Natural products that reduce rotavirus infectivity identified by a cell-based moderate-throughput screening assay
© Shaneyfelt et al; licensee BioMed Central Ltd. 2006
Received: 22 June 2006
Accepted: 01 September 2006
Published: 01 September 2006
There is widespread interest in the use of innate immune modulators as a defense strategy against infectious pathogens. Using rotavirus as a model system, we developed a cell-based, moderate-throughput screening (MTS) assay to identify compounds that reduce rotavirus infectivity in vitro, toward a long-term goal of discovering immunomodulatory agents that enhance innate responses to viral infection.
A natural product library consisting of 280 compounds was screened in the assay and 15 compounds that significantly reduced infectivity without cytotoxicity were identified. Time course analysis of four compounds with previously characterized effects on inflammatory gene expression inhibited replication with pre-treatment times as minimal as 2 hours. Two of these four compounds, α-mangostin and 18-β-glycyrrhetinic acid, activated NFκB and induced IL-8 secretion. The assay is adaptable to other virus systems, and amenable to full automation and adaptation to a high-throughput format.
Identification of several compounds with known effects on inflammatory and antiviral gene expression that confer resistance to rotavirus infection in vitro suggests the assay is an appropriate platform for discovery of compounds with potential to amplify innate antiviral responses.
Interest in the use of innate immune modulating agents recently has increased in the context of developing effective biodefense strategies. Increasing natural disease resistance by administration of agonists that stimulate pathogen recognition receptors and gene expression pathways is an approach that would provide broad protection from infection without need for pathogen-specific vaccines. Stimulating broadly reactive immune responses to viral, as well as bacterial and protozoan pathogens, has shown efficacy in animal models and is the subject of recent reviews that address the utility of immune potentiators in developing infectious disease defense strategies [1, 2]. The number of such compounds currently in clinical development lends support for this approach .
To be seriously considered as a candidate antiviral drug or innate immune agonist, rapid and quantitative assessment of activity and toxicity in cell culture are prerequisite to lead compound development . Therefore, development of screening assays in cell lines that can simultaneously support virus replication and be responsive to measures of inhibition of virus replication are a priority. Cell-based high-throughput screening (HTS) assays that test for compounds active against hepatitis C virus (HCV), HIV and SARS coronavirus utilize recombinant viruses and changes in reporter gene expression in engineered cell lines to measure antiviral activity [5–8]. For example, a dual replicon assay system that combined reporter gene assays and FRET was used to screen for drugs effective against HCV . Advantages of these cited assays include rapid readout and an ability to perform screens with viruses that must be handled under elevated biosafety conditions.
The intent of the studies reported here was to establish a cell-based screening assay that could identify compounds that inhibit virus replication by inducing antiviral gene expression pathways. Although the assay could identify compounds that target distinct steps of the virus replication cycle, we are interested in those that stimulate cellular responses necessary to confer initial resistance to rotavirus infection. The theory behind this approach is that amplification of the antiviral response will override virus-encoded immune evasion strategies and restrict replication to subclinical levels.
Rotaviruses are responsible for the majority of childhood morbidity and mortality from viral gastroenteritis . Several rotavirus vaccines in clinical trials show promising efficacy, suggesting that a long-term goal of rotavirus gastroenteritis becoming a vaccine preventable disease is attainable . However, the significant mortality associated with rotavirus illness in the developing world suggests approaches to enhance the antiviral immune response and consequent natural resistance to rotavirus infection need to be explored. The importance of this issue and the significant amount of data available on rotavirus replication led us to use this virus as a model system. We report development of an assay that uses unmodified adherent epithelial cells to measure reductions in rotavirus infectivity in response to treatment of cells with a variety of compounds. A natural product library consisting of 280 compounds derived from plant extracts was screened, and several compounds that inhibited rotavirus infectivity in a dose-dependent manner were identified. All of the compounds that passed the designated criteria of a hit have been reported to have direct effects on inflammatory or antiviral gene expression or on virus replication. Development of such a platform to screen compounds for the ability to diminish virus replication easily can be applied to other virus systems where direct measurement of activity in epithelial cells is desirable.
Screening assay development
Plate uniformity assessments were performed according to the recommendations of the NIH Chemical Genomics Center in order to determine whether our assay was suitable for eventual adaptation to a high-throughput format. The Z' coefficient was established with mock infected cells representing the minimum signal value, virus infected cells representing the maximum signal value, and the IFNα/IFNγ mixture as the midrange signal. Experiments consisting of three plates each run on three consecutive days were performed and an example of the data from one day is shown in Figure 3. No evidence of significant drift or edge effect was observed. The Z' coefficient measures the quality of an HTS assay by comparing data variation and the signal dynamic range, and a value of > 0.5 establishes the assay as excellent for screening [12, 13]. The Z' values ranged from 0.69 – 0.82 for individual plates, and the aggregate Z' value from all plates and all days was 0.64. A signal-to-noise value of 4.96 was calculated with the aggregate data from all plates. All of the statistical criteria for intra-plate assessment were met .
