gms | German Medical Science

Objectives: Heterotopic ossification (HO) describes the formation of bone in areas where it usually does not occur, such as soft tissues, and may develop most commonly after tissue and joint injuries or spinal and traumatic brain injuries. HOs can significantly limit the patient’s quality of life by causing pain and restrictions in the range of motion of a joint up to its ankylosis. While preventive measures such as irradiation or the administration of NSAIDs after total hip arthroplasty are currently established strategies to reduce the incidence of HOs, many times the only option to treat symptomatic HOs is surgical resection. Here, we set out to develop a possible new cell-based approach of treating HOs.

Methods: Our group has developed engineered osteoclasts (iRANK cells) with ectopic bone resorbing activity. Osteoclast (OC) differentiation and bone resorption of murine myeloid stem cells transfected with an inducible, ligand-independent receptor activator of NF-kappaB (RANK) can be activated using a chemical inducer of dimerization (CID) and is tested on mineral resorption assays and bone in vitro. Using an in vivo HO model, iRANK cells are injected onto bone nodules and following iRANK activation bone volume is monitored using µCT.

Following feasibility studies with murine cells we aim to replicate this approach with human cells. However, expandability of human primary OC precursors such as PBMCs is limited and therefore restricts upscaling of OC production. iPSCs can be propagated indefinitely and represent an alternative approach that allows for upscaling. OC differentiation protocols including mesodermal, hematopoietic and osteoclast differentiation from iPSCs were compared and tested. Staining for TRAP and Cathepsin K in conjunction with PCR and mineral resorption assays is used to confirm successful differentiation of human OCs.

Results and conclusion: Murine iRANK cells demonstrate mineral resorption following activation with a CID on mineral resorption assays. Notably, osteoclastogenesis of iRANK cells is shown to be osteoprotegerin (OPG)-independent when tested with OPG concentrations up to 50 nM. When treating HOs with iRANK cells in a murine in vivo HO model, bone volume and mineral density was significantly reduced within a 14-day time frame in comparison to controls. In transition to human cells, iPSCs derived from different tissue origins were differentiated to OCs using an embryoid body-based and monolayer-based approach. Embryoid body-based differentiation yielded large numbers of hematopoietic progenitor cells that could be differentiated into large, multinucleated OCs.

iRANK cells represent a potential new approach of treating HOs. While initial results seem promising, many obstacles and challenges still need to be solved on its way to clinical translation.