4. DISCUSSION
Discussion
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
PCV2 is a widespread virus that causes an array of diseases and syndromes in pigs (PCVAD)
(Grau‐Roma et al., 2010). Currently, the only antiviral strategy is prevention by vaccination and no drug
exists for controlling the disease by limiting viremia. Thus, there is an unmet need to develop antivirals
that could effectively inhibit PCV2 replication and this study was aimed at addressing this issue through
the establishment of a primary screening assay in an effort to facilitate drug discovery against PCV2
replication. The assay was developed by monitoring the expression of Rep protein through IFA to assess
PCV2 replication. Screening assays in drug discovery are generally divided into cell‐based and cell‐free
formats. In this study, a whole‐cell approach was chosen for development primarily because of
availability of materials; cell lines and virus stocks were readily available and protocols were already
established. Before commencing, materials had to be standardized by obtaining a uniform batch of (1)
stable cell line highly permissive to infection and supports high titer growth of the virus, (2) large
amounts of antibodies that effectively recognize the target, and (3) large volume of infectious, high titer
virus.
Two cell lines considered for the screening assay were PK15‐C1 and 3D4/31 because of their
widespread use in producing PCV2 in cultures. Porcine kidney (PK15) cells are the most widely used for
growing PCV2 according to literature survey. Moreover, a cell line derived from a subpopulation of PK15
cells that were shown highly permissive to PCV2 infection had been previously generated (Zhu et al.,
2007) and was mainly used in this study. The porcine‐derived transformed monocytic cell line 3D4/31, is
another cell line chosen because it had been previously utilized in studies of PCV2 infection biochemistry
and dynamics (Misinzo et al., 2005; Misinzo et al., 2006), thus showing that the cells are permissive to
PCV2 infection. Cell line optimized for use in the screening assay should be permissive to infection,
support viral replication and exhibit high infection rates with the virus. 3D4/31 and PK15‐C1 cells were
grown in monolayer and incubated with stock PCV2. Infected cells at early time points, i.e. 24 and 48
HPI, did not show signs of CPE, consistent with the literature (Allan et al., 1998). Significant cell death
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
th
was observed from 72 HPI onwards; on the 4 day following infection 50% of cells had died, and on the
8th day 90% had lifted off. To determine whether PCV2 infection caused these observations, medium
obtained from both cell lines grown with the virus was subjected to either PCR or IFA. PCV2‐specific PCR
of DNA extracted from virus‐containing media resulted to the amplification of a 250 bp portion of the
virus ORF2 (Liu et al., 2005). Moreover, harvested media used to infect freshly seeded cells resulted to
positive staining for Rep protein by IFA. These observations confirmed presence of PCV2 in the collected
media from both cell lines but it was insufficient proof of virus replication within cells.
To confirm PCV2 replication, infected cell lysates were probed for Rep protein by western blot
and detected in situ by IFA. The two splice variants of Rep protein – Rep (37 kDa) and Rep’ (20 kDa),
were detected by Western blot in PK15‐C1 but not in 3D4/31 lysates. When infected cells were
processed for IFA, however, both cell lines stained positive for Rep expression, although infection rates
were significantly less for 3D4/31 than PK15‐C1 cells. Furthermore, 3D4/31 cells exhibited less than 5%
infection rate at the highest MOI tested (15, corresponding to infection with 105 TCID50), while PK15‐C1
exhibited at least 10% infection rate at the lowest MOI (0.17, corresponding to infection with 103
TCID50). These data confirm that both cells supported PCV2 replication, although PK15‐C1 was more
permissive to PCV2 infection compared to 3D4/31. High infection rate was observed in PK15‐C1 cells
infected at low MOI (0.17), while a low infection rate was observed in 3D4/31 infected at high MOI (15).
As a result, PK15‐C1 cells were chosen for optimizing the screening platform.
