Enhanced efferocytosis by dendritic cells underlies memory T-cell expansion and susceptibility to autoimmune disease in CD300f-deficient mice

Efferocytosis: Unveiling its potential in autoimmune disease and treatment strategies

Efferocytosis is a crucial process whereby phagocytes engulf and eliminate apoptotic cells (ACs). This intricate process can be categorized into four steps: (1) ACs release "find me" signals to attract phagocytes, (2) phagocytosis is directed by "eat me" signals emitted by ACs, (3) phagocytes engulf and internalize ACs, and (4) degradation of ACs occurs. Maintaining immune homeostasis heavily relies on the efficient clearance of ACs, which eliminates self-antigens and facilitates the generation of anti-inflammatory and immunosuppressive signals that maintain immune tolerance. However, any disruptions occurring at any of the efferocytosis steps during apoptosis can lead to a diminished efficacy in removing apoptotic cells. Factors contributing to this inefficiency encompass dysregulation in the release and recognition of "find me" or "eat me" signals, defects in phagocyte surface receptors, bridging molecules, and other signaling pathways. The inadequate clearance of ACs can result in their rupture and subsequent release of self-antigens, thereby promoting immune responses and precipitating the onset of autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, and multiple sclerosis. A comprehensive understanding of the efferocytosis process and its implications can provide valuable insights for developing novel therapeutic strategies that target this process to prevent or treat autoimmune diseases.

Crosstalk between efferocytic myeloid cells and T-cells and its relevance to atherosclerosis

The interplay between myeloid cells and T-lymphocytes is critical to the regulation of host defense and inflammation resolution. Dysregulation of this interaction can contribute to the development of chronic inflammatory diseases. Important among these diseases is atherosclerosis, which refers to focal lesions in the arterial intima driven by elevated apolipoprotein B-containing lipoproteins, notably low-density lipoprotein (LDL), and characterized by the formation of a plaque composed of inflammatory immune cells, a collection of dead cells and lipids called the necrotic core, and a fibrous cap. As the disease progresses, the necrotic core expands, and the fibrous cap becomes thin, which increases the risk of plaque rupture or erosion. Plaque rupture leads to a rapid thrombotic response that can give rise to heart attack, stroke, or sudden death. With marked lowering of circulating LDL, however, plaques become more stable and cardiac risk is lowered-a process known as atherosclerosis regression. A critical aspect of both atherosclerosis progression and regression is the crosstalk between innate (myeloid cells) and adaptive (T-lymphocytes) immune cells. Myeloid cells are specialized at clearing apoptotic cells by a process called efferocytosis, which is necessary for inflammation resolution. In advanced disease, efferocytosis is impaired, leading to secondary necrosis of apoptotic cells, inflammation, and, most importantly, defective tissue resolution. In regression, efferocytosis is reawakened aiding in inflammation resolution and plaque stabilization. Here, we will explore how efferocytosing myeloid cells could affect T-cell function and vice versa through antigen presentation, secreted factors, and cell-cell contacts and how this cellular crosstalk may contribute to the progression or regression of atherosclerosis.

Efferocytosis in dendritic cells: an overlooked immunoregulatory process

Efferocytosis, the process of engulfing and removing apoptotic cells, plays an essential role in preserving tissue health and averting undue inflammation. While macrophages are primarily known for this task, dendritic cells (DCs) also play a significant role. This review delves into the unique contributions of various DC subsets to efferocytosis, highlighting the distinctions in how DCs and macrophages recognize and handle apoptotic cells. It further explores how efferocytosis influences DC maturation, thereby affecting immune tolerance. This underscores the pivotal role of DCs in orchestrating immune responses and sustaining immune equilibrium, providing new insights into their function in immune regulation.

Pathogenic rickettsiae utilize the phosphatidylserine binding receptor CD300f on macrophages for host invasion and pathogenesis

