Dead Cells Induce Innate Anergy via Mertk after Acute Viral Infection

The protective role of Mertk in JEV-induced encephalitis by maintaining the integrity of blood-brain barrier

Japanese encephalitis is an acute infectious disease of the central nervous system caused by neurotropic Japanese encephalitis virus (JEV). As a member of TAM (Tyro3, Axl and Mertk) family, Mertk has involved in multiple biological processes by engaging with its bridging ligands Gas6 and Protein S, including invasion of pathogens, phagocytosis of apoptotic cells, inflammatory response regulation, and the maintenance of blood brain barrier (BBB) integrity. However, its role in encephalitis caused by JEV infection has not been studied in detail. Here, we found that Mertk-/- mice exhibited higher mortality and more rapid disease progression than wild-type mice after JEV challenge. There were no significant differences in viral load and cytokines expression level in peripheral tissues between Wild type and Mertk-/- mice. Furthermore, the absence of Mertk had little effect on the inflammatory response and immunopathological damage while it can cause an increased viral load in the brain. For the in vitro model of BBB, Mertk was shown to maintain the integrity of the BBB. In vivo, Mertk-/- mice exhibited higher BBB permeability and lower BBB integrity. Taken together, our findings demonstrate that Mertk acts as a protective factor in the development of encephalitis induced by JEV infection, which is mainly associated with its beneficial effect on BBB integrity, rather than its regulation of inflammatory response.

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.

Semaphorin3B promotes an anti-inflammatory and pro-resolving phenotype in macrophages from rheumatoid arthritis patients in a MerTK-dependent manner

Previous works from our group show that Semaphorin3B (Sema3B) is reduced in RA and plays a protective role in a mouse arthritis model. In turn, MerTK plays a protective function in murine arthritis models, is expressed by synovial tissue macrophages and is linked to remission in patients with RA. In this study, we examined the role of Sema3B in the phenotypic characteristics of RA macrophages and the implication of MerTK. Peripheral blood monocytes from RA patients were differentiated into IFN-γ (RA MØIFN-γ) or M-CSF (RA MØM-CSF) macrophages and stimulated with LPS, Sema3B or their combination. Alternatively, RA fibroblast like synoviocytes (FLS) were stimulated with RA MØIFN-γ and RA MØM-CSF supernatants. Gene expression was determined by qPCR and protein expression and activation by flow cytometry, ELISA and western blot. Sema3B down-regulated the expression of pro-inflammatory mediators, in both RA MØIFN-γ and RA MØM-CSF. We observed a similar reduction in RA FLS stimulated with the supernatant of Sema3B-treated RA MØIFN-γ and RA MØM-CSF. Sema3B also modulated cell surface markers in macrophages towards an anti-inflammatory phenotype. Besides, MerTK expression and activation was up-regulated by Sema3B, just as GAS6 expression, Resolvin D1 secretion and the phagocytic activity of macrophages. Importantly, the inhibition of MerTK and neuropilins 1 and 2 abrogated the anti-inflammatory effect of Sema3B. Our data demonstrate that Sema3B modulates the macrophage characteristics in RA, inducing a skewing towards an anti-inflammatory/pro-resolving phenotype in a MerTK-dependant manner. Therefore, here we identify a new mechanism supporting the protective role of Sema3B in RA pathogenesis.

Lupus Nephritis Biomarkers: A Critical Review

Lupus nephritis (LN), a major complication in individuals diagnosed with systemic lupus erythematosus, substantially increases morbidity and mortality. Despite marked improvements in the survival of patients with severe LN over the past 50 years, complete clinical remission after immunosuppressive therapy is achieved in only half of the patients. Therefore, timely detection of LN is vital for initiating prompt therapeutic interventions and improving patient outcomes. Biomarkers have emerged as valuable tools for LN detection and monitoring; however, the complex role of these biomarkers in LN pathogenesis remains unclear. Renal biopsy remains the gold standard for the identification of the histological phenotypes of LN and guides disease management. However, the molecular pathophysiology of specific renal lesions remains poorly understood. In this review, we provide a critical, up-to-date overview of the latest developments in the field of LN biomarkers.

