-affinity receptor for leukotriene B4 (BLT1), and expression of this receptor is essential for their accumulation in the lung and development of airway hyperresponsiveness and airway inflammation. A similar subset of CD8+/BLT1+/IL-13+ T cells has also been identified in the bronchoalveolar lavage fluid of asthmatic subjects, suggesting a pathogenic role for this unique subset of CD8+ T cells in asthma.

Role of the LTB4/BLT1 pathway in allergen-induced airway hyperresponsiveness and inflammation

LTB4, a proinflammatory lipid mediator generated from arachidonic acid through the action of 5-lipoxygenase, has been known for over two decades and is implicated in a wide variety of inflammatory disorders. BLT1, a G-protein-coupled receptor, has recently been identified as a high affinity receptor specific for LTB4. Recent studies in allergen-induced airway hyperresponsiveness and inflammation using mice lacking BLT1 have shown crucial new roles for leukotriene B4 and BLT1 in Th2 cytokine IL-13 production from lung T cells and recruitment of antigen-specific effector CD8+ T cells, suggesting novel mechanisms for their actions. The leukotriene B4-BLT1 pathway is an important target for the treatment of bronchial asthma.

STAT1 in peripheral tissue differentially regulates homing of antigen-specific Th1 and Th2 cells

Th1 and Th2 effector CD4+ T cells orchestrate distinct counterregulatory biological responses. To deliver effective tissue Th1- and Th2-type responses, Th1 and Th2 cell recruitment into tissue must be differentially regulated. We show that tissue-derived STAT1 controls the trafficking of adoptively transferred, Ag-specific, wild-type Th1 cells into the lung. Trafficking of Th1 and Th2 cells is differentially regulated as STAT6, which regulates Th2 cell trafficking, had no effect on the trafficking of Th1 cells and STAT1 deficiency did not alter Th2 cell trafficking. We demonstrate that STAT1 control of Th1 cell trafficking is not mediated through T-bet. STAT1 controls the recruitment of Th1 cells through the induction of CXCL9, CXCL10, CXCL11, and CXCL16, whose expression levels in the lung were markedly decreased in STAT1-/- mice. CXCL10 replacement partially restored Th1 cell trafficking in STAT1-deficient mice in vivo, and deficiency in CXCR3, the receptor for CXCL9, CXCL10, and CXCL11, impaired the trafficking of adoptively transferred Th1 cells in wild-type mice. Our work identifies that STAT1 in peripheral tissue regulates the homing of Ag-specific Th1 cells through the induction of a distinct subset of chemokines and establishes that Th1 and Th2 cell trafficking is differentially controlled in vivo by STAT1 and STAT6, respectively.

Targeted Disruption of Leukotriene B4 Receptors BLT1 and BLT2: A Critical Role for BLT1 in Collagen-Induced Arthritis in Mice

Leukotriene B(4) mediates diverse inflammatory diseases through the G protein-coupled receptors BLT1 and BLT2. In this study, we developed mice deficient in BLT1 and BLT2 by simultaneous targeted disruption of these genes. The BLT1/BLT2 double-deficient mice developed normally and peritoneal exudate cells showed no detectable responses to leukotriene B(4) confirming the deletion of the BLT1/BLT2 locus. In a model of collagen-induced arthritis on the C57BL/6 background, the BLT1/BLT2(-/-) as well as the previously described BLT1(-/-) animals showed complete protection from disease development. The disease severity correlated well with histopathology, including loss of joint architecture, inflammatory cell infiltration, fibrosis, pannus formation, and bone erosion in joints of BLT1/BLT2(+/+) animals and a total absence of disease pathology in leukotriene receptor-deficient mice. Despite these differences, all immunized BLT1(-/-) and BLT1/BLT2(-/-) animals had similar serum levels of anti-collagen Abs relative to BLT1/BLT2(+/+) animals. Thus, BLT1 may be a useful target for therapies directed at treating inflammation associated with arthritis.

The leukotriene B4 receptor (BLT1) is required for effector CD8+ T cell-mediated, mast cell-dependent airway hyperresponsiveness

Studies in both humans and rodents have suggested that CD8+ T cells contribute to the development of airway hyperresponsiveness (AHR) and that leukotriene B4 (LTB4) is involved in the chemotaxis of effector CD8+ T cells (T(EFF)) to the lung by virtue of their expression of BLT1, the receptor for LTB4. In the present study, we used a mast cell-CD8-dependent model of AHR to further define the role of BLT1 in CD8+ T cell-mediated AHR. C57BL/6+/+ and CD8-deficient (CD8-/-) mice were passively sensitized with anti-OVA IgE and exposed to OVA via the airways. Following passive sensitization and allergen exposure, C57BL/6+/+ mice developed altered airway function, whereas passively sensitized and allergen-exposed CD8-/- mice failed to do so. CD8-/- mice reconstituted with CD8+ T(EFF) developed AHR in response to challenge. In contrast, CD8-/- mice reconstituted with BLT1-deficient effector CD8+ T cells did not develop AHR. The induction of increased airway responsiveness following transfer of CD8+ T(EFF) or in wild-type mice could be blocked by administration of an LTB4 receptor antagonist confirming the role of BLT1 in CD8+ T cell-mediated AHR. Together, these data define the important role for mast cells and the LTB4-BLT1 pathway in the development of CD8+ T cell-mediated allergic responses in the lung.

