Stem cells inside and outside of the lung for lung repair: roles, functions and existing problems

doi:10.3969/j.issn.2095-4344.0675

Abstract

Abstract:

BACKGROUND: Stem cells have the ability to repair local tissue damage, which opens the possibility of cell therapy for most irreversible diseases, including chronic lung disease.
OBJECTIVE: To elucidate the effect of stem cells in the repair of lung tissue.
METHODS: The first author used the computer to retrieve CNKI and Medline databases using the keywords of “lung stem cells, tracheal and bronchial epithelial stem cells; transplantation; stem cells in the bronchial and terminal bronchial epithelium; stem cells at the bronchoalveolar junction; alveolar epithelial stem cells, interstitial lung stem cells” in Chinese and English, respectively. Approximately 500 articles were initially retrieved, and finally 47 eligible articles were included.
RESULTS AND CONCLUSION: Stem cells inside and outside of the lung are the basis of lung injury, repair and remodeling. The lung stem cells in the body are mostly in a resting state. When the lung tissue is damaged or cultured in vitro, the stem cells from the lung or other tissues divide and proliferate. In addition, it is still unknown whether apoptosis, migration and differentiation will be triggered by other factors during the repair of lung tissue or even under physiological conditions. Therefore, many problems regarding stem cells for chronic lung diseases remain to be solved.

中国组织工程研究杂志出版内容重点：干细胞；骨髓干细胞；造血干细胞；脂肪干细胞；肿瘤干细胞；胚胎干细胞；脐带脐血干细胞；干细胞诱导；干细胞分化；组织工程

Key words: Lung, Stem Cells, Lung Diseases, Stem Cell Transplantation, Tissue Engineering

CLC Number:

R394.2

Chen Yan, Liu Xian. Stem cells inside and outside of the lung for lung repair: roles, functions and existing problems[J]. Chinese Journal of Tissue Engineering Research, 2018, 22(33): 5408-5412.

