Juying Lai

Aspirin-Exacerbated Respiratory Disease Involves a Cysteinyl Leukotriene–Driven IL-33–Mediated Mast Cell Activation Pathway

Aspirin-exacerbated respiratory disease (AERD), a severe eosinophilic inflammatory disorder of the airways, involves overproduction of cysteinyl leukotrienes (cysLTs), activation of airway mast cells (MCs), and bronchoconstriction in response to nonselective cyclooxygenase inhibitors that deplete homeostatic PGE2. The mechanistic basis for MC activation in this disorder is unknown. We now demonstrate that patients with AERD have markedly increased epithelial expression of the alarmin-like cytokine IL-33 in nasal polyps, as compared with polyps from aspirin-tolerant control subjects. The murine model of AERD, generated by dust mite priming of mice lacking microsomal PGE2 synthase (ptges−/− mice), shows a similar upregulation of IL-33 protein in the airway epithelium, along with marked eosinophilic bronchovascular inflammation. Deletion of leukotriene C4 synthase, the terminal enzyme needed to generate cysLTs, eliminates the increased IL-33 content of the ptges−/− lungs and sharply reduces pulmonary eosinophilia and basal secretion of MC products. Challenges of dust mite–primed ptges−/− mice with lysine aspirin induce IL-33–dependent MC activation and bronchoconstriction. Thus, IL-33 is a component of a cysLT-driven innate type 2 immune response that drives pathogenic MC activation and contributes substantially to AERD pathogenesis.

KIT Inhibition by Imatinib in Patients with Severe Refractory Asthma

BACKGROUND Mast cells are present in the airways of patients who have severe asthma despite glucocorticoid treatment; these cells are associated with disease characteristics including poor quality of life and inadequate asthma control. Stem cell factor and its receptor, KIT, are central to mast‐cell homeostasis. We conducted a proof‐of‐principle trial to evaluate the effect of imatinib, a KIT inhibitor, on airway hyperresponsiveness, a physiological marker of severe asthma, as well as on airway mast‐cell numbers and activation in patients with severe asthma. METHODS We conducted a randomized, double‐blind, placebo‐controlled, 24‐week trial of imatinib in patients with poorly controlled severe asthma who had airway hyperresponsiveness despite receiving maximal medical therapy. The primary end point was the change in airway hyperresponsiveness, measured as the concentration of methacholine required to decrease the forced expiratory volume in 1 second by 20% (PC20). Patients also underwent bronchoscopy. RESULTS Among the 62 patients who underwent randomization, imatinib treatment reduced airway hyperresponsiveness to a greater extent than did placebo. At 6 months, the methacholine PC20 increased by a mean (±SD) of 1.73±0.60 doubling doses in the imatinib group, as compared with 1.07±0.60 doubling doses in the placebo group (P=0.048). Imatinib also reduced levels of serum tryptase, a marker of mast‐cell activation, to a greater extent than did placebo (decrease of 2.02±2.32 vs. 0.56±1.39 ng per milliliter, P=0.02). Airway mast‐cell counts declined in both groups. Muscle cramps and hypophosphatemia were more common in the imatinib group than in the placebo group. CONCLUSIONS In patients with severe asthma, imatinib decreased airway hyperresponsiveness, mast‐cell counts, and tryptase release. These results suggest that KIT‐dependent processes and mast cells contribute to the pathobiologic basis of severe asthma. (Funded by the National Institutes of Health and others; ClinicalTrials.gov number, NCT01097694.)

Leukotriene E4 elicits respiratory epithelial cell mucin release through the G-protein–coupled receptor, GPR99

Significance Leukotriene E4 (LTE4), a lipid mediator detected in asthma exacerbations triggered by allergen, viruses, and aspirin, elicits airflow obstruction and lung inflammation in asthmatics. GPR99 is the recently identified high-affinity receptor for LTE4 and is resistant to blockade by commercially available cysteinyl leukotriene (cysLT) receptor antagonists. Here we find that GPR99 is expressed in respiratory epithelial cells and mediates mucin release and submucosal swelling in response to LTE4 or to cysLTs elicited by Alternaria, a common airborne fungus. Furthermore, among cysLT receptors, only GPR99 regulates baseline numbers of mucin-containing goblet cells. These studies demonstrate a unique role for GPR99 in epithelial cell homeostasis and activation and indicate that epithelial cells may be a dominant site of LTE4 action in the lung. Cysteinyl leukotrienes (cysLTs), leukotriene C4 (LTC4), LTD4, and LTE4 are proinflammatory lipid mediators with pathobiologic function in asthma. LTE4, the stable cysLT, is a weak agonist for the type 1 and type 2 cysLT receptors (CysLTRs), which constrict airway smooth muscle, but elicits airflow obstruction and pulmonary inflammation in patients with asthma. We recently identified GPR99 as a high-affinity receptor for LTE4 that mediates cutaneous vascular permeability. Here we demonstrate that a single intranasal exposure to extract from the respiratory pathogen Alternaria alternata elicits profound epithelial cell (EpC) mucin release and submucosal swelling in the nasal mucosa of mice that depends on cysLTs, as it is absent in mice deficient in the terminal enzyme for cysLT biosynthesis, LTC4 synthase (LTC4S). These mucosal changes are associated with mast cell (MC) activation and absent in MC-deficient mice, suggesting a role for MCs in control of EpC function. Of the three CysLTRs, only GPR99-deficient mice are fully protected from EpC mucin release and swelling elicited by Alternaria or by intranasal LTE4. GPR99 expression is detected on lung and nasal EpCs, which release mucin to doses of LTE4 one log lower than that required to elicit submucosal swelling. Finally, mice deficient in MCs, LTC4S, or GPR99 have reduced baseline numbers of goblet cells, indicating an additional function in regulating EpC homeostasis. These results demonstrate a novel role for GPR99 among CysLTRs in control of respiratory EpC function and suggest that inhibition of LTE4 and of GPR99 may have therapeutic benefits in asthma.

Platelet-Driven Leukotriene C4–Mediated Airway Inflammation in Mice Is Aspirin-Sensitive and Depends on T Prostanoid Receptors

Cysteinyl leukotrienes (cysLTs) are bronchoconstricting lipid mediators that amplify eosinophilic airway inflammation by incompletely understood mechanisms. We recently found that LTC4, the parent cysLT, potently activates platelets in vitro and induces airway eosinophilia in allergen-sensitized and -challenged mice by a platelet- and type 2 cysLT receptor–dependent pathway. We now demonstrate that this pathway requires production of thromboxane A2 and signaling through both hematopoietic and lung tissue–associated T prostanoid (TP) receptors. Intranasal administration of LTC4 to OVA-sensitized C57BL/6 mice markedly increased the numbers of eosinophils in the bronchoalveolar lavage fluid, while simultaneously decreasing the percentages of eosinophils in the blood by a TP receptor–dependent mechanism. LTC4 upregulated the expressions of ICAM-1 and VCAM-1 in an aspirin-sensitive and TP receptor–dependent manner. Both hematopoietic and nonhematopoietic TP receptors were essential for LTC4 to induce eosinophil recruitment. Thus, the autocrine and paracrine functions of thromboxane A2 act downstream of LTC4/type 2 cysLT receptor signaling on platelets to markedly amplify eosinophil recruitment through pulmonary vascular adhesion pathways. The findings suggest applications for TP receptor antagonists in cases of asthma with high levels of cysLT production.