Is it time for a new classification of mast cells? What do we know about mast cell heterogeneity?

Mast cells (MCs) are derived from committed precursors that leave the hematopoietic tissue, migrate in the blood, and colonize peripheral tissues where they terminally differentiate under microenvironment stimuli. They are distributed in almost all vascularized tissues where they act both as immune effectors and housekeeping cells, contributing to tissue homeostasis. Historically, MCs were classified into 2 subtypes, according to tryptic enzymes expression. However, MCs display a striking heterogeneity that reflects a complex interplay between different microenvironmental signals delivered by various tissues, and a differentiation program that decides their identity. Moreover, tissue-specific MCs show a trained memory, which contributes to shape their function in a specific microenvironment. In this review, we summarize the current state of our understanding of MC heterogeneity that reflects their different tissue experiences. We describe the discovery of unique cell molecules that can be used to distinguish specific MC subsets in vivo, and discuss how the improved ability to recognize these subsets provided new insights into the biology of MCs. These recent advances will be helpful for the understanding of the specific role of individual MC subsets in the control of tissue homeostasis, and in the regulation of pathological conditions such as infection, autoimmunity, and cancer.

MHC class II cell-autonomously regulates self-renewal and differentiation of normal and malignant B cells.

Best known for presenting antigenic peptides to CD4+ T cells, major histocompatibility complex class II (MHC II) also transmits or may modify intracellular signals. Here, we show that MHC II cell-autonomously regulates the balance between self-renewal and differentiation in B-cell precursors, as well as in malignant B cells. Initiation of MHC II expression early during bone marrow B-cell development limited the occupancy of cycling compartments by promoting differentiation, thus regulating the numerical output of B cells. MHC II deficiency preserved stem cell characteristics in developing pro-B cells in vivo, and ectopic MHC II expression accelerated hematopoietic stem cell differentiation in vitro. Moreover, MHC II expression restrained growth of murine B-cell leukemia cell lines in vitro and in vivo, independently of CD4+ T-cell surveillance. Our results highlight an important cell-intrinsic contribution of MHC II expression to establishing the differentiated B-cell phenotype.

Mast cell chymase protects against acute ischemic kidney injury by limiting neutrophil hyperactivation and recruitment.

Here we investigated the role of murine mast cell protease 4 (MCPT4), the functional counterpart of human mast cell chymase, in an experimental model of renal ischemia reperfusion injury, a major cause of acute kidney injury. MCPT4-deficient mice had worsened kidney function compared to wildtype mice. MCPT4 absence exacerbated pathologic neutrophil infiltration in the kidney and increased kidney myeloperoxidase expression, cell death and necrosis. In kidneys with ischemia reperfusion injury, when compared to wildtype mice, MCPT4-deficient mice showed increased surface expression of adhesion molecules necessary for leukocyte extravasation including neutrophil CD162 and endothelial cell CD54. In vitro, human chymase mediated the cleavage of neutrophil expressed CD162 and also CD54, P- and E-Selectin expressed on human glomerular endothelial cells. MCPT4 also dampened systemic neutrophil activation after renal ischemia reperfusion injury as neutrophils expressed more CD11b integrin and produced more reactive oxygen species in MCPT4-deficient mice. Accordingly, after renal injury, neutrophil migration to an inflammatory site distal from the kidney was increased in MCPT4-deficient versus wildtype mice. Thus, contrary to the described overall aggravating role of mast cells, one granule-released mediator, the MCPT4 chymase, exhibits a potent anti-inflammatory function in renal ischemia reperfusion injury by controlling neutrophil extravasation and activation thereby limiting associated damage.

Early Phase Mast Cell Activation Determines the Chronic Outcome of Renal Ischemia-Reperfusion Injury.

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.

Mast cells contribute to autoimmune diabetes by releasing interleukin-6 and failing to acquire a tolerogenic IL-10+ phenotype.

