Cognate microglia-T cell interactions shape the functional regulatory T cell pool in experimental autoimmune encephalomyelitis pathology

Drivers and shapers of macrophages specification in the developing brain

The brain harbors two major macrophage populations: microglia reside within the brain parenchyma, while border-associated macrophages (BAMs) are situated at central nervous system (CNS) interfaces. BAMs can be further classified into distinct subsets based on their localization: perivascular macrophages surround blood vessels, meningeal macrophages reside in the leptomeninges, dura macrophages in the dura mater, and choroid plexus macrophages are confined to the choroid plexus. The environmental factors and molecular mechanisms driving the specification of these macrophage populations are still being elucidated. Deciphering the communication pathways between CNS macrophages and their tissue niches during development, homeostasis, and pathologic conditions offers significant potential for treating a wide range of brain disorders, from neurodevelopmental and neuroinflammatory diseases to neurovascular and neurodegenerative conditions. With this short review, we will address the current understanding and knowledge gaps in the field, as well as the future directions for the upcoming years.

Targeting microglia-Th17 feed-forward loop to suppress autoimmune neuroinflammation

Microglia and Th17 cells are the major immunopathogenic cells in multiple sclerosis and its animal model of immune aspects, experimental autoimmune encephalomyelitis (EAE). While studies have highlighted the distinct roles of microglia and Th17 cells in EAE, it remains unclear whether microglia, as potential professional antigen-presenting cells, activate and stabilize the effector program of EAE-pathogenic Th17 cells in vivo; and if so, whether the Th17 could in turn reinforce the active state of the microglia. Our data demonstrate in an array of mouse models, including active/passive-EAE and transgenic mice, a microglia-Th17 feed-forward activation loop drives EAE disease progression through a mechanism dependent on both MHC-II, proinflammatory cytokines, inflammatory chemokines as well as STING→NF-κB pathway in the microglia and effector cytokines produced by the pathogenic Th17 cells. We also captured and identified the molecular properties of the feed-forward loop, which are two-cell entities of microglia-Th17, and proved them as the functional units of antigen presentation and bi-directional activation between the two cell types. Moreover, ACT001, an orphan drug to treat glioblastoma, disrupts this feed-forward activation loop by inhibiting the STING→NF-κB pathway in microglia, thereby alleviating EAE. These findings emphasize the importance of interactions and bi-directional activations between microglia and Th17 in the autoimmune neuroinflammation, and provide rationale for further investigation on ACT001 as therapeutic option for autoimmune inflammatory diseases driven by similar mechanisms.

Paeoniflorin Directly Targets ENO1 to Inhibit M1 Polarization of Microglia/Macrophages and Ameliorates EAE Disease

The chronic autoimmune disease multiple sclerosis (MS) now remains incurable. Paeoniflorin (PF), which is a monoterpene glucoside obtained from Paeonia lactiflora Pall, is recognized for neuroprotective and anti-inflammatory properties. However, the precise mechanism by which PF regulates MS is unclear. This work aims to elucidate the underlying mechanisms of PF in EAE, a well established animal model of MS, and to discover the target proteins that PF directly acts on. Our results revealed that PF administration can significantly attenuate the clinical symptoms of EAE and alleviate the central nervous system (CNS) inflammatory environment by inhibiting M1-type microglia/macrophages. Mechanistically, PF was found to directly interact with the glycolytic enzyme α-enolase (ENO1), inhibiting its enzymatic activity and expression to impair glucose metabolism, thereby suppressing microglia/macrophage M1 polarization and ameliorating CNS inflammation. Significantly, Eno1 knockdown in microglia/macrophages diminished their pro-inflammatory phenotype, while treatment with ENOBlock or the specific knockout of Eno1 in microglia led to EAE remission, underscoring the critical role of ENO1 in EAE progression. This study uncovers the molecular mechanism of PF in treating EAE, linking the anti-inflammatory property of PF to the glucose metabolism process, which will broaden the prospective applications of PF.

Central TYK2 inhibition identifies TYK2 as a key neuroimmune modulator

GWAS have identified tyrosine kinase 2 (TYK2) variants in multiple inflammatory disorders, specifically a protective hypomorphic TYK2 allele (P1104A) in multiple sclerosis (MS). Impaired TYK2 signaling within the central nervous system (CNS) may impart the protective effects of TYK2 P1104A allele in MS. We deployed brain-penetrant TYK2 inhibitors (cTYK2i) alongside the peripherally restricted TYK2 inhibitor (pTYK2i; BMS-986165) to untangle the contributions of central TYK2 inhibition in diverse models of neuroinflammation. While pTYK2i had little impact, cTYK2i reduced clinical score, lymphoid cell infiltration, and cytokines/chemokines in experimental autoimmune encephalomyelitis (EAE). Microglial activation was attenuated in cTYK2i-treated EAE spinal cords and circulating neurofilament light (NfL) was reduced in plasma and cerebral spinal fluid (CSF). Additionally, cTYK2i was protective in an antibody-mediated mouse model of primary progressive MS (PPMS). Finally, we demonstrate TYK2 inhibition has a robust impact on a unique subset of activated astrocytes termed Interferon-Responsive-Reactive-Astrocytes (IRRA). The data presented herein identify a key role for CNS TYK2 signaling in regulating neuroinflammation and solidify TYK2 as a potential therapeutic target for MS.

