Human IgM antibody rHIgM22 promotes phagocytic clearance of myelin debris by microglia

The interplay of inflammation and remyelination: rethinking MS treatment with a focus on oligodendrocyte progenitor cells

BACKGROUND

Multiple sclerosis (MS) therapeutic goals have traditionally been dichotomized into two distinct avenues: immune-modulatory-centric interventions and pro-regenerative strategies. Oligodendrocyte progenitor cells (OPCs) were regarded for many years solely in concern to their potential to generate oligodendrocytes and myelin in the central nervous system (CNS). However, accumulating data elucidate the multifaceted roles of OPCs, including their immunomodulatory functions, positioning them as cardinal constituents of the CNS's immune landscape.

MAIN BODY

In this review, we will discuss how the two therapeutic approaches converge. We present a model by which (1) an inflammation is required for the appropriate pro-myelinating immune function of OPCs in the chronically inflamed CNS, and (2) the immune function of OPCs is crucial for their ability to differentiate and promote remyelination. This model highlights the reciprocal interactions between OPCs' pro-myelinating and immune-modulating functions. Additionally, we review the specific effects of anti- and pro-inflammatory interventions on OPCs, suggesting that immunosuppression adversely affects OPCs' differentiation and immune functions.

CONCLUSION

We suggest a multi-systemic therapeutic approach, which necessitates not a unidimensional focus but a harmonious balance between OPCs' pro-myelinating and immune-modulatory functions.

Frequency of an intrathecal IgM synthesis and MRZ reaction in children with MS

BACKGROUND

Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the CNS. An intrathecal IgM synthesis is associated with a more rapid progression of MS and the intrathecal immune response to measles -, rubella -and varicella zoster virus (MRZR) which, if present, increases the likelihood of a diagnosis of MS in adults.

OBJECTIVE

To evaluate the frequency of an intrathecal IgM synthesis and MRZR in children with MS. MethodsChildren with MS and a data set including clinical and treatment history, MRI at onset, in addition to a CSF analysis, and determination of antibody index (AI) of measles, rubella, and zoster antibodies, were eligible. The presence of an intrathecal IgM synthesis and/or a positive MRZ reaction were compared to biomarkers of a more progressive disease course.

RESULTS

In 75 children with MS, OCBs were present in 93.3 %). 49,2 % experienced their first relapse within 6 months. 50.7 % had a total lesion load of more than 10 lesions in the first brain MRI. Spinal lesions were identified in 64 %. 23.5 % had a positive MRZR and 40.3 % an intrathecal IgM synthesis. No significant associations were detected between the presence of an intrathecal IgM synthesis and MRZR and parameters including the relapse rate in the first two years.

CONCLUSION

An intrathecal IgM synthesis and a positive MRZR are found in a subset of MS children but are not associated with markers associated with a poor prognosis.

Tackling myelin deficits in neurodevelopmental disorders using drug delivery systems

Interest in myelin and its roles in almost all brain functions has been greatly increasing in recent years, leading to countless new studies on myelination, as a dominant process in the development of cognitive functions. Here, we explore the unique role myelin plays in the central nervous system and specifically discuss the results of altered myelination in neurodevelopmental disorders. We present parallel developmental trajectories involving myelination that correlate with the onset of cognitive impairment in neurodevelopmental disorders and discuss the key challenges in the treatment of these chronic disorders. Recent developments in drug repurposing and nano/micro particle-based therapies are reviewed as a possible pathway to circumvent some of the main hurdles associated with early intervention, including patient's adherence and compliance, side effects, relapse, and faster route to possible treatment of these disorders. The strategy of drug encapsulation overcomes drug solubility and metabolism, with the possibility of drug targeting to a specific compartment, reducing side effects upon systemic administration.

Immune response of BV-2 microglial cells is impacted by peroxisomal beta-oxidation

