IL-34 is a tissue-restricted ligand of CSF1R required for the development of Langerhans cells and microglia. Nat Immunol. 2012;13:753-760. [PMID: 22729249 DOI: 10.1038/ni.2360]

Limited proliferation capacity of aortic intima resident macrophages requires monocyte recruitment for atherosclerotic plaque progression

Early atherosclerosis depends upon responses by immune cells resident in the intimal aortic wall. Specifically, the healthy intima is thought to be populated by vascular dendritic cells (DCs) that, during hypercholesterolemia, initiate atherosclerosis by being the first to accumulate cholesterol. Whether these cells remain key players in later stages of disease is unknown. Using murine lineage-tracing models and gene expression profiling, we reveal that myeloid cells present in the intima of the aortic arch are not DCs but instead specialized aortic intima resident macrophages (MacAIR) that depend upon colony-stimulating factor 1 and are sustained by local proliferation. Although MacAIR comprise the earliest foam cells in plaques, their proliferation during plaque progression is limited. After months of hypercholesterolemia, their presence in plaques is overtaken by recruited monocytes, which induce MacAIR-defining genes. These data redefine the lineage of intimal phagocytes and suggest that proliferation is insufficient to sustain generations of macrophages during plaque progression.

Negative feedback control of neuronal activity by microglia

Microglia, the brain’s resident macrophages, help to regulate brain function by removing dying neurons, pruning non-functional synapses, and producing ligands that support neuronal survival¹. Here we show that microglia are also critical modulators of neuronal activity and associated behavioural responses in mice. Microglia respond to neuronal activation by suppressing neuronal activity, and ablation of microglia amplifies and synchronizes the activity of neurons, leading to seizures. Suppression of neuronal activation by microglia occurs in a highly region-specific fashion and depends on the ability of microglia to sense and catabolize extracellular ATP, which is released upon neuronal activation by neurons and astrocytes. ATP triggers the recruitment of microglial protrusions and is converted by the microglial ATP/ADP hydrolysing ectoenzyme CD39 into AMP; AMP is then converted into adenosine by CD73, which is expressed on microglia as well as other brain cells. Microglial sensing of ATP, the ensuing microglia-dependent production of adenosine, and the adenosine-mediated suppression of neuronal responses via the adenosine receptor A₁R are essential for the regulation of neuronal activity and animal behaviour. Our findings suggest that this microglia-driven negative feedback mechanism operates similarly to inhibitory neurons and is essential for protecting the brain from excessive activation in health and disease.

CNS Macrophages and Infant Infections

The central nervous system (CNS) harbors its own immune system composed of microglia in the parenchyma and CNS-associated macrophages (CAMs) in the perivascular space, leptomeninges, dura mater, and choroid plexus. Recent advances in understanding the CNS resident immune cells gave new insights into development, maturation and function of its immune guard. Microglia and CAMs undergo essential steps of differentiation and maturation triggered by environmental factors as well as intrinsic transcriptional programs throughout embryonic and postnatal development. These shaping steps allow the macrophages to adapt to their specific physiological function as first line of defense of the CNS and its interfaces. During infancy, the CNS might be targeted by a plethora of different pathogens which can cause severe tissue damage with potentially long reaching defects. Therefore, an efficient immune response of infant CNS macrophages is required even at these early stages to clear the infections but may also lead to detrimental consequences for the developing CNS. Here, we highlight the recent knowledge of the infant CNS immune system during embryonic and postnatal infections and the consequences for the developing CNS.

Pediatric PK/PD Phase I Trial of Pexidartinib in Relapsed and Refractory Leukemias and Solid Tumors Including Neurofibromatosis Type I-Related Plexiform Neurofibromas

PURPOSE

Simultaneously targeting the tumor and tumor microenvironment may hold promise in treating children with refractory solid tumors. Pexidartinib, an oral inhibitor of tyrosine kinases including colony stimulating factor 1 receptor (CSF-1R), KIT, and FLT3, is FDA approved in adults with tenosynovial giant cell tumor. A phase I trial was conducted in pediatric and young adult patients with refractory leukemias or solid tumors including neurofibromatosis type 1-related plexiform neurofibromas.

PATIENTS AND METHODS

A rolling six design with dose levels (DL) of 400 mg/m², 600 mg/m², and 800 mg/m² once daily for 28-day cycles (C) was used. Response was assessed at regular intervals. Pharmacokinetics and population pharmacokinetics were analyzed during C1.

RESULTS

Twelve patients (4 per DL, 9 evaluable) enrolled on the dose-escalation phase and 4 patients enrolled in the expansion cohort: median (lower, upper quartile) age 16 (14, 16.5) years. No dose-limiting toxicities were observed. Pharmacokinetics appeared linear over three DLs. Pharmacokinetic modeling and simulation determined a weight-based recommended phase II dose (RP2D). Two patients had stable disease and 1 patient with peritoneal mesothelioma (C49+) had a sustained partial response (67% RECIST reduction). Pharmacodynamic markers included a rise in plasma macrophage CSF (MCSF) levels and a decrease in absolute monocyte count.

CONCLUSIONS

Pexidartinib in pediatric patients was well tolerated at all DL tested, achieved target inhibition, and resulted in a weight-based RPD2 dose.

The role of Langerhans cells in epidermal homeostasis and pathogenesis of psoriasis

The skin is the main barrier between the human body and the outside world, which not only plays the role of a physical barrier but also functions as the first line of defence of immunology. Langerhans cells (LCs), as dendritic cells (DC) that play an important role in the immune system, are mainly distributed in the epidermis. This review focuses on the role of these epidermal LCs in regulating skin threats (such as microorganisms, ultraviolet radiation and allergens), especially psoriasis. Since human and mouse skin DC subsets share common ontogenetic characteristics, we can further explore the role of LCs in psoriatic inflammation.

To Kill a Microglia: A Case for CSF1R Inhibitors

Microglia, the brain's immune sentinels, have garnered much attention in recent years. Researchers have begun to identify the manifold roles that these cells play in the central nervous system (CNS), and this work has been greatly facilitated by microglial depletion paradigms. The varying degrees of spatiotemporal manipulation afforded by such techniques allow microglial ablation before, during, and/or following insult, injury, or disease. We review the major methods of microglial depletion, including toxin-based, genetic, and pharmacological approaches, which differ in key factors including depletion onset, duration, and off-target effects. We conclude that pharmacological CSF1R inhibitors afford the most extensive versatility in manipulating microglia, making them ideal candidates for future studies investigating microglial function in health and disease.

