IL-33 Induces Cellular and Exosomal miR-146a Expression as a Feedback Inhibitor of Mast Cell Function

IL-33 is an inflammatory cytokine that promotes allergic disease by activating group 2 innate lymphoid cells, Th2 cells, and mast cells. IL-33 is increased in asthmatics, and its blockade suppresses asthma-like inflammation in mouse models. Homeostatic control of IL-33 signaling is poorly understood. Because the IL-33 receptor, ST2, acts via cascades used by the TLR family, similar feedback mechanisms may exist. MicroRNA (miR)-146a is induced by LPS-mediated TLR4 signaling and serves as a feedback inhibitor. Therefore, we explored whether miR-146a has a role in IL-33 signaling. IL-33 induced cellular and exosomal miR-146a expression in mouse bone marrow-derived mast cells (BMMCs). BMMCs transfected with a miR-146a antagonist or derived from miR-146a knockout mice showed enhanced cytokine expression in response to IL-33, suggesting that miR-146a is a negative regulator of IL-33-ST2 signaling. In vivo, miR-146a expression in plasma exosomes was elevated after i.p. injection of IL-33 in wild-type but not mast cell-deficient KitW-sh/W-sh mice. Finally, KitW-sh/W-sh mice acutely reconstituted with miR-146a knockout BMMCs prior to IL-33 challenge had elevated plasma IL-6 levels compared with littermates receiving wild-type BMMCs. These results support the hypothesis that miR-146a is a feedback regulator of IL-33-mediated mast cell functions associated with allergic disease.

High-dimensional comparison of monocytes and T cells in post-COVID and idiopathic pulmonary fibrosis

Introduction

Up to 30% of hospitalized COVID-19 patients experience persistent sequelae, including pulmonary fibrosis (PF).

Methods

We examined COVID-19 survivors with impaired lung function and imaging worrisome for developing PF and found within six months, symptoms, restriction and PF improved in some (Early-Resolving COVID-PF), but persisted in others (Late-Resolving COVID-PF). To evaluate immune mechanisms associated with recovery versus persistent PF, we performed single-cell RNA-sequencing and multiplex immunostaining on peripheral blood mononuclear cells from patients with Early- and Late-Resolving COVID-PF and compared them to age-matched controls without respiratory disease.

Results and discussion

Our analysis showed circulating monocytes were significantly reduced in Late-Resolving COVID-PF patients compared to Early-Resolving COVID-PF and non-diseased controls. Monocyte abundance correlated with pulmonary function forced vital capacity and diffusion capacity. Differential expression analysis revealed MHC-II class molecules were upregulated on the CD8 T cells of Late-Resolving COVID-PF patients but downregulated in monocytes. To determine whether these immune signatures resembled other interstitial lung diseases, we analyzed samples from Idiopathic Pulmonary Fibrosis (IPF) patients. IPF patients had a similar marked decrease in monocyte HLA-DR protein expression compared to Late-Resolving COVID-PF patients. Our findings indicate decreased circulating monocytes are associated with decreased lung function and uniquely distinguish Late-Resolving COVID-PF from Early-Resolving COVID-PF, IPF, and non-diseased controls.

The interplay of fibroblasts, the extracellular matrix, and inflammation in scar formation

Various forms of fibrosis, comprising tissue thickening and scarring, are involved in 40% of deaths across the world. Since the discovery of scarless functional healing in fetuses prior to a certain stage of development, scientists have attempted to replicate scarless wound healing in adults with little success. While the extracellular matrix (ECM), fibroblasts, and inflammatory mediators have been historically investigated as separate branches of biology, it has become increasingly necessary to consider them as parts of a complex and tightly regulated system that becomes dysregulated in fibrosis. With this new paradigm, revisiting fetal scarless wound healing provides a unique opportunity to better understand how this highly regulated system operates mechanistically. In the following review, we navigate the four stages of wound healing (hemostasis, inflammation, repair, and remodeling) against the backdrop of adult versus fetal wound healing, while also exploring the relationships between the ECM, effector cells, and signaling molecules. We conclude by singling out recent findings that offer promising leads to alter the dynamics between the ECM, fibroblasts, and inflammation to promote scarless healing. One factor that promises to be significant is fibroblast heterogeneity and how certain fibroblast subpopulations might be predisposed to scarless healing. Altogether, reconsidering fetal wound healing by examining the interplay of the various factors contributing to fibrosis provides new research directions that will hopefully help us better understand and address fibroproliferative diseases, such as idiopathic pulmonary fibrosis, liver cirrhosis, systemic sclerosis, progressive kidney disease, and cardiovascular fibrosis.

