Harnessing and delivering microbial metabolites as therapeutics via advanced pharmaceutical approaches 通过先进的制药方法利用和提供微生物代谢物作为治疗药物
Lindsey M. Williams, Shijie Cao* Lindsey M. Williams， 曹世杰*Department of Pharmaceutics, School of Pharmacy, University of Washington, Seattle, WA 98195, United States 华盛顿大学药学院药剂学系，西雅图，华盛顿 98195，美国
A R T I C L E I N F O
Available online 16 February 2024 2024 年 2 月 16 日在线提供
Associate editor: Dr. Nina Isoherranen 副主编： Nina Isoherranen 博士
Microbial metabolites have emerged as key players in the interplay between diet, the gut microbiome, and host health. Two major classes, short-chain fatty acids (SCFAs) and tryptophan (Trp) metabolites, are recognized to regulate inflammatory, immune, and metabolic responses within the host. Given that many human diseases are associated with dysbiosis of the gut microbiome and consequent reductions in microbial metabolite production, the administration of these metabolites represents a direct, multi-targeted treatment. While a multitude of preclinical studies showcase the therapeutic potential of both SCFAs and Trp metabolites, they often rely on high doses and frequent dosing regimens to achieve systemic effects, thereby constraining their clinical applicability. To address these limitations, a variety of pharmaceutical formulations approaches that enable targeted, delayed, and/or sustained microbial metabolite delivery have been developed. These approaches, including enteric encapsulations, esterification to dietary fiber, prodrugs, and nanoformulations, pave the way for the next generation of microbial metabolite-based therapeutics. In this review, we first provide an overview of the roles of microbial metabolites in maintaining host homeostasis and outline how compromised metabolite production contributes to the pathogenesis of inflammatory, metabolic, autoimmune, allergic, infectious, and cancerous diseases. Additionally, we explore the therapeutic potential of metabolites in these disease contexts. Then, we provide a comprehensive and up-to-date review of the pharmaceutical strategies that have been employed to enhance the therapeutic efficacy of microbial metabolites, with a focus on SCFAs and Trp metabolites. 微生物代谢物已成为饮食、肠道微生物组和宿主健康之间相互作用的关键参与者。短链脂肪酸 （SCFA） 和色氨酸 （Trp） 代谢物两大类被认为可调节宿主内的炎症、免疫和代谢反应。鉴于许多人类疾病与肠道微生物组的生态失调以及随之而来的微生物代谢物产生的减少有关，这些代谢物的施用代表了一种直接的、多靶点的治疗。虽然大量临床前研究展示了 SCFA 和 Trp 代谢物的治疗潜力，但它们通常依赖于高剂量和频繁的给药方案来实现全身效果，从而限制了它们的临床适用性。为了解决这些限制，已经开发了多种药物制剂方法，以实现靶向、延迟和/或持续的微生物代谢物递送。这些方法，包括肠溶包埋、酯化为膳食纤维、前药和纳米制剂，为下一代基于微生物代谢物的疗法铺平了道路。在这篇综述中，我们首先概述了微生物代谢物在维持宿主稳态中的作用，并概述了受损的代谢物产生如何导致炎症、代谢、自身免疫、过敏、感染和癌性疾病的发病机制。此外，我们还探讨了代谢物在这些疾病背景下的治疗潜力。然后，我们对用于增强微生物代谢物治疗效果的药物策略进行了全面和最新的回顾，重点是 SCFA 和 Trp 代谢物。