Effects of polybrene and retronectin as transduction enhancers on the development and phenotypic characteristics of VHH-based CD19-redirected CAR T cells: a comparative investigation

Chimeric antigen receptor T cells (CAR T cells) have improved the prognosis of patients with certain hematologic malignancies. However, broader clinical application of this type of therapy is dependent on production protocols. We characterized VHH-based CD19-redirected CAR T cells generated using the transduction enhancers (TEs) polybrene or retronectin. The proliferation rate of activated T cells transduced using polybrene concentrations > 6 mg/mL decreased compared with untreated group. There was a direct relationship between polybrene concentration and transduction efficacy. Moreover, we demonstrated the proliferation of retronectin-transduced T cells increased in a dose-dependent manner (4-20 μg/mL). Whereas, different retronectin concentrations did not mediate a significant increase in T cell transduction rate. Moreover, lentiviral transduction rate was also dependent on the concentration of lentiviruses. At optimized TE concentrations, multiplicity of infection (MOI) of > 10 decreased living T cell transduction rate. Additionally, we demonstrated that CAR T cell phenotype is highly affected by TE type. Naïve T cell differentiation to central memory T cell was observed in the beginning of the expansion process and effector memory T cells became the predominant subset in the second week of expansion. Importantly, retronectin increased the proliferation of CAR T cells alongside medicating higher transduction rates, resulting in more naïve and central memory T cells. We demonstrated that a higher percentage of CAR T cells were generated using retronectin (with a less differentiated phenotype) making retronectin a more effective TE than polybrene for long-term CAR T cell processing in preclinical or clinical studies.

Innate immune memory in inflammatory arthritis

The concept of immunological memory was demonstrated in antiquity when protection against re-exposure to pathogens was observed during the plague of Athens. Immunological memory has been linked with the adaptive features of T and B cells; however, in the past decade, evidence has demonstrated that innate immune cells can exhibit memory, a phenomenon called 'innate immune memory' or 'trained immunity'. Innate immune memory is currently being defined and is transforming our understanding of chronic inflammation and autoimmunity. In this Review, we provide an up-to-date overview of the memory-like features of innate immune cells in inflammatory arthritis and the crosstalk between chronic inflammatory milieu and cell reprogramming. Aberrant pro-inflammatory signalling, including cytokines, regulates the metabolic and epigenetic reprogramming of haematopoietic progenitors, leading to exacerbated inflammatory responses and osteoclast differentiation, in turn leading to bone destruction. Moreover, imprinted memory on mature cells including terminally differentiated osteoclasts alters responsiveness to therapies and modifies disease outcomes, commonly manifested by persistent inflammatory flares and relapse following medication withdrawal.

Stress induces behavioral abnormalities by increasing expression of phagocytic receptor MERTK in astrocytes to promote synapse phagocytosis

Childhood neglect and/or abuse can induce mental health conditions with unknown mechanisms. Here, we identified stress hormones as strong inducers of astrocyte-mediated synapse phagocytosis. Using in vitro, in vivo, and human brain organoid experiments, we showed that stress hormones increased the expression of the Mertk phagocytic receptor in astrocytes through glucocorticoid receptor (GR). In post-natal mice, exposure to early social deprivation (ESD) specifically activated the GR-MERTK pathway in astrocytes, but not in microglia. The excitatory post-synaptic density in cortical regions was reduced in ESD mice, and there was an increase in the astrocytic engulfment of these synapses. The loss of excitatory synapses, abnormal neuronal network activities, and behavioral abnormalities in ESD mice were largely prevented by ablating GR or MERTK in astrocytes. Our work reveals the critical roles of astrocytic GR-MERTK activation in evoking stress-induced abnormal behaviors in mice, suggesting GR-MERTK signaling as a therapeutic target for stress-induced mental health conditions.

Influenza A Virus-Induced circRNA circMerTK Negatively Regulates Innate Antiviral Responses

