Modulation of Hematopoietic Lineage Specification Impacts TREM2 Expression in Microglia-Like Cells Derived From Human Stem Cells

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

Human pluripotent stem cell (hPSC)-derived microglia for the study of brain disorders. A comprehensive review of existing protocols

Microglia, resident immune cells of the brain that originate from the yolk sac, play a critical role in maintaining brain homeostasis by monitoring and phagocytosing pathogens and cellular debris in the central nervous system (CNS). While they share characteristics with myeloid cells, they are distinct from macrophages. In response to injury, microglia release pro-inflammatory factors and contribute to brain homeostasis through activities such as synapse pruning and neurogenesis. To better understand their role in neurological disorders, the generation of in vitro models of human microglia has become essential. These models, derived from patient-specific induced pluripotent stem cells (iPSCs), provide a controlled environment to study the molecular and cellular mechanisms underlying microglia-mediated neuroinflammation and neurodegeneration. The incorporation or generation of microglia into three-dimensional (3D) organoid cultures provides a more physiologically relevant environment that offers further opportunities to study microglial dynamics and disease modeling. This review describes several protocols that have been recently developed for the generation of human-induced microglia. Importantly, it highlights the promise of these in vitro models in advancing our understanding of brain disorders and facilitating personalized drug screening.

The molecular determinants of microglial developmental dynamics

Microglia constitute the largest population of parenchymal macrophages in the brain and are considered a unique subset of central nervous system glial cells owing to their extra-embryonic origins in the yolk sac. During development, microglial progenitors readily proliferate and eventually colonize the entire brain. In this Review, we highlight the origins of microglial progenitors and their entry routes into the brain and discuss the various molecular and non-molecular determinants of their fate, which may inform their specific functions. Specifically, we explore recently identified mechanisms that regulate microglial colonization of the brain, including the availability of space, and describe how the expansion of highly proliferative microglial progenitors facilitates the occupation of the microglial niche. Finally, we shed light on the factors involved in establishing microglial identity in the brain.

Human iPSC-derived glia models for the study of neuroinflammation

Neuroinflammation is a complex biological process that plays a significant role in various brain disorders. Microglia and astrocytes are the key cell types involved in inflammatory responses in the central nervous system. Neuroinflammation results in increased levels of secreted inflammatory factors, such as cytokines, chemokines, and reactive oxygen species. To model neuroinflammation in vitro, various human induced pluripotent stem cell (iPSC)-based models have been utilized, including monocultures, transfer of conditioned media between cell types, co-culturing multiple cell types, neural organoids, and xenotransplantation of cells into the mouse brain. To induce neuroinflammatory responses in vitro, several stimuli have been established that can induce responses in either microglia, astrocytes, or both. Here, we describe and critically evaluate the different types of iPSC models that can be used to study neuroinflammation and highlight how neuroinflammation has been induced and measured in these cultures.

Analysis of Aβ-induced neurotoxicity and microglial responses in simple two- and three-dimensional human iPSC-derived cortical culture systems

The extracellular accumulation of amyloid-β (Aβ) in plaques and associated neurodegeneration are the pathological hallmarks of Alzheimer's disease (AD). These plaques are surrounded by microglia-the resident tissue macrophages of the brain parenchyma that originate from primitive macrophages from the embryonic yolk sac. Microglia, including a unique subpopulation called "disease-associated microglia" (DAM), are strongly implicated in AD pathology; however, their exact function and physiology remain largely unknown. Notably, simple cell and tissue culture systems that adequately recreate the brain microenvironment and can simulate critical aspects of AD pathology could fundamentally contribute to elucidating microglial function in disease development and progression. Thus, we added human-induced pluripotent stem cell (hiPSC)-induced primitive macrophages (hiMacs) to hiPSC-induced cortical neurons (cell model) and cortical organoids (tissue model). The treatment of these culture systems with the O-acyl isopeptide of Aβ_1-42, which reverts to natural extracellular Aβ_1-42 at neutral pH and starts self-aggregation, caused the degeneration of hiPSC-induced cortical neurons in 2D culture and within cortical organoid cultures. Notably, the hiMacs phagocytosed extracellular Aβ and exhibited a DAM-like phenotype. In both cell and tissue organoid culture systems, neurodegeneration was attenuated by the addition of hiMacs. Moreover, in cortical organoids, Aβ plaques formed more circular and fewer hotspot-like morphological structures in the vicinity of hiMacs. These findings demonstrate the utility of simple hiPSC-induced cortical cell and tissue culture systems supplemented with hiMacs for elucidating critical aspects of AD pathology, such as microglial function and physiology. Adopting such systems in routine research practice may lead to the development of novel therapeutic strategies for AD.

