Exogenous mesenchymal stromal cells in osteochondral repair
Despite osteoarthritis (OA) representing a large burden for health and social care systems, affecting approximately 10 million people in the UK alone, there remains no effective intervention capable of regenerating the damaged joint cartilage in OA and current clinical management for end-stage disease remains joint replacement surgery. There is therefore a need for a change in healthcare strategies to manage early disease, a shift that is likely to include cell-based regenerative therapies. Regenerative medicine and cell therapies are poised to have a tremendous impact on the future of medicine by delivering more effective and personalised treatment than are currently available today. Mesenchymal stromal cells (MSCs) are adult-derived, multipotent cells which have immunomodulatory effects, and can differentiate into a variety of cell types, including cartilage progenitor cells, making MSCs a candidate for musculoskeletal cell therapy. However, there has been several challenges to translating basic science research into new therapies and two pivotal questions remain unanswered:
• Where does the MSC therapy go after administration? • What is its precise mechanism of action?
To address these questions, the effects of an intra-articular injection of human bone marrow derived MSCs into a mouse knee osteochondral injury model were investigated in young C57Bl/6 mice, aged C57Bl/6 mice and GNL3 heterozygote mice. MSC treatment was associated with improved osteochondral tissue repair in young C57Bl/6 mice but this was more limited in aged C57BL/6 mice and GNL3 heterozygote mice. Cage activity monitoring was used to assess mouse recovery following surgery, and the administration of human MSCs was associated with significantly improved recovery following injury when compared to untreated controls. To better understand the mechanisms of action of an MSC therapy, a novel gene reporter system which utilises the organic anion transporting protein, Oatp1a1, was used for cell tracking. Using a lentiviral vector system, transfected MSCs expressing this cell surface protein can transport several imageable small molecules across the cell membrane, including the MRI contrast agent, gadolinium-ethoxybenzyl-DTPA (Gd-EOB-DTPA, PrimovistTM) which is licensed for clinical use. Cells were successfully imaged in vitro and in vivo as a proof-of-concept for this approach in the musculoskeletal system.
Whilst imaging provided circumstantial evidence of MSC therapy fate, single cell RNAseq of retrieved cells (following injection into the mouse after osteochondral injury) was performed to elucidate what transcriptomic changes were important to driving tissue repair over time. Here, the data demonstrated that the exogenous human MSC therapy can be retrieved from both the repair tissue (in the epiphysis) and synovial tissue of the damaged knee joint at both 1 and 4 weeks after intra-articular injection. The retrieved human MSC therapy from the mouse joint tissue showed spatial and temporal transcriptional heterogeneity which was pronounced between the two tissue sources. Supported by mass cytometry data, the paracrine role of MSC treatment was further emphasises, as MSC therapy was associated with the induction of regulatory T cells (Tregs) in the mouse knee joint tissues. As Tregs are involved in skeletal homeostasis, more work needs to be performed to investigate the crosstalk that exists between MSCs and Tregs.
Overall, the presented work shows that exogenous human MSC therapy in the mouse may improve both histological and clinical outcomes following osteochondral injury. Following intra-articular injection of human MSCs, the transcriptomes of the retrieved human cells were studied for the first time and their heterogeneity described. Subpopulations with different functional roles may be implicated in the different phases of osteochondral repair. As tissue digestion and isolation protocols are refined, it is likely that rarer populations may be retrieved from the mouse osteochondral model, but the data presented here offers insights into the interaction between the MSC therapy and the host cells, opening new avenues for the role which MSCs can play as a cell therapy in osteochondral repair.