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BioMed Central
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Virology Journal
Open Access
Research
Chloroquine is a potent inhibitor of SARS coronavirus infection and
spread
Martin J Vincent1, Eric Bergeron2, Suzanne Benjannet2, Bobbie R Erickson1,
Pierre E Rollin1, Thomas G Ksiazek1, Nabil G Seidah2 and Stuart T Nichol*1
Address: 1Special Pathogens Brach, Division of Viral and Rickettsial Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road,
Atlanta, Georgia, 30333, USA and 2Laboratory of Biochemical Neuroendocrinology, Clinical Research Institute of Montreal, 110 Pine Ave West,
Montreal, QCH2W1R7, Canada
Email: Martin J Vincent - mvincent@cdc.gov; Eric Bergeron - bergere@ircm.qc.ca; Suzanne Benjannet - benjans@ircm.qc.ca;
Bobbie R Erickson - BErickson1@cdc.gov; Pierre E Rollin - PRollin@cdc.gov; Thomas G Ksiazek - TKsiazek@cdc.gov;
Nabil G Seidah - seidahn@ircm.qc.ca; Stuart T Nichol* - SNichol@cdc.gov
* Corresponding author
severe acute respiratory syndrome coronaviruschloroquineinhibitiontherapy
Abstract
Background: Severe acute respiratory syndrome (SARS) is caused by a newly discovered
coronavirus (SARS-CoV). No effective prophylactic or post-exposure therapy is currently available.
Results: We report, however, that chloroquine has strong antiviral effects on SARS-CoV infection
of primate cells. These inhibitory effects are observed when the cells are treated with the drug
either before or after exposure to the virus, suggesting both prophylactic and therapeutic
advantage. In addition to the well-known functions of chloroquine such as elevations of endosomal
pH, the drug appears to interfere with terminal glycosylation of the cellular receptor, angiotensin-
converting enzyme 2. This may negatively influence the virus-receptor binding and abrogate the
infection, with further ramifications by the elevation of vesicular pH, resulting in the inhibition of
infection and spread of SARS CoV at clinically admissible concentrations.
Conclusion: Chloroquine is effective in preventing the spread of SARS CoV in cell culture.
Favorable inhibition of virus spread was observed when the cells were either treated with
chloroquine prior to or after SARS CoV infection. In addition, the indirect immunofluorescence
assay described herein represents a simple and rapid method for screening SARS-CoV antiviral
compounds.
Background
Severe acute respiratory syndrome (SARS) is an emerging
disease that was first reported in Guangdong Province,
China, in late 2002. The disease rapidly spread to at least
30 countries within months of its first appearance, and
concerted worldwide efforts led to the identification of
the etiological agent as SARS coronavirus (SARS-CoV), a
novel member of the family Coronaviridae [1]. Complete
genome sequencing of SARS-CoV [2,3] confirmed that
this pathogen is not closely related to any of the
Published: 22 August 2005
Virology Journal 2005, 2:69 doi:10.1186/1743-422X-2-69
Received: 12 July 2005
Accepted: 22 August 2005
This article is available from: http://www.virologyj.com/content/2/1/69
© 2005 Vincent et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Virology Journal 2005, 2:69 http://www.virologyj.com/content/2/1/69
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previously established coronavirus groups. Budding of the
SARS-CoV occurs in the Golgi apparatus [4] and results in
the incorporation of the envelope spike glycoprotein into
the virion. The spike glycoprotein is a type I membrane
protein that facilitates viral attachment to the cellular
receptor and initiation of infection, and angiotensin-con-
verting enzyme-2 (ACE2) has been identified as a func-
tional cellular receptor of SARS-CoV [5]. We have recently
shown that the processing of the spike protein was
effected by furin-like convertases and that inhibition of
this cleavage by a specific inhibitor abrogated cytopathic-
ity and significantly reduced the virus titer of SARS-CoV
[6].