Reenacting Neuroectodermal Exposure of Hematopoietic Progenitors Enables Scalable Production of Cryopreservable iPSC-Derived Human Microglia

Human microglia, as innate immune cells of the central nervous system (CNS), play a central role in the pathogenesis of a large number of neurological and psychiatric disorders. However, experimental access to primary human microglia for biomedical applications such as disease modeling is extremely limited. While induced pluripotent stem cells (iPSCs) could provide an alternative source of microglia, the reenactment of their complex ontogenesis with a yolk sac origin and subsequent priming upon CNS invasion has remained a challenge. Here, we report a developmentally informed in vitro differentiation method for large-scale production and cryopreservation of iPSC-derived microglia (iPSdMiG). Specifically, iPSCs were propagated in conditions yielding both yolk sac hematopoietic derivatives and early neuroepithelial cells. To enable large-scale production, we implemented 3D bioreactor-based dynamic culture conditions and the use of novel mesh macrocarriers. Under these conditions, microglia could be harvested across a time period of at least 6 weeks, with 1 × 10⁶ iPSCs giving rise to up to 45 × 10⁶ iPSdMiG. The transcriptomic profile of iPSdMiG showed high similarity to adult human microglia, and harvested cells were immunopositive for typical microglial markers. In addition, iPSdMiG were able to secrete pro-inflammatory cytokines, engaged in phagocytotic activity, produced reactive oxygen species and lent themselves to co-culture studies in neural 2D and 3D systems. Importantly, iPSdMiG were efficiently cryopreserved, enabling the establishment of donor-specific microglia cell banks for disease modeling, drug discovery and eventually cell therapy. Main points. Scalable generation of iPSC-derived multi-lineage embryoid bodies on macrocarriers, reproducibly releasing microglia exhibiting characteristic markers and function. Cells are transcriptomically similar to primary human microglia and cryopreservable.

TREM2 is associated with tumor immunity and implies poor prognosis in glioma

Triggering receptor expressed on myeloid cells 2 (TREM2) is expressed in myeloid cells of the central nervous system (CNS), which mediate the immunological response in a variety of diseases. Uncertain is the function of TREM2 in glioma and tumor immune responses. In this research, the expression patterns of TREM2 in glioma were analyzed, along with its prognostic value and functional roles. TREM2 expression is increased in glioblastomas, gliomas with a mesenchymal subtype, gliomas with wild-type isocitrate dehydrogenase, and gliomas without 1p/19q deletion, all of which suggest the aggressiveness and poor prognosis of gliomas. Gene ontology, KEGG, and Gene set variation analyses indicated that TREM2 may serve as an immune response mediator. However, the function of T cells against tumor cells was negatively correlated with TREM2, suggesting that TREM2 may suppress tumor immunity. Further investigation demonstrated a correlation between TREM2 expression and immune checkpoint expression. CIBERSORT research revealed a link between a higher TREM2 expression level and the enrichment of tumor-associated macrophages, especially M2 subtype. Single-cell analysis and multiple immunohistochemical staining results showed that microglia and macrophage cells expressed TREM2. Immunofluorescent staining indicated that knocking down the expression of TREM2 would result in a decrease in M2 polarization. TREM2 was discovered to be an independent prognostic factor in glioma. In conclusion, our findings revealed that TREM2 was significantly expressed in microglia and macrophage cells and was intimately associated with the tumor immune microenvironment. Thus, it is expected that small-molecule medications targeting TREM2 or monoclonal antibodies would enhance the efficacy of glioma immunotherapy.

