Chronic relapsing experimental allergic encephalomyelitis: CNS plaque development in unsuppressed and suppressed animals

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.

Schwann cell remyelination of the central nervous system: why does it happen and what are the benefits?

Myelin sheaths, by supporting axonal integrity and allowing rapid saltatory impulse conduction, are of fundamental importance for neuronal function. In response to demyelinating injuries in the central nervous system (CNS), oligodendrocyte progenitor cells (OPCs) migrate to the lesion area, proliferate and differentiate into new oligodendrocytes that make new myelin sheaths. This process is termed remyelination. Under specific conditions, demyelinated axons in the CNS can also be remyelinated by Schwann cells (SCs), the myelinating cell of the peripheral nervous system. OPCs can be a major source of these CNS-resident SCs-a surprising finding given the distinct embryonic origins, and physiological compartmentalization of the peripheral and central nervous system. Although the mechanisms and cues governing OPC-to-SC differentiation remain largely undiscovered, it might nevertheless be an attractive target for promoting endogenous remyelination. This article will (i) review current knowledge on the origins of SCs in the CNS, with a particular focus on OPC to SC differentiation, (ii) discuss the necessary criteria for SC myelination in the CNS and (iii) highlight the potential of using SCs for myelin regeneration in the CNS.

Experimental in vivo and in vitro models of multiple sclerosis: EAE and beyond

Although the primary cause of multiple sclerosis (MS) is unknown, the widely accepted view is that aberrant (auto)immune responses possibly arising following infection(s) are responsible for the destructive inflammatory demyelination and neurodegeneration in the central nervous system (CNS). This notion, and the limited access of human brain tissue early in the course of MS, has led to the development of autoimmune, viral and toxin-induced demyelination animal models as well as the development of human CNS cell and organotypic brain slice cultures in an attempt to understand events in MS. The autoimmune models, collectively known as experimental autoimmune encephalomyelitis (EAE), and viral models have shaped ideas of how environmental factors may trigger inflammation, demyelination and neurodegeneration in the CNS. Understandably, these models have also heavily influenced the development of therapies targeting the inflammatory aspect of MS. Demyelination and remyelination in the absence of overt inflammation are better studied in toxin-induced demyelination models using cuprizone and lysolecithin. The paradigm shift of MS as an autoimmune disease of myelin to a neurodegenerative disease has required more appropriate models reflecting the axonal and neuronal damage. Thus, secondary progressive EAE and spastic models have been crucial to develop neuroprotective approaches. In this review the current in vivo and in vitro experimental models to examine pathological mechanisms involved in inflammation, demyelination and neuronal degeneration, as well as remyelination and repair in MS are discussed. Since this knowledge is the basis for the development of new therapeutic approaches for MS, we particularly address whether the currently available models truly reflect the human disease, and discuss perspectives to further optimise and develop more suitable experimental models to study MS.

Spontaneous remyelination following prolonged inhibition of alpha4 integrin in chronic EAE

Inhibition of alpha(4)beta(1) integrin blocks immune cell influx into the CNS providing benefit to patients with multiple sclerosis and in animal model systems. We have used this mechanism to examine whether the presence of inflammatory cells suppresses spontaneous myelin repair in experimental autoimmune encephalomyelitis. We observed (1) 87% of plaques showed remyelination after 40 days of treatment; (2) myelin repair occurred in half of the total lesion area; (3) half of the animals regained motor function. There was no significant repair or gain of motor function in vehicle-treated animals. Therefore, prolonged inhibition of CNS inflammation, in the absence of targeted myelin repair, facilitates mechanisms of spontaneous remyelination.

Episodic demyelination and subsequent remyelination within the murine central nervous system: changes in axonal calibre

Exposure of young adult C57BL/6 mice to cuprizone in the diet initiated profound and synchronous demyelination of the corpus callosum, which was virtually complete by 4 weeks of exposure. Interestingly, even in the face of a continued exposure to cuprizone, there was spontaneous remyelination 2 weeks later. This remyelination preferentially involved smaller calibre axons. There was a suggestion of yet another cycle of demyelination (at 10 weeks) and remyelination (at 12 weeks), but by 16 weeks of exposure, the regenerative capacity was exhausted and the animals were near death. The relapsing-remitting pattern suggests this may be a useful model for certain human demyelinating disorders. In contrast to the above chronic model, the corpus callosum from mice exposed to cuprizone for only 6 weeks continued to remyelinate, with 67% of the axons being myelinated or remyelinated at 10 weeks. Interestingly, a significant reduction in the mean value for axonal diameter was observed during acute demyelination. Upon remyelination, however, the axonal calibre distribution returned to near-normal. In contrast, when mice were maintained on a cuprizone diet for 16 weeks, the mean value for axonal diameter was reduced to 60% of normal. These results provide further evidence that the interactions between oligodendrocytes and axons alter axonal calibre.

