Elevation of Gas6 protein concentration in cerebrospinal fluid of patients with chronic inflammatory demyelinating polyneuropathy (CIDP)

COVID-19 Biomarkers at the Crossroad between Patient Stratification and Targeted Therapy: The Role of Validated and Proposed Parameters

Clinical knowledge about SARS-CoV-2 infection mechanisms and COVID-19 pathophysiology have enormously increased during the pandemic. Nevertheless, because of the great heterogeneity of disease manifestations, a precise patient stratification at admission is still difficult, thus rendering a rational allocation of limited medical resources as well as a tailored therapeutic approach challenging. To date, many hematologic biomarkers have been validated to support the early triage of SARS-CoV-2-positive patients and to monitor their disease progression. Among them, some indices have proven to be not only predictive parameters, but also direct or indirect pharmacological targets, thus allowing for a more tailored approach to single-patient symptoms, especially in those with severe progressive disease. While many blood test-derived parameters quickly entered routine clinical practice, other circulating biomarkers have been proposed by several researchers who have investigated their reliability in specific patient cohorts. Despite their usefulness in specific contexts as well as their potential interest as therapeutic targets, such experimental markers have not been implemented in routine clinical practice, mainly due to their higher costs and low availability in general hospital settings. This narrative review will present an overview of the most commonly adopted biomarkers in clinical practice and of the most promising ones emerging from specific population studies. Considering that each of the validated markers reflects a specific aspect of COVID-19 evolution, embedding new highly informative markers into routine clinical testing could help not only in early patient stratification, but also in guiding a timely and tailored method of therapeutic intervention.

Gas6/TAM Axis Involvement in Modulating Inflammation and Fibrosis in COVID-19 Patients

Gas6 (growth arrest-specific gene 6) is a widely expressed vitamin K-dependent protein that is involved in many biological processes such as homeostatic regulation, inflammation and repair/fibrotic processes. It is known that it is the main ligand of TAMs, a tyrosine kinase receptor family of three members, namely MerTK, Tyro-3 and Axl, for which it displays the highest affinity. Gas6/TAM axis activation is known to be involved in modulating inflammatory responses as well as fibrotic evolution in many different pathological conditions. Due to the rapidly evolving COVID-19 pandemic, this review will focus on Gas6/TAM axis activation in SARS-CoV-2 infection, where de-regulated inflammatory responses and fibrosis represent a relevant feature of severe disease manifestation. Furthermore, this review will highlight the most recent scientific evidence supporting an unsuspected role of Axl as a SARS-CoV-2 infection driver, and the potential therapeutic advantages of the use of existing Axl inhibitors in COVID-19 management. From a physiological point of view, the Gas6/TAM axis plays a dual role, fostering the tissue repair processes or leading to organ damage and loss of function, depending on the prevalence of its anti-inflammatory or profibrotic properties. This review makes a strong case for further research focusing on the Gas6/TAM axis as a pharmacological target to manage different disease conditions, such as chronic fibrosis or COVID-19.

Baseline Plasma Gas6 Protein Elevation Predicts Adverse Outcomes in Hospitalized COVID-19 Patients

