Patterns of oligodendroglia pathology in multiple sclerosis

Oligodendrocyte and axon pathology in clinically silent multiple sclerosis lesions

Oligodendrocyte and axon pathology was studied in 11 autopsy cases of clinically silent multiple sclerosis. A total of 54 lesions, either demyelinated or late remyelinated, were distributed through the whole brain and spinal cord with 39% of the lesions located in periventricular areas. Determination of axon density revealed an average reduction of 64% and 59% in demyelinated and remyelinated lesions with an extreme variation between different plaques and cases. Oligodendrocytes were identified by immunocytochemistry for myelin oligodendrocyte glycoprotein (MOG) and in situ hybridization for proteolipid protein (PLP) mRNA. Oligodendrocytes were almost completely lost in demyelinated lesions; remyelinated lesions revealed preservation of a considerable number of oligodendrocytes within the lesions. At the border between plaques and the periplaque white matter, similar oligodendrocyte numbers as in remyelinated lesions were found. Different factors including lesion site, axonal preservation and remyelination may thus contribute to the clinical nonappearance of multiple sclerosis lesions.

Genetically determined failure of activation-induced apoptosis of autoreactive T cells as a cause of multiple sclerosis

I postulate that multiple sclerosis is an autoimmune disease that involves genetically determined failure of activation-induced apoptosis of autoreactive T cells in the central nervous system. Activation of central-nervous-system-reactive T cells in peripheral lymphoid organs by exposure to crossreacting antigens or superantigens derived from common infectious agents may trigger attacks of multiple sclerosis. In normal individuals these activated T cells are deleted by activation-induced apoptosis, but in individuals predisposed to multiple sclerosis they survive, proliferate, and damage the central nervous system. The clinical course of multiple sclerosis may vary according to the antigens in the central nervous system being targeted: targeting of myelin antigens leads to a relapsing-remitting course of clinical recovery due to remyelination or other mechanisms; targeting of axonal antigens leads to a progressive course from onset because axonal regeneration is limited in the central nervous system. This hypothesis can account for many characteristics of multiple sclerosis and has predictions that can be tested.

p75 neurotrophin receptor expression on adult human oligodendrocytes: signaling without cell death in response to NGF

Oligodendrocytes (OLs) are the primary targets in the autoimmune disease multiple sclerosis (MS). Cell receptors belonging to the tumor necrosis factor receptor (TNF-R) superfamily, such as TNF receptors and fas, are implicated in signaling the injury response of OLs. The p75 neurotrophin receptor (p75(NTR)), another member of the TNF-R superfamily, has been reported to mediate nerve growth factor (NGF)-induced apoptosis in some neural systems. To address the potential relationship between p75(NTR) signaling and OL injury, we assayed adult human OLs cultured under several different conditions for p75(NTR) and tyrosine kinase receptor trkA expression, for NGF-mediated apoptosis, and for NGF-mediated jun kinase (JNK) or nuclear factor (NF) kappaB activation. In the current study, we have found expression of p75(NTR) on cultured adult CNS-derived human OLs but not on other glial cells. TrkA was not detected on these OLs in any of the culture conditions tested. Treatment of the OLs with varying concentrations of NGF over a range of time periods resulted in no significant increase in numbers of terminal transferase (TdT)-mediated d-uridine triphosphate (UTP)-biotin nick-end labeling positive OLs, whereas significant cell death was observed using TNF-alpha. Furthermore, unlike TNF-alpha treatment, NGF treatment did not significantly activate JNK, although both TNF-alpha and NGF induced nuclear translocation of NF-kappaB. These findings contrast with the recent report of NGF-mediated apoptosis in the OLs of neonatal rats matured in vitro, which express p75(NTR) but not trkA (), and suggest that, at least in humans, p75(NTR) signaling may mediate responses other than apoptosis of OLs.

Cytokines, signal transduction, and inflammatory demyelination: review and hypothesis

The mechanism of focal demyelination in multiple sclerosis has been a long-standing enigma of this disorder. Cytokines, a diverse family of signalling molecules, are viewed as potential mediators of the process based on clinical observations and studies with animal models and tissue/cell culture systems. Myelin and oligodendrocyte (OL) destruction occur in cultured preparations subjected to cytokines such as tumor necrosis factor-alpha (TNF alpha) and lymphotoxin (LT). Many studies have shown these and other cytokines to be elevated at lesion sites and in the CSF of multiple sclerosis (MS) patients, with similar findings in animal models. Some variability in the nature of MS lesion formation has been reported, both OLs and myelin being primary targets. To account for myelin destruction in the presence of apparently functional OLs we hypothesize that cytokines such as TNF alpha and LT alpha contribute to myelin damage through triggering of specific reactions within the myelin sheath. We further propose that neutral sphingomyelinase (SMase) is one such enzyme, two forms of which have been detected in purified myelin. An additional event is accumulation of cholesterol ester, apparently a downstream consequence of cytokine-induced SMase. The resulting lipid changes are viewed as potentially destabilizing to myelin, which may render it more vulnerable to attack by invading and resident phagocytes.

