MERTK tyrosine kinase receptor together with TIM4 phosphatidylserine receptor mediates distinct signal transduction pathways for efferocytosis and cell proliferation

Radiation-based immunogenic vaccine combined with a macrophage "checkpoint inhibitor" for boosting innate and adaptive immunity against metastatic colon cancers

Immunogenic dying tumor cells hold promising prospects as cancer vaccines to activate systemic immunity against both primary and metastatic tumors. Especially, X-ray- induced dying tumor cells are rich in highly immunogenic tumor-associated antigens and self-generated dsDNA as potent adjuvants. However, we found that the X-ray induction process can result in the excessive exposure of phosphatidylserine in cancer vaccines, which can specifically bind with the MerTK receptor on macrophages, acting as a "checkpoint" to facilitate immune silence in the tumor microenvironment. Therefore, we developed a novel strategy combining X-ray-induced cancer vaccines with UNC2250, a macrophage MerTK "checkpoint inhibitor," for treating peritoneal carcinomatosis in colon cancer. By incorporating UNC2250 into the treatment regimen, immunosuppressive efferocytosis of macrophages, which relies on MerTK-directed recognition of phosphatidylserine on vaccines, was effectively blocked. Consequently, the immune analysis revealed that this combination strategy promoted the maturation of dendritic cells and M1-like repolarization of macrophages, thereby simultaneously eliciting robust adaptive and innate immunity. This innovative approach utilizing X-ray-induced vaccines combined with a checkpoint inhibitor may provide valuable insights for developing effective cancer vaccines and immunotherapies targeting colon cancer.

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.

Efferocytosis is restricted by axon guidance molecule EphA4 via ERK/Stat6/MERTK signaling following brain injury

BACKGROUND

Efferocytosis is a process that removes apoptotic cells and cellular debris. Clearance of these cells alleviates neuroinflammation, prevents the release of inflammatory molecules, and promotes the production of anti-inflammatory cytokines to help maintain tissue homeostasis. The underlying mechanisms by which this occurs in the brain after injury remain ill-defined.

METHODS

We used GFP bone marrow chimeric knockout (KO) mice to demonstrate that the axon guidance molecule EphA4 receptor tyrosine kinase is involved in suppressing MERTK in the brain to restrict efferocytosis of resident microglia and peripheral-derived monocyte/macrophages.

RESULTS

Single-cell RNAseq identified MERTK expression, the primary receptor involved in efferocytosis, on monocytes, microglia, and a subset of astrocytes in the damaged cortex following brain injury. Loss of EphA4 on infiltrating GFP-expressing immune cells improved functional outcome concomitant with enhanced efferocytosis and overall protein expression of p-MERTK, p-ERK, and p-Stat6. The percentage of GFP+ monocyte/macrophages and resident microglia engulfing NeuN+ or TUNEL+ cells was significantly higher in KO chimeric mice. Importantly, mRNA expression of Mertk and its cognate ligand Gas6 was significantly elevated in these mice compared to the wild-type. Analysis of cell-specific expression showed that p-ERK and p-Stat6 co-localized with MERTK-expressing GFP + cells in the peri-lesional area of the cortex following brain injury. Using an in vitro efferocytosis assay, co-culturing pHrodo-labeled apoptotic Jurkat cells and bone marrow (BM)-derived macrophages, we demonstrate that efferocytosis efficiency and mRNA expression of Mertk and Gas6 was enhanced in the absence of EphA4. Selective inhibitors of ERK and Stat6 attenuated this effect, confirming that EphA4 suppresses monocyte/macrophage efferocytosis via inhibition of the ERK/Stat6 pathway.

CONCLUSIONS

Our findings implicate the ERK/Stat6/MERTK axis as a novel regulator of apoptotic debris clearance in brain injury that is restricted by peripheral myeloid-derived EphA4 to prevent the resolution of inflammation.

Hijacking homeostasis: Regulation of the tumor microenvironment by apoptosis

Cancers are genetically driven, rogue tissues which generate dysfunctional, obdurate organs by hijacking normal, homeostatic programs. Apoptosis is an evolutionarily conserved regulated cell death program and a profoundly important homeostatic mechanism that is common (alongside tumor cell proliferation) in actively growing cancers, as well as in tumors responding to cytotoxic anti-cancer therapies. Although well known for its cell-autonomous tumor-suppressive qualities, apoptosis harbors pro-oncogenic properties which are deployed through non-cell-autonomous mechanisms and which generally remain poorly defined. Here, the roles of apoptosis in tumor biology are reviewed, with particular focus on the secreted and fragmentation products of apoptotic tumor cells and their effects on tumor-associated macrophages, key supportive cells in the aberrant homeostasis of the tumor microenvironment. Historical aspects of cell loss in tumor growth kinetics are considered and the impact (and potential impact) on tumor growth of apoptotic-cell clearance (efferocytosis) as well as released soluble and extracellular vesicle-associated factors are discussed from the perspectives of inflammation, tissue repair, and regeneration programs. An "apoptosis-centric" view is proposed in which dying tumor cells provide an important platform for intricate intercellular communication networks in growing cancers. The perspective has implications for future research and for improving cancer diagnosis and therapy.

