The mammalian intestines harbor trillions of commensal microorganisms composed of thousands of species that are collectively called gut microbiota. Among the microbiota, bacteria are the predominant microorganism, with viruses, protozoa, and fungi (mycobiota) making up a relatively smaller population. The microbial communities play fundamental roles in the maturation and orchestration of the immune landscape in health and disease. Primarily, the gut microbiota modulates the immune system to maintain homeostasis and plays a crucial role in regulating the pathogenesis and pathophysiology of inflammatory, neuronal, and metabolic disorders. The microbiota modulates the host immune system through direct interactions with immune cells or indirect mechanisms such as producing short-chain acids and diverse metabolites. Numerous researchers have put extensive efforts into investigating the role of microbes in immune regulation, discovering novel immunomodulatory microbial species, identifying key effector molecules, and demonstrating how microbes and their key effector molecules mechanistically impact the host immune system. Consequently, recent studies suggest that several microbial species and their immunomodulatory molecules have therapeutic applicability in preclinical settings of multiple disorders. Nonetheless, it is still unclear why and how a handful of microorganisms and their key molecules affect the host immunity in diverse diseases. This review mainly discusses the role of microbes and their metabolites in T helper cell differentiation, immunomodulatory function, and their modes of action.

中文翻译：

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T 辅助细胞分化和功能中的微生物成分和效应分子。

哺乳动物的肠道内藏有数以万亿计的共生微生物，这些微生物由数千种物种组成，统称为肠道微生物群。在微生物群中，细菌是主要的微生物，病毒、原生动物和真菌（菌群）构成相对较小的种群。微生物群落在健康和疾病免疫景观的成熟和协调中发挥着重要作用。首先，肠道微生物群调节免疫系统以维持体内平衡，并在调节炎症、神经元和代谢紊乱的发病机制和病理生理学中发挥关键作用。微生物群通过与免疫细胞的直接相互作用或间接机制（例如产生短链酸和多种代谢物）来调节宿主免疫系统。许多研究人员已投入大量精力研究微生物在免疫调节中的作用，发现新的免疫调节微生物物种，鉴定关键效应分子，并证明微生物及其关键效应分子如何机械地影响宿主免疫系统。因此，最近的研究表明，几种微生物物种及其免疫调节分子在多种疾病的临床前环境中具有治疗适用性。尽管如此，目前仍不清楚少数微生物及其关键分子为何以及如何影响多种疾病的宿主免疫力。本综述主要讨论微生物及其代谢产物在 T 辅助细胞分化、免疫调节功能及其作用模式中的作用。