The Role of Microglia during West Nile Virus Infection of the Central Nervous System

Encephalitis resulting from viral infections is a major cause of hospitalization and death worldwide. West Nile Virus (WNV) is a substantial health concern as it is one of the leading causes of viral encephalitis in the United States today. WNV infiltrates the central nervous system (CNS), where it directly infects neurons and induces neuronal cell death, in part, via activation of caspase 3-mediated apoptosis. WNV infection also induces neuroinflammation characterized by activation of innate immune cells, including microglia and astrocytes, production of inflammatory cytokines, breakdown of the blood-brain barrier, and infiltration of peripheral leukocytes. Microglia are the resident immune cells of the brain and monitor the CNS for signs of injury or pathogens. Following infection with WNV, microglia exhibit a change in morphology consistent with activation and are associated with increased expression of proinflammatory cytokines. Recent research has focused on deciphering the role of microglia during WNV encephalitis. Microglia play a protective role during infections by limiting viral growth and reducing mortality in mice. However, it also appears that activated microglia are triggered by T cells to mediate synaptic elimination at late times during infection, which may contribute to long-term neurological deficits following a neuroinvasive WNV infection. This review will discuss the important role of microglia in the pathogenesis of a neuroinvasive WNV infection. Knowledge of the precise role of microglia during a WNV infection may lead to a greater ability to treat and manage WNV encephalitis.

Microglia and Astrocytes in Disease: Dynamic Duo or Partners in Crime?

Microglia-astrocyte interactions represent a delicate balance affecting neural cell functions in health and disease. Tightly controlled to maintain homeostasis during physiological conditions, rapid and prolonged departures during disease, infection, and following trauma drive multiple outcomes: both beneficial and detrimental. Recent sequencing studies at the bulk and single-cell level in humans and rodents provide new insight into microglia-astrocyte communication in homeostasis and disease. However, the complex changing ways these two cell types functionally interact has been a barrier to understanding disease initiation, progression, and disease mechanisms. Single cell sequencing is providing new insights; however, many questions remain. Here, we discuss how to bridge transcriptional states to specific functions so we can develop therapies to mediate negative effects of altered microglia-astrocyte interactions.

The M-CSF receptor in osteoclasts and beyond

Colony-stimulating factor 1 receptor (CSF1R, also known as c-FMS) is a receptor tyrosine kinase. Macrophage colony-stimulating factor (M-CSF) and IL-34 are ligands of CSF1R. CSF1R-mediated signaling is crucial for the survival, function, proliferation, and differentiation of myeloid lineage cells, including osteoclasts, monocytes/macrophages, microglia, Langerhans cells in the skin, and Paneth cells in the intestine. CSF1R also plays an important role in oocytes and trophoblastic cells in the female reproductive tract and in the maintenance and maturation of neural progenitor cells. Given that CSF1R is expressed in a wide range of myeloid cells, altered CSF1R signaling is implicated in inflammatory, neoplastic, and neurodegenerative diseases. Inhibiting CSF1R signaling through an inhibitory anti-CSF1R antibody or small molecule inhibitors that target the kinase activity of CSF1R has thus been a promising therapeutic strategy for those diseases. In this review, we cover the recent progress in our understanding of the various roles of CSF1R in osteoclasts and other myeloid cells, highlighting the therapeutic applications of CSF1R inhibitors in disease conditions.

Drugs directed at a key signaling receptor involved in breaking down bone tissue could help treat diseases marked by pathological bone loss and destruction. In a review article, Kyung-Hyun Park-Min and colleagues from the Hospital for Special Surgery in New York, USA, discuss the essential roles played by the colony-stimulating factor 1 receptor (CSF1R) protein in the survival, function, proliferation and differentiation of myeloid lineage stem cells in the bone marrow, including bone-resorbing osteoclasts. They explore the links between the CSF1R-mediated signaling pathway and diseases such as cancer and neurodegeneration. The authors largely focus on bone conditions, highlighting mouse studies in which CSF1R-blocking drugs were shown to ameliorate bone loss and inflammatory symptoms in models of arthritis, osteoporosis and metastatic cancer. Clinical trials are ongoing to test therapeutic applications.

IL-34 Actions on FOXP3+ Tregs and CD14+ Monocytes Control Human Graft Rejection

Cytokines are major players regulating immune responses toward inflammatory and tolerogenic results. In organ and bone marrow transplantation, new reagents are needed to inhibit tissue destructive mechanisms and eventually induce immune tolerance without overall immunosuppression. IL-34 is a cytokine with no significant homology with any other cytokine but that acts preferentially through CSF-1R, as CSF-1 does, and through PTPζ and CD138. Although IL-34 and CSF-1 share actions, a detailed analysis of their effects on immune cells needs further research. We previously showed that both CD4⁺ and CD8⁺ FOXP3⁺ Tregs suppress effector T cells through the production of IL-34, but not CSF-1, and that this action was mediated through antigen-presenting cells. We showed here by single-cell RNAseq and cytofluorimetry that different subsets of human monocytes expressed different levels of CSF-1R, CD138, and PTPζ and that both CD4⁺ and CD8⁺ FOXP3⁺ Tregs expressed higher levels of CSF-1R than conventional T cells. The effects of IL-34 differed in the survival of these different subpopulations of monocytes and RNAseq analysis showed several genes differentially expressed between IL-34, CSF-1, M0, M1, and also M2 macrophages. Acute graft-vs.-host disease (aGVHD) in immunodeficient NSG mice injected with human PBMCs was decreased when treated with IL-34 in combination with an anti-CD45RC mAb that depleted conventional T cells. When IL-34-differentiated monocytes were used to expand Tregs in vitro, both CD4⁺ and CD8⁺ FOXP3⁺ Tregs were highly enriched and this effect was superior to the one obtained with CSF-1. Human CD8⁺ Tregs expanded in vitro with IL-34-differentiated allogeneic monocytes suppressed human immune responses in an NSG mouse aGVHD model humanized with hPBMCs. Overall, we showed that IL-34 induced the differentiation of human monocytes with a particular transcriptional profile and these cells favored the development of potent suppressor FOXP3⁺ Tregs.

The Role of Immune Factors in Shaping Fetal Neurodevelopment

Fetal neurodevelopment in utero is profoundly shaped by both systemic maternal immunity and local processes at the maternal-fetal interface. Immune pathways are a critical participant in the normal physiology of pregnancy and perturbations of maternal immunity due to infections during this period have been increasingly linked to a diverse array of poor neurological outcomes, including diseases that manifest much later in postnatal life. While experimental models of maternal immune activation (MIA) have provided groundbreaking characterizations of the maternal pathways underlying pathogenesis, less commonly examined are the immune factors that serve pathogen-independent developmental functions in the embryo and fetus. In this review, we explore what is known about the in vivo role of immune factors in fetal neurodevelopment during normal pregnancy and provide an overview of how MIA perturbs the proper orchestration of this sequence of events. Finally, we discuss how the dysregulation of immune factors may contribute to the manifestation of a variety of neurological disorders.

