The small GTPase RAB1B promotes antiviral innate immunity by interacting with TNF receptor-associated factor 3 (TRAF3)

Innate immune detection of viral nucleic acids during viral infection activates a signaling cascade that induces type I and type III IFNs as well as other cytokines, to generate an antiviral response. This signaling is initiated by pattern recognition receptors, such as the RNA helicase retinoic acid-inducible gene I (RIG-I), that sense viral RNA. These sensors then interact with the adaptor protein mitochondrial antiviral signaling protein (MAVS), which recruits additional signaling proteins, including TNF receptor-associated factor 3 (TRAF3) and TANK-binding kinase 1 (TBK1), to form a signaling complex that activates IFN regulatory factor 3 (IRF3) for transcriptional induction of type I IFNs. Here, using several immunological and biochemical approaches in multiple human cell types, we show that the GTPase-trafficking protein RAB1B up-regulates RIG-I pathway signaling and thereby promotes IFN-β induction and the antiviral response. We observed that RAB1B overexpression increases RIG-I-mediated signaling to IFN-β and that RAB1B deletion reduces signaling of this pathway. Additionally, loss of RAB1B dampened the antiviral response, indicated by enhanced Zika virus infection of cells depleted of RAB1B. Importantly, we identified the mechanism of RAB1B action in the antiviral response, finding that it forms a protein complex with TRAF3 to facilitate the interaction of TRAF3 with mitochondrial antiviral signaling protein. We conclude that RAB1B regulates TRAF3 and promotes the formation of innate immune signaling complexes in response to nucleic acid sensing during RNA virus infection.

Time elapsed between Zika and dengue virus infections affects antibody and T cell responses

Zika virus (ZIKV) and dengue virus (DENV) are co-endemic in many parts of the world, but the impact of ZIKV infection on subsequent DENV infection is not well understood. Here we show in rhesus macaques that the time elapsed after ZIKV infection affects the immune response to DENV infection. We show that previous ZIKV exposure increases the magnitude of the antibody and T cell responses against DENV. The time interval between ZIKV and subsequent DENV infection further affects the immune response. A mid-convalescent period of 10 months after ZIKV infection results in higher and more durable antibody and T cell responses to DENV infection than a short period of 2 months. In contrast, previous ZIKV infection does not affect DENV viremia or pro-inflammatory status. Collectively, we find no evidence of a detrimental effect of ZIKV immunity in a subsequent DENV infection. This supports the implementation of ZIKV vaccines that could also boost immunity against future DENV epidemics.

High correlation between Zika virus NS1 antibodies and neutralizing antibodies in selected serum samples from normal healthy Thais

Despite the widespread presence of the mosquito transmitted Zika virus (ZIKV) over much of Southeast Asia, the number of reported cases remains low. One possibility is that residents in Southeast Asia are immunologically protected, although the nature of any such protection remains unclear. This study sought to investigate the presence of antibodies directed to ZIKV NS1 protein in a selected sub-set of samples from a well characterized cohort of serum samples from normal, healthy Thais that had been previously characterized for the presence of neutralizing antibodies to ZIKV, DENV 1-4, and JEV. Because of similarities in molecular weight between the flavivirus E and NS1 proteins, an immunoblot system was established in which the NS1 antigen was not denatured, allowing detection of the dimer form of NS1, distinctly clear from the migration position of the E and NS1 monomer proteins. The results showed that antibodies to ZIKV NS1 protein were only detected in samples with ZIKV neutralizing antibodies (27/30 samples), and no sample (0/30) with a ZIKV plaque reduction neutralization test (PRNT)90 < 20 showed evidence of anti-ZIKV NS1 antibodies. The high correlation between the presence of ZIKV NS1 antibodies and ZIKV PRNT suggests that immunological protection against ZIKV infection in Thailand arises from prior exposure to ZIKV, and not through cross neutralization.