Mast cells (MCs) are innate immune cells that exert positive and negative immune modulatory functions capable to enhance or limit the intensity and/or duration of adaptive immune responses. Although MCs are crucial to regulate T cell immunity, their action in the pathogenesis of autoimmune diseases is still debated. Here we demonstrate that MCs play a crucial role in T1D pathogenesis so that their selective depletion in conditional MC knockout NOD mice protects them from the disease. MCs of diabetic NOD mice are overly inflammatory and secrete large amounts of IL-6 that favors differentiation of IL-17-secreting T cells at the site of autoimmunity. Moreover, while MCs of control mice acquire an IL-10+ phenotype upon interaction with FoxP3+ Treg cells, MCs of NOD mice do not undergo this tolerogenic differentiation. Our data indicate that overly inflammatory MCs unable to acquire a tolerogenic IL-10+ phenotype contribute to the pathogenesis of autoimmune T1D.

Mast cells control the expansion and differentiation of IL-10-competent B cells.

The discovery of B cell subsets with regulatory properties, dependent on IL-10 production, has expanded our view on the mechanisms that control inflammation. Regulatory B cells acquire the ability to produce IL-10 in a stepwise process: first, they become IL-10 competent, a poised state in which B cells are sensitive to trigger signals but do not actually express the Il-10 gene; then, when exposed to appropriate stimuli, they start producing IL-10. Even if the existence of IL-10-competent B cells is now well established, it is not yet known how different immune cell types cross talk with B cells and affect IL-10-competent B cell differentiation and expansion. Mast cells (MCs) contribute to the differentiation and influence the effector functions of various immune cells, including B lymphocytes. In this study, we explored whether MCs could play a role in the expansion of IL-10-competent B cells and addressed the in vivo relevance of MC deficiency on the generation of these cells. We show that MCs can expand IL-10-competent B cells, but they do not directly induce IL-10 production; moreover, the absence of MCs negatively affects IL-10-competent B cell differentiation. Noteworthy, our findings reveal that the CD40L/CD40 axis plays a significant role in MC-driven expansion of IL-10-competent B cells in vitro and highlight the importance of MC CD40L signaling in the colon.

The aryl hydrocarbon receptor modulates acute and late mast cell responses.

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.

CD4+ T cell memory is impaired by species-specific cytotoxic differentiation, but not by TCF-1 loss.

CD4+ T cells are typically considered as 'helper' or 'regulatory' populations that support and orchestrate the responses of other lymphocytes. However, they can also develop potent granzyme (Gzm)-mediated cytotoxic activity and CD4+ cytotoxic T cells (CTLs) have been amply documented both in humans and in mice, particularly in the context of human chronic infection and cancer. Despite the established description of CD4+ CTLs, as well as of the critical cytotoxic activity they exert against MHC class II-expressing targets, their developmental and memory maintenance requirements remain elusive. This is at least in part owing to the lack of a murine experimental system where CD4+ CTLs are stably induced. Here, we show that viral and bacterial vectors encoding the same epitope induce distinct CD4+ CTL responses in challenged mice, all of which are nevertheless transient in nature and lack recall properties. Consistent with prior reports, CD4+ CTL differentiation is accompanied by loss of TCF-1 expression, a transcription factor considered essential for memory T cell survival. Using genetic ablation of Tcf7, which encodes TCF-1, at the time of CD4+ T cell activation, we further show that, contrary to observations in CD8+ T cells, continued expression of TCF-1 is not required for CD4+ T cell memory survival. Whilst Tcf7-deficient CD4+ T cells persisted normally following retroviral infection, the CD4+ CTL subset still declined, precluding conclusive determination of the requirement for TCF-1 for murine CD4+ CTL survival. Using xenotransplantation of human CD4+ T cells into murine recipients, we demonstrate that human CD4+ CTLs develop and persist in the same experimental conditions where murine CD4+ CTLs fail to persist. These observations uncover a species-specific defect in murine CD4+ CTL persistence with implications for their use as a model system.

Single-cell dynamics of mast cell-CD4+ CD25+ regulatory T cell interactions.