Reliable biomarkers allowing early patients’ stratification for the risk of adverse outcomes in COVID-19 are lacking. Gas6, together with its tyrosine kinase receptors named TAM, is involved in the regulation of immune homeostasis, fibrosis, and thrombosis. Our aim was to evaluate whether Gas6, sAxl, and sMerTK could represent early predictors of disease evolution either towards a negative (death or need of ICU admission) or a positive (discharge and/or clinical resolution within the first 14 days of hospitalization) outcome. To this purpose, between January and May 2021 (corresponding to third pandemic wave in Italy), 139 consecutive SARS-CoV-2 positive patients were enrolled in a prospective observational study. Plasma levels of these molecules were measured by ELISA at the time of hospitalization and after 7 and 14 days. We observed that higher plasma Gas6 concentrations at hospital admission were associated with a worsening in clinical conditions while lower sMerTK concentrations at baseline and after 7 days of hospitalization were associated with a more favorable outcome. At multivariate analysis, after correction for demographic and COVID-19 severity variables (NEWS2 and PiO2/FiO2), only Gas6 measured at baseline predicted an adverse prognosis with an odds ratio of 1.03 (C.I. 1.01-10.5). At ROC curve analysis, baseline Gas6 levels higher than 58.0 ng/ml predicted a severe disease evolution with 53.3% sensitivity and 77.6% specificity (area under the curve 0.653, $p=0.01$ , likelihood ratio of 2.38, IQR: 1.46-3.87). Taken together, these results support the hypothesis that a dysregulation in the Gas6/TAM axis could play a relevant role in modulating the course of COVID-19 and suggest that plasma Gas6 may represent a promising prognostic laboratory parameter for this condition.

Therapeutic aspects of the Axl/Gas6 molecular system

Axl receptor tyrosine kinase (RTK) and its ligand, growth arrest-specific protein 6 (Gas6), are involved in several biological functions and participate in the development and progression of a range of malignancies and autoimmune disorders. In this review, we present this molecular system from a drug discovery perspective, highlighting its therapeutic implications and challenges that need to be addressed. We provide an update on Axl/Gas6 axis biology, exploring its role in fields ranging from angiogenesis, cancer development and metastasis, immune response and inflammation to viral infection. Finally, we summarize the molecules that have been developed to date to target the Axl/Gas6 molecular system for therapeutic and diagnostic applications.

Role of Gas6 and TAM Receptors in the Identification of Cardiopulmonary Involvement in Systemic Sclerosis and Scleroderma Spectrum Disorders

Background

Few biomarkers are available for early identification of pulmonary arterial hypertension (PAH) and interstitial lung disease (ILD) in systemic sclerosis (SS) and scleroderma spectrum disorders (SSD).

Aims

To evaluate Gas6, sAxl, and sMer as biomarkers for cardiopulmonary complications of SS and SSD.

Methods

In a cross-sectional observational study, we recruited 125 consecutive patients, affected by SS and SSD and referred to a tertiary-level pulmonary hypertension outpatient clinic. All patients underwent a comprehensive evaluation for identification of PAH and ILD. Gas6, sMer, and sAxl concentrations were measured with ELISA protocols, and concentrations were compared according to PAH or ILD.

Results

Nineteen subjects had pulmonary hypertension (PH) (14 PAH), and 39 had ILD (6 severe). Plasma sMer was increased in PAH (18.6 ng/ml IQR [11.7-20.3]) with respect to the absence (12.4 [8.0-15.8]) or other form of pulmonary hypertension (9.6 [7.4-12.5]; K–W variance p < 0.04). Conversely, Gas6 and sAxl levels were slightly increased in mild ILD (25.8 ng/ml [19.5-32.1] and 24.6 [20.1-32.5]) and reduced in severe ILD (16.6 [15.0-22.1] and 15.5 [14.9-22.4]) in comparison to no evidence of ILD (23.4 [18.8-28.1] and 21.6 [18.1-28.4]; K–W, p ≤ 0.05). Plasma sMer ≥ 19 ng/ml has 50% sensitivity and 92% specificity in PAH identification (area under the ROC curve (AUC) 0.697, p < 0.03). Values of Gas6 ≤ 24.5 ng/ml and of sAxl ≤ 15.5 ng/ml have 100% and 67% sensitivity and 47% and 86% specificity, respectively, in identifying severe ILD (Gas6 AUC 0.787, p < 0.001; sAxl AUC 0.705, p < 0.05).

Conclusions

The assay of Gas6 sAxl and sMer may be useful to help in the identification of PAH and ILD in SS and SSD patients. The Gas6/TAM system seems to be relevant in cardiopulmonary complications of SS and SSD and merits further investigations.