Trillions of microbes reside in the human body, rendering humans composite organisms consisting of both human and microbial cells. A dynamic collection of mutualistic, commensal, and pathogenic microorganisms in the human gastrointestinal (GI) tract together contribute the gut microbiome, an additional genome comprising millions of genes. These genes encode for a vast and diverse assortment of proteins that regulate both microbial and host functions. Recent groundbreaking studies in the field of gut microbiome research have helped to illuminate the complexity of the host-gut microbe interface. In these studies, microbial metabolites produced by commensal bacteria during their digestion of dietary components have emerged as key players in the interplay between diet, the gut microbiome, and host health. Two major classes of microbial metabolites, short-chain fatty acids (SCFAs) and tryptophan (Trp) metabolites, are now understood to be crucial regulators of host immune, metabolic, and neuroendocrine processes. In fact, alterations in microbial metabolite production are hypothesized to contribute to the development and progression of a multitude of human diseases. 数万亿微生物存在于人体中，使人类成为由人类和微生物细胞组成的复合生物体。人类胃肠道 （GI） 道中共生微生物、共生微生物和病原微生物的动态集合共同构成了肠道微生物组，这是一个由数百万个基因组成的附加基因组。这些基因编码种类繁多的蛋白质，这些蛋白质调节微生物和宿主功能。肠道微生物组研究领域最近的开创性研究有助于阐明宿主-肠道微生物界面的复杂性。在这些研究中，共生菌在消化膳食成分过程中产生的微生物代谢物已成为饮食、肠道微生物组和宿主健康之间相互作用的关键参与者。两大类微生物代谢物，短链脂肪酸 （SCFA） 和色氨酸 （Trp） 代谢物，现在被认为是宿主免疫、代谢和神经内分泌过程的关键调节因子。事实上，据推测，微生物代谢物产生的改变会导致多种人类疾病的发展和发展。
Altered microbial metabolite production is an inherent consequence of gut dysbiosis, which occurs when the balance between helpful and harmful bacteria is disrupted. In recent years, the emergence of evidence linking gut dysbiosis to a wide range of human diseases has sparked the development of microbiome-based therapeutic approaches. Classic approaches to counteract gut dysbiosis, including probiotics supplementation and fecal microbiota transplantation (FMT), involve administering live bacteria. Although these approaches have shown promise in various disease contexts, their widespread use has been limited by numerous challenges (Suez & Elinav, 2017). For orally administered probiotics, controversy exists as to whether the bacteria can survive their transit through the stomach to reach the GI tract in sufficient amounts. Additionally, the relevance of specific strains in treating specific diseases has not been clearly defined. On the other hand, FMT is often accomplished using intrarectal delivery. Although this ensures that the bacteria are delivered to the distal GI tract, it is not a preferred administration route for most patients. Further, introducing an entire microbial community is associated with risks, as pathogens could also be transferred, producing unpredictable effects in their new host. Other microbiome-based therapeutic approaches include supplementation with prebiotics, which are fermentable substrates designed to promote the expansion of beneficial bacteria, or a combination of probiotics and prebiotics (synbiotics) (L. Du et al., 2022). While the administration of prebiotics can positively modulate the composition of the gut microbiota, it requires large amounts of supplementary material to be consumed. 