Mast cells (MCs) influence intercellular communication during inflammation by secreting cytoplasmic granules that contain diverse mediators. Here, we have demonstrated that MCs decode different activation stimuli into spatially and temporally distinct patterns of granule secretion. Certain signals, including substance P, the complement anaphylatoxins C3a and C5a, and endothelin 1, induced human MCs rapidly to secrete small and relatively spherical granule structures, a pattern consistent with the secretion of individual granules. Conversely, activating MCs with anti-IgE increased the time partition between signaling and secretion, which was associated with a period of sustained elevation of intracellular calcium and formation of larger and more heterogeneously shaped granule structures that underwent prolonged exteriorization. Pharmacological inhibition of IKK-β during IgE-dependent stimulation strongly reduced the time partition between signaling and secretion, inhibited SNAP23/STX4 complex formation, and switched the degranulation pattern into one that resembled degranulation induced by substance P. IgE-dependent and substance P-dependent activation in vivo also induced different patterns of mouse MC degranulation that were associated with distinct local and systemic pathophysiological responses. These findings show that cytoplasmic granule secretion from MCs that occurs in response to different activating stimuli can exhibit distinct dynamics and features that are associated with distinct patterns of MC-dependent inflammation.

# The following authors contributed equally Life Sciences Reporting Summary. Further information on experimental design and reagents is available in the Life Sciences Reporting Summary. Data availability. The data that support the findings of this study are available from the corresponding author Nicolas Gaudenzio upon request.

Mast cells (MCs) are cells of hematopoietic origin that normally reside in mucosal tissues, often near epithelial cells, glands, smooth muscle cells, and nerves. Best known for their contributions to pathology during IgE-associated disorders such as food allergy, asthma, and anaphylaxis, MCs are also thought to mediate IgE-associated effector functions during certain parasite infections. However, various MC populations also can be activated to express functional programs – such as secreting pre-formed and/or newly synthesized biologically active products – in response to encounters with products derived from diverse pathogens, other host cells (including leukocytes and structural cells), damaged tissue, or the activation of the complement or coagulation systems, as well as by signals derived from the external environment (including animal toxins, plant products, and physical agents). In this review, we will discuss evidence suggesting that MCs can perform diverse effector and immunoregulatory roles that contribute to homeostasis or pathology in mucosal tissues.

Reber et al. reveal a protective function for neutrophils after lethal endotoxin challenge using a novel mouse model of diphtheria toxin–inducible neutropenia. Neutrophil expression of myeloperoxidase (MPO) is necessary for this protection. These findings imply that neutrophils protect the host by limiting the extent of LPS-induced pathology in an MPO-dependent manner.

Mast cells regulate the immunological responses causing allergy and autoimmunity, and contribute to the tumor microenvironment through generation and secretion of a broad array of preformed, granule-stored and de novo synthesized bioactive compounds. The release and production of mast cell mediators is the result of a coordinated signaling machinery, followed by the FcεRI and FcγR antigen ligation. In this review, we present the latest understanding of FcεRI and FcγR signaling, required for the canonical mast cell activation during allergic responses and anaphylaxis. We then describe the cooperation between the signaling of FcR and other recently characterized membrane-bound receptors (i.e., IL-33R and thymic stromal lymphopoietin receptor) and their role in the chronic settings, where mast cell activation is crucial for the development and the sustainment of chronic diseases, such as asthma or airway inflammation. Finally, we report how the FcR activation could be used as a therapeutic approach to treat allergic and atopic diseases by mast cell inactivation. Understanding the magnitude and the complexity of mast cell signaling is necessary to identify the mechanisms underlying the potential effector and regulatory roles of mast cells in the biology and pathology of those disease settings in which mast cells are activated.Keywords: FcεRI r FcγR r IL-33 r mast cells r TSLP IntroductionMast cells originate in the BM from a lineage-specific multipotent hematopoietic progenitor, circulate as CD34 + precursors until they migrate to tissues and mature into effector cells in the proximity of organs and blood vessels. Mast cells respond to antigenic stimulation through the cross-linking of immunoglobulin E (IgE) bound to high-affinity receptors for IgE (FcεRI) and the activation of the FcγR after IgG binding (reviewed in [1,2]). Upon activation, mast cells release either preformed, granule-stored mediators, such as histamine and proteases, or newly generated mediators, such as eicosanoids, cytokines, and chemokines [3]. Although the basic molecular mechanisms of Ig:Fc activation have been extensively studied in the past 15 years [4][5][6], new molecules regulating the Correspondence: Dr. Barbara Frossi e-mail: barbara.frossi@uniud.it FcR-signaling cascades have recently been characterized, suggesting not only a still vivid interest in the field of mast cell activation and signaling, but also in identifying putative new therapeutic targets for intervention in mast cell dependent disorders. This review will focus on three aspects of mast cell activation/function. First, we give an essential but complete overview of the FcR-mediated activation in mast cells and report on recent advances in IgE-and IgG-mediated signaling. For clarity, we will dissect and analyze the signals derived from each pathway, with particular attention to the newly identified positive or negative molecular players describing -when possible -their implication toward degranulation and cytokine production. Second, we highlight the growing interest aroun...