Blk as a Master Regulator of Marginal Zone B Cell Development and Activation

描述（由申请方提供）：对血源性病原体的体液应答由称为边缘区（MZ）B细胞的脾B细胞特化亚群介导。这些细胞在进入脾脏时被战略性地定位以遇到外来抗原，迅速产生有效的抗体应答，因此代表了对抗血源性病原体的第一道防线。虽然已知MZ B细胞发育和功能依赖于通过多种表面受体的信号传导，所述表面受体包括B细胞抗原受体（BCR）、Notch、肿瘤坏死因子受体（TNFR）家族、G蛋白偶联受体（GPCR）家族、toll样受体（TLR）家族和1型干扰素受体（IFNR）家族，目前仍不清楚这些不同的信号通路如何以及何时相互作用以产生功能性MZ B细胞。在最近发表的一项研究中，我们报道了酪氨酸激酶（SFK）Src家族的一个B细胞特异性成员Blk在成熟脾B细胞亚群中的差异表达，MZ B细胞表达的Blk是滤泡（FO）B细胞的两倍。重要的是，Blk表达水平的这种差异反映了MZ和FO B细胞发育中对Blk的不同需求，因为Blk缺陷型和Blk单倍体不足型小鼠均表现出MZ发育缺陷，而FO B细胞发育不缺陷。此外，尽管与Blk+/+小鼠相比，Blk+/-和Blk-/-小鼠中存在较少的MZ B细胞，但Blk突变MZ B细胞在体外和体内均对BCR刺激具有高应答性。总之，这些数据证明了先前未记载的Blk在MZ B细胞的发育和活化中的作用。我们的长期目标是确定高水平的Blk控制MZ B细胞发育和功能的机制。由于SFK参与多种信号传导途径，我们假设增加Blk水平可提高不同类型表面受体对它的利用，从而提供了一种在多种信号传导途径之间进行通信的机制，这些信号传导途径有助于MZ B细胞的生成和维持。该R21授权申请的目的是通过确定在MZ B细胞发育期间哪些信号传导途径上调Blk表达以及在MZ B细胞发育和活化期间哪些信号传导途径需要Blk活性来测试该新假设。我们期望所提出的研究将不仅确定Blk是否是BCR、Notch、GPCR、TNFR、TLR和IFNR型信号通路中的关键组分，而且还将阐明这些不同的信号通路在MZ B细胞发育和活化期间如何以及何时相互作用。这项研究很重要，因为通过了解它们正常发育和功能的分子要求，它将促进特异性靶向MZ B细胞的疫苗和疗法的开发。

DESCRIPTION (provided by applicant): Humoral responses to blood-borne pathogens are mediated by a specialized subset of splenic B cells known as marginal zone (MZ) B cells. These cells, which are strategically positioned to encounter foreign antigen as it enters the spleen, rapidly produce an effective antibody response and, therefore, represent a first line of defense against blood-borne pathogens. While it is known that MZ B cell development and function are dependent on signaling through a variety of surface receptors including the B cell antigen receptor (BCR), Notch, the tumor necrosis factor receptor (TNFR) family, the G-protein-coupled receptor (GPCR) family, the toll-like receptor (TLR) family, and the type 1 interferon receptor (IFNR) family, it is still not understood how and when these diverse signaling pathways interact to generate functional MZ B cells. In a recently published study, we reported that Blk, a B cell-specific member of the Src family of tyrosine kinases (SFKs), is differentially expressed in mature splenic B cell subsets, with MZ B cells expressing twice as much Blk as follicular (FO) B cells. Importantly, this difference in Blk expression levels reflected a differential requirement fr Blk in MZ and FO B cell development, as both Blk-deficient and Blk-haploinsufficient mice exhibited defects in the development of MZ but not FO B cells. Moreover, although there were fewer MZ B cells in Blk+/- and Blk-/- mice compared to Blk+/+ mice, Blk mutant MZ B cells were hyper-responsive to BCR stimulation, both in vitro and in vivo. Together, these data demonstrate a previously undocumented role for Blk in the development and activation of MZ B cells. Our long-term goal is to determine the mechanism(s) by which high levels of Blk control MZ B cell development and function. Since SFKs participate in a multitude of signaling pathways, we hypothesize that increasing Blk levels improves its utilization by different classes of surface receptors, thereby providing a mechanism for communication among the diverse signaling pathways that contribute to the generation and maintenance of MZ B cells. The objective of this R21 grant application is to test this novel hypothesis by determining which signaling pathways upregulate Blk expression during MZ B cell development and which require Blk activity during MZ B cell development and activation. We expect that the proposed study will not only determine whether Blk is a key component in the BCR, Notch, GPCR, TNFR, TLR and type IFNR signaling pathways but will also shed light on how and when these diverse signaling pathways interact during MZ B cell development and activation. This study is important because, by understanding the molecular requirements for their proper development and function, it will facilitate development of vaccines and therapies that specifically target MZ B cells.