Hsp90α and Hsp90β proteins are strongly expressed in MM but not in MGUS or normal plasma cells. Immunohistochemical analysis of Hsp90α and Hsp90β protein expression in normal PCs (A) versus PCs from patients with MGUS or MM (B). Immunofluorescence images of sections of human tonsil (To), bone marrow (BM), and lymph node (LN) stained for CD138 (Cy-2, green), Hsp90β (Cy-3, red), and nuclei (DAPI, blue). CD138⁺ PCs of human tonsil and bone marrow express no Hsp90β, which is, however, expressed in ECs (*) and myeloblasts (Δ). Staining of lymph nodes shows strong Hsp90β expression in extrafollicular activated B cells (Δ) and weak expression in CD138⁺ PCs. The expression of Hsp90α was identical to that of Hsp90β (not shown). (B) Representative sections of BM biopsies from patients with MGUS (top panel) and MM (bottom panels 1 and 2). MGUS and MM(b) show staining for Hsp90α or Hsp90β (both with Cy-3, red) and CD138⁺ (Cy-2, green). Row MM(a) shows Hsp90α and Hsp90β expression staining using a secondary biotinylated antibody, a streptavidin/horseradish peroxidase complex, and 3,3 diaminobenzidine/H₂O₂ as the chromogen together with hematoxylin as the counterstain. PCs of MGUS express no Hsp90α and Hsp90β, which are, however, expressed in megakaryocytes (*). In contrast, Hsp90α and Hsp90β are strongly expressed in the cytoplasm of MM cells.