Anti-MAG Polyneuropathy: Characterization of the Monoclonal Gammopathy and Clonal B-Cell Population

Polyneuropathy due to antibodies to myelin-associated glycoprotein (MAG) is a rare disease with an estimated prevalence of 1 per 100,000. Symptoms start in the sixth or seventh decade of life, presenting sensory polyneuropathy with sensory ataxia, paresthesia, mild motor deficit, and tremor of upper extremities. The neurophysiological features are compatible with length-dependent demyelination. It is caused by monoclonal gammopathy of the IgM type produced by clonal B cells. In recent years the monoclonal anti-CD20 antibody has become the preferred first-line treatment, but with limited efficacy. We identified a cohort of 42 patients diagnosed with anti-MAG polyneuropathy. Clinical data were collected retrospectively, and flow cytometry files and immune histochemistry slides were reassessed. We report that in most cases of anti-MAG polyneuropathy, the monoclonal IgM is kappa-restricted, the B-cell population shows lymphoplasmacytic differentiation, MYD88^L265P mutation is a frequent finding, and the clonal B-cell population includes plasma cells. These findings most likely explain the low efficacy of rituximab monotherapy. We found the burden of symptoms to be high among patients with anti-MAG polyneuropathy as most of our patients had received intravenous immunoglobulin infusions and/or B-cell-directed treatment. We suggest that future treatment protocols for anti-MAG polyneuropathy should incorporate plasma cell-directed drugs. Furthermore, we found complement receptor 1 (CD35) to be down regulated on clonal B lymphocytes but not on normal B lymphocytes in these patients. We suggest that this may be a common feature of clonal B lymphocytes in chronic lymphoproliferative diseases.