The human anti-myelin-associated glycoprotein IgM system

Myelin protein zero is naturally processed in the B cells of monoclonal gammopathy of undetermined significance of immunoglobulin M isotype: aberrant triggering of a patient's T cells

BACKGROUND

Monoclonal gammopathy of undetermined significance of immunoglobulin M isotype is a condition with clonally expanded B cells, recently suggested to have an infectious origin. This monoclonal gammopathy is frequently associated with polyneuropathy and antibodies against myelin protein zero, whereas the role of the T cells remains largely unknown. We analyzed protein zero-specific B cells, as antigen-presenting cells, and their capacity to activate T helper cells.

DESIGN AND METHODS

We used a well-characterized monoclonal gammopathy of undetermined significance-derived B-cell line, TJ2, expressing anti-protein zero immunoglobulin M. The ability of TJ2 cells to bind, endocytose, process, and present protein zero was investigated by receptor-clustering and immunofluorescence. The activation of protein zero-specific autologous T cells was studied by measuring interleukin-2 and interferon-gamma with flow cytometry, immunobeads, and enzyme-linked immunospot assays.

RESULTS

Surface-receptor clustering and endocytosis of receptor-ligand (immunoglobulin M/protein zero) complexes were pronounced after exposure to protein zero. Naturally processed or synthetic protein zero peptide (194-208)-pulsed TJ2 cells significantly induced interleukin-2 secretion from autologous T cells compared to control antigen-pulsed cells (P<0.001). The numbers of interferon-gamma-producing T helper cells, including CD4(+)/CD8(+) cells, were also significantly increased (P=0.0152). Affinity-isolated naturally processed myelin peptides were potent interferon-gamma stimulators for autologous peripheral blood mononuclear cells, but not for control peripheral blood mononuclear cells.

CONCLUSIONS

We show for the first time that myelin protein zero is naturally processed in B cells from monoclonal gammopathy of undetermined significance of immunoglobulin M isotype, acting as aberrant antigen-presenting cells in activation of a patient's T helper cells. Our findings cast new light on the important role of autoreactive protein zero-specific B cells in the induction of the pathogenic T-cell responses found in nerve lesions of patients with monoclonal gammopathy of undetermined significance with peripheral neuropathy.

How useful are anti-neural IgM antibodies in the diagnosis of chronic immune-mediated neuropathies?

Antibodies against several neural antigens have been associated with different chronic immune-mediated neuropathies but their practical clinical relevance remains unclear. To determine the possible diagnostic usefulness of these antibodies we reviewed the clinical correlate of IgM antibodies to the myelin-associated glycoprotein (MAG), sulfatide, the gangliosides GM1, GM2, GD1a and GD1b in 539 consecutive patients examined for neuropathy or related diseases in our Neuropathy Clinics and tested for these antibodies in our laboratory since 1985. 302 patients (56%) had an established diagnosis of definite or possible chronic immune-mediated neuropathy while 237 had a neuropathy of non-immune-mediated origin or of unknown aetiology or a closely related disease. Antibodies to one or more antigen were more frequent (chi(2)=63.32; p<0.00001) in patients with chronic immune-mediated neuropathy (37.7%) than with other neuropathy or related diseases (7.2%) and their presence was associated in 87% of the patients with an immune-mediated neuropathy, incrementing by 31% the probability of having this form. Testing for MAG permitted to identify 24.8% of patients with an immune-mediated neuropathy, GM1 an additional 9.9%, while GM2, GD1b, GD1a and sulfatide altogether an additional 3% of the patients. Concerning clinical correlations, all 75 patients with anti-MAG IgM had neuropathy and IgM monoclonal gammopathy (PN+IgM) with a positive predictive value for this neuropathy of 100%. A similarly high predictive value for neuropathy (91.4%) was observed among 269 patients with IgM monoclonal gammopathy including 103 patients without neuropathy. Anti-sulfatide IgM, though rare, were also significantly and constantly associated with PN+IgM and permitted to identify few patients not bearing anti-MAG IgM. Anti-GM1 IgM were significantly associated with multifocal motor neuropathy (MMN) (29.2%) but where also found in a few patients with other immune or non-immune neuropathies or related diseases with a positive predictive value for MMN of 25.5%. Anti-GM2 IgM were also significantly associated with MMN and increased the sensitivity (36.2%) for MMN obtained with anti-GM1 IgM only, without affecting its specificity and positive predictive value. Anti-GD1a, GD1b, though not significantly more frequent in patients with immune-mediated neuropathy, were associated in 80 to 100% of patients with these neuropathies. In conclusion anti-neural IgM antibodies may help in identifying patients with a chronic immune-mediated neuropathy, even if only anti-MAG and anti-sulfatide IgM appear to be strictly associated with a definite clinical syndrome.

