Competition Between (Mono)Clonal Plasma Cells and Normal Cells for Potentially Overlapping Bone Marrow Niches Is Associated with a Progressively Altered Cellular Distribution In MGUS Vs. Myeloma

Abstract 617

Disappearance of normal bone marrow (BM) plasma cells (N-PC) predicts malignant transformation of monoclonal gammopathy of undetermined significance (MGUS) and smoldering myeloma (SMM) into symptomatic myeloma (MM). The homing, behavior and survival of N-PC, but also CD34⁺ hematopoietic stem/precursor cells (HPC), B-cell precursors, and (mono)clonal/aberrant PC (M-PC) largely depends on their interaction with SDF-1 expressing BM stromal cell niches. Accordingly, it can be hypothesized that a certain degree of competition among BM B-cell precursors, CD34⁺ HPC, and PC exist for the same BM niches. Thus, progressive replacement of normal cells by M-PC could help to explain the occurrence of cytopenias and hypogammaglobulinemia in MM patients. However, this hypothesis has not been investigated in depth neither in MM nor in SMM and MGUS.

In this study we analyze by 8-color multiparameter flow cytometry the distribution and competitive migration capacity of B-cell precursors, CD34⁺ HPC, N-PC and M-PC in the BM and PB of patients with MGUS (n=60), SMM (n=47) and MM (n=87) at diagnosis plus 12 MM cases studied after high-dose therapy/autologous stem cell transplantation (MM POST-HDT/ASCT) vs. healthy adults aged >60 years (HA; n=26).

The percentage of BM M-PC as well as the number of M-PC from all BMPC found at diagnosis significantly (p<.001) increased from MGUS to SMM and MM patients. Circulating M-PC were also detected at diagnosis in the PB of MGUS (21%) vs. SMM (69%) and MM (75%) at increasing M-PC counts (p≤.002). Interestingly, PB vs. BM M-PC showed significantly lower amounts of sideward light scatter (p<.001), together with lower levels of CD38 (p=.001), CD40 (p=.006), CD56 (p=.03) and CD138 (p=.02) expression. Focusing on the CXCR4-SDF1 axis, we found that the proportion of CXCR4⁺ PC slightly decreased (p>.05) from N-PC from HA (42%) to M-PC from MGUS (39%), SMM (37%) and MM (35%) patients at diagnosis, while MM POST-HDT/ASCT cases showed the lowest values (28%). In contrast, plasma levels of SDF-1 increased from HA (1150 pg/mL) to MGUS (1352 pg/mL), SMM (1656 pg/mL) and MM (1778 pg/mL; p=.04 vs. HA) patients, returning to almost normal levels in MM POST-HDT/ASCT (1331 pg/mL). Thus, an inverse correlation trend (r²=.12; p=.05) between the proportion of CXCR4⁺ PC and SDF-1 plasma levels was found.

Concerning the distribution of the normal cell populations, BM pro-B and pre-B cell precursors were significantly decreased in MM (p=.001) patients vs. HA, while it was normal in MGUS (p>.05) and SMM (p>.05). Despite the number of N-PC in the BM was significantly lower among MM and SMM vs. MGUS cases (p<.001), the distribution of circulating N-PC in PB was normal in all three groups of patients (p>.05 vs. HA). Interestingly however, the proportion of CXCR4⁺ PB N-PC progressively increased from HA (11%) to MGUS (14%), SMM (15%) and MM (21%; p=.05 vs. HA), while MM POST-HDT/ASCT cases showed the lowest median percentage of CXCR4⁺ PB N-PC (5%).

CD34⁺ HPC were found to be depleted in the BM of MM (0.3%; p=.001) and SMM (0.4%; p=.002) patients vs. HA (0.9%) while MGUS (0.8%) and MM POST-HDT/ASCT (1.1%) patients showed normal CD34⁺ HPC numbers. Conversely, PB CD34⁺ HPC progressively increase from HA to MGUS, SMM and MM cases (p=.008 and p=.06 vs. HA, respectively). By contrast, in MM POST-HDT/ASCT patients the number of PB CD34⁺ HPC returned to normal/lower levels.

Ex-vivo competition assays between BM B-cells, PC and CD34⁺ HPC for SDF-1 induced migration showed that in HA, CD34⁺ HPC displayed the highest migration potential in the presence of SDF-1, followed by pre-B cell precursors; conversely N-PC barely migrated. No significant differences were found for the migration of all cell populations analyzed between MGUS and SMM patients vs. HA, except for M-PC that showed an impaired migration in the presence of SDF-1. In turn, the migration potential of CD34⁺ HPC (p=.04), and pre-B cell precursors (p=.02) was markedly reduced in symptomatic MM, particularly at lower SDF-1 concentrations (30nM). Most interestingly, the migration of M-PC from symptomatic MM was markedly increased at both concentrations of SDF-1 used: median of 2.9% and 1.0% for SDF-1 concentrations of 30nM and 70nM, respectively.

Overall, these findings provide evidence about the role of progressive competition and replacement of normal BM cells by M-PC in determining transformation of pre-malignant MGUS and SMM into symptomatic MM.