Apoptotic cells induce Mer tyrosine kinase-dependent blockade of NF-kappaB activation in dendritic cells

Phosphatidylserine-functional polydimethylsiloxane substrates regulate macrophage M2 polarization via modulus-dependent NF-κB/PPARγ pathway

Macrophages, highly plastic innate immune cells, critically influence the success of implantable devices by responding to biochemical and physical cues. However, the mechanisms underlying their synergistic regulation of macrophage polarization on implant surfaces remain poorly understood. Therefore, we constructed anti-inflammatory phosphatidylserine (PS) modified polydimethylsiloxane (PDMS) substrates with low, medium, and high modulus (1-100 kPa) to investigate the combined effects and underlying mechanisms of substrate modulus and biochemical signal on macrophage polarization. The introduction of PS on the PDMS surface not only significantly enhanced the polarization of M0 to M2 but also potently suppressed lipopolysaccharide (LPS)-induced M1 activation, with this effect further potentiated by a reduction in substrate modulus. In vivo subcutaneous implantation experiments also corroborated the synergistic effect of PS functionalization and low modulus PDMS in inhibiting M1 activation and promoting M2 polarization. Notably, reduced modulus led to decreased integrin αV/β3 clustering and cytoskeletal protein aggregation, ultimately diminishing YAP activation and nuclear translocation. Concomitantly, this disruption of the Piezo1-cytoskeletal protein positive feedback loop resulted in reduced p65/IκB phosphorylation and inflammation, while concurrently promoting PPARγ expression. Overall, our findings underscore the pivotal role of substrate modulus in modulating PS-mediated biomaterial-cell interactions, synergistically potentiating PS-induced M2 macrophage polarization, thus paving the way for the design of advanced immunomodulatory biomaterials.

Nanoparticle targeting of neutrophil glycolysis prevents lung ischemia-reperfusion injury

Neutrophils exacerbate pulmonary ischemia-reperfusion injury (IRI) resulting in poor short and long-term outcomes for lung transplant recipients. Glycolysis powers neutrophil activation, but it remains unclear if neutrophil-specific targeting of this pathway will inhibit IRI. Lipid nanoparticles containing the glycolysis flux inhibitor 2-deoxyglucose (2-DG) were conjugated to neutrophil-specific Ly6G antibodies (NP-Ly6G[2-DG]). Intravenously administered NP-Ly6G(2-DG) to mice exhibited high specificity for circulating neutrophils. NP-Ly6G(2-DG)-treated neutrophils were unable to adapt to hypoglycemic conditions of the lung airspace environment as evident by the loss of demand-induced glycolysis, reductions in glycogen and ATP content, and an increased vulnerability to apoptosis. NP-Ly6G(2-DG) treatment inhibited pulmonary IRI following hilar occlusion and orthotopic lung transplantation. IRI protection was associated with less airspace neutrophil extracellular trap generation, reduced intragraft neutrophilia, and enhanced alveolar macrophage efferocytotic clearance of neutrophils. Collectively, our data show that pharmacologically targeting glycolysis in neutrophils inhibits their activation and survival leading to reduced pulmonary IRI.

Protective Role of MerTK in Diabetic Peripheral Neuropathy via Inhibition of the NF-κB Signaling Pathway

INTRODUCTION

Diabetic peripheral neuropathy (DPN) impacts patient quality of life. In such patients, increased expression of mer tyrosine kinase (MerTK) has been demonstrated; however, its mechanism of action remains unclear. In this study, type 2 diabetes mellitus (T2DM) and DPN models were established in Sprague Dawley rats via low-dose streptozotocin and a high-fat diet and the mode of action of MerTK was examined.

METHODS

MerTK-specific inhibitors were administered by gavage once daily for 2 weeks. Sciatic nerve conduction velocity and nerve structure were measured. The levels of MerTK, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and relevant biochemical indexes were detected.

RESULTS

The study revealed upregulation of MerTK expression in T2DM and more so in DPN groups. Inhibiting MerTK led to reduced nerve conduction velocity and further deterioration of sciatic nerve structure, as evidenced by structural morphology. Concurrently, serum levels of total cholesterol, glycated hemoglobin, and triglyceride significantly increased. Moreover, levels of NF-κB increased in both serum and nerve tissue, alongside a significant rise in TNF-α and IL-1β expressions. MerTK could bind to the inhibitor of kappa B kinase beta (Ikbkb) in Schwann cells, establishing Ikbkb as a precursor to NF-κB activation.

DISCUSSION

Inhibition of MerTK exacerbates neuropathy, indicating its protective role in DPN by suppressing the NF-κB pathway, highlighting a potential new target for its diagnosis and treatment.

ACK1 and BRK non-receptor tyrosine kinase deficiencies are associated with familial systemic lupus and involved in efferocytosis

Systemic Lupus Erythematosus (SLE) is an autoimmune disease, the pathophysiology and genetic basis of which are incompletely understood. Using a forward genetic screen in multiplex families with systemic lupus erythematosus (SLE) we identified an association between SLE and compound heterozygous deleterious variants in the non-receptor tyrosine kinases (NRTKs) ACK1 and BRK. Experimental blockade of ACK1 or BRK increased circulating autoantibodies in vivo in mice and exacerbated glomerular IgG deposits in an SLE mouse model. Mechanistically, non-receptor tyrosine kinases (NRTKs) regulate activation, migration, and proliferation of immune cells. We found that the patients' ACK1 and BRK variants impair efferocytosis, the MERTK-mediated anti-inflammatory response to apoptotic cells, in human induced Pluripotent Stem Cell (hiPSC)-derived macrophages, which may contribute to SLE pathogenesis. Overall, our data suggest that ACK1 and BRK deficiencies are associated with human SLE and impair efferocytosis in macrophages.

Duodenal mucosa of untreated celiac disease patients has altered expression of the GAS6 and PROS1 and the negative regulator tyrosine kinase TAM receptors subfamily

Celiac disease (CD) is an immune-driven disease characterized by tissue damage in the small intestine of genetically-susceptible individuals. We evaluated here a crucial immune regulatory pathway involving TYRO3, AXL, and MERTK (TAM) receptors and their ligands PROS1 and GAS6 in duodenal biopsies of controls and CD patients. We found increased GAS6 expression associated with downregulation of PROS1 and variable TAM receptors levels in duodenum tissue of CD patients. Interestingly, CD3+ lymphocytes, CD68+, CD11c+ myeloid and epithelial cells, showed differential expressions of TAM components comparing CD vs controls. Principal component analysis revealed a clear segregation of two groups of CD patients based on TAM components and IFN signaling. In vitro validation demonstrated that monocytes, T lymphocytes and epithelial cells upregulated TAM components in response to IFN stimulation. Our findings highlight a dysregulated TAM axis in CD related to IFN signaling and contribute to a deeper understanding of the pathophysiology of CD.

