Zika virus (ZIKV) is an emerging mosquito-borne flavivirus of significant public health concern. ZIKV shares a high degree of sequence and structural homology compared with other flaviviruses, including dengue virus (DENV), resulting in immunological cross-reactivity. Improving our current understanding of the extent and characteristics of this immunological cross-reactivity is important, as ZIKV is presently circulating in areas that are highly endemic for dengue. To assess the magnitude and functional quality of cross-reactive immune responses between these closely related viruses, we tested acute and convalescent sera from nine Thai patients with PCRconfirmed DENV infection against ZIKV. All of the sera tested were cross-reactive with ZIKV, both in binding and in neutralization. To deconstruct the observed serum cross-reactivity in depth, we also characterized a panel of DENV-specific plasmablast-derived monoclonal antibodies (mAbs) for activity against ZIKV. Nearly half of the 47 DENV-reactive mAbs studied bound to both whole ZIKV virion and ZIKV lysate, of which a subset also neutralized ZIKV. In addition, both sera and mAbs from the dengue-infected patients enhanced ZIKV infection of Fc gamma receptor (FcγR)-bearing cells in vitro. Taken together, these findings suggest that preexisting immunity to DENV may impact protective immune responses against ZIKV. In addition, the extensive cross-reactivity may have implications for ZIKV virulence and disease severity in DENV-experienced populations. (2-4). However, the more recent outbreaks have caused severe neurological complications including GuillainBarré Syndrome in adults and an increase in congenital microcephaly and other adverse birth outcomes in Brazil (5-7). The Pan American Health Organization has reported that as of May 2016, local transmission of ZIKV had spread to over 38 countries or territories in the Americas. In addition, a recent WHO report states that 44 new countries are experiencing their first ZIKV outbreak since 2015. Despite the improving surveillance of the virus, accurate diagnosis has been challenging given the similarities in the clinical presentation of ZIKV to other arboviral infections endemic in these regions, among other factors.During the viremic period, ZIKV can be found in patient blood, saliva, urine, and other bodily fluids early after symptom onset (8-10). During the Yap Islands epidemic in 2007, anti-ZIKV IgM ELISAs and ZIKV plaque reduction neutralization titer (PRNT) assays were performed to confirm infection in RT-PCR negative cases (2, 8). However, as these studies showed, the cross-reactivity between ZIKV and other flaviviruses makes confirmation of infection difficult, especially when patients may have had flavivirus exposures before their suspected ZIKV infection (2,8). Given the overlapping presence of DENV and other flaviviruses in a majority of ZIKV epidemic regions (11), there are great challenges in serology-based testing of flavivirus-immune patients (12).The DENV envelope (E) protein, considered a major imun...

h Humoral immune responses are thought to play a major role in dengue virus-induced immunopathology; however, little is known about the plasmablasts producing these antibodies during an ongoing infection. Herein we present an analysis of plasmablast responses in patients with acute dengue virus infection. We found very potent plasmablast responses that often increased more than 1,000-fold over the baseline levels in healthy volunteers. In many patients, these responses made up as much 30% of the peripheral lymphocyte population. These responses were largely dengue virus specific and almost entirely made up of IgG-secreting cells, and plasmablasts reached very high numbers at a time after fever onset that generally coincided with the window where the most serious dengue virus-induced pathology is observed. The presence of these large, rapid, and virus-specific plasmablast responses raises the question as to whether these cells might have a role in dengue immunopathology during the ongoing infection. These findings clearly illustrate the need for a detailed understanding of the repertoire and specificity of the antibodies that these plasmablasts produce. Dengue virus causes an infection with symptoms ranging from a mild fever to severe hemorrhagic fever with vascular leakage that ranges in severity from minor subcutaneous bleeding to severe gastrointestinal bleeding (5,28,34). A striking epidemiological and immunological characteristic of dengue fever (DF) is that the severe immunopathology is more likely to occur in individuals who have previously been infected with a heterologous dengue virus serotype (8,29,32). While the exact mechanism of this phenomenon remains to be fully elucidated, several hypotheses have been developed over the last few decades to explain the reason for the exacerbated pathology observed in these patients. One of the main hypotheses revolves around a mechanism referred to as antibody-dependent enhancement (ADE) (14). This hypothesis suggests that during a secondary infection, cross-reactive yet poorly cross-neutralizing antibodies produced against a previously encountered serotype will mediate an increased infectivity, in addition to altering the host range of target cells. This mechanism has been extensively studied in vitro (6,17,20), and its importance in vivo is beginning to be elucidated (2, 10, 27). Another proposed hypothesis (22,23) suggests that an enhanced infection together with a potent T cell-mediated recall response produces massive amounts of effector mediators (4, 11-13, 15, 16, 25), a so-called cytokine storm, that is responsible for the observed immunopathology. These two mechanisms are not mutually exclusive and may in fact work in concert to cause the immunopathology of dengue disease.While human T cell responses during acute dengue virus infection have been studied in some detail, much less is known about the B cell responses. Early studies in dengue patients showed that increases in immunoglobulin-containing cells could be observed during infection and that these cel...

Dengue virus (DENV) infection in the presence of reactive, non-neutralizing IgG (RNNIg) is the greatest risk factor for dengue hemorrhagic fever (DHF) or shock syndrome (DSS). Progression to DHF/DSS is attributed to antibody-dependent enhancement (ADE); however, since only a fraction of infections occurring in the presence of RNNIg advance to DHF/DSS, the presence of RNNIg alone cannot account for disease severity. We discovered that DHF/DSS patients respond to infection by producing IgGs with enhanced affinity for the activating Fc receptor IIIA due to afucosylated Fc glycans and IgG1 subclass. RNNIg enriched for afucosylated IgG1 triggered platelet reduction in vivo and was a significant risk factor for thrombocytopenia (OR 11.00, p=0.0139). Thus, therapeutics and vaccines restricting production of afucosylated, IgG1 RNNIg during infection may prevent ADE of DENV disease.