Central Nervous System Macrophages in Health and Disease

The central nervous system (CNS) has a unique set of macrophages that seed the tissue early during embryonic development. Microglia reside in the parenchyma, and border-associated macrophages are present in border regions, including the meninges, perivascular spaces, and choroid plexus. CNS-resident macrophages support brain homeostasis during development and steady state. In the diseased brain, however, the immune landscape is altered, with phenotypic and transcriptional changes in resident macrophages and the invasion of blood-borne monocytes, which differentiate into monocyte-derived macrophages upon entering the CNS. In this review, we focus on the fate and function of the macrophage compartment in health, neurodegenerative conditions such as amyloidosis, and neuroinflammation as observed in multiple sclerosis and infection. We discuss our current understanding that monocyte-derived macrophages contribute to neuropathology whereas native macrophages play a neuroprotective role in disease.

Monocyte-derived macrophages act as reinforcements when microglia fall short in Alzheimer's disease

The central nervous system (CNS) is endowed with its own resident innate immune cells, the microglia. They constitute approximately 10% of the total cells within the CNS parenchyma and act as 'sentinels', sensing and mitigating any deviation from homeostasis. Nevertheless, under severe acute or chronic neurological injury or disease, microglia are unable to contain the damage, and the reparative activity of monocyte-derived macrophages (MDMs) is required. The failure of the microglia under such conditions could be an outcome of their prolonged exposure to hostile stimuli, leading to their exhaustion or senescence. Here, we describe the conditions under which the microglia fall short, focusing mainly on the context of Alzheimer's disease, and shed light on the functions performed by MDMs. We discuss whether and how MDMs engage in cross-talk with the microglia, why their recruitment is often inadequate, and potential ways to augment their homing to the brain in a well-controlled manner.

Glial polarization in neurological diseases: Molecular mechanisms and therapeutic opportunities

Glial cell polarization plays a pivotal role in various neurological disorders. In response to distinct stimuli, glial cells undergo polarization to either mitigate neurotoxicity or facilitate neural repair following injury, underscoring the importance of glial phenotypic polarization in modulating central nervous system function. This review presents an overview of glial cell polarization, focusing on astrocytes and microglia. It explores the involvement of glial polarization in neurological diseases such as Alzheimer's disease, Parkinson's disease, stroke, epilepsy, traumatic brain injury, amyotrophic lateral sclerosis, multiple sclerosis and meningoencephalitis. Specifically, it emphasizes the role of glial cell polarization in disease pathogenesis through mechanisms including neuroinflammation, neurodegeneration, calcium signaling dysregulation, synaptic dysfunction and immune response. Additionally, it summarizes various therapeutic strategies including pharmacological treatments, dietary supplements and cell-based therapies, aimed at modulating glial cell polarization to ameliorate brain dysfunction. Future research focused on the spatio-temporal manipulation of glial polarization holds promise for advancing precision diagnosis and treatment of neurological diseases.

Sp3 ameliorated experimental autoimmune encephalomyelitis by triggering Socs3 in Th17 cells

INTRODUCTION

Although it is believed that chronic inflammatory and degenerative diseases of the central nervous system are mediated by autoimmune Th17 cells, the underlying mechanisms remain largely unexplored. Recent studies and our research have revealed that Sp3 was blocked in multiple sclerosis (MS) patients and experimental autoimmune encephalomyelitis (EAE). However, it remained unclear why it is silent and how it regulates Th17 cell differentiation in MS.

OBJECTIVES

This study aimed to explore the impact of Sp3 on Th17 cell-mediated EAE and the underlying mechanism.

METHODS

The effect of Sp3 on the clinical symptoms of EAE was evaluated by scoring, histochemistry, and fast blue (FB) techniques, scRNA-seq data analysis, flow cytometry, ELISA, PCR, WB, immunofluorescence and reporter gene techniques were used to explore the molecular mechanism of Sp3 regulating Th17 cell differentiation.

RESULTS

Injection of overexpression Sp3 lentivirus could significantly ameliorate the EAE progress and clinical symptoms and prevent the polarization of Th1 and Th17 cells both in vivo and in vitro. We confirmed the occurrence of EAE in Sp3+/+CD4Cre mice and Sp3+/- knockout mice. Furthermore, we identified Sp3 as a target of miR-223, which is found to be upregulated in the blood of MS patients, as well as in EAE and Th17 cells. Moreover, knockdown of miR-223 led to a marked improvement in EAE symptoms and a suppression of Th1 and Th17 cell polarization in vivo and in vitro. Mechanistically, Sp3 significantly suppressed RORγt expression and the phosphorylation of Stat3 and Smad2/3 by directly upregulating Socs3. Interestingly, Socs3 was found to regulate Sp3 expression in response to TGF-β1 via a feedback loop. Moreover, Socs3 modulated phospho-Smad2/3 by binding to and degrading the transforming growth factor-β receptor II (TβRII).

CONCLUSION

Thus, our study suggests a novel mechanism involving miR-223/Sp3/Socs3/TGF-β signaling as a potential therapeutic strategy for targeting Th17 cells in immunotherapy.

