Widenings of the myelin lamellae in a typical Guillain-Barré syndrome

Role of cytokines and Toll-like receptors in the immunopathogenesis of Guillain-Barré syndrome

Guillain-Barré syndrome (GBS) is an autoimmune disease of the peripheral nervous system, mostly triggered by an aberrant immune response to an infectious pathogen. Although several infections have been implicated in the pathogenesis of GBS, not all such infected individuals develop this disease. Moreover, infection with a single agent might also lead to different subtypes of GBS emphasizing the role of host factors in the development of GBS. The host factors regulate a broad range of inflammatory processes that are involved in the pathogenesis of autoimmune diseases including GBS. Evidences suggest that systemically and locally released cytokines and their involvement in immune-mediated demyelination and axonal damage of peripheral nerves are important in the pathogenesis of GBS. Toll-like receptors (TLRs) link innate and adaptive immunity through transcription of several proinflammatory cytokines. TLR genes may increase susceptibility to microbial infections; an attenuated immune response towards antigen and downregulation of cytokines occurs due to mutation in the gene. Herein, we discuss the crucial role of host factors such as cytokines and TLRs that activate the immune response and are involved in the pathogenesis of the disease.

Myelin architecture: zippering membranes tightly together

Rapid nerve conduction requires the coating of axons by a tightly packed multilayered myelin membrane. In the central nervous system, myelin is formed from cellular processes that extend from oligodendrocytes and wrap in a spiral fashion around an axon, resulting in the close apposition of adjacent myelin membrane bilayers. In this review, we discuss the physical principles underlying the zippering of the plasma membrane of oligodendrocytes at the cytoplasmic and extracellular leaflet. We propose that the interaction of the myelin basic protein with the cytoplasmic leaflet of the myelin bilayer triggers its polymerization into a fibrous network that drives membrane zippering and protein extrusion. In contrast, the adhesion of the extracellular surfaces of myelin requires the down-regulation of repulsive components of the glycocalyx, in order to uncover weak and unspecific attractive forces that bring the extracellular surfaces into close contact. Unveiling the mechanisms of myelin membrane assembly at the cytoplasmic and extracelluar sites may help to understand how the myelin bilayers are disrupted and destabilized in the different demyelinating diseases.

Anti-astrocyte Autoantibodies in Guillain–Barré Syndrome—Possible Involvement in the Pathophysiology of a Psychosyndrome?

Guillain-Barre syndrome (GBS) is an autoimmune disorder of the peripheral nervous system. However, GBS patients frequently display a psychosyndrome, indicating an accompanying involvement of the central nervous system (CNS), although the cause is not understood. Hepatic encephalopathy is possibly the best characterized example of a psychosyndrome, and astrocyte dysfunction appears to play a major role. We hypothesized that if compromised astrocyte function is of general importance in the pathomechanism of a psychosyndrome, then astrocytes should be involved in the CNS dysfunctions of GBS patients as well. Using immunocytochemistry, we found anti-astrocyte autoantibodies in about 60% of GBS patients and few or none in control groups. This finding was confirmed by Western blots. Our data are consistent with the suspected importance of an impaired astrocyte function in the pathomechanism of a psychosyndrome.

IgG Neuropathy: An Immunoelectron Microscopic Study

There are many potential mechanisms of peripheral nerve impairment by a monoclonal IgG dysglobulinemia. In this study, using electron microscopy, we observed widening of the myelin lamellae comparable to that commonly described in IgM neuropathies with antimyelin-associated glycoprotein activity. Such features have yet to be described in IgG neuropathies. In addition, we observed deposits of a granular material in the interstitial tissue of the nerve. By immunoelectron microscopy, we demonstrated the presence of the IgG in the lesions of myelin and the endoneurial space. A direct link between monoclonal dysglobulinemia (regardless of type) and polyneuropathy should be confirmed by nerve biopsy, because the result may influence treatment.

Paraproteinemic neuropathies

The occurrence of a peripheral neuropathy (PN) in association with a monoclonal gammopathy is quite common and suggests that monoclonal proteins may play a pathogenetic role in peripheral nervous system damage. In fact, paraproteinemic PN constitute an heterogeneous group of disorders related to various pathogenetic factors, and the histopathologic features in peripheral nerve biopsies differ from one condition to another. In several well defined disorders, the responsibility of the monoclonal component in the development of the PN has been evidenced. This is the case for most of the PN associated with an IgM monoclonal gammopathy, either a monoclonal gammopathy of undetermined significance (MGUS) or Waldenstrom's macroglobulinemia. The responsibility of the monoclonal protein in the occurrence of amyloid neuropathy related to multiple myeloma is also recognized. However, most IgG or IgA MGUS, as well as the monoclonal component in POEMS syndrome, have an uncertain causal relationship with the neuropathy. PN associated with monoclonal cryoglobulin (type 1) are occasional and differ from those associated with mixed cryoglobulins (types 2 or 3).

Schmidt-Lanterman's incisures--the principal target of autoimmune attack in demyelinating Guillain-Barré syndrome?

