Blood Research Institute, BloodCenter of Wisconsin, Milwaukee, WI.
Immunology Research Group, Department of Medical Biology, The Arctic University of Norway, Tromsø, Norway.
Blood Adv. 2018 Nov 13;2(21):3001-3011. doi: 10.1182/bloodadvances.2018023341.
Antibodies to platelet-specific antigens are responsible for 2 clinically important bleeding disorders: posttransfusion purpura and fetal/neonatal alloimmune thrombocytopenia (FNAIT). The human platelet-specific alloantigen 1a/1b (HPA-1a/1b; also known as Pl) alloantigen system of human platelet membrane glycoprotein (GP) IIIa is controlled by a Leu33Pro polymorphism and is responsible for ∼80% of the cases of FNAIT. Local residues surrounding polymorphic residue 33 are suspected to have a profound effect on alloantibody binding and subsequent downstream effector events. To define the molecular requirements for HPA-1a alloantibody binding, we generated transgenic mice that expressed murine GPIIIa (muGPIIIa) isoforms harboring select humanized residues within the plexin-semaphorin-integrin (PSI) and epidermal growth factor 1 (EGF1) domains and examined their ability to support the binding of a series of monoclonal and polyclonal HPA-1a-specific antibodies. Humanizing the PSI domain of muGPIIIa was sufficient to recreate the HPA-1a epitope recognized by some HPA-1a-specific antibodies; however, humanizing distinct amino acids within the linearly distant but conformationally close EGF1 domain was required to enable binding of others. These results reveal the previously unsuspected complex heterogeneity of the polyclonal alloimmune response to this clinically important human platelet alloantigen system. High-resolution mapping of this alloimmune response may improve diagnosis of FNAIT and should facilitate the rational design and selection of contemplated prophylactic and therapeutic anti-HPA-1a reagents.
血小板特异性抗原的抗体是导致 2 种临床重要的出血性疾病的原因：输血后紫癜和胎儿/新生儿同种免疫性血小板减少症（FNAIT）。人类血小板特异性同种抗原 1a/1b（HPA-1a/1b；也称为 Pl）同种抗原系统是由人类血小板膜糖蛋白（GP）IIIa 的 Leu33Pro 多态性控制的，负责约 80%的 FNAIT 病例。多态性残基 33 周围的局部残基被怀疑对同种抗体结合和随后的下游效应事件有深远影响。为了定义 HPA-1a 同种抗体结合的分子要求，我们生成了表达小鼠 GPIIIa（muGPIIIa）同种型的转基因小鼠，这些同种型在 plexin-semaphorin-integrin（PSI）和表皮生长因子 1（EGF1）结构域内携带选择性的人源化残基，并检查了它们支持一系列单克隆和多克隆 HPA-1a 特异性抗体结合的能力。人源化 muGPIIIa 的 PSI 结构域足以重新创建一些 HPA-1a 特异性抗体识别的 HPA-1a 表位；然而，需要在线性上相隔但构象上接近的 EGF1 结构域内人源化不同的氨基酸，才能使其他抗体结合。这些结果揭示了对这种临床重要的人类血小板同种抗原系统的多克隆同种免疫反应的先前未被怀疑的复杂异质性。对这种同种免疫反应的高分辨率作图可能会改善 FNAIT 的诊断，并应有助于合理设计和选择预期的预防性和治疗性抗 HPA-1a 试剂。
