Increased soluble phagocytic receptors sMer, sTyro3 and sAxl and reduced phagocytosis in juvenile-onset systemic lupus erythematosus

Role and Mechanisms of Tyro3 in Podocyte Biology and Glomerular Disease

Background

Podocyte loss occurs in both primary and secondary glomerular diseases, leading to the progression of kidney disease. A large body of evidence suggests that apoptosis and detachment are the mechanisms mediating the reduction in podocyte numbers in glomerular diseases. Recent studies demonstrate a renal protective effect of protein S (PS) through the activation of Tyro3, one of the TAM receptors. Tyro3 is predominantly expressed in podocytes within the kidney, and its expression increases in early diabetic kidney disease (DKD) but decreases in patients with progressive DKD and focal segmental glomerulosclerosis (FSGS). Glomerular expression of Tyro3 also correlates with the progression of DKD and predicts the progression of primary glomerular diseases. High glucose increases Tyro3 expression, while TNF-α suppresses the expression of PS and Tyro3. PS has anti-inflammatory and antiapoptotic effects in podocytes, likely via the activation of the Akt pathway and the inhibition of NF-kB activation. In vivo, the knockout of PS or Tyro3 exacerbates podocyte loss and glomerular disease, while the overexpression of PS and Tyro3 attenuates the injury in mice with DKD and FSGS. Tyro3 agonists have also been shown to protect podocytes from injury in these animal models.

Summary

Tyro3 plays a critical role in podocyte biology and glomerular disease. Tyro3 agonists could potentially be developed as a new therapy for glomerular disease.

Key Message

The aim of this review article was to summarize the role and mechanisms mediating the protective effects of Tyro3 in podocyte biology and glomerular disease. Additionally, we discuss the possibility of developing Tyro3 agonists as potential treatment for glomerular diseases.

Immunological role of Gas6/TAM signaling in hemostasis and thrombosis

The immune system is an emerging regulator of hemostasis and thrombosis. The concept of immunothrombosis redefines the relationship between coagulation and immunomodulation, and the Gas6/Tyro3-Axl-MerTK (TAM) signaling pathway builds the bridge across them. During coagulation, Gas6/TAM signaling pathway not only activates platelets, but also promotes thrombosis through endothelial cells and vascular smooth muscle cells involved in inflammatory responses. Thrombosis appears to be a common result of a Gas6/TAM signaling pathway-mediated immune dysregulation. TAM TK and its ligands have been found to be involved in coagulation through the PI3K/AKT or JAK/STAT pathway in various systemic diseases, providing new perspectives in the understanding of immunothrombosis. Gas6/TAM signaling pathway serves as a breakthrough target for novel therapeutic strategies to improve disease management. Many preclinical and clinical studies of TAM receptor inhibitors are in process, confirming the pivotal role of Gas6/TAM signaling pathway in immunothrombosis. Therapeutics targeting the TAM receptor show potential both in anticoagulation management and immunotherapy. Here, we review the immunological functions of the Gas6/TAM signaling pathway in coagulation and its multiple mechanisms in diseases identified to date, and discuss the new clinical strategies that may generated by these roles.

Efferocytosis by macrophages in physiological and pathological conditions: regulatory pathways and molecular mechanisms

Efferocytosis is defined as the highly effective phagocytic removal of apoptotic cells (ACs) by professional or non-professional phagocytes. Tissue-resident professional phagocytes ("efferocytes"), such as macrophages, have high phagocytic capacity and are crucial to resolve inflammation and aid in homeostasis. Recently, numerous exciting discoveries have revealed divergent (and even diametrically opposite) findings regarding metabolic immune reprogramming associated with efferocytosis by macrophages. In this review, we highlight the key metabolites involved in the three phases of efferocytosis and immune reprogramming of macrophages under physiological and pathological conditions. The next decade is expected to yield further breakthroughs in the regulatory pathways and molecular mechanisms connecting immunological outcomes to metabolic cues as well as avenues for "personalized" therapeutic intervention.

Therapeutic targeting of the functionally elusive TAM receptor family

The TAM receptor family of TYRO3, AXL and MERTK regulates tissue and immune homeostasis. Aberrant TAM receptor signalling has been linked to a range of diseases, including cancer, fibrosis and viral infections. Specifically, the dysregulation of TAM receptors can enhance tumour growth and metastasis due to their involvement in multiple oncogenic pathways. For example, TAM receptors have been implicated in the epithelial-mesenchymal transition, maintaining the stem cell phenotype, immune modulation, proliferation, angiogenesis and resistance to conventional and targeted therapies. Therapeutically, multiple TAM receptor inhibitors are in preclinical and clinical development for cancers and other indications, with those targeting AXL being the most clinically advanced. Although there has been notable clinical advancement in recent years, challenges persist. This Review aims to provide both biological and clinical insights into the current therapeutic landscape of TAM receptor inhibitors, and evaluates their potential for the treatment of cancer and non-malignant diseases.

A variety of death modes of neutrophils and their role in the etiology of autoimmune diseases

Neutrophils are important in the context of innate immunity and actively contribute to the progression of diverse autoimmune disorders. Distinct death mechanisms of neutrophils may exhibit specific and pivotal roles in autoimmune diseases and disease pathogenesis through the orchestration of immune homeostasis, the facilitation of autoantibody production, the induction of tissue and organ damage, and the incitement of pathological alterations. In recent years, more studies have provided in-depth examination of various neutrophil death modes, revealing nuances that challenge conventional understanding and underscoring their potential clinical utility in diagnosis and treatment. This review explores the multifaceted processes and characteristics of neutrophil death, with a focus on tailored investigations within various autoimmune diseases. It also highlights the potential interplay between neutrophil death and the landscape of autoimmune disorders. The review encapsulates the pertinent pathways implicated in various neutrophil death mechanisms across diverse autoimmune diseases while also charts possible avenues for future research.