Differential induction of BLT receptor expression on human endothelial cells by lipopolysaccharide, cytokines, and leukotriene B4

Leukotriene (LT) B4 is a powerful chemotactic and immune modulating agent that signals via two receptors denoted BLT1 and BLT2. Here we report that BLT1 and BLT2 are expressed at low levels in an apparently silent state in human umbilical vein endothelial cells (HUVEC). However, treatment with LPS leads to a >10 fold increase in the levels of BLT1 mRNA without any significant effects on BLT2 mRNA. In parallel, LPS also increases the amounts of BLT1 protein. Tumor necrosis factor-alpha (TNF-alpha) increases the expression of BLT2 mRNA approximately 6 times above basal levels with only a modest increase in BLT1 mRNA. Interleukin-1beta causes variable and parallel increases of both BLT1 and BLT2 mRNA. The natural ligand LTB4 also increases BLT1, but not BLT2, mRNA and protein expression. Along with the induction of BLT1 and/or BLT2, HUVEC acquire the capacity to respond to LTB4 with increased levels of intracellular calcium and these signals can be blocked by isotype selective BLT antagonists, CP-105696 and LY-255283. In addition, treatment of HUVEC with LTB4 causes increased release of both nitrite, presumably reflecting nitric oxide (NO), and monocyte chemoattractant protein-1. Our data indicate that expression of functional BLT receptors may occur at the surface of endothelial cells in response to LPS, cytokines, and ligand, which in turn may have functional consequences during the early vascular responses to inflammation. Moreover, the results point to BLT receptors as potential targets for pharmacological intervention in LT-dependent inflammatory diseases such as asthma, rheumatoid arthritis, and arteriosclerosis.

Cysteinyl leukotriene 2 receptor and protease-activated receptor 1 activate strongly correlated early genes in human endothelial cells

Cysteinyl leukotrienes (cysLT), i.e., LTC4, LTD4, and LTE4, are lipid mediators derived from the 5-lipoxygenase pathway, and the cysLT receptors cysLT1-R/cysLT2-R mediate inflammatory tissue reactions. Although endothelial cells (ECs) predominantly express cysLT2-Rs, their role in vascular biology remains to be fully understood. To delineate cysLT2-R actions, we stimulated human umbilical vein EC with LTD4 and determined early induced genes. We also compared LTD4 effects with those induced by thrombin that binds to protease-activated receptor (PAR)-1. Stringent filters yielded 37 cysLT2-R- and 34 PAR-1-up-regulated genes (>2.5-fold stimulation). Most LTD4-regulated genes were also induced by thrombin. Moreover, LTD4 plus thrombin augmented gene expression when compared with each agonist alone. Strongly induced genes were studied in detail: Early growth response (EGR) and nuclear receptor subfamily 4 group A transcription factors; E-selectin; CXC ligand 2; IL-8; a disintegrin-like and metalloprotease (reprolysin type) with thrombospondin type 1 motif 1 (ADAMTS1); Down syndrome critical region gene 1 (DSCR1); tissue factor (TF); and cyclooxygenase 2. Transcripts peaked at approximately 60 min, were unaffected by a cysLT1-R antagonist, and were superinduced by cycloheximide. The EC phenotype was markedly altered: LTD4 induced de novo synthesis of EGR1 protein and EGR1 localized in the nucleus; LTD4 up-regulated IL-8 formation and secretion; and LTD4 raised TF protein and TF-dependent EC procoagulant activity. These data show that cysLT2-R activation results in a proinflammatory EC phenotype. Because LTD4 and thrombin are likely to be formed concomitantly in vivo, cysLT2-R and PAR-1 may cooperate to augment vascular injury.

Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways

Atherosclerosis is a chronic disease of the arterial wall where both innate and adaptive immunoinflammatory mechanisms are involved. Inflammation is central at all stages of atherosclerosis. It is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Cells involved in the atherosclerotic process secrete and are activated by soluble factors, known as cytokines. Important recent advances in the comprehension of the mechanisms of atherosclerosis provided evidence that the immunoinflammatory response in atherosclerosis is modulated by regulatory pathways, in which the two anti-inflammatory cytokines interleukin-10 and transforming growth factor-β play a critical role. The purpose of this review is to bring together the current information concerning the role of cytokines in the development, progression, and complications of atherosclerosis. Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Based on our current knowledge of the role of cytokines in atherosclerosis, we propose some novel therapeutic strategies to combat this disease. In addition, we discuss the potential of circulating cytokine levels as biomarkers of coronary artery disease.

Downregulation of leukotriene biosynthesis by thymoquinone attenuates airway inflammation in a mouse model of allergic asthma

Chronic airway inflammation is a key feature of bronchial asthma. Leukotrienes are potent inflammatory mediators that play a role in the pathophysiology of asthma, and their levels are elevated in the airways in response to allergen challenge. We examined the anti-inflammatory effect of thymoquinone (TQ), the active principle in the volatile oil of Nigella sativa seeds, on leukotriene (LT) biosynthesis in a mouse model of allergic asthma. Mice sensitized and challenged with ovalbumin (OVA) antigen had an increased amounts of leukotriene B4 and C4, Th2 cytokines, and eosinophils in bronchoalveolar lavage (BAL) fluid. In addition, there was also a marked increase in lung tissue eosinophilia and goblet cell numbers. Administration of TQ before OVA challenge inhibited 5-lipoxygenase, the main enzyme in leukotriene biosynthesis, expression by lung cells and significantly reduced the levels of LTB4 and LTC4. This was accompanied by a marked decrease in Th2 cytokines and BAL fluid and lung tissue eosinophilia, all of which are characteristics of airway inflammation. These results demonstrate the anti-inflammatory effect of TQ in experimental asthma.

