Myeloid cell iron uptake pathways and paramagnetic rim formation in multiple sclerosis

Recent developments in multiple sclerosis neuropathology

PURPOSE OF REVIEW

Neuropathological studies in human brain tissue are indispensable for our understanding of disease mechanisms in multiple sclerosis (MS). They inform concepts of lesion evolution, tissue regeneration and disease progression, and ideally reveal new disease mechanisms and therapeutic targets. Here we review recent neuropathological studies that have advanced our knowledge of MS pathogenesis.

RECENT FINDINGS

Recent cohort studies support the notion that different clinical MS disease phenotypes share underlying pathological features, and that clinical and pathological heterogeneity is derived from a variable combination of innate and adaptive inflammation, demyelinating activity, and neuroaxonal loss. Importantly, emerging technologies for spatial transcriptome analysis enable an unprecedented glimpse into the cellular composition and molecular mechanisms involved in lesion evolution. These promising technologies will help identify the identification of molecular hubs governing tissue damage and regeneration.

SUMMARY

Recent neuropathological studies helped to identify tissue correlates of disability and disease progression. Substantial progress in molecular brain tissue analysis revealed the complexity of MS-related tissue features. Close collaboration between tissue-based, molecular, bioinformatic, pharmacologic, imaging and clinical experts is needed to continue to advance the field, particularly for the benefit of people with progressive MS.

Paramagnetic rim lesion formation is predicted by the initial gadolinium-enhancing lesion diameter

BACKGROUND

Paramagnetic rim lesions (PRLs) are a magnetic resonance imaging (MRI) marker of compartmentalized intraparenchymal inflammation.

OBJECTIVES

The primary objective was to investigate clinical, demographic, and MRI factors that may be predictive of the future formation of PRL.

METHODS

This is a retrospective analysis of longitudinal data. Patients were included if they had ⩾1 gadolinium-enhancing lesion on any historical MRI and follow-up scan(s) ⩾6 months afterward on a standardized 3T MRI using the filtered phase component of a susceptibility-sensitive sequence ("SWAN"). Regression and machine-learning models were used to identify the predictive ability of demographic, clinical, immunological, treatment-related, and MRI predictors of PRL formation.

RESULTS

A total of 64 patients having 229 contrast-enhancing lesions (CELs) were included. Among all predictors, the diameter of the initial enhancing lesion was the most influential for determining subsequent PRL formation; every millimeter increase in diameter increased the risk of PRL formation by 44%. Other factors did not contribute additional information; the administration of steroids was not associated with any effect.

CONCLUSIONS

The long-axis diameter of a CEL is the best translational predictor of subsequent PRL formation at follow-up. This measure holds promise as a method to identify patients at high risk of chronic active lesion formation during the acute inflammatory window.

Multiple Sclerosis: Glial Cell Diversity in Time and Space

Multiple sclerosis (MS) is the most prevalent human inflammatory disease of the central nervous system with demyelination and glial scar formation as pathological hallmarks. Glial cells are key drivers of lesion progression in MS with roles in both tissue damage and repair depending on the surrounding microenvironment and the functional state of the individual glial subtype. In this review, we describe recent developments in the context of glial cell diversity in MS summarizing key findings with respect to pathological and maladaptive functions related to disease-associated glial subtypes. A particular focus is on the spatial and temporal dynamics of glial cells including subtypes of microglia, oligodendrocytes, and astrocytes. We contextualize recent high-dimensional findings suggesting that glial cells dynamically change with respect to epigenomic, transcriptomic, and metabolic features across the inflamed rim and during the progression of MS lesions. In summary, detailed knowledge of spatially restricted glial subtype functions is critical for a better understanding of MS pathology and its pathogenesis as well as the development of novel MS therapies targeting specific glial cell types.

Role of innate immune cells in multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune, inflammatory and neurodegenerative disease affecting the central nervous system (CNS). MS is associated with a complex interplay between neurodegenerative and inflammatory processes, mostly attributed to pathogenic T and B cells. However, a growing body of preclinical and clinical evidence indicates that innate immunity plays a crucial role in MS promotion and progression. Accordingly, preclinical and clinical studies targeting different innate immune cells to control MS are currently under study, highlighting the importance of innate immunity in this pathology. Here, we reviewed recent findings regarding the role played by innate immune cells in the pathogenesis of MS. Additionally, we discuss potential new treatments for MS based on targets against innate immune components.