Some arthropod-borne obligate intracellular rickettsiae are among the most virulent human pathogens. Upon entry, Rickettsia species modulate immune (e.g., macrophages; MΦ) and non-immune cell (e.g., endothelial cells) responses to create a habitable environment for host colonization. In particular, MΦ play a crucial role in either terminating an infection at an early stage or succumbing to bacterial replication and colonization. However, our understanding on how Rickettsia species modulate crucial cellular processes within MΦ, including phagocytosis, and host cell defenses, to establish an intracytosolic replication niche, remain poorly defined. In this study, we describe a previously unappreciated mechanism, in which pathogenic rickettsiae infection is mediated by the phosphatidylserine (PS)-binding receptor, CD300f. We found that CD300f -/- mice but not wild-type (WT) C57BL/6J mice were protected against R. typhi - or R. rickettsii [ Shelia Smith ]-induced fatal rickettsiosis. Adoptative transfer studies further revealed that CD300f-expressing bone marrow-derived macrophages (BMDMΦ) are important mediators to control rickettsiosis in WT mice. Mechanistical analysis, using WT or CD300f -/- BMDMΦ, showed that CD300f facilitates the engulfment of both pathogenic R. typhi and R. rickettsii species, likely via a PS-mediated mechanism. Furthermore, CD300f was involved in the intracytosolic replication of both pathogenic rickettsiae by differentially modulating the anti-inflammatory Interleukin (IL)-10 and anti-rickettsial IL-1α and IL-1β cytokine responses. Collectively, our findings describe a previously unappreciated role for the efferocytic receptor, CD300f, to facilitate engulfment and the intracellular survival of pathogenic rickettsiae within the host.

Significance Statement

Vector-borne diseases, which are transmitted by hematophagous arthropods, like ticks and fleas, present a perilous threat to public health. In fact, tick- and flea-borne rickettsial diseases are on the rise globally and our current inadequate understanding on how Rickettsia interacts with their mammalian host has significantly impaired the development of effective interventions against pathogenic rickettsial infections. Here, we identified the phosphatidylserine (PS)-receptor, CD300f, as an important mediator of pathogenic rickettsiae infection in vivo and in vitro . Specifically, we showed that CD300f-expressing macrophages facilitate rickettsial infection by differentially modulating anti-inflammatory Interleukin (IL)-10 and anti-rickettsial IL-1α and IL-1β cytokine responses. In sum, our data described CD300f as an important regulator of rickettsial infection and may present a target for therapeutic intervention.

Efferocytosis: An accomplice of cancer immune escape

The clearance of apoptotic cells by efferocytes such as macrophages and dendritic cells is termed as "efferocytosis", it plays critical roles in maintaining tissue homeostasis in multicellular organisms. Currently, available studies indicate that efferocytosis-related molecules and pathways are tightly associated with cancer development, metastasis and treatment resistance, efferocytosis also induces an immunosuppressive tumor microenvironment and assists cancer cells escape from immune surveillance. In this study, we reviewed the underlying mechanisms of efferocytosis in mediating the occurrence of cancer immune escape, and then emphatically summarized the strategies of using efferocytosis as therapeutic target to enhance the anti-tumor efficacies of immune checkpoint inhibitors, hoping to provide powerful evidences for more effective therapeutic regimens of malignant tumors.

CD300f immune receptor contributes to healthy aging by regulating inflammaging, metabolism, and cognitive decline

Emerging evidence suggests that immune receptors may participate in many aging-related processes such as energy metabolism, inflammation, and cognitive decline. CD300f, a TREM2-like lipid-sensing immune receptor, is an exceptional receptor as it integrates activating and inhibitory cell-signaling pathways that modulate inflammation, efferocytosis, and microglial metabolic fitness. We hypothesize that CD300f can regulate systemic aging-related processes and ultimately healthy lifespan. We closely followed several cohorts of two strains of CD300f-/- and WT mice of both sexes for 30 months and observed an important reduction in lifespan and healthspan in knockout mice. This was associated with systemic inflammaging, increased cognitive decline, reduced brain glucose uptake observed by 18FDG PET scans, enrichment in microglial aging/neurodegeneration phenotypes, proteostasis alterations, senescence, increased frailty, and sex-dependent systemic metabolic changes. Moreover, the absence of CD300f altered macrophage immunometabolic phenotype. Taken together, we provide strong evidence suggesting that myeloid cell CD300f immune receptor contributes to healthy aging.

CD300f signalling induces inhibitory human monocytes/macrophages

The CD300 glycoproteins are a family of related leucocyte surface molecules that regulate the immune response via their paired triggering and inhibitory receptors. Here we studied CD300f, an apoptotic cell receptor, and how it modulates the function of human monocytes and macrophages. We showed that CD300f signalling by crosslinking with anti-CD300f mAb (DCR-2) suppressed monocytes causing upregulation of the inhibitory molecule, CD274 (PD-L1) and their inhibition of T cell proliferation. Furthermore, CD300f signalling drove macrophages preferentially towards M2-type with upregulation of CD274, which was further enhanced by IL-4. CD300f signalling activates the PI3K/Akt pathway in monocytes. Inhibition of PI3K/Akt signalling resulting from CD300f crosslinking leads to downregulation of CD274 expression on monocytes. These findings highlight the potential use of CD300f blockade in cancer immune therapy to target immune suppressive macrophages in the tumour microenvironment, a known resistance mechanism to PD-1/PD-L1 checkpoint inhibitors.