Development of a high-capacity homogeneous fluorescent assay for the measurement of leukotriene B4

Current immunoassays for the measurement of leukotriene B(4) (LTB(4)) typically utilize an enzyme-linked immunosorbent assay (ELISA) format that requires multiple incubations and washing steps and often expensive immunoassay kits. We have developed a bead-based, mix and read, indirect fluorescence-linked immunosorbent assay utilizing fluorometric microvolume assay technology (FMAT). The assay employs a monoclonal anti-LTB(4) antibody-coated onto goat antimouse antibody coupled polystyrene beads and an AlexaFluor-647-coupled LTB(4) ligand. Because the FMAT measurement is made only in the portion of the well volume containing the settled beads coated with AF647-LTB(4), the free label in the solution is not measured. Similarly, substances present in plasma that interfere with other immunoassays are largely ignored. The assay is robust (Z=0.8; S/N=250) and can be measured in the presence of relatively high concentrations of dimethyl sulfoxide or serum. It is inexpensive (<0.10 dollars/assay) and amenable to robotics and has a sensitivity comparable to that of the most sensitive ELISA assays; the concentration of LTB(4) giving 50% inhibition (IC(50)) was ca. 55pg/ml. Cross-reactivity in the FMAT assay was comparable to that of the ELISA assay with significant cross-reactivity found only with 20-hydroxy LTB(4) and 12-epi LTB(4). Measurements of LTB(4) determined by FMAT were equivalent to those measured by standard ELISA in samples of ionophore-stimulated human neutrophils or whole blood.

Activation of neutrophils in cutaneous T-cell lymphoma

PURPOSE

Cutaneous T-cell lymphoma (CTCL) is a spectrum of disease of unknown etiology defined by infiltrates of activated and malignant T cells in the skin. In working with blood from CTCL patients, we noticed frequent activation of neutrophils; therefore, we tested the hypothesis that neutrophils are activated in CTCL subjects compared with normal healthy controls.

EXPERIMENTAL DESIGN

Using peripheral blood of 44 subjects with CTCL and 15 normal controls, we examined three measures of neutrophil activation. These are the presence of neutrophils of reduced buoyant density, the presence of primed neutrophils in a stimulated chemiluminescence assay, and changes in surface markers by flow cytometry. In addition, we tested plasma interleukin-8 (IL-8) and leukotriene B4 (LTB4) levels using ELISA.

RESULTS

A significantly larger fraction of hypodense neutrophils was observed in CTCL subjects compared with normals (10.6 +/- 1.7% versus 1.5 +/- 0.4%). Stimulated chemiluminescence was also significantly increased in CTCL, and analysis of neutrophil surface markers using flow cytometry showed significantly increased CD11b and CD66b and decreased CD62L, consistent with neutrophil activation. These changes were present even in early stages of CTCL. We further found that plasma IL-8 and LTB4 levels are elevated in CTCL, which could form a feedback loop contributing to disease pathophysiology.

CONCLUSIONS

CTCL is associated with systemic neutrophil activation, even in early disease, and a feedback loop between neutrophils and T cells mediated by IL-8 and LTB4 is a potential contribution to the pathophysiology of CTCL.

Immune cell migration in inflammation: present and future therapeutic targets

The burgeoning field of leukocyte trafficking has created new and exciting opportunities in the clinic. Trafficking signals are being defined that finely control the movement of distinct subsets of immune cells into and out of specific tissues. Because the accumulation of leukocytes in tissues contributes to a wide variety of diseases, these 'molecular codes' have provided new targets for inhibiting tissue-specific inflammation, which have been confirmed in the clinic. However, immune cell migration is also critically important for the delivery of protective immune responses to tissues. Thus, the challenge for the future will be to identify the trafficking molecules that will most specifically inhibit the key subsets of cells that drive disease processes without affecting the migration and function of leukocytes required for protective immunity.

Leukotriene receptors in atherosclerosis

Leukotriene-forming enzymes are expressed within atherosclerotic lesions and locally produced leukotrienes exert pro-inflammatory actions within the vascular wall by means of cell surface receptors of the BLT and CysLT receptor subtypes. The migration and accumulation of inflammatory cells that follow leukotriene receptor activation have been implicated in atherosclerosis initiation and progression. Leukotriene receptors are in addition expressed on endothelial and vascular smooth muscle cells, associated with intimal hyperplasia in early atherosclerosis and restenotic lesions after angioplasty. Taken together, recent evidence suggests that leukotriene receptors may be a potential target in the treatment of atherosclerosis and in the prevention of restenosis after coronary interventions.

Mechanisms of lymphocyte migration in autoimmune disease

The recruitment of leukocytes to inflamed tissues plays an essential role in combating infection and promoting wound healing. However, in autoimmune diseases such as multiple sclerosis and diabetes, leukocytes enter tissues and contribute to inappropriate inflammatory responses, which cause tissue injury and dysfunction. In diseases of this type, lymphocytes play critical roles in initiating and maintaining these aberrant inflammatory responses. The aim of this review is to examine the mechanisms whereby T-lymphocytes enter tissues in autoimmune diseases and to compare these mechanisms between various organs and diseases. An overview of the mechanisms of leukocyte recruitment and the techniques used to study leukocyte trafficking is provided, focusing on the use of intravital microscopy as a tool to assess the functional microvasculature in vivo. We also discuss the series of tissue homing events which allow naïve lymphocytes to first enter lymph nodes and undergo activation, then subsequently to home to the peripheral organ where their cognate antigen is present. Finally, we examine mechanisms of leukocyte recruitment in diseases such as multiple sclerosis, autoimmune diabetes, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease and asthma.

Physiologic assessment of allergic rhinitis in mice: role of the high-affinity IgE receptor (FcepsilonRI)

BACKGROUND

There have been few reports using animal models to study the development of allergic rhinitis. Characterization of such a model in mice would be advantageous given the availability of reagents and gene-manipulated strains.

OBJECTIVE

We sought to develop a murine model of allergic rhinitis in the absence of lower airway changes.

METHODS

After sensitization and challenge, both wild-type and FcepsilonRI-deficient mice were studied for their ability to develop early- and late-phase nasal responses. In the invasive approach, direct measurements of nasal airway resistance (R(NA)) were obtained; in the noninvasive approach using whole-body plethysmography, respiratory frequency and expiratory and inspiratory times were monitored. In both approaches, nasal responses were determined either acutely after challenge (early phase) or 24 hours after challenge (late phase).