Computationally Designed Peptides for Zika Virus Detection: An Incremental Construction Approach

Herein, and in contrast to current production of anti-Zika virus antibodies, we propose a semi-combinatorial virtual strategy to select short peptides as biomimetic antibodies/binding agents for the detection of intact Zika virus (ZIKV) particles. The virtual approach was based on generating different docking cycles of tetra, penta, hexa, and heptapeptide libraries by maximizing the discrimination between the amino acid motif in the ZIKV and dengue virus (DENV) envelope protein glycosylation site. Eight peptides, two for each length (tetra, penta, hexa, and heptapeptide) were then synthesized and tested vs. intact ZIKV particles by using a direct enzyme linked immunosorbent assay (ELISA). As a reference, we employed a well-established anti-ZIKV antibody, the antibody 4G2. Three peptide-based assays had good detection limits with dynamic range starting from 10⁵ copies/mL of intact ZIKV particles; this was one order magnitude lower than the other peptides or antibodies. These three peptides showed slight cross-reactivity against the three serotypes of DENV (DENV-1, -2, and -3) at a concentration of 10⁶ copies/mL of intact virus particles, but the discrimination between the DENV and ZIKV was lost when the coating concentration was increased to 10⁷ copies/mL of the virus. The sensitivity of the peptides was tested in the presence of two biological matrices, serum and urine diluted 1:10 and 1:1, respectively. The detection limits decreased about one order of magnitude for ZIKV detection in serum or urine, albeit still having for two of the three peptides tested a distinct analytical signal starting from 10⁶ copies/mL, the concentration of ZIKV in acute infection.

Postnatal Zika virus infection of nonhuman primate infants born to mothers infected with homologous Brazilian Zika virus

Recent data in a nonhuman primate model showed that infants postnatally infected with Zika virus (ZIKV) were acutely susceptible to high viremia and neurological damage, suggesting the window of vulnerability extends beyond gestation. In this pilot study, we addressed the susceptibility of two infant rhesus macaques born healthy to dams infected with Zika virus during pregnancy. Passively acquired neutralizing antibody titers dropped below detection limits between 2 and 3 months of age, while binding antibodies remained detectable until viral infection at 5 months. Acute serum viremia was comparatively lower than adults infected with the same Brazilian isolate of ZIKV (n = 11 pregnant females, 4 males, and 4 non-pregnant females). Virus was never detected in cerebrospinal fluid nor in neural tissues at necropsy two weeks after infection. However, viral RNA was detected in lymph nodes, confirming some tissue dissemination. Though protection was not absolute and our study lacks an important comparison with postnatally infected infants born to naïve dams, our data suggest infants born healthy to infected mothers may harbor a modest but important level of protection from postnatally acquired ZIKV for several months after birth, an encouraging result given the potentially severe infection outcomes of this population.

High specificity and sensitivity of Zika EDIII-based ELISA diagnosis highlighted by a large human reference panel

BACKGROUND

Zika virus (ZIKV) and Dengue virus (DENV) are often co-endemic. The high protein-sequence homology of flaviviruses renders IgG induced by and directed against them highly cross-reactive against their antigen(s), as observed on a large set of sera, leading to poorly reliable sero-diagnosis.

METHODS

We selected Domain III of the ZIKV Envelope (ZEDIII) sequence, which is virus specific. This recombinant domain was expressed and purified for the specific detection of ZEDIII-induced IgG by ELISA from ZIKV-RT-PCR-positive, ZIKV-IgM-positive, flavivirus-positive but ZIKV-negative, or flavivirus-negative sera. We also assessed the reactivity of ZEDIII-specific human antibodies against EDIII of DENV serotype 4 (D4EDIII) as a specific control. Sera from ZEDIII-immunized mice were also tested.

RESULTS

Cross-reactivity of IgG from 5,600 sera against total inactivated DENV or ZIKV was high (71.0% [69.1; 72.2]), whereas the specificity and sensitivity calculated using a representative cohort (242 sera) reached 90% [84.0; 95.8] and 92% [84.5; 99.5], respectively, using a ZEDIII-based ELISA. Moreover, purified human IgG against D2EDIII or D4EDIII did not bind to ZEDIII and we observed no D4EDIII reactivity with ZIKV-induced mouse polyclonal IgGs.