Mudskipper interleukin-34 modulates the functions of monocytes/macrophages via the colony-stimulating factor-1 receptor 1

Interleukin-34 (IL-34) is a novel cytokine that plays an important role in innate immunity and inflammatory processes by binding to the colony-stimulating factor-1 receptor (CSF-1R). However, information on the function of IL-34 in fish remains limited. In the present study, we identified an IL-34 homolog from mudskippers (Boleophthalmus pectinirostris). In silico analysis showed that the mudskipper IL-34 (BpIL-34) was similar to other known IL-34 variants in sequence and structure and was most closely related to an orange-spotted grouper (Epinephelus coioides) homolog. BpIL-34 transcripts were constitutively expressed in various tissues, with the highest level of expression found in the brain. Edwardsiella tarda infection significantly up-regulated the mRNA expression of BpIL-34 in the mudskipper tissues. The recombinant mature BpIL-34 peptide (rBpIL-34) was purified and used to produce anti-rBpIL-34 IgG. Western blot analysis combined with PNGase F digestion revealed that native BpIL-34 in monocytes/macrophages (MOs/MФs) was N-glycosylated. In vitro, rBpIL-34 treatment enhanced the phagocytotic and bactericidal activity of mudskipper MOs/MФs, as well as the mRNA expression of pro-inflammatory cytokines like tumor necrosis factor α (BpTNF-α) and BpIL-1β in these cells. Furthermore, the knockdown of mudskipper CSF-1R1 (BpCSF-1R1), but not mudskipper BpCSF-1R2, significantly inhibited the rBpIL-34-mediated enhanced effect on MO/MФ function. In conclusion, our results indicate that mudskipper BpIL-34 modulates the functions of MOs/MФs via BpCSF-1R1.

Origin and Differentiation of Nerve-Associated Macrophages

The mature peripheral nervous system is a steady network structure yet shows remarkable regenerative properties. The interaction of peripheral nerves with myeloid cells has largely been investigated in the context of damage, following trauma or infection. Recently, specific macrophages dedicated to homeostatic peripheral nerves have come into focus. These macrophages are defined by tissue and nerve type, are seeded in part prenatally, and self-maintain via proliferation. Thus, they are markedly distinct from monocyte-derived macrophages invading after local disturbance of nerve integrity. The phenotypic and transcriptional adaptation of macrophages to the discrete nervous niche may exert axon guidance and nerve regeneration and thus contribute to the stability of the peripheral nervous network. Deciphering these conserved macrophage-nerve interactions offers new translational perspectives for chronic diseases of the peripheral nervous system, such as diabetic neuropathy and pain.

Microglia: A Central Player in Depression

Microglia are the major immune cells in the central nervous system and play a key role in the normal function of the brain. Microglia exhibit functional diversity, and they control the inflammation in central nervous system through releasing inflammatory cytokine, clearing apoptotic cells via phagocytosis, regulating synaptic plasticity and the formation of neural network by synapse pruning. Recent studies have strongly indicated that the microglial dysfunction is associated with a variety of neuropsychiatric diseases such as depression, which have been termed as "microgliopathy". The emergency of advanced technologies and tools has enabled us to comprehensively understand the role of microglia in physiology and pathology, and growing studies have targetted microglia to explore the treatment of neuropsychiatric diseases. Here, we describe the key progress of microglia research, and review the recent developments in the understanding of the role of microglia in physiology and etiology of depression.

Effects of IL-34 on Macrophage Immunological Profile in Response to Alzheimer's-Related Aβ42 Assemblies

Interleukin-34 (IL-34) is a recently discovered cytokine that acts as a second ligand of the colony stimulating factor 1 receptor (CSF1R) in addition to macrophage colony-stimulating factor (M-CSF). Similar to M-CSF, IL-34 also stimulates bone marrow (BM)-derived monocyte survival and differentiation into macrophages. Growing evidence suggests that peripheral BM-derived monocyte/macrophages (BMMO) play a key role in the physiological clearance of cerebral amyloid β-protein (Aβ). Aβ₄₂ forms are especially neurotoxic and highly associated with Alzheimer's disease (AD). As a ligand of CSF1R, IL-34 may be relevant to innate immune responses in AD. To investigate how IL-34 affects macrophage phenotype in response to structurally defined and stabilized Aβ₄₂ oligomers and preformed fibrils, we characterized murine BMMO cultured in media containing M-CSF, IL-34, or regimens involving both cytokines. We found that the immunological profile and activation phenotype of IL-34-stimulated BMMO differed significantly from those cultured with M-CSF alone. Specifically, macrophage uptake of fibrillar or oligomeric Aβ₄₂ was markedly reduced following exposure to IL-34 compared to M-CSF. Surface expression of type B scavenger receptor CD36, known to facilitate Aβ recognition and uptake, was modified following treatment with IL-34. Similarly, IL-34 macrophages expressed lower levels of proteins involved in both Aβ uptake (triggering receptor expressed on myeloid cells 2, TREM2) as well as Aβ-degradation (matrix metallopeptidase 9, MMP-9). Interestingly, intracellular compartmentalization of Aβ visualized by staining of early endosome antigen 1 (EEA1) was not affected by IL-34. Macrophage characteristics associated with an anti-inflammatory and pro-wound healing phenotype, including processes length and morphology, were also quantified, and macrophages stimulated with IL-34 alone displayed less process elongation in response to Aβ₄₂ compared to those cultured with M-CSF. Further, monocytes treated with IL-34 alone yielded fewer mature macrophages than those treated with M-CSF alone or in combination with IL-34. Our data indicate that IL-34 impairs monocyte differentiation into macrophages and reduces their ability to uptake pathological forms of Aβ. Given the critical role of macrophage-mediated Aβ clearance in both murine models and patients with AD, future work should investigate the therapeutic potential of modulating IL-34 in vivo to increase macrophage-mediated Aβ clearance and prevent disease development.

Role of Macrophages and Microglia in Zebrafish Regeneration

Currently, there is no treatment for recovery of human nerve function after damage to the central nervous system (CNS), and there are limited regenerative capabilities in the peripheral nervous system. Since fish are known for their regenerative abilities, understanding how these species modulate inflammatory processes following injury has potential translational importance for recovery from damage and disease. Many diseases and injuries involve the activation of innate immune cells to clear damaged cells. The resident immune cells of the CNS are microglia, the primary cells that respond to infection and injury, and their peripheral counterparts, macrophages. These cells serve as key modulators of development and plasticity and have been shown to be important in the repair and regeneration of structure and function after injury. Zebrafish are an emerging model for studying macrophages in regeneration after injury and microglia in neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. These fish possess a high degree of neuroanatomical, neurochemical, and emotional/social behavioral resemblance with humans, serving as an ideal simulator for many pathologies. This review explores literature on macrophage and microglial involvement in facilitating regeneration. Understanding innate immune cell behavior following damage may help to develop novel methods for treating toxic and chronic inflammatory processes that are seen in trauma and disease.