Extracellular vesicles arising from apoptosis: forms, functions, and applications

Extracellular vesicles (EVs) are lipid bilayer-enclosed subcellular bodies produced by most, if not all cells. Research over the last two decades has recognised the importance of EVs in intercellular communication and horizontal transfer of biological material. EVs range in diameter from tens of nanometres up to several micrometres and are able to transfer a spectrum of biologically active cargoes - from whole organelles, through macromolecules including nucleic acids and proteins, to metabolites and small molecules - from their cells of origin to recipient cells, which may consequently become physiologically or pathologically altered. Based on their modes of biogenesis, the most renowned EV classes are (1) microvesicles, (2) exosomes (both produced by healthy cells), and (3) EVs from cells undergoing regulated death by apoptosis (ApoEVs). Microvesicles bud directly from the plasma membrane, while exosomes are derived from endosomal compartments. Current knowledge of the formation and functional properties of ApoEVs lags behind that of microvesicles and exosomes, but burgeoning evidence indicates that ApoEVs carry manifold cargoes, including mitochondria, ribosomes, DNA, RNAs, and proteins, and perform diverse functions in health and disease. Here we review this evidence, which demonstrates substantial diversity in the luminal and surface membrane cargoes of ApoEVs, permitted by their very broad size range (from around 50 nm to >5 μm; the larger often termed apoptotic bodies), strongly suggests their origins through both microvesicle- and exosome-like biogenesis pathways, and indicates routes through which they interact with recipient cells. We discuss the capacity of ApoEVs to recycle cargoes and modulate inflammatory, immunological, and cell fate programmes in normal physiology and in pathological scenarios such as cancer and atherosclerosis. Finally, we provide a perspective on clinical applications of ApoEVs in diagnostics and therapeutics. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Targeted degradation of MERTK and other TAM receptor paralogs by heterobifunctional targeted protein degraders

TAM receptors (TYRO3, AXL, and MERTK) comprise a family of homologous receptor tyrosine kinases (RTK) that are expressed across a range of liquid and solid tumors where they contribute to both oncogenic signaling to promote tumor proliferation and survival, as well as expressed on myeloid and immune cells where they function to suppress host anti-tumor immunity. In recent years, several strategies have been employed to inhibit TAM kinases, most notably small molecule tyrosine kinase inhibitors and inhibitory neutralizing monoclonal antibodies (mAbs) that block receptor dimerization. Targeted protein degraders (TPD) use the ubiquitin proteasome pathway to redirect E3 ubiquitin ligase activity and target specific proteins for degradation. Here we employ first-in-class TPDs specific for MERTK/TAMs that consist of a cereblon E3 ligase binder linked to a tyrosine kinase inhibitor targeting MERTK and/or AXL and TYRO3. A series of MERTK TPDs were designed and investigated for their capacity to selectively degrade MERTK chimeric receptors, reduce surface expression on primary efferocytic bone marrow-derived macrophages, and impact on functional reduction in efferocytosis (clearance of apoptotic cells). We demonstrate proof-of-concept and establish that TPDs can be tailored to either selectivity degrades MERTK or concurrently degrade multiple TAMs and modulate receptor expression in vitro and in vivo. This work demonstrates the utility of proteome editing, enabled by tool degraders developed here towards dissecting the therapeutically relevant pathway biology in preclinical models, and the ability for TPDs to degrade transmembrane proteins. These data also provide proof of concept that TPDs may serve as a viable therapeutic strategy for targeting MERTK and other TAMs and that this technology could be expanded to other therapeutically relevant transmembrane proteins.

Damage-mediated macrophage polarization in sterile inflammation

Most of the leading causes of death, such as cardiovascular diseases, cancer, dementia, neurodegenerative diseases, and many more, are associated with sterile inflammation, either as a cause or a consequence of these conditions. The ability to control the progression of inflammation toward tissue resolution before it becomes chronic holds significant clinical potential. During sterile inflammation, the initiation of inflammation occurs through damage-associated molecular patterns (DAMPs) in the absence of pathogen-associated molecules. Macrophages, which are primarily localized in the tissue, play a pivotal role in sensing DAMPs. Furthermore, macrophages can also detect and respond to resolution-associated molecular patterns (RAMPs) and specific pro-resolving mediators (SPMs) during sterile inflammation. Macrophages, being highly adaptable cells, are particularly influenced by changes in the microenvironment. In response to the tissue environment, monocytes, pro-inflammatory macrophages, and pro-resolution macrophages can modulate their differentiation state. Ultimately, DAMP and RAMP-primed macrophages, depending on the predominant subpopulation, regulate the balance between inflammatory and resolving processes. While sterile injury and pathogen-induced reactions may have distinct effects on macrophages, most studies have focused on macrophage responses induced by pathogens. In this review, which emphasizes available human data, we illustrate how macrophages sense these mediators by examining the expression of receptors for DAMPs, RAMPs, and SPMs. We also delve into the signaling pathways induced by DAMPs, RAMPs, and SPMs, which primarily contribute to the regulation of macrophage differentiation from a pro-inflammatory to a pro-resolution phenotype. Understanding the regulatory mechanisms behind the transition between macrophage subtypes can offer insights into manipulating the transition from inflammation to resolution in sterile inflammatory diseases.

Internalization of apoptotic cells during efferocytosis requires Mertk-mediated calcium influx

Phagocytosis of apoptotic cells, called efferocytosis, requires calcium inside and outside of phagocytes. Due to its necessity, calcium flux is sophisticatedly modulated, and the level of intracellular calcium in phagocytes is ultimately elevated during efferocytosis. However, the role of elevated intracellular calcium in efferocytosis remains elusive. Here, we report that Mertk-mediated intracellular calcium elevation is necessary for internalization of apoptotic cells during efferocytosis. Drastic depletion of intracellular calcium abrogated the internalization step of efferocytosis by delaying phagocytic cup extension and closure. Especially, the defect of phagocytic cup closure for internalization of apoptotic cells was caused by impaired F-actin disassembly and the attenuated interaction of Calmodulin with myosin light chain kinase (MLCK), leading to diminished myosin light chain (MLC) phosphorylation. Genetic and pharmacological impairment of the Calmodulin-MLCK-MLC axis or Mertk-mediated calcium influx also resulted in inefficient efferocytosis due to a defect in internalization of the targets. Taken together, our observations imply that intracellular calcium elevation through Mertk-mediated calcium influx facilitates efferocytosis by inducing myosin II-mediated contraction and F-actin disassembly required for internalization of apoptotic cells.