CONCLUSIONS

We developed a ZEDIII-based ELISA that can discriminate between past or current DENV and ZIKV infections, allowing the detection of a serological scar from other flaviviruses. This could be used to confirm exposure of pregnant women or to follow the spread of an endemic disease.

Experimental Infection of Pregnant Female Sheep with Zika Virus During Early Gestation

Zika virus (ZIKV) is a vertically and sexually transmissible virus resulting in severe congenital malformation. The goal of this study was to develop an ovine model of ZIKV infection. Between 28–35 days gestation (DG), four pregnant animals were infected with two doses of 6 × 10⁶ PFU of ZIKV; four control animals received PBS. Animals were evaluated for 45 days (D) post-infection (PI) and necropsies were performed. Viral RNA was detected in infected ewe peripheral blood mononuclear cells (PBMC) during the first week PI; however, all fluids and tissues were negative upon culture. Anti-ZIKV IgM (1:400) and neutralizing antibodies were detected in all infected animals. Clinical disease, virus, or ZIKV antibodies were not detected in control ewes. After two weeks PI, fetal loss occurred in two infected animals, and at necropsy, three infected animals had placental petechiation and ecchymosis and one had hydramnion. Fetal morphometrics revealed smaller cranial circumference to crown-rump length ratios (p < 0.001) and relative brain weights (p = 0.038) in fetuses of infected animals compared with control fetuses. Immunophenotyping indicated an increase in B cells (p = 0.012) in infected sheep. Additionally, in vitro experiments using both adult and fetal cell lines demonstrated that ovine cells are highly permissive to ZIKV infection. In conclusion, ZIKV infection of pregnant sheep results in a change in fetal growth and gestational outcomes.

Viral priming of cell intrinsic innate antiviral signaling by the unfolded protein response

The innate response to a pathogen is critical in determining the outcome of the infection. However, the interplay of different cellular responses that are activated following viral infection and their contribution to innate antiviral signalling has not been clearly established. This work shows that flaviviruses, including Dengue, Zika, West Nile and Tick-borne encephalitis viruses, activate the unfolded protein response before transcription of interferon regulatory factor 3 induced genes. Infection in conditions of unfolded protein response priming leads to early activation of innate antiviral responses and cell intrinsic inhibition of viral replication, which is interferon regulatory factor 3 dependent. These results demonstrate that the unfolded protein response is not only a physiological reaction of the cell to viral infection, but also synergizes with pattern recognition sensing to mount a potent antiviral response.

Protective to a T: The Role of T Cells during Zika Virus Infection

CD4 and CD8 T cells are an important part of the host's capacity to defend itself against viral infections. During flavivirus infections, T cells have been implicated in both protective and pathogenic responses. Given the recent emergence of Zika virus (ZIKV) as a prominent global health threat, the question remains as to how T cells contribute to anti-ZIKV immunity. Furthermore, high homology between ZIKV and other, co-circulating flaviviruses opens the possibility of positive or negative effects of cross-reactivity due to pre-existing immunity. In this review, we will discuss the CD4 and CD8 T cell responses to ZIKV, and the lessons we have learned from both mouse and human infections. In addition, we will consider the possibility of whether T cells, in the context of flavivirus-naïve and flavivirus-immune subjects, play a role in promoting ZIKV pathogenesis during infection.