New Insights into the Role of Tyro3, Axl, and Mer Receptors in Rheumatoid Arthritis

Rheumatoid Arthritis (RA) is the most common chronic inflammatory autoimmune disease involving joints. Among several pathogenic mechanisms, the impairment of homeostatic regulators of inflammation seems to be critically important to sustain persistent infiltration and activation of immune and stromal cells within the diseased synovium. Tyrosine kinase receptors Tyro3, Axl, and Mer are members of the TAM family. Upon binding their ligands Growth Arrest-Specific gene 6 (Gas6) and Protein S (ProS1), TAM receptors (TAMs) exert numerous and diverse biologic functions. Activated Axl and Mer, for instance, can negatively regulate the inflammatory cascade and mediate phagocytosis of apoptotic cells, contributing to prevent the development of autoimmunity. Thus, a role for TAMs has been hypothesized in RA. In this review, we will summarise unmet clinical needs in RA, depict the biology of TAMs and TAM ligands, focussing on their ability to regulate the immune system and inflammation cascade, and finally offer an overview of the state-of-the-art literature about the putative role of TAM axis in RA.

Gas6/TAM Axis in Sepsis: Time to Consider Its Potential Role as a Therapeutic Target

Tyrosine kinase receptors are transmembrane proteins involved in cell signaling and interaction. Among them, the TAM family (composed by Tyro 3, Axl, and Mer) represents a peculiar subgroup with an important role in many physiological and pathological conditions. Despite different mechanisms of activation (e.g., protein S and Galactin-3), TAM action is tightly related to their common ligand, a protein named growth arrest-specific 6 (Gas6). Since the expression of both TAM and Gas6 is widely distributed among tissues, any alteration of one of these components can lead to different pathological conditions. Moreover, as they are indispensable for homeostasis maintenance, in recent years a growing interest has emerged regarding their role in the regulation of the inflammatory process. Due to this involvement, many authors have demonstrated the pivotal role of the Gas6/TAM axis in both sepsis and the sepsis-related inflammatory responses. In this narrative review, we highlight the current knowledge as well as the last discoveries on TAM and Gas6 implication in different clinical conditions, notably in sepsis and septic shock. Lastly, we underline not only the feasible use of Gas6 as a diagnostic and prognostic biomarker in certain systemic acute conditions but also its potential therapeutic role in these life-threatening diseases.

Putative Cellular and Molecular Roles of Zika Virus in Fetal and Pediatric Neuropathologies

Although the World Health Organization declared an end to the recent Zika virus (ZIKV) outbreak and its association with adverse fetal and pediatric outcome, on November 18, 2016, the virus still remains a severe public health threat. Laboratory experiments thus far supported the suspicions that ZIKV is a teratogenic agent. Evidence indicated that ZIKV infection cripples the host cells' innate immune responses, allowing productive replication and potential dissemination of the virus. In addition, studies suggest potential transplacental passage of the virus and subsequent selective targeting of neural progenitor cells (NPCs). Depletion of NPCs by ZIKV is associated with restricted brain growth. And while microcephaly can result from infection at any gestational stages, the risk is greater during the first trimester. Although a number of recent studies revealed some of specific molecular and cellular roles of ZIKV proteins of this mosquito-borne flavivirus, the mechanisms by which it produces it suspected pathophysiological effects are not completely understood. Thus, this review highlights the cellular and molecular evidence that implicate ZIKV in fetal and pediatric neuropathologies.

Gas6 as a predictor of esophageal varices in patients affected by hepatitis C virus related-chronic liver disease

AIM

Plasma Gas6 was tested as an alternative to Baveno VI criteria (liver stiffness <20 kPa and platelet count >150 × 10⁹/l) in an endoscopy-sparing strategy.

METHODS

 A total of 160 patients with chronic hepatitis C and advanced fibrosis/cirrhosis underwent, on the same occasion, liver elastography, upper endoscopy, a platelet count and serum Gas6 measurement.