Microglia are crucial for brain homeostasis, and dysfunction of these cells is a key driver in most neurodegenerative diseases, including peroxisomal leukodystrophies. In X-linked adrenoleukodystrophy (X-ALD), a neuroinflammatory disorder, very long-chain fatty acid (VLCFA) accumulation due to impaired degradation within peroxisomes results in microglial defects, but the underlying mechanisms remain unclear. Using CRISPR/Cas9 gene editing of key genes in peroxisomal VLCFA breakdown (Abcd1, Abcd2, and Acox1), we recently established easily accessible microglial BV-2 cell models to study the impact of dysfunctional peroxisomal β-oxidation and revealed a disease-associated microglial-like signature in these cell lines. Transcriptomic analysis suggested consequences on the immune response. To clarify how impaired lipid degradation impacts the immune function of microglia, we here used RNA-sequencing and functional assays related to the immune response to compare wild-type and mutant BV-2 cell lines under basal conditions and upon pro-inflammatory lipopolysaccharide (LPS) activation. A majority of genes encoding proinflammatory cytokines, as well as genes involved in phagocytosis, antigen presentation, and co-stimulation of T lymphocytes, were found differentially overexpressed. The transcriptomic alterations were reflected by altered phagocytic capacity, inflammasome activation, increased release of inflammatory cytokines, including TNF, and upregulated response of T lymphocytes primed by mutant BV-2 cells presenting peptides. Together, the present study shows that peroxisomal β-oxidation defects resulting in lipid alterations, including VLCFA accumulation, directly reprogram the main cellular functions of microglia. The elucidation of this link between lipid metabolism and the immune response of microglia will help to better understand the pathogenesis of peroxisomal leukodystrophies.

The roles of microglia and astrocytes in myelin phagocytosis in the central nervous system

Myelination is an important process in the central nervous system (CNS). Oligodendrocytes (OLs) extend multiple layers to densely sheath on axons, composing the myelin to achieve efficient electrical signal conduction. The myelination during developmental stage maintains a balanced state. However, numerous CNS diseases including neurodegenerative and cerebrovascular diseases cause demyelination and disrupt the homeostasis, resulting in inflammation and white matter deficits. Effective clearance of myelin debris is needed in the region of demyelination, which is a key step for remyelination and tissue regeneration. Microglia and astrocytes are the major resident phagocytic cells in the brain, which may play different or collaborative roles in myelination. Microglia and astrocytes participate in developmental myelination through engulfing excessive unneeded myelin. They are also involved in the clearance of degenerated myelin debris for accelerating remyelination, or engulfing healthy myelin sheath for inhibiting remyelination. This review focuses on the roles of microglia and astrocytes in phagocytosing myelin in the developmental brain and diseased brain. In addition, the interaction between microglia and astrocytes to mediate myelin engulfment is also summarized.

Identification of the Lipid Antigens Recognized by rHIgM22, a Remyelination-Promoting Antibody

Failure of the immune system to discriminate myelin components from foreign antigens plays a critical role in the pathophysiology of multiple sclerosis. In fact, the appearance of anti-myelin autoantibodies, targeting both proteins and glycolipids, is often responsible for functional alterations in myelin-producing cells in this disease. Nevertheless, some of these antibodies were reported to be beneficial for remyelination. Recombinant human IgM22 (rHIgM22) binds to myelin and to the surface of O4-positive oligodendrocytes, and promotes remyelination in mouse models of chronic demyelination. Interestingly, the identity of the antigen recognized by this antibody remains to be elucidated. The preferential binding of rHIgM22 to sulfatide-positive cells or tissues suggests that sulfatide might be part of the antigen pattern recognized by the antibody, however, cell populations lacking sulfatide expression are also responsive to rHIgM22. Thus, we assessed the binding of rHIgM22 in vitro to purified lipids and lipid extracts from various sources to identify the antigen(s) recognized by this antibody. Our results show that rHIgM22 is indeed able to bind both sulfatide and its deacylated form, whereas no significant binding for other myelin sphingolipids has been detected. Remarkably, binding of rHIgM22 to sulfatide in lipid monolayers can be positively or negatively regulated by the presence of other lipids. Moreover, rHIgM22 also binds to phosphatidylinositol, phosphatidylserine and phosphatidic acid, suggesting that not only sulfatide, but also other membrane lipids might play a role in the binding of rHIgM22 to oligodendrocytes and to other cell types not expressing sulfatide.

Remyelinating strategies: What can be learned from normal brain development

Multiple sclerosis (MS) is a neuroinflammatory demyelinating and neurodegenerative disease of the central nervous system (CNS). Immunomodulatory therapies are effective in reducing relapses, however, there is no remedy for progressive disease emphasizing the need for regenerative strategies. Chronic demyelination causes axonal injury and loss which is a key component of neurodegeneration and permanent disability in MS. New oligodendrocyte progenitor cells (OPCs) proliferate in response to inflammatory demyelination representing the potential for remyelination to protect axons and preserve neuronal function. The majority of remyelinating therapies have targeted intrinsic signaling processes in oligodendrocytes to promote differentiation or utilized methods for transplantation of oligodendrocytes. Here, we discuss specific roles of microglia in contributing to normal myelin development and the significance of these functions for remyelinating strategies.

