Department of Microbiology and Immunology, Thomas Jefferson University Philadelphia, PA, USA.
Immun Inflamm Dis. 2015 Sep;3(3):247-64. doi: 10.1002/iid3.64. Epub 2015 Jun 4.
In vitro studies have demonstrated that the immunoreceptor tyrosine-based inhibitory motif (ITIM) of the inhibitory Fc receptor FcγRIIB is critical for mediating attenuation of signaling via immunoreceptor tyrosine-based activation motif (ITAM) containing receptors, such as the B cell antigen receptor (BCR), when FcγRIIB is co-cross-linked to these activation receptors. To test the role of the FcγRIIB ITIM motif in regulation of the B cell immune response in vivo, we constructed lines of transgenic mice expressing a form of FcγRIIB with an inactivating tyrosine (Y) to phenylalanine (F) mutation in the ITIM motif. Detailed studies of one of these lines, in which the mutant FcγRIIB was expressed on B cells and other cell types that normally express this receptor, were performed. No quantitative differences in germinal center (GC) B cell responses were observed between the mutant FcγRIIB transgenic line and control mice. However, serum antibody and antibody forming cell responses were often observed to be elevated in the ITIM mutant FcγRIIB transgenic mice as compared to controls, though not to the same extent as mice deficient in expression of FcγRIIB. Moreover, primary B cells from the ITIM mutant FcγRIIB line did not display the same level of augmented BCR signaling as primary FcγRIIB deficient B cells under conditions inducing co-cross-linking of FcγRIIB and the BCR. In total, these data suggest that a functional ITIM motif is not required for all in vivo inhibitory activity of this receptor. However, we also found that the transgenic ITIM mutant FcγRIIB receptor was expressed at abnormal levels in several hematopoietic lineages. Thus, confirmation of our findings will require the generation and analysis of mice in which an ITIM mutant form of FcγRIIB is expressed in vivo as is the endogenous receptor.
体外研究表明，抑制性 Fc 受体 FcγRIIB 的免疫受体酪氨酸抑制基序（ITIM）对于介导通过包含免疫受体酪氨酸激活基序（ITAM）的受体（如 B 细胞抗原受体（BCR））的信号转导衰减至关重要，当 FcγRIIB 与这些激活受体共交联时。为了测试 FcγRIIB ITIM 基序在体内调节 B 细胞免疫反应中的作用，我们构建了表达一种 FcγRIIB 形式的转基因小鼠，该形式的 FcγRIIB 在 ITIM 基序中具有失活的酪氨酸（Y）到苯丙氨酸（F）突变。对其中一条线进行了详细研究，其中突变型 FcγRIIB 在通常表达这种受体的 B 细胞和其他细胞类型上表达。在突变型 FcγRIIB 转基因线和对照小鼠之间，未观察到生发中心（GC）B 细胞反应的定量差异。然而，与对照相比，经常观察到突变型 FcγRIIB 转基因小鼠的血清抗体和抗体形成细胞反应升高，尽管不如 FcγRIIB 表达缺失的小鼠升高程度高。此外，在诱导 FcγRIIB 和 BCR 共交联的条件下，来自 ITIM 突变型 FcγRIIB 系的原代 B 细胞并未显示出与原发性 FcγRIIB 缺乏 B 细胞相同水平的增强 BCR 信号。总的来说，这些数据表明，功能性 ITIM 基序对于该受体的所有体内抑制活性并非必需。然而，我们还发现，在几种造血谱系中，转基因 ITIM 突变型 FcγRIIB 受体的表达水平异常。因此，需要生成和分析在体内表达 ITIM 突变型 FcγRIIB 形式的受体和内源性受体的小鼠来证实我们的发现。
