Development of a HiBiT-tagged reporter hepatitis E virus and its utility as an antiviral drug screening platform

Plasmacytoid dendritic cell sensing of hepatitis E virus is shaped by both viral and host factors

Type I and III interferons critically protect the host against viral infection. Previous studies showed that IFN responses are suppressed in cells infected by hepatitis E virus (HEV). Here, we studied the anti-HEV function of IFN secreted by plasmacytoid dendritic cells (pDCs), specialized producers of IFNs. We showed that pDCs co-cultured with HEV-replicating cells secreted IFN in a cell contact-dependent manner. This pDC response required the endosomal nucleic acid sensor TLR7 and adhesion molecules. IFNs secreted by pDCs reduced viral spread. Intriguingly, ORF2, the capsid protein of HEV, can be produced in various forms by the infected cells, and we wanted to study their role in anti-HEV immune response. During infection, a fraction of ORF2 localizes into the nucleus, and glycosylated forms of ORF2 are massively secreted by infected cells. We showed that glycosylated ORF2 potentiates the recognition of infected cells by pDCs, by regulating cell contacts. On the other hand, nuclear ORF2 triggers immune response by IRF3 activation. Together, our results suggest that pDCs may be essential to control HEV replication.

Antiviral resistance and barrier integrity at the maternal-fetal interface restrict hepatitis E virus from crossing the placental barrier

Hepatitis E virus (HEV) genotype 1 (HEV-1) infection in pregnant women is associated with adverse outcomes of pregnancy including fulminant hepatic failure, fetal loss, premature birth, and neonatal mortality, although the underlying mechanisms remain largely unclear. In this study, we first demonstrated that HEV-1 robustly infects pregnant gerbils and causes pregnancy-associated adverse outcomes, which were recorded in 4/6 HEV-1-infected but only 1/5 in PBS-inoculated pregnant gerbils. However, vertical transmission of HEV-1 from mothers to newborns is not evident, as HEV-1 RNA was not detected in uterus tissues or in newborn pups. To further determine whether HEV-1 can cross the placental barrier, we established an in vitro blood-placental barrier by coculturing human placental trophoblast cells (BeWo) and umbilical vein endothelial cells (HUVEC) in Transwell inserts. By using the placental barrier under the conditions in this study, we showed that quasi-enveloped or nonenveloped HEV-1, HEV-3, or HEV-4 virions do not readily cross the barrier prior to 4 d postinoculation when it has high barrier integrity. Importantly, we demonstrated that the placental barrier induces local antiviral resistance at the maternal-fetal interface, that interactions between maternal- and fetal-derived cocultured cells are important for induction of antiviral resistance, and that anti-HEV resistance can be transferred to nonplacental HepG2 liver cells. We also revealed that the main effectors of antiviral resistance at the placental barrier are type III interferons (IFN-λ1, λ2/3) and the chemokine CXCL10. The findings have important implications in understanding the mechanisms leading to HEV-1-associated maternal and fetal adverse outcomes in pregnant women.

Development of a reporter HBoV1 strain for antiviral drug screening and life cycle studies

Human bocavirus 1 (HBoV1; family: Parvoviridae) causes a wide spectrum of respiratory diseases in children and gastroenteritis in adults. A lack of sensitive cell lines and efficient animal models hinders research on HBoV, including the development of anti-HBoV drugs or vaccines. Although the construction of a wild-type HBoV1 infectious clone has been reported, generating HBoV1 infectious clone carrying foreign reporter genes with suitable insertion sites in its genome while retaining replicative ability remains challenging. Here, HBoV1 infectious clones harboring the 11-amino-acid HiBiT tag at five distinct insertion sites were constructed and evaluated. Only the recombinant HBoV1 carrying the HiBiT tag in the N-terminus of the NS1 protein (HBoV1-HiBiTNS1) displayed comparable characteristics to wild-type HBoV1 as determined via the analysis of viral DNA copy number, NanoLuc activity, viral protein expression, and the formation of replication intermediates. Notably, the replication kinetics of HBoV1-HiBiTNS1 could be examined by monitoring NanoLuc activity, which was noted to be correlated with the viral DNA level. Additionally, we successfully applied HiBiT-tagged HBoV1 for the evaluation of antiviral drug activity and identified ivermectin (EC50 ​= ​2.27 ​μM) as a potent anti-HBoV1 replication drug. Overall, our study demonstrated that the HBoV1-HiBiTNS1 reporter can serve as a convenient platform for screening candidate drugs targeting HBoV1 replication and may also be useful for investigating the life cycle of the virus.

Monitoring of hepatitis E virus infection and replication by functional tagging of the ORF2 protein

Background and Aims

Hepatitis E virus (HEV) infection is a leading cause of acute hepatitis worldwide. Understanding of the mechanisms underlying productive HEV infection remains incomplete and would benefit from technological advances improving current model systems.