Due to the severity of SARS-CoV infection, the potential
for rapid spread of the disease, and the absence of proven
effective and safe in vivo inhibitors of the virus, it is impor-
tant to identify drugs that can effectively be used to treat
or prevent potential SARS-CoV infections. Many novel
therapeutic approaches have been evaluated in laboratory
studies of SARS-CoV: notable among these approaches are
those using siRNA [7], passive antibody transfer [8], DNA
vaccination [9], vaccinia or parainfluenza virus expressing
the spike protein [10,11], interferons [12,13], and mono-
clonal antibody to the S1-subunit of the spike glycopro-
tein that blocks receptor binding [14]. In this report, we
describe the identification of chloroquine as an effective
pre- and post-infection antiviral agent for SARS-CoV.
Chloroquine, a 9-aminoquinoline that was identified in
1934, is a weak base that increases the pH of acidic vesi-
cles. When added extracellularly, the non-protonated por-
tion of chloroquine enters the cell, where it becomes
protonated and concentrated in acidic, low-pH
organelles, such as endosomes, Golgi vesicles, and lyso-
somes. Chloroquine can affect virus infection in many
ways, and the antiviral effect depends in part on the extent
to which the virus utilizes endosomes for entry. Chloro-
quine has been widely used to treat human diseases, such
as malaria, amoebiosis, HIV, and autoimmune diseases,
without significant detrimental side effects [15]. Together
with data presented here, showing virus inhibition in cell
culture by chloroquine doses compatible with patient
treatment, these features suggest that further evaluation of
chloroquine in animal models of SARS-CoV infection
would be warranted as we progress toward finding effec-
tive antivirals for prevention or treatment of the disease.
Results
Preinfection chloroquine treatment renders Vero E6 cells
refractory to SARS-CoV infection
In order to investigate if chloroquine might prevent SARS-
CoV infection, permissive Vero E6 cells [1] were pre-
treated with various concentrations of chloroquine (0.1–
10 µM) for 20–24 h prior to virus infection. Cells were
then infected with SARS-CoV, and virus antigens were vis-
ualized by indirect immunofluorescence as described in
Materials and Methods. Microscopic examination (Fig.
1A) of the control cells (untreated, infected) revealed
extensive SARS-CoV-specific immunostaining of the mon-
olayer. A dose-dependant decrease in virus antigen-posi-
tive cells was observed starting at 0.1 µM chloroquine, and
concentrations of 10 µM completely abolished SARS-CoV
infection. For quantitative purposes, we counted the
number of cells stained positive from three random loca-
tions on a slide. The average number of positively stained
control cells was scored as 100% and was compared with
the number of positive cells observed under various chlo-
roquine concentrations (Fig. 1B). Pretreatment with 0.1,
1, and 10 µM chloroquine reduced infectivity by 28%,
53%, and 100%, respectively. Reproducible results were
obtained from three independent experiments. These data
demonstrated that pretreatment of Vero E6 cells with
chloroquine rendered these cells refractory to SARS-CoV
infection.
Postinfection chloroquine treatment is effective in
preventing the spread of SARS-CoV infection
In order to investigate the antiviral properties of chloro-
quine on SARS-CoV after the initiation of infection, Vero
E6 cells were infected with the virus and fresh medium
supplemented with various concentrations of chloro-
quine was added immediately after virus adsorption.
Infected cells were incubated for an additional 16–18 h,
after which the presence of virus antigens was analyzed by
indirect immunofluorescence analysis. When chloro-
quine was added after the initiation of infection, there was
a dramatic dose-dependant decrease in the number of
virus antigen-positive cells (Fig. 2A). As little as 0.1–1 µM
chloroquine reduced the infection by 50% and up to 90–
94% inhibition was observed with 33–100 µM concentra-
tions (Fig. 2B). At concentrations of chloroquine in excess
of 1 µM, only a small number of individual cells were ini-
tially infected, and the spread of the infection to adjacent
cells was all but eliminated. A half-maximal inhibitory
effect was estimated to occur at 4.4 ± 1.0 µM chloroquine
(Fig. 2C). These data clearly show that addition of chloro-
quine can effectively reduce the establishment of infection
and spread of SARS-CoV if the drug is added immediately
following virus adsorption.
