Masking of phosphatidylserine inhibits apoptotic cell engulfment and induces autoantibody production in mice

Protein S and Gas6 induce efferocytosis of HIV-1-infected cells

Efferocytosis, the phagocytic clearance of apoptotic cells, can provide host protection against certain types of viruses by mediating phagocytic clearance of infected cells undergoing apoptosis. It is known that HIV-1 induces apoptosis and HIV-1-infected cells are efferocytosed by macrophages, although its molecular mechanisms are unknown. To elucidate the roles that efferocytosis of HIV-1-infected cells play in clearance of infected cells, we sought to identify molecules that mediate these processes. We found that protein S, present in human serum, and its homologue, Gas6, can mediate phagocytosis of HIV-1-infected cells by bridging receptor tyrosine kinase Mer, expressed on macrophages, to phosphatidylserine exposed on infected cells. Efferocytosis of live infected cells was less efficient than dead infected cells; however, a significant fraction of live infected cells were phagocytosed over 12 h. Our results suggest that efferocytosis not only removes dead cells, but may also contribute to macrophage removal of live virus producing cells.

15-Deoxy-Δ12,14-Prostaglandin J2 Exerts Proresolving Effects Through Nuclear Factor E2-Related Factor 2-Induced Expression of CD36 and Heme Oxygenase-1

AIMS

15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂) has been shown to rescue cells from inflammatory insults and to participate in the resolution of acute inflammation. In this study, we investigated molecular mechanisms underlying proresolving effects of 15d-PGJ₂.

RESULTS

15d-PGJ₂ injected into the peritoneum of mice facilitated the resolution of zymosan A-induced peritonitis. 15d-PGJ₂ administration reduced the number of total leukocytes and attenuated polymorphonuclear leukocyte infiltration. Furthermore, 15d-PGJ₂ increased the proportion of macrophages engulfing apoptotic neutrophils, a process called efferocytosis. In addition, when the thioglycollate-elicited mouse peritoneal macrophages were stimulated with 15d-PGJ₂, their efferocytic activity was amplified. In another experiment, RAW264.7 murine macrophages exposed to 15d-PGJ₂ conducted phagocytic clearance of apoptotic cells to a greater extent than the control cells. Under these conditions, expression of CD36 and heme oxygenase-1 (HO-1) was enhanced along with increased accumulation of the nuclear factor E2-related factor 2 (Nrf2) in the nucleus. Knockdown of Nrf2 abolished 15d-PGJ₂-induced expression of CD36 and HO-1, and silencing of CD36 and HO-1 attenuated 15d-PGJ₂-induced efferocytosis. Moreover, peritoneal macrophages isolated from Nrf2-null mice failed to upregulate 15d-PGJ₂-induced expression of CD36 and HO-1 and to mediate efferocytosis. Unlike 15d-PGJ₂, its nonelectrophilic analog 9,10-dihydro-15d-PGJ₂ lacking the α,β-unsaturated carbonyl group could not induce CD36 expression and efferocytosis.

INNOVATION

15d-PGJ₂, as one of the terminal products of cyclooxygenase-2, exerts proresolving effects through induction of efferocytosis. The results of this study suggest that 15d-PGJ₂ possesses a therapeutic value in the management of inflammatory disorders.

CONCLUSION

15d-PGJ₂ facilitates resolution of inflammation by inducing Nrf2-induced expression of CD36 and HO-1 in macrophages. Antioxid. Redox Signal. 27, 1412-1431.

Tissue-Resident Macrophages Are Locally Programmed for Silent Clearance of Apoptotic Cells

Although apoptotic cells (ACs) contain nucleic acids that can be recognized by Toll-like receptors (TLRs), engulfment of ACs does not initiate inflammation in healthy organisms. Here we identified macrophage populations that continually engulf ACs in distinct tissues and found that these macrophages share characteristics compatible with immunologically silent clearance of ACs; such characteristics include high expression of AC recognition receptors, low expression of TLR9, and reduced TLR responsiveness to nucleic acids. Removal of the macrophages from tissues resulted in loss of many of these characteristics and the ability to generate inflammatory responses to AC-derived nucleic acids, suggesting that cues from the tissue microenvironment program macrophages for silent AC clearance. The transcription factors KLF2 and KLF4 control the expression of many genes within this AC clearance program. The coordinated expression of AC receptors with genes that limit responses to nucleic acids might ensure maintenance of homeostasis and thus represent a central feature of tissue macrophages.

Apoptotic Cells Induced Signaling for Immune Homeostasis in Macrophages and Dendritic Cells

Inefficient and abnormal clearance of apoptotic cells (efferocytosis) contributes to systemic autoimmune disease in humans and mice, and inefficient chromosomal DNA degradation by DNAse II leads to systemic polyarthritis and a cytokine storm. By contrast, efficient clearance allows immune homeostasis, generally leads to a non-inflammatory state for both macrophages and dendritic cells (DCs), and contributes to maintenance of peripheral tolerance. As many as 3 × 10⁸ cells undergo apoptosis every hour in our bodies, and one of the primary “eat me” signals expressed by apoptotic cells is phosphatidylserine (PtdSer). Apoptotic cells themselves are major contributors to the “anti-inflammatory” nature of the engulfment process, some by secreting thrombospondin-1 (TSP-1) or adenosine monophosphate and possibly other immune modulating “calm-down” signals that interact with macrophages and DCs. Apoptotic cells also produce “find me” and “tolerate me” signals to attract and immune modulate macrophages and DCs that express specific receptors for some of these signals. Neither macrophages nor DCs are uniform, and each cell type may variably express membrane proteins that function as receptors for PtdSer or for opsonins like complement or opsonins that bind to PtdSer, such as protein S and growth arrest-specific 6. Macrophages and DCs also express scavenger receptors, CD36, and integrins that function via bridging molecules such as TSP-1 or milk fat globule-EGF factor 8 protein and that differentially engage in various multi-ligand interactions between apoptotic cells and phagocytes. In this review, we describe the anti-inflammatory and pro-homeostatic nature of apoptotic cell interaction with the immune system. We do not review some forms of immunogenic cell death. We summarize the known apoptotic cell signaling events in macrophages and DCs that are related to toll-like receptors, nuclear factor kappa B, inflammasome, the lipid-activated nuclear receptors, Tyro3, Axl, and Mertk receptors, as well as induction of signal transducer and activator of transcription 1 and suppressor of cytokine signaling that lead to immune system silencing and DC tolerance. These properties of apoptotic cells are the mechanisms that enable their successful use as therapeutic modalities in mice and humans in various autoimmune diseases, organ transplantation, graft-versus-host disease, and sepsis.

Deficiency in milk fat globule-epidermal growth factor-factor 8 exacerbates organ injury and mortality in neonatal sepsis

INTRODUCTION

Neonatal sepsis is a systemic inflammation occurring in neonates because of a proven infection within the first 28days of birth. It is the third leading cause of morbidity and mortality in the newborns. The mechanism(s) underlying the systemic inflammation in neonatal sepsis has not been completely understood. We hypothesize that the deficiency of milk fat globule-epidermal growth factor-factor 8 (MFG-E8), a protein commonly found in human milk, could be responsible for the increased inflammatory response leading to morbidity and mortality in neonatal sepsis.

METHODS

Male and female newborn mice aged 5-7days were injected intraperitoneally with 0.9mg/g body weight cecal slurry (CS). At 10h after CS injection, they were euthanized, and blood, lungs and gut tissues were obtained for further analyses. Control newborn mice underwent similar procedures with the exception of the CS injection. In duplicate newborn mice after CS injection, they were returned to their respective cages with their mothers and were closely monitored for 7days and survival rate recorded.