RESULTS

After challenge of sensitized mice, R(NA) significantly increased. In parallel, respiratory frequency significantly decreased and was highly correlated with the increases in R(NA). Sensitized wild-type mice had an early-phase nasal response and persistent nasal blockage (late-phase response) after allergen challenge. In contrast, sensitized and challenged FcepsilonRI alpha-chain-deficient mice did not have an early-phase nasal reaction and exhibited reduced nasal blockage and lower IL-13 levels in nasal tissue homogenates.

CONCLUSIONS

These data indicate that FcepsilonRI is essential to development of an early-phase nasal response and contributes to the development of the late-phase nasal response. These invasive and noninvasive approaches provide new opportunities to evaluate the mechanisms underlying the development of nasal responses to allergen and to assess various therapeutic interventions.

Mast cell survival and activation by IgE in the absence of antigen: a consideration of the biologic mechanisms and relevance

Mast cells are not only major effector cells in allergy and host defense against parasites and bacteria but also important cellular components in other immune responses. Recent studies on the effects of monomeric IgE on mast cell survival and activation have made an impact on our view of the IgE binding to its high-affinity receptors, Fc epsilonRI. Traditionally, IgE binding to Fc epsilonRI has been considered as a passive action of "sensitization" before receptor aggregation by Ag. However, recent studies indicate that at high concentrations some monoclonal IgEs have effects on mast cells similar to or identical to those induced by IgE+Ag stimulation. These effects may be due to induction of Fc epsilonRI aggregation by these IgEs in the absence of Ag. This review will synthesize recent findings of the heterogeneity of IgEs in their ability to induce survival and activation events, their mechanisms, the potential in vivo significance of IgE-Fc epsilonRI interactions, and the implications of the mouse studies to human diseases.

Absence of leukotriene B4 receptor 1 confers resistance to airway hyperresponsiveness and Th2-type immune responses

Bronchial asthma is an increasingly common disorder that remains poorly understood and difficult to manage. The disease is characterized by airway hyperresponsiveness, chronic inflammation, and mucus overproduction. Based on the finding that leukotriene B4 receptor 1 (BLT1) is expressed highly in Th2 lymphocytes, we analyzed the roles of BLT1 using an OVA-induced bronchial asthma model. BLT1-null mice did not develop airway hyperresponsiveness, eosinophilic inflammation, and hyperplasia of goblet cells. Attenuated symptoms were accompanied by reduced IgE production, and accumulation of IL-5 and IL-13 in bronchoalveolar lavage fluid, suggesting attenuated Th2-type immune response in BLT1-null mice. Peribronchial lymph node cells of sensitized BLT1-null mice showed much attenuated proliferation and production of Th2 cytokines upon re-stimulation with Ag in vitro. Thus, LTB4-BLT1 axis is required for the development of Th2-type immune response, and blockade of LTB4 functions through BLT1 would be novel and useful in the effort to ameliorate bronchial asthma and related Th2-biased immune disorders.

Leukotriene B4 signaling through NF-kappaB-dependent BLT1 receptors on vascular smooth muscle cells in atherosclerosis and intimal hyperplasia

Leukotriene B(4) (LTB(4)), a potent leukocyte chemoattractant derived from the 5-lipoxygenase metabolism of arachidonic acid, exerts its action by means of specific cell surface receptors, denoted BLT(1) and BLT(2). In this study, BLT(1) receptor proteins were detected in human carotid artery atherosclerotic plaques, colocalizing with markers for macrophages, endothelial cells, and vascular smooth muscle cells (SMC). Challenge of human coronary artery SMC with either LTB(4) or U75302, a partial agonist that is selective for the BLT(1) receptor, induced an approximately 4-fold increase of whole-cell currents by using the patch-clamp technique, indicating that these cells express functional BLT(1) receptors. LTB(4) induced migration and proliferation of SMC in vitro, and treatment with the BLT receptor antagonist BIIL 284 (10 mg/kg, once daily) for 14 days after carotid artery balloon injury in vivo inhibited intimal hyperplasia in rats. In the latter model, SMC derived from the intima exhibited increased levels of BLT(1) receptor mRNA compared with medial SMC. BLT receptor up-regulation in the intima in vivo, as well as that induced by IL-1beta in vitro, were prevented by transfection with a dominant-negative form of Ikappa kinase beta carried by adenovirus, indicating that BLT(1) receptor expression depends on NF-kappaBeta. These results show that LTB(4) activates functional BLT(1) receptors on vascular SMC, inducing chemotaxis and proliferation, and that BLT(1) receptors were up-regulated through an Ikappa kinase beta/NF-kappaB-dependent pathway. Inhibition of LTB(4)/BLT(1) signaling during the response to vascular injury reduced intimal hyperplasia, suggesting this pathway as a possible target for therapy.

Cytosolic phospholipase A2 alpha-deficient mice are resistant to experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE), a Th1-mediated inflammatory disease of the central nervous system (CNS), is a model of human multiple sclerosis. Cytosolic phospholipase A2alpha (cPLA2alpha), which initiates production of prostaglandins, leukotrienes, and platelet-activating factor, is present in EAE lesions. Using myelin oligodendrocyte glycoprotein (MOG) immunization, as well as an adoptive transfer model, we showed that cPLA2alpha-/- mice are resistant to EAE. Histologic examination of the CNS from MOG-immunized mice revealed extensive inflammatory lesions in the cPLA2alpha+/- mice, whereas the lesions in cPLA2alpha-/- mice were reduced greatly or completely absent. MOG-specific T cells generated from WT mice induced less severe EAE in cPLA2alpha-/- mice compared with cPLA2alpha+/- mice, which indicates that cPLA2alpha plays a role in the effector phase of EAE. Additionally, MOG-specific T cells from cPLA2alpha-/- mice, transferred into WT mice, induced EAE with delayed onset and lower severity compared with EAE that was induced by control cells; this indicates that cPLA2alpha also plays a role in the induction phase of EAE. MOG-specific T cells from cPLA2alpha-/- mice were deficient in production of Th1-type cytokines. Consistent with this deficiency, in vivo administration of IL-12 rendered cPLA2alpha-/- mice susceptible to EAE. Our data indicate that cPLA2alpha plays an important role in EAE development and facilitates differentiation of T cells toward the Th1 phenotype.

