| Abstract: | 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 PCR-confirmed 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.Zika virus (ZIKV) is a mosquito-borne virus belonging to the Flaviviridae family of single-stranded positive-sense RNA viruses. First isolated in Uganda in 1947 (1), this virus remained largely dormant for the next six decades until it reemerged as the cause of an epidemic on Yap Islands, Micronesia in 2007 (2). ZIKV has since then been linked with several outbreaks in the Pacific and Americas, along with sporadic human cases in Africa and Asia (3, 4). Until its appearance in French Polynesia in 2013 and more recently in Brazil in 2015, ZIKV infection was primarily associated with mild self-limiting illness, with symptoms similar to and often milder than dengue virus (DENV) or Chikungunya virus (CHIKV) infections (2–4). However, the more recent outbreaks have caused severe neurological complications including Guillain–Barré 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 imunodominant target for antibody responses in dengue patients (13–15), bears greater than 50% homology to ZIKV E protein (16). In addition to complicating the serology-based diagnosis of ZIKV infection, this raises an interesting question about the biological implications of the cross-reactivity on protection, virulence, and immunopathology of ZIKV infections. At present, the effect of preexisting immunity to DENV or other flaviviruses on immune responses induced by ZIKV infection is unknown. To this end, we were interested in determining the degree to which dengue-induced antibodies cross-react with ZIKV in terms of binding, virus neutralization, and antibody-dependent enhancement (ADE) of ZIKV infection, both at the serum and single-cell level.In this study, we provide an analysis of the cross-reactivity of acute and convalescent dengue-immune sera against ZIKV. The sera were collected from nine patients admitted to Siriraj Hospital in Bangkok, Thailand with confirmed DENV infection. Both acute and convalescent sera showed high binding titers to ZIKV lysate and could also neutralize ZIKV in vitro. To understand the origin and characteristics of these cross-reactive serum responses, we also analyzed a panel of plasmablast-derived DENV-reactive monoclonal antibodies (mAbs). Of the 47 mAbs tested, nearly half (22/47) bound to ZIKV lysate and an additional four to the whole virus. Seven of these mAbs also neutralized ZIKV in vitro. Five sera and a subset of the mAbs were also tested for ADE activity using the FcγR-bearing monocytic U937 cell line. All sera and ZIKV-reactive mAbs tested enhanced infection in vitro, whereas two DENV-specific but ZIKV-nonreactive mAbs did not. The data presented here have important implications for clinical diagnosis given that the current ZIKV outbreak in the Americas and the Caribbean is largely ongoing in dengue-endemic areas. Equally important, these findings set the stage for more in-depth studies that explore how preexisting flavivirus immunity may shape immune responses to ZIKV infection. |