Methods

We exploited transposon-mediated random insertion and selection of viable clones to identify sites in the HEV open reading frame 2 (ORF2) protein, corresponding to the viral capsid, allowing for the insertion of reporter sequences in a functional context.

Results

Short sequence insertions (5 amino acids) were tolerated at four distinct sites in the C-terminal region of the ORF2 protein, without significantly affecting viral capsid expression and subcellular localization as well as virus production. Full-length HEV genomes harboring larger sequence insertions such as an HA epitope tag, a highly sensitive miniaturized luciferase reporter (HiBiT) or a split GFP at these sites conserved their ability to produce infectious virus, with about a 1-log decrease in viral titers. Findings were confirmed in two different HEV genotype 3 clones. In addition, we demonstrate that HiBiT-tagged HEV, offering rapid and several-log amplitude detection, can be used for the evaluation of antiviral drugs and neutralizing antibodies.

Conclusions

We describe a convenient, quantitative and potentially scalable system for the monitoring of HEV infection and replication in tissue culture.

Impact and implications

Hepatitis E virus infection is one of the most frequent causes of acute hepatitis and jaundice worldwide. As treatment options are limited and a vaccine is not universally available, the development of molecular tools to facilitate the identification of new therapeutic strategies is crucial. Based on a screening approach to identify viable insertion sites in the viral genome, we describe a versatile system for preparing recombinant viruses harboring split-reporter tags, i.e. luciferase and GFP. Proof-of-concept experiments revealed that convenient and quantitative monitoring of viral infection and replication is possible with this system, allowing for the evaluation of antiviral drugs and neutralizing antibodies.

Neutralizing Antibody Screening Using NanoBiT-Based Virus-like Particles of Foot-and-Mouth Disease Type Asia1 Enhances Biosafety and Sensitivity

BACKGROUND/OBJECTIVES

Foot-and-mouth disease (FMD) is a highly contagious class 1 animal disease that affects cloven-hoofed animals, such as cattle, pigs, and goats. Diagnosis and research on live FMD virus (FMDV) typically require biosafety level 3 facilities, which are challenging to maintain due to strict protocols and high costs. The development of NanoBiT-based assays has accelerated in response to the coronavirus disease pandemic, providing safer alternatives for viral research, and is now applicable for general laboratories. This study aimed to develop a NanoBiT-based virus-like particle (VLP) assay for the rapid and safe screening of neutralizing antibodies against FMDV Asia1 Shamir (AS).

METHODS

We developed an AS VLP with an inserted HiBiT tag that enabled the detection of entry into LgBiT cells through luminescence signals.

RESULTS

HiBiT-tagged AS VLPs mixed with anti-serum and introduced into LgBiT-expressing cells led to a reduction in luciferase activity. Therefore, we established a NanoBiT-based viral neutralizing antibody test (VNT) that demonstrated a high correlation (R2 = 0.881) with the traditional gold standard VNT.

CONCLUSIONS

The assay demonstrated high sensitivity and could be performed in BL-2 facilities, offering a safer and more efficient alternative to traditional assays while reducing the need to handle live viruses in high-containment facilities. This method provides a valuable tool for rapid screening of neutralizing antibodies and can be adapted for broader applications in FMDV research.

A novel high-throughput screen identifies phenazine-1-carboxylic acid as an inhibitor of African swine fever virus replication in primary porcine alveolar macrophages

African swine fever (ASF), caused by African swine fever virus (ASFV), has resulted in significant economic impacts on the global swine industry. Currently, there is no safe and effective commercial vaccine available for ASFV. Thus, the development of effective and readily available therapeutics for ASF is urgently needed. To conduct high-throughput screening (HTS) for anti-ASFV drugs, we initially developed a recombinant dual-reporter virus (rASFV-Gluc/EGFP) using the virulent strain ASFV HLJ/18 (ASFV-WT). The enhanced green fluorescent protein (EGFP)- and Gaussia luciferase (Gluc)-encoding genes were incorporated downstream of the ASFV MGF300-4L gene without disrupting viral genes. The growth kinetics, hemadsorption, and transmission electron microscopy analysis of rASFV-Gluc/EGFP in primary porcine alveolar macrophages (PAMs) revealed that rASFV-Gluc/EGFP exhibits similar biological characteristics to ASFV-WT. Furthermore, analysis of Gluc activities, fluorescence, and next-generation sequencing indicated that rASFV-Gluc/EGFP maintains good genetic stability after 20 consecutive passages in PAMs. Using the HTS platform established with rASFV-Gluc/EGFP, we screened and identified phenazine-1-carboxylic acid (PCA) as an effective inhibitor of ASFV replication from 246 small molecule compounds in PAMs. Importantly, PCA was found to reduce ASFV replication by as much as 100-fold at a concentration of 25 μM. Overall, this study suggests that rASFV-Gluc/EGFP is suitable for rapid screening of anti-ASFV drugs. Importantly, we showed that PCA has significant anti-ASFV activity in PAMs.

