Frontline Science: Mast cells regulate neutrophil homeostasis by influencing macrophage clearance activity

The Basophil IL-18 Receptor Precisely Regulates the Host Immune Response and Malaria-Induced Intestinal Permeability and Alters Parasite Transmission to Mosquitoes without Effect on Gametocytemia

We have recently demonstrated that basophils are protective against intestinal permeability during malaria and contribute to reduced parasite transmission to mosquitoes. Given that IL-18 is an early cytokine/alarmin in malaria and has been shown to activate basophils, we sought to determine the role of the basophil IL-18R in this protective phenotype. To address this, we infected control [IL18r ^flox/flox or basoIL-18R (+)] mice and mice with basophils lacking the IL-18R [IL18r ^flox/flox × Basoph8 or basoIL-18R (-)] with Plasmodium yoelii yoelii 17XNL, a nonlethal strain of mouse malaria. Postinfection (PI), intestinal permeability, ileal mastocytosis, bacteremia, and levels of ileal and plasma cytokines and chemokines were measured through 10 d PI. BasoIL-18R (-) mice exhibited greater intestinal permeability relative to basoIL-18R (+) mice, along with increased plasma levels of proinflammatory cytokines at a single time point PI, day 4 PI, a pattern not observed in basoIL-18R (+) mice. Surprisingly, mosquitoes fed on basoIL-18R (-) mice became infected less frequently than mosquitoes fed on basoIL-18R (+) mice, with no difference in gametocytemia, a pattern that was distinct from that observed previously with basophil-depleted mice. These findings suggest that early basophil-dependent protection of the intestinal barrier in malaria is mediated by IL-18, and that basophil IL-18R-dependent signaling differentially regulates the inflammatory response to infection and parasite transmission.

Mast Cell–Tumor Interactions: Molecular Mechanisms of Recruitment, Intratumoral Communication and Potential Therapeutic Targets for Tumor Growth

Mast cells (MCs) are tissue-resident immune cells that are important players in diseases associated with chronic inflammation such as cancer. Since MCs can infiltrate solid tumors and promote or limit tumor growth, a possible polarization of MCs to pro-tumoral or anti-tumoral phenotypes has been proposed and remains as a challenging research field. Here, we review the recent evidence regarding the complex relationship between MCs and tumor cells. In particular, we consider: (1) the multifaceted role of MCs on tumor growth suggested by histological analysis of tumor biopsies and studies performed in MC-deficient animal models; (2) the signaling pathways triggered by tumor-derived chemotactic mediators and bioactive lipids that promote MC migration and modulate their function inside tumors; (3) the possible phenotypic changes on MCs triggered by prevalent conditions in the tumor microenvironment (TME) such as hypoxia; (4) the signaling pathways that specifically lead to the production of angiogenic factors, mainly VEGF; and (5) the possible role of MCs on tumor fibrosis and metastasis. Finally, we discuss the novel literature on the molecular mechanisms potentially related to phenotypic changes that MCs undergo into the TME and some therapeutic strategies targeting MC activation to limit tumor growth.

Regulation of emergency granulopoiesis during infection

During acute infectious and inflammatory conditions, a large number of neutrophils are in high demand as they are consumed in peripheral organs. The hematopoietic system rapidly responds to the demand by turning from steady state to emergency granulopoiesis to expedite neutrophil generation in the bone marrow (BM). How the hematopoietic system integrates pathogenic and inflammatory stress signals into the molecular cues of emergency granulopoiesis has been the subject of investigations. Recent studies in the field have highlighted emerging concepts, including the direct sensing of pathogens by BM resident or sentinel hematopoietic stem and progenitor cells (HSPCs), the crosstalk of HSPCs, endothelial cells, and stromal cells to convert signals to granulopoiesis, and the identification of novel inflammatory molecules, such as C/EBP-β, ROS, IL-27, IFN-γ, CXCL1 with direct effects on HSPCs. In this review, we will provide a detailed account of emerging concepts while reassessing well-established cellular and molecular players of emergency granulopoiesis. While providing our views on the discrepant results and theories, we will postulate an updated model of granulopoiesis in the context of health and disease.

Bacterial and Fungal Toll-Like Receptor Activation Elicits Type I IFN Responses in Mast Cells

Next to their role in IgE-mediated allergic diseases and in promoting inflammation, mast cells also have antiinflammatory functions. They release pro- as well as antiinflammatory mediators, depending on the biological setting. Here we aimed to better understand the role of mast cells during the resolution phase of a local inflammation induced with the Toll-like receptor (TLR)-2 agonist zymosan. Multiple sequential immunohistology combined with a statistical neighborhood analysis showed that mast cells are located in a predominantly antiinflammatory microenvironment during resolution of inflammation and that mast cell-deficiency causes decreased efferocytosis in the resolution phase. Accordingly, FACS analysis showed decreased phagocytosis of zymosan and neutrophils by macrophages in mast cell-deficient mice. mRNA sequencing using zymosan-induced bone marrow-derived mast cells (BMMC) revealed a strong type I interferon (IFN) response, which is known to enhance phagocytosis by macrophages. Both, zymosan and lipopolysaccharides (LPS) induced IFN-β synthesis in BMMCs in similar amounts as in bone marrow derived macrophages. IFN-β was expressed by mast cells in paws from naïve mice and during zymosan-induced inflammation. As described for macrophages the release of type I IFNs from mast cells depended on TLR internalization and endosome acidification. In conclusion, mast cells are able to produce several mediators including IFN-β, which are alone or in combination with each other able to regulate the phagocytotic activity of macrophages during resolution of inflammation.

Should mast cells be considered therapeutic targets in multiple sclerosis?

Mast cells are immune cells of the myeloid lineage that are found throughout the body, including the central nervous system. They perform many functions associated with innate and specific immunity, angiogenesis, and vascular homeostasis. Moreover, they have been implicated in a series of pathologies (e.g., hypersensitivity reactions, tumors, and inflammatory disorders). In this review, we propose that this cell could be a relevant therapeutic target in multiple sclerosis, which is a central nervous system degenerative disease. To support this proposition, we describe the general biological properties of mast cells, their contribution to innate and specific immunity, and the participation of mast cells in the various stages of multiple sclerosis and experimental autoimmune encephalomyelitis development. The final part of this review is dedicated to an overview of the available mast cells immunomodulatory drugs and their activity on multiple sclerosis and experimental autoimmune encephalomyelitis, including our own experience related to the effect of ketotifen fumarate on experimental autoimmune encephalomyelitis evolution.

The Theranostics Role of Mast Cells in the Pathophysiology of Rosacea

Rosacea is a chronic inflammatory cutaneous disorder that adversely affects patient's health and quality of life due to the complex course and the need for repeated treatment. The exact molecular mechanisms of rosacea are unclear. Mast cells are innate immune cells that can be found in virtually all tissues. Recently, increasing evidence has indicated that mast cells have important effects on the pathogenesis of rosacea. In this review article, we describe recent advances of skin mast cells in the development of rosacea. These studies suggested that mast cells can be an important immune cell that connected innate immunity, nerves, and blood vessels in the development of rosacea. Moreover, we review the inhibition of mast cells for the potential treatment of rosacea.