Allogeneic primary mesenchymal stem/stromal cell aggregates within poly(N-isopropylacrylamide-co-acrylic acid) hydrogel for osteochondral regeneration

Abstract Osteoarthritis (OA) is an inflammation-related chronic disease that causes progressive degeneration of cartilage, which might even extend to subchondral bones. Due to the unique physiological structural differences between cartilage and subchondral bone, it is challenging to restore the full function of an osteochondral defect. In this study, a thermosensitive poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAAm-AA)) hydrogel was used as a carrier for allogeneic primary mesenchymal stem/stromal cells to determine the therapeutic efficacy of the cell-hydrogel hybrid on osteochondral regeneration. At a similar mechanical strength, p(NIPAAm-AA) hydrogel facilitates formation of cell aggregates of allogeneic primary MSCs in situ, while the fibrin hydrogel supports cell binding and the development of a spindle morphology. While the p(NIPAAm-AA) hydrogel did not induce any inflammatory effects, the fibrin hydrogel was found to elicit a pro-inflammatory response. MSC aggregates promotes expression of paracrine signaling-related genes (TGFB1, VEGF, CXCL12, IGF1, BMP2, BMP7, WNT3A, CTNNB1) as well as expression of chondrogenesis-related genes (SOX9, ACAN, and COL2A1) and generation of extracellular matrices (ECMs) (type 2 collagen and glycosaminoglycans) in chondrogenic induction medium. The functional enhancement of paracrine effects by MSC aggregates (i.e. the immunomodulatory effect and the stimulation of stem/progenitor cells homing) and induced differentiation of MSCs might play a synergistic role in the generation of the neo-cartilage and subchondral bone at an osteochondral defect site in vivo. Delivery of primary allogeneic MSCs in the context of a thermosensitive p(NIPAAm-AA) hydrogel to an osteochondral defect site may be a novel and promising strategy for osteochondral regeneration.