Multiple sclerosis is an inflammatory demyelinating disease of the CNS in which activated immune cells invade the central nervous system and cause inflammation, neurodegeneration, and tissue damage. The underlying cause is currently unknown. Current research in neuropathology, neuroimmunology, neurobiology, and neuroimaging, together with clinical neurology, provide support for the notion that MS is not a single disease but rather a spectrum. [1]
There are three clinical phenotypes: relapsing-remitting MS (RRMS), characterized by periods of neurological worsening following by remissions; secondary-progressive MS (SPMS), in which there is gradual progression of neurological dysfunction with fewer or no relapses; and primary-progressive MS (MS), in which neurological deterioration is observed from onset.
Pathophysiology is a convergence of pathology with physiology. Pathology is the medical discipline that describes conditions typically observed during a disease state; whereas physiology is the biological discipline that describes processes or mechanisms operating within an organism. Referring to MS, the physiology refers to the different processes that lead to the development of the lesions and the pathology refers to the condition associated with the lesions.
Multiple sclerosis can be pathologically defined as the presence of distributed glial scars (or sclerosis) in the central nervous system disseminated in time (DIT) and space (DIS). [2] The gold standard for MS diagnosis is pathological correlation, though given its limited availability, other diagnosis methods are normally used. [3] The scleroses that define the disease are the remainders of previous demyelinating lesions in the CNS white matter of a patient (encephalomyelitis) showing special characteristics, such as confluent instead of perivenous demyelination. [4]
There are three phases for how an unknown underlying condition may cause damage in MS:
Multiple sclerosis differs from other idiopathic inflammatory demyelinating diseases in its confluent subpial cortical lesions. These types of lesions are the most specific finding for MS, being exclusively present in MS patients, though currently they can only be detected at autopsy. [12]
Most MS findings take place inside the white matter, and lesions appear mainly in a periventricular distribution (clustered around the ventricles of the brain). Apart from white matter demyelination, the cortex and deep gray matter (GM) nuclei can be affected, together with diffuse injury of the NAWM. [13] GM atrophy is independent of classical MS lesions and is associated with physical disability, fatigue, and cognitive impairment in MS [14]
At least five characteristics are present in CNS tissues of MS patients: Inflammation beyond classical white matter lesions, intrathecal Ig production with oligoclonal bands, an environment fostering immune cell persistence, and a disruption of the blood–brain barrier outside of active lesions. [15] The scars that give the name to the condition are produced by astrocytes healing old lesions. [16] MS is active even during remission periods. [17]
Follicle-like aggregates in the meninges are formed only in secondary progressive MS. [18] and correlate with the degree of subpial cortical demyelination and brain atrophy, suggesting that they might contribute to cortical pathology in SPMS [18]
These ectopic lymphoid follicles are composed mainly of EBV infected B-cells. [19]
Four different damage patterns have been identified in patient's brain tissues. The original report[ citation needed ] suggests that there may be several types of MS with different immune causes, and that MS may be a family of several diseases. Though originally a biopsy was required to classify the lesions of a patient, since 2012 it is possible to classify them by a blood test [20] looking for antibodies against 7 lipids, three of which are cholesterol derivatives [21] Cholesterol crystals are believed to both impair myelin repair and aggravate inflammation. [22] [23]
It is believed that they may correlate with differences in disease type and prognosis, and perhaps with different responses to treatment. In any case, understanding lesion patterns can provide information about differences in disease between individuals and enable doctors to make more effective treatment decisions.[ citation needed ]
According to one of the researchers involved in the original research, "Two patterns (I and II) showed close similarities to T-cell-mediated or T-cell plus antibody-mediated autoimmune encephalomyelitis, respectively. The other patterns (III and IV) were highly suggestive of a primary oligodendrocyte dystrophy, reminiscent of virus- or toxin-induced demyelination rather than autoimmunity."
