In the majority of patients multiple sclerosis starts with a relapsing remitting course (RRMS), which may at later times transform into secondary progressive disease (SPMS). In a minority of patients ...the relapsing remitting disease is skipped and the patients show progression from the onset (primary progressive MS, PPMS). Evidence obtained so far indicate major differences between RRMS and progressive MS, but no essential differences between SPMS and PPMS, with the exception of a lower incidence in the global load of focal white matter lesions and in particular in the presence of classical active plaques in PPMS. We suggest that in MS patients two types of inflammation occur, which develop in parallel but partially independent from each other. The first is the focal bulk invasion of T- and B-lymphocytes with profound blood brain barrier leakage, which predominately affects the white matter, and which gives rise to classical active demyelinated plaques. The other type of inflammation is a slow accumulation of T-cells and B-cells in the absence of major blood brain barrier damage in the connective tissue spaces of the brain, such as the meninges and the large perivascular Virchow Robin spaces, where they may form aggregates or in most severe cases structures in part resembling tertiary lymph follicles. This type of inflammation is associated with the formation of subpial demyelinated lesions in the cerebral and cerebellar cortex, with slow expansion of pre-existing lesions in the white matter and with diffuse neurodegeneration in the normal appearing white or gray matter. The first type of inflammation dominates in acute and relapsing MS. The second type of inflammation is already present in early stages of MS, but gradually increases with disease duration and patient age. It is suggested that CD8
T-lymphocytes remain in the brain and spinal cord as tissue resident cells, which may focally propagate neuroinflammation, when they re-encounter their cognate antigen. B-lymphocytes may propagate demyelination and neurodegeneration, most likely by producing soluble neurotoxic factors. Whether lymphocytes within the brain tissue of MS lesions have also regulatory functions is presently unknown. Key open questions in MS research are the identification of the target antigen recognized by tissue resident CD8
T-cells and B-cells and the molecular nature of the soluble inflammatory mediators, which may trigger tissue damage.
Glia cells are mediators as well as targets of the chronic inflammatory process in the central nervous system of multiple sclerosis (MS) patients. They are involved in the control of autoimmunity, in ...the propagation and termination of the inflammatory reaction, in the induction of demyelination and neurodegeneration, and in remyelination and scaring. Demyelination, as well as neuronal and GLIA cell damage are induced by different immunological mechanisms including components of the adaptive and innate immune system. Oxidative injury resulting in mitochondrial dysfunction is one important mechanism of tissue injury. It is in part driven by the inflammatory response and the production of oxygen radicals mainly in microglia and macrophages. With increasing age of the patients and disease progression, oxidative injury is further amplified by additional mechanisms including central nervous system damage related microglia activation, progressive mitochondrial damage, and age‐dependent iron accumulation within the human central nervous system. The inflammatory mechanisms associated with lesion formation in MS are to a large extent reflected in experimental models of inflammatory demyelination, such as autoimmune encephalomyelitis. This is not the case for the amplification mechanisms of oxidative injury, which mainly operate in the progressive stage of the disease. GLIA 2014;62:1816–1830
Main Points:
Widespread primary demyelination is the most specific pathological feature of multiple sclerosis in all disease stages.
Active tissue injury occurs on a background of inflammation, microglia activation, oxidative injury and mitochondrial damage.
Abstract In this review the differences in pathology and disease mechanisms between early and late stages of multiple sclerosis are discussed. The data suggest that affection of the brain is ...different, depending on the location of lesions, on the stage of the disease, when lesions arise, and on inter-individual differences between patients. We suggest that in the early stage of the disease new lesions are formed by new waves of inflammatory cells, entering the central nervous system from the circulation and giving rise to focal demyelinated plaques in the white and gray matter. In contrast, at late stages of the disease inflammation decreases, but the susceptibility of the target tissue for neurodegeneration increases. New data suggest that mitochondrial injury, mediated through oxidative injury, is in the center of the pathogenetic events leading to brain damage in multiple sclerosis patients.
Highly effective anti-inflammatory therapies have so far been developed for patients with relapsing/remitting multiple sclerosis, which also show some benefits in the early progressive stage of the ...disease. However, treatment options for patients, who have entered the progressive phase, are still limited. Disease starts as an inflammatory process, which induces focal demyelinating lesions in the gray and white matter. This stage of the disease dominates in the relapsing phase, extends into the early stages of progressive disease, and can be targeted by current anti-inflammatory treatments. In parallel, inflammation accumulates behind a closed or repaired blood brain barrier, and this process peaks in the late relapsing and early progressive stage and then declines. Some data suggest that this process may be targeted by immune ablation and hematopoietic stem cell transplantation. In the late stage, inflammation may decline to levels seen in age-matched controls, but age and disease burden–related neurodegeneration ensues. Such neurodegeneration affects the damaged brain and spinal cord, in which functional reserve capacity is exhausted, giving rise to further disability progression. Anti-inflammatory treatments are unlikely to be beneficial in this stage of the disease, but neuroprotective and repair-inducing strategies may still be effective.