Figures/Tables 1

2.1 肺外干细胞参与修复肺组织损伤骨髓是机体最大的干细胞库，肺外干细胞主要来源是骨髓池。潜在参与肺损伤修复的细胞主要包括骨髓间充质干细胞、内皮细胞和造血干细胞，在肺部感染或急性肺损伤或骨髓动员剂作用下，以上细胞从骨髓池外流并发生定向迁移，以特定的分化形式参与损伤肺组织的修复过程。既往研究证实，在小鼠肺气肿模型肺泡再生过程中，骨髓动员剂粒细胞集落刺激因子、造血生长因子或肾上腺髓质蛋白可诱导肺毛细血管腔骨髓源性内皮祖细胞的增加。然而，骨髓源性细胞究竟是分化为肺泡细胞还是与定居细胞融合有待进一步证实。在细菌性肺炎和急性肺损伤患者中，循环内皮祖细胞数目显著增加，而且增加的数量与疾病预后相关，提示骨髓源性祖细胞在炎性刺激作用下释放到循环中，并且这些细胞促进炎症消退和损伤肺组织修复。骨髓源性间充质细胞对肺泡再生促进作用在弹性蛋白酶诱导的肺气肿模型上得到很好的验证[7]。在临床干细胞治疗中，间充质干细胞是细胞治疗的重要候选细胞。间充质干细胞易于从骨髓和其他组织中分离。同种异体间充质干细胞由于其低表达主要组织相容性复合物Ⅰ和组织相容性复合物Ⅱ型蛋白且缺失T细胞的共刺激分子而易于为受体耐受。因此，同种异体间充质干细胞应用在理论上可行，间充质干细胞可以储存到治疗时使用，且无伦理学争议。近年在美国，已有超过100例间充质干细胞临床试验注册并开展[8]。如上所述，间充质干细胞能够减轻肺组织损伤并促进修复。这些有益的效应是基于间充质干细胞调节免疫系统以及产生生长因子和细胞因子(如表皮细胞生长因子、造血生长因子和前列腺素E2)的能力。鉴于以上抗炎效应，间充质干细胞治疗严重肺疾病，包括急性肺损伤、慢性阻塞性肺疾病、肺动脉高压、哮喘和肺纤维化的潜力已有广泛研究。同时，在实验模型中，静脉或气管注射骨髓细胞或骨髓源性间充质干细胞可减轻脂多糖诱导的小鼠肺损伤，博来霉素诱导的炎症、胶原沉积和纤维化也在气管或静脉输注间充质干细胞后减轻，其作用机制主要涉及以增加抗炎递质、减少促炎递质分泌为导向的免疫调节效应、以分泌生长因子为导向的气血屏障修复效应，肺泡水肿液清除效应和肺泡上皮细胞凋亡抑制效应、以分泌生长因子为导向的气血屏障修复效应，肺泡水肿液清除效应和肺泡上皮细胞凋亡抑制效应，因此，间充质干细胞在急性肺损伤修复与再生中具有重要的临床价值。在内毒素所致急性肺损伤模型中，肺组织损伤包括细胞凋亡和坏死[9]，这就需要正常的修复细胞替代并维持器官内环境稳定。既往有研究证实骨髓间充质干细胞在肺损伤微环境可塑性很强，能够分化为肺泡Ⅰ型和Ⅱ型上皮细胞、成纤维细胞、内皮细胞、支气管上皮细胞等多种类型的肺组织细胞。而且，对于骨髓重建的绿色荧光蛋白小鼠在脂多糖注射后7 d，扁平的绿色荧光蛋白阳性骨髓间充质干细胞出现在肺泡壁，这些细胞分子标志角蛋白或CD34内皮细胞呈阳性染色。这就提示骨髓间充质干细胞可分化或与肺泡上皮细胞、血管内皮细胞融合，显示移植的骨髓间充质干细胞可能参与了肺损伤的修复过程[10]。然而，随着观察时间的延长，骨髓间充质干细胞逐渐减少。此外，骨髓源性单个核细胞治疗能够改善急性肺损伤的炎症损伤和纤维化进程。尽管目前对肺内定植的骨髓源性细胞数目、停留时间以及旁分泌调节尚有诸多争议[11]，但根据以上结果显示，骨髓源性单个核细胞最初迁移到损伤器官并分化或与器官实质细胞融合，随着骨髓源性单个核细胞定植于损伤器官，便难以或不能分化或发育成新的细胞，此时主要作用应该是对损伤局部微环境的调节，刺激内源性修复反应。另一方面，新近研究认为，静脉输注骨髓间充质干细胞能够显著改善新生小鼠高氧所致的肺损伤，逆转肺泡表面积病理性减少和呼吸功能减弱[12]，进一步研究发现，用间充质干细胞条件培养基同样具有类似的治疗效果[13]，其作用机制有待进一步证实。还有研究表明，间充质干细胞对急性肺损伤的修复效应可能是通过刺激支气管肺泡干细胞增殖所致[14]。 2.2 肺内干祖细胞参与修复肺组织损伤干细胞治疗各种肺脏病研究显示，肺组织自身的干细胞和肺外组织来源的干细胞均参与肺损伤组织修复。然而，基于呼吸道上皮本身极低的生长更新率和有限的再生能力的认识，既往应用外源性干细胞并取得一定的治疗效果，但外源性干细胞在肺组织内的修复和再生作用是有限，难以产生足够的气管上皮细胞或肺泡上皮细胞，目前尚难以通过促进损伤肺组织的修复和再生作用达到治疗肺脏疾病的目的。事实上，哺乳动物体内许多器官组织内都存留少量的内源性成体干细胞/祖细胞，它们分布于特定的微环境-微龛内，是维持正常器官组织稳定和修复损伤组织的重要细胞来源。研究表明，成年小鼠的气管、支气管、细支气管和肺泡内都分布具有一定分化能力的干/祖细胞[15]。人、大鼠、家兔等哺乳动物肺组织也证实存在类似的干/祖细胞的分布。