Crosstalk between efferocytic myeloid cells and T-cells and its relevance to atherosclerosis

The interplay between myeloid cells and T-lymphocytes is critical to the regulation of host defense and inflammation resolution. Dysregulation of this interaction can contribute to the development of chronic inflammatory diseases. Important among these diseases is atherosclerosis, which refers to focal lesions in the arterial intima driven by elevated apolipoprotein B-containing lipoproteins, notably low-density lipoprotein (LDL), and characterized by the formation of a plaque composed of inflammatory immune cells, a collection of dead cells and lipids called the necrotic core, and a fibrous cap. As the disease progresses, the necrotic core expands, and the fibrous cap becomes thin, which increases the risk of plaque rupture or erosion. Plaque rupture leads to a rapid thrombotic response that can give rise to heart attack, stroke, or sudden death. With marked lowering of circulating LDL, however, plaques become more stable and cardiac risk is lowered-a process known as atherosclerosis regression. A critical aspect of both atherosclerosis progression and regression is the crosstalk between innate (myeloid cells) and adaptive (T-lymphocytes) immune cells. Myeloid cells are specialized at clearing apoptotic cells by a process called efferocytosis, which is necessary for inflammation resolution. In advanced disease, efferocytosis is impaired, leading to secondary necrosis of apoptotic cells, inflammation, and, most importantly, defective tissue resolution. In regression, efferocytosis is reawakened aiding in inflammation resolution and plaque stabilization. Here, we will explore how efferocytosing myeloid cells could affect T-cell function and vice versa through antigen presentation, secreted factors, and cell-cell contacts and how this cellular crosstalk may contribute to the progression or regression of atherosclerosis.

Efferocytosis in dendritic cells: an overlooked immunoregulatory process

Efferocytosis, the process of engulfing and removing apoptotic cells, plays an essential role in preserving tissue health and averting undue inflammation. While macrophages are primarily known for this task, dendritic cells (DCs) also play a significant role. This review delves into the unique contributions of various DC subsets to efferocytosis, highlighting the distinctions in how DCs and macrophages recognize and handle apoptotic cells. It further explores how efferocytosis influences DC maturation, thereby affecting immune tolerance. This underscores the pivotal role of DCs in orchestrating immune responses and sustaining immune equilibrium, providing new insights into their function in immune regulation.

GAS6/TAM Axis as Therapeutic Target in Liver Diseases

TAM (TYRO3, AXL, and MERTK) protein tyrosine kinase membrane receptors and their vitamin K-dependent ligands GAS6 and protein S (PROS) are well-known players in tumor biology and autoimmune diseases. In contrast, TAM regulation of fibrogenesis and the inflammation mechanisms underlying metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis, and, ultimately, liver cancer has recently been revealed. GAS6 and PROS binding to phosphatidylserine exposed in outer membranes of apoptotic cells links TAMs, particularly MERTK, with hepatocellular damage. In addition, AXL and MERTK regulate the development of liver fibrosis and inflammation in chronic liver diseases. Acute hepatic injury is also mediated by the TAM system, as recent data regarding acetaminophen toxicity and acute-on-chronic liver failure have uncovered. Soluble TAM-related proteins, mainly released from activated macrophages and hepatic stellate cells after hepatic deterioration, are proposed as early serum markers for disease progression. In conclusion, the TAM system is becoming an interesting pharmacological target in liver pathology and a focus of future biomedical research in this field.

Anti-Inflammatory Neutrophils Reprogram Macrophages toward a Pro-Healing Phenotype with Increased Efferocytosis Capacity

Following myocardial infarction (MI), blood neutrophils quickly and extensively infiltrate the heart, where they are temporally polarized into pro-inflammatory (N1) and anti-inflammatory (N2) subpopulations. Neutrophil transmigration is rapidly followed by the accrual of macrophages (MACs), which are believed to undergo local phenotypic transformations from pro-inflammatory to pro-healing MACs that mediate inflammation resolution. We hypothesized that N2 neutrophils can reprogram MACs toward a healing phenotype with increased efferocytosis capacity. To examine this, human neutrophils isolated from healthy subjects were polarized in N1 and N2 neutrophils, and their secretome was added to human MACs derived from THP monocytes. The impact of neutrophil factors on macrophages was investigated using qPCR, ELISA, Western blot, immunofluorescence, or an efferocytosis assay. The results show that the MACs exposed to N2 neutrophil secretome exhibited (i) increased expression of the anti-inflammatory molecules CD206, TGF-β, and IL-10 and the nuclear factors associated with reparatory macrophages (PPARγ, Nur77, and KLF4); (ii) enhanced expression of efferocytosis receptors (MerTK, CD36, CX3CR1, and integrins αv/β5) and of the bridge molecules Mfage8 and Gas6; and (iii) enhanced efferocytosis. In conclusion, factors released by N2 neutrophils induce a pro-healing phenotype of MACs by upregulating anti-inflammatory molecules and efferocytosis receptors and ensuing the efferocytosis capacity. The data suggest that molecular therapy to foster N2 polarization, which boosts macrophages' pro-healing phenotype, could be a promising strategy to speed up inflammation resolution and tissue repair.

The Tolerogenic Influence of Dexamethasone on Dendritic Cells Is Accompanied by the Induction of Efferocytosis, Promoted by MERTK

Many treatments for autoimmune diseases, caused by the loss of immune self-tolerance, are broadly immunosuppressive. Dendritic cells (DCs) can be induced to develop anti-inflammatory/tolerogenic properties to suppress aberrant self-directed immunity by promoting immune tolerance in an antigen-specific manner. Dexamethasone can generate tolerogenic DCs and upregulates MERTK expression. As MERTK can inhibit inflammation, we investigated whether dexamethasone's tolerogenic effects are mediated via MERTK, potentially providing a novel therapeutic approach. Monocyte-derived DCs were treated with dexamethasone, and with and without MERTK ligands or MERTK inhibitors. Flow cytometry was used to assess effects of MERTK modulation on co-stimulatory molecule expression, efferocytosis, cytokine secretion and T cell proliferation. The influence on expression of Rab17, which coordinates the diversion of efferocytosed material away from cell surface presentation, was assessed. Dexamethasone-treated DCs had upregulated MERTK expression, decreased expression of co-stimulatory molecules, maturation and proliferation of co-cultured T cells and increased uptake of myelin debris. MERTK ligands did not potentiate these properties, whilst specific MERTK inhibition only reversed dexamethasone's effect on myelin uptake. Cells undergoing efferocytosis had higher Rab17 expression. Dexamethasone-enhanced efferocytosis in DCs is MERTK-dependent and could exert its tolerogenic effects by increasing Rab17 expression to prevent the presentation of efferocytosed material on the cell surface to activate adaptive immune responses.

1-Monopalmitin promotes lung cancer cells apoptosis through PI3K/Akt pathway in vitro

Lung cancer is the leading cause of cancer-related death worldwide and non-small cell lung cancer (NSCLC) represents 85%. Mougeotia nummuloides and Spirulina major have been reported to possess anticancer properties. 1-Monopalmitin (1-Mono) is the principle active constituent in these natural plants. It is debating whether 1-Mono exerts antitumor effects. Therefore, we explored the role of 1-Mono in lung cancer in vitro. Results showed that 1-Mono significantly inhibited A549 and SPC-A1 cell proliferation, induced G2/M arrest and caspase-dependent apoptosis. Moreover, it suppressed the protein expression of inhibitors of apoptosis proteins (IAPs). It was further demonstrated that 1-Mono activated the PI3K/Akt pathway, suppression of PI3K/Akt activities with LY294002 and Wortmannin partially attenuated 1-Mono-mediated anticancer activities, indicating that 1-Mono-induced antitumor effects is dependent on PI3K/Akt pathway. 1-Mono induced cytoprotective autophagy since autophagy inhibitor Chloroquine dramatically enhanced 1-Mono-induced cytotoxicity. In summary, our results showed 1-Mono kills lung cancer through PI3K/Akt pathway, providing novel options for lung cancer administration.