Paraproteinaemic neuropathies

In this article, we review the main clinical and pathological features of paraproteinaemic neuropathies and discuss recent experimental findings. Further knowledge of the disease process at the molecular level has allowed a better characterization of clinical syndromes and has given new insights into their pathogenesis. The most convincing evidence for a causal relationship can be drawn from IgM monoclonal gammopathies with specificities directed against carbohydrate determinants of the myelin associated glycoprotein (MAG). There remain however, many unresolved questions, such as how monoclonal anti-MAG IgM antibodies cross the blood-nerve barrier and trigger a chronic demyelinating polyneuropathy while the central nervous system is essentially spared. IgM paraproteins with specificity for other molecules, such as neurofilaments, sulphatide, gangliosides, chondroitin sulphate and tubulin, have also been identified, but their pathogenetic importance remains to be elucidated. Other paraproteinaemic neuropathies such as IgG and IgA neuropathies have to be considered separately. The paraneoplastic endocrine and cytokine manifestations of rare osteosclerotic myelomas provide valuable insights into the interaction between the immune and the nervous system. The antigen-specificity of IgG and IgA monoclonal antibodies are only poorly characterized but some have been found to be directed against endoneurial determinants and a few against axonal proteins such as neurofilaments.

Microheterogeneity of anti-myelin-associated glycoprotein antibodies

Antibodies to the myelin-associated glycoprotein (MAG) are implicated in the pathogenesis of an acquired demyelinating polyneuropathy. We studied IgM affinity to MAG in 18 patients with anti-MAG antibodies. Binding of sera was tested for anti-MAG immunoreactivity in central nervous system (CNS) by ELISA and in CNS and peripheral nervous system (PNS) by Western blot analysis. Furthermore, immunohistochemical characterization of IgM binding on sural nerve tissue was investigated using the indirect peroxidase method. Western blot analysis revealed that all sera detected MAG in central myelin, but only eight in peripheral myelin. Anti-MAG-IgM-ELISA-titers correlated significantly (p<0.0001) with PNS-Western blot results. By indirect immunoperoxidase immunohistochemistry, 12 sera stained myelin sheaths, while 6 sera showed no staining. These results demonstrate considerable variations in antibody binding strength to MAG between PNS myelin and CNS myelin. The relevance of these differences for the pathogenesis of the neuropathy and clinical impairment remains to be demonstrated.

Anti-MAG IgM penetration into myelinated fibers correlates with the extent of myelin widening

The purpose of this study was to evaluate the relationship between immunoglobulin M (IgM) antibodies penetration into myelinated peripheral nerve fibers and the widening of the peripheral myelin sheaths in anti-myelin-associated glycoprotein (anti-MAG) demyelinating IgM monoclonal polyneuropathy. Demyelinating polyneuropathy with monoclonal IgM is often associated with anti-MAG autoantibodies, which are thought to initiate the disease with IgM deposits usually present on the myelin sheaths. We analyzed nerve biopsies from 12 patients with an anti-MAG demyelinating neuropathy by confocal and electron microscopy. The total number of nerve fibers and the proportion of IgM-associated fibers were quantified after immunohistochemical staining. The affinities of IgM were examined by analyzing the binding pattern of serum IgM on normal peripheral nerve sections. Ultrastructural examinations of the biopsies showed a good correlation between in situ widened myelin sheaths and the IgM penetration level into myelinated fibers. The terminal complement complex appears not be involved in the penetration of IgM into the myelinated fibers. Our findings suggest a causative role of the IgM anti-MAG antibodies in the ultrastructural modifications of the myelin sheaths. The basement membrane and myelin components appear to be the major targets of the IgM monoclonal antibodies. However, the pathogenic mechanism whereby IgM antibodies reach their targets and induce nerve damage are still unclear.