Inhibition of Mertk Signaling Enhances Bone Healing after Tooth Extraction

Regeneration of alveolar bone is an essential step in restoring healthy function following tooth extraction. Growth of new bone in the healing extraction socket can be variable and often unpredictable when systemic comorbidities are present, leading to the need for additional therapeutic targets to accelerate the regenerative process. One such target is the TAM family (Tyro3, Axl, Mertk) of receptor tyrosine kinases. These proteins have been shown to help resolve inflammation and maintain bone homeostasis and thus may have therapeutic benefits in bone regeneration following extraction. Treatment of mice with a pan-TAM inhibitor (RXDX-106) led to accelerated alveolar bone fill following first molar extraction in a mouse model without changing immune infiltrate. Treatment of human alveolar bone mesenchymal stem cells with RXDX-106 upregulated Wnt signaling and primed the cells for osteogenic differentiation. Differentiation of human alveolar bone mesenchymal stem cells with osteogenic media and TAM-targeted inhibitor RXDX-106 (pan-TAM), ASP-2215 (Axl specific), or MRX-2843 (Mertk specific) showed enhanced mineralization with pan-TAM or Mertk-specific inhibitors and no change with Axl-specific inhibitor. First molar extractions in Mertk-/- mice had increased alveolar bone regeneration in the extraction socket relative to wild type controls 7 d postextraction. Flow cytometry of 7-d extraction sockets showed no difference in immune cell numbers between Mertk-/- and wild type mice. RNAseq of day 7 extraction sockets showed increased innate immune-related pathways and genes associated with bone differentiation in Mertk-/- mice. Together, these results indicate that TAM receptor signaling, specifically through Mertk, can be targeted to enhance bone regeneration after injury.

Pediatric systemic lupus erythematosus: phagocytic defect and oxidase activity of neutrophils

BACKGROUND

There are limited data on neutrophil function in pediatric-onset systemic lupus erythematosus (pSLE) patients. This study aimed to evaluate phagocytosis and oxidase activity of neutrophils in patients with pSLE.

PATIENTS AND METHODS

Eighty-seven patients with pSLE and 44 controls were enrolled. Phagocytic activity was assayed using pHrodoTMRed E. coli BioParticles Phagocytosis Kit by flow cytometry. Determination of NADPH oxidase activity was carried out by Dihyrdrorhodamine-123 (DHR-123) flow cytometry assay.

RESULTS

Phagocytic activity of patients' neutrophils (mean 76.59%) was lower than that in controls (91.30%) (p < 0.001). Median delta median fluorescence intensity (ΔMFI) and stimulation index (SI) in patients (ΔMFI: 0.09; SI: 2.79) were also decreased compared to controls (ΔMFI: 0.18; SI: 5.00) (p < 0.002; p < 0.001 respectively). Disease activity showed an inverse correlation with phagocytic activity. Oxidase activity was also significantly low (SI DHR < 40) in 16% of patients. No significant correlation was found between oxidative burst and disease activity.

CONCLUSION

Neutrophil function is impaired in patients with pSLE, as evidenced by the markedly reduced phagocytic activity. Phagocytic activity is also inversely correlated with disease activity. The oxidative activity was also reduced but not significantly.

IMPACT

Neutrophil phagocytic function is impaired in pediatric-onset systemic lupus erythematosus (pSLE). There is an inverse correlation between disease activity in pSLE and phagocytic activity. NADPH oxidase activity in patients with pSLE did not show significant correlation with disease activity.

Dynamics of phagocytosis mediated by phosphatidylserine

Phagocytosis triggered by the phospholipid phosphatidylserine (PS) is key for the removal of apoptotic cells in development, tissue homeostasis and infection. Modulation of PS-mediated phagocytosis is an attractive target for therapeutic intervention in the context of atherosclerosis, neurodegenerative disease, and cancer. Whereas the mechanisms of target recognition, lipid and protein signalling, and cytoskeletal remodelling in opsonin-driven modes of phagocytosis are increasingly well understood, PS-mediated phagocytosis has remained more elusive. This is partially due to the involvement of a multitude of receptors with at least some redundancy in functioning, which complicates dissecting their contributions and results in complex downstream signalling networks. This review focusses on the receptors involved in PS-recognition, the signalling cascades that connect receptors to cytoskeletal remodelling required for phagocytosis, and recent progress in our understanding of how phagocytic cup formation is coordinated during PS-mediated phagocytosis.

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.

Two-Front War on Cancer-Targeting TAM Receptors in Solid Tumour Therapy

Simple Summary

In recent years, many studies have shown the importance of TAM kinases in both normal and neoplastic cells. In this review, we present and discuss the role of the TAM family (AXL, MERTK, TYRO3) of receptor tyrosine kinases (RTKs) as a dual target in cancer, due to their intrinsic roles in tumour cell survival, migration, chemoresistance, and their immunosuppressive roles in the tumour microenvironment. This review presents the potential of TAMs as emerging therapeutic targets in cancer treatment, focusing on the distinct structures of TAM receptor tyrosine kinases. We analyse and compare different strategies of TAM inhibition, for a full perspective of current and future battlefields in the war with cancer.