Control of myeloid cell density in barrier tissues

The interface between the mammalian host and its environment is formed by barrier tissues, for example, of the skin, and the respiratory and the intestinal tracts. On the one hand, barrier tissues are colonized by site-adapted microbial communities, and on the other hand, they contain specific myeloid cell networks comprising macrophages, dendritic cells, and granulocytes. These immune cells are tightly regulated in function and cell number, indicating important roles in maintaining tissue homeostasis and immune balance in the presence of commensal microorganisms. The regulation of myeloid cell density and activation involves cell-autonomous 'single-loop circuits' including autocrine mechanisms. However, an array of microenvironmental factors originating from nonimmune cells and the microbiota, as well as the microanatomical structure, impose additional layers of regulation onto resident myeloid cells. This review discusses models integrating these factors into cell-specific programs to instruct differentiation and proliferation best suited for the maintenance and renewal of immune homeostasis in the tissue-specific environment.

Increased Levels of IL-34 in Acquired Immune-Mediated Neuropathies

Interleukin (IL)-34 is ligand for the colony-stimulating factor (CSF)-1 receptor. This cytokine has fundamental roles the pathogenesis of a number of autoimmune and neurologic disorders. However, its role in the pathogenesis of acute and chronic inflammatory demyelinating polyneuropathies (AIDP and CIDP) has not been assessed yet. We measured serum levels of IL-34 33 CIDP cases, 16 AIDP cases, and 33 control subjects using commercial ELISA kits. IL-34 levels were significantly higher in both AIDP (44.87 ± 4.38) and CIDP (44.87 ± 4.38) groups compared with healthy subjects (30.10 ± 1.05) (P = 0.046 and P = 0.01, respectively). Differences between female subgroups were insignificant. However, levels of this cytokine were significantly higher in male subjects with CIDP compared with male controls (P = 0.042). Thus, levels of this cytokine might be regarded as biomarkers for these kinds of autoimmune disorders. Future studies are needed to verify these results and find the molecular mechanism of participation of IL-34 in the pathogenesis of AIDP/CIDP.

Targeting Microglial Population Dynamics in Alzheimer's Disease: Are We Ready for a Potential Impact on Immune Function?

Alzheimer’s disease (AD) is the most common form of dementia, affecting two-thirds of people with dementia in the world. To date, no disease-modifying treatments are available to stop or delay the progression of AD. This chronic neurodegenerative disease is dominated by a strong innate immune response, whereby microglia plays a central role as the main resident macrophage of the brain. Recent genome-wide association studies (GWASs) have identified single-nucleotide polymorphisms (SNPs) located in microglial genes and associated with a delayed onset of AD, highlighting the important role of these cells on the onset and/or progression of the disease. These findings have increased the interest in targeting microglia-associated neuroinflammation as a potentially disease-modifying therapeutic approach for AD. In this review we provide an overview on the contribution of microglia to the pathophysiology of AD, focusing on the main regulatory pathways controlling microglial population dynamics during the neuroinflammatory response, such as the colony-stimulating factor 1 receptor (CSF1R), its ligands (the colony-stimulating factor 1 and interleukin 34) and the transcription factor PU.1. We also discuss the current therapeutic strategies targeting proliferation to modulate microglia-associated neuroinflammation and their potential impact on peripheral immune cell populations in the short and long-term. Understanding the effects of immunomodulatory approaches on microglia and other immune cell types might be critical for developing specific, effective, and safe therapies for neurodegenerative diseases.

Microglia: sculptors of neuropathic pain?

Neuropathic pain presents a huge societal and individual burden. The limited efficacy of current analgesics, diagnostic markers and clinical trial outcome measures arises from an incomplete understanding of the underlying mechanisms. A large and growing body of evidence has established the important role of microglia in the onset and possible maintenance of neuropathic pain, and these cells may represent an important target for future therapy. Microglial research has further revealed their important role in structural remodelling of the nervous system. In this review, we aim to explore the evidence for microglia in sculpting nervous system structure and function, as well as their important role in neuropathic pain, and finally integrate these studies to synthesize a new model for microglia in somatosensory circuit remodelling, composed of six key and inter-related mechanisms. Summarizing the mechanisms through which microglia modulate nervous system structure and function helps to frame a better understanding of neuropathic pain, and provide a clear roadmap for future research.

Immune responses to stroke: mechanisms, modulation, and therapeutic potential

Stroke is the second leading cause of death worldwide and a leading cause of disability. Most strokes are caused by occlusion of a major cerebral artery, and substantial advances have been made in elucidating how ischemia damages the brain. In particular, increasing evidence points to a double-edged role of the immune system in stroke pathophysiology. In the acute phase, innate immune cells invade brain and meninges and contribute to ischemic damage, but may also be protective. At the same time, danger signals released into the circulation by damaged brain cells lead to activation of systemic immunity, followed by profound immunodepression that promotes life-threatening infections. In the chronic phase, antigen presentation initiates an adaptive immune response targeted to the brain, which may underlie neuropsychiatric sequelae, a considerable cause of poststroke morbidity. Here, we briefly review these pathogenic processes and assess the potential therapeutic value of targeting immunity in human stroke.

Decidual stromal cells maintain decidual macrophage homeostasis by secreting IL-24 in early pregnancy

PROBLEM

The state of self-renewal and self-maintain of decidual macrophages would be important for immune homeostasis at the maternal-fetal interface. The roles of interleukin (IL)-24 derived from decidual stromal cells (DSCs) on decidual macrophages have not been explored.

METHOD OF STUDY

IL-24 expression in DSCs was interfered by lentivirus, and the transcription levels of IL-24 in DSCs were verified by real time (RT)-PCR. The levels of IL-24 receptors were determined by flow cytometry assays. The effect of recombination human IL-24 (rhIL-24) on the differentiation and apoptosis of macrophages was analyzed by flow cytometry in vitro. The viability of macrophages was detected by Cell Counting Kit-8 assays.

RESULTS

The growth of DSCs was not affected obviously only by IL-24 knockdown while the growth of knockdown DSCs was inhibited significantly after co-cultured with decidual macrophages. The levels of IL-24 receptors (IL-20R1 and IL-22R1) were moderately to highly expressed on decidual macrophages and human macrophage cell line U937. The differentiation of decidual macrophages treated by rhIL-24 or co-cultured with IL-24 knockdown DSCs was not affected. Both apoptosis and viability of U937 cells were promoted by rhIL-24. The ratio of Bcl-2/Bax was down-regulated and Ki-67 was up-regulated by IL-24 treatment. The expression of Bcl-2/Bax was up-regulated while Ki-67 was down-regulated in U937 cells after co-cultured by IL-24 knockdown DSCs.

CONCLUSION

IL-24 secreted by DSCs promotes the renewal and homeostasis of decidual macrophages possibly via down-regulating the ratio of Bcl-2/Bax and up-regulating of the expression of Ki-67 in early pregnancy.