RESULTS

 A total of 74/160 (46%) patients had esophageal varices, that were small (diameter <5 mm) in 57/160 (34%) and large in 17/160 (11%) cases. A total of 34/160 (21%) patients satisfied Baveno VI criteria, according to which screening for esophageal varices could have been omitted; 1/34 had large varices (sensitivity 94%). A plasma Gas6 value <45 ng/ml, detected in 34/160 (21%) patients, was also 94% sensitive.

CONCLUSION

Plasma Gas6 might represent a feasible alternative to Baveno VI criteria when transient elastography is unavailable/unsuccessful.

Fibronectin connecting segment-1 peptide inhibits pathogenic leukocyte trafficking and inflammatory demyelination in experimental models of chronic inflammatory demyelinating polyradiculoneuropathy

The molecular determinants of pathogenic leukocyte migration across the blood-nerve barrier (BNB) in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) are unknown. Specific disease modifying therapies for CIDP are also lacking. Fibronectin connecting segment-1 (FNCS1), an alternatively spliced fibronectin variant expressed by microvascular endothelial cells at sites of inflammation in vitro and in situ, is a counterligand for leukocyte α₄ integrin (also known as CD49d) implicated in pathogenic leukocyte trafficking in multiple sclerosis and inflammatory bowel disease. We sought to determine the role of FNCS1 in CIDP patient leukocyte trafficking across the BNB in vitro and in severe chronic demyelinating neuritis in vivo using a representative spontaneous murine CIDP model. Peripheral blood mononuclear leukocytes from 7 untreated CIDP patients were independently infused into a cytokine-treated, flow-dependent in vitro BNB model system. Time-lapse digital video microscopy was performed to visualize and quantify leukocyte trafficking, comparing FNCS1 peptide blockade to relevant controls. Fifty 24-week old female B7-2 deficient non-obese diabetic mice with spontaneous autoimmune peripheral polyneuropathy (SAPP) were treated daily with 2mg/kg FNCS1 peptide for 5days via intraperitoneal injection with appropriate controls. Neurobehavioral measures of disease severity, motor nerve electrophysiology assessments and histopathological quantification of inflammation and morphometric assessment of demyelination were performed to determine in vivo efficacy. The biological relevance of FNCS1 and CD49d in CIDP was evaluated by immunohistochemical detection in affected patient sural nerve biopsies. 25μM FNCS1 peptide maximally inhibited CIDP leukocyte trafficking at the human BNB in vitro. FNCS1 peptide treatment resulted in significant improvements in disease severity, motor electrophysiological parameters of demyelination and histological measures of inflammatory demyelination. Microvessels demonstrating FNCS1 expression and CD49d+ leukocytes were seen within the endoneurium of patient nerve biopsies. Taken together, these results imply a role for FNCS1 in pathogenic leukocyte trafficking in CIDP, providing a potential target for therapeutic modulation.

Growth Arrest Specific 6 Concentration is Increased in the Cerebrospinal Fluid of Patients with Alzheimer's Disease

Growth arrest specific 6 (Gas6) has neurotrophic and neuroinflammatory functions, and may play a role in Alzheimer's disease (AD). In keeping with this hypothesis, we observed that cerebrospinal fluid (CSF) Gas6 is increased in AD patients compared to controls (63 versus 67 subjects; median value 13.3 versus 9.1 ng/ml; p < 0.0001). Thereafter, we assessed whether CSF Gas6 concentration was correlated to the following parameters: disease duration, MMSE score two years after clinical diagnosis, AD CSF biomarkers, and years of formal schooling. We detected an inverse correlation between CSF Gas6 levels at diagnosis and both disease duration (p < 0.0001) and decrease in the MMSE score two years later (p < 0.0001). Conversely, we found no correlation between CSF Gas6 and both AD biomarkers and years of formal schooling. In conclusion, our results suggest that upregulation of CSF Gas6 may be part of a defensive response aimed at counteracting AD progression.