尽管目前缺乏严格的内源性肺干/祖细胞标记，且分离培养较为困难，这种干/祖细胞的分类方法尚有一定争论，但对其在维持肺结构稳定和肺组织修复方面的作用已获得较广泛认可[16]。在肺损伤修复过程中，肺内干/祖细胞如气管和支气管干细胞、细支气管干细胞、肺泡干细胞、肺泡Ⅱ型上皮细胞对于修复肺内环境稳定，参与损伤区组织修复扮演了重要角色[17]。在执行气体交换的主体区域肺泡壁的组成细胞中，肺泡Ⅰ型和Ⅱ型上皮细胞覆盖肺泡腔的大部分区域。在肺损伤发生时，表面积较大的肺泡Ⅰ型上皮细胞损伤、坏死，数目占绝对优势的肺泡Ⅱ型上皮细胞能够分化并替代肺泡Ⅰ型上皮。研究证实，在肺内炎性刺激条件下，可导致肺泡Ⅰ型上皮损伤，肺泡Ⅱ型上皮可能分化并替代受损的肺泡Ⅰ型上皮[18]。目前有研究进一步认为，在肺泡Ⅱ型上皮中存在形态结构不同的干细胞亚群，在终末细支气管、肺泡管连接处、肺泡壁均有分布[19]。因此，在肺损伤结构重塑过程中，如何有效调动肺泡Ⅱ型上皮细胞的修复潜能，从数量、分布和细胞转化路径分析无疑具有绝对的权重优势。应用绿色荧光蛋白嵌合小鼠实验发现，肺损伤后再生的肺泡由骨髓源性和非骨髓源性细胞组成。肺泡Ⅱ型上皮细胞能够修复损伤的肺泡上皮。然而，肺内源性干细胞替代损伤的肺泡Ⅱ型上皮的潜能尚不清楚[20]。最近认为，小鼠干细胞抗原阳性细胞可能是肺内源性干细胞[21]。曾有报道称弹性蛋白酶诱导的肺损伤可增加具有干细胞标记的细胞数目，在弹性蛋白酶作用下，细胞数量显著增加，如何有效增加肺内源性干细胞的数目可能是有效修复损伤肺组织的关键环节之一。激活信号通路显著增加肺干细胞的数量，而锂等药理学调控物可使肺组织中的关键干细胞群进行强制性扩增和分化，毫无疑问，这将为以肺干细胞为切入点修复损伤肺脏的设想提供了新的可能[22]。内源性组织干细胞被认为有助于修复组织和器官。这些未分化的细胞局限于每个组织中的细胞龛内，而在肠道和皮肤中是罕见的。在肺中，内源性上皮干细胞和祖细胞被认为有助于上皮的维持和损伤修复。一些已发表的研究有争议，对于小鼠、人肺内源性肺上皮干细胞或祖细胞的识别和(或)功能的研究还没有统一的一致性[23]。内源性组织特异性干细胞最早被认为是具有自我更新能力的细胞，它能产生子细胞，称为“过境扩增细胞”，从而产生更特化或分化的细胞。组织特异性干细胞具有广泛的分化潜能。与此相反，成人内源性祖细胞被认为是没有自我更新能力但能分化成更特化细胞的组织特异性细胞[24]。在这种能力下，这些细胞被称为“兼性祖细胞”，这种兼性祖细胞可以进入有丝分裂细胞池进行组织损伤修复[25]。因此，兼性祖细胞池作为一个大的、广泛分布的修复细胞池，具有补充组织干细胞的修复能力[26]。另外，兼性前体池可用于常规的组织平衡和再生，而干细胞只在更极端的损伤情况下发挥作用[27]。肺是一个复杂的器官，不同类型的上皮细胞多分布在几个不同的区域微环境[28]。进一步跟踪人肺上皮干细胞或祖细胞增殖和迁移是必要的[29]。在小鼠等动物肺损伤模型中，干细胞诱导肺上皮细胞增殖和祖细胞增殖[30]。某些损伤发生于肺上皮的特定区域，例如，二氧化硫、臭氧和二氧化氮吸入损伤气管和大气道上皮，已被用于研究近端呼吸道。给予博莱霉素损伤支气管上皮和肺泡上皮，已被用来研究更远端气道和肺泡区[31]。这些模型为小鼠肺上皮干细胞或祖细胞的区域特异性提供了强有力的证据[32]。至少有5群干细胞被发现在肺部损伤后修复小鼠气道上皮细胞，分别是兼性祖细胞、基底干细胞、肺泡Ⅰ型上皮细胞、肺泡Ⅱ型上皮细胞和表皮祖细胞。超微结构分析表明，近端到远端轴都具有上皮分泌细胞[33]。然而，它们表达不同的分泌蛋白。最重要的是，近端球杆样细胞来源于不同的祖细胞，而不是远端的气道细胞[34]。气管和大气道包含2个主要的上皮细胞谱系：基底层和纤毛细胞层。研究表明，二氧化硫损伤后基底细胞能在近侧气道内产生肺细胞和纤毛细胞[35]。细胞信号通路、组织因子调节基底上皮细胞的功能和转录[36-40]。最近的数据表明，在小鼠中，兼性祖细胞池在正常的稳态中替代大部分气道上皮细胞，有助于细胞损伤后的组织修复[41]。相反，脉冲追踪研究已经证明，肺干细胞是一种具有自我更新的细胞类型，是大鼠纤毛呼吸道细胞的祖细胞[42-43]。关于干细胞是否参与上呼吸道上皮修复的研究缺乏共识。尽管如此，动物模型以及体外分析表明，基底细胞可修复小鼠上呼吸道组织干细胞或兼性祖细胞[44]。总的来说，气管和大气道的数据虽然有限，但却为肺组织特异性干细胞提供了迄今为止最有力的证据。干细胞可以作为祖细胞，修复损伤气管上皮[45]。最近发现，气管黏膜下腺管细胞的干/祖细胞在缺氧缺血性损伤后能再生为黏膜下腺管、导管和表面上皮[46]，充分证明了肺内源性组织干细胞对肺组织损伤修复的作用。"

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