改变的微生物代谢物产生是肠道菌群失调的固有后果，当有益细菌和有害细菌之间的平衡被破坏时，就会发生这种情况。近年来，将肠道菌群失调与多种人类疾病联系起来的证据的出现引发了基于微生物组的治疗方法的发展。抵消肠道菌群失调的经典方法，包括补充益生菌和粪便微生物群移植 （FMT），涉及使用活细菌。尽管这些方法在各种疾病环境中显示出前景，但它们的广泛使用受到许多挑战的限制（Suez & Elinav，2017）。对于口服益生菌，关于细菌是否能在通过胃运输以足够量到达胃肠道后存活下来存在争议。此外，特定菌株在治疗特定疾病中的相关性尚未明确定义。另一方面，FMT 通常是通过直肠内分娩完成的。虽然这确保了细菌被输送到远端胃肠道，但它并不是大多数患者的首选给药途径。此外，引入整个微生物群落与风险有关，因为病原体也可能被转移，从而在其新宿主中产生不可预测的影响。其他基于微生物组的治疗方法包括补充益生元，益生元是旨在促进有益细菌扩增的可发酵底物，或益生菌和益生元的组合（合生元）（L. Du et al.， 2022）。虽然益生元的给药可以积极调节肠道微生物群的组成，但它需要消耗大量的补充材料。
For of all the aforementioned microbiome-based therapeutic strategies, interindividual variability in gut microbial composition is a significant limiting factor. The existing microbial community influences the efficacy of prebiotics, as they must be microbially degraded to produce their effects, and also influences the ability of live bacteria to effectively colonize the gut. As such, classical applications of microbiome-based therapeutics have resulted in limited and highly variable clinical efficacies. Growing evidence supports that microbial metabolite-based therapeutics can overcome these caveats. As microbial metabolites largely contribute to the beneficial effects of commensal gut microbes, their 对于上述所有基于微生物组的治疗策略，肠道微生物组成的个体间变异是一个重要的限制因素。现有的微生物群落会影响益生元的功效，因为它们必须经过微生物降解才能产生效果，也影响活细菌有效定植肠道的能力。因此，基于微生物组的疗法的经典应用导致临床疗效有限且高度可变。越来越多的证据表明，基于微生物代谢物的疗法可以克服这些警告。由于微生物代谢物在很大程度上有助于共生肠道微生物的有益作用，因此它们的
administration may provide a more direct method of promoting host homeostasis. Their efficacies are not as dependent on the composition of the gut microbiota, and the potential for unwanted effects resulting from transplanting an entirely new microbial community are avoided. Moreover, while the phylogenetic composition of the human microbiota exhibits a high degree of interindividual variability, microbial metabolic pathways are largely stable among individuals, indicating that administering metabolites may be more universally applicable than targeting phylogeny (Huttenhower et al., 2012). Other noteworthy advantages of microbial metabolites include their relative ease of production, especially compared to administration of live bacteria that are not readily culturable, ability to be administered in specified doses, and suitability for administration by various routes. Lastly, microbial metabolites regulate diverse aspects of host physiology. Given the multifactorial nature of many chronic human diseases, novel therapeutics capable of targeting multiple features of disease pathogenesis, such as microbial metabolites, offer the opportunity to greatly improve clinical outcomes. 给药可能提供一种更直接的促进宿主稳态的方法。它们的功效不依赖于肠道微生物群的组成，并且避免了移植全新微生物群落可能导致的不良影响。此外，虽然人类微生物群的系统发育组成表现出高度的个体间变异性，但微生物代谢途径在个体之间基本稳定，这表明施用代谢物可能比靶向系统发育更普遍适用（Huttenhower et al.， 2012）。微生物代谢物的其他值得注意的优点包括它们相对容易生产，特别是与不易培养的活细菌相比，能够以指定剂量给药，以及适合通过各种途径给药。最后，微生物代谢物调节宿主生理学的各个方面。鉴于许多慢性人类疾病的多因素性质，能够针对疾病发病机制的多种特征（例如微生物代谢物）的新型疗法为大大改善临床结果提供了机会。
