Multiple myeloma predominantly grows in bone, causing extensive destruction. Despite targeted therapies, relapse is common and the disease remains incurable. To develop more effective treatments we need an improved understanding of myeloma cell engraftment, dormancy and reactivation in the skeleton. We hypothesise that myeloma cells engage in an endosteal niche in which they reside in a dormant state, resist chemotherapy and can be reactivated through changes in the local environment, contributing to disease relapse. To address this, we have developed intravital imaging to study tumor cell colonization of the endosteal niche and tumour cell dormancy and reactivation. 5TGM1eGFP murine myeloma cells were labelled with the membrane dye DiD. In vitro, DiD label is lost through division, distinguishing dormant (DiD+ve/GFP+ve) from proliferating cells (DiDNeg/GFP+ve). Myeloma cells were injected into C57BLKalwRij mice and treated with melphalan (3 times/week, 5mg/kg days 14-28), sRANKL (daily days 4-6) or vehicle. Using intravital microscopy, individual dormant DiD+ve/GFP+ve cells were visualised at 7, 14, 21, or 28 days post injection and located in endosteal niches. By day 14, a limited number of myeloma cells were activated to form growing DiDNeg/GFP+ve colonies which were localised distant from bone surfaces. Melphalan treatment reduced tumour burden (<97%), however dormant DiD+ve/GFP+ve tumour cells remained. Following removal of melphalan treatment, tumor burden increased and DiD+ve/GFP+ve cells reduced, indicating that re-activation had occurred. Lastly, sRANKL stimulation of osteoclast activity reduced dormant (DiD+ve/GFP+ve) cells, suggesting osteoclast driven increased reactivation of dormant tumour cells. Taken together, these data show that dormant tumour cells, which reside in endosteal niches, resist chemotherapy and are available to repopulate the tumour. Importantly, we demonstrate that increased osteoclast remodelling of the endosteal niche reactivates tumour cells in the skeleton. These data provide insights into the fate of dormant cells, mechanisms behind drug resistance and identifies new mechanisms for disease relapse.
Disclosure: The authors declared no competing interests. This work was supported by Cancer Council NSW.