The four identified patterns are: [24]
These differences are noticeable only in early lesions [38] and the heterogeneity was controversial for some time because some research groups thought that these four patterns could be a consequence of the age of the lesions. [39] Nevertheless, after some debate among research groups, the four patterns model is accepted and the exceptional case found by Prineas has been classified as NMO [40] [41]
For some investigation teams this means that MS is an immunopathogenetically heterogeneous disease. The latter hypothesis is further corroborated by a recent study that demonstrated significant differences in routine cerebrospinal fluid findings between patients with pattern I lesions and patients with non-pattern I lesions, including a lack of CSF-restricted oligoclonal bands, in most pattern II and III patients. [42] Finally, some previously diagnosed with pattern II MS were later found to have in fact MOG-IgG-related encephalomyelitis, suggesting that both the current clinicoradiological diagnostic criteria for MS and the histopathological criteria for MS may be insufficiently specific. This was already indicated by previous studies that found a relatively high rate of false diagnoses of MS among patients with AQP4-IgG-positive neuromyelitis optica spectrum disorders or MOG encephalomyelitis. Currently antibodies to lipids and peptides in sera, detected by microarrays, can be used as markers of the pathological subtype given by brain biopsy. [43]
Another development in this area is the finding that some lesions present mitochondrial defects that could distinguish types of lesions. [44]
In multiple sclerosis, inflammation, demyelination, and neurodegeneration are observed together. Some clinical trials have shown that the inflammation produces both the relapses and the demyelination, and that neurodegeneration (axonal transection) is independent from inflammation, produces the accumulative disability, and advances even when inflammation and demyelination are delayed. [45] It seems that neurodegeneration is produced by damaged mitochondria, which in turn comes from activated microglia. [46]
Currently it is unknown what the primary cause of MS is; if MS is a heterogeneous disease, the lesion development process would not be unique. In particular, some PPMS patients having a special clinical course named rapidly progressive multiple sclerosis could have a special genetic cause [47] and a different development process.
Several types of damage appear in the brain: normal appearing white matter (NAWM) and characteristic lesions. Changes in NAWM include axonal injury without demyelination, low-grade inflammation, and microglial and astrocytic activation [48]
MS lesions develop inside NAWM areas. Their shape is influenced by their activity [49]
The most accepted sequence of events is first NAWM appearance, then the so-called pre-active lesions, with activated microglia, and finally the BBB (blood-brain barrier) breakdown, which enables the entry of T-cells to the CNS. This marks the beginning of an autoimmune attack which destroys myelin in active lesions. [50] When the attack is resolved, a characteristic glial scar is formed by astrocytes.
Current models can be divided into two categories: inside-out and outside-in. In the former, it is hypothesized that a problem in CNS cells produces an immune response that destroys myelin and subsequently breaks the BBB. In latter, an external factor produces BBB leaks, enters the CNS, and destroys myelin and axons. [51] Whatever the underlying condition for MS is, it appears that damage is triggered by an unknown soluble factor in the CSF, potentially produced in meningeal areas; this factor can diffuse into the cortical parenchyma and destroy myelin either directly or indirectly through microglia activation. [12]
The evolution of a preactive lesion is related to microglia reactivity. Increased expression of pro-inflammatory cell surface markers have been observed in NAWM and "initial" lesions, corresponding to a so-called loss of homeostatic microglial equilibrium. [52]
Some authors report active lesion formation before BBB breakdown; [53] others point to adipsin as a factor of the breakdown. [54]
MS lesions are driven mainly by T-cells. It has been found recently that B-cells are also involved. [55]
The blood–brain barrier (BBB) is a protective barrier that denies the entrance of foreign material into the nervous system. BBB disruption is the moment in which penetration of the barrier by lymphocytes occur and has been considered one of the early problems in MS lesions. [56]
The BBB is composed of endothelial cells which line the blood vessel walls of the central nervous system. Compared to normal endothelial cells, the cells lining the BBB are connected by occludin and claudin which form tight junctions in order to create a barrier to keep out larger molecules such as proteins. In order to pass through, molecules must be taken in by transport proteins or an alteration in the BBB permeability must occur, such as interactions with associated adaptor proteins like ZO-1, ZO-2 and ZO-3. [57]
