Yin Xiaoxue, Li Xiaoyu, Mu Liangliang, Bai Hao, Yang Yanjian, Chen Nuo, Wu Liting, Fu Shengli, Li Jun, Ying Wantao, Ye Jianmin
School of Life Sciences, South China Normal University, Institute of Modern Aquaculture Science and Engineering, Guangdong Provincial Key Laboratory for Healthy and Safe Aquaculture, Guangzhou, People's Republic of China.
State Key Laboratory of Proteomics, Beijing Proteome Research Center, Beijing Institute of Life Omics, Beijing, People's Republic of China.
J Immunol. 2022 Aug 1;209(3):593-605. doi: 10.4049/jimmunol.2100921.
Teleost tetramer IgM is the predominant Ig in the immune system and plays essential roles in host defense against microbial infection. Due to variable disulfide polymerization of the monomeric subunits, tetrameric IgM possesses considerable structural diversity. Previous work indicated that the teleost IgM H chain was fully occupied with complex-type N-glycans. However, after challenge with trinitrophenyl (TNP) Ag, the complex N-glycans in the Asn-509 site of Oreochromis niloticus IgM H chain transformed into high mannose. This study, therefore, was conducted to examine the functional roles of the affinity-related high-mannose modification in tilapia IgM. The TNP-specific IgM Ab affinity maturation was revealed in tilapia over the response. A positive correlation between TNP-specific IgM affinity and its disulfide polymerization level of isomeric structure was demonstrated. Mass spectrometric analysis indicated that the relationship between IgM affinity and disulfide polymerization was associated with the Asn-509 site-specific high-mannose modification. Furthermore, the increase of high mannose content promoted the combination of IgM and mannose receptor (MR) on the surface of phagocytes. Moreover, the increased interaction of IgM and MR amplified the phagocytic ability of phagocytes to Streptococcus agalactiae. To our knowledge, this study demonstrates that site-specific high-mannose modification associates with IgM Ab affinity and its structural disulfide polymerization and amplifies the phagocytosis of phagocytes by the combination of IgM and MR. The present study provides evidence for understanding the association of IgM structure and function during the evolution of the immune system.
硬骨鱼的四聚体IgM是免疫系统中的主要免疫球蛋白，在宿主抵御微生物感染中发挥着重要作用。由于单体亚基可变的二硫键聚合作用，四聚体IgM具有相当大的结构多样性。先前的研究表明，硬骨鱼IgM重链完全被复合型N-聚糖占据。然而，在用三硝基苯基（TNP）抗原攻击后，尼罗罗非鱼IgM重链Asn-509位点的复合型N-聚糖转变为高甘露糖型。因此，本研究旨在探讨罗非鱼IgM中与亲和力相关的高甘露糖修饰的功能作用。在罗非鱼的免疫反应过程中揭示了TNP特异性IgM抗体亲和力成熟现象。证实了TNP特异性IgM亲和力与其异构体结构的二硫键聚合水平之间存在正相关。质谱分析表明，IgM亲和力与二硫键聚合之间的关系与Asn-509位点特异性高甘露糖修饰有关。此外，高甘露糖含量的增加促进了IgM与吞噬细胞表面甘露糖受体（MR）的结合。而且，IgM与MR相互作用的增强放大了吞噬细胞对无乳链球菌的吞噬能力。据我们所知，本研究表明位点特异性高甘露糖修饰与IgM抗体亲和力及其结构二硫键聚合相关，并通过IgM与MR的结合放大了吞噬细胞的吞噬作用。本研究为理解免疫系统进化过程中IgM结构与功能的关联提供了证据。
