前言:
间充质干细胞治疗的有效性与安全性已在多种疾病中得到证实。然而成体来源的间充质干细胞其体外扩增能力有限性、分离细胞的高异质性导致的临床结果的不稳定性等问题亟待解决。将iPSC通过特定实验路线诱导分化为iPSC衍生的间充质样MSC(iPSC-MSC),是最有潜力替代成体MSC的产品。许多研究证明iPSC可以通过多种分化策略获得iPSC-MSC,且经研究表明iPSC-MSC与成体MSC相比,其增殖能力、免疫调节能力、生物学效能等特点均优于成体MSC或与之相当。笔者试图通过检索相关文献资料,从iPSC-MSC制备的方法学研究、iMSC产品在不同疾病模型中的应用、临床级iPSC衍生细胞制品的最新研究进展以及新一代的iPSC衍生产品的开发策略等四个方面,与广大读者探讨此类细胞治疗产品的开发前景,抛砖引玉,以期展开对类似管线的开发前景的讨论,从而丰富产品开发思路。
再生医学和细胞治疗已成为当今世界各国先进生物医疗技术关注的焦点,其中间充质干细胞(Mesenchymal stem cell, MSC)疗法潜力无限,已被证明在多种疾病的临床试验研究中具备安全性和有效性,广受关注。MSC属于成体多能干细胞,因其多向分化潜能高、免疫原性低及伦理问题少和来源广泛等特点,一直作为干细胞研究的重要方向[1]。MSC具有免疫调节、抗炎、支持造血功能、组织修复等功能特性[2],可应用于自身免疫性疾病如红斑狼疮、炎性疾病如骨关节炎、衰老相关疾病、感染性疾病的治疗[3]。然而由于不同的MSC细胞来源、供体情况不同、培养体系差异等造成的细胞异质性、体外扩增能力的有限性、分离提取的难度与局限性等[4],使得MSC细胞疗法在临床应用方面存在一些限制和效果的不稳定[5]。
由iPSC获得iPSC-MSC的方法学研究进展
笔者汇总了目前市面上的主流iPSC衍生MSC(iMSC)的制备方法[6]。分化方向大致可归类为三个方面,主流分化策略包括:MSC培养基替换诱导法、信号通路调节诱导法及拟胚体法等[7]。在下文中,对每种报道的研究方案使用的关键物料与方法细节进行了详细描述,各项研究也对衍生的iMSC进行了细胞表面标志物与多向分化潜能的评估,同时也针对每种方案的整体分化时间做了统计,以期找到一种高效且安全的可用于工业化生产的分化方案。
(1)MSC培养基替换诱导法
通过将iPSC培养基更换为MSC培养体系,能够诱导iPSC的自发分化,起到对细胞的诱导和筛选作用。MSC基础培养基成分包括高/低糖DMEM、KO-DMEM(特定成分敲除的DMEM)、DMEM-F12和α-MEM等,同时配合加入FBS、KOSR、L-谷氨酰胺、P/S、非必需氨基酸和FGF等。
Kang[8]等人报道,将iPSC培养基直接替换为含有10% FBS的DMEM培养基作用两周,传代后在预包被培养皿中对细胞持续传代,可获得成纤维细胞形态的iMSC细胞[9]。类似的诱导条件,使用α-MEM培养基[20]也能够获得。利用本法的整体诱导效率较低,如获得合格表型的iMSC(高表达 CD73、CD44、CD105、CD90;低表达CD19、CD34、CD45、HLA-DR等),且具备三系分化能力[10],时间都要长达30-50天。
(2)信号通路调节剂诱导iMSC形成
通路抑制剂法是iPSCs诱导MSCs的一类常用方法,可快速高效的将iPSC诱导至MSC细胞表型并具有间充质干细胞功能。用于iPSC-MSCs诱导的抑制剂包括SB203580(p38-MAPK抑制剂)、SB431542(TGF-β抑制剂)、CHIR(GSK3抑制剂)、β-巯基乙醇、b-FGF等。
-SB203580
SB203580是一种吡啶咪唑类p38 MAPK抑制剂,对GAK、CK1、RIP2、c-Raf 和 GSK3等激酶也有抑制活性。p38-MAPK途径可促进MSC分化为表皮样细胞[11],还可以抑制IL-2介导的T细胞增殖作用,在体内参与多种炎症反应或诱导细胞自噬。有文章报道SB203580能够促进iPSC细胞向中胚层表型的转化[12],通过将iPSC悬浮形成拟胚体(EB),并在培养体系中加入一定浓度SB203580诱导拟胚体,将诱导后的拟胚体贴壁培养传代,28天后获得表面标志物合格的iMSC细胞,且传代20代后核型完整且细胞未衰老。