T cells promote microglia-mediated synaptic elimination and cognitive dysfunction during recovery from neuropathogenic flaviviruses

T cells clear virus from the CNS and dynamically regulate brain functions, including spatial learning, through cytokine signaling. Here we determined whether hippocampal T cells that persist after recovery from infection with West Nile virus (WNV) or Zika virus (ZIKV) impact hippocampal-dependent learning and memory. Using newly established models of viral encephalitis recovery in adult animals, we show that in mice that have recovered from WNV or ZIKV infection, T cell-derived interferon-γ (IFN-γ) signaling in microglia underlies spatial-learning defects via virus-target-specific mechanisms. Following recovery from WNV infection, mice showed presynaptic termini elimination with lack of repair, while for ZIKV, mice showed extensive neuronal apoptosis with loss of postsynaptic termini. Accordingly, animals deficient in CD8+ T cells or IFN-γ signaling in microglia demonstrated protection against synapse elimination following WNV infection and decreased neuronal apoptosis with synapse recovery following ZIKV infection. Thus, T cell signaling to microglia drives post-infectious cognitive sequelae that are associated with emerging neurotropic flaviviruses.

Differential Shedding and Antibody Kinetics of Zika and Chikungunya Viruses, Brazil

In seroconversion panels obtained from patients from Brazil, diagnostic testing for Zika virus infection was improved by combining multiple antibody isotypes, techniques, and antigens, but sensitivity remained suboptimal. In contrast, chikungunya virus diagnostic testing was unambiguous. Recurrent recent arbovirus infections suggested by serologic data and unspecific symptoms highlight the need for exhaustive virologic testing.

Zika Virus IgM Detection and Neutralizing Antibody Profiles 12-19 Months after Illness Onset

Data on the duration of detectable Zika virus–specific IgM in infected persons are limited. Neutralizing antibody cross-reactivity occurs between Zika virus and related flaviviruses, but the degree to which this confounds diagnosis is uncertain. We tested serum specimens collected 12–19 months after illness onset from patients with confirmed Zika virus disease for Zika virus IgM and Zika virus and dengue virus neutralizing antibodies. Among 62 participants, 45 (73%) had detectable Zika virus IgM and 12 (19%) had an equivocal result. Although all patients tested had Zika virus neutralizing antibodies, 39 (63%) also had neutralizing antibodies against dengue virus; of those, 12 (19%) had <4-fold difference between Zika virus and dengue virus titers, and 5 (8%) had dengue virus titer >4-fold higher than Zika virus titer. Prolonged detection of IgM and neutralizing antibody cross-reactivity make it difficult to determine the timing of Zika virus infection and differentiate between related flaviviruses.

Axl Promotes Zika Virus Entry and Modulates the Antiviral State of Human Sertoli Cells

Zika virus (ZIKV) is unique among mosquito-borne flaviviruses in its ability to be sexually transmitted. Persistent ZIKV infection in the testes, which are immune privileged organs, long after peripheral clearance suggests involvement of immunosuppressive pathways; however, the underlying mechanisms remain undetermined. We recently demonstrated that ZIKV infects human Sertoli cells (SC), the major cell type of the seminiferous epithelium responsible for maintaining the immune privileged compartment of seminiferous tubules. Recent reports have identified the TAM (Tyro3, Axl, Mer) receptor tyrosine kinase Axl as an entry receptor and/or immune modulator for ZIKV in a cell type-specific manner. Interestingly, the seminiferous epithelium exhibits high basal expression of the Axl receptor where it is involved in clearance of apoptotic germ cells and immunosuppression. Here, we show that Axl was highly expressed in SC compared to Leydig cells (LC) that correlated with robust ZIKV infection of SC, but not LC. Further, neutralization of Axl receptor and its ligand Gas6 strongly attenuated virus entry in SC. However, inhibition of Axl kinase did not affect ZIKV entry but instead led to decreased protein levels of suppressor of cytokine signaling 1 (SOCS1) and SOCS3, increased expression of interferon-stimulated genes (ISGs), and reduced ZIKV replication. Similarly, treatment of multicellular human testicular organoids with an Axl kinase inhibitor attenuated ZIKV replication and increased ISG expression. Together, our data demonstrate that Axl promotes ZIKV entry and negatively regulates the antiviral state of SC to augment ZIKV infection of the testes and provides new insights into testis antiviral immunity and ZIKV persistence.IMPORTANCE Recent Zika virus (ZIKV) outbreaks have identified sexual transmission as a new route of disease spread not reported for other flaviviruses. ZIKV crosses the blood-testis barrier and establishes infection in seminiferous tubules, the site for spermatozoa development. Currently, there are no therapies to treat ZIKV infection, and the immune mechanisms underlying testicular persistence are unclear. We found that multiple human testicular cell types, except Leydig cells, support ZIKV infection. Axl receptor, which plays a pivotal role in maintaining the immunosuppressive milieu of the testis, is highly expressed in Sertoli cells and augments ZIKV infection by promoting virus entry and negatively regulating the antiviral state. By using testicular organoids, we further describe the antiviral role of Axl inhibition. The significance of our research lies in defining cross talk between Axl and type I interferon signaling as an essential mechanism of immune control that can inform therapeutic efforts to clear ZIKV from the testis.