A wave of monocytes is recruited to replenish the long-term Langerhans cell network after immune injury

A dense population of embryo-derived Langerhans cells (eLCs) is maintained within the sealed epidermis without contribution from circulating cells. When this network is perturbed by transient exposure to ultraviolet light, short-term LCs are temporarily reconstituted from an initial wave of monocytes but thought to be superseded by more permanent repopulation with undefined LC precursors. However, the extent to which this process is relevant to immunopathological processes that damage LC population integrity is not known. Using a model of allogeneic hematopoietic stem cell transplantation, where alloreactive T cells directly target eLCs, we have asked whether and how the original LC network is ultimately restored. We find that donor monocytes, but not dendritic cells, are the precursors of long-term LCs in this context. Destruction of eLCs leads to recruitment of a wave of monocytes that engraft in the epidermis and undergo a sequential pathway of differentiation via transcriptionally distinct EpCAM⁺ precursors. Monocyte-derived LCs acquire the capacity of self-renewal, and proliferation in the epidermis matched that of steady-state eLCs. However, we identified a bottleneck in the differentiation and survival of epidermal monocytes, which, together with the slow rate of renewal of mature LCs, limits repair of the network. Furthermore, replenishment of the LC network leads to constitutive entry of cells into the epidermal compartment. Thus, immune injury triggers functional adaptation of mechanisms used to maintain tissue-resident macrophages at other sites, but this process is highly inefficient in the skin.

Microglia Biology: One Century of Evolving Concepts

Microglia were first recognized as a distinct cell population in the CNS one century ago. For a long time, they were primarily considered to be phagocytes responsible for removing debris during CNS development and disease. More recently, advances in imaging and genetics and the advent of single-cell technologies provided new insights into the much more complex and fascinating biology of microglia. The ontogeny of microglia was identified, and their functions in health and disease were better defined. Although many questions about microglia and their roles in human diseases remain unanswered, the prospect of targeting microglia for the treatment of neurological and psychiatric disorders is tantalizing.

Peripheral nerve resident macrophages share tissue-specific programming and features of activated microglia

Whereas microglia are recognized as fundamental players in central nervous system (CNS) development and function, much less is known about macrophages of the peripheral nervous system (PNS). Here, by comparing gene expression across neural and conventional tissue-resident macrophages, we identified transcripts that were shared among neural resident macrophages as well as selectively enriched in PNS macrophages. Remarkably, PNS macrophages constitutively expressed genes previously identified to be upregulated by activated microglia during aging, neurodegeneration, or loss of Sall1. Several microglial activation-associated and PNS macrophage-enriched genes were also expressed in spinal cord microglia at steady state. We further show that PNS macrophages rely on IL-34 for maintenance and arise from both embryonic and hematopoietic precursors, while their expression of activation-associated genes did not differ by ontogeny. Collectively, these data uncover shared and unique features between neural resident macrophages and emphasize the role of nerve environment for shaping PNS macrophage identity.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

Dendritic cells and the skin environment

The skin is inhabited by several immune cell populations that serve as a first line of defence against pathogen invasion. Amongst these populations are dendritic cells, which play an essential sentinel function by taking up antigen or infectious agents and transporting them to the lymph node for T cell recognition and the priming of immune responses. In this review, we briefly summarise recent advances showing how skin dendritic cells are connected to a network of epithelial and stromal cells, which provide structural support, growth factors, spatial cues, contact with the external environment and the skin microbiome, and favour interactions with other immune cells. We propose that this network creates a unique skin environment that may condition dendritic cell phenotype and function.

Zebrafish macrophage developmental arrest underlies depletion of microglia and reveals Csf1r-independent metaphocytes

Macrophages derive from multiple sources of hematopoietic progenitors. Most macrophages require colony-stimulating factor 1 receptor (CSF1R), but some macrophages persist in the absence of CSF1R. Here, we analyzed mpeg1:GFP-expressing macrophages in csf1r-deficient zebrafish and report that embryonic macrophages emerge followed by their developmental arrest. In larvae, mpeg1+ cell numbers then increased showing two distinct types in the skin: branched, putative Langerhans cells, and amoeboid cells. In contrast, although numbers also increased in csf1r-mutants, exclusively amoeboid mpeg1+ cells were present, which we showed by genetic lineage tracing to have a non-hematopoietic origin. They expressed macrophage-associated genes, but also showed decreased phagocytic gene expression and increased epithelial-associated gene expression, characteristic of metaphocytes, recently discovered ectoderm-derived cells. We further demonstrated that juvenile csf1r-deficient zebrafish exhibit systemic macrophage depletion. Thus, csf1r deficiency disrupts embryonic to adult macrophage development. Zebrafish deficient for csf1r are viable and permit analyzing the consequences of macrophage loss throughout life.

The effects of statins on microglial cells to protect against neurodegenerative disorders: A mechanistic review

Microglia are the primary innate immune system cells in the central nervous system (CNS). They are crucial for the immunity, neurogenesis, synaptogenesis, neurotrophic support, phagocytosis of cellular debris, and maintaining the CNS integrity and homeostasis. Invasion by pathogens as well as in CNS injuries and damages results in activation of microglia known as microgliosis. The activated microglia have the capacity to release proinflammatory mediators leading to neuroinflammation. However, uncontrolled neuroinflammation can give rise to various neurological disorders (NDs), especially the neurodegenerative diseases including Parkinson's disease (PD) and related disorders, Alzheimer's disease (AD) and other dementias, multiple sclerosis (MS), Huntington's disease (HD), spinocerebellar ataxia (SCA), spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), and stroke. Statins (HMG-CoA reductase inhibitors) are among the most widely prescribed medications for the management of hypercholesterolemia worldwide. It can be used for primary prevention in healthy individuals who are at higher risk of cardiovascular and coronary heart diseases as well as the secondary prevention in patients with cardiovascular and coronary heart diseases disease. A growing body of evidence has indicated that statins have the potential to attenuate the proinflammatory mediators and subsequent NDs by controlling the microglial activation and consequent reduction in neuroinflammatory mediators. In this review, we have discussed the recent studies on the effects of statins on microglia activation and neuroinflammation.

Modeling Psychiatric Disorder Biology with Stem Cells

PURPOSE OF REVIEW

We review the ways in which stem cells are used in psychiatric disease research, including the related advances in gene editing and directed cell differentiation.

RECENT FINDINGS

The recent development of induced pluripotent stem cell (iPSC) technologies has created new possibilities for the study of psychiatric disease. iPSCs can be derived from patients or controls and differentiated to an array of neuronal and non-neuronal cell types. Their genomes can be edited as desired, and they can be assessed for a variety of phenotypes. This makes them especially interesting for studying genetic variation, which is particularly useful today now that our knowledge on the genetics of psychiatric disease is quickly expanding. The recent advances in cell engineering have led to powerful new methods for studying psychiatric illness including schizophrenia, bipolar disorder, and autism. There is a wide array of possible applications as illustrated by the many examples from the literature, most of which are cited here.

