Multiple Myeloma Stem Cells and Plasma Cells Display Distinct Drug Sensitivities.

We recently demonstrated that multiple myeloma (MM) is organized in a hierarchical manner in which clonogenic MM progenitors or stem cells resembling post-germinal B cells give rise to MM plasma cells (PC). To study the potential biologic differences between MM stem cells and MM PC, we examined each cellular subset for characteristics found in normal stem cells as well as their responses to various antitumor agents. The human MM cell lines RPMI 8226 and NCI-H929 were initially studied as we previously found that they recapitulate clinical MM specimens and consist of distinct cell populations based on the expression of the PC surface antigen CD138; CD138⁺ cells resemble typical MM PC, whereas CD138^neg cells express B cell surface antigens and have greater clonogenic capacity. Examination of these cellular subpopulations by flow cytometry demonstrated that CD138^neg cells were smaller and less granular by light scatter than CD138⁺ PC and expressed higher levels of the intracellular enzyme aldehyde dehydrogenase that is present in normal hematopoietic progenitors with self-renewal potential. Furthermore, cells expressing the side population phenotype after staining with the DNA binding dye Hoechst 33342 were exclusively CD138^neg. We also investigated the effects of different clinically applicable agents on CD138⁺ and CD138^neg cells. CD138⁺ and CD138^neg cells isolated from RPMI 8226 and NCI-H929 cells by fluorescence activated cell sorting were treated with dexamethasone (dex, 100nM), bortezomib (velcade, 10nM), CC5013 (revlimid, 1μM), rituximab (10μg/ml) or alemtuzumab (campath,10μg/ml) for 72 hours followed by plating in methylcellulose to assess clonogenic capacity. CD138⁺ PC were significantly inhibited by dex (27 ± 11% recovery compared to untreated control cells), velcade (14 ± 6%) and revlimid (44 ± 27%), whereas rituximab (92 ± 25%) and campath (97 ± 18%) had little activity. In contrast, clonogenic growth of CD138^neg cells was not significantly inhibited by dex (82 ± 19%), velcade (88 ± 29%), or revlimid (91 ± 14%), but was significantly decreased by rituximab (63 ± 22%) and campath (47 ± 27%). Similarly, clonogenic MM growth of CD138^neg cells from 4 clinical MM samples was not affected by dex (84 ± 9%), velcade (82 ± 24%), or revlimid (93 ± 11%), but was significantly inhibited by rituximab (19 ± 7%) or campath (15 ± 11%). Clonogenic MM precursors may be distinguished from MM PC by a variety of biological parameters typically expressed by normal stem cells. Furthermore, these cellular subsets have different susceptibilities to a variety of clinical agents, and agents with activity against MM PC may be ineffective against MM stem cells. Moreover, agents without activity agasint MM PC may have major activity against MM stem cells. The divergent sensitivities of MM stem cells and PC may explain the dramatic, but transient, responses seen with many agents. Therapeutic strategies that result in long-term remissions may require the inhibition of both MM PC to reduce clinical symptoms and MM stem cells responsible for relapse.