Combining freshly isolated chondroprogenitor cells from the infrapatellar fat pad with a growth factor delivery hydrogel as a putative single stage therapy for articular cartilage repair.

Growth factor delivery systems incorporating chondroprogenitor cells are an attractive potential treatment option for damaged cartilage. The rapid isolation, processing, and implantation of therapeutically relevant numbers of autologous chondroprogenitor cells, all performed "in-theatre" during a single surgical procedure, would significantly accelerate the clinical translation of such tissue engineered implants by avoiding the time, financial and regulatory challenges associated with in vitro cell expansion, and differentiation. The first objective of this study was to explore if rapid adherence to a specific substrate could be used as a simple means to quickly identify a subpopulation of chondroprogenitor cells from freshly digested infrapatellar fat pad (IFP) tissue. Adhesion of cells to tissue culture plastic within 30 min was examined as a mechanism of isolating subpopulations of cells from the freshly digested IFP. CD90, a cell surface marker associated with cell adhesion, was found to be more highly expressed in rapidly adhering cells (termed "RA" cells) compared to those that did not adhere (termed "NA" cells) in this timeframe. The NA subpopulation contained a lower number of colony forming cells, but overall had a greater chondrogenic potential but a diminished osteogenic potential compared to the RA subpopulation and unmanipulated freshly isolated (FI) control cells. When cultured in agarose hydrogels, NA cells proliferated faster than RA cells, accumulating significantly higher amounts of total sGAG and collagen. Finally, we sought to determine if cartilage tissue could be engineered by seeding such FI cells into a transforming growth factor-β3 delivery hydrogel. In such a system, both RA and NA cell populations demonstrated an ability to proliferate and produced a matrix rich in sGAG (∼2% w/w) that stained positively for type II collagen; however, the tissues were comparable to that generated using FI cells. Therefore, while the results of these in vitro studies do not provide strong evidence to support the use of selective substrate adhesion as a means to isolate chondroprogenitor cells, the findings demonstrate the potential of combining a growth factor delivery hydrogel and FI IFP cells as a single stage therapy for cartilage defect repair.