Mesenchymal stem cell (MSC) exosome was previously shown to be effective in repairing critical size osteochondral defects in an immunocompetent rat model. Here we investigate the cellular processes modulated by MSC exosomes and the mechanism of action underlying the exosome-mediated responses in cartilage repair. We observed that exosome-mediated repair of osteochondral defects was characterised by increased cellular proliferation and infiltration, enhanced matrix synthesis and a regenerative immune phenotype. Using chondrocyte cultures, we could attribute the rapid cellular proliferation and infiltration during exosome-mediated cartilage repair to exosomal CD73-mediated adenosine activation of AKT and ERK signalling. Inhibitors of AKT or ERK phosphorylation suppressed exosome-mediated increase in cell proliferation and migration but not matrix synthesis. The role of exosomal CD73 was confirmed by the attenuation of AKT and ERK signalling by AMPCP, a CD73 inhibitor and theophylline, an adenosine receptor antagonist. Exosome-treated defects also displayed a regenerative immune phenotype characterised by a higher infiltration of CD163⁺ regenerative M2 macrophages over CD86⁺ M1 macrophages, with a concomitant reduction in pro-inflammatory synovial cytokines IL-1β and TNF-α. Together, these observations demonstrated that the efficient osteochondral regeneration by MSC exosomes was effected through a coordinated mobilisation of multiple cell types and activation of several cellular processes.

间充质干细胞（MSC）外泌体先前已被证明在具有免疫功能的大鼠模型中可有效修复临界大小的骨软骨缺损。在这里，我们研究了由MSC外泌体调节的细胞过程以及软骨修复中外泌体介导的反应的潜在作用机制。我们观察到，外泌体介导的骨软骨缺损修复的特征在于增加的细胞增殖和浸润，增强的基质合成和再生的免疫表型。使用软骨细胞培养，我们可以将外泌体介导的软骨修复过程中快速的细胞增殖和浸润归因于外泌体CD73介导的AKT和ERK信号传导的腺苷活化。AKT或ERK磷酸化抑制剂可抑制外来体介导的细胞增殖和迁移增加，但不能抑制基质合成。外泌体CD73的作用已被AMPCP（一种CD73抑制剂）和茶碱（一种腺苷受体拮抗剂）减弱了AKT和ERK信号传导所证实。外来体治疗的缺陷还表现出再生免疫表型，其特征在于CD163的浸润度更高⁺再生的M2巨噬细胞超过CD86 ⁺ M1巨噬细胞，并伴有促炎性滑膜细胞因子IL-1β和TNF-α降低。总之，这些观察结果表明，MSC外泌体的有效骨软骨再生是通过多种细胞类型的协调动员和几种细胞过程的激活来实现的。