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.

Default polyfunctional T helper 1 response to ample signal 1 alone.

CD4+ T cells integrate well-defined signals from the T-cell receptor (TCR) (signal 1) and a host of costimulatory molecules (signal 2) to initiate clonal expansion and differentiation into diverse functional T helper (Th) subsets. However, our ability to guide the expansion of context-appropriate Th subsets by deploying these signals in vaccination remains limited. Using cell-based vaccines, we selectively amplified signal 1 by exclusive presentation of an optimized peptide:MHC II (pMHC II) complex in the absence of classic costimulation. Contrary to expectations, amplified signal 1 alone was strongly immunogenic and selectively expanded high-affinity TCR clonotypes, despite delivering intense TCR signals. In contrast to natural infection or standard vaccines, amplified signal 1, presented by a variety of professional and nonprofessional antigen-presenting cells (APCs), induced exclusively polyfunctional Th1 effector and memory cells, which protected against retroviral infection and tumor challenge, and expanded tumor-reactive CD4+ T cells otherwise rendered unresponsive in tumor-bearing hosts. Together, our findings uncover a default Th1 response to ample signal 1 and offer a means to selectively prime such protective responses by vaccination.

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

The receptor tyrosine kinase cKit and its ligand stem cell factor are essential for mast cells (MC) development and survival. Strains with mutations affecting the Kit gene display a profound MC deficiency in all tissues and have been extensively used to investigate the role of MC in both physiologic and pathologic conditions. However, these mice present a variety of abnormalities in other immune cell populations that can affect the interpretation of MC-related responses. C57BL/6 KitW-sh are characterized by an aberrant extramedullary myelopoiesis and systemic neutrophilia. MC deficiency in KitW-sh mice can be selectively repaired by engraftment with in vitro-differentiated MC to validate MC-specific functions. Nevertheless, the impact of MC reconstitution on other immune populations has never been evaluated in detail. Here, we specifically investigated the neutrophil compartment in primary and secondary lymphoid organs of C57BL/6 KitW-sh mice before and after MC reconstitution. We found that, albeit not apparently affecting neutrophils phenotype or maturation, MC reconstitution of KitW-sh mice restored the number of neutrophils at a level similar to that of wild-type C57BL/6 mice. In vitro and ex vivo experiments indicated that MC can influence neutrophil clearance by increasing macrophages' phagocytic activity. Furthermore, the G-CSF/IL-17 axis was also influenced by the presence or absence of MC in KitW-sh mice. These data suggest that MC play a role in the control of neutrophil homeostasis and that this aspect should be taken into account in the interpretation of results obtained using KitW-sh mice.

Correlates of Follicular Helper Bias in the CD4 T Cell Response to a Retroviral Antigen.

CD4+ T cell differentiation is influenced by a plethora of intrinsic and extrinsic factors, providing the immune system with the ability to tailor its response according to specific stimuli. Indeed, different classes of pathogens may induce a distinct balance of CD4+ T cell differentiation programmes. Here, we report an uncommonly strong bias toward follicular helper (Tfh) differentiation of CD4+ T cells reactive with a retroviral envelope glycoprotein model antigen, presented in its natural context during retroviral infection. Conversely, the response to the same antigen, presented in different immunization regimens, elicited a response typically balanced between Tfh and T helper 1 cells. Comprehensive quantitation of variables known to influence Tfh differentiation revealed the closest correlation with the strength of T cell receptor (TCR) signaling, leading to PD-1 expression, but not with surface TCR downregulation, irrespective of TCR clonotypic avidity. In contrast, strong TCR signaling leading to TCR downregulation and induction of LAG3 expression in high TCR avidity clonotypes restrained CD4+ T cell commitment and further differentiation. Finally, stunted Th1 differentiation, correlating with limited IL-2 availability in retroviral infection, provided permissive conditions for Tfh development, suggesting that Tfh differentiation is the default program of envelope-reactive CD4+ T cells.