Toward a better understanding of T cell dysregulation in autism: An integrative review

Autism spectrum disorder (ASD) is a highly heterogeneous disorder characterized by impairments in social, communicative, and restrictive behaviors. Over the past 20 years, research has highlighted the role of the immune system in regulating neurodevelopment and behavior. In ASD, immune abnormalities are frequently observed, such as elevations in pro-inflammatory cytokines, alterations in immune cell frequencies, and dysregulated mechanisms of immune suppression. The adaptive immune system - the branch of the immune system conferring cellular immunity - may be involved in the etiology of ASD. Specifically, dysregulated T cell activity, characterized by altered cellular function and increased cytokine release, presence of inflammatory phenotypes and altered cellular signaling, has been consistently observed in several studies across multiple laboratories and geographic regions. Similarly, mechanisms regulating their activation are also disrupted. T cells at homeostasis coordinate the healthy development of the central nervous system (CNS) during early prenatal and postnatal development, and aid in CNS maintenance into adulthood. Thus, T cell dysregulation may play a role in neurodevelopment and the behavioral and cognitive manifestations observed in ASD. Outside of the CNS, aberrant T cell activity may also be responsible for the increased frequency of immune based conditions in the ASD population, such as allergies, gut inflammation and autoimmunity. In this review, we will discuss the current understanding of T cell biology in ASD and speculate on mechanisms behind their dysregulation. This review also evaluates how aberrant T cell biology affects gastrointestinal issues and behavior in the context of ASD.

Cathepsin C exacerbates EAE by promoting the expansion of Tfh cells and the formation of TLSs in the CNS

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) mediated by CD4+ T helper (Th) cells, and characterized by immune cell infiltration, demyelination and neurodegeneration, with no definitive cure available. Thus, it is pivotal and imperative to acquire more profound comprehension of the underlying mechanisms implicated in MS. Dysregulated immune responses are widely believed to play a primary role in the pathogenesis of MS. Recently, a plethora of studies have demonstrated the involvement of T follicular helper (Tfh) cells and tertiary lymphoid-like structures (TLSs) in the pathogenesis and progression of MS. Cathepsin C (CatC) is a cysteine exopeptidase which is crucial for the activation of immune-cell-associated serine proteinases in many inflammatory diseases in peripheral system, such as rheumatoid arthritis and septicemia. We have previously demonstrated that CatC is involved in neuroinflammation and exacerbates demyelination in both cuprizone-induced and experimental autoimmune encephalomyelitis (EAE) mouse models. However, the underlying immunopathological mechanism remains elusive. In the present study, we established a recombinant myelin oligodendrocyte glycoprotein 35-55 peptide-induced EAE model using conditional CatC overexpression mice to investigate the effects of CatC on the alteration of CD4+ Th subsets, including Th1, Th2, Th17, Tfh and T regulatory cells. Our findings demonstrated that CatC particularly enhanced the population of Tfh cell in the brain, resulting in the earlier onset and more severe chronic syndrome of EAE. Furthermore, CatC promoted the formation of TLSs in the brain, leading to persistent neuroinflammation and exacerbating the severity of EAE in the chronic phase. Conversely, treatment with AZD7986, a specific inhibitor of CatC, effectively attenuated the syndrome of EAE and its effects caused by CatC both in vivo and in vitro. These findings provide a novel insight into the critical role of CatC in innate and adaptive immunity in EAE, and specific inhibitor of CatC, AZD7986, may contribute to potential therapeutic strategies for MS.

RiboTag-based RNA-Seq uncovers oligodendroglial lineage-specific inflammation in autoimmune encephalomyelitis

Oligodendroglial lineage cells (OLCs) are critical for neuronal support functions, including myelination and remyelination. Emerging evidence reveals their active roles in neuroinflammation, particularly in conditions like Multiple Sclerosis (MS). This study explores the inflammatory translatome of OLCs during the early onset of experimental autoimmune encephalomyelitis (EAE), an established MS model. Using RiboTag-based RNA sequencing in genetically modified Olig2-Cre RiboTag mice, we identified 1,556 upregulated and 683 downregulated genes in EAE OLCs. Enrichment analysis indicated heightened immune-related pathways, such as cytokine signaling, interferon responses, and antigen presentation, while downregulated genes were linked to neuronal development and myelination. Notably, OLCs expressed cytokines/chemokines, and their receptor, highlighting their active involvement in neuroinflammatory signaling. Functional studies demonstrated that interferon-gamma (IFN-γ) signaling in OLCs exacerbates EAE pathology by enhancing antigen presentation and chemokine production, whereas interferon-beta (IFN-β) signaling showed minimal impact. These findings provide novel insights into the inflammatory role of OLCs in EAE and suggest therapeutic potential in targeting OLC-mediated neuroinflammation for MS and related disorders.

The emerging role of microglia in the development and therapy of multiple sclerosis

Microglia are innate immune cells that maintain homeostasis of the central nervous system (CNS) and affect various neurodegenerative diseases, especially multiple sclerosis (MS). MS is an autoimmune disease of the CNS characterized by persistent inflammation, diffuse axonal damage, and microglia activation. Recent studies have shown that microglia are extremely related to the pathological state of MS and play an important role in the development of MS. This article reviews the multiple roles of microglia in the progression of MS, including the regulatory role of microglia in inflammation, remyelination, oxidative stress, the influence of phagocytosis and antigen-presenting capacity of microglia, and the recent progress by using microglia as a target for MS therapy. Microglia modulation may be a potential way for better MS therapy.

