M2 Macrophages in Crystal Storing Histiocytosis Associated with Plasma Cell Myeloma.

Crystal Storing Histiocytosis (CSH) is a rare entity occurring in association with underlying lymphoproliferative disorders and plasma cell neoplasms. It denotes accumulation of immunoglobulin crystals in the histiocyte cytoplasm. In this study, we report a patient with plasma cell myeloma who presented with bilateral comminuted femur fractures. Histological examination of fracture tissue revealed hypercellular (~100%) marrow with extensive involvement by sheets of histiocytes with abundant eosinophilic cytoplasm admixed with scattered plasma cells. Intracytoplasmic diamond and rhomboid crystals within histiocytes were demonstrated by electron microscopy. Immunohistochemistry highlighted monotypic plasma cells with kappa restriction, representing 20-30% of marrow cellularity; however, non-polarizable cytoplasmic striations in histiocytes were negative for light chain expression. A diagnosis of crystal-storing histiocytosis associated with plasma cell myeloma was rendered. Further evaluation of these macrophages is positive for CD163 and COX2 and shows pSTAT3 with variable nuclear staining in some histiocytes. This case demonstrates that numerous M2 macrophages are present as crystal storing histiocytosis; and this knowledge might convey prognostic and therapeutic significance for the patients with crystal storing histiocytosis.