Efferocytosis is restricted by axon guidance molecule EphA4 via ERK/Stat6/Mertk signaling following brain injury

Background

Efferocytosis is a process that removes apoptotic cells and cellular debris. Clearance of these cells alleviates neuroinflammation and prevents the release of inflammatory molecules and promotes the production of anti-inflammatory cytokines to help maintain tissue homeostasis. The underlying mechanisms by which this occurs in the brain after injury remains ill-defined.

Methods

We demonstrate using GFP bone marrow chimeric knockout (KO) mice, that the axon guidance molecule EphA4 receptor tyrosine kinase is involved in suppressing Mertk signaling in the brain to restrict the function of efferocytosis on resident microglia and peripheral-derived monocyte/macrophages.

Results

Single-cell RNAseq identified Mertk expression, the primary receptor involved in efferocytosis, on monocytes, microglia, and a subset of astrocytes in the damaged cortex following brain injury. Loss of EphA4 on infiltrating GFP-expressing immune cells improved functional outcome concomitant with enhanced efferocytosis, and overall protein expression of p-Mertk, p-ERK, and p-Stat6. The percentage of GFP+ monocyte/macrophages and resident microglia engulfing NeuN+ or TUNEL+ cells was significantly higher in KO chimeric mice. Importantly, mRNA expression of Mertk and its cognate ligand Gas6 was significantly elevated in these mice compared to wild-type. Analysis of cell-specific expression showed that p-ERK and p-Stat6 co-localized with Mertk-expressing GFP + cells in the peri-lesional area of the cortex following brain injury. Using an in vitro efferocytosis assay, co-culturing pHrodo-labeled apoptotic Jurkat cells and bone marrow (BM)-derived macrophages, we demonstrate that efferocytosis efficiency and mRNA expression of Mertk and Gas6 was enhanced in the absence of EphA4. Select inhibitors of ERK and Stat6 attenuated this effect confirming that EphA4 suppresses monocyte/macrophage efferocytosis via inhibition of the ERK/Stat6 pathway.

Conclusions

Our findings implicate the Mertk/ERK/Stat6 axis as a novel regulator of apoptotic debris clearance in brain injury that is restricted by peripheral myeloid-derived EphA4 to prevent the resolution of inflammation.

Constructing Mal-Efferocytic Macrophage Model and Its Atherosclerotic Spheroids and Rat Model for Therapeutic Evaluation

Efferocytosis, responsible for apoptotic cell clearance, is an essential factor against atherosclerosis. It is reported that efferocytosis is severely impaired in fibroatheroma, especially in vulnerable thin cap fibroatheroma. However, there is a shortage of studies on efferocytosis defects in cell and animal models. Here, the impacts of oxidized low density lipoprotein (ox-LDL) and glut 1 inhibitor (STF31) on efferocytosis of macrophages are studied, and an evaluation system is constructed. Through regulating the cell ratios and stimulus, three types of atherosclerotic spheroids are fabricated, and a necrotic core emerges with surrounding apoptotic cells. Rat models present a similar phenomenon in that substantial apoptotic cells are uncleared in time in vulnerable plaque, and the model period is shortened to 7 weeks. Mechanism studies reveal that ox-LDL, through mRNA and miRNA modulation, downregulates efferocytosis receptor (PPARγ/LXRα/MerTK), internalization molecule (SLC29a1), and upregulates the competitive receptor CD300a that inhibits efferocytosis receptor-ligand binding process. The foam cell differentiation has also confirmed that CD36 and Lp-PLA2 levels are significantly elevated, and macrophages present an interesting transition into prothrombic phenotype. Collectively, the atherosclerotic models featured by efferocytosis defect provide a comprehensive platform to evaluate the efficacy of medicine and biomaterials for atherosclerosis treatment.

PFKFB2-mediated glycolysis promotes lactate-driven continual efferocytosis by macrophages

Resolving-type macrophages prevent chronic inflammation by clearing apoptotic cells through efferocytosis. These macrophages are thought to rely mainly on oxidative phosphorylation, but emerging evidence suggests a possible link between efferocytosis and glycolysis. To gain further insight into this issue, we investigated molecular-cellular mechanisms involved in efferocytosis-induced macrophage glycolysis and its consequences. We found that efferocytosis promotes a transient increase in macrophage glycolysis that is dependent on rapid activation of the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 2 (PFKFB2), which distinguishes this process from glycolysis in pro-inflammatory macrophages. Mice transplanted with activation-defective PFKFB2 bone marrow and then subjected to dexamethasone-induced thymocyte apoptosis exhibit impaired thymic efferocytosis, increased thymic necrosis, and lower expression of the efferocytosis receptors MerTK and LRP1 on thymic macrophages compared with wild-type control mice. In vitro mechanistic studies revealed that glycolysis stimulated by the uptake of a first apoptotic cell promotes continual efferocytosis through lactate-mediated upregulation of MerTK and LRP1. Thus, efferocytosis-induced macrophage glycolysis represents a unique metabolic process that sustains continual efferocytosis in a lactate-dependent manner. The differentiation of this process from inflammatory macrophage glycolysis raises the possibility that it could be therapeutically enhanced to promote efferocytosis and resolution in chronic inflammatory diseases.

Cystathionine gamma-lyase (Cth) induces efferocytosis in macrophages via ERK1/2 to modulate intestinal barrier repair

BACKGROUND

The inflammatory response induced by intestinal ischaemia‒reperfusion injury (I/R) is closely associated with infectious complications and mortality in critically ill patients, and the timely and effective clearance of apoptotic cells is an important part of reducing the inflammatory response. Studies have shown that the efferocytosis by phagocytes plays an important role. Recently, studies using small intestine organoid models showed that macrophage efferocytosis could promote the repair capacity of the intestinal epithelium. However, no studies have reported efferocytosis in the repair of I/R in animal models.

RESULTS

We used an in vivo efferocytosis assay and discovered that macrophage efferocytosis played an indispensable role in repairing and maintaining intestinal barrier function after I/R. In addition, the specific molecular mechanism that induced macrophage efferocytosis was Cth-ERK1/2 dependent. We found that Cth drove macrophage efferocytosis in vivo and in vitro. Overexpression/silencing Cth promoted/inhibited the ERK1/2 pathway, respectively, which in turn affected efferocytosis and mediated intestinal barrier recovery. In addition, we found that the levels of Cth and macrophage efferocytosis were positively correlated with the recovery of intestinal function in clinical patients.