Update of cellular responses to the efferocytosis of necroptosis and pyroptosis

Cell death is a basic physiological process that occurs in all living organisms. A few key players in these mechanisms, as well as various forms of cell death programming, have been identified. Apoptotic cell phagocytosis, also known as apoptotic cell clearance, is a well-established process regulated by a number of molecular components, including 'find-me', 'eat-me' and engulfment signals. Efferocytosis, or the rapid phagocytic clearance of cell death, is a critical mechanism for tissue homeostasis. Despite having similar mechanism to phagocytic clearance of infections, efferocytosis differs from phagocytosis in that it induces a tissue-healing response and is immunologically inert. However, as field of cell death has rapid expanded, much attention has recently been drawn to the efferocytosis of additional necrotic-like cell types, such as necroptosis and pyroptosis. Unlike apoptosis, this method of cell suicide allows the release of immunogenic cellular material and causes inflammation. Regardless of the cause of cell death, the clearance of dead cells is a necessary function to avoid uncontrolled synthesis of pro-inflammatory molecules and inflammatory disorder. We compare and contrast apoptosis, necroptosis and pyroptosis, as well as the various molecular mechanisms of efferocytosis in each type of cell death, and investigate how these may have functional effects on different intracellular organelles and signalling networks. Understanding how efferocytic cells react to necroptotic and pyroptotic cell uptake can help us understand how to modulate these cell death processes for therapeutic purposes.

The interaction of CD300lf and ceramide reduces the development of periodontitis by inhibiting osteoclast differentiation

AIM

The regulation of osteoclasts (OCs) by inhibitory immunoreceptors maintains bone homeostasis and is considered an important determinant of the extent of periodontal pathology. The aim of this study was to investigate the role of the inhibitory immunoreceptor CD300lf and its ligand ceramide in osteoclastogenesis in periodontitis.

MATERIALS AND METHODS

The expression of CD300lf was measured in vitro and in a ligature-induced periodontitis model. The effect of CD300lf ablation on osteoclastogenesis was examined in ligature-retained and ligature removal periodontitis models. The effect of ceramide, the ligand of CD300lf, was examined in osteoclastogenesis in vitro and in vivo by smearing 20 μg of ceramide dissolved in carboxymethylcellulose on teeth and gingiva every other day in an experimental periodontitis model and ligature removal model.

RESULTS

CD300lf expression was downregulated during osteoclastogenesis. Ablation of CD300lf in the ligature-induced periodontitis model increased the number of OCs and exacerbated bone damage. Bone resorption caused by CD300lf ablation was reversible following ligature removal. CD300lf-ceramide binding suppressed osteoclastogenesis in vitro and inhibited alveolar bone loss in a mouse periodontitis model.

CONCLUSIONS

Our findings reveal that CD300lf-ceramide binding plays a critical negative role in alveolar bone loss in periodontitis by inhibiting OCs differentiation.

Caspase-9 driven murine model of selective cell apoptosis and efferocytosis

Apoptosis and efficient efferocytosis are integral to growth, development, and homeostasis. The heterogeneity of these mechanisms in different cells across distinct tissues renders it difficult to develop broadly applicable in vivo technologies. Here, we introduced a novel inducible caspase-9 (iCasp9) mouse model which allowed targeted cell apoptosis and further facilitated investigation of concomitant efferocytosis. We generated iCasp9+/+ mice with conditional expression of chemically inducible caspase-9 protein that is triggered in the presence of Cre recombinase. In vitro, bone marrow cells from iCasp9+/+ mice showed expression of the iCasp9 protein when transduced with Cre-expressing adenovirus. Treatment of these cells with the chemical dimerizer (AP20187/AP) resulted in iCasp9 processing and cleaved caspase-3 upregulation, indicating successful apoptosis induction. The in vivo functionality and versatility of this model was demonstrated by crossing iCasp9+/+ mice with CD19-Cre and Osteocalcin (OCN)-Cre mice to target CD19+ B cells or OCN+ bone-lining osteoblasts. Immunofluorescence and/or immunohistochemical staining in combination with histomorphometric analysis of EGFP, CD19/OCN, and cleaved caspase-3 expression demonstrated that a single dose of AP effectively induced apoptosis in CD19+ B cells or OCN+ osteoblasts. Examination of the known efferocytes in the target tissues showed that CD19+ cell apoptosis was associated with infiltration of dendritic cells into splenic B cell follicles. In the bone, where efferocytosis remains under-explored, the use of iCasp9 provided direct in vivo evidence that macrophages are important mediators of apoptotic osteoblast clearance. Collectively, this study presented the first mouse model of iCasp9 which achieved selective apoptosis, allowing examination of subsequent efferocytosis. Given its unique feature of being controlled by any Cre-expressing mouse lines, the potential applications of this model are extensive and will bring forth more insights into the diversity of mechanisms and cellular effects induced by apoptosis including the physiologically important efferocytic process that follows.

