Department of Public Health and Caring Science/Molecular Geriatrics, Rudbecklaboratoriet, Uppsala University, Dag Hammarskjölds Väg 20, Uppsala, Sweden.
Department of Pharmaceutical Biosciences/Protein Drug Design, BMC, Uppsala University, Husargatan 3, Uppsala, Sweden.
Mol Imaging Biol. 2021 Oct;23(5):665-675. doi: 10.1007/s11307-021-01591-3. Epub 2021 Feb 23.
The triggering receptor expressed on myeloid cells 2 (TREM2) is expressed by brain microglia. Microglial activation, as observed in Alzheimer's disease (AD) as well as in transgenic mice expressing human amyloid-beta, appears to increase soluble TREM2 (sTREM2) levels in CSF and brain. In this study, we used two different transgenic mouse models of AD pathology and investigated the potential of TREM2 to serve as an in vivo biomarker for microglial activation in AD.
We designed and generated a bispecific antibody based on the TREM2-specific monoclonal antibody mAb1729, fused to a single-chain variable fragment of the transferrin receptor binding antibody 8D3. The 8D3-moiety enabled transcytosis of the whole bispecific antibody across the blood-brain barrier. The bispecific antibody was radiolabeled with I-125 (ex vivo) or I-124 (PET) and administered to transgenic AD and wild-type (WT) control mice. Radioligand retention in the brain of transgenic animals was compared to WT mice by isolation of brain tissue at 24 h or 72 h, or with in vivo PET at 24 h, 48 h, and 72 h. Intrabrain distribution of radiolabeled mAb1729-scFv8D3 was further studied by autoradiography, while ELISA was used to determine TREM2 brain concentrations.
Transgenic animals displayed higher total exposure, calculated as the AUC based on SUV determined at 24h, 48h, and 72h post injection, of PET radioligand [I]mAb1729-scFv8D3 than WT mice. However, differences were not evident in single time point PET images or SUVs. Ex vivo autoradiography confirmed higher radioligand concentrations in cortex and thalamus in transgenic mice compared to WT, and TREM2 levels in brain homogenates were considerably higher in transgenic mice compared to WT.
Antibody-based radioligands, engineered to enter the brain, may serve as PET radioligands to follow changes of TREM2 in vivo, but antibody formats with faster systemic clearance to increase the specific signal in relation to that from blood in combination with antibodies showing higher affinity for TREM2 must be developed to further progress this technique for in vivo use.
髓系细胞表达的触发受体 2（TREM2）在脑小胶质细胞中表达。在阿尔茨海默病（AD）以及表达人淀粉样蛋白-β的转基因小鼠中观察到的小胶质细胞激活似乎会增加 CSF 和脑内可溶性 TREM2（sTREM2）的水平。在这项研究中，我们使用了两种不同的 AD 病理转基因小鼠模型，并研究了 TREM2 作为 AD 中小胶质细胞激活的体内生物标志物的潜力。
我们设计并生成了一种基于 TREM2 特异性单克隆抗体 mAb1729 的双特异性抗体，与转铁蛋白受体结合抗体 8D3 的单链可变片段融合。8D3 部分使整个双特异性抗体能够穿过血脑屏障进行转胞吞作用。用 I-125（体外）或 I-124（PET）放射性标记双特异性抗体，并将其施用于转基因 AD 和野生型（WT）对照小鼠。通过在 24 小时或 72 小时分离脑组织，或在 24 小时、48 小时和 72 小时进行体内 PET，比较转基因动物和 WT 小鼠脑中放射性配体的保留情况。通过放射自显影进一步研究放射性标记 mAb1729-scFv8D3 的脑内分布，而 ELISA 用于确定 TREM2 脑浓度。
与 WT 小鼠相比，转基因动物在注射后 24h、48h 和 72h 基于 SUV 确定的 AUC 计算的总暴露（[I]mAb1729-scFv8D3）更高。然而，在单个时间点的 PET 图像或 SUV 中没有明显差异。体外放射自显影证实，与 WT 相比，转基因小鼠皮质和丘脑的放射性配体浓度更高，而与 WT 相比，转基因小鼠脑匀浆中的 TREM2 水平要高得多。
设计用于进入大脑的抗体放射性配体可作为 PET 放射性配体，用于在体内跟踪 TREM2 的变化，但必须开发具有更快系统清除率的抗体格式，以增加与血液相关的特异性信号，并结合对 TREM2 具有更高亲和力的抗体，以进一步推进该技术在体内的应用。
