Dissecting unique functions of IgM and IgD BCRs in tolerance and autoimmunity

项目总结：“剖析免疫球蛋白M和免疫球蛋白受体在耐受和自身免疫中的独特功能” 典型的自身免疫性疾病系统性红斑狼疮(SLE)的特点是抗- 核自身抗体(Ana)不仅是疾病的关键生物标志物，而且还发挥着核心作用 在发病机制上。令人惊讶的是，尽管中枢耐受机制，如受体编辑和删除 自身反应性B细胞在骨髓发育过程中，高达30%的成熟人类B细胞仍含有 有一定程度的抗核抗体反应性。这表明，额外的外周耐受机制必须保持这种 有潜在危险的克隆人被控制住了。我们最近鉴定了一种新的荧光报告鼠系， Nur77-EGFP，体内对抗原依赖的B细胞受体(BCR)信号敏感。最高级别的 来自这些小鼠的荧光幼稚B细胞识别内源性抗原，并对ANA- 反应性。因此，Nur77-EGFP报告者在不同的、正常的谱系中识别ANA反应的B细胞， 使我们能够追踪和分离这些细胞。我们已经证明，这些ANA反应性B细胞下调 IgM同型bcr的表达，导致对IgM刺激的反应减弱。然而，天真的B 细胞唯一地表达第二种BCR同种异型IGD，具有与IgM相同的表位结合域。与之形成鲜明对比的是 IgM、IGD在ANA反应性B细胞中的表达和信号转导不受干扰。IgM下调如何限制 尽管免疫球蛋白高表达，自身抗原的自身免疫反应？每个bcr亚型单独能够 调节B细胞发育和对模型抗原免疫的反应。与此形成对比的是 明显的冗余，我们有新的初步数据表明B细胞缺乏IgM，只表达IGD 完全被阻止在SLE小鼠模型中促进自身抗体的产生。它有 最近的研究表明，对单价抗原唯一敏感的是IgM，而不是IgD。然而，相关的 系统性红斑狼疮中的核酸相关自身抗原被认为是膜相关的多价抗原。 这表明，IgM的其他独特特性可能对自身抗体的产生也很重要。在这 我们打算确定IgM和IGD对B细胞的不同调节机制 健康和疾病的背景。为此：(1)我们将利用免疫球蛋白M或免疫球蛋白D缺陷的小鼠 探索单独表达每种同型的B细胞如何被招募来分泌致病自身抗体 具有不同疾病机制的系统性自身免疫模型(LYN-/-和BAFF-TG)。(2)我们会 确定IgM和IGD BCRs如何通过传输到内体隔室来招募共刺激信号 TLRs7/9，并通过MHCII将抗原提呈给T细胞。(3)我们将探索IgM的差示偶联 而IGD到B细胞的刺激性和抑制性共受体改变下游信号转导并促进B细胞 宽容。这些研究将共同定义IgM下调如何限制B细胞对自身的反应 抗原。这一耐受检查点可能代表着治疗干预的一个新靶点。 在系统性红斑狼疮和其他自身抗体介导的疾病患者中建立免疫稳态。

Project Summary: “Dissecting unique functions of IgM and IgD BCRs in tolerance and autoimmunity” The prototypical autoimmune disease systemic lupus erythematosus (SLE) is characterized by anti- nuclear autoantibodies (ANAs) that serve not only as critical biomarkers of disease, but also play a central role in pathogenesis. Surprisingly, despite central tolerance mechanisms such as receptor editing and deletion of autoreactive B cells during bone marrow development, up to 30% of mature human B cells nevertheless harbor some degree of ANA-reactivity. This suggests that additional peripheral tolerance mechanisms must keep such potentially dangerous clones in check. We have recently characterized a novel fluorescent reporter mouse line, Nur77-eGFP, that is sensitive to antigen-dependent B cell receptor (BCR) signaling in vivo. The most highly fluorescent naïve B cells from these mice have recognized endogenous antigen, and are enriched for ANA- reactivity. The Nur77-eGFP reporter thus identifies ANA-reactive B cells in a diverse, normal repertoire, enabling us to track and isolate such cells. We have shown that these ANA-reactive B cells down-regulate expression of the IgM isotype BCR, resulting in dampened responses to IgM stimulation. However, naïve B cells uniquely express a second BCR isotype, IgD, with the same epitope-binding domain as IgM. In contrast to IgM, IgD expression and signaling in ANA-reactive B cells are unperturbed. How does IgM downregulation limit autoimmune responses to self-antigens despite high IgD expression? Each BCR isotype alone is capable of mediating B cell development and responses to immunization with model antigens. In contrast to such apparent redundancy, we have new preliminary data to show that B cells lacking IgM and expressing only IgD are completely prevented from contributing to autoantibody production in a mouse model of SLE. It has recently been shown that IgM, but not IgD, is uniquely sensitive to monovalent antigens. However, relevant nucleic acid-associated autoantigens in SLE are thought to be membrane associated and multivalent suggesting that other distinctive properties of IgM may be important for autoantibody production. In this proposal we intend to identify the mechanisms by which IgM and IgD differentially regulate B cells in the context of health and disease. To do so: (1) We will take advantage of mice deficient for either IgM or IgD to explore how B cells expressing each isotype alone are recruited to secrete pathogenic autoantibodies in two models of systemic autoimmunity with distinct disease mechanisms (Lyn-/- and BAFF Tg). (2) We will determine how the IgM and IgD BCRs traffic to endosomal compartments to recruit costimulatory signals via TLRs7/9 and via MHCII presentation of antigen to T cells. (3) We will explore how differential coupling of IgM and IgD to B cell stimulatory and inhibitory co-receptors alters downstream signaling and contributes to B cell tolerance. Together these studies will define how IgM down-modulation limits B cell responses to self- antigens. This tolerance checkpoint may represent a novel target for therapeutic intervention to re- establish immune homeostasis in patients with SLE and other autoantibody-mediated diseases.