Escherichia coli Mastitis in Dairy Cattle: Etiology, Diagnosis, and Treatment Challenges

Bovine mastitis is an inflammation of the udder tissue parenchyma that causes pathological changes in the glandular tissue and abnormalities in milk leading to significant economic losses to the dairy industry across the world. Mammary pathogenic Escherichia (E.) coli (MPEC) is one of the main etiologic agents of acute clinical mastitis in dairy cattle. MPEC strains have virulence attributes to resist the host innate defenses and thrive in the mammary gland environment. The association between specific virulence factors of MPEC with the severity of mastitis in cattle is not fully understood. Furthermore, the indiscriminate use of antibiotics to treat mastitis has resulted in antimicrobial resistance to all major antibiotic classes in MPEC. A thorough understanding of MPEC’s pathogenesis and antimicrobial susceptibility pattern is required to develop better interventions to reduce mastitis incidence and prevalence in cattle and the environment. This review compiles important information on mastitis caused by MPEC (e.g., types of mastitis, host immune response, diagnosis, treatment, and control of the disease) as well as the current knowledge on MPEC virulence factors, antimicrobial resistance, and the dilemma of MPEC as a new pathotype. The information provided in this review is critical to identifying gaps in knowledge that will guide future studies to better design diagnostic, prevent, and develop therapeutic interventions for this significant dairy disease.

Importance of Efferocytosis in COVID-19 Mortality

COVID-19 is a generally benign coronavirus disease that can spread rapidly, except for those with a group of risk factors. Since the pathogenesis responsible for the severity of the disease has not been clearly revealed, effective treatment alternatives has not been developed. The hallmark of the SARS-CoV-2-infected cells is apoptosis. Apoptotic cells are cleared through a sterile process defined as efferocytosis by professional and nonprofessional phagocytic cells. The disease would be rapidly brought under control in the organism that can achieve effective efferocytosis, which is also a kind of innate immune response. In the risk group, the efferocytic process is defective. With the addition of the apoptotic cell load associated with SARS-COV-2 infection, failure to achieve efferocytosis of dying cells can initiate secondary necrosis, which is a highly destructive process. Uncontrolled inflammation and coagulation abnormalities caused by secondary necrosis reason in various organ failures, lung in particular, which are responsible for the poor prognosis. Following the short and simplified information, this opinion paper aims to present possible treatment options that can control the severity of COVID-19 by detailing the mechanisms that can cause defective efferocytosis.

Efferocytosis and Its Role in Inflammatory Disorders

Efferocytosis is the effective clearance of apoptotic cells by professional and non-professional phagocytes. The process is mechanically different from other forms of phagocytosis and involves the localization, binding, internalization, and degradation of apoptotic cells. Defective efferocytosis has been demonstrated to associate with the pathogenesis of various inflammatory disorders. In the current review, we summarize recent findings with regard to efferocytosis networks and discuss the relationship between efferocytosis and different immune cell populations, as well as describe how efferocytosis helps resolve inflammatory response and modulate immune balance. Our knowledge so far about efferocytosis suggests that it may be a useful target in the treatment of numerous inflammatory diseases.

