Binding of serum IgA of multiple myeloma to normal peripheral nerve

Serum from a 60-year-old man with multiple myeloma (monoclonal IgA-lambda) and peripheral sensorimotor neuropathy showed immunohistochemical binding to normal human endoneurium in dilutions up to 1:6000 (IgA = 1 mg/dl). The binding was shown to be specific for IgA-lambda and it was negative for IgA-kappa, IgG and IgM. Absorption of the serum with peripheral myelin eliminated the myelin immunostaining. Peptic digestion of the serum failed to eliminate immunostaining of the endoneurium. Immunoblot analysis revealed reactivity of the monoclonal IgA with 58,000, 43,000 and 18,500 dalton components of human nerve endoneurium. Sera from two patients with monoclonal IgA without peripheral neuropathy, from one patient with peripheral neuropathy and lung cancer, from one patient with stroke and from six normal subjects failed to stain myelin in sections of peripheral nerves or to react in immunoblots at comparable serum concentrations. Axonal staining was seen with some of the control sera. The relationship of the findings to the pathogenesis of peripheral neuropathy in multiple myeloma is discussed.