RESULTS

At 10h after CS injection, serum LDH in the MFG-E8 knockout (KO) newborn mice was significantly increased by 58% and serum IL-6, IL-1β and TNF-α in the MFG-E8KO newborn mice were also significantly increased by 56%, 65%, and 105%, respectively, from wild type (WT) newborn mice. There were no significant difference between WT control and MFG-E8 control newborn mice. The lung architecture was severely damaged and a significant 162% increase in injury score was observed in the CS MFG-E8KO newborn mice. The MPO, TUNEL staining, and cytokine levels in the lungs and the intestine in CS MFG-E8KO newborn mice were significantly increased from CS WT newborn mice. Similarly, intestinal integrity was also compromised in the CS MFG-E8KO newborn mice. In a survival study, while the mortality rate within 7days was only 29% in the CS WT newborn mice, 80% of the CS MFG-E8KO newborn mice died during the same time period with the majority of mortality occurring within 48h.

CONCLUSION

The deficiency in MFG-E8 caused increases in inflammation, tissue injury, neutrophil infiltration and apoptosis, which led to morbidity and mortality in murine neonatal sepsis. These studies suggest that MFG-E8 has a protective role in fighting against neonatal sepsis.

Clearance of Dying Cells by Phagocytes: Mechanisms and Implications for Disease Pathogenesis

The efficient clearance of apoptotic cells is an evolutionarily conserved process crucial for homeostasis in multicellular organisms. The clearance involves a series of steps that ultimately facilitates the recognition of the apoptotic cell by the phagocytes and the subsequent uptake and processing of the corpse. These steps include the phagocyte sensing of "find-me" signals released by the apoptotic cell, recognizing "eat-me" signals displayed on the apoptotic cell surface, and then intracellular signaling within the phagocyte to mediate phagocytic cup formation around the corpse and corpse internalization, and the processing of the ingested contents. The engulfment of apoptotic cells by phagocytes not only eliminates debris from tissues but also produces an anti-inflammatory response that suppresses local tissue inflammation. Conversely, impaired corpse clearance can result in loss of immune tolerance and the development of various inflammation-associated disorders such as autoimmunity, atherosclerosis, and airway inflammation but can also affect cancer progression. Recent studies suggest that the clearance process can also influence antitumor immune responses. In this review, we will discuss how apoptotic cells interact with their engulfing phagocytes to generate important immune responses, and how modulation of such responses can influence pathology.

Elevated serum MFG-E8 level is possibly associated with the presence of high-intensity cerebral lesions on magnetic resonance imaging in patients with systemic lupus erythematosus

Human milk fat globule-EGF factor 8 (MFG-E8), also known as lactadherin, is a secreted glycoprotein that plays essential roles in the clearance of apoptotic cells and angiogenesis. It has been reported that serum MFG-E8 levels are higher in systemic lupus erythematosus (SLE) patients compared with in healthy controls; however, a previous study reported no correlation between serum MFG-E8 levels and SLE disease activity. The objective of this study was to assess serum MFG-E8 levels and their clinical associations in patients with SLE. Serum MFG-E8 levels in 49 Japanese patients with SLE, eight with cutaneous LE, and 28 healthy controls were examined. Serum MFG-E8 levels in SLE patients were significantly higher than those in cutaneous LE patients and healthy individuals. In addition, serum MFG-E8 levels were positively correlated with the SLE Disease Activity Index score, which reflects the disease activity of SLE. Notably, the frequency of the presence of high-intensity cerebral lesions on MRI in the SLE patients with elevated serum MFG-E8 levels was significantly higher than that in SLE patients with normal serum MFG-E8 levels. These findings suggest that elevated serum MFG-E8 levels may be associated with cerebrovascular diseases or neuropsychiatric SLE in patients with SLE, and that the measurement of serum MFG-E8 levels in SLE patients is useful for risk stratification of cerebrovascular disease or cerebrovascular disease-related neuropsychiatric SLE.

Mesenchymal stem cells-derived MFG-E8 accelerates diabetic cutaneous wound healing

BACKGROUND

Diabetic wounds are intractable due to complex factors, such as the inhibition of angiogenesis, dysfunction of phagocytosis by macrophages and abnormal inflammatory responses. It is recognized that mesenchymal stem cells (MSCs) promote wound healing in diabetic mice. We previously demonstrated that MSCs produce large amounts of MFG-E8.

OBJECT

The objective was to ascertain the role of MSCs-derived MFG-E8 in murine diabetic wounds.

METHODS

MFG-E8 WT/KO MSCs or rMFG-E8 were subcutaneously injected around the wound in diabetic db/db mice, and wound areas were analyzed. Quantification of angiogenesis, infiltrating inflammatory cells, apoptotic cells at the wound area was performed by immunofluorescence staining and real-time PCR. Phagocytosis assay was performed using peritoneal macrophages from WT or db/db mice.

RESULTS

MFG-E8 expression in granulation tissue in diabetic mice was significantly reduced compared with that in non-diabetic mice. We next examined the effect of subcutaneous injection of MFG-E8 WT/KO MSCs around the wound. Diabetic wound healing was significantly accelerated by the injection of MSCs. Diabetic wound healing in MFG-E8 KO MSCs-injected wounds was significantly delayed compared to that in WT MSCs-injected wounds. The numbers of CD31⁺ EC and NG2⁺ pericytes, as well as M2 macrophages in wounds in KO MSCs-injected mice were significantly decreased. MFG-E8 WT MSCs treatment suppressed the number of apoptotic cells and TNF-α⁺ cells in wounds. In an in vitro assay, MFG-E8 WT MSCs-conditioned medium enhanced phagocytosis of apoptotic cells by peritoneal macrophages from diabetic mice.

CONCLUSION

MSCs-derived MFG-E8 might accelerate diabetic wound healing by promoting angiogenesis, the clearance of apoptotic cells, and the infiltration of M2 macrophages, and by suppressing inflammatory cytokines in wound area.

Efferocytosis Signaling in the Regulation of Macrophage Inflammatory Responses

Since the pioneering work of Elie Metchnikoff and the discovery of cellular immunity, the phagocytic clearance of cellular debris has been considered an integral component of resolving inflammation and restoring function of damaged and infected tissues. We now know that the phagocytic clearance of dying cells (efferocytosis), particularly by macrophages and other immune phagocytes, has profound consequences on innate and adaptive immune responses in inflamed tissues. These immunomodulatory effects result from an array of molecular signaling events between macrophages, dying cells, and other tissue-resident cells. In recent years, many of these molecular pathways have been identified and studied in the context of tissue inflammation, helping us better understand the relationship between efferocytosis and inflammation. We review specific types of efferocytosis-related signals that can impact macrophage immune responses and discuss their relevance to inflammation-related diseases.

Cigarette smoke attenuates phagocytic ability of macrophages through down-regulating Milk fat globule-EGF factor 8 (MFG-E8) expressions

Chronic obstructive pulmonary disease (COPD) is one of the most common inflammatory diseases resulting from habitual smoking. Impaired clearance of apoptotic cell by airway macrophages contributes to lung inflammation. Milk fat globule-EGF factor 8 (MFG-E8), as a link between apoptotic cells and phagocytes, facilitates clearance of apoptotic cells and attenuates inflammation. We sought to investigate altered expression and potential role of MFG-E8 in COPD. In this study, apoptosis was increased and the level of MFG-E8 was decreased while HMGB1 expression was increased in lung tissues of CS-exposed mice. Compared with CS-exposed WT mice, more apoptotic cells were accumulated in lung tissues of CS-exposed MFG-E8 deficiency mice. Exposure of a range of macrophages to cigarette smoke extract (CSE) resulted in decreased MFG-E8 expression. Administration of rmMFG-E8 ameliorated phagocytic ability of RAW264.7 cells and suppressed inflammatory response induced by CS-exposure. 10% CSE stimulation suppressed Rac1 membrane localization in RAW264.7 cells which was restored by administration of rmMFG-E8. MFG-E8 deficiency diminished uptake of apoptotic thymocytes by peritoneal macrophages upon CSE exposure. Overall, the findings in current work provide a novel target for diagnosing and treating COPD.

Programmed cell death and the immune system

More than 50 years ago, cells were observed to die during insect development via a process that was named 'programmed cell death'. Later, a similar cell death process was found to occur in humans, and the process was renamed 'apoptosis'. In the 1990s, a number of apoptosis-regulating molecules were identified, and apoptosis was found to have essential roles in the immune system. In this Timeline article, we highlight the key events that have demonstrated the importance of programmed cell death processes, including apoptosis and programmed necrosis, in the immune system.