Targeting cytokine networks in neuroinflammatory diseases

In neuroinflammatory diseases, systemic (blood-borne) leukocytes invade the central nervous system (CNS) and lead to tissue damage. A causal relationship between neuroinflammatory diseases and dysregulated cytokine networks is well established across several preclinical models. Cytokine dysregulation is also observed as an inadvertent effect of cancer immunotherapy, where it often leads to neuroinflammation. Neuroinflammatory diseases can be separated into those in which a pathogen is at the centre of the immune response and those of largely unknown aetiology. Here, we discuss the pathophysiology, cytokine networks and therapeutic landscape of 'sterile' neuroinflammatory diseases such as multiple sclerosis (MS), neuromyelitis optica spectrum disorder (NMOSD), neurosarcoidosis and immune effector cell-associated neurotoxicity syndrome (ICANS) triggered by cancer immunotherapy. Despite successes in targeting cytokine networks in preclinical models of neuroinflammation, the clinical translation of targeting cytokines and their receptors has shown mixed and often paradoxical responses.

Mononuclear phagocytes in autoimmune neuroinflammation

A healthy mammalian central nervous system (CNS) harbors a diverse population of leukocytes including members of the mononuclear phagocyte system (MPS). Exerting their specific functions, CNS tissue-resident macrophages as well as associated monocytes and dendritic cells (DCs) maintain CNS homeostasis. Under neuroinflammatory conditions, leukocytes from the systemic immune compartment invade the CNS. This review focuses on the newly discovered roles of the MPS in autoimmune neuroinflammation elicited by encephalitogenic T cells. We propose that CNS-associated DCs act as gatekeepers and antigen-presenting cells that guide the adaptive immune response while bone marrow (BM)-derived monocytes contribute to immunopathology and tissue damage. By contrast, CNS-resident macrophages primarily support tissue function and promote the repair and maintenance of CNS functions.

BRD4 expression in microglia supports recruitment of T cells into the CNS and exacerbates EAE

In EAE, a mouse model of multiple sclerosis, immunization with MOG autoantigen results in the generation of Th1/Th17 T cells in the periphery. MOG-specific T cells then invade into the central nervous system (CNS), resulting in neuronal demyelination. Microglia, innate immune cells in the CNS are known to regulate various neuronal diseases. However, the role of microglia in EAE has remained elusive. BRD4 is a BET protein expressed in microglia, whether BRD4 in microglia contributes to EAE has not been determined. We show that EAE pathology was markedly reduced with microglia-specific Brd4 conditional knockout (cKO). In these mice, microglia- T cell interactions were greatly reduced, leading to the lack of T cell reactivation. Microglia specific transcriptome data showed downregulation of genes required for interaction with and reactivation of T cells in Brd4 cKO samples. In summary, BRD4 plays a critical role in regulating microglia function in normal and EAE CNS.

Summary

This study demonstrates that in a EAE model, microglia-specific Brd4 conditional knockout mice were defective in expressing genes required for microglia- T cells interaction and those involved in neuroinflammation, and demyelination resulting in fewer CNS T cell invasion and display marked reduction in EAE pathology.

Epigenetic adaptation drives monocyte differentiation into microglia-like cells upon engraftment into the retina

The identification of specific markers for microglia has been a long-standing challenge. Recently, markers such as P2ry12, TMEM119, and Fcrls have been proposed as microglia-specific and widely used to explore microglial functions within various central nervous system (CNS) contexts. The specificity of these markers was based on the assumption that circulating monocytes retain their distinct signatures even after infiltrating the CNS. However, recent findings reveal that infiltrating monocytes can adopt microglia-like characteristics while maintaining a pro-inflammatory profile upon permanent engraftment in the CNS.In this study, we utilize bone marrow chimeras, single-cell RNA sequencing, ATAC-seq, flow cytometry, and immunohistochemistry to demonstrate that engrafted monocytes acquire expression of established microglia markers-P2ry12, TMEM119, Fcrls-and the pan-myeloid marker Iba1, which has been commonly mischaracterized as microglia-specific. These changes are accompanied by alterations in chromatin accessibility and shifts in chromatin binding motifs that are indicative of microglial identity. Moreover, we show that engrafted monocytes dynamically regulate the expression of CX3CR1, CCR2, Ly6C, and transcription factors PU.1, CTCF, RUNX, AP-1, CEBP, and IRF2, all of which are crucial for shaping microglial identity. This study is the first to illustrate that engrafted monocytes in the retina undergo both epigenetic and transcriptional changes, enabling them to express microglia-like signatures. These findings highlight the need for future research to account for these changes when assessing the roles of monocytes and microglia in CNS pathology.

