第二军医大学学报  2017, Vol. 38 Issue (12): 1556-1561   PDF    
巨噬细胞在骨折愈合过程中作用的研究进展
周潇逸1, 吴随一2, 张子程1, 王飞1, 杨依林1, 徐锡明3, 李明1, 魏显招1     
1. 第二军医大学长海医院脊柱外科, 上海 200433;
2. 第二军医大学学员旅学员3队, 上海 200433;
3. 第二军医大学长征医院脊柱外科, 上海 200003
摘要: 炎性反应对骨折愈合初期血肿形成、组织碎片清除及纤维组织生成具有重要作用。M1、M2型巨噬细胞分别通过促炎、抗炎作用参与骨折愈合过程。巨噬细胞分泌的肿瘤坏死因子α、白细胞介素等细胞因子可调节成骨/破骨细胞活性。骨组织生物工程学研究通过功能型组织工程调节骨折局部炎性反应,可促进血管新生及骨折愈合。因此,探究巨噬细胞在骨折愈合过程中的作用可以为促进骨折愈合提供新思路。
关键词: 组织工程     巨噬细胞     骨折     炎症     愈合    
Research progress on role of macrophage in fracture healing
ZHOU Xiao-yi1, WU Sui-yi2, ZHANG Zi-cheng1, WANG Fei1, YANG Yi-lin1, XU Xi-ming3, LI Ming1, WEI Xian-zhao1     
1. Department of Spine Surgery, Changhai Hospital, Second Military Medical University, Shanghai 200433, China;
2. The Third Student Team, Student Brigade, Second Military Medical University, Shanghai 200433, China;
3. Department of Spine Surgery, Changzheng Hospital, Second Military Medical University, Shanghai 200003, China
Supported by National Natural Science Foundation of China (81301554), Natural Science Foundation of Shanghai (13ZR1450200), and Scientific Project of Shanghai Municipal Health and Family Planning Commission (20154Y0018).
Abstract: Inflammatory reaction plays an important role in hematoma formation, tissue debris removal and fibrous tissue formation during the first stage of fracture healing. M1 and M2 macrophages participate in the process of fracture healing through pro-inflammatory and anti-inflammatory effects, respectively. Tumor necrosis factor α, interleukin and other cytokines secreted by macrophages can regulate the activities of osteoblasts and osteoclasts. Bone tissue bioengineering studies found that functional tissue engineering can promote angiogenesis and fracture healing by regulating inflammatory reaction. Therefore, exploring the role of macrophages in the process of fracture healing may provide new insights for promoting fracture healing.
Key words: tissue engineering     macrophage     fracture     inflammation     healing    

随着社会现代化和人口老龄化的发展,创伤性和骨质疏松性骨折的发生率显著提高。据2007年流行病学统计,对于创伤性骨折交通事故因素占总体因素的45%[1]。绝经25年以上的女性的骨折发生率高达39.62%[2]。约有5%~10%的骨折患者会出现延迟愈合或骨不连等并发症,延长了患者的康复周期,严重增加了家庭及社会的经济负担[3-4]。因此,深入了解骨再生的各个阶段及其调控作用对研究骨折愈合的内在机制具有重要意义。

骨折愈合过程包括炎性反应期、骨痂形成期及骨重塑期。骨再生过程始于炎性反应,骨折部位炎性反应过强、过弱或炎症迁延不愈都可能导致骨折延迟愈合甚至骨不连的发生。

骨折初期局部血管破坏导致血肿发生[5]。最初24 h中性粒细胞富集至骨折区域并释放白细胞介素(interleukin,IL)-6、化学趋化因子受体2(CC chemokine receptor type 2, CCR2)等招募炎性细胞单核巨噬细胞浸润[6]。巨噬细胞通过细胞吞噬作用清除最初的纤维组织和坏死细胞,破骨细胞则负责吸收坏死的骨折片段。此外,巨噬细胞还可通过分泌多种炎性介质和趋化因子[肿瘤坏死因子(tumor necrosis factor,TNF)-α、IL-1β、IL-6、单核细胞趋化蛋白-1(monocyte chemotactic protein 1,MCP-1]来招募局部骨髓、骨膜及毛细血管内的成纤维细胞、成骨细胞前体和间充质干细胞(mesenchymal stem cell,MSC)。在1周内血肿逐渐由肉芽组织取代,且出现新生血管及感觉神经纤维,这两者的出现对于骨折愈合十分重要[7]。随后骨折区域逐渐形成软骨痂,为骨重塑阶段提供必要的稳定性[8]。这一过程中巨噬细胞因其一致性高、可塑性强的特点在组织重塑的免疫调节中发挥重要作用[9]。因此,研究骨折愈合与巨噬细胞之间的关系将有助于理解骨再生初期炎性反应中各类分子的调控机制,为进一步研究促进骨折愈合的机制提供依据。

