Interleukin‐10 (IL‐10) is a pleiotropic immunoregulatory cytokine with a chondroprotective effect that is elevated in cartilage and synovium in patients with osteoarthritis. However, the role of IL‐10 during endochondral bone formation and its mechanism of action have not been elucidated.

IL‐10^–/– mice and IL‐10–treated tibial organ cultures were used to study loss and gain of IL‐10 functions, respectively, during endochondral bone formation. Primary chondrocytes from the long bones of mouse embryos were cultured with and without IL‐10. To assess the role of IL‐10 in chondrogenic differentiation, we conducted mesenchymal cell micromass cultures.

The lengths of whole skeletons from IL‐10^–/– mice were similar to those of their wild‐type littermates, although their skull diameters were smaller. The tibial growth plates of IL‐10^–/– mice showed shortening of the proliferating zone. Treatment with IL‐10 significantly increased tibial lengths in organ culture. IL‐10 also induced chondrocyte proliferation and hypertrophic differentiation in primary chondrocytes in vitro. Mechanistically, IL‐10 activated STAT‐3 and the Smad1/5/8 and ERK‐1/2 MAP kinase pathways and induced the expression of bone morphogenetic protein 2 (BMP‐2) and BMP‐6 in primary chondrocytes. Furthermore, the blocking of BMP signaling attenuated the IL‐10–mediated induction of cyclin D1 and RUNX‐2 in primary chondrocytes and suppressed Alcian blue and alkaline phosphatase staining in mesenchymal cell micromass cultures.

These results indicate that IL‐10 acts as a stimulator of chondrocyte proliferation and chondrogenic or hypertrophic differentiation via activation of the BMP signaling pathway.