CYP7B1 deficiency impairs myeloid cell activation in autoimmune disease of the central nervous system

Dysregulation of cholesterol metabolism underlies neurodegenerative disease and is increasingly implicated in neuroinflammatory diseases, such as multiple sclerosis (MS). Cytochrome P450 family 7 subfamily B member 1 (CYP7B1) is a key enzyme in alternative cholesterol metabolism. A recessive mutation in the gene CYP7B1 is known to cause a neurodegenerative disease, hereditary spastic paraplegia type 5 and oxysterol accumulation. However, the role of CYP7B1 in neuroinflammation has been little revealed. In this study, we induced experimental autoimmune encephalomyelitis (EAE), as a murine model of MS, using CYP7B1 homozygous knockout (KO) mice. We found that CYP7B1 deficiency can significantly attenuate EAE severity. CYP7B1 deficiency is sufficient to reduce leukocyte infiltration into the central nervous system, suppress proliferation of pathogenic CD4+ T cells, and decrease myeloid cell activation during EAE. Additionally, live-animal imaging targeting translocator protein expression, an outer mitochondrial membrane protein biomarker of neuroinflammation, showed that CYP7B1 deficiency results in suppressed neuroinflammation. Using human monocyte-derived microglia-like cellular disease model and primary microglia of CYP7B1 KO mice, we also found that activation of microglia of CYP7B1 deficiency was impaired. These cumulative results suggest that CYP7B1 can regulate neuroinflammation, thus providing potential new targets for therapeutic intervention.

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

Pin1 maintains the effector program of pathogenic Th17 cells in autoimmune neuroinflammation

Th17 cells mediated immune response is the basis of a variety of autoimmune diseases, including multiple sclerosis and its mouse model of immune aspects, experimental autoimmune encephalomyelitis (EAE). The gene network that drives both the development of Th17 and the expression of its effector program is dependent on the transcription factor RORγt. In this report, we showed that Peptidylprolyl Cis/Trans Isomerase, NIMA-Interacting 1 (Pin1) formed a complex with RORγt, and enhanced its transactivation activity, thus sustained the expression of the effector genes as well as RORγt in the EAE-pathogenic Th17 cells. We first found out that PIN1 was highly expressed in the samples from patients of multiple sclerosis, and the expression of Pin1 by the infiltrating lymphocytes in the central nerve system of EAE mice was elevated as well. An array of experiments with transgenic mouse models, cellular and molecular assays was included in the study to elucidate the role of Pin1 in the pathology of EAE. It turned out that Pin1 promoted the activation and maintained the effector program of EAE-pathogenic Th17 cells in the inflammation foci, but had little effect on the priming of Th17 cells in the draining lymph nodes. Mechanistically, Pin1 stabilized the phosphorylation of STAT3 induced by proinflammatory stimuli, and interacted with STAT3 in the nucleus of Th17 cells, which resulted in the increased expression of Rorc. Moreover, Pin1 formed a complex with RORγt, and enhanced the transactivation of RORγt to the +11 kb enhancer of Rorc, which enforced and maintained the expression of both Rorc and the effector program of pathogenic Th17 cells in EAE. Finally, the inhibition of Pin1, by genetic knockdown or by small molecule inhibitor, deceased the population of Th17 cells and the neuroinflammation, and alleviated the symptoms of EAE. These findings suggest that Pin1 is a potential therapeutic target for MS and other autoimmune inflammatory diseases.

Myeloid cell replacement is neuroprotective in chronic experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease characterized by demyelination of the central nervous system (CNS). Autologous hematopoietic cell transplantation (HCT) shows promising benefits for relapsing-remitting MS in open-label clinical studies, but the cellular mechanisms underlying its therapeutic effects remain unclear. Using single-nucleus RNA sequencing, we identify a reactive myeloid cell state in chronic experimental autoimmune encephalitis (EAE) associated with neuroprotection and immune suppression. HCT in EAE mice results in an increase of the neuroprotective myeloid state, improvement of neurological deficits, reduced number of demyelinated lesions, decreased number of effector T cells and amelioration of reactive astrogliosis. Enhancing myeloid cell incorporation after a modified HCT further improved these neuroprotective effects. These data suggest that myeloid cell manipulation or replacement may be an effective therapeutic strategy for chronic inflammatory conditions of the CNS.

Regulatory T cells limit age-associated retinal inflammation and neurodegeneration

BACKGROUND

Ageing is the principal risk factor for retinal degenerative diseases, which are the commonest cause of blindness in the developed countries. These conditions include age-related macular degeneration or diabetic retinopathy. Regulatory T cells play a vital role in immunoregulation of the nervous system by limiting inflammation and tissue damage in health and disease. Because the retina was long-considered an immunoprivileged site, the precise contribution of regulatory T cells in retinal homeostasis and in age-related retinal diseases remains unknown.

METHODS

Regulatory T cells were selectively depleted in both young (2-4 months) and aged (18-23 months) FoxP3-DTR mice. We evaluated neuroretinal degeneration, gliosis, subretinal space phagocyte infiltration, and retinal pigmented epithelium morphology through immunofluorescence analysis. Subsequently, aged Treg depleted animals underwent adoptive transfer of both young and aged regulatory T cells from wild-type mice, and the resulting impact on neurodegeneration was assessed. Statistical analyses employed included the U-Mann Whitney test, and for comparisons involving more than two groups, 1-way ANOVA analysis followed by Bonferroni's post hoc test.

RESULTS

Our study shows that regulatory T cell elimination leads to retinal pigment epithelium cell dysmorphology and accumulation of phagocytes in the subretinal space of young and aged mice. However, only aged mice experience retinal neurodegeneration and gliosis. Surprisingly, adoptive transfer of young but not aged regulatory T cells reverse these changes.