Microglia versus Monocytes: Distinct Roles in Degenerative Diseases of the Retina

Unlike in the healthy mammalian retina, macrophages in retinal degenerative states are not solely comprised of microglia but may include monocyte-derived recruits. Recent studies have applied transgenics, lineage-tracing, and transcriptomics to help decipher the distinct roles of these two cell types in the diseasesettings of inherited retinal degenerations and age-related macular degeneration.Literature discussed here focuses on the ectopic presence of both macrophage types in the extracellular site surrounding the outer aspect ofphotoreceptor cells (i.e.,the subretinal space), which is crucially involved in the pathobiology. From these studies we propose a working model in which perturbed photoreceptor states cause microglial dominant migration to the subretinal space as a protective response, whereas the abundant presence ofmonocyte-derived cells there instead drives and accelerates pathology. The latter, we propose, is underpinned by specific genetic and nongenetic determinants that lead to a maladaptive macrophage state.

Characterization of a p53/miR-34a/CSF1R/STAT3 Feedback Loop in Colorectal Cancer

BACKGROUND & AIMS

The miR-34a gene is a direct target of p53 and is commonly silenced in colorectal cancer (CRC). Here we identified the receptor tyrosine kinase CSF1R as a direct miR-34a target and characterized CSF1R as an effector of p53/miR-34a-mediated CRC suppression.

METHODS

Analyses of TCGA-COAD and three other CRC cohorts for association of mRNA expression and signatures with patient survival and molecular subtypes. Bioinformatics identification and experimental validation of miRNA and transcription factor targets. Functional analysis of factors/pathways in the regulation of epithelial-mesenchymal transition (EMT), invasion, migration, acquired chemo-resistance and metastasis. Analyses of protein expression and CpG methylation within primary human colon cancer samples.

RESULTS

In primary CRCs increased CSF1R, CSF1 and IL34 expression was associated with poor patient survival and a mesenchymal-like subtype. CSF1R displayed an inverse correlation with miR-34a expression. This was explained by direct inhibition of CSF1R by miR-34a. Furthermore, p53 repressed CSF1R via inducing miR-34a, whereas SNAIL induced CSF1R both directly and indirectly via repressing miR-34a in a coherent feed-forward loop. Activation of CSF1R induced EMT, migration, invasion and metastasis of CRC cells via STAT3-mediated down-regulation of miR-34a. 5-FU resistance of CRC cells was mediated by CpG-methylation of miR-34a and the resulting elevated expression of CSF1R. In primary CRCs elevated expression of CSF1R was detected at the tumor invasion front and was associated with CpG methylation of the miR-34a promoter as well as distant metastasis.

CONCLUSIONS

The reciprocal inhibition between miR-34a and CSF1R and its loss in tumor cells may be relevant for therapeutic and prognostic approaches towards CRC management.

The developing immune network in human prenatal skin

Establishment of a well‐functioning immune network in skin is crucial for its barrier function. This begins in utero alongside the structural differentiation and maturation of skin, and continues to expand and diversify across the human lifespan. The microenvironment of the developing human skin supports immune cell differentiation and has an overall anti‐inflammatory profile. Immunologically inert and skewed immune populations found in developing human skin promote wound healing, and as such may play a crucial role in the structural changes occurring during skin development.

Microglia-Mediated Regulation of Neuropathic Pain: Molecular and Cellular Mechanisms

Pain is a defense system that responds rapidly to harmful internal and external stimuli through the somatosensory neuronal pathway. However, damage to the nervous system through cancer, diabetes, infection, autoimmune disease, chemotherapy or trauma often leads to neuropathic pain, a debilitating chronic pain condition. Neuropathic pain is not simply a temporal continuum of acute nociceptive signals from the periphery, but rather due to pathologically altered functions in the nervous system, which shift the net neuronal excitatory balance toward excitation. Although alterations were long thought to be a result of changes in neurons, but an increasing body of evidence over the past decades indicates the necessity and sufficiency of microglia, the tissue-resident macrophages of the spinal cord and brain, for nerve injury-induced malfunction of the nervous system. In this review article, I describe our current understanding of the molecular and cellular mechanisms underlying the role of microglia in the pathogenesis of neuropathic pain and discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia.

Interleukin-33 Signaling Controls the Development of Iron-Recycling Macrophages

Splenic red pulp macrophages (RPMs) contribute to erythrocyte homeostasis and are required for iron recycling. Heme induces the expression of SPIC transcription factor in monocyte-derived macrophages and promotes their differentiation into RPM precursors, pre-RPMs. However, the requirements for differentiation into mature RPMs remain unknown. Here, we have demonstrated that interleukin (IL)-33 associated with erythrocytes and co-cooperated with heme to promote the generation of mature RPMs through activation of the MyD88 adaptor protein and ERK1/2 kinases downstream of the IL-33 receptor, IL1RL1. IL-33- and IL1RL1-deficient mice showed defective iron recycling and increased splenic iron deposition. Gene expression and chromatin accessibility studies revealed a role for GATA transcription factors downstream of IL-33 signaling during the development of pre-RPMs that retained full potential to differentiate into RPMs. Thus, IL-33 instructs the development of RPMs as a response to physiological erythrocyte damage with important implications to iron recycling and iron homeostasis.

CSF1R Is Required for Differentiation and Migration of Langerhans Cells and Langerhans Cell Histiocytosis

Langerhans cell histiocytosis (LCH) is a rare disorder characterized by tissue accumulation of CD1a⁺CD207⁺ LCH cells. In LCH, somatic mutations of the BRAF ^V600E gene have been detected in tissue LCH cells, bone marrow CD34⁺ hematopoietic stem cells, circulating CD14⁺ monocytes, and BDCA1⁺ myeloid dendritic cells (DC). Targeting BRAF ^V600E in clonal Langerhans cells (LC) and their precursors is a potential treatment option for patients whose tumors have the mutation. The development of mouse macrophages and LCs is regulated by the CSF1 receptor (CSF1R). In patients with diffuse-type tenosynovial giant cell tumors, CSF1R inhibition depletes tumor-associated macrophages (TAM) with therapeutic efficacy; however, CSF1R signaling in LCs and LCH has not been investigated. We found through IHC and flow cytometry that CSF1R is normally expressed on human CD1a⁺CD207⁺ LCs in the epidermis and stratified epithelia. LCs that were differentiated from CD14⁺ monocytes, BDCA1⁺ DCs, and CD34⁺ cord blood progenitors expressed CSF1R that was downregulated upon maturation. Immature LCs migrated toward CSF1, but not IL34. Administration of the c-FMS/CSF1R kinase inhibitors GW2580 and BLZ945 significantly reduced human LC migration. In LCH clinical samples, LCH cells (including BRAF ^V600E cells) and TAMs retained high expression of CSF1R. We also detected the presence of transcripts for its ligand, CSF1, but not IL34, in all tested LCH cases. CSF1R and CSF1 expression in LCH, and their role in LC migration and differentiation, suggests CSF1R signaling blockade as a candidate rational approach for treatment of LCH, including the BRAF ^V600E and wild-type forms of the disease.