Chronic stage multiple sclerosis lesions contain a relatively quiescent population of oligodendrocyte precursor cells

In the past decade, considerable progress has been made in the understanding of the biology of rodent oligodendrocyte precursor cells and their role in the generation of oligodendrocytes in the developing and adult rodent CNS. Much less is known about human oligodendrocyte lineage cells and about the reasons for the failure of the regeneration of the oligodendrocyte population during chronic stages of multiple sclerosis (MS). In particular, the fate of the oligodendrocyte precursor population in MS has remained elusive. The present study examined the possibility that oligodendrocyte regeneration ultimately fails because of the local destruction of both oligodendrocytes and their precursor cells. Analysis of chronic stage MS tissue suggested that this is not the case, because all chronic MS lesions studied contained significant numbers of oligodendrocyte precursor cells, identified as process-bearing cells that bound the O4 antibody but not antibodies to GalC and GFAP. The oligodendrocyte precursor cells appeared, however, to be relatively quiescent, because none expressed the nuclear proliferation antigen recognized by the Ki-67 antibody, and because most lesions lacked myelinating oligodendrocytes in their centers. Thus, it appears that the regeneration of the oligodendrocyte population fails during chronic stages of MS because of the inability of oligodendrocyte precursor cells to proliferate and differentiate rather than because of the local destruction of all oligodendrocyte lineage cells. The identification of ways of stimulating the endogenous oligodendrocyte precursor population to expand and generate remyelinating cells may represent an alternative to transplantation of oligodendrocyte lineage cells to promote myelin repair in MS.

Nitric oxide synthase inhibitor, aminoguanidine, reduces inflammation and demyelination produced by Theiler's virus infection

This study evaluated effects of the inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AG), on the neuropathology and clinical disease produced by Theiler's murine encephalomyelitis virus (TMEV) DA strain infection. Treatment with AG was started on day 7, 14, 28 or 66 post-inoculation and continued for a minimum of 21 days. Inflammation, demyelination and axonal necrosis were scored in a blinded fashion on spinal cord sections from each mouse. Reduction in inflammation, demyelination and axonal necrosis was observed in mice treated with AG. Apoptosis within the spinal cord parenchyma and perivascular cuffs was significantly decreased. AG treatment resulted in delayed onset of clinical disease.

Nerve growth factor signal transduction in mature pig oligodendrocytes

It has previously been shown that nerve growth factor (NGF) is of functional significance for mature pig oligodendrocytes (OLs) in culture. The present data give evidence for the expression of TrkA, the so-called high-affinity NGF receptor, and of p75NTR, the so-called low-affinity NGF receptor. TrkA is upregulated during culturing, in contrast to the p75 receptor. Exposure of OLs to NGF induces an autophosphorylation of TrkA via its intrinsic tyrosine kinase. K-252a inhibits the TrkA autophosphorylation, which reduces the OL process formation to control levels. To the tyrosine-phosphorylated sites of TrkA several proteins, such as phospholipase C-gamma1, the adaptor protein SHC, the phosphotyrosine phosphatase SH-PTP2 (SYP) associate via their SH2 phosphotase SH-PTP2 domain. The association of SHC to TrkA is shown by co-immunoprecipitation. Indirect evidence for a possible activation of PLC-gamma1 is given by an NGF-induced increase of oligodendroglial [Ca2+]i. Downstream from TrkA, a mitogen-activated protein kinase cascade, which includes Erk1 and Erk2, is operating. An in-gel myelin basic protein kinase assay revealed that NGF activates predominantly Erk1. Finally, it is shown that NGF stimulates expression of c-fos.

Repair of myelin disease: strategies and progress in animal models

Myelin disorders form an important group of human neurological diseases that are as yet incurable. Recent studies on experimental remyelination have suggested that it might be feasible to repair the CNS, either by transplanting normal myelinating cells or by enhancing endogenous repair. Progress in animal models, particularly in transplanting cells of the oligodendrocyte lineage, has resulted in significant focal remyelination and physiological evidence of restoration of function. These data suggest that focal lesions in multiple sclerosis could be repaired by the transplantation of myelin-forming cells. Future therapies could involve both transplantation and promotion of endogenous repair, and the two approaches could be combined with ex vivo manipulation of the donor tissue.

Myelin contains neutral sphingomyelinase activity that is stimulated by tumor necrosis factor-alpha

Purified myelin from mouse brain was found to contain two forms of neutral sphingomyelinase, one Mg2+ dependent and the other Mg2+ independent. The former had a pH optimum of 7.5 and Km of 0.35 mM, whereas the corresponding values for the latter were pH 8.0 and Km 3.03 mM. Specific activity of the Mg(2+)-dependent enzyme showed a rostral-caudal gradient, ranging from 75 nmol/mg protein/hr in myelin from cerebral hemispheres to 21 nmol/mg protein/hr in myelin from spinal cord. Relative specific activity was approximately 20% that of brain stem or cerebral hemisphere homogenate. Treatment of myelin with taurocholate or high salt concentration did not significantly reduce activity of the Mg(2+)-dependent enzyme. The activity of that enzyme did not change with time or in the presence or absence of protease inhibitors; by contrast, that of Mg(2+)-independent enzyme decreased sharply in the absence of protease inhibitors but rose in their presence. To test for the effect of tumor necrosis factor-alpha (TNF alpha) on myelin sphingomyelinase, mouse brain myelin was labeled in vivo by intracerebral injection of [3H]acetate into 18-20-day-old mice. After 40 hr, brain stems were removed, minced, and treated with TNF alpha in Krebs-Ringer solution, after which myelin was immediately isolated. Separation and counting of individual lipids revealed TNF alpha treatment to cause increased labeling of myelin ceramide and cholesterol ester with concomitant decrease in myelin sphingomyelin. Western blotting of myelin proteins using antibodies to the two TNF alpha receptors as probes revealed the presence of the p75 receptor. Implications of these findings in relation to possible mechanisms of autoimmune demyelination are discussed.