Electron microscopic analysis indicated the appearance of
significant amounts of extracellular virus particles 5–6 h
after infection [16]. Since we observed antiviral effects by
chloroquine immediately after virus adsorption, we fur-
ther extended the analysis by adding chloroquine 3 and 5
h after virus adsorption and examined for the presence of
virus antigens after 20 h. We found that chloroquine was
still significantly effective even when added 5 h after infec-
tion (Fig. 3); however, to obtain equivalent antiviral
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effect, a higher concentration of chloroquine was required
if the drug was added 3 or 5 h after adsorption.
Ammonium chloride inhibits SARS-CoV infection of Vero
E6 cells
Since chloroquine inhibited SARS-CoV infection when
added before or after infection, we hypothesized that
another common lysosomotropic agent, NH4Cl, might
also function in a similar manner. Ammonium chloride
has been widely used in studies addressing endosome-
mediated virus entry. Coincidently, NH4Cl was recently
shown to reduce the transduction of pseudotype viruses
decorated with SARS-CoV spike protein [17,18]. In an
attempt to examine if NH4Cl functions similarly to chlo-
roquine, we performed infection analyses in Vero E6 cells
before (Fig. 4A) and after (Fig. 4B) they were treated with
various concentrations of NH4Cl. In both cases, we
observed a 93–99% inhibition with NH4Cl at ≥ 5 mM.
These data indicated that NH4Cl (≥ 5 mM) and chloro-
quine (≥ 10 µM) are very effective in reducing SARS-CoV
infection. These results suggest that effects of chloroquine
and NH4Cl in controlling SARS CoV infection and spread
might be mediated by similar mechanism(s).
Effect of chloroquine and NH4Cl on cell surface expression
of ACE2
We performed additional experiments to elucidate the
mechanism of SARS-CoV inhibition by chloroquine and
NH4Cl. Since intra-vesicular acidic pH regulates cellular
functions, including N-glycosylation trimming, cellular
trafficking, and various enzymatic activities, it was of
interest to characterize the effect of both drugs on the
processing, glycosylation, and cellular sorting of SARS-
CoV spike glycoprotein and its receptor, ACE2. Flow
cytometry analysis was performed on Vero E6 cells that
were either untreated or treated with highly effective anti-
SARS-CoV concentrations of chloroquine or NH4Cl. The
results revealed that neither drug caused a significant
change in the levels of cell-surface ACE2, indicating that
the observed inhibitory effects on SARS-CoV infection are
not due to the lack of available cell-surface ACE2 (Fig.
5A). We next analyzed the molecular forms of endog-
Prophylactic effect of chloroquineFigure 1
Prophylactic effect of chloroquine. Vero E6 cells pre-treated with chloroquine for 20 hrs. Chloroquine-containing media
were removed and the cells were washed with phosphate buffered saline before they were infected with SARS-CoV (0.5 mul-
tiplicity of infection) for 1 h. in the absence of chloroquine. Virus was then removed and the cells were maintained in Opti-
MEM (Invitrogen) for 16–18 h in the absence of chloroquine. SARS-CoV antigens were stained with virus-specific HMAF, fol-
lowed by FITC-conjugated secondary antibodies. (A) The concentration of chloroquine used is indicated on the top of each
panel. (B) SARS-CoV antigen-positive cells at three random locations were captured by using a digital camera, the number of
antigen-positive cells was determined, and the average inhibition was calculated. Percent inhibition was obtained by considering
the untreated control as 0% inhibition. The vertical bars represent the range of SEM.
