Mesenchymal stem cells for orthopedic injury
Although there's much work being done regarding the use of mesenchymal stem cells for orthopedic injury, most studies are
in vitro with relatively few in vivo studies evaluating the use of mesenchymal stem cells in surgically created or naturally
occurring musculoskeletal disease. Some concerns that clinicians may have regarding the use of stem cells include their viability,
proliferation and ability to renew and regenerate into viable tissue after injection, allowing horses to heal with little
scar formation and enabling their return to previous levels of performance.
Successful use of intra-articularly administered mesenchymal stem cells has shown promising results in tissue healing of experimentally
produced lesions in dogs and sheep. According to one study of experimentally induced equine femoropatellar joint lesions,
in vivo implantation of mesenchymal stem cells within autogenous fibrin plugs yielded increased fibrous tissue deposition
that contained predominantly collagen type II.2 However, this lasted only 30 days. But seven months later, the control and treated lesions were indistinguishable.
Researchers at Louisiana State University's School of Veterinary Medicine found that equine BMSCs had superior chondrogenic
potential compared with adipose-derived cells in the presence of stimulatory growth factors.3
Besides the viability of stem cells and their ability to convert to various tissue cells for repair (i.e., their pluripotent,
self-renewing capability toward tissue regeneration), there's data as to their ability to directly affect the inflammatory
process since it appears that stem cell therapy in horses may also provide impact as an immunomodulator.4 Tissue repair and disease improvement as assisted by mesenchymal stem cells has been shown to be related to an individual's
immune system. It's been found that mesenchymal stem cells have effects on various immunomodulators (e.g., B cells, T cells,
natural killer cells, dendritic cells, macrophages, neutrophils) that can affect the humoral immune response. As they regenerate
tissue, stem cells could also help by reducing the immune response, making it easier for them to be effective as opposed to
classical tissue grafting or tissue transplantation regarding a concern for tissue rejection.
Boone's AAEP fellow research
Boone explored various aspects of regenerative medicine as a portion of her work, including the research for which she received
the AAEP fellowship. As a clinician using various regenerative medicine therapies, Boone had a lot of questions as she injected
horses intra-articularly with mesenchymal stem cells.
The basis for her PhD work was her curiosity about what happened to the cells after they were injected into a joint: Where
do they go? Do they live in the synovial fluid? Do they develop toward an articular cartilage phenotype? Can they start to
produce cartilage if they're exposed to synovial fluid? What's the proper vehicle of delivery?
Boone also became interested in how to manipulate the cells and how to use them in an allogenic manner. She concentrated on
two major types of articular disease that the veterinarians at the University of Georgia were treating—osteoarthritis (OA),
an ongoing inflammatory process that leads to articular cartilage degeneration, and osteochondritis dissecans (OCD) lesions,
or subchondral bone cysts that have abnormalities to their articular cartilage but don't necessarily have the degree of on-going
inflammation within the joint.
Originally, Boone wanted to try to meld the two, but she says she is now "very much more interested in the fact that the cells
we inject also have immunomodulatory anti-inflammatory properties. Can we use those properties to help modulate the inflammatory
environment within the joint and then use them to repair articular cartilage? Is there a way to make them useful in an ongoing
OA horse, which I think is much more enticing to people, but also fixing articular defects that are an interest, looking at
the inflammatory environment?"
The first project of Boone's PhD work looked at harvesting synovial fluid from horses and examining the survivability, proliferation
and chondrogenesis of stem cells in allogenic synovial fluid in vitro.
"If I take cell lines from a group of several horses and add synovial fluid that's not their own, will they survive, will
they increase in their numbers and can they promote cartilage?" Boone asked. "We use various levels of synovial fluid, because
in vitro in the synovial fluid environment you don't have that ongoing process of replenishment. In vitro, obviously you have
a limited amount of growth factors—nutrients that will be depleted over time."
During this first study, she looked at the response during a 72-hour period—in some cases out to five days—to various percentages
of media supplementation with synovial fluid up to 100 percent; she introduced only synovial fluid. In this study, she says,
"We were using normal synovial fluid, though we're planning on trying to use OA synovial fluid as well."