Like classical microbiome-based therapeutic approaches, there are challenges associated with administering microbial metabolites. As their beneficial effects are maximal in the distal gut, a major challenge lies in shielding them from premature absorption and metabolism in the upper GI tract after oral administration. Rapid metabolism also limits their efficacy after parenteral administration, resulting high dosage requirements and frequent administrations. Pharmaceutical formulations approaches aiming to provide controlled and targeted microbial metabolite delivery have greatly contributed to the advancement of microbial metabolite-based therapeutics. Drug delivery systems such as enteric coating, esterification to dietary fiber, prodrugs, and nanoformulations have enabled the targeted and/or sustained delivery of microbial metabolites to their sites of action, resulting in improved therapeutic efficacy and reduced side effects. In this review, we first overview the roles of the two major classes of microbial metabolites, SCFAs and Trp metabolites, in maintaining host homeostasis. Then, we describe how gut dysbiosis in various disease contexts associates with dysregulated microbial metabolite production and outline preclinical and clinical studies demonstrating the ability of these metabolites to alleviate disease progression. Finally, we overview the novel pharmaceutical approaches to harness and deliver microbial metabolites that have brought us closer to realizing their therapeutic potential. 与经典的基于微生物组的治疗方法一样，微生物代谢物的给药也存在挑战。由于它们的有益作用在远端肠道中最大，因此一个主要挑战在于保护它们在口服给药后免受上消化道的过早吸收和代谢。快速代谢也限制了它们在肠外给药后的疗效，导致高剂量要求和频繁给药。旨在提供受控和靶向微生物代谢物递送的药物制剂方法极大地促进了基于微生物代谢物的疗法的进步。药物递送系统，如肠溶衣、膳食纤维酯化反应、前体药物和纳米制剂，能够靶向和/或持续地将微生物代谢物递送到其作用部位，从而提高治疗效果并减少副作用。在这篇综述中，我们首先概述了两大类微生物代谢物 SCFA 和 Trp 代谢物在维持宿主稳态中的作用。然后，我们描述了各种疾病背景下的肠道菌群失调如何与失调的微生物代谢物产生相关联，并概述了临床前和临床研究，证明了这些代谢物缓解疾病进展的能力。最后，我们概述了利用和递送微生物代谢物的新型药物方法，这些方法使我们更接近实现其治疗潜力。
2. Roles of microbial metabolites in maintaining host homeostasis 2. 微生物代谢物在维持宿主稳态中的作用
Microbial metabolites are intermediates or end-products resulting from microbial metabolism of exogenous food molecules, endogenous host-derived compounds, or microbiota-derived compounds. These small molecules coordinate communication among gut microbes and between gut microbes and their host. The two most extensively studied classes are short-chain fatty acids (SCFAs) and tryptophan (Trp) metabolites, though there are many other products that play important roles in microbiota-host communication. 微生物代谢物是外源性食物分子、内源性宿主衍生化合物或微生物群衍生化合物的微生物代谢产生的中间体或终产物。这些小分子协调肠道微生物之间以及肠道微生物与其宿主之间的通讯。研究最广泛的两类是短链脂肪酸 （SCFA） 和色氨酸 （Trp） 代谢物，尽管还有许多其他产品在微生物群-宿主通讯中起着重要作用。
SCFAs are the end-products of bacterial fermentation of the otherwise indigestible carbohydrates of dietary fibers. The three major SCFAs produced are acetate, propionate, and butyrate. As energy sources, butyrate and propionate are largely utilized by the colonic epithelium and liver, respectively, while acetate reaches the systemic SCFA 是膳食纤维中原本难以消化的碳水化合物进行细菌发酵的最终产物。产生的三种主要 SCFA 是乙酸盐、丙酸盐和丁酸盐。作为能量来源，丁酸盐和丙酸盐分别主要被结肠上皮和肝脏利用，而乙酸盐则到达全身
