I. Bernabei 1, D. Ehirchiou 1, S. Nasi 1, V. Chobaz 1, M. Castelblanco 1, A. So 1, L. Zhang 2, N. Busso 1
1. Service of Rheumatology, Department Of Musculoskeletal Medicine, Centre Hospitalier Universitaire Vaudois and University of Lausanne, Switzerland 2. Department of Physiology, Center for Vascular and Inflammatory Diseases, University of Maryland School of Medicine, Baltimore, Maryland, USA
Purpose: Osteoarthritis (OA) is a progressive joint disease that is strongly associated with calcium-containing crystal formation (mineralization) by chondrocytes leading ultimately to joint calcification. However, this calcification process is poorly understood and treatments targeting underlying disease mechanisms lacking. CD11b integrin (CD11b/CD18, Mac-1, or αMβ2 ) is a member of the beta 2 integrin family of adhesion receptors, critically involved in the development of several inflammatory diseases, including collagen-induced arthritis , where CD11b-deficient mice exhibited increased cartilage degradation as compared to that of WT control animals. However, the functional significance of CD11b integrin signaling in the pathophysiology of chondrocytes is unknown.
Methods: Cartilage from human and surgically-induced murine OA knee joints was dissected. Primary chondrocytes were isolated from human cartilage or cartilage of wild-type (WT) and CD11b-deficient (KO) newborn mice. CD11b expression in chondrocytes and cartilage was assessed by flow cytometry and immunohistochemistry respectively. Chondrocyte mineralization was assessed by alizarin-red staining and calcium-content assay. Murine IL-6 cytokine and collagen X were assayed by qRT-PCR and ELISA. Wild-type and CD11b-deficient mice (n=5 per group) were subjected to meniscectomy (MNX, the surgical model of OA). Two months after MNX, histological analysis of knee joint sections were performed and examined for inflammation and cartilage degradation, using OARSI scores.
Results: We found that CD11b was expressed in both undamaged and damaged human and murine articular cartilage, both in extracellular matrix and in chondrocytes. Primary murine CD11b KO chondrocytes had increased mineralization and alkaline phosphatase (Alp) activity and secreted more IL-6 compared to control wild-type cells. In addition, in the same conditions the expression of collagen X was increased by more than 10-fold in CD11b-deficient cells. Conversely, the CD11b activator LA1 reduced chondrocyte mineralization, IL-6 production and collagen X expression. In the surgically-induced murine knee osteoarthritis, the deficiency of CD11b led to more severe OA (medial cartilage damage in CD11b: 5.6+/-1.8, in WT: 1.2 +/- 0.5, p<0.05, inflammation in CD11b: 2.8 +/- 0.2, in WT: 1.4 +/- 0.5).
Conclusions: Altogether, these data demonstrate that CD11b is protective in in vitro and in vivo models of OA.
References: 1. Stevanin M et al. CD11b regulates the Treg/Th17 balance in murine arthritis via IL-6.Eur J Immunol. 2017 47(4):637-64.