Abstract

Receptor tyrosine kinases (RTKs) are transmembrane receptors that bind growth factors and cytokines and contain a regulated kinase activity within their cytoplasmic domain. RTKs play an important role in signal transduction in both normal and malignant cells, and their encoding genes belong to the most frequently affected genes in cancer cells. The TAM family proteins (TYRO3, AXL, and MERTK) are involved in diverse biological processes: immune regulation, clearance of apoptotic cells, platelet aggregation, cell proliferation, survival, and migration. Recent studies show that TAMs share overlapping functions in tumorigenesis and suppression of antitumour immunity. MERTK and AXL operate in innate immune cells to suppress inflammatory responses and promote an immunosuppressive tumour microenvironment, while AXL expression correlates with epithelial-to-mesenchymal transition, metastasis, and motility in tumours. Therefore, TAM RTKs represent a dual target in cancer due to their intrinsic roles in tumour cell survival, migration, chemoresistance, and their immunosuppressive roles in the tumour microenvironment (TME). In this review, we discuss the potential of TAMs as emerging therapeutic targets in cancer treatment. We critically assess and compare current approaches to target TAM RTKs in solid tumours and the development of new inhibitors for both extra- and intracellular domains of TAM receptor kinases.

Defective Efferocytosis in a Murine Model of Sjögren's Syndrome Is Mediated by Dysfunctional Mer Tyrosine Kinase Receptor

Sjögren's syndrome (SjS) is a chronic autoimmune disease primarily involving the exocrine glands in which the involvement of the innate immune system is largely uncharacterized. Mer signaling has been found to be protective in several autoimmune diseases but remains unstudied in SjS. Here, we investigated the role of Mer signaling in SjS. Mer knockout (MerKO) mice were examined for SjS disease criteria. SjS-susceptible (SjS^S) C57BL/6.NOD-Aec1Aec2 mice were assessed for defective Mer signaling outcomes, soluble Mer (sMer) levels, A disintegrin and metalloprotease 17 (ADAM17) activity, and Rac1 activation. In addition, SjS patient plasma samples were evaluated for sMer levels via ELISA, and sMer levels were correlated to disease manifestations. MerKO mice developed submandibular gland (SMG) lymphocytic infiltrates, SMG apoptotic cells, anti-nuclear autoantibodies (ANA), and reduced saliva flow. Mer signaling outcomes were observed to be diminished in SjS^S mice, as evidenced by reduced Rac1 activation in SjS^S mice macrophages in response to apoptotic cells and impaired efferocytosis. Increased sMer was also detected in SjS^S mouse sera, coinciding with higher ADAM17 activity, the enzyme responsible for cleavage and inactivation of Mer. sMer levels were elevated in patient plasma and positively correlated with focus scores, ocular staining scores, rheumatoid factors, and anti-Ro60 levels. Our data indicate that Mer plays a protective role in SjS, similar to other autoimmune diseases. Furthermore, we suggest a series of events where enhanced ADAM17 activity increases Mer inactivation and depresses Mer signaling, thus removing protection against the loss of self-tolerance and the onset of autoimmune disease in SjS^S mice.

Axl is related to inflammation in hemodialysis patients

BACKGROUND

Hemodialysis (HD) patients often develop chronic inflammation, which is associated with an increased risk of cardiovascular complications and mortality. Axl and its ligand, growth arrest 6 (Gas6), have been reported to play key roles in regulating the immune response. However, the function of Axl in HD patients has not been clarified.

METHODS

In the present study, we enrolled 130 HD patients and 117 normal controls (NCs) and evaluated the levels of inflammatory markers, soluble Axl (sAxl), membrane Axl (mAxl), and Gas6 in all participants. The potential downstream cascades of Gas6-Axl signaling in HD patients were identified by quantitative real time polymerase chain reaction and western blotting.

RESULTS

The levels of inflammatory cytokines-tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ)-plasma sAxl, and Gas6, were significantly increased in HD patients compared to NCs. Additionally, sAxl was positively associated with the inflammatory factor, interleukin-6 (IL-6), in HD patients. Moreover, we found that mAxl in CD14⁺ mononuclear cells and CD19⁺ B cells was increased upon HD. Furthermore, we discovered that the metalloproteinase ADAM17, also called TACE, contributed to the cleavage of mAxl into sAxl, and not ADAM10, in the peripheral blood mononuclear cells (PBMCs) of HD patients. The upregulation of Gas6-mAxl signaling caused the activation of the STAT1-SOCS3 pathway in the PBMCs of HD patients. After two years follow-up, patients with lower sAxl levels had longer survival time than those with higher sAxl levels.

CONCLUSION

Our results suggest that Axl may play a significant role in systemic inflammation in HD patients.

Anti-Tyro3 IgG Associates with Disease Activity and Reduces Efferocytosis of Macrophages in New-Onset Systemic Lupus Erythematosus

Background

Systemic lupus erythematosus (SLE) is a disease characterized by the production of a large number of autoantibodies. Defected phagocytosis of macrophage plays an important role in innate immunity in the pathogenesis of SLE. Tyro3 is a receptor responsible for the recognition of apoptotic cells during efferocytosis by macrophages. To investigate the role of Tyro3 receptor in macrophages' efferocytosis of apoptotic cells in SLE, we aimed to reveal the clinical relevance and impact of Tyro3 autoantibody on SLE.

Methods

The serum levels of IgG-type autoantibody against Tyro3 receptor were detected in new-onset, treatment-naïve SLE patients (n = 70), rheumatoid arthritis (RA) (n = 24), primary Sjögren's Syndrome (pSS) (n = 21), and healthy controls (HCs) (n = 70) using enzyme-linked immunosorbent assay (ELISA). The effects of purified Tyro3 autoantibody from SLE patients on the efferocytosis of human monocyte-derived macrophages were measured by flow cytometry and immunofluorescence.

