Establishment of a cell-based assay for screening of compounds inhibiting very early events in the cytomegalovirus replication cycle and characterization of a compound identified using the assay

Characterization of a thiourea derivative that targets viral transactivators of cytomegalovirus and herpes simplex virus type 1

Although currently available antivirals against certain herpesviruses are effective, the development of resistance during long-term use has necessitated the search for seed compounds that work against novel target molecules. In this report, we identified a thiourea derivative compound, 147B3, that inhibits the infection of human cytomegalovirus (HCMV) in fibroblasts and herpes simplex virus type 1 (HSV-1) in Vero cells at a 50% effective concentration of 0.5 μM and 1.9 μM, respectively. Characterization of the compound provided the following clues regarding its mode of action. 1) Time-of-addition and block-release assays showed that 147B3 behaved similarly to ganciclovir. 2) 147B3 reduced the expression of early and late but not immediate-early gene products and the accumulation of viral genomic DNA in both HCMV-infected and HSV-1-infected cells. 3) 147B3 inhibited the HCMV IE2-dependent activation of viral early gene promoters. 4) Four HSV-1 clones resistant to 147B3 were isolated and next-generation sequencing analysis of their genome DNA revealed that all of them had a mutation(s) in the infected cell protein 4 (ICP4) gene, which encodes a viral transcriptional factor. 5) Although 147B3 did not reduce the amount of ICP4 in an immunoblotting analysis, it changed the localization of the ICP4 from the speckles in the nuclei to diffused dots in the cytoplasm. 6) 147B3 did not affect the localization of promyelocytic leukemia (PML) bodies. Our findings suggest that 147B3 targets viral transactivators, potentially through their interaction with factors required for the viral gene expression system.

Validation and Characterization of Five Distinct Novel Inhibitors of Human Cytomegalovirus

The critical consequences of human cytomegalovirus (HCMV) infection in the transplant population and in congenitally infected infants, the limited treatment options for HCMV, and the rise of resistant mutants toward existing therapies has fueled the search for new anti-HCMV agents. A pp28-luciferase recombinant HCMV was used as a reporter system for high-throughput screening of HCMV inhibitors. Approximately 400 000 compounds from existing libraries were screened. Subsequent validation assays using resynthesized compounds, several virus strains, and detailed virology assays resulted in the identification of five structurally unique and selective HCMV inhibitors, active at sub to low micromolar concentrations. Further characterization revealed that each compound inhibited a specific stage of HCMV replication. One compound was also active against herpes simplex virus (HSV1 and HSV2), and another compound was active against Epstein-Barr virus (EBV). Drug combination studies revealed that all five compounds were additive with ganciclovir or letermovir. Future studies will focus on optimization of these new anti-HCMV compounds along with mechanistic studies.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Titration of cell-associated varicella-zoster virus with the MV9G reporter cell line for antiviral studies

Titration of the cell-associated virus (CAV) of varicella-zoster virus (VZV) is essential for antiviral studies. A VZV reporter cell line, MV9G, generated in our previous study expresses firefly luciferase upon CAV infection in a dose-dependent manner, suggesting that use of the cell line for titration is feasible. In this study, MeWo cells infected with VZV vaccine Oka (vOka) strain or with clinical isolates obtained from patients with varicella or zoster were used as CAV. A co-culture of MV9G cells with the virus-infected MeWo cells were set up and optimized for titration of CAV. Luciferase activities of MV9G cells measured as relative light units (RLUs) of chemiluminescence correlated well (r > 0.9, p < 0.05) both with quantities of viral DNAs measured by TaqMan PCR and with numbers of viral foci detected by immunostaining with a monoclonal antibody against VZV IE62. In addition, the usefulness of MV9G for antiviral studies was exemplified by treatment of the VZV-infected cells with various concentrations of acyclovir. Thus, the reporter cell-based titration of CAV by measuring the induced RLUs may be a reliable way to estimate viral foci and viral DNAs.

Characterization of phenyl pyrimidine derivatives that inhibit cytomegalovirus immediate-early gene expression

Background

Previously, we established a reporter cell line for human cytomegalovirus and screened anti-human cytomegalovirus compounds using the cell line. In this study, we characterized one of the identified compounds, 2,4-diamino-6–(4-methoxyphenyl)pyrimidine (coded as 35C10).