NOX2 inhibition stabilizes vulnerable plaques by enhancing macrophage efferocytosis via MertK/PI3K/AKT pathway

NADPH oxidases 2 (NOX2) is the main source of ROS in macrophages, which plays a critical role in the formation of atherosclerosis. However, effects of NOX2 inhibition on established vulnerable plaques and the potential role involved remain unclear. The purpose of this study is to investigate the latent mechanism of NOX2-triggered vulnerable plaque development. We generated a vulnerable carotid plaque model induced by carotid branch ligation and renal artery constriction, combined with a high-fat diet in ApoE^-/- mice. NOX2 specific inhibitor, GSK2795039 (10 mg/kg/day by intragastric administration for 8 weeks) significantly prevented vulnerable plaque, evaluated by micro-ultrasound imaging parameters. A profile of less intraplaque hemorrhage detection, increased collagen-lipid ratio, fibrous cap thickness and less necrotic core formation were also found in GSK2795039 treated group. Mechanistically, reduced 4-HNE, in situ lesional apoptosis and enhanced efferocytosis were involved in mice treated with NOX2 inhibitor. Further analysis in mouse macrophages confirmed the role of NOX2 inhibition in enhancing macrophage efferocytosis by regulating the MertK/PI3K/AKT pathway. In summary, our data defined previously few recognized roles of NOX2 in vulnerable plaque pathogenesis and an undescribed NOX2-ROS-MerTK axis acts involved in regulating macrophage efferocytosis in the formation of rupture-prone vulnerable plaques.

LXR signaling controls homeostatic dendritic cell maturation

Dendritic cells (DCs) mature in an immunogenic or tolerogenic manner depending on the context in which an antigen is perceived, preserving the balance between immunity and tolerance. Whereas the pathways driving immunogenic maturation in response to infectious insults are well-characterized, the signals that drive tolerogenic maturation during homeostasis are still poorly understood. We found that the engulfment of apoptotic cells triggered homeostatic maturation of type 1 conventional DCs (cDC1s) within the spleen. This maturation process could be mimicked by engulfment of empty, nonadjuvanted lipid nanoparticles (LNPs), was marked by intracellular accumulation of cholesterol, and was highly specific to cDC1s. Engulfment of either apoptotic cells or cholesterol-rich LNPs led to the activation of the liver X receptor (LXR) pathway, which promotes the efflux of cellular cholesterol, and repressed genes associated with immunogenic maturation. In contrast, simultaneous engagement of TLR3 to mimic viral infection via administration of poly(I:C)-adjuvanted LNPs repressed the LXR pathway, thus delaying cellular cholesterol efflux and inducing genes that promote T cell-mediated immunity. These data demonstrate that conserved cellular cholesterol efflux pathways are differentially regulated in tolerogenic versus immunogenic cDC1s and suggest that administration of nonadjuvanted cholesterol-rich LNPs may be an approach for inducing tolerogenic DC maturation.

Role of transcription factors in apoptotic cells clearance

The human body generates 10-100 billion cells every day, and the same number of cells die to maintain homeostasis. The genetically controlled, autonomously ordered cell death mainly proceeds by apoptosis. Apoptosis is an important way of programmed cell death in multicellular organisms, timely and effective elimination of apoptotic cells plays a key role in the growth and development of organisms and the maintenance of homeostasis. During the clearance of apoptotic cells, transcription factors bind to specific target promoters and act as activators or repressors to regulate multiple genes expression, how transcription factors regulate apoptosis is an important and poorly understood aspect of normal development. This paper summarizes the regulatory mechanisms of transcription factors in the clearance of apoptotic cells to date.

Gsk3β regulates the resolution of liver ischemia/reperfusion injury via MerTK

Although glycogen synthase kinase β (Gsk3β) has been shown to regulate tissue inflammation, whether and how it regulates inflammation resolution versus inflammation activation is unclear. In a murine liver, partial warm ischemia/reperfusion injury (IRI) model, we found that Gsk3β inhibitory phosphorylation increased at both the early-activation and late-resolution stages of the disease. Myeloid Gsk3β deficiency not only alleviated liver injuries, it also facilitated the restoration of liver homeostasis. Depletion of Kupffer cells prior to the onset of liver ischemia diminished the differences between the WT and Gsk3β-KO mice in the activation of liver IRI. However, the resolution of liver IRI remained accelerated in Gsk3β-KO mice. In CD11b-DTR mice, Gsk3β-deficient BM-derived macrophages (BMMs) facilitated the resolution of liver IRI as compared with WT cells. Furthermore, Gsk3β deficiency promoted the reparative phenotype differentiation in vivo in liver-infiltrating macrophages and in vitro in BMMs. Gsk3 pharmacological inhibition promoted the resolution of liver IRI in WT, but not myeloid MerTK-deficient, mice. Thus, Gsk3β regulates liver IRI at both activation and resolution stages of the disease. Gsk3 inactivation enhances the proresolving function of liver-infiltrating macrophages in an MerTK-dependent manner.

Leishmania donovani Impedes Antileishmanial Immunity by Suppressing Dendritic Cells via the TIM-3 Receptor

An immunological hallmark of visceral leishmaniasis (VL), caused by Leishmania donovani, is profound immunosuppression. However, the molecular basis for this immune dysfunction has remained ill defined. Since dendritic cells (DCs) normally initiate antileishmanial immune responses, we investigated whether DCs are dysregulated during L. donovani infection and assessed its role in immunosuppression. Accordingly, we determined the regulatory effect of L. donovani on DCs. Notably, it is still unclear whether L. donovani activates or suppresses DCs. In addition, the molecular mechanism and the relevant receptor (or receptors) mediating the immunoregulatory effect of L. donovani on DCs are largely undefined. Here, we report that L. donovani inhibited DC activation/maturation by transmitting inhibitory signals through the T cell immunoglobulin and mucin protein-3 (TIM-3) receptor and thereby suppressed antileishmanial immune responses. L. donovani in fact triggered TIM-3 phosphorylation in DCs, which in turn recruited and activated a nonreceptor tyrosine kinase, Btk. Btk then inhibited DC activation/maturation by suppressing the NF-κB pathway in an interleukin-10 (IL-10)-dependent manner. Treatment with TIM-3-specific blocking antibody or suppressed expression of TIM-3 or downstream effector Btk made DCs resistant to the inhibitory effects of L. donovani. Adoptive transfer experiments further demonstrated that TIM-3-mediated L. donovani-induced inhibition of DCs plays a crucial role in the suppression of the antileishmanial immune response in vivo. These findings identify TIM-3 as a new regulator of the antileishmanial immune response and demonstrate a unique mechanism for host immunosuppression associated with L. donovani infection. IMPORTANCE Visceral leishmaniasis (VL), a poverty-related disease caused by Leishmania donovani, is ranked by the World Health Organization as the second largest killer parasitic disease in the world. The protective immune response against VL is primarily regulated by dendritic cells (DCs), which upon activation/maturation initiate an antileishmanial immune response. However, it remains obscure whether L. donovani promotes or inhibits DC activation. In addition, the receptor through which L. donovani exerts immunoregulatory effect on DCs is ill defined. Here, we for the first time report that L. donovani inhibits DC activation and maturation via the T cell immunoglobulin and mucin protein-3 (TIM-3) receptor and thereby attenuates the capacity of DCs to trigger antileishmanial immune responses in vivo. In fact, we demonstrate here that suppression of TIM-3 expression in DCs augments antileishmanial immunity. Our study uncovers a unique mechanism by which L. donovani subverts host immune responses and suggests TIM-3 as a potential new target for immunotherapy against VL.

Drugging the efferocytosis process: concepts and opportunities

The daily removal of billions of apoptotic cells in the human body via the process of efferocytosis is essential for homeostasis. To allow for this continuous efferocytosis, rapid phenotypic changes occur in the phagocytes enabling them to engulf and digest the apoptotic cargo. In addition, efferocytosis is actively anti-inflammatory and promotes resolution. Owing to its ubiquitous nature and the sheer volume of cell turnover, efferocytosis is a point of vulnerability. Aberrations in efferocytosis are associated with numerous inflammatory pathologies, including atherosclerosis, cancer and infections. The recent exciting discoveries defining the molecular machinery involved in efferocytosis have opened many avenues for therapeutic intervention, with several agents now in clinical trials.