Expression of alphaB-crystallin in glia cells during lesional development in multiple sclerosis

The small heat shock protein alphaB-crystallin was recently identified as a dominant human T-cell antigen in myelin derived from multiple sclerosis (MS) patients. Using immunohistochemical techniques, oligodendrocytes as well as astrocytes in MS lesions were shown to express alphaB-crystallin. In the present study we examined the expression of alphaB-crystallin, human natural killer cell marker (HNK-1; as a marker for immature oligodendrocytes) and heat shock protein 60 (hsp60) in glia cells at different stages of MS lesion development i.e. in early active lesions, late active lesions and inactive lesions. The results demonstrate that already at the earliest stages of lesional development a subpopulation of oligodendrocytes express detectable levels of alphaB-crystallin. In active lesions about 5-10% of all oligodendrocytes were found to express alphaB-crystallin, whereas in inactive lesions the relative number of alphaB-crystallin-expressing oligodendrocytes was approximately tenfold less. For astrocytes the relative number of alphaB-crystallin-expressing cells was 40-50% for all three types of lesions. Also, alphaB-crystallin-expressing oligodendrocytes and astrocytes displayed different patterns of distribution in lesional areas. These data suggest different regulatory pathways for alphaB-crystallin expression in either type of glia cell. No correlation was found between expression patterns of HNK-1 and alphaB-crystallin indicating that the subpopulation of alphaB-crystallin-expressing oligodendrocytes consisted of both mature and immature oligodendrocytes. In addition, no correlation was found between expression of hsp60 and alphaB-crystallin in MS lesions suggesting different regulatory pathways for either hsp.

T-cell apoptosis in autoimmune diseases: termination of inflammation in the nervous system and other sites with specialized immune-defense mechanisms

We have studied T-cell apoptosis in animal models of human autoimmune disorders of the nervous system and in other tissues devoid of specialized immune-defense mechanisms. Our data suggest that the CNS has high potential for elimination of T-cell-dependent inflammation, whereas this mechanism is less effective in the PNS, and is almost absent in other tissues such as muscle and skin. Interestingly, several conventional and novel immunotherapeutic approaches, such as glucocorticosteroid and high-dose antigen therapy, induce T-cell apoptosis in situ. In vitro experiments suggest different scenarios for the mechanisms by which specific cellular and humoral elements in the nervous system synergize and sensitize T cells for apoptosis in vivo. We also discuss regulatory, proapoptotic mechanisms, such as the Fas-FasL system and galectin-I, that have been utilized in other tissues to mediate immune protection.

High levels of cerebrospinal fluid IgM binding to myelin basic protein are associated with early benign course in multiple sclerosis

We assessed human myelin basic protein (MBP) binding IgM levels in CSF. MBP is the most studied putative antigen in multiple sclerosis (MS) and immune responses against it may be involved in the demyelination process. We also correlated these levels with EDSS score and other parameters of disease progression and prognosis, both at the time of CSF analysis and during follow-up. CSF IgM anti-MBP levels were assayed by measuring total IgM levels with solid-phase ELISA in CSF samples from 66 patients with relapsing-remitting MS, 11 subjects without neurological diseases, 20 patients with non-inflammatory neurological diseases and 7 patients with lymphocytic meningitis, before and after immunoabsorption with human MBP. Confirmation of IgM binding specificity was performed by immunoblotting of positive CSF samples onto MBP coated-nitrocellulose sheets. Clinical evaluation (disability score, number and time of attacks) was performed during a mean follow-up of 2.7 +/- 1.1 years. 23 of the 66 relapsing-remitting MS patients (33.8%) had elevated IgM anti-MBP levels. In this patient subgroup, IgM anti-MBP levels correlated with the IgM index (r = 0.71; P = 0.0001), but not with CSF/serum albumin (r = 0.08; P = 0.72). In the first year of follow-up, patients with low IgM anti-MBP suffered from more numerous attacks than those with elevated levels (0.86 +/- 0.63 versus 0.43 +/- 0.58; P = 0.017). Patients with high IgM binding to MBP had a first attack during follow-up in a significantly higher time than those with low binding (28.87 +/- 4.7 versus 17 +/- 2.6 months, respectively; P = 0.005) and reached a decrease of 0.5 EDSS point significantly faster than those with low IgM (16.17 +/- 1.2 versus 29.7 +/- 2.6 months, respectively; P = 0.0002). A similar significant finding was observed when the time to reach low disability score (EDSS < or = 2.0) was analyzed (10.7 +/- 2.57 +/- 3.3 months, respectively; P = 0.014). These findings demonstrate that in a subgroup of MS patients, elevated CSF levels of IgM anti-MBP are associated with early favorable course and therefore suggest that IgM binding to MBP could be a possible prognostic marker in relapsing-remitting MS to select early MS patients for future trials.