Assessing human iPSC-derived microglia identity and function by immunostaining, phagocytosis, calcium activity, and inflammation assay

To understand how potential gene manipulations affect in vitro microglia, we provide a set of short protocols to evaluate microglia identity and function. We detail steps for immunostaining to determine microglia identity. We describe three functional assays for microglia: phagocytosis, calcium response following ATP stimulation, and cytokine expression upon inflammatory stimuli. We apply these protocols to human induced-pluripotent-stem-cell (hiPSC)-derived microglia, but they can be also applied to other in vitro microglial models including primary mouse microglia. For complete details on the use and execution of this protocol, please refer to Bartalska et al. (2022).¹.

Human microglial models to study HIV infection and neuropathogenesis: a literature overview and comparative analyses

HIV persistence in the CNS despite antiretroviral therapy may cause neurological disorders and poses a critical challenge for HIV cure. Understanding the pathobiology of HIV-infected microglia, the main viral CNS reservoir, is imperative. Here, we provide a comprehensive comparison of human microglial culture models: cultured primary microglia (pMG), microglial cell lines, monocyte-derived microglia (MDMi), stem cell-derived microglia (iPSC-MG), and microglia grown in 3D cerebral organoids (oMG) as potential model systems to advance HIV research on microglia. Functional characterization revealed phagocytic capabilities and responsiveness to LPS across all models. Microglial transcriptome profiles of uncultured pMG showed the highest similarity to cultured pMG and oMG, followed by iPSC-MG and then MDMi. Direct comparison of HIV infection showed a striking difference, with high levels of viral replication in cultured pMG and MDMi and relatively low levels in oMG resembling HIV infection observed in post-mortem biopsies, while the SV40 and HMC3 cell lines did not support HIV infection. Altogether, based on transcriptional similarities to uncultured pMG and susceptibility to HIV infection, MDMi may serve as a first screening tool, whereas oMG, cultured pMG, and iPSC-MG provide more representative microglial culture models for HIV research. The use of current human microglial cell lines (SV40, HMC3) is not recommended.

Human stem cell models of neurodegeneration: From basic science of amyotrophic lateral sclerosis to clinical translation

Neurodegenerative diseases are characterized by progressive cell loss leading to disruption of the structure and function of the central nervous system. Amyotrophic lateral sclerosis (ALS) was among the first of these disorders modeled in patient-specific iPSCs, and recent findings have translated into some of the earliest iPSC-inspired clinical trials. Focusing on ALS as an example, we evaluate the status of modeling neurodegenerative diseases using iPSCs, including methods for deriving and using disease-relevant neuronal and glial lineages. We further highlight the remaining challenges in exploiting the full potential of iPSC technology for understanding and potentially treating neurodegenerative diseases such as ALS.

Roles of microglia in Alzheimer's disease and impact of new findings on microglial heterogeneity as a target for therapeutic intervention

Microglia are specialized macrophages that reside within the central nervous system and play key roles in brain immunity, development and homeostasis. Recent studies also revealed functions of microglia in neuroprotection and neuroinflammation, leading to the discovery that microglia are involved in several brain pathologies including Alzheimer's disease (AD). However, the beneficial and detrimental actions of this intriguing cell population can be challenging to dissect: the advent of single-cell and single-nucleus transcriptomic technologies has revolutionized our understanding of the heterogeneity of multiple cell types and is now being applied to the study of microglia in health and disease. Here, we review recent findings on microglial biology, focusing on insights from single cell transcriptomic studies and the heterogeneity that they reveal, and consider the impact of these findings on our understanding of AD. We also discuss how microglia might represent a next-generation therapeutic target for treatment of AD and other neuroinflammatory conditions.