Circular RNAs (circRNAs) are an important subclass of noncoding RNAs implicated in the regulation of multiple biological processes. However, the functional involvement of circRNAs in the pathogenesis of influenza A viruses (IAVs) remains largely unknown. Here, we employed RNA sequencing (RNA-Seq) to examine the differentially expressed circRNAs in mouse lung tissues challenged or not challenged with IAV to evaluate the impact of viral infection on circRNAs in vivo. We observed that 413 circRNAs exhibited significantly altered levels following IAV infection. Among these, circMerTK, the derivative of myeloid-epithelial-reproductive tyrosine kinase (MerTK) pre-mRNA, was highly induced by IAV. Interestingly, circMerTK expression was also increased upon infection with multiple DNA and RNA viruses in human and animal cell lines, and thus it was selected for further studies. Poly(I:C) and interferon β (IFN-β) stimulated circMerTK expression, while RIG-I knockout and IFNAR1 knockout cell lines failed to elevate circMerTK levels after IAV infection, demonstrating that circMerTK is regulated by IFN signaling. Furthermore, circMerTK overexpression or silencing accelerated or impeded IAV and Sendai virus replication, respectively. Silencing circMerTK enhanced the production of type I IFNs and interferon-stimulating genes (ISGs), whereas circMerTK overexpression suppressed their expression at both the mRNA and protein levels. Notably, altering circMerTK expression had no effect on the MerTK mRNA level in cells infected or not infected with IAV, and vice versa. In addition, human circMerTK and mouse homologs functioned similarly in antiviral responses. Together, these results identify circMerTK as an enhancer of IAV replication through suppression of antiviral immunity. IMPORTANCE CircRNAs are an important class of noncoding RNAs characterized by a covalently closed circular structure. CircRNAs have been proven to impact numerous cellular processes, where they conduct specialized biological activities. In addition, circRNAs are believed to play a crucial role in regulating immune responses. Nevertheless, the functions of circRNAs in the innate immunity against IAV infection remain obscure. In this study, we employed transcriptomic analysis to investigate the alterations in circRNAs expression following IAV infection in vivo. It was found that expression of 413 circRNAs was significantly altered, of which 171 were upregulated, and 242 were downregulated following the IAV infection. Interestingly, circMerTK was identified as a positive regulator of IAV replication in both human and mouse hosts. CircMerTK was shown to influence IFN-β production and its downstream signaling, enhancing IAV replication. This finding provides new insights into the critical roles of circRNAs in regulating antiviral immunity.

Targeting Human Proteins for Antiviral Drug Discovery and Repurposing Efforts: A Focus on Protein Kinases

Despite the great technological and medical advances in fighting viral diseases, new therapies for most of them are still lacking, and existing antivirals suffer from major limitations regarding drug resistance and a limited spectrum of activity. In fact, most approved antivirals are directly acting antiviral (DAA) drugs, which interfere with viral proteins and confer great selectivity towards their viral targets but suffer from resistance and limited spectrum. Nowadays, host-targeted antivirals (HTAs) are on the rise, in the drug discovery and development pipelines, in academia and in the pharmaceutical industry. These drugs target host proteins involved in the virus life cycle and are considered promising alternatives to DAAs due to their broader spectrum and lower potential for resistance. Herein, we discuss an important class of HTAs that modulate signal transduction pathways by targeting host kinases. Kinases are considered key enzymes that control virus-host interactions. We also provide a synopsis of the antiviral drug discovery and development pipeline detailing antiviral kinase targets, drug types, therapeutic classes for repurposed drugs, and top developing organizations. Furthermore, we detail the drug design and repurposing considerations, as well as the limitations and challenges, for kinase-targeted antivirals, including the choice of the binding sites, physicochemical properties, and drug combinations.

Mer-tyrosine kinase: a novel susceptibility gene for SLE related end-stage renal disease

OBJECTIVE

Lupus nephritis (LN) is a common and severe manifestation of SLE. The genetic risk for nephritis and progression to end-stage renal disease (ESRD) in patients with LN remains unclear. Herein, we aimed to identify novel genetic associations with LN, focusing on subphenotypes and ESRD.

METHODS

We analysed genomic data on 958 patients with SLE (discovery cohort: LN=338) with targeted sequencing data from 1832 immunological pathway genes. We used an independent multiethnic cohort comprising 1226 patients with SLE (LN=603) as a replication dataset. Detailed functional annotation and functional epigenomic enrichment analyses were applied to predict functional effects of the candidate variants.

RESULTS

A genetic variant (rs56097910) within the MERTK gene was associated with ESRD in both cohorts, meta-analysis OR=5.4 (2.8 to 10.6); p=1.0×10-6. We observed decreased methylation levels in peripheral blood cells from SLE patients with ESRD, compared with patients without renal SLE (p=2.7×10-4), at one CpG site (cg16333401) in close vicinity to the transcription start site of MERTK and located in a DNAse hypersensitivity region in T and B cells. Rs56097910 is linked to altered MERTK expression in kidney tissue in public eQTL databases. Two loci were replicated for association with proliferative LN: PRDM1 (rs6924535, pmeta=1.6×10-5, OR=0.58) and APOA1BP (NAXE) (rs942960, pmeta=1.2×10-5, OR=2.64).