Zika Virus-Immune Plasmas from Symptomatic and Asymptomatic Individuals Enhance Zika Pathogenesis in Adult and Pregnant Mice

Preexisting immunity against dengue virus or West Nile virus was previously reported to mediate antibody-dependent enhancement (ADE) of Zika virus (ZIKV) infection in a mouse model. We show here that ZIKV-immune plasma samples from both symptomatic and asymptomatic individuals mediated ZIKV ADE of infection in vitro and in mice. In a lethal infection model with a viral inoculum 10 times higher, both ADE and protection were observed, depending on the amount of infused immune plasma. In a vertical-transmission model, ZIKV-immune plasma infused to timed pregnant mice increased fetal demise and decreased the body weight of surviving fetuses. Depletion of IgG from an immune plasma abolished ADE of infection, and the presence of purified IgG alone mediated ADE of infection. Higher viral loads and proinflammatory cytokines were detected in mice treated with ZIKV-immune plasma samples compared to those receiving control plasma. Together, these data show that passive immunization with homotypic ZIKV antibodies, depending on the concentration, could either worsen or limit a subsequent ZIKV infection.IMPORTANCE Antibody-dependent enhancement (ADE) of virus infection is common to many viruses and is problematic when plasma antibody levels decline to subneutralizing concentrations. ADE of infection is especially important among flaviviruses, many of which are the cause of global health problems. Recently, human plasma samples immune to heterologous flaviviruses were shown to promote Zika virus (ZIKV) infection. Here we showed in immunocompromised mouse models that homologous immune plasma samples protect mice from subsequent infection at high antibody concentrations but that they mediate ADE of infection and increase ZIKV pathogenesis in adult mice and fetal demise during pregnancy at low concentrations.

Aedes aegypti NeSt1 Protein Enhances Zika Virus Pathogenesis by Activating Neutrophils

Saliva from the mosquito vector of flaviviruses is capable of changing the local immune environment, leading to an increase in flavivirus-susceptible cells at the infected bite site. In addition, an antibody response to specific salivary gland (SG) components changes the pathogenesis of flaviviruses in human populations. To investigate whether antigenic SG proteins are capable of enhancing infection with Zika virus (ZIKV), a reemerging flavivirus primarily transmitted by the Aedes aegypti mosquito, we screened for antigenic SG proteins using a yeast display library and demonstrate that a previously undescribed SG protein we term neutrophil stimulating factor 1 (NeSt1) activates primary mouse neutrophils ex vivo Passive immunization against NeSt1 decreases pro-interleukin-1β and CXCL2 expression, prevents macrophages from infiltrating the bite site, protects susceptible IFNAR-/- IFNGR-/- (AG129) mice from early ZIKV replication, and ameliorates virus-induced pathogenesis. These findings indicate that NeSt1 stimulates neutrophils at the mosquito bite site to change the immune microenvironment, allowing a higher level of early viral replication and enhancing ZIKV pathogenesis.IMPORTANCE When a Zika virus-infected mosquito bites a person, mosquito saliva is injected into the skin along with the virus. Molecules in this saliva can make virus infection more severe by changing the immune system to make the skin a better place for the virus to replicate. We identified a molecule that activates immune cells, called neutrophils, to recruit other immune cells, called macrophages, that the virus can infect. We named this molecule neutrophil-stimulating factor 1 (NeSt1). When we used antibodies to block NeSt1 in mice and then allowed Zika virus-infected mosquitoes to feed on these mice, they survived much better than mice that do not have antibodies against NeSt1. These findings give us more information about how mosquito saliva enhances virus infection, and it is possible that a vaccine against NeSt1 might protect people against severe Zika virus infection.

