IgMs are the first antibodies produced by the immune system upon encounter of a possible pathogen and are one of five antibody subclasses in humans. For IgG, the most intensively studied antibody class, the N-linked glycosylation site located in the Fc-domain is directly involved in high affinity binding to the respective receptors and initiation of corresponding immune response. IgM molecules have five N-glycosylation sites and one N-glycosylation site in the J-chain, which can be incorporated in IgM or IgA molecules. There is only limited knowledge available concerning the function of these N-glycosylations in IgMs. To address this question, we produced IgM molecules lacking a particular N-glycosylation site and tested these variants as well as IgA molecules for binding to the known receptors: the polymeric immunoglobulin receptor (pIgR), the dual receptor for IgA and IgM, FcαμR, and the specific receptor for IgM, FcμR. The single glycosylation sites did not show an impact on expression and multimerization, except for variant N402Q, which could not be expressed. In SPR measurements, no major impact on the binding to the receptors by particular glycosylation sites could be detected. In cellular assays, deglycosylated variants showed some alterations in induction of CDC activity. Most strikingly, we observed also binding of IgA to the FcμR in the same affinity range as IgM, suggesting that this might have a physiological role. To further substantiate the binding of IgA to FcμR we used IgA from different origins and were able to confirm binding of IgA preparations to the FcμR.

中文翻译：

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IgM 和 IgA 免疫球蛋白的功能研究：与 pIgR、FcαμR、FcμR 和 CDC 活性的结合

IgM 是免疫系统在遇到可能的病原体时产生的第一种抗体，是人类五种抗体亚类之一。对于 IgG（研究最深入的抗体类型），位于 Fc 结构域的 N 连接糖基化位点直接参与与相应受体的高亲和力结合并引发相应的免疫反应。IgM 分子在 J 链上有 5 个 N-糖基化位点和 1 个 N-糖基化位点，可以掺入 IgM 或 IgA 分子中。关于 IgM 中这些 N-糖基化的功能，目前所知有限。为了解决这个问题，我们生产了缺乏特定 N-糖基化位点的 IgM 分子，并测试了这些变体以及 IgA 分子与已知受体的结合：聚合免疫球蛋白受体 (pIgR)、IgA 和 IgM 的双重受体、FcαμR、以及IgM的特异性受体FcμR。除了无法表达的变体 N402Q 外，单个糖基化位点并未显示出对表达和多聚化的影响。在 SPR 测量中，未检测到特定糖基化位点对受体结合的重大影响。在细胞测定中，去糖基化变体在 CDC 活性的诱导方面显示出一些改变。最引人注目的是，我们还观察到 IgA 与 FcμR 的结合与 IgM 的亲和力范围相同，表明这可能具有生理作用。为了进一步证实 IgA 与 FcμR 的结合，我们使用了不同来源的 IgA，并且能够确认 IgA 制剂与 FcμR 的结合。