CONCLUSION

We identified a novel genetic risk locus, MERTK, associated with SLE-ESRD using the data from two large SLE cohorts. Through DNA methylation analysis and functional annotation, we showed that the risk could be mediated through regulation of gene expression. Our results suggest that variants in the MERTK gene are important for the risk of developing SLE-ESRD and suggest a role for PRDM1 and APOA1BP in proliferative LN.

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.

Glucocorticoids as Regulators of Macrophage-Mediated Tissue Homeostasis

Our immune system has evolved as a complex network of cells and tissues tasked with maintaining host homeostasis. This is evident during the inflammatory responses elicited during a microbial infection or traumatic tissue damage. These responses seek to eliminate foreign material or restore tissue integrity. Even during periods without explicit disturbances, the immune system plays prominent roles in tissue homeostasis. Perhaps one of the most studied cells in this regard is the macrophage. Tissue-resident macrophages are a heterogenous group of sensory cells that respond to a variety of environmental cues and are essential for organ function. Endogenously produced glucocorticoid hormones connect external environmental stress signals with the function of many cell types, producing profound changes in immune cells, including macrophages. Here, we review the current literature which demonstrates specific effects of glucocorticoids in several organ systems. We propose that tissue-resident macrophages, through glucocorticoid signaling, may play an underappreciated role as regulators of organ homeostasis.

ODC (Ornithine Decarboxylase)-Dependent Putrescine Synthesis Maintains MerTK (MER Tyrosine-Protein Kinase) Expression to Drive Resolution

OBJECTIVE

ODC (ornithine decarboxylase)-dependent putrescine synthesis promotes the successive clearance of apoptotic cells (ACs) by macrophages, contributing to inflammation resolution. However, it remains unknown whether ODC is required for other arms of the resolution program. Approach and Results: RNA sequencing of ODC-deficient macrophages exposed to ACs showed increases in mRNAs associated with heightened inflammation and decreases in mRNAs related to resolution and repair compared with WT (wild type) macrophages. In zymosan peritonitis, myeloid ODC deletion led to delayed clearance of neutrophils and a decrease in the proresolving cytokine, IL (interleukin)-10. Nanoparticle-mediated silencing of macrophage ODC in a model of atherosclerosis regression lowered IL-10 expression, decreased efferocytosis, enhanced necrotic core area, and reduced fibrous cap thickness. Mechanistically, ODC deletion lowered basal expression of MerTK (MER tyrosine-protein kinase)-an AC receptor-via a histone methylation-dependent transcriptional mechanism. Owing to lower basal MerTK, subsequent exposure to ACs resulted in lower MerTK-Erk (extracellular signal-regulated kinase) 1/2-dependent IL-10 production. Putrescine treatment of ODC-deficient macrophages restored the expression of both MerTK and AC-induced IL-10.

CONCLUSIONS

These findings demonstrate that ODC-dependent putrescine synthesis in macrophages maintains a basal level of MerTK expression needed to optimally resolve inflammation upon subsequent AC exposure. Graphic Abstract: A graphic abstract is available for this article.

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.

Expression of TAM-R in Human Immune Cells and Unique Regulatory Function of MerTK in IL-10 Production by Tolerogenic DC

Tumor-infiltrating myeloid cells are a key component of the immune infiltrate often correlated with a poor prognosis due to their capacities to sustain an immunosuppressive environment. Among membrane receptors implicated in myeloid cell functions, Tyro3, Axl, and MerTK, which are a family of tyrosine kinase receptors (TAM-R), have been described in the regulation of innate cell functions. Here, we have identified MerTK among TAM-R as the major marker of both human M2 macrophages and tolerogenic dendritic cells (DC). In situ, MerTK expression was found within the immune infiltrate in multiple solid tumors, highlighting its potential role in cancer immunity. TAM-R ligands Gas6 and PROS1 were found to be constitutively produced by myeloid cells in vitro. Importantly, we describe a novel function of MerTK/PROS1 axis in the regulation of IL-10 production by tolerogenic DC. Finally, the analysis of TAM-R expression within the lymphoid compartment following activation revealed that MerTK, but not Axl or Tyro3, is expressed on activated B lymphocytes and regulatory T cells, as well as CD4⁺ and CD8⁺ T cells. Thus, our findings deepen the implication of MerTK in the regulation of myeloid cell-mediated immunosuppression and identified new cellular targets expressing MerTK that could participate in the antitumor immune response.