Fragile TIM-4-expressing tissue resident macrophages are migratory and immunoregulatory

Modulating Lipid-Polymer Nanoparticles' Physicochemical Properties to Alter Macrophage Uptake

Macrophage uptake of nanoparticles is highly dependent on the physicochemical characteristics of those nanoparticles. Here, we have created a collection of lipid-polymer nanoparticles (LPNPs) varying in size, stiffness, and lipid makeup to determine the effects of these factors on uptake in murine bone marrow-derived macrophages. The LPNPs varied in diameter from 232 to 812 nm, in storage modulus from 21.2 to 287 kPa, and in phosphatidylserine content from 0 to 20%. Stiff, large nanoparticles with a coating containing phosphatidylserine were taken up by macrophages to a much higher degree than any other formulation (between 9.3× and 166× higher than other LPNPs). LPNPs with phosphatidylserine were taken up most by M2-polarized macrophages, while those without were taken up most by M1-polarized macrophages. Differences in total LPNP uptake were not dependent on endocytosis pathway(s) other than phagocytosis. This work acts as a basis for understanding how the interactions between nanoparticle physicochemical characteristics may act synergistically to facilitate particle uptake.

Tissue-Resident Macrophages in Solid Organ Transplantation: Harmful or Protective?

Transplanted organs carry donor immune cells into the recipient, the majority of which are tissue-resident macrophages (TRMs). The role they play in guiding the fate of the transplanted organ toward acceptance or rejection remains elusive. TRMs originate from both embryonic and bone marrow-derived precursors. Embryo-derived TRMs retain the embryonic capability to proliferate, so they are able to self-renew and, theoretically, persist for extended periods of time after transplantation. Bone marrow-derived TRMs do not proliferate and must constantly be replenished by adult circulating monocytes. Recent studies have aimed to clarify the different roles and interactions between donor TRMs, recipient monocytes, and monocyte-derived macrophages (MFs) after organ transplantation. This review aims to shed light on how MFs affect the fate of a transplanted organ by differentiating between the role of donor TRMs and that of MFs derived from graft infiltrating monocytes.

Targeting Macrophages in Organ Transplantation: A Step Toward Personalized Medicine

Organ transplantation remains the most optimal strategy for patients with end-stage organ failure. However, prevailing methods of immunosuppression are marred by adverse side effects, and allograft rejection remains common. It is imperative to identify and comprehensively characterize the cell types involved in allograft rejection, and develop therapies with greater specificity. There is increasing recognition that processes mediating allograft rejection are the result of interactions between innate and adaptive immune cells. Macrophages are heterogeneous innate immune cells with diverse functions that contribute to ischemia-reperfusion injury, acute rejection, and chronic rejection. Macrophages are inflammatory cells capable of innate allorecognition that strengthen their responses to secondary exposures over time via "trained immunity." However, macrophages also adopt immunoregulatory phenotypes and may promote allograft tolerance. In this review, we discuss the roles of macrophages in rejection and tolerance, and detail how macrophage plasticity and polarization influence transplantation outcomes. A comprehensive understanding of macrophages in transplant will guide future personalized approaches to therapies aimed at facilitating tolerance or mitigating the rejection process.

Apoptosis recognition receptors regulate skin tissue repair in mice

Apoptosis and clearance of apoptotic cells via efferocytosis are evolutionarily conserved processes that drive tissue repair. However, the mechanisms by which recognition and clearance of apoptotic cells regulate repair are not fully understood. Here, we use single-cell RNA sequencing to provide a map of the cellular dynamics during early inflammation in mouse skin wounds. We find that apoptotic pathways and efferocytosis receptors are elevated in fibroblasts and immune cells, including resident Lyve1+ macrophages, during inflammation. Interestingly, human diabetic foot wounds upregulate mRNAs for efferocytosis pathway genes and display altered efferocytosis signaling via the receptor Axl and its ligand Gas6. During early inflammation in mouse wounds, we detect upregulation of Axl in dendritic cells and fibroblasts via TLR3-independent mechanisms. Inhibition studies in vivo in mice reveal that Axl signaling is required for wound repair but is dispensable for efferocytosis. By contrast, inhibition of another efferocytosis receptor, Timd4, in mouse wounds decreases efferocytosis and abrogates wound repair. These data highlight the distinct mechanisms by which apoptotic cell detection coordinates tissue repair and provides potential therapeutic targets for chronic wounds in diabetic patients.

Immune Regulation of the Liver Through the PCSK9/CD36 Pathway During Heart Transplant Rejection

BACKGROUND

PCSK9 (proprotein convertase subtilisin/kexin 9), which is mainly secreted by the liver, is not only a therapeutic target for hyperlipidemia and cardiovascular disease, but also has been implicated in the immune regulation of infections and tumors. However, the role of PCSK9 and the liver in heart transplant rejection (HTR) and the underlying mechanisms remain unclear.

METHODS

We assessed serum PCSK9 expression in both murine and human recipients during HTR and investigated the effect of PCSK9 ablation on HTR by using global knockout mice and a neutralizing antibody. Moreover, we performed multiorgan histological and transcriptome analyses, and multiomics and single-cell RNA-sequencing studies of the liver during HTR, as well. We further used hepatocyte-specific Pcsk9 knockout mice to investigate whether the liver regulated HTR through PCSK9. Last, we explored the regulatory effect of the PCSK9/CD36 pathway on the phenotype and function of macrophages in vitro and in vivo.

RESULTS

Here, we report that murine and human recipients have high serum PCSK9 levels during HTR. PCSK9 ablation prolonged cardiac allograft survival and attenuated the infiltration of inflammatory cells in the graft and the expansion of alloreactive T cells in the spleen. Next, we demonstrated that PCSK9 was mainly produced and significantly upregulated in the recipient liver, which also showed a series of signaling changes, including changes in the TNF-α (tumor necrosis factor α) and IFN-γ (interferon γ) signaling pathways and the bile acid and fatty acid metabolism pathways. We found mechanistically that TNF-α and IFN-γ synergistically promoted PCSK9 expression in hepatocytes through the transcription factor SREBP2 (sterol regulatory element binding protein 2). Moreover, in vitro and in vivo studies indicated that PCSK9 inhibited CD36 expression and fatty acid uptake by macrophages and strengthened the proinflammatory phenotype, which facilitated their ability to promote proliferation and IFN-γ production by donor-reactive T cells. Last, we found that the protective effect of PCSK9 ablation against HTR is dependent on the CD36 pathway in the recipient.

CONCLUSIONS

This study reveals a novel mechanism for immune regulation by the liver through the PCSK9/CD36 pathway during HTR, which influences the phenotype and function of macrophages and suggests that the modulation of this pathway may be a potential therapeutic target to prevent HTR.

Multiple Shades of Gray-Macrophages in Acute Allograft Rejection

Long-term results following solid organ transplantation do not mirror the excellent short-term results achieved in recent decades. It is therefore clear that current immunosuppressive maintenance protocols primarily addressing the adaptive immune system no longer meet the required clinical need. Identification of novel targets addressing this shortcoming is urgently needed. There is a growing interest in better understanding the role of the innate immune system in this context. In this review, we focus on macrophages, which are known to prominently infiltrate allografts and, during allograft rejection, to be involved in the surge of the adaptive immune response by expression of pro-inflammatory cytokines and direct cytotoxicity. However, this active participation is janus-faced and unspecific targeting of macrophages may not consider the different subtypes involved. Under this premise, we give an overview on macrophages, including their origins, plasticity, and important markers. We then briefly describe their role in acute allograft rejection, which ranges from sustaining injury to promoting tolerance, as well as the impact of maintenance immunosuppressants on macrophages. Finally, we discuss the observed immunosuppressive role of the vitamin-like compound tetrahydrobiopterin and the recent findings that suggest the innate immune system, particularly macrophages, as its target.