Central nervous system remyelination clinical application of basic neuroscience principles

Studies in both humans and experimental animals have demonstrated that myelin repair in the CNS is a normal physiological response to myelin damage, similar to tissue injury elsewhere in the body. The unanswered question is why myelin repair is incomplete in multiple sclerosis patients. In this paper we review the morphological characteristics of remyelination, discuss the available animal models of CNS demyelination and their usefulness to identify the molecular, cellular, and morphological events involved in CNS myelin repair, examine the use of immunosuppression, immunoglobulins, protein growth factors, and glial cell transplantation at the primary experimental therapies designed to promote CNS remyelination, and address the potential electrophysiological and clinical benefits of myelin repair in the CNS.

Spontaneous and induced remyelination in multiple sclerosis and the Theiler's virus model of central nervous system demyelination

Remyelination in the central nervous system, originally thought to occur rarely, if ever, is now an established phenomena in multiple sclerosis patients. However, the extent of myelin repair is incomplete and limited. Experimental models of central nervous system demyelination provide an opportunity to study the cellular and molecular events involved in remyelination. These models may provide some clue to why remyelination in multiple sclerosis is incomplete as well as suggest potential methods to stimulate central nervous system repair. In this review we examine the morphological aspects of central nervous system remyelination and discuss both spontaneous and induced remyelination in multiple sclerosis and experimental models of central nervous system demyelination. We give special emphasis to the Theiler's virus model of central nervous system demyelination and its usefulness to identify therapeutic agents to promote remyelination. The role of immunoglobulins in promoting remyelination in both the Theiler's model system and in multiple sclerosis is discussed. Finally, we examine the potential physiological role of demyelination and remyelination and its relationship with clinical manifestations of central nervous system disease.

Central axons in injured cat spinal cord recover electrophysiological function following remyelination by Schwann cells

Axonal morphometry of the lesion site was studied at 3 months after standardized weight-drop contusion injury of the thoracic spinal cord in adult cats. From a sample of 25 injured animals, 12 examples were found in which all surviving axons in the dorsal column were remyelinated by Schwann cells, at the level of the lesion. The dorsolateral tracts were also peripherally myelinated in 6 of these cases, and there was no central myelination in complete transverse sections through the lesion in four animals. In these cases, Schwann cell myelination was prevalent for several millimeters on either side of the lesion center. The extent of Schwann cell invasion correlated with the intensity of injury, measured by overall axon loss. Cortical somatosensory evoked potentials (CSEP) were recorded from all animals before and at intervals for 12 weeks after injury. CSEP to hindlimb (tibial nerve) stimulation were lost immediately at injury but some recovery took place during the first month. The extent of CSEP recovery correlated negatively but weakly with overall axon loss. Clear SEP were recorded at 3 months post-injury in 3 of the animals in which the dorsal columns were remyelinated by Schwann cells; in one of these, the dorsolateral funiculi were also peripherally myelinated. In another, oligodendrocyte myelination was absent from the entire transverse section of the lesion site. Thus, abnormal remyelination by cells of the peripheral nervous system, which is known to occur in a variety of central demyelinating conditions, is capable of restoring effective action potential conduction in mammalian spinal cord sensory tracts.

Relapsing subacute demyelinating encephalomyelitis in rats during the course of coronavirus JHM infection

Temperature-sensitive mutants of the murine coronavirus JHM induced a subacute demyelinating encephalomyelitis (SDE) in young rats. Neurological symptoms were associated with marked lesions of primary demyelination in the white matter of the central nervous system (CNS), and developing after an incubation time of several weeks to months. Many rats survived this infection and recovered completely from this CNS disease. Among 43 survivors of SDE, 9 rats developed a relapse 27-153 days after onset of the first attack. Neuropathological examination of these animals revealed areas of fresh demyelination together with old remyelinated lesions. Viral antigens were detectable in the neighbourhood of fresh lesions and in some cases infectious virus was re-isolated from rats revealing low antibody titers to JHM virus. These results demonstrate that mutants of JHM virus can induce a relapsing demyelinating disease process, associated with a persistent infection, which possesses some similarities to chronic experimental allergic encephalomyelitis.

