Role of mesenchymal stem cells in osteoarthritis therapy

Osteoarthritis (OA) is the most common joint disease encountered worldwide. OA of the knees is particularly common, with radiographic OA of the tibiofemoral compartment occurring in 5-15% of people aged 35-74 years. Currently, adult stem cells are being evaluated for various therapeutic approaches, including OA. However, the efficacy of mesenchymal stem cells (MSC) in the treatment of OA is controversial. Recently, major advances in the development of animal models for cell therapy and cell-based gene therapy have been accomplished in my laboratory. To further explore the therapeutic potential of regenerative treatment protocols, it is necessary to trace the fate of individual donor or manipulated cells in the host organism. However, immune-mediated rejection of labeled cells is a general problem in transplantation studies using cells labeled with any immunogenic marker, and also in gene therapy protocols. In proof-of-principle experiments, we were able to establish a syngeneic rat model for long-term histological cell tracking in the absence of immune-mediated rejection of labeled cells in immunocompetent animals by inducing specific tolerance to the marker gene through neonatal tolerization. In addition, we found that the genetic marker human placental alkaline phosphatase (hPLAP) provides superb detection quality in hard tissues, because this heat stable marker enzyme survives not only paraffin but also methylmethacrylate embedding. Recently, we generated a novel in vivo cell tracking system consisting of a transgenic donor and a corresponding marker tolerant transgenic recipient mouse line on the same inbred background. The donor line ubiquitously expresses wild-type hPLAP, whereas the recipient line expresses a mutated form of the marker (hPLAP E429G) which can be easily distinguished from the wild-type form by its heat sensitivity. Because only a single amino acid is different between wild-type and mutated hPLAP, hPLAP E429G- transgenic mice are tolerant to hPLAP. The patents for these inventions are pending. In the proposed project we intend to 1) generate a hPLAP/hPLAPE429G transgenic marker tolerant rat model, using the same technology that we established for mice, and to employ this model to study 2) the factors governing the attachment of intra-articularly injected MSC to damaged cartilage, and 3) the long-term survival and efficacy of intra-articularly injected MSC in OA rats, as well as the migration of intra-articularly injected MSC to other organs. In hPLAP E429G marker tolerant recipients, OA of the knee will be induced by destabilization of the medial meniscus. MSC isolated and expanded from syngeneic hPLAP transgenic rats will be used as genetically labeled cells. We are positive that the proposed project will significantly increase our understanding of the behavior, efficacy and safety of autologous MSC in OA therapy. Therefore, this project may have important implications for clinical medicine.

Adult stem cells are currently being evaluated for various therapeutic approaches, including osteoarthritis (OA), the most common joint disease encountered worldwide. However, the efficacy of mesenchymal stem cells (MSC) in cartilage regeneration is still controversial. To further explore the therapeutic potential of cell-based regenerative treatment protocols, it is necessary to trace the fate of individual donor or manipulated cells in the host organism. However, immune-mediated rejection of labeled cells is a general problem in transplantation studies using cells labeled with any immunogenic marker. In the current project, we have developed a novel animal model in which genetically labeled cells can be tracked in an immunocompetent host over long periods of time in the complete absence of immune-mediated rejection. Our model is almost fully equivalent to the situation in a patient treated with autologous stem cells. Using our novel animal model, we were able to show that MSC injected into the knee are able to attach to focal cartilage lesions, and to positively influence cartilage regeneration. Interestingly, however, the attached MSC did not give rise to cartilage-producing cells. Rather, the MSC orchestrated the recruitment and differentiation of endogenous, host- derived cells into cartilage producing cells. In addition, our experiments showed that MSC attach to cartilage lesions through specific membrane proteins, so-called integrins. Thus, the current project significantly contributed to our understanding of the mechanisms underlying the therapeutic effect of MSC in cartilage lesions. In the long run, our project may lead to significant improvements in MSC-based regenerative therapies of articular cartilage.