CONCLUSION

Cth can activate the ERK1/2 signalling pathway, induce macrophage efferocytosis, and thus promote intestinal barrier repair. Video Abstract.

Absence of MerTK disrupts spermatogenesis in an age-dependent manner

Spermatogenesis is a highly specialized cell differentiation process regulated by the testicular microenvironment. During the process of spermatogenesis, phagocytosis performs an essential role in male germ cell development, and its dysfunction in the testis can cause reproduction defects. MerTK, as a critical protein of phagocytosis, facilitates the removal of apoptotic substrates from the retina and ovaries through cooperation with several phagocytosis receptors. However, its role in mammalian spermatogenesis remains undefined. Here, we found that 30-week-old MerTK^-/- male mice developed oligoasthenospermia due to abnormal spermatogenesis. These mice showed damaged seminiferous tubule structure, as well as altered spermatogonia proliferation and differentiation. We also found that Sertoli cells from MerTK^-/- mice had decreased phagocytic activity on apoptotic germ cells in vitro. Moreover, a transcriptomic analysis demonstrated that the pivotal genes involved in spermatid differentiation and development changed expression. These results indicate that MerTK is crucial for spermatogenesis, as it regulates the crosstalk between germ cells and Sertoli cells. This provides us insight into the molecular mechanism of MerTK on spermatogenesis and its implications for the diagnosis and treatment of human male infertility.

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

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

Macrophage: Key player in the pathogenesis of autoimmune diseases

The macrophage is an essential part of the innate immune system and also serves as the bridge between innate immunity and adaptive immune response. As the initiator and executor of the adaptive immune response, macrophage plays an important role in various physiological processes such as immune tolerance, fibrosis, inflammatory response, angiogenesis and phagocytosis of apoptotic cells. Consequently, macrophage dysfunction is a vital cause of the occurrence and development of autoimmune diseases. In this review, we mainly discuss the functions of macrophages in autoimmune diseases, especially in systemic lupus erythematosus (SLE), rheumatic arthritis (RA), systemic sclerosis (SSc) and type 1 diabetes (T1D), providing references for the treatment and prevention of autoimmune diseases.

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.

N-glycosylation stabilizes MerTK and promotes hepatocellular carcinoma tumor growth

Despite the evidences of elevated expression of Mer tyrosine kinase (MerTK) in multiple human cancers, mechanisms underlying the oncogenic roles of MerTK in hepatocellular carcinoma (HCC) remains undefined. We explored the functional effects of MerTK and N-Glycosylated MerTK on HCC cell survival and tumor growth. Here, we show that MerTK ablation increases reactive oxygen species (ROS) production and promotes the switching from glycolytic metabolism to oxidative phosphorylation in HCC cells, thus suppressing HCC cell proliferation and tumor growth. MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote oncogenic transformation. Moreover, we observed that nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress. Pathologically, tissue microarray (TMA) data indicate that MerTK is a pivotal prognostic factor for HCC. Our data strongly support the roles of MerTK N-glycosylation in HCC tumorigenesis and suggesting N-glycosylation inhibition as a potential HCC therapeutic strategy.

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MerTK promotes the switching from oxidative phosphorylation to glycolytic metabolism in HCC cells.

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MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote HCC tumor growth.

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The nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress.

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MerTK is a pivotal prognostic factor for HCC and its N-glycosylation inhibition is a potential HCC therapeutic strategy.

Interaction between macrophages and ferroptosis

Abstract

Ferroptosis, a newly discovered iron-dependent cell death pathway, is characterized by lipid peroxidation and GSH depletion mediated by iron metabolism and is morphologically, biologically and genetically different from other programmed cell deaths. Besides, ferroptosis is usually found accompanied by inflammatory reactions. So far, it has been found participating in the development of many kinds of diseases. Macrophages are a group of immune cells that widely exist in our body for host defense and play an important role in tissue homeostasis by mediating inflammation and regulating iron, lipid and amino acid metabolisms through their unique functions like phagocytosis and efferocytosis, cytokines secretion and ROS production under different polarization. According to these common points in ferroptosis characteristics and macrophages functions, it’s obvious that there must be relationship between macrophages and ferroptosis. Therefore, our review aims at revealing the interaction between macrophages and ferroptosis concerning three metabolisms and integrating the application of certain relationship in curing diseases, mostly cancer. Finally, we also provide inspirations for further studies in therapy for some diseases by targeting certain resident macrophages in distinct tissues to regulate ferroptosis.

Facts

Ferroptosis is considered as a newly discovered form characterized by its nonapoptotic and iron-dependent lipid hydroperoxide, concerning iron, lipid and amino acid metabolisms.

Ferroptosis has been widely found playing a crucial part in various diseases, including hepatic diseases, neurological diseases, cancer, etc.

Macrophages are phagocytic immune cells, widely existing and owning various functions such as phagocytosis and efferocytosis, cytokines secretion and ROS production.

Macrophages are proved to participate in mediating metabolisms and initiating immune reactions to maintain balance in our body.

Recent studies try to treat cancer by altering macrophages’ polarization which damages tumor microenvironment and induces ferroptosis of cancer cells.

Open questions

How do macrophages regulate ferroptosis of other tissue cells specifically?

Can we use the interaction between macrophages and ferroptosis in treating diseases other than cancer?

What can we do to treat diseases related to ferroptosis by targeting macrophages?

Is the use of the relationship between macrophages and ferroptosis more effective than other therapies when treating diseases?