A Window into New Insights on Progression Independent of Relapse Activity in Multiple Sclerosis: Role of Therapies and Current Perspective

In multiple sclerosis (MS), there is significant evidence indicating that both progression independent of relapse activity (PIRA) and relapse-related worsening events contribute to the accumulation of progressive disability from the onset of the disease and throughout its course. Understanding the compartmentalized pathophysiology of MS would enhance comprehension of disease progression mechanisms, overcoming the traditional distinction in phenotypes. Smoldering MS activity is thought to be maintained by a continuous interaction between the parenchymal chronic processes of neuroinflammation and neurodegeneration and the intrathecal compartment. This review provides a comprehensive and up-to-date overview of the neuropathological and immunological evidence related to the mechanisms underlying PIRA phenomena in MS, with a focus on studies investigating the impact of currently available therapies on these complex mechanisms.

Association between paramagnetic rim lesions and pulvinar iron depletion in persons with multiple sclerosis

BACKGROUND

The deep gray matter (DGM), especially the pulvinar, and the white matter surrounding chronic active lesions have demonstrated depleted iron levels, indicating a possible mechanistic link. However, no studies have investigated the potential relationship between these phenomena.

OBJECTIVES

The study aimed to determine whether PRLs were associated with pulvinar iron depletion and, if so, whether this relationship was spatially mediated.

METHODS

This retrospective analysis included 139 people with MS (pwMS) and 43 healthy controls (HCs) scanned at 3T MRI at baseline and after 5.4 ± 0.6 years. Pulvinar iron concentrations (cFe) and iron masses (mFe) were estimated from quantitative susceptibility maps and tested for associations with PRLs. A separate cohort of 96 pwMS with PRLs and propensity-matched HCs was included to evaluate peri‑plaque normal-appearing white matter (NAWM) abnormalities.

RESULTS

PRL number was associated with greater decline in pulvinar cFe (β = -0.265, p = 0.005) and mFe (β = -0.256, p = 0.006). Peri-plaque NAWM susceptibility was increased 11 mm surrounding PRLs, outside which shorter PRL-to-pulvinar distance was associated with greater decline in pulvinar cFe (β = 0.380, p = 0.005) and mFe (β = 0.348, p = 0.022).

CONCLUSIONS

Our findings support a spatially-mediated relationship between PRLs and chronic pulvinar iron depletion.

Paramagnetic rim lesions are associated with inner retinal layer thinning and progression independent of relapse activity in multiple sclerosis

BACKGROUND AND PURPOSE

Paramagnetic rim lesions (PRLs) are chronic active lesions associated with a severe disease course in multiple sclerosis (MS). This study was undertaken to investigate an association between retinal layer thinning (annualized loss of peripapillary retinal nerve fiber layer [aLpRNFL] and ganglion cell-inner plexiform layer [aLGCIPL]) and PRLs in patients with MS (pwMS).

METHODS

In this study, pwMS with brain magnetic resonance imaging and ≥2 optical coherence tomography scans were included. Cox proportional hazard regression models were performed using progression independent of relapse activity (PIRA) as the dependent variable, and aLpRNFL, aLGCIPL, or the number of PRLs as independent variables, adjusted for covariates.

RESULTS

We analyzed data from 97 pwMS (mean age = 35.2 years [SD = 9.9], 71.1% female, median disease duration = 2.3 years [interquartile range = 0.9-9.0]). The number of PRLs was associated with aLpRNFL and aLGCIPL. PIRA was observed in 18 (18.6%) pwMS, with aLpRNFL (hazard ratio [HR] = 1.44 per %/year), aLGCIPL (HR = 1.61 per %/year), and the number of PRLs (HR = 1.24 per PRL) being associated with increased risk of PIRA.

CONCLUSIONS

The number of PRLs is associated with inner retinal layer thinning and increased risk of PIRA. A combination of PRLs and retinal layer thinning could serve as a surrogate for pwMS at highest risk of disability progression.

