Mechanisms of Mast Cell Activation in Severe Asthma: Beyond IgE

Mast cells (MCs) are present in all heathy human tissues and exhibit marked heterogeneity with respect to their development, mediator content, ultrastructure, and function (1). They secrete a vast arsenal of biological substances including the classical autacoid mediators (histamine, prostaglandin D2, and leukotriene C4), proteases, and cytokines. They respond rapidly to a multitude of perceived tissue insults with the initiation of a coordinated program of inflammation and repair. Potentially beneficial MC roles include protection against infection (2), kidney injury (3), envenomation (4), and in some circumstances, cancer progression (5). However, when a tissue insult is repeated or continuous, the effects of MC mediators are potentially deleterious, and it is not surprising that MCs are implicated in the pathophysiology of many diverse diseases including asthma (1) and idiopathic pulmonary fibrosis (6). MCs release their mediators in response to all stimuli considered important for the development of asthma, day-today symptoms, and exacerbations (reviewed in Reference 1). The biological activity of their mediators can account for all of the physiological and pathological abnormalities present in asthmatic airways. Importantly, there is unequivocal evidence of the ongoing release of MC mediators in the airways of people with both mild and severe asthma, and in both T2-high and T2-low severe asthma (1, 7). MCs are unique among leukocytes in that they infiltrate the airway submucosal glands and airway smooth muscle bundles in asthmatic airways, and infiltrate the airway epithelium, placing activated MCs within these dysfunctional airway elements (1). For decades the investigation of MC signaling focused on allergen-driven IgE-dependent activation via the high-affinity IgE receptor FceRI. However, human MCs respond to a wide variety of stimuli including cytokines/growth factors, lipids, nucleotides, complement, proteases, products of infection (human MCs express all Toll-like receptors), pollutants, physical stimuli, occupational agents, drugs, and cell–cell signals (1) (Figure 1). The historic work of Okumura using human adult peripheral blood progenitor–derived MCs showed that although there are core changes in MC gene expression after activation via FceRI, LPS, and IFN-g, there are also many stimulus-specific changes, and these are modified with combined stimulation (8). It is likely that many stimuli contribute to MC activation within the airways of an individual over time, but to date, there has been little information on the activity of IgE-independent pathways in asthmatic airways. In this issue of the Journal, Tiotiu and colleagues (pp. 397–411) have used gene set variation analysis to interrogate MC activation signatures in the sputum of the U-BIOPRED (Unbiased Biomarkers for the Prediction of Respiratory Disease Outcomes) asthma and healthy volunteer cohort, cross-validated with the ADEPT (Airways Disease Endotyping for Personalized Therapeutics) cohort (with which there was moderate agreement) (9). They looked at the relationship of these activation signatures with asthma severity and sputum inflammation and also mapped them to three molecular clusters of severe asthma they had identified previously, described as transcriptome-associated clusters (TACs) 1–3 (10). The gene sets used were obtained from previous studies using cultured human blood progenitor–derivedMCs that were unstimulated, FceRIactivated (single and multiple activation rounds), LPS-activated, IFNg–activated, or IL-33–activated. There was also a signature fromMCs identified in mild asthma bronchial biopsies by single-cell sequencing. In severe asthma, the FceRI-activated, single-cell sequencing signature frommild asthma, LPS-, and IL-33–dependent signatures were enriched compared with heathy controls. These signatures were also enriched in the subset of patients with severe asthma who were eosinophilic, but the LPSand IL-33–dependent signatures were enriched further in people deemed neutrophilic or mixed granulocytic defined by sputum percentage cell counts. FceR1-related signatures were enriched predominantly in people expressing the T-helper cell type 2 eosinophilic TAC1molcular phenotype, whereas LPSand IL-33–dependent signatures were enriched in the TAC2 neutrophil inflammasome–associated phenotype. These sameMC signatures were similarly enriched in individuals with asthma with airflow obstruction compared with those without. The MC activation signatures correlated poorly with tissueMC numbers, supporting previous work indicating that MC activation and their tissue localization is more important than absolute numbers. This is a comprehensive data analysis and the first study in severe asthma to examineMC gene expression signatures generated by different stimuli. The authors have uncovered several interesting associations between stimulus-specific MC gene signatures, clinical features of asthma, andmolecular phenotypes, and this represents an important advance in our understanding of IgE-independent factors that may be drivingMCs in severe asthma. The study also provides further evidence that MCs are activated across clinical and molecular severe asthma phenotypes. There are some limitations to the study. Gene set variation analysis has problems because not all genes expressed are necessarily specific for one cell type, although the authors took steps to exclude genes that overlap with other leukocytes. The gene This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0. For commercial usage and reprints, please e-mail Diane Gern (dgern@thoracic.org).