The Canadian Blood Services, Toronto, Canada.
PLoS One. 2013 Aug 5;8(8):e71882. doi: 10.1371/journal.pone.0071882. Print 2013.
Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by a low platelet count and the production of anti-platelet antibodies. The majority of ITP patients have antibodies to platelet integrin α(IIb)β₃ (GPIIbIIIa) which can direct platelet phagocytosis by macrophages. One effective treatment for patients with ITP is intravenous immunoglobulin (IVIg) which rapidly reverses thrombocytopenia. The exact mechanism of IVIg action in human patients is unclear, although in mouse models of passive ITP, IVIg can rapidly increase platelet counts in the absence of adaptive immunity. Another antibody therapeutic that can similarly increase platelet counts independent of adaptive immunity are CD44 antibodies. Toll-like receptors (TLRs) are pattern recognition receptors which play a central role in helping direct the innate immune system. Dendritic cells, which are notable for their expression of TLRs, have been directly implicated in IVIg function as an initiator cell, while CD44 can associate with TLR2 and TLR4. We therefore questioned whether IVIg, or the therapeutic CD44 antibody KM114, mediate their ameliorative effects in a manner dependent upon normal TLR function. Here, we demonstrate that the TLR4 agonist LPS does not inhibit IVIg or KM114 amelioration of antibody-induced thrombocytopenia, and that these therapeutics do not ameliorate LPS-induced thrombocytopenia. IVIg was able to significantly ameliorate murine ITP in C3H/HeJ mice which have defective TLR4. All known murine TLRs except TLR3 utilize the Myd88 adapter protein to drive TLR signaling. Employing Myd88 deficient mice, we found that both IVIg and KM114 ameliorate murine ITP in Myd88 deficient mice to the same extent as normal mice. Thus both IVIg and anti-CD44 antibody can mediate their ameliorative effects in murine passive ITP independent of the Myd88 signaling pathway. These data help shed light on the mechanism of action of IVIg and KM114 in the amelioration of murine ITP.
免疫性血小板减少症（ITP）是一种自身免疫性出血性疾病，其特征为血小板计数降低和抗血小板抗体的产生。大多数 ITP 患者的抗体针对血小板整合素 α(IIb)β₃（GPIIbIIIa），该抗体可引导巨噬细胞吞噬血小板。静脉注射免疫球蛋白（IVIg）是治疗 ITP 的有效方法，可迅速逆转血小板减少症。IVIg 在人类患者中的确切作用机制尚不清楚，尽管在被动性 ITP 的小鼠模型中，IVIg 可在无适应性免疫的情况下迅速增加血小板计数。另一种可类似地独立于适应性免疫增加血小板计数的抗体治疗药物是 CD44 抗体。Toll 样受体（TLR）是模式识别受体，在帮助指导固有免疫系统方面发挥着核心作用。树突状细胞因其表达 TLR 而引人注目，它们直接参与 IVIg 作为起始细胞的功能，而 CD44 可与 TLR2 和 TLR4 结合。因此，我们质疑 IVIg 或治疗性 CD44 抗体 KM114 是否以依赖于正常 TLR 功能的方式介导其改善作用。在这里，我们证明 TLR4 激动剂 LPS 不会抑制 IVIg 或 KM114 改善抗体诱导的血小板减少症，并且这些治疗药物不会改善 LPS 诱导的血小板减少症。IVIg 能够在 TLR4 缺陷的 C3H/HeJ 小鼠中显著改善小鼠 ITP。除 TLR3 外，所有已知的小鼠 TLR 均利用 Myd88 衔接蛋白来驱动 TLR 信号传导。使用 Myd88 缺陷型小鼠，我们发现 IVIg 和 KM114 均可在 Myd88 缺陷型小鼠中改善 ITP，其效果与正常小鼠相同。因此，IVIg 和抗 CD44 抗体均可在不依赖于 Myd88 信号通路的情况下在小鼠被动性 ITP 中介导其改善作用。这些数据有助于阐明 IVIg 和 KM114 在改善小鼠 ITP 中的作用机制。