circulation in the highest concentrations (den Besten et al., 2013). SCFAs signal via activation of GPCRs, such as GPR43 (FFAR2), GPR41 (FFAR3) and GPR109A, and via inhibition of histone deacetylase (HDAC). While acetate is more selective for GPR43, butyrate preferentially binds GPR41, and propionate is the most potent agonist of both receptors (Le Poul et al., 2003). Butyrate is the only SCFA that can activate GPR109A (J. K. Tan, McKenzie, Mariño, Macia, & Mackay, 2017). For HDAC inhibition, in vitro studies in both epithelial and immune cells have shown that butyrate is the most potent, followed by propionate, then acetate (Waldecker, Kautenburger, Daumann, Busch, & Schrenk, 2008; Zou et al., 2021). 最高浓度的循环（den Besten et al.， 2013）。SCFA 通过激活 GPCR（如 GPR43 （FFAR2）、GPR41 （FFAR3） 和 GPR109A）以及抑制组蛋白脱乙酰酶 （HDAC） 来发出信号。虽然乙酸盐对 GPR43 更具选择性，但丁酸盐优先结合 GPR41，而丙酸盐是两种受体中最有效的激动剂（Le Poul et al.， 2003）。丁酸盐是唯一可以激活 GPR109A 的 SCFA（J. K. Tan， McKenzie， Mariño， Macia， & Mackay， 2017）。对于 HDAC 抑制，上皮细胞和免疫细胞的体外研究表明，丁酸盐是最有效的，其次是丙酸盐，然后是乙酸盐（Waldecker， Kautenburger， Daumann， Busch， & Schrenk， 2008;Zou 等人，2021 年）。
In recent years, Trp metabolites have also emerged as key metabolites produced in the colon as a result of microbial Trp catabolism. These metabolites include indole, tryptamine, indole-3-pyruvic acid (IPyA), indole-3-lactic acid (ILA), indole-3-acrylic acid (IA), indole-3propionic acid (IPA), indole-3-ethanol (IE), indole-3-aldehyde (IAld), indole-3-acetic acid (IAA), and others, which predominantly act as ligands of the aryl hydrocarbon receptor (AHR) (Roager & Licht, 2018). Microbial catabolism of Trp reduces the amount Trp available for the indoleamine 2,3-dioxygenase (IDO)-1 enzyme expressed in host epithelial and immune cells, which catalyzes the production of kynurenine (Kyn) (Agus, Planchais, & Sokol, 2018). Both SCFAs and Trp metabolites have been found to play crucial roles in regulating the functions of the intestinal epithelium, the immune system, and other processes in diverse tissues throughout the body (Fig. 1). In this section, we aim to outline the key regulatory mechanisms by which these microbial metabolites regulate host homeostasis. 近年来，Trp 代谢物也成为微生物 Trp 分解代谢在结肠中产生的关键代谢物。这些代谢物包括吲哚、色胺、吲哚-3-丙酮酸（IPyA）、吲哚-3-乳酸：，吲哚-3-丙烯酸：、吲哚-3-丙酸：：-3-丙酸，吲哚-3-乙醇：（IE）、吲哚-3-醛：：-3-乙酸（IAA）等，主要作为芳烃受体（AHR）的配体（Roager & Licht（）：（3-丙烯酸）（IAl））等。Trp 的微生物分解代谢减少了在宿主上皮细胞和免疫细胞中表达的吲哚胺 2,3-双加氧酶（IDO）-1 酶的 Trp 可用量，从而催化犬尿氨酸（Kyn）的产生（Agus， Planchais 和 Sokol，2018）。已发现 SCFA 和 Trp 代谢物在调节肠上皮、免疫系统和全身不同组织的其他过程的功能中起着至关重要的作用（图 1）。在本节中，我们旨在概述这些微生物代谢物调节宿主稳态的关键调节机制。
2.1. Effects on the intestinal epithelium 2.1. 对肠上皮的影响
2.1.1. SCFAs 2.1.1. SCFA
