Regulation of Persistent Ab Responses by Fc Receptors

描述(申请人提供)：免疫球蛋白Fc部分的受体(FcGammaRs)广泛分布在造血细胞上，通过将抗体介导的反应与效应器和调节细胞的活性联系起来，在免疫调节中发挥重要作用。小鼠FcGammaR FcGammaRIIB是一种低亲和力的受体，因此只以免疫复合体(IC)的形式与IgG结合。该受体包含一个基于酪氨酸的免疫受体抑制基序(ITIM)，当通过IC结合与免疫受体酪氨酸激活基序的受体(如B细胞抗原受体(BCR)共交联时，ITIM是一种有效的信号转导抑制因子。在之前的资助阶段，我们发现在生发中心(GC)反应中，FcGammaRIIB的表达水平发生了显著变化，在GC B细胞上下调，在滤泡树突状细胞(FDCs)上上调。此外，FcGammaRIIB缺陷小鼠在GC反应中表现出扰动。我们还发现，FcGammaRIIB在B细胞系上的同源交联会诱导凋亡反应，这不需要ITIM基序的存在。最后，观察到C57BL/6背景的FcGammaRIIB缺陷小鼠产生高滴度的抗核抗原抗体，并出现系统性自身免疫性疾病。后者的发现与之前的建议一致，即FcGammaRIIB的异常表达可能导致B细胞耐受性的丧失。 这些发现推动了假设的发展，即FcGammaRIIB在调节GC反应的结果中发挥核心作用。然而，已经提出的不同假设归因于FcGammaRIIB在GC B细胞和FDCs上的表达和功能的重要性以及该受体的1C捕获、BCR抑制和诱导凋亡的能力的差异。因此，在下一个资助期，我们建议：1)分析B细胞中FcGammaRIIB选择性缺陷小鼠的GC反应、记忆B细胞发育和对全身自身免疫的易感性；2)对FcGammaRIIB的ITIM信号通路中完全或选择性B细胞和FDC缺陷的小鼠进行类似的研究；以及，3)利用一种新的表达染色质/芳香酸“双反应”BCRs的VH“敲门”系来确定在GC反应过程中，整体或选择性FcGammaRIIB缺陷小鼠是否会改变自身反应性B细胞的负选择。这些研究的结果将为以下方面提供重要的新见解：不同类型细胞中FcGammaRIIB功能和表达的调节如何协调免疫记忆的正常发展和耐受性的维持，以及这些调节通路的干扰如何有助于自身免疫性疾病的发展。

DESCRIPTION (provided by applicant): Receptors for the Fc portion of IgG (FcgammaRs) are widely distributed on hematopoetic cells and play an important role in immune regulation by linking antibody-mediated responses with effector and regulatory cell activity. The murine FcgammaR FcgammaRIIB is a low affinity receptor and thus binds IgGs only in the form of immune complexes (ICs). This receptor contains an immunoreceptor tyrosine based inhibitory motif (ITIM) that is a potent inhibitor of signal transduction via receptors with immunoreceptor tyrosine activation motifs, such as the B cell antigen receptor (BCR), when co-crosslinked with the BCR via IC binding. In the previous funding period we discovered that levels of FcgammaRIIB expression are dramatically altered during the germinal center (GC) reaction, being down regulated on GC B cells and upregulated on follicular dendritic cells (FDCs). Moreover, FcgammaRIIB deficient mice display perturbations in the GC response. We also found that homologous cross-linking of FcgammaRIIB on B cell lines results in induction of an apoptotic response, and this does not require the presence of the ITIM motif. Finally, mice with an FcgammaRIIB deficiency on a C57BL/6 background were observed to develop high liters on anti-nuclear antigen antibody, and systemic autoimmune disease. The latter finding is consistent with previous suggestions that dysregulated expression of FcgammaRIIB can contribute to loss of B cell tolerance. These findings have motivated the development of hypotheses asserting that FcgammaRIIB plays a central role in regulating the outcome of the GC reaction. However, the different hypotheses that have been forwarded ascribe widely divergent levels of importance to FcgammaRIIB expression and function on GC B cells versus FDCs, as well as to the 1C trapping, BCR inhibitory, and apoptosis inducing capacities of this receptor. Therefore, in the next funding period we propose to: 1) analyze the GC response, memory B cell development and predisposition to systemic autoimmunity in mice with selective deficiencies of FcgammaRIIB in B cells versus stromal elements including FDCs; 2) perform analogous studies on mice with either complete or selective B cell and FDC deficiencies in the ITIM signaling pathway of FcgammaRIIB; and, 3) utilize a novel VH "knockin" line of mice expressing chromatin/arsonate "dual reactive" BCRs to determine if global or selective FcgammaRIIB deficiencies alter negative selection of autoreactive B cells during the GC response. The results of these studies will provide important new insights into how regulation of the functions and expression of FcgammaRIIB in various cell types are orchestrated towards the normal development of immune memory and maintenance of tolerance, and how perturbations in these regulatory pathways can contribute to the development of autoimmune disease.