Purification of flavivirus VLPs by a two-step chomatographic process

Flaviviruses are enveloped viruses with positive-sense, single-stranded RNA, which are most commonly transmitted by infected mosquitoes. Zika virus (ZIKV) and yellow fever virus (YFV) are flaviviruses that have caused significant outbreaks in the last few years. Since there is no approved vaccine against ZIKV, and since the existing YF attenuated vaccine presents disadvantages related to limited supply and to rare, but fatal adverse effects, there is an urgent need for new vaccines to control these diseases. Virus-like particles (VLPs) represent a recombinant platform to produce safe and immunogenic vaccines. Thus, based on our experience of expressing in recombinant mammalian cells VLPs of most flaviviruses circulating in the Americas, this work focused on the evaluation of chromatographic purification processes for zika and yellow-fever VLPs. The clarified cell culture supernatant was processed by a membrane-based anion-exchange chromatography and then a multimodal chromatographic step. With this process, it was possible to obtain the purified VLPs with a yield (including the clarification step) of 66.4% for zika and 68.1% for yellow fever. DNA clearance was in the range of 99.8-99.9%, providing VLP preparations that meet the WHO limit for this critical contaminant. Correct size and morphology of the purified VLPs were confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The promising results obtained for both zika and yellow fever VLPs indicate that this process could be potentially applied also to VLPs of other flaviviruses.

Differential human antibody repertoires following Zika infection and the implications for serodiagnostics and disease outcome

Zika virus (ZIKV) outbreak in Americas led to extensive efforts to develop vaccines and ZIKV-specific diagnostics. In the current study, we use whole genome phage display library spanning the entire ZIKV genome (ZIKV-GFPDL) for in-depth immune profiling of IgG and IgM antibody repertoires in serum and urine longitudinal samples from individuals acutely infected with ZIKV. We observe a very diverse IgM immune repertoire encompassing the entire ZIKV polyprotein on day 0 in both serum and urine. ZIKV-specific IgG antibodies increase 10-fold between day 0 and day 7 in serum, but not in urine; these are highly focused on prM/E, NS1 and NS2B. Differential antibody affinity maturation is observed against ZIKV structural E protein compared with nonstructural protein NS1. Serum antibody affinity to ZIKV-E protein inversely correlates with ZIKV disease symptoms. Our study provides insight into unlinked evolution of immune response to ZIKV infection and identified unique targets for ZIKV serodiagnostics.

The possible role of cross-reactive dengue virus antibodies in Zika virus pathogenesis

Zika virus (ZIKV) has been known for decades to circulate in Africa and Asia. However, major complications of a ZIKV infection have recently become apparent for reasons that are still not fully elucidated. One of the hypotheses for the seemingly increased pathogenicity of ZIKV is that cross-reactive dengue antibodies can enhance a ZIKV infection through the principle of antibody-dependent enhancement (ADE). Recently, ADE in ZIKV infection has been studied, but conclusive evidence for the clinical importance of this principle in a ZIKV infection is lacking. Conversely, the widespread circulation of ZIKV in dengue virus (DENV)-endemic regions raises new questions about the potential contribution of ZIKV antibodies to DENV ADE. In this review, we summarize the results of the evidence to date and elaborate on other possible detrimental effects of cross-reactive flavivirus antibodies, both for ZIKV infection and the risk of ZIKV-related congenital anomalies, DENV infection, and dengue hemorrhagic fever.

