Nature of Intracytoplasmic Crystalline Inclusions in Myeloma Cells (Morphologic, Cytochemical, Ultrastructural, and Immunofluorescent Studies

The authors describe a 70-year-old woman with multiple myeloma and adult Fanconi syndrome. A monoclonal protein of IgA heavy-chain class and kappa light-chain class was demonstrable in the serum. Urine immunoelectrophoresis showed the presence of kappa light chains. Bone marrow aspirate showed increased plasma cells with large bundles of pink-staining Auer-rod-like crystals in their cytoplasm. These crystals failed to stain with Sudan black B, peroxidase, esterase, and PAS, but showed strong acid phosphatase and beta-glucuronidase positivity. Ultrastructural studies showed them to have a fibrillar and an unusual cross-striated pattern. Immunofluorescent studies showed strong IgA and kappa activity in the cytoplasm of the tumor cells, but the fluorescence was absent in the region of the crystals, which were identified easily by their negative birefringence. The authors interpret these observations to indicate that the intracytoplasmic crystals in this case are of lysosomal origin.