Apoptosis in Cellular Society: Communication between Apoptotic Cells and Their Neighbors

Apoptosis is one of the cell-intrinsic suicide programs and is an essential cellular behavior for animal development and homeostasis. Traditionally, apoptosis has been regarded as a cell-autonomous phenomenon. However, recent in vivo genetic studies have revealed that apoptotic cells actively influence the behaviors of surrounding cells, including engulfment, proliferation, and production of mechanical forces. Such interactions can be bidirectional, and apoptosis is non-autonomously induced in a cellular community. Of note, it is becoming evident that active communication between apoptotic cells and living cells contributes to physiological processes during tissue remodeling, regeneration, and morphogenesis. In this review, we focus on the mutual interactions between apoptotic cells and their neighbors in cellular society and discuss issues relevant to future studies of apoptosis.

Mesenchymal stem cells: The roles and functions in cutaneous wound healing and tumor growth

Mesenchymal stem cells (MSCs) are bone marrow-derived non-hematopoietic progenitor cells. MSCs are able to differentiate into various types of cells, including chondrocytes, adipocytes, osteocytes, myocytes, endothelial cells, and keratinocytes. There is increasing evidence that MSCs might be located external to the vasculature, and that perivascular cells in the skin, generally called as "pericytes", might include MSCs. It has been suggested that MSCs localized around blood vessels might migrate into wounds and contribute to the restoration of injured tissues. Many studies have demonstrated that intravenous or intradermal administration of MSCs enhanced cutaneous wound healing, such as acute incisional and excisional wounds, diabetic ulcers, radiation ulcers, and burns in animals and humans. Several mechanisms of the acceleration of wound healing by MSCs have been identified, including the enhancement of angiogenesis by secretion of pro-angiogenic factors and the differentiation into endothelial cells and/or pericytes, M2 macrophages polarization, the recruitment of endogenous stem/progenitor cells, extracellular matrix production and remodeling, and immunosuppressive effects. Since the microenvironments of wounds and/or injured tissues are similar to those of tumors, MSCs also play similar roles in malignant tumors, such as the enhancement of angiogenesis, M2 macrophages polarization, and immunosuppressive effects. In addition, the mechanisms of homing of MSCs might have a commonality in the pathogenesis of wound healing and tumors. Thus, the regulating factors of MSCs, including MFG-E8, could be a therapeutic target and lead to the establishment of new therapeutic approaches for both intractable wound healing and tumors.

Mfge8 regulates enterocyte lipid storage by promoting enterocyte triglyceride hydrolase activity

The small intestine has an underappreciated role as a lipid storage organ. Under conditions of high dietary fat intake, enterocytes can minimize the extent of postprandial lipemia by storing newly absorbed dietary fat in cytoplasmic lipid droplets. Lipid droplets can be subsequently mobilized for the production of chylomicrons. The mechanisms that regulate this process are poorly understood. We report here that the milk protein Mfge8 regulates hydrolysis of cytoplasmic lipid droplets in enterocytes after interacting with the αvβ3 and αvβ5 integrins. Mice deficient in Mfge8 or the αvβ3 and αvβ5 integrins accumulate excess cytoplasmic lipid droplets after a fat challenge. Mechanistically, interruption of the Mfge8-integrin axis leads to impaired enterocyte intracellular triglyceride hydrolase activity in vitro and in vivo. Furthermore, Mfge8 increases triglyceride hydrolase activity through a PI3 kinase/mTORC2-dependent signaling pathway. These data identify a key role for Mfge8 and the αvβ3 and αvβ5 integrins in regulating enterocyte lipid processing.

Phosphatidylserine Converts Immunogenic Recombinant Human Acid Alpha-Glucosidase to a Tolerogenic Form in a Mouse Model of Pompe Disease

Development of unwanted immune responses against therapeutic proteins is a major clinical complication. Recently, we have shown that exposure of Factor VIII in the presence of phosphatidylserine (PS) induces antigen-specific hyporesponsiveness to Factor VIII rechallenge, suggesting that PS is not immune suppressive, but rather immune regulatory in that PS converts an immunogen to a tolerogen. Since PS is exposed in the outer leaflet during apoptosis, we hypothesize that PS imparts tolerogenic activity to this natural process. Thus, immunization with PS containing liposomes would mimic this natural process. Here, we investigate the immune regulatory effects of PS in inducing tolerance toward recombinant human acid alpha-glucosidase (rhGAA). rhGAA was found to complex with PS liposomes through hydrophobic interactions, and incubation PS-rhGAA with dendritic cells resulted in the increased secretion of transforming growth factor-β. Immunization with PS-rhGAA or O-phospho-L-serine-rhGAA led to a reduction in anti-rhGAA antibody response which persisted despite rechallenge with free rhGAA. Importantly, the titer levels in a majority of these animals remained unchanged after rechallenge and can be considered nonresponders. These data provide evidence that PS liposomes can be used to induce tolerance toward therapeutic proteins, in general.

Role of milk fat globule-epidermal growth factor 8 in osteoimmunology

Milk fat globule-epidermal growth factor 8 (MFG-E8) is a glycoprotein that is abundantly expressed in various tissues and has a pivotal role in the phagocytic clearance of apoptotic cells. However, MFG-E8 has also gained significant attention because of its wide range of functions in autoimmunity, inflammation and tissue homeostasis. More recently, MFG-E8 has been identified as a critical regulator of bone homeostasis, being expressed in both, osteoblasts and osteoclasts. In addition, it was shown that MFG-E8 fulfils an active role in modulating inflammatory processes, suggesting an anti-inflammatory role of MFG-E8 and proposing it as a novel therapeutic target for inflammatory diseases. This concise review focusses on the expression and regulation of MFG-E8 in the context of inflammatory bone diseases, highlights its role in the pathophysiology of osteoimmune diseases and discusses the therapeutic potential of MFG-E8.

Functional Role of Milk Fat Globule-Epidermal Growth Factor VIII in Macrophage-Mediated Inflammatory Responses and Inflammatory/Autoimmune Diseases

Inflammation involves a series of complex biological processes mediated by innate immunity for host defense against pathogen infection. Chronic inflammation is considered to be one of the major causes of serious diseases, including a number of autoimmune/inflammatory diseases, cancers, cardiovascular diseases, and neurological diseases. Milk fat globule-epidermal growth factor 8 (MFG-E8) is a secreted protein found in vertebrates and was initially discovered as a critical component of the milk fat globule. Previously, a number of studies have reported that MFG-E8 contributes to various biological functions including the phagocytic removal of damaged and apoptotic cells from tissues, the induction of VEGF-mediated neovascularization, the maintenance of intestinal epithelial homeostasis, and the promotion of mucosal healing. Recently, emerging studies have reported that MFG-E8 plays a role in inflammatory responses and inflammatory/autoimmune diseases. This review describes the characteristics of MFG-E8-mediated signaling pathways, summarizes recent findings supporting the roles of MFG-E8 in inflammatory responses and inflammatory/autoimmune diseases, and discusses MFG-E8 targeting as a potential therapeutic strategy for the development of anti-inflammatory/autoimmune disease drugs.

TIM-3 Regulates Distinct Functions in Macrophages

The transmembrane protein TIM-3 is a type I protein expressed by sub-types of lymphoid cells, such as lymphocytes Th1, Th17, Tc1, NK, as well as in myeloid cells. Scientific evidence indicates that this molecule acts as a negative regulator of T lymphocyte activation and that its expression is modified in viral infections or autoimmune diseases. In addition to evidence from lymphoid cells, the function of TIM-3 has been investigated in myeloid cells, such as monocytes, macrophages, and dendritic cells (DC), where studies have demonstrated that it can regulate cytokine production, cell activation, and the capture of apoptotic bodies. Despite these advances, the function of TIM-3 in myeloid cells and the molecular mechanisms that this protein regulates are not yet fully understood. This review examines the most recent evidence concerning the function of TIM-3 when expressed in myeloid cells, primarily macrophages, and the potential impact of that function on the field of basic immunology.