Rheumatoid factor IgM autoantibodies control IgG homeostasis

Rheumatoid arthritis is an autoimmune disease characterized by joint inflammation due to autoantibodies targeting multiple self-proteins. Most patients with poor prognosis show elevated titers of IgM antibodies specifically binding to IgG. Such autoreactive antibodies are referred to as rheumatoid factor (RF). However, their biological function and contribution to disease progression remains elusive. We have recently shown that autoreactive antibodies are present in healthy individuals and play an important role in regulating physiological processes. This regulatory mechanism is determined by the class and affinity of the autoreactive antibody, as low-affinity autoreactive IgM neutralizes the recognized autoantigen while high-affinity IgM protects its autoantigen from degradation. Here, we show that RFs possessing a high affinity and mono-specificity to IgG have a stabilizing effect on IgG, whereas low-affinity polyreactive RFs neutralize IgG in vivo. These results suggest that autoreactive IgM antibodies recognizing IgG play a crucial role in regulating IgG homeostasis and that a disbalance between IgM-mediated IgG degradation and stabilization might affect the onset and progression of autoimmune diseases. Consequently, restoring this balance using low-affinity anti-IgG IgM might be a promising therapeutic approach for autoimmune diseases involving autoreactive IgG.

Optic Neuritis in Multiple Sclerosis—A Review of Molecular Mechanisms Involved in the Degenerative Process

Multiple sclerosis is a central nervous system inflammatory demyelinating disease with a wide range of clinical symptoms, ocular involvement being frequently marked by the presence of optic neuritis (ON). The emergence and progression of ON in multiple sclerosis is based on various pathophysiological mechanisms, disease progression being secondary to inflammation, demyelination, or axonal degeneration. Early identification of changes associated with axonal degeneration or further investigation of the molecular processes underlying remyelination are current concerns of researchers in the field in view of the associated therapeutic potential. This article aims to review and summarize the scientific literature related to the main molecular mechanisms involved in defining ON as well as to analyze existing data in the literature on remyelination strategies in ON and their impact on long-term prognosis.

A double-blind, placebo-controlled, single-ascending-dose intravenous infusion study of rHIgM22 in subjects with multiple sclerosis immediately following a relapse

Background

Recombinant human immunoglobulin M22 (rHIgM22) has promoted remyelination in animal models and was well tolerated in people with clinically stable multiple sclerosis.

Objective

Safety/tolerability of a single rHIgM22 dose was investigated following an acute relapse and to determine whether this enhanced CNS/CSF concentrations.

Methods

Adults (N = 27) with acute relapse were assigned to rHIgM22 (0.5 or 2.0 mg/kg) or placebo. Study included screening/steroid administration periods and 10 study visits over 6 months. rHIgM22 CSF concentrations were assessed on days 2 and 29. Pharmacokinetic and safety samples were taken for up to 60 days. Assessments included adverse events and other clinical measures. Brain magnetic resonance imaging was performed with/without gadolinium.

Results

rHIgM22 CSF levels were consistent with dose-dependent concentration on both days 2 and 29. Infusion was generally well tolerated during an acute relapse. Immunogenicity was mild. Most adverse events did not appear to be dose dependent, were mild/moderate, and were events often associated with multiple sclerosis.

Conclusion

Although limited by high variability and small sample size, the data suggest enhanced CNS uptake associated with a drop in CSF levels. This study demonstrated safety of an antibody directed to myelin and oligodendrocytes in the course of active demyelinating disease. Further research into rHIgM22 is warranted.

ClinicalTrials.gov: NCT02398461 https://clinicaltrials.gov/ct2/show/study/NCT02398461?term=M22&draw=2&rank=8

Breaking the barriers to remyelination in multiple sclerosis

Chronically demyelinated axons are rendered susceptible to degeneration through loss of trophic support from oligodendrocytes and myelin, and this process underlies disability progression in multiple sclerosis. Promoting remyelination is a promising neuroprotective therapeutic strategy, but to date, has not been achieved through simply promoting oligodendrocyte precursor cell differentiation, and it is clear that a detailed understanding of the molecular mechanisms underlying failed remyelination is required to guide future therapeutic approaches. In multiple sclerosis, remyelination is impaired by extrinsic inhibitory cues in the lesion microenvironment including secreted effector molecules released from compartmentalized immune cells and reactive glia, as well as by intrinsic defects in oligodendrocyte lineage cells, most notably increased metabolic demands causing oxidative stress and accelerated cellular senescence. Promising advances in our understanding of the cellular and molecular mechanisms underlying these processes offers hope for strategically designed interventions to facilitate remyelination thereby resulting in robust clinical benefits.