The controversy surrounding the pathogenesis of the multiple sclerosis lesion

The main issues in multiple sclerosis research revolve around four fundamental questions. (1) What initiates the disease-that is, autoimmune T cells, a virus, or a toxin? (2) Is the inflammatory response primary to the development of demyelination, or is it a secondary response to injury? (3) Is the oligodendrocyte, the myelin-producing cell, the primary target? (4) How can myelin repair be promoted? This review focuses on the controversies revolving around these important questions. Although many investigators believe that T-cell receptors on CD4+ cells interact with myelin antigens to initiate an inflammatory cascade that leads to myelin destruction, others maintain that a viral agent may have a direct or indirect role in the pathogenesis of multiple sclerosis. The concept that the immune system contributes to the tissue destruction in multiple sclerosis is generally accepted; however, the debate about cause versus consequence of the pathologic process remains unresolved, as does the identification of the initial event or focus of the damage. Electron microscopic studies have disclosed evidence of remyelination (albeit often incomplete) in lesions of multiple sclerosis. Enhanced understanding of the factors limiting remyelination could help formulate strategies to promote repair. By innovative experimental design and application of available molecular techniques, the answers to these questions may provide insights on how to prevent or treat multiple sclerosis.

Cell death and birth in multiple sclerosis brain

The hallmark of the brain pathology in multiple sclerosis is the white matter plaque, characterized by myelin destruction and oligodendrocyte loss. To examine the role that cell death plays in the development of MS lesions, we used the in situ TUNEL technique, a method that sensitively detects DNA fragmentation associated with death at the single cell level. We found that patchy areas within acute MS lesions have massive numbers of inflammatory and glial cells undergoing cell death. The punched out areas of some long-standing chronic lesions also had labeled glial cells showing that the attack was not a single event. Immunocytochemical identification of the dying cells with glial specific marker co-labeling showed that 14-40% were the myelin-sustaining oligodendroglial cell. Confocal microscopic evaluation of fluorescein-labeled TUNEL positive cells revealed nuclei with morphologic characteristics of apoptosis, and electrophoresed MS brain DNA produced a ladder characteristic of apoptotic DNA cleavage confirming that substantial numbers of labeled cells, but not necessarily all, were dying by apoptotic mechanisms rather than cell necrosis. Companion studies using a marker for cell proliferation on MS lesions revealed that unexpectedly large populations of perivascular inflammatory cells and parenchymal glial cells had entered the cell proliferation cycle. These findings establish that two opposing glial cell responses - relentless cell death and coincident brisk cellular proliferation - are important features of MS pathology. In the end, however, glial cell loss prevails, and we suspect apoptosis may be the critical death mechanism responsible for the depletion of myelin observed in this condition.

Multiple sclerosis: oligodendrocytes display cell death-related molecules in situ but do not undergo apoptosis

To investigate whether apoptosis is involved in the fate of oligodendrocytes in chronic multiple sclerosis lesions, the pro-apoptotic molecules fas and tumor necrosis factor receptors and the anti-apoptotic molecule bcl-2 were examined by immunohistochemistry, and DNA fragmentation was assessed by an end labeling technique. Fas and both tumor necrosis factor receptors were preferentially expressed on oligodendrocytes in multiple sclerosis lesions, this phenotype being more evident at the lesion edge. The ligand for fasL, was constitutively present at high levels on microglia. The anti-apoptotic molecule bcl-2 was selectively expressed on oligodendrocytes in silent lesions and on astrocytes in active lesions. These molecules were also detected in control material, albeit at lower levels. In chronic active lesions, a few inflammatory cells displayed fas reactivity, whereas the majority expressed bcl-2. DNA fragmentation was found in a number of infiltrating cells and some microglia, whereas, with one possible exception, oligodendrocytes showed no evidence of apoptosis. Thus, while apoptosis is involved in the elimination of infiltrating cells, it plays little or no role in oligodendrocyte depletion in multiple sclerosis, a process that may be related to a lytic pathway. In addition, microglia constitutively displayed the ligand for fas, and appeared to be the major effector cell population in the central nervous system.

Promotion of endogenous remyelination in multiple sclerosis

Studies in both human and experimental models demonstrate that myelin repair occurs in the central nervous system and is a normal physiologic response to myelin injury. However, remyelination in MS is often incomplete and limited. The outcome of an actively demyelinating lesion depends on the balance between factors promoting myelin destruction and myelin repair. Experimental models of CNS demyelination provide an opportunity to investigate the morphologic, cellular and molecular mechanisms involved in remyelination. This review focuses on experiments using the Theiler's virus model of demylination which indicate that manipulation of the immune response has the potential to promote endogenous CNS remyelination and functional recovery in MS.

Remyelination in multiple sclerosis

Stimulation of remyelination in multiple sclerosis lesions may be one possible therapeutic approach. However, since the pathology and pathogenesis of multiple sclerosis may be heterogeneous, it is important to analyze in what patients and at what stages of the disease such a therapy can be reasonable and feasible. Spontaneous remyelination is pronounced in some MS patients. The extent of remyelination correlates well with the presence of oligodendrocytes in the lesions. Yet, the source and origin of the remyelinating cells is still unclear. These may be derived from either undifferentiated progenitor cells or, in part from mature oligodendrocytes, that have escaped destruction during lesional activity. Complete remyelination of plaques is possible, leading to the formation of 'shadow plaques'. No reliable data are available at present on the frequency of remyelination in different forms of MS. However, most studies agree that remyelination is especially prominent at the early stages of the disease, whereas it is sparse after several years of disease duration. In addition, very little remyelination is found in cases of primary progressive MS.