Before concluding that 3D4/31 cells are not permissive to PCV2 infection, however, caveat must
be considered. One possible explanation for the observed disparity in infection rates between the two
cell lines is differential virus adaptation. PCV2 used in the experiment had undergone more than 10
passaged and expanded in PK15‐C1, and was more adapted to it compared to 3D4/31 cells. Meanwhile,
PCV2 had been passaged less than five times in the monocytic cells. Lower adaptation possibly resulted
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
to less infection and consequently reduced levels of replication. This hypothesis is supported by previous
studies (Yu et al., 2007) where tissues and cells collected from PCV2‐infected pigs were stained for
presence of PCV2 antigens. The group concluded that monocytic cells were a main site of viral
persistence but not for viral replication. Further adaptation of the virus would be needed to increase
infection rates in 3D4/31 cells, but this is already beyond the scope of this study.
Growth curve for PK15‐C1 was established to determine the time point at which DNA replication
at S phase occurs. Whole genome content was extracted and the quantity was plotted against duration
of incubation. It was observed that PK15‐C1 cells have DNA doubling time of 15.5 hours, and subsequent
infections with PCV2 were performed less than 15.5. hours post‐seeding. Previous studies on PCV2
infection dynamics revealed that PCV2 replication is dependent on enzymes expressed at S phase and
viral replication occurs normally after mitosis (Tischer et al., 1987). To expedite infection, cells have to
be infected prior to mitosis, preferably before S phase. Alternatively, infected cells could be treated to
glucosamine to circumvent the need to participate in cellular mitosis prior to onset of viral genome
replication.
The second prerequisite before commencing the screening assay is large volumes of antibody.
The monoclonal antibody #4 was previously generated by conventional hybridoma technology and was
shown to recognize both splice variants of the Rep protein (Meng et al., 2010). Medium collected from
several rounds of hybridoma cell culture was pooled (1 liter) and frozen in small volumes. Aliquots were
tested for Rep protein recognition by western blot and yielded positive results with strong staining.
Antibodies were used only once for uniformity of results. FITC‐conjugated antibodies used in the study
were also purchased in large volumes (10 ml) of the same batch and lot to reduce variability between
experiments. Antibodies were tested and found capable of recognizing its target.
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4. DISCUSSION
The third pre‐condition that must be satisfied was production of large volume of infectious PCV2
with high titer. Initial attempts to produce high titer virus were unsuccessful and titer did not exceed
104 TCID50/ml. It was observed that virus production in roller bottles also did not improve titer.
Superinfection of persistently infected PK15‐C1 cells was found to reduce viral titers, because treatment
with trypsin was lethal to infected cells. This was in contrast to previously observed titer enhancement
by superinfection, where a three‐fold increase in virus titer was obtained (Allan et al., 1998; O'Dea et al.,
2008). This disparity in results from superinfection studies could be attributed to the difference in cell
lines used in this study compared with those done by Allan et al. and O’Dea et al. These groups used the
parental PK15 cells comprised of a heterogenous mixture of slow‐ and fast‐growing cells displaying a
wide range of PCV2 infection permissivity, while this current study used the subclone PK15‐C1, which is
mainly comprised of slow‐growing cells highly permissive to PCV2 infection (Zhu et al., 2007). Virus titer
was significantly enhanced (100‐fold) only when pooled virus medium was concentrated by
ultracentrifugation; titer reached 106 TCID50/ ml and infection rate in PK15‐C1 using the concentrated
virus was greater than 50% when infected with 105 TCID50 (50 MOI). Passage of concentrated virus in
PK15‐C1 cells did not significantly alter the titer, and pooled virus media was frozen in small aliquots
(‐80o C) until further use.
Once all the necessary materials had been obtained, development of the screening assay was
performed. Detection of PCV2 infection and replication through IFA of Rep protein is a well‐established
method abundantly found in the literature (Allan et al., 1998; Liu et al., 2005; Meng et al., 2010).
Fluorescence detection system was adopted over colorimetric detection because of the inherent
sensitivity of fluorescent dyes. The IFA had been shown to work in 96‐well plate format without
difficulty, and scaling down to the 384‐well plate format was expected to be straightforward. Optimizing
required tweaking three parameters: 1) cell seeding density, 2) MOI, and 3) duration of infection prior to
fixation.