Oxidative injury plays a major role in brain damage in many age-related human brain diseases and is particularly pronounced in the progressive stage of multiple sclerosis. In the latter it is related ...to the chronic inflammatory process and is amplified by brain changes due to aging and accumulation of disease burden. It induces demyelination and neurodegeneration by direct oxidation of lipids, proteins and DNA as well as by the induction of mitochondrial injury, which results in energy deficiency and further amplification of oxygen radical production. It affects neurons and all types of glia cells, but neurons and oligodendrocytes are most vulnerable. Difference in the susceptibility for oxidative injury between different cellular components of the central nervous system appears to be due to cell type specific differences in anti-oxidant defense mechanisms, iron loading, cellular susceptibility to apoptosis induction and energy demand. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
•Oxidative injury is a major mechanism of demyelination and neurodegeneration in the multiple sclerosis brain.•Oxidative injury is induced by oxidative burst in macrophages and microglia and amplified by mitochondrial damage and iron liberation within lesions.•Neurons and different glia cells respond in a different way to oxidative stress•The patterns and extent of oxidative damage seen in the MS brain are currently not reflected in animals models of inflammatory demyelination
Summary A better understanding of the pathological mechanisms that drive neurodegeneration in individuals with multiple sclerosis is needed to develop therapies that will effectively treat patients ...in the primary and secondary progressive stages of the disease. We propose that the inflammatory demyelinating disease process in early multiple sclerosis triggers a cascade of events that lead to neurodegeneration and are amplified by pathogenic mechanisms related to brain ageing and accumulated disease burden. Key elements driving neurodegeneration include microglia activation, chronic oxidative injury, accumulation of mitochondrial damage in axons, and age-related iron accumulation in the human brain. Altered mitochondrial function in axons might be of particular importance. This process leads to chronic cell stress and imbalance of ionic homoeostasis, resulting in axonal and neuronal death. The evidence suggests that treatment of progressive multiple sclerosis should be based on a combination of anti-inflammatory, regenerative, and neuroprotective strategies.
► Multiple sclerosis is an inflammatory disease leading to demyelination and neurodegeneration. ► Mechanisms of innate and adaptive immunity are involved in its pathogenesis. ► Mitochondrial injury ...mediated by reactive oxygen and nitric oxide species is important. ► Mechanisms of tissue injury are only partly reflected in models of autoimmune encephalomyelitis.
Studies aimed to elucidate the pathogenesis of the disease and to find new therapeutic options for multiple sclerosis (MS) patients heavily rely on experimental autoimmune encephalomyelitis (EAE) as a suitable experimental model. This strategy has been highly successful for the inflammatory component of the disease, but had so far little success in the development of neuroprotective therapies, which are also effective in the progressive stage of the disease. Here we discuss opportunities and limitations of EAE models for MS research and provide an overview on the complex mechanisms leading to demyelination and neurodegeneration in this disease. We suggest that the underlying mechanisms involve adaptive and innate immunity. However, mitochondrial injury, resulting in energy failure, is a key element of neurodegeneration in MS and is apparently driven by radical production in activated microglia.
Multiple sclerosis is a complex and heterogeneous, most likely autoimmune, demyelinating disease of the central nervous system (CNS). Although a number of histological classification systems for CNS ...lesions have been used by different groups in recent years, no uniform classification exists. In this paper, we propose a simple and unifying classification of MS lesions incorporating many elements of earlier histological systems that aims to provide guidelines for neuropathologists and researchers studying MS lesions to allow for better comparison of different studies performed with MS tissue, and to aid in understanding the pathogenesis of the disease. Based on the presence/absence and distribution of macrophages/microglia (inflammatory activity) and the presence/absence of ongoing demyelination (demyelinating activity), we suggest differentiating between active, mixed active/inactive, and inactive lesions with or without ongoing demyelination. Active lesions are characterized by macrophages/microglia throughout the lesion area, whereas mixed active/inactive lesions have a hypocellular lesion center with macrophages/microglia limited to the lesion border. Inactive lesions are almost completely lacking macrophages/microglia. Active and mixed active/inactive lesions can be further subdivided into lesions with ongoing myelin destruction (demyelinating lesions) and lesions in which the destruction of myelin has ceased, but macrophages are still present (post-demyelinating lesions). This distinction is based on the presence or absence of myelin degradation products within the cytoplasm of macrophages/microglia. For this classification of MS lesions, identification of myelin with histological stains such as luxol fast blue-PAS or by immunohistochemistry using antibodies against myelin basic-protein (MBP) or proteolipid-protein (PLP), as well as, detection of macrophages/microglia by, e.g., anti-CD68 is sufficient. Active and demyelinating lesions may be further subdivided into the early and late demyelinating lesions. The former is defined by the presence in macrophages of major and small molecular weight myelin proteins, such as cyclic nucleotide diphosphoesterase (CNP), myelin oligodendrocyte glycoprotein (MOG), or myelin-associated protein (MAG), whereas macrophages in the latter demonstrate merely the presence of the major myelin proteins MBP or PLP. We discuss the histological features and staining techniques required to classify MS lesions, and, in addition, describe the histological hallmarks of cortical pathology and diffuse white matter changes, as well as of remyelination.
In multiple sclerosis, currently approved disease-modifying treatments are effective in modulating peripheral immunity, and coherently, in reducing clinical/radiological relapses, but still, they ...perform poorly in preventing disease progression and overall disability accrual. This review provides an up-to-date overview of the neuropathology of progressive multiple sclerosis, including a summary of the main mechanisms of disease progression.
Clinical progression in multiple sclerosis is likely related to the accumulation of neuro-axonal loss in a lifelong inflammatory CNS environment (both adaptive and innate) and relative un-balance between damage, repair and brain functional reserve. A critical driver appears to be the T-cell and B-cell-mediated compartmentalized inflammation within the leptomeninges and within the parenchyma. Recent perspective highlighted also the role of the glial response to such lifelong inflammatory injury as the critical player for both pathological and clinical outcomes.
The neuropathological and biological understanding of disease progression in multiple sclerosis have progressed in the last few years. As a consequence, new therapeutic approaches are emerging outside the modulation of T-cell activity and/or the depletion of B cells.
PDF
