肠道菌群与结核杆菌感染相关的研究进展
Research Progress on the Relationship between Gut Microbiota and Mycobacterium Tuberculosis Infection

作者: 李百远 , 李元军 :延安大学附属医院,陕西 延安;

关键词: 肠道菌群肺结核菌群失调免疫Gut Microbiota Tuberculosis Dysbacteriosis Immune

摘要:
随着全基因组测序的发展,肠道菌群成为学术界研究热点。肠道菌群参与人体多种生理活动,并与多种疾病发生有关。肠道菌群与呼吸道疾病,尤其是与结核病之间的研究颇受关注。研究证实肠道菌群失调可影响宿主对结核分枝杆菌的易感性,感染结核分枝杆菌会导致宿主肠道菌群的改变。两者通过宿主免疫反应、肠道菌群代谢产物等相互影响。因此,本文就结核杆菌感染与肠道菌群的关系及相关作用机制等进行综述,为结核病治疗提供新思路。

Abstract: With the development of whole genome sequencing, gut microbiota has become the focus of academic research. It is known that gut microbiota is involved in a variety of physiological activities of the human body, and is related to the occurrence of a variety of diseases. Much attention has been paid to the relationship between gut microbiota and respiratory diseases, especially tuberculosis. Studies have confirmed that gut dysbacteriosis can affect the host susceptibility to mycobacterium tuberculosis, infection with mycobacterium tuberculosis will lead to changes in the host gut microbiota. The two interact with each other through host immune response, gut microbiota metabolites and so on. Therefore, the relationship and the relevant mechanism were reviewed in order to provide new ideas for the treatment of tuberculosis.

1. 引言

结核病(Tuberculosis, TB)是由结核分枝杆菌(Mycobacterium Tuberculosis, Mtb)引起的一种具有潜在致命性的慢性传染病,是作为世界头号传染病杀手。根据2019年WHO公布的全球结核病报告 [1] ,2018年全球估计有1000万人罹患TB,多数为潜伏感染,这意味着宿主和环境因素在决定Mtb的结果(包括耐药和结核复发)方面至关重要。肠道菌群的组成和活性从出生起便与宿主共同发展,相互作用。肠道菌群通过影响细胞信号传导、参与机体免疫应答、参与多种宿主代谢途径的调节等机制影响疾病的发生发展。大量研究证明 [2] 肠道菌群与哮喘、慢性阻塞性肺疾病、肺囊性纤维化等呼吸道疾病有关。肺结核也不例外。肠道菌群失调可影响宿主对Mtb的易感性,Mtb感染可影响宿主肠道菌群。另外,Mtb感染的严重程度与肠道菌群相关 [3] 。因此,本文就肠道菌群与Mtb感染的相互影响及具体作用机制等进行综述。

2. 肠道菌群概述

全基因组测序的出现使得人们对人体不同腔表面菌群的相互连接、组成、多样性、代谢物和生物活性有了更加深入的了解。人类肠道中大约有1014个细菌 [4] ,这些肠道菌群的代谢过程及代谢产物参与人体多种生理活动,例如肠道菌群代谢产物短链脂肪酸(Short-chain fatty acids, SCFAs)可降低结肠pH值、抑制病原体生长、提供能量,且与肥胖、胰岛素抵抗、结直肠癌等有关 [5] 。胆碱代谢产物可调节脂质代谢和葡萄糖动态平衡,与肥胖、糖尿病和心血管疾病有关 [6] 。同时,宿主主要通过先天和适应性免疫,如上皮分泌的防御素、IgA等反过来影响和维持肠道菌群的稳态,二者处于“共生”状态。例如,维生素D信号通过调节潘氏细胞a防御素维持肠道菌群,并改善动物模型中的代谢紊乱和肝脂肪变性 [7] 。不平衡的微生物群,也就是所谓的“生态失调”,可导致各种退化性疾病,包括肥胖、糖尿病、脂肪肝、肝硬化和某些癌症等,并影响多种疾病的进展 [5] 。

同样,肺部菌群在维持宿主稳态中也起着重要作用。肺部菌群通过影响TLR2介导的促炎细胞因子的产生、中性粒细胞募集、以及抗菌肽的释放(如Th17细胞刺激β-防御素2的释放)等,影响粘膜免疫和诱导免疫耐受 [8] 。Brown等 [9] 研究证实肺部菌群能够通过IL-17和NOD2刺激肺内粒细胞–巨噬细胞集落刺激因子(GM-CSF)的产生,增强呼吸道防御,通过细胞外信号调节激酶信号传导,并刺激肺泡巨噬细胞杀死并清除病原体,从而保护肺部免受肺炎链球菌、肺炎克雷伯菌感染。

