“Almost all of the research on using stem cells in joints is focused on trying to treat focal defects in cartilage,” said Farshid Guilak, PhD, Laszlo Ormandy professor and vice-chair of the departments of orthopaedic surgery, biomedical engineering, and mechanical engineering & materials science at Duke University Medical Center. Speaking at the 2014 AAOS Now Forum on the use of stem cells in orthopaedics, Dr. Guilak compared focal defects to “a pothole in the road.”

The real problem, he noted, is osteoarthritis (OA), which is a much more challenging goal. “Can we use a stem-cell based approach to treat OA?” he asked. “We can try; one way is tissue engineering, which combines cells, scaffolds, and biologically active molecules to start to regenerate cartilage and maybe to restore damaged or diseased tissues.”

Dr. Guilak outlined a number of challenges researchers face in using stem cells to treat OA, including the following:

• OA is not a focal defect; treating it requires treating the entire joint. Although treating focal defects presents challenges in integrating the old tissue and the new tissue, removing and resurfacing the entire cartilage surface could address these issues.
• Because harvesting cartilage cells from the joint itself damages native tissues, researchers must find readily accessible sources of stem cells.
• The difficulty in restoring the original geometry of the joint must be addressed, possibly through anatomically based three-dimensional (3-D) scaffolds.
• Because current scaffolds do not possess adequate mechanical properties, novel structures must be developed.
• The inflammatory component of OA must be addressed, possibly by using the scaffold to deliver anti-inflammatory drugs.
• Getting new materials approved by the U.S. Food and Drug Administration is a lengthy process, pushing researchers to use already approved biomaterials.

Making cartilage
Dr. Guilak noted that adipose stem cells, derived from body fat, could be used to make cartilaginous tissues. More recently, he said, induced pluripotent stem cells (iPSCs), which can be created from adult cells, have also been used to make large volumes of genetically defined cells, such as cartilage cells.

“When you work in tissue engineering,” said Dr. Guilak, “growing chondrocytes from stem cells in gels is really good for the cells; they love being in a hydrated environment. But although the gels are excellent for keeping the cells alive, they have mechanical properties similar to gelatin. They don’t hold up very well under load.

“Nonwoven fibrous polymers are also great for growing cells, but don’t have the right mechanical properties. Two-dimensional woven polymers have good properties, but are limited to a single layer of fibers,” he explained.

Dr. Guilak noted that his laboratory has built a table-top machine that produces a 3-D weaving (Fig. 1) “so we can make biomaterial scaffolds that have the strength of fibers and the flexibility of a textile, but are porous and completely defined in their architecture.” Such a scaffold, he said, can be designed to match the properties of cartilage, and, “once cells are placed in the gel with it, they behave very much like cartilage when they are implanted.”

This type of scaffold can be molded into a structure that is the shape and size of the actual joint. The scaffold can then be seeded to grow cartilage cells and used to resurface the bone. Dr. Guilak is currently conducting a preclinical study to see whether this theory actually works.

Treating the inflammatory component
Dr. Guilak also discussed the use of scaffolds as delivery mechanisms for growth factors or therapeutic drugs. The fact that OA has an inflammatory component is well known, and that inflammatory component may have a detrimental impact on chondrogenesis.

Ongoing research is showing that including a lentivirus can cause cells to make growth factors and other materials. “We can get chondrogenesis without adding any growth factors,” said Dr. Guilak. “We can coat one layer of fibers with the lentivirus to induce cartilage formation and another layer of fibers with a different virus that includes bone morphogenetic protein (BMP-2). When we put the two layers together, we have one set of cells, one media, one scaffold—but we get this gradient of bone to cartilage.”

Scaffolds can also be used to deliver the interleukin-1 receptor antagonist (IL-1Ra), an important natural anti-inflammatory protein. “If the scaffold sees any IL-1—even levels as low as 100 picograms per mL—it wipes out chondrogenesis. Even though the stem cells are anti-inflammatory, they can’t overcome the impact of the IL-1,” explained Dr. Guilak.

Researchers in his lab are working to develop ways to deliver IL-1Ra and, just as importantly, to control its delivery. Their goal is to custom design a stem cell that expresses transgenes only when they are needed. “Our idea,” said Dr. Guilak, “is to take the cell and insert a new gene that produces an IL-1 inhibitor anytime we give it a chemical signal, which in this case is doxycycline.

“Basically, we can then turn the genes on and off like a faucet so they can provide controlled delivery of anti-inflammatory drugs within the joint. This is a crucial concept of what we can do; if the stem cells aren’t smart enough on their own, we can reprogram them and make them do what we want them to,” said Dr. Guilak.

Final take-aways
Dr. Guilak presented the following “take-aways” on the potential for treating OA with stem cells:

• Stem cells may have significant potential for treating OA using cell therapy or tissue engineering.
• OA is a whole joint (or systemic) disease and thus may require a whole joint or systemic therapy.
• Cell therapy approaches may provide anti-inflammatory effects in addition to tissue
  regeneration.
• Complex scaffolds can provide mechanical function as well as cell-instructive signals; optimal scaffold and cell sources remain to be determined.
• Cells can be customized to provide new functions.

Disclosure information: Dr. Guilak—Cytex Therapeutics, Inc.; Osteoarthritis; Cartilage Journal of Biomechanics.

Frank B. Kelly, MD, is a member of the AAOS Now editorial board and cochaired the 2014 AAOS Now Forum; Mary Ann Porucznik is managing editor of AAOS Now.