Effect of Streptococcus uberis on Gamma Delta T Cell Phenotype in Bovine Mammary Gland

Simple Summary

Bovine mastitis (inflammation of the mammary gland) is still an important problem for dairy farmers. This disease causes great financial losses across the world. The common method of treating mastitis is through the use of antibiotics. Antibiotic treatment should be minimized because of increasing antibiotic resistance. Streptococcus uberis (S. uberis) is one of the most important pathogens that causes bovine mastitis. This bacterium is able to hide and survive inside of epithelial cells. In this situation, antibiotic treatment is not efficient. Therefore, it is necessary to study the pathogenesis of mastitis that is caused by S. uberis to better understand how to treat this disease. In this study, we investigated a special type of lymphocytes—γδ T cells. The results of our study show that those cells may play a role in terminating inflammation in the mammary glands of cattle.

Abstract

In this study, we focused analyzing γδ T cells during bovine mammary gland inflammation induced by Streptococcus uberis. A mammary gland cell suspension was obtained using lavage 24, 48, 72, and 168 h after intramammary-induced infection. The proportion of lymphocytes increased during the entire week in which inflammation was present. The γδ T cells were also elevated during inflammation, reaching their peak at 72 h following induced inflammation. The percentage of apoptotic lymphocytes continually increased, with the highest proportion occurring 168 h after S. uberis infection. The results show that γδ T cells may be involved in the resolution of inflammation in bovine mammary glands, with the apoptosis of those cells potentially playing an important role.

Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

TIM4 expression by dendritic cells mediates uptake of tumor-associated antigens and anti-tumor responses

Acquisition of cell-associated tumor antigens by type 1 dendritic cells (cDC1) is essential to induce and sustain tumor specific CD8+ T cells via cross-presentation. Here we show that capture and engulfment of cell associated antigens by tissue resident lung cDC1 is inhibited during progression of mouse lung tumors. Mechanistically, loss of phagocytosis is linked to tumor-mediated downregulation of the phosphatidylserine receptor TIM4, that is highly expressed in normal lung resident cDC1. TIM4 receptor blockade and conditional cDC1 deletion impair activation of tumor specific CD8+ T cells and promote tumor progression. In human lung adenocarcinomas, TIM4 transcripts increase the prognostic value of a cDC1 signature and predict responses to PD-1 treatment. Thus, TIM4 on lung resident cDC1 contributes to immune surveillance and its expression is suppressed in advanced tumors.

CD300a and CD300f molecules regulate the function of leukocytes

The CD300 molecule family is a type I transmembrane glycoprotein expressed on cell membrane of human and other mammals, and of its eight members, only CD300a and CD300f are classified as inhibitory receptors. CD300a and CD300f play an important role in regulating the function of leukocytes, such as activation, proliferation, differentiation, migration and immunity function. They are considered as potential targets for studying the development and progression of inflammation, infection and other diseases. Here, we review the expression and regulatory mechanisms of CD300a and CD300f on leukocytes, as well as their effects on relevant diseases.

Autoantigen-specific immune tolerance in pathological and physiological cell death: Nanotechnology comes into view

Apoptotic cells are tolerogenic and can present self-antigens in the absence of inflammation, to antigen-presenting cells by the process of efferocytosis, resulting in anergy and depletion of immune effector cells. This tolerance is essential to maintain immune homeostasis and prevent systemic autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus. Consequently, effective efferocytosis can result in the induction of immune tolerance mediated via triggering modulatory lymphocytes and anti-inflammatory responses. Furthermore, several distinct soluble factors, receptors and pathways have been found to be involved in the efferocytosis, which are able to regulate immune tolerance by lessening antigen presentation, inhibition of T-cell proliferation and induction of regulatory T-cells. Some newly developed nanotechnology-based approaches can induce antigen-specific immunological tolerance without any systemic immunosuppression. These strategies have been explored to reverse autoimmune responses induced against various protein antigens in different diseases. In this review, we describe some nanotechnology-based approaches for the maintenance of self-tolerance using the apoptotic cell clearance process (efferocytosis) that may be able to induce immune tolerance and treat autoimmune diseases.

Regulation of efferocytosis as a novel cancer therapy

Efferocytosis is a physiologic phagocytic clearance of apoptotic cells, which modulates inflammatory responses and the immune environment and subsequently facilitates immune escape of cancer cells, thus promoting tumor development and progression. Efferocytosis is an equilibrium formed by perfect coordination among “find-me”, “eat-me” and “don’t-eat-me” signals. These signaling pathways not only affect the proliferation, invasion, metastasis, and angiogenesis of tumor cells but also regulate adaptive responses and drug resistance to antitumor therapies. Therefore, efferocytosis-related molecules and pathways are potential targets for antitumor therapy. Besides, supplementing conventional chemotherapy, radiotherapy and other immunotherapies with efferocytosis-targeted therapy could enhance the therapeutic efficacy, reduce off-target toxicity, and promote patient outcome.