Intestinal epithelial cells (IECs) are the first cells to absorb SCFAs via facilitated diffusion through monocarboxylate transporter (MCT)-1 and sodium-dependent (S)MCT-1, where they fulfill as much as 60-70% of IEC energy requirements (den Besten et al., 2013). In humans, molar fractions of SCFAs in the hepatic portal vein were found to be ∼70:20:10\sim 70: 20: 10 for acetate:propionate:butyrate, compared to ~60:20:20 in the intestinal lumen, indicating preferential metabolism of butyrate by IECs (Cummings, Pomare, Branch, Naylor, & Macfarlane, 1987). Oxidative metabolism of butyrate and other SCFAs contributes to establishing the hypoxic colonic environment that is ideal for anaerobic bacteria that produce SCFAs via dietary fiber fermentation. Additionally, butyrate was found to modulate peroxisome proliferator-activated receptor gamma\gamma (PPAR- gamma\gamma ) expression to drive epithelial beta\beta-oxidation (Byndloss et al., 2017). Oxygen consumption by IECs also leads to HIF stabilization, which plays a fundamental role in maintaining epithelial barrier integrity (Kelly et al., 2015). Via GPR43 and GPR109A signaling, SCFAs activate a well-characterized molecular mechanism of epithelial barrier maintenance, namely IL-18 upregulation resulting from NLRP3 inflammasome activation (Macia et al., 2015). Additionally, SCFAs are known to promote the assembly of tight junction protein (TJP) complexes and regulate TJP expression (Pérez-Reytor, Puebla, Karahanian, & García, 2021). As we will describe in the next section, fortification of intestinal barrier functions is a crucial mechanism by which SCFAs alleviate the progression of various diseases. 肠上皮细胞 （IEC） 是第一个通过单羧酸转运蛋白 （MCT）-1 和钠依赖性 （S）MCT-1 的促进扩散吸收 SCFA 的细胞，它们满足高达 60-70% 的 IEC 能量需求（den Besten 等人，2013 年）。在人类中，发现肝门静脉中 SCFAs 的摩尔分数是 ∼70:20:10\sim 70: 20: 10 乙酸盐：丙酸盐：丁酸盐，而肠腔中的比例为~60：20：20，这表明 IECs 优先代谢丁酸盐（Cummings， Pomare， Branch， Naylor， & Macfarlane， 1987）。丁酸盐和其他 SCFA 的氧化代谢有助于建立缺氧结肠环境，这是通过膳食纤维发酵产生 SCFA 的厌氧细菌的理想选择。此外，发现丁酸盐可调节过氧化物酶体增殖物激活受体 gamma\gamma （PPAR- gamma\gamma ） 表达以驱动上皮 beta\beta 氧化（Byndloss et al.， 2017）。IEC 的耗氧量也会导致 HIF 稳定，这在维持上皮屏障完整性方面起着重要作用（Kelly 等人，2015 年）。通过 GPR43 和 GPR109A 信号传导，SCFA 激活上皮屏障维持的明确分子机制，即 NLRP3 炎性小体激活导致的 IL-18 上调（Macia 等人，2015 年）。此外，已知 SCFAs 可以促进紧密连接蛋白（TJP）复合物的组装并调节 TJP 表达（Pérez-Reytor， Puebla， Karahanian 和 García， 2021）。正如我们将在下一节中描述的那样，强化肠道屏障功能是 SCFA 缓解各种疾病进展的重要机制。
Besides regulating epithelial barrier integrity, SCFAs also modulate secretion of soluble mediators by IECs. Butyrate activates TGF- beta\beta production by IECs via HDAC inhibition, which supports immune tolerance by promoting the expansion of anti-inflammatory regulatory T cells (Tregs) (Martin-Gallausiaux et al., 2018). In the context of infections, however, SCFAs acutely stimulate mechanisms that afford protective immunity against pathogens. SCFA-mediated induction of cytokines, chemokines, and antimicrobial peptides (AMPs) in IECs are mediated through GPR41 and GPR43, which activate mitogen/extracellular signal protein kinase (MEK)/extracellular signal regulated kinase 1//21 / 2 (ERK) and p38/mitogen activated protein (MAP) kinase pathways (M. H. 除了调节上皮屏障完整性外，SCFA 还调节 IEC 对可溶性介质的分泌。丁酸盐通过 HDAC 抑制激活 IEC 产生的 TGF- beta\beta ，它通过促进抗炎调节性 T 细胞 （Treg） 的扩增来支持免疫耐受（Martin-Gallausiaux 等人，2018 年）。然而，在感染的情况下，SCFA 会急性刺激提供针对病原体的保护性免疫机制。SCFA 介导的 IEC 中细胞因子、趋化因子和抗菌肽 （AMP） 的诱导是通过 GPR41 和 GPR43 介导的，它们激活丝裂原/细胞外信号蛋白激酶 （MEK）/细胞外信号调节激酶 1//21 / 2 （ERK） 和 p38/丝裂原活化蛋白 （MAP） 激酶通路 （MH。