Engineered Apoptosis-Bioinspired Nanoparticles Initiate Immune Cascade for Cancer Immunotherapy of Malignant Ascites

Malignant ascites (MA) is a common symptom of peritoneal metastasis in liver cancer. Cancer immunotherapy can modulate immune cells to induce antitumor immune efficiency. Reprogramming tumor immune microenvironment (TIME) is a momentous strategy to overcome immunosuppression and achieve immune functional normalization. Inspired by the inherent apoptotic bodies and vesicles, we proposed and systematically studied engineered apoptosis-bioinspired nanoparticles (EBN) for cancer immunotherapy of MA. Using both in vitro and in vivo experimental validations, we elucidated that EBN could be efficiently engulfed by the tumor-associated macrophages (TAMs) and manipulate their polarization. Moreover, a boosted immune cascade response as a result of heightening cytotoxic T-lymphocytes (CTLs) activity was investigated. Based on these results, EBN was confirmed to have strong immune cascade activation capability. Remarkably, the injection of EBN further reduced ascites volume and reformed immune cell subtypes, compared to the injection of either PBS or free TMP195 alone. In short, this novel nanodrug delivery system (NDDS) represents a prospective immunotherapeutic approach for clinical therapeutics of hepatoma ascites and other malignant effusion.

Apoptosis recognition receptors regulate skin tissue repair in mice

Apoptosis and clearance of apoptotic cells via efferocytosis are evolutionarily conserved processes that drive tissue repair. However, the mechanisms by which recognition and clearance of apoptotic cells regulate repair are not fully understood. Here, we use single-cell RNA sequencing to provide a map of the cellular dynamics during early inflammation in mouse skin wounds. We find that apoptotic pathways and efferocytosis receptors are elevated in fibroblasts and immune cells, including resident Lyve1 + macrophages, during inflammation. Interestingly, human diabetic foot wounds upregulate mRNAs for apoptotic genes and display increased and altered efferocytosis signaling via the receptor Axl. During early inflammation in mouse wounds, we detect upregulation of Axl in dendritic cells and fibroblasts via TLR3-independent mechanisms. Inhibition studies in vivo in mice reveal that Axl signaling is required for wound repair but is dispensable for efferocytosis. By contrast, inhibition of another efferocytosis receptor, Timd4, in mouse wounds decreases efferocytosis and abrogates wound repair. These data highlight the distinct mechanisms by which apoptotic cell detection coordinates tissue repair and provides potential therapeutic targets for chronic wounds in diabetic patients.

Macrophages in cardiac remodelling after myocardial infarction

Myocardial infarction (MI), as a result of thrombosis or vascular occlusion, is the most prevalent cause of morbidity and mortality among all cardiovascular diseases. The devastating consequences of MI are compounded by the complexities of cellular functions involved in the initiation and resolution of early-onset inflammation and the longer-term effects related to scar formation. The resultant tissue damage can occur as early as 1 h after MI and activates inflammatory signalling pathways to elicit an immune response. Macrophages are one of the most active cell types during all stages after MI, including the cardioprotective, inflammatory and tissue repair phases. In this Review, we describe the phenotypes of cardiac macrophage involved in MI and their cardioprotective functions. A specific subset of macrophages called resident cardiac macrophages (RCMs) are derived from yolk sac progenitor cells and are maintained as a self-renewing population, although their numbers decrease with age. We explore sophisticated sequencing techniques that demonstrate the cardioprotective properties of this cardiac macrophage phenotype. Furthermore, we discuss the interactions between cardiac macrophages and other important cell types involved in the pathology and resolution of inflammation after MI. We summarize new and promising therapeutic approaches that target macrophage-mediated inflammation and the cardioprotective properties of RCMs after MI. Finally, we discuss future directions for the study of RCMs in MI and cardiovascular health in general.

T-cell immunoglobulin- and mucin-domain-containing molecule-4 maintains adipose tissue homeostasis by orchestrating M2 macrophage polarization via nuclear factor kappa B pathway

Obesity is generally associated with low-grade inflammation. Adipose tissue macrophages (ATMs) orchestrate metabolic inflammation. The classical (M1-like) or alternative (M2-like) activation of ATMs is functionally coupled with the metabolic status of fat tissues. It has been found that T-cell immunoglobulin- and mucin-domain-containing molecule-4 (Tim-4) inhibits inflammation by regulating macrophages. However, the exact role of Tim-4 in macrophage polarization and obesity remains unknown. Here, we identified Tim-4 as a critical switch governing macrophage M1/M2 polarization and energy homeostasis. Tim-4 deletion led to spontaneous obesity in elder mice and promoted obesity severity of db/db mice. Obesity microenvironment enhanced the expression of Tim-4 in white adipose tissue and ATMs. In vitro, we detected an increase in M1-like cells and decrease in M2-like cells in both peritoneal macrophages and bone marrow-derived macrophages from Tim-4 knockout mice. Mechanistically, we demonstrated that Tim-4 promoted M2-like macrophages polarization via suppressing nuclear factor kappa B (NF-κB) signaling pathway. In addition, we found that Tim-4 promoted TLR4 internalization, which might contribute to regulation of NF-κB signaling. Collectively, these results indicated that Tim-4 maintained adipose tissue homeostasis by regulating macrophage polarization via NF-κB pathway, which would provide a new target for obesity intervention.

Function and characteristics of TIM‑4 in immune regulation and disease (Review)

T‑cell/transmembrane immunoglobulin and mucin domain containing 4 (TIM‑4) is a phosphatidylserine receptor that is mainly expressed on antigen‑presenting cells and is involved in the recognition and efferocytosis of apoptotic cells. TIM‑4 has been found to be expressed in immune cells such as natural killer T, B and mast cells and to participate in multiple aspects of immune regulation, suggesting that TIM‑4 may be involved in a variety of immune‑related diseases. Recent studies have confirmed that TIM‑4 is also abnormally expressed in a variety of malignant tumor cells and is closely associated with the occurrence and development of tumors and the tumor immune microenvironment. The present study aimed to describe the expression and functional characteristics of TIM‑4 in detail and to comprehensively discuss its role in pathophysiological processes such as infection, allergy, metabolism, autoimmunity and tumor immunity. The current review provided a comprehensive understanding of the functions and characteristics of TIM‑4, as well as novel ideas for the diagnosis and treatment of diseases.

A Microbiota-Dependent Subset of Skin Macrophages Protects Against Cutaneous Bacterial Infection

Microbiota is essential to the development and functional maturation of the immune system. The effects of the gut microbiota on myeloid cells remote from the gut, especially the skin remain unclear. Transcriptomic analysis revealed that type I interferon (IFN) signaling was down-regulated in the skin of germ-free mice compared to that in specific pathogen-free mice. The decrease in type I IFN signaling was closely related to the presence of microbiota and macrophage-specific marker CD169. The absence of CD169⁺ macrophages resulted in increased bacterial burden and impaired immune responses against Staphylococcus aureus skin infection. CD169⁺ macrophages mediated the recruitment of γδ T cells as well as the activation of γδ T cells via interleukin (IL)-23. Our findings demonstrate the role of the microbiota in establishment of a specific myeloid cell subset expressing CD169 in the skin and provide evidence of a specific mechanism by which this subset protects against bacterial skin infection.

