The Tissue Fibrinolytic System Contributes to the Induction of Macrophage Function and CCL3 during Bone Repair in Mice

Macrophages play crucial roles in repair process of various tissues. However, the details in the role of macrophages during bone repair still remains unknown. Herein, we examined the contribution of the tissue fibrinolytic system to the macrophage functions in bone repair after femoral bone defect by using male mice deficient in plasminogen (Plg–/–), urokinase-type plasminogen activator (uPA–/–) or tissue-type plasminogen activator (tPA–/–) genes and their wild-type littermates. Bone repair of the femur was delayed in uPA–/– mice until day 6, compared with wild-type (uPA+/+) mice. Number of Osterix-positive cells and vessel formation were decreased in uPA–/– mice at the bone injury site on day 4, compared with those in uPA+/+ mice. Number of macrophages and their phagocytosis at the bone injury site were reduced in uPA–/– and Plg–/–, but not in tPA–/– mice on day 4. Although uPA or plasminogen deficiency did not affect the levels of cytokines, including TNF-α, IL-1β, IL-6, IL-4 and IFN-γ mRNA in the damaged femur, the elevation in CCL3 mRNA levels was suppressed in uPA–/– and Plg–/–, but not in tPA–/– mice. Neutralization of CCL3 antagonized macrophage recruitment to the site of bone injury and delayed bone repair in uPA+/+, but not in uPA–/– mice. Our results provide novel evidence that the tissue fibrinolytic system contributes to the induction of macrophage recruitment and CCL3 at the bone injury site, thereby, leading to the enhancement of the repair process.巨噬细胞（Macrophages）在多种组织的修复进程中发挥关键作用。然而，巨噬细胞在骨修复过程中的具体功能机制仍未明确。本研究中，我们通过使用纤溶酶原（plasminogen，Plg）缺陷型（Plg–/–）、尿激酶型纤溶酶原激活物（urokinase-type plasminogen activator，uPA）缺陷型（uPA–/–）或组织型纤溶酶原激活物（tissue-type plasminogen activator，tPA）缺陷型（tPA–/–）雄性小鼠及其野生型同窝对照小鼠，探究了组织纤维蛋白溶解系统对股骨骨缺损后骨修复过程中巨噬细胞功能的调控贡献。与野生型（uPA+/+）小鼠相比，uPA–/–小鼠的股骨骨修复进程在术后第6天前均出现显著延迟。术后第4天，uPA–/–小鼠骨损伤部位的Osterix阳性细胞数量及血管生成水平均低于uPA+/+小鼠。术后第4天，uPA–/–及Plg–/–小鼠骨损伤部位的巨噬细胞数量及其吞噬功能均显著降低，但tPA–/–小鼠未观察到该异常变化。尽管uPA或Plg缺陷并未对受损股骨内肿瘤坏死因子-α（TNF-α）、白细胞介素-1β（IL-1β）、白细胞介素-6（IL-6）、白细胞介素-4（IL-4）及干扰素-γ（IFN-γ）等细胞因子的mRNA表达水平造成明显影响，但uPA–/–及Plg–/–小鼠体内趋化因子CCL3的mRNA表达上调被显著抑制，而tPA–/–小鼠未出现该现象。对CCL3进行中和阻断可抑制野生型（uPA+/+）小鼠骨损伤部位的巨噬细胞招募，并延迟其骨修复进程，但该处理对uPA–/–小鼠无显著作用。本研究结果证实，组织纤维蛋白溶解系统可通过促进骨损伤部位的巨噬细胞招募及CCL3表达，进而增强骨修复过程，为相关研究提供了全新的实验依据。