来自美国基因科技公司(GENENTECH INC.)的研究人员在新研究中绕过抗体合成的常规法则，合成了一种能够通过血脑屏障的且具有双特异性的新型基因工程抗体。这一新技术或可在将来推动科学家们开发出基于抗体的脑病治疗新方法。研究人员在5月25日的《科学—转化医学》（Science Translational Medicine）同时发表了两篇文章，详述地描述了这一抗体的设计过程。
抗体是生物体内免疫系统用于中和有害的外源物质的一种特异的蛋白质，捕获天然抗体或人工制备高度特异性的抗体一直是蛋白质组学研究中的热点领域。近年来各个研究机构及企业的研究人员也在致力于构建具有多靶向性的抗体。
“合成双特异性抗体的新浪潮即将来到，”Genentech公司的神经生物学家Ryan Watts说道：“它们将成为这一领域中的研究热点。”
血脑屏障是机体参与固有免疫的一个重要的内部屏障，它能够阻挡病原生物和其他大分子由血循环进入脑组织和脑室。从而保护脑组织避免循环血液中有毒物质的损害。由于抗体通常无法通过血脑屏障，因此在大脑中抗体的浓度约比在血液中要低一千倍。
在新研究中，Watts等合成了一种能通过血脑屏障且具有双蛋白质靶向性的抗体。其靶向的β分泌酶（β-secretase）是当前阿尔茨海默症治疗的一个重要的药物开发靶点。过去的研究证实β-secretase在大脑的淀粉样肽生成中起重要作用。
这一抗体靶向的第二种蛋白质则是转铁蛋白受体（transferrin receptor）。在正常情况转铁蛋白受体可通过与转铁蛋白的相互作用介导大脑中的铁摄取。研究人员利用转铁蛋白将抗体输送至大脑中，从而确保其能在大脑中作用于β-secretase。
研究人员证实这种双特性抗体能在阿尔茨海默症模型小鼠中很好地发挥作用。在每天接受一次抗体注射后，小鼠大脑中的β淀粉样蛋白的浓度下降了47%。
“我们设计合成这一新型抗体是基于一种独特的理念，挑战了抗体工程中的一个重要的法则，”Watts说。
抗体与抗原之间的相互作用力通常被称之为抗体的亲和力（affinity）。亲和力越高则表明抗体与抗原之间的相互作用力越强。长期以来大多数的抗体工程技术人员都致力于合成出具有高亲和力的抗体，从而确保抗体能与抗原紧密的结合。
Watts和另一位抗体工程人员Mark Dennis在最初是希望能够合成出对转铁蛋白具有高亲和力的抗体。然而他们却发现这些高亲和力的抗体被阻止在血管中无法通过脑组织。Dennis由此推测抗体有可能是被转铁蛋白受体截留在了血管中，于是他开始设计合成低亲和力的抗体。正如Dennis所预想的一样，合成的低亲和力抗体能够更广泛地分布到大脑中。
“这是一个极好的范例，它表明在考虑多特异性蛋白质治疗时，我们必须抛开某些我们从单克隆抗体中学习到的规则和模式，”这一研究的负责人David Hilbert说。
目前Hilbert正在努力研发能同时识别5种不同靶蛋白的多特异性抗体。Hilbert认为多特异性抗体的设计理念还可运用到其他的领域，例如癌干细胞中。科学家们通常根据其表达的细胞表面标志蛋白来识别癌干细胞。然而由于这些表面标志有时候也存在于其他的一些健康细胞中。常规的高亲和力单克隆抗体会在靶向癌细胞的同时杀伤健康细胞，而低亲和力的多靶向性抗体则能够更具有选择性地靶向癌干细胞。（生物谷Bioon.com）
生物谷推荐原文：
Science Translational Medicine DOI： 10.1126/scitranslmed.3002254
A Therapeutic Antibody Targeting BACE1 Inhibits Amyloid-β Production in Vivo
Atwal, Jasvinder K.; Chen, Yongmei; Chiu, Cecilia; Mortensen, Deborah L.; Meilandt, William J.; Liu, Yichin; Heise, Christopher E.; Hoyte, Kwame; Luk, Wilman; Lu, Yanmei; Peng, Kun; Wu, Ping; Rouge, Lionel; Zhang, Yingnan; Lazarus, Robert A.; Scearce-Levie, Kimberly; Wang, Weiru; Wu, Yan; Tessier-Lavigne, Marc; Watts, Ryan J.
Reducing production of amyloid-β (Aβ) peptide by direct inhibition of the enzymes that process amyloid precursor protein (APP)is a central therapeutic strategy for treating Alzheimer’s disease. However, small-molecule inhibitors of the β-secretase(BACE1) and γ-secretase APP processing enzymes have shown a lack of target selectivity and poor penetrance of the blood-brainbarrier (BBB). Here, we have developed a high-affinity, phage-derived human antibody that targets BACE1 (anti-BACE1) and isanti-amyloidogenic. Anti-BACE1 reduces endogenous BACE1 activity and Aβ production in human cell lines expressing APP andin cultured primary neurons. Anti-BACE1 is highly selective and does not inhibit the related enzymes BACE2 or cathepsin D.Competitive binding assays and x-ray crystallography indicate that anti-BACE1 binds noncompetitively to an exosite on BACE1and not to the catalytic site. Systemic dosing of mice and nonhuman primates with anti-BACE1 resulted in sustained reductionsin peripheral Aβ peptide concentrations. Anti-BACE1 also reduces central nervous system Aβ concentrations in mouse and monkey,consistent with a measurable uptake of antibody across the BBB. Thus, BACE1 can be targeted in a highly selective manner throughpassive immunization with anti-BACE1, providing a potential approach for treating Alzheimer’s disease. Nevertheless, therapeuticsuccess with anti-BACE1 will depend on improving antibody uptake into the brain.
Science Translational Medicine DOI： 10.1126/scitranslmed.3002230
Boosting Brain Uptake of a Therapeutic Antibody by Reducing Its Affinity for a Transcytosis Target
Yu, Y. Joy; Zhang, Yin; Kenrick, Margaret; Hoyte, Kwame; Luk, Wilman; Lu, Yanmei; Atwal, Jasvinder; Elliott, J. Michael; Prabhu, Saileta; Watts, Ryan J.; Dennis, Mark S.
Monoclonal antibodies have therapeutic potential for treating diseases of the central nervous system, but their accumulationin the brain is limited by the blood-brain barrier (BBB). Here, we show that reducing the affinity of an antibody for thetransferrin receptor (TfR) enhances receptor-mediated transcytosis of the anti-TfR antibody across the BBB into the mousebrain where it reaches therapeutically relevant concentrations. Anti-TfR antibodies that bind with high affinity to TfR remainassociated with the BBB, whereas lower-affinity anti-TfR antibody variants are released from the BBB into the brain and showa broad distribution 24 hours after dosing. We designed a bispecific antibody that binds with low affinity to TfR and withhigh affinity to the enzyme β-secretase (BACE1), which processes amyloid precursor protein into amyloid-β (Aβ) peptides includingthose associated with Alzheimer’s disease. Compared to monospecific anti-BACE1 antibody, the bispecific antibody accumulatedin the mouse brain and led to a greater reduction in brain Aβ after a single systemic dose. TfR-facilitated transcytosis ofthis bispecific antibody across the BBB may enhance its potency as an anti-BACE1 therapy for treating Alzheimer’s disease.