The role of efferocytosis in neuro-degenerative diseases

Efferocytosis has a critical role in maintaining tissues and organs' homeostasis by removing apoptotic cells. It is essential for human health, and disturbances in efferocytosis may result indifferent illnesses. In case of inadequate clearance of the dead cells, the content in the cells would be released. In fact, it induces some damages to the tissue and leads to the prolonged inflammation, so unsuitable phagocytosis of the apoptotic cells is involved in occurrence as well as expansion of numerous human chronic inflammatory diseases. Studies have shown age dependence of the neuro-degenerative diseases, which are largely due to the neuro-inflammation and the loss of neurons and thus cause the brain's functional disorders. Efferocytosis is coupled to anti-inflammatory responses that contribute to the elimination of the dying neurons in neuro-degenerative diseases, so its disruption may make a risk factor in numerous human chronic inflammatory diseases such as multiple sclerosis, Alzheimer's disease, glioblastoma, and Rett syndrome. This study is a review of the efferocytosis molecular pathways and their role in neuro-degenerative diseases in order to discover a new treatment option to cure patients.

The role of salivary gland macrophages in infection, disease and repair

Macrophages are mononuclear innate immune cells which have become of increasing interest in the fields of disease and regeneration, as their non-classical functions have been elucidated in addition to their classical inflammatory functions. Macrophages can regulate tissue remodeling, by both mounting and reducing inflammatory responses; and exhibit direct communication with other cells to drive tissue turnover and cell replacement. Furthermore, macrophages have recently become an attractive therapeutic target to drive tissue regeneration. The major salivary glands are glandular tissues that are exposed to pathogens through their close connection with the oral cavity. Moreover, there are a number of diseases that preferentially destroy the salivary glands, causing irreversible injury, highlighting the need for a regenerative strategy. However, characterization of macrophages in the mouse and human salivary glands is sparse and has been mostly determined from studies in infection or autoimmune pathologies. In this review, we describe the current literature around salivary gland macrophages, and speculate about the niches they inhabit and how their role in development, regeneration and cancer may inform future therapeutic advances.

Tolerogenic properties of CD206+ macrophages appeared in the sublingual mucosa after repeated antigen-painting

The sublingual mucosa (SLM) in the oral cavity is utilized as the site for sublingual immunotherapy to induce tolerance against allergens. We previously reported that CD206+ round-type macrophage-like cells were induced in the SLM after repeated antigen (e.g. cedar pollen or fluorescein isothiocyanate (FITC))-painting. In this study, we examined the phenotypic and functional properties of CD206+ cells induced by repeated FITC-painting on the SLM. CD206+ cells after the repeated FITC-painting possessed a macrophage-like CD11b+Ly6C+ F4/80+CD64+ phenotype and expressed TIM-4, which was expressed in tolerogenic tissue-resident macrophages, at a high level. SLM CD206+ cells preferentially expressed molecules related to endocytosis and homeostatic processes, including the novel B7 family of immune checkpoint molecules, as assessed by microarray analyses. SLM CD206+ cells showed preferential expression of M2-related genes such as Fizz1, Aldh1a1 and Aldh1a2 but not Ym-1 and Arginase-1. A CD206+ cell-rich status inhibited OVA-specific CD4+ T-cell responses but reciprocally enhanced the proportion of both IL-10+CD4+ cells and Foxp3+ regulatory T-cells in regional lymph nodes. Co-culture of CD206+ cells with dendritic cells (DCs) showed that IL-12 production was suppressed in DCs concurrent with the decline of the MHC class IIhiCD86+ population, which was restored by neutralization of IL-10. These results demonstrate SLM CD206+ cells show the feature of tolerogenic macrophages and down-regulate the antigen-presenting cell function of mature DCs resulting in the inhibition of CD4+ T-cell responses.

The Evolving Roles of Cardiac Macrophages in Homeostasis, Regeneration, and Repair

Macrophages were first described as phagocytic immune cells responsible for maintaining tissue homeostasis by the removal of pathogens that disturb normal function. Historically, macrophages have been viewed as terminally differentiated monocyte-derived cells that originated through hematopoiesis and infiltrated multiple tissues in the presence of inflammation or during turnover in normal homeostasis. However, improved cell detection and fate-mapping strategies have elucidated the various lineages of tissue-resident macrophages, which can derive from embryonic origins independent of hematopoiesis and monocyte infiltration. The role of resident macrophages in organs such as the skin, liver, and the lungs have been well characterized, revealing functions well beyond a pure phagocytic and immunological role. In the heart, recent research has begun to decipher the functional roles of various tissue-resident macrophage populations through fate mapping and genetic depletion studies. Several of these studies have elucidated the novel and unexpected roles of cardiac-resident macrophages in homeostasis, including maintaining mitochondrial function, facilitating cardiac conduction, coronary development, and lymphangiogenesis, among others. Additionally, following cardiac injury, cardiac-resident macrophages adopt diverse functions such as the clearance of necrotic and apoptotic cells and debris, a reduction in the inflammatory monocyte infiltration, promotion of angiogenesis, amelioration of inflammation, and hypertrophy in the remaining myocardium, overall limiting damage extension. The present review discusses the origin, development, characterization, and function of cardiac macrophages in homeostasis, cardiac regeneration, and after cardiac injury or stress.

Innovative Systems to Deliver Allergen Powder for Epicutaneous Immunotherapy

Allergy is a disorder owing to hyperimmune responses to a particular kind of substance like food and the disease remains a serious healthcare burden worldwide. This unpleasant and sometimes fatal allergic disease has been tackled vigorously by allergen-specific immunotherapy over a century, but the progress made so far is far from satisfactory for some allergies. Herein, we introduce innovative, allergen powder-based epicutaneous immunotherapies (EPIT), which could potentially serve to generate a new stream of technological possibilities that embrace the features of super safety and efficacious immunotherapy by manipulating the plasticity of the skin immune system via sufficient delivery of not only allergens but also tolerogenic adjuvants. We attempt to lay a framework to help understand immune physiology of the skin, epicutaneous delivery of powdered allergy, and potentials for tolerogenic adjuvants. Preclinical and clinical data are reviewed showing that deposition of allergen powder into an array of micropores in the epidermis can confer significant advantages over intradermal or subcutaneous injection of aqueous allergens or other epicutaneous delivery systems to induce immunological responses toward tolerance at little risk of anaphylaxis. Finally, the safety, cost-effectiveness, and acceptability of these novel EPITs are discussed, which offers the perspective of future immunotherapies with all desirable features.

Macrophages in Organ Transplantation

Current immunosuppressive therapy has led to excellent short-term survival rates in organ transplantation. However, long-term graft survival rates are suboptimal, and a vast number of allografts are gradually lost in the clinic. An increasing number of animal and clinical studies have demonstrated that monocytes and macrophages play a pivotal role in graft rejection, as these mononuclear phagocytic cells recognize alloantigens and trigger an inflammatory cascade that activate the adaptive immune response. Moreover, recent studies suggest that monocytes acquire a feature of memory recall response that is associated with a potent immune response. This form of memory is called “trained immunity,” and it is retained by mechanisms of epigenetic and metabolic changes in innate immune cells after exposure to particular ligands, which have a direct impact in allograft rejection. In this review article, we highlight the role of monocytes and macrophages in organ transplantation and summarize therapeutic approaches to promote tolerance through manipulation of monocytes and macrophages. These strategies may open new therapeutic opportunities to increase long-term transplant survival rates in the clinic.