Microglia Development and Maturation and Its Implications for Induction of Microglia-Like Cells from Human iPSCs

Microglia are resident immune cells of the central nervous system and play critical roles during the development, homeostasis, and pathologies of the brain. Originated from yolk sac erythromyeloid progenitors, microglia immigrate into the embryonic brain parenchyma to undergo final postnatal differentiation and maturation driven by distinct chemokines, cytokines, and growth factors. Among them, TGFβ1 is an important regulator of microglial functions, mediating homeostasis, anti-inflammation, and triggering the expression of microglial homeostatic signature genes. Since microglia studies are mainly based on rodent cells and the isolation of homeostatic microglia from human tissue is challenging, human-induced pluripotent stem cells have been successfully differentiated into microglia-like cells recently. However, employed differentiation protocols strongly vary regarding used cytokines and growth factors, culture conditions, time span, and cell yield. Moreover, the incomplete differentiation of human microglia can hamper the similarity to primary human microglia and dramatically influence the outcome of follow-up studies with these differentiated cells. This review summarizes the current knowledge of the molecular mechanisms driving rodent microglia differentiation in vivo, further compares published differentiation protocols, and highlights the potential of TGFβ as an essential maturation factor.

Human amniotic fluid stem cells are able to form embryoid body-like aggregates which performs specific functions: morphological evidences

Human second trimester Amniotic Fluid Stem Cells (hAFSCs) harbour the potential to differentiate into cells of each of the three germ layers and to form Embryoid Body (EB)-like aggregates, without inducing teratoma formation and with no ethical concerns. However, in spite of the number of reports on hAFSCs-EBs and their characterization, a thorough evaluation in light and electron microscopy of morphological and morphometric features of hAFSCs-EBs development in vitro has not been reported yet. Apart from a superficial layer of epithelial-like flat cells, displaying rare microvilli on the free surface, hAFSCs-EBs enclose inner material, abundant in vesicles and secretory granules, showing early characteristics of connective extracellular matrix dispersed among different types of inner cells. The observation of a number of microvesicles mainly represented by microparticles and, to a lower extent, by exosomes indicates the presence of a complex cellular communication system within this structure. According to morphological analysis, after 7 days of in vitro culture hAFSCs-EB appears as a well-organized corpuscle, sufficiently young to be a carrier of stemness and at the same time, when appropriately stimulated, able to differentiate. In fact, 7-day hAFSCs-EB represents itself an initial cellular transformation towards a specialized structure both in recording and in providing different stimuli from the surrounding environment, organizing structures and cells towards a differentiation fate.

IL-34 and CSF-1, deciphering similarities and differences at steady state and in diseases

Although IL-34 and CSF-1 share actions as key mediators of monocytes/macrophages survival and differentiation, they also display differences that should be identified to better define their respective roles in health and diseases. IL-34 displays low sequence homology with CSF-1 but has a similar general structure and they both bind to a common receptor CSF-1R, although binding and subsequent intracellular signaling shows differences. CSF-1R expression has been until now mainly described at a steady state in monocytes/macrophages and myeloid dendritic cells, as well as in some cancers. IL-34 has also 2 other receptors, protein-tyrosine phosphatase zeta (PTPζ) and CD138 (Syndecan-1), expressed in some epithelium, cells of the central nervous system (CNS), as well as in numerous cancers. While most, if not all, of CSF-1 actions are mediated through monocyte/macrophages, IL-34 has also other potential actions through PTPζ and CD138. Additionally, IL-34 and CSF-1 are produced by different cells in different tissues. This review describes and discusses similarities and differences between IL-34 and CSF-1 at steady state and in pathological situations and identifies possible ways to target IL-34, CSF-1, and its receptors.

