Background In healthy cartilage, there is a balance between anabolic and catabolic activities of chondrocytes that maintains the functional integrity of the extracellular matrix. However, during osteoarthritis (OA), chondrocytes become more catabolically active and express increased levels of matrix degrading enzymes, such as MMPs and ADAMTSs. Increased MMP and ADAMTS activity results in a net loss of the extracellular matrix and therefore leads to cartilage damage. Previously, we found that the anti-inflammatory cytokine Interleukin 37 (IL37) is able to counter-regulate the catabolic status of chondrocytes by reducing the IL1β-driven expression of pro-inflammatory cytokines and catabolic enzymes.

Objectives The goal of this study was to investigate, in human OA cartilage explants, the effect of IL37 on sulfated glycosaminoglycans (GAG) content and synthesis of extracellular matrix molecules and cartilage degrading enzymes to investigate its therapeutic potential in OA.

Methods Human cartilage was obtained from eighteen OA patients undergoing total knee or hip arthroplasty. Biopsy punches of 4 mm in diameter were made to equalize explant size. After culturing overnight, explants were incubated for 48 h with three doses (1, 10 or 100 ng/ml) of recombinant-human IL37 (rhIL37). In the supernatant of the explant cultures, GAG release was measured with the DMB assay and levels of the ARGS neoepitope, which is one of the products of aggrecan degradation by ADAMTS5, were detected using Western Blot. Furthermore, gene and protein expression of extracellular matrix molecules and cartilage degrading enzymes were measured. Nitric oxide (NO), an important effector molecule that may suppress cartilage matrix synthesis, levels were measured in the supernatant of the explants culture using Griess reagens.

Results Adding rhIL37 (100 ng/ml) to OA cartilage explants caused a highly significant reduction in GAG release to the supernatant of, on average, 32% in eighteen donors (Figure 1). Gene expression of the matrix molecules aggrecan and collagen type II was not affected, indicating that this effect of rhIL37 was not due to a loss of aggrecan synthesis. Another mechanism to prevent GAG release in cartilage is via inhibition of NO synthesis, but NO levels in the supernatant were comparable between rhIL37 treated groups and the control group. In contrast, after addition of rhIL37, ARGS neoeptiope levels, which reflect ADAMTS5 activity, were dose-dependently down regulated in the supernatant. Furthermore, protein analysis of the explants showed that rhIL37 reduced ADAMTS5 levels. These data indicate that IL37 interferes with the amount active matrix degrading enzymes in the cartilage matrix. However, gene expression of ADAMTS5 was not affected by rhIL37, indicating that the effect of IL37 on ADAMTS5 is post translational.

Conclusions Our data show that rhIL37 reduces GAG release by OA cartilage explants. The mechanism behind this protective effect of IL37 probably runs via a reduction in ADAMTS5 abundance in the cartilage matrix, which is the main aggrecanase involved in OA This effect of IL37 on ADAMTS5 is probably post translational. Our data indicate that IL37 can maintain cartilage matrix integrity under OA conditions and is able to reduce the severity of cartilage destruction during OA.

Disclosure of Interest None declared