基于肠道与肺部菌群在维持宿主稳态和疾病发展中作用,肠–肺轴近年来受到广泛关注。作为连接肠道与肺部的纽带,肠–肺轴是由肠道和肺部微生物群的不同微生物组成之间的复杂相互作用以及局部和长期的免疫效应共同作用的结果。已知肠–肺轴参与哮喘、肺部感染、肺结核、慢性阻塞性肺疾病、囊性纤维化等多种呼吸道疾病的调控,影响疾病发展演变。Mtb被认为是肺部菌群失调后迅速定植的最重要的病原菌之一 [10] ,Mtb感染后启动人体细胞免疫应答,并通过肠–肺轴影响肠道菌群。肠道菌群通过免疫反应、菌群代谢物影响Mtb的感染和预后。

3. 肠道菌群代谢物对TB的影响

3.1. 吲哚丙酸阻断Mtb中的色氨酸生物合成

人类微生物群产生多种分子,包括非核糖体肽、硫肽、抗生素、细菌素和氨基酸代谢物。吲哚丙酸(Indole propionic acid,IPA)是人类肠道细菌产生的第一个抗结核代谢物 [11] 。IPA存在于人血清中,已被证明可抑制β-淀粉样纤维的形成,并可作为神经保护剂对抗多种氧化毒素。已有研究证明,IPA可影响小鼠的肠道屏障和宿主免疫 [12] 。另外,IPA可干扰Mtb的生长。IPA是色氨酸(Trp)的脱氨基产物,是这种重要的芳香氨基酸的一个紧密的结构类似物。在结核分枝杆菌中,Trp的从头合成是通过反馈抑制来调节的:Trp作为邻氨基苯甲酸合成酶(TrpE)的变构抑制剂,催化Trp生物合成途径的第一步。而IPA可通过模拟该酶的生理变构抑制剂Trp来抑制TrpE,从而在TrpE催化步骤阻断结核分枝杆菌Trp的生物合成 [11] 。TrpE或将成为抗结核治疗的新靶点。

3.2. SCFAs通过影响宿主免疫反应增加Mtb易感性

SCFAs作为肠道微生物发酵不可消化膳食纤维的主要代谢物,除了提供能量来源外,还可调节多种细胞过程,包括基因表达、细胞的分化、增殖和凋亡 [13] 。G蛋白偶联受体(GPCRs)的激活、组蛋白脱乙酰化酶的抑制、组蛋白乙酰转移酶活性的刺激等信号通路,以及稳定缺氧诱导因子都与SCFAs有关 [14] 。作为肠道菌群和免疫系统之间的重要纽带,SCFAs对维持肠道内稳态至关重要。一项关于糖尿病增加TB易感性的研究认为 [15] ,丁酸(SCFAs的一种)可拮抗Mtb引起的巨噬细胞的促炎反应并促进IL-10的表达。而抗炎细胞因子IL-10可能在补体C4对结核分枝杆菌的免疫应答中起重要作用。糖尿病作为一种代谢性疾病,其与肠道菌群关系密切。有研究表明 [16] ,糖尿病患者肠道菌群的组成发生了改变,这随后改变了他们体内的SCFAs水平,然后SCFAs通过激活GPCRs和抑制组蛋白脱乙酰化酶等作用于免疫和内皮细胞,从而发挥免疫活性,影响宿主对Mtb的反应。多项流行病学证据表明糖尿病患者发生Mtb感染和罹患TB的风险更高 [17] ,更加肯定了肠道菌群失调增加Mtb易感性。

4. Mtb感染对肠道菌群的影响

肠道菌群在Mtb感染期间受到调节,并可能对宿主免疫状态的变化做出应答 [18] 。Winglee等 [19] 使用16S rRNA基因测序研究了气溶胶感染CDC1551Mtb菌株对Balb/c小鼠肠道菌群的影响,对其进行监测直至死亡。在感染后的前6天内,所有小鼠的菌群Shannon多样性指数(计算菌群多样性的指数)突然下降,特别是Bacteroidales (拟杆菌目)和Clostridiales (梭菌目)的菌群显著减少。研究发现,在感染前和未感染的样本中Bacteroides (类杆菌属)存在显著差异(P < 0.001)。已有研究表明 [20] ,Bacteroides可抑制NF-kB途径的激活以及诱导产生IL-10的T细胞中发挥重要的抗炎作用。此外,Bacteroides fragilis (脆弱类杆菌)可纠正全身性T细胞缺陷和辅助性T细胞1/2 (Th1/Th2)失衡,Bacteroides fragilis产生的多糖A (PSA)能够激活T细胞依赖性免疫反应,影响宿主免疫系统 [21] 。为排除Mtb菌株的选择对菌群变化的影响,研究人员进一步对菌株H37Rv进行观察,结果同样表明小鼠的肠道菌群随Mtb感染而发生显著变化。