The BBB is compromised due to active recruitment of lymphocytes and monocytes and their migration across the barrier. Release of chemokines allow for the activation of adhesion molecules on the lymphocytes and monocytes, resulting in an interaction with the endothelial cells of the BBB which then activate the expression of matrix metalloproteinases to degrade the barrier. [57] This results in disruption of the BBB, causing an increase in barrier permeability due to the degradation of tight junctions which maintain barrier integrity. Inducing the formation of tight junctions can restore BBB integrity and reduces its permeability, which can be used to reduce the damage caused by lymphocyte and monocyte migration across the barrier as restored integrity would restrict their movement. [58]
After barrier breakdown symptoms may appear, such as swelling. Activation of macrophages and lymphocytes and their migration across the barrier may result in direct attacks on myelin sheaths within the central nervous system, leading to the characteristic demyelination event observed in MS. [59] After demyelination has occurred, the degraded myelin sheath components, such as myelin basic proteins and Myelin oligodendrocyte glycoproteins, are then used as identifying factors to facilitate further immune activity upon myelin sheaths. Further activation of cytokines is also induced by macrophage and lymphocyte activity, promoting inflammatory activity as well continued activation of proteins such as matrix metalloproteinases, which have detrimental effect on BBB integrity. [60]
Recently it has been found that BBB damage happens even in non-enhancing lesions. [61] MS has an important vascular component. [62]
Postmortem studies of the BBB, especially the vascular endothelium, show immunological abnormalities. Microvessels in periplaque areas coexpressed HLA-DR and VCAM-1, some others HLA-DR and urokinase plasminogen activator receptor, and others HLA-DR and ICAM-1. [63]
The damaged white matter is known as "Normal-appearing white matter" (NAWM) and is where lesions appear. [10] These lesions form in NAWM before blood–brain barrier breakdown. [64]
BBB can be broken centripetally (the most normal) or centrifugally. [65] Several possible biochemical disrupters were proposed. Some hypotheses about how the BBB is compromised revolve around the presence of compounds in the blood that could interact with vessels only in the NAWM areas. The permeability of two cytokines, Interleukin 15 and LPS, may be involved in BBB breakdown. [66] Breakdown is responsible for monocyte infiltration and inflammation in the brain. [67] Monocyte migration and LFA-1-mediated attachment to brain microvascular endothelia is regulated by SDF-1alpha through Lyn kinase. [68]
Using iron nanoparticles, involvement of macrophages in BBB breakdown can be detected. [69] A special role is played by Matrix metalloproteinases. These increase BBB T-cell permeability, specially in the case of MMP-9 [60] and are supposedly related to the mechanism of action of interferons. [70]
Whether BBB dysfunction is the cause or the consequence of MS [71] is disputed, because activated T-Cells can cross a healthy BBB when they express adhesion proteins. [72] Apart from that, activated T-Cells can cross a healthy BBB when they express adhesion proteins. [72] (Adhesion molecules could also play a role in inflammation [73] ) In particular, one of these adhesion proteins involved is ALCAM (Activated Leukocyte Cell Adhesion Molecule, also called CD166), and is under study as therapeutic target. [74] Another protein involved is CXCL12, [75] which is found also in brain biopsies of inflammatory elements, [76] and which could be related to the behavior of CXCL13 under methylprednisolone therapy. [77] Some molecular biochemical models for relapses have been proposed. [78]
Normally, gadolinium enhancement is used to show BBB disruption on MRIs. [79] Abnormal tight junctions are present in both SPMS and PPMS. They appear in active white matter lesions and in gray matter in SPMS. They persist in inactive lesions, particularly in PPMS. [80]
A uric acid deficiency was implicated in this process. Uric acid added in physiological concentrations (i.e. achieving normal concentrations) is therapeutic in MS by preventing BBB breakdown through inactivation of peroxynitrite. [81] The low level of uric acid found in people with MS is manifestedly causative rather than a tissue damage consequence in the white matter lesions, [82] but not in the grey matter lesions. [83] Uric acid levels are lower during relapses. [84]
It is not known what causes MS. Several problems appear together with the white matter lesions, like cortical lesions and normal-appearing tissues. Several theories have been proposed to explain it.