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enous ACE2 in untreated Vero E6 cells and in cells that
were pre-incubated for 1 h with various concentrations of
either NH4Cl (2.5–10 mM) or chloroquine (1 and 10 µM)
and labeled with 35S-(Met) for 3 h in the presence or
absence of the drugs (Fig. 5B and 5C). Under normal con-
ditions, we observed two immunoreactive ACE2 forms,
migrating at ~105 and ~113 kDa, respectively (Fig. 5B,
lane 1). The ~105-kDa protein is endoglycosidase H sen-
sitive, suggesting that it represents the endoplasmic retic-
ulum (ER) localized form, whereas the ~113-kDa protein
is endoglycosidase H resistant and represents the Golgi-
modified form of ACE2 [19]. The specificity of the anti-
body was confirmed by displacing the immunoreactive
protein bands with excess cold-soluble human recom-
binant ACE2 (+ rhACE2; Fig. 5B, lane 2). When we ana-
lyzed ACE2 forms in the presence of NH4Cl, a clear
stepwise increase in the migration of the ~113-kDa pro-
tein was observed with increasing concentrations of
NH4Cl, with a maximal effect observed at 10 mM NH4Cl,
resulting in only the ER form of ACE2 being visible on the
gel (Fig. 5B, compare lanes 3–5). This suggested that the
trimming and/or terminal modifications of the N-glyco-
sylated chains of ACE2 were affected by NH4Cl treatment.
In addition, at 10 mM NH4Cl, the ER form of ACE2
migrated with slightly faster mobility, indicating that
NH4Cl at that concentration might also affect core glyco-
Post-infection chloroquine treatment reduces SARS-CoV infection and spreadFigure 2
Post-infection chloroquine treatment reduces SARS-CoV infection and spread. Vero E6 cells were seeded and
infected as described for Fig. 1 except that chloroquine was added only after virus adsorption. Cells were maintained in Opti-
MEM (Invitrogen) containing chloroquine for 16–18 h, after which they were processed for immunofluorescence. (A) The con-
centration of chloroquine is indicated on the top. (B) Percent inhibition and SEM were calculated as in Fig. 1B. (C) The effec-
tive dose (ED50) was calculated using commercially available software (Grafit, version 4, Erithacus Software).
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sylation. We also examined the terminal glycosylation sta-
tus of ACE2 when the cells were treated with chloroquine
(Fig. 5C). Similar to NH4Cl, a stepwise increase in the
electrophoretic mobility of ACE2 was observed with
increasing concentrations of chloroquine. At 25 µM chlo-
roquine, the faster electrophoretic mobility of the Golgi-
modified form of ACE2 was clearly evident. On the basis
of the flow cytometry and immunoprecipitation analyses,
Timed post-infection treatment with chloroquineFigure 3
Timed post-infection treatment with chloroquine. This experiment is similar to that depicted in Fig. 2 except that cells
were infected at 1 multiplicity of infection, and chloroquine (10, 25, and 50 µM) was added 3 or 5 h after infection.
NH4Cl inhibits SARS-CoV during pre or post infection treatmentFigure 4
NH4Cl inhibits SARS-CoV during pre or post infection treatment. NH4Cl was added to the cells either before (A) or
after (B) infection, similar to what was done for chloroquine in Figs 1 and 2. Antigen-positive cells were counted, and the
results were presented as in Fig. 1B.
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it can be inferred that NH4Cl and chloroquine both
impaired the terminal glycosylation of ACE2, while
NH4Cl resulted in a more dramatic effect. Although ACE2
is expressed in similar quantities at the cell surface, the
variations in its glycosylation status might render the
ACE2-SARS-CoV interaction less efficient and inhibit
virus entry when the cells are treated with NH4Cl and
chloroquine.