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4. DISCUSSION
For uniformity of results, it was necessary to have an even monolayer of cells per well and any
unnecessary perturbation that might lead to disruption of the cell sheet must be avoided. It was
previously observed that induction with glucosamine leads to enhancement of infection but with
concomitant cytotoxicity (Tischer et al., 1987; Allan and Ellis, 2000). It was therefore prudent to test
whether the benefits outweighed the risks of glucosamine treatment in 384‐well plates. Comparison of
cells treated and untreated with glucosamine yielded strong evidence for cytotoxicity. The central
portion of treated wells was devoid of cells in contrast to the more confluent cell sheets observed in
untreated wells. Significant glucosamine‐induced cytotoxicity was observed at various MOI (1, 2, 3, and
4) and different incubation periods (48 and 60 hours). Similar results were obtained at other seeding
densities. Although the cytotoxicity of glucosamine could be empirically reduced by shortening the
durationof cell contact and thorough washing with PBS prior to replacement with fresh media, doing this
was not an easy task. Due to the small area of the wells in 384‐well plates, however, glucosamine could
not be completely removed without disrupting the cell sheet. Washing with buffer also resulted to
mechanical disruption causing cells to lift off the wells. Most importantly, enhancement of infection was
not observed in glucosamine‐induced cells. Infection rates in cells treated with glucosamine were not
significantly higher in comparison to those observed in untreated cells. Thus, the benefits of
glucosamine induction did not outweigh its inherent cytotoxicity and was considered inessential to the
assay being developed. Further optimizations precluded the use of glucosamine.
Aside from an even monolayer of cells, another important prerequisite for the assay was
infection rate of at least 50%. It is necessary to achieve this minimum infection rate in untreated cells so
that % inhibition of replication that results from the screening would yield unequivocal results.
Achieving this requirement depended on the interplay of three factors: (1) seeding density, (2) MOI at
the time of infection, and (3) duration of infection prior to fixation. Seeding at insufficient density would
result to low cell confluence at the time of assay; doing so at excessive density would result to very high
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
density of cells growing on top of each other. Infection at excessively high MOI would result to greater
cell death and consequently larger regions devoid of cells on the well; conversely, infection with
insufficient MOI would result to infection rates less than 50%. Balance also had to be achieved with
duration of infection. Although higher infection rates could be achieved if cells were infected for a
longer duration, incubation for too long would result to most of the infected cells lifting off and dying
as a consequence of virus‐induced apoptosis (Liu et al., 2005). On the other hand, prolonging the
duration of incubation of cells infected at low MOI would result to overgrowth and and formation of
multiple layers.
Initial tests performed at low MOI (< 10) resulted to low infection rate (< 20%), even after the
duration of incubation was extended from 48 to 72 hours. Although prolonged incubation resulted to
higher infection rates for all seeding densities, these were less than 50% and cells seeded at higher
density formed stacks of cells with multiple layers. Infection with higher MOI (> 10), alternatively, did
not result to higher infection rates and cell death became more evident. This consequently led to larger
areas denuded of cells, and this phenomenon was more prominent in wells incubated for longer
durations. For cells infected with 10 MOI, infection rates observed at 48 HPI were lower than in cells
infected with < 1 MOI. At 60 HPI, infection rate was twice as much as that of cells fixed at 48 HPI, and
this was consistent among all trials; furthermore, the observed infection rates were higher compared to
cells infected with < 1 MOI. One hypothesis to explain this phenomenon is that progeny virus within
cells had not yet been released at 48 HPI. The observed higher infection rate at 60 HPI might be due to
re‐infection of cells with the released virus. The greater amount of cell death observed at 60 HPI
compared to 48 HPI supports this hypothesis. Moreover, it was recently published that host cell
apoptosis induced by PCV2 ORF3 is essential for virus exit and concomitant release of progeny virus into
the environment leading to increased infection rates and viremia (Karuppannan and Kwang, 2010) The
group also observed that the initial amount of cell‐free virus in the culture measured by quantitative
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
real‐time PCR was low compared to the amount of virus associated with cells. However, the levels
reached equivalence at later time points in infection, i.e. between 48‐72 HPI, indicating that progeny
virus must have been released within this time frame.