Some areas that appear normal under normal MRI look abnormal under special MRI, like magnetisation transfer MTR-MRI. These are called Normal Appearing White Matter (NAWM) and Normal Appearing Grey Matter (NAGM). The cause why the normal appearing areas appear in the brain is unknown, but seems clear that they appear mainly in the ventricles and that they predict the course of the disease. [85]
Given that MS lesions begin inside the NAWM areas, these areas are expected to be produced by the same underlying condition that produces the lesions, and therefore the ultimate MS underlying condition, whatever it is. [86] Historically, several theories about how these areas appear have been presented:
The search for an auto-antigen has taken a long time, but at least there is one reported. It is the enzyme GDP-L-fucose synthase. [87] [88]
This theory in part could also explain why some patients report amelioration under dietary treatment.
Human endogenous retroviruses (HERVs) have been reported in MS for several years. In fact, one of the families, Human Endogenous Retrovirus-W was first discovered while studying MS patients.
Recent research as of 2019 point to one of the HERV-W viruses (pHEV-W), and specifically one of the proteins of the viral capsid that has been found to activate microglia in vitro. Activated microglia in turn produces demyelination. [89] Some interactions between the Epstein-Barr virus and the HERVs could be the trigger of the MS microglia reactions. [90] Supporting this study, a monoclonal antibody against the viral capside (Temelimab) has shown good results in trials in phase IIb. [91]
Whatever the underlying primary condition is, it is expected to be a soluble factor in the CSF, [12] maybe an unknown cytokine or ceramide, or a combination of them. Also B-cells and microglia could be involved. [114] [115] In particular, it is known that B-cells of MS patients secrete an unknown toxin against oligodendrocytes [116]
It has been reported several times that CSF of some MS patients can damage myelin in culture [117] [118] [119] [120] [121] and mice [122] [123] and ceramides have been recently brought into the stage. [124] Whatever the problem is, it produces apoptosis of neurons respecting astrocytes [125]
In 2012 it was reported that a subset of MS patients have a seropositive anti-Kir4.1 status, [126] which can represent up to a 47% of the MS cases, and the study has been reproduced by at least two other groups. [127] [128]
In 2016 a similar association was reported for anti-Anoctamin-2 [129]
If the existence of any of these subsets of MS is confirmed, the situation would be similar to what happened for Devic Disease and Aquaporin-4 [ citation needed ]. MS could be considered a heterogeneous condition or a new medical entity will be defined for these cases.
Some authors propose a primary neurodegenerative factor. Maybe the strongest argument supporting this theory comes from the comparison with NMO. Though autoimmune demyelination is strong, axons are preserved, showing that the standard model of a primary demyelination cannot be hold. [130] The theory of the trans-synaptic degeneration, is compatible with other models based in the CSF biochemistry. [131]
Others propose an oligodendrocyte stress as primary dysfunction, which activates microglia creating the NAWM areas [132] and others propose a yet-unknown intrinsic CNS trigger induces the microglial activation and clustering, which they point out could be again axonal injury or oligodendrocyte stress. [133]
Finally, other authors point to a cortical pathology which starts in the brain external layer (pial surface) and progresses extending into the brain inner layers [134]
If as expected MS is an heterogeneous disease and the lesion development process would not be unique. In particular, some PPMS patients have been found to have a special genetic variant named rapidly progressive multiple sclerosis [47] which would behave differently from what here is explained.
It is due to a mutation inside the gene NR1H3, an arginine to glutamine mutation in the position p.Arg415Gln, in an area that codifies the protein LXRA.