1 巨噬细胞的分类及其生物学功能

巨噬细胞根据激活方式可分为促炎型(M1型)和抗炎型(M2型)巨噬细胞。

骨折后会出现一个由M1型巨噬细胞主导的快速炎性反应,接下来则是由M2型巨噬细胞主导的成骨细胞募集反应,且M1、M2型均对成骨具有促进作用。Loi等[10]研究发现,将巨噬细胞与前成骨细胞共培养,碱性磷酸酶(alkaline phosphatase,ALP)、骨钙蛋白(osteocalcin,OC)、骨桥蛋白(osteopotin,OPN)等成骨相关基因的表达水平及基质的矿化均明显上调。

γ干扰素(interferon γ,INF-γ)介导的经典激活途径活化的M1型巨噬细胞通过分泌TNF-α、IL-1β、IL-6等细胞因子及巨噬细胞炎性蛋白1α(macrophage inflammatory protein-1α,MIP-1α)等趋化因子调控白细胞浸润,促进炎性反应发生和骨折局部微生物与组织碎片的清除。此外,IL-4、IL-13等细胞因子可以诱导巨噬细胞通过替代激活途径向M2型巨噬细胞极化,从而调节并抑制炎性反应。M2型巨噬细胞分泌的血管内皮生长因子(vascular endothelial growth factor,VEGF)和基质金属蛋白酶(matrix metalloproteinase,MMP)对骨折愈合阶段的血管新生有关键作用[11-13]

最新的研究发现巨噬细胞极化与年龄密切相关,Gibon等[14]研究发现老年小鼠M1型巨噬细胞分别暴露于IFN-γ和脂多糖(lipopolysaccharide, LPS)后,TNF-α的mRNA表达和蛋白分泌均显著上调,但老化的M1型巨噬细胞在暴露于LPS后IL-1受体拮抗剂(IL-1ra)的分泌并不会增加。

体外实验发现M1型巨噬细胞能够抑制MSC活性,而M2型巨噬细胞则具有相反作用[15]。巨噬细胞在骨折愈合中的作用已被多项敲除巨噬细胞功能的动物实验所证实。Chang等[16]研究发现骨内膜巨噬细胞可在年轻大鼠骨膜表面的成骨细胞周围形成龛状结构,通过fas诱导巨噬细胞凋亡而敲除大鼠巨噬细胞能显著抑制成骨细胞形成。Vi等[17]使用同一模型发现巨噬细胞功能敲除后大鼠早期骨骼发育迟缓,MSC数量及其分化为成骨细胞的能力均显著下降。在胫骨骨折模型中,敲除巨噬细胞功能的大鼠骨痂形成、骨质沉积显著减少,且被大量纤维组织取而代之。由此可见巨噬细胞在骨折愈合初期炎性反应阶段和中期骨再生阶段均起到十分重要的作用。

2 巨噬细胞与成骨/破骨细胞的关系 2.1 炎性反应在骨折愈合中的作用

骨折初期巨噬细胞可分泌多种细胞因子,其中促炎/抗炎信号的时空变化在骨折愈合过程中发挥重要作用。例如,TNF-α、IL-1、IL-6、IL-11、IL-17等促炎细胞因子能够增加破骨细胞活性及其分化能力,促进骨吸收,同时抑制胶原和成骨发生;抗炎因子IL-10和IL-13则具有相反的生理作用。

骨折修复始于骨折部位的早期炎性反应,炎性反应募集单核细胞、巨噬细胞及MSC等至骨折部位,此时募集的各类细胞和细胞因子对骨折修复具有重要意义。研究发现局部TNF治疗仅在骨折后24 h内对促进骨折愈合有效,进一步说明了早期炎性反应是骨折修复的关键,因此在骨折早期过早应用抗炎药物如类固醇等不利于骨折预后修复[18-19]

2.2 骨细胞可调节炎性细胞

骨折的初始阶段局部的炎性信号由大量炎性细胞介导,而在伤后的3~7 d内炎性因子则转由局部成骨细胞、软骨细胞等骨细胞分泌[20-21]。这一机制的存在使部分骨折愈合可以在炎性细胞缺失的条件下进行。在体实验研究报道了骨修复过程在一些特定炎性细胞缺失的情况下仍能够正常甚至加速进行,Grøcgaard等[22]通过注射抗白细胞羊血清建立多核白细胞减少的小鼠模型,其骨折愈合能力与注射普通羊血清的小鼠比较,差异并无统计学意义。

成骨细胞和破骨细胞对骨髓内造血干细胞的迁移和增殖具有调控作用。甲状旁腺素(parathyroid hormone,PTH)是调节骨代谢的重要肽类激素之一,也是成骨细胞的一类亚群,研究显示PTH可以通过产生造血生长因子(如Notch信号通路配体Jagged1等)促进造血干细胞增殖[23]。Kollet等[24]通过RANKL途径激活破骨细胞,后者可通过趋化因子受体-4 (chemokine receptor-4,CXCR4)及MMP-9依赖途径招募未成熟的造血干细胞前体,证实了破骨细胞具有促进造血干细胞迁移的能力。