Methods

50% Effective concentrations (EC50s) and 50% cytotoxic concentrations (CC50s) of 35C10 and its derivatives in human fibroblasts were determined by X-gal staining of the cells infected with human cytomegalovirus Towne strain expressing β-galactosidase.

Results

EC50 and CC50 of 35C10 were 4.3 µM and >200 µM, respectively. Among several 35C10 derivatives, only one lacking 4-amino group of pyrimidine showed a similar EC50. 35C10 weakly inhibited murine cytomegalovirus, herpes simplex virus type 1, and varicella-zoster virus. A “time of addition” experiment suggested that 35C10 inhibited an early phase of the infection. Although 35C10 did not inhibit viral attachment to the cells nor the delivery of viral DNA to the nuclei, it decreased the number of infected cells expressing immediate-early 1 and 2 (IE1/IE2) proteins. 35C10 also inhibited the activation of a promoter for TRL4 in the reporter cells upon human cytomegalovirus infection, but not in the same reporter cells transfected with a plasmid expressing IE2.

Conclusion

Our findings suggest that 35C10 is a novel compound that inhibits IE gene expression in human cytomegalovirus-infected cells.

Modular cell-based platform for high throughput identification of compounds that inhibit a viral interferon antagonist of choice

Viral interferon (IFN) antagonists are a diverse class of viral proteins that counteract the host IFN response, which is important for controlling viral infections. Viral IFN antagonists are often multifunctional proteins that perform vital roles in virus replication beyond IFN antagonism. The critical importance of viral IFN antagonists is highlighted by the fact that almost all viruses encode one of these proteins. Inhibition of viral IFN antagonists has the potential to exert pleiotropic antiviral effects and thus this important protein class represents a diverse plethora of novel therapeutic targets. To exploit this, we have successfully developed and executed a novel modular cell-based platform that facilitates the safe and rapid screening for inhibitors of a viral IFN antagonist of choice. The platform is based on two reporter cell-lines that provide a simple method to detect activation of IFN induction or signaling via an eGFP gene placed under the control of the IFNβ or an ISRE-containing promoter, respectively. Expression of a target IFN antagonist in the appropriate reporter cell-line will block the IFN response and hence eGFP expression. We hypothesized that addition of a compound that inhibits IFN antagonist function will release the block imposed on the IFN response and hence restore eGFP expression, providing a measurable parameter for high throughput screening (HTS). We demonstrate assay proof-of-concept by (i) exploiting hepatitis C virus (HCV) protease inhibitors to inhibit NS3-4A's capacity to block IFN induction and (ii) successfully executing two HTS targeting viral IFN antagonists that block IFN signaling; NS2 and IE1 from human respiratory syncytial virus (RSV) and cytomegalovirus (CMV) respectively, two clinically important viruses for which vaccine development has thus far been unsuccessful and new antivirals are required. Both screens performed robustly and Z' Factor scores of >0.6 were achieved. We identified (i) four hit compounds that specifically inhibit RSV NS2's ability to block IFN signaling by mediating STAT2 degradation and exhibit modest antiviral activity and (ii) two hit compounds that interfere with IE1 transcription and significantly impair CMV replication. Overall, we demonstrate assay proof-of-concept as we target viral IFN antagonists from unrelated viruses and demonstrate its suitability for HTS.

Vaccine Development for Cytomegalovirus

The development of a cytomegalovirus (CMV) vaccine has become a top priority due to its potential cost-effectiveness and associated public health benefits. However, there are a number of challenges facing vaccine development including the following: (1) CMV has many mechanisms for evading immune responses , and natural immunity is not perfect, (2) the immune correlates for protection are unclear, (3) a narrow range of CMV hosts limits the value of animal models, and (4) the placenta is a specialized organ formed transiently and its immunological status changes with time. In spite of these limitations, several types of CMV vaccine candidate, including live-attenuated, DISC , subunit, DNA, vectored, and peptide vaccines, have been developed or are currently under development. The recognition of the pentameric complex as the major neutralization target and identification of various strategies to block viral immune response evasion mechanisms have opened new avenues to CMV vaccine development. Here, we discuss the immune correlates for protection, the characteristics of the various vaccine candidates and their clinical trials, and the relevant animal models.