We used immunocytochemical staining of peripheral (trigeminal) nerve to screen sera of patients with Guillain-Barré syndrome (GBS) for the presence of autoantibodies, using sera from patients with other neurological diseases and healthy volunteers as controls. Most sera mildly reacted with axons, myelin sheaths, or sensory neurons without correlation to a specific disease. A characteristic staining, however, was found in 23 demyelinating cases (89%) out of 26 investigated GBS sera. With these sera, dark, oval and often paired small blobs were observed throughout the sections. A similar picture was rarely observed with sera from patients with other disorders or healthy controls. Using immunocytochemical marker proteins and high light microscopic resolution, the blobs were identified as Schmidt-Lanterman's incisures (SLIs). Further investigations will be necessary to identify the corresponding antigen and to answer the question, whether these antibodies represent an epiphenomenon or play a role in the causative mechanism of the disease.

Myelin modifications in 8 cases of peripheral neuropathy with Waldenström's macroglobulinemia and anti-MAG activity

Characteristic myelin modifications in patients with IgM monoclonal gammopathy and anti-MAG activity have mainly been studied in cases of undetermined significance, but also exist in cases with indolent Waldenström's macroglobulinemia, i.e., when lymphoplasmocytic infiltration in bone marrow is 15% or more, without any visceral involvement. Since 1983, the authors have examined nerve biopsies from 8 cases with Waldenström's macroglobulinelia by direct immunofluorescence examination on frozen sections and ultrastructural examination. At direct immunofluorescence, fixation of anti-IgM serum on myelinated fibers was present in 7 cases. At ultrastructural examination, a widening of some myelin lamellae at the periphery of a few fibers was visible in 8 cases. A few fibers with hypermyelination were present in 5 cases. In 2 of these 5 cases widening of some myelin lamellae was present in numerous fibers, 88% in one of them. Frequently, there was a major widening of some myelin lamellae with dilated lamellae present in the inner part of the myelin sheath. Certain lamellae were more dilated, up to 50 nm. Occasionally, enlarged lamellae were not compacted with each other. The authors also examined nerve biopsies from 36 patients with IgM monoclonal gammopathy of undetermined significance and anti-MAG activity, but found only one case with major widening of some myelin lamellae. Five other cases with major widening of some myelin lamellae, 4 Waldenström's macroglobulinemia and 1 IgM monoclonal gammopathy of undetermined significance, have been reported. Given that demyelinating neuropathies are far more numerous in cases with IgM monoclonal gammopathy of undetermined significance, it is likely that cases of indolent Waldenström's macroglobulinemia are prone to develop major myelin modifications, possibly due to another mechanism, added to the classic anti-MAG activity.

The antibody repertoire in experimental allergic neuritis: evidence for PMP-22 as a novel neuritogen

Experimental allergic neuritis (EAN) is an autoimmune disease that serves as an animal model for the Guillain-Barré syndrome (GBS). In both disorders there is still great uncertainty as to the significance and diversity of autoantibodies involved. We focused on the characterization of serum antibody production in response to various peripheral nervous system (PNS) myelin proteins during the course of actively induced EAN in Lewis rats. These data were compared with EAN induced by adoptive transfer of P2-specific CD4+ T cells (AT-EAN) and with inoculation with complete Freund's adjuvant (CFA) alone. Semiquantitative Western blotting was applied to measure serum IgM and IgG titers against specific myelin proteins, including P2, P0, myelin basic protein (MBP), myelin-associated glycoprotein (MAG) and PMP-22. Considerable differences in the dynamics of antibody titers against individual myelin proteins were observed in active EAN and after inoculation with CFA alone. Our data suggest a pathogenic role of IgM antibodies against HNK adhesion carbohydrate epitope expressing PNS proteins P0, MAG and PMP-22. Among these, PMP-22, a novel candidate neuritogen may be of particular relevance. Thus, we provide evidence for the involvement of antibody-mediated immune response in actively induced EAN and a basis for similar studies on related human disorders such as GBS or other demyelinating neuropathies.

Motor conduction parameters in neuropathies associated with anti-MAG antibodies and other types of demyelinating and axonal neuropathies

We measured residual latency (RL), motor conduction velocity (MCV), and terminal latency index (TLI) in 15 patients with neuropathy and anti-MAG or SGPG antibodies and compared these to values obtained in 103 patients with other types of polyneuropathy (PN) and to 57 normal subjects. Ten patients had anti-MAG antibody titers of 25,600 or higher, and 5 had titers between 800 and 12,600. Patients with the highest titers had longer RL, slower MCV and shorter TLI than those with lower titers, acute or chronic inflammatory demyelinating PN, hereditary neuropathy, and metabolic or axonal neuropathy. In contrast F-wave latencies did not contribute to the differentiation between the groups of demyelinating neuropathies. RL and TLI correlated best with anti-MAG antibody titers, whereas there was a poor correlation with anti-SGPG titers suggesting that MAG more than SGPG may be the antigen in PN, and that the distal nerves are affected more than their proximal segments. The RL rather than TLI turned out to be the best variable to classify the demyelinating type of anti-MAG neuropathy. Sural nerve biopsy in 5 of the patients with the highest titer of anti-MAG antibodies showed deposits of IgM and C3 on the myelin sheaths, pronounced demyelination and widening of the myelin lamellae. In 4 of the patients with lower titers demyelination was absent or less pronounced.