Imaging of remyelination

Now that the concept of remyelination is gaining acceptance in MS, there is a need for a paraclinical marker to monitor remyelination in MS, for example to study the effect of therapeutical interventions. At present there is no known imaging marker for remyelination. While positron emission tomography (PET) has ample sensitivity and specificity, an appropriate tracer is locking and spatial resolution is poor. Magnetic resonance (MR) imaging has superior sensitivity and spatial resolution, but the standard techniques lack specificity. Conventional T2-weighted images will most probably still appear abnormal in an area of remyelination. Among several newer MR techniques that are more specific, magnetization transfer contrast is the best candidate with regards to imaging remyelination. There is a strong need for an appropriate animal model and for histopathological confirmation to be able to further develop both PET and MR.

Apoptosis in acute and chronic central nervous system disease induced by Theiler's murine encephalomyelitis virus

Apoptosis has been observed in neural development and in various neurological diseases, including viral infection and multiple sclerosis. Theiler's murine encephalomyelitis virus is divided into two subgroups based on neurovirulence: the highly neurovirulent GDVII strain produces an acute fatal polioencephalomyelitis in mice, whereas the attenuated DA strain produces demyelination with virus persistence preceded by an acute infection. TUNEL combined with immunocytochemistry was used to detect apoptosis in the central nervous system and to characterize which cell types were involved during the acute stage in both GDVII and DA virus infection and during the chronic stage in DA virus infection. We found that during the acute stage, apoptosis was induced in neurons in both virus infections. However, the number of apoptotic neurons was much greater in GDVII virus-infected mice than in DA virus-infected mice (P < 0.01). During the chronic stage of DA virus infection, apoptotic cells were detected only in the spinal cord white matter. Some of these cells were dual labeled for fragmented DNA and carbonic anhydrase II, an oligodendrocyte marker. Our results indicate that apoptosis of neurons could be responsible for the fatal outcome in GDVII virus infection. In contrast, apoptosis of oligodendrocytes can contribute to the chronic demyelinating DA virus infection.

The effect of insulin-like growth factors on brain myelination and their potential therapeutic application in myelination disorders

Degenerative disorders of the cerebral white matter, leukodystrophies and demyelination diseases, are characterized by the faulty formation or excessive breakdown of myelin. Insulin-like growth factors (IGFs) promote the proliferation of oligodendrocytes as well as their myelin synthesis. IGF-I overexpressing mice show a significant increase in brain weight associated with increased myelin content. In contrast, the brains of IGF-binding protein-1 transgenic mice show a dramatic decrease in myelination. Furthermore, IGFs and IGF-binding proteins are among the factors that are induced by brain injury and have neuroprotective effects. IGFs also induce neurite growth and survival, in particular in glial cells of the peripheral nervous system. In demyelinating diseases, IGF-I may be useful for reducing myelin breakdown and promoting myelin regeneration. These observations may lead to new therapeutic applications for IGFs, for example promoting remyelination or limiting damage following brain injury.

Patterns of oligodendrocyte pathology in coronavirus-induced subacute demyelinating encephalomyelitis in the Lewis rat

Intracerebral infection of rats with JHM coronavirus induces a chronic inflammatory demyelinating disease, which in many respects mimicks the pathology of multiple sclerosis. We investigated the patterns of demyelination and oligodendrocyte pathology in this model. In early stages of the disease infection of oligodendrocytes was associated with a downregulation of expression of mRNA for proteolipid protein in the absence of myelin destruction. When demyelinating lesions were formed infected oligodendrocytes were destroyed by necrosis, whereas oligodendrocytes that did not contain detectable virus antigen or RNA were in part dying by apoptosis. At this stage of the disease remyelination of the lesions was pronounced. At later stages after infection virus antigen was nearly completely cleared from the lesions. In spite of the lack of detectable virus, ongoing demyelination and unspecific tissue destruction occurred, and oligodendrocytes were mainly destroyed by apoptosis. These late lesions revealed only minimal central remyelination, but they were frequently repaired by Schwann cells. Our studies suggest that the mechanisms of myelin destruction in this model of virus-induced demyelination are complex and that the patterns of tissue damage may change during the course of the disease.

Basic mechanisms of brain inflammation

The mechanisms, how the immune system surveys the nervous tissue and how brain inflammation is regulated are essential questions for therapy of neuroimmunological diseases. The nervous system is continuously patrolled by hematogenous cells, which may pass the blood brain barrier in an activated state. When these cells find their respective target antigen in the CNS compartment, an inflammatory reaction is started through the secretion of proinflammatory cytokines. This leads to the upregulation of endothelial adhesion molecules and the local production of chemokines, which in concert facilitate the entry of inflammatory effector cells into the lesions. T-lymphocytes are effectively removed from inflammatory brain lesions by local apoptosis. In addition some lymphatic drainage of the nervous system allows the removal of effector cells from the lesions and their migration into regional lymph nodes. In summary these data suggest that the immune surveillance of the central nervous system is much more tightly controlled compared to that in other organs.

