CONTROL OF PERSISTENT ANTIBODY RESPONSE BY FC RECEPTORS

抗原特异性抗体的长期持续和高度特异性记忆B细胞的产生是公认的体液免疫反应的特征。生发中心（GC）的特殊微环境可能是这些途径的关键，特别是滤泡树突状细胞（FDC）长时间以免疫复合物（ic）的形式保留免疫原性抗原的能力。有人提出，这种形式的抗原在初级免疫反应期间驱动高亲和力B细胞受体体细胞突变体的选择，并在此后的较长时间内维持记忆B细胞区室的稳定性。缺乏激活Fc受体FcgammaRI和fcgammarii的小鼠在fdc上显示出显著增强的IC沉积和保留。然而，这不会导致原发性免疫反应的扰动。在Aim 1中，我们提出验证这样的假设，即这种增强的IC沉积和保留在fdc上将导致骨髓中持久性血清抗体、记忆B细胞和抗体形成细胞（AFCs）水平升高。我们还将确定在随后的第二抗原免疫中，免疫球蛋白在FDC上的保留是否稳定，以及当FDC上同源抗原水平较高时，持久血清抗体的亲和力是否较低。在记忆的产生和维持过程中，B细胞的活性也可能通过ic与抗原特异性B细胞通过抑制性FcR FcgammaRIIB直接相互作用来调节。缺乏这种FcR的小鼠在免疫后显示血清抗体和AFC水平升高，这与这一观点以及过去对这种FcR功能的体外研究一致。然而，由于FcgammaRIIB是最广泛表达的FcR，在许多髓系谱系和fdc中都有发现，因此这一结论仍是初步的。在目的2中，我们提出验证在FcgammaRIIB中观察到的抗体产生表达失调的假设。缺陷小鼠是由于B细胞上缺乏这种FcR。最后，最近的数据表明BCR-FcgammaRIIB共连接抑制BCR信号的机制是通过将肌醇多磷酸磷酸酶SHIP募集到FcgammaRIIB细胞质尾部的ITIM基序上介导的。在ITIM基序中Y大于F变化的FcgammaRIIB突变形式，排除了SHIP结合所需的Y磷酸化，不会介导转染细胞系中BCR信号传导的抑制。在Aim 3中，我们提出验证itimm - ship途径是体内抑制B细胞活性所必需的唯一FcgammaRIIB启动途径的假设，使用在B细胞腔室中表达FcgammaRIIB y -大于F突变体的工程小鼠。

Long-term persistence of antigen-specific antibody and the generation of highly specific memory B cells are well recognized features of the humoral immune response. The specialized microenvironment of the germinal center (GC) may be key to these pathways, in particular, the ability of the follicular dendritic cells (FDC) to retain immunogenic antigen in the form of immune complexes (ICs) for extended periods. It has been proposed that this form of antigen both drives the selection of high affinity B cell receptor somatic mutants during the primary immune response, and maintains the stability of the memory B cell compartment over extended periods thereafter. Mice deficient in the activation Fc receptors FcgammaRI and FcgammaRIII display dramatically enhanced IC deposition and retention on FDCs. However, this does not lead to perturbation of the primary immune response. In Aim 1, we propose to test the hypothesis that this enhanced IC deposition and retention on FDCs will result in elevated levels of persistent serum antibody, memory B cells, and antibody forming cells (AFCs) in the bone marrow. We will also determine whether retention of ICs on FDCs is stable to subsequent immunization with a second antigen, and whether the affinity of persistent serum antibody is lower when cognate antigen levels on FDC are higher. B cell activity during the generation and maintenance of memory may also be regulated by direct interaction of ICs with antigen specific B cells via the inhibitory FcR, FcgammaRIIB. Mice deficient in this FcR display elevated serum antibody and AFC levels after immunization, consistent with this idea and with past in vitro studies on the functioning of this FcR. However, since FcgammaRIIB is the most widely expressed FcR, found on many myeloid lineages and FDCs, this conclusion remains tentative. In Aim 2 we propose to test the hypothesis that the dysregulated expression of antibody production observed in FcgammaRIIB. deficient mice is due to absence of this FcR on B cells. Finally, recent data have shown that the mechanism of inhibition of BCR signaling upon BCR-FcgammaRIIB co-ligation is mediated by recruitment of the inositol polyphosphate phosphatase SHIP to the ITIM motif in the cytoplasmic tail of FcgammaRIIB. A mutant form of FcgammaRIIB with a Y-greater than F change in the ITIM motif, precluding the Y phosphorylation necessary for SHIP binding, does not mediate inhibition of BCR signaling in transfected cell lines. In Aim 3, we propose to test the hypothesis that the ITIM-SHIP pathway is the only FcgammaRIIB initiated pathway necessary for inhibition of B cell activity in vivo, using mice engineered to express the FcgammaRIIB Y-greater than F mutant in the B cell compartment.