Regulation and function of macrophage colony-stimulating factor (CSF1) in the chicken immune system

Macrophage colony-stimulating factor (CSF1) is an essential growth factor to control the proliferation, differentiation and survival of cells of the macrophage lineage in vertebrates. We have previously produced a recombinant chicken CSF1-Fc fusion protein and administrated it to birds which produced a substantial expansion of tissue macrophage populations. To further study the biology of CSF1 in the chicken, here we generated anti-chicken CSF1 antibodies (ROS-AV181 and 183) using CSF1-Fc as an immunogen. The specific binding of each monoclonal antibody was confirmed by ELISA, Western blotting and immunohistochemistry on tissue sections. Using the anti-CSF1 antibodies, we show that chicken bone marrow derived macrophages (BMDM) express CSF1 on their surface, and that the level appears to be regulated further by exogenous CSF1. By capture ELISA circulating CSF1 levels increased transiently in both layer and broiler embryos around the day of hatch. The levels of CSF1 in broilers was higher than in layers during the first week after hatch. Antibody ROS-AV183 was able to block CSF1 biological activity in vitro and treatment of hatchlings using this neutralising antibody in vivo impacted on some tissue macrophage populations, but not blood monocytes. After anti-CSF1 treatment, CSF1R-transgene reporter expressing cells were reduced in the bursa of Fabricius and cecal tonsil and TIM4⁺ Kupffer cells in the liver were almost completely ablated. Anti-CSF1 treatment also produced a reduction in overall bone density, trabecular volume and TRAP⁺ osteoclasts. Our novel neutralising antibody provides a new tool to study the roles of CSF1 in birds.

Eavesdropping on the conversation between immune cells and the skin epithelium

The skin epithelium covers our body and serves as a vital interface with the external environment. Here, we review the context-specific interactions between immune cells and the epithelium that underlie barrier fitness and function. We highlight the mechanisms by which these two systems engage each other and how immune-epithelial interactions are tuned by microbial and inflammatory stimuli. Epithelial homeostasis relies on a delicate balance of immune surveillance and tolerance, breakdown of which results in disease. In addition to their canonical immune functions, resident and recruited immune cells also supply the epithelium with instructive signals to promote repair. Decoding the dialogue between immunity and the epithelium therefore has great potential for boosting barrier function or mitigating inflammatory epithelial diseases.

How microbiota shape microglial phenotypes and epigenetics

Among the myeloid cells in the central nervous system (CNS) microglia are the main representatives of the innate immune system. Microglial fulfil tasks beyond phagocytosing debris and host defense against invading microorganism. During brain development microglia guide for example neurons for proper CNS formation, in adulthood they maintain tissue homeostasis and in aging microglia may become pro-inflammatory and finally exhausted. Recently, several endogenous and exogenous factors were identified that essentially shape the microglial phenotype during both steady-state and pathological conditions. On the one hand, microglia receive inputs from CNS-endogenous sources for example, via crosstalk with other glial cells and neurons but on the other hand microglia are also highly modulated by external signals. Among them, host microbiota-the host's resident bacteria-are vital regulators of the CNS innate immune system. This review summarizes key extrinsic and intrinsic factors, with special focus on the host microbiota, that essentially influence microglia functions and states during development, homeostasis, and disease.

A role for IL-34 in osteolytic disease of multiple myeloma

Multiple myeloma (MM) is a hematological malignancy that grows in multiple sites of the axial skeleton and causes debilitating osteolytic disease. Interleukin-34 (IL-34) is a newly discovered cytokine that acts as a ligand of colony-stimulating factor-1 (CSF-1) receptor and can replace CSF-1 for osteoclast differentiation. In this study, we identify IL-34 as an osteoclastogenic cytokine that accelerates osteolytic disease in MM. IL-34 was found to be expressed in the murine MM cell line MOPC315.BM, and the expression of IL-34 was enhanced by stimulation with proinflammatory cytokines or by bone marrow (BM) stromal cells. MM-cell-derived IL-34 promoted osteoclast formation from mouse BM cells in vitro. Targeting Il34 by specific small interfering RNA impaired osteoclast formation in vitro and attenuated osteolytic disease in vivo. In BM aspirates from MM patients, the expression levels of IL-34 in CD138⁺ populations vary among patients from high to weak to absent. MM cell-derived IL-34 promoted osteoclast formation from human CD14⁺ monocytes, which was reduced by a neutralizing antibody against IL-34. Taken together, this study describes for the first time the expression of IL-34 in MM cells, indicating that it may enhance osteolysis and suggesting IL-34 as a potential therapeutic target to control pathological osteoclastogenesis in MM patients.

Microglial regional heterogeneity and its role in the brain

Microglia have been recently shown to manifest a very interesting phenotypical heterogeneity across different regions in the mammalian central nervous system (CNS). However, the underlying mechanism and functional meaning of this phenomenon are currently unclear. Baseline diversities of adult microglia in their cell number, cellular and subcellular structures, molecular signature as well as relevant functions have been discovered. But recent transcriptomic studies using bulk RNAseq and single-cell RNAseq have produced conflicting results on region-specific signatures of microglia. It is highly speculative whether such spatial heterogeneity contributes to varying sensitivities of individual microglia to the same physiological and pathological signals in different CNS regions, and hence underlie their functional relevance for CNS disease development. This review aims to thoroughly summarize up-to-date knowledge on this specific topic and provide some insights on the potential underlying mechanisms, starting from microgliogenesis. Understanding regional heterogeneity of microglia in the context of their diverse neighboring neurons and other glia may provide an important clue for future development of innovative therapies for neuropsychiatric disorders.

