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

项目摘要：“剖析IgM和IgD BCR在耐受性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）的特征是抗- 核自身抗体（ANA）不仅作为疾病的关键生物标志物，而且还发挥核心作用 在发病机理上。令人惊讶的是，尽管有中枢耐受机制，如受体编辑和缺失， 尽管在骨髓发育过程中存在自身反应性B细胞，但高达30%的成熟人类B细胞仍然具有 一定程度的ANA反应性。这表明，额外的外周耐受机制必须保持这种耐受性。 潜在危险的克隆人被控制住了我们最近表征了一种新的荧光报告小鼠系， Nur 77-eGFP，其在体内对抗原依赖性B细胞受体（BCR）信号传导敏感。最高的 来自这些小鼠的荧光幼稚B细胞已识别内源性抗原，并富含ANA- 反应性因此，Nur 77-eGFP报告基因鉴定了不同的正常库中的ANA反应性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同种型BCR的表达，导致对IgM刺激的应答减弱。不过，天真的B 细胞独特地表达第二种BCR同种型IgD，其具有与IgM相同的表位结合结构域。相比 ANA反应性B细胞中的IgM、IgD表达和信号传导未受干扰。IgM下调如何限制 尽管IgD高表达，但对自身抗原的自身免疫反应？每种BCR同种型单独能够 介导B细胞发育和对模型抗原免疫的应答。与此相反， 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 在SLE的小鼠模型中完全防止了自身抗体的产生。它有 最近显示IgM而不是IgD对单价抗原是唯一敏感的。但相关 SLE中的核酸相关自身抗原被认为是膜相关的和多价的 这表明IgM的其它独特性质对于自身抗体的产生可能是重要的。在这 我们打算确定IgM和IgD差异调节B细胞的机制， 健康和疾病的背景。为了做到这一点：（1）我们将利用IgM或IgD缺陷的小鼠， 探索单独表达每种同种型的B细胞是如何被招募来分泌两种致病性自身抗体的。 具有不同疾病机制的全身性自身免疫模型（林恩-/-和BAFF Tg）。(2)我们将 确定IgM和IgD BCR如何运输到内体区室，以通过以下途径募集共刺激信号： TLR 7/9和通过MHCII将抗原呈递给T细胞。(3)我们将探索IgM如何差异偶联 和IgD对B细胞刺激性和抑制性共受体的作用改变下游信号传导，并有助于B细胞 宽容这些研究将共同确定IgM下调如何限制B细胞对自身免疫应答的反应。 抗原这种耐受性检查点可能代表了治疗干预的新靶点，以重新治疗。 在SLE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.