Kim, Kang, Park, Yanagisawa, & Kim, 2013; Schauber et al., 2003; Y. Zhao et al., 2018). Kim， Kang， Park， Yanagisawa， & Kim， 2013;Schauber 等人，2003 年;Y. Zhao et al.， 2018）。
2.1.2. Trp metabolites 2.1.2. Trp 代谢物
In recent years, growing evidence has revealed the roles of microbially-produced Trp metabolites in maintaining intestinal epithelial homeostasis. The microbial Trp metabolite indole was found to both enhance barrier functions and attenuate inflammation in IECs (Bansal, Alaniz, Wood, & Jayaraman, 2010). Indole increased the expression of TJP, which corresponded with increased transepithelial resistance. Other Trp metabolites have also been shown to positively modulate intestinal barrier functions. IE, IPyA, and IAld were shown to modulate the contractility of the actin cytoskeleton via AHR signaling to protect against barrier disruption (Scott, Fu, & Chang, 2020). Interestingly, IAld was recently found to reverse the deleterious effects of aging on the intestinal epithelium by increasing proliferation and goblet cell differentiation (Powell et al., 2020). In this study, IAld’s effect on goblet cell differentiation was dependent on upregulation of IL-10, which is largely produced by anti-inflammatory Tregs. Additionally, AHR activation by IA was found to promote both goblet cell differentiation and mucus production (Wlodarska et al., 2017). On the other hand, evidence suggests that IPA promotes epithelial integrity by signaling via the pregnane XX receptor (PXR) (Dodd et al., 2017; Venkatesh et al., 2014). IPAmediated activation of PXR signaling restrains immune responses in the intestinal mesenchyme, thereby reducing inflammation and dampening fibrosis (Flannigan et al., 2023). Trp metabolites may also contribute to establishing anti-inflammatory IEC phenotypes as both IPA and IAld were found to induce the expression of IL-10R1, the receptor for IL-10 (Alexeev et al., 2018). 近年来，越来越多的证据表明微生物产生的 Trp 代谢物在维持肠上皮稳态中的作用。发现微生物 Trp 代谢物吲哚既能增强屏障功能，又能减轻 IECs 的炎症（Bansal， Alaniz， Wood， & Jayaraman， 2010）。吲哚增加 TJP 的表达，这与跨上皮抵抗增加相对应。其他 Trp 代谢物也被证明可以正向调节肠道屏障功能。IE、IPyA 和 IAld 被证明可以通过 AHR 信号传导调节肌动蛋白细胞骨架的收缩力，以防止屏障破坏（Scott、Fu 和 Chang，2020 年）。有趣的是，最近发现 IAld 通过增加增殖和杯状细胞分化来逆转衰老对肠上皮细胞的有害影响（Powell 等人，2020 年）。在这项研究中，IAld 对杯状细胞分化的影响取决于 IL-10 的上调，而 IL-10 主要由抗炎 Treg 产生。此外，发现 IA 的 AHR 激活可促进杯状细胞分化和粘液产生（Wlodarska 等人，2017 年）。另一方面，有证据表明 IPA 通过孕烷 XX 受体 （PXR） 发出信号传导来促进上皮完整性（Dodd et al.， 2017;Venkatesh et al.， 2014）。IPA 介导的 PXR 信号传导激活抑制肠道间充质中的免疫反应，从而减少炎症并抑制纤维化（Flannigan 等人，2023 年）。Trp 代谢物也可能有助于建立抗炎 IEC 表型，因为发现 IPA 和 IAld 都诱导了 IL-10 受体 IL-10R1 的表达（Alexeev 等人，2018 年）。
2.2. Effects on the immune system 2.2. 对免疫系统的影响
2.2.1. SCFAs 2.2.1. SCFA