5. 抗结核治疗对肠道菌群的影响

5.1. 抗生素诱导的肠道菌群失调影响肺泡巨噬细胞对Mtb的免疫

抗生素诱导的肠道菌群失调可增加感染性疾病的易感性。Becattini等 [22] 发现,肠道免疫系统可防止Listeria monocytogenes (单核细胞增多性李斯特氏菌)在肠腔的定植,并防止细菌的全身传播。在接种该菌前,小鼠低剂量口服抗生素可促进肠道中Listeria monocytogenes的生长。Sekirov等 [23] 研究证实,给小鼠使用广谱抗生素可增加肠道对Salmonella typhimurium (鼠伤寒沙门氏菌)的易感性。肠道菌群失调不仅影响肠道感染,而且还会损害包括呼吸道在内的其他屏障部位的免疫力。广谱抗生素治疗削弱了流感感染后的抗病毒免疫,并与CD8+ T细胞分泌的TNF-a和IFN-γ减少有关 [24] 。此外,类似的广谱抗生素方案可降低肺部对Mtb感染的免疫力 [25] 肺泡巨噬细胞的杀菌能力是Mtb感染早期免疫的核心,早期Mtb感染期间最大限度地发挥肺泡巨噬细胞的功能对于预防TB至关重要。

抗结核药物虽可控制Mtb的生长,但不能防止再次感染,抗结核治疗成功后的个体在再次感染时罹患TB的风险是未经治疗个体的4倍 [26] ,推测可能是由于长期抗结核治疗改变肠道菌群稳态,进而影响肺泡巨噬细胞的免疫功能 [27] 。Khan N等 [28] 发现给予异烟肼(INH)或吡嗪酰胺(PYZ)处理小鼠的Mtb易感性与肺泡巨噬细胞代谢受损和杀菌活性缺陷有关。给予INH/PYZ处理小鼠的肺泡巨噬细胞的细菌负荷较对照小鼠明显增高。Cohen等人 [29] 利用H37Rv菌株感染小鼠模型证明,在感染早期,Mtb主要针对肺泡巨噬细胞。感染Mtb后,小鼠肺泡巨噬细胞MHCII表达水平、TNF-a和IL-1β的产生均显著降低 [28] ;同时,肺泡巨噬细胞将代谢从氧化磷酸化转变为糖酵解,这是产生促炎细胞因子(如IL-1β)和有效杀菌能力所必需的 [29] 。抗结核治疗介导的肠道菌群失调促进了肺泡巨噬细胞的氧化磷酸化,进而影响其杀菌活性。

5.2. 抗结核治疗改变特定具有免疫学意义的肠道菌群

与通常使用的广谱抗生素不同,大多数用于治疗TB的一线药物都是具有Mtb特异性靶点的窄谱抗菌剂。抗结核药物在通过特异性靶点发挥抗菌活性同时,可导致宿主肠道菌群失调。Wipperman等 [30] 基于基因测序研究了未感染Mtb、结核潜伏感染(LTBI) (包括治疗、治愈)、正在接受抗结核(抗结核治疗方案:2HRZE/4HR)治疗的活动性肺结核、已治愈活动性肺结核的患者的肠道菌群,并以Shannon指数衡量菌群多样性。结果发现,接受抗结核治疗的活动性肺结核患者的肠道菌群多样性总体上与未感染Mtb组或LTBI组没有显著差异。然而,抗结核治疗导致特定肠道菌群显著丧失。与LTBI组相比,抗结核治疗导致Blautia (步劳特氏菌属)减少了10倍,Lactobacillus (乳杆菌)和Coprococcus (球菌)减少了200多倍,Ruminococcus (瘤胃球菌)减少了675倍。在Actinomyces (放线菌)门中,Bifidobacterium (双歧杆菌)的数量减少了近20倍。上述部分细菌已经被证实具有免疫功能,并与Mtb感染介导的免疫相关。已知Ruminococcus和Coprococcus可调节外周细胞因子的产生,包括IL-1和IFN-γ2 [31] ;Bifidobacterium可以在小鼠体内诱导Th17介导的免疫反应 [32] 。来自国内的一项研究表明 [33] ,在抗结核治疗期间,Clostridiales (梭状芽孢杆菌)属成员的相对丰度显著下降。Stefka等 [34] 认为含有Clostridiales的肠道微生物群落可调节免疫功能和上皮对食物抗原的渗透性,以防止过敏原过敏,维持肠道免疫稳态,对机体免疫功能亦有一定益处。除此之外,缺失的肠道菌群的代谢产物对免疫功能的影响不容忽视。作为促进肠道免疫稳态的经典例子,SCFAs可作为肠上皮细胞的能量来源,调节细胞因子的产生,诱导调节性T细胞的增殖 [35] 。基于以上研究,抗结核药物诱导的肠道菌群失调消耗了多种具有免疫学意义的肠道细菌,导致菌群代谢产物的缺失等,均可能对宿主免疫功能产生影响。

6. 结语与展望

TB患者肠道中的特定菌群或许将成为预测TB感染状态及预后的潜在生物标志物,更深入地了解两者之间的关系对优化TB治疗策略以治疗疾病至关重要。随着生物医学发展以及新一代测序技术的出现,肠道菌群与呼吸系统疾病之间相关分子机制愈加明确。在不久的将来,饮食、益生菌或更精准的调节或将成为临床辅助诊断及治疗疾病的重要手段。

文章引用: 李百远 , 李元军 (2020) 肠道菌群与结核杆菌感染相关的研究进展。 临床医学进展, 10, 975-980. doi: 10.12677/ACM.2020.106148

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