Results

The serum levels of IgG-type autoantibody against Tyro3 receptor were significantly elevated in patients with SLE compared to RA, pSS, and HCs (all p < 0.0001). The levels of anti-Tyro3 IgG were positively associated with the SLE disease activity index (SLEDAI) score (r = 0.254, p = 0.034), erythrocyte sedimentation rate (ESR) (r = 0.430, p < 0.001), C-reactive protein (CRP) (r = 0.246, p = 0.049), and immunoglobulin G (IgG) (r = 0.408, p = 0.001) and negatively associated with haemoglobin (Hb) (r = -0.294, p = 0.014). ROC curves illustrated that the anti-Tyro3 antibody could differentiate patients with SLE from HCs. Furthermore, flow cytometry and immunofluorescence demonstrated that purified anti-Tyro3 IgG inhibited the efferocytosis of macrophages (p = 0.004 and 0.044, respectively) compared with unconjugated human IgG.

Conclusions

These observations indicated that autoantibody against Tyro3 was associated with disease activity and could impair efferocytosis of macrophages. It might be a potential novel disease biomarker and might be involved in the pathogenesis of SLE.

Contributions of Major Cell Populations to Sjögren's Syndrome

Sjögren’s syndrome (SS) is a female dominated autoimmune disease characterized by lymphocytic infiltration into salivary and lacrimal glands and subsequent exocrine glandular dysfunction. SS also may exhibit a broad array of extraglandular manifestations including an elevated incidence of non-Hodgkin’s B cell lymphoma. The etiology of SS remains poorly understood, yet progress has been made in identifying progressive stages of disease using preclinical mouse models. The roles played by immune cell subtypes within these stages of disease are becoming increasingly well understood, though significant gaps in knowledge still remain. There is evidence for distinct involvement from both innate and adaptive immune cells, where cells of the innate immune system establish a proinflammatory environment characterized by a type I interferon (IFN) signature that facilitates propagation of the disease by further activating T and B cell subsets to generate autoantibodies and participate in glandular destruction. This review will discuss the evidence for participation in disease pathogenesis by various classes of immune cells and glandular epithelial cells based upon data from both preclinical mouse models and human patients. Further examination of the contributions of glandular and immune cell subtypes to SS will be necessary to identify additional therapeutic targets that may lead to better management of the disease.

Defective Neutrophil Function in Patients with Sepsis Is Mostly Restored by ex vivo Ascorbate Incubation

Background

Neutrophil function is essential for effective defence against bacterial infections but is defective in patients with sepsis. Ascorbate or vitamin C, which is low in the plasma of patients with sepsis, is stored inside human neutrophils and is essential for their normal function.

Objective

This study aimed to determine if ascorbate treatment ex vivo improved neutrophil function in patients with sepsis.

Patients and Methods

Human blood neutrophils were isolated from 20 patients with sepsis and 20 healthy age-matched controls. Neutrophils were incubated with or without ascorbate (1, 5, 10, 20 and 40 mM) for periods up to 2h. Chemotaxis was evaluated using a chemotactic chamber in response to the chemoattractant, fMLP. Phagocytosis (uptake of pHrodo red stained S. aureus) and apoptosis (annexin-V/propidium iodide staining) were measured by flow cytometry. Neutrophil extracellular trap (NET) formation was detected and quantified using DAPI, anti-myeloperoxidase and anti-neutrophil elastase immuno-fluorescence staining. Quantifluor detected the amount of dsDNA in NET supernatants, while quantitative PCR identified changes in expression of PADI4 gene.

Results

Chemotactic and phagocytic activities were decreased in patients with sepsis but increased after treatment with the high concentrations of ascorbate. Apoptosis was increased in the sepsis patients but not altered by ascorbate treatment. Spontaneous NET formation was observed in patients with sepsis. A quantity of 1mM ascorbate decreased spontaneous NETosis to that of normal, healthy neutrophils, while high concentrations of ascorbate (>10mM) further promoted NET formation.

Conclusion

Dysregulated neutrophil function was observed in patients with sepsis which could contribute to disease pathology and outcomes. Exposure to ascorbate could reverse some of these changes in function. These novel discoveries raise the possibility that ascorbate treatment could be used as an adjunctive therapy that could result in improved neutrophil function during sepsis.

Juvenile-onset systemic lupus erythematosus: Update on clinical presentation, pathophysiology and treatment options

Juvenile-onset systemic lupus erythematosus (jSLE) accounts for up to 20% of all SLE patients. Key differences between juvenile- and adult-onset (aSLE) disease include higher disease activity, earlier development of damage, and increased use of immunosuppressive treatment in jSLE suggesting (at least partial) infectivity secondary to variable pathomechanisms. While the exact pathophysiology of jSLE remains unclear, genetic factors, immune complex deposition, complement activation, hormonal factors and immune cell dysregulation are involved to variable extents, promising future patient stratification based on immune phenotypes. Though less effective and potentially toxic, jSLE patients are treated based upon evidence from studies in aSLE cohorts. Here, age-specific clinical features of jSLE, underlying pathomechanisms, treatment options and disease outcomes will be addressed. Future directions to improve the care of jSLE patients, including implementation of the Single Hub and Access point for pediatric Rheumatology in Europe (SHARE) recommendations, biomarkers, treat to target and personalized medicine approaches are discussed.

Increased sMer, but not sAxl, sTyro3, and Gas6 relate with active disease in juvenile systemic lupus erythematosus

INTRODUCTION/OBJECTIVES

Tyro3, Axl, and Mer (TAM) receptors and ligands mediate apoptotic bodies engulfment which alteration has been related with juvenile systemic lupus erythematosus (JSLE) pathogenesis. Thus, the aim was to determine their soluble levels.

METHODS

Serum sTyro3, sAxl, sMer, and Gas6 levels were measured using ELISA in 67 JSLE patients, 12 juvenile idiopathic arthritis (JIA) inflammatory and 20 healthy controls and related with SLEDAI-2K score, anti-dsDNA antibody, ESR, CRP, C3, C4 levels, and nephritis.