The type 1 cysteinyl leukotriene receptor triggers calcium influx and chemotaxis in mouse alpha beta- and gamma delta effector T cells

Linker for activation of T cells (LAT) is essential for T cell activation. Mice with mutations of distinct LAT tyrosine residues (LatY136F and Lat3YF) develop lymphoproliferative disorders involving TCR alphabeta or gammadelta T cells that trigger symptoms resembling allergic inflammation. We analyzed whether these T cells share a pattern of gene expression that may account for their pathogenic properties. Both LatY136F alphabeta and Lat3YF gammadelta T cells expressed high levels of the type 1 cysteinyl leukotriene receptor (CysLT(1)). Upon binding to the 5(S)-hydroxy-6(R)-S-cysteinylglycyl-7,9-trans-11,14-cis-eicosatetraenoic acid (LTD(4)) cysteinyl leukotriene, CysLT(1) induced Ca(2+) flux and caused chemotaxis in both LatY136F alphabeta and Lat3YF gammadelta T cells. Wild-type in vitro-activated T cells, but not resting T cells, also migrated toward LTD(4) however with a lower magnitude than T cells freshly isolated from LatY136F and Lat3YF mice. These results suggest that CysLT(1) is likely involved in the recruitment of activated alphabeta and gammadelta T cells to inflamed tissues.

Group IB secretory phospholipase A2 stimulates leukotriene B4 production by a unique mechanism in human neutrophils

We found that group IB secretory phospholipase A(2) (sPLA(2)-IB) stimulates leukotriene B4 (LTB4) production in the absence of cytochalasin B in human neutrophils. Although LTB4 production has been reported to be associated with arachidonic acid release, the exogenous addition of sPLA(2)-IB did not induce this release from human neutrophils, suggesting that sPLA(2)-IB stimulates LTB4 production without affecting arachidonic acid. Moreover, the intracellular signaling events induced by sPLA(2)-IB included an increase in intracellular Ca(2+), which is required for LTB4 production. sPLA(2)-IB also stimulated mitogen-activated protein kinase ERK, but its activity was not required for LTB4 production. In terms of functional aspects, the supernatant of sPLA(2)-IB-stimulated human neutrophils caused chemotactic migration, which was almost completely inhibited by preincubating these cells with three different 5-lipoxygenase inhibitors (MK-886, AA-861, or NDGA). Taken together, we suggest that sPLA(2)-IB plays a role in the modulation of inflammatory and immune responses by inducing LTB4 production in human neutrophils.

The leukotriene B4 lipid chemoattractant receptor BLT1 defines antigen-primed T cells in humans

We have recently shown that the leukotriene B(4) (LTB(4))-BLT1 pathway is important in early effector T-cell recruitment in mouse models of inflammation. Here we characterize the phenotype and function of human peripheral blood BLT1(+) T cells in health and illustrate their involvement in asthma and acute infection. In healthy individuals, BLT1(+) T cells are a rare peripheral blood T-cell population enriched for the activation markers CD38 and HLA-DR. Compared with BLT1(-) T cells, a larger proportion of peripheral blood BLT1(+) T cells express the effector cytokines IFNgamma and IL-4 and inflammatory chemokine receptors, CCR1, CCR2, CCR6, and CXCR1. Consequently, in healthy individuals peripheral blood BLT1(+) T cells are a rare antigen-primed T-cell subset with unique phenotypic, migratory, and functional properties. BLT1 expression on T cells is tightly regulated by inflammation and only transiently expressed after naive T-cell activation by dendritic cells. Although rare in the peripheral blood of healthy individuals, BLT1(+) T cells are markedly increased in frequency in the peripheral blood in response to acute Epstein-Barr virus (EBV) infection and moderately increased in the airways of asymptomatic allergic asthmatics. Our studies provide novel insights into the LTB(4)-BLT1 lipid chemoattractant pathway in human T-cell responses, and how it may link innate and adaptive immunity.

The greater chemotactic network for lymphocyte trafficking: chemokines and beyond

PURPOSE OF REVIEW

This review examines our current understanding of the chemoattractant network for lymphocyte trafficking and discusses the recent findings in this area.

RECENT FINDINGS

The universe of chemoattractants is expanding. Although the approximately 40 chemokines by themselves provide an extensive network of chemotactic signals, it is now clear that nonchemokine molecules such as lipid mediators, pathogen-derived products, antimicrobial peptides, complement products, and other normal constituents of our body are also chemotactic and regulate lymphocyte trafficking. The past several years has witnessed important progress in the area of lymphocyte trafficking: CD45+CD4+CD3- lymphoid tissue inducer cells express CXCR5 and CCR7 and migrate toward lymphoid tissue--organizing stromal cells. Sphingosine 1-phosphate and homeostatic chemokines regulate lymphocyte localization in lymph nodes and egress to circulation. Upon antigen priming, regulatory T cells rapidly upregulate CXCR5 to migrate and suppress germinal center T and B cells. Plasmacytoid dendritic cells can migrate to lymph nodes through high endothelial venule cells, and chemokines and nonchemokine G-protein--coupled seven-transmembrane domain receptor ligands such as chemerin can regulate pDC migration. Gut dendritic cells and retinoic acid induce the expression of alpha4beta7 and CCR9 on T cells for their homing to the gut. Leukotriene B4 and interleukin-8, known chemoattractants for myeloid cells, are also selective chemoattractants for cytotoxic effector T cells.

SUMMARY

Immune cells migrate within the vast and dense network of chemoattractants. This greater chemotactic network comprehensively controls lymphoid organogenesis, homeostatic immune cell migration, and effector cell dispatch to clear pathogens.