Methods for monitoring the progression of cell death, cell disassembly and cell clearance

Cell death through apoptosis, necrosis, necroptosis and pyroptosis, as well as the clearance of dead cells are crucial biological processes in the human body. Likewise, disassembly of dying cells during apoptosis to generate cell fragments known as apoptotic bodies may also play important roles in regulating cell clearance and intercellular communication. Recent advances in the field have led to the development of new experimental systems to identify cells at different stages of cell death, measure the levels of apoptotic cell disassembly, and monitor the cell clearance process using a range of in vitro, ex vivo and in vivo models. In this article, we will provide a comprehensive review of the methods for monitoring the progression of cell death, cell disassembly and cell clearance.

CD300f immunoreceptor is associated with major depressive disorder and decreased microglial metabolic fitness

A role for microglia in neuropsychiatric diseases, including major depressive disorder (MDD), has been postulated. Regulation of microglial phenotype by immune receptors has become a central topic in many neurological conditions. We explored preclinical and clinical evidence for the role of the CD300f immune receptor in the fine regulation of microglial phenotype and its contribution to MDD. We found that a prevalent nonsynonymous single-nucleotide polymorphism (C/T, rs2034310) of the human CD300f receptor cytoplasmic tail inhibits the protein kinase C phosphorylation of a threonine and is associated with protection against MDD, mainly in women. Interestingly, CD300f^-/- mice displayed several characteristic MDD traits such as augmented microglial numbers, increased interleukin 6 and interleukin 1 receptor antagonist messenger RNA, alterations in synaptic strength, and noradrenaline-dependent and persistent depressive-like and anhedonic behaviors in females. This behavioral phenotype could be potentiated inducing the lipopolysaccharide depression model. RNA sequencing and biochemical studies revealed an association with impaired microglial metabolic fitness. In conclusion, we report a clear association that links the function of the CD300f immune receptor with MDD in humans, depressive-like and anhedonic behaviors in female mice, and altered microglial metabolic reprogramming.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

Mouse LIMR3/CD300f is a negative regulator of the antimicrobial activity of neutrophils

Leukocyte mono-immunoglobulin-like receptor (LMIR)/CD300 proteins comprise a family of immunoglobulin-like receptors that are widely expressed on the immune cell surface in humans and mice. In general, LMIR3/CD300f suppresses the inflammatory response, but it can occasionally promote it. However, the precise roles of LMIR3 in the function of neutrophils remain to be elucidated. In the present study, we investigated LMIR3 expression in mature and immature neutrophils, and evaluated the effects of LMIR3 deficiency in mouse neutrophils. Our results indicated that bone marrow (BM) neutrophils expressed LMIR3 on their cell surface during cell maturation and that surface LMIR3 expression increased in response to Pseudomonas aeruginosa infection in a TLR4/MyD88-dependent manner. LMIR3-knockout (KO) neutrophils displayed significantly increased hypochlorous acid production, and elastase release, as well as significantly augmented cytotoxic activity against P. aeruginosa and Candida albicans; meanwhile, inhibitors of elastase and myeloperoxidase offset this enhanced antimicrobial activity. Furthermore, LMIR3-KO mice were significantly more resistant to Pseudomonas peritonitis and systemic candidiasis, although this may not be entirely due to the enhanced activity of neutrophils. These results demonstrate that LMIR3/CD300f deficiency augments the antimicrobial activity of mouse neutrophils.

Liposome Preparation for the Analysis of Lipid-Receptor Interaction and Efferocytosis

Efficient phagocytosis of apoptotic cells (efferocytosis) is essential for immune homeostasis. Phospholipids exposed on the surface of apoptotic cells, such as phosphatidylserine, supply important "eat-me" signals. Liposomes are lipid bilayer vesicles that can be generated from one or several types of phospholipids of interest. Thus, these vesicles offer versatility, flexibility, and, importantly, a three-dimensional structure for studying the interaction between lipids and their receptors as well as the lipid-receptor interaction-mediated signaling events controlling efferocytosis by cells like professional phagocytes. Here, we describe methods to prepare liposomes, perform liposome-based lipid-receptor binding assays, use liposomes to block efferocytosis, and utilize liposome-coated beads as apoptotic cell surrogates for phagocytosis. © 2018 by John Wiley & Sons, Inc.