Fluvastatin enhances IL-33-mediated mast cell IL-6 and TNF production

Statins are HMG-CoA reductase inhibitors prescribed for lowering cholesterol. They can also inhibit inflammatory responses by suppressing isoprenylation of small G proteins. Consistent with this, we previously found that fluvastatin suppresses IgE-mediated mast cell function. However, some studies have found that statins induced pro-inflammatory cytokines in macrophages and NK cells. In contrast to IgE signaling, we show that fluvastatin augments IL-33-induced TNF and IL-6 production by mast cells. This effect required the key mast cell growth factor, stem cell factor (SCF). Treatment of IL-33-activated mast cells with mevalonic acid or isoprenoids reduced fluvastatin effects, suggesting fluvastatin acts at least partly by reducing isoprenoid production. Fluvastatin also enhanced IL-33-induced NF-κB transcriptional activity and promoted neutrophilic peritonitis in vivo, a response requiring mast cell activation. Other statins tested did not enhance IL-33 responsiveness. Therefore, this work supports observations of unexpected pro-inflammatory effects of some statins and suggests mechanisms by which this may occur. Because statins are candidates for repurposing in inflammatory disorders, our work emphasizes the importance of understanding the pleiotropic and possible unexpected effects of these drugs.

Lactate Is a Metabolic Mediator That Shapes Immune Cell Fate and Function

Lactate and the associated H⁺ ions are still introduced in many biochemistry and general biology textbooks and courses as a metabolic by-product within fast or oxygen-independent glycolysis. However, the role of lactate as a fuel source has been well-appreciated in the field of physiology, and the role of lactate as a metabolic feedback regulator and distinct signaling molecule is beginning to gain traction in the field of immunology. We now know that while lactate and the associated H⁺ ions are generally immunosuppressive negative regulators, there are cell, receptor, mediator, and microenvironment-specific effects that augment T helper (Th)17, macrophage (M)2, tumor-associated macrophage, and neutrophil functions. Moreover, we are beginning to uncover how lactate and H⁺ utilize different transporters and signaling cascades in various immune cell types. These immunomodulatory effects may have a substantial impact in cancer, sepsis, autoimmunity, wound healing, and other immunomodulatory conditions with elevated lactate levels. In this article, we summarize the known effects of lactate and H⁺ on immune cells to hypothesize potential explanations for the divergent inflammatory vs. anti-inflammatory effects.

T. gondii infection induces IL-1R dependent chronic cachexia and perivascular fibrosis in the liver and skeletal muscle

Cachexia is a progressive muscle wasting disease that contributes to death in a wide range of chronic diseases. Currently, the cachexia field lacks animal models that recapitulate the long-term kinetics of clinical disease, which would provide insight into the pathophysiology of chronic cachexia and a tool to test therapeutics for disease reversal. Toxoplasma gondii (T. gondii) is a protozoan parasite that uses conserved mechanisms to infect rodents and human hosts. Infection is lifelong and has been associated with chronic weight loss and muscle atrophy in mice. We have recently shown that T. gondii-induced muscle atrophy meets the clinical definition of cachexia. Here, the longevity of the T. gondii-induced chronic cachexia model revealed that cachectic mice develop perivascular fibrosis in major metabolic organs, including the adipose tissue, skeletal muscle, and liver by 9 weeks post-infection. Development of cachexia, as well as liver and skeletal muscle fibrosis, is dependent on intact signaling through the type I IL-1R receptor. IL-1α is sufficient to activate cultured fibroblasts and primary hepatic stellate cells (myofibroblast precursors in the liver) in vitro, and IL-1α is elevated in the sera and liver of cachectic, suggesting a mechanism by which chronic IL-1R signaling could be leading to cachexia-associated fibrosis.

Lactic Acid Inhibits Lipopolysaccharide-Induced Mast Cell Function by Limiting Glycolysis and ATP Availability

Sepsis has a well-studied inflammatory phase, with a less-understood secondary immunosuppressive phase. Elevated blood lactate and slow lactate clearance are associated with mortality; however, regulatory roles are unknown. We hypothesized that lactic acid (LA) contributes to the late phase and is not solely a consequence of bacterial infection. No studies have examined LA effects in sepsis models in vivo or a mechanism by which it suppresses LPS-induced activation in vitro. Because mast cells can be activated systemically and contribute to sepsis, we examined LA effects on the mast cell response to LPS. LA significantly suppressed LPS-induced cytokine production and NF-κB transcriptional activity in mouse bone marrow-derived mast cells and cytokine production in peritoneal mast cells. Suppression was MCT-1 dependent and reproducible with sodium lactate or formic acid. Further, LA significantly suppressed cytokine induction following LPS-induced endotoxemia in mice. Because glycolysis is linked to inflammation and LA is a byproduct of this process, we examined changes in glucose metabolism. LA treatment reduced glucose uptake and lactate export during LPS stimulation. LA effects were mimicked by glycolytic inhibitors and reversed by increasing ATP availability. These results indicate that glycolytic suppression and ATP production are necessary and sufficient for LA effects. Our work suggests that enhancing glycolysis and ATP production could improve immune function, counteracting LA suppressive effects in the immunosuppressive phase of sepsis.