Valnoctamide Inhibits Cytomegalovirus Infection in Developing Brain and Attenuates Neurobehavioral Dysfunctions and Brain Abnormalities

Cytomegalovirus (CMV) is the most common infectious cause of brain defects and neurological dysfunction in developing human babies. Due to the teratogenicity and toxicity of available CMV antiviral agents, treatment options during early development are markedly limited. Valnoctamide (VCD), a neuroactive mood stabilizer with no known teratogenic activity, was recently demonstrated to have anti-CMV potential. However, it is not known whether this can be translated into an efficacious therapeutic effect to improve CMV-induced adverse neurological outcomes. Using multiple models of CMV infection in the developing mouse brain, we show that subcutaneous low-dose VCD suppresses CMV by reducing the level of virus available for entry into the brain and by acting directly within the brain to block virus replication and dispersal. VCD during the first 3 weeks of life restored timely acquisition of neurological milestones in neonatal male and female mice and rescued long-term motor and behavioral outcomes in juvenile male mice. CMV-mediated brain defects, including decreased brain size, cerebellar hypoplasia, and neuronal loss, were substantially attenuated by VCD. No adverse side effects on neurodevelopment of uninfected control mice receiving VCD were detected. Treatment of CMV-infected human fetal astrocytes with VCD reduced both viral infectivity and replication by blocking viral particle attachment to the cell, a mechanism that differs from available anti-CMV drugs. These data suggest that VCD during critical periods of neurodevelopment can effectively suppress CMV replication in the brain and safely improve both immediate and long-term neurological outcomes.SIGNIFICANCE STATEMENT Cytomegalovirus (CMV) can irreversibly damage the developing brain. No anti-CMV drugs are available for use during fetal development, and treatment during the neonatal period has substantial limitations. We studied the anti-CMV actions of valnoctamide (VCD), a psychiatric sedative that appears to lack teratogenicity and toxicity, in the newborn mouse brain, a developmental period that parallels that of an early second-trimester human fetus. In infected mice, subcutaneous VCD reaches the brain and suppresses viral replication within the CNS, rescuing the animals from CMV-induced brain defects and neurological problems. Treatment of uninfected control animals exerts no detectable adverse effects. VCD also blocks CMV replication in human fetal brain cells.

Characterization of a thienylcarboxamide derivative that inhibits the transactivation functions of cytomegalovirus IE2 and varicella zoster virus IE62

Previously we established reporter cell lines for human cytomegalovirus (HCMV) and varicella zoster virus (VZV) and identified several antiviral compounds against these viruses using the reporter cells. In this study, we found that one of the identified anti-HCMV compounds, a thienylcarboxamide derivative (coded as 133G4), was effective against not only HCMV but also VZV. The following findings indicate that 133G4 inhibits the activation of early gene promoters by HCMV IE2 and VZV IE62: i) 133G4 decreased the expression of HCMV early and late genes but not that of HCMV IE1/IE2 in HCMV-infected cells, ii) 133G4 inhibited the activation of several HCMV early gene promoters of transiently-transfected plasmids in HCMV-infected cells, and iii) in transient transfection assays, 133G4 decreased the activation of HCMV (or VZV) early gene promoters by HCMV IE2 (or VZV IE62) in the absence of other viral protein expression. The inhibition of early gene activation was observed in the human and African green monkey cell lines but not in the rodent cell lines, and the compound was not effective against murine CMV. In addition, VZV IE62 activated HCMV early promoters, and 133G4 still inhibited such promoter activation. Therefore, we hypothesized that 133G4 targets a cellular factor used commonly in activation of human herpesvirus promoters and examined whether 133G4 affects the functions of cellular proteins USF1, TBP, Med25 and EAP, the involvement of which in VZV IE62-dependent viral gene activation has been well characterized. Our experimental results using one-hybrid and bimolecular fluorescence complementation assays demonstrated that 133G4 did not inhibit the recruitment of USF1 or TBP to their binding sites, nor inhibited the direct interactions of VZV IE62 with Med25 and EAP. Thus, 133G4 is a unique anti-VZV and -HCMV compound, which warrants further studies to find out its inhibitory mechanism.