Before the "cytokine storm": Boosting efferocytosis as an effective strategy against SARS-CoV-2 infection and associated complications

The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is responsible for the ongoing COVID-19 pandemic, and causes many health complications, including major lung diseases. Besides investigations into the virology of SARS-CoV-2, understanding the immunological routes underlying the clinical manifestations of COVID-19 is important for developing effective therapeutic interventions. The clearance of SARS-CoV-2-infected apoptotic cells by professional efferocytes, through a process termed as 'efferocytosis', is essential for maintaining tissue homeostasis, and reducing the chances of health complications caused by SARS-CoV-2 infection. In this review, we focus on the cellular events leading to engagement of the SARS-CoV-2 with type 2 alveolar cells, and how SARS-COV-2 infection impairs the macrophage anti-inflammatory programming. We also discuss accounts of impaired efferocytosis, and the “cytokine storm” which occur concomitantly with the SARS-CoV-2 infection. Finally, we propose how targeting impaired efferocytosis, due to the SARS-CoV-2 infection, may be a beneficial therapeutic strategy to combat COVID-19, and its complications.

Mertk: An emerging target in cancer biology and immuno-oncology

Mertk, a type I Receptor Tyrosine Kinase (RTK) and member of the TAM (Tyro3, Axl, and Mertk) family of homologous tyrosine kinases, has important roles in signal transduction both homeostatically on normal cells as well as patho-physiologically on both tumor-associated macrophages and malignant cells by its overexpression in a wide array of cancers. The main ligands of Mertk are Vitamin K-modified endogenous proteins Gas6 and Protein S (ProS1), heterobifunctional modular proteins that bind Mertk via two carboxyl-terminal laminin-like globular (LG) domains, and an N-terminal Gla domain that binds anionic phospholipids, whereby externalized phosphatidylserine (PS) on stressed viable and caspase-activated apoptotic cells is most emblematic. Recent studies indicate that Vitamin K-dependent γ-carboxylation on the N-terminal Gla domain of Gas6 and Protein S is necessary for PS binding and Mertk activation, implying that Mertk is preferentially active in tissues where there is high externalized PS, such as the tumor microenvironment (TME) and acute virally infected tissues. Once stimulated, activated Mertk can provide a survival advantage for cancer cells as well as drive compensatory proliferation. On monocytes and tumor-associated macrophages, Mertk promotes efferocytosis and acts as an inhibitory receptor that impairs host anti-tumor immunity, functioning akin to a myeloid checkpoint inhibitor. In recent years, inhibition of Mertk has been implicated in a dual role to enhance the sensitivity of cancer cells to cytotoxic agents along with improving host anti-tumor immunity with anti-PD-1/PD-L1 immunotherapy. Here, we examine the rationale of Mertk-targeted immunotherapies, the current and potential therapeutic strategies, the clinical status of Mertk-specific therapies, and potential challenges and obstacles for Mertk-focused therapies.

Targeting MERTK and AXL in EGFR Mutant Non-Small Cell Lung Cancer

MERTK and AXL are members of the TAM family of receptor tyrosine kinases and are abnormally expressed in 69% and 93% of non-small cell lung cancers (NSCLCs), respectively. Expression of MERTK and/or AXL provides a survival advantage for NSCLC cells and correlates with lymph node metastasis, drug resistance, and disease progression in patients with NSCLC. The TAM receptors on host tumor infiltrating cells also play important roles in the immunosuppressive tumor microenvironment. Thus, MERTK and AXL are attractive biologic targets for NSCLC treatment. Here, we will review physiologic and oncologic roles for MERTK and AXL with an emphasis on the potential to target these kinases in NSCLCs with activating EGFR mutations.

Study of the Role of the Tyrosine Kinase Receptor MerTK in the Development of Kidney Ischemia-Reperfusion Injury in RCS Rats

Renal ischaemia reperfusion (I/R) triggers a cascade of events including oxidative stress, apoptotic body and microparticle (MP) formation as well as an acute inflammatory process that may contribute to organ failure. Macrophages are recruited to phagocytose cell debris and MPs. The tyrosine kinase receptor MerTK is a major player in the phagocytosis process. Experimental models of renal I/R events are of major importance for identifying I/R key players and for elaborating novel therapeutical approaches. A major aim of our study was to investigate possible involvement of MerTK in renal I/R. We performed our study on both natural mutant rats for MerTK (referred to as RCS) and on wild type rats referred to as WT. I/R was established by of bilateral clamping of the renal pedicles for 30' followed by three days of reperfusion. Plasma samples were analysed for creatinine, aspartate aminotransferase (ASAT), lactate dehydrogenase (LDH), kidney injury molecule -1 (KIM-1), and neutrophil gelatinase-associated lipocalin (NGAL) levels and for MPs. Kidney tissue damage and CD68-positive cell requirement were analysed by histochemistry. monocyte chemoattractant protein-1 (MCP-1), myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS), and histone 3A (H3A) levels in kidney tissue lysates were analysed by western blotting. The phagocytic activity of blood-isolated monocytes collected from RCS or WT towards annexin-V positive bodies derived from cultured renal cell was assessed by fluorescence-activated single cell sorting (FACS) and confocal microscopy analyses. The renal I/R model for RCS rat described for the first time here paves the way for further investigations of MerTK-dependent events in renal tissue injury and repair mechanisms.

MERTK on mononuclear phagocytes regulates T cell antigen recognition at autoimmune and tumor sites

Understanding mechanisms of immune regulation is key to developing immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes during peripheral T cell regulation in type 1 diabetes and melanoma. MERTK expression and activity in mononuclear phagocytes in the pancreatic islets promoted islet T cell regulation, resulting in reduced sensitivity of T cell scanning for cognate antigen in prediabetic islets. MERTK-dependent regulation led to reduced T cell activation and effector function at the disease site in islets and prevented rapid progression of type 1 diabetes. In human islets, MERTK-expressing cells were increased in remaining insulin-containing islets of type 1 diabetic patients, suggesting that MERTK protects islets from autoimmune destruction. MERTK also regulated T cell arrest in melanoma tumors. These data indicate that MERTK signaling in mononuclear phagocytes drives T cell regulation at inflammatory disease sites in peripheral tissues through a mechanism that reduces the sensitivity of scanning for antigen leading to reduced responsiveness to antigen.

Post-translational modifications of the ligands: Requirement for TAM receptor activation

The Tyro3, Axl, and MerTK (TAM) receptors are three homologous Type I Receptor Tyrosine Kinases that have important homeostatic functions in multicellular organisms by regulating the clearance of apoptotic cells (efferocytosis). Pathologically, TAM receptors are overexpressed in a wide array of human cancers, and often associated with aggressive tumor grade and poor overall survival. In addition to their expression on tumor cells, TAMs are also expressed on infiltrating myeloid-derived cells in the tumor microenvironment, where they appear to act akin to negative immune checkpoints that impair host anti-tumor immunity. The ligands for TAMs are two endogenous proteins, Growth Arrest-Specific 6 (Gas6) and Protein S (Pros1), that function as bridging molecules between externalized phosphatidylserine (PtdSer) on apoptotic cells and the TAM ectodomains. One interesting feature of TAMs biology is that their ligand proteins require specific post-translational modifications to acquire activities. This chapter summarized these important modifications and explained the molecular mechanisms behind such phenomenon. Current evidences suggest that these modifications help Gas6/Pros1 to achieve optimal PtdSer-binding capacities. In addition, this chapter included recent discovery of regulating machineries of PtdSer dynamic across the plasma membrane, as well as their potential impacts in the tumor microenvironment. Taken together, this review highlights the importance of the upstream PtdSer and Gas6 in regulating TAMs' function and hope to provide researchers with new perspectives to inspire future studies of TAM receptors in human disease models.