MFG-E8 Drives Melanoma Growth by Stimulating Mesenchymal Stromal Cell-Induced Angiogenesis and M2 Polarization of Tumor-Associated Macrophages

Secretion of the powerful angiogenic factor MFG-E8 by pericytes can bypass the therapeutic effects of anti-VEGF therapy, but the mechanisms by which MFG-E8 acts are not fully understood. In this study, we investigated how this factor acts to promote the growth of melanomas that express it. We found that mouse bone marrow-derived mesenchymal stromal cells (MSC) expressed a substantial amount of MFG-E8. To assess its expression from this cell type, we implanted melanoma cells and MSC derived from wild type (WT) or MFG-E8 deficient [knockout (KO)] into mice and monitored tumor growth. Tumor growth and M2 macrophages were each attenuated in subjects coimplanted with KO-MSC compared with WT-MSC. In both xenograft tumors and clinical specimens of melanoma, we found that MFG-E8 expression was heightened near blood vessels where MSC could be found. Through in vitro assays, we confirmed that WT-MSC-conditioned medium was more potent at inducing M2 macrophage polarization, compared with KO-MSC-conditioned medium. VEGF and ET-1 expression in KO-MSC was significantly lower than in WT-MSC, correlating in vivo with reduced tumor growth and numbers of pericytes and M2 macrophages within tumors. Overall, our results suggested that MFG-E8 acts at two levels, by increasing VEGF and ET-1 expression in MSC and by enhancing M2 polarization of macrophages, to increase tumor angiogenesis. Cancer Res; 76(14); 4283-92. ©2016 AACR.

Molecular Mechanisms Underlying the Regulation of the MFG-E8 Gene Promoter Activity in Physiological and Inflammatory Conditions

Milk fat globule-EGF factor 8 (MFG-E8) is expressed by macrophages and plays an important role in attenuating inflammation and maintaining tissue homeostasis. Previously, we and others found that lipopolysaccharide (LPS) inhibits MFG-E8 gene expression in macrophages. Here, we characterized the 5'-flanking region of the mouse MFG-E8 gene. To functionally analyze the upstream regulatory region of the MFG-E8 gene, a series of luciferase reporter gene constructs containing deleted or mutated regulatory elements were prepared. Using the luciferase assay, we revealed that Sp1 binding motifs within the proximal promoter region were necessary for full activity of the MFG-E8 promoter, whereas AP-1 like binding sequence at -372 played a role in governing the promoter activity at a homeostatic level. With chromatin immunoprecipitation assay, we showed that Sp1 and c-Jun physically interact with the MFG-E8 promoter region in vivo. In addition, Sp1 was found to regulate the MFG-E8 promoter activity positively and c-Jun negatively. Furthermore, we demonstrated that LPS inhibited MFG-E8 promoter activity via targeting Sp1 and AP-1-like motifs in the 5'-flanking region. Collectively, our data indicate that Sp1 and AP-1-related factors are involved in the regulation of MFG-E8 gene transcription by targeting their binding sites in the 5'-flanking region under physiological and inflammatory states.

Rescuing macrophage normal function in spinal cord injury with embryonic stem cell conditioned media

Background

Macrophages play an important role in the inflammatory responses involved with spinal cord injury (SCI). We have previously demonstrated that infiltrated bone marrow-derived macrophages (BMDMs) engulf myelin debris, forming myelin-laden macrophages (mye-Mϕ). These mye-Mϕ promote disease progression through their pro-inflammatory phenotype, enhanced neurotoxicity, and impaired phagocytic capacity for apoptotic cells. We thus hypothesize that the excessive accumulation of mye-Mϕ is the root of secondary injury, and that targeting mye-Mϕ represents an efficient strategy to improve the local inflammatory microenvironment in injured spinal cords and to further motor neuron function recovery. In this study, we administer murine embryonic stem cell conditioned media (ESC-M) as a cell-free stem cell based therapy to treat a mouse model of SCI.

Results

We showed that BMDMs, but not microglial cells, engulf myelin debris generated at the injury site. Phagocytosis of myelin debris leads to the formation of mye-Mϕ in the injured spinal cord, which are surrounded by activated microglia cells. These mye-Mϕ are pro-inflammatory and lose the normal macrophage phagocytic capacity for apoptotic cells. We therefore focus on how to trigger lipid efflux from mye-Mϕ and thus restore their function. Using ESC-M as an immune modulating treatment for inflammatory damage after SCI, we rescued mye-Mϕ function and improved functional locomotor recovery. ESC-M treatment on mye-Mϕ resulted in improved exocytosis of internalized lipids and a normal capacity for apoptotic cell phagocytosis. Furthermore, when ESC-M was administered intraperitoneally after SCI, animals exhibited significant improvements in locomotor recovery. Examination of spinal cords of the ESC-M treated mice revealed similar improvements in macrophage function as well as a shift towards a more anti-inflammatory environment at the lesion and parenchyma.

Conclusions

The embryonic stem cell conditioned media can be used as an effective treatment for SCI to resolve inflammation and improve functional recovery while circumventing the complications involved in whole cell transplantation.

The role of airway macrophages in apoptotic cell clearance following acute and chronic lung inflammation

Acute and chronic inflammatory responses in the lung are associated with the accumulation of large quantities of immune and structural cells undergoing apoptosis, which need to be engulfed by phagocytes in a process called ‘efferocytosis’. Apoptotic cell recognition and removal from the lung is mediated predominantly by airway macrophages, though immature dendritic cells and non-professional phagocytes, such as epithelial cells and mesenchymal cells, can also display this function. Efficient clearance of apoptotic cells from the airways is essential for successful resolution of inflammation and the return to lung homeostasis. Disruption of this process leads to secondary necrosis of accumulating apoptotic cells, release of necrotic cell debris and subsequent uncontrolled inflammatory activation of the innate immune system by the released ‘damage associated molecular patterns’ (DAMPS). To control the duration of the immune response and prevent autoimmune reactions, anti-inflammatory signalling cascades are initiated in the phagocyte upon apoptotic cell uptake, mediated by a range of receptors that recognise specific phospholipids or proteins externalised on, or secreted by, the apoptotic cell. However, prolonged activation of apoptotic cell recognition receptors, such as the family of receptor tyrosine kinases Tyro3, Axl and MerTK (TAM), may delay or prevent inflammatory responses to subsequent infections. In this review, we will discuss recent advances in our understanding of the mechanism controlling apoptotic cell recognition and removal from the lung in homeostasis and during inflammation, the contribution of defective efferocytosis to chronic inflammatory lung diseases, such as chronic obstructive pulmonary disease, asthma and cystic fibrosis, and implications of the signals triggered by apoptotic cells in the susceptibility to pulmonary microbial infections.

Exposure of phosphatidylserine on the cell surface

Phosphatidylserine (PtdSer) is a phospholipid that is abundant in eukaryotic plasma membranes. An ATP-dependent enzyme called flippase normally keeps PtdSer inside the cell, but PtdSer is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation. Once exposed to the cell surface, PtdSer acts as an 'eat me' signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets. The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers. Indeed, their identity as well as the mechanism of the PtdSer exposure to the cell surface has only recently been revealed. Here, we describe how PtdSer is exposed in apoptotic cells and in activated platelets, and discuss PtdSer exposure in other biological processes.

Microenvironmental Effects of Cell Death in Malignant Disease

Although apoptosis is well recognized as a cell death program with clear anticancer roles, accumulating evidence linking apoptosis with tissue repair and regeneration indicates that its relationship with malignant disease is more complex than previously thought. Here we review how the responses of neighboring cells in the microenvironment of apoptotic tumor cells may contribute to the cell birth/cell death disequilibrium that provides the basis for cancerous tissue emergence and growth. We describe the bioactive properties of apoptotic cells and consider, in particular, how apoptosis of tumor cells can engender a range of responses including pro-oncogenic signals having proliferative, angiogenic, reparatory, and immunosuppressive features. Drawing on the parallels between wound healing, tissue regeneration and cancer, we propose the concept of the "onco-regenerative niche," a cell death-driven generic network of tissue repair and regenerative mechanisms that are hijacked in cancer. Finally, we consider how the responses to cell death in tumors can be targeted to provide more effective and long-lasting therapies.