CD300 family receptors regulate eosinophil survival, chemotaxis, and effector functions

The CD300 family of receptors is an evolutionary conserved receptor family that belongs to the Ig superfamily and is expressed predominantly by the myeloid lineage. Over the past couple of years, accumulating data have shown that eosinophils express various Ig superfamily receptors that regulate key checkpoints in their biology including their maturation, transition from the bone marrow to the peripheral blood, migration, adhesion, survival, and effector functions in response to numerous activating signals such as IL-4, IL-33, and bacteria. In this review, we will present the emerging roles of CD300 family receptors and specifically CD300a and CD300f in the regulation of these eosinophil activities. The structure and expression pattern of these molecules will be discussed and their involvement in suppressing or co-activating eosinophil functions in health and disease will be illustrated.

Effect of Specific Mutations in Cd300 Complexes Formation; Potential Implication of Cd300f in Multiple Sclerosis

Herein, we have used bioinformatics tools to predict five clusters defining ligand-binding sites on the extracellular domain of human CD300b receptor, presumably involved in the formation of both homodimers and heterodimers with other CD300 family members. Site-directed mutagenesis revealed residues glutamic acid 28 and glutamine 29 in cluster 5 to be necessary for the formation of CD300b complexes. Surprisingly, the disruption of cluster 2 and 4 reconstituted the binding capability lost by the mutation of residues glutamic acid 28 to alanine, glutamine 29 to alanine (E28A-Q29G). We identified a missense mutation arginine 33 to glutamine (R33Q) in CD300f by direct sequencing of exon 2 in peripheral blood samples from 50 patients with multiple sclerosis (MS). Levels of expression of CD300f were almost undetectable on monocytes from the patient bearing the R33Q mutation compared with healthy individuals. Whereas R33Q mutation had no effect in the formation of CD300f complexes, the inhibition of protein synthesis with cycloheximide indicated that CD300f R33Q is less stable than native CD300f. Finally, we report that the levels of expression of CD300f on the surface of classical and intermediate monocytes from MS patients are significantly lower when compared to the same cell populations in healthy individuals.

Monocytes Phenotype and Cytokine Production in Human Immunodeficiency Virus-1 Infected Patients Receiving a Modified Vaccinia Ankara-Based HIV-1 Vaccine: Relationship to CD300 Molecules Expression

A modified vaccinia Ankara-based HIV-1 vaccine clade B (MVA-B) has been tested for safety and immunogenicity in low-risk human immunodeficiency virus (HIV)-uninfected individuals and as a therapeutic vaccine in HIV-1-infected individuals on combined antiretroviral therapy (cART). As a therapeutic vaccine, MVA-B was safe and broadly immunogenic; however, patients still showed a viral rebound upon treatment interruption. Monocytes are an important part of the viral reservoir and several studies suggest that they are partly responsible for the chronic inflammation observed in cART-treated HIV-infected people. The CD300 family of receptors has an important role in several diseases, including viral infections. Monocytes express CD300a, c, e, and f molecules and lipopolysaccharide (LPS) and other stimuli regulate their expression. However, the expression and function of CD300 receptors on monocytes in HIV infection is still unknown. In this work, we investigated for the first time the expression of CD300 molecules and the cytokine production in response to LPS on monocytes from HIV-1-infected patients before and after vaccination with MVA-B. Our results showed that CD300 receptors expression on monocytes from HIV-1-infected patients correlates with markers of HIV infection progression and immune inflammation. Specifically, we observed a positive correlation between the expression of CD300e and CD300f receptors on monocytes with the number of CD4+ T cells of HIV-1-infected patients before vaccination. We also saw a positive correlation between the expression of the inhibitory receptor CD300f and the expression of CD163 on monocytes from HIV-1-infected individuals before and after vaccination. In addition, monocytes exhibited a higher cytokine production in response to LPS after vaccination, almost at the same levels of monocytes from healthy donors. Furthermore, we also described a correlation in the expression of CD300e and CD300f receptors with TNF-α production in response to LPS, only in monocytes of HIV-1-infected patients before vaccination. Altogether, our results describe the impact of HIV-1 and of the MVA-B vaccine in cytokine production and monocytes phenotype.