Several biomarkers for diagnosis, disease evolution and response to medication (current or expected) are under research. While most of them are still under research, there are some of them already well stablished:
Multiple sclerosis (MS) is an autoimmune disease in which the insulating covers of nerve cells in the brain and spinal cord are damaged. Being a demyelinating disease, MS disrupts the ability of parts of the nervous system to transmit signals, resulting in a range of signs and symptoms, including physical, mental, and sometimes psychiatric problems. Symptoms include double vision, vision loss, eye pain, muscle weakness, and loss of sensation or coordination. MS takes several forms, with new symptoms either occurring in isolated attacks or building up over time. In relapsing forms of MS, between attacks, symptoms may disappear completely, although some permanent neurological problems often remain, especially as the disease advances. In progressive forms of MS, bodily function slowly deteriorates once symptoms manifest and will steadily worsen if left untreated.
Oligoclonal bands (OCBs) are bands of immunoglobulins that are seen when a patient's blood serum, or cerebrospinal fluid (CSF) is analyzed. They are used in the diagnosis of various neurological and blood diseases. Oligoclonal bands are present in the CSF of more than 95% of patients with clinically definite multiple sclerosis.
A demyelinating disease refers to any disease affecting the nervous system where the myelin sheath surrounding neurons is damaged. This damage disrupts the transmission of signals through the affected nerves, resulting in a decrease in their conduction ability. Consequently, this reduction in conduction can lead to deficiencies in sensation, movement, cognition, or other functions depending on the nerves affected.
Neuromyelitis optica spectrum disorders (NMOSD) are a spectrum of autoimmune diseases characterized by acute inflammation of the optic nerve and the spinal cord (myelitis). Episodes of ON and myelitis can be simultaneous or successive. A relapsing disease course is common, especially in untreated patients.
Myelin basic protein (MBP) is a protein believed to be important in the process of myelination of nerves in the nervous system. The myelin sheath is a multi-layered membrane, unique to the nervous system, that functions as an insulator to greatly increase the velocity of axonal impulse conduction. MBP maintains the correct structure of myelin, interacting with the lipids in the myelin membrane.
Experimental autoimmune encephalomyelitis, sometimes experimental allergic encephalomyelitis (EAE), is an animal model of brain inflammation. It is an inflammatory demyelinating disease of the central nervous system (CNS). It is mostly used with rodents and is widely studied as an animal model of the human CNS demyelinating diseases, including multiple sclerosis (MS) and acute disseminated encephalomyelitis (ADEM). EAE is also the prototype for T-cell-mediated autoimmune disease in general.
Multiple sclerosis and other demyelinating diseases of the central nervous system (CNS) produce lesions and glial scars or scleroses. They present different shapes and histological findings according to the underlying condition that produces them.
Remyelination is the process of propagating oligodendrocyte precursor cells to form oligodendrocytes to create new myelin sheaths on demyelinated axons in the Central nervous system (CNS). This is a process naturally regulated in the body and tends to be very efficient in a healthy CNS. The process creates a thinner myelin sheath than normal, but it helps to protect the axon from further damage, from overall degeneration, and proves to increase conductance once again. The processes underlying remyelination are under investigation in the hope of finding treatments for demyelinating diseases, such as multiple sclerosis.
Inflammatory demyelinating diseases (IDDs), sometimes called Idiopathic (IIDDs) due to the unknown etiology of some of them, are a heterogenous group of demyelinating diseases - conditions that cause damage to myelin, the protective sheath of nerve fibers - that occur against the background of an acute or chronic inflammatory process. IDDs share characteristics with and are often grouped together under Multiple Sclerosis. They are sometimes considered different diseases from Multiple Sclerosis, but considered by others to form a spectrum differing only in terms of chronicity, severity, and clinical course.
Baló's concentric sclerosis is a disease in which the white matter of the brain appears damaged in concentric layers, leaving the axis cylinder intact. It was described by József Mátyás Baló who initially named it "leuko-encephalitis periaxialis concentrica" from the previous definition, and it is currently considered one of the borderline forms of multiple sclerosis.