The Forgotten Brother: The Innate-like B1 Cell in Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease of the central nervous system (CNS), traditionally considered a chronic autoimmune attack against the insulating myelin sheaths around axons. However, the exact etiology has not been identified and is likely multi-factorial. Recently, evidence has been accumulating that implies that autoimmune processes underlying MS may, in fact, be triggered by pathological processes initiated within the CNS. This review focuses on a relatively unexplored immune cell-the "innate-like" B1 lymphocyte. The B1 cell is a primary-natural-antibody- and anti-inflammatory-cytokine-producing cell present in the healthy brain. It has been recently shown that its frequency and function may differ between MS patients and healthy controls, but its exact involvement in the MS pathogenic process remains obscure. In this review, we propose that this enigmatic cell may play a more prominent role in MS pathology than ever imagined. We aim to shed light on the human B1 cell in health and disease, and how dysregulation in its delicate homeostatic role could impact MS. Furthermore, novel therapeutic avenues to restore B1 cells' beneficial functions will be proposed.

Autoreactive antibodies control blood glucose by regulating insulin homeostasis

The random nature of antibody repertoire generation includes the potential of producing autoantibodies recognizing self-structures. It is believed that establishing immunological tolerance and prevention of autoimmune diseases require the removal of antibody specificities recognizing self. Using insulin as a common and physiologically important autoantigen, we show that anti-insulin antibodies associated with autoimmune diabetes can readily be detected in mice and humans and are involved in the physiological regulation of blood glucose levels. Importantly, human high-affinity, anti-insulin IgM antibodies protect insulin from autoimmune degradation by anti-insulin IgG antibodies. Thus, in contrast to the proposed negative selection, self-recognition and the production of highly autoreactive IgM antibodies are important for tolerance induction.

Homeostasis of metabolism by hormone production is crucial for maintaining physiological integrity, as disbalance can cause severe metabolic disorders such as diabetes mellitus. Here, we show that antibody-deficient mice and immunodeficiency patients have subphysiological blood glucose concentrations. Restoring blood glucose physiology required total IgG injections and insulin-specific IgG antibodies detected in total IgG preparations and in the serum of healthy individuals. In addition to the insulin-neutralizing anti-insulin IgG, we identified two fractions of anti-insulin IgM in the serum of healthy individuals. These autoreactive IgM fractions differ in their affinity to insulin. Interestingly, the low-affinity IgM fraction (anti-insulin IgM^low) neutralizes insulin and leads to increased blood glucose, whereas the high-affinity IgM fraction (anti-insulin IgM^high) protects insulin from neutralization by anti-insulin IgG, thereby preventing blood glucose dysregulation. To demonstrate that anti-insulin IgM^high acts as a protector of insulin and counteracts insulin neutralization by anti-insulin IgG, we expressed the variable regions of a high-affinity anti-insulin antibody as IgG and IgM. Remarkably, the recombinant anti-insulin IgM^high normalized insulin function and prevented IgG-mediated insulin neutralization. These results suggest that autoreactive antibodies recognizing insulin are key regulators of blood glucose and metabolism, as they control the concentration of insulin in the blood. Moreover, our data suggest that preventing autoimmune damage and maintaining physiological homeostasis requires adaptive tolerance mechanisms generating high-affinity autoreactive IgM antibodies during memory responses.

The Role of Distinct Subsets of Macrophages in the Pathogenesis of MS and the Impact of Different Therapeutic Agents on These Populations

Multiple sclerosis (MS) is a demyelinating inflammatory disorder of the central nervous system (CNS). Besides the vital role of T cells, other immune cells, including B cells, innate immune cells, and macrophages (MФs), also play a critical role in MS pathogenesis. Tissue-resident MФs in the brain’s parenchyma, known as microglia and monocyte-derived MФs, enter into the CNS following alterations in CNS homeostasis that induce inflammatory responses in MS. Although the neuroprotective and anti-inflammatory actions of monocyte-derived MФs and resident MФs are required to maintain CNS tolerance, they can release inflammatory cytokines and reactivate primed T cells during neuroinflammation. In the CNS of MS patients, elevated myeloid cells and activated MФs have been found and associated with demyelination and axonal loss. Thus, according to the role of MФs in neuroinflammation, they have attracted attention as a therapeutic target. Also, due to their different origin, location, and turnover, other strategies may require to target the various myeloid cell populations. Here we review the role of distinct subsets of MФs in the pathogenesis of MS and different therapeutic agents that target these cells.