Macrophages use apoptotic cell-derived methionine and DNMT3A during efferocytosis to promote tissue resolution

Efferocytosis, the clearance of apoptotic cells (ACs) by macrophages, is critical for tissue resolution, with defects driving many diseases. Mechanisms of efferocytosis-mediated resolution are incompletely understood. Here, we show that AC-derived methionine regulates resolution through epigenetic repression of the extracellular signal-regulated kinase 1/2 (ERK1/2) phosphatase Dusp4. We focus on two key efferocytosis-induced pro-resolving mediators, prostaglandin E2 (PGE2) and transforming growth factor beta 1 (TGF-β1), and show that efferocytosis induces prostaglandin-endoperoxide synthase 2/cyclooxygenase 2 (Ptgs2/COX2), leading to PGE2 synthesis and PGE2-mediated induction of TGF-β1. ERK1/2 phosphorylation/activation by AC-activated CD36 is necessary for Ptgs2 induction, but this is insufficient owing to an ERK-DUSP4 negative feedback pathway that lowers phospho-ERK. However, subsequent AC engulfment and phagolysosomal degradation lead to Dusp4 repression, enabling enhanced p-ERK and induction of the Ptgs2-PGE2-TGF-β1 pathway. Mechanistically, AC-derived methionine is converted to S-adenosylmethionine, which is used by DNA methyltransferase-3A (DNMT3A) to methylate Dusp4. Bone-marrow DNMT3A deletion in mice blocks COX2/PGE2, TGF-β1, and resolution in sterile peritonitis, apoptosis-induced thymus injury and atherosclerosis. Knowledge of how macrophages use AC-cargo and epigenetics to induce resolution provides mechanistic insight and therapeutic options for diseases driven by impaired resolution.

Single-Cell Sequencing Reveals an Intrinsic Heterogeneity of the Preovulatory Follicular Microenvironment

The follicular microenvironment, including intra-follicular granulosa cells (GCs), is responsible for oocyte maturation and subsequent ovulation. However, the functions of GCs and cellular components of the follicular microenvironment in preovulatory follicles have not been extensively explored. Here, we surveyed the single-cell transcriptome of the follicular microenvironment around MII oocytes in six human preovulatory follicles in in vitro fertilization. There were six different cell types in the preovulatory follicles, including GCs and various immune cells. In GCs, we identified nine different functional clusters with different functional transcriptomic profiles, including specific clusters involved in inflammatory responses and adhesive function. Follicular macrophages are involved in immune responses, extracellular matrix remoulding and assist GCs in promoting the oocyte meiotic resumption. Interestingly, we observed that the specific terminal state subcluster of GCs with high levels of adhesive-related molecules should result in macrophage recruitment and residence, further contributing to an obvious heterogeneity of the immune cell proportion in preovulatory follicles from different patients. Our results provide a comprehensive understanding of the transcriptomic landscape of the preovulatory follicular microenvironment at the single-cell level. It provides valuable insights into understanding the regulation of the oocyte maturation and ovulation process, offering potential clues for the diagnosis and treatment of oocyte-maturation-related and ovulation-related diseases.

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

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

[Patterns of immunological reactions in the pathogenesis of chronic heart failure: review]

The immune system is essential for maintaining the homeostasis. At present, there is convincing evidence for participation of the immune system in the pathogenesis of cardiovascular pathology, including the final step of cardiovascular continuum, heart failure. Objective difficulties in understanding subtle processes of loss of the normal cardiac structure and function are based on the diversity of pathogenetic factors of development and progression of chronic heart failure (CHF) and the involvement of most organs and body systems. Russian and international scientists actively study issues of immune homeostasis, including the efficacy of current immune therapy. At the same time, available reports are largely uncompiled and reflect isolated parts of the immunopathogenesis of cardiovascular diseases. This review focuses on comprehensive elucidation of major patterns of immune processes in the CHF pathogenesis to form an integral view of the problem under study.

Efferocytosis induces macrophage proliferation to help resolve tissue injury

Apoptotic cell clearance by macrophages (efferocytosis) promotes resolution signaling pathways, which can be triggered by molecules derived from the phagolysosomal degradation of apoptotic cells. We show here that nucleotides derived from the hydrolysis of apoptotic cell DNA by phagolysosomal DNase2a activate a DNA-PKcs-mTORC2/Rictor pathway that increases Myc to promote non-inflammatory macrophage proliferation. Efferocytosis-induced proliferation expands the pool of resolving macrophages in vitro and in mice, including zymosan-induced peritonitis, dexamethasone-induced thymocyte apoptosis, and atherosclerosis regression. In the dexamethasone-thymus model, hematopoietic Rictor deletion blocked efferocytosing macrophage proliferation, apoptotic cell clearance, and tissue resolution. In atherosclerosis regression, silencing macrophage Rictor or DNase2a blocked efferocyte proliferation, apoptotic cell clearance, and plaque stabilization. In view of previous work showing that other types of apoptotic cell cargo can promote resolution in individual efferocytosing macrophages, the findings here suggest that signaling-triggered apoptotic cell-derived nucleotides can amplify this benefit by increasing the number of these macrophages.

Lifting the innate immune barriers to antitumor immunity

The immune system evolved for adequate surveillance and killing of pathogens while minimizing host damage, such as due to chronic or exaggerated inflammation and autoimmunity. This is achieved by negative regulators and checkpoints that limit the magnitude and time course of the immune response. Tumor cells often escape immune surveillance and killing. Therefore, disrupting the brakes built into the immune system should effectively boost the anticancer immune response. The success of anti-CTLA4, anti-PD-1 and anti-PD-L1 have firmly established this proof of concept. Since the response rate of anti-CTLA4, anti-PD-1 and anti-PD-L1 is still limited, there is an intense effort for the identification of new targets and development of approaches that can expand the benefits of immunotherapy to a larger patient pool. Additional T cell checkpoints are obvious targets; however, here we focus on the unusual suspects—cells that function to initiate and guide T cell activity. Innate immunity is both an obligate prerequisite for the initiation of adaptive immune responses and a requirement for the recruitment of activated T cells to the site of action. We discuss some of the molecules present in innate immune cells, including natural killer cells, dendritic cells, macrophages, myeloid-derived suppressor cells, endothelial cells and stromal cells, that can activate or enhance innate immune cell functions, and more importantly, the inhibitors or checkpoints present in these cells that restrain their functions. Boosting innate immunity, either by enhancing activator functions or, preferably, by blocking the inhibitors, may represent a new anticancer treatment modality or at least function as adjuvants to T cell checkpoint inhibitors.