A Role of TMEM16E Carrying a Scrambling Domain in Sperm Motility

Transmembrane protein 16E (TMEM16E) belongs to the TMEM16 family of proteins that have 10 transmembrane regions and appears to localize intracellularly. Although TMEM16E mutations cause bone fragility and muscular dystrophy in humans, its biochemical function is unknown. In the TMEM16 family, TMEM16A and -16B serve as Ca(2+)-dependent Cl(-) channels, while TMEM16C, -16D, -16F, -16G, and -16J support Ca(2+)-dependent phospholipid scrambling. Here, we show that TMEM16E carries a segment composed of 35 amino acids homologous to the scrambling domain in TMEM16F. When the corresponding segment of TMEM16A was replaced by this 35-amino-acid segment of TMEM16E, the chimeric molecule localized to the plasma membrane and supported Ca(2+)-dependent scrambling. We next established TMEM16E-deficient mice, which appeared to have normal skeletal muscle. However, fertility was decreased in the males. We found that TMEM16E was expressed in germ cells in early spermatogenesis and thereafter and localized to sperm tail. TMEM16E(-/-) sperm showed no apparent defect in morphology, beating, mitochondrial function, capacitation, or binding to zona pellucida. However, they showed reduced motility and inefficient fertilization of cumulus-free but zona-intact eggs in vitro. Our results suggest that TMEM16E may function as a phospholipid scramblase at inner membranes and that its defect affects sperm motility.

An Apoptotic 'Eat Me' Signal: Phosphatidylserine Exposure

Apoptosis and the clearance of apoptotic cells are essential processes in animal development and homeostasis. For apoptotic cells to be cleared, they must display an 'eat me' signal, most likely phosphatidylserine (PtdSer) exposure, which prompts phagocytes to engulf the cells. PtdSer, which is recognized by several different systems, is normally confined to the cytoplasmic leaflet of the plasma membrane by a 'flippase'; apoptosis activates a 'scramblase' that quickly exposes PtdSer on the cell surface. The molecules that flip and scramble phospholipids at the plasma membrane have recently been identified. Here we discuss recent findings regarding the molecular mechanisms of apoptotic PtdSer exposure and the clearance of apoptotic cells.

Clearance of Apoptotic Cells and Pyrenocytes

Apoptotic cells are engulfed and digested by macrophages to maintain homeostasis in animals. If dead cells are not engulfed swiftly, they undergo secondary necrosis and release intracellular components that activate the immune system. Apoptotic cells are efficiently cleared due to phosphatidylserine (PtdSer) exposed on the cell surface that acts as an "eat me" signal. PtdSer is exposed through the activation of phospholipid scramblase and the inactivation of phospholipid flippase, which are both caspase-mediated events. Macrophages express a variety of molecules to recognize PtdSer, and use a sophisticated mechanism to engulf apoptotic cells. In red blood cells, the nucleus is lost when it is extruded as a pyrenocyte during definitive erythropoiesis. These pyrenocytes (nuclei surrounded by plasma membrane) also expose PtdSer on their surface and are efficiently engulfed by macrophages in a PtdSer-dependent manner. Macrophages transfer the engulfed apoptotic cell or pyrenocyte into lysosomes, where the components of the dead cell or pyrenocyte are degraded. If lysosomes cannot digest the DNA from apoptotic cells or pyrenocytes, the undigested DNA accumulates in the lysosome and activates macrophages to produce type I interferon (IFN) via a STING-dependent pathway; in embryos, this causes severe anemia. Here, we discuss how macrophages clear apoptotic cells and pyrenocytes.

Role of milk fat globule-epidermal growth factor 8 in colonic inflammation and carcinogenesis

BACKGROUND

Milk fat globule-epidermal growth factor 8 (MFG-E8) promotes phagocytic clearance of apoptotic cells to maintain normal tissue homeostasis. However, its functions in intestinal inflammation and carcinogenesis are unknown.

METHODS

Experimental colitis was induced in MFG-E8 knockout (KO) and wild-type (WT) mice by dextran sodium sulfate (DSS) administration. Colon tissues were used for assessments of colitis activity and epithelial proliferation. A mouse colitis-associated cancer (CAC) model was induced by intraperitoneal injection of azoxymethane (AOM) and then the animals were given a single administration of DSS. A sporadic colon cancer model was established by repeated intraperitoneal injections of AOM. The role of MFG-E8 in epithelial proliferation with or without treatment of siRNA targeting α(v)-integrin was examined in vitro using a WST-1 assay.

RESULTS

The severity of colitis in KO mice was greater than that in WT mice, while the proliferative potential of colonic epithelial cells in KO mice was lower during the regenerative phase. In both CAC and sporadic models, tumor size in KO was lower as compared to WT mice, while decreased tumor incidence was only found in the CAC model. In vitro findings showed that MFG-E8 promotes epithelial cell proliferation, and treatment with a siRNA targeting α(v)-integrin reduced the proliferation of Colon-26 cells stimulated with recombinant MFG-E8.

CONCLUSIONS

MFG-E8 promotes tumor growth regardless of the presence or absence of colonic inflammation, whereas colon tumor development is initiated by MFG-E8 under inflammatory conditions. These MFG-E8 functions may be dependent on integrin-mediated cellular signaling.

Exposure of FVIII in the Presence of Phosphatidyl Serine Reduces Generation of Memory B-Cells and Induces Regulatory T-Cell-Mediated Hyporesponsiveness in Hemophilia A Mice

A major complication of replacement therapy with Factor VIII (FVIII) for hemophilia A (HA) is the development of unwanted immune responses. Previous studies showed that administration of FVIII in the presence of phosphatidyl serine (PS) reduced the development of anti-FVIII antibodies in HA mice. However, the impact of PS-mediated effects on immunological memory, such as generation of memory B-cells, is not clear. The effect of PS on memory B-cells was therefore investigated using adoptive transfer approach in FVIII(-/-) HA mice. Adoptive transfer of memory B-cells from a PS-FVIII-treated group to naïve mice followed by challenge of the recipient mice with FVIII showed a significantly reduced anti-FVIII antibody response in the recipient mice, compared with animals that received memory B-cells from free FVIII and FVIII-charge matched phosphatidyl glycerol (PG) group. The decrease in memory B-cell response is accompanied by an increase in FoxP3 expressing regulatory T-cells (Tregs). Flow cytometry studies showed that the generation of Tregs is higher in PS-treated animals as compared with FVIII and FVIII-PG treated animals. The PS-mediated hyporesponsiveness was found to be antigen-specific. The PS-FVIII immunization showed hyporesponsiveness toward FVIII rechallenge but not against ovalbumin (OVA) rechallenge, an unrelated antigen. This demonstrates that PS reduces immunologic memory of FVIII and induces antigen-specific peripheral tolerance in HA mice.

Nonenzymatic Reactions above Phospholipid Surfaces of Biological Membranes: Reactivity of Phospholipids and Their Oxidation Derivatives

Phospholipids play multiple and essential roles in cells, as components of biological membranes. Although phospholipid bilayers provide the supporting matrix and surface for many enzymatic reactions, their inherent reactivity and possible catalytic role have not been highlighted. As other biomolecules, phospholipids are frequent targets of nonenzymatic modifications by reactive substances including oxidants and glycating agents which conduct to the formation of advanced lipoxidation end products (ALEs) and advanced glycation end products (AGEs). There are some theoretical studies about the mechanisms of reactions related to these processes on phosphatidylethanolamine surfaces, which hypothesize that cell membrane phospholipids surface environment could enhance some reactions through a catalyst effect. On the other hand, the phospholipid bilayers are susceptible to oxidative damage by oxidant agents as reactive oxygen species (ROS). Molecular dynamics simulations performed on phospholipid bilayers models, which include modified phospholipids by these reactions and subsequent reactions that conduct to formation of ALEs and AGEs, have revealed changes in the molecular interactions and biophysical properties of these bilayers as consequence of these reactions. Then, more studies are desirable which could correlate the biophysics of modified phospholipids with metabolism in processes such as aging and diseases such as diabetes, atherosclerosis, and Alzheimer's disease.

