The effect of mesenchymal stem cells on the process of regulating the function of immune system cells and their application in bone fracture healing

  Annually, millions of bone fractures occur around the world. Many of these fractures cause non-welded or defective ones in the absence of treatment, and therefore such fractures require medical treatment interventions. In this review study, the process of immune system performance and it's cytokines on bone tissue repair was investigated. Also, the effect of mesenchymal stem cells on the process of both interaction with immune system cells and bone tissue repair was studied. A review of databases (PubMed, Google Scholar, and Scopus) was conducted to identify related studies by the end of 2017. A set of keywords including "mesenchymal stem cells", "immune system", "inflammation", "bone repair", and "tissue engineering" was used. The immune system and initial inflammation have a very important role in the repair process immediately after the breakdown event. However, the long-term inflammation can act as a deterrent in bone repair. So far, the exact process and the mechanism of immunity based on the secreted cytokines from inflammatory cells have remained unknown in bone repair. Some of these inflammatory mediators act as stimulants in the formation of new bone tissue, while others act as inhibitors. From the past, stem cell therapy has been considered as one of the most attractive ways to repair damaged tissues, especially in bone tissue engineering. Mesenchymal stem cells, as one of the stem cells groups, not only can attract to the category of osteoblastic precursor cells, but also can regulate the immune system through some mechanisms.