Mast cells in health and disease
MCs, first described in 1878, are an important part of the immune system . They arise from pluripotent bone marrow progenitor cells and mature under the influence of c-kit ligand and stem cell factor (SCF) and other growth factors (e. g. cytokines) provided by the microenvironment of the tissue in which they are to establish . They are central to initial pathogen response and can mainly be found in the skin and the mucous membranes . MCs are convoluted in numerous physiological and pathological conditions, including angiogenesis, tissue remodeling, wound healing, IgE-dependent allergic disease, infection-induced immune response, and autoimmune inflammatory disease . They contain granules with a wide variety of biologically active substances, including prostaglandins/leukotrienes, vasoactive amines, such as histamine, and proteases [mainly mast cell tryptase (MCT) and chymas] . Like other immune cells, e. g. T lymphocytes , macrophages , and neutrophils , MCs can be encountered in different functional subpopulations, depending on their granular content . MCT cells exclusively express MCT within their granules and can primarily be found in lung tissue , while MCTC cells also contain chymase and carboxypeptidase A3 in addition to MCT within their granules (mainly located in the skin and mucous membranes) .
Different mast cell functions during tumor progression
MCs play a critical role during several malignoma progression. Tumor-promoting [9, 10] and tumor-inhibiting [11–13] effects have been delineated in various tumor entities. These properties depend on whether MCs contact the tumor cells or reside in the tumor microenvironment (TME) [14–16]. In Hodgkin's lymphoma [17, 18], malignant melanoma [19, 20], and various types of carcinoma, such as esophageal carcinoma , adenocarcinoma of the lung , and carcinomas of the gastrointestinal tract [22, 23], a poorer patients` prognosis has increased MC density in the TME. It may be due to MCs’ releasing angiogenic factors (e. g., VEGF) from their granules into the TME, thus supporting angiogenesis. The histamine release can induce tumor cell proliferation [24, 25]. Matrix metalloproteinases (MMPs) and proteases (mainly MCT and chymase) are released, modulating the extracellular matrix and promoting tumor invasion and metastasis [11, 24, 25]. MCs can inhibit the immune response by releasing IL-10 and TGF-β, thus promoting tumor spread .
Contrarily, high intratumoral MC density (close contact between both cell types) has been linked to improved prognosis in patients with prostate carcinoma [26, 27], colorectal carcinoma [28, 29], and clear cell renal cell carcinoma . MCs have a TNF-induced cytotoxic effect on tumor cells  and promote tumor cell apoptosis . Releases of CCL5, CXCL8, CXCL10, and IL-6 can recruit and activate various immune cells, thereby inhibiting tumor growth . MCs can generate reactive oxygen (ROO-, OH-, H2O2) and nitrogen (ONOO-) radicals, besides granzymes, inhibiting tumor growth in high concentrations or having a direct cytotoxic effect [30, 31].
An example of MC localization in the TME and partly intratumorally is shown in Figure 1 using an oral squamous cell carcinoma (OSCC).
The effects of mast cell-derived extracellular vesicles during cancer progression
EV is a general term that refers to membrane structures released by all cell types through different biogenesis pathways . EVs are involved in a variety of physiological and pathological processes by transporting bioactive molecules between cells . Most of their effects are mediated by microRNAs (miRNAs) that modulate gene expression in target cells and initiate epigenetic reprogramming . In addition, EV contain a variety of immunomodulatory molecules such as cytokines, costimulatory/inhibitory molecules, and growth factors, and participate in immune system activation, inhibition, and modulation .
MCs are more often localized in the TME, than intratumorally . In the tumor’s surroundings, they can stimulate the immune system by EV secretion, primarily through T- and B-cell activation , resulting in tumor cell eradication [35, 36].
It has been shown that MC-derived EVs, express CD63, OX40L, and these EVs promote naive CD4+ T cell proliferation and enhance the differentiation of Th2 cells via ligation of OX40L and OX40 between EV and those T cells . Moreover, MC-derived EV harbored immunologically relevant molecules, such as MHC class II, CD86, LFA-1, and ICAM-1 .
In addition to the activating effect on the immune system, MCs can directly impact tumor cells via EV. The EV isolated from the human MC line HMC-1 contained and transferred KIT protein but not the c-KIT mRNA to human lung adenocarcinoma cells (A549). Then, it activated KIT-SCF signal transduction, increasing cyclin D1 expression and accelerating the proliferation in those cells .
Shuttle miRNAs emerge as mediators of the interaction between MCs and tumor cells . MCs secrete EV, transferring shuttle miRNAs into hepatocellular carcinoma cells and inhibiting metastasis by blocking the ERK1/2 pathway . Nineteen differentially expressed miRNAs were identified through miRNA microarray analysis in MC-derived EV, and most likely, miR‑490 was involved in HepG2 cell migration regulation .
MC’s anti-proliferative effect via EVs was also detected for melanoma cells . It primarily depended on endocytosis of MCT (as EV cargo) by melanoma cells, followed by MCT transit to the nucleus . Here, MCT led to clipping of histone-3 and degradation of lamin B1, resulting in extensive nuclear remodeling . In addition, MCT degraded hnRNP A2/B1, a protein involved in mRNA stabilization and interaction with noncoding RNAs. Subsequently, the expression of the oncogene EGR1 and several noncoding RNAs, including oncogenic species, was downregulated .
Additional functions of mast cell-derived extracellular vesicles of interest in tumor progression
In addition to the described MC/tumor cell interactions via EV, other functions likely to play a role in tumor progression have been reported.
