Integrative studies on cartilage tissue engineering and joint homeostasis

Abstract

The impact of cartilage injury to the joint is often larger than the initial clinical symptoms suggest. Through an alteration in joint homeostasis and biomechanical loading, cartilage lesions may accelerate osteoarthritis onset. Although good clinical results are achieved in patients treated by the autologous chondrocyte implantation (ACI), hyaline cartilage is not consequently formed. Cytokines and growth factors from the surrounding tissues all influence the chondrocyte’s matrix-forming capacity and clinical outcome of cartilage repair. The overall aim of this thesis therefore is ‘to enhance chondrocyte regenerative capacity during ACI by exogenous modulation of cell phenotype and by addressing joint homeostasis’. Cartilage matrix metabolism may be influenced externally by modifying the membrane on which chondrocytes are replanted. Collagen type I and type II coating both improved the quality of in vitro generated neocartilage. Another external stimulus, parathyroid hormone (PTH), earlier inhibited hypertrophic differentiation in mesenchymal stem cells (MSCs) but could not inhibit hypertrophic differentiation in the healthy chondrocytes used here. Internal factors may also influence cartilage matrix metabolism. Choosing chondrocytes of young human ankle donors in stead of of older donors resulted in an even higher expression of hypertrophic differentiation markers, although final cartilage quality was similar. Overall, knee chondrocytes produced the highest quality cartilage. Addressing joint homeostasis may also influence cartilage quality. The ‘Autologous Conditioned Serum’ (ACS) therapy is based on intra-articular administration of autologous IL-1RA, in theory inhibiting the degradative action of intra-articular IL-1β. ACS was investigated in 22 human donors and found to contain chondrodegradative in addition to chondroprotective cytokines. Although expected, it did not improve proteoglycan incorporation in comparison to control, nor were injected cytokine levels of ACS detectable in the joint. In an investigation of clinical scores of previously placebo-treated patients who were now treated with ACS, no further clinical improvement was demonstrated. When discussing the impact of cartilage repairing technologies, it is important to enable comparison of histological data. Several scoring systems exist, however consensus thus far is lacking. The Histological-Histochemical Grading System (HHGS) and the Osteoarthritis Research Society International (OARSI) system are recommended for evaluation of normal to osteoarthritic cartilage tissue. The ICRS-II score is the most suitable for in vivo repaired cartilage. For evaluation of in vitro engineered cartilage, the ‘Bern score’ is preferred. An alternative method to influence cartilage quality is by altering biomechanical loading. The effect of the high tibial osteotomy (HTO), a procedure in which the biomechanical leg axis is altered, on cartilage quality is unknown. Delayed gadolinium enhanced magnetic resonance imaging of cartilage (dGEMRIC) enabled non-invasive assessment of cartilage glycosaminoglycan content in 10 HTO patients. Although a decrease in pain was observed, cartilage glycosaminoglycan content did not improve. In conclusion, the outcome of cartilage repair techniques is likely to be further enhanced provided that the joint homeostasis is optimized. While hypertrophic differentiation may form a threat during microfracture, it does not seem to influence ACI. Differences in joint homeostasis after trauma and OA should be kept in mind while developing intra-articular therapies, and non-invasive evaluation tools may improve follow-up after cartilage repair interventions.