Myelin Repair: From Animal Models to Humans

It is widely thought that brain repair does not occur, but myelin regeneration provides clear evidence to the contrary. Spontaneous remyelination may occur after injury or in multiple sclerosis (MS). However, the efficiency of remyelination varies considerably between MS patients and between the lesions of each patient. Myelin repair is essential for optimal functional recovery, so a profound understanding of the cells and mechanisms involved in this process is required for the development of new therapeutic strategies. In this review, we describe how animal models and modern cell tracing and imaging methods have helped to identify the cell types involved in myelin regeneration. In addition to the oligodendrocyte progenitor cells identified in the 1990s as the principal source of remyelinating cells in the central nervous system (CNS), other cell populations, including subventricular zone-derived neural progenitors, Schwann cells, and even spared mature oligodendrocytes, have more recently emerged as potential contributors to CNS remyelination. We will also highlight the conditions known to limit endogenous repair, such as aging, chronic inflammation, and the production of extracellular matrix proteins, and the role of astrocytes and microglia in these processes. Finally, we will present the discrepancies between observations in humans and in rodents, discussing the relationship of findings in experimental models to myelin repair in humans. These considerations are particularly important from a therapeutic standpoint.

Tissues: the unexplored frontier of antibody mediated immunity

Pathogen-specific immunity evolves in the context of the infected tissue. However, current immune correlates analyses and vaccine efficacy metrics are based on immune functions from peripheral cells. Less is known about tissue-resident mechanisms of immunity. While antibodies represent the primary correlate of immunity following most clinically approved vaccines, how antibodies interact with localized, compartment-specific immune functions to fight infections, remains unclear. Emerging data demonstrate a unique community of immune cells that reside within different tissues. These tissue-specific immunological communities enable antibodies to direct both expected and unexpected local attack strategies to control, disrupt, and eliminate infection in a tissue-specific manner. Defining the full breadth of antibody effector functions, how they selectively contribute to control at the site of infection may provide clues for the design of next-generation vaccines able to direct the control, elimination, and prevention of compartment specific diseases of both infectious and non-infectious etiologies.

Remyelination-Promoting DNA Aptamer Conjugate Myaptavin-3064 Binds to Adult Oligodendrocytes In Vitro

We previously applied Systematic Evolution of Ligands by EXponential enrichment (SELEX) technology to identify myelin-specific DNA aptamers, using crude mouse central nervous system myelin as bait. This selection identified a 40-nucleotide aptamer (LJM-3064). Multiple biotinylated LJM-3064 molecules were conjugated to a streptavidin core to mimic a multimeric immunoglobulin M (IgM) antibody, generating 3064-BS-streptavidin (Myaptavin-3064). We previously showed that Myaptavin-3064 induces remyelination in the Theiler’s murine encephalomyelitis virus (TMEV) model of chronic spinal cord demyelination. While details of target binding and the mechanism of action remain unclear, we hypothesized that Myaptavin-3064 induces remyelination by binding to oligodendrocytes (OLs). We now report the results of binding assays using the human oligodendroglioma (HOG) cell line, applying both flow cytometry and immunocytochemistry (IC) to assay aptamer conjugate binding to cells. IC assays were applied to compare aptamer conjugate binding to primary embryonic mouse mixed cortical cultures and primary adult rat mixed glial cultures. We show that Myaptavin-3064 binds to HOG cells, with increased binding upon differentiation. In contrast, a negative control aptamer conjugate, 3060-BS, which did not promote central nervous system (CNS) remyelination, does not bind to HOG cells. Myaptavin-3064 did not bind to lung (L2) or kidney (BHK) cell lines. Total internal reflection fluorescence (TIRF) imaging indicates that Myaptavin-3064 binds at the cell membrane of live cells. In addition to HOG cells, Myaptavin-3064 binds to adult rat OLs, but not to embryonic mouse mixed cortical cultures. These data support the hypothesis that Myaptavin-3064 binds to a surface molecule on both rodent and human OLs in a manner that triggers a remyelination signal pathway.

Microglial Phagocytosis-Rational but Challenging Therapeutic Target in Multiple Sclerosis

Multiple sclerosis (MS) is the most common autoimmune and demyelinating disease of the central nervous system (CNS), characterized, in the majority of cases, by initial relapses that later evolve into progressive neurodegeneration, severely impacting patients’ motor and cognitive functions. Despite the availability of immunomodulatory therapies effective to reduce relapse rate and slow disease progression, they all failed to restore CNS myelin that is necessary for MS full recovery. Microglia are the primary inflammatory cells present in MS lesions, therefore strongly contributing to demyelination and lesion extension. Thus, many microglial-based therapeutic strategies have been focused on the suppression of microglial pro-inflammatory phenotype and neurodegenerative state to reduce disease severity. On the other hand, the contribution of myelin phagocytosis advocating the neuroprotective role of microglia in MS has been less explored. Indeed, despite the presence of functional oligodendrocyte precursor cells (OPCs), within lesioned areas, MS plaques fail to remyelinate as a result of the over-accumulation of myelin-toxic debris that must be cleared away by microglia. Dysregulation of this process has been associated with the impaired neuronal recovery and deficient remyelination. In line with this, here we provide a comprehensive review of microglial myelin phagocytosis and its involvement in MS development and repair. Alongside, we discuss the potential of phagocytic-mediated therapeutic approaches and encourage their modulation as a novel and rational approach to ameliorate MS-associated pathology.