BLT1-mediated T cell trafficking is critical for rejection and obliterative bronchiolitis after lung transplantation

Leukotriene B₄ is a lipid mediator that recently has been shown to have potent chemotactic activity for effector T lymphocytes mediated through its receptor, BLT1. Here, we developed a novel murine model of acute lung rejection to demonstrate that BLT1 controls effector CD8⁺ T cell trafficking into the lung and that disruption of BLT1 signaling in CD8⁺ T cells reduces lung inflammation and mortality in the model. In addition, we used BLT1-deficient mice and a BLT1 antagonist in two tracheal transplant models of lung transplantation to demonstrate the importance of BLT1 for the recruitment of T cells into tracheal allografts. We also show that BLT1-mediated CD8⁺ T cell recruitment plays an important role in the development of airway fibroproliferation and obliteration. Finally, in human studies of lung transplant recipients, we found that BLT1 is up-regulated on T lymphocytes isolated from the airways of patients with obliterative bronchiolitis. These data demonstrate that BLT1 contributes to the development of lung rejection and obliterative bronchiolitis by mediating effector T lymphocyte trafficking into the lung. This is the first report that describes a pathologic role for BLT1-mediated T lymphocyte recruitment in disease and identifies BLT1 as a potential therapeutic target after lung transplantation.

Leukotriene modifiers as potential therapeutics for cardiovascular disease

Owing to their anti-inflammatory properties, leukotriene modifiers have been the primary therapeutics in asthma management for several years. Although blocking the inflammatory component of human disease is a long-standing and established concept, the use of leukotriene modifiers in treating the inflammatory component of cardiovascular disease encompassing atherosclerosis, myocardial infarction, stroke and aortic aneurysm has, surprisingly, only been seriously contemplated in the past few years. As reviewed here, several exciting studies have recently contributed to this expanding area of interest, and so far one leukotriene modifier has entered Phase II clinical trials to assess its potential for reducing the risk of heart attacks.

Lipoxins and lipoxin analogs in asthma

The pathobiology of asthma is characterized by production of eicosanoids, a diverse family of bioactive fatty acids that play important roles in regulating airway inflammation and reactivity. Lipoxins (LXs) are products of arachidonic acid metabolism that are distinct from leukotrienes (LTs) and prostaglandins (PGs) in structure and function. Unlike the pro-inflammatory PGs and LTs, LXs display counter-regulatory actions. Cell-type specific biological actions have been uncovered for LXs and LX stable analogs that promote resolution of acute inflammatory responses. At least two classes of receptors, CysLT1 receptors and LXA4 receptors (named ALX), can interact with LXA4 and LXA4 analogs to mediate their biological actions. LXs are generated during asthma and LXA4 signaling blocks asthmatic responses in humans and experimental model systems. Of interest, respiratory diseases of increased severity, such as aspirin-intolerant asthma, cystic fibrosis and steroid-dependent, severe asthma, display defective generation of these protective lipid signals. Together, these findings indicate a pivotal role for LXs in mediating airway homeostasis.

Antibody-antigen interaction in the airway drives early granulocyte recruitment through BLT1

Antibody-antigen interactions in the airway initiate inflammation in acute asthma exacerbations. This inflammatory response is characterized by the recruitment of granulocytes into the airways. In murine models of asthma, granulocyte recruitment into the lung contributes to the development of airway hyperresponsiveness (AHR), mucus production, and airway remodeling. Leukotriene B4 is a mediator released following antigen challenge that has chemotactic activity for granulocytes, mediated through its receptor, BLT1. We investigated the role of BLT1 in granulocyte recruitment following antigen challenge. Wild-type mice and BLT1-/- mice were sensitized and challenged with ovalbumin (OVA) to induce acute allergic airway inflammation. In addition, to explore the relevance to antibody-antigen interactions, we injected OVA bound to anti-OVA IgG1 or anti-OVA IgE intratracheally into naïve wild-type and BLT1-/- mice. Cell composition of the lungs, cytokine levels, histology, and AHR were determined. After sensitization and challenge with ovalbumin, there was significantly reduced neutrophil and eosinophil recruitment into the airways of BLT1-/- mice compared with wild-type animals after one or two daily antigen challenges, but this difference was not seen after three or four daily antigen challenges. Mucus production and AHR were not affected. Intratracheal injection of OVA bound to IgG1 or IgE induced neutrophil recruitment into the airways in wild-type mice but not in the BLT1-/- mice. We conclude that BLT1 mediates early recruitment of granulocytes into the airway in response to antigen-antibody interactions in a murine model of acute asthma.

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.

Chemokine receptor CCR7 guides T cell exit from peripheral tissues and entry into afferent lymphatics

T cell circulation between peripheral tissues and the lymphoid compartment is critical for immunosurveillance and host defense. However, the factors that determine whether T cells remain in peripheral tissue or return to the circulation are undefined. Here we demonstrate that the chemokine receptor CCR7 is a critical signal that determines T cell exit from peripheral tissue. Both CCR7(-) and CCR7(+) effector T cells entered mouse asthmatic lung and while CCR7(-) T cells accumulated, CCR7(+) T cells continued to migrate into afferent lymph. Delivery of both CCR7(+) and CCR7(-) T cells directly into the airways showed that only CCR7(+) T cells exited the lung and entered draining lymph nodes. Our study establishes a molecular basis for T cell exit from peripheral tissues.

Lipids as bioeffectors in the immune system

Lipids, in addition to serving as fuel stores and structural components of cell membranes, act as effectors and second messengers in a variety of biological processes including those associated with the immune system. These lipid mediators and regulators differ in structural composition and exert a diverse array of effects on cellular functional activities including those linked to homeostasis, immune responsiveness, and inflammation. They function as intercellular mediators and at the intracellular level act as critical conduits of external stimuli in signal transduction cascades. Lipid derived messengers and their receptors also may interact with other signaling molecules. Exogenous compounds such as cannabinoids share functionally relevant receptor binding domains with those for endogenous lipid signaling ligands and have the potential to alter transductional cascades linked to immune functional activities.