A Luciferase Gene Driven by an Alphaherpesviral Promoter Also Responds to Immediate Early Antigens of the Betaherpesvirus HCMV, Allowing Comparative Analyses of Different Human Herpesviruses in One Reporter Cell Line

Widely used methods for quantification of human cytomegalovirus (HCMV) infection in cell culture such as immunoblotting or plaque reduction assays are generally restricted to low throughput and require time-consuming evaluation. Up to now, only few HCMV reporter cell lines have been generated to overcome these restrictions and they are afflicted with other limitations because permanently expandable cell lines are normally not fully permissive to HCMV. In this work, a previously existing epithelial cell line hosting a luciferase gene under control of a Varicella-zoster virus promoter was adopted to investigate HCMV infection. The cells were susceptible to different HCMV strains at infection efficiencies that corresponded to their respective degree of epithelial cell tropism. Expression of early and late viral antigens, formation of nuclear inclusions, release of infectious virus progeny, and focal growth indicated productive viral replication. However, viral release and spread occurred at lower levels than in primary cell lines which appears to be due to a malfunction of virion morphogenesis during the nuclear stage. Expression of the luciferase reporter gene was specifically induced in HCMV infected cultures as a function of the virus dose and dependent on viral immediate early gene expression. The level of reporter activity accurately reflected infection efficiencies as determined by viral antigen immunostaining, and hence could discriminate the cell tropism of the tested virus strains. As proof-of-principle, we demonstrate that this cell line is applicable to evaluate drug resistance of clinical HCMV isolates and the neutralization capacity of human sera, and that it allows comparative and simultaneous analysis of HCMV and human herpes simplex virus type 1. In summary, the permanent epithelial reporter cell line allows robust, rapid and objective quantitation of HCMV infection and it will be particularly useful in higher throughput analyses as well as in comparative analyses of different human herpesviruses.

Mood stabilizers inhibit cytomegalovirus infection

Cytomegalovirus (CMV) infection can generate debilitating disease in immunocompromised individuals and neonates. It is also the most common infectious cause of congenital birth defects in infected fetuses. Available anti-CMV drugs are partially effective but are limited by some toxicity, potential viral resistance, and are not recommended for fetal exposure. Valproate, valpromide, and valnoctamide have been used for many years to treat epilepsy and mood disorders. We report for the first time that, in contrast to the virus-enhancing actions of valproate, structurally related valpromide and valnoctamide evoke a substantial and specific inhibition of mouse and human CMV in vitro. In vivo, both drugs safely attenuate mouse CMV, improving survival, body weight, and developmental maturation of infected newborns. The compounds appear to act by a novel mechanism that interferes with CMV attachment to the cell. Our work provides a novel potential direction for CMV therapeutics through repositioning of agents already approved for use in psychiatric disorders.

A dark-to-bright reporter cell for classical swine fever virus infection

Current methods to quantitate classical swine fever virus (CSFV) infectivity in cell culture are time-consuming and labor-intensive. This study described the generation of a dark-to-bright fluorescent reporter cells to facilitate in vitro studies of CSFV infection and replication. This assay was based on a novel reporter cell stably expressing the enhanced green fluorescent protein (EGFP) fused in-frame to a quenching peptide via a special recognition sequence of the CSFV NS3 protease. Chromophore maturation of EGFP can be prevented by quenching peptide until the quenching peptide was specifically cleaved by NS3 protease during CSFV infection, making it a dark-to-bright reporter of CSFV infection. The result demonstrated that the CSFV-infected cells were clearly distinguishable from mock-infected cells and cells infected with other viruses. There was a strong correlation between the fluorescence intensity and viral RNA replication in CSFV-infected cells. The cell enabled rapid and sensitive detection of CSFV infection and viral replication in cell culture. The best time to examine the fluorescence in CSFV-infected cells was at 48h post-inoculation. These data suggested that the cells can be used as a reporter cell in CSFV infection assays. This reporter cell provides a sensitive method for the detection and isolation of CSFV and it will be useful for the screening of antiviral drugs or neutralizing antibody assays.

Development of a high-throughput assay to measure the neutralization capability of anti-cytomegalovirus antibodies