Lactic acid suppresses IgE-mediated mast cell function in vitro and in vivo

Mast cells have functional plasticity affected by their tissue microenvironment, which greatly impacts their inflammatory responses. Because lactic acid (LA) is abundant in inflamed tissues and tumors, we investigated how it affects mast cell function. Using IgE-mediated activation as a model system, we found that LA suppressed inflammatory cytokine production and degranulation in mouse peritoneal mast cells, data that were confirmed with human skin mast cells. In mouse peritoneal mast cells, LA-mediated cytokine suppression was dependent on pH- and monocarboxylic transporter-1 expression. Additionally, LA reduced IgE-induced Syk, Btk, and ERK phosphorylation, key signals eliciting inflammation. In vivo, LA injection reduced IgE-mediated hypothermia in mice undergoing passive systemic anaphylaxis. Our data suggest that LA may serve as a feedback inhibitor that limits mast cell-mediated inflammation.

Polymer scaffold architecture is a key determinant in mast cell inflammatory and angiogenic responses

Implanted polymer scaffolds can induce inflammation leading to the foreign body response (FBR), fibrosis, and implant failure. Thus, it is important to understand how immune cells interact with scaffolds to mitigate inflammation and promote a regenerative response. We previously demonstrated that macrophage phenotype is modulated by fiber and pore diameters of an electrospun scaffold. However, it is unclear if this effect is consistent among other innate immune cells. Mast cells are inflammatory sentinels that play a vital role in the FBR of implanted biomaterials, as well as angiogenesis. We determined if altering electrospun scaffold architecture modulates mast cell responses, with the goal of promoting regenerative cell-scaffold interactions. Polydioxanone (PDO) scaffolds were made from 60 mg/mL or 140 mg/mL PDO solutions, yielding structures with divergent fiber and pore diameters. Mouse mast cells plated on these scaffolds were activated with IL-33 or lipopolysaccharide (LPS). Relative to the 60 mg/mL scaffold, 140 mg/mL scaffolds yielded less IL-6 and TNF, and greater VEGF secretion. Pores >4-6 μm elicited less IL-6 and TNF secretion. IL-33-induced VEGF regulation was more complex, showing effects of both pore size and fiber diameter. These data indicate parameters that can predict mast cell responses to scaffolds, informing biomaterial design to increase wound healing and diminish implant rejection. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 884-892, 2019.

Macrophage and Fibroblast Interactions in Biomaterial-Mediated Fibrosis

Biomaterial-mediated inflammation and fibrosis remain a prominent challenge in designing materials to support tissue repair and regeneration. Despite the many biomaterial technologies that have been designed to evade or suppress inflammation (i.e., delivery of anti-inflammatory drugs, hydrophobic coatings, etc.), many materials are still subject to a foreign body response, resulting in encapsulation of dense, scar-like extracellular matrix. The primary cells involved in biomaterial-mediated fibrosis are macrophages, which modulate inflammation, and fibroblasts, which primarily lay down new extracellular matrix. While macrophages and fibroblasts are implicated in driving biomaterial-mediated fibrosis, the signaling pathways and spatiotemporal crosstalk between these cell types remain loosely defined. In this review, the role of M1 and M2 macrophages (and soluble cues) involved in the fibrous encapsulation of biomaterials in vivo is investigated, with additional focus on fibroblast and macrophage crosstalk in vitro along with in vitro models to study the foreign body response. Lastly, several strategies that have been used to specifically modulate macrophage and fibroblast behavior in vitro and in vivo to control biomaterial-mediated fibrosis are highlighted.