CONCLUSION

Our findings demonstrate an essential role for regulatory T cells in maintaining age retinal homeostasis and preventing age-related neurodegeneration. This previously undescribed role of regulatory T cells in limiting retinal inflammation, RPE/choroid epithelium damage and subsequently photoreceptor loss with age, opens novel avenues to explore regulatory T cell neuroprotective and anti-inflammatory properties as potential therapeutic approaches for age-related retinal diseases.

Non-invasive in vivo imaging of brain and retinal microglia in neurodegenerative diseases

Microglia play crucial roles in immune responses and contribute to fundamental biological processes within the central nervous system (CNS). In neurodegenerative diseases, microglia undergo functional changes and can have both protective and pathogenic roles. Microglia in the retina, as an extension of the CNS, have also been shown to be affected in many neurological diseases. While our understanding of how microglia contribute to pathological conditions is incomplete, non-invasive in vivo imaging of brain and retinal microglia in living subjects could provide valuable insights into their role in the neurodegenerative diseases and open new avenues for diagnostic biomarkers. This mini-review provides an overview of the current brain and retinal imaging tools for studying microglia in vivo. We focus on microglia targets, the advantages and limitations of in vivo microglia imaging approaches, and applications for evaluating the pathogenesis of neurological conditions, such as Alzheimer's disease and multiple sclerosis.

CD4 T-cell aging exacerbates neuroinflammation in a late-onset mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset progressive neurodegenerative disorder characterized by the loss of upper and lower motor neurons in the brain and spinal cord. Accumulating evidence suggests that ALS is not solely a neuronal cell- or brain tissue-autonomous disease and that neuroinflammation plays a key role in disease progression. Furthermore, whereas both CD4 and CD8 T cells were observed in spinal cords of ALS patients and in mouse models of the disease, their role in the neuroinflammatory process, especially considering their functional changes with age, is not fully explored. In this study, we revealed the structure of the CD4 T-cell compartment during disease progression of early-onset SOD1G93A and late-onset SOD1G37R mouse models of ALS. We show age-related changes in the CD4 T-cell subset organization between these mutant SOD1 mouse models towards increased frequency of effector T cells in spleens of SOD1G37R mice and robust infiltration of CD4 T cells expressing activation markers and the checkpoint molecule PD1 into the spinal cord. The frequency of infiltrating CD4 T cells correlated with the frequency of infiltrating CD8 T cells which displayed a more exhausted phenotype. Moreover, RNA-Seq and immunohistochemistry analyses of spinal cords from SOD1G37R mice with early clinical symptoms demonstrated immunological trajectories reminiscent of a neurotoxic inflammatory response which involved proinflammatory T cells and antigen presentation related pathways. Overall, our findings suggest that age-related changes of the CD4 T cell landscape is indicative of a chronic inflammatory response, which aggravates the disease process and can be therapeutically targeted.

Not just Glia-Dissecting brain macrophages in the mouse

Macrophages have emerged as critical cellular components of the central nervous system (CNS), promoting development, maintenance, and immune defense of the CNS. Here we will review recent advances in our understanding of brain macrophage heterogeneity, including microglia and border-associated macrophages, focusing on the mouse. Emphasis will be given to the discussion of strengths and limitations of the experimental approaches that have led to the recent insights and hold promise to further deepen our mechanistic understanding of brain macrophages that might eventually allow to harness their activities for the management of CNS pathologies.

Translatome Profiling of Tissue-Resident Macrophages Using the RiboTag Approach

Global gene expression profiling has provided valuable insights into the specific contributions of different cell types to various physiological processes. Notably though, both bulk and single-cell transcriptomics require the prior retrieval of the cells from their tissue context to be analyzed. Isolation protocols for tissue macrophages are, however, notoriously inefficient and, moreover, prone to introduce considerable bias and artifacts. Here, we will discuss a valuable alternative, originally introduced by Amieux and colleagues. This so-called RiboTag approach allows, in combination with respective macrophage-specific Cre transgenic lines, to retrieve macrophage translatomes from crude tissue extracts. We will review our experience with this ingenious method, focusing on the study of brain macrophages, including microglia and border-associated cells. We will elaborate on the advantages of the RiboTag approach that render it a valuable complement to standard cell sorting-based profiling strategies, especially for the investigation of tissue macrophages.

Macrophage phenotypes and functions: resolving inflammation and restoring homeostasis

Inflammation must be tightly regulated to both defend against pathogens and restore tissue homeostasis. The resolution of inflammatory responses is a dynamic process orchestrated by cells of the immune system. Macrophages, tissue-resident innate immune cells, are key players in modulating inflammation. Here, we review recent work highlighting the importance of macrophages in tissue resolution and the return to homeostasis. We propose that enhancing macrophage pro-resolution functions represents a novel and widely applicable therapeutic strategy to dampen inflammation, promote repair, and restore tissue integrity and function.

Spatial transcriptomics and neurofilament light chain reveal changes in lesion patterns in murine autoimmune neuroinflammation

OBJECTIVE

Ongoing neuroaxonal damage is a major contributor to disease progression and long-term disability in multiple sclerosis. However, spatio-temporal distribution and pathophysiological mechanisms of neuroaxonal damage during acute relapses and later chronic disease stages remain poorly understood.