Induction of recurrent experimental allergic encephalomyelitis with myelin basic protein

Recurrent experimental allergic encephalomyelitis (EAE) was induced in Lewis rats by inoculation of human myelin basic protein (MBP) and complete Freund's adjuvant (CFA). The animals developed clinical disease characterized by unsteadiness, ataxic gait, and abnormal posturing of the limbs. Spontaneous remissions and relapses were noted for periods up to 120 days. Histologically there were perivascular infiltrates of mononuclear cells, especially prominent in the cerebellar white matter. There was little evidence of demyelination. This study demonstrates that relapsing EAE may be induced with MBP in the rat. Sensitization with other myelin antigens is not required, although immunity to such antigens may be necessary to induce demyelination. It is postulated that relapsing EAE may be associated with a defect in suppressor cell function.

Demyelinating disease as the assumed cause of hearing loss and vertigo. A case report with light- and electron-microscopic findings

A patient initially presenting typical symptoms of idiopathic sudden deafness later developed disabling episodic vertigo, which led to translabyrinthine eighth nerve transection. Morphological examination of the removed cochlear nerve specimen revealed a demyelinating process in the neuroglial portion of the nerve. The major part of the peripheral, neurolemmal portion of the cochlear nerve was normal. The inferior vestibular nerve was fibrotic. The major part of the superior vestibular nerve was normal. Some of its peripheral bundles showed increased endoneurial fibrosis. It is suggested that a demyelinating process was the cause of the patient's symptoms.

The demonstration of recurrent demyelination and remyelination of axons in the central nervous system

A model for studying recurrent demyelination and remyelination in the central nervous system was developed by means of repeated administration of Cuprizone to mice. In contrast to the demyelination seen during the first course of Cuprizone, the recurrent demyelination was markedly protracted, displayed features of a "dying-back" gliopathy, and resulted in a greatly reduced inflammatory and glial reaction. The repeat remyelination also occurred at a slower tempo, varied markedly in completeness, and was associated with a diminished regeneration of oligodendrocytes. These results demonstrated that axons of the central nervous system in this model can be recurrently remyelinated if oligodendrocytes are available, that regeneration of oligodendrocytes is dependent upon the tissue reaction to demyelination, and that remyelinated axons are not more susceptible to demyelination than normal ones.

Chronic relapsing experimental allergic encephalomyelitis. Correlation of circulating lymphocyte fluctuations with disease activity in suppressed and unsuppressed animals

Groups of juvenile Strain 13 guinea pigs sensitized for chronic relapsing experimental allergic encephalomyelitis (EAE) with isogeneic central nervous system (CNS) tissue in complete Freund's adjuvant (CFA) were either left to develop late-onset chronic EAE (unsuppressed), or given a series of injections of bovine myelin basic protein (MBP) in incomplete Freund's adjuvant (IFA) to suppress the disease. All unsuppressed animals developed disease and all suppressed animals remained healthy over a 27-month period of study. some unsuppressed and suppressed animals were rechallenged with CNS tissue in CFA 12 or 26 months post-inoculation (PI). Unsuppressed animals all became sick 2-4 weeks after rechallenge, while rechallenged, suppressed animals were protected, indicating that the suppression was permanent. Pathologic findings in the CNS complemented the clinical changes. Circulating lymphocyte studies were performed on animals from all groups. Early (active, high-affinity rosetting) T cell levels in unsuppressed animals showed significant decreases during exacerbations (P less than 0.01) and normal values during remissions. After rechallenge, circulating early T cells decreased in unsuppressed animals with the development of signs. In suppressed animals, early T cells showed significant elevations during, and for a short time after, the period of suppressive injections, and normal values afterwards. These levels did not change significantly after rechallenge. Late (total, 24 hour rosetting) T cell and B cell values showed minor fluctuations only which did not correlate with disease activity. These results indicate that chronic relapsing EAE can be successfully suppressed with MBP in IFA, that this suppression is permanent and that the immunologic findings presented correlate well with the clinical and pathologic facets of the disease. the findings are presented in terms of their relevance to multiple sclerosis.