To confirm that ACE2 undergoes terminal sugar modifica-
tions and that the terminal glycosylation is affected by
NH4Cl or chloroquine treatment, we performed immuno-
preipitation of 35S-labeled ACE2 and subjected the immu-
Effect of lysomotropic agents on the cell-surface expression and biosynthesis of ACE2Figure 5
Effect of lysomotropic agents on the cell-surface expression and biosynthesis of ACE2. (A) Vero E6 cells were cul-
tured for 20 h in the absence (control) or presence of chloroquine (10 µM) or NH4Cl (20 mM). Cells were labeled with anti-
ACE2 (grey histogram) or with a secondary antibody alone (white histogram). (B) Biosynthesis of ACE2 in untreated cells or
in cells treated with NH4Cl. Vero E6 cells were pulse-labeled for 3 h with 35S-Met, and the cell lysates were immunoprecipi-
tated with an ACE2 antibody (lane 1). Preincunbation of the antibody with recombinant human ACE2 (rhACE2) completely
abolished the signal (lane 2). The positions of the endoglycosidase H-sensitive ER form and the endoglycosidase H-resistant
Golgi form of ACE2 are emphasized. Note that the increasing concentration of NH4Cl resulting in the decrease of the Golgi
form of ACE2. (C) A similar experiment was performed in the presence of the indicated concentrations of chloroquine. Note
the loss of terminal glycans with increasing concentrations of chloroquine. (D) The terminal glycosidic modification of ACE2
was evaluated by neuraminidase treatment of immunoprecipitated ACE2. Here cells were treated with 1–25 µM concentra-
tions of chloroquine during starvation, pulse, and 3-h chase.
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noprecipitates to neuraminidase digestion. Proteins were
resolved using SDS-PAGE (Fig 5D). It is evident from the
slightly faster mobility of the Golgi form of ACE2 after
neuraminidase treatment (Fig 5D, compare lanes 1 and
2), that ACE2 undergoes terminal glycosylation; however,
the ER form of ACE2 was not affected by neuraminidase.
Cells treated with 10 µM chloroquine did not result in a
significant shift; whereas 25 µM chloroquine caused the
Golgi form of ACE2 to resolve similar to the neuramini-
dase-treated ACE2 (Fig 5D, compare lanes 5 and 6). These
data provide evidence that ACE2 undergoes terminal gly-
cosylation and that chloroquine at anti-SARS-CoV con-
centrations abrogates the process.
Effect of chloroquine and NH4Cl on the biosynthesis and
processing of SARS-CoV spike protein
We next addressed whether the lysosomotropic drugs
(NH4Cl and chloroquine) affect the biosynthesis, glyco-
sylation, and/or trafficking of the SARS-CoV spike glyco-
protein. For this purpose, Vero E6 cells were infected with
SARS-CoV for 18 h. Chloroquine or ammonium chloride
was added to these cells during while they were being
starved (1 h), labeled (30 min) or chased (3 h). The cell
lysates were analyzed by immunoprecipitation with the
SARS-specific polyclonal antibody (HMAF). The 30-min
pulse results indicated that pro-spike (proS) was synthe-
sized as a ~190-kDa precursor (proS-ER) and processed
into ~125-, ~105-, and ~80-kDa proteins (Fig. 6A, lane 2),
a result identical to that in our previous analysis [6].
Except for the 100 µM chloroquine (Fig. 6A, lane 3), there
was no significant difference in the biosynthesis or
processing of the virus spike protein in untreated or chlo-
roquine-treated cells (Fig. 6A, lanes 4–6). It should be
noted that chloroquine at 100 µM resulted in an overall
decrease in biosynthesis and in the levels of processed
virus glycoprotein. In view of the lack of reduction in the
Effects of NH4Cl and chloroquine (CQ) on the biosynthesis, processing, and glycosylation of SARS-CoV spike proteinFigure 6
Effects of NH4Cl and chloroquine (CQ) on the biosynthesis, processing, and glycosylation of SARS-CoV spike
protein. Vero E6 cells were infected with SARS-CoV as described in Fig. 2. CQ or NH4Cl was added during the periods of
starvation (1 h) and labeling (30 min) with 35S-Cys and followed by chase for 3 h in the presence of unlabeled medium. Cells
were lysed in RIPA buffer and immunoprecipitated with HMAF. Virus proteins were resolved using 3–8% NuPAGE gel (Invitro-
gen). The cells presented were labeled for 30 min (A) and chased for 3 h (B). The migration positions of the various spike
molecular forms are indicated at the right side, and those of the molecular standards are shown to the left side. proS-ER and
proS-Golgi are the pro-spike of SARS-Co in the ER and Golgi compartments, respectively and proS-ungly is the unglycosylated
pro-spike ER.