-SB431542
SB431542是TGF-β信号通路抑制剂,TGF信号通路是维持干细胞多能性的主要信号通路,抑制TGF通路可以诱导ESC和iPSC分化为MSC或神经元细胞[13]。Glaeser等人使用20 μM的SB431542经10-14天将ESC分步诱导为神经嵴细胞,再转换为有血清培养体系将神经嵴细胞向MSC样细胞转化[14]。Lee和Sun[15]等人在整个iMSC诱导过程中均加入10 μM SB431542,传代若干次后均获得具有三系分化能力的iMSC细胞系。不同研究者使用的SB431542作用浓度从1 μM向20 μM不等[16],对iMSC的诱导效率未见明显差异。
-CHIR-99021
CHIR-99021是一种高度特异的糖原合成酶激酶-3(GSK-3)抑制剂,被证明可以促进BALB/c小鼠ES细胞的自我更新并维持其多能性。CHIR-99021涉及包括Wnt/β-catenin、TGF-β、Nodal和MAPK等多种信号通路[17]。不同浓度的CHIR99021常与 SB431542联合应用于iMSC的诱导分化。Harada等人[18]使用10 μM SB431542+1 μM CHIR99021的化合物组合,按iPSC-神经嵴细胞-MSC样细胞的路线在21天左右成功诱导出间充质表型细胞。Winston等人[19]则使用4 μM SB431542+4 μM CHIR99021+10 ng/mL β-FGF在25天左右获得了iMSC。信号通路调节剂的加入与MSC培养基直接转换相比,诱导iMSC成熟的效率更高,整体分化时间被有效缩短,且获得的细胞无论从表型、三系分化能力及基因组表达方面,均与原代MSC细胞无明显差异[20]。
-β-巯基乙醇
β-巯基乙醇也是iMSC诱导过程中的常用添加物。Liang、McGrath、Hua和Lim等人,分别在iPSC培养体系中加入了浓度不同的β-巯基乙醇(终浓度分别为55 uM,0.1 mM,110mM[21]),30天左右获得成纤维样的iMSC细胞,经鉴定细胞表面标志物与三系分化能力与BM-MSC相似。
(3)拟胚体法
通过拟胚体(EB)介导的iMSC分化路径,能够在有限的培养体积中收获更多的目的细胞,有利于工业化和成本控制。然而培养过程整体需要的时间较长,且产生的细胞存在一定的异质性。研究者们通常在拟胚体形成及iMSC爬出阶段,在培养体系中加入化合物通路调节剂,以提高细胞向iMSC的富集效率。
Chen YS使用含有20% KOSR及0.1 μg/mL bFGF的DMEM-F12培养基,将iPSC悬浮培养形成EB,之后体系中加入10 μM浓度的SB431542以促进细胞爬出,接着经细胞传代获得iMSC
[22]。Zhou等[23]开发了一种在生物反应器中大规模扩增胚胎干细胞的方法,将ESC悬浮培养后获得大量EB,接着使用含有KOSR的体系诱导EB分化,进一步诱导EB爬出,获得ES-MSC。研究者还进一步优化诱导体系,开发了诱导分化的新配方,在体系中添加一定浓度的b-FGF与TGFβ-1促进ES-MSC的爬出与成熟[24]。另外,包括Activin A,VEGF,BMP4,RA[25]等细胞因子也应用于经拟胚体分化的诱导过程中,分化天数在20-30天时间不等。
由于篇幅所限,在下一篇讨论中,我们将对iMSC产品在不同疾病模型中的应用、临床级iPSC衍生细胞制品的最新研究进展以及新一代的iPSC衍生产品的开发策略这些方面进行进一步的探讨,笔者期待各位读者的反馈与进一步交流。
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