Multiple sclerosis: Fas signaling in oligodendrocyte cell death

Fas is a cell surface receptor that transduces cell death signals when cross-linked by agonist antibodies or by fas ligand. In this study, we examined the potential of fas to contribute to oligodendrocyte (OL) injury and demyelination as they occur in the human demyelinating disease multiple sclerosis (MS). Immunohistochemical study of central nervous system (CNS) tissue from MS subjects demonstrated elevated fas expression on OLs in chronic active and chronic silent MS lesions compared with OLs in control tissue from subjects with or without other neurologic diseases. In such lesions, microglia and infiltrating lymphocytes displayed intense immunoreactivity to fas ligand. In dissociated glial cell cultures prepared from human adult CNS tissue, fas expression was restricted to OLs. Fas ligation with the anti-fas monoclonal antibody M3 or with the fas-ligand induced rapid OL cell membrane lysis, assessed by LDH release and trypan blue uptake and subsequent cell death. In contrast to the activity of fas in other cellular systems, dying OLs did not exhibit evidence of apoptosis, assessed morphologically and by terminal transferase-mediated d-uridine triphosphate-biotin nick-end-labeling staining for DNA fragmentation. Other stimuli such as C2-ceramide were capable of inducing rapid apoptosis in OLs. Antibodies directed at other surface molecules expressed on OLs or the M33 non-activating anti-fas monoclonal antibody did not induce cytolysis of OLs. Our results suggest that fas-mediated signaling might contribute in a novel cytolytic manner to immune-mediated OL injury in MS.

Macrophages in multiple sclerosis

Macrophages are important effector cells involved in the pathogenesis of demyelination in multiple sclerosis (MS). Macrophage differentiation was studied in a series of 158 MS plaques from 43 patients obtained at different stages of the disease. Macrophages were identified by immunocytochemistry using a panel of antibodies recognizing different formalin- and paraffin-resistant macrophage activation antigens. The number of cells stained with each antibody was related to the demyelinating activity of the lesions as detected by the presence of myelin degradation products as well as to the category of MS tissue. Highest numbers of macrophages were observed in actively demyelinating and early remyelinating lesions using immunocytochemistry for the panmacrophage marker Ki-M1P. Lower numbers were encountered in inactive, demyelinated or late remyelinated lesions. The acute stage inflammatory macrophage markers MRP14 and 27E10 were selectively expressed in early and late active lesions, thus allowing the identification of actively demyelinating lesions. The chronic stage inflammatory macrophage marker 25F9, in contrast, showed a continuous expression also in inactive lesions. The different types of MS tissue revealed significant differences in their macrophage response. The most intense macrophage infiltration was seen in acute MS cases whereas lesions of early and late chronic MS showed lower macrophage levels. These findings indicate a differentiated pattern of macrophage activation in MS depending on the stage of the demyelinating activity as well as on the category of MS tissue. Furthermore, these macrophage markers give new parameters for staging the inflammatory and demyelinating activity of MS lesions.

Induction of myelination in the central nervous system by electrical activity

The oligodendrocyte is the myelin-forming cell in the central nervous system. Despite the close interaction between axons and oligodendrocytes, there is little evidence that neurons influence myelinogenesis. On the contrary, newly differentiated oligodendrocytes, which mature in culture in the total absence of neurons, synthesize the myelin-specific constituents of oligodendrocytes differentiated in vivo and even form myelin-like figures. Neuronal electrical activity may be required, however, for the appropriate formation of the myelin sheath. To investigate the role of electrical activity on myelin formation, we have used highly specific neurotoxins, which can either block (tetrodotoxin) or increase (alpha-scorpion toxin) the firing of neurons. We show that myelination can be inhibited by blocking the action potential of neighboring axons or enhanced by increasing their electrical activity, clearly linking neuronal electrical activity to myelinogenesis.

Jun expression is found in neurons located in the vicinity of subacute plaques in patients with multiple sclerosis

The expression of members of the Fos and Jun families is examined by immunohistochemistry in the brains of two patients with multiple sclerosis (MS). Strong c-Jun immunoreactivity is observed in the cytoplasm of neurons located in the vicinity of subacute plaques, but not in neurons of brain compartments not compromised by MS and in the neighborhood of chronic plaques. Strong c-Jun immunoreactivity also contrast with weak c-Jun immunoreactivity of corresponding neurons in control brains. In addition, punctate Jun D immunoreactivity is observed in the neuropil of the same areas that express c-Jun. No immunoreaction is found to c-Fos, Fos-related antigens and Jun B in these areas. The present results suggest that selective Jun neuronal expression in the vicinity of subacute plaques is a consistent reaction to demyelination and axonal damage.

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.