RESULTS

JSLE patients with active disease (SLEDAI-2K> 4) had significantly increased sMer levels compared with healthy controls (median 8.4 vs. 6.0 ng/mL, p = 0.009) and inactive disease patients (5.2 ng/mL, p = 0.0003). sMer levels correlated with SLEDAI-2K (r = 0.44; p = 0.0004) and ESR (r = 0.24; p = 0.04), while sAxl correlated with SLEDAI-2K (r = 0.33; p = 0.008) and C4 levels (r = - 0.24; p = 0.04). JSLE patients taking glucocorticoid had increased sAxl and sMer levels. Moreover, sAxl correlated with sMer and sTyro3 levels. Patients with nephritis and those with focal or diffuse proliferative glomerulonephritis had these protein levels similar to healthy controls and patients without renal involvement. sTyro3 levels of JSLE patients taking glucocorticoid were decreased, and correlated with Gas6 and sAxl, while Gas6 levels correlated with age upon enrollment. JIA controls had protein levels similar to healthy controls and JSLE patients.

CONCLUSIONS

This study reinforces that sMer is increased in active JSLE patients, yet sMer and sAxl correlates with disease activity parameters, and their alterations are disease-specific. However, further studies are needed to determine exact roles of sTyro3 and Gas6 in disease pathogenesis. Key Points • sMer and sAxl serum levels are related with active disease in JSLE patients • sMer correlated with SLEDAI-2K score in JSLE • sTyro3, sAxl, sMer and Gas6 levels did not related with nephritis in JSLE patients • sTyro3 and Gas6 exact roles in JSLE are not established and further studies are needed.

Myeloid SOCS3 Deficiency Regulates Angiogenesis via Enhanced Apoptotic Endothelial Cell Engulfment

Mononuclear phagocytes, such as macrophages and microglia, are key regulators of organ homeostasis including vascularization processes. Here, we investigated the role of the suppressor of cytokine signaling 3 (SOCS3) in myeloid cells as a regulator of mononuclear phagocyte function and their interaction with endothelial cells in the context of sprouting angiogenesis. As compared to SOCS3-sufficient counterparts, SOCS3-deficient microglia and macrophages displayed an increased phagocytic activity toward primary apoptotic endothelial cells, which was associated with an enhanced expression of the opsonin growth arrest-specific 6 (Gas6), a major prophagocytic molecule. Furthermore, we found that myeloid SOCS3 deficiency significantly reduced angiogenesis in an ex vivo mouse aortic ring assay, which could be reversed by the inhibition of the Gas6 receptor Mer. Together, SOCS3 in myeloid cells regulates the Gas6/Mer-dependent phagocytosis of endothelial cells, and thereby angiogenesis-related processes. Our findings provide novel insights into the complex crosstalk between mononuclear phagocytes and endothelial cells, and may therefore provide a new platform for the development of new antiangiogenic therapies.

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.

Gas6/TAM Signaling Components as Novel Biomarkers of Liver Fibrosis

Liver fibrosis consists in the accumulation of extracellular matrix components mainly derived from activated hepatic stellate cells. This is commonly the result of chronic liver injury repair and represents an important health concern. As liver biopsy is burdened with many drawbacks, not surprisingly there is great interest to find new reliable noninvasive methods. Among the many are new potential fibrosis biomarkers under study, some of the most promising represented by the growth arrest-specific gene 6 (Gas6) serum protein and its family of tyrosine kinase receptors, namely, Tyro3, Axl, and MERTK (TAM). Gas6/TAM system (mainly, Axl and MERTK) has in fact recently emerged as an important player in the progression of liver fibrosis. This review is aimed at giving an overall perspective of the roles played by these molecules in major chronic liver diseases. The most promising findings up to date acknowledge that both Gas6 and its receptor serum levels (such as sAxl and, probably, sMERTK) have been shown to potentially allow for easy and accurate measurement of hepatic fibrosis progression, also providing indicative parameters of hepatic dysfunction. Although most of the current scientific evidence is still preliminary and there are no in vivo validation studies on large patient series, it still looks very promising to imagine a possible future prognostic role for these biomarkers in the multidimensional assessment of a liver patient. One may also speculate on a potential role for this system targeting (e.g., with small molecule inhibitors against Axl) as a therapeutic strategy for liver fibrosis management, always bearing in mind that any such therapeutic approach might face toxicity.

Efferocytosis and autoimmune disease

An enormous number of cells in the body die by apoptosis during development and under homeostasis. Apoptotic cells are swiftly engulfed by macrophages and digested into units. This removal of apoptotic cells is called ‘efferocytosis’. For efferocytosis, macrophages recognize phosphatidylserine (PtdSer) exposed on the cell surface as an ‘eat me’ signal. In healthy cells, PtdSer is exclusively localized to the inner leaflet of the plasma membrane by the action of flippases. When cells undergo apoptosis, caspase cleaves flippases to inactivate them, while it cleaves pro-scramblases to active scramblases, which quickly translocate PtdSer to the cell surface. The PtdSer is then recognized by PtdSer-binding proteins or by PtdSer receptors on macrophages, which subsequently engulf the apoptotic cells. When efferocytosis fails, apoptotic cells can rupture, releasing cellular materials that can evoke an autoimmune response. Thus, a defect in the PtdSer-exposing or PtdSer-recognizing processes triggers autoimmunity, leading to a systemic lupus erythematosus-type autoimmune disease.