Neuropathies associated with monoclonal gammopathies

Approximately 10% of patients with idiopathic peripheral neuropathy have an associated serum monoclonal gammopathy or M-protein. This represents six times the incidence of M-proteins found in the general population. In 5% of idiopathic peripheral neuropathy patients the M-protein is associated with an identifiable plasma cell dyscrasia. Sclerotic myeloma is particularly important to recognize because treatment may result in amelioration of the neuropathy and remission of the tumor. Patients with primary systemic amyloidosis often have preferential small fiber involvement with a dissociated sensory loss and autonomic dysfunction. The nerve root infiltration of lymphoproliferative disorders may simulate a polyradiculoneuropathy. In cases without an identifiable cause for the M-protein, referred to as monoclonal gammopathy of undetermined significance (MGUS), the pathophysiologic basis for the neuropathy is poorly defined in most cases. A role for M-proteins with antibody activity to myelin-associated glycoprotein is provocative. This review summarizes current knowledge of this important group of disorders.

Sensorimotor polyneuropathy and monoclonal IgM: a review of clinical and immunological features

Main clinical and biological features of peripheral neuropathies associated with monoclonal IgM are reviewed. Special emphasis is given to the structure of these monoclonal auto-antibodies which points to an antigen (or some other kind of selective pressure) driven process.

A human monoclonal IgG reactive with a private idiotope of a monoclonal IgM with autoantibody activity against myelin-associated glycoprotein

One hundred and thirteen sera from patients with monoclonal IgG were tested for reactivity against a panel of 13 human monoclonal IgM having various autoantibody activities: 6 to myelin-associated glycoprotein (MAG), 2 to vimentin intermediate filament protein and 5 to red blood cell antigens [cold agglutinins with specificity directed to I antigen (3 cases), i antigen (1 case) or Pr antigen (1 case)]. One IgG was found to react with a monoclonal IgM with anti-MAG activity. This reactivity was characterized as idiotypic and directed against a private idiotope of the monoclonal IgM. This work provides further evidence for the existence of anti-idiotypic antibody activity of monoclonal Ig occurring in human B cell neoplasias.

Expression of a public idiotype by human monoclonal IgM directed to myelin-associated glycoprotein and characterization of the variability subgroup of their heavy and light chains

Most studies using rabbit or mouse antisera failed to detect CRI between human IgM directed to MAG. We show here that 9 of 10 such IgM express a public CRI as defined by a nonhuman primate antiserum. Shared idiotype is likely involved in (or close to) the combining site of those IgM since antiidiotypic serum inhibited the binding of IgM to MAG and reacted with IgM having different variable regions of light and heavy chains. Partial aminoterminal sequence of heavy and light chains showed that anti-MAG IgM use either lambda chains (one IgM) or kappa light chains (six IgM) of different variability subgroups (V kappa IV in three instances, V kappa I in two, and V kappa II in one), whereas heavy chains belong to the VHIII (six IgM) or to the VHII (1 IgM) subgroup. These features distinguish these IgM from other human monoclonal IgM with a defined antibody activity, such as rheumatoid factors or cold agglutinins.

Anti-myelin-associated glycoprotein IgM antibody titers in neuropathy associated with macroglobulinemia

Twenty-seven patients with neuropathy and IgM monoclonal gammopathy were tested for antigen specificity of the M-protein and for anti-myelin-associated glycoprotein (MAG) IgM levels by immunoblot. In 16 patients (59.2%) the M-protein reacted with MAG and with cross-reactive glycoconjugates. Anti-MAG IgM titers in these patients ranged between 1:12,800 and 1:100,000. A fainter IgM reactivity with MAG and related glycoconjugates was detected in 3 additional patients with neuropathy, but also in 8 of 24 patients with IgM M-protein without neuropathy (33.3%). This reactivity was not due to the M-protein and corresponded to antibody titers of 1:400 or less in all but 1 patient with a titer of 1:3,200. Low titers of anti-MAG IgM (1:200 or less) were also detected in 17 of 101 control patients without IgM M-proteins (16.8%), while 1 patient with neuropathy of unknown cause had anti-MAG IgMK titers of 1:25,600. In 1 patient with neuropathy and IgM M-protein that was not anti-MAG, the M-protein bound to other antigens in nerve, while in 6, other possible causes or mechanisms for the neuropathy were found. In this study, high titers of anti-MAG IgM antibodies were always associated with neuropathy. The presence of low levels of anti-MAG IgM in a significant proportion of controls suggests that monoclonal expansion of naturally occurring B-cell clones secreting anti-MAG IgM may be responsible for the high incidence of this antigen specificity of the M-protein.