2.3 巨噬细胞与成骨再生关系密切

巨噬细胞与破骨细胞关系密切,骨折损伤后可被固有免疫细胞,尤其是巨噬细胞迅速识别,并启动下游一系列级联反应,以纤维结缔组织取代血肿[25]。Alexander等[26]及Raggatt等[27]在小鼠骨折模型初期敲除巨噬细胞功能可导致骨折愈合迁延不愈。在另一项斑马鱼实验中,选择性敲除巨噬细胞功能可致鱼骨质疏松、尾鳍退化[28]。这些研究都提示巨噬细胞对初期骨再生具有重要作用。

多种类型来源的巨噬细胞可以在体外被诱导分化成破骨细胞,成熟巨噬细胞能够作为破骨细胞前体参与成骨修复过程[29-30]。此外,不同类型的外界刺激诱使巨噬细胞分泌不同的细胞因子,如TNF-α、IL-6、IL-1等,刺激/阻止破骨细胞增殖分化,从而调控骨再生。TNF-α和IL-1通过下调成骨细胞骨保护素(osteoprotegerin,OPG)、促进核因子κB受体活化因子配体(receptor activator for nuclear factor-κB ligand,RANKL)的表达促进破骨细胞分化成熟,且二者能够发挥协同作用[31-32]。其中,TNF-α可以通过诱导骨细胞凋亡促进破骨细胞增殖[33]。同时,TNF将M2型巨噬细胞[由巨噬细胞集落刺激因子(macrophage-colony stimulating factor,M-CSF)诱导]转化为M1型巨噬细胞,M1型巨噬细胞可增强破骨细胞的形成潜力,从而增强破骨细胞的作用[34]。Wu等[35]研究发现在骨坏死早期坏死骨周围TNF-α高表达并伴随大量M1型巨噬细胞聚集,发生骨坏死的进一步连锁反应;骨坏死晚期TNF-α表达降低,大量M2型巨噬细胞聚集,为新生组织的生成和修复提供必需的组织微环境。有学者通过敲除TNF-α受体发现TNF-α在招募破骨细胞、MSC以及诱导软骨细胞凋亡中发挥重要作用,TNF-α的持续高水平表达则会导致慢性炎症发生和骨量减少,甚至产生类风湿性关节炎样症状,延缓骨折愈合[36-37];而IL-1刺激成骨细胞产生IL-6,从而促进VEGF生成,参与骨折局部的初级软骨痂和血管新生,促进骨修复[38-39]

研究表明OPN在巨噬细胞中表达,其功能之一是可促进巨噬细胞向炎性部位趋化[40-41]。McKee等[41]发现骨折后巨噬细胞和成骨细胞分泌的OPN在骨折愈合中具有重要的作用:巨噬细胞通过吞噬作用清除骨碎片,并在骨折处形成黏合线,从而整合新生骨的修复。此外,巨噬细胞还能通过调控成骨细胞基质形成及矿化沉积参与成骨微环境组成,促进成骨细胞分化及骨再生。

3 巨噬细胞参与骨重塑过程

骨重塑过程是骨形成(由成骨细胞介导)与骨吸收(由破骨细胞介导)之间的动态平衡过程。Watari等[42]通过敲除N-myc下游调控基因NDRG1阻止巨噬细胞系分化。小鼠血清中M-CSF及相关细胞因子在NDRG1敲除小鼠中明显下降,导致骨髓MSC向破骨细胞、M1/M2型巨噬细胞及树突状细胞分化能力减弱。此外,细胞实验还发现NDRG1敲除后使血管新生能力显著降低,提示巨噬细胞在骨重塑及炎性血管新生中发挥重要作用。

骨科生物组织工程学通常使用生物可降解支架覆盖组织诱导生物材料以促进成骨发生[43]。尽管组织工程学研究目前主要集中于促进成骨(如Wnt)及血管新生(如VEGF)的各类信号通路,仍有越来越多的学者将方向转为研究调节炎性反应的仿生材料[17, 44-45]。Spiller等[46]发现M1型和M2型巨噬细胞可通过诱导内皮细胞出芽促进血管新生。通过“重组工程”技术,此类细胞可被直接覆在功能型支架上参与局部组织的生理过程,如招募干细胞并诱导其分化,辅助新骨形成并降解支架。这种细胞疗法可促进干细胞向成骨细胞分化,且MSC自身的免疫调节功能可激活巨噬细胞向M2型转化,从而促进局部血管新生。骨折愈合过程中的免疫调节功能与骨折区域内血管新生及成骨分化和破骨吸收都具有密切联系,利用这一特点进行组织工程改造,研发生物材料已经成为目前的研究热点。

4 展望

有关巨噬细胞与成骨修复之间的关系是一个新的研究领域,近年来多侧重于研究不同动物模型骨代谢过程中巨噬细胞的不同类型及参与情况。然而,巨噬细胞如何调控骨代谢和骨再生的机制目前仍不明确,未来可主要针对这一机制进行进一步探究。此外,巨噬细胞在不同组织内,尤其在骨髓内可表达多种亚型。不同亚型的巨噬细胞需要以其特异的标志物准确区分,而目前仅以巨噬细胞的组织定位和普通标志物定义大部分巨噬细胞亚型在一定程度上缺少准确性,因此,探寻不同亚型巨噬细胞的特异性标志物也将成为未来研究的一大方向。

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