Neuron-Specific Vitamin D Signaling Attenuates Microglia Activation and CNS Autoimmunity

Low vitamin D during childhood is associated with an increased risk of developing multiple sclerosis (MS) as an adult. Given that vitamin D has anti-inflammatory properties, it has been postulated that the relationship between MS and low vitamin D is due to immune dysregulation. Since the vitamin D receptor (VDR) is expressed in many cell types, this study investigated an alternative hypothesis-neuron-specific VDR signaling induces anti-inflammatory molecules that protect the central nervous system from autoimmunity. Using media from neurons treated with calcitriol, the active form of vitamin D3, LPS-activated microglia had a reduction in pro-inflammatory molecules, and a reciprocal induction of anti-inflammatory molecules. Since IL-34 is critical to the homeostasis of microglia, and was previously shown to be induced in endothelial cells by vitamin D, we investigated IL-34 as the potential anti-inflammatory molecule induced in neurons by vitamin D. Treatment of LPS-activated microglia with IL-34 reduced pro-inflammatory cytokine production and enhanced the expression of anti-inflammatory transcripts. However, neutralizing IL-34 in vitamin D neuronal conditioned media only impacted IL-6 and not the broader anti-inflammatory phenotype of microglia. To mimic low vitamin D in children, we used a neuron-specific inducible mouse model in which VDR was partially deleted in juvenile mice. Partial deletion of VDR in neurons during early life resulted in exacerbated CNS autoimmunity in adult mice. Overall, the study illustrated that vitamin D signaling in neurons promotes an anti-inflammatory state in microglia, and low vitamin D in early life may enhance CNS autoimmunity.

Role of Interleukin-34 in Cancer

Cross-talk between cancer cells and the immune cells occurring in the tumor microenvironment is crucial in promoting signals that foster tumor growth and metastasis. Both cancer cells and immune cells secrete various interleukins (IL), which, either directly or indirectly, stimulate cancer-cell proliferation, survival, and diffusion, as well as contribute to sculpt the immune microenvironment, thereby amplifying tumorigenic stimuli. IL-34, a cytokine produced by a wide range of cells, has been initially involved in the control of differentiation, proliferation, and survival of myeloid cells. More recent studies documented the overexpression of IL-34 in several cancers, such as hepatocarcinoma, osteosarcoma, multiple myeloma, colon cancer, and lung cancer, and showed that tumor cells can produce and functionally respond to this cytokine. In this review, we summarize the multiple roles of IL-34 in various cancers, with the aim to better understand the relationship between the expression of this cytokine and cancer behavior and to provide new insights for exploring a new potential therapeutic target.

Brain Parenchymal and Extraparenchymal Macrophages in Development, Homeostasis, and Disease

Microglia are parenchymal macrophages of the CNS; as professional phagocytes they are important for maintenance of the brain's physiology. These cells are generated through primitive hematopoiesis in the yolk sac and migrate into the brain rudiment after establishment of embryonic circulation. Thereafter, microglia develop in a stepwise fashion, reaching complete maturity after birth. In the CNS, microglia self-renew without input from blood monocytes. Recent RNA-sequencing studies have defined a molecular signature for microglia under homeostasis. However, during disease, microglia undergo remarkable phenotypic changes, which reflect the acquisition of specialized functions tailored to the pathological context. In addition to microglia, the brain-border regions host populations of extraparenchymal macrophages with disparate origins and phenotypes that have recently been delineated. In this review we outline recent findings that provide a deeper understanding of both parenchymal microglia and extraparenchymal brain macrophages in homeostasis and during disease.

Human iPSC-derived microglia: A growing toolset to study the brain's innate immune cells

Recent advances in the generation of microglia from human induced pluripotent stem cells (iPSCs) have provided exciting new approaches to examine and decipher the biology of microglia. As these techniques continue to evolve to encompass more complex in situ and in vivo paradigms, so too have they begun to yield novel scientific insight into the genetics and function of human microglia. As such, researchers now have access to a toolset comprised of three unique "flavors" of iPSC-derived microglia: in vitro microglia (iMGs), organoid microglia (oMGs), and xenotransplanted microglia (xMGs). The goal of this review is to discuss the variety of research applications that each of these techniques enables and to highlight recent discoveries that these methods have begun to uncover. By presenting the research paradigms in which each model has been successful, as well as the key benefits and limitations of each approach, it is our hope that this review will help interested researchers to incorporate these techniques into their studies, collectively advancing our understanding of human microglia biology.

Role of TREM2 in Alzheimer's Disease and its Consequences on β- Amyloid, Tau and Neurofibrillary Tangles

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Alzheimer's Disease (AD) is age-related neurodegenerative disorder recognized by a steadily gradual cognitive decline that has devastating personal and socioeconomic implications. Recently, some genetic factors for AD have been identified which attracted wide attention of researchers in different areas of AD biology and possible new therapeutic targets. Alternative forms of triggering receptor expressed on myeloid cells 2 (TREM2) genes are examples of such risk factors, which contribute higher risk for developing AD. Comprehending TREM2 function pledge to provide salient insight into how neuroinflammation contributes to AD pathology. The dearth of microglial TREM2 shepherd to augmented tau pathology is couple with frequent enhancement of activated neuronal stress kinases. The involvement of TREM2 in the regulation of tau-associated innate immune response of the CNS has clearly demonstrated through these findings. However, whether decrease level of TREM2 assists pathology of tau through changed clearance and pathological escalation of tau or through direct contact between microglia and neuron and any alternative possible mechanisms need to examine. This review briefly summarizes distinct functional roles of TREM2 in AD pathology and highlights the TREM2 gene regulation. We have also addressed the impact of TREM2 on β-amyloid plaques and tau pathology in Alzheimer’s disease.

Schwann cells orchestrate peripheral nerve inflammation through the expression of CSF1, IL-34, and SCF in amyotrophic lateral sclerosis

Distal axonopathy is a recognized pathological feature of amyotrophic lateral sclerosis (ALS). In the peripheral nerves of ALS patients, motor axon loss elicits a Wallerian-like degeneration characterized by denervated Schwann cells (SCs) together with immune cell infiltration. However, the pathogenic significance of denervated SCs accumulating following impaired axonal growth in ALS remains unclear. Here, we analyze SC phenotypes in sciatic nerves of ALS patients and paralytic SOD1^G93A rats, and identify remarkably similar and specific reactive SC phenotypes based on the pattern of S100β, GFAP, isolectin and/or p75^NTR immunoreactivity. Different subsets of reactive SCs expressed colony-stimulating factor-1 (CSF1) and Interleukin-34 (IL-34) and closely interacted with numerous endoneurial CSF-1R-expressing monocyte/macrophages, suggesting a paracrine mechanism of myeloid cell expansion and activation. SCs bearing phagocytic phenotypes as well as endoneurial macrophages expressed stem cell factor (SCF), a trophic factor that attracts and activates mast cells through the c-Kit receptor. Notably, a subpopulation of Ki67+ SCs expressed c-Kit in the sciatic nerves of SOD1^G93A rats, suggesting a signaling pathway that fuels SC proliferation in ALS. c-Kit+ mast cells were also abundant in the sciatic nerve from ALS donors but not in controls. Pharmacological inhibition of CSF-1R and c-Kit with masitinib in SOD1^G93A rats potently reduced SC reactivity and immune cell infiltration in the sciatic nerve and ventral roots, suggesting a mechanism by which the drug ameliorates peripheral nerve pathology. These findings provide strong evidence for a previously unknown inflammatory mechanism triggered by SCs in ALS peripheral nerves that has broad application in developing novel therapies.