Infection by human cytomegalovirus (CMV) elicits a strong humoral immune response and robust anti-CMV antibody production. Diagnosis of virus infection can be carried out by using a variety of serological assays; however, quantification of serum antibodies against CMV may not present an accurate measure of a patient's ability to control a virus infection. CMV strains that express green fluorescent protein (GFP) fusion proteins can be used as screening tools for evaluating characteristics of CMV infection in vitro. In this study, we employed a CMV virus strain, AD169, that ectopically expresses a yellow fluorescent protein (YFP) fused to the immediate-early 2 (IE2) protein product (AD169IE2-YFP) to quantify a CMV infection in human cells. We created a high-throughput cell-based assay that requires minimal amounts of material and provides a platform for rapid analysis of the initial phase of virus infection, including virus attachment, fusion, and immediate-early viral gene expression. The AD169IE2-YFP cell infection system was utilized to develop a neutralization assay with a monoclonal antibody against the viral surface glycoprotein gH. The high-throughput assay was extended to measure the neutralization capacity of serum from CMV-positive subjects. These findings describe a sensitive and specific assay for the quantification of a key immunological response that plays a role in limiting CMV dissemination and transmission. Collectively, we have demonstrated that a robust high-throughput infection assay can analyze the early steps of the CMV life cycle and quantify the potency of biological reagents to attenuate a virus infection.

Identification of a varicella-zoster virus replication inhibitor that blocks capsid assembly by interacting with the floor domain of the major capsid protein

A novel anti-varicella-zoster virus compound, a derivative of pyrazolo[1,5-c]1,3,5-triazin-4-one (coded as 35B2), was identified from a library of 9,600 random compounds. This compound inhibited both acyclovir (ACV)-resistant and -sensitive strains. In a plaque reduction assay under conditions in which the 50% effective concentration of ACV against the vaccine Oka strain (V-Oka) in human fibroblasts was 4.25 μM, the 50% effective concentration of 35B2 was 0.75 μM. The selective index of the compound was more than 200. Treatment with 35B2 inhibited neither immediate-early gene expression nor viral DNA synthesis. Twenty-four virus clones resistant to 35B2 were isolated, all of which had a mutation(s) in the amino acid sequence of open reading frame 40 (ORF40), which encodes the major capsid protein (MCP). Most of the mutations were located in the regions corresponding to the "floor" domain of the MCP of herpes simplex virus 1. Treatment with 35B2 changed the localization of MCP in the fibroblasts infected with V-Oka but not in the fibroblasts infected with the resistant clones, although it did not affect steady-state levels of MCP. Overexpression of the scaffold proteins restored the normal MCP localization in the 35B2-treated infected cells. The compound did not inhibit the scaffold protein-mediated translocation of MCP from the cytoplasm to the nucleus. Electron microscopic analysis demonstrated the lack of capsid formation in the 35B2-treated infected cells. These data indicate the feasibility of developing a new class of antivirals that target the herpesvirus MCPs and inhibit normal capsid formation by a mechanism that differs from those of the known protease and encapsidation inhibitors. Further biochemical studies are required to clarify the precise antiviral mechanism.

Early inhibitors of human cytomegalovirus: state-of-art and therapeutic perspectives

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, mainly transplant recipients and AIDS patients, and is the most frequent cause of congenital malformations in newborn children. To date, few drugs are licensed for the treatment of HCMV infections, most of which target the viral DNA polymerase and suffer from many drawbacks, including long-term toxicity, low potency, and poor bioavailability. In addition, the emergence of drug-resistant viral strains is becoming an increasing problem for disease management. Finally, none of the current anti-HCMV drugs have been approved for the treatment of congenital infections. For all these reasons, there is still a strong need for new anti-HCMV drugs with novel mechanisms of action. The first events of the virus replication cycle, including attachment, entry, immediate-early gene expression, and immediate-early functions—in particular that of Immediate-Early 2 protein—represent attractive targets for the development of novel antiviral compounds. Such inhibitors would block not only the expression of viral immediate-early proteins, which play a key role in the pathogenesis of HCMV infection, but also the host immunomodulation and the changes to cell physiology induced by the first events of virus infection. This review describes the current knowledge on the initial phases of HCMV replication, their validation as potential novel antiviral targets, and the development of compounds that block such processes.

Recombinant luciferase-expressing human cytomegalovirus (CMV) for evaluation of CMV inhibitors

Recombinant Towne CMV expressing luciferase under the control of CMV-DNA polymerase (POL) or the late pp28 (UL99) promoters were evaluated for potential application in high-throughput screening of anti-viral compounds. POL-and pp28-luciferase displayed maximal expression 48 and 72 hours post infection, respectively. The pp28-luciferase virus achieved a wider dynamic range of luciferase expression (6-7 logs) and was selected for testing of inhibition by five anti-viral compounds. Luciferase expression highly correlated with plaque reduction and real-time PCR. The pp28-luciferase reporter system is rapid, reproducible, and highly sensitive. It may be applied to screening of novel anti-CMV compounds.