The Akt-mTORC1 pathway mediates Axl receptor tyrosine kinase-induced mesangial cell proliferation

Glomerulonephritis (GN), an important pathologic feature of many renal diseases, is frequently characterized by mesangial cell proliferation. We and others have previously shown that the TAM family receptor tyrosine kinases Axl, Mer, and Tyro-3 contribute to cell survival, proliferation, migration, and clearance of apoptotic cells (ACs); that Axl contributes to GN by promoting mesangial cell proliferation; and that small molecule inhibition of Axl ameliorates nephrotoxic serum-induced GN in mice. We now show that stimulation of renal mesangial cell Axl causes a modest increase in intracellular Ca²⁺ and activates NF-κB, mTOR, and the mTOR-containing mTORC1 complex, which phosphorylates the ribosomal protein S6. Axl-induction of Akt activation is upstream of NF-κB and mTOR activation, which are mutually codependent. Axl-induced NF-κB activation leads to Bcl-xl up-regulation. Axl is more important than Mer at mediating AC phagocytosis by mesangial cells, but less important than Mer at mediating phagocytosis of ACs by peritoneal macrophages. Taken together, our data suggest the possibility that Axl mediates mesangial cell phagocytosis of ACs and promotes mesangial cell proliferation by activating NF-κB and mTORC1.

Tyro3, Axl, Mertk receptor-mediated efferocytosis and immune regulation in the tumor environment

Three structurally related tyrosine receptor cell surface kinases, Tyro3, Axl, and Mertk (TAM) have been recognized to modulate immune function, tissue homeostasis, cardiovasculature, and cancer. The TAM receptor family appears to operate in adult mammals across multiple cell types, suggesting both widespread and specific regulation of cell functions and immune niches. TAM family members regulate tissue homeostasis by monitoring the presence of phosphatidylserine expressed on stressed or apoptotic cells. The detection of phosphatidylserine on apoptotic cells requires intermediary molecules that opsonize the dying cells and tether them to TAM receptors on phagocytes. This complex promotes the engulfment of apoptotic cells, also known as efferocytosis, that leads to the resolution of inflammation and tissue healing. The immune mechanisms dictating these processes appear to fall upon specific family members or may involve a complex of different receptors acting cooperatively to resolve and repair damaged tissues. Here, we focus on the role of TAM receptors in triggering efferocytosis and its consequences in the regulation of immune responses in the context of inflammation and cancer.

Myeloid cell-derived PROS1 inhibits tumor metastasis by regulating inflammatory and immune responses via IL-10

Stimulation of TAM (TYRO3, AXL, and MERTK) receptor tyrosine kinases promotes tumor progression through numerous cellular mechanisms. TAM cognate ligands GAS6 and PROS1 (for TYRO3 and MERTK) are secreted by host immune cells, an interaction which may support tumor progression. Here, we revealed an unexpected antimetastatic role for myeloid-derived PROS1: suppressing metastatic potential in lung and breast tumor models. Pros1 deletion in myeloid cells led to increased lung metastasis, independent of primary tumor infiltration. PROS1-cKO bone marrow-derived macrophages (BMDMs) led to elevated TNF-α, IL-6, Nos2, and IL-10 via modulation of the Socs3/NF-κB pathway. Conditioned medium from cKO BMDMs enhanced EMT, ERK, AKT, and STAT3 activation within tumor cells and promoted IL-10-dependent invasion and survival. Macrophages isolated from metastatic lungs modulated T cell proliferation and function, as well as expression of costimulatory molecules on DCs in a PROS1-dependent manner. Inhibition of MERTK kinase activity blocked PROS1-mediated suppression of TNF-α and IL-6 but not IL-10. Overall, using lung and breast cancer models, we identified the PROS1/MERTK axis within BMDMs as a potent regulator of adaptive immune responses with a potential to suppress metastatic seeding and revealed IL-10 regulation by PROS1 to deviate from that of TNF-α and IL-6.

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

Cbl-b Is Upregulated and Plays a Negative Role in Activated Human NK Cells

The E3 ubiquitin ligase Cbl-b has been characterized as an intracellular checkpoint in T cells; however, the function of Cbl-b in primary human NK cells, an innate immune anti-tumor effector cell, is not well defined. In this study, we show that the expression of Cbl-b is significantly upregulated in primary human NK cells activated by IL-15, IL-2, and the human NK cell-sensitive tumor cell line K562 that lacks MHC class I expression. Pretreatment with JAK or AKT inhibitors prior to IL-15 stimulation reversed Cbl-b upregulation. Downregulation of Cbl-b resulted in significant increases in granzyme B and perforin expression, IFN-γ production, and cytotoxic activity against tumor cells. Collectively, we demonstrate upregulation of Cbl-b and its inhibitory effects in IL-15/IL-2/K562-activated human NK cells, suggesting that Cbl-b plays a negative feedback role in human NK cells.

MERTK Inhibition: Potential as a Treatment Strategy in EGFR Tyrosine Kinase Inhibitor-Resistant Non-Small Cell Lung Cancer

Epidermal growth factor tyrosine kinase inhibitors (EGFR-TKIs) are currently the most effective treatment for non-small cell lung cancer (NSCLC) patients, who carry primary EGFR mutations. However, the patients eventually develop drug resistance to EGFR-TKIs after approximately one year. In addition to the acquisition of the EGFR T790M mutation, the activation of alternative receptor-mediated signaling pathways is a common mechanism for conferring the insensitivity of EGFR-TKI in NSCLC. Upregulation of the Mer receptor tyrosine kinase (MERTK), which is a member of the Tyro3-Axl-MERTK (TAM) family, is associated with a poor prognosis of many cancers. The binding of specific ligands, such as Gas6 and PROS1, to MERTK activates phosphoinositide 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK) cascades, which are the signaling pathways shared by EGFR. Therefore, the inhibition of MERTK can be considered a new therapeutic strategy for overcoming the resistance of NSCLC to EGFR-targeted agents. Although several small molecules and monoclonal antibodies targeting the TAM family are being developed and have been described to enhance the chemosensitivity and converse the resistance of EGFR-TKI, few have specifically been developed as MERTK inhibitors. The further development and investigation of biomarkers which can accurately predict MERTK activity and the response to MERTK inhibitors and MERTK-specific drugs are vitally important for obtaining appropriate patient stratification and increased benefits in clinical applications.

Targeting Tumor-Associated Macrophages in the Pediatric Sarcoma Tumor Microenvironment

For many pediatric sarcoma patients, multi-modal therapy including chemotherapy, radiation, and surgery is sufficient to cure their disease. However, event-free and overall survival rates for patients with more advanced disease are grim, necessitating the development of novel therapeutic approaches. Within many pediatric sarcomas, the normal immune response, including recognition and destruction of cancer cells, is lost due to the highly immune suppressive tumor microenvironment (TME). In this setting, tumor cells evade immune detection and capitalize on the immune suppressed microenvironment, leading to unchecked proliferation and metastasis. Recent preclinical and clinical approaches are aimed at understanding this immune suppressive microenvironment and employing cancer immunotherapy in an attempt to overcome this, by renewing the ability of the immune system to recognize and destroy cancer cells. While there are several factors that drive the attenuation of immune responses in the sarcoma TME, one of the most remarkable are tumor associated macrophage (TAMs). TAMs suppress immune cytolytic function, promote tumor growth and metastases, and are generally associated with a poor prognosis in most pediatric sarcoma subtypes. In this review, we summarize the mechanisms underlying TAM-facilitated immune evasion and tumorigenesis and discuss the potential therapeutic application of TAM-focused drugs in the treatment of pediatric sarcomas.