Chronic active lesions in multiple sclerosis: classification, terminology, and clinical significance

In multiple sclerosis (MS), increasing disability is considered to occur due to persistent, chronic inflammation trapped within the central nervous system (CNS). This condition, known as smoldering neuroinflammation, is present across the clinical spectrum of MS and is currently understood to be relatively resistant to treatment with existing disease-modifying therapies. Chronic active white matter lesions represent a key component of smoldering neuroinflammation. Initially characterized in autopsy specimens, multiple approaches to visualize chronic active lesions (CALs) in vivo using advanced neuroimaging techniques and postprocessing methods are rapidly emerging. Among these in vivo imaging correlates of CALs, paramagnetic rim lesions (PRLs) are defined by the presence of a perilesional rim formed by iron-laden microglia and macrophages, whereas slowly expanding lesions are identified based on linear, concentric lesion expansion over time. In recent years, several longitudinal studies have linked the occurrence of in vivo detected CALs to a more aggressive disease course. PRLs are highly specific to MS and therefore have recently been incorporated into the MS diagnostic criteria. They also have prognostic potential as biomarkers to identify patients at risk of early and severe disease progression. These developments could significantly affect MS care and the evaluation of new treatments. This review describes the latest knowledge on CAL biology and imaging and the relevance of CALs to the natural history of MS. In addition, we outline considerations for current and future in vivo biomarkers of CALs, emphasizing the need for validation, standardization, and automation in their assessment.

Opportunities and challenges of single-cell and spatially resolved genomics methods for neuroscience discovery

Over the past decade, single-cell genomics technologies have allowed scalable profiling of cell-type-specific features, which has substantially increased our ability to study cellular diversity and transcriptional programs in heterogeneous tissues. Yet our understanding of mechanisms of gene regulation or the rules that govern interactions between cell types is still limited. The advent of new computational pipelines and technologies, such as single-cell epigenomics and spatially resolved transcriptomics, has created opportunities to explore two new axes of biological variation: cell-intrinsic regulation of cell states and expression programs and interactions between cells. Here, we summarize the most promising and robust technologies in these areas, discuss their strengths and limitations and discuss key computational approaches for analysis of these complex datasets. We highlight how data sharing and integration, documentation, visualization and benchmarking of results contribute to transparency, reproducibility, collaboration and democratization in neuroscience, and discuss needs and opportunities for future technology development and analysis.

Cell type mapping reveals tissue niches and interactions in subcortical multiple sclerosis lesions

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system. Inflammation is gradually compartmentalized and restricted to specific tissue niches such as the lesion rim. However, the precise cell type composition of such niches, their interactions and changes between chronic active and inactive stages are incompletely understood. We used single-nucleus and spatial transcriptomics from subcortical MS and corresponding control tissues to map cell types and associated pathways to lesion and nonlesion areas. We identified niches such as perivascular spaces, the inflamed lesion rim or the lesion core that are associated with the glial scar and a cilia-forming astrocyte subtype. Focusing on the inflamed rim of chronic active lesions, we uncovered cell-cell communication events between myeloid, endothelial and glial cell types. Our results provide insight into the cellular composition, multicellular programs and intercellular communication in tissue niches along the conversion from a homeostatic to a dysfunctional state underlying lesion progression in MS.

Should We Consider Neurodegeneration by Itself or in a Triangulation with Neuroinflammation and Demyelination? The Example of Multiple Sclerosis and Beyond

Neurodegeneration is preeminent in many neurological diseases, and still a major burden we fail to manage in patient's care. Its pathogenesis is complicated, intricate, and far from being completely understood. Taking multiple sclerosis as an example, we propose that neurodegeneration is neither a cause nor a consequence by itself. Mitochondrial dysfunction, leading to energy deficiency and ion imbalance, plays a key role in neurodegeneration, and is partly caused by the oxidative stress generated by microglia and astrocytes. Nodal and paranodal disruption, with or without myelin alteration, is further involved. Myelin loss exposes the axons directly to the inflammatory and oxidative environment. Moreover, oligodendrocytes provide a singular metabolic and trophic support to axons, but do not emerge unscathed from the pathological events, by primary myelin defects and cell apoptosis or secondary to neuroinflammation or axonal damage. Hereby, trophic failure might be an overlooked contributor to neurodegeneration. Thus, a complex interplay between neuroinflammation, demyelination, and neurodegeneration, wherein each is primarily and secondarily involved, might offer a more comprehensive understanding of the pathogenesis and help establishing novel therapeutic strategies for many neurological diseases and beyond.