METHODS

Here, we applied immunohistochemistry, single-molecule array, spatial transcriptomics, and microglia/axon co-cultures to gain insight into spatio-temporal neuroaxonal damage in experimental autoimmune encephalomyelitis (EAE).

RESULTS

Association of spinal cord white matter lesions and blood-based neurofilament light (sNfL) levels revealed a distinct, stage-dependent anatomical pattern of neuroaxonal damage: in chronic EAE, sNfL levels were predominately associated with anterolateral lumbar lesions, whereas in early EAE sNfL showed no correlation with lesions in any anatomical location. Furthermore, neuroaxonal damage in late EAE was largely confined to white matter lesions but showed a widespread distribution in early EAE. Following this pattern of neuroaxonal damage, spatial transcriptomics revealed a widespread cyto- and chemokine response at early disease stages, whereas late EAE was characterized by a prominent glial cell accumulation in white matter lesions. These findings were corroborated by immunohistochemistry and microglia/axon co-cultures, which further revealed a strong association between CNS myeloid cell activation and neuroaxonal damage both in vivo and in vitro.

INTERPRETATION

Our findings indicate that CNS myeloid cells may play a crucial role in driving neuroaxonal damage in EAE. Moreover, neuroaxonal damage can progress in a stage-dependent centripetal manner, transitioning from normal-appearing white matter to focal white matter lesions. These insights may contribute to a better understanding of neurodegeneration and elevated sNfL levels observed in multiple sclerosis patients at different disease stages.

A tale of two cells: Regulatory T cell-microglia cross-talk in the ischemic brain

Regulatory T cells exert a beneficial immunomodulatory effect on poststroke neuroinflammation that is amplified by microglial cells.

How myeloid cells shape experimental autoimmune encephalomyelitis: At the crossroads of outside-in immunity

Experimental autoimmune encephalomyelitis (EAE) is an animal model of central nervous system (CNS) autoimmunity. It is most commonly used to mimic aspects of multiple sclerosis (MS), a demyelinating disorder of the human brain and spinal cord. The innate immune response displays one of the core pathophysiological features linked to both the acute and chronic stages of MS. Hence, understanding and targeting the innate immune response is essential. Microglia and other CNS resident MUs, as well as infiltrating myeloid cells, diverge substantially in terms of both their biology and their roles in EAE. Recent advances in the field show that antigen presentation, as well as disease-propagating and regulatory interactions with lymphocytes, can be attributed to specific myeloid cell types and cell states in EAE lesions, following a distinct temporal pattern during disease initiation, propagation and recovery. Furthermore, single-cell techniques enable the assessment of characteristic proinflammatory as well as beneficial cell states, and identification of potential treatment targets. Here, we discuss the principles of EAE induction and protocols for varying experimental paradigms, the composition of the myeloid compartment of the CNS during health and disease, and systematically review effects on myeloid cells for therapeutic approaches in EAE.

Foxp3+ regulatory T cells in the central nervous system and other nonlymphoid tissues

Foxp3+ regulatory T (Treg) cells are indispensable for the maintenance of immunologic self-tolerance as well as for the confinement of autoimmune inflammation after the breach of self-tolerance. In order to fulfill these tasks, Treg cells operate in secondary lymphoid tissues and nonlymphoid tissues. The conditions for Treg cell stability and for their modes of action are different according to their compartment of residence. In addition, Treg cells initiate residency programs to inhabit niches in nonlympoid tissues (NLT) in steady state and after re-establishment of previously deflected homeostasis for extended periods of time. These NLT Treg cells are different from lymphoid tissue residing Treg cells and are functionally specialized to subserve not only immune functions but support intrinsic functions of their tissue of residence. This review will highlight current ideas about the functional specialization of NLT Treg cells in particular in the central nervous system (CNS) and discuss challenges that we are facing in an effort to exploit the power of NLT Treg cells for maintenance of tissue homeostasis and perhaps also tissue regeneration.

Microglial expression of CD83 governs cellular activation and restrains neuroinflammation in experimental autoimmune encephalomyelitis

Microglial activation during neuroinflammation is crucial for coordinating the immune response against neuronal tissue, and the initial response of microglia determines the severity of neuro-inflammatory diseases. The CD83 molecule has been recently shown to modulate the activation status of dendritic cells and macrophages. Although the expression of CD83 is associated with early microglia activation in various disease settings, its functional relevance for microglial biology has been elusive. Here, we describe a thorough assessment of CD83 regulation in microglia and show that CD83 expression in murine microglia is not only associated with cellular activation but also with pro-resolving functions. Using single-cell RNA-sequencing, we reveal that conditional deletion of CD83 results in an over-activated state during neuroinflammation in the experimental autoimmune encephalomyelitis model. Subsequently, CD83-deficient microglia recruit more pathogenic immune cells to the central nervous system, deteriorating resolving mechanisms and exacerbating the disease. Thus, CD83 in murine microglia orchestrates cellular activation and, consequently, also the resolution of neuroinflammation.

