Wolfgang Brück

Die klinisch-neuropathologische Multiple Sklerose (MS)-Forschung konnte in den letzten Jahren deutliche Fortschritte verzeichnen. Wesentliche Erkenntnisse ergaben sich vor allem aus der systematischen Untersuchung früher MS-Läsionen hirnbiopsierter Patienten. Hier steht der Befund einer differenziellen Myelin- und Oligodendrozytenschädigung beim individuellen MS-Patienten im Vordergrund. Ein weiterer Paradigmenwechsel ergab sich durch den Befund, dass das Ausmaß der Schädigung der Nervenzellfortsätze die Langzeitprognose der Patienten bestimmt. Großes Augenmerk wird auf die Erforschung der endogenen Reparaturkapazität und deren therapeutischer Stimulation gelegt. Im Folgenden werden der aktuelle Stand der neuropathologischen MS-Forschung sowie wesentliche Forschungsansätze im Überblick dargestellt. Recent years saw important advances in clinical and neuropathological MS research. Large systematical studies on early MS lesions underline the fact that multiple mechanisms of myelin and oligodendrocyte damage may lead to the formation of an inflammatory-demyelinated lesion. A further paradigm shift was induced by the finding that axonal loss is a key determinant of permanent clinical disability. In addition to identifying mediators of axonal damage and developing axon- and neuroprotective strategies, current research focuses on the elucidation and stimulation of endogenous repair processes. A short overview of current topics and future directions of MS research is given.

Multiple sclerosis is characterized morphologically by the key features demyelination, inflammation, gliosis and axonal damage. In recent years, it has become more evident that axonal damage is the major morphological substrate of permanent clinical disability. In our study, we investigated the occurrence of acute axonal damage determined by immunocytochemistry for amyloid precursor protein (APP) which is produced in neurones and accumulates at sites of recent axon transection or damage. The numbers of APP‐positive axons in multiple sclerosis lesions were correlated with the disease duration and course. Most APP‐positive axons were detected within the first year after disease onset, but acute axonal damage was also detected to a minor degree in lesions of patients with a disease duration of 10 years and more. This effect was not due to the lack of active demyelinating lesions in the chronic disease stage. Late remyelinated lesions (so‐called shadow plaques) did not show signs of axon destruction. The number of inflammatory cells showed a decrease over time similar to that of the number of APP‐positive axons. There was a significant correlation between the extent of axon damage and the numbers of CD8‐positive cytotoxic T cells and macrophages/microglia. Our results indicate that a putative axon‐protective treatment should start as early as possible and include strategies preventing T cell/macrophage‐mediated axon destruction and leading to remyelination of axons.

The activation of macrophages/microglia in multiple sclerosis (MS) lesions plays a central role in the effector phase of myelin breakdown. The precise patterns of macrophage/microglia activation during demyelination have not yet been defined. The growth and activating factor macrophage‐colony stimulating factor (M‐CSF) and its specific receptor (M‐CSFR) may be involved in this process. The present study investigated the expression of M‐CSF and M‐CSFR mRNA by in situ hybridization in 60 lesions from 32 MS patients. In the control and periplaque white matter, microglia was almost completely M‐CSFR positive. Irrespective of the demyelinating activity, an increased number of cells expressed M‐CSF or M‐CSFR mRNA within the lesions. However, despite the tremendous increase in macrophages/microglia within the lesions, the relative number of these cells expressing M‐CSF or M‐CSFR decreased. There was no correlation of M‐CSF or M‐CSFR expression with active myelin breakdown. The correlation between the clinical course and the expression of M‐CSF or M‐CSFR mRNA revealed significant differences with the lowest expression in primary progressive MS. These results suggest a downregulation of M‐CSF and M‐CSFR inside the MS plaque probably due to the high amount of macrophage‐derived cytokines or mediators. Nevertheless, the differences in the relative number of cells expressing the M‐CSF/M‐CSFR pathway implicate that this pathway may be an important contributory factor in different forms of MS pathology. GLIA 40:121–129, 2002.

The present review will focus on the current knowledge of the pathology of primary progressive multiple sclerosis lesions. Multiple sclerosis (MS) is an inflammatory demyelinating disease with a broad clinical variability. The main disease courses are relapsing-remitting, secondary progressive and primary progressive MS. Pathological studies examining the specific underlying pathology of a defined clinical subtype are rare. Here, we focus on the phatological characteristics of the MS lesions and summarize the current findings of the phatology of primary progressive MS with respect to inflammation, oligodendrocyte/myelin pathology, axon destruction and immunopathology of the lesions.