Progressive Changes in CXCR4 Expression That Define Thymocyte Positive Selection Are Dispensable For Both Innate and Conventional αβT-cell Development

The ordered migration of immature thymocytes through thymic microenvironments generates both adaptive MHC restricted αβT-cells and innate CD1d-restricted iNKT-cells. While several chemokine receptors and ligands control multiple stages of this process, their involvement during early thymocyte development often precludes direct analysis of potential roles during later developmental stages. For example, because of early lethality of CXCR4-/- mice, and stage-specific requirements for CXCR4 in thymus colonisation and pre-TCR mediated selection, its role in thymic positive selection is unclear. Here we have examined CXCR4-CXCL12 interactions during the maturation of CD4+CD8+ thymocytes, including downstream stages of iNKT and αβT-cell development. We show CXCL12 expression is a common feature of cortical thymic epithelial cells, indicating widespread availability throughout the cortex. Moreover, CXCR4 expression by CD4+CD8+ pre-selection thymocytes is progressively downregulated following both MHC and CD1d-restricted thymic selection events. However, using CD4Cre-mediated deletion to bypass its involvement in CD4-CD8- thymocyte development, we show CXCR4 is dispensable for the maintenance and intrathymic positioning of CD4+CD8+ thymocytes, and their ability to generate mature αβT-cells and CD1d-restricted iNKT-cells. Collectively, our data define dynamic changes in CXCR4 expression as a marker for intrathymic selection events, and show its role in T-cell development is restricted to pre-CD4+CD8+ stages.

Dendritic cells expressing immunoreceptor CD300f are critical for controlling chronic gut inflammation

Proinflammatory cytokine overproduction and excessive cell death, coupled with impaired clearance of apoptotic cells, have been implicated as causes of failure to resolve gut inflammation in inflammatory bowel diseases. Here we have found that dendritic cells expressing the apoptotic cell-recognizing receptor CD300f play a crucial role in regulating gut inflammatory responses in a murine model of colonic inflammation. CD300f-deficient mice failed to resolve dextran sulfate sodium-induced colonic inflammation as a result of defects in dendritic cell function that were associated with abnormal accumulation of apoptotic cells in the gut. CD300f-deficient dendritic cells displayed hyperactive phagocytosis of apoptotic cells, which stimulated excessive TNF-α secretion predominantly from dendritic cells. This, in turn, induced secondary IFN-γ overproduction by colonic T cells, leading to prolonged gut inflammation. Our data highlight a previously unappreciated role for dendritic cells in controlling gut homeostasis and show that CD300f-dependent regulation of apoptotic cell uptake is essential for suppressing overactive dendritic cell-mediated inflammatory responses, thereby controlling the development of chronic gut inflammation.

Norovirus Regulation by Host and Microbe

Norovirus (NoV) infection is the leading cause of epidemic gastroenteritis globally, and can lead to detrimental chronic infection in immunocompromised hosts. Despite its prevalence as a cause of diarrheal illness, the study of human NoVs (HNoVs) has historically been limited by a paucity of models. The use of murine NoV (MNoV) to interrogate mechanisms of host control of viral infection has facilitated the exploration of different genetic mouse models, revealing roles for both innate and adaptive immunity in viral regulation. MNoV studies have also recently identified important interactions between the commensal microbiota and NoV with clear extensions to HNoVs. In this review, we discuss the most current understanding of how the host, the microbiome, and their interactions regulate NoV infections.

The expression and function of human CD300 receptors on blood circulating mononuclear cells are distinct in neonates and adults

Neonates are more susceptible to infections than adults. This susceptibility is thought to reflect neonates' qualitative and quantitative defects in the adaptive and innate immune responses. Differential expression of cell surface receptors may result in altered thresholds of neonatal immune cell activation. We determined whether the expression and function of the lipid-binding CD300 family of receptors are different on neonatal immune cells compared to adult immune cells. A multiparametric flow cytometry analysis was performed to determine the expression of CD300 receptors on adult peripheral blood mononuclear cells and neonatal cord blood mononuclear cells. The expression of the CD300a inhibitory receptor was significantly reduced on cells from the newborn adaptive immune system, and neonatal antigen presenting cells exhibited a different CD300 receptors expression pattern. We also found differential LPS-mediated regulation of CD300 receptors expression on adult monocytes compared to cord blood monocytes, and that CD300c and CD300e-mediated activation was quantitatively different in neonatal monocytes. This is the first complete study examining the expression of CD300 receptors on human neonatal immune cells compared with adult immune cells. Significant differences in the expression and function of CD300 receptors may help to explain the peculiarities and distinctness of the neonatal immune responses.