Compounds from a natural product library reduce rotavirus infectivity
Compounds that reduce rotavirus infectivity without cytotoxicity
Manufacturer's ID Number
Known Functions (Select references)
-Antiviral activity against a number of DNA and RNA viruses 
-Inhibits gap junction communication
-Inhibits acute inflammation after topical or oral administration 
-Reduces neutrophil infiltration 
-Reduces COX-2 and TNF-α expression 
-Activates PPAR-γ in macrophages 
-Increases expression of type I interferon receptors 
-Used in treatment of skin infections, wounds and diarrhea in Southeast Asia 
-γ-Mangostin inhibits NF-κB activation and COX-2 expression 
-α-Mangostin preferentially inhibits growth of HL60 cells 
-Induces caspase-9 and -3 activation in HL60 cells 
-Antiviral activity against Tobacco Mosaic Virus 
-Causes a decrease in (poly rI) (poly rC) stimulated interferon response in VSV challenged mice 
-Reverses the effect of endotoxin-enhanced host resistance 
-Tyrosine kinase inhibitor 
-Down-regulates iNOS 
-Inhibits NF-κB activation 
-Inhibits COX-2 induction 
-Inhibits LPS induced IL-1β, IL-6, and TNF-α production in monocytes
-Inhibits NF-κB activation 
-Activates transient receptor potential vanilloid-1 (TRPV-1) 
-GABA receptor antagonist
-Antibacterial activity against E. coli, Staph. aureus and Myc. smegmatis 
-Antimalarial activity 
-Inhibitory activity against multidrug-resistant strains of Staphylococcus aureus 
-activates large-conductance Ca2+-activated K+ channel α-subunit 
-inhibits NFκB activation by preventing induction of IκB kinase 
Time course of inhibitory effects
α-mangostin and 18 β-glycyrrhetinic acid activate NFκB and induce IL-8 secretion
We designed a MTS assay capable of identifying compounds that reduce rotavirus infectivity, toward a long-term goal of discovering compounds that activate innate immune signaling pathways to reduce the disease impact of acute viral infections. We screened a library consisting of 280 natural products purified from plant extracts, and several compounds were identified that reproducibly inhibited rotavirus replication without cytotoxicity. The assay has been validated statistically, as well as by the observation that compounds selected for further study were purchased from different sources and showed reproducible inhibition of virus replication. In addition, follow-up assays measured reductions in infectivity in response to compound treatment in the standard rotavirus IF assay and the degrees of replication inhibition and dose-dependence correlated with the values obtained in the MTS assay. We calculated a true hit rate for the natural product library of 5% and a false positive rate of ~20%. We performed numerous optimization assays to reduce variability presented by variation in growth properties of our cell line. We also have initiated a screen of a 10,000 compound synthetic chemical library, and after screening ~2500 compounds, our true hit rate is 1.8% (data not shown). These true hit percentages yield a large, but not prohibitive, number of compounds for follow-up studies both in vitro and in animal models. Together the data suggest our assay is an effective platform for screening candidate compounds for antiviral activity in adherent epithelial cells.
In this study, we sought to identify compounds with innate immune modulating effects that resulted in cellular resistance to rotavirus infection. The format of the assay, that is, readout of reduction in infectivity, does not distinguish between compounds that act on antiviral signaling pathways and those that may target specific steps in the virus replication cycle, such as entry or replicase activity. However, two of the four compounds selected for follow-up study activated NFκB and induced IL-8 secretion. Moreover, the majority of compounds called a hit in the screen for which data is available has been previously described to affect inflammatory or anti-inflammatory gene expression or pathogen growth in vitro (Table 1). These observations strongly support the assertion that our assay is appropriate, but not exclusive, for discovery of immune potentiators, as we intended.