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biosynthesis and processing of the spike glycoprotein in
the presence of chloroquine concentrations (10 and 50
µM) that caused large reductions in SARS-CoV replication
and spread, we conclude that the antiviral effect is proba-
bly not due to alteration of virus glycoprotein biosynthe-
sis and processing. Similar analyses were performed with
NH4Cl, and the data suggested that the biosynthesis and
processing of the spike protein were also not negatively
affected by NH4Cl (Fig. 6A, lanes 7–12). Consistent with
our previous analysis [6], we observed the presence of a
larger protein, which is referred to here as oligomers.
Recently, Song et al. [20] provided evidence that these are
homotrimers of the SARS-CoV spike protein and were
incorporated into the virions. Interestingly, the levels of
the homotrimers in cells treated with 100 µM chloroquine
and 40 and 20 mM NH4Cl (Fig. 6A, lanes 3, 9, and 10)
were slightly lower than in control cells or cells treated
with lower drug concentrations.
The data obtained from a 30-min pulse followed by a 3-h
chase (Fig. 6B, lanes 2 and 8) confirmed our earlier obser-
vation that the SARS-CoV spike protein precursor (proS-
ER) acquires Golgi-specific modifications (proS-Golgi)
resulting in a ~210-kDa protein [6]. Chloroquine at 10,
25, and 50 µM had no substantial negative impact on the
appearance of the Golgi form (Fig. 6B, compare lane 2 to
lanes 4–6). Only at 100 µM chloroquine was a reduction
in the level of the Golgi-modified pro-spike observed
(lane 3). On the other hand, NH4Cl abrogated the appear-
ance of Golgi-modified forms at ≥10 mM (compare lane
8 with 9–11) and had a milder effect at 1 mM (lane 12).
These data clearly demonstrate that the biosynthesis and
proteolytic processing of SARS-CoV spike protein are not
affected at chloroquine (25 and 50 µM) and NH4Cl (1
mM) doses that cause virus inhibitory effects. In addition,
with 40, 20, and 10 mM NH4Cl, there was an increased
accumulation of proS-ER with a concomitant decrease in
the amount of oligomers (Fig. 6B, lanes 9–11). When we
examined the homotrimers, we found that chloroquine at
100 µM and NH4Cl at 40 and 20 mM resulted in slightly
faster mobility of the trimers (Fig. 6B, lanes 3, 9, and 10),
but lower drug doses, which did exhibit significant antivi-
ral effects, did not result in appreciable differences. These
data suggest that the newly synthesized intracellular spike
protein may not be a major target for chloroquine and
NH4Cl antiviral action. The faster mobility of the trimer at
certain higher concentration of the drugs might be due the
effect of these drugs on the terminal glycosylation of the
trimers.
Discussion
We have identified chloroquine as an effective antiviral
agent for SARS-CoV in cell culture conditions, as evi-
denced by its inhibitory effect when the drug was added
prior to infection or after the initiation and establishment
of infection. The fact that chloroquine exerts an antiviral
effect during pre- and post-infection conditions suggest
that it is likely to have both prophylactic and therapeutic
advantages. Recently, Keyaerts et al. [21] reported the anti-
viral properties of chloroquine and identified that the
drug affects SARS-CoV replication in cell culture, as evi-
denced by quantitative RT-PCR. Taken together with the
findings of Keyaerts et al. [21], our analysis provides fur-
ther evidence that chloroquine is effective against SARS-
CoV Frankfurt and Urbani strains. We have provided
evidence that chloroquine is effective in preventing SARS-
CoV infection in cell culture if the drug is added to the
cells 24 h prior to infection. In addition, chloroquine was
significantly effective even when the drug was added 3–5
h after infection, suggesting an antiviral effect even after
the establishment of infection. Since similar results were
obtained by NH4Cl treatment of Vero E6 cells, the under-
lying mechanism(s) of action of these drugs might be
similar.