Inhibition of Atherogenesis in BLT1-Deficient Mice Reveals a Role for LTB4 and BLT1 in Smooth Muscle Cell Recruitment

Background— It is known that 5-lipoxygenase and its product, leukotriene B4 (LTB 4 ), are highly expressed in several human pathologies, including atherosclerotic plaque. LTB 4 signals primarily through its high-affinity G protein-coupled receptor BLT1, which is expressed on specific leukocyte subsets. BLT1 receptor expression and function on other atheroma-associated cell types is unknown.

Methods and Results— To directly assess the role of the LTB 4 -BLT1 pathway in atherogenesis, we bred BLT1 −/− mice into the atherosclerosis-susceptible apoE −/− strain. Compound-deficient apoE −/− /Blt1 −/− mice fed a Western-type diet had a marked reduction in plaque formation compared with apoE −/− controls. Immunohistochemical analysis of atherosclerotic lesions in compound-deficient mice revealed a striking decrease in smooth muscle cells (SMCs) and significant decreases in macrophages and T cells. We report here novel evidence of the expression and function of BLT1 on vascular SMCs. LTB 4 triggered SMC chemotaxis, which was pertussis toxin sensitive in Blt1 +/+ SMCs and absent in Blt1 −/− cells, suggesting that BLT1 was the dominant receptor mediating effector functions through a G protein-coupled signaling pathway. Furthermore, BLT1 colocalized with SMCs in human atherosclerotic lesions.

Conclusions— These new findings extend the role of inducible BLT1 to nonleukocyte populations and suggest an important target for intervention to modulate the response to vascular injury.

Leukotriene B4 receptor and the function of its helix 8

More than 30 lipid ligands, which express their biological activities through cognate G-protein-coupled receptors (GPCRs), have been reported. Among them, leukotriene B(4) (LTB(4)) is a potent lipid mediator involved in host defense, inflammation, and the immune responses. Two GPCRs for LTB(4) (BLT1 and BLT2) have been cloned and analyzed. Recent studies using genetically engineered mice suggest that BLT1 plays an important role in several inflammatory diseases including ischemic reperfusion tissue injury, atherosclerosis, and bronchial asthma. BLT1 is also a good tool to study the molecular mechanism of GPCR activation and inactivation in vitro. In this brief review, we focus on the biological and biochemical properties of BLT1 with special attention to the putative helix 8 of the receptor.

Characterization of a Mouse Second Leukotriene B4 Receptor, mBLT2

Leukotriene B4 (LTB4) is a potent chemoattractant and activator for granulocytes and macrophages and is considered to be an inflammatory mediator. Two G-protein-coupled receptors for LTB4, BLT1 and BLT2, have been cloned from human and shown to be high and low affinity LTB4 receptors, respectively. To reveal the biological roles of BLT2 using mouse disease models, we cloned and characterized mouse BLT2. Chinese hamster ovary cells stably expressing mouse BLT2 exhibited specific binding to LTB4, LTB4-induced calcium mobilization, inhibition of adenylyl cyclase, and phosphorylation of extracellular signal-regulated kinase. We found that Compound A (4'-{[pentanoyl (phenyl) amino] methyl}-1, 1'-biphenyl-2-carboxylic acid) was a BLT2-selective agonist and induced Ca(2+) mobilization and phosphorylation of extracellular signal-regulated kinase through BLT2, whereas it had no effect on BLT1. 12-epi LTB4 exhibited a partial agonistic activity against mBLT1 and mBLT2, whereas 6-trans-12-epi LTB4 did not. Northern blot analysis showed that mouse BLT2 is expressed highly in small intestine and skin in contrast to the ubiquitous expression of human BLT2. By in situ hybridization and the reverse transcriptase polymerase chain reaction, we demonstrated that mouse BLT2 is expressed in follicular and interfollicular keratinocytes. Compound A, LTB4, and 12-epi LTB4 all induced phosphorylation of extracellular signal-regulated kinase in primary mouse keratinocytes. Furthermore, Compound A and LTB4 induced chemotaxis in primary mouse keratinocytes. These data suggest the presence of functional BLT2 in primary keratinocytes.

Leukotriene B4, an activation product of mast cells, is a chemoattractant for their progenitors

Mast cells are tissue-resident cells with important functions in allergy and inflammation. Pluripotential hematopoietic stem cells in the bone marrow give rise to committed mast cell progenitors that transit via the blood to tissues throughout the body, where they mature. Knowledge is limited about the factors that release mast cell progenitors from the bone marrow or recruit them to remote tissues. Mouse femoral bone marrow cells were cultured with IL-3 for 2 wk and a range of chemotactic agents were tested on the c-kit(+) population. Cells were remarkably refractory and no chemotaxis was induced by any chemokines tested. However, supernatants from activated mature mast cells induced pronounced chemotaxis, with the active principle identified as leukotriene (LT) B(4). Other activation products were inactive. LTB(4) was highly chemotactic for 2-wk-old cells, but not mature cells, correlating with a loss of mRNA for the LTB(4) receptor, BLT1. Immature cells also accumulated in vivo in response to intradermally injected LTB(4). Furthermore, LTB(4) was highly potent in attracting mast cell progenitors from freshly isolated bone marrow cell suspensions. Finally, LTB(4) was a potent chemoattractant for human cord blood-derived immature, but not mature, mast cells. These results suggest an autocrine role for LTB(4) in regulating tissue mast cell numbers.

