Toll-like receptor signaling in the generation of B1b cell memory

描述（由申请人提供）：传染病是世界范围内死亡的主要原因，疫苗接种是控制传染病的最有效手段。疫苗接种产生的记忆B细胞的持久性对于保护性免疫的持久性至关重要，但其长期维持所需的分子机制尚不清楚。在没有T细胞帮助的情况下产生的B细胞应答被认为是短暂的，并且在产生B细胞记忆方面是低效的。使用赫氏疏螺旋体菌血症的实验模型，我们发现了B1 B细胞（成熟B细胞的一个亚群）在缺乏T细胞帮助的情况下在持久记忆反应中的新作用。具体地说，我们发现B1 B细胞与B的消退同时扩增。hermsii菌血症并持续较长时间。恢复期小鼠的B1 B细胞产生特异性抗体应答，并赋予持久的免疫力，这表明保护性应答与传统B细胞记忆的情况下B1 B细胞扩增和持久性相对应。对T细胞非依赖性抗原如细菌多糖的B细胞应答主要通过交联B细胞抗原受体（BCR）产生。虽然肺炎球菌多糖也能被B1 b细胞识别，但它不会在X-连锁免疫缺陷小鼠（xid）中诱导抗体应答，这些小鼠在编码布鲁顿酪氨酸激酶（Btk）的基因中存在突变，这是最佳BCR介导的信号传导所必需的。相反，B. hermsii在xid小鼠中不仅诱导特异性抗体应答，而且诱导B1 b细胞的选择性扩增。这些扩增的B1 b细胞持续很长时间，赋予恢复期xid小鼠对再感染的抵抗力。这些数据表明，免疫刺激机制以外的BCR信号可以发挥重要作用，在产生和维持T细胞非依赖性B1 b细胞记忆。我们找到了那个B。hermsii能够激活Toll样受体（TLR），Btk和MyD 88（TLR家族多个成员所需的衔接蛋白）缺陷的小鼠在建立保护性应答中严重受损，表明通过BCR和TLR的协调信号传导是关键的。我们还发现TLR和BCR信号传导途径协同作用以在体外上调B1 B细胞上的B淋巴细胞刺激因子（BLyS，也称为BAFF）的主要受体BR 3和TACI。此外，我们发现用全菌免疫也诱导BR 3在体内扩增的抗原特异性B1 b细胞上上调。BLyS通过接合BR 3诱导促存活分子Mcl-1和Bcl-XL以及参与细胞代谢适应性和B细胞稳态的激酶的表达。这些发现使我们假设，对BCR和TLR刺激都有反应的B1 B细胞表达更高水平的BR 3和TACI，并且优先维持在幼稚B细胞或仅由BCR交联刺激的B细胞（如在普通多糖抗原的情况下）之上。确定TLR和BLyS信号在B1 b细胞扩增和长期维持中的作用将为有效疫苗的产生提供新的方法。

DESCRIPTION (provided by applicant): Infectious diseases are the leading cause of death worldwide and vaccination is the most effective means to control them. Persistence of memory B cells generated by vaccination is critical for the longevity of protective immunity, but the molecular mechanism required for their long-term maintenance is unknown. B cell responses generated in the absence of T cell-help had been considered short-lived and inefficient in generating B cell memory. Using the experimental model of Borrelia hermsii bacteremia, we discovered a novel role for B1b cells, a subset of mature B cells, in long-lasting memory responses in the absence of T cell-help. Specifically, we found that B1b cells expand concurrently with the resolution of B. hermsii bacteremia and persist for long time. B1b cells from convalescent mice but not from na¿ve mice generate a specific antibody response and confer long-lasting immunity, indicating that the protective response corresponds to B1b cell expansion and persistence as in the case of conventional B cell memory. B cell responses to T cell-independent antigens such as bacterial polysaccharides are generated primarily by cross-linking B cell antigen receptors (BCR). Although Pneumococcal polysaccharide is also recognized by B1b cells, it does not induce antibody responses in X-linked immunodeficient mice (xid) mice, which have a mutation in gene encoding for Bruton's tyrosine kinase (Btk), which is required for optimal BCR-mediated signaling. In contrast, B. hermsii induces not only a specific antibody response but also a selective expansion of B1b cells in xid mice. These expanded B1b cells persist for long time, conferring upon the convalescent xid mice resistance to re-infection. These data suggest that immunostimulatory mechanisms other than BCR signaling can play an important role in the generation and maintenance of T cell-independent B1b cell memory. We found that B. hermsii is capable of activating Toll-like receptors (TLRs) and mice deficient in both Btk and MyD88, an adaptor protein required by multiple members of the TLR family, are severely impaired in mounting protective responses indicating that a coordinated signaling through BCR and TLR is critical. We have also found that TLR and BCR signaling pathways synergize to upregulate BR3 and TACI, the major receptors for B Lymphocyte Stimulator (BLyS, also known as BAFF), on B1b cells in vitro. Furthermore, we found that immunization with whole bacteria also induces BR3 up-regulation on expanded antigen-specific B1b cells in vivo. BLyS, by engaging BR3, induces the expression of the pro-survival molecules Mcl-1 and Bcl-XL and kinases involved in cellular metabolic fitness and B cell homeostasis. These findings led us to hypothesize that B1b cells that have responded to both BCR and TLR stimulation express higher levels of BR3 and TACI and are preferentially maintained over na¿ve B cells or B cells that are stimulated by BCR cross-linking alone as in the case of plain polysaccharide antigens. Identifying the role of TLR and BLyS signaling in B1b cell expansion and long-term maintenance will provide novel approaches for the generation of effective vaccines.