Leukoencephalopathy with vanishing white matter is an autosomal recessive neurological disease. The cause of the disease are mutations in any of the 5 genes encoding subunits of the translation initiation factor eIF2B: EIF2B1, EIF2B2, EIF2B3, EIF2B4, or EIF2B5. The disease belongs to a family of conditions called the Leukodystrophies.
Research in multiple sclerosis may find new pathways to interact with the disease, improve function, curtail attacks, or limit the progression of the underlying disease. Many treatments already in clinical trials involve drugs that are used in other diseases or medications that have not been designed specifically for multiple sclerosis. There are also trials involving the combination of drugs that are already in use for multiple sclerosis. Finally, there are also many basic investigations that try to understand the disease better and in the future may help to find new treatments.
Tumefactive multiple sclerosis is a condition in which the central nervous system of a person has multiple demyelinating lesions with atypical characteristics for those of standard multiple sclerosis (MS). It is called tumefactive as the lesions are "tumor-like" and they mimic tumors clinically, radiologically and sometimes pathologically.
Chronic cerebrospinal venous insufficiency is a term invented by Italian researcher Paolo Zamboni in 2008 to describe compromised flow of blood in the veins draining the central nervous system. Zamboni hypothesized that it might play a role in the cause or development of multiple sclerosis (MS). Zamboni also devised a surgical procedure which the media nicknamed a liberation procedure or liberation therapy, involving venoplasty or stenting of certain veins. Zamboni's ideas about CCSVI are very controversial, with significantly more detractors than supporters, and any treatments based on his ideas are considered experimental.
Theiler's murine encephalomyelitis virus (TMEV) is a single-stranded RNA murine cardiovirus from the family Picornaviridae. It has been used as a mouse model for studying virally induced paralysis, as well as encephalomyelitis comparable to multiple sclerosis. Depending on the mouse and viral strain, viral pathogenesis can range from negligible, to chronic or acute encephalomyelitis.
Neuroinflammation is inflammation of the nervous tissue. It may be initiated in response to a variety of cues, including infection, traumatic brain injury, toxic metabolites, or autoimmunity. In the central nervous system (CNS), including the brain and spinal cord, microglia are the resident innate immune cells that are activated in response to these cues. The CNS is typically an immunologically privileged site because peripheral immune cells are generally blocked by the blood–brain barrier (BBB), a specialized structure composed of astrocytes and endothelial cells. However, circulating peripheral immune cells may surpass a compromised BBB and encounter neurons and glial cells expressing major histocompatibility complex molecules, perpetuating the immune response. Although the response is initiated to protect the central nervous system from the infectious agent, the effect may be toxic and widespread inflammation as well as further migration of leukocytes through the blood–brain barrier may occur.
Multiple sclerosis (MS) can be pathologically defined as the presence of distributed glial scars (scleroses) in the central nervous system that must show dissemination in time (DIT) and in space (DIS) to be considered MS lesions.
MOG antibody disease (MOGAD) or MOG antibody-associated encephalomyelitis (MOG-EM) is an inflammatory demyelinating disease of the central nervous system. Serum anti-myelin oligodendrocyte glycoprotein antibodies are present in up to half of patients with an acquired demyelinating syndrome and have been described in association with a range of phenotypic presentations, including acute disseminated encephalomyelitis, optic neuritis, transverse myelitis, and neuromyelitis optica.
Several biomarkers for diagnosis of multiple sclerosis, disease evolution and response to medication are under research. While most of them are still under research, there are some of them already well stablished:
Helga (Elga) De Vries is a Dutch neuroimmunologist and a Full Professor in the Department of Molecular Cell Biology and Immunology at Amsterdam University Medical Centers in Amsterdam, The Netherlands. De Vries is a leader in the field of blood brain barrier research. She founded the Dutch Blood Brain Barrier Network and is the President of the International Brain Barrier Society. De Vries’ research explores the interactions between the brain and the immune system and she specifically looks at neurovascular biology in the context of neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease.
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