Maturation of the medaka immune system depends on reciprocal interactions between the microbiome and the intestinal tract

Interaction between host immune system and intestinal microorganisms have been studied in many animals including fish. However, detailed analysis has not been done in medaka, one of the established fish models biological studies. Here we investigate effect of immunodeficiency on microbiota composition and effect of gut bacteria on intestinal epithelial development and immune responses. Chronological analysis of intestinal metagenome of il2rg mutant medaka shows gradual decrease of evenness of OTUs, mainly caused by overwhelm by Aeromonadaceae family. Infection of wild type medaka with high dose of this opportunistic bacteria causes sign of inflammatory response, suggesting harmful effect of Aeromonadaceae on adult il2rg mutant. In addition, we establish germfree condition of larval medaka and observe large absorptive vacuoles in intestinal epithelial cells, indicating block of epithelial maturation. Transcriptome analysis shows decrease of expression of genes involved in defense response including antimicrobial peptide gene hepcidin which were expressed upon LPS stimulation in normal larvae. These results show that reciprocal interactions between the microbiome and the intestinal tract are required for the maturation of the medaka immune system.

宿主免疫系统与肠道微生物的互作已在包括鱼类在内的多种动物中得到广泛研究，但作为经典鱼类生物研究模型之一的青鳉（medaka），其相关细节分析尚未开展。本研究旨在探究免疫缺陷对微生物群组成的影响，以及肠道细菌对肠道上皮发育与免疫应答的作用。对il2rg突变青鳉的肠道宏基因组进行时序分析后发现，其操作分类单元（OTUs）的群落均匀度逐渐降低，该现象主要由气单胞菌科（Aeromonadaceae）的过度增殖所致。以高剂量的该条件致病菌感染野生型青鳉，可诱导炎症反应发生，提示气单胞菌科对成年il2rg突变青鳉具有有害作用。此外，本研究成功建立了青鳉幼鱼的无菌（germfree）培养体系，并观察到其肠道上皮细胞内存在大型吸收空泡，表明上皮细胞的成熟过程受到阻滞。转录组分析显示，正常幼鱼经脂多糖（LPS）刺激后可表达的免疫防御相关基因，包括抗菌肽基因铁调素（hepcidin），其表达量出现下调。上述结果表明，微生物群与肠道之间的双向互作是青鳉免疫系统成熟所必需的。