The Mi15 monoclonal antibody (anti-syndecan-1) is a reliable marker for quantifying plasma cells in paraffin-embedded bone marrow biopsy specimens

Plasmocyte selective monoclonal antibodies (MAb) recognizing syndecan-1 have recently been described. They belong to a new cluster, CD138. Using the MAb MI15, we investigated the expression of syndecan-1 in routinely paraffin-embedded tissues. Nontumoral lymph nodes (25 cases) and bone marrow biopsy specimens (63 cases) showed strong membrane staining of plasma cells only, allowing accurate analysis of the nuclear structure. The MI15 positivity correlated with kappa and lambda light chain expression in the cytoplasm. The percentages of plasma cells calculated in bone marrow biopsy specimens after MI15 staining were, respectively, 2.1% (range, 1% to 4%) in normal bone marrows, 8.5% (range, 5 to 17) in reactive plasmocytosis, and 4.66% in monoclonal gammapathy of undetermined significance (MGUS) patients (range, 1 to 13), in the same range but slightly higher than those obtained on smears or on hematoxylin and eosin (H&E)-stained sections. In multiple myeloma (40 cases), all plasma cell types were marked, and Mi15 MAb gave additional information in 8 of 40 (20%) patients. In lymph nodes, Mi15 MAb reacted with Reed-Sternberg cells of classical Hodgkin's disease in 23 of 31 cases (74%) with variable intensity. In contrast, nodular lymphocyte predominance Hodgkin's disease (10 cases), most B cell lymphomas (88 of 107 cases) and all T cell lymphomas (30 cases) were negative. In B cell lymphomas, plasmocytomas (8 cases), plasmocytic lymphomas (2 cases), and 5 of 13 cases of immunoblastic lymphoma with plasmocytoid differentiation were stained. In lymphoplasmocytoid lymphomas (4 lymph nodes and 20 bone marrow biopsy specimens), only mature plasma cells were positive. Moreover, a wide distribution of syndecan-1 was observed in normal and tumoral epithelial tissues. Finally, Mi15 MAb appears to be a reliable marker for identifying and quantifying normal and tumoral plasma cells in paraffin-embedded bone marrow and lymph node samples.