Veterans Affairs Medical Center and the Department of Medicine, University of Tennessee Health Science Center, Memphis, Tennessee, United States of America.
PLoS One. 2013 Oct 25;8(10):e78719. doi: 10.1371/journal.pone.0078719. eCollection 2013.
The non-immune binding of immunoglobulins by bacteria is thought to contribute to the pathogenesis of infections. M-related proteins (Mrp) are group A streptococcal (GAS) receptors for immunoglobulins, but it is not known if this binding has any impact on virulence. To further investigate the binding of immunoglobulins to Mrp, we engineered mutants of an M type 4 strain of GAS by inactivating the genes for mrp, emm, enn, sof, and sfbX and tested these mutants in IgG-binding assays. Inactivation of mrp dramatically decreased the binding of human IgG, whereas inactivation of emm, enn, sof, and sfbx had only minor effects, indicating that Mrp is a major IgG-binding protein. Binding of human immunoglobulins to a purified, recombinant form of Mrp indicated that it selectively binds to the Fc domain of human IgG, but not IgA or IgM and that it preferentially bound subclasses IgG₁>IgG₄>IgG₂>IgG₃. Recombinant proteins encompassing different regions of Mrp were engineered and used to map its IgG-binding domain to its A-repeat region and a recombinant protein with 3 A-repeats was a better inhibitor of IgG binding than one with a single A-repeat. A GAS mutant expressing Mrp with an in-frame deletion of DNA encoding the A-repeats had a dramatically reduced ability to bind human IgG and to grow in human blood. Mrp exhibited host specificity in binding IgG; human IgG was the best inhibitor of the binding of IgG followed by pig, horse, monkey, and rabbit IgG. IgG from goat, mouse, rat, cow, donkey, chicken, and guinea pig were poor inhibitors of binding. These findings indicate that Mrp preferentially binds human IgG and that this binding contributes to the ability of GAS to resist phagocytosis and may be a factor in the restriction of GAS infections to the human host.
细菌对免疫球蛋白的非免疫结合被认为有助于感染的发病机制。M 相关蛋白（Mrp）是 A 组链球菌（GAS）的免疫球蛋白受体，但尚不清楚这种结合是否对毒力有任何影响。为了进一步研究免疫球蛋白与 Mrp 的结合，我们通过失活 mrp、emm、enn、sof 和 sfbX 基因对 M 型 4 株 GAS 进行了工程突变，并在 IgG 结合测定中测试了这些突变体。Mrp 的失活显著降低了人 IgG 的结合，而 emm、enn、sof 和 sfbX 的失活仅有轻微影响，表明 Mrp 是主要的 IgG 结合蛋白。人免疫球蛋白与人源 Mrp 重组形式的结合表明，它选择性地与人 IgG 的 Fc 结构域结合，而不与 IgA 或 IgM 结合，并且它优先结合 IgG₁>IgG₄>IgG₂>IgG₃ 亚类。设计了涵盖 Mrp 不同区域的重组蛋白，并用于将其 IgG 结合结构域映射到其 A 重复区，并且具有 3 个 A 重复的重组蛋白比具有单个 A 重复的重组蛋白更能抑制 IgG 结合。表达缺失编码 A 重复的 DNA 的 Mrp 的 GAS 突变体与人 IgG 结合的能力显著降低，并且在人血液中生长的能力降低。Mrp 在结合 IgG 时表现出宿主特异性；人 IgG 是抑制 IgG 结合的最佳抑制剂，其次是猪、马、猴和兔 IgG。来自山羊、小鼠、大鼠、牛、驴、鸡和豚鼠的 IgG 是较差的结合抑制剂。这些发现表明 Mrp 优先结合人 IgG，并且这种结合有助于 GAS 抵抗吞噬作用的能力，并且可能是 GAS 感染局限于人类宿主的一个因素。
