Analysis of the neutralizing antibody response following flavivirus infection

登革热病毒（DENV）是黄病毒属的一种蚊媒成员，由于其广泛分布和在人类中引起严重疾病的能力，对全球公共卫生产生影响。每年，估计有3.9亿人感染登革出血热，其临床表现从自限性急性发热性疾病（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）不等。登革热病毒在自然界中有四种相关血清型，每一种血清型都能引起登革热病毒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相关。报告的登革出血热病例数在过去二十年中急剧增加，现在每年超过25万例。因此，迫切需要为DENV的所有四种血清型开发一种安全有效的疫苗。

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.