Astrocytes and microglia in neurodegenerative diseases: Lessons from human in vitro models

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Astrocytes and microglia key fulfil homeostatic and immune functions in the CNS.

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Dysfunction of these cell types is implicated in neurodegenerative diseases.

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Understanding cellular autonomy and early pathogenic changes is a key goal.

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New human iPSC models will inform on disease mechanisms and therapy development.

Both astrocytes and microglia fulfil homeostatic and immune functions in the healthy CNS. Dysfunction of these cell types have been implicated in the pathomechanisms of several neurodegenerative diseases. Understanding the cellular autonomy and early pathological changes in these cell types may inform drug screening and therapy development. While animal models and post-mortem tissue have been invaluable in understanding disease processes, the advent of human in vitro models provides a unique insight into disease biology as a manipulable model system obtained directly from patients. Here, we discuss the different human in vitro models of astrocytes and microglia and outline the phenotypes that have been recapitulated in these systems.

Early-Onset Familial Alzheimer Disease Variant PSEN2 N141I Heterozygosity is Associated with Altered Microglia Phenotype

BACKGROUND

Early-onset familial Alzheimer disease (EOFAD) is caused by heterozygous variants in the presenilin 1 (PSEN1), presenilin 2 (PSEN2), and APP genes. Decades after their discovery, the mechanisms by which these genes cause Alzheimer's disease (AD) or promote AD progression are not fully understood. While it is established that presenilin (PS) enzymatic activity produces amyloid-β (Aβ), PSs also regulate numerous other cellular functions, some of which intersect with known pathogenic drivers of neurodegeneration. Accumulating evidence suggests that microglia, resident innate immune cells in the central nervous system, play a key role in AD neurodegeneration.

OBJECTIVE

Previous work has identified a regulatory role for PS2 in microglia. We hypothesized that PSEN2 variants lead to dysregulated microglia, which could further contribute to disease acceleration. To mimic the genotype of EOFAD patients, we created a transgenic mouse expressing PSEN2 N141I on a mouse background expressing one wildtype PS2 and two PS1 alleles.

RESULTS

Microglial expression of PSEN2 N141I resulted in impaired γ-secretase activity as well as exaggerated inflammatory cytokine release, NFκB activity, and Aβ internalization. In vivo, PS2 N141I mice showed enhanced IL-6 and TREM2 expression in brain as well as reduced branch number and length, an indication of "activated" morphology, in the absence of inflammatory stimuli. LPS intraperitoneal injection resulted in higher inflammatory gene expression in PS2 N141I mouse brain relative to controls.

CONCLUSION

Our findings demonstrate that PSEN2 N141I heterozygosity is associated with disrupted innate immune homeostasis, suggesting EOFAD variants may promote disease progression through non-neuronal cells beyond canonical dysregulated Aβ production.

Pathological Changes in Alzheimer's Disease Analyzed Using Induced Pluripotent Stem Cell-Derived Human Microglia-Like Cells

Microglia constitute the majority of innate immune cells in the brain, and their dysfunction is associated with various central nervous system diseases. Human microglia are extremely difficult to obtain experimentally, thereby limiting studies on their role in complex diseases. Microglia derived from human stem cells provide new tools to assess the pathogenesis of complex diseases and to develop effective treatment methods. This study aimed to develop a reliable method to derive human microglial-like cells (iMGLs) from induced pluripotent stem cells (iPSCs) expressing microglia-specific markers IBA1 and TMEM119 and respond to lipopolysaccharide (LPS) stimulation. Thereafter, we compared iMGL functions from Alzheimer's disease (AD) patients and cognitive normal controls (CNCs). AD-iMGLs displayed stronger phagocytic ability with or without stimulation. High LPS concentrations (>2μg/ml) caused death in CNC-iMGLs, while AD-iMGLs did not display significant cell death. Cytokine analysis revealed that TNF-α, IL-6, and IL-10 secreted by AD-iMGLs were significantly increased upon LPS stimulation compared to those in CNC-iMGLs. The present results indicate that AD-iMGLs exhibit significant inflammatory characteristics and can reflect some pathological changes in microglia in AD, thereby providing new valuable tools to screen candidate drugs for AD and to elucidate the mechanisms underlying AD pathogenesis.