Role of Rab13, Protein Kinase A, and Zonula Occludens-1 in Hepatitis E Virus Entry and Cell-to-Cell Spread: Comparative Analysis of Quasi-Enveloped and Non-Enveloped Forms

Hepatitis E virus (HEV) exists in two distinct forms: a non-enveloped form (neHEV), which is present in feces and bile, and a quasi-enveloped form (eHEV), found in circulating blood and culture supernatants. This study aimed to elucidate the roles of Ras-associated binding 13 (Rab13) and protein kinase A (PKA) in the entry mechanisms of both eHEV and neHEV, utilizing small interfering RNA (siRNA) and chemical inhibitors. The results demonstrated that the entry of both viral forms is dependent on Rab13 and PKA. Further investigation into the involvement of tight junction (TJ) proteins revealed that the targeted knockdown of zonula occludens-1 (ZO-1) significantly impaired the entry of both eHEV and neHEV. In addition, in ZO-1 knockout (KO) cells inoculated with either viral form, HEV RNA levels in culture supernatants did not increase, even up to 16 days post-inoculation. Notably, the absence of ZO-1 did not affect the adsorption efficiency of eHEV or neHEV, nor did it influence HEV RNA replication. In cell-to-cell spread assays, ZO-1 KO cells inoculated with eHEV showed a lack of expression of HEV ORF2 and ORF3 proteins. In contrast, neHEV-infected ZO-1 KO cells showed markedly reduced ORF2 and ORF3 protein expression within virus-infected foci, compared to non-targeting knockout (NC KO) cells. These findings underscore the crucial role of ZO-1 in facilitating eHEV entry and mediating the cell-to-cell spread of neHEV in infected cells.

Integration of HiBiT into enteroviruses: A universal tool for advancing enterovirus virology research

The utilization of enteroviruses engineered with reporter genes serves as a valuable tool for advancing our understanding of enterovirus biology and its applications, enabling the development of effective therapeutic and preventive strategies. In this study, our initial attempts to introduce a NanoLuc luciferase (NLuc) reporter gene into recombinant enteroviruses were unsuccessful in rescuing viable progenies. We hypothesized that the size of the inserted tag might be a determining factor in the rescue of the virus. Therefore, we inserted the 11-amino-acid HiBiT tag into the genomes of enterovirus A71 (EV-A71), coxsackievirus A10 (CVA10), coxsackievirus A7 (CVA7), coxsackievirus A16 (CVA16), namely EV-A71-HiBiT, CVA16-HiBiT, CVA10-HiBiT, CVA7-HiBiT, and observed that the HiBiT-tagged viruses exhibited remarkably high rescue efficiency. Notably, the HiBiT-tagged enteroviruses displayed comparable characteristics to the wild-type viruses. A direct comparison between CVA16-NLuc and CVA16-HiBiT recombinant viruses revealed that the tiny HiBiT insertion had minimal impact on virus infectivity and replication kinetics. Moreover, these HiBiT-tagged enteroviruses demonstrated high genetic stability in different cell lines over multiple passages. In addition, the HiBiT-tagged viruses were successfully tested in antiviral drug assays, and the sensitivity of the viruses to drugs was not affected by the HiBiT tag. Ultimately, our findings provide definitive evidence that the integration of HiBiT into enteroviruses presents a universal, convenient, and invaluable method for advancing research in the realm of enterovirus virology. Furthermore, HiBiT-tagged enteroviruses exhibit great potential for diverse applications, including the development of antivirals and the elucidation of viral infection mechanisms.

Three Distinct Reporter Systems of Hepatitis E Virus and Their Utility as Drug Screening Platforms

The hepatitis E virus (HEV) is increasingly acknowledged as the primary cause of acute hepatitis. While most HEV infections are self-limiting, cases of chronic infection and fulminant hepatitis necessitate the administration of anti-HEV medications. However, there is a lack of specific antiviral drugs designed for HEV, and the currently available drug (ribavirin) has been associated with significant adverse effects. The development of innovative antiviral drugs involves targeting distinct steps within the viral life cycle: the early step (attachment and internalization), middle step (translation and RNA replication), and late step (virus particle formation and virion release). We recently established three HEV reporter systems, each covering one or two of these steps. Using these reporter systems, we identified various potential drug candidates that target different steps of the HEV life cycle. Through rigorous in vitro testing using our robust cell culture system with the genotype 3 HEV strain (JE03-1760F/P10), we confirmed the efficacy of these drugs, when used alone or in combination with existing anti-HEV drugs. This underscores their significance in the quest for an effective anti-HEV treatment. In the present review, we discuss the development of the three reporter systems, their applications in drug screening, and their potential to advance our understanding of the incompletely elucidated HEV life cycle.