There has been a resurgence of interest in natural products as drug candidates for a variety of reasons including increased interest in infectious disease prevention and therapy, a lower percentage of chemical properties that negatively affect permeation and absorption, and the propensity of natural products to act by affecting protein-protein interactions . Important for the studies described here, natural products are known to modulate immune responses and cell signaling pathways. Table 1 lists compounds designated hits in this study, along with some of the reported functions. Compounds were grouped into those chosen for further study (Group A), and those known to stimulate or repress inflammatory responses or innate immune signaling pathways and two with unknown functions (Group B). Also included in Group B are compounds with reported antiviral, antibacterial, or anti-protozoan activity. We have not yet deciphered mechanisms by which these compounds inhibit rotavirus replication in vitro. However, the fact that several of the compounds, for example, abietic acid, genistein, and capsaicin, interfere with NFκB activation and cyclo-oxygenase 2 (COX2) expression is noteworthy (see Table 1 for references). NFκB activation is an important regulator of COX2 expression; COX2 activity and COX-mediated prostaglandin synthesis is necessary for rotavirus infectivity in CaCo-2 intestinal cells . Interestingly, the addition of prostaglandin E2 (PGE2) restored infectivity reduced by the COX inhibitor  and three of the compounds we chose for follow-up studies all have an inhibitory effect on either synthesis or release of PGE2 (see Table 1 references). The ability of α-mangostin and 18 β-glycyrrhetinic acid to inhibit rotavirus replication when both compounds activate NFκB is most likely because antiviral states are established upon treatment of the cells with each compound prior to infection. The definitive mechanisms by which the compounds identified in this natural product library screen warrant further investigation.
We intend this MTS assay to serve as a platform for discovery of candidate adjuvants that will be effective against acute viral infections at mucosal surfaces. Rotavirus is an ideal model system for these purposes for several reasons. First, rotaviruses cause gastrointestinal illness in most mammalian species and so their relevance as mucosal pathogens is clear. Second, these viruses are well characterized with respect to structure, antigenicity, and mechanisms of virus replication, and thus an excellent resource for mechanistic follow-up studies is available. Third, rotavirus is promiscuous in its tropism for cultured cell lines, and multiple cell lines of different types and species of origin, including primary cell lines , support productive virus replication. The ability to propagate rotavirus in a variety of cell types supports high throughput applications that study general as well as cell-type specific innate immune responses. Fourth, both small and large animal models of natural infection allow relatively rapid evaluation of the efficacy of candidate compounds in vivo. Finally, the fact that rotavirus employs a mechanism to down-regulate antiviral gene expression allows consideration of possible evasion strategies when selecting and testing candidate immune potentiators [21, 22]. Current development efforts include expansion of screening studies to other lytic RNA viruses such as influenza virus, and adaptation of the assay to a fully automated format.
Cells, virus, and rotavirus monoclonal antibodies
MA104 monkey kidney epithelial cells were maintained in M199 medium (MediaTech Cell Grow) supplemented with 5% fetal bovine serum (FBS; Atlanta Biologicals). Isolation and cultivation of G serotype 6 (G6) bovine rotavirus strain NCDV has been described . Monoclonal antibody (MAb) E4 reacts with major structural protein VP6 of most group A rotavirus strains . MAb A6M recognizes VP6 and was generated by immunizing mice with G6 bovine rotavirus strain B641 and screening hybridoma supernatants for antibodies that react with rotavirus specific proteins.
Reagents and chemicals
A library containing 0.5 mg each of 280 natural products purified from plant extracts was purchased from TimTec (TimTec Corporation). Compounds were reconstituted in 500 μl of dimethyl sulfoxide (DMSO) and the library was stored at -80°C. Individual compounds α-mangostin (TimTec or Indofine), 18-β-glycyrrhetinic acid (18-BGA; Fluka), abietic acid (AA; Sigma), and all-trans-retinoic acid (ATRA; Sigma) were reconstituted in DMSO to final stock concentrations of 25 mg/ml.
Recombinant human interferon-α (IFN-α; BioSource International, Inc) was diluted to a concentration of 1 × 105 U/ml in phosphate buffered saline (PBS) containing 0.1% bovine serum albumin (BSA). Recombinant human interferon-γ (IFN-γ) and recombinant human interleukin-2 (IL-2; Peprotech Inc) were diluted to 2 × 106 U/ml and 10 μg/ml, respectively, in PBS.
Horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG (H+L), HRP-conjugated goat anti-mouse IgG (H+L) F(ab')2 fragments, and FITC-conjugated goat anti-mouse IgG were purchased from Jackson ImmunoResearch Laboratories.
Compound treatments and virus infections
Working stock solutions of each compound from the library were prepared to twice the desired final concentrations of 20 μg/ml, 10 μg/ml, and 2 μg/ml in serum-free M199. Working stock solutions of assay controls consisted of 2%, 1% DMSO, and 0.2% DMSO, a mixture of 200 U/ml each of IFN-α and IFN-γ, and 2 ng/ml of IL-2.
MA104 cells were cultured to confluence in 96-well black-walled plates (Costar). The culture media was decanted and replaced with 50 μl of M199. 50 μl of 2X control and experimental stock solutions were added to respective wells, in triplicate, and plates were incubated for 4 hours at 37°C. Following the 4 hour incubation, the contents of each plate were removed and 8.9 × 105 ffu/well of trypsin-activated NCDV in 0% M199 was added to appropriate wells. Mock infected wells received 50 μl of 0% M199. 50 μl of fresh 2X control and experimental compounds were added and at 18 hours post-infection, the cells were fixed for 10 minutes with 80% acetone.
