Over 50 million adults in the U.S. are estimated to be affected by some form of arthritis, a very painful, debilitating condition in which the cartilage that provides cushioning within bone joints gradually degrades. Health care costs of treating arthritis in California alone has been estimated at over $12 billion and that figure is already over a decade old. Unfortunately, the body doesn’t do a good job at healing cartilage in the joint so doctors rely mostly on masking symptoms with pain management therapy and, in severe cases, resorting to surgery.

Illustration of damaged cartilage within an osteoarthritic hip joint Image: Wikipedia/Open Stax

Mesenchymal stem cells (MSCs) – found in bone marrow, fat and blood – give rise to several cell types including cartilage-producing cells called chondrocytes. For that reason, they hold a lot of promise to restore healthy joints for arthritis sufferers. While there is growing evidence that injection of MSCs into joint cartilage is effective, it is still not clear how exactly the stem cells work. Do they take up residence in the cartilage, and give rise to new cartilage production in the joint? Or do they simply release proteins and molecules that stimulate other cells within the joint to restore cartilage? These are important questions to ask when it comes to understanding what tweaks you can make to your cell therapy to optimize its safety and effectiveness. Using some clever genetic engineering techniques in animal models, a research team at the University of Veterinary Medicine in Vienna, Austria report this week in JCI Insights that they’ve uncovered an answer.

Tracking the fate of a stem cell treatment after they’ve been injected into an animal, requires the attachment of some sort of “beacon” to the cells. A number of methods exist to accomplish this feat and they all rely on creating transgenic animals engineered to carry a gene that produces a protein label on the cells. For instance, cells from mice or rats engineered to carry the luciferase gene from fireflies, will glow and can be tracked in live animals. So, in this scenario, MSCs from a genetically-engineered donor animal are injected into the joints of a recipient animal which lacks this protein marker. This technique allows the researchers to observe what happens to the labeled cells.

There’s a catch, though. The protein marker carried along with the injected cells is seen as foreign to the immune system of the animal that receives the cells. As a result, the cells will be rejected and destroyed. To get around that problem, the current practice is to use recipient animals bred to have a limited immune response so that the injected cells survive. But solving this problem adds yet another: the immune system plays a key role in the mechanisms of arthritis so removing the effects of it in this experiment will likely lead to misinterpretations of the results.

So, the research team did something clever. They genetically engineered both the donor and recipient mice to carry the same protein marker but with an ever-so-slight difference in their genetic code. The genetic difference in the protein marker was large enough to allow the team to track the donor stem cells in the recipient animals, but similar enough to avoid rejection from the immune system. With all these components of the experiment in place, the researchers were able to show that the MSCs release protein factors to help the body repair its own cartilage damage and not by directly replacing the cartilage-producing cells.