Distinct patterns of multiple sclerosis pathology indicates heterogeneity on pathogenesis

Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system. The hallmark of its pathology is the demyelinated plaque with reactive glial scar formation. However, a detailed analysis of the patterns of demyelination, oligodendroglia cell pathology and the reaction of other tissue components suggests that the pathogenesis of myelin destruction in this disease may be heterogeneous. In this review we present a new classification scheme of lesional activity on the basis of the molecular composition of myelin degradation products in macrophages. When these criteria are used, different patterns of demyelination can be distinguished, including demyelination with relative preservation of oligodendrocytes, myelin destruction with concomitant and complete destruction of oligodendrocytes or primary destruction or disturbance of myelinating cells with secondary demyelination. Furthermore, in some cases a primary selective demyelination may be followed by a secondary oligodendrocyte loss in the established lesions. Finally, some extraordinarily severe conditions may result in destructive lesions with loss of myelin, oligodendrocytes, axons and astrocytes. This heterogeneity of plaque pathology is discussed in the context of recent experimental models of inflammatory demyelination, which show that different immunological pathways may lead to the formation of demyelinated plaques that reveal the diverse structural aspects described above. Our data indicate, that the demyelinated plaques of multiple sclerosis may reflect a common pathological end point of a variety of different immunological mechanisms of myelin destruction in this disease.

Multiple sclerosis: an altered immune response or an altered stress response?

The pathogenesis of multiple sclerosis (MS), the major neurological disease of young adults in the Western world, is still poorly understood, and no effective therapy to block MS is available as yet. The clinical symptoms of MS result from inflammatory damage to the insulating myelin sheath of axons in the CNS and-at later stages-to axons themselves. A local autoimmune process involving activation of helper T cells against CNS protein components is likely to be crucial in this development. Especially at the first stages of MS, therapies aimed at the selective downregulation of MS-specific autoimmune responses may contribute to controlling the disease. Key to the success of such approaches is the identification of CNS proteins that are the target of local T cell responses. We recently identified the small heat-shock protein alpha B-crystallin as the single immunodominant myelin antigen in MS-affected myelin. This review discusses the functional and therapeutic implications of this finding along with other data on MS, and hypothesizes that an inappropriate stress response within the CNS itself is crucial as an initiating event in disease development.

Macrophage subtyping in the determination of age of injection sites

Determination of the age of injection marks in skin may be of particular interest in the investigation of drug abuse-related fatalities. The aim of our study was to assess the value of macrophage subtyping by antibody-based markers in the determination of the age of injection marks. Immunohistochemical investigations were performed with the antibodies Ki-M1P, 27E10, MRP14, MRP8 and 25F9. Monocytes/macrophages in acute lesions (several hours to 2 days old) expressed proteins detectable with the antibodies 27E10 and MRP14 and showed acute erythrophagia. An additional reaction with the antibody MRP8 was seen in lesions a few days old. An antigen recognized by the antibody 25F9 was found in tissue macrophages, multinucleated giant cells of active granulomas and siderophages. The expression of the 25F9 detectable antigen was absent in inactive granulomas and siderophages, whereas the macrophages were always detectable with the pan-macrophage marker Ki-M1P.

The role of macrophage subpopulations in autoimmune disease of the central nervous system

In this review the role of various subpopulations of macrophages in the pathogenesis of experimental autoimmune encephalomyetitis is discussed. Immunohistochemistry with macrophage markers shows that in this disease different populations of macrophages (i.e. perivascular cells, microglia and infiltrating blood-borne macrophages) are present in the central nervous system. These subpopulations partially overlap in some functional activity while other activities seem to be restricted to a distinct subpopulation, indicating that these subpopulations have different roles in the pathogenesis of encephalomyelitis. The studies discussed in this review reveal that immunocytochemical and morphological studies, combined with new techniques such as in situ nick translation and experimental approaches like the use of bone marrow chimeras and macrophage depletion techniques, give valuable information about the types and functions of cells involved in central nervous system inflammation. The review is divided in three parts. In the first part the experimental autoimmune encephalomyelitis model is introduced. The second part gives an overview of the origin, morphology and functions of the various subpopulations. In the third part the role of these subpopulations is discussed in relation to the various stages (i.e. preclinical, clinical and recovery) of the experimental disease.

Apoptosis in brain-specific autoimmune disease

Recent neuropathological studies of experimental autoimmune encephalomyelitis have focused attention on the high number of cells in the lesions that show typical morphological features of apoptosis. Surprisingly, it has turned out that the vast majority of apoptotic cells are T lymphocytes and that they actually represent the antigen-specific T-cell population responsible for the induction of the disease. Taken together, these data suggest that clearance of autoimmune inflammation in the nervous system is accomplished by the destruction of the antigen-specific T-cell population within the lesions. This may explain the low level of central nervous system specific T-cell memory formation, as well as previously unexplained phenomena of 'epitope spreading', in autoimmune inflammation of the nervous system.

Monocyte/macrophage differentiation in early multiple sclerosis lesions

Monocyte/macrophage differentiation was studied in biopsy samples of multiple sclerosis (MS) lesions obtained in the early course of the disease. Macrophages were identified by immunocytochemistry using a panel of antibodies recognizing different macrophage-activation antigens. The number of cells stained with each antibody was related to the demyelinating activity of the lesions as detected by the presence of myelin degradation products. The pan-macrophage marker Ki-M1P revealed the highest numbers of macrophages in early and late active lesions. Lower numbers were encountered in inactive, demyelinated, or remyelinated lesions. The acute stage inflammatory macrophage markers MRP14 and 27E10 were expressed in either only early active (MRP14) or early and late active (27E10) lesions, thus allowing the identification of actively demyelinating lesions. The chronic stage inflammatory macrophage marker 25F9, in contrast, showed increasing expression with decreasing lesional activity. These findings indicate a differentiated pattern of macrophage activation in MS lesions and allow the staging of demyelinating lesions in routinely fixed and paraffin-embedded tissue.