Astrocytes and Microglia as Major Players of Myelin Production in Normal and Pathological Conditions

Myelination is an essential process that consists of the ensheathment of axons by myelin. In the central nervous system (CNS), myelin is synthesized by oligodendrocytes. The proliferation, migration, and differentiation of oligodendrocyte precursor cells constitute a prerequisite before mature oligodendrocytes extend their processes around the axons and progressively generate a multilamellar lipidic sheath. Although myelination is predominately driven by oligodendrocytes, the other glial cells including astrocytes and microglia, also contribute to this process. The present review is an update of the most recent emerging mechanisms involving astrocyte and microglia in myelin production. The contribution of these cells will be first described during developmental myelination that occurs in the early postnatal period and is critical for the proper development of cognition and behavior. Then, we will report the novel findings regarding the beneficial or deleterious effects of astroglia and microglia, which respectively promote or impair the endogenous capacity of oligodendrocyte progenitor cells (OPCs) to induce spontaneous remyelination after myelin loss. Acute delineation of astrocyte and microglia activities and cross-talk should uncover the way towards novel therapeutic perspectives aimed at recovering proper myelination during development or at breaking down the barriers impeding the regeneration of the damaged myelin that occurs in CNS demyelinating diseases.

The pro-remyelination properties of microglia in the central nervous system

Microglia are resident macrophages of the CNS that are involved in its development, homeostasis and response to infection and damage. Microglial activation is a common feature of neurological disorders, and although in some instances this activation can be damaging, protective and regenerative functions of microglia have been revealed. The most prominent example of the regenerative functions is a role for microglia in supporting regeneration of myelin after injury, a process that is critical for axonal health and relevant to numerous disorders in which loss of myelin integrity is a prevalent feature, such as multiple sclerosis, Alzheimer disease and motor neuron disease. Although drugs that are intended to promote remyelination are entering clinical trials, the mechanisms by which remyelination is controlled and how microglia are involved are not completely understood. In this Review, we discuss work that has identified novel regulators of microglial activation - including molecular drivers, population heterogeneity and turnover - that might influence their pro-remyelination capacity. We also discuss therapeutic targeting of microglia as a potential approach to promoting remyelination.

Activated human astrocyte-derived extracellular vesicles modulate neuronal uptake, differentiation and firing

Astrocytes in the central nervous system (CNS) provide supportive neural functions and mediate inflammatory responses from microglia. Increasing evidence supports their critical roles in regulating brain homoeostasis in response to pro-inflammatory factors such as cytokines and pathogen/damage-associated molecular pattern molecules in infectious and neurodegenerative diseases. However, the underlying mechanisms of the trans-cellular communication are still unclear. Extracellular vesicles (EVs) can transfer a large diversity of molecules such as lipids, nucleic acids and proteins for cellular communications. The purpose of this study is to characterize the EVs cargo proteins derived from human primary astrocytes (ADEVs) under both physiological and pathophysiological conditions. ADEVs were isolated from human primary astrocytes after vehicle (CTL) or interleukin-1β (IL-1β) pre-treatment. Label-free quantitative proteomic profiling revealed a notable up-regulation of proteins including actin-associated molecules, integrins and major histocompatibility complex in IL-1β-ADEVs compared to CTL-ADEVs, which were involved in cellular metabolism and organization, cellular communication and inflammatory response. When fluorescently labelled ADEVs were added into primary cultured mouse cortical neurons, we found a significantly increased neuronal uptake of IL-1β-ADEVs compared to CTL-ADEVs. We further confirmed it is likely due to the enrichment of surface proteins in IL-1β-ADEVs, as IL-1β-ADEVs uptake by neurons was partially suppressed by a specific integrin inhibitor. Additionally, treatment of neurons with IL-1β-ADEVs also reduced neurite outgrowth, branching and neuronal firing. These findings provide insight for the molecular mechanism of the ADEVs' effects on neural uptake, neural differentiation and maturation, and its alteration in inflammatory conditions.

