Articular cartilage-derived progenitor cells for cartilage tissue engineering
Authors:   , Abinzano Florencia 
Published: February 2024

Articular cartilage damage is a persistent problem in the orthopedic field, particularly prevalent among young, active patients. Cartilage defects are found in over 60% of exploratory knee surgeries due to knee complaints, and can lead to pain and early osteoarthritis. Once damaged, cartilage tissue has limited self-repair capabilities. Current treatments often result in fibrous repair tissue with inferior mechanical properties. Cell-based approaches, like autologous chondrocyte implantation, show promise but suffer from inconsistency and high failure rates. Articular cartilage-derived progenitor cells (ACPCs), a subpopulation of chondrocytes residing mainly on the cartilage surface, offer a potential solution. These cells can be expanded in high numbers while maintaining chondrogenic potential and resisting hypertrophic dedifferentiation.

This thesis aims to optimize ACPC stimulation and utilize advanced delivery systems for enhanced cartilage repair. Hydrogels, known for their aqueous nature and modifiability, were tested as potential delivery systems. ACPC-laden GelMA-Tyr hydrogels facilitated chondrogenesis and exhibited high viability, printability, and potential for growth factor delivery, by using a safe crosslinking system for the surrounding tissue. Moreover, the effects of hypoxia were explored on equine ACPCs and mesenchymal stromal cells (MSCs) in agarose gels, revealing ACPCs' superior GAGs and type II collagen production independently of oxygen tension. Since cell-cell contact seems to maximize ACPC matrix production, a hydrogel modified with N-Cadherin mimetic adhesion sites that simulate these interactions was studied. Cartilage matrix production was only achieved under continuous bone morphogenetic protein 9 (BMP-9) stimulation, and the addition of the HAVDI peptide did not lead to significant differences in chondrogenic performance. However, hydrogels that are chondro-permissive and cell-friendly are unable to bear the high loads from the joint.

To address the mechanical challenge, gel-free, low-volume melt electrowritten reinforcing scaffolds that permit cell-cell contact were investigated as an alternative delivery system. BMP-9 supplementation enhanced equine ACPC chondrogenesis. Then, a high density of BMP-9 stimulated ACPCs were combined with a PCL melt electrowritten mesh, which led to a significantly higher Young’s modulus after culture. These promising cartilage constructs, combined with an osteal anchor, were tested in an orthotopic equine model. However, the failure of the bone component meant that the evaluation of the cartilage construct was severely compromised, and further testing will be necessary. To move closer to clinical translation, these results were confirmed with human ACPCs derived from osteoarthritic patients. Here, one dose of BMP-9 stimulation was enough to produce high amounts of GAGs and type II collagen, in pellet cultures, as well as combined with the reinforcing meshes.

This work underscores the potential of ACPCs to overcome limitations in cell-based cartilage treatments. By strategically modulating biochemical and physical cues to induce chondrogenesis, the inherent capabilities of ACPCs were harnessed. The advantages of ACPCs over chondrocytes and MSCs warrant additional in-depth studies, including in vivo experiments followed by clinical trials. Establishing ACPCs as a new gold standard for autologous cartilage repair could offer a promising avenue for patients suffering from cartilage-related issues.