Expression of TrkB by MM. (A) RT-PCR demonstrating TrkB mRNA expression by 8 of 9 HMCLs (lanes 1-9) and by 2 primary MM samples (lanes 11-12) but not by a Waldenstrom cell line (lane 10). No PCR products were evident in the absence of RT (not shown). HMCLs were (1) NCI H929, (2) LP1, (3) RPMI 8226, (4) ARP1, (5) U266, (6) OPM1, (7) UTMC2, (8) KMS11, and (9) JJN3. (B) Western blot of whole-cell lysates stained with anti-Trk (C-14), demonstrating TrkB expression by 4 HMCLs. RPMI 8226 constitutively express TrkB, whereas JJN3, OPM1, and U266 express TrkB after 24-hour exposure to 100 nM dexamethasone. (C) Immunofluorescence demonstrating TrkB expression by primary MM plasma cells. Representative cytospins of BM mononuclear cells freshly isolated from a patient with MM and stained with antibodies to TrkB (rhodamine) and CD138 (FITC). CD138⁺ plasma cells show strong immunoreactivity for TrkB, seen in the merged image. (Insets) No staining is evident when nonimmune rabbit IgG (for TrkB) or mouse IgG (for CD138) are used as primary antibodies.