In this study, we demonstrated that injecting PDACs into myelomatous osteolytic bone lesions effectively prevented bone loss and promoted bone formation by inhibiting osteoclast formation and stimulating differentiation of the host’s osteoblasts. Intralesional PDAC cytotherapy also resulted in inhibiting growth of H929 myeloma cells and primary myeloma cells categorized by global gene expression classification [31
] as high risk. Furthermore, PDACs had no effect on subcutaneous growth of H929 myeloma cells in SCID mice, and PDACs did not confer a growth advantage to myeloma cells cocultured with PDACs or the supportive MSCs [41
]. Along with the reported potential for PDACs to enhance engraftment of hematopoietic stem cells [26
], our results provide proof-of-concept that PDAC cytotherapy is a promising approach for treating myeloma bone disease and controlling myeloma progression.
Our study revealed that, like MSC [40
], exogenous PDACs were not detectable in vivo for long periods of time; the majority of these cells disappeared 3–5 weeks after injection. Clinically, this phenomenon might be advantageous because it limits the duration of the intervention. Biologically, these results indicate that, like MSCs, PDACs can be expanded and undergo many passages in vitro, but in vivo they lack typical stem cell properties such as self-renewal potential and differentiation into mesenchymal lineages (e.g., osteoblasts). Rather, these cells stimulate bone metabolism by acting as bystander cells that increase endogenous osteoblastogenesis and inhibit osteoclastogenesis, presumably by producing soluble factors and by cell-cell interactions. Interestingly, the ability of PDACs to increase bone mass and subsequently reduce myeloma burden seem to be sustained even when most PDACs have already disappeared, consistent with similar “touch-and-go” mechanisms implied for MSCs [22
Despite a safety concern that mesenchymal cells, rich with cytokines and chemokines, may stimulate tumor growth [42
], our findings revealed that growth rates of myeloma cell lines were lower when cocultured with PDACs than with MSCs from fetal bone or MM patients. In vivo, PDACs and MSCs suppressed myeloma growth in bone, but injection of PDACs into subcutaneous tumors had no effect on growth of myeloma cells. Antitumor effects of bone marrow MSCs were recently demonstrated in Kaposi’s sarcoma, a highly inflammatory angiogenic malignancy, and were attributed to contact-dependent inhibition of Akt activity in a xenograft model and coculture setting [43
]. The mechanisms by which PDACs inhibit myeloma cell growth in bones can only be partially delineated from our study. We and others have demonstrated that inhibition of bone resorption and stimulation of bone formation by pharmaceutical and osteoblast-activating agents or MSC cytotherapy is associated with inhibition of myeloma burden in experimental models [9
]. PDACs are likely to affect osteoclastogenesis and osteoblastogenesis: two unregulated physiological processes in myelomatous bone [46
]. Our in vitro study demonstrated that PDACs directly inhibit osteoclast formation. Osteoblastogenesis is impaired in MM, particularly as a consequence of suppression of Wnt signaling in bone through production of Wnt inhibitors such as DKK1 [10
]. PDACs may help restore critical signaling pathways associated with osteoblast deactivation in MM, resulting in increased bone formation.
Similar to MSCs [22
], PDACs may also exert immunomodulatory properties, mostly immunosuppressive effects in vivo. Our study used SCID mice that lack T and B lymphocytes but still harbor other immune cells such as NK cells and macrophages, which may be responsible for the rapid clearance of PDACs. The consequences of the interactions of PDACs with host macrophages on the properties, survival, and function of both cell types are yet to be determined.
Our study strongly suggests that intralesional PDAC cytotherapy will help recover large osteolytic lesions in patients with MM and suppress myeloma growth in these lesions. This is of particular interest because, as we have reported, relapses occur in pre-existing lesions in most patients with focal myeloma lesions, suggesting that these lesions may harbor “dormant myeloma cells” [1
]. The consequences of systemic injection of PDACs on myeloma bone disease and tumor growth are still under investigation. Our experimental data indicate that the majority of PDACs are trapped in the lungs after i.v. administration, but a few of these cells do traffic into implanted myelomatous bones in SCID-rab mice, particularly, when myeloma burden is well established. These results suggest that PDACs are attracted by chemokines produced by myeloma cells or by conditions induced as a consequence of myeloma growth in bone (e.g., wound-like conditions). MSCs were previously reported to home to tumor sites [47
], and although various chemokines and their receptors have been implicated in this process, the common notion is that a complex set of factors attract MSCs to tumor sites [42
]. An ongoing study is underway to develop approaches to improve trafficking of PDACs or MSCs to myelomatous bone, increasing their systemic therapeutic potential during active disease or maintenance therapy.