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导致死亡的主要原因，而接种疫苗是控制传染病的最有效手段。免疫产生的记忆B细胞的持久性对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细胞反应被认为是短暂的，并且在产生B细胞记忆方面效率低下。利用hermsii伯氏疏螺旋体菌血症的实验模型，我们发现成熟B细胞的一个亚群B1b细胞在缺乏T细胞帮助的情况下，在持久记忆反应中发挥了新的作用。具体来说，我们发现B1b细胞与hermsii杆菌血症的溶解同时扩增并持续很长时间。来自恢复期小鼠的B1b细胞产生特异性抗体反应，并赋予持久的免疫，这表明保护性反应与传统B细胞记忆的情况下B1b细胞的扩增和持久性相对应。B细胞对T细胞非依赖性抗原（如细菌多糖）的反应主要通过交联B细胞抗原受体（BCR）产生。尽管肺炎球菌多糖也能被B1b细胞识别，但它不会在x连锁免疫缺陷小鼠（xid）中诱导抗体反应，这些小鼠编码布鲁顿酪氨酸激酶（Btk）的基因发生突变，而布鲁顿酪氨酸激酶是最佳bcr介导的信号传导所必需的。相比之下，B. hermsii在xid小鼠中不仅诱导特异性抗体反应，而且诱导B1b细胞选择性扩增。这些扩增的B1b细胞持续存在很长时间，赋予恢复期小鼠抵抗再感染的能力。这些数据表明，除了BCR信号外，免疫刺激机制在T细胞非依赖性B1b细胞记忆的产生和维持中发挥重要作用。我们发现，hermsii能够激活toll样受体（TLRs），缺乏Btk和MyD88（一种TLR家族多个成员所需的接头蛋白）的小鼠，在建立保护性反应方面严重受损，这表明通过BCR和TLR协调的信号传导至关重要。我们还发现，TLR和BCR信号通路协同作用，上调B1b细胞上B淋巴细胞刺激剂（BLyS，也称为BAFF）的主要受体BR3和TACI。此外，我们发现全菌免疫在体内也能诱导扩增的抗原特异性B1b细胞上BR3的上调。BLyS通过参与BR3诱导促生存分子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刺激均有反应的B1b细胞表达更高水平的BR3和TACI，并且在普通多糖抗原的情况下，比单纯的B细胞或仅受BCR交联刺激的B细胞更优先维持BR3和TACI。确定TLR和BLyS信号在B1b细胞扩增和长期维持中的作用将为产生有效疫苗提供新方法。

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.