EphA2 Interacts with Tim-4 through Association between Its FN3 Domain and the IgV Domain of Tim-4

Tim-4 promotes the engulfment of apoptotic cells or exogenous particles by securing them on phagocytes. It is unable to transduce signals by itself but helps other engulfment receptors sense and internalize them. However, the identity of the engulfment receptors collaborating with Tim-4 is still incompletely understood. In this study, we searched for a candidate transmembrane protein with a FN3 domain, important for interaction with Tim-4, in silico and investigated whether it indeed interacts with Tim-4 and is involved in Tim-4-mediated phagocytosis. We found that EphA2 containing a FN3 domain in the extracellular region interacted with Tim-4, which was mediated by the IgV domain of Tim-4 and the FN3 domain of EphA2. Nevertheless, we found that EphA2 expression failed to alter Tim-4-mediated phagocytosis of apoptotic cells or polystyrene beads. Taken together, our findings suggest that EphA2, a new Tim-4 interacting protein, may intervene in a Tim-4-mediated cellular event even if it is not phagocytosis of endogenous or exogenous particles and vice versa.

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

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

Mechanisms of Macrophage Plasticity in the Tumor Environment: Manipulating Activation State to Improve Outcomes

Macrophages are a specialized class of innate immune cells with multifaceted roles in modulation of the inflammatory response, homeostasis, and wound healing. While developmentally derived or originating from circulating monocytes, naïve macrophages can adopt a spectrum of context-dependent activation states ranging from pro-inflammatory (classically activated, M1) to pro-wound healing (alternatively activated, M2). Tumors are known to exploit macrophage polarization states to foster a tumor-permissive milieu, particularly by skewing macrophages toward a pro-tumor (M2) phenotype. These pro-tumoral macrophages can support cancer progression by several mechanisms including immune suppression, growth factor production, promotion of angiogenesis and tissue remodeling. By preventing the adoption of this pro-tumor phenotype or reprogramming these macrophages to a more pro-inflammatory state, it may be possible to inhibit tumor growth. Here, we describe types of tumor-derived signaling that facilitate macrophage reprogramming, including paracrine signaling and activation of innate immune checkpoints. We also describe intervention strategies targeting macrophage plasticity to limit disease progression and address their implications in cancer chemo- and immunotherapy.

Recent advances in dead cell clearance during acute lung injury and repair

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are clinical syndromes that cause significant mortality in clinical settings and morbidity among survivors accompanied by huge healthcare costs. Lung-resident cell dysfunction/death and neutrophil alveolitis accompanied by proteinous edema are the main pathological features of ALI/ARDS. While understanding of the mechanisms underlying ALI/ARDS pathogenesis is progressing and potential treatments such as statin therapy, nutritional strategies, and mesenchymal cell therapy are emerging, poor clinical outcomes in ALI/ARDS patients persist. Thus, a better understanding of lung-resident cell death and neutrophil alveolitis and their mitigation and clearance mechanisms may provide new therapeutic strategies to accelerate lung repair and improve outcomes in critically ill patients. Macrophages are required for normal tissue development and homeostasis as well as regulating tissue injury and repair through modulation of inflammation and other cellular processes. While macrophages mediate various functions, here we review recent dead cell clearance (efferocytosis) mechanisms mediated by these immune cells for maintaining tissue homeostasis after infectious and non-infectious lung injury.

Participation of ABCA1 Transporter in Pathogenesis of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is the important medical and social problem. According to modern concepts, COPD is a chronic inflammatory disease, macrophages play a key role in its pathogenesis. Macrophages are heterogeneous in their functions, which is largely determined by their immunometabolic profile, as well as the features of lipid homeostasis, in which the ATP binding cassette transporter A1 (ABCA1) plays an essential role. The objective of this work is the analysis of the ABCA1 protein participation and the function of reverse cholesterol transport in the pathogenesis of COPD. The expression of the ABCA1 gene in lung tissues takes the second place after the liver, which indicates the important role of the carrier in lung function. The participation of the transporter in the development of COPD consists in provision of lipid metabolism, regulation of inflammation, phagocytosis, and apoptosis. Violation of the processes in which ABCA1 is involved may be a part of the pathophysiological mechanisms, leading to the formation of a heterogeneous clinical course of the disease.

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.

TAM kinases as regulators of cell death

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.

Targeting Tumor-Associated Macrophages in the Pediatric Sarcoma Tumor Microenvironment

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

Cell Death in the Tumor Microenvironment: Implications for Cancer Immunotherapy

The physiological fate of cells that die by apoptosis is their prompt and efficient removal by efferocytosis. During these processes, apoptotic cells release intracellular constituents that include purine nucleotides, lysophosphatidylcholine (LPC), and Sphingosine-1-phosphate (S1P) that induce migration and chemo-attraction of phagocytes as well as mitogens and extracellular membrane-bound vesicles that contribute to apoptosis-induced compensatory proliferation and alteration of the extracellular matrix and the vascular network. Additionally, during efferocytosis, phagocytic cells produce a number of anti-inflammatory and resolving factors, and, together with apoptotic cells, efferocytic events have a homeostatic function that regulates tissue repair. These homeostatic functions are dysregulated in cancers, where, aforementioned events, if not properly controlled, can lead to cancer progression and immune escape. Here, we summarize evidence that apoptosis and efferocytosis are exploited in cancer, as well as discuss current translation and clinical efforts to harness signals from dying cells into therapeutic strategies.