Drug delivery to macrophages: A review of targeting drugs and drug carriers to macrophages for inflammatory diseases

Macrophages play a key role in defending against foreign pathogens, healing wounds, and regulating tissue homeostasis. Driving this versatility is their phenotypic plasticity, which enables macrophages to respond to subtle cues in tightly coordinated ways. However, when this coordination is disrupted, macrophages can aid the progression of numerous diseases, including cancer, cardiovascular disease, and autoimmune disease. The central link between these disorders is aberrant macrophage polarization, which misguides their functional programs, secretory products, and regulation of the surrounding tissue microenvironment. As a result of their important and deterministic roles in both health and disease, macrophages have gained considerable attention as targets for drug delivery. Here, we discuss the role of macrophages in the initiation and progression of various inflammatory diseases, summarize the leading drugs used to regulate macrophages, and review drug delivery systems designed to target macrophages. We emphasize strategies that are approved for clinical use or are poised for clinical investigation. Finally, we provide a prospectus of the future of macrophage-targeted drug delivery systems.

Sphingosine 1-Phosphate Receptors and Metabolic Enzymes as Druggable Targets for Brain Diseases

The central nervous system is characterized by a high content of sphingolipids and by a high diversity in terms of different structures. Stage- and cell-specific sphingolipid metabolism and expression are crucial for brain development and maintenance toward adult age. On the other hand, deep dysregulation of sphingolipid metabolism, leading to altered sphingolipid pattern, is associated with the majority of neurological and neurodegenerative diseases, even those totally lacking a common etiological background. Thus, sphingolipid metabolism has always been regarded as a promising pharmacological target for the treatment of brain disorders. However, any therapeutic hypothesis applied to complex amphipathic sphingolipids, components of cellular membranes, has so far failed probably because of the high regional complexity and specificity of the different biological roles of these structures. Simpler sphingosine-based lipids, including ceramide and sphingosine 1-phosphate, are important regulators of brain homeostasis, and, thanks to the relative simplicity of their metabolic network, they seem a feasible druggable target for the treatment of brain diseases. The enzymes involved in the control of the levels of bioactive sphingoids, as well as the receptors engaged by these molecules, have increasingly allured pharmacologists and clinicians, and eventually fingolimod, a functional antagonist of sphingosine 1-phosphate receptors with immunomodulatory properties, was approved for the therapy of relapsing-remitting multiple sclerosis. Considering the importance of neuroinflammation in many other brain diseases, we would expect an extension of the use of such analogs for the treatment of other ailments in the future. Nevertheless, many aspects other than neuroinflammation are regulated by bioactive sphingoids in healthy brain and dysregulated in brain disease. In this review, we are addressing the multifaceted possibility to address the metabolism and biology of bioactive sphingosine 1-phosphate as novel targets for the development of therapeutic paradigms and the discovery of new drugs.

Optimization of a 40-mer Antimyelin DNA Aptamer Identifies a 20-mer with Enhanced Properties for Potential Multiple Sclerosis Therapy

We previously reported the in vitro selection and characterization of a DNA aptamer capable of stimulating remyelination in a mouse model of multiple sclerosis. This aptamer was selected for its ability to bind to suspensions of crude murine myelin in vitro. Our initial studies in vitro and in vivo involved a 40-nucleotide derivative (LJM-3064) of the original 100-nucleotide aptamer. LJM-3064 retained robust myelin-binding properties. Structural characterization of LJM-3064 revealed that the guanosine-rich 5' half of the sequence forms different G-quadruplex-type structures that are variably stable in the presence of physiologically relevant ions. We hypothesized that this structured domain is sufficient for myelin binding. In this study, we confirm that a 20-nucleotide DNA, corresponding to the 5' half of LJM-3064, retains myelin-binding properties. We then optimize this minimal myelin-binding aptamer via systematic evolution of ligands by exponential enrichment after sparse rerandomization. We report a sequence variant (LJM-5708) of the 20-nucleotide myelin-binding aptamer with enhanced myelin-binding properties and the ability to bind cultured human oligodendroglioma cells in vitro, providing the first evidence of cross-species reactivity of this myelin-binding aptamer. As our formulation of DNA aptamers for in vivo remyelination therapy involves conjugation to streptavidin, we verified that the myelin-binding properties of LJM-5708 were retained in conjugates to avidin, streptavidin, and neutravidin. DNA aptamer LJM-5708 is a lead for further preclinical development of remyelinating aptamer technologies.

