Analysis of the neutralizing antibody response following flavivirus infection

登革病毒(DENV)是黄病毒属中一种由蚊子传播的成员，由于其广泛的分布和在人类中引起严重疾病的能力，对公共卫生具有全球影响。据估计，每年有3.9亿人感染登革热病毒，其临床表现从自限性急性发热病(登革热)到以血浆渗漏和休克为特征的潜在致命综合征(登革出血热；DHF)。四种相关的DENV血清型在自然界中循环，每一种都能够引起DENV相关疾病的全部谱系。前瞻性临床研究清楚地表明，连续感染两种DENV血清型与更严重的病程有关。在过去20年中，报告的DHF病例数量急剧增加，现在每年超过25万例。因此，迫切需要开发一种针对所有四种血清型DENV的安全有效的疫苗。 中和抗体在预防黄病毒感染方面发挥着重要作用。抗体已经被映射到E蛋白(DI-DIII)的所有三个结构域，这些结构域显示出不同程度的中和效力，并通过多种效应机制提供保护。获得中和抗体是疫苗研制的主要目标。使这些努力复杂化的是，疫苗要求同时对四种不同的病毒产生保护，这些病毒虽然在抗原性上相关，但只共享一些被认为有助于病毒中和的抗体结合决定因素。因此，针对一种血清类型的DENV产生的抗体可能与另一种血清类型的病毒粒子反应，但通常亲和力和功能效力降低。 矛盾的是，抗体也可能在增强病毒感染和加重疾病方面发挥作用。抗体依赖的感染增强(ADE)描述了在抗体或免疫血清的亚中和浓度存在的情况下，携带Fc受体的细胞的感染显著增加。ADE和DENV感染的临床结局之间最直接的联系来自对婴儿在出生第一年观察到的原发感染后出现的异常大量的严重DENV病例的调查。在更广泛的背景下，一次感染一种血清型DENV引起的抗体可能与二次感染期间引入的相关病毒结合，亲和力降低，导致病毒粒子与化学计量比结合，该化学计量比不允许病毒中和，但可以支持ADE。发展一种免疫反应，引起针对疫苗中存在的所有四种血清型病毒的中和抗体的保护性水平，是降低ADE风险的关键因素。 四价减毒活疫苗的所有四种成分在接种者体内的免疫原性可能不是一样的，这使得保护性四价反应的发展变得复杂。据报道，在这方面，DENV毒株之间的干扰和传染性水平参差不齐。了解四价疫苗每个成分的免疫原性是疫苗开发和确定适当的保护相关性的一个重要方面，特别是因为目前尚不清楚需要多少血清型特异性反应才能保护所有四种血清型的DENV。然而，由于存在结合疫苗不同成分共享的交叉反应决定因素的抗体，剖析四价疫苗每种成分的具体贡献在技术上具有挑战性。目前还没有E蛋白上重要的类型特异性和基团反应性表位的详细图谱。改进的方法学区分和量化四价疫苗每一组分的功能贡献，将有助于改进疫苗免疫原性的估计，更精确地关联保护，并成为研究疫苗成败的强大调查工具。

Dengue virus (DENV) is a mosquito-borne member of the Flavivirus genus that has a global impact on public heath due to its widespread distribution and the ability to cause severe disease in humans. Each year, an estimated 390 million individuals are infected by DENV, with clinical manifestations ranging from a self-limiting acute febrile illness (dengue fever) to a potentially fatal syndrome characterized by plasma leakage and shock (dengue hemorrhagic fever; DHF). Four related serotypes of DENV circulate in nature, each capable of causing the full spectrum of DENV-related disease. Prospective clinical studies clearly demonstrate that sequential infection with two DENV serotypes is associated with a more severe disease course. The number of DHF cases reported has increased dramatically during the past twenty years, and now exceeds 250,000 cases annually. Thus, there is an urgent need for the development of a safe and effective vaccine for all four serotypes of DENV. Neutralizing antibodies play an important role in protection against flavivirus infection. Antibodies have been mapped to all three structural domains of the E protein (DI-DIII) that exhibit varying degrees of neutralization potency and confer protection by multiple effector mechanisms. Eliciting neutralizing antibody is a major goal of vaccine development. Complicating these efforts is a requirement for vaccines to simultaneously elicit protection against four different viruses that while antigenically related, share only some of the antibody-binding determinants thought to contribute to virus neutralization. Thus, antibodies raised against one serotype of DENV may react with virions of another serotype, but often with reduced affinity and functional potency. Paradoxically, antibodies may also play a role in enhancing virus infection and exacerbating disease. Antibody-dependent enhancement of infection (ADE) describes a dramatic increase in infection of Fc-receptor-bearing cells in the presence of sub-neutralizing concentrations of antibody or immune sera. The most direct link between ADE and the clinical outcome of DENV infection comes from investigations of the unusually large number of severe DENV cases following primary infection observed in infants during the first year of life. In a broader context, antibodies elicited by primary infection with one serotype of DENV may bind related viruses introduced during secondary infection with reduced avidity, resulting in engagement of the virion with a stoichiometry that does not permit virus neutralization but can support ADE. The development of an immune response that elicits protective levels of neutralizing antibodies against all four serotypes of virus present in the vaccine is a key factor for reducing the risk of ADE. The development of a protective tetravalent response is complicated by the possibility that all four components of a live attenuated tetravalent vaccine may not be equally immunogenic in the vaccinee. Interference and uneven levels of infectivity among DENV strains in this context has been reported. Understanding the immunogenicity of each component of a tetravalent vaccine is an important aspect of vaccine development and identifying appropriate correlates of protection, particularly because it is presently unclear how many serotype-specific responses will be required for protection from all four serotypes of DENV. However, dissecting the specific contribution of each element of a tetravalent vaccine is technically challenging due to the presence of antibodies that bind cross-reactive determinants shared by different components of the vaccine. A detailed map of the functionally important type-specific and group-reactive epitopes on the E protein is not presently available. Improved methodology that distinguishes and quantifies the functional contribution of each component of a tetravalent vaccine would allow for improvements in estimates of vaccine immunogenicity, a more precise correlate of protection, and a powerful investigational tool to study vaccine success and failure.