Autoimmunity Related to IgM Monoclonal Gammopathy of Undetermined Significance

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Abstract
In eight of 10 consecutive cases of IgM monoclonal gammopathy of undetermined significance (MGUS), the M‐protein had specificity towards various tissues as estimated by direct and indirect immunofluorescence studies of skin and/or sural nerve biopsies. Five of the cases had neuropathy. In three of them, including two siblings with a demyelinating peripheral neuropathy, the IgM was bound to the myelin‐associated glycoprotein (MAG) of peripheral nerves. One had axonal neuropathy with IgM activity against the peri‐ and endoneurium, while another case with post‐infectious neuritis had IgM activity against structures in the endoneurium but no IgM autoimmunity in the direct fluorescence test. The latter improved clinically in parallel with a decrease in the M‐protein indicating a pathogenetic role of the autoantibody. In three other cases, the IgM was bound to connective tissue structures, two of them also had plasma antibodies against the peri‐ and endoneurium in the indirect fluorescence test. Finally, two cases showed no reaction of the M‐protein against any tissue structures. Since an autoimmune pathogenesis is suspected, the HLA types of seven patients are reported.