Apart from the probable role of chloroquine on SARS-
CoV replication, the mechanisms of action of chloroquine
on SARS-CoV are not fully understood. Previous studies
have suggested the elevation of pH as a mechanism by
which chloroquine reduces the transduction of SARS-CoV
pseudotype viruses [17,18]. We examined the effect of
chloroquine and NH4Cl on the SARS-CoV spike proteins
and on its receptor, ACE2. Immunoprecipitation results of
ACE2 clearly demonstrated that effective anti-SARS-CoV
concentrations of chloroquine and NH4Cl also impaired
the terminal glycosylation of ACE2. However, the flow
cytometry data demonstrated that there are no significant
differences in the cell surface expression of ACE2 in cells
treated with chloroquine or NH4Cl. On the basis of these
results, it is reasonable to suggest that the pre-treatment
with NH4Cl or chloroquine has possibly resulted in the
surface expression of the under-glycosylated ACE2. In the
case of chloroquine treatment prior to infection, the
impairment of terminal glycosylation of ACE2 may result
in reduced binding affinities between ACE2 and SARS-
CoV spike protein and negatively influence the initiation
of SARS-CoV infection. Since the biosynthesis, processing,
Golgi modification, and oligomerization of the newly
synthesized spike protein were not appreciably affected by
anti-SARS-CoV concentrations of either chloroquine or
NH4Cl, we conclude that these events occur in the cell
independent of the presence of the drugs. The potential
contribution of these drugs in the elevation of endosomal
pH and its impact on subsequent virus entry or exit could
not be ruled out. A decrease in SARS-CoV pseudotype
transduction in the presence of NH4Cl was observed and
was attributed to the effect on intracellular pH [17,18].
When chloroquine or NH4Cl are added after infection,
these agents can rapidly raise the pH and subvert on-going
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fusion events between virus and endosomes, thus inhibit-
ing the infection.
In addition, the mechanism of action of NH4Cl and chlo-
roquine might depend on when they were added to the
cells. When added after the initiation of infection, these
drugs might affect the endosome-mediated fusion, subse-
quent virus replication, or assembly and release. Previous
studies of chloroquine have demonstrated that it has mul-
tiple effects on mammalian cells in addition to the eleva-
tion of endosomal pH, including the prevention of
terminal glycosyaltion of immunoglobulins [22]. When
added to virus-infected cells, chloroquine inhibited later
stages in vesicular stomatitis virus maturation by inhibit-
ing the glycoprotein expression at the cell surface [23],
and it inhibited the production of infectious HIV-1 parti-
cles by interfering with terminal glycosylation of the glyc-
oprotein [24,25]. On the basis of these properties, we
suggest that the cell surface expression of under-glyco-
sylated ACE2 and its poor affinity to SARS-CoV spike pro-
tein may be the primary mechanism by which infection is
prevented by drug pretreatment of cells prior to infection.
On the other hand, rapid elevation of endosomal pH and
abrogation of virus-endosome fusion may be the primary
mechanism by which virus infection is prevented under
post-treatment conditions. More detailed SARS CoV
spike-ACE2 binding assays in the presence or absence of
chloroquine will be performed to confirm our findings.
Our studies indicate that the impact of NH4Cl and chloro-
quine on the ACE2 and spike protein profiles are signifi-
cantly different. NH4Cl exhibits a more pronounced effect
than does chloroquine on terminal glycosylation, high-
lighting the novel intricate differences between chloro-
quine and ammonium chloride in affecting the protein
transport or glycosylation of SARS-CoV spike protein and
its receptor, ACE2, despite their well-established similar
effects of endosomal pH elevation.
The infectivity of coronaviruses other than SARS-CoV are
also affected by chloroquine, as exemplified by the
human CoV-229E [15]. The inhibitory effects observed on
SARS-CoV infectivity and cell spread occurred in the pres-
ence of 1–10 µM chloroquine, which are plasma concen-
trations achievable during the prophylaxis and treatment
of malaria (varying from 1.6–12.5 µM) [26] and hence are
well tolerated by patients. It recently was speculated that
chloroquine might be effective against SARS and the
authors suggested that this compound might block the
production of TNFα, IL6, or IFNγ [15]. Our data provide
evidence for the possibility of using the well-established
drug chloroquine in the clinical management of SARS.
