الفيروسية في وقت مبكر فحوصات دخول لتحديد وتقييم المركبات المضادة للفيروسات
Summary
Here, we present a protocol that examines specific steps of the viral entry to identify and evaluate novel antiviral agents.
Abstract
Cell-based systems are useful for discovering antiviral agents. Dissecting the viral life cycle, particularly the early entry stages, allows a mechanistic approach to identify and evaluate antiviral agents that target specific steps of the viral entry. In this report, the methods of examining viral inactivation, viral attachment, and viral entry/fusion as antiviral assays for such purposes are described, using hepatitis C virus as a model. These assays should be useful for discovering novel antagonists/inhibitors to early viral entry and help expand the scope of candidate antiviral agents for further drug development.
Introduction
Viral infections are a constant threat to the public health and a significant cause of epidemic diseases, morbidity, and deaths worldwide. Specific modes of control against viral infections include vaccine development and antiviral therapies. While vaccine efforts have proven successful in immunizing against several viruses, many viral pathogens remain without a protective vaccine including dengue virus (DENV), human cytomegalovirus (HCMV), hepatitis C virus (HCV), human immunodeficiency virus (HIV), and respiratory syncytial virus (RSV)1-5. Antivirals, on the other hand, play an important role for the management of these viral infections when prophylactic vaccines are unavailable. However, to date, only few licensed and cost-effective antiviral drugs are available compared to the number of viral pathogens that threatens the public health. More importantly, due to an increase in global travel and rapid urbanization, the situation is aggravated by risks of epidemic outbreaks from emerging/re-emerging viral infections that are being introduced into non-indigenous areas6. Recent outbreaks caused by severe acute respiratory syndrome (SARS) virus, influenza viruses (H1N1, H5N1, H3N2, and H7N9), DENV, West Nile virus (WNV), measles virus (MV), Middle East Respiratory Syndrome (MERS) virus, and Ebola virus6-12 are among the examples reflecting the need for antivirals development when immunization and/or therapeutics are unavailable. In addition, there is always a potential risk of generating drug-resistant infections with currently used antivirals. Thus, the continuous development and expansion of the scope of antivirals to these emerging/re-emerging viral infections are necessary to provide better management strategies and safeguard the public health.
Most antiviral therapies consist of direct acting antivirals (DAAs) which target a specific viral protein or cofactor that mediates important steps in the viral life cycle. For example, the nucleoside analogue Acyclovir inhibits herpesvirus DNA polymerase, protease inhibitors Boceprevir and Telaprevir antagonize the HCV NS3, and Oseltamivir and Zanamivir are neuraminidase inhibitors that block the release of influenza virus particles from infected cells13-15. There are however very few licensed viral entry inhibitors including Enfuvirtide, which targets HIV gp41 to prevent fusion, and Maraviroc, which blocks the HIV co-receptor CCR5, thereby preventing viral entry16. Exploring novel antagonists/inhibitors to viral entry could help provide additional therapeutics for prophylactic or therapeutic use, such as in combination with other antivirals with a different mechanism of action to better manage viral infections17-19.
Identification of antivirals can involve structure-guided drug design and candidate drug screening-based strategy. Methods for assessing antiviral activity of test agents include biochemical assays of enzymatic activity and evaluation by cell-based systems20-23. In cell-based systems, the viral life cycle can be dissected into distinct stages of infection, such as entry events (attachment, fusion, uncoating), the replication phase (viral genome replication and protein translation), and virion egress (assembly, maturation, and release). Since the assays can be adapted to investigate each specific stage using various tools and methods, this approach allows identification/examination of potential candidate antivirals with a specific mechanism of action targeting the distinct stage analyzed. For instance, to analyze drug effect specifically on the free virus particles prior to binding to the host cell, a ‘viral inactivation assay’ can be performed. In this assay, the virus is allowed to incubate with the test drug and then diluted to titrate out the drug before infecting a cell monolayer. Additional steps such as viral attachment and entry/fusion stages can also be analyzed individually, by shifting the temperature during the infection. For many enveloped viruses, viral entry/fusion at the host cell membrane is greatly facilitated at 37 °C, but is typically precluded at 4 °C which does not affect virus binding24-29. Finally, the use of reporter viruses or cell systems could facilitate these studies and permit a high-throughput analysis.
We previously employed the cell-based approach and dissected the early entry of various enveloped viruses for the examination of antiviral compounds that potentially antagonize viral entry30,31. Herein, the various methods used, including viral inactivation, viral attachment, and viral entry/fusion assays, are described.