Lactic acid suppresses LPS-induced cytokine production in mast cells by limiting glycolysis and ATP availability

Despite a mortality rate of 30–50%, there are no targeted molecular treatments for sepsis. Our understanding of the secondary immunosuppressive phase is recent, and it is unclear whether many mediators are helpful or detrimental to patient survival. Elevated blood lactate and slow lactate clearance are associated with mortality, however, any regulatory roles are currently unknown. Because mast cells can be activated systemically and contribute to sepsis, this project aims to elucidate lactic acid effects on mast cell activation and signaling. Lactic acid significantly suppressed LPS-induced cytokine and chemokine production as well as NFκB transcription in bone marrow derived and peritoneal mast cells. These effects were dependent upon pH and MCT-1 transporter activity. Further, lactic acid significantly suppressed cytokine and chemokine induction following LPS-induced septic shock in vivo. We also examined the role of lactic acid as a feedback regulator of glucose metabolism. Lactic acid treatment reduced glucose uptake, lactate export, and hexokinase 2 expression following LPS activation. The glycolytic inhibitors 2-deoxyglucose, sodium oxamate, or dichloracetate mimicked lactic acid effects, suppressing LPS-induced cytokine production and NFκB transcription. Furthermore, adding ATP to the culture media at the time of LPS activation reversed lactic acid effects. These results indicate that suppressing glycolysis and ATP production are necessary and sufficient for lactic acid effects. Future studies should target enhancing glycolysis and ATP production to improve immune function and counteract lactic acid effects in sepsis.

TGF-β1 Suppresses IL-33-Induced Mast Cell Function

TGF-β1 is involved in many pathological conditions, including autoimmune disorders, cancer, and cardiovascular and allergic diseases. We have previously found that TGF-β1 can suppress IgE-mediated mast cell activation of human and mouse mast cells. IL-33 is a member of the IL-1 family capable of inducing mast cell responses and enhancing IgE-mediated activation. In this study, we investigated the effects of TGF-β on IL-33-mediated mast cell activation. Bone marrow-derived mast cells cultured in TGF-β1, β2, or β3 showed reduced IL-33-mediated production of TNF, IL-6, IL-13, and MCP-1 in a concentration-dependent manner. TGF-β1 inhibited IL-33-mediated Akt and ERK phosphorylation as well as NF-κB- and AP-1-mediated transcription. These effects were functionally important, as TGF-β1 injection suppressed IL-33-induced systemic cytokines in vivo and inhibited IL-33-mediated cytokine release from human mast cells. TGF-β1 also suppressed the combined effects of IL-33 and IgE-mediated activation on mouse and human mast cells. The role of IL-33 in the pathogenesis of allergic diseases is incompletely understood. These findings, consistent with our previously reported effects of TGF-β1 on IgE-mediated activation, demonstrate that TGF-β1 can provide broad inhibitory signals to activated mast cells.

Galectin-1 promotes an M2 macrophage response to polydioxanone scaffolds

Regulating soft tissue repair to prevent fibrosis and promote regeneration is central to creating a microenvironment conducive to soft tissue development. Macrophages play an important role in this process. The macrophage response can be modulated using biomaterials, altering cytokine and growth factor secretion to promote regeneration. Electrospun polydioxanone (PDO) fiber scaffolds promoted an M2 phenotype when macrophages were cultured on large diameter, highly porous scaffolds, but an M1 phenotype on smaller diameter fibers. In this study, we investigated whether incorporation of galectin-1, an immunosuppressive protein that enhances muscle regeneration, could promote the M2 response. Galectin-1 was incorporated into large and small fiber PDO scaffolds during electrospinning. Galectin-1 incorporation increased arginase-1 and reduced iNOS and IL-6 production in mouse bone-marrow derived macrophages compared with PDO alone for both scaffold types. Inhibition of ERK mitogen-activated protein kinase did not alter galectin-1 effects on arginase-1 and iNOS expression, but reversed IL-6 suppression, indicating that IL-6 is mediated by a different mechanism. Our results suggest that galectin-1 can be used to modulate macrophage commitment to a pro-regenerative M2 phenotype, which may positively impact tissue regeneration when using small diameter PDO scaffolds. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2562-2571, 2017.

