Mast cells derived from systemic mastocytosis exhibit an increased responsiveness to hyperosmolarity
Authors: Katarina Lyberg, Maria Ekoff, Christine Möller Westerberg, Camilla Engblom, Barbro Dahlén, Theo Gülen, Gunnar Nilsson
Journal: Allergy (2022)

Systemic mastocytosis (SM) is a disease characterized by increased number of aberrant mast cells in one or several organs and increased systemic levels of mast cell (MC) mediators.1 Indolent SM (ISM) is the most common form of SM, constituting approximately 80% of the patients diagnosed with SM. Individuals with ISM often have mediator mediated symptoms, most commonly from the skin, the gastrointestinal tract, cardiovascular, and respiratory system, but also in the form of anaphylaxis. Although basal mediator levels, including serum tryptase and metabolites of histamine and prostaglandin D2 in the urine, are increased at steady state,1-3 the symptoms often come as spells without any obvious trigger suggesting an intrinsic defect causing a hyper-reactive state of the mast cells, or an endogenous trigger. We have previously addressed the hypothesis of a hyper-reactive MC phenotype in ISM by in vivo provocation.2 None of the used triggers mounted a response that was different between ISM patients and healthy volunteers (HV). To further investigate the hypothesis of a hyper-reactive MC phenotype, we also developed MCs in vitro from 14 ISM patients and 13 HV (same subjects as included in2) (Tables S1 and S2). Peripheral blood was obtained, and CD34-selected progenitor cells were cultured under MC promoting conditions4 (see supplement). When the cells were mature, they were plated and exposed to IgE-receptor activation, morphine, or mannitol-induced hyperosmolarity, representing three distinct activation pathways (see supplement). Histamine (as a measurement of degranulation) and PGD2 (newly synthesized lipid mediator); that is, two prominent MC mediators, released through two different routes that are increased in ISM, were measured. We did not observe any difference in in vitro growth and development of MCs over a 6-week period between cells from ISM patients and HV (Figure 1). This result stands in contrast to a study where a significant increase in MC growth from CD34-selected progenitor cells from ISM patients was described.5 An explanation could be the different culture protocols used in the two studies.5 The in vitro developed MCs (>90% tryptase positive) were plated and exposed to different MC secretagogues: the calcium ionophore A23187, morphine, anti-IgE, and mannitol. The release of histamine was comparable between MCs derived from ISM and HV in response to all tested secretagogues (Figure 2A). In contrast, MCs derived from ISM showed a significantly increased release of PGD2 in response to mannitol, but not to the other tested triggers (Figure 2B). It has been reported previously that the release of β-hexosaminidase (released through degranulation) after IgE-receptor activation is the same from MCs derived from ISM as from HV.5 However, in that study, they neither investigated the secretion of PGD2, nor other type of secretagogues. Our study provides the first evidence that MCs derived from ISM exhibit an aberrant response profile to mannitol-induced hyperosmolarity, with no change in degranulation but an increased synthesis and secretion of PGD2, the main eicosanoid produced by MCs. Mannitol is clinically used to measure bronchoconstriction in individuals with asthma. Individuals with mastocytosis have not been reported to have increased risk for asthma and airway hyperresponsiveness, and in our previously published study, we did not observe any increased bronchoconstriction after mannitol challenge in those with mastocytosis.2 Here, we used mannitol as a stimulus to mimic hyperosmolarity. Cells sense physical changes through the receptor family transient receptor potential vanilloid type 1–4 (TRPV).6 Thus, one could speculate that MCs sense osmolarity changes through TRPV and that this pathway is altered in mastocytosis patients, resulting in an increased synthesis and release of PGD2. A hyper-reactive MC phenotype in ISM is still elusive, but our data indicate that an intrinsic defect in these cells could affect other signaling pathways than the commonly studied downstream of the IgE-receptor and that other mediator releasing systems than degranulation, that is, newly synthesized mediators, should be studied. We thank Ingrid Delin for technical assistance with the analysis of PGD2. This study was supported by grants from the Swedish Research Council – Medicine and Health, the Swedish Cancer Society, the Swedish Heart Lung foundation, Ellen, Walter and Lennart Hesselman’s foundation, the Konsul TH C Bergh foundation, and through the regional agreement on medical training and clinical research (ALF) between Stockholm County Council and the Karolinska Institutet. The authors report no conflict of interest. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.