Binding to the neonatal Fc receptor (FcRn) extends serum half-life of IgG, and antagonizing this interaction is a promising therapeutic approach in IgG-mediated autoimmune diseases. Fc-MST-HN, designed for enhanced FcRn binding capacity, has not been evaluated in the context of a full-length antibody, and the structural properties of the attached Fab regions might affect the FcRn-mediated intracellular trafficking pathway. Here we present a comprehensive comparative analysis of the IgG salvage pathway between two full-size IgG1 variants, containing wild type and MST-HN Fc fragments, and their Fc-only counterparts. We find no evidence of Fab-regions affecting FcRn binding in cell-free assays, however, cellular assays show impaired binding of full-size IgG to FcRn, which translates into improved intracellular FcRn occupancy and intracellular accumulation of Fc-MST-HN compared to full size IgG1-MST-HN. The crystal structure of Fc-MST-HN in complex with FcRn provides a plausible explanation why the Fab disrupts the interaction only in the context of membrane-associated FcRn. Importantly, we find that Fc-MST-HN outperforms full-size IgG1-MST-HN in reducing IgG levels in cynomolgus monkeys. Collectively, our findings identify the cellular membrane context as a critical factor in FcRn biology and therapeutic targeting.

与新生儿 Fc 受体 (FcRn) 的结合可延长 IgG 的血清半衰期，而拮抗这种相互作用是治疗 IgG 介导的自身免疫性疾病的一种有前途的治疗方法。为增强 FcRn 结合能力而设计的 Fc-MST-HN 尚未在全长抗体的背景下进行评估，并且所连接的 Fab 区域的结构特性可能会影响 FcRn 介导的细胞内运输途径。在这里，我们对包含野生型和 MST-HN Fc 片段的两种全尺寸 IgG1 变体及其仅 Fc 对应物的 IgG 补救途径进行了综合比较分析。我们没有发现 Fab 区域影响 FcRn 在无细胞试验中结合的证据，然而，细胞试验显示全尺寸 IgG 与 FcRn 的结合受损，与全尺寸 IgG1-MST-HN 相比，这转化为改进的 Fc-MST-HN 细胞内 FcRn 占据和细胞内积累。与 FcRn 复合的 Fc-MST-HN 的晶体结构提供了 Fab 仅在膜相关 FcRn 的背景下破坏相互作用的合理解释。重要的是，我们发现 Fc-MST-HN 在降低食蟹猴 IgG 水平方面优于全尺寸 IgG1-MST-HN。总的来说，我们的研究结果将细胞膜环境确定为 FcRn 生物学和治疗靶向的关键因素。我们发现 Fc-MST-HN 在降低食蟹猴 IgG 水平方面优于全尺寸 IgG1-MST-HN。总的来说，我们的研究结果将细胞膜环境确定为 FcRn 生物学和治疗靶向的关键因素。我们发现 Fc-MST-HN 在降低食蟹猴 IgG 水平方面优于全尺寸 IgG1-MST-HN。总的来说，我们的研究结果将细胞膜环境确定为 FcRn 生物学和治疗靶向的关键因素。