Deciphering the Function and Regulation of the B Cell Intrinsic a2,6-Sialylation Network

糖基化是一种重要的翻译后修饰，对许多蛋白质的稳定性和功能起着至关重要的作用。在第一个资助期，P3项目的重点是研究a2,6唾液酸残基在稳态和激活状态下对小鼠B细胞发育和功能的影响。为了详细研究这一点，我们建立了一个B细胞特异性敲除ST6Gal1的体内模型，这种酶将末端a2,6唾液酸残基添加到抗体和其他细胞表面受体上的糖结构上。使用这种小鼠模型可以进行一些有趣的观察。首先，我们能够证明IgG唾液化发生在B细胞内，而不是IgG分泌后，正如最近提出的那样。此外，通过研究这些小鼠的B细胞发育，我们发现成熟B细胞亚群的数量，包括滤泡B细胞、边缘区B细胞、生发中心B细胞和脾脏的浆母细胞/浆细胞和腹膜腔的B2细胞都大大减少。与产生抗体的B细胞亚群发育的缺陷一致，我们注意到血清IgG水平严重降低。令人惊讶的是，ST6Gal1缺失对IgG亚类的特异性作用在稳态期间变得明显。因此，虽然血清IgG2c几乎不存在，但IgG1， IgG2b和IgG3水平仅受到轻微影响，这表明在稳定状态下，a2,6链唾液酸残基对IgG亚类转换有影响。然而，接种T细胞非依赖型和T细胞依赖型抗原后，IgG1/IgG2c或IgG2c抗原特异性抗体反应可分别引起降低但明显可检测的水平。这可能表明，通过toll样受体或其他共刺激分子的共刺激信号可能能够克服稳定状态下IgG2c开关的缺陷。在第二个资助期，我们希望以第一个资助期的结果为基础，研究B细胞亚群和单细胞水平，在那里和何时表达决定a2,6-连接蛋白水平的酶。对B细胞激活和类别转换至关重要的不同信号通路如何受到a2,6-链唾液酸残基缺乏的影响，以及在近交小鼠模型中观察到的表型如何通过在人B细胞中敲低或敲除ST6Gal1的人源化小鼠的产生转化为人类免疫系统。

Glycosylation is an essential post-translational modification and plays a critical role for the stability and function of many proteins. In the first funding period the focus of project P3 was to study the impact of a2,6 sialic acid residues on mouse B cell development and function during steady state and activation. To study this in detail, we generated an in vivo model for a B cell specific knockout of ST6Gal1, the enzyme that adds terminal a2,6 sialic acid residues to sugar structures present on antibodies and other cell surface receptors. Using this mouse model allowed to make several interesting observations. Firstly, we were able to demonstrate that IgG sialylation occurs within B cells and not post IgG secretion, as suggested more recently. Moreover, by investigating B cell development in these mice, we show that the number of mature B cell subsets including follicular B cells, marginal zone B cells, germinal centre B cells and plasma blasts/plasma cells in the spleen and B2 cells in the peritoneal cavity was strongly reduced. In line with this defect in development of antibody producing B cell subsets, we noted a severe reduction in serum IgG levels. Surprisingly, a strong IgG subclass specific effect of a deletion of ST6Gal1 became evident during the steady-state. Thus, while serum IgG2c was virtually absent, IgG1, IgG2b and IgG3 levels were only slightly affected, suggesting an impact of a2,6-linked sialic acid residues on IgG subclass switching during the steady state. Upon vaccination with T cell-independent and T cell- dependent antigens, however, a reduced but clearly detectable level of IgG1/IgG2c or IgG2c antigen-specific antibody responses could be elicited, respectively. This may suggest, that co-stimulatory signals via toll-like receptors or other co-stimulatory molecules may be able to overcome the defect in IgG2c switching during the steady state. In the second funding period we would like build on the results of the first funding period and study on a B cell subpopulation and single cell level where and when enzymes determining the level of a2,6-linked protein sialyation are expressed, how different signaling pathways critical for B cell activation and class-switching are affected by a lack of a2,6-linked sialic acid residues and how the phenotypes observed in inbred mouse models can be translated to the human immune system via the generation of humanized mice with a knockdown or knockout of ST6Gal1 in human B cells.