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.

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.

Inflammatory neuropathies: pathology, molecular markers and targets for specific therapeutic intervention

Inflammatory neuropathies encompass groups of heterogeneous disorders characterized by pathogenic immune-mediated hematogenous leukocyte infiltration of peripheral nerves, nerve roots or both, with resultant demyelination or axonal degeneration or both. Inflammatory neuropathies may be divided into three major disease categories: Guillain-Barré syndrome (particularly the acute inflammatory demyelinating polyradiculoneuropathy variant), chronic inflammatory demyelinating polyradiculoneuropathy and nonsystemic vasculitic neuropathy (or peripheral nerve vasculitis). Despite major advances in molecular biology, pathology and genetics, the pathogenesis of these disorders remains elusive. There is insufficient knowledge on the mechanisms of hematogenous leukocyte trafficking into the peripheral nervous system to guide the development of specific molecular therapies for immune-mediated inflammatory neuropathies compared to disorders such as psoriasis, inflammatory bowel disease, rheumatoid arthritis or multiple sclerosis. The recent isolation and characterization of human endoneurial endothelial cells that form the blood-nerve barrier provides an opportunity to elucidate leukocyte-endothelial cell interactions critical to the pathogenesis of inflammatory neuropathies at the interface between the systemic circulation and peripheral nerve endoneurium. This review discusses our current knowledge of the classic pathological features of inflammatory neuropathies, attempts at molecular classification and genetic determinants, the utilization of in vitro and in vivo animal models to determine pathogenic mechanisms at the interface between the systemic circulation and the peripheral nervous system relevant to these disorders and prospects for future potential molecular pathology biomarkers and targets for specific therapeutic intervention.

Serum growth arrest-specific protein 6 levels are elevated in adult-onset Still's disease

We investigated the growth arrest-specific protein 6 in adult-onset Still's disease. Serums were collected from 52 adult-onset Still's disease patients with follow-up samples of 21 patients. The growth arrest-specific protein 6 levels in adult-onset Still's disease were higher compared to those in the normal controls (25.37±7.71 vs. 19.86±5.01 ng/mL, p<0.001). However, growth arrest-specific protein 6 did not correlate with disease activity. Also, growth arrest-specific protein 6 was not decreased after activity was resolved in the follow-up. The growth arrest-specific protein 6 in adult-onset Still's disease patients were higher than the normal controls. However, growth arrest-specific protein 6 was not correlated with disease activity.

Gas6 enhances axonal ensheathment by MBP+ membranous processes in human DRG/OL promyelinating co-cultures

The molecular requirements for human myelination are incompletely defined, and further study is needed to fully understand the cellular mechanisms involved during development and in demyelinating diseases. We have established a human co-culture model to study myelination. Our earlier observations showed that addition of human γ-carboxylated growth-arrest-specific protein 6 (Gas6) to human oligodendrocyte progenitor cell (OPC) cultures enhanced their survival and maturation. Therefore, we explored the effect of Gas6 in co-cultures of enriched OPCs plated on axons of human fetal dorsal root ganglia explant. Gas6 significantly enhanced the number of myelin basic protein-positive (MBP⁺) oligodendrocytes with membranous processes parallel with and ensheathing axons relative to co-cultures maintained in defined medium only for 14 days. Gas6 did not increase the overall number of MBP⁺ oligodendrocytes/culture; however, it significantly increased the length of MBP⁺ oligodendrocyte processes in contact with and wrapping axons. Multiple oligodendrocytes were in contact with a single axon, and several processes from one oligodendrocyte made contact with one or multiple axons. Electron microscopy supported confocal Z-series microscopy demonstrating axonal ensheathment by MBP⁺ oligodendrocyte membranous processes in Gas6-treated co-cultures. Contacts between the axonal and oligodendrocyte membranes were evident and multiple wraps of oligodendrocyte membrane around the axon were visible supporting a model system in which to study events in human myelination and aspects of non-compact myelin formation.

