Luca Danelli

Ischemia–reperfusion injury (IRI) is an important cause of acute kidney injury that can lead to end-stage renal failure. Although the ensuing inflammatory response can restore homeostasis, a consecutive maladaptive repair and persistent inflammation represent important risk factors for postischemic chronic kidney disease development. In this study, we investigated the role of mast cells in both the early and late phases of the inflammatory response in experimental models of acute and chronic renal IRI using our recently developed mouse model that allows conditional ablation of mast cells. Depletion of mast cells prior to IRI resulted in improved renal function due to diminished local inflammatory cytokine/chemokine levels and neutrophil recruitment to the kidneys after the acute injury phase (48 h post-IRI). Furthermore, although not completely protected, mast cell–depleted mice displayed less organ atrophy and fibrosis than did wild-type mice during the chronic phases (2 and 6 wk post-IRI) of disease development. Conversely, mast cell ablation after the acute phase of IRI had no impact on organ atrophy, tubular necrosis, or fibrosis. Thus, our results suggest a deleterious role of mast cells during the acute inflammatory phase of IRI promoting subsequent fibrosis development, but not during the chronic phase of the disease.

Cross-linking of mast cell (MC) IgE receptors (FcεRI) triggers degranulation of secretory granules (SGs) and the release of many allergic and inflammatory mediators. Although degranulation depends crucially on microtubule dynamics, the molecular machinery that couples SGs to microtubule-dependent transport is poorly understood. In this study, we demonstrate that mice lacking Kif5b (the heavy chain of kinesin-1) in hematopoietic cells are less sensitive to IgE-mediated, passive, systemic anaphylaxis. After IgE-induced stimulation, bone marrow–derived MCs from Kif5b knockout mice exhibited a marked reduction in SG translocation toward the secretion site. In contrast, a lack of Kif5b did not affect cytokine secretion, early FcεRI-initiated signaling pathways, or microtubule reorganization upon FcεRI stimulation. We identified Slp3 as the critical effector linking kinesin-1 to Rab27b-associated SGs. Kinesin-1 recruitment to the Slp3/Rab27b effector complex was independent of microtubule reorganization but occurred only upon stimulation requiring phosphatidylinositol 3-kinase (PI3K) activity. Our findings demonstrate that PI3K-dependent formation of a kinesin-1/Slp3/Rab27b complex is critical for the microtubule-dependent movement of SGs required for MC degranulation.

Acute myocardial infarction (MI) is a severe ischemic disease responsible for heart failure and sudden death. Inflammatory cells orchestrate postischemic cardiac remodeling after MI. Studies using mice with defective mast/stem cell growth factor receptor c-Kit have suggested key roles for mast cells (MCs) in postischemic cardiac remodeling. Because c-Kit mutations affect multiple cell types of both immune and nonimmune origin, we addressed the impact of MCs on cardiac function after MI, using the c-Kit–independent MC-deficient (Cpa3^Cre/+) mice. In response to MI, MC progenitors originated primarily from white adipose tissue, infiltrated the heart, and differentiated into mature MCs. MC deficiency led to reduced postischemic cardiac function and depressed cardiomyocyte contractility caused by myofilament Ca²⁺ desensitization. This effect correlated with increased protein kinase A (PKA) activity and hyperphosphorylation of its targets, troponin I and myosin-binding protein C. MC-specific tryptase was identified to regulate PKA activity in cardiomyocytes via protease-activated receptor 2 proteolysis. This work reveals a novel function for cardiac MCs modulating cardiomyocyte contractility via alteration of PKA-regulated force–Ca²⁺ interactions in response to MI. Identification of this MC-cardiomyocyte cross-talk provides new insights on the cellular and molecular mechanisms regulating the cardiac contractile machinery and a novel platform for therapeutically addressable regulators.

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor whose activity is modulated by xenobiotics as well as physiological ligands. These compounds may modulate inflammatory responses and contribute to the rising prevalence of allergic diseases observed in industrialized countries. Mast cells (MCs), located within tissues at the boundary of the external environment, represent a potential target of AhR ligands. In this study, we report that murine and human MCs constitutively express AhR, and its activation by the high-affinity ligand 6-formylindolo[3,2-b]carbazole (FICZ) determines a boost in degranulation. On the contrary, repeated exposure to FICZ inhibits MC degranulation. Accordingly, histamine release, in an in vivo passive systemic anaphylactic model, is exacerbated by a single dose and is attenuated by repetitive stimulation of AhR. FICZ-exposed MCs produce reactive oxygen species and IL-6 in response to cAMP-dependent signals. Moreover, AhR-activated MCs produce IL-17, a critical player in chronic inflammation and autoimmunity, suggesting a novel pathway for MC activation in the pathogenesis of these diseases. Indeed, histological analysis of patients with chronic obstructive pulmonary disease revealed an enrichment in AhR/IL-6 and AhR/IL-17 double-positive MCs within bronchial lamina propria. Thus, tissue-resident MCs could translate external chemical challenges through AhR by modulating allergic responses and contributing to the generation of inflammation-related diseases.

Mast cells (MCs) are currently recognized as effector cells in many settings of the immune response, including host defense, immune regulation, allergy, chronic inflammation, and autoimmune diseases. MC pleiotropic functions reflect their ability to secrete a wide spectrum of preformed or newly synthesized biologically active products with pro-inflammatory, anti-inflammatory and/or immunosuppressive properties, in response to multiple signals. Moreover, the modulation of MC effector phenotypes relies on the interaction of a wide variety of membrane molecules involved in cell–cell or cell-extracellular-matrix interaction. The delivery of co-stimulatory signals allows MC to specifically communicate with immune cells belonging to both innate and acquired immunity, as well as with non-immune tissue-specific cell types. This article reviews and discusses the evidence that MC membrane-expressed molecules play a central role in regulating MC priming and activation and in the modulation of innate and adaptive immune response not only against host injury, but also in peripheral tolerance and tumor-surveillance or -escape. The complex expression of MC surface molecules may be regarded as a measure of connectivity, with altered patterns of cell–cell interaction representing functionally distinct MC states. We will focalize our attention on roles and functions of recently discovered molecules involved in the cross-talk of MCs with other immune partners.

The biological behavior of immune cells is determined by their intrinsic properties and interactions with other cell populations within their microenvironment. Several studies have confirmed the existence of tight spatial interactions between mast cells (MCs) and Tregs in different settings. For instance, we have recently identified the functional cross‐talk between MCs and Tregs, through the OX40L–OX40 axis, as a new mechanism of reciprocal influence. However, there is scant information regarding the single‐cell dynamics of this process. In this study, time‐lapse video microscopy revealed direct interactions between Tregs and MCs in both murine and human cell co‐cultures, resulting in the inhibition of the MC degranulation response. MCs incubated with WT, but not OX40‐deficient, Tregs mediated numerous and long‐lasting interactions and displayed different morphological features lacking the classical signs of exocytosis. MC degranulation and Ca²⁺ mobilization upon activation were inhibited by Tregs on a single‐cell basis, without affecting overall cytokine secretion. Transmission electron microscopy showed ultrastructural evidence of vesicle‐mediated secretion reconcilable with the morphological pattern of piecemeal degranulation. Our results suggest that MC morphological and functional changes following MC–Treg interactions can be ascribed to cell–cell contact and represent a transversal, non‐species‐specific mechanism of immune response regulation. Further research, looking at the molecular composition of this interaction will broaden our understanding of its contribution to immunity.