Evaluation of alternative endpoints for ZIKV vaccine efficacy trials

Zika virus (ZIKV) infection during pregnancy is associated with microcephaly and other birth defects, collectively termed Congenital Zika Syndrome (CZS). During the epidemic in 2015-16, ZIKV spread through the Americas and quickly joined the list of other known teratogenic pathogens, TORCH. Multiple ZIKV vaccines have been developed for protection of pregnant women and women of childbearing age. However, ZIKV infection incidence has since waned substantially, and adverse birth outcomes are rare outcomes of infection. Studying a vaccine's protective efficacy against CZS in a large phase III clinical trial may be infeasible in such times of low incidence. Should trials be initiated, researchers may resort to alternative clinical endpoints. In this study, we simulate a variety of vaccine clinical trial scenarios to evaluate the feasibility of the CZS endpoint in vaccine studies and compare CZS to other potential outcomes: ZIKV infection detected through weekly, biweekly, or monthly testing and laboratory-confirmed, symptomatic Zika Virus Disease. We compare the sample size required for 80% statistical power to detect vaccine efficacy and trial duration for each scenario. Our results show the feasibility of CZS clinical endpoints depends on the timing of simulated clinical trials in the course of a seasonal epidemic, due to CZS risk varying with trimester of infection. This result highlights additional considerations needed when designing vaccine efficacy trials of protection against teratogenic pathogens.

Zika Virus-Specific IgY Results Are Therapeutic Following a Lethal Zika Virus Challenge without Inducing Antibody-Dependent Enhancement

The Zika virus (ZIKV) is a newly emerged pathogen in the Western hemisphere. It was declared a global health emergency by the World Health Organization in 2016. There have been 223,477 confirmed cases, including 3720 congenital syndrome cases since 2015. ZIKV infection symptoms range from asymptomatic to Gullain⁻Barré syndrome and extensive neuropathology in infected fetuses. Passive and active vaccines have been unsuccessful in the protection from or the treatment of flaviviral infections due to antibody-dependent enhancement (ADE). ADE causes an increased viral load due to an increased monocyte opsonization by non-neutralizing, low-avidity antibodies from a previous dengue virus (DENV) infection or from a previous exposure to ZIKV. We have previously demonstrated that polyclonal avian IgY generated against whole-killed DENV-2 ameliorates DENV infection in mice while not inducing ADE. This is likely due to the inability of the Fc portion of IgY to bind to mammalian Fc receptors. We have shown here that ZIKV oligoclonal IgY is able to neutralize the virus in vitro and in IFNAR-/- mice. The concentration of ZIKV-specific IgY yielding 50% neutralization (NT50) was 25 µg/mL. The exposure of the ZIKV, prior to culture with ZIKV-specific IgY or 4G2 flavivirus-enveloped IgG, demonstrated that the ZIKV-specific IgY does not induce ADE. ZIKV IgY was protective in vivo when administered following a lethal ZIKV challenge in 3-week-old IFNAR-/- mice. We propose polyclonal ZIKV-specific IgY may provide a viable passive immunotherapy for a ZIKV infection without inducing ADE.

Impact of pre-existing dengue immunity on human antibody and memory B cell responses to Zika

Little is known about enduring memory B cell (MBC) responses to Zika virus (ZIKV) and their relationship with circulating antibodies. Here we comprehensively assess MBC frequency and specificity alongside serum binding and neutralizing antibody responses to ZIKV ~2 weeks and ~8 months postinfection in 31 pediatric subjects with 0, 1 or >1 prior infections with the related dengue virus (DENV). ZIKV infection elicits a robust type-specific MBC response, and the majority of late convalescent anti-ZIKV serum neutralizing activity is attributable to ZIKV-specific antibodies. The number of prior DENV infections does not influence type-specific or cross-reactive MBC responses, although ZIKV has the highest cross-reactivity with DENV3. DENV cross-reactive MBCs expanded by ZIKV infection decline in number and proportion by late convalescence. Finally, ZIKV induces greater cross-reactivity in the MBC pool than in serum antibodies. Our data suggest immunity to DENV only modestly shapes breadth and magnitude of enduring ZIKV antibody responses.