Increased concentrations of plasma growth arrest-specific 6 and its soluble tyrosine kinase receptor sAxl in Taiwanese women with pelvic inflammatory disease

BACKGROUNDS

To investigate the concentrations of plasma growth arrest-specific protein 6 (Gas6) and its soluble tyrosine kinase receptor sAxl in women with pelvic inflammatory disease (PID) and their association with clinical outcomes of PID.

METHODS

Blood specimens were consecutively collected from the 64 patients with PID before and after treatment and 70 healthy women in university hospital. Concentrations of plasma Gas6 and sAxl were detected using enzyme-linked immunosorbent assay.

RESULTS

The concentration of plasma Gas6 and sAxl was significantly increased in the patients with PID compared to the healthy controls, and then reduced significantly after treatment. Gas6 was significantly correlated with sAxl. When we selected 7.5 and 15.2 ng/ml as the cutoff concentration of plasma Gas6 and sAxl to detect PID respectively, the sensitivities of Gas6 and sAxl were 76.6% and 75.0%. When Gas6 and sAxl were combined, the sensitivity rose to 92.2%. They were not related to the incidences of tuboovarian abscesses and surgery, which were, however, significantly associated with length of hospital stay.

CONCLUSIONS

Novel application of Gas6 or sAxl in combination had a high sensitivity to detect PID and is important in order to prevent severe sequelae.

Growth arrest specific gene 6 protein concentration in cerebrospinal fluid correlates with relapse severity in multiple sclerosis

Background. Growth arrest specific gene 6 (Gas6) protein enhances survival of oligodendrocytes and neurons, and it is involved in autoimmunity. Therefore, we aimed to verify whether cerebrospinal-fluid (CSF) Gas6 concentration may represent a biomarker of disease activity in multiple sclerosis. Methods. Sixty-five patients who underwent a spinal tap during relapse of relapsing/remitting multiple sclerosis (RR-MS)(McDonald-criteria) were studied. Forty patients affected by noninflammatory/nonautoimmune neurological diseases served as controls. Relapse was defined according to Schumacher criteria. Symptoms were grouped according to Kurtzke-Functional System (FS). Clinical characteristics of the relapse, duration, Expanded-Disability-Status Scale (EDSS) change, number of FS involved, completeness of recovery, age, steroid therapy, were categorised. Patients were followed at 6-month intervals to assess relapse rate and EDSS progression. Gas6 was measured (CSF, plasma) by in-house-enzyme-linked immunoassay (ELISA). Results. Higher CSF Gas6 concentrations were observed in relapses lasting ≤60 days (8.7 ± 3.9 ng/mL) versus >60 days (6.5 ± 2.6) or controls (6.5 ± 2.4; P = 0.05), with ≤2 FS involved (8.5 ± 3.8) versus >2 FS (5.6 ± 2.5) (P < 0.05) and EDSS change ≤2.5 points (8.8 ± 3.7) versus >2.5 (6.5 ± 3.5) (P = 0.04). Conversely, CSF Gas6 was not predictive of the completeness of recovery. Plasma and CSF concentrations were not related (R² = 0.0003), and neither were predictive of relapse rate or EDSS progression after first relapse. Conclusions. CSF concentration of Gas6 is inversely correlated with the severity of relapse in RR-MS patients but does not predict the subsequent course of the disease.

Serum growth arrest-specific protein 6 levels are a reliable biomarker of disease activity in systemic lupus erythematosus

PURPOSE

Growth arrest-specific protein 6 (Gas6) has been suggested to be a biomarker of disease activity in patients with systemic lupus erthematosus (SLE). We investigated the clinical significance of this protein in Korean SLE.

METHODS

Blood samples were collected from 150 SLE patients and 50 normal controls (NC). In addition, follow-up samples were collected from 50 SLE patients.

