B1 cell-mediated immunity in Human Immune System mice

描述(由申请人提供)：为了了解解决菌血症所需的保护性免疫反应的细胞学基础，我们一直在研究赫氏疏螺旋体感染的小鼠模型。利用这个实验系统，我们已经确定了B1B细胞亚群在保护性免疫中的独特作用。具体地说，我们发现B1B细胞在Hermsii菌血症消退的同时扩张，并且扩张的细胞在恢复期的小鼠中持续了相当长的时间，并有助于形成一种新的T细胞非依赖性B细胞记忆。这一开创性的发现已经用几种重要的人类病原体进行了概括。有趣的是，在这些细菌感染系统中，B1B细胞识别的抗原不仅包括多糖，还包括细菌外膜蛋白。尽管小鼠模型在阐明免疫学机制方面的力量是毋庸置疑的，但目前尚不清楚人类是否存在B1B细胞介导的免疫的功能等价物。最近，在人脐血和成人外周血中发现了一种新的B细胞群(CD19+CD20+CD27+CD43+CD70-CD69-)，它具有小鼠B1细胞的几个关键特征。为了研究人类B1细胞的功能，我们利用CD34+人造血干细胞(HSC)移植的NOD/SCID/IL-2R？null(NSG)小鼠，称为“人免疫系统”小鼠(HIS小鼠)。我们发现，HIS小鼠产生了包括B1细胞在内的几个人类B细胞亚群，这表明小鼠的微环境能够支持从人类HSCs发育成人类B1细胞。我们还发现，Hermsii在HIS小鼠中的感染会导致反复发作的菌血症，这是复发发烧的标志。此外，原发菌血症的消退与产生对赫氏杆菌特异的人类免疫球蛋白M反应是同步的。在小鼠感染模型中，B1B细胞对H因子结合蛋白A(FhBA)产生特异性抗体反应，FhBA是赫氏杆菌的一种外膜蛋白。有趣的是，来自感染Hermsii的个体或来自感染B.hermsii的HIS小鼠的IgM显示出相同的FhBA反应。在HIS小鼠中，抗人CD20耗尽B细胞会导致Hermsii和FhBA特异性抗体反应的丧失，并导致持续性菌血症。综上所述，这些发现表明，在人类和HIS小鼠中存在小鼠B1B细胞的功能等价物。尽管在HIS小鼠中感染Hermsii的特征反映了人类复发热的特征，但尚不清楚人类B细胞的哪些亚群在功能上负责控制B.hermsii。这项建议的目的是：(1)确定人类在HIS小鼠中与小鼠B1B细胞的等价物；(2)确定HIS小鼠中扩大的人类B细胞种群是否持续存在，并增强对B.hermsii的保护性反应。研究HIS小鼠B细胞亚群保护性TI反应的基础不仅将验证我们目前基于小鼠模型对B1B细胞的理解，而且还将作为一个翻译平台，有助于我们探索B1B细胞在人类许多细菌感染中的潜在作用。

DESCRIPTION (provided by applicant): To understand the cellular basis for the protective immune responses required for the resolution of bacteremia, we have been studying the murine model of Borrelia hermsii infection. Using this experimental system we have identified a unique role for B1b cell subset in protective immunity. Specifically we found that B1b cells expand concurrent with the resolution of B. hermsii bacteremia and the expanded cells persist in convalescent mice for a remarkably long time and contribute to a novel T cell-independent B cell memory. This seminal finding has been recapitulated using several important human pathogens. Interestingly, the antigens recognized by B1b cells in these bacterial infection systems include not only polysaccharides, but also bacterial outer-membrane proteins. Although the power of murine models in elucidating immunological mechanisms is unquestioned, it is not known whether a functional equivalent of B1b cell-mediated immunity exists in humans. Recently, a novel population of B cells (CD19+CD20+CD27+CD43+CD70-CD69-) was identified in human umbilical cord blood and adult peripheral blood that has several key characteristics of murine B1 cells. To investigate the function of human B1 cells, we have utilized CD34+ human hematopoeitic stem cell (HSC)-engrafted NOD/SCID/IL-2R??null (NSG) mice, referred to as "Human Immune System" mice (HISmice). We have found that HISmice generate several subsets of human B cells including B1 cells suggesting that the murine microenvironment is capable of supporting the development of human B1 cells from human HSCs. We also found that B. hermsii infection in HISmice results in recurrent episodes of bacteremia, a hallmark of relapsing fever. Moreover, the resolution of the primary episode of bacteremia was concurrent with the generation of a B. hermsii-specific human IgM response. In the murine infection model, B1b cells mount a specific antibody response to Factor H binding protein A, (FhBA), an outer-membrane protein of B. hermsii. Interestingly, IgM from B. hermsii-infected individuals or from B. hermsii-infected HISmice display an identical FhBA reactivity. Depletion of B cells by anti- human CD20 in HISmice results in loss of B. hermsii- and FhBA-specific antibody responses and in persistent bacteremia. Together, these findings suggest that functional equivalents of murine B1b cells exist in humans and HISmice. Although the characteristics of B. hermsii infection in HISmice mirror those of human relapsing fever, it is not known which subsets of human B cells are functionally responsible for controlling B. hermsii. The Aims of this proposal are to: (1) identify the human equivalents of murine B1b cells in HISmice and (2) to determine whether expanded human B cell population in HISmice persist and confer a heightened protective response to B. hermsii. Investigating the basis for protective TI responses conferred by B cell subsets in HISmice will not only validate our current understanding of B1b cells based on murine models, but will also serve as a translational platform, which can help us to explore the potential role for B1b cells in a number of bacterial infections in humans.