Glial cells as therapeutic targets in progressive multiple sclerosis

Introduction: Multiple sclerosis is a serious demyelinating disease of the central nervous system (CNS) with treatments generally restricted to immunosuppression to reduce attack rate and for symptom management. Glial cells may be useful targets for future CNS regenerative therapies to reverse disease. Areas covered: In this review, the authors cover currently available multiple sclerosis treatments and examine potential upcoming therapies targeting glial cells. The potential for new therapeutic approaches in the treatment of progressive multiple sclerosis is examined. Expert opinion: Microglia, astrocytes, and oligodendrocytes are each promising targets for the disease-altering treatment of multiple sclerosis. Though challenging, the opportunities presented have great potential for CNS regeneration and further investigation of glial cells in therapy is warranted. Patient-specific combinatorial therapy targeting the three glial cell types is expected to be the future of MS treatment.

IgM Natural Autoantibodies in Physiology and the Treatment of Disease

Antibodies are vital components of the adaptive immune system for the recognition and response to foreign antigens. However, some antibodies recognize self-antigens in healthy individuals. These autoreactive antibodies may modulate innate immune functions. IgM natural autoantibodies (IgM-NAAs) are a class of primarily polyreactive immunoglobulins encoded by germline V-gene segments which exhibit low affinity but broad specificity to both foreign and self-antigens. Historically, these autoantibodies were closely associated with autoimmune disease. Nevertheless, not all human autoantibodies are pathogenic and compelling evidence indicates that IgM-NAAs may exert a spectrum of effects from injurious to protective depending upon cellular and molecular context. In this chapter, we review the current state of knowledge regarding the potential physiological and therapeutic roles of IgM-NAAs in different disease conditions such as atherosclerosis, cancer, and autoimmune disease. We also describe the discovery of two reparative IgM-NAAs by our laboratory and delineate their proposed mechanisms of action in central nervous system (CNS) disease.

Human Remyelination Promoting Antibody Stimulates Astrocytes Proliferation Through Modulation of the Sphingolipid Rheostat in Primary Rat Mixed Glial Cultures

Remyelination promoting human IgMs effectively increase the number of myelinated axons in animal models of multiple sclerosis. Hence, they ultimately stimulate myelin production by oligodendrocytes (OLs); however, their exact mechanism of action remains to be elucidated, and in particular, it remains unclear whether they are directly targeting OLs, or their action is mediated by effects on other cell types. We assessed the effect of remyelination promoting antibody rHIgM22 on the proliferative response and on the ceramide/sphingosine 1-phosphate rheostat in mixed glial cell cultures (MGCs). rHIgM22 treatment caused a time-dependent increase in PDGFαR protein in MGCs. Forty-eight hours of treatment with rHIgM22 induced a dose-dependent proliferative response (evaluated as total cell number and as EdU(+) cell number) in MGCs. When the proliferation response of MGCs to rHIgM22 was analyzed as a function of the cell types, the most significant proliferative response was associated with GLAST(+) cells, i.e., astrocytes. In many cell types, the balance between different sphingolipid mediators (the "sphingolipid rheostat"), in particular ceramide and sphingosine 1-phosphate, is critical in determining the cell fate. rHIgM22 treatment in MGCs induced a moderate but significant inhibition of total acidic sphingomyelinase activity (measured in vitro on cell lysates), the main enzyme responsible for the stimulus-mediated production of ceramide, when treatment was performed in serum containing medium, but no significant differences were observed when antibody treatment was performed in the absence of serum. Moreover, rHIgM22 treatment, either in the presence or in absence of serum, had no effects on ceramide levels. On the other hand, rHIgM22 treatment for 24 h induced increased production and release of sphingosine 1-phosphate in the extracellular milieu of MGC. Release of sphingosine 1-phosphate upon rHIgM22 treatment was strongly reduced by a selective inhibitor of PDGFαR. Increased sphingosine 1-phosphate production does not seem to be mediated by regulation of the biosynthetic enzymes, sphingosine kinase 1 and 2, since protein levels of these enzymes and phosphorylation of sphingosine kinase 1 were unchanged upon rHIgM22 treatment. Instead, we observed a significant reduction in the levels of sphingosine 1-phosphate lyase 1, one of the key catabolic enzymes. Remarkably, rHIgM22 treatment under the same experimental conditions did not induce changes in the production and/or release of sphingosine 1-phosphate in pure astrocyte cultures. Taken together, these data suggest that rHIgM22 indirectly influences the proliferation of astrocytes in MGCs, by affecting the ceramide/sphingosine 1-phosphate balance. The specific cell population directly targeted by rHIgM22 remains to be identified, however our study unveils another aspect of the complexity of rHIgM22-induced remyelinating effect.