Relation of clinical, serological, morphological, and electrophysiological findings in galactocerebroside-induced experimental allergic neuritis

Schwann cells, but not Oligodendrocytes, Depend Strictly on Dynamin 2 Function

Myelination requires extensive plasma membrane rearrangements, implying that molecules controlling membrane dynamics play prominent roles. The large GTPase dynamin 2 (DNM2) is a well-known regulator of membrane remodeling, membrane fission, and vesicular trafficking. Here, we genetically ablated Dnm2 in Schwann cells (SCs) and in oligodendrocytes of mice. Dnm2 deletion in developing SCs resulted in severely impaired axonal sorting and myelination onset. Induced Dnm2 deletion in adult SCs caused a rapidly-developing peripheral neuropathy with abundant demyelination. In both experimental settings, mutant SCs underwent prominent cell death, at least partially due to cytokinesis failure. Strikingly, when Dnm2 was deleted in adult SCs, non-recombined SCs still expressing DNM2 were able to remyelinate fast and efficiently, accompanied by neuropathy remission. These findings reveal a remarkable self-healing capability of peripheral nerves that are affected by SC loss. In the central nervous system, however, we found no major defects upon Dnm2 deletion in oligodendrocytes.

Tissue resident macrophages are sufficient for demyelination during peripheral nerve myelin induced experimental autoimmune neuritis?

The contribution of resident endoneurial tissue macrophages versus recruited monocyte derived macrophages to demyelination and disease during Experimental Autoimmune Neuritis (EAN) was investigated using passive transfer of peripheral nerve myelin (PNM) specific serum antibodies or adoptive co-transfer of PNM specific T and B cells from EAN donors to leukopenic and normal hosts. Passive transfer of PNM specific serum antibodies or adoptive co-transfer of myelin specific T and B cells into leukopenic recipients resulted in a moderate reduction in nerve conduction block or in the disease severity compared to the normal recipients. This was despite at least a 95% decrease in the number of circulating mononuclear cells during the development of nerve conduction block and disease and a 50% reduction in the number of infiltrating endoneurial macrophages in the nerve lesions of the leukopenic recipients. These observations suggest that during EAN in Lewis rats actively induced by immunization with peripheral nerve myelin, phagocytic macrophages originating from the resident endoneurial population may be sufficient to engage in demyelination initiated by anti-myelin antibodies in this model.

The immunocompetence of Schwann cells

Schwann cells are the myelinating glial cells of the peripheral nervous system that support and ensheath axons with myelin to enable rapid saltatory signal propagation in the axon. Immunocompetence, however, has only recently been recognized as an important feature of Schwann cells. An autoimmune response against components of the peripheral nervous system triggers disabling inflammatory neuropathies in patients and corresponding animal models. The immune system participates in nerve damage and disease manifestation even in non-inflammatory hereditary neuropathies. A growing body of evidence suggests that Schwann cells may modulate local immune responses by recognizing and presenting antigens and may also influence and terminate nerve inflammation by secreting cytokines. This review summarizes current knowledge on the interaction of Schwann cells with the immune system, which is involved in diseases of the peripheral nervous system.

From bench to bedside--experimental rationale for immune-specific therapies in the inflamed peripheral nerve

Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy are autoimmune-mediated inflammatory diseases of the PNS. In recent years, substantial progress has been made towards understanding the immune mechanisms that underlie these conditions, in large part through the study of experimental models. Here, we review the available animal models that partially mimic human Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy, and discuss the wide range of therapeutic approaches that have been successfully established in these models of inflammatory neuropathies. Transfer of this preclinical knowledge to patients has been far less successful, and inflammatory neuropathies are still associated with significant morbidity and mortality. We will summarize successful therapeutic trials in human autoimmune neuropathies to provide a vantage point for the transfer of experimental treatment strategies to clinical practice in immune-mediated diseases of the peripheral nerve.

Animal models of immune-mediated neuropathies

PURPOSE OF REVIEW

This article gives an overview on animal models for immune-mediated demyelinating disorders of the peripheral nervous system. As insight into human disease is mainly based on biopsy material and ex-vivo analysis, an understanding of the pathogenetic mechanism of these complex and heterogeneous disorders is mainly based on animal models.