Lactic acid suppresses LPS-induced cytokine production in vitro and in vivo

Sepsis is a considerable health burden worldwide with no molecular-based drug treatments. Following the cytokine storm, a hypoinflammatory phase is observed in patients, characterized by impaired cytokine production and glycolysis in immune cells. Lactic acid levels are associated with mortality in patients; however, any regulatory role of lactic acid is currently unknown. We hypothesized that lactic acid contributes to the late phase of sepsis and is not solely a consequence of bacterial infection. As a product of glycolysis, lactic acid may feedback to inhibit glycolysis and overall energy production. Therefore, our purpose was to examine the effects of lactic acid on LPS-induced cytokine production and glycolysis in vitro and on LPS-induced septic shock in vivo. In bone marrow derived and peritoneal mast cells, lactic acid significantly suppressed cytokine and chemokine production at concentrations ≥ 6mM (IL-6) and 12.5 mM (TNF and MCP-1). These effects were dependent upon pH and MCT-1 transport. Additionally, lactic acid attenuated glucose uptake, lactate production, and the expression of the glycolytic enzyme hexokinase 2 following LPS-activation. Extending our in vitro results, lactic acid significantly suppressed IL-6, TNF-α, and MCP-1 induction in multiple in vivo experiments using LPS-induced septic shock. Interestingly, lactic acid did not affect temperature change or observational score. These findings suggest that elevated lactic acid levels may contribute to reduced cytokine concentrations and impaired immune cell metabolism observed in sepsis patients. This information may improve our understanding of immunosuppression in the late phase of sepsis and could reveal new molecular targets for treatment.

Lactic Acid Suppresses IL-33-Mediated Mast Cell Inflammatory Responses via Hypoxia-Inducible Factor-1α-Dependent miR-155 Suppression

Lactic acid (LA) is present in tumors, asthma, and wound healing, environments with elevated IL-33 and mast cell infiltration. Although IL-33 is a potent mast cell activator, how LA affects IL-33-mediated mast cell function is unknown. To investigate this, mouse bone marrow-derived mast cells were cultured with or without LA and activated with IL-33. LA reduced IL-33-mediated cytokine and chemokine production. Using inhibitors for monocarboxylate transporters (MCT) or replacing LA with sodium lactate revealed that LA effects are MCT-1- and pH-dependent. LA selectively altered IL-33 signaling, suppressing TGF-β-activated kinase-1, JNK, ERK, and NF-κB phosphorylation, but not p38 phosphorylation. LA effects in other contexts have been linked to hypoxia-inducible factor (HIF)-1α, which was enhanced in bone marrow-derived mast cells treated with LA. Because HIF-1α has been shown to regulate the microRNA miR-155 in other systems, LA effects on miR-155-5p and miR-155-3p species were measured. In fact, LA selectively suppressed miR-155-5p in an HIF-1α-dependent manner. Moreover, overexpressing miR-155-5p, but not miR-155-3p, abolished LA effects on IL-33-induced cytokine production. These in vitro effects of reducing cytokines were consistent in vivo, because LA injected i.p. into C57BL/6 mice suppressed IL-33-induced plasma cytokine levels. Lastly, IL-33 effects on primary human mast cells were suppressed by LA in an MCT-dependent manner. Our data demonstrate that LA, present in inflammatory and malignant microenvironments, can alter mast cell behavior to suppress inflammation.

Dexamethasone rapidly suppresses IL-33-stimulated mast cell function by blocking transcription factor activity

Mast cells are critical effectors of allergic disease and can be activated by IL-33, a proinflammatory member of the IL-1 cytokine family. IL-33 worsens the pathology of mast cell-mediated diseases, but therapies to antagonize IL-33 are still forthcoming. Because steroids are the mainstay of allergic disease treatment and are well known to suppress mast cell activation by other stimuli, we examined the effects of the steroid dexamethasone on IL-33-mediated mast cell function. We found that dexamethasone potently and rapidly suppressed cytokine production elicited by IL-33 from murine bone marrow-derived and peritoneal mast cells. IL-33 enhances IgE-mediated mast cell cytokine production, an activity that was also antagonized by dexamethasone. These effects were consistent in human mast cells. We additionally observed that IL-33 augmented migration of IgE-sensitized mast cells toward antigen. This enhancing effect was similarly reversed by dexamethasone. Simultaneous addition of dexamethasone with IL-33 had no effect on the phosphorylation of MAP kinases or NFκB p65 subunit; however, dexamethasone antagonized AP-1- and NFκB-mediated transcriptional activity. Intraperitoneal administration of dexamethasone completely abrogated IL-33-mediated peritoneal neutrophil recruitment and prevented plasma IL-6 elevation. These data demonstrate that steroid therapy may be an effective means of antagonizing the effects of IL-33 on mast cells in vitro and in vivo, acting partly by suppressing IL-33-induced NFκB and AP-1 activity.