Tomosyn functions as a PKCδ-regulated fusion clamp in mast cell degranulation.

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family proteins mediate membrane fusion critical for vesicular transport and cellular secretion. Mast cells rely on SNARE-mediated membrane fusion for degranulation stimulated by crosslinking of immunoglobulin E (IgE) bound to the Fcε receptor (FcεRI). We investigated the mechanisms downstream of receptor activation that control degranulation. We found that the SNARE binding protein tomosyn-1 (also known as STXBP5) inhibited FcεRI-stimulated degranulation of mast cells. After mast cell activation, tomosyn-1 was phosphorylated on serine and threonine residues, dissociated from the SNARE protein syntaxin 4 (STX4), and associated with STX3. We identified PKCδ as the major kinase required for tomosyn-1 threonine phosphorylation and for regulation of the interaction with STXs. Incubation with high IgE concentrations increased tomosyn-1 abundance in cultured mast cells. Similarly, in basophils from allergic patients with high amounts of serum IgE, the abundance of tomosyn-1 was increased as compared to that in patients with normal IgE concentrations. Our findings identified tomosyn-1 as an inhibitor of mast cell degranulation that required PKCδ to switch its interaction with STX partners during fusion. We suggest that the IgE-mediated increase in tomosyn-1 abundance in allergic patients may represent a counterregulatory mechanism to limit disease development.

Lyn and Fyn function as molecular switches that control immunoreceptors to direct homeostasis or inflammation.

Immunoreceptors can transduce either inhibitory or activatory signals depending on ligand avidity and phosphorylation status, which is modulated by the protein kinases Lyn and Fyn. Here we show that Lyn and Fyn control immune receptor signaling status. SHP-1 tyrosine 536 phosphorylation by Lyn activates the phosphatase promoting inhibitory signaling through the immunoreceptor. By contrast, Fyn-dependent phosphorylation of SHP-1 serine 591 inactivates the phosphatase, enabling activatory immunoreceptor signaling. These SHP-1 signatures are relevant in vivo, as Lyn deficiency exacerbates nephritis and arthritis in mice, whereas Fyn deficiency is protective. Similarly, Fyn-activating signature is detected in patients with lupus nephritis, underlining the importance of this Lyn-Fyn balance. These data show how receptors discriminate negative from positive signals that respectively result in homeostatic or inflammatory conditions.Src-family kinases Fyn and Lyn are signaling components downstream of ITAM-bearing antigen receptors. Here the authors show that by phosphorylating SHP-1 at different residues, Lyn and Fyn can have opposing regulatory effects on ITAM receptors.

Mast Cells and MCPT4 Chymase Promote Renal Impairment after Partial Ureteral Obstruction.

Obstructive nephropathy constitutes a major cause of pediatric renal progressive disease. The mechanisms leading to disease progression are still poorly understood. Kidney fibrotic lesions are reproduced using a model of partial unilateral ureteral obstruction (pUUO) in newborn mice. Based on data showing significant mast cell (MC) infiltration in patients, we investigated the role of MC and murine MCPT4, a MC-released chymase, in pUUO using MC- (Wsh/sh), MCPT4-deficient (Mcpt4-/-), and wild-type (WT) mice. Measurement of kidney length and volume by magnetic resonance imaging (MRI) as well as postmortem kidney weight revealed hypotrophy of operated right kidneys (RKs) and compensatory hypertrophy of left kidneys. Differences between kidneys were major for WT, minimal for Wsh/sh, and intermediate for Mcpt4-/- mice. Fibrosis development was focal and increased only in WT-obstructed kidneys. No differences were noticed for local inflammatory responses, but serum CCL2 was significantly higher in WT versus Mcpt4-/- and Wsh/sh mice. Alpha-smooth muscle actin (αSMA) expression, a marker of epithelial-mesenchymal transition (EMT), was high in WT, minimal for Wsh/sh, and intermediate for Mcpt4-/- RK. Supernatants of activated MC induced αSMA in co-culture experiments with proximal tubular epithelial cells. Our results support a role of MC in EMT and parenchyma lesions after pUUO involving, at least partly, MCPT4 chymase. They confirm the importance of morphologic impairment evaluation by MRI in pUUO.

