Immunophenotypic analysis of neoplastic cells in follicular dendritic cell sarcoma

The existence of a sarcoma derived from the antigen-presenting follicular dendritic cells (FDCs) has been assumed but never confirmed. This report describes a tumor from axillary lymph nodes of a 39-year-old male in which the morphologic, enzyme histochemical, and immune phenotypic data are consistent with the malignant cells being of FDC origin. Morphologically, the tumor showed a tendency toward bi- and polynucleation with areas of storiform growth pattern, resulting in an initial diagnosis of malignant fibrous histiocytoma. However, the tumor cells had interconnections with well-developed desmosomes. Enzyme histochemistry revealed strong 5'-nucleotidase activity. Immunohistologic analysis showed that tumor cells expressed two of three FDC-specific antigens (Ki-M4, BU-10, and R4/23) strongly. Virtually all myelomonocytic markers were absent. Like normal FDCs, the leukocyte antigens CD35, CD19, CD21, and CD23 were expressed strongly and CD4 antigen weakly. No staining was evident for CD45 antigen, and no nonhemopoietic cell markers were expressed. The origin of the normal FDC is obscure. Given some evidence suggesting a bone marrow origin for the FDC, this cell may represent a lineage distinct from other known cell lineages derived from bone marrow stem cells since its immune phenotype differs considerably from them.