Ultrastructural characterisation of the M protein in nerve biopsy of patients with POEMS syndrome.

To learn more about the mechanisms of nerve lesions in POEMS syndrome, nerve specimens from four patients were studied with an immunogold method at the ultrastructural level to detect and localise the M protein in the different nerve compartments. An indirect immunolabelling technique was applied on 4% PFA fixed and LR White embedded nerve specimens. Antisera against IgG, IgA, IgM, and lambda and kappa light chains were used as primary antisera. Morphological studies disclosed an important axonal loss in association with the demyelinative process. Endoneurial deposits of immunoglobulins were found in all cases. In the patient with the most severe form of neuropathy, diffuse deposits were present in the endoneurial space, especially in the subperineurial area. In the other patients, occasional deposits of the M protein were found in the myelin sheath (n=2); or between cells (n=1). No deposit was found in the axons. The class of the M protein labelled in the nerve corresponded to that detected in the serum in three of four patients, with labelling of two heavy chains in one patient. Immunolabelling of the M protein on the myelin sheath, Schwann cells, and in the endoneurial space favour a direct role of the M component in the lesions of nerve fibres, and justify active treatment of the plasmacytic proliferation.