信号调节蛋白α通过影响肠道微环境调节胆汁酸代谢的机制

Bile acids are synthesized in the liver cells, which are discharged to the extrahepatic bile duct and the small intestine with the bile. Bile acid metabolism plays an important role in maintaining the homeostasis of the body. Bile acid production excessive or excretion disorder can cause cholestasis, liver damage and systemic multiple organ damage. Intestinal bacteria also play an important role in bile acid metabolism, through the enterohepatic circulation of bile acid content and the composition effect. The imbalance of intestinal flora will affect the physiological activities and immune response of the organism. On the other hand, the immune status of intestinal micro environment has an important regulatory function on the distribution of intestinal flora. We found that the signal regulatory protein alpha (SIRPa) knockout mice had the spontaneous phenotype of gallbladder enlargement, mild jaundice, and elevated serum total bile acid level. Therefore, the regulatory role of SIRP alpha on the bile acid metabolism is investigated. SIRPa is mainly expressed in myeloid cells, so we also established the animal model of myeloid cell specific knock out of SIRPa. This study will demostrate the mechanism of SIRPa on regulating the intestinal immune microenvironment and intestinal flora distribution, which profoundly affects the metabolism of bile acids. At the same time, the value of SIRP alpha as a target for the intervention of bile stasis liver disease will be estimated.

胆汁酸主要在肝细胞内合成，随胆汁排出至肝外胆道及小肠，并通过肠肝循环重复利用。胆汁酸代谢对维持机体稳态具有重要作用。胆汁酸排泌障碍或产生过量会造成胆汁淤积，引起肝损伤及全身多器官损害。肠道菌群对胆汁酸代谢也起到重要作用，通过肝肠循环影响胆汁酸的含量和成分。肠道菌群的失衡会影响机体多种生理活动和免疫应答状态。另一方面，肠道微环境免疫状态对肠道菌群分布具有重要调节功能。实验中我们发现信号调节蛋白α（SIRPα）敲除小鼠具有胆囊增大、轻微黄疸、血清总胆汁酸水平升高等自发表型。因此，我们设计课题研究SIRPα对胆汁酸代谢的调节作用和机制。SIRPα是主要表达在髓系细胞的抑制性膜分子，故我们也建立了髓系细胞特异性敲除的动物模型。课题将深入解析SIRPα是如何通过调节肠道免疫微环境和肠道菌群的相互作用进而影响胆汁酸代谢过程，同时对SIRPα作为胆汁淤积性肝病干预靶点的价值也将做初步探究。

我们通过TALEN技术构建了SIRPα全身性敲除小鼠，通过流式检测小鼠脾脏、肠道固有层和淋巴结组织中DCs、巨噬细胞（Macropages, Mφ）和T细胞各亚群数量和比例，探究SIRPα在肠道免疫稳态中的作用。为了进一步明确SIRPα的功能，我们采用Cas9/RNA 基因打靶技术，使用Loxp/Loxp-Cre系统构建了DCs和Mφ上SIRPα特异性敲除小鼠，通过流式检测肠道淋巴组织中免疫细胞亚群的数量和比例，深入探讨DCs和Mφ等不同来源的SIRPα对于肠道免疫系统的固有调节作用。研究发现：1.小鼠脾脏、小肠固有层和肠系膜淋巴结中Mφ均高表达SIRPα，而在DCs亚群中存在分化；2. SIRPα-KO小鼠脾脏CD4+ cDCs特异性减少，肠道固有层和肠系膜淋巴结中CD103+CD11b+ DCs数量和比例特异性减少；3. 造血细胞来源的SIRPα参与肠道DCs亚群的调节；4. DCs和Mφ上SIRPα特异性缺失均会导致脾脏CD4+ cDCs、肠系膜淋巴结和小肠固有层中CD103+CD11b+ DCs数量的特异性减少；5. SIRPα依赖的CD103+CD11b+ DCs影响肠道Th17/Th1细胞的产生；6. 阻断SIRPα-CD47通路能够增强SLAMF7依赖的肠道CD103+CD11b+ DCs和Th17/Th1的吞噬；7. SIRPα-KO小鼠肠黏膜上皮菌群数量增加且抗菌功能减弱，肠上皮细胞异常激活，胆汁酸代谢负反馈调节通路受抑制；8. SIRPα的缺失加重小鼠DSS诱导的肠炎症状。综上所述，我们发现SIRPα依赖的CD103+CD11b+ DCs在肠道免疫和代谢稳态中扮演着至关重要的角色。另外，我们大胆推测CD103+CD11b+ DCs可能会参与除胆汁酸代谢之外的其他代谢稳态，这亟待更广泛的研究。最近CD47中和抗体用于肿瘤治疗研究成为热点，在肿瘤治疗过程中阻断SIRPα-CD47通路同样也会引起系统免疫和代谢的改变，这无疑会对治疗效果产生影响，因此对于SIRPα和CD103+CD11b+ DCs的理解需要更深入的探索。