研究熱帶念珠菌對於腸道免疫反應的影響

Abstract

真菌在發炎性腸道疾病和其他慢性發炎性疾病中具有關鍵作用。熱帶念珠菌為一種腸道共生真菌，對於其如何引發宿主免疫反應來調節腸道平衡仍所知甚少。在本研究中，透過對小鼠使用廣效抗生素，我建立了一個研究宿主與真菌相互作用的動物模型。進而發現熱帶念珠菌標準菌株 (MYA-3404) 在化學誘導和細菌感染模型中都有利於抵抗結腸炎。在機制方面，定植在腸道中的熱帶念珠菌促進了 IL-17A 和 IL-22 的產生，進而增強了腸道屏障功能和腸上皮細胞的增殖。而 T 細胞亞群（Th17 和 T 細胞）和 ILC3 則刺激這兩種細胞激素的產生。熱帶念珠菌定植小鼠的糞便樣本的代謝物組分析顯示其維生素 B3 代謝發生了變化，透過菸鹼醯胺酶將菸鹼醯胺轉化為菸鹼酸。管餵菸鹼醯胺會加劇結腸炎症狀，而補充菸鹼酸則可透過 IL-17A 減輕腸道發炎。添加熱帶念珠菌可以緩解在實驗性結腸炎下施用菸鹼醯胺的作用。菸鹼醯胺酶抑制劑處理消除了熱帶念珠菌腸道定植所帶來的保護作用。此外，接種熱帶念珠菌增加血漿中的γ-谷氨酰亮氨酸表現量。注射 γ-谷氨酰亮氨酸的小鼠透過增加 IL-17A、IL-22 表現和上皮細胞增殖重現了對結腸炎的有益效果。有趣的是，與標準菌株相比，從念珠菌血症患者分離出的臨床菌株缺少對實驗性結腸炎的保護功能以及維生素 B3 和 γ-谷氨酰亮氨酸的代謝。整體而言，我的研究結果強調了熱帶念珠菌透過維生素 B3 和 γ-谷氨酰亮氨酸代謝途徑誘導 IL-17A 和 IL-22產生，以增強腸道發炎時保護效果。我的結果闡明了對共生真菌-代謝物組-免疫細胞軸調節腸道發炎的理解，並為開發真菌作為益生菌產品來治療發炎性腸道疾病患者提供了證據。

Mycobiota plays a critical role in inflammatory bowel disease and other chronic inflammatory conditions. However, how Candida tropicalis - a commensal gut fungus drives the host immune response to regulate intestinal homeostasis remains poorly unknown. In the present study, I established a model of investigating host-fungi interaction by administering broad-spectrum antibiotics for conventional mice. I demonstrated that the C. tropicalis reference strain (MYA-3404) displays favorable benefits against colitis in both chemical-induced and bacterial infection models. Mechanistically, C. tropicalis colonization in the gut promoted IL-17A and IL-22 production to enhance gut barrier function and the proliferation of intestinal epithelial cells. T cell subsets (Th17 and T cells) and ILC3s stimulated the production of these two cytokines. Metabolomic profile of fecal samples from mice with C. tropicalis colonization identified the alteration in vitamin B3 metabolism with the conversion of nicotinamide to nicotinic acid via nicotinamidase activity. Oral gavage with nicotinamide exacerbated colitis symptoms, while nicotinic acid supplementation alleviated intestinal inflammation through an IL-17A-dependent pathway. C. tropicalis supplementation rescued the consequences of nicotinamide administration under experimental colitis. Nicotinamidase inhibitor treatment abrogated the protective benefits conferred by intestinal C. tropicalis colonization. In addition, C. tropicalis inoculation increased g-glutamylleucine levels in plasma. Administration mice with g-glutamylleucine recapitulated the beneficial effects against colitis by enhancing IL-17A, IL-22, and epithelial cell proliferation. Interestingly, a clinical strain isolated from a candidemia patient lacked the protective function against experimental colitis and the metabolism of vitamin B3 and g-glutamylleucine in contrast to the reference strain. Overall, my findings underscore the role of C. tropicalis as an inducer of IL-17A and IL-22 production via vitamin B3 and g-glutamylleucine metabolic pathways to enhance intestinal inflammation protection. My results have shed light on the understanding of the commensal fungi-metabolome-immune cells axis to regulate intestinal inflammation and provide evidence for developing fungi as probiotic products to treat patients with inflammatory bowel disease (IBD).