MerTK Expression and ERK Activation Are Essential for the Functional Maturation of Osteopontin-Producing Reparative Macrophages After Myocardial Infarction

Background We previously reported that osteopontin plays an essential role in accelerating both reparative fibrosis and clearance of dead cells (efferocytosis) during tissue repair after myocardial infarction (MI) and galectin-3^hiCD206⁺ macrophages is the main source of osteopontin in post-MI heart. Interleukin-10- STAT3 (signal transducer and activator of transcription 3)-galectin-3 axis is essential for Spp1 (encoding osteopontin) transcriptional activation in cardiac macrophages after MI. Here, we investigated the more detailed mechanism responsible for functional maturation of osteopontin-producing macrophages. Methods and Results In post-MI hearts, Spp1 transcriptional activation occurred almost exclusively in MerTK (Mer tyrosine kinase)⁺ galectin-3^hi macrophages. The induction of MerTK on galectin-3^hi macrophages is essential for their functional maturation including efferocytosis and Spp1 transcriptional activity. MerTK⁺galectin-3^hi macrophages showed a strong activation of both STAT3 and ERK (extracellular signal-regulated kinase). STAT3 inhibition suppressed the differentiation of osteopontin-producing MerTK⁺galectin-3^hi macrophages, however, STAT3 activation was insufficient at inducing Spp1 transcriptional activity. ERK inhibition suppressed Spp1 transcriptional activation without affecting MerTK or galectin-3 expression. Concomitant activation of ERK is required for transcriptional activation of Spp1. In Il-10 knockout enhanced green fluorescent protein-Spp1 knock-in mice subjected to MI, osteopontin-producing macrophages decreased but did not disappear entirely. Interleukin-10 and macrophage colony-stimulating factor synergistically activated STAT3 and ERK and promoted the differentiation of osteopontin-producing MerTK⁺galectin-3^hi macrophages in bone marrow-derived macrophages. Coadministration of anti-interleukin-10 plus anti-macrophage colony-stimulating factor antibodies substantially reduced the number of osteopontin-producing macrophages by more than anti-interleukin-10 antibody alone in post-MI hearts. Conclusions Interleukin-10 and macrophage colony-stimulating factor act synergistically to activate STAT3 and ERK in cardiac macrophages, which in turn upregulate the expression of galectin-3 and MerTK, leading to the functional maturation of osteopontin-producing macrophages.

Single-cell RNA sequencing uncovers heterogenous transcriptional signatures in macrophages during efferocytosis

Efferocytosis triggers cellular reprogramming, including the induction of mRNA transcripts which encode anti-inflammatory cytokines that promote inflammation resolution. Our current understanding of this transcriptional response is largely informed from analysis of bulk phagocyte populations; however, this precludes the resolution of heterogeneity between individual macrophages and macrophage subsets. Moreover, phagocytes may contain so called "passenger" transcripts that originate from engulfed apoptotic bodies, thus obscuring the true transcriptional reprogramming of the phagocyte. To define the transcriptional diversity during efferocytosis, we utilized single-cell mRNA sequencing after co-cultivating macrophages with apoptotic cells. Importantly, transcriptomic analyses were performed after validating the disappearance of apoptotic cell-derived RNA sequences. Our findings reveal new heterogeneity of the efferocytic response at a single-cell resolution, particularly evident between F4/80+ MHCIILO and F4/80- MHCIIHI macrophage sub-populations. After exposure to apoptotic cells, the F4/80+ MHCIILO subset significantly induced pathways associated with tissue and cellular homeostasis, while the F4/80- MHCIIHI subset downregulated these putative signaling axes. Ablation of a canonical efferocytosis receptor, MerTK, blunted efferocytic signatures and led to the escalation of cell death-associated transcriptional signatures in F4/80+ MHCIILO macrophages. Taken together, our results newly elucidate the heterogenous transcriptional response of single-cell peritoneal macrophages after exposure to apoptotic cells.

Sensing and clearance of apoptotic cells

Macrophages specifically engulf apoptotic cells but not healthy cells. Phosphatidylserine (PtdSer) is localized at the inner leaflet of plasma membranes as a result of the action of flippases (ATP11A and 11C). When cells undergo apoptosis, caspase 3 cleaves and inactivates the flippases, while simultaneously cleaving XKR8 to activate its phospholipid scramblase activity. PtdSer is thus swiftly and irreversibly exposed to the cell surface as an 'eat me' signal. Tissue resident macrophages recognize the apoptotic cells using a PtdSer-receptor TIM4 and engulf them with TAM tyrosine-kinase receptors, and integrins. The PtdSer 'eat me' signal appears to override 'don't eat me' signals in most cases.

LC3-Associated Phagocytosis (LAP): A Potentially Influential Mediator of Efferocytosis-Related Tumor Progression and Aggressiveness

One aim of cancer therapies is to induce apoptosis of tumor cells. Efficient removal of the apoptotic cells requires coordinated efforts between the processes of efferocytosis and LC3-associated phagocytosis (LAP). However, this activity has also been shown to produce anti-inflammatory and immunosuppressive signals that can be utilized by live tumor cells to evade immune defense mechanisms, resulting in tumor progression and aggressiveness. In the absence of LAP, mice exhibit suppressed tumor growth during efferocytosis, while LAP-sufficient mice show enhanced tumor progression. Little is known about how LAP or its regulators directly affect efferocytosis, tumor growth and treatment responses, and identifying the mechanisms involved has the potential to lead to the discovery of novel approaches to target cancer cells. Also incompletely understood is the direct effect of apoptotic cancer cells on LAP. This is particularly important as induction of apoptosis by current cytotoxic cancer therapies can potentially stimulate LAP following efferocytosis. Herein, we highlight the current understanding of the role of LAP and its relationship with efferocytosis in the tumor microenvironment with a view to presenting novel therapeutic strategies.