Autophagic adaptation to oxidative stress alters peritoneal residential macrophage survival and ovarian cancer metastasis

Tumor-associated macrophages (TAMs) affect cancer progression and therapy. Ovarian carcinoma often metastasizes to the peritoneal cavity. Here, we found 2 peritoneal macrophage subsets in mice bearing ID8 ovarian cancer based on T cell immunoglobulin and mucin domain containing 4 (Tim-4) expression. Tim-4⁺ TAMs were embryonically originated and locally sustained while Tim-4^– TAMs were replenished from circulating monocytes. Tim-4⁺ TAMs, but not Tim-4^– TAMs, promoted tumor growth in vivo. Relative to Tim-4^– TAMs, Tim-4⁺ TAMs manifested high oxidative phosphorylation and adapted mitophagy to alleviate oxidative stress. High levels of arginase-1 in Tim-4⁺ TAMs contributed to potent mitophagy activities via weakened mTORC1 activation due to low arginine resultant from arginase-1–mediated metabolism. Furthermore, genetic deficiency of autophagy element FAK family-interacting protein of 200 kDa resulted in Tim-4⁺ TAM loss via ROS-mediated apoptosis and elevated T cell immunity and ID8 tumor inhibition in vivo. Moreover, human ovarian cancer–associated macrophages positive for complement receptor of the immunoglobulin superfamily (CRIg) were transcriptionally, metabolically, and functionally similar to murine Tim-4⁺ TAMs. Thus, targeting CRIg⁺ (Tim-4⁺) TAMs may potentially treat patients with ovarian cancer with peritoneal metastasis.

Ovarian tumor-associated peritoneal macrophages have potent mitochondria activity, which enables reliance on the mitophagy pathway to alleviate oxidative stress and facilitate survival in the tumor microenvironment.

Role of donor macrophages after heart and lung transplantation

Since the 1960s, heart and lung transplantation has remained the optimal therapy for patients with end-stage disease, extending and improving quality of life for thousands of individuals annually. Expanding donor organ availability and immunologic compatibility is a priority to help meet the clinical demand for organ transplant. While effective, current immunosuppression is imperfect as it lacks specificity and imposes unintended adverse effects such as opportunistic infections and malignancy that limit the health and longevity of transplant recipients. In this review, we focus on donor macrophages as a new target to achieve allograft tolerance. Donor organ-directed therapies have the potential to improve allograft survival while minimizing patient harm related to global suppression of recipient immune responses. Topics highlighted include the role of ontogenically distinct donor macrophage populations in ischemia-reperfusion injury and rejection, including their interaction with allograft-infiltrating recipient immune cells and potential therapeutic approaches. Ultimately, a better understanding of how donor intrinsic immunity influences allograft acceptance and survival will provide new opportunities to improve the outcomes of transplant recipients.

Adaptive features of innate immune cells and their relevance to graft rejection

PURPOSE OF REVIEW

Allograft rejection involves both innate and adaptive immune cells, and the adaptive immune cells have dominated transplant studies for decades. Recent studies have identified surprising new features for the innate immune cells, including memory recall responses, which may have significant implications in further improvement of transplant outcomes.

RECENT FINDINGS

Transplant survival is excellent in the short-term, but the long-term graft outcomes are not so, and most grafts are continuously lost to chronic rejection in the clinic. In both animal models and clinical settings, graft loss to chronic rejection is often dominated by innate immune cells, especially macrophages and natural killer (NK) cells in the grafts. Recent studies suggest that innate immune cells can acquire features of adaptive cells in that they either directly sense allogeneic nonself or become 'trained' in the allogeneic milieu, where they show features of memory recall responses. In certain models, targeting the adaptive features of such innate immune cells can promote long-term allograft survival. These findings may open new therapeutic opportunities in promoting transplant survival in the clinic.

SUMMARY

The discovery of donor specificity and memory recall responses of certain innate immune cells, which are prominently featured in chronic allograft rejection, may open novel therapeutic opportunities in transplantation, as well as in treatment of cancers and autoimmune diseases.

Tim-4 in Health and Disease: Friend or Foe?

T-cell immunoglobulin and mucin domain containing 4 (Tim-4) is a phosphatidylserine receptor and is selectively expressed on antigen presenting cells. Recently, Tim-4 was reported to be expressed on iNKT cells, B1 cells, and tumor cells, suggesting it has multiple biological functions. In this review, we mainly summarize the expression and regulation of Tim-4 in immune cells including T cells, macrophages, dendritic cells, NKT cells, B cells, and mast cells. The expression of Tim-4 in these cells implies that Tim-4 might participate in immune related diseases. Emerging evidence emphasizes a substantial role for Tim-4 in maintaining homeostasis by regulating various immune responses, including viral infection, allergy, autoimmunity, and tumor immunity. Here, we collectively evaluated the role of Tim-4 in health and diseases. This summary will be extremely useful to fully understand the function of Tim-4 in the pathogenesis of immune related diseases, which would provide novel clues for the diagnosis and treatment of diseases.

Serum soluble sialic acid-binding immunoglobulin-like lectin-7 concentration as an indicator of liver macrophage activation and advanced fibrosis in patients with non-alcoholic fatty liver disease

AIM

Non-alcoholic fatty liver disease (NAFLD) is a leading cause of liver disease worldwide. Because liver fibrosis is associated with the long-term prognosis of patients with NAFLD, there is an urgent need for non-invasive markers of liver fibrosis. Sialic acid-binding immunoglobulin-like lectin-7 (Siglec-7) is an immunomodulatory molecule expressed on various immune cells, including macrophages, which plays a key role in liver inflammation and fibrosis in NAFLD. We aimed to determine whether serum levels of soluble Siglec-7 (sSiglec-7) could have utility at a marker of fibrosis in this patient population.

METHODS

We examined serum samples from 93 NAFLD patients and 19 healthy donors for macrophage-associated protein, including sSiglec-7, soluble CD163, and YKL-40, and examined their correlation with liver fibrosis scores, tissue elastography, and histological findings. Independent factors associated with advanced fibrosis were analyzed using a logistic regression model and a decision tree. To clarify the source of sSiglec-7, we examined its expression in liver tissue-derived macrophages and cultured monocyte-derived macrophages.

RESULTS

Serum sSiglec-7 levels were significantly higher in NAFLD patients compared with healthy donors, and correlated positively with sCD163 and YKL-40 levels. Serum sSiglec-7 was an independent diagnostic marker with high specificity (96.3%) for advanced fibrosis (F3 and F4) in NAFLD patients. Siglec-7 was mainly expressed on CCR2⁺ macrophages in the liver, and sSiglec-7 production by monocyte-derived macrophages in vitro was increased after stimulation by pro-inflammatory factors.

CONCLUSIONS

Elevated serum sSiglec-7 could serve as an independent marker with high specificity for advanced liver fibrosis in patients with NAFLD.

The role of phosphatidylserine recognition receptors in multiple biological functions

Apoptotic cells are rapidly engulfed and degraded by phagocytes through efferocytosis. Efferocytosis is a highly regulated process. It is triggered upon the activation of caspase-dependent apoptosis, which in turn promotes the expression of "eat me" signals on the surface of dying cells and the release of soluble "find me" signals for the recruitment of phagocytes. To date, many "eat me" signals have been recognized, including phosphatidylserine (PS), intercellular adhesion molecule-3, carbohydrates (e.g., amino sugars, mannose) and calreticulin. Among them, PS is the most studied one. PS recognition receptors are different functionally active receptors expressed by phagocytes. Various PS recognition receptors with different structure, cell type expression, and ability to bind to PS have been recognized. Although PS recognition receptors do not fall into a single classification or family of proteins due to their structural differences, they all share the common ability to activate downstream signaling pathways leading to the production of anti-inflammatory mediators. In this review, available evidence regarding molecular mechanisms underlying PS recognition receptor-regulated clearance of apoptotic cells is discussed. In addition, some efferocytosis-independent biological functions of PS recognition receptors are reviewed.