Autoimmunity in neuromuscular disease

A number of confounding factors can be identified from the search for autoimmune mechanisms over the last 2 decades that may be relevant for future studies. (1) An apparently homogeneous clinical disorder may represent more than one disease process and thereby imply antibody/antigen heterogeneity as, for example, in MG with and without detectable anti-AChR antibodies. In some cases, physiologic studies allow the different forms of the disease to be distinguished as in AIDP and acute inflammatory axonal polyneuropathy. (2) A homogeneous disorder (e.g., LEMS) may have at least two different triggering mechanisms (SCLC and an unknown stimulus). (3) Antigen density may be too low to be detected by the immunohistologic techniques available, as initially occurred in MG and LEMS. (4) Autoantibodies may be detected that are irrelevant to the primary disease, such as anti-striated muscle antibodies in MG. (5) Poor antibody cross-reactivity between species may mean that the pathogenic antibody is undetected in binding assays or in experimental passive transfer studies. For example, anti-AChR antibody in MG shows less than 5% reactivity with Torpedo AChR. (6) A poor regenerative capacity of the target antigen may mean that reduction of circulating autoantibodies by either plasma exchange or ISD treatment is not associated with detectable clinical improvement, as may be the case in SSN in which DRG cells appear to be the target. TABLE 5 summarizes the extent to which the data reviewed have established a role for pathogenic antibodies in the light of the postulates for autoimmunity set out earlier and ranks the disorders accordingly. Only in MG with detectable anti-AChR antibody are all the postulates met, including definition of the antigen, experimental passive transfer by the IgG fraction of MG sera, active immunization of experimental animals, and propagation. In both LEMS and the IgM kappa anti-MAG demyelinating neuropathy the antigen is known, although better characterized in LEMS; the epitopes are not yet defined in either. Data relating to passive transfer are more extensive in LEMS, however; systemic passive transfer of anti-MAG has not yet been reported. In neither condition is an animal model available. In the demyelinating neuropathies, the case for autoimmunity is less complete. Neither in AIDP nor in CIDP is the antigen known, and thus the relevance of the different EAN disorders is uncertain. Current evidence thus rests on the demonstration of serum IgM antibodies that react with peripheral nerve myelin and fix complement and on the intraneural passive transfer studies.(ABSTRACT TRUNCATED AT 400 WORDS)

An immunoglobulin M monoclonal antibody, recognizing a subset of acetylcholinesterase molecules from electric organs of Electrophorus and Torpedo, belongs to the HNK-1 anti-carbohydrate family

An immunoglobulin M (IgM) monoclonal antibody (mAb Elec-39), obtained against asymmetric acetylcholinesterase (AChE) from Electrophorus electric organs, also reacts with a fraction of globular AChE (amphiphilic G2 form) from Torpedo electric organs. This antibody does not react with asymmetric AChE from Torpedo electric organs or with the enzyme from other tissues of Electrophorus or Torpedo. The corresponding epitope is removed by endoglycosidase F, showing that it is a carbohydrate. The subsets of Torpedo G2 that react or do not react with Elec-39 (Elec-39+ and Elec-39-) differ in their electrophoretic mobility under nondenaturing conditions; the Elec-39+ component also binds the lectins from Pisum sativum and Lens culinaris. Whereas the Elec-39- component is present at the earliest developmental stages examined, an Elec-39+ component becomes distinguishable only around the 70-mm stage. Its proportion increases progressively, but later than the rapid accumulation of the total G2 form. In immunoblots, mAb Elec-39 recognizes a number of proteins other than AChE from various tissues of several species. The specificity of Elec-39 resembles that of a family of anti-carbohydrate antibodies that includes HNK-1, L2, NC-1, NSP-4, as well as IgMs that occur in human neuropathies. Although some human neuropathy IgMs that recognize the myelin-associated glycoprotein did not react with Elec-39+ AChE, mAbs HNK-1, NC-1, and NSP-4 showed the same selectivity as Elec-39 for Torpedo G2 AChE, but differed in the formation of immune complexes.