Although normal screening assays do not usually employ higher than 10 MOI, it was deemed
necessary to explore the possibility of obtaining high infection rates (at least 50%). An increase in
infection rate was expected at MOI > 10, since a dose response was previously observed when
performing PCV2 titration in 96‐well plates. However, this was not observed when the assay was scaled
down to 384‐well plates. One possible explanation was inefficient infection due to suboptimal contact
between virus and cells. To rule out this possibility, virus infection was expedited by gentle
centrifugation of 384‐well plates previously seeded with cells at 200 RPM for 1 minute during infection.
It was observed that due to the small diameter of the wells in a 384‐well plate, addition of protein‐rich
liquids such as culture medium (5% FBS) often resulted to bubble formation. Bubbles forming at the
interface between the medium and cell surface pose a significant barrier to virus infection, because the
virus could not directly contact the cells. In some instances it was impossible to determine by ocular
inspection whether a bubble had been introduced by pipetting since the wells were too small.
Centrifugation of plates forces the medium towards the bottom of the plate, hence effectively removing
any introduced bubble. In spite of this, no significant improvement in infection rate was observed
suggesting that the low infection rate was not due to bubble formation.
A second attempt to increase infection rate by enhancing host‐virus interaction was by pre‐
incubating PCV2 and PK15‐C1 in suspension prior to seeding. The rocking motion during incubation
would aid virus‐cell contact and facilitate infection. This also did not lead to significant increase in
infection rates, but instead resulted to greater cell death. The number of cells attached to the bottom of
the plate 24 hours post‐seeding was significantly less than the number of attached cells in plates seeded
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
and infected normally. Whether cell death was due to enhanced virus infection or to mechanical
shearing during pre‐incubation with the virus could not be determined.
From the previous experiments, it was confirmed that the observed low infection rate was not
due to inefficient virus‐host contact. Another possibility was that the fluorescence intensity of FITC was
weak and could not be efficiently detected by the imaging software, hence resulting to the perceived
low infection rates despite high MOI. Fluorescein is the most widely used green‐fluorescent labeling dye
and reaches maximum excitation at 494 nm, which is close to that emitted by argon‐ion laser (488 nm).
Fluorescein has several derivatized dyes such as FITC, whose quantum yield reaches maximum of 0.57.
Quantum yield is a measure of fluorescence efficiency and is defined as the number of emitted photons
per number of photons of absorbed (LifeTechnologies, 2010). Alexa Fluor dyes are modified
fluorophores that reach higher quantum yields; Alexa Fluor 546 has a yield of 0.79 and therefore
exhibits a more intense fluorescence emission. When FITC and AlexaFluor 546 were compared,
fluorescence was more intense in the wells labeled with Alexa Fluor 546 but the actual infection rates
did not differ significantly. Infection rates remained the same although cells were seeded at the same
density and infected with the same MOI. This suggested that the low infection rates previously observed
with FITC was not due to weak fluorescence emission. This showed that although quantum yield was not
high, FITC was sufficient for signal detection in the screening format.
In summary, development of a cell‐based assay with fluorescence detection in a 384‐well plate
format was not successful. Despite optimization with cell seeding density, MOI, and duration of
incubation prior to fixation, infection rates in PK15‐C1 cells did not reach the minimum requirement of
50%. Enhancement of infection rates by glucosamine was also not observed. Infection rates greater
than 50% could be achieved, however, using the concentrated virus (106 TCID5‐0/ml) in the 96‐well plate
format at 16 MOI. But in the 384‐well plate format, infection rates failed to reach 50% even at 25 MOI.
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
Both inefficient virus‐host interaction and low fluorescence intensity had been ruled out as possible
causes of the observed low infection rates.