Incomplete clearance of apoptotic cells in systemic lupus erythematosus: pathogenic role and potential biomarker

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with repeated inflammation against multiple organs. Although its pathophysiology is not yet unveiled, uncleared apoptotic cells and their accumulation in tissue contribute to the autoimmune disturbance in SLE. Apoptosis is a programmed cell death process, which maintains tissue homeostasis and inhibits the development of any further immune response against apoptotic remnants. Earlier studies revealed that various 'eat-me' signals on apoptotic cells, bridging molecules and their receptors on phagocytes play a role in such a complicated process. Tyro3-Axl-Mer receptors, their bridging molecules, milk fat globulin epidermal growth factor-8, T-cell immunoglobulin mucin domain protein family, scavenger receptors, C1q, and pentraxins were found to be abnormal in SLE. In this review, apoptosis and clearance of its remnants are summarized, and the molecules involved in the incomplete clearance of apoptotic cells in SLE are discussed.

Prospects of apoptotic cell-based therapies for transplantation and inflammatory diseases

Apoptotic cell removal or interactions of early-stage apoptotic cells with immune cells are associated with an immunomodulatory microenvironment that can be harnessed to exert therapeutic effects. While the involved immune mechanisms are still being deciphered, apoptotic cell infusion has been tested in different experimental models where inflammation is deregulated. This includes chronic and acute inflammatory disorders such as arthritis, contact hypersensitivity and acute myocardial infarction. Apoptotic cell infusion has also been used in transplantation settings to prevent or treat acute and chronic rejection, as well as to limit acute graft-versus-host disease associated with allogeneic hematopoietic cell transplantation. Here, we review the mechanisms involved in apoptotic cell-induced immunomodulation and data obtained in preclinical models of transplantation and inflammatory diseases.

Polarization of prostate cancer-associated macrophages is induced by milk fat globule-EGF factor 8 (MFG-E8)-mediated efferocytosis

Tumor cells secrete factors that modulate macrophage activation and polarization into M2 type tumor-associated macrophages, which promote tumor growth, progression, and metastasis. The mechanisms that mediate this polarization are not clear. Macrophages are phagocytic cells that participate in the clearance of apoptotic cells, a process known as efferocytosis. Milk fat globule- EGF factor 8 (MFG-E8) is a bridge protein that facilitates efferocytosis and is associated with suppression of proinflammatory responses. This study investigated the hypothesis that MFG-E8-mediated efferocytosis promotes M2 polarization. Tissue and serum exosomes from prostate cancer patients presented higher levels of MFG-E8 compared with controls, a novel finding in human prostate cancer. Coculture of macrophages with apoptotic cancer cells increased efferocytosis, elevated MFG-E8 protein expression levels, and induced macrophage polarization into an alternatively activated M2 phenotype. Administration of antibody against MFG-E8 significantly attenuated the increase in M2 polarization. Inhibition of STAT3 phosphorylation using the inhibitor Stattic decreased efferocytosis and M2 macrophage polarization in vitro, with a correlating increase in SOCS3 protein expression. Moreover, MFG-E8 knockdown tumor cells cultured with wild-type or MFG-E8-deficient macrophages resulted in increased SOCS3 expression with decreased STAT3 activation. This suggests that SOCS3 and phospho-STAT3 act in an inversely dependent manner when stimulated by MFG-E8 and efferocytosis. These results uncover a unique role of efferocytosis via MFG-E8 as a mechanism for macrophage polarization into tumor-promoting M2 cells.

Apoptotic cell capture by DCs induces unexpectedly robust autologous CD4+ T-cell responses

Apoptotic cells represent an important source of self-antigens and their engulfment by dendritic cells (DCs) is usually considered to be related to tolerance induction. We report here an unexpectedly high level of human CD4(+) T-cell proliferation induced by autologous DCs loaded with autologous apoptotic cells, due to the activation of more than 10% of naive CD4(+) T cells. This proliferation is not due to an increase in the costimulatory capacity of DCs, but is dependent on apoptotic cell-associated material processed through an endo-lysosomal pathway and presented on DC MHC class II molecules. Autologous CD4(+) T cells stimulated with apoptotic cell-loaded DCs exhibit suppressive capacities. However, in the presence of bacterial lipopolysaccharide, apoptotic cell-loaded DCs induce the generation of IL-17-producing cells. Thus, apoptotic cell engulfment by DCs may lead to increased autologous responses, initially generating CD4(+) T cells with suppressive capacities able to differentiate into Th17 cells in the presence of a bacterial danger signal such as LPS.

MFG-E8 regulates angiogenesis in cutaneous wound healing

Our research group recently demonstrated that pericytes are major sources of the secreted glycoprotein and integrin ligand lactadherin (MFG-E8) in B16 melanoma tumors, and that MFG-E8 promotes angiogenesis via enhanced PDGF-PDGFRβ signaling mediated by integrin-growth factor receptor crosstalk. However, sources of MFG-E8 and its possible roles in skin physiology are not well characterized. The objective of this study was to characterize the involvement of MFG-E8 in skin wound healing. In the dermis of normal murine and human skin, accumulations of MFG-E8 were found around CD31(+) blood vessels, and MFG-E8 colocalized with PDGFRβ(+), αSMA(+), and NG2(+) pericytes. MFG-E8 protein and mRNA levels were elevated in the dermis during full-thickness wound healing in mice. MFG-E8 was diffusely present in granulation tissue and was localized around blood vessels. Wound healing was delayed in MFG-E8 knockout mice, compared with the wild type, and myofibroblast and vessel numbers in wound areas were significantly reduced in knockout mice. Inhibition of MFG-E8 production with siRNA attenuated the formation of capillary-like structures in vitro. Expression of MFG-E8 in fibrous human granulation tissue with scant blood vessels was less than that in granulation tissue with many blood vessels. These findings suggest that MFG-E8 promotes cutaneous wound healing by enhancing angiogenesis.

Feasibility of lactadherin-bearing clinically available microbubbles as ultrasound contrast agent for angiogenesis

OBJECTIVES

Phagocytosis of apoptotic cells is carried out through bridging of phosphatidylserine (PS)-expressing apoptotic cells and integrin αvβ3-expressing phagocytes with lactadherin. The objective of this study was to examine whether microbubbles targeted to integrin αvβ3 could be produced by conjugating a PS-containing clinically available ultrasound contrast agent with lactadherin.

MATERIALS AND METHODS

PS-containing perfluorobutane-filled microbubbles were incubated with R-phycoerythrin (PE)-labeled lactadherin, and the presence of PE-positive bubbles was examined by FACS analysis. Secondly, the attachment of lactadherin to integrin αvβ3-expressing cells (human umbilical vein endothelial cells (HUVEC)) was also examined by FACS analysis. Finally, the adhesion of PS-containing bubbles to HUVEC was examined using a parallel plate flow chamber. The number of adherent bubbles with or without the intermediation of lactadherin was compared.

RESULTS

The more lactadherin was added to the bubble suspension, the more PE-positive bubbles were detected. The size of bubbles was not increased even after conjugation with lactadherin (2.90 ± 0.04 vs. 2.81 ± 0.02 μm). Binding between lactadherin and HUVEC was also confirmed by FACS analysis. The parallel plate flow chamber study revealed that the number of PS-containing bubbles adherent to HUVEC was increased about five times by the intermediation of lactadherin (12.1 ± 6.0 to 58.7 ± 33.1 bubbles).

CONCLUSION

Because integrin αvβ3 is well-known to play a key role in angiogenesis, the complex of PS-containing bubbles and lactadherin has feasibility as a clinically translatable targeted ultrasound contrast agent for angiogenesis.