RESULTS

Serum Gas6 levels of SLE patients (43.01 ± 28.02 ng/mL) were higher than those of NC (20.15 ± 9.23 ng/mL, p<0.001). When evaluated sensitivity and specificity of the Gas6 for diagnosing SLE using ROC curves, the sensitivity and specificity were 72.7 % and 84 % with a cut-off value of 25.3 ng/mL. In the ROC analysis of Gas6, anti-dsDNA antibody, ESR, complement 3 and complement 4 to identify patients with active lupus, area under the curve (AUC) of Gas6 was highest with 0.763. Serum Gas6 levels were significantly higher in the patients with serositis (70.04 ± 30.85 ng/mL) and renal disorder (65.66 ± 32.28 ng/mL) compared to those without (41.88 ± 27.44 ng/mL, p=0.033, 40.3 ± 26.33 ng/mL, p=0.001, respectively). Gas6 levels were correlated positively with anti-dsDNA antibody (r=0.199, p=0.015), ESR (r=0.204, p=0.013) and SLEDAI (r=0.512, p<0.001). In addition, serum Gas6 levels were correlated negatively with hemoglobin (r= -0.165, p=0.043), lymphocyte count (r= -0.165, p=0.043), complement 3 (r= -0.343, p<0.001) and complement 4 (r= -0.316, p<0.001). Furthermore, change in serum Gas6 levels was correlated with change in SLEDAI levels in the SLE patients that were followed up (r=0.524, p<0.001).

CONCLUSION

These results suggest that serum Gas6 can be a reliable clinical marker for monitoring disease activity and treatment response in SLE.

Loss of the receptor tyrosine kinase Axl leads to enhanced inflammation in the CNS and delayed removal of myelin debris during experimental autoimmune encephalomyelitis

Background

Axl, together with Tyro3 and Mer, constitute the TAM family of receptor tyrosine kinases. In the nervous system, Axl and its ligand Growth-arrest-specific protein 6 (Gas6) are expressed on multiple cell types. Axl functions in dampening the immune response, regulating cytokine secretion, clearing apoptotic cells and debris, and maintaining cell survival. Axl is upregulated in various disease states, such as in the cuprizone toxicity-induced model of demyelination and in multiple sclerosis (MS) lesions, suggesting that it plays a role in disease pathogenesis. To test for this, we studied the susceptibility of Axl-/- mice to experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis.

Methods

WT and Axl-/- mice were immunized with myelin oligodendrocyte glycoprotein (MOG)_35-55 peptide emulsified in complete Freund's adjuvant and injected with pertussis toxin on day 0 and day 2. Mice were monitored daily for clinical signs of disease and analyzed for pathology during the acute phase of disease. Immunological responses were monitored by flow cytometry, cytokine analysis and proliferation assays.

Results

Axl-/- mice had a significantly more severe acute phase of EAE than WT mice. Axl-/- mice had more spinal cord lesions with larger inflammatory cuffs, more demyelination, and more axonal damage than WT mice during EAE. Strikingly, lesions in Axl-/- mice had more intense Oil-Red-O staining indicative of inefficient clearance of myelin debris. Fewer activated microglia/macrophages (Iba1+) were found in and/or surrounding lesions in Axl-/- mice relative to WT mice. In contrast, no significant differences were noted in immune cell responses between naïve and sensitized animals.

Conclusions

These data show that Axl alleviates EAE disease progression and suggests that in EAE Axl functions in the recruitment of microglia/macrophages and in the clearance of debris following demyelination. In addition, these data provide further support that administration of the Axl ligand Gas6 could be therapeutic for immune-mediated demyelinating diseases.