Since the IFA for Rep was previously shown to work in the 96‐well plate format, POC trial of
reference drugs for PCV2 replication inhibition was performed. Testing drugs entailed determining the
possible cytotoxic effects and the maximum concentration tolerated by the cell lines being used. In
order to setup a cytotoxicity assay, a standard curve of FI was initially generated at various seeding
densities and without the drug. FI was plotted against duration of incubation with alamar blue. Cells
seeded at 6000 per well was amenable for incubation with alamar blue for less than 4 hours and
generates a linear response in FI. The same results were obtained when absorbance at 570 nm was
measured.
The NF‐ĸB inhibitor Caffeic Acid Phenethyl Ester (CAPE) was previously shown to inhibit PCV2
replication in vitro at an approximate EC50 of 15 µg/ml through an unidentified mechanism of action
(Wei et al., 2008). In this study, this drug was added at the time of infection with PCV2 at 15 MOI. No
adverse cytotoxicity was observed at the tested concentrations(0.01, 0.05, 0.10, 0.50, 1.0, 5.0, and 10.0
µM) and only 6% growth control was determined at 10 µM. This suggested that CAPE did not induce
significant death in PK15‐C1 cells, even at maximum concentration of 10.0 µM. To test whether PCV2
replication was inhibited, infected cells treated with CAPE were fixed and processed for IFA. Cells were
treated with the drugs at various time points: 1) 4 hours prior to (T = ‐4), 2) during (T = 0), and 3) 4 hours
after (T = 4) infection. The infection rates observed in the treated cells did not differ from drug‐treated
cells for all time points tested. The experiment was repeated several times but low infection rates were
still obtained. Replication inhibition tests were also performed using other reference drugs such as
U0126 and Mycophenolic Acid (MPA), and similar ambiguous results were obtained due to low infection
rates. U0126 is a MEK 1 /2 inhibitor found to inhibit PCV2 replication at EC50 of less than 10 µM (Wei and
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
Liu, 2009); although never been tested against PCV2, MPA is a potent and reversible inhibitor of Inosine
Monophosphate Dehydrogenase (MPDH) that had been tried on a diversity of viruses (Cline et al., 1969;
Panattoni et al., 2007). At this point, it is premature to draw conclusions whether these reference drugs
failed to inhibit PCV2 replication, precisely because the basal infection rates in untreated cells were
unexpectedly low (<20%) and a clear reduction in infection could not be assessed properly.
Since in both 384‐ and 96‐well plate formats the infection rates failed the minimum target of
50%, the IFA format was abandoned and efforts were turned towards an assay format that didn’t
require a minimum infection rate. ELISA had been used previously as a platform for HTS (Lavery et al.,
2001; John et al., 2005; Zuck et al., 2005). Although IFA and ELISA were very similar formats, some steps
had to be added when performing ELISA to ensure minimal background signals; endogenous peroxidase
had to be quenched by addition of freshly prepared 1% hydrogen peroxide prior to blocking. This was
found to significantly reduce background signals when compared with unquenched cells. Fixed cells had
to be blocked with 5% BSA in order to reduce nonspecific binding of #4 mAb to the cells. Stringent
washing with detergent also had to be performed after every antibody incubation step.
One major difference between signals obtained from ELISA and IFA was that FI in the latter was
counted per individual cell, whereas in the former an average absorbance reading for the whole well
was measured. Therefore, great care must be taken to ensure that background signal is at minimum
and sample signals are at maximum levels. If background signals were too high, there would be greater
chances that the average signal from the untreated wells would not significantly differ from the treated
wells. This would lead to overestimated false‐negative rate, and therefore lower the chances of finding
hits.
There are many parameters for evaluating assay performance or sensitivity, and in this study
S/N and S/B ratios were used because these could easily be derived from the raw data. S/B ratio gives a
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
straightforward description of the difference between the signals from the samples and controls, while
S/N ratio gives further information on variation in control signals. To test whether cell‐based ELISA could
be a sensitive, high‐performance assay for detection of PCV2 replication, S/N and S/B ratios derived
from setups with various cell seeding densities, MOI, secondary antibody dilutions, duration of
incubation prior to fixation, and duration of incubation with the OPD substrate were compared. S/N
and S/B ratios were at maximum when cells were incubated in substrate for 60 minutes. Therefore, 1
hour was concluded the optimum duration of incubation with substrate for signal detection. This was
longer than the usual substrate incubation period in normal ELISA, but was necessary because of low
amounts of detected antigen in the cells. When samples were incubated for 90 minutes, however,
nonspecific signals became more prominent, hence resulting to lower S/N ratios.