TGFβ inhibits IL-33-mediated mast cell activation in vitro and in vivo

TGFβ is involved in many pathological conditions, including autoimmune disorders, cancer, and cardiovascular and allergic diseases. We have previously found that TGFβ suppresses IgE-mediated mast cell activation in human and mouse mast cells in vitro. IL-33 is a recently discovered member of the IL-1 family that functions as an “alarmin” stimulating mast cell responses and enhancing IgE-mediated activation. How TGFβ affects IL-33 signaling is now known. We find that mouse bone marrow-derived mast cells cultured in TGFβ -1, -2, or -3 showed reduced IL-33-mediated production of TNF, IL-6, IL-13 and MCP-1, in a concentration-dependent manner. Furthermore, TGFβ also reduced expression of the IL-33 receptor ST2, as well as IL-33-mediated TAK1, IKB and ERK phosphorylation. TGF-β1 injection suppressed IL-33-mediated production of systemic inflammatory cytokines in vivo. The role of IL-33 in the pathogenesis of allergic diseases is incompletely understood. These findings, consistent with our previously reported effects of TGFβ on IgE-mediated activation, demonstrate that TGFβ can provide broad and substantial inhibitory signals to activated mast cells. These data suggest possible therapeutic strategies to maintain mast cell homeostasis in autoimmune and allergic diseases.

Fluvastatin induces mast cell apoptosis and autophagy: effects on primary and transformed mast cells

Mast cells are best known for their role in inflammatory and allergic disease. Mast cell leukemia is a rare subtype of acute myelogenous leukemia, most of which express the c-Kit mutation, D816V, that yields factor-independent growth. Understanding and treating mast cell hyperplasia and leukemia could be beneficial to both inflammatory and malignant diseases. Recently the family of statin drugs, widely employed as HMG CoA Reductase inhibitors to lower serum cholesterol, have also been found to be immunosuppressive. However, statin effects on mast cell survival are not known. We report that Fluvastatin induces apoptosis in mouse bone marrow-derived mast cells (BMMC) and P815 mastocytoma cells in a dose- and time-dependent manner. Apoptosis was evidenced by DNA fragmentation and caspase activation. Importantly, the mastocytoma line P815, which expresses mutant c-Kit, was readily killed by Fluvastatin, with an IC50 of 5–10μM and apoptosis peaking at 72 hours. An up-regulation of the mutated c-Kit receptor was seen at 12 hours, followed by mitochondrial membrane instability and Caspase-9 activation within 24–28 hours. Finally, preliminary evidence of autophagy was detected during fluvastatin treatment. Interestingly, autophagy appeared to be cytoprotective in primary mast cells but cytotoxic in P815 mastocytoma cells. These preliminary data support the theory that statin family drugs induce mast cell autophagy and apoptosis, lending new therapeutic approaches for suppressing growth of both normal and transformed mast cells.

Lactic acid suppresses cytokine production and expression of miR-155 and 146a following LPS activation in mast cells

A considerable health burden, sepsis is a clinical inflammatory condition with mortality rates from 30–50% and no effective treatments. Septic patients and animal models show increased blood lactate concentrations and microRNA(miR)-155 and miR-146a expression, but how these factors are related is not clear. Lactic acid has been shown to have a suppressive effect on LPS activation and subsequent NFkB induction in macrophages and dendritic cells, but mast cells have not been examined. Our data suggests that lactic acid suppresses LPS-induced cytokine and chemokine production in bone marrow derived mast cells (BMMCs) and peritoneal mast cells. This effect is dependent upon the concentration of lactic acid, pH, and transport through MCT-1. Additionally, it is known that miR-155 and 146a affect cytokine production downstream of NFkB and TLR4, however the effects of lactic acid on these miRs is unknown. Our results show that LPS enhances miR-155 and miR-146a expression, an effect that is antagonized by lactic acid. Interestingly, miR-155 is historically considered pro-inflammatory, while miR-146a is considered anti-inflammatory. Current studies seek to determine the contribution of these miRs to septic inflammation and its modulation by lactic acid.

Lactic acid inhibits IL-33-mediated mast cell inflammatory responses via HIF-1α suppressing miR-155 expression

Lactic acid (LA) is elevated in tumors, asthma, and wound healing, environments that also include mast cells and IL-33. However, how LA affects mast cell function is unknown. Therefore we evaluated how LA modifies the IL-33-mediated mast cell response. When bone marrow derived mast cells were cultured with LA, we noted reduced IL-33-mediated cytokine production, an effect that was both monocarboxylate transporter (MCT)1- and pH-dependent. LA selectively decreased IL-33-induced TAK1, JNK, ERK, and NFkB phosphorylation, but not p38 activation. Additionally, LA increased HIF-1α expression. Since HIF-1α has been shown to regulate the pro-inflammatory microRNA miR-155, we examined miR-155 expression. miR-155-5p was reduced by LA, an effect that was reversed by HIF-1α antagonism. More importantly, miR-155-5p overexpression abolished the suppressive effects of LA. Additionally, the negative regulator SOCS1, a known miR-155 target, was elevated by LA addition. This suggests that LA employs HIF-1a to suppress miR-155, indirectly increasing SOCS-1 and limiting cytokine production. These data were recapitulated in vivo, since C57BL/6 mice injected with LA showed less IL-33-induced plasma cytokine levels than control mice. Lastly, LA suppressed IL-33-mediated human skin mast cell activation, an effect that was also MCT1-dependent. Our data demonstrate that lactic acid, present in inflammatory and malignant microenvironments, alters mast cell function to suppress inflammation.