Recent advancements in role of TAM receptors on efferocytosis, viral infection, autoimmunity, and tissue repair

Evolutionarily conserved highly regulated process of apoptosis has been a major physiological process throughout the entire evolutionary history of living beings that has impacted the process of evolution itself. One of the key features of this highly researched field of science is the process of phosphatidylserine (PS) externalization by the different membrane bound enzymes. The process is a result of series of biological events and is associated with various biological outcomes depending on the proper recognition of this ligand. In this review, we will briefly summarize the recent advancement in the field pertaining to the set of receptors, known as TAM (Tyro3, Axl and Mertk) receptors, for their influence in the recognition of various PS externalization events and mediation of pathological outcomes such as autoimmunity, cancer, and tissue repair.

Role of Vitamin K-Dependent Factors Protein S and GAS6 and TAM Receptors in SARS-CoV-2 Infection and COVID-19-Associated Immunothrombosis

The vitamin K-dependent factors protein S (PROS1) and growth-arrest-specific gene 6 (GAS6) and their tyrosine kinase receptors TYRO3, AXL, and MERTK, the TAM subfamily of receptor tyrosine kinases (RTK), are key regulators of inflammation and vascular response to damage. TAM signaling, which has largely studied in the immune system and in cancer, has been involved in coagulation-related pathologies. Because of these established biological functions, the GAS6-PROS1/TAM system is postulated to play an important role in SARS-CoV-2 infection and progression complications. The participation of the TAM system in vascular function and pathology has been previously reported. However, in the context of COVID-19, the role of TAMs could provide new clues in virus-host interplay with important consequences in the way that we understand this pathology. From the viral mimicry used by SARS-CoV-2 to infect cells, to the immunothrombosis that is associated with respiratory failure in COVID-19 patients, TAM signaling seems to be involved at different stages of the disease. TAM targeting is becoming an interesting biomedical strategy, which is useful for COVID-19 treatment now, but also for other viral and inflammatory diseases in the future.

Negative regulation of interleukin 1β expression in response to DnaK from Pseudomonas aeruginosa via the PI3K/PDK1/FoxO1 pathways

Interleukin (IL)-1β is crucial for a wide range of inflammatory responses. Previously, we reported that IL-1β is produced in response to Pseudomonas aeruginosa-derived DnaK via NF-κB and JNK pathways; however, the signaling pathways that counter the process to maintain IL-1β homeostasis are unknown. Here, we show that DnaK-mediated expression of IL1β is increased markedly in macrophages upon blockade of PI3K/PDK1. This was verified by measuring released IL-1β protein. The negative effect of PI3K on IL-1β production was dependent on suppression of both NF-κB and JNK activation. Intriguingly, PDK1 (an underlying mediator of PI3K) acted as an upstream regulator for the activation of NF-κB, but downregulated JNK activation. Furthermore, production of IL-1β and activation of JNK were triggered by inhibition of phosphorylated FoxO1; phosphorylation of FoxO1 was controlled by PDK1 signaling in response to DnaK. Thus, IL-1β production is modulated by P. aeruginosa-derived DnaK via cross-talk between JNK and PI3K/PDK1/FoxO1 pathways.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

Pancreatic islets engineered with a FasL protein induce systemic tolerance at the induction phase that evolves into long-term graft-localized immune privilege

We have previously shown that pancreatic islets engineered to transiently display a modified form of FasL protein (SA-FasL) on their surface survive indefinitely in allogeneic recipients without a need for chronic immunosuppression. Mechanisms that confer long-term protection to allograft are yet to be elucidated. We herein demonstrated that immune protection evolves in two distinct phases; induction and maintenance. SA-FasL-engineered allogeneic islets survived indefinitely and conferred protection to a second set of donor-matched, but not third-party, unmanipulated islet grafts simultaneously transplanted under the contralateral kidney capsule. Protection at the induction phase involved a reduction in the frequency of proliferating alloreactive T cells in the graft-draining lymph nodes, and required phagocytes and TGF-β. At the maintenance phase, immune protection evolved into graft site-restricted immune privilege as the destruction of long-surviving SA-FasL-islet grafts by streptozotocin followed by the transplantation of a second set of unmanipulated islet grafts into the same site from the donor, but not third party, resulted in indefinite survival. The induced immune privilege required both CD4⁺ CD25⁺ Foxp3⁺ Treg cells and persistent presence of donor antigens. Engineering cell and tissue surfaces with SA-FasL protein provides a practical, efficient, and safe means of localized immunomodulation with important implications for autoimmunity and transplantation.

Tumor microenvironmental influences on dendritic cell and T cell function: A focus on clinically relevant immunologic and metabolic checkpoints

Cancer immunotherapy is fast becoming one of the most promising means of treating malignant disease. Cancer vaccines, adoptive cell transfer therapies, and immune checkpoint blockade have all shown varying levels of success in the clinical management of several cancer types in recent years. However, despite the clinical benefits often achieved by these regimens, an ongoing problem for many patients is the inherent or acquired resistance of their cancer to immunotherapy. It is now appreciated that dendritic cells and T lymphocytes both play key roles in antitumor immune responses and that the tumor microenvironment presents a number of barriers to the function of these cells that can ultimately limit the success of immunotherapy. In particular, the engagement of several immunologic and metabolic checkpoints within the hostile tumor microenvironment can severely compromise the antitumor functions of these important immune populations. This review highlights work from both preclinical and clinical studies that has shaped our understanding of the tumor microenvironment and its influence on dendritic cell and T cell function. It focuses on clinically relevant targeted and immunotherapeutic strategies that have emerged from these studies in an effort to prevent or overcome immune subversion within the tumor microenvironment. Emphasis is also placed on the potential of next-generation combinatorial regimens that target metabolic and immunologic impediments to dendritic cell and T lymphocyte function as strategies to improve antitumor immune reactivity and the clinical outcome of cancer immunotherapy going forward.

Funerals and Feasts: The Immunological Rites of Cell Death

The immune system functions as a vanguard against pathogens and toxins. While it is mostly considered to be activated on the basis of self versus non-self recognition, injury/infection and damage are unavoidably associated with cell death. Does cell death play a role in the regulation of the immune response? Cell death, for better or for worse, is an omnipresent process in all stages of life that are observed throughout most tissues in multicellular organisms. From development to homeostasis in adult organisms, cells commit to scheduled death, while cases of injury and infection result in unscheduled cell death. Novel understanding of the molecular mechanisms that govern cell death demonstrate that, in fact, a plethora of molecular processes participate in directed dying. Parallel to the molecular modalities directing cell death are machineries employed by the organism to respond to dying cells, including either eliciting an inflammatory or immunological response or altogether avoiding it. Disturbing the careful coupling of these two processes is often met with pathology - on one hand a failure to respond to cell death may contribute to the lack of proper immune response or defective development, and on the other hand exaggerated or aberrant response to cell death can trigger unregulated inflammation, autoimmunity, or fibrosis/scarring. Here we review the molecular mechanisms and associated effector responses that accompany some of the most well-known cell death modalities - with an emphasis on efferocytosis, a process by which the dead cell is recognized and engulfed. In doing so, we highlight the TAM (TYRO3, AXL, MERTK) family of receptor tyrosine kinases (RTKs) that functions dually in the recognition and engulfment of dead cells, and as an important negative regulator of inflammation.