RECENT FINDINGS

Besides experimental autoimmune neuritis in rats, recent efforts to establish this model in mice are discussed. In addition, models for spontaneous autoimmune neuropathies and secondary immune reactions in degenerative disorders of the peripheral nervous system are reviewed.

SUMMARY

Recently described animal models offer the possibility to analyse the complex interaction of genetic and immunological factors. The entire panel of animal models for immune-mediated disorders of the peripheral nervous system provides a rational basis for studying the mechanisms of pathogenesis and new immunotherapeutic strategies for human autoimmune demyelinating neuropathies.

Functional abnormalities in P0-deficient mice resemble human hereditary neuropathies linked to P0 gene mutations

Mutations in the gene encoding the transmembranous cell adhesion molecule, myelin protein zero (P0), have been reported in patients with Charcot-Marie-Tooth disease types 1B and 3 (Déjérine-Sottas disease). We have previously shown that the targeted deletion of the P0 gene in mice results in impairment of sciatic nerve conduction, and we now extend our detailed electrophysiologic investigation to the facial nerve. In concordance with histologic investigations which revealed severe hypomyelination in peripheral nerves we found the typical electrophysiologic signs of severe dysmyelination in both the facial and sciatic nerves in mice homozygously deficient for the expression of P0 (P0 -/- mice). As compared to control mice (P0+/+), nerve conduction velocities were reduced to below 10% and compound muscle action potential (CMAP) amplitudes to below 25%, while CMAP duration and excitation thresholds were markedly increased. Surprisingly, nerve conduction changes in mice heterozygously deficient for P0 (P0+/-) were only mild, were detected only in the sciatic nerve, and occurred not before 5-7 months of age. They were more prominent at age 12-13 months. Thus, P0 -/- mice resemble severe human inherited neuropathies like Charcot-Marie-Tooth disease type 3 (Déjérine-Sottas disease) with onset early in life, whereas the P0 +/- mice may resemble the milder form, CMT1B.

Guillain-Barré syndrome: clinical and immunological aspects

Immune-mediated PNS disorders comprise a significant segment of diseases of the nervous system. Studies on GBS as a prototype of these disorders and its experimental model EAN have helped to elucidate some of the mechanisms responsible for myelin injury in the PNS. These mechanisms, although partially understood have been useful in implementing therapies such as plasmapheresis and IVIG and various other immunomodulators. The question of whether an infectious agent such as a virus can directly damage the myelin sheath and/or Schwann cells or whether the agent triggers an immune response against self through antigenic mimicry remains unanswered. The association between C. jejuni infection and GBS has opened new areas of investigation in understanding the immunopathogenesis of the disease. Similar observations with other environmental factors may be made in the future, pointing to the possibility that GBS may not be caused by a single agent but could be the result of an immunological attack on the PNS myelin assembly by a variety of agents or factors. Regardless of the etiology, if the myelin injury is aggravated by product of the immune cells, such as various cytokines, neutralization of these factors could help lessen the burden of injury to the nerves. Future research in autoimmune disorders of the PNS needs to focus on identifying environmental factors that directly, or indirectly through antigen mimicry, damage the PNS myelin. In parallel, further understanding of the immunopathogenesis by dissecting the immunological phenomenon at the systemic and local levels, especially the role of cytokines, growth factors, and adhesion molecules will pave the way for more rational therapies, even if the causative factors are not known. Studies in laboratory animals have demonstrated the efficacy of selective immunotherapy through modulation of the trimolecular complex, i.e., T cell receptor, MHC/molecule, and antigen. Immunological tolerance, presumably through deletion of autoreactive clones, clonal anergy, or active suppression, has proven effective in animals. Other modes of immunotherapy such as nonspecific depletion of T or B cells or down-regulation of activated cells have also been shown to abolish or decrease the severity of experimental autoimmune neurological disorders, including EAN. These immunotherapeutic modalities may become applicable to human autoimmune neuropathies.

Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy: immune mechanisms and update on current therapies

The relation between Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy is discussed. Most likely they represent parts of a continuum, arbitrarily separated by their time course. Within the concept of chronic inflammatory demyelinating polyneuropathy the presence of a monoclonal gammopathy of undetermined significance is discussed. The pathogenesis of inflammatory demyelinating polyneuropathies has not been elucidated yet, but involvement of the immune system has been firmly established. Preceding infections, especially with Campylobacter jejuni, and the analysis of antiganglioside antibodies lend new support to the hypothesis of molecular mimicry between epitopes on infectious agents and peripheral nerve constituents as one of the mechanisms in Guillain-Barré syndrome. In the future, a further classification of individual patients based on clinical, epidemiological, electrophysiological, pathological, microbiological, and immunological criteria may give a basis for more individualized treatment strategies. In Guillain-Barré syndrome the efficacy of high-dose intravenous immune globulin treatment was established after earlier positive findings with plasma exchange; immune globulins are easier to administer and may be superior. Even with these treatments it should be anticipated that one fourth of patients after immune globulin treatment and one third of patients after plasma exchange will show further deterioration in the first 2 weeks after onset of treatment. Despite this, just one treatment course usually is indicated in the individual patient, and no valid arguments were found to switch to the other treatment modality. In chronic inflammatory demyelinating polyneuropathy, prednisone, plasma exchange, and immune globulins are effective in a proportion of patients. The last two are equally effective. Patients may respond to one of these if a previous treatment failed, and here switching therapy may be effective due to the chronic course of the disease. Complexity and costs make plasma exchange the last choice. Whether prednisone or immune globulin is the first choice depends on the speed of recovery and the estimation of long-term loss of quality of life due to side effects of prednisone versus the costs of immune globulins. The mechanism of immune globulins in inflammatory polyneuropathies is discussed. There is evidence that idiotypic-antiidiotypic interaction may play a role, but several other mechanisms also may be involved.

Sural nerve biopsy in chronic inflammatory demyelinating polyradiculoneuropathy

We compared histologic features of sural nerve biopsies in 14 patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) with those in other forms of neuropathy. In CIDP endoneurial pericapillary cellular infiltrates were found in 4 patients (29%), onion bulbs in 5 patients (36%), and predominant demyelination in 7 patients (50%). None of these abnormalities was specific, but cellular infiltrates and onion bulbs appear to be diagnostically useful when combined with clinical information. To detect macrophage infiltration of myelin, cell nuclei were counter-stained in 20 teased fiber preparations. Nine patients with CIDP had a significantly higher mean number of cells per centimeter of teased fiber than 11 patients with other neuropathies. Despite overlap, significant infiltration of myelin detected by this method suggests CIDP in an appropriate clinical setting.

Humoral factors in inflammatory disorders of the central and peripheral nervous system

The immune-mediated disorders of the central and peripheral nervous systems all involve humoral mechanisms in which immunoglobulin, complement and/or other mediators are implicated. The disorders can be characterized by pattern of disease activity, site of immunoglobulin synthesis, humoral mechanism of injury, and presence of one or more concurrent cellular mechanisms of injury. Control of the humoral responses is presumably T-cell dependent. While these disorders should be viewed as having both humoral and cellular components, the specific components of each disorder might predict the response to specific therapeutic approaches.

Humoral factors in peripheral nerve disease

Humoral factors including soluble substances transported by the blood stream and factors released at a target tissue may play a role in diseases of the peripheral nervous system. Various criteria have to be met in order to accept humoral factors as potential pathogens. In this review these general criteria are discussed, including the evidence provided by plasma exchange therapy, demonstration of circulating or deposited autoantibodies and immune complexes, identification of antigenic molecules, animal model diseases, passive transfer experiments, and the demonstration of circulating factors not directed against specific targets. In acute, chronic, and chronic relapsing inflammatory polyneuropathies, and in the polyneuropathy associated with monoclonal gammopathy, humoral factors have been identified, but their exact pathogenic role is not fully understood. In the Lambert-Eaton myasthenic syndrome, a disorder of the motor nerve terminal, pathogenic IgG-antibodies have been demonstrated by passive transfer experiments. In the experimental animal model disorders, the acute and chronic variants of experimental allergic neuritis, humoral factors including antibodies to myelin basic proteins and galactocerebroside and nonspecific humoral factors may all contribute to the ultimate peripheral nerve damage, but their relative importance in relation to cell-mediated immune reactions is not yet clear.