Macrophages/microglia are the key effector cells in myelin removal. Differences exist in the amount and time course of myelin uptake in the central (CNS) and peripheral nervous system (PNS), the basis of this difference, however, is not yet clarified. In the present experiments we studied the phagocytosis rate of CNS or PNS myelin by macrophages and microglia in vitro. Additionally, the effects of intravenous immunoglobulins (IVIg) on this process were investigated. In the PNS experiments, sciatic nerves were cocultured with peritoneal macrophages. Optic nerve fragments were used to characterize the myelin‐removing properties of microglia. Cocultures with peritoneal macrophages aimed at investigating the differences in phagocytosis between resident microglia and added macrophages. The myelin phagocytosis in sciatic nerve fragments was higher than in optic nerves, indicating differences in the myelin uptake rate between peripheral macrophages and microglia. IVIg increased the phagocytosis of PNS myelin by macrophages, but not by microglia in optic nerves. The addition of peritoneal macrophages to optic nerve fragments did not lead to an increase in the phagocytosis of CNS myelin either. The IVIg induced phagocytosis of PNS myelin by peripheral macrophages was associated with an increased expression of macrophage Fc receptors measured by FACS. Blocking of Fc receptors by anti‐Fc receptor antibody reduced the IVIg induced PNS myelin phagocytosis to basic levels, indicating that the induced but not the basic myelin uptake by macrophages is Fc receptor dependent. In contrast to peripheral macrophages, IVIg did not increase Fc receptor density on microglia. These data indicate that phagocytosis of PNS and CNS myelin by macrophages or microglia is differentially regulated. Local factors within the CNS or PNS may affect this process by modulating the surface receptor profile and activation state of the phagocytic cell or the structure of the myelin sheath.

There has been tremendous progress in the immunomodulatory treatment of multiple sclerosis (MS) during recent years. With the introduction of interferon-β, glatiramer acetate and mitoxantrone (recently registered for MS in the US), there are at least three therapeutic strategies that have proven effective in large phase III studies. However, not all patients with MS respond well to treatment with these drugs. This may largely be a consequence of disease heterogeneity. From a clinical perspective, patients with different disease courses show different treatment responses. Patients with relapsing-remitting MS are more likely to respond to immunomodulatory therapy than those with a progressive disease course. Studies of patients with secondary progressive MS have yielded inconsistent results and, so far, there has been no positive phase III study of immunomodulatory therapy in patients with primary progressive MS. Pathological evidence indicates that subtyping based on clinical findings alone does not reflect actual disease heterogeneity. In a large series of biopsy and autopsy specimens, at least four subtypes could be identified with respect to oligodendrocyte/myelin pathology and immunopathology. As long as the only method of identifying subtypes of disease is histopathology, differential therapy will remain a future goal. Thus, there is an urgent need for in vivo markers of immunopathogenesis in an individual patient that would allow treatment to be specifically directed towards a given pathological focus. However, at least from a theoretical point of view, some therapeutic approaches appear very attractive. Plasmapheresis and/or intravenous immunoglobulins could most plausibly be the best approach for the immunopathological subtype of MS, which is characterised by antibody and complement deposition next to demyelinated axons, in order to remove antibodies. The subtype of MS that is associated with heavy macrophage activation, T cell infiltration and expression of inflammatory mediator molecules, including tumour necrosis factor-α, may be most likely to respond to immunomodulation with interferon-β or glatiramer acetate. There are other subtypes of MS in which viral infection or oligodendrocyte degeneration, rather than autoimmunity, appear to play a role. It is possible that these could benefit from antiviral therapy, oligodendrocyte protection or oligodendrocyte transplantation, although therapies based on these latter approaches have yet to be developed.

We describe histopathological changes in a 2‐year‐old boy who died from myelinopathia centralis diffusa. Despite extensive white matter destruction, surprisingly high numbers of oligodendrocytes expressing proteolipid protein mRNA were detected. In an active demyelinating lesion in the brainstem, oligodendrocytes showed typical signs of apoptosis. We suggest that death of mature oligodendrocytes is the critical event in the disease.

Expression of apoptosis‐associated proteins p53, bcl‐2, bax, and caspase‐3/CPP32, activation of caspase‐3, and modification of proteins via poly(ADP‐ribosyl)ation was studied in pontosubicular neuron necrosis (PSN), a form of perinatal brain damage revealing the morphological hallmarks of neuronal apoptosis. Immunoreactivity for p53 was completely absent. The majority of cells stained with the bax and procaspase‐3 antibodies did not show morphological signs of apoptosis. In contrast, an antibody against activated caspase‐3 almost exclusively stained cells with apoptotic morphology. Poly(ADP‐ribosyl)ated proteins were only rarely detected in cells with apoptotic morphology. The expression patterns of bax, procaspase‐3, bcl‐2, and p53 in PSN were similar to that found in age‐matched control brains. However, activated caspase‐3 and poly‐ADP‐ribosylated proteins were exclusively found in apoptotic cells. These data indicate that detection of active caspase‐3 is a reliable marker for apoptosis in formalin‐fixed human tissue, and that neuronal apoptosis in pontosubicular neuron necrosis is accompanied by a pronounced activation of caspase‐3.