Kinesin-1 controls mast cell degranulation and anaphylaxis through PI3K-dependent recruitment to the granular Slp3/Rab27b complex.

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.

Mast cells regulate myofilament calcium sensitization and heart function after myocardial infarction.

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 Ca(2+) 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-Ca(2+) 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.

Mast cell/MDSC a liaison immunosuppressive for tumor microenvironment.

The instauration of an immunosuppressive microenvironment is a key event in cancer development and progression. Here, we discuss increasing evidences of the crosstalk between myeloid-derived suppressor cells (MDSCs) and mast cells (MCs) as a new fuel for the cancer immunosuppressive machinery.

Mast Cells Infiltrating Inflamed or Transformed Gut Alternatively Sustain Mucosal Healing or Tumor Growth.

Mast cells (MC) are immune cells located next to the intestinal epithelium with regulatory function in maintaining the homeostasis of the mucosal barrier. We have investigated MC activities in colon inflammation and cancer in mice either wild-type (WT) or MC-deficient (Kit(W-sh)) reconstituted or not with bone marrow-derived MCs. Colitis was chemically induced with dextran sodium sulfate (DSS). Tumors were induced by administering azoxymethane (AOM) intraperitoneally before DSS. Following DSS withdrawal, Kit(W-sh) mice showed reduced weight gain and impaired tissue repair compared with their WT littermates or Kit(W-sh) mice reconstituted with bone marrow-derived MCs. MCs were localized in areas of mucosal healing rather than damaged areas where they degraded IL33, an alarmin released by epithelial cells during tissue damage. Kit(W-sh) mice reconstituted with MC deficient for mouse mast cell protease 4 did not restore normal mucosal healing or reduce efficiently inflammation after DSS withdrawal. In contrast with MCs recruited during inflammation-associated wound healing, MCs adjacent to transformed epithelial cells acquired a protumorigenic profile. In AOM- and DSS-treated WT mice, high MC density correlated with high-grade carcinomas. In similarly treated Kit(W-sh) mice, tumors were less extended and displayed lower histologic grade. Our results indicate that the interaction of MCs with epithelial cells is dependent on the inflammatory stage, and on the activation of the tissue repair program. Selective targeting of MCs for prevention or treatment of inflammation-associated colon cancer should be timely pondered to allow tissue repair at premalignant stages or to reduce aggressiveness at the tumor stage.

Mast cells boost myeloid-derived suppressor cell activity and contribute to the development of tumor-favoring microenvironment.

Inflammation plays crucial roles at different stages of tumor development and may lead to the failure of immune surveillance and immunotherapy. Myeloid-derived suppressor cells (MDSC) are one of the major components of the immune-suppressive network that favors tumor growth, and their interaction with mast cells is emerging as critical for the outcome of the tumor-associated immune response. Herein, we showed the occurrence of cell-to-cell interactions between MDSCs and mast cells in the mucosa of patients with colon carcinoma and in the colon and spleen of tumor-bearing mice. Furthermore, we demonstrated that the CT-26 colon cancer cells induced the accumulation of CD11b(+)Gr1(+) immature MDSCs and the recruitment of protumoral mast cells at the tumor site. Using ex vivo analyses, we showed that mast cells have the ability to increase the suppressive properties of spleen-derived monocytic MDSCs, through a mechanism involving IFNγ and nitric oxide production. In addition, we demonstrated that the CD40:CD40L cross-talk between the two cell populations is responsible for the instauration of a proinflammatory microenvironment and for the increase in the production of mediators that can further support MDSC mobilization and tumor growth. In light of these results, interfering with the MDSC:mast cell axis could be a promising approach to abrogate MDSC-related immune suppression and to improve the antitumor immune response.