Serum milk fat globule epidermal growth factor 8 elevation may subdivide systemic lupus erythematosus into two pathophysiologically distinct subsets

Objective

Impaired clearance of apoptotic cells is a potential trigger of systemic lupus erythematosus (SLE). Milk fat globule epidermal growth factor 8 (MFG-E8) plays an important role in the clearance of dying cells. Previously, we reported serum MFG-E8 was elevated in some SLE patients. Here we further investigated the prevalence of MFG-E8 in active SLE and other autoimmune diseases and also tried to clarify the characteristics of MFG-E8-positive and -negative SLE.

Methods

Serum MFG-E8 was measured in 40 active non-treated SLE patients, 104 disease controls and 104 healthy controls by ELISA. Clinical characteristics and serum cytokine profiles were compared between MFG-E8-positive and MFG-E8-negative SLE patients.

Results

Prevalence of MFG-E8 was significantly higher in SLE patients (40%) than in various controls ( p < 0.05). MFG-E8 level became negative after treatment, and increased again upon relapse. When compared, MFG-E8-positive SLE patients showed higher immune complex ( p = 0.021) and lower complement ( p = 0.004 for CH50). In contrast, MFG-E8-negative SLE patients tended to show higher CRP ( p = 0.094). There was a positive correlation between MFG-E8 level and immune complex level ( rs = 0.49, p = 0.049). TNF-α ( p = 0.019), IFN-γ ( p = 0.031) and IL-10 ( p = 0.013) were significantly higher in MFG-E8-positive SLE.

Conclusion

MFG-E8-positive SLE and -negative SLE may have different clinical features, the one with stronger immunological response and the other with stronger inflammatory response, and those two groups may be two distinct subtypes of SLE driven by different mechanisms. Further, MFG-E8 could be used as a biomarker for diagnosis and monitoring of disease activity in certain SLE patients.

Role of phosphatidylserine receptors in enveloped virus infection

We recently demonstrated that a soluble protein, Gas6, can facilitate viral entry by bridging viral envelope phosphatidylserine to Axl, a receptor tyrosine kinase expressed on target cells. The interaction between phosphatidylserine, Gas6, and Axl was originally shown to be a molecular mechanism through which phagocytes recognize phosphatidylserine exposed on dead cells. Since our initial report, several groups have confirmed that Axl/Gas6, as well as other phosphatidylserine receptors, facilitate entry of dengue, West Nile, and Ebola viruses. Virus binding by viral envelope phosphatidylserine is now a viral entry mechanism generalized to many families of viruses. In addition to Axl/Gas6, various molecules are known to recognize phosphatidylserine; however, the effects of these molecules on virus binding and entry have not been comprehensively evaluated and compared. In this study, we examined most of the known human phosphatidylserine-recognizing molecules, including MFG-E8, TIM-1, -3, and -4, CD300a, BAI1, and stabilin-1 and -2, for their abilities to facilitate virus binding and infection. Using pseudotyped lentiviral vectors, we found that a soluble phosphatidylserine-binding protein, MFG-E8, enhances transduction. Cell surface receptors TIM-1 and -4 also enhance virus binding/transduction. The extent of enhancement by these molecules varies, depending on the type of pseudotyping envelope proteins. Mutated MFG-E8, which binds viral envelope phosphatidylserine without bridging virus to cells, but, surprisingly, not annexin V, which has been used to block phagocytosis of dead cells by concealing phosphatidylserine, efficiently blocks these phosphatidylserine-dependent viral entry mechanisms. These results provide insight into understanding the role of viral envelope phosphatidylserine in viral infection.

IMPORTANCE

Envelope phosphatidylserine has previously been shown to be important for replication of various envelope viruses, but details of this mechanism(s) were unclear. We were the first to report that a bifunctional serum protein, Gas6, bridges envelope phosphatidylserine to a cell surface receptor, Axl. Recent studies demonstrated that many envelope viruses, including vaccinia, dengue, West Nile, and Ebola viruses, utilize Axl/Gas6 to facilitate their entry, suggesting that the phosphatidylserine-mediated viral entry mechanism can be shared by various enveloped viruses. In addition to Axl/Gas6, various molecules are known to recognize phosphatidylserine; however, the effects of these molecules on virus binding and entry have not been comprehensively evaluated and compared. In this study, we examined most human phosphatidylserine-recognizing molecules for their abilities to facilitate viral infection. The results provide insights into the role(s) of envelope phosphatidylserine in viral infection, which can be applicable to the development of novel antiviral reagents that block phosphatidylserine-mediated viral entry.

Mfge8 promotes obesity by mediating the uptake of dietary fats and serum fatty acids

Fatty acids are integral mediators of energy storage, membrane formation and cell signaling. The pathways that orchestrate uptake of fatty acids remain incompletely understood. Expression of the integrin ligand Mfge8 is increased in human obesity and in mice on a high-fat diet, but its role in obesity is unknown. We show here that Mfge8 promotes the absorption of dietary triglycerides and the cellular uptake of fatty acid and that Mfge8-deficient (Mfge8(-/-)) mice are protected from diet-induced obesity, steatohepatitis and insulin resistance. Mechanistically, we found that Mfge8 coordinates fatty acid uptake through αvβ3 integrin- and αvβ5 integrin-dependent phosphorylation of Akt by phosphatidylinositide-3 kinase and mTOR complex 2, leading to translocation of Cd36 and Fatp1 from cytoplasmic vesicles to the cell surface. Collectively, our results imply a role for Mfge8 in regulating the absorption and storage of dietary fats, as well as in the development of obesity and its complications.

Follicular dendritic cells: origin, phenotype, and function in health and disease

Follicular dendritic cells (FDCs) were originally identified by their specific morphology and by their ability to trap immune-complexed antigen in B cell follicles. By virtue of the latter as well as the provision of chemokines, adhesion molecules, and trophic factors, FDCs participate in the shaping of B cell responses. Importantly, FDCs also supply tingible body macrophages (TBMs) with the eat-me-signaling molecule milk fat globule-EGF factor 8 (Mfge8), thereby enabling the disposal of apoptotic B cells. Recent studies have provided fundamental insights into the multiple functions of FDCs in both physiological and pathophysiological contexts and into their origin. Here we review these findings, and discuss current concepts related to FDC histogenesis both in lymphoid organs and in inflammatory lymphoneogenesis.

Citrullination of autoantigens implicates NETosis in the induction of autoimmunity

Tolerance blocks the expression of autoantibodies, whereas autoimmunity promotes it. How tolerance breaks and autoantibody production begins thus are crucial questions for understanding and treatment of autoimmune diseases. Evidence implicates cell death and autoantigen modifications in the initiation of autoimmune reactions. One form of neutrophil cell death called NETosis deserves attention because it requires the post-translational modification of histones and results in the extracellular release of chromatin. NETosis received its name from NET, the acronym given to Neutrophil Extracellular Trap. The extracellular chromatin incorporates histones in which arginines have been converted to citrullines by peptidylarginine deiminase IV (PAD4). The deiminated chromatin may function to capture or 'trap' bacterial pathogens, thus generating an extracellular complex of deiminated histones and bacterial cell adjuvants. The complex of bacterial antigens and deiminated chromatin may be internalised by host phagocytes during acute inflammatory conditions, as arise during bacterial infections or chronic autoinflammatory disorders. The uptake and processing of deiminated chromatin together with bacterial adjuvants by phagocytes may induce the presentation of modified histone epitopes and co-stimulation, thus yielding a powerful stimulus to break tolerance. Autoantibodies to deiminated histones are prevalent in Felty's syndrome patients and are present in systemic lupus erythematosus (SLE) and patients with rheumatoid arthritis (RA). These observations clearly implicate histone deimination as an epigenetic mark that can act as an autoantibody stimulant.

Decreased apopto-phagocytic gene expression in the macrophages of systemic lupus erythematosus patients

The clearance of apoptotic cells has an important role in the maintenance of tissue homeostasis and in the protection of tissues from the inflammatory and immunogenic contents of dying cells. A defect in the recognition and phagocytosis of apoptotic cells contributes to the development of chronic inflammation and autoimmune disorders. We have observed that compared with healthy donors, differentiated macrophages from patients with untreated systemic lupus erythematosus (SLE) showed decreased phagocytosis of apoptotic neutrophils. A TaqMan Low Density Array was designed to determine the mRNA expression levels of 95 apopto-phagocytic genes in differentiated non-phagocytosing and phagocytosing macrophages. In the macrophages of clinically and immunoserologically active SLE patients, 39 genes were expressed at lower levels than in the control macrophages. When inactive patients were compared with those with minor immunoserological abnormalities or patients in an immunoserologically active state, a relationship was observed between the altered gene expression profile and the disease state. In the macrophages of patients with engulfing apoptotic cells, an upregulation of genes involved in inflammation, autophagy, and signaling was observed. These results indicate that novel immune-pathological pathways are involved in SLE and suggest targets for potential therapeutic modulation.