Conclusion
Chloroquine, a relatively safe, effective and cheap drug
used for treating many human diseases including malaria,
amoebiosis and human immunodeficiency virus is effec-
tive in inhibiting the infection and spread of SARS CoV in
cell culture. The fact that the drug has significant inhibi-
tory antiviral effect when the susceptible cells were treated
either prior to or after infection suggests a possible pro-
phylactic and therapeutic use.
Methods
SARS-CoV infection, immunofluorescence, and
immunoprecipitation analyses
Vero E6 cells (an African green monkey kidney cell line)
were infected with SARS-CoV (Urbani strain) at a multi-
plicity of infection of 0.5 for 1 h. The cells were washed
with PBS and then incubated in OPTI-MEM (Invitrogen)
medium with or without various concentrations of either
chloroquine or NH4Cl (both from Sigma). Immunofluo-
rescence staining was performed with SARS-CoV-specific
hyperimmune mouse ascitic fluid (HMAF) [8] followed
by anti-mouse fluorescein-coupled antibody.
Eighteen hours after infection, the virus-containing super-
natants were removed, and the cells were pulsed with 35S-
(Cys) for 30 min and chased for 3 h before lysis in RIPA
buffer. Clarified cell lysates and media were incubated
with HMAF, and immunoprecipitated proteins were sepa-
rated by 3–8% NuPAGE gel (Invitrogen); proteins were
visualized by autoradiography. In some experiments, cells
were chased for 3 h with isotope-free medium. Clarified
cell supernatants were also immunoprecipitated with
SARS-CoV-specific HMAF.
ACE2 flow cytometry analysis and biosynthesis
Vero E6 cells were seeded in Dulbecco's modified Eagle
medium (Invitrogen) supplemented with 10% fetal
bovine serum. The next day, the cells were incubated in
Opti-MEM (Invitrogen) in the presence or absence of 10
µM chloroquine or 20 mM NH4Cl. To analyze the levels
of ACE2 at the cell surface, cells were incubated on ice
with 10 µg/mL affinity-purified goat anti-ACE2 antibody
(R&D Systems) and then incubated with FITC-labeled
swine anti-goat IgG antibody (Caltag Laboratories).
Labeled cells were analyzed by flow cytometry with a FAC-
SCalibur flow cytometer (BD Biosciences). For ACE2 bio-
synthesis studies, Vero E6 cells were pulsed with 250 µCi
35S-(Met) (Perkin Elmer) for 3 h with the indicated con-
centrations of chloroquine or NH4Cl and then lysed in
RIPA buffer. Clarified lysates were immunoprecipitated
with an affinity-purified goat anti-ACE2 antibody (R&D
systems), and the immunoprecipitated proteins were sep-
arated by SDS-polyacrylamide gel electrophoresis.
Competing interests
The author(s) declare that they have no competing
interests.
Virology Journal 2005, 2:69 http://www.virologyj.com/content/2/1/69
Page 10 of 10
(page number not for citation purposes)
Authors' contributions
MV did all the experiments pertaining to SARS CoV infec-
tion and coordinated the drafting of the manuscript. EB
and SB performed experiments on ACE2 biosynthesis and
FACS analysis. BE performed data acquisition from the
immunofluorescence experiments. PR and TK provided
critical reagents and revised the manuscript critically. NS
and SN along with MV and EB participated in the plan-
ning of the experiments, review and interpretation of data
and critical review of the manuscript. All authors read and
approved the content of the manuscript.
Acknowledgements
We thank Claudia Chesley and Jonathan Towner for critical reading of the
manuscript. This work was supported by a Canadian PENCE grant (T3),
CIHR group grant #MGC 64518, and CIHR grant #MGP-44363 (to NGS).
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