Protocol
Representative Results
Discussion
In this report the methods to identify and evaluate antiviral compounds based on a mechanistic approach of dissecting the early viral entry events were described. Specifically, the assays allowed us to examine the effect of test compounds on free virus particles, viral attachment, and viral entry/fusion. Critical steps were implemented to distinctly evaluate the drug effect on the specific stage of early viral entry. For instance, in the ‘viral inactivation assay’, the dilution of the virus-drug mixture to sub-therapeutic concentration prevents significant interaction between the test compound and the host cell surface by ‘titrating out’ the drug. This ensures that the inhibitory effect observed on the subsequent infection of the host cell is due to a direct interaction between the test compound and the cell-free virions, rather than an effect from the test compound on host cell membrane or membrane-associated molecules, including viral receptors30. Similarly, the shift in temperature between 4 °C (which allows for virus binding but not entry) and 37 °C (which facilitates virus entry/fusion) in the ‘viral attachment assay’ and ‘viral entry/fusion assay’ are crucial to determine the test compound’s effect on each of these specific events. This is feasible due to the temperature sensitivity of enveloped viruses during these steps in the infection24-29. It is therefore important that the assays are performed at the indicated temperature to ensure the accuracy of the results; for example, by carrying out the experiment on ice to maintain at 4 °C and by placing the sample directly in a 37 °C incubator for the temperature shift. In addition, the use of negative (ex. DMSO solvent for drug preparation) and positive (ex. heparin treatment) controls also help further establish the assays’ accuracy. The utility and applicability of such methods have been demonstrated in many antiviral studies26,30,31,40,41. Note that while heparin is included as a control for all three assays in the context of the representative results, it typically blocks the initial virus binding rather than the ensuing fusion/entry step (as reflected by the data in Figure 2). Additional controls could also be used, such as neutralizing antibodies directed against the virus (for viral inactivation assay), antibodies that mask the cell surface receptors for the virus (for viral attachment assay), and membrane fusion inhibitors (for viral fusion/entry assay).
The assays described in this report, which are specific to the early stages of the viral infection, are useful in terms of application as secondary tests to characterize the mechanism of action of candidate drugs from primary screens which typically target the viral infection more broadly. Alternatively, they could also be incorporated in primary screens if one is specifically looking for inhibitors of early viral entry, including virus inactivating agents, viral attachment antagonists, and inhibitors to viral entry/fusion. In this case, their use allows a more focused and precise screen analysis for the identification of mechanism-specific antiviral candidates, which, in turn, would expedite downstream drug development.
The use of cell-based assays in identifying antiviral agents provides several important advantages compared to biochemical assays, including revealing potential off-target effects (such as cytotoxicity) and adding physiological relevance to the bioactivity of the test agents42. These issues are important considerations for deciding whether a candidate agent is of value for continuation in subsequent phases of drug development. Similarly, the early viral entry-specific assays described in this report allow examination of the drug effect on the distinct viral entry stage at the cellular level, and more specifically in the context of an authentic viral infection in vitro. The results obtained from such assays would therefore help better predict the antiviral efficacy of the test compounds and also identify potentially unwanted off-target effects against the host cell. One potential limitation though, is that an in vitro cell-based assay may not completely reflect the actual in vivo entry step in the context of a natural viral infection. Nonetheless, the assays presented in this protocol do serve as an analytical platform for mechanism-based identification and evaluation of novel antiviral agents.
The development of reporter viruses or reporter cell systems to quantitate the amount of viral infection has greatly facilitated cell-based screening and evaluation of antiviral compounds. Examples include the use of recombinant viruses carrying a reporter gene or by means of recombinant human cell lines containing a reporter gene driven by the specific virus promoter31,43. In this report, the infection from luciferase-tagged HCV can be easily monitored by quantitating the reporter signal, thus facilitating data analysis. By incorporating these useful reporter-based tools, the early viral entry assays described here can essentially be adapted into high-throughput format for mechanism-based screening of small molecule libraries.
In conclusion, a protocol was described for assays dissecting the early viral entry as a means of identifying and evaluating mechanism-specific antiviral compounds. Such assays would be useful for discovering novel antagonists/inhibitors to viral entry and help expand the scope of antiviral agents for development as prophylactic and/or therapeutic treatments.
Divulgations
The authors have nothing to disclose.
Acknowledgements
This study is supported by funding from Taipei Medical University Hospital (102TMU-TMUH-19) and the Ministry of Science and Technology of Taiwan (MOST103-2320-B-038-031-MY3).
Materials
| DMEM | GIBCO | 11995-040 | |
| FBS | GIBCO | 26140-079 | |
| Penicillin-Streptomycin | GIBCO | 15070-063 | |
| Amphotericin B | GIBCO | 15290-018 | |
| DMSO | Sigma | D5879 | |
| In vitro toxicology assay kit, XTT-based | Sigma | TOX2 | |
| PBS pH 7.4 | GIBCO | 10010023 | |
| Microplate reader | Bio-Tek Instrument, Inc. | ELx800 | |
| Microcentrifuge | Thermo Scientific | 75002420 | |
| BioLux Gaussia luciferase assay kit | New England Biolabs | E3300L | |
| Luminometer | Promega | GloMax-20/20 | |
| Sodium citrate, dihydrate | Sigma | 71402 | |
| Potassium chloride | Sigma | P5405 |
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