The many shades of macrophages in regulating transplant outcome

The shift of emphasis from short-term to long-term graft outcomes has led to renewed interests in how the innate immune cells regulate transplant survival, an area that is traditionally dominated by T cells in the adaptive system. This shift is driven largely by the limited efficacy of current immunosuppression protocols which primarily target T cells in preventing chronic graft loss, as well as by the rapid advance of basic sciences in the realm of innate immunity. In fact, the innate immune cells have emerged as key players in the allograft response in various models, contributing to both graft rejection and graft acceptance. Here, we focus on the macrophages, highlighting their diversity, plasticity and emerging features in transplant models, as well as recent developments in our studies of diverse subsets of macrophages. We also discuss challenges, unsolved questions, and emerging approaches in therapeutically modulating macrophages in further improvement of transplant outcomes.

IL-4/IL-13 polarization of macrophages enhances Ebola virus glycoprotein-dependent infection

BACKGROUND

Ebolavirus (EBOV) outbreaks, while sporadic, cause tremendous morbidity and mortality. No therapeutics or vaccines are currently licensed; however, a vaccine has shown promise in clinical trials. A critical step towards development of effective therapeutics is a better understanding of factors that govern host susceptibility to this pathogen. As macrophages are an important cell population targeted during virus replication, we explore the effect of cytokine polarization on macrophage infection.

METHODS/MAIN FINDINGS

We utilized a BSL2 EBOV model virus, infectious, recombinant vesicular stomatitis virus encoding EBOV glycoprotein (GP) (rVSV/EBOV GP) in place of its native glycoprotein. Macrophages polarized towards a M2-like anti-inflammatory state by combined IL-4 and IL-13 treatment were more susceptible to rVSV/EBOV GP, but not to wild-type VSV (rVSV/G), suggesting that EBOV GP-dependent entry events were enhanced by these cytokines. Examination of RNA expression of known surface receptors that bind and internalize filoviruses demonstrated that IL-4/IL-13 stimulated expression of the C-type lectin receptor DC-SIGN in human macrophages and addition of the competitive inhibitor mannan abrogated IL-4/IL-13 enhanced infection. Two murine DC-SIGN-like family members, SIGNR3 and SIGNR5, were upregulated by IL-4/IL-13 in murine macrophages, but only SIGNR3 enhanced virus infection in a mannan-inhibited manner, suggesting that murine SIGNR3 plays a similar role to human DC-SIGN. In vivo IL-4/IL-13 administration significantly increased virus-mediated mortality in a mouse model and transfer of ex vivo IL-4/IL-13-treated murine peritoneal macrophages into the peritoneal cavity of mice enhanced pathogenesis.

SIGNIFICANCE

These studies highlight the ability of macrophage polarization to influence EBOV GP-dependent virus replication in vivo and ex vivo, with M2a polarization upregulating cell surface receptor expression and thereby enhancing virus replication. Our findings provide an increased understanding of the host factors in macrophages governing susceptibility to filoviruses and identify novel murine receptors mediating EBOV entry.

Costimulation Blockade in Transplantation

T cells play a pivotal role in orchestrating immune responses directed against a foreign (allogeneic) graft. For T cells to become fully activated, the T-cell receptor (TCR) must interact with the major histocompatibility complex (MHC) plus peptide complex on antigen-presenting cells (APCs), followed by a second "positive" costimulatory signal. In the absence of this second signal, T cells become anergic or undergo deletion. By blocking positive costimulatory signaling, T-cell allo-responses can be aborted, thus preventing graft rejection and promoting long-term allograft survival and possibly tolerance (Alegre ML, Najafian N, Curr Mol Med 6:843-857, 2006; Li XC, Rothstein DM, Sayegh MH, Immunol Rev 229:271-293, 2009). In addition, costimulatory molecules can provide negative "coinhibitory" signals that inhibit T-cell activation and terminate immune responses; strategies to promote these pathways can also lead to graft tolerance (Boenisch O, Sayegh MH, Najafian N, Curr Opin Organ Transplant 13:373-378, 2008). However, T-cell costimulation involves an incredibly complex array of interactions that may act simultaneously or at different times in the immune response and whose relative importance varies depending on the different T-cell subsets and activation status. In transplantation, the presence of foreign alloantigen incites not only destructive T effector cells but also protective regulatory T cells, the balance of which ultimately determines the fate of the allograft (Lechler RI, Garden OA, Turka LA, Nat Rev Immunol 3:147-158, 2003). Since the processes of alloantigen-specific rejection and regulation both require activation of T cells, costimulatory interactions may have opposing or synergistic roles depending on the cell being targeted. Such complexities present both challenges and opportunities in targeting T-cell costimulatory pathways for therapeutic purposes. In this chapter, we summarize our current knowledge of the various costimulatory pathways in transplantation and review the current state and challenges of harnessing these pathways to promote graft tolerance (summarized in Table 10.1).

CCL2/MCP-1 signaling drives extracellular matrix turnover by diverse macrophage subsets

Macrophage plasticity, cellular origin, and phenotypic heterogeneity are perpetual challenges for studies addressing the biology of this pivotal immune cell in development, homeostasis, and tissue remodeling/repair. Consequently, a myriad of macrophage subtypes has been described in these contexts. To facilitate the identification of functional macrophage subtypes in vivo, here we used a flow cytometry-based assay that allows for detailed phenotyping of macrophages engaged in extracellular matrix (ECM) degradation. Of the five macrophage subtypes identified in the remodeling dermis by using this assay, collagen degradation was primarily executed by Ly6C⁻CCR2⁺ and Ly6C⁻CCR2^low macrophages via mannose receptor-dependent collagen endocytosis, while Ly6C⁺CCR2⁺ macrophages were the dominant fibrin-endocytosing cells. Unexpectedly, the CCL2/MCP1-CCR2 signaling axis was critical for both collagen and fibrin degradation, while collagen degradation was independent of IL-4Ra signaling. Furthermore, the cytokine GM-CSF selectively enhanced collagen degradation by Ly6C⁺CCR2⁺ macrophages. This study reveals distinct subsets of macrophages engaged in ECM turnover and identifies novel wound healing-associated functions for CCL2 and GM-CSF inflammatory cytokines.

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Phenotypically diverse subsets of dermal macrophages undertake the degradation of extracellular matrix

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C-C motif chemokine Ligand 2 (CCL2) signaling is critical for macrophage-mediated endocytosis of collagen and fibrin.

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Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) and Interleukin (IL)-13 stimulate collagen endocytosis.

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The wound healing-associated IL4-IL4 Receptor a (IL4Ra) signaling is dispensable for collagen endocytosis by macrophages.

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The mannose receptor is the principal endocytic collagen receptor utilized by resident dermal macrophages.

Allergy inhibitory receptor-1 inhibits autoantibody production via upregulation of apoptotic debris clearance by macrophages

AIM

Allergy inhibitory receptor-1 (Allergin-1) is a newly identified immune regulatory molecule thought to influence autoantibody production. Autoantibody production, like that observed in Allergin-1-deficient mice, is crucial in the pathogenesis of several autoimmune diseases such as systemic lupus erythematosus. The purpose of this study is to clarify the regulatory role of Allergin-1-mediated autoantibody production using a murine model of thymocytic anaphylaxis.

METHODS

C57BL/6 (WT) and Allergin-1-deficient mice were treated with apoptotic cells from naive thymocytes stimulated by dexamethasone. Antibody titers of total or immunoglobulin G (IgG) subclass of anti-double-stranded DNA (anti-dsDNA) and anti-histone antibody from serum were measured using an enzyme-linked immunosorbent assay. Macrophages from wild-type (WT) or Allergin-1-deficient mice were co-cultured with fluorescence-labeled apoptotic thymocytes or fluorogenic reagent and resultant phagocytic activity was quantified by with flow cytometry.