Immunological tolerance towards harmless bacterial- and foodderived enteric antigens, as well as self-antigens, is essential for maintaining host homeostasis. Disruption of immune tolerance mechanisms can lead to the development of chronic inflammation, autoimmunity, and allergies. SCFAs support immune tolerance by modulating both adaptive ( TT cells and BB cells) and innate (dendritic cells (DCs) and macrophages) immune cells towards more tolerogenic phenotypes. 对无害的细菌和食物来源的肠道抗原以及自身抗原的免疫耐受性对于维持宿主稳态至关重要。免疫耐受机制的破坏会导致慢性炎症、自身免疫和过敏的发展。SCFA 通过调节适应性 （ TT 细胞和 BB 细胞） 和先天性 （树突状细胞 （DC） 和巨噬细胞） 免疫细胞对更耐耐受性的表型来支持免疫耐受。
Regulatory T cells (Tregs) negatively regulate the activities of effector T cells, such as Th1, Th2 and Th17 cells, and thus are essential for maintaining immune tolerance. SCFA production by gut commensals promotes the expansion of Foxp3 ^(+){ }^{+}Tregs and their production of the anti-inflammatory cytokine IL-10 (Arpaia et al., 2013; Furusawa et al., 2013; Smith et al., 2013). They accomplish this, in part, by functioning as HDAC inhibitors to epigenetically regulate histone acetylation at the Foxp3 locus. Additionally, SCFA-mediated upregulation of TGF- beta\beta, a Treg-polarizing cytokine, contributes to Treg induction. As we will discuss in the next section, SCFAs also inhibit the differentiation of proinflammatory effector T cells in a variety of disease contexts. Through these mechanisms, SCFAs drive the Treg/T effector cell balance towards a more tolerogenic profile. 调节性 T 细胞 （Treg） 负向调节效应 T 细胞（如 Th1、Th2 和 Th17 细胞）的活性，因此对维持免疫耐受至关重要。肠道共生体产生的 SCFA 促进 Foxp3 ^(+){ }^{+} Treg 的扩增及其抗炎细胞因子 IL-10 的产生（Arpaia 等人，2013 年;Furusawa et al.， 2013;Smith et al.， 2013）。它们部分是通过作为 HDAC 抑制剂在 Foxp3 基因座上表观遗传调节组蛋白乙酰化来实现这一点。此外，SCFA 介导的 TGF- beta\beta （一种 Treg 极化细胞因子）的上调有助于 Treg 诱导。正如我们将在下一节中讨论的那样，SCFA 还可以在各种疾病环境中抑制促炎效应 T 细胞的分化。通过这些机制，SCFA 驱动 Treg/T 效应细胞平衡朝着更耐受的特征发展。
Additionally, tolerogenic T cell profiles are established by SCFAs through their regulation of innate immune cells. SCFAs have consistently been found to promote tolerogenic DC and macrophage phenotypes, rendering them more potent inducers of Tregs and weaker inducers of effector T cells (Corrêa-Oliveira, Fachi, Vieira, Sato, & Vinolo, 2016). A key feature of tolerogenic DCs and macrophages is their production of retinoic acid (RA). Evidence supports both GPCR signaling and HDAC inhibition as mechanisms by which SCFAs activate RA production in these cell types (Kaisar, Pelgrom, van der Ham, Yazdanbakhsh, & Everts, 2017; Wu et al., 2017). SCFA treatment also results in decreased co-stimulatory marker expression (Millard et al., 2002) and more anti-inflammatory cytokine secretion profiles (Chang, Hao, Offermanns, & Medzhitov, 2014; Nastasi et al., 2017). Notably, 此外，SCFA 通过调节先天免疫细胞来建立耐受性原 T 细胞谱。SCFAs 一直被发现可以促进耐受性强的 DC 和巨噬细胞表型，使它们成为更有效的 Tregs 诱导剂和较弱的效应 T 细胞诱导剂（Corrêa-Oliveira， Fachi， Vieira， Sato， & Vinolo， 2016）。致耐受性 DC 和巨噬细胞的一个关键特征是它们产生视黄酸 （RA）。证据支持 GPCR 信号传导和 HDAC 抑制是 SCFA 在这些细胞类型中激活 RA 产生的机制（Kaisar， Pelgrom， van der Ham， Yazdanbakhsh， & Everts， 2017;Wu et al.， 2017）。SCFA 治疗还导致共刺激标志物表达降低（Millard 等人，2002）和更多的抗炎细胞因子分泌谱（Chang，Hao，Offermanns 和 Medzhitov，2014;Nastasi et al.， 2017）。特别是