Antibodies may bind to bacterial pathogens or their toxins to control infections, and their effector activity is mediated through the recruitment of complement component C1q or the engagement with Fcγ receptors (FcγRs). For bacterial pathogens that rely on a single toxin to cause disease, immunity correlates with toxin neutralization. Most other bacterial pathogens, including Staphylococcus aureus, secrete numerous toxins and evolved multiple mechanisms to escape opsonization and complement killing. Several vaccine candidates targeting defined surface antigens of S. aureus have failed to meet clinical endpoints. It is unclear that such failures can be solely attributed to the poor selection of antibody targets. Thus far, studies to delineate antibody-mediated uptake and killing of Gram-positive pathogens remain extremely limited. Here, we exploit 3F6-hIgG1, a human monoclonal antibody that binds and neutralizes the abundant surface-exposed Staphylococcal protein A (SpA). We find that galactosylation of 3F6-hIgG1 that favors C1q recruitment is indispensable for opsonophagocytic killing of staphylococci and for protection against bloodstream infection in animals. However, the simple removal of fucosyl residues, which results in reduced C1q binding and increased engagement with FcγR, maintains the opsonophagocytic killing and protective attributes of the antibody. We confirm these results by engineering 3F6-hIgG1 variants with biased binding toward C1q or FcγRs. While the therapeutic benefit of monoclonal antibodies against infectious disease agents may be debatable, the functional characterization of such antibodies represents a powerful tool for the development of correlates of protection that may guide future vaccine trials.

抗体可以与细菌病原体或其毒素结合以控制感染，并且其效应子活性是通过募集补体成分C1q或与Fcγ受体（FcγRs）介导的。对于依靠单一毒素引起疾病的细菌病原体，免疫力与毒素中和相关。大多数其他细菌性病原体，包括金黄色葡萄球菌，都分泌大量毒素，并进化出多种机制来逃避调理作用和补充杀伤作用。几种靶向金黄色葡萄球菌表面抗原的候选疫苗未能达到临床指标。尚不清楚此类失败是否仅归因于抗体靶标的选择不当。迄今为止，描述抗体介导的摄取和杀死革兰氏阳性病原体的研究仍然非常有限。在这里，我们利用3F6-hIgG1，这是一种人类单克隆抗体，可以结合并中和大量表面暴露的葡萄球菌蛋白A（SpA）。我们发现，有利于C1q募集的3F6-hIgG1的半乳糖基化对于调理吞噬葡萄球菌并防止动物受到血流感染是必不可少的。但是，岩藻糖基残基的简单去除会导致C1q结合减少并与FcγR的结合增加，从而维持了抗体的调理吞噬作用和保护特性。我们通过工程化具有偏向C1q或FcγRs的3F6-hIgG1变体来证实这些结果。尽管针对传染病原药的单克隆抗体的治疗优势尚有争议，但此类抗体的功能表征是开发保护性相关因子的有力工具，可指导未来的疫苗试验。