The relation between BTK expression and iron accumulation of myeloid cells in multiple sclerosis

Activation of Bruton's tyrosine kinase (BTK) has been shown to play a crucial role in the proinflammatory response of B cells and myeloid cells upon engagement with B cell, Fc, Toll-like receptor, and distinct chemokine receptors. Previous reports suggest BTK actively contributes to the pathogenesis of multiple sclerosis (MS). The BTK inhibitor Evobrutinib has been shown to reduce the numbers of gadolinium-enhancing lesions and relapses in relapsing-remitting MS patients. In vitro, BTK inhibition resulted in reduced phagocytic activity and modulated BTK-dependent inflammatory signaling of microglia and macrophages. Here, we investigated the protein expression of BTK and CD68 as well as iron accumulation in postmortem control (n = 10) and MS (n = 23) brain tissue, focusing on microglia and macrophages. MS cases encompassed active, chronic active, and inactive lesions. BTK+ and iron+ cells positively correlated across all regions of interests and, along with CD68, revealed highest numbers in the center of active and at the rim of chronic active lesions. We then studied the effect of BTK inhibition in the human immortalized microglia-like HMC3 cell line in vitro. In particular, we loaded HMC3 cells with iron-dextran and subsequently administered the BTK inhibitor Evobrutinib. Iron treatment alone induced a proinflammatory phenotype and increased the expression of iron importers as well as the intracellular iron storage protein ferritin light chain (FTL). BTK inhibition of iron-laden cells dampened the expression of microglia-related inflammatory genes as well as iron-importers, whereas the iron-exporter ferroportin was upregulated. Our data suggest that BTK inhibition not only dampens the proinflammatory response but also reduces iron import and storage in activated microglia and macrophages with possible implications on microglial iron accumulation in chronic active lesions in MS.

Association of Levels of CSF Osteopontin With Cortical Atrophy and Disability in Early Multiple Sclerosis

BACKGROUND AND OBJECTIVES

To evaluate CSF inflammatory markers with accumulation of cortical damage as well as disease activity in patients with early relapsing-remitting MS (RRMS).

METHODS

CSF levels of osteopontin (OPN) and 66 inflammatory markers were assessed using an immune-assay multiplex technique in 107 patients with RRMS (82 F/25 M, mean age 35.7 ± 11.8 years). All patients underwent regular clinical assessment and yearly 3T MRI scans for 2 years while 39 patients had a 4-year follow-up. White matter lesion number and volume, cortical lesions (CLs) and volume, and global cortical thickness (CTh) were evaluated together with the 'no evidence of disease activity' (NEDA-3) status, defined by no relapses, no disability worsening, and no MRI activity, including CLs.

RESULTS

The random forest algorithm selected OPN, CXCL13, TWEAK, TNF, IL19, sCD30, sTNFR1, IL35, IL16, and sCD163 as significantly associated with changes in global CTh. OPN and CXCL13 were most related to accumulation of atrophy after 2 and 4 years. In a multivariate linear regression model on CSF markers, OPN (p < 0.001), CXCL13 (p = 0.001), and sTNFR1 (p = 0.024) were increased in those patients with accumulating atrophy (adjusted R-squared 0.615). The 10 markers were added in a model that included all clinical, demographic, and MRI variables: OPN (p = 0.002) and IL19 (p = 0.022) levels were confirmed to be significantly increased in patients developing more CTh change over the follow-up (adjusted R-squared 0.619). CXCL13 and OPN also revealed the best association with NEDA-3 after 2 years, with OPN significantly linked to disability accumulation (OR 2.468 [1.46-5.034], p = 0.004) at the multivariate logistic regression model.

DISCUSSION

These data confirm and expand our knowledge on the prognostic role of the CSF inflammatory profile in predicting changes in cortical pathology and disease activity in early MS. The data emphasize a crucial role of OPN.

Iron scavenging and myeloid cell polarization

Myeloid cells that populate all human organs and blood are a versatile class of innate immune cells. They are crucial for sensing and regulating processes as diverse as tissue homeostasis and inflammation and are frequently characterized by their roles in either regulating or promoting inflammation. Recent studies in cultured cells and mouse models highlight the role of iron in skewing the functional properties of myeloid cells in tissue damage and repair. Here, we review certain emerging concepts on how iron influences and determines myeloid cell polarization in the context of its uptake, storage, and metabolism, including in conditions such as multiple sclerosis (MS), sickle cell disease, and tumors.