Macrophages Orchestrate Hematopoietic Programs and Regulate HSC Function During Inflammatory Stress

The bone marrow contains distinct cell types that work in coordination to generate blood and immune cells, and it is the primary residence of hematopoietic stem cells (HSCs) and more committed multipotent progenitors (MPPs). Even at homeostasis the bone marrow is a dynamic environment where billions of cells are generated daily to replenish short-lived immune cells and produce the blood factors and cells essential for hemostasis and oxygenation. In response to injury or infection, the marrow rapidly adapts to produce specific cell types that are in high demand revealing key insight to the inflammatory nature of "demand-adapted" hematopoiesis. Here we focus on the role that resident and monocyte-derived macrophages play in driving these hematopoietic programs and how macrophages impact HSCs and downstream MPPs. Macrophages are exquisite sensors of inflammation and possess the capacity to adapt to the environment, both promoting and restraining inflammation. Thus, macrophages hold great potential for manipulating hematopoietic output and as potential therapeutic targets in a variety of disease states where macrophage dysfunction contributes to or is necessary for disease. We highlight essential features of bone marrow macrophages and discuss open questions regarding macrophage function, their role in orchestrating demand-adapted hematopoiesis, and mechanisms whereby they regulate HSC function.

Mertk Interacts with Tim-4 to Enhance Tim-4-Mediated Efferocytosis

Apoptotic cells expressing phosphatidylserine (PS) on their cell surface are directly or indirectly recognized by phagocytes through PS-binding proteins. The PS-binding protein Tim-4 secures apoptotic cells to phagocytes to facilitate the engulfment of apoptotic cells. However, the molecular mechanism by which Tim-4 transduces signals to phagocytes during Tim-4-mediated efferocytosis is incompletely understood. Here, we report that Tim-4 collaborates with Mertk during efferocytosis through a biochemical interaction with Mertk. Proximal localization between the two proteins in phagocytes was observed by immunofluorescence and proximal ligation assays. Physical association between Tim-4 and Mertk, which was mediated by an interaction between the IgV domain of Tim-4 and the fibronectin type-III domain of Mertk, was also detected with immunoprecipitation. Furthermore, the effect of Mertk on Tim-4-mediated efferocytosis was abolished by GST-Mertk^FnIII, a soluble form of the fibronectin type-III domain of Mertk that disrupts the interaction between Tim-4 and Mertk. Taken together, the results from our study suggest that a physical interaction between Tim-4 and Mertk is necessary for Mertk to enhance efferocytosis mediated by Tim-4.

Efferocytosis and Its Associated Cytokines: A Light on Non-tumor and Tumor Diseases?

Billions of cells undergo turnover and die via apoptosis throughout our lifetime. A prompt clearance of these apoptotic cells and debris by phagocytic cells, a process known as efferocytosis, is important in maintaining tissue homeostasis. Accordingly, impaired efferocytosis due to the defective clearance and disrupted stages can lead to a growing number of inflammation- and immune-related diseases. Although numerous studies have shown the mechanisms of efferocytosis, its role in disorders, such as non-tumor and tumor diseases, remains poorly understood. This review summarizes the processes and signal molecules in efferocytosis, and efferocytosis-related functions in non-tumor (e.g., atherosclerosis, lung diseases) and tumor diseases (e.g., breast cancer, prostate cancer), as well as describes the role of involved cytokines. Of note, there is a dual role of efferocytosis in the abovementioned disorders, and a paradoxical effect among non-tumor and tumor diseases in terms of inflammation resolution, immune response, and disease progression. Briefly, intact efferocytosis and cytokines promote tissue repair, while they contribute to tumor progression via the tumor microenvironment and macrophage politzerization. Additionally, this review provides potential targets associated with TAM (TYRO3, AXL, MERTK) receptors and cytokines, such as tumor necrosis factor α and CXCL5, suggesting potential novel therapeutic ways in treating diseases.

Phosphatidylserine recognition and Rac1 activation are required for Müller glia proliferation, gliosis and phagocytosis after retinal injury

Müller glia, the principal glial cell type in the retina, have the potential to reenter the cell cycle after retinal injury. In mammals, proliferation of Müller glia is followed by gliosis, but not regeneration of neurons. Retinal injury is also accompanied by phagocytic removal of degenerated cells. We here investigated the possibility that proliferation and gliosis of Müller glia and phagocytosis of degenerated cells may be regulated by the same molecular pathways. After N-methyl-N–nitrosourea-induced retinal injury, degenerated photoreceptors were eliminated prior to the infiltration of microglia/macrophages into the outer nuclear layer, almost in parallel with cell cycle reentry of Müller glia. Inhibition of microglia/macrophage activation with minocycline did not affect the photoreceptor clearance. Accumulation of lysosomes and rhodopsin-positive photoreceptor debris within the cytoplasm of Müller glia indicated that Müller glia phagocytosed most photoreceptor debris. Pharmacological inhibition of phosphatidylserine and Rac1, key regulators of the phagocytic pathway, prevented cell cycle reentry, migration, upregulation of glial fibrillary acidic protein, and phagocytic activity of Müller glia. These data provide evidence that phosphatidylserine and Rac1 may contribute to the crosstalk between different signaling pathways activated in Müller glia after injury.

Contribution of Macrophage Efferocytosis to Liver Homeostasis and Disease

The clearance of apoptotic cells is pivotal for both maintaining tissue homeostasis and returning to homeostasis after tissue injury as part of the regenerative resolution response. The liver is known for its capacity to remove aged and damaged cells from the circulation and can serve as a graveyard for effector T cells. In particular Kupffer cells are active phagocytic cells, but during hepatic inflammatory responses incoming neutrophils and monocytes may contribute to pro-inflammatory damage. To stimulate resolution of such inflammation, myeloid cell function can change, via sensing of environmental changes in the inflammatory milieu. Also, the removal of apoptotic cells via efferocytosis and the signaling pathways that are activated in macrophages/phagocytes upon their engulfment of apoptotic cells are important for a return to tissue homeostasis. Here, we will discuss, how efferocytosis mechanisms in hepatic macrophages/phagocytes may regulate tissue homeostasis and be involved in tissue regeneration in liver disease.