Efferocytosis in health and disease

The clearance of apoptotic cells by professional and non-professional phagocytes - a process termed 'efferocytosis' - is essential for the maintenance of tissue homeostasis. Accordingly, defective efferocytosis underlies a growing list of chronic inflammatory diseases. Although much has been learnt about the mechanisms of apoptotic cell recognition and uptake, several key areas remain incompletely understood. This Review focuses on new discoveries related to how phagocytes process the metabolic cargo they receive during apoptotic cell uptake; the links between efferocytosis and the resolution of inflammation in health and disease; and the roles of efferocytosis in host defence. Understanding these aspects of efferocytosis sheds light on key physiological and pathophysiological processes and suggests novel therapeutic strategies for diseases driven by defective efferocytosis and impaired inflammation resolution.

LXR Suppresses Inflammatory Gene Expression and Neutrophil Migration through cis-Repression and Cholesterol Efflux

The activation of liver X receptor (LXR) promotes cholesterol efflux and repression of inflammatory genes with anti-atherogenic consequences. The mechanisms underlying the repressive activity of LXR are controversial and have been attributed to cholesterol efflux or to transrepression of activator protein-1 (AP-1) activity. Here, we find that cholesterol efflux contributes to LXR repression, while the direct repressive functions of LXR also play a key role but are independent of AP-1. We use assay for transposase-accessible chromatin using sequencing (ATAC-seq) to show that LXR reduces chromatin accessibility in cis at inflammatory gene enhancers containing LXR binding sites. Targets of this repressive activity are associated with leukocyte adhesion and neutrophil migration, and LXR agonist treatment suppresses neutrophil recruitment in a mouse model of sterile peritonitis. These studies suggest a model of repression in which liganded LXR binds in cis to canonical nuclear receptor binding sites and represses pro-atherogenic leukocyte functions in tandem with the induction of LXR targets mediating cholesterol efflux.

Thomas et al. show the roles of cholesterol efflux and direct repression in anti-inflammatory effects of LXR and establish the mechanism of LXR cis-repression using ATAC-seq. LXR agonists suppress neutrophil migration genes and neutrophil recruitment during inflammation, highlighting a potential role for these compounds in the control of neutrophil-predominant inflammatory conditions.

Angiotensin II deteriorates advanced atherosclerosis by promoting MerTK cleavage and impairing efferocytosis through the AT1R/ROS/p38 MAPK/ADAM17 pathway

Vulnerable plaques in advanced atherosclerosis have defective efferocytosis. The role of ANG II in the progression of atherosclerosis is not fully understood. Herein, we investigated the effects and the underlying mechanisms of ANG II on macrophage efferocytosis in advanced atherosclerosis. ANG II decreased the surface expression of Mer tyrosine kinase (MerTK) in macrophages through a disintegrin and metalloproteinase17 (ADAM17)-mediated shedding of the soluble form of MerTK (sMer) in the medium, which led to efferocytosis suppression. ANG II-activated ADAM17 required reactive oxygen species (ROS) and p38 MAPK phosphorylation. Selective angiotensin II type 1 receptor (AT1R) blocker losartan suppressed ROS production, and ROS scavenger N-acetyl-l-cysteine (NAC) prevented p38 MAPK phosphorylation. In addition, mutant MERTKΔ483-488was resistant to ANG II-induced MerTK shedding and efferocytosis suppression. The advanced atherosclerosis model that is characterized by larger necrotic cores, and less collagen content was established by feeding apolipoprotein E knockout (ApoE−/−) mice with a high-fat diet for 16 wk. NAC and losartan oral administration prevented atherosclerotic lesion progression. Meanwhile, the inefficient efferocytosis represented by decreased macrophage-associated apoptotic cells and decreased MerTK+CD68+double-positive macrophages in advanced atherosclerosis were prevented by losartan and NAC. Additionally, the serum levels of sMer were increased and positively correlated with the upregulated levels of ANG II in acute coronary syndrome (ACS) patients. In conclusion, ANG II promotes MerTK shedding via AT1R/ROS/p38 MAPK/ADAM17 pathway in macrophages, which led to defective efferocytosis and atherosclerosis progression. Defining the molecular mechanisms of defective efferocytosis may provide a promising prognosis and therapy for ACS patients.

Triple Therapy with MerTK and PD1 Inhibition Plus Radiotherapy Promotes Abscopal Antitumor Immune Responses

PURPOSE

Radiotherapy (RT) traditionally has been used for local tumor control in the treatment of cancer. The recent discovery that radiotherapy can have anticancer effects on the immune system has led to recognition of its ability to sensitize the tumor microenvironment to immunotherapy. However, radiation can also prompt adverse immunosuppressive effects that block aspects of systemic response at other tumor sites. Our hypothesis was that inhibition of the MER proto-oncogene tyrosine kinase (MerTK) in combination with anti-programmed cell death-1 (α-PD1) checkpoint blockade will enhance immune-mediated responses to radiotherapy.

EXPERIMENTAL DESIGN

We tested the efficacy of this triple therapy (Radiation + α-PD1 + α-MerTK mAbs) in 129Sv/Ev mice with bilateral lung adenocarcinoma xenografts. Primary tumors were treated with stereotactic radiotherapy (36 Gy in 3 12-Gy fractions), and tumors were monitored for response.

RESULTS

The triple therapy significantly delayed abscopal tumor growth, improved survival rates, and reduced numbers of lung metastases. We further found that the triple therapy increased the activated CD8⁺ and NK cells populations measured by granzyme B expression with upregulation of CD8⁺CD103⁺ tissue-resident memory cells (T_RM) within the abscopal tumor microenvironment relative to radiation only.

CONCLUSIONS

The addition of α-PD1 + α-MerTK mAbs to radiotherapy could alter the cell death to be more immunogenic and generate adaptive immune response via increasing the retention of T_RM cells in the tumor islets of the abscopal tumors which was proven to play a major role in survival of non-small cell lung cancer patients.

Glycan-Modified Apoptotic Melanoma-Derived Extracellular Vesicles as Antigen Source for Anti-Tumor Vaccination

Tumors that lack T cell infiltration are less likely to respond to immune checkpoint inhibition and could benefit from cancer vaccination for the initiation of anti-tumor T cell responses. An attractive vaccine strategy is in vivo targeting of dendritic cells (DCs), key initiators of antigen-specific T cell responses. In this study we generated tumor-derived apoptotic extracellular vesicles (ApoEVs), which are potentially an abundant source of tumor-specific neo-antigens and other tumor-associated antigens (TAAs), and which can be manipulated to express DC-targeting ligands for efficient antigen delivery. Our data demonstrates that by specifically modifying the glycocalyx of tumor cells, high-mannose glycans can be expressed on their cell surface and on extracellular vesicles derived after the induction of apoptosis. High-mannose glycans are the natural ligands of dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), a dendritic cell associated C-type lectin receptor (CLR), which has the ability to efficiently internalize its cargo and direct it to both major histocompatibility complex (MHC)-I and MHC-II pathways for the induction of CD8⁺ and CD4⁺ T cell responses, respectively. Compared to unmodified ApoEVs, ApoEVs carrying DC-SIGN ligands are internalized to a higher extent, resulting in enhanced priming of tumor-specific CD8⁺ T cells. This approach thus presents a promising vaccination strategy in support of T cell-based immunotherapy of cancer.