Generating microglia from human pluripotent stem cells: novel in vitro models for the study of neurodegeneration

Microglia play an essential role for central nervous system (CNS) development and homeostasis and have been implicated in the onset, progression, and clearance of numerous diseases affecting the CNS. Previous in vitro research on human microglia was restricted to post-mortem brain tissue-derived microglia, with limited availability and lack of scalability. Recently, the first protocols for the generation of microglia from human pluripotent stem cells have become available, thus enabling the implementation of powerful platforms for disease modeling, drug testing, and studies on cell transplantation. Here we give a detailed and comprehensive overview of the protocols available for generating microglia from human pluripotent stem cells, highlighting the advantages, drawbacks, and operability and placing them into the context of current knowledge of human embryonic development. We review novel insights into microglia biology and the role of microglia in neurological diseases as drawn from the new methods and provide an outlook for future lines of research involving human pluripotent stem cell-derived microglia.

Studying tissue macrophages in vitro: are iPSC-derived cells the answer?

Macrophages are immune cells with important roles in tissue homeostasis, inflammation and pathologies. Hence, macrophage populations represent promising targets for modern medicine. Exploiting the potential of macrophage-targeted therapies will require a thorough understanding of the mechanisms controlling their development, specialization and maintenance throughout their lifespan. Macrophages have been studied in vitro for many years, but recent advances in the field of macrophage biology have called into question the validity of traditional approaches. New models, such as recent innovations in generating macrophages from induced pluripotent stem cells (iPSCs), must take into account the impact of heterogeneity in the origin and tissue-specific functions of macrophages. Here, we discuss these protocols and argue for a better understanding of the type of macrophages made in vitro; we also encourage recognition of the importance of tissue identity of macrophages, which cannot be recapitulated by cytokine-dependent protocols. We suggest that a two-step model - in which iPSC-derived macrophages are first generated based on their ontogeny and then conditioned by their tissue-specific environment - offers immense potential for generating biologically relevant macrophages for future studies.

Functionalization of Brain Region-specific Spheroids with Isogenic Microglia-like Cells

Current brain spheroids or organoids derived from human induced pluripotent stem cells (hiPSCs) still lack a microglia component, the resident immune cells in the brain. The objective of this study is to engineer brain region-specific organoids from hiPSCs incorporated with isogenic microglia-like cells in order to enhance immune function. In this study, microglia-like cells were derived from hiPSCs using a simplified protocol with stage-wise growth factor induction, which expressed several phenotypic markers, including CD11b, IBA-1, CX3CR1, and P2RY12, and phagocytosed micron-size super-paramagnetic iron oxides. The derived cells were able to upregulate pro-inflammatory gene (TNF-α) and secrete anti-inflammatory cytokines (i.e., VEGF, TGF-β1, and PGE2) when stimulated with amyloid β42 oligomers, lipopolysaccharides, or dexamethasone. The derived isogenic dorsal cortical (higher expression of TBR1 and PAX6) and ventral (higher expression of NKX2.1 and PROX1) spheroids/organoids displayed action potentials and synaptic activities. Co-culturing the microglia-like cells (MG) with the dorsal (D) or ventral (V) organoids showed differential migration ability, intracellular Ca2+ signaling, and the response to pro-inflammatory stimuli (V-MG group had higher TNF-α and TREM2 expression). Transcriptome analysis exhibited 37 microglia-related genes that were differentially expressed in MG and D-MG groups. In addition, the hybrid D-MG spheroids exhibited higher levels of immunoreceptor genes in activating members, but the MG group contained higher levels for most of genes in inhibitory members (except SIGLEC5 and CD200). This study should advance our understanding of the microglia function in brain-like tissue and establish a transformative approach to modulate cellular microenvironment toward the goal of treating various neurological disorders.

