Impact of dietary tryptophan on the gut microbiome and autoimmune neuroinflammation

多发性硬化症(MS)是年轻人中最常见的神经系统疾病，被认为主要是由聚集在中枢神经系统(CNS)内并攻击髓鞘蛋白的自身反应性T细胞推动的。最近的临床前数据表明，饮食成分和肠道微生物组组成，可能通过这两个成分之间的严格调控相互作用，塑造自身反应性T细胞反应。必需氨基酸色氨酸(Trp)及其代谢产物被认为是重要的免疫调节剂。我们实验室以前的研究表明，饮食中Trp的耗竭可以完全消除实验性自身免疫性脑脊髓炎(EAE)，这是一种MS的小鼠模型，临床症状的缺乏伴随着T细胞缺乏进入中枢神经系统，但Trp耗竭的小鼠仍然能够启动抗原特异性T细胞反应。当在无菌小鼠中诱导EAE时，在Trp缺乏的小鼠中也检测到了自身免疫性神经炎的临床迹象。用16S rDNA测序对肠道微生物组组成的分析表明，从饮食中省略色氨酸会导致促炎分类群的扩张。与此相一致，我们在色氨酸耗竭的小鼠中发现了轻微但明显的肠道炎症。然而，肠道炎症不足以预防中枢神经系统自身免疫，因为小剂量葡聚糖硫酸钠(DSS)诱导轻微肠道炎症的小鼠仍然容易诱发EAE。基于这些数据，我们假设饮食中色氨酸的缺乏通过肠道微生物群限制了自身免疫性神经炎症。肠道微生物群组成的改变可能通过直接作用或通过抗原提呈细胞影响T细胞的分化和运输。我们的目标是验证这一假说，并确定在色氨酸缺失时介导对EAE诱导抗性的分子关键分子。为此，我们将分析确定的物种对粪便移植调节中枢神经系统自身免疫的影响。在第二个工作包中，我们想要检查饮食色氨酸耗竭对肠道代谢组的影响，并发现可以增加或限制抗原特异性T细胞在中枢神经系统内积累的代谢物。为了更好地了解EAE诱导的关键过程中涉及的细胞成分，我们将跟踪体内自身反应性T细胞的时空分布，并分析基因表达谱以确定相关的信号通路。此外，我们想要描述抗原提呈细胞的表型，它可能诱导自身反应性T细胞的可逆“再分化”。识别局部肠道微环境中的分子靶标将增加这一图景。在临床上更相关的复发-缓解型多发性硬化症模型中验证饮食色氨酸耗竭的影响将有助于我们将研究结果转化为治疗方法。

Multiple sclerosis (MS), the most common neurological disorder among young adults, is thought to be mainly driven by autoreactive T cells that accumulate within the central nervous system (CNS) and attack myelin proteins. Recent preclinical data suggest that both dietary constituents and the gut microbiome composition, potentially via a tightly regulated interplay between the two components, shape autoreactive T cell responses. The essential amino acid tryptophan (trp) and its metabolites have been identified as important immune modulators.Previous studies performed in our lab demonstrated that dietary trp depletion results in complete abrogation of experimental autoimmune encephalomyelitis (EAE), a murine model for MS. The absence of clinical signs was accompanied by a lack of T cell infiltration into the CNS, however, trp-depleted mice were still able to mount an antigen-specific T cell response. When EAE was induced in germ-free mice, clinical signs of autoimmune neuroinflammation were also detected in trp-depleted mice. Analysis of the gut microbiome composition by 16S rDNA sequencing revealed that omission of trp from the diet induces expansion of pro-inflammatory taxa. In line with this, we found mild, but evident gut inflammation in trp-depleted mice. However, intestinal inflammation was not sufficient to prevent CNS autoimmunity, as mice treated with low-dose dextran sulfate sodium (DSS) to induce a mild gut inflammation were still susceptible to EAE induction. Based on these data we hypothesize that dietary trp depletion limits autoimmune neuroinflammation via the gut microbiome. Alterations in the gut microbiome composition may affect T cell differentiation and trafficking either by direct effects or via antigen-presenting cells. We aim to verify this hypothesis and identify molecular key players that mediate resistance to EAE induction upon trp deprivation. To this end we will analyse the effects of defined species on the regulation of CNS autoimmunity by faecel transplantation. In a second work package we would like to examine the impact of dietary trp depletion on the gut metabolome and to discover metabolites that either augment or limit accumulation of antigen-specific T cells within the CNS. To obtain a better understanding of the cellular components that are involved in key processes of EAE induction, we will track spatial and temporal distribution of autoreactive T cells in vivo and analyse gene expression profiles to identify relevant signaling pathways. Additionally we would like to characterize the phenotype of antigen-presenting cells that may induce a reversible "re-differentiation" of autoreactive T cells. Identification of molecular targets within the local gut microenvironment will add to the picture. Verification of the effects of dietary trp depletion in the clinically more relevant relapsing-remitting MS model will help us to translate findings into therapeutic approaches.