For example, EVs released from mouse MCs exposed to oxidative stress differ in their mRNA content . These EVs can influence the other cells’ responses to oxidative stress by endowing recipient cells with resistance to it, as evidenced by attenuated loss of cell viability . Additionally, microarray studies disclosed that mRNA content in EVs differentiated not only between EVs and donor cells but also between EVs derived from cells grown under varied conditions (oxidative stress vs. normal conditions). Exposure to UV light impaired the biological functions of EV released under oxidative stress .
In systemic mastocytosis (SM), a rare disease characterized by MC accumulation in the skin or internal organs, EV exhibited a MC signature, transferred KIT to hepatic stellate cells (HSC), and promoted proliferation, cytokine production, and differentiation . These effects were reduced by inhibiting or neutralizing KIT and reiterated by forced expression of KIT or constitutively active D816V-KIT, a gain-of-function variant associated with SM .
The in vitro and in vivo results showed that lipopolysaccharide (LPS)-stimulated MC-derived EVs were absorbed by non-stimulated MCs and these EVs induced TNF-α expression in a TLR4, JNK, and P38 MAPK-dependent manner . These data suggested EV-mediated propagation of the proinflammatory response between MCs .
Furthermore, EVs released from MCs contained active and latent transforming growth factor β-1 (TGFβ-1) on their surface . The TGFβ-1’s latent form was linked to EVs via heparinase II and pH-sensitive elements. These vesicles entered the endocytic compartment of recipient human mesenchymal stem cells (MSCs) within 60 minutes of exposure. EV-associated TGFβ-1 was retained in the endosomal compartments while signaling, causing prolonged cellular signaling comparable to that of free TGFβ-1. These EVs induced a migratory phenotype in primary MSCs, relying on SMAD-dependent signaling pathways .
Effects of tumor-derived extracellular vesicles on mast cells
Besides the impact MCs have on surrounding cells through the EVs’ release, they can be influenced by EVs .
The TME contains multiple soluble factors driving MC recruitment and activation . Nonetheless, MCs can directly be activated by cancer cells under conditions recapitulating cell-to-cell contact .
The MC tryptase’s release can promote tumor cell metastasis . Lung adenocarcinoma cell line A549 released EV absorbable by MCs. These A549 EVs transferred SCF to MCs and subsequently induced MC activation through SCF-KIT signal transduction, leading to MC degranulation and tryptase release. Tryptase accelerated the proliferation and migration of human umbilical vein endothelial cells (HUVEC) through the JAK-STAT signaling pathway .
A comparative protein profiling analysis of EV derived from human colorectal carcinoma cell line LIM1215, murine MCs, and human urine revealed 394 unique EV proteins. Of them, 112 proteins (28 %) contained signal peptides and substantial enrichment of proteins with coiled-coil, RAS, and MIRO domains . This comparative analysis’ striking finding was the presence of host cell-specific (LIM1215-EV) proteins, such as A33, cadherin-17, carcinoembryonic antigen, epithelial cell surface antigen (EpCAM), proliferating cell nuclear antigen, epidermal growth factor receptor, mucin 13, misshaped-like kinase 1, keratin 18, mitogen-activated protein kinase 4, claudins (1, 3, and 7), centrosomal protein 55 kDa, and ephrin-B1 and -B2. In addition, the presence of the enzyme phospholipid scramblase, involved in remodeling the transbilayer lipid distribution of the membrane, was discovered. The LIM1215-specific exosomal proteins provided insights into the biology of colon cancer and were potential diagnostic biomarkers .
MCs can directly be activated by cancer cell-derived EVs by a CD73- and adenosine-dependent mechanism . It has been shown that exposure of MCs to EVs derived from pancreatic cancer cells or non–small cell lung carcinoma cells (NSCLC) results in MC activation, discernible by the increased phosphorylation of the ERK1/2 MAP kinases . Like activating by cancer cell contact, activation by EVs depended on the ectoenzyme CD73 mediating the extracellular formation of adenosine and signaling by the A3 adenosine receptor. The activation by either cell contact or EVs regulated the expression of angiogenic and tissue remodeling genes, including IL8, IL6, VEGF, and amphiregulin. These findings depicted that intratumorally localized MCs and peripheral MCs could be activated and reprogrammed in the TME either by contact with the cancer cells or by their released EVs .
Tumor-derived EV from NSCLC cells interacted with human MCs and activated to release several cytokines and chemokines, including TNF-α and MCP-1/CCL2, also enhanced their chemotactic and chemokinetic activity . Lung cancer‑derived EV was a probable mediator of MC activation in the TME . PKH67-labeled EV isolated from NSCLC cell lines were internalized by MCs and activated to release cytokines and impact their migratory ability .
Tumor-derived EVs can influence MC activity and angiogenesis in the TME . Tumor-derived EVs induced CCL18 production by MCs, associated with extensive angiogenesis, both in vitro and in vivo . CCL18 secreted from MCs activated by NSCLC- derived EVs increased the migration of human HUVECs, tube formation, and endothelial-to-mesenchymal transition (EMT), thus promoting angiogenesis .
Figure 2 gives a graphical overview of the MC/TC interactions via EV certainly known so far.
Mast cells are a primary component of the tumor microenvironment. They can positively or negatively impact tumor progression. The interaction between mast cells and tumor cells can occur either via direct cell-cell contacts or over more distant places via the exchange of EV. The EVs’ blocking or influencing their release offers an interesting new approach for targeted tumor therapy.