Gas6/TAM Receptors in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multiorgan autoimmune disease associated with impaired immune system regulation. The exact mechanisms of SLE development remain to be elucidated. TAM receptor tyrosine kinases (RTKs) are important for apoptotic cell clearance, immune homeostasis, and resolution of immune responses. TAM deficiency leads to lupus-like autoimmune diseases. Activation of TAM receptors leads to proteolytic cleavage of the receptors, generating soluble forms of TAM. Circulating TAM receptors have an immunoregulatory function and may also serve as biomarkers for disease prognosis. Here, we review the biological function and signaling of TAM RTKs in the development and pathogenesis of lupus and lupus nephritis. Targeting Gas6/TAM pathways may be of therapeutic benefit. A discussion of potential TAM activation and inhibition in the treatment of lupus and lupus nephritis is included.

Sjogren's Syndrome and TAM Receptors: A Possible Contribution to Disease Onset

Sjogren's syndrome (SS) is a chronic, progressive autoimmune disease featuring both organ-specific and systemic manifestations, the most frequent being dry mouth and dry eyes resulting from lymphocytic infiltration into the salivary and lacrimal glands. Like the related autoimmune disease systemic lupus erythematosus (SLE), SS patients and mouse models display accumulation of apoptotic cells and a Type I interferon (IFN) signature. Receptor tyrosine kinases (RTKs) of the Tyro3, Axl, and Mer (TAM) family are present on the surface of macrophages and dendritic cells and participate in phagocytosis of apoptotic cells (efferocytosis) and inhibition of Type I IFN signaling. This review examines the relationship between TAM receptor dysfunction and SS and explores the potential contributions of TAM defects on macrophages to SS development.

Innate immune-responses and their role in driving autoimmunity

Autoimmunity and autoimmune diseases were always considered to be driven mainly by adaptive immune responses, namely by auto-reactive B and T cell over-activity. The continuous stimulation of dendritic cells by autoantigens increases B cell activity, driving auto-reactive B cells to increase the production of autoantibodies and of pro-inflammatory cytokines. On the other hand, a subset of dendritic cells is established being of tolerogenic properties thus becoming important in maintaining self-tolerance. However, early innate immune responses are continuously appreciated to be highly important in the development of immune-mediated inflammation in general and autoimmunity in particular. The innate immune system is a complex network of structured cells/proteins such as antigen presenting cells (macrophages and dendritic cells), the complement cascade, and many receptors/cytokines/proteins. Of these, one may mention the high expression of toll-like receptors 7 and 9 in antigen presenting cells, and B cells of systemic lupus erythematosus patients contributing to the expansion of auto-reactive B cells. C-reactive protein (CRP) and C1q are crucially important for efficient uptake of apoptotic cells. However, CRP is appreciated to have a role in maintaining anti-inflammatory responses and in altering autoimmunity. Natural killer cells (NK) are responsible for cytotoxicity responses but some of them (mainly CD56high), are important in maintaining peripheral self-tolerance, thus considered to be immune-regulatory cells. In this review we will cover most of the new data on innate immune system and discuss its importance in the development of autoimmunity. New treatments were developed following our better understanding of these pathways, the targeting of which, opened new therapeutic avenues in treating autoimmune diseases.

The Dual Role of TAM Receptors in Autoimmune Diseases and Cancer: An Overview

Receptor tyrosine kinases (RTKs) regulate cellular processes by converting signals from the extracellular environment to the cytoplasm and nucleus. Tyro3, Axl, and Mer (TAM) receptors form an RTK family that plays an intricate role in tissue maintenance, phagocytosis, and inflammation as well as cell proliferation, survival, migration, and development. Defects in TAM signaling are associated with numerous autoimmune diseases and different types of cancers. Here, we review the structure of TAM receptors, their ligands, and their biological functions. We discuss the role of TAM receptors and soluble circulating TAM receptors in the autoimmune diseases systemic lupus erythematosus (SLE) and multiple sclerosis (MS). Lastly, we discuss the effect of TAM receptor deregulation in cancer and explore the therapeutic potential of TAM receptors in the treatment of diseases.

Protective Role of the MER Tyrosine Kinase via Efferocytosis in Rheumatoid Arthritis Models

Objective

Rheumatoid arthritis (RA) is a chronic and progressive joint disease. It appears that anti-inflammatory feedback mechanisms that could restrain joint inflammation and restore homeostasis are insufficient to perform this control. In this study, we investigated the contribution of the MER tyrosine kinase-mediated anti-inflammatory response on arthritis and whether targeting MER could be a valid approach to treat RA.

Methods

KRN serum transfer arthritis (KRN STA) was induced in either Mertk-deficient mice or in mice that adenovirally overexpressed Pros1. Human synovial micromasses were treated with MER-specific antibodies or PROS1. Collagen-induced arthritis (CIA) mice were treated with MER-specific agonistic antibodies or by viral overexpression of Pros1.

Results

Mertk^−/− mice showed exacerbated arthritis pathology, whereas Pros1 overexpression diminished joint pathology in KRN STA. Human synovial micromasses challenged with MER-specific antibodies enhanced the secretion of inflammatory cytokines, whereas stimulating MER with PROS1 reduced the secretion of these cytokines, confirming the protective role of MER. Next, we treated CIA mice with MER-specific agonistic antibodies, and this unexpectedly resulted in exacerbated arthritis pathology. This was associated with increased numbers of apoptotic cells in their knee joints and higher serum levels of interleukin (IL)-16C, a cytokine released by secondary necrotic neutrophils. Apoptotic cell numbers and IL-16C levels were enhanced during arthritis in Mertk^−/− mice and reduced in Pros1-overexpressing mice.

Conclusion

MER plays a protective role during joint inflammation and activating MER by its ligand PROS1 ameliorates disease. Treatment of mice with MER receptor agonistic antibodies is deleterious due to its counterproductive effect of blocking efferocytosis in the arthritic joint.

