Immunoglobulin (Ig) M antibody against myelin associated glycoprotein (MAG): A comparison of methods

Polyneuropathy Associated with IgM Monoclonal Gammopathy; Advances in Genetics and Treatment, Focusing on Anti-MAG Antibodies

With increasing age, the chances of developing either MGUS or polyneuropathy increase as well. In some cases, there is a causative relationship between the IgM M-protein and polyneuropathy. In approximately half of these cases, IgM targets the myelin-associated glycoprotein (MAG). This results in chronic polyneuropathy with slowly progressive, predominantly sensory neurological deficits and distally demyelinating features in nerve conduction studies. Despite the disease being chronic and developing slowly, it can cause considerable impairment. We reviewed English medical publications between 1980 and May 2022 on IgM gammopathy-associated polyneuropathy, with special attention to studies addressing the pathophysiology or treatment of anti-MAG polyneuropathy. Treatment options have been limited to a temporizing effect of intravenous immunoglobulins in some patients and a more sustained effect of rituximab but in only 30 to 55 percent of patients. An increase in our knowledge concerning genetic mutations, particularly the MYD88L265P mutation, led to the development of novel targeted treatment options such as BTK inhibitors. Similarly, due to the increasing knowledge of the pathophysiology of anti-MAG polyneuropathy, new treatment options are emerging. Since anti-MAG polyneuropathy is a rare disease with diverse symptomatology, large trials with good outcome measures are a challenge.

Central nervous system involvement in patients with monoclonal gammopathy and polyneuropathy

BACKGROUND AND PURPOSE

To evaluate clinical presentation of patients with the clinical triad of monoclonal gammopathy, polyneuropathy and signs of CNS involvement.

METHODS

Nineteen patients with monoclonal protein (M-protein, 9 IgM, 10 IgG) were studied. Clinical examination, MRI, cerebrospinal fluid analysis and immune reactivity against myelin-associated glycoprotein and gangliosides in serum were obtained. By immunohistochemistry, different binding patterns of M-proteins to human CNS tissue were investigated.

RESULTS

Nine out of 19 patients (four IgM, five IgG) showed one or more clinical signs of CNS involvement. Clinical features associated with signs of CNS pathology were disease duration and greater concentration of IgM paraprotein. The IgM M-protein of two patients strongly stained the cortex/cerebellar neurons in human brain sections.

CONCLUSION

Our results complement previous reports that some patients with monoclonal gammopathy and polyneuropathy can develop solitary or disseminated signs of CNS involvement. It indicates that pathological effects of M-proteins are not necessarily restricted to the peripheral nervous system. The specificity and affinity of circulating M-protein to antigens in the CNS might be critical for the development of different clinical phenotypes.

Further comparisons of assays for detecting MAG IgM autoantibodies

Anti-MAG antibodies are commonly found in the sera of patients with demyelinating sensorimotor neuropathy and IgM paraproteinemia. Our objective here was to compare MAG results obtained by two different laboratories using similar methods (Western blot, EIA, IFA). Western blot (WB) employing MAG from monkey was less sensitive (72.5%) than myelin IFA (92.5%; monkey nerve) and EIA (97.5%; human MAG) when compared to WB using human MAG and is most likely due to methodology (not antigen source). EIA detected low titers of MAG IgM antibodies in suspected patient sera (negative by other methods) that were also SGPG IgM-positive. Patients having low titers by EIA, but negative by WB may have other autoimmune neuropathies without demyelination.