GAS6 enhances repair following cuprizone-induced demyelination

Growth arrest-specific protein 6 (gas6) activities are mediated through the Tyro3, Axl, and Mer family of receptor tyrosine kinases. Gas6 is expressed and secreted by a wide variety of cell types, including cells of the central nervous system (CNS). In this study, we tested the hypothesis that administration of recombinant human Gas6 (rhGas6) protein into the CNS improves recovery following cuprizone withdrawal. After a 4-week cuprizone diet, cuprizone was removed and PBS or rhGas6 (400 ng/ml, 4 µg/ml and 40 µg/ml) was delivered by osmotic mini-pump into the corpus callosum of C57Bl6 mice for 14 days. Nine of 11 (82%) PBS-treated mice had abundant lipid-associated debris in the corpus callosum by Oil-Red-O staining while only 4 of 19 (21%) mice treated with rhGas6 had low Oil-Red-O positive droplets. In rhGas6-treated mice, SMI32-positive axonal spheroids and APP-positive deposits were reduced in number relative to PBS-treated mice. Compared to PBS, rhGas6 enhanced remyelination as revealed by MBP immunostaining and electron microscopy. The rhGas6-treated mice had more oligodendrocytes expressing Olig1 in the cytoplasm, indicative of oligodendrocyte progenitor cell maturation. Relative to PBS-treated mice, rhGas6-treated mice had fewer activated microglia in the corpus callosum by Iba1 immunostaining. The data show that rhGas6 treatment resulted in more efficient repair following cuprizone-induced injury.

Plasma concentrations of Gas6 and soluble Axl correlate with disease and predict mortality in patients with critical limb ischemia

INTRODUCTION

Critical limb ischemia (CLI) is a severe peripheral arterial disease, characterized by rest pain, ulcers and gangrene in the legs. Gas6 is a vitamin K-dependent protein, which binds and activates the tyrosine kinase receptor Axl. Gas6-mediated Axl-signaling influences endothelial activation, neointima formation and immune regulation. Axl can be cleaved and soluble Axl (sAxl) is detectable in circulation.

DESIGN AND METHODS

We quantified plasma concentrations of Gas6 and sAxl in 189 CLI patients and 204 controls.

RESULTS

Gas6 and sAxl concentrations were increased in the CLI patients (p<0.0001) and correlated to C-reactive protein, interleukin-6, tumor necrosis factor alpha and neopterin. Patients who died within 3years of sampling (n=84) had increased concentrations of Gas6 and sAxl as compared to survivors (p=0.0009 and p=0.0011).

CONCLUSIONS

Plasma concentrations of Gas6 and sAxl correlate to inflammation and predict survival. This indicates that Gas6 and sAxl have a role in CLI, presumably connected to the inflammatory process.

Survival and migration of human dendritic cells are regulated by an IFN-alpha-inducible Axl/Gas6 pathway

Axl, a prototypic member of the transmembrane tyrosine kinase receptor family, is known to regulate innate immunity. In this study, we show that Axl expression is induced by IFN-alpha during human dendritic cell (DC) differentiation from monocytes (IFN/DC) and that constitutively Axl-negative, IL-4-differentiated DC (IL-4/DC) can be induced to up-regulate Axl by IFN-alpha. This effect is inhibited by TLR-dependent maturation stimuli such as LPS, poly(I:C), TLR7/8 ligand, and CD40L. LPS-induced Axl down-regulation on the surface of human IFN-alpha-treated DC correlates with an increased proteolytic cleavage of Axl and with elevated levels of its soluble form. GM6001 and TAPI-1, general inhibitors of MMP and ADAM family proteases, restored Axl expression on the DC surface and diminished Axl shedding. Furthermore, stimulation of Axl by its ligand, Gas6, induced chemotaxis of human DC and rescued them from growth factor deprivation-induced apoptosis. Our study provides the first evidence that Gas6/Axl-mediated signaling regulates human DC activities, and identifies Gas6/Axl as a new DC chemotaxis pathway. This encourages one to explore whether dysregulation of this novel pathway in human DC biology is involved in autoimmunity characterized by high levels of IFN-alpha.