MicroRNA networks associated with active systemic juvenile idiopathic arthritis regulate CD163 expression and anti-inflammatory functions in macrophages through two distinct mechanisms

Systemic juvenile idiopathic arthritis (SJIA) is a severe childhood arthropathy with features of autoinflammation. Monocytes and macrophages in SJIA have a complex phenotype with both pro- and anti-inflammatory properties that combine features of several well characterized in vitro conditions used to activate macrophages. An important anti-inflammatory phenotype is expression of CD163, a scavenger receptor that sequesters toxic pro-inflammatory complexes that is highly expressed in both active SJIA and macrophage activation syndrome (MAS). CD163 is most strongly up-regulated by IL-10 (M(IL-10)), and not by other conditions that reflect features seen in SJIA monocytes such as M(LPS+IC). MicroRNA plays key roles in integrating cellular signals such as those in macrophage polarization, and as such we hypothesize microRNAs regulate macrophage functional responses in SJIA including CD163 expression. We find that 2 microRNAs previously found to be elevated in active SJIA, miR-125a-5p and miR-181c, significantly reduced macrophage CD163 expression through 2 distinct mechanisms. Neither microRNA was elevated in M(IL-10) with robust CD163 expression, but were instead induced in M(LPS+IC) where they restricted CD163 mRNA expression. Mir-181 species directly targeted CD163 mRNA for degradation. In contrast, miR-125a-5p functions indirectly, as transcriptome analysis of miR-125a-5p overexpression identified "cytokine-cytokine receptor interactions" as the most significantly repressed gene pathway, including decreased IL10RA, required for IL-10-mediated CD163 expression. Finally, overexpression of miR-181c inhibited CD163 anti-inflammatory responses to hemoglobin or high mobility group box 1 (HMGB1) complexes. Together, these data show that microRNA utilizes multiple mechanisms to integrate well-characterized polarization phenotypes and regulate macrophage functional properties seen in SJIA.

Gas6 as a putative noninvasive biomarker of hepatic fibrosis

AIM

To evaluate serum growth arrest-specific gene 6 (Gas6) concentration as a biomarker of liver fibrosis progression.

MATERIALS & METHODS

One hundred and thirteen consecutive patients affected by chronic liver disease underwent transient elastography, Gas6 measurement and, if clinically indicated, liver biopsy.

RESULTS

Gas6 concentration was directly correlated to liver stiffness (r = 0.67; p < 0.0001) and was significantly higher in patients with advanced fibrosis (Ishak 4-5; p < 0.001). A plasma concentration <30 ng/ml Gas6 ruled out fibrosis with 84% sensitivity and 56% specificity, while values >42 ng/ml identified severe fibrosis with a sensitivity of 64% and a specificity of 95%; the diagnostic accuracy was comparable to that of transient elastography.

CONCLUSION

Gas6 is a novel biomarker of liver fibrosis, with a potential clinical and pathophysiological relevance.

Molecular Pathways: Receptor Ectodomain Shedding in Treatment, Resistance, and Monitoring of Cancer

Proteases known as sheddases cleave the extracellular domains of their substrates from the cell surface. The A Disintegrin and Metalloproteinases ADAM10 and ADAM17 are among the most prominent sheddases, being widely expressed in many tissues, frequently overexpressed in cancer, and promiscuously cleaving diverse substrates. It is increasingly clear that the proteolytic shedding of transmembrane receptors impacts pathophysiology and drug response. Receptor substrates of sheddases include the cytokine receptors TNFR1 and IL6R; the Notch receptors; type-I and -III TGFβ receptors; receptor tyrosine kinases (RTK) such as HER2, HER4, and VEGFR2; and, in particular, MET and TAM-family RTKs AXL and Mer (MerTK). Activation of receptor shedding by mechanical cues, hypoxia, radiation, and phosphosignaling offers insight into mechanisms of drug resistance. This particularly holds for kinase inhibitors targeting BRAF (such as vemurafenib and dabrafenib) and MEK (such as trametinib and cobimetinib), along with direct sheddase inhibitors. Receptor proteolysis can be detected in patient fluids and is especially relevant in melanoma, glioblastoma, lung cancer, and triple-negative breast cancer where RTK substrates, MAPK signaling, and ADAMs are frequently dysregulated. Translatable strategies to exploit receptor shedding include combination kinase inhibitor regimens, recombinant decoy receptors based on endogenous counterparts, and, potentially, immunotherapy. Clin Cancer Res; 23(3); 623-9. ©2016 AACR.

Accurate Determination of Soluble Axl by Enzyme-Linked Immunosorbent Assay

Levels of soluble Axl (sAxl) are routinely assessed in human sera by sandwich enzyme-linked immunosorbent assay (ELISA). Although sAxl values are suggested to diagnose different types of disorders, no uniform ELISA method is available, allowing the reliable interassay comparison between results. Furthermore, little is known about the stability of sAxl under storage conditions, which is a relevant parameter for biomedical trials. The evaluation of sAxl stability under various stress conditions and the determination of proper conditions to use the sAxl ELISA for routine clinical applications are of great interest. In this study, serum samples were subjected to freeze-thaw cycles and incubation at different temperatures to analyze the stability of sAxl by ELISA. Dilution and spike-in experiments were carried out to examine the impact of serum and diluent components on the ELISA performance. Various diluents and media were employed to resolve masking effects of the serum. The assay components were further optimized for long-term usability by treatment with stabilizers and validation under temperature stress. Indeed, sAxl showed long-term stability in serum during freeze-thaw cycles and incubation under temperature stress conditions. The dilution experiments revealed that unknown components in the serum caused masking effects that can be reduced by proper dilutions. The assay performance was further increased by using a standardized buffer system to dilute serum samples. Stabilization of coated plates and of streptavidin-horseradish peroxidase allowed long-term storage for up to 6 months. In sum, our data demonstrate proper ELISA conditions, allowing the accurate analysis of sAxl levels in human serum.