IL-10-induced miR-155 targets SOCS1 to enhance IgE-mediated mast cell function

Interleukin 10 (IL-10) is an important regulatory cytokine that modulates a wide range of immune cells. While it is best known for its ability to suppress inflammation, IL-10 has been found to be pathogenic in several human and animal studies of inflammatory diseases. There is a considerable gap in our understanding of the molecular mechanisms behind the pro-inflammatory effects of IL-10. Long term (3–4 day) IL-10 treatment has been shown to suppress mast cell function. In this study we report the effect of short-term (24 hour) IL-10 treatment, which enhanced IgE-mediated histamine and cytokine production both in vitro and in vivo. This pro-inflammatory effect was consistent in mouse and human skin mast cells. IL-10 enhanced activation of the key FcɛRI signaling proteins Stat5, JNK, and ERK. We demonstrate that IL-10 effects are dependent on Stat3 activation, eliciting miR-155 expression, with a resulting loss of SOCS-1. The importance of miR-155 was demonstrated by the inability of IL-10 to enhance anaphylaxis in miR-155-deficient mice. Taken together, our results reveal an IL-10-induced, Stat3-miR-155 signaling pathway that can promote mast cell inflammatory responses.

IL-10-Induced miR-155 Targets SOCS1 To Enhance IgE-Mediated Mast Cell Function

IL-10 is an important regulatory cytokine that modulates a wide range of immune cells. Whereas it is best known for its ability to suppress immune responses, IL-10 has been found to be pathogenic in several human and animal studies of immune-mediated diseases. There is a considerable gap in our understanding of the molecular mechanisms behind the stimulatory effects of IL-10 during allergic inflammation. IL-10 treatment has been shown to suppress mast cell TNF production. In this study, we report that whereas TNF secretion was reduced, IL-10 surprisingly enhanced IgE-mediated protease and cytokine production both in vitro and in vivo. This stimulatory effect was consistent in mouse and human skin mast cells. IL-10 enhanced activation of the key FcεRI signaling proteins Stat5, JNK, and ERK. We demonstrate that IL-10 effects are dependent on Stat3 activation, eliciting miR-155 expression, with a resulting loss of suppressor of cytokine signaling-1. The importance of miR-155 was demonstrated by the inability of IL-10 to enhance anaphylaxis in miR-155-deficient mice. Taken together, our results reveal an IL-10-induced, Stat3-miR-155 signaling pathway that can promote mast cell responses.

Fluvastatin Suppresses Mast Cell and Basophil IgE Responses: Genotype-Dependent Effects

Mast cell (MC)- and basophil-associated inflammatory diseases are a considerable burden to society. A significant portion of patients have symptoms despite standard-of-care therapy. Statins, used to lower serum cholesterol, have immune-modulating activities. We tested the in vitro and in vivo effects of statins on IgE-mediated MC and basophil activation. Fluvastatin showed the most significant inhibitory effects of the six statins tested, suppressing IgE-induced cytokine secretion among mouse MCs and basophils. The effects of fluvastatin were reversed by mevalonic acid or geranylgeranyl pyrophosphatase, and mimicked by geranylgeranyl transferase inhibition. Fluvastatin selectively suppressed key FcεRI signaling pathways, including Akt and ERK. Although MCs and basophils from the C57BL/6J mouse strain were responsive to fluvastatin, those from 129/SvImJ mice were completely resistant. Resistance correlated with fluvastatin-induced upregulation of the statin target HMG-CoA reductase. Human MC cultures from eight donors showed a wide range of fluvastatin responsiveness. These data demonstrate that fluvastatin is a potent suppressor of IgE-mediated MC activation, acting at least partly via blockade of geranyl lipid production downstream of HMG-CoA reductase. Importantly, consideration of statin use for treating MC-associated disease needs to incorporate genetic background effects, which can yield drug resistance.