The Application and Interpretation of IgG Avidity and IgA ELISA Tests to Characterize Zika Virus Infections

In the absence of viremia, the diagnostics of Zika virus (ZIKV) infections must rely on serological techniques. In order to improve the serological diagnosis of ZIKV, ZIKV-IgA and ZIKV-IgG avidity assays were evaluated. Forty patients returning from ZIKV endemic areas, with confirmed or suspected ZIKV infections were studied. Samples were classified as early acute, acute and late acute according to the number of days post illness onset. Low avidity IgG was only detected at acute and late acute stages and IgA mostly at the early acute and acute stages. The date of sampling provides useful information and can help to choose the best technique to use at a determined moment in time and to interpret low avidity IgG and IgA results, improving the serological diagnosis of ZIKV.

Impact of preexisting dengue immunity on Zika virus emergence in a dengue endemic region

The clinical outcomes associated with Zika virus (ZIKV) in the Americas have been well documented, but other aspects of the pandemic, such as attack rates and risk factors, are poorly understood. We prospectively followed a cohort of 1453 urban residents in Salvador, Brazil, and, using an assay that measured immunoglobulin G3 (IgG3) responses against ZIKV NS1 antigen, we estimated that 73% of individuals were infected during the 2015 outbreak. Attack rates were spatially heterogeneous, varying by a factor of 3 within a community spanning 0.17 square kilometers. Preexisting high antibody titers to dengue virus were associated with reduced risk of ZIKV infection and symptoms. The landscape of ZIKV immunity that now exists may affect the risk for future transmission.

John AL. Maternal immunity and antibodies to dengue virus promote infection and Zika virus-induced microcephaly in fetuses

Zika virus (ZIKV), an emergent flaviviral pathogen, has been linked to microcephaly in neonates. Although the risk is greatest during the first trimester of pregnancy in humans, timing alone cannot explain why maternal ZIKV infection leads to severe microcephaly in some fetuses, but not others. The antigenic similarities between ZIKV and dengue virus (DENV), combined with high levels of DENV immunity among ZIKV target populations in recent outbreaks, suggest that anti-DENV maternal antibodies could promote ZIKV-induced microcephaly. We demonstrated maternal-to-fetal ZIKV transmission, fetal infection, and disproportionate microcephaly in immunocompetent mice. We show that DENV-specific antibodies in ZIKV-infected pregnant mice enhance vertical ZIKV transmission and result in a severe microcephaly-like syndrome, which was dependent on the neonatal Fc receptor, FcRN. This novel immune-mediated mechanism of vertical transmission of viral infection is of special concern because ZIKV epidemic regions are also endemic to DENV.

Control of Acute Arboviral Infection by Natural Killer Cells

The recent explosive pandemic of chikungunya virus (CHIKV) followed by Zika (ZIKV) virus infections occurring throughout many countries represents the most unexpected arrival of arthropod-borne viral diseases in the past 20 years. Transmitted through the bite of Aedes mosquitoes, the clinical picture associated with these acute arbovirus infections, including Dengue (DENV), CHIKV and ZIKV, ranges from classical febrile illness to life-threatening disease. Whereas ZIKV and CHIKV-mediated infections have previously been recognized as relatively benign diseases, in contrast to Dengue fever, recent epidemic events have brought waves of increased morbidity and mortality leading to a serious public health problem. Although the host immune response plays a crucial role in controlling infections, it may also promote viral spread and immunopathology. Here, we review recent developments in our understanding of the immune response, with an emphasis on the early antiviral immune response mediated by natural killer cells and emphasize their Janus-faced effects in the control of arbovirus infection and pathogenesis. Improving our understanding knowledge on of the mechanisms that control viral infection is crucial in the current race against the globalization of arbovirus epidemics.