Biological functions and clinical implications of interleukin-34 in inflammatory diseases

Interleukin (IL)-34 is a recently discovered cytokine and ligand of the colony-stimulating factor (CSF)-1 receptor. Although CSF-1 and IL-34 share similar biological properties, their expression patterns and downstream signaling pathways are distinct. IL-34 can influence differentiation and has functions in multiple cell types (e.g., dendritic cells, monocytes, macrophages). In the pathological conditions, IL-34 is induced by pro-inflammatory stimuli (e.g., cytokines, pathogen-associated molecular patterns, and infection). Current evidence shows that IL-34 is a critical player in inflammatory response and is involved in the pathogenesis of inflammatory autoimmune dysfunction. Therefore, IL-34 may be a promising clinical biomarker and therapeutic target for treating inflammatory related disorders. In this article, we review the advances in biological functions of IL-34 and our understanding of its role in the development of inflammatory diseases as well as therapeutic applications.

Interleukin-34, a comprehensive review

IL-34 is a novel cytokine that was identified in 2008 in a comprehensive proteomic analysis as a tissue-specific ligand of CSF-1 receptor (CSF-1R). IL-34 exists in all vertebrates including fish, amphibians, birds, and mammals, showing high conservation among species. Structurally, IL-34 belongs to the short-chain helical hematopoietic cytokine family but shows no apparent consensus structural domains, motifs, or sequence homology with other cytokines. IL-34 is synthesized as a secreted homodimeric glycoprotein that binds to the extracellular domains of CSF-1R and receptor-type protein-tyrosine phosphatase-zeta (PTP-ζ) in addition to the chondroitin sulfate chains of syndecan-1. These interactions result in activating several signaling pathways that regulate major cellular functions, including proliferation, differentiation, survival, metabolism, and cytokine/chemokine expression in addition to cellular adhesion and migration. In the steady state, IL-34 contributes to the development and maintenance of specific myeloid cell subsets in a tissue-specific manner: Langerhans cells in the skin and microglia in the brain. In pathological conditions, changes in IL-34 expression-increased or decreased-are involved in disease pathogenesis and correlate with progression, severity, and chronicity. One decade after its discovery, IL-34 has been introduced as a newcomer to the big family of interleukins with specific physiological functions, critical pathological roles, and promising clinical applications in disease diagnosis and treatment. In this review, we celebrate the 10th anniversary of IL-34 discovery, introducing its biological characteristics, and discussing the importance of IL-34 signaling network in health and disease.

Generating tissue-resident macrophages from pluripotent stem cells: Lessons learned from microglia

Over the past decades, the importance of the immune system in a broad scope of pathologies, has drawn attention towards tissue-resident macrophages, such as microglia in the brain. To enable the study of for instance microglia, it is crucial to recreate in vitro (and in vivo) assays. However, very fast loss of tissue-specific features of primary tissue resident macrophages, including microglia, upon in vitro culture has complicated such studies. Moreover, limited knowledge of macrophage developmental pathways and the role of local 'niche factors', has hampered the generation of tissue-resident macrophages from pluripotent stem cells (PSC). Recent data on the ontogeny of tissue-resident macrophages, combined with bulk and single cell RNAseq studies have identified the distinct origins and gene profile of microglia compared to other myeloid cells. As a result, over the past years, protocols have been published to create hPSC-derived microglia-'like' cells, as these cells are considered potential new therapeutic targets for therapies to treat neurodegenerative diseases. In this review we will provide an overview of different approaches taken to generate human microglia in vitro, taking into account their origin, and resemblance to their in vivo counterpart. Finally, we will discuss cell-extrinsic (culture conditions) and intrinsic factors (transcriptional machinery and epigenetics) that we believe can improve future differentiation protocols of tissue-resident macrophages from stem cells.