Lactic acid attenuates LPS-induced cytokine production in mast cells (INC6P.320)

Elevated lactate levels and attenuated lactate clearance following hospital admission have been associated with increased mortality in sepsis patients. While some effects of lactate on LPS stimulation have been examined in monocytes and macrophages, mast cells have not been examined. Therefore, the purpose of this study was to examine the effect of lactic acid on LPS-induced mast cell activation. Methods: Murine bone marrow derived mast cells were cultured in medium with or without lactic acid for 24 hours. Additionally, sodium lactate and formic acid were examined. Following the pre-incubation period, LPS was added and supernatants were collected after 16 hours for cytokine ELISA analysis. Results: Pre-incubation with lactic acid significantly reduced cytokine (TNF, IL-6) and chemokine (MCP-1, MIP-1a) secretion following LPS stimulation. Sodium lactate pre-incubation did not change cytokine production. However formic acid, which has a similar pKa to lactic acid, also reduced cytokine production compared to the control. Discussion: Lactic acid attenuates LPS-induced mast cell activation, an effect that appears to be dependent upon pH rather than acid chemical structure. Our results suggest that lactic acid may play a suppressive role in mast cell activation during sepsis.

Lactic Acid suppresses IL-33-mediated mast cell inflammatory responses while increasing VEGF production: an Akt- and miR-155-dependent pathway (INM6P.336)

Lactic acid (LA) is present in tumors, asthma, and wound healing, environments that also include mast cells and IL-33. The goal of this work is to evaluate how LA modifies IL-33 activation in mast cells. Bone marrow derived mast cells (BMMC) were cultured in the presence of LA and activated with IL-33. BMMC cultured in LA had reduced IL-33-mediated cytokine production. However, LA increased IL-33-mediated VEGF production. Using an MCT1 inhibitor or replacing LA with sodium lactate revealed that LA effects are MCT1 and pH-dependent. Examining IL-33 signaling demonstrated that LA decreased TAK1, JNK, and NFkB phosphorylation, yet increased Akt phosphorylation. An Akt inhibitor blocked VEGF production, but surprisingly enhanced IL-33-mediated cytokine production. Because miR-155 has recently been reported to suppress Akt activation in mast cells, we examined LA effects on miR-155. LA decreased miR-155-5p expression in BMMC. Moreover, BMMC transfected with miR-155-5p were resistant to LA-mediated suppression. Our data demonstrate that lactic acid, present in inflammatory and malignant microenvironments, can alter mast cell behavior to suppress inflammation while increasing angiogenesis.

Characterizing bone marrow-derived mast cell interaction with electrospun bioresorbable vascular grafts (INM2P.436)

Upon implantation, the regenerative potential of a vascular graft will be dependent on the body’s innate immune response. Thus, it is imperative to evaluate and regulate how innate immune cells interact with a vascular graft’s structural architecture. Different pore-sizes and fiber diameters of electrospun grafts should modulate mast cell activation and secretion, thus dictating their innate ability for inflammation or angiogenesis and regeneration. Grafts were electrospun with two different polymer concentrations on a solid mandrel, an air-impedance mandrel (increasing scaffold porosity), and altered by compression (decreasing scaffold porosity). Murine bone marrow-derived mast cells (BMMC) were seeded on scaffolds, and supernatants and cell lysates were collected after LPS and IL-33-mediated activation. ELISA analysis demonstrated that BMMC released TNFα and IL-6 at significantly greater concentrations on grafts with decreased fiber diameter and porosity. This was matched by greater NFκB activation, as measured by western blotting. In contrast, significantly greater VEGF secretion was observed on grafts with the largest fiber diameter and porosity. These results indicate that graft architecture regulates mast cell interactions and subsequent potential for healing, with increased fiber diameter and porosity promoting an anti-inflammatory and angiogenic response.

The role of lactic acid on mast cell function (HYP3P.404)

Lactic acid is produced from pyruvate during anaerobic respiration due to a high demand and low supply of oxygen. Therefore, it stands to reason that hypoxic tissues such as tumors or asthmatic bronchioles would have increased lactic acid production relative to their healthy counterparts. Mast cells are also prevalent in these tissues. The purpose of this research is to determine the effect lactic acid can have on mast cell function. Murine bone marrow derived mast cells were cultured in the presence of various amounts of L-(+)-lactic acid (0-25mM). BMMCs were activated with either IgE+antigen or IL-33. Cytokine and chemokine production was measured using ELISAs. Our results showed that IgE-mediated cytokine/chemokine production increased with the amount of lactic acid, with significantly enhanced MCP-1, IL-6 and TNFα at 12.5mM lactic acid. However, IL-33-induced cytokine/chemokine production was decreased in the presence of lactic acid, with significant suppression of MCP-1 and IL-6 at 12.5mM lactic acid. These data suggest that a hypoxic environment can alter mast cell inflammatory responses in a stimulus-dependent manner, and warrant further study into molecular mechanisms.