Alpha defensin-1 attenuates surgically induced osteoarthritis in association with promoting M1 to M2 macrophage polarization

Macrophages play an important part in the pathogenesis of osteoarthritis (OA). Our objective was to determine the effects of α-defensin-1 on macrophage polarization and consequently OA.OA synovial tissue and synovial fluid were assessed for the presence of M1 (CD68+CD16+CD206-) and M2 (CD68+CD206+CD16-) macrophages by flow cytometry. M0, M1, and M2 macrophages were co-cultured with OA chondrocytes to determine their influence on chondrogenic phenotype. Polarization of THP-1 activated monocytes from M1 to M2 in response to α-defensin-1 was evaluated by flow cytometry, RT-PCR and RNA sequencing. Effects of intra-articular α-defensin-1 in vivo were evaluated in a rat meniscal/ligamentous injury (MLI) model.The quantity of M1 exceeded M2 polarized macrophages in human OA synovial tissue (mean difference 26.1% [13.6-38.6%], P < 0.001) and fluid (mean difference 10.5% [5.0-16.1%], P = 0.003). M1 to M2 polarization in vitro was most effectively promoted with 10 ng/mL α-defensin-1. Compared with untreated macrophages, the α-defensin-1 polarized macrophages modified co-cultured OA chondrocytes from a pro-catabolic state to a pro-anabolic (regenerative-like) state based on expression of COL2A1, ACN, MMP3, MMP13 and ADAMTS5. Intra-articular α-defensin-1 decreased severity of cartilage damage and synovitis in the MLI rat model. RNAseq analyses suggested insulin and Toll-like receptor signaling pathways in the chondroprotective α-defensin-1 mechanism of action.α-defensin-1 promotes M1 to M2 macrophage polarization in vitro, has beneficial effects on chondrocytes indirectly via M2 macrophage polarization, and attenuates the severity of OA in vivo, suggesting it might be a candidate treatment for OA.