（通讯员：潘奕如）近日，我院张永安教授领衔的鱼类免疫与病害防控团队在鱼类抗体亚型及相应B细胞亚群的组成及功能研究方面的最新研究结果以“An Atlas of Grass Carp IgM⁺ B Cells in Homeostasis and Bacterial Infection Helps to Reveal the Unique Heterogeneity of B Cells in Early Vertebrates”为题在Journal of Immunology发表。该研究以我国主养经济鱼类--草鱼为研究对象，首次发现草鱼具有三个IgM亚型，并通过单细胞转录组技术揭示了草鱼IgM⁺ B细胞的表达谱以及单个IgM⁺ B细胞共表达多个IgM亚型的新机制。

细菌感染前后草鱼头肾中IgM⁺ B细胞的单细胞图谱

已有研究表明硬骨鱼类B细胞为原始的B细胞类型，既可以像哺乳动物B2细胞一样产生抗原特异性抗体，也可以像哺乳动物B1细胞一样发挥吞噬、杀菌以及抗原递呈功能。然而，由于抗体等工具的缺乏，对硬骨鱼类B细胞的研究还停留在初级阶段，硬骨鱼类B细胞亚群的组成和功能有何独特性尚待深入解析。此外，与哺乳动物具有5类抗体（IgG、IgM、IgA、IgD、IgE）不同，硬骨鱼类仅具有3类抗体（IgM、IgT、IgD），其中仅IgM在系统免疫中起主要作用。研究表明大西洋鲑（Salmo salar）和褐鳟（Salmo trutta）具有2个IgM亚型，那么，硬骨鱼类是否普遍具有多个IgM亚型？不同IgM亚型产生的细胞机制是什么？为了回答上述科学问题，研究团队以草鱼为研究对象，测序并组装了雌核发育草鱼的染色体水平基因组（Wu et al. BMC Genomics, 2022），发现草鱼基因组中存在3个IgM重链基因（Igμ），随后从RNA和蛋白水平证实了3个Igμ基因都可表达形成功能性的IgM亚类。进一步流式分选了细菌感染和对照草鱼头肾中的IgM+ B细胞并进行了单细胞转录组测序分析，共鉴定出6种不同的IgM+ B细胞亚群，包括（未）成熟B细胞、先天性B细胞、增殖性B细胞、浆细胞、CD22+细胞和CD34+细胞。针对不同IgM+ B细胞亚群进行了功能差异分析，预测了不同IgM+ B细胞亚群可能存在的功能，同时比较了细菌感染前后不同IgM+ B细胞亚群的变化。值得注意的是，在单细胞转录组水平上，不同Igμ基因可以共表达于单个IgM+ B细胞内，依据不同Igμ基因的表达模式可以将浆细胞细分为IgM1high浆细胞、IgM3high浆细胞、IgM2highIgM3high浆细胞和IgM1highIgM2highIgM3high浆细胞，细菌感染后IgM1highIgM2highIgM3high浆细胞的比例显著增加了，通过流式分选单个IgM+ B细胞结合可变区测序进一步证实了草鱼单个IgM+ B细胞可以共表达VHDJH重排的Igμ基因，暗示硬骨鱼类B细胞可能通过共表达多个IgM亚型来应对感染。此研究结果从低等脊椎动物的视角为经典免疫学范式“一个B细胞产生一种抗体”提出了新的挑战。

51菠菜网博士生潘奕如为论文第一作者，张旭杰副教授和张永安教授为论文的共同通讯作者。上海海洋大学的邹曙明教授和郑国栋博士为研究提供了雌核发育的草鱼。我校邬长松博士和博士研究生钟雅琴也参与了研究工作。该研究得到了国家自然科学基金、现代农业产业技术体系、国家重点研发计划等课题资助。

审核人：张旭杰、张永安

【英文摘要】

Teleost B cells are primitive lymphocytes with both innate and adaptive immune functions. However, the heterogeneity and differentiation trajectory of teleost B cells remain largely unknown. Here, the landscape of grass carp (gc) IgM⁺ B cells was revealed by single-cell RNA sequencing. The results showed that gcIgM⁺ B cells mainly comprise six populations: (im)mature B cells, innate B cells, proliferating B cells, plasma cells, CD22⁺ cells, and CD34⁺ cells, among which innate B cells and proliferating B cells were uncommon B cell subsets with, to our knowledge, new characteristics. Remarkably, three functional IgMs were discovered in grass carp, and a significant percentage of gcIgM⁺ B cells, especially plasma cells, expressed multiple Igμs (Igμ1, Igμ2, and/or Igμ3). More importantly, through single-cell sorting combined with Sanger sequencing, we found that distinct V_HDJ_H recombination patterns of Igμs were present in single IgM⁺ B cells, indicating that individual teleost B cells might produce multiple antibodies by co-expressing rearranged IgM subclass genes. Moreover, the percentage of IgM1^highIgM2^highIgM3^high plasma cells increased significantly after bacterial infection, suggesting that individual plasma cells might tend to produce multiple IgMs to resist the infection in teleost fish. In summary, to our knowledge, this study not only uncovered the unique heterogeneity of B cells in early vertebrates, but also provided significant new evidence supporting recently proposed “one cell-multiple antibodies’’ paradigm, challenging the classical rule of “one B cell-one antibody”.

论文链接：https://journals.aai.org/jimmunol/article-abstract/211/6/964/265857/An-Atlas-of-Grass-Carp-IgM-B-Cells-in-Homeostasis?redirectedFrom=fulltext