Cells incubated in 1:200 dilution of secondary antibody generally yielded better S/N and S/B
ratios compared to cells incubated in 1:100 dilution of secondary antibody. The trend was observed
regardless of MOI or duration of infection prior to fixation and was probably due to reduced nonspecific
binding of antibodies and therefore less background signals obtained with higher dilutions of HRP‐
conjugated antibody.
When considering the effect of seeding density and MOI on the S/N and S/B values, no general
trend could be obtained although there was a perceived increase in S/B values as seeding density was
increased and infection was done at 10 MOI. This trend was reversed at 20 MOI, but these differences
failed to reach significance. Data for the S/N ratios were even more confounding, and none of the
differences reached significance. One possible reason for inconsistent results was the low infection rates
of cells. Infection rates at 15 MOI assessed by IFA was less than 5%. This was mainly due to the stringent
blocking and washing required of ELISA. In comparison with cells processed for IFA, infection rates of as
high as 30% could be observed. Thus, blocking and stringent washing acted like double‐edged swords;
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
while these significantly reduced background signals, binding of antibody to Rep protein might have
been inadvertently hindered as well. This resulted to low sensitivity and insufficient separation between
the positive and control signals.
Another parameter for assessing assay robustness and repeatability is the Z’ factor (Zhang et al.,
1999; Sui and Wu, 2007). Since it takes into consideration the variability of values both from the control
and sample signals, it is the usual parameter of choice for determining whether the screening assay
would be amenable to HTS. A Z’ factor of > 0.5 indicates amenability for HTS, while a value between 0
and 0.5 indicates faulty reagents or suboptimal assay, and a negative value indicates that the assay
results to variable data and not amenable for HTS (Inglese et al., 2007; Sui and Wu, 2007). Z values for
cell‐based ELISA was not calculated for the cell‐based ELISA assay since the number of replicates and
trials performed were insufficient for comparison.
The unexplained sudden drop in infection rate obtained was the main impediment in the
progress of this study. Initial infection rates using the concentrated virus (106 TCID50/ml) was > 50% in
the 96‐well plate format at 15 MOI. When the assay was scaled down to 384‐well plates, infection rates
failed to reach 50% despite high MOI (25). Furthermore, reverting to 96‐well plates for reference drug
testing at 15 MOI yielded the same low infection rate. This could be due to either the virus losing
infectivity and virulence or the cell line becoming less permissive to PCV2 infection.
In order to address the possibility of cells losing permissiveness to PCV2 infection, PK15‐C1 could
be re‐cloned from the original parental stock and used immediately for screening studies. Other cell
lines possibly highly permissive to PCV2 infection could also be discovered. Although the cell line most
widely used for propagating PCV2 is PK15, other non‐porcine cell lines such as simian cos‐7 (Liu et al.,
2005), human epithelial and alveolar cells (Chaiyakul et al., 2010) had previously been used. Moreover,
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
new porcine‐derived cell lines recently established for propagation of swine viruses (Ferrari et al., 2003)
can be employed, although PCV2 was not included in the panel of tested viruses. It would be interesting
to grow PCV2 using the Newborn Swine Kidney (NSK) and Newborn Pig Trachea (NPTr) cells and
determine whether improved infection rates and viral titers could be obtained. For these swine cell
lines, no methodology exists for growing PCV2 and such protocols have to be established which could be
a time‐consuming process. Another likely candidate would be pig lymphocytic cells, since in vivo studies
in pigs showed that PCV2 preferentially infect and replicate within these cells during early stages of
infection (Yu et al, 2007). Staining for PCV2 antigens from samples derived from infected pigs also
showed high localization in lymph nodes (Chae, 2004), suggesting high tropism of PCV2 towards
lymphocytes.