RESULTS

After apoptotic cells injection, antibody titers of total and IgG3 anti-dsDNA and total anti-histone from serum were significantly increased in Allergin-1-deficient versus WT mice. Phagocytic activity was significantly lower in macrophages from Allergin-1-deficient mice versus WT mice.

CONCLUSION

Allergin-1 might play an inhibitory role in autoantibody production via upregulation of macrophage phagocytosis.

Cigarette Smoke Extract Promotes TIM4 Expression in Murine Dendritic Cells Leading to Th2 Polarization through ERK-Dependent Pathways

Smoking is considered to be the main source of indoor pollution, and it has been identified as an important environmental factor contributing to asthma onset. We know that T helper 2 (Th2) response plays a crucial role in the process of asthma disease. We have investigated the reaction of cigarette smoke extract (CSE) on Th polarization which is controlled by dendritic cells (DCs). Stimulated by CSE, immature DCs from murine bone marrow showed upregulated levels of TIM4. Cocultured with CD4+ T cells, stimulated DCs increased the ratio of IL-4+ versus IFN-γ+ of CD4+ T cells. This suggests a differentiation towards Th2 response. Moreover, antibodies against TIM4 reversed the upexpression of the IL-4+/IFN-γ+ ratio provoked by CSE, indicating that the Th2 polarization which was induced by CSE is via TIM4 mechanisms. CSE could activate mitogen-activated protein kinase pathways like ERK and p38. Upregulation of TIM4 expression by CSE stimulation was found to be inhibited by an ERK inhibitor but not p38 and JNK. In conclusion, DC-induced Th2 polarization is a hallmark of CSE allergy, and this aspect can be explained by CSE-induced TIM4 expression.

Comparison of the characteristics of macrophages derived from murine spleen, peritoneal cavity, and bone marrow

Macrophages have a diverse set of functions based upon their activation states. The activation states, including resting (M0) and polarizing (M1 and M2) states, of macrophages derived from the mouse bone marrow, spleen, and peritoneal cavity (BMs, SPMs, and PCMs, respectively) were compared. We evaluated the macrophage yield per mouse and compared the surface markers major histocompatibility complex (MHC) II and CD86 by flow cytometry. The relative mRNA levels of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, mannose receptor (MR), and Ym1 in the M0, M1, and M2 states were also compared using real-time polymerase chain reaction (PCR) analysis. Bone marrow yielded the most macrophages with the best homogeneity, but they were polarized toward the M2 phenotype. All three types of macrophages had the capacity to polarize into the M1 and M2 states, but SPMs had a stronger capacity to polarize into M1. The three types of macrophages showed no differences in their capacity to polarize into the M2 state. Therefore, the three types of macrophages have distinct characteristics regardless of their resting or polarizing states. Although bone marrow can get large amounts of homogeneous macrophages, the macrophages cannot replace tissue-derived macrophages.

Self-repopulating recipient bone marrow resident macrophages promote long-term hematopoietic stem cell engraftment

Distinct subsets of resident tissue macrophages are important in hematopoietic stem cell niche homeostasis and erythropoiesis. We used a myeloid reporter gene (Csf1r-eGFP) to dissect the persistence of bone marrow and splenic macrophage subsets following lethal irradiation and autologous hematopoietic stem cell transplantation in a mouse model. Multiple recipient bone marrow and splenic macrophage subsets survived after autologous hematopoietic stem cell transplantation with organ-specific persistence kinetics. Short-term persistence (5 weeks) of recipient resident macrophages in spleen paralleled the duration of extramedullary hematopoiesis. In bone marrow, radiation-resistant recipient CD169⁺ resident macrophages and erythroid-island macrophages self-repopulated long-term after transplantation via autonomous cell division. Posttransplant peak expansion of recipient CD169⁺ resident macrophage number in bone marrow aligned with the persistent engraftment of phenotypic long-term reconstituting hematopoietic stem cells within bone marrow. Selective depletion of recipient CD169⁺ macrophages significantly compromised the engraftment of phenotypic long-term reconstituting hematopoietic stem cells and consequently impaired hematopoietic reconstitution. Recipient bone marrow resident macrophages are essential for optimal hematopoietic stem cell transplantation outcomes and could be an important consideration in the development of pretransplant conditioning therapies and/or chemoresistance approaches.

Sex Differences in Macrophage Functions in Middle-Aged Rats: Relevance of Estradiol Level and Macrophage Estrogen Receptor Expression

The aim of this study was to examine the influence of sex on age-related changes in phenotype and functional capacity of rat macrophages. The potential role of estradiol as a contributing factor to a sex difference in macrophage function with age was also examined. Thioglycollate-elicited peritoneal macrophages derived from the young (2 months old) and the naturally senescent intact middle-aged (16 months old) male and female rats were tested for cytokine secretion and antimicrobial activity (NO and H₂O₂ production and myeloperoxidase activity). Serum concentration of estradiol and the expression of estrogen receptor (ER)α and ERβ on freshly isolated peritoneal macrophages were also examined. Decreased secretion of IL-1β and IL-6 by macrophages from middle-aged compared to the young females was accompanied with the lesser density of macrophage ERα expression and the lower systemic level of estradiol, whereas the opposite was true for middle-aged male rats. Macrophages in the middle-aged females, even with the diminished circulating estradiol levels, produce increased amount of IL-6, and comparable amounts of IL-1β, TNF-α, and NO to that measured in macrophages from the middle-aged males. Age-related changes in macrophage phenotype and the antimicrobial activity were independent of macrophage ERα/ERβ expression and estradiol level in both male and female rats. Although our study suggests that the sex difference in the level of circulating estradiol may to some extent contribute to sex difference in macrophage function of middle-aged rats, it also points to more complex hormonal regulation of peritoneal macrophage activity in females.

Macrophage subpopulations and their impact on chronic allograft rejection versus graft acceptance in a mouse heart transplant model

Macrophages infiltrating the allografts are heterogeneous, consisting of proinflammatory (M1 cells) as well as antiinflammatory and fibrogenic phenotypes (M2 cells); they affect transplantation outcomes via diverse mechanisms. Here we found that macrophage polarization into M1 and M2 subsets was critically dependent on tumor necrosis factor receptor-associated factor 6 (TRAF6) and mammalian target of rapamycin (mTOR), respectively. In a heart transplant model we showed that macrophage-specific deletion of TRAF6 (LysM^Cre Traf6 ^fl/fl ) or mTOR (LysM^Cre Mtor^fl/fl ) did not affect acute allograft rejection. However, treatment of LysM^Cre Mtor^fl/fl recipients with CTLA4-Ig induced long-term allograft survival (>100 days) without histological signs of chronic rejection, whereas the similarly treated LysM^Cre Traf6 ^fl/fl recipients developed severe transplant vasculopathy (chronic rejection). The presentation of chronic rejection in CTLA4-Ig-treated LysM^Cre Traf6 ^fl/fl mice was similar to that of CTLA4-Ig-treated wild-type B6 recipients. Mechanistically, we found that the graft-infiltrating macrophages in LysM^Cre Mtor^fl/fl recipients expressed high levels of PD-L1, and that PD-L1 blockade readily induced rejection of otherwise survival grafts in the LysM^Cre Mtor^fl/fl recipients. Our findings demonstrate that targeting mTOR-dependent M2 cells is critical for preventing chronic allograft rejection, and that graft survival under such conditions is dependent on the PD-1/PD-L1 coinhibitory pathway.