The Gas6/TAM System and Multiple Sclerosis

Growth arrest specific 6 (Gas6) is a multimodular circulating protein, the biological actions of which are mediated by the interaction with three transmembrane tyrosine kinase receptors: Tyro3, Axl, and MerTK, collectively named TAM. Over the last few decades, many progresses have been done in the understanding of the biological activities of this highly pleiotropic system, which plays a role in the regulation of immune response, inflammation, coagulation, cell growth, and clearance of apoptotic bodies. Recent findings have further related Gas6 and TAM receptors to neuroinflammation in general and, specifically, to multiple sclerosis (MS). In this paper, we review the biology of the Gas6/TAM system and the current evidence supporting its potential role in the pathogenesis of MS.

The Role of TAM Family Receptors in Immune Cell Function: Implications for Cancer Therapy

The TAM receptor protein tyrosine kinases-Tyro3, Axl, and Mer-are essential regulators of immune homeostasis. Guided by their cognate ligands Growth arrest-specific gene 6 (Gas6) and Protein S (Pros1), these receptors ensure the resolution of inflammation by dampening the activation of innate cells as well as by restoring tissue function through promotion of tissue repair and clearance of apoptotic cells. Their central role as negative immune regulators is highlighted by the fact that deregulation of TAM signaling has been linked to the pathogenesis of autoimmune, inflammatory, and infectious diseases. Importantly, TAM receptors have also been associated with cancer development and progression. In a cancer setting, TAM receptors have a dual regulatory role, controlling the initiation and progression of tumor development and, at the same time, the associated anti-tumor responses of diverse immune cells. Thus, modulation of TAM receptors has emerged as a potential novel strategy for cancer treatment. In this review, we discuss our current understanding of how TAM receptors control immunity, with a particular focus on the regulation of anti-tumor responses and its implications for cancer immunotherapy.

Opposing Roles of Tyrosine Kinase Receptors Mer and Axl Determine Clinical Outcomes in Experimental Immune-Mediated Nephritis

Glomerulonephritis is one of the most severe manifestations of systemic lupus erythematosus, with considerable morbidity and mortality. There remains a major unmet need for successful management of lupus nephritis. TAM family receptor tyrosine kinases (Mer and Axl) play an important role in the maintenance of immune homeostasis in the kidney. Mer is constitutively expressed in the glomeruli; Axl expression is inducible in glomeruli under inflammatory conditions. To investigate the distinct functions of Axl and Mer in lupus nephritis, we compared the severity of nephrotoxic serum glomerulonephritis in wild-type (WT), Axl-knockout (KO), Mer-KO, and Axl/Mer-KO mice. Mer-KO mice developed severe glomerulonephritis, with significantly decreased survival and increased blood urea nitrogen levels compared with WT mice given the same treatment. However, nephrotoxic serum-treated Axl-KO mice had significantly increased survival rates and improved renal function compared with similarly treated WT, Mer-KO, and Axl/Mer-KO mice. Interestingly, mice lacking both Axl and Mer developed kidney inflammation comparable to WT mice. Western blot analysis revealed significantly increased Stat3 phosphorylation and caspase-1 activation in the kidneys of nephritic Mer-KO mice. In contrast, Axl-deficient nephrotoxic serum-injected mice showed decreased Akt phosphorylation and Bcl-xL upregulation. Thus, the reciprocal activation of Axl and Mer receptor tyrosine kinases has a major impact on the outcome of renal inflammation.

Defects in lysosomal maturation facilitate the activation of innate sensors in systemic lupus erythematosus

Defects in clearing apoptotic debris disrupt tissue and immunological homeostasis, leading to autoimmune and inflammatory diseases. Herein, we report that macrophages from lupus-prone MRL/lpr mice have impaired lysosomal maturation, resulting in heightened ROS production and attenuated lysosomal acidification. Impaired lysosomal maturation diminishes the ability of lysosomes to degrade apoptotic debris contained within IgG-immune complexes (IgG-ICs) and promotes recycling and the accumulation of nuclear self-antigens at the membrane 72 h after internalization. Diminished degradation of IgG-ICs prolongs the intracellular residency of nucleic acids, leading to the activation of Toll-like receptors. It also promotes phagosomal membrane permeabilization, allowing dsDNA and IgG to leak into the cytosol and activate AIM2 and TRIM21. Collectively, these events promote the accumulation of nuclear antigens and activate innate sensors that drive IFNα production and heightened cell death. These data identify a previously unidentified defect in lysosomal maturation that provides a mechanism for the chronic activation of intracellular innate sensors in systemic lupus erythematosus.

Axl and Mer Receptor Tyrosine Kinases: Distinct and Nonoverlapping Roles in Inflammation and Cancer?

The receptor tyrosine kinases Axl and Mer subserve the process of termination of proinflammatory signaling and have key roles in both the resolution of inflammation and restoration of homeostasis. Axl functions prominently under conditions of tissue stress or in response to infection, whereas Mer has a major role in maintenance of homeostasis within tissues. Distinct patterns of expression of Axl and Mer underlie their clearly defined functional roles during the initiation and progression of inflammation. Axl and Mer are expressed by tumor cells and by infiltrating inflammatory cells and the regulation of cellular function via Axl and Mer signaling is also important for tumorigenesis, tumor progression, and metastasis. In this review, we consider the divergent functions of Axl and Mer in the context of inflammatory processes within tumors and the implications for development of therapeutic agents targeting these receptors.