The recruitment of p47(phox) and Rac2G12V at the phagosome is transient and phosphatidylserine dependent

BACKGROUND INFORMATION

During phagocytosis, neutrophils internalise pathogens in a phagosome and produce reactive oxygen species (ROS) by the NADPH oxidase to kill the pathogen. The cytosolic NADPH oxidase subunits p40(phox), p47(phox), p67(phox) and Rac2 translocate to the phagosomal membrane to participate in enzyme activation. The kinetics of this recruitment and the underlying signalling pathways are only partially understood. Anionic phospholipids, phosphatidylserine (PS) and phosphoinositides (PPI) provide an important attachment site for numerous proteins, including several oxidase subunits.

RESULTS

We investigated the kinetics of p47(phox) and Rac2 phagosomal membrane recruitment. Both subunits are known to interact with anionic phospholipids; we therefore addressed the role of PS in this recruitment. Phagosomal accumulation of p47(phox) and Rac2 tagged with fluorescent proteins was analysed by videomicroscopy. We used the C2 domain of lactadherin (lactC2) that interacts strongly and specifically with PS to monitor intracellular PS localisation and to decrease PS accessibility. During phagocytosis of opsonised zymosan, p47(phox) and constitutively active Rac2G12V briefly translocated to the phagosomal membrane, whereas ROS production continued for a longer period. However, in the presence of lactC2, Rac2G12V recruitment was inhibited and the kinetics of p47(phox) recruitment and detachment were delayed. A reduced phagosomal ROS production was also observed during the first 7 min following the phagosome closure.

CONCLUSIONS

These results suggest that p47(phox) and Rac2 accumulate only transiently at the phagosome at the onset of NADPH activity and detach from the phagosome before the end of ROS production. Furthermore, lactC2, by masking PS, interfered with the phagosomal recruitment of p47(phox) and Rac2 and disturbed NADPH oxidase activity. Thus, PS appears as a modulator of NADPH oxidase activation.

Identification of MFG-E8 as a novel therapeutic target for diseases

INTRODUCTION

Milk fat globule-epidermal growth factor-factor 8 (MFG-E8), a peripheral membrane glycoprotein has been widely studied in recent years due to its omnipresent locations, express and multiple functions. Traditionally, MFG-E8 was identified as an outstanding factor for phagocytosis of apoptotic cells and a significant factor in immune systems. Recent studies with some new findings have shed an interesting light on the old peripheral membrane glycoprotein in various diseases.

AREAS COVERED

In inflammatory and the systemic lupus erythematosus-type autoimmune disease, age-related diseases and tumors, MFG-E8 plays a multifunctional role in attenuating inflammation and improving prognosis, healing wound and remodeling arterial and enhancing tumorigenicity and cancer metastasis. This review provides a comprehensive view on the latest advances in the field. The summarized knowledge will help to explore the potential therapeutic roles of MFG-E8 and to design MFG-E8-based strategies for the treatment of these diseases.

EXPERT OPINION

Though the exact roles of MFG-E8 have not been fully elucidated in diseases. MFG-E8 may serve as a promising therapeutic strategy.

Annexin A1 on the surface of early apoptotic cells suppresses CD8+ T cell immunity

Prevention of an immune response against self-antigens derived from apoptotic cells is essential to preclude autoimmune and chronic inflammatory diseases. Here, we describe apoptosis induced externalization of endogenous cytosolic annexin 1 initiating an anti-inflammatory effector mechanism that suppresses the immune response against antigens of apoptotic cells. Cytosolic annexin 1 rapidly translocated to the apoptotic cell surface and inhibited dendritic cell (DC) activation induced by Toll like receptors (TLR). Annexin 1-inhibited DC showed strongly reduced secretion of pro-inflammatory cytokines (e.g. TNF and IL-12) and costimulatory surface molecules (e.g. CD40 and CD86), while anti-inflammatory mediators like PD-L1 remained unchanged. T cells stimulated by such DC lacked secretion of interferon-γ (IFN-γ) and TNF but retained IL-10 secretion. In mice, annexin 1 prevented the development of inflammatory DC and suppressed the cellular immune response against the model antigen ovalbumin (OVA) expressed in apoptotic cells. Furthermore, annexin 1 on apoptotic cells compromised OVA-specific tumor vaccination and impaired rejection of an OVA-expressing tumor. Thus, our results provide a molecular mechanism for the suppressive activity of apoptotic cells on the immune response towards apoptotic cell-derived self-antigens. This process may play an important role in prevention of autoimmune diseases and of the immune response against cancer.

Prolonged apoptotic cell accumulation in germinal centers of Mer-deficient mice causes elevated B cell and CD4+ Th cell responses leading to autoantibody production

Mer receptor tyrosine kinase is a member of the Tyro-3/Axl/Mer (TAM) subfamily of receptor tyrosine kinases, and its expression on phagocytes facilitates their clearance of apoptotic cells (ACs). Mer expression in germinal centers (GCs) occurs predominantly on tingible body macrophages. B and T cells do not express Mer. In this study, we show that Mer deficiency ((Mer(-/-)) resulted in the long-term accumulation of ACs primarily in GCs and not in the T cell zone, marginal zone, or red pulp areas of the spleen. AC accumulation in GCs led to augmented Ab-forming cell, GC, and IgG2 Ab responses in Mer(-/-) mice, which were sustained for at least 80 d. Enhanced responses in Mer(-/-) mice were due to increased activation and proliferation of B cells and CD4(+) Th cells, including follicular helper T cells, which resulted in high titers of anti-nuclear Abs in Mer(-/-) mice compared with wild-type controls. Secondary IgG-producing Ab-forming cell, total IgG, and IgG2 Ab responses were also increased in Mer(-/-) mice. Finally, compared with wild-type controls, Mer(-/-) mice had increased percentage of IFN-γ-producing CD4(+) Th cells and elevated levels of Th1 (i.e., IL-2 and IFN-γ) and proinflammatory (i.e., TNF and IL-6) cytokines, consistent with elevated levels of Th1-biased IgG2 Abs in Mer(-/-) mice. Together, our results demonstrate that Mer deficiency induces prolonged accumulation of ACs in GCs, resulting in dysregulation of GC B cell and CD4(+) Th cell responses and Th1 cytokine production, leading to alteration of B cell tolerance and the development of autoantibodies.

MFGE8 does not orchestrate clearance of apoptotic neurons in a mouse model of Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms implicating microglial cells and inflammatory processes. Yet, how apoptotic DNs get removed by professional phagocytes and how this process modulates inflammatory processes are still unresolved issues. In this study, we investigated the role of MFGE8, a soluble factor involved in phagocytic recognition, in apoptotic DN clearance and neuroinflammation in PD. We report that glial expression of MFGE8 is enhanced in post-mortem PD brains compared to control individuals. Then, in vivo functional analysis of Mfge8 was assessed in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mouse model of PD using wild-type (WT) and Mfge8-deficient mice. Neuropathological analysis consisted in evaluating (i) the loss of nigral DN and striatal DN terminals, (ii) the extent of glial cell activation and (iii) the number of apoptotic profiles. In vivo microglial phagocytic activity was further assessed by measuring the engulfment of apoptotic DN preloaded with fluorescent latex beads. Here we show that Mfge8 deficiency neither impact the phagocytic clearance of apoptotic bodies nor change the overall neuropathological parameters (DN cell loss and glial cell activation). In summary, our data argue that MFGE8 is not likely involved in the phagocytic clearance of neuronal debris associated with nigrostriatal pathway injury.