New Imaging Markers in Multiple Sclerosis and Related Disorders: Smoldering Inflammation and the Central Vein Sign

Concepts of multiple sclerosis (MS) biology continue to evolve, with observations such as "progression independent of disease activity" challenging traditional phenotypic categorization. Iron-sensitive, susceptibility-based imaging techniques are emerging as highly translatable MR imaging sequences that allow for visualization of at least 2 clinically useful biomarkers: the central vein sign and the paramagnetic rim lesion (PRL). Both biomarkers demonstrate high specificity in the discrimination of MS from other mimics and can be seen at 1.5 T and 3 T field strengths. Additionally, PRLs represent a subset of chronic active lesions engaged in "smoldering" compartmentalized inflammation behind an intact blood-brain barrier.

[XVI Post-ECTRIMS Meeting: review of the new developments presented at the 2023 ECTRIMS Congress (I)]

The XVI Post-ECTRIMS meeting took place in Seville on 20 and 21 October 2023. This meeting was attended by neurologists specialising in multiple sclerosis (MS) from Spain, who shared a summary of the most interesting innovations at the ECTRIMS congress, which had taken place in Milan the previous week. The aim of this article is to summarise new developments related to the pathogenesis, diagnosis and prognosis of MS. The contributions of innate immunity and central nervous system resident cells, including macrophages and microglia in MS pathophysiology and as therapeutic targets were discussed. Compartmentalised intrathecal inflammation was recognised as central to understanding the progression of MS, and the relationship between inflammatory infiltrates and disease progression was highlighted. Perspectives in demyelinating pathologies were reviewed, focusing on neuromyelitis optica and myelin oligodendrocyte glycoprotein antibody-associated disease, highlighting their pathophysiological and diagnostic differences compared to MS. Advances in neuroimaging were also discussed, and especially the analysis of active chronic lesions, such as paramagnetic rim lesions. In the absence of clinical improvements in trials of remyelinating treatments, methodological strategies to optimise the design of future studies were proposed. Breakthroughs in detecting the prodromal phase of MS, the use of biomarkers in body fluids to assess activity, progression and treatment response, and research on progression independent of flares were addressed. The need to define criteria for radiologically isolated syndrome and to clarify the concept was also discussed.

The neuropathobiology of multiple sclerosis

Chronic low-grade inflammation and neuronal deregulation are two components of a smoldering disease activity that drives the progression of disability in people with multiple sclerosis (MS). Although several therapies exist to dampen the acute inflammation that drives MS relapses, therapeutic options to halt chronic disability progression are a major unmet clinical need. The development of such therapies is hindered by our limited understanding of the neuron-intrinsic determinants of resilience or vulnerability to inflammation. In this Review, we provide a neuron-centric overview of recent advances in deciphering neuronal response patterns that drive the pathology of MS. We describe the inflammatory CNS environment that initiates neurotoxicity by imposing ion imbalance, excitotoxicity and oxidative stress, and by direct neuro-immune interactions, which collectively lead to mitochondrial dysfunction and epigenetic dysregulation. The neuronal demise is further amplified by breakdown of neuronal transport, accumulation of cytosolic proteins and activation of cell death pathways. Continuous neuronal damage perpetuates CNS inflammation by activating surrounding glia cells and by directly exerting toxicity on neighbouring neurons. Further, we explore strategies to overcome neuronal deregulation in MS and compile a selection of neuronal actuators shown to impact neurodegeneration in preclinical studies. We conclude by discussing the therapeutic potential of targeting such neuronal actuators in MS, including some that have already been tested in interventional clinical trials.

Novel and Emerging Treatments to Target Pathophysiological Mechanisms in Various Phenotypes of Multiple Sclerosis

The objective is to comprehensively review novel pharmacotherapies used in multiple sclerosis (MS) and the possibilities they may carry for therapeutic improvement. Specifically, we discuss pathophysiological mechanisms worth targeting in MS, ranging from well known targets, such as autoinflammation and demyelination, to more novel and advanced targets, such as neuroaxonal damage and repair. To set the stage, a brief overview of clinical MS phenotypes is provided, followed by a comprehensive recapitulation of both clinical and paraclinical outcomes available to assess the effectiveness of treatments in achieving these targets. Finally, we discuss various promising novel and emerging treatments, including their respective hypothesized modes of action and currently available evidence from clinical trials. SIGNIFICANCE STATEMENT: This comprehensive review discusses pathophysiological mechanisms worth targeting in multiple sclerosis. Various promising novel and emerging treatments, including their respective hypothesized modes of action and currently available evidence from clinical trials, are reviewed.