In order to determine whether cell line permissiveness was at fault for the reduced virus
infection, further studies were performed using the original PK15‐C1 clones obtained in 2005 (Zhu et al.,
2007) to propagate PCV2 at low MOI (1.6). This also failed to increase the virus titer, suggesting that the
virus must have lost its virulence.
One method to address the issue of possible decrease in viral infectivity is the production of
highly concentrated virus by ultracentrifugation. Subsequent passage in PK15‐C1 cells could yield a large
volume of stock with the same titer and possibly more virulent PCV2. Serial passaging of virus in cell
lines had been shown to either attenuate parasites and viruses (Ebert, 1998; Badgett et al., 2002) or
make them more highly pathogenic (Clark, 1978; Kaul et al., 2009) due to mutations. For PCV2, evidence
of enhanced replication in cell culture was observed after serial passage (120 times) in PK15 cells caused
by two point mutations in the Cap sequence (Fenaux et al., 2004). Moreover, serial passage of live wild‐
type PCV2 in postweaned pigs could also enhance virulence. These methods were not tested in the
study due to time constraints. Another possible solution is to reconstitute the PCV2 virus from the
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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4. DISCUSSION
plasmid vector. The PCV2 Beijing strain used in this study (GenBank Accession No. AY847748) was
previously cloned in M13 plasmid. The whole viral genome excised from the plasmid could be self‐
ligated to produce the virus and afterwards used to infect PK15‐C1 cells, with the hope of obtaining
higher infection rates1.
In summary, the goal of developing a cell‐based method for primary screening of potential
inhibitors of PCV2 replication and subsequent validation using reference compounds that inhibit PCV2
replication were not met due to factors such as unexplained, sudden decrease in infection rates and
time constraints in troubleshooting. It is recommended for further studies to generate higher titers of
PCV2 and verify stability of cells in supporting high infection rates (> 50%) before proceeding with the
assay development. A stable virus stock consistently inducing >50% infection rates in cultures also
needs to be obtained. Without these, assay development in primary screening for PCV2 replication
inhibitors would not yield satisfactory results.
1
This is currently being done at the time of this manuscript revision. However due to time constraints, the results
of this experiment could not be included in this document
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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5. BIBLIOGRAPHY
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6. APPENDICES
Appendices
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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6. APPENDICES
6.1 Effect of glucosamine treatment on infection rates at various seeding
densities
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
(I)
(J)
(K)
(L)
(M)
(N)
(O)
(P)
Figure 6.1 | Effect of Glucosamine Treatment on Infection Rates in Cells Seeded at 2000 per
well. Cells were infected with PCV2 BJW at MOI 1 (A, B, I, J), 2 (C, D, K, L), 3 (E, F, M, N), and 4
(G, H, O, P) and fixed at 48 HPI (A‐H) and 60 HPI (I‐P). Infected cells were either untreated (A, C,
E, G, I, K, M, O) or treated (B, D, F, H, J, L, N, P) with glucosamine. Images are representative of
3 independent trials. Green (FITC) – PCV2 Rep protein; Blue (DAPI) – cell nuclei
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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6. APPENDICES
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
(I)
(J)
(K)
(L)
(M)
(N)
(O)
(P)
Figure 6.2 | Effect of Glucosamine Treatment on Infection Rates in Cells Seeded at 3000 per
well. Cells were infected with PCV2 BJW at MOI 1 (A, B, I, J), 2 (C, D, K, L), 3 (E, F, M, N), and 4
(G, H, O, P) and fixed at 48 HPI (A‐H) and 60 HPI (I‐P). Infected cells were either untreated (A, C,
E, G, I, K, M, O) or treated (B, D, F, H, J, L, N, P) with glucosamine. Images are representative of
3 independent trials. Green (FITC) – PCV2 Rep protein; Blue (DAPI) – cell nuclei
Cell‐Based Screening Assay for Inhibitors of Porcine Circovirus Type 2 (PCV2) Replication
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