The small heat-shock protein alpha B-crystallin as candidate autoantigen in multiple sclerosis

The identification of key antigens in human autoimmune diseases is a crucial step towards the development of specific intervention. The autoantigen(s) relevant to multiple sclerosis (MS) probably reside in myelin of the central nervous system, the target of the disease. Here we examine proliferative responses of human peripheral blood T cells to the complete collection of myelin proteins fractionated by reversed-phase high-performance liquid chromatography. Myelin isolated from MS-affected brain contained a single protein fraction to which T cells from MS patients and from healthy controls showed dominant responses. This highly immunogenic protein was identified as alpha B-crystallin, a small heat-shock protein. Immunohistochemical examination of MS lesions revealed the presence of oligodendrocytes and astrocytes with raised alpha B-crystallin expression, which were not found in unaffected myelin. Our findings indicate that alpha B-crystallin serves as immunodominant myelin antigen to human T cells when expressed at the elevated levels found in active MS lesions.

Insulin-like growth factor I treatment reduces demyelination and up-regulates gene expression of myelin-related proteins in experimental autoimmune encephalomyelitis

To compare effects of insulin-like growth factor I (IGF-I) and placebo treatment on lesions that resemble those seen during active demyelination in multiple sclerosis, we induced experimental autoimmune encephalomyelitis in Lewis rats with an emulsion containing guinea pig spinal cord and Freund's adjuvant. On day 12-13, pairs of rats with the same degree of weakness were given either IGF-I or placebo intravenously twice daily for 8 days. After 8 days of placebo or IGF-I (200 micrograms/day or 1 mg/day) treatment, the spinal cord lesions were studied by in situ hybridization and with immunocytochemical and morphological methods. IGF-I produced significant reductions in numbers and areas of demyelinating lesions. These lesions contained axons surrounded by regenerating myelin segments instead of demyelinated axons seen in the placebo-treated rats. Relative mRNA levels for myelin basic protein, proteolipid protein (PLP), and 2',3'-cyclic nucleotide 3'-phosphodiesterase in lesions of IGF-I-treated rats were significantly higher than they were in placebo-treated rats. PLP mRNA-containing oligodendroglia also were more numerous and relative PLP mRNA levels per oligodendrocyte were higher in lesions of IGF-I-treated rats. Finally, a significantly higher proportion of proliferating cells were oligodendroglia-like cells in lesions of IGF-I-treated rats. We think that IGF-I effects on oligodendrocytes, myelin protein synthesis, and myelin regeneration reduced lesion severity and promoted clinical recovery in this experimental autoimmune encephalomyelitis model. These IGF-I actions may also benefit patients with multiple sclerosis.

Insulin-like growth factor I treatment reduces clinical deficits and lesion severity in acute demyelinating experimental autoimmune encephalomyelitis

Our goal was to test the effects of insulin-like growth factor I (IGF-I) treatment on clinical deficits, lesion number and lesion size in acute demyelinating experimental autoimmune encephalomyelitis (EAE) induced in Lewis rats with an emulsion containing guinea pig spinal cord. In this EAE model, there is severe immune-mediated demyelination, which resembles that seen in actively demyelinating MS lesions. On day 12-13 after EAE induction, a total of 23 pairs of rats with the same mild degree of tail and hind limb weakness were given either intravenous IGF-I or placebo twice daily for 8 days. The daily IGF-I dose used in the first trial was 200 micrograms (about 0.6 mg kg-1) and in the second and third trials was 1 mg (about 3.0 mg kg-1). IGF-I treatment reduced permeability of the blood-spinal cord barrier to Evans blue-albumin. Maximum clinical deficit scores of IGF-I-treated rats were significantly lower and treated rats recovered faster than controls. IGF-I treatment produced significant reductions in weight loss and hind limb weakness. Treatment also improved treadmill walking, stride length and climbing performance. Morphometric analysis showed that spinal cord inflammatory lesions were significantly smaller and fewer in IGF-I-treated rats. The higher IGF-I dose produced a greater reduction in clinical and pathological deficits. We conclude that IGF-I treatment promotes clinical recovery by reducing EAE-induced blood-spinal cord barrier changes and the associated immune-mediated inflammatory lesions. Our results suggest that IGF-I may be useful in treating patients with multiple sclerosis and other demyelinating diseases.

Cell death in Alzheimer's disease evaluated by DNA fragmentation in situ

Loss of nerve cells is a hallmark of the pathology of Alzheimer's disease (AD), yet the patterns of cell death are unknown. By analyzing DNA fragmentation in situ we found evidence for cell death not only of nerve cells but also of oligodendrocytes and microglia in AD brains. In average, 30 times more brain cells showed DNA fragmentation in AD as compared to age-matched controls. Nuclear alterations suggestive of apoptosis were rare in degenerating cells. Even though the majority of degenerating cells were not located within amyloid deposits and did not contain neurofibrillary tangles, neurons situated within areas of amyloid deposits or affected by neurofibrillary degeneration revealed a higher risk of DNA fragmentation and death than cells not exposed to these AD changes.