The wells were blocked for one hour at room temperature with 100 μl of PBS containing 3% bovine serum albumin (w/v) and 0.05% Tween-20. The plates were washed one time with 400 μl of wash buffer consisting of PBS and 0.05% Tween 20. 50 μl of 20 μg/ml A6M or E4 (1:25 hybridoma supernatant) in PBS containing 0.05% Tween 20 and 0.5% dry milk was added to the wells and incubated for one hour at room temperature. The plates were washed four times with wash buffer, then 50 μl of a 1:500 dilution of HRP-conjugated F(ab')2 in 0.5% Blotto was added and the plates were incubated for one hour at room temperature. Following a final wash, 100 μl of BM Chemiluminescence ELISA Substrate (Roche Diagnostics) was added and reactions were allowed to proceed for 4 minutes to reach a steady state of enzymatic activity. Signals were measured on a ThermoElectron Fluroskan (ThermoElectron Cooperation) with an integration time of 1,000 ms.
Cell viability assay
Cell viability assays for compound toxicity were set up similar to compound screening except cells were not infected over the course of the experiments. Cell viability was measured with the CellTiter-Glo Luminescent Cell Viability Assay (Promega) according to instructions provided by the manufacturer.
Statistics and criteria for "hit" designation
Compounds that showed a greater than 60% decrease in signal at 5 μg/ml when compared to the 0.5% DMSO control, and p < 0.05 as determined by a one-tailed student's t test, were selected for secondary screening. Additionally, compounds that showed a greater than 90% decrease in signal when compared to the 1% DMSO control and p < 0.05 also were selected for further screening. Compounds that showed a greater than 10% decrease in signal and a p < 0.05 in the cell viability assay were determined to be toxic.
Plate uniformity assessment
The recommendations of the National Institutes of Health Chemical Genomics Center's Assay Guidance Manual Version 4.1 for plate uniformity assessments were followed. Three separate experiments consisting of three plates each were performed on three different days. Confluent MA104 cells were pretreated for 4 hours with 100 μl of either media or a mixture of 100 U/ml each of IFN-α and IFN-γ. All media was serum free and contained 1% DMSO.
The pre-treatment media was decanted and the cells were infected with 50 μl of 8.9 × 105 pfu/ml of trypsin-activated NCDV. Mock infected wells were treated with 50 μl of serum free media. 50 μl of fresh 2X treatment media containing 2% DMSO was added to the respective wells and the plates were incubated for 18 hours. The plates were fixed, labeled and quantified according to the assay procedure described above. All calculations were performed using the Assay Guidance Manual's spreadsheet .
Immunofluorescent focus assay
Immunofluorescent assays (IF) for rotavirus infectivity were performed as previously described . Cells were cultured in 96-well plates and were mock infected or infected with NCDV at approximately 150 ffu/well. Pre- or post-treatment with compounds was performed for the indicated times as described above. 18 hours post-infection, the cells were fixed for 10 minutes with 80% acetone. Incubations with primary and secondary antibody were as described above, except the secondary antibody was FITC-conjugated goat anti-mouse IgG. Fluorescent foci were counted by microscopy (Nikon Eclipse TE300).
NFκB and IL-8 assays
MA104 cells in 100 × 20 mm culture dishes were treated with 7.5 μg/ml of selected compounds in serum-free M199 containing 0.03% DMSO. The effects of the presence of virus on NFκB activation and IL-8 secretion in the assays was tested by infecting cells with trypsin-activated NCDV at an moi of 10 pfu/cell at the time of compound treatment. In all cases, incubation periods were 6 hours.
NF-κB activation was quantified with TransAM NF-κB p50 Transcription Factor Assay Kit (Active Motif). Nuclear extracts were prepared with CelLytic NuCLEAR Extraction Kit (Sigma) following the manufacturer's protocol. Nuclear protein concentration was determined with the Dc Protein Assay (Bio-Rad Laboratories), and 20 μg of nuclear protein was used in the assay. Statistical analysis of the data was performed by two-factor ANOVA.
IL-8 secretion was measured with the Quantikine Human IL-8 system (R & D Systems) following the manufacturer's protocol. Compound treatments and infections were performed as described above, except that the incubation periods were extended to 24 hours. The data were analyzed by student's t test with a threshold of significance set to p < 0.001.
This work was supported by DoD contract W9113M-04-1-0010, USDA NRI/CGP 02657 to MEH, the Montana Agriculture Experiment Station, and PHS grant P20 RR020185. ADB gratefully acknowledges financial support from the Arnold and Mabel Beckman Foundation administered through the MSU Beckman Scholars Program.
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