Interleukin-37 Protects Stem Cell-Based Cartilage Formation in an Inflammatory Osteoarthritis-Like Microenvironment

Osteoarthritis (OA) is characterized by progressive loss of articular cartilage. Because cartilage has limited regenerative capacity, there is need for methods to repair damaged cartilage. Human mesenchymal stem cells (MSCs) have potential to differentiate into chondrocytes and can be used for cartilage tissue engineering in vitro and in vivo . However, the inflammatory microenvironment of a damaged joint impairs cartilage formation. The goal of this study was to investigate the potential protective effect of the anti-inflammatory cytokine interleukin-37 (IL37) on MSC-based cartilage formation in an OA-like microenvironment. To study the effect of IL37, IL37 was overexpressed using an adenovirus. Subsequently, MSCs were cultured in a three-dimensional pellet culture model. After 10 days of differentiation, MSCs were exposed to interleukin-1β (IL1β) or OA synovium-conditioned medium (OAS-CM) to create an inflammatory microenvironment. Cartilage formation was determined at day 14 by measuring sulfated glycosaminoglycan (sGAG) content and performing histology. Messenger RNA (mRNA) levels of pro-inflammatory cytokines, catabolic enzymes, and hypertrophy markers were measured using RT-PCR. Finally, the type II collagen:type I collagen (Col2:Col1) ratio was determined on protein level by Western Blot and immunohistochemistry. IL1β and OAS-CM significantly reduced sGAG content of differentiating MSCs by 85% and 40%, respectively. Strikingly, IL37 increased sGAG content of IL1β- and OAS-CM-treated pellets by 1.5 fold. This was reflected on histology by a more intense red safranin-O staining. On gene expression, IL37 significantly reduced IL1β- and OAS-CM-induced IL1B , IL6 , and IL8 gene expression and suppressed both matrix metalloproteinase 3 (MMP3) gene and protein expression. IL37 did not alter the expression of hypertrophy markers. On protein level, IL37 induced enhanced Col2:Col1 ratio, which is indicative for improved hyaline cartilage formation. In this study, we demonstrated that IL37 partly rescued IL1β- and OAS-CM-impaired cartilage formation of MSCs. This effect was correlated to a lower MMP3 expression, which provides a possible explanation. Furthermore, IL37 increased the Col2:Col1 ratio without increasing the expression of hypertrophy markers. These data suggest IL37 as a new strategy to be implemented in stem cell-based cartilage repair for OA-affected joints, to improve cartilage formation.