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 hyper-responsiveness, 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.)

Background Prostaglandin D2 (PGD2) is the dominant cyclooxygenase product of mast cells and is an effector of aspirin-induced respiratory reactions in aspirin-exacerbated respiratory disease (AERD). Objective We evaluated the role of the innate cytokine thymic stromal lymphopoietin (TSLP) acting on mast cells to generate PGD2 and facilitate tissue eosinophilia and nasal polyposis in AERD. Methods Urinary eicosanoids were measured in aspirin-tolerant controls and patients with AERD. Nasal polyp specimens from subjects with AERD and chronic rhinosinusitis were analyzed via qPCR, western blot, and immunohistochemistry. Human cord blood-derived and peripheral blood-derived mast cells were stimulated with TSLP in vitro to assess PGD2 generation. Results Urinary levels of a stable PGD2 metabolite (uPGD-M) were 2-fold higher in subjects with AERD relative to controls, and increased further during aspirin-induced reactions. Peak uPGD-M levels during aspirin reactions correlated with reductions in blood eosinophil counts and lung function, and with increases in nasal congestion. Mast cells sorted from nasal polyps expressed PGD2 synthase (hPGDS) mRNA at higher levels than did eosinophils from the same tissue. Whole nasal polyp TSLP mRNA expression correlated strongly with mRNA encoding hPGDS (r = .75), the mast cell-specific marker carboxypeptidase A3 (r = .74), and uPGD-M (r=0.74). The cleaved, active form of TSLP was increased in AERD nasal polyps relative to aspirin-tolerant controls. Recombinant TSLP induced PGD2 generation by cultured human mast cells. Conclusions Our study demonstrates that mast cell-derived PGD2 is a major effector of type 2 immune responses driven by TSLP, and suggests that dysregulation of this innate system contributes significantly to the pathophysiology of AERD.

Cysteinyl leukotrienes (cysLTs), leukotriene C 4 (LTC 4 ), LTD 4 , and LTE 4 are proinflammatory lipid mediators with pathobiologic function in asthma. LTE 4 , 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 LTE 4 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, LTC 4 synthase (LTC 4 S). 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 LTE 4 . GPR99 expression is detected on lung and nasal EpCs, which release mucin to doses of LTE 4 one log lower than that required to elicit submucosal swelling. Finally, mice deficient in MCs, LTC 4 S, 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 LTE 4 and of GPR99 may have therapeutic benefits in asthma.C ysteinyl leukotrienes (cysLTs), leukotriene C 4 (LTC 4 ), LTD 4 , and LTE 4 are lipid mediators detected during asthma exacerbations triggered by allergen (1), aspirin (2, 3), and respiratory viruses (4). The cysLTs elicit vascular permeability, inflammation, and bronchoconstriction through three G-protein-coupled receptors. The type 1 cysLT receptor (CysLTR), CysLT 1 R, is the highaffinity receptor for LTD 4 and the dominant CysLTR mediating airway smooth muscle constriction (5-8). The type 2 CysLTR, CysLT 2 R, has prominent effects on the vascular endothelium (9-12) and also elicits bronchial constriction (13,14). LTE 4 , the stable cysLT (15-18), is a weak agonist for CysLT 1 R and CysLT 2 R in transfected cells (5, 19), but elicits airflow obstruction in patients with asthma (20-22). Moreover, LTE 4 has comparable activity to LTC 4 and LTD 4 in eliciting a wheal and flare response in human skin (23), and LTE 4 elicits cutaneous vascular permeability in mice lacking both CysLT 1 R and CysLT 2 R, suggesting the existence of a high-affinity receptor for LTE 4 , which was recently identified as GPR99 (24,25). However, the mechanism by which LTE 4 induces lung pathobiology and the role of GPR99 remain poorly understood.The cysLTs are derived from arachidonate through the serial enzymatic actions of 5-lipoxygenase and leukotriene C 4 synthase (LTC 4 S). LTC 4 , the terminal product of intrac...