Fluorescent leukotriene B4: potential applications

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation that acts primarily via a seven-transmembrane-spanning, G-protein-coupled receptor denoted BLT1. Here, we describe the synthesis and characterization of fluorescent analogs of LTB4 that are easy to produce, inexpensive, and without the disadvantages of a radioligand. Fluorescent LTB4 is useful for labeling LTB4 receptors for which no antibodies are available and for performing one-step fluorescence polarization assays conducive to high-throughput screening. We found that orange and green fluorescent LTB4 were full agonists that activated the LTB4 receptor BLT1 with EC50 values of 68 and 40 nM, respectively (4.5 nM for unmodified LTB4). Flow cytometric measurements and confocal imaging showed that fluorescent LTB4 colocalized with BLT1. Fluorescence polarization measurements showed that orange fluorescent LTB4 bound to BLT1 with a Kd of 66 nM and that this binding could be displaced by unlabeled LTB4 and other BLT1-specific ligands. Fluorescent LTB4 analogs were also able to displace tritiated LTB4. Orange fluorescent LTB4 binding to enhanced green fluorescent protein-tagged BLT1 could be observed using fluorescence resonance energy transfer. In addition to being a useful alternative to radiolabeled LTB4, the unique properties of fluorescently labeled LTB4 allow a variety of detection technologies to be used.

Leukotriene B4 receptor-1 is essential for allergen-mediated recruitment of CD8+ T cells and airway hyperresponsiveness

Recent studies in both human and rodents have indicated that in addition to CD4+ T cells, CD8+ T cells play an important role in allergic inflammation. We previously demonstrated that allergen-sensitized and -challenged CD8-deficient (CD8-/-) mice develop significantly lower airway hyperresponsiveness (AHR), eosinophilic inflammation, and IL-13 levels in bronchoalveolar lavage fluid compared with wild-type mice, and that all these responses were restored by adoptive transfer of in vivo-primed CD8+ T cells or in vitro-generated effector CD8+ T cells (T(EFF)). Recently, leukotriene B4 and its high affinity receptor, BLT1, have been shown to mediate in vitro-generated T(EFF) recruitment into inflamed tissues. In this study we investigated whether BLT1 is essential for the development of CD8+ T cell-mediated allergic AHR and inflammation. Adoptive transfer of in vivo-primed BLT1+/+, but not BLT1-/-, CD8+ T cells into sensitized and challenged CD8-/- mice restored AHR, eosinophilic inflammation, and IL-13 levels. Moreover, when adoptively transferred into sensitized CD8-/- mice, in vitro-generated BLT1+/+, but not BLT1-/-, T(EFF) accumulated in the lung and mediated these altered airway responses to allergen challenge. These data are the first to show both a functional and an essential role for BLT1 in allergen-mediated CD8+ T(EFF) recruitment into the lung and development of AHR and airway inflammation.

Requirement for leukotriene B4 receptor 1 in allergen-induced airway hyperresponsiveness

RATIONALE

Leukotriene B4 (LTB4) is a rapidly synthesized, early leukocyte chemoattractant that signals via its cell surface receptor, leukotriene B4 receptor 1 (BLT1), to attract and activate leukocytes during inflammation. A role for the LTB4-BLT1 pathway in allergen-induced airway hyperresponsiveness and inflammation is not well defined.

OBJECTIVES

To define the role of the LTB4 receptor (BLT1) in the development of airway inflammation and altered airway function.

METHODS

BLT1-deficient (BLT1 -/-) mice and wild-type mice were sensitized to ovalbumin by intraperitoneal injection and then challenged with ovalbumin via the airways. Airway responsiveness to inhaled methacholine, bronchoalveolar lavage fluid cell composition and cytokine levels, and lung inflammation and goblet cell hyperplasia were assessed.

RESULTS

Compared with wild-type mice, BLT1 -/- mice developed significantly lower airway responsiveness to inhaled methacholine, lower goblet cell hyperplasia in the airways, and decreased interleukin (IL)-13 production both in vivo, in the bronchoalveolar lavage fluid, and in vitro, after antigen stimulation of lung cells in culture. Intracellular cytokine staining of lung cells revealed that bronchoalveolar lavage IL-13 levels and numbers of IL-13(+)/CD4+ and IL-13(+)/CD8+ T cells were also reduced in BLT1 -/- mice. Reconstitution of sensitized and challenged BLT1 -/- mice with allergen-sensitized BLT1 +/+ T cells fully restored the development of airway hyperresponsiveness. In contrast, transfer of naive T cells failed to do so.

CONCLUSION

These data suggest that BLT1 expression on primed T cells is required for the full development of airway hyperresponsiveness, which appears to be associated with IL-13 production in these cells.

TLR3-induced activation of mast cells modulates CD8+ T-cell recruitment

Mast cells play an important role in host defense against various pathogens, but their role in viral infection has not been clarified in detail. dsRNA, synthesized by various types of viruses and mimicked by polyinosinic-polycytidylic acid (poly(I:C)) is recognized by Toll-like receptor 3 (TLR3). In this study, we demonstrate that poly(I:C) injection in vivo potently stimulates peritoneal mast cells to up-regulate a number of different costimulatory molecules. Therefore, we examined the expression and the functional significance of TLR3 activation in mast cells. Mast cells express TLR3 on the cell surface and intracellularly. After stimulation of mast cells with poly(I:C) and Newcastle disease virus (NDV), TLR3 is phosphorylated and the expression of key antiviral response cytokines (interferon beta, ISG15) and chemokines (IP10, RANTES) is upregulated. Interestingly, mast cells activated via TLR3-poly(I:C) potently stimulate CD8+ T-cell recruitment. Indeed, mast-cell-deficient mice (KitW/KitW-v) given an intraperitoneal injection of poly(I:C) show a decreased CD8+ T-cell recruitment, whereas granulocytes normally migrate to the peritoneal cavity. Mast-cell reconstitution of KitW/KitW-v mice normalizes the CD8+ T-cell influx. Thus, mast cells stimulated through engagement of TLR3 are potent regulators of CD8+ T-cell activities in vitro and in vivo.