Phosphatidylserine receptor-targeting therapies for the treatment of cancer

Asymmetric distribution of phospholipids across the plasma membrane is a unique characteristic of eukaryotic cells. Phosphatidylcholine and sphingomyelin are exposed in the outer leaflet, and phosphatidylserine (PS) is predominantly located in the inner leaflet. Redistribution of PS to the cell surface can be observed in several physiological conditions, such as apoptosis and platelet activation, or in pathological conditions, such as the release of microvesicles/exosomes from tumor tissues. PS binding to the phosphatidylserine receptor (PSR) on immune cells initiates immunosuppressive pathways that can lead to immune evasion by cancer cells. Conversely, PSR activation of cancer cells plays an important role in their survival, proliferation and metastasis. Herein, we briefly summarize both recent advances in our understanding of the pathological roles of PS and its receptor in cancer biology, as well as relevant pharmacological approaches.

Silica Particles Mediate Phenotypic and Functional Alteration of Dendritic Cells and Induce Th2 Cell Polarization

During silicosis, immune cells, including macrophages, T cells, B cells, and NK cells, participate in fibrosis development through alteration of the immune status. Dendritic cells (DCs) are professional antigen-presenting cells (APCs) with a key role in initiating immune responses and sustaining immune tolerance to maintain homeostasis. The relative contribution of DCs to silicosis progression is not well-documented. In the current study, we investigated the phenotypic and functional alterations of peripheral blood mononuclear cell (PBMC)-derived DCs of Sprague-Dawley (SD) rat during immune responses to silica exposure. We established models for direct and indirect exposure of DCs to silica by either treating DCs with silica or coculturing them with alveolar macrophages (AMs) treated with silica, respectively. The functional activity of DCs was analyzed by measuring their expression of costimulatory molecules, fluorescent microparticle uptake, cytokine production, and ability to mediate T cell polarization in vitro. In vivo, we demonstrated that silica could induce DC migration in response to silica exposure. Our results show that cytokine production by DCs was increased in response to direct silica direct exposure, while indirect silica exposure led to reduced cytokine levels. Moreover, the phagocytic capacity of DCs increased in cocultures after silica exposure. Gene and protein expression analyses showed that silica exposure altered the expression levels of Toll-like receptor pathway proteins and inflammatory factors. DC surface expression of the costimulatory molecules, CD80, CD86, and major histocompatibility complex, was inhibited by exposure to silica, which mediated a Th2-polarizing response in vitro. In rats, silica exposure induced migration of DCs from the peripheral blood into the alveoli. These results demonstrate that direct and indirect exposure to silica particles alter the phenotype and function of DCs, thereby regulating immune responses. Such changes may contribute to the development of silicosis by altering DC phenotype, function, and migration and by influencing the balance between Th1 and Th2 cells.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

Antibody Cross-Linking of CD14 Activates MerTK and Promotes Human Macrophage Clearance of Apoptotic Neutrophils: the Dual Role of CD14 at the Crossroads Between M1 and M2c Polarization

Mer receptor tyrosine kinase (MerTK) is key for efficient phagocytosis of apoptotic neutrophils (ANs) and homeostasis of IL-10 production by human anti-inflammatory M2c monocytes/macrophages. We asked whether stimulation of M2c surface receptors contributes in turn to MerTK activation. For this purpose, human monocytes/macrophages were differentiated under M1, M2a, and M2c polarizing conditions. The effects of antibody-mediated cross-linking of M2c receptors (i.e., CD14, CD16, CD32, CD163, CD204) on MerTK phosphorylation and phagocytosis of ANs were tested. MerTK expression was also studied by flow cytometry and western blot in the presence of LPS and in M2c-derived microvesicles (MVs). Antibody cross-linking of either CD14 or CD32/FcγRII led to Syk activation and MerTK phosphorylation in its two distinct glycoforms (175-205 and 135-155 kDa). Cross-linked CD14 enhanced efferocytosis by M2c macrophages and enabled M1 and M2a cells to clear ANs efficiently. In M1 conditions, LPS abolished surface MerTK expression on CD14^bright cell subsets, so disrupting the anti-inflammatory pathway. In M2c cells, instead, MerTK was diffusely and brightly co-expressed with CD14, and was also detected in M2c macrophage-derived MVs; in these conditions, LPS only partially downregulated MerTK on cell surfaces, while the smaller MerTK glycoform contained in MVs remained intact. Altogether, cooperation between CD14 and MerTK may foster the clearance of ANs by human monocytes/macrophages. CD14 stands between M1-related LPS co-receptor activity and M2c-related MerTK-dependent response. MerTK interaction with CD32/FcγRII, its detection in M2c MVs, and the differential localization and LPS susceptibility of MerTK glycoforms add further new elements to the complexity of the MerTK network.

Dynamics of Axl Receptor Shedding in Hepatocellular Carcinoma and Its Implication for Theranostics

Signaling of the receptor tyrosine kinase Axl and its ligand Gas6 is crucially involved in the development of liver fibrosis and hepatocellular carcinoma (HCC) by activation of hepatic stellate cells and modulation of hepatocyte differentiation. Shedding of Axl’s ectodomain leads to the release of soluble Axl (sAxl), which is increased in advanced fibrosis and in early-to-late stage HCC in the presence and absence of cirrhosis. Here, we focus on the dynamics of Axl receptor shedding and delineate possible scenarios how Axl signaling might act as driver of fibrosis progression and HCC development. Based on experimental and clinical data, we discuss the consequences of modifying Axl signaling by sAxl cleavage, as well as cellular strategies to escape from antagonizing effects of Axl shedding by the involvement of the hepatic microenvironment. We emphasize a correlation between free Gas6 and free sAxl levels favoring abundant Gas6/Axl signaling in advanced fibrosis and HCC. The raised scenario provides a solid basis for theranostics allowing the use of sAxl as an accurate diagnostic biomarker of liver cirrhosis and HCC, as well as Axl receptor signaling for therapeutic intervention in stratified HCC patients.

MerTK signaling in macrophages promotes the synthesis of inflammation resolution mediators by suppressing CaMKII activity

Inflammation resolution counterbalances excessive inflammation and restores tissue homeostasis after injury. Failure of resolution contributes to the pathology of numerous chronic inflammatory diseases. Resolution is mediated by endogenous specialized proresolving mediators (SPMs), which are derived from long-chain fatty acids by lipoxygenase (LOX) enzymes. 5-LOX plays a critical role in the biosynthesis of two classes of SPMs: lipoxins and resolvins. Cytoplasmic localization of the nonphosphorylated form of 5-LOX is essential for SPM biosynthesis, whereas nuclear localization of phosphorylated 5-LOX promotes proinflammatory leukotriene production. We previously showed that MerTK, an efferocytosis receptor on macrophages, promotes SPM biosynthesis by increasing the abundance of nonphosphorylated, cytoplasmic 5-LOX. We now show that activation of MerTK in human macrophages led to ERK-mediated expression of the gene encoding sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2), which decreased the cytosolic Ca2+ concentration and suppressed the activity of calcium/calmodulin-dependent protein kinase II (CaMKII). This, in turn, reduced the activities of the mitogen-activated protein kinase (MAPK) p38 and the kinase MK2, resulting in the increased abundance of the nonphosphorylated, cytoplasmic form of 5-LOX and enhanced SPM biosynthesis. In a zymosan-induced peritonitis model, an inflammatory setting in which macrophage MerTK activation promotes resolution, inhibition of ERK activation delayed resolution, which was characterized by an increased number of neutrophils and decreased amounts of SPMs in tissue exudates. These findings contribute to our understanding of how MerTK signaling induces 5-LOX-derived SPM biosynthesis and suggest a therapeutic strategy to boost inflammation resolution in settings where defective resolution promotes disease progression.