"Cell-based approaches for soft-tissue regeneration have been around since the 1980s, when autologous chondrocytes were first used to treat cartilage defects,” said Scott A. Rodeo, MD, co-chief, sports medicine and shoulder service, at the Hospital for Special Surgery and professor of orthopaedic surgery at Weill Cornell Medical College in New York City.
Speaking at the AAOS Now-sponsored forum on “Stem Cells in Orthopaedics: Myth, Miracle, or Something In-Between,” Dr. Rodeo reviewed several cell-based approaches for treating soft tissues such as muscles, tendons, ligaments, and menisci. In addition to marrow- or adipose-derived mesenchymal stem cells (MSCs), noted Dr. Rodeo, other cell sources may contribute to tissue repair, including differentiated cells (skin or tendon), placenta-derived cells, and the intrinsic stem cell niche found in many tissues.
Cells die regularly, noted Dr. Rodeo, and the loss of cells impairs healing. Programmed cell death occurs through the following two processes:
- apoptosis, which is the process of cell self-destruction marked by the fragmentation of nuclear DNA to eliminate damaged or unwanted cells
- autophagy, which is self-digestion by a cell of nonessential organelles through the action of enzymes originating within the same cell
Both processes are found in degenerative tendinopathy, including conditions such as chronic lateral epicondylitis (tennis elbow).
Dr. Rodeo cited several studies showing the potential of stem cells to improve tendon healing. In a horse model, injecting adipose-derived stem cells into a collagenase-induced tendinitis resulted in reduced inflammatory cell infiltrate and significant improvement in tendon fiber architecture and organization, including a more normal crimp pattern.
Studies in his own lab focused on rotator cuff repair in a rat model, using bone-marrow–derived MSCs in a fibrin carrier. “We verified that the cells were present and metabolically active, but the cells alone didn’t do much. There were no differences in fibrocartilage formation at the healing attachment side, in collagen fiber organization, or in biomechanical strength,” he reported.
Concluding that the repair site may lack the cellular and/or molecular signals necessary to induce appropriate differentiation of transplanted cells, Dr. Rodeo noted that cell-based strategies may need to be combined with growth and differentiation factors to be effective.
One of those factors may be the membrane type 1-matrix metalloproteinase (MT1-MMP). This molecule is expressed during embryonic development and mediates the development of the junction between calcified and uncalcified cartilage. Studies conducted by Dr. Rodeo’s lab using a rat model found that using MT1-MMP improved fibrocartilage formation and tendon repair strength at the bone-tendon insertion site.
Another way to enhance cell-based approaches may be through the use of scaffolds as delivery vehicles, noted Dr. Rodeo. He cited a rabbit study that compared a polyglycolic acid scaffold seeded with marrow-derived MSCs with just the scaffold alone to augment rotator cuff repair. The MSC group had more consistent restoration of the fibrocartilage transition zone, a better ratio of collagen 1 to collagen 3, and better tensile strength than the empty scaffold at 16 weeks.
“What about other sources for tendon repair, apart from MSCs, bone marrow, and adipose-derived cells?” asked Dr. Rodeo. “What about using adult-derived, differentiated cells as another example of cell-based approaches?” He cited two studies that used tenocytes—cells derived from tendon—in the treatment of lateral epicondylitis. In both studies, the use of tenocytes resulted in positive structural changes.
Similarly, studies using MSCs isolated from rotator cuff tissue and from the subacromial bursa have demonstrated that these cells have multipotent features and may be potential sources for MSCs in tendon repair. “Can we actually stimulate these intrinsic cells and do they actually contribute to the repair or can they contribute to the repair?” asked Dr. Rodeo. Based on preclinical data from recent studies, this may be possible.
Dr. Rodeo also examined the role of MSCs in meniscal healing. A rabbit study reported in 2010 demonstrated the potential for cell-based approaches to work, and a 2013 human study found that the synovial fluid of patients with meniscal injuries contained a significantly larger number of MSCs than that obtained from uninjured volunteers.
To determine whether the synovium or synovial cells have a role in tissue healing, a recent study looked at their impact in a rabbit model with a massive meniscal defect. It confirmed the multipotentiality of the colony-forming cells and found that injecting synovial-derived MSCs enhanced meniscal regeneration. “The size of the meniscus in the treated group was larger than that in the control group,” said Dr. Rodeo. “Importantly, articular cartilage and subchondral bone were better preserved in the MSC-treated group, so the regenerated tissue seemed to have some function as far as protecting the articular surface.”
Dr. Rodeo also noted that the meniscus has an intrinsic stem cell niche similar to other tissues, citing a rabbit model that looked at meniscal regeneration induced by the intra-articular injection of meniscus stem cells. “They found that transplanting allogeneic meniscus-derived stem cells (MeSCs) promoted new tissue formation with better defined shape and more mature matrix. More importantly, transplantation of MeSCs protected joint surface cartilage; the controls had extensive joint surface irregularities. These researchers concluded that intra-articular injection of MeSCs might be a successful strategy for both articular cartilage protection and meniscus regeneration.”
Just this year, a randomized, double-blind, controlled study in humans supported that theory, noted Dr. Rodeo. “Essentially, they found no real safety issues—no ectopic formation or immunologic reaction,” he said. “They did find increased meniscal volume, and patients with arthritic changes who received the MeSCs experienced a significant reduction in pain compared with controls.”
“We’ve known for many years that muscle has satellite cells that contribute to some degree of muscle regeneration, but clearly the existence of nonsatellite cells—a stem cell type population in muscles—is becoming evident,” continued Dr. Rodeo. “It may be that these stem cells regulate the satellite cells. These muscle stem cells increase in response to exercise, so they appear to have a regulatory role for satellite cells, ultimately stimulating them to lead to muscle regeneration.”
In conclusion, Dr. Rodeo noted that various options for “cell-based” therapy exist, beyond just stromal cells from marrow or adipose tissue. “We should try to consider these other cell sources. How can we stimulate that intrinsic cell niche, which we know is there, to get away from allograft cells? Clearly, we need more than just the cells; it’s a complex biology so we may need to augment them with growth factors, platelet-rich plasma, and other cytokines.”
Disclosure information: Dr. Rodeo— Smith and Nephew, Pluristem Therapeutics Inc., Rotation Medical, Cayenne Medical.
Frank B. Kelly, MD, is a member of the AAOS Now editorial board and served as cochair of the AAOS Now forum on stem cells in orthopaedics. Mary Ann Porucznik is managing editor of AAOS Now. She can be reached at firstname.lastname@example.org
- Cell-based approaches to tissue healing have been used for more than 30 years.
- Multiple sources—including adult differentiated (skin, tendon), intrinsic cell niche, allogeneic, and placental cells—should be considered when discussing the use of cell-based therapies in soft-tissue healing.
- Cell-based therapies may need to be augmented with additional factors to be effective in treating soft-tissue injuries.
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