The transcriptional signature of human ovarian carcinoma macrophages is associated with extracellular matrix reorganization

Macrophages occur as resident cells of fetal origin or as infiltrating blood monocyte-derived cells. Despite the critical role of tumor-associated macrophages (TAMs) in tumor progression, the contribution of these developmentally and functionally distinct macrophage subsets and their alteration by the tumor microenvironment are poorly understood. We have addressed this question by comparing TAMs from human ovarian carcinoma ascites, resident peritoneal macrophages (pMPHs) and monocyte-derived macrophages (MDMs). Our study revealed striking a similarity between TAMs and pMPHs, which was considerably greater that the resemblance of TAMs and MDMs, including their transcriptomes, their inflammation-related activation state, the presence of receptors mediating immune functions and the expression of tumor-promoting mediators. Consistent with these results, TAMs phagocytized bacteria, presented peptide antigens and activated cytotoxic T cells within their pathophysiological environment. These observations support the notion that tumor-promoting properties of TAMs may reflect, at least to some extent, normal features of resident macrophages rather than functions induced by the tumor microenvironment. In spite of these surprising similarities between TAMs and pMPHs, bioinformatic analyses identified a TAM-selective signature of 30 genes that are upregulated relative to both pMPHs and MDMs. The majority of these genes is linked to extracellular matrix (ECM) remodeling, supporting a role for TAMs in cancer cell invasion and ovarian cancer progression.

Apoptosis and Clearance of Apoptotic Cells

The human body generates 10-100 billion cells every day, and the same number of cells die to maintain homeostasis in our body. Cells infected by bacteria or viruses also die. The cell death that occurs under physiological conditions mainly proceeds by apoptosis, which is a noninflammatory, or silent, process, while pathogen infection induces necroptosis or pyroptosis, which activates the immune system and causes inflammation. Dead cells generated by apoptosis are quickly engulfed by macrophages for degradation. Caspases are a large family of cysteine proteases that act in cascades. A cascade that leads to caspase 3 activation mediates apoptosis and is responsible for killing cells, recruiting macrophages, and presenting an "eat me" signal(s). When apoptotic cells are not efficiently engulfed by macrophages, they undergo secondary necrosis and release intracellular materials that represent a damage-associated molecular pattern, which may lead to a systemic lupus-like autoimmune disease.

Mannose receptor high, M2 dermal macrophages mediate nonhealing Leishmania major infection in a Th1 immune environment

The origin and functional specialization of dermal macrophages in cutaneous infections have been little studied. In this paper, we show that a strain of Leishmania major (L. major Seidman [LmSd]) that produces nonhealing cutaneous lesions in conventionally resistant C57BL/6 mice was more efficiently taken up by M2-polarized bone marrow (BM)-derived macrophages (BMDMs) in vitro and by mannose receptor (MR)^hi dermal macrophages in vivo compared with a healing strain (L. major Friedlin V1). Both in steady and in T helper type 1 (Th1) cell-driven inflammatory states, the MR^hi dermal macrophages showed M2 characteristics. The dermal macrophages were radio resistant and not replaced by monocytes or adult BM-derived cells during infection, but were locally maintained by IL-4 and IL-10. Notably, the favored infection of M2 BMDMs by LmSd in vitro was MR dependent, and genetic deletion of MR or selective depletion of MR^hi dermal macrophages by anti-CSF-1 receptor antibody reversed the nonhealing phenotype. We conclude that embryonic-derived, MR^hi dermal macrophages are permissive for parasite growth even in a strong Th1-immune environment, and the preferential infection of these cells plays a crucial role in the severity of cutaneous disease.

Diverse roles of TIM4 in immune activation: implications for alloimmunity

PURPOSE OF REVIEW

T-cell immunoglobulin and mucin domain-containing molecule (TIM)4 is a costimulatory molecule and phosphatidylserine receptor. Its dominant function varies according to the expressing cell and site of activation. In recent years, studies have identified its role in diverse disease processes and increasingly in alloimmunity. Herein, we will comprehensively review the literature on TIM4 and outline its function in shaping the alloimmune response.

RECENT FINDINGS

TIM4 expression on dendritic cells increases following transplantation. Blockade of TIM4 in vivo leads to increased differentiation of regulatory T cells and improved allograft survival. TIM4 binds phosphatidylserine-expressing apoptotic cells. Previously thought of as a tethering molecule, recent studies have demonstrated that TIM4 interacts with integrins to mediate uptake of apoptotic cells. TIM4 B cells have recently been identified, which produce high levels of IFNγ and promote allograft rejection. Targeting these B cells improved allograft survival and promoted the development of TIM1 regulatory B cells.

SUMMARY

TIM4 is expressed in niche compartments and has many immunological effects. However, inhibition of TIM4 has been demonstrated to prolong allograft survival, through varied mechanisms. A unifying explanation for the role of TIM4 in alloimmunity remains to be found, but this pathway appears to hold considerable promise in transplantation.

CRIg, a tissue-resident macrophage specific immune checkpoint molecule, promotes immunological tolerance in NOD mice, via a dual role in effector and regulatory T cells

How tissue-resident macrophages (TRM) impact adaptive immune responses remains poorly understood. We report novel mechanisms by which TRMs regulate T cell activities at tissue sites. These mechanisms are mediated by the complement receptor of immunoglobulin family (CRIg). Using animal models for autoimmune type 1 diabetes (T1D), we found that CRIg⁺ TRMs formed a protective barrier surrounding pancreatic islets. Genetic ablation of CRIg exacerbated islet inflammation and local T cell activation. CRIg exhibited a dual function of attenuating early T cell activation and promoting the differentiation of Foxp3⁺ regulatory (Treg) cells. More importantly, CRIg stabilized the expression of Foxp3 in Treg cells, by enhancing their responsiveness to interleukin-2. The expression of CRIg in TRMs was postnatally regulated by gut microbial signals and metabolites. Thus, environmental cues instruct TRMs to express CRIg, which functions as an immune checkpoint molecule to regulate adaptive immunity and promote immune tolerance.

Therapeutic application of T regulatory cells in composite tissue allotransplantation

With growing number of cases in recent years, composite tissue allotransplantation (CTA) has been improving the quality of life of patient who seeks reconstruction and repair of damaged tissues. Composite tissue allografts are heterogeneous. They are composed of a variety of tissue types, including skin, muscle, vessel, bone, bone marrow, lymph nodes, nerve, and tendon. As a primary target of CTA, skin has high antigenicity with a rich repertoire of resident cells that play pivotal roles in immune surveillance. In this regard, understanding the molecular mechanisms involved in immune rejection in the skin would be essential to achieve successful CTA. Although scientific evidence has proved the necessity of immunosuppressive drugs to prevent rejection of allotransplanted tissues, there remains a lingering dilemma due to the lack of specificity of targeted immunosuppression and risks of side effects. A cumulative body of evidence has demonstrated T regulatory (Treg) cells have critical roles in induction of immune tolerance and immune homeostasis in preclinical and clinical studies. Presently, controlling immune susceptible characteristics of CTA with adoptive transfer of Treg cells is being considered promising and it has drawn great interests. This updated review will focus on a dominant form of Treg cells expressing CD4⁺CD25⁺ surface molecules and a forkhead box P3 transcription factor with immune tolerant and immune homeostasis activities. For future application of Treg cells as therapeutics in CTA, molecular and cellular characteristics of CTA and immune rejection, Treg cell development and phenotypes, Treg cell plasticity and stability, immune tolerant functions of Treg cells in CTA in preclinical studies, and protocols for therapeutic application of Treg cells in clinical settings are addressed in this review. Collectively, Treg cell therapy in CTA seems feasible with promising perspectives. However, the extreme high immunogenicity of CTA warrants caution.

