Interaction between receptor activator of nuclear factor B ligand (RANKL) and RANK/ osteoprotegerin (OPG) plays a dominant role in osteoclast activation and possibly in plasma cell survival in multiple myeloma (MM). We measured soluble RANKL (sRANKL), OPG, and bone remodeling markers in 121 patients with newly diagnosed MM to evaluate their role in bone disease and survival. Serum levels of sRANKL were elevated in patients with MM and correlated with bone disease. The sRANKL/OPG ratio was also increased and correlated with markers of bone resorption, osteolytic lesions, and markers of disease activity. The sRANKL /OPG ratio, C-reactive protein (CRP), and ␤ 2 -microglobulin were the only independent prognostic factors predicting survival in multivariate analysis. We generated a prognostic index based on these factors that divided our patients into 3 risk groups. The low-risk group had a 96% probability of survival at 5 years, whereas the intermediate-risk and the high-risk groups had probabilities of survival of 52% and 0%, respectively. Not only do these results confirm for the first time in humans the importance of sRANKL/OPG in the development of bone disease, they also highlight the role of this pathway in the biology of plasma cell growth as reflected by its influence on survival. (

No abstract

IntroductionMultiple myeloma (MM) is an incurable plasma cell (PC) malignancy of the bone marrow (BM). Although acquired genetic events and the tumor microenvironment are well-established regulators of myeloma cell survival and proliferation pathways, the identity and functional properties of the myeloma-propagating cells have been a matter of controversy. 1,2 The terminal differentiation of normal mature B lymphocytes to immunoglobulin (Ig)-secreting PCs entails conversion of antigennaive to antigen-experienced B cells in the germinal center of secondary lymphoid organs and their subsequent differentiation to either memory B cells or PCs. 3,4 Each stage of B-cell differentiation can be defined by surface markers with naive and memory B cells expressing CD19 and terminally differentiated normal and malignant PCs, but not B cells, expressing CD138 (Syndecan-1). 5,6 Given this linear B-cell lineage developmental process, it was suggested that myeloma cell growth is sustained by a minority of cells more immature than the PC. This hypothesis is supported by the presence of CD19 ϩ CD138 Ϫ clonotypic B cells (ie, cells sharing the same Ig heavy chain [IgH] complementarity region 3 [CDR3] sequence with the myeloma PCs) in peripheral blood (PB) and BM of patients with MM. 7-10 Indeed, because CD138 Ϫ but not CD138 ϩ PCs were found to lead to myeloma engraftment in NOD/SCID mice, it was proposed that CD138 Ϫ cells were the principal myeloma-propagating or "myeloma stem" cells [11][12][13][14] Earlier studies, For personal use only. on May 9, 2018. by guest www.bloodjournal.org From though, using a huSCID mouse model, had concluded that mature PCs (defined as CD38 hi CD45 Ϫ ), and not the CD19 ϩ B-cell fraction, contained the entire myeloma-propagating activity, 15 whereas more recently, CD19 Ϫ CD138 Ϫ as well as CD138 ϩ cells engrafted SCID-rab mice with myeloma. 16 Furthermore, whereas earlier studies reported that the myeloma side population is enriched in clonogenic activity and identifies with CD138 Ϫ but not CD138 ϩ myeloma cells, 13 recent evidence shows that both CD138 ϩ and CD138 Ϫ cells are included in the highly clonogenic myeloma side population. 17 Whether these discrepancies result from different animal models and phenotypic definitions of PC is not clear. Here, through a detailed phenotypic and genetic analysis of primary human myeloma cells and a prospective, dynamic ex vivo and in vivo study of the constituents of the myeloma cellular architecture, we show that a phenotypic and functional interconvertible state between CD138 ϩ and CD138 Ϫ cells underpins myelomapropagating activity and clinical drug resistance. Methods Patient and normal donor BM and PB samplesPatient BM and PB samples were obtained after written informed consent and appropriate institutional ethics committee approval. Patient characteristics are shown in supplemental Table 1 (available on the Blood Web site; see the Supplemental Materials link at the top of the online article). Diagnosis, remission, and relapse of MM were defined accordi...