Systemically Injected Mesenchymal Stem Cells Traffic to Myelomatous Bone and Inhibit Bone Disease, and Intralesionally Promotes Bone Formation and Delays Myeloma Progression During the Disease Active Stage or Remission

Abstract 980

Mesenchymal stem cells (MSCs) cytotherapy has been clinically tested in various applications including bone regeneration, autoimmune diseases, and cancer. The aims of the study were to test the effect of MSCs cytotherapy on myeloma (MM) bone disease and tumor growth, and determine their ability to traffic into myelomatous bone during active disease stage and induction of remission by melphalan. We exploited the SCID-rab model for MM and a novel human myeloma cell line, Hg, established through our previously reported procedure (Xin et al., BJH 2007). Hg myeloma cells do not grow in culture but are capable of sequential passaging in SCID-rab mice. Microarray analysis revealed similar gene expression profiling between the Hg cell line and the original patient's myeloma plasma cells, and that these cells are classified in the MMSET subgroup and express DKK1, indicating their authenticity and clinical relevancy. The human fetal MSCs were transduced with a luciferase/GFP reporter in a lentiviral vector for in vivo tracking. In Hg-bearing hosts (5 mice/group), intra-bone injection of MSCs (1×10⁶ cells/mouse) increased bone mineral density (BMD) by 21±4% while in control, diluent injected bones it was reduced by 14±5% (p<0.001). Increased bone formation by MSCs was associated with reduced tumor growth by 50% (p<0.01). Bioluminescence analysis revealed disappearance of the majority of the intralesionally injected MSCs within 4 weeks, suggesting that MSCs exert their effects on bone remodeling as a bystander cells (trophic effect). To test effect on relapse, remission was induced by treating Hg-bearing hosts with a total of 4 subcutaneous injections of melphalan (10 mg/kg/4 days), followed by intra-bone injection of diluent or MSCs (10 mice/group). Three weeks post-cytotherapy BMD was increased by 23±5% in bones injected with MSCs and reduced by 23±3% in bones injected with diluent (p<0.001). Eleven weeks post-cytotherapy, BMD was reduced by 33±6% and 9±7% in bones injected with diluent and MSCs, respectively (p<0.03). Following melphalan treatment circulating immunoglobulin (Ig) level (MM burden) was undetected while at 2 weeks after cytotherapy it was detected in 80% and 30% of hosts injected with diluent and MSCs, respectively. At experiment's end, Ig levels were significantly lower by 6 folds in hosts treated with MSCs than diluent (p<0.01). To further validate clinical relevancy, MSCs or diluent were intravenously injected into Hg-bearing hosts. In contrast to a single injection, 4 weekly, intravenous injections of MSCs (10 mice/group) prevented reduction of the BMD of the myelomatous bone while in control hosts the BMD was reduced by 14±3% (p<0.006 vs. pretreatment). MM growth was not affected by single or multiple intravenous injections of MSCs. Ex vivo imaging of tissues from hosts with active MM or treated with melphalan detected MSCs in implanted bones and murine lungs indicating that myeloma cells or conditions induced by MM or melphalan attract MSCs to myelomatous bones. We conclude that intra-bone injection of MSCs effectively promotes bone formation and delays MM progression during the disease active stage or remission. We also conclude that exogenous MSCs are capable of trafficking to myelomatous bone and that systemic, weekly injections of MSCs inhibit MM bone disease.