Microorganisms have coevolved diverse mechanisms to impair host defenses. A major one, superantigens, can result in devastating effects on the immune system. While all known superantigens induce vast immune cell proliferation and come from opportunistic pathogens, recently, proteins with similar broad specificity to antibody variable (V) domain families were identified in a commensal microbiota. These proteins, identified in the human commensal Ruminococcus gnavus, are called immunoglobulin-binding protein (Ibp) A and B and have been shown to activate B cells in vitro expressing either human VH3 or murine VH5/6/7. Here, we provide molecular and functional studies revealing the basis of this Ibp/immunoglobulin (Ig) interaction. The crystal structure and biochemical assays of a truncated IbpA construct in complex with mouse VH5 antigen-binding fragment (Fab) shows a binding of Ig heavy chain framework residues to the Ibp Domain D and the C-terminal heavy chain binding domain (HCBD). We used targeted mutagenesis of contact residues and affinity measurements and performed studies of the Fab-IbpA complex to determine the stoichiometry between Ibp and VH domains, suggesting Ibp may serve to cluster full-length IgA antibodies in vivo. Furthermore, in vitro stimulation experiments indicate that binding of the Ibp HCBD alone is sufficient to activate responsive murine B cell receptors. The presence of these proteins in a commensal microbe suggest that binding a broad repertoire of immunoglobulins, particularly in the gut/microbiome environment, may provide an important function in the maintenance of host/microbiome homeostasis contrasting with the pathogenic role of structurally homologous superantigens expressed by pathogens.

微生物共同进化出多种机制来削弱宿主的防御能力。其中一种主要因素是超级抗原，它会对免疫系统造成毁灭性影响。虽然所有已知的超级抗原都会诱导大量的免疫细胞增殖，并且来自机会性病原体，但最近，在共生微生物群中发现了与抗体可变 (V) 结构域家族具有相似广泛特异性的蛋白质。这些蛋白质在人类共生瘤胃球菌中被鉴定为免疫球蛋白结合蛋白 (Ibp) A 和 B，并已被证明可以在体外激活表达人 VH3 或鼠 VH5/6/7 的 B 细胞。在这里，我们提供分子和功能研究，揭示了 Ibp/免疫球蛋白 (Ig) 相互作用的基础。与小鼠 VH5 抗原结合片段 (Fab) 复合的截短 IbpA 构建体的晶体结构和生化测定显示 Ig 重链框架残基与 Ibp 结构域 D 和 C 端重链结合结构域 (HCBD) 的结合。我们使用接触残基的靶向诱变和亲和力测量，并对 Fab-IbpA 复合物进行研究，以确定 Ibp 和 VH 结构域之间的化学计量，表明 Ibp 可能有助于在体内聚集全长 IgA 抗体。此外，体外刺激实验表明，单独结合 Ibp HCBD 就足以激活反应性小鼠 B 细胞受体。这些蛋白质在共生微生物中的存在表明，结合广泛的免疫球蛋白，特别是在肠道/微生物组环境中，可能在维持宿主/微生物组稳态方面提供重要功能，这与由免疫球蛋白表达的结构同源超抗原的致病作用形成鲜明对比。病原体。