T Cell Cosignaling Molecules in Transplantation

The ultimate outcome of alloreactivity versus tolerance following transplantation is potently influenced by the constellation of cosignaling molecules expressed by immune cells during priming with alloantigen, and the net sum of costimulatory and coinhibitory signals transmitted via ligation of these molecules. Intense investigation over the last two decades has yielded a detailed understanding of the kinetics, cellular distribution, and intracellular signaling networks of cosignaling molecules such as the CD28, TNF, and TIM families of receptors in alloimmunity. More recent work has better defined the cellular and molecular mechanisms by which engagement of cosignaling networks serve to either dampen or augment alloimmunity. These findings will likely aid in the rational development of novel immunomodulatory strategies to prolong graft survival and improve outcomes following transplantation.

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.

Mouse macrophages show different requirements for phosphatidylserine receptor Tim4 in efferocytosis

Protein S (ProS) and growth arrest-specific 6 (Gas6) bind to phosphatidylserine (PtdSer) and induce efferocytosis upon binding TAM-family receptors (Tyro3, Axl, and Mer). Here, we produced mouse ProS, Gas6, and TAM-receptor extracellular region fused to IgG fragment crystallizable region in HEK293T cells. ProS and Gas6 bound Ca²⁺ dependently to PtdSer (K_d 20-40 nM), Mer, and Tyro3 (K_d 15-50 nM). Gas6 bound Axl strongly (K_d < 1.0 nM), but ProS did not bind Axl. Using NIH 3T3-based cell lines expressing a single TAM receptor, we showed that TAM-mediated efferocytosis was determined by the receptor-binding ability of ProS and Gas6. Tim4 is a membrane protein that strongly binds PtdSer. Tim4 alone did not support efferocytosis, but enhanced TAM-dependent efferocytosis. Resident peritoneal macrophages, Kupffer cells, and CD169⁺ skin macrophages required Tim4 for TAM-stimulated efferocytosis, whereas efferocytosis by thioglycollate-elicited peritoneal macrophages or primary cultured microglia was TAM dependent, but not Tim4 dependent. These results indicate that TAM and Tim4 collaborate for efficient efferocytosis in certain macrophage populations.

Macrophages as Effectors of Acute and Chronic Allograft Injury

Organ transplants give a second chance of life to patients with end-stage organ failure. However, the immuno-logical barriers prove to be very challenging to overcome and graft rejection remains a major hurdle to long-term transplant survival. For decades, adaptive immunity has been the focus of studies, primarily based on the belief that T cells are necessary and sufficient for rejection. With better-developed immunosuppressive drugs and protocols that effectively control adaptive cells, innate immune cells have emerged as key effector cells in triggering graft injury and have therefore attracted much recent attention. In this review, we discuss current understanding of macrophages and their role in transplant rejection, their dynamics, distinct phenotypes, locations, and functions. We also discuss novel therapeutic approaches under development to target macrophages in transplant recipients.

New insights into the multidimensional concept of macrophage ontogeny, activation and function. Nat Immunol. 2016;17:34-40. [PMID: 26681460 DOI: 10.1038/ni.3324]

Macrophages have protective roles in immunity to pathogens, tissue development, homeostasis and repair following damage. Maladaptive immunity and inflammation provoke changes in macrophage function that are causative of disease. Despite a historical wealth of knowledge about macrophages, recent advances have revealed unknown aspects of their development and function. Following development, macrophages are activated by diverse signals. Such tissue microenvironmental signals together with epigenetic changes influence macrophage development, activation and functional diversity, with consequences in disease and homeostasis. We discuss here how recent discoveries in these areas have led to a multidimensional concept of macrophage ontogeny, activation and function. In connection with this, we also discuss how technical advances facilitate a new roadmap for the isolation and analysis of macrophages at high resolution.

The Mononuclear Phagocyte System in Organ Transplantation

The mononuclear phagocyte system (MPS) comprises monocytes, macrophages and dendritic cells (DCs). Over the past few decades, classification of the cells of the MPS has generated considerable controversy. Recent studies into the origin, developmental requirements and function of MPS cells are beginning to solve this problem in an objective manner. Using high-resolution genetic analyses and fate-mapping studies, three main mononuclear phagocyte lineages have been defined, namely, macrophage populations established during embryogenesis, monocyte-derived cells that develop during adult life and DCs. These subsets and their diverse subsets have specialized functions that are largely conserved between species, justifying the introduction of a new, universal scheme of nomenclature and providing the framework for therapeutic manipulation of immune responses in the clinic. In this review, we have commented on the implications of this novel MPS classification in solid organ transplantation.

Contrasting Roles of Islet Resident Immunoregulatory Macrophages and Dendritic Cells in Experimental Autoimmune Type 1 Diabetes

The innate immune system critically shapes diabetogenic adaptive immunity during type 1 diabetes (T1D) pathogenesis. While the role of tissue-infiltrating monocyte-derived macrophages in T1D is well established, the role of their tissue-resident counterparts remains undefined. We now demonstrate that islet resident macrophages (IRMs) from non-autoimmune mice have an immunoregulatory phenotype and powerfully induce FoxP3+ Tregs in vitro. The immunoregulatory phenotype and function of IRMs is compromised by TLR4 activation in vitro. Moreover, as T1D approaches in NOD mice, the immunoregulatory phenotype of IRMs is diminished as is their relative abundance compared to immunostimulatory DCs. Our findings suggest that maintenance of IRM abundance and their immunoregulatory phenotype may constitute a novel therapeutic strategy to prevent and/or cure T1D.

Blockade of Tim-1 and Tim-4 Enhances Atherosclerosis in Low-Density Lipoprotein Receptor-Deficient Mice

OBJECTIVE

T cell immunoglobulin and mucin domain (Tim) proteins are expressed by numerous immune cells, recognize phosphatidylserine on apoptotic cells, and function as costimulators or coinhibitors. Tim-1 is expressed by activated T cells but is also found on dendritic cells and B cells. Tim-4, present on macrophages and dendritic cells, plays a critical role in apoptotic cell clearance, regulates the number of phosphatidylserine-expressing activated T cells, and is genetically associated with low low-density lipoprotein and triglyceride levels. Because these functions of Tim-1 and Tim-4 could affect atherosclerosis, their modulation has potential therapeutic value in cardiovascular disease.

APPROACH AND RESULTS

ldlr(-/-) mice were fed a high-fat diet for 4 weeks while being treated with control (rat immunoglobulin G1) or anti-Tim-1 (3D10) or -Tim-4 (21H12) monoclonal antibodies that block phosphatidylserine recognition and phagocytosis. Both anti-Tim-1 and anti-Tim-4 treatments enhanced atherosclerosis by 45% compared with controls by impairment of efferocytosis and increasing aortic CD4(+)T cells. Consistently, anti-Tim-4-treated mice showed increased percentages of activated T cells and late apoptotic cells in the circulation. Moreover, in vitro blockade of Tim-4 inhibited efferocytosis of oxidized low-density lipoprotein-induced apoptotic macrophages. Although anti-Tim-4 treatment increased T helper cell (Th)1 and Th2 responses, anti-Tim-1 induced Th2 responses but dramatically reduced the percentage of regulatory T cells. Finally, combined blockade of Tim-1 and Tim-4 increased atherosclerotic lesion size by 59%.

CONCLUSIONS

Blockade of Tim-4 aggravates atherosclerosis likely by prevention of phagocytosis of phosphatidylserine-expressing apoptotic cells and activated T cells by Tim-4-expressing cells, whereas Tim-1-associated effects on atherosclerosis are related to changes in Th1/Th2 balance and reduced circulating regulatory T cells.