Centre for Innovation, Canadian Blood Services, Ottawa, ON, Canada.
Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON, Canada.
Blood. 2021 Apr 15;137(15):2114-2124. doi: 10.1182/blood.2020009497.
Monoclonal immunoglobulin G (IgG) antibodies to CD44 (anti-CD44) are anti-inflammatory in numerous murine autoimmune models, but the mechanisms are poorly understood. Anti-CD44 anti-inflammatory activity shows complete therapeutic concordance with IV immunoglobulin (IVIg) in treating autoimmune disease models, making anti-CD44 a potential IVIg alternative. In murine immune thrombocytopenia (ITP), there is no mechanistic explanation for anti-CD44 activity, although anti-CD44 ameliorates disease similarly to IVIg. Here, we demonstrate a novel anti-inflammatory mechanism of anti-CD44 that explains disease amelioration by anti-CD44 in murine ITP. Macrophages treated with anti-CD44 in vitro had dramatically suppressed phagocytosis through FcγRs in 2 separate systems of IgG-opsonized platelets and erythrocytes. Phagocytosis inhibition by anti-CD44 was mediated by blockade of the FcγR IgG binding site without changing surface FcγR expression. Anti-CD44 of different subclasses revealed that FcγR blockade was specific to receptors that could be engaged by the respective anti-CD44 subclass, and Fc-deactivated anti-CD44 variants lost all FcγR-inhibiting activity. In vivo, anti-CD44 functioned analogously in the murine passive ITP model and protected mice from ITP when thrombocytopenia was induced through an FcγR that could be engaged by the CD44 antibody's subclass. Consistent with FcγR blockade, Fc-deactivated variants of anti-CD44 were completely unable to ameliorate ITP. Together, anti-CD44 inhibits macrophage FcγR function and ameliorates ITP consistent with an FcγR blockade mechanism. Anti-CD44 is a potential IVIg alternative and may be of particular benefit in ITP because of the significant role that FcγRs play in human ITP pathophysiology.
单克隆免疫球蛋白 G (IgG) 抗 CD44（抗-CD44）在许多鼠类自身免疫模型中具有抗炎作用，但机制尚不清楚。抗-CD44 的抗炎活性与 IV 免疫球蛋白 (IVIg) 在治疗自身免疫疾病模型中具有完全的治疗一致性，使其成为 IVIg 的潜在替代品。在鼠类免疫性血小板减少症 (ITP) 中，尽管抗-CD44 与 IVIg 相似地改善疾病，但没有抗-CD44 活性的机制解释。在这里，我们展示了抗-CD44 的一种新的抗炎机制，该机制解释了抗-CD44 在鼠类 ITP 中的疾病改善作用。体外用抗-CD44 处理的巨噬细胞通过 2 种独立的 IgG 调理血小板和红细胞系统，显著抑制吞噬作用。抗-CD44 通过阻断 FcγR IgG 结合位点而不改变表面 FcγR 表达来抑制吞噬作用。不同亚类的抗-CD44 表明 FcγR 阻断是针对可以与各自抗-CD44 亚类结合的受体的特异性阻断，并且 Fc 失活的抗-CD44 变体失去了所有 FcγR 抑制活性。在体内，抗-CD44 在鼠类被动性 ITP 模型中发挥类似作用，并在通过可以与 CD44 抗体的亚类结合的 FcγR 诱导血小板减少症时保护小鼠免受 ITP 影响。与 FcγR 阻断一致，Fc 失活的抗-CD44 变体完全不能改善 ITP。总之，抗-CD44 抑制巨噬细胞 FcγR 功能，并改善 ITP，这与 FcγR 阻断机制一致。抗-CD44 是 IVIg 的潜在替代品，由于 FcγR 在人类 ITP 病理生理学中发挥重要作用，因此在 ITP 中可能特别有益。