Interferon-gamma ameliorates experimental autoimmune encephalomyelitis by inducing homeostatic adaptation of microglia

Compelling evidence has shown that interferon (IFN)-γ has dual effects in multiple sclerosis and in its animal model of experimental autoimmune encephalomyelitis (EAE), with results supporting both a pathogenic and beneficial function. However, the mechanisms whereby IFN-γ may promote neuroprotection in EAE and its effects on central nervous system (CNS)-resident cells have remained an enigma for more than 30 years. In this study, the impact of IFN-γ at the peak of EAE, its effects on CNS infiltrating myeloid cells (MC) and microglia (MG), and the underlying cellular and molecular mechanisms were investigated. IFN-γ administration resulted in disease amelioration and attenuation of neuroinflammation associated with significantly lower frequencies of CNS CD11b⁺ myeloid cells and less infiltration of inflammatory cells and demyelination. A significant reduction in activated MG and enhanced resting MG was determined by flow cytometry and immunohistrochemistry. Primary MC/MG cultures obtained from the spinal cord of IFN-γ-treated EAE mice that were ex vivo re-stimulated with a low dose (1 ng/ml) of IFN-γ and neuroantigen, promoted a significantly higher induction of CD4⁺ regulatory T (Treg) cells associated with increased transforming growth factor (TGF)-β secretion. Additionally, IFN-γ-treated primary MC/MG cultures produced significantly lower nitrite in response to LPS challenge than control MC/MG. IFN-γ-treated EAE mice had a significantly higher frequency of CX3CR1^high MC/MG and expressed lower levels of program death ligand 1 (PD-L1) than PBS-treated mice. Most CX3CR1^highPD-L1^lowCD11b⁺Ly6G^- cells expressed MG markers (Tmem119, Sall2, and P2ry12), indicating that they represented an enriched MG subset (CX3CR1^highPD-L1^low MG). Amelioration of clinical symptoms and induction of CX3CR1^highPD-L1^low MG by IFN-γ were dependent on STAT-1. RNA-seq analyses revealed that in vivo treatment with IFN-γ promoted the induction of homeostatic CX3CR1^highPD-L1^low MG, upregulating the expression of genes associated with tolerogenic and anti-inflammatory roles and down-regulating pro-inflammatory genes. These analyses highlight the master role that IFN-γ plays in regulating microglial activity and provide new insights into the cellular and molecular mechanisms involved in the therapeutic activity of IFN-γ in EAE.

Roles and regulation of microglia activity in multiple sclerosis: insights from animal models

As resident macrophages of the CNS, microglia are critical immune effectors of inflammatory lesions and associated neural dysfunctions. In multiple sclerosis (MS) and its animal models, chronic microglial inflammatory activity damages myelin and disrupts axonal and synaptic activity. In contrast to these detrimental effects, the potent phagocytic and tissue-remodelling capabilities of microglia support critical endogenous repair mechanisms. Although these opposing capabilities have long been appreciated, a precise understanding of their underlying molecular effectors is only beginning to emerge. Here, we review recent advances in our understanding of the roles of microglia in animal models of MS and demyelinating lesions and the mechanisms that underlie their damaging and repairing activities. We also discuss how the structured organization and regulation of the genome enables complex transcriptional heterogeneity within the microglial cell population at demyelinating lesions.

Augmentation of a neuroprotective myeloid state by hematopoietic cell transplantation

Multiple sclerosis (MS) is an autoimmune disease associated with inflammatory demyelination in the central nervous system (CNS). Autologous hematopoietic cell transplantation (HCT) is under investigation as a promising therapy for treatment-refractory MS. Here we identify a reactive myeloid state in chronic experimental autoimmune encephalitis (EAE) mice and MS patients that is surprisingly associated with neuroprotection and immune suppression. HCT in EAE mice leads to an enhancement of this myeloid state, as well as clinical improvement, reduction of demyelinated lesions, suppression of cytotoxic T cells, and amelioration of reactive astrogliosis reflected in reduced expression of EAE-associated gene signatures in oligodendrocytes and astrocytes. Further enhancement of myeloid cell incorporation into the CNS following a modified HCT protocol results in an even more consistent therapeutic effect corroborated by additional amplification of HCT-induced transcriptional changes, underlining myeloid-derived beneficial effects in the chronic phase of EAE. Replacement or manipulation of CNS myeloid cells thus represents an intriguing therapeutic direction for inflammatory demyelinating disease.

The endogenous repertoire harbors self-reactive CD4+ T cell clones that adopt a follicular helper T cell-like phenotype at steady state

The T cell repertoire of healthy mice and humans harbors self-reactive CD4+ conventional T (Tconv) cells capable of inducing autoimmunity. Using T cell receptor profiling paired with in vivo clonal analysis of T cell differentiation, we identified Tconv cell clones that are recurrently enriched in non-lymphoid organs following ablation of Foxp3+ regulatory T (Treg) cells. A subset of these clones was highly proliferative in the lymphoid organs at steady state and exhibited overt reactivity to self-ligands displayed by dendritic cells, yet were not purged by clonal deletion. These clones spontaneously adopted numerous hallmarks of follicular helper T (TFH) cells, including expression of Bcl6 and PD-1, exhibited an elevated propensity to localize within B cell follicles at steady state, and produced interferon-γ in non-lymphoid organs following sustained Treg cell depletion. Our work identifies a naturally occurring population of self-reactive TFH-like cells and delineates a previously unappreciated fate for self-specific Tconv cells.