5  Conclusion
Chloroquine has been shown to be capable of inhibiting the in vitro replication of several coronaviruses. Recent publications support the hypothesis that chloroquine can improve the clinical outcome of patients infected by SARS-CoV-2. The multiple molecular mechanisms by which chloroquine can achieve such results remain to be further explored. Since SARS-CoV-2 was found a few days ago to utilise the same cell surface receptor ACE2 (expressed in lung, heart, kidney and intestine) as SARS-CoV-1 [85,86] (Table 1 ), it may be hypothesised that chloroquine also interferes with ACE2 receptor glycosylation thus preventing SARS-CoV-2 binding to target cells. Wang and Cheng reported that SARS-CoV and MERS-CoV upregulate the expression of ACE2 in lung tissue, a process that could accelerate their replication and spread [85]. Although the binding of SARS-CoV to sialic acids has not been reported so far (it is expected that Betacoronavirus adaptation to humans involves progressive loss of hemagglutinin-esterase lectin activity), if SARS-CoV-2 like other coronaviruses targets sialic acids on some cell subtypes, this interaction will be affected by chloroquine treatment [87,88]. Today, preliminary data indicate that chloroquine interferes with SARS-CoV-2 attempts to acidify the lysosomes and presumably inhibits cathepsins, which require a low pH for optimal cleavage of SARS-CoV-2 spike protein [89], a prerequisite to the formation of the autophagosome [49]. Obviously, it can be hypothesised that SARS-CoV-2 molecular crosstalk with its target cell can be altered by chloroquine through inhibition of kinases such as MAPK. Chloroquine could also interfere with proteolytic processing of the M protein and alter virion assembly and budding (Fig. 1 ). Finally, in COVID-19 disease this drug could act indirectly through reducing the production of pro-inflammatory cytokines and/or by activating anti-SARS-CoV-2 CD8+ T-cells.
Table 1 Human coronavirus (HCoV) receptors/co-receptors as possible targets for chloroquine-induced inhibition of the virus replication cycle
Coronavirus Receptora May also bind Replication cycle inhibited by chloroquineb
Alphacoronavirus
 HCoV-229E Aminopeptidase N (APN)/CD13 Yes
 HCoV-NL63 Angiotensin-converting enzyme 2 (ACE2) ?
Heparan sulfate proteoglycansc
Betacoronavirus
 HCoV-OC43 HLA class Id, IFN-inducible transmembrane (IFITM) proteins in endocytic vesiclese Sialic acid (O-acetylated sialic acid)f Yes
 SARS-CoV-1 Angiotensin-converting enzyme 2 (ACE2) DC-SIGN/CD209, DC-SIGNr, DC-SIGN-related lectin LSECting Yes
 HCoV-HKU1 HLA class Ih Sialic acid (O-acetylated sialic acid) ?
 MERS-CoV i Dipeptidyl peptidase 4 (DPP4)/CD26 Yes
 SARS-CoV-2 ACE2i Sialic acid? Yes
HLA, human leukocyte antigen.
a Adapted from Graham et al. [91].
b Chloroquine could interfere with receptor (ACE2) glycosylation and/or sialic acid biosynthesis.
c According to Milewska et al. [92].
d According to Collins [93].
e According to Zhao et al. [94].
f According to Vlasak et al. [95].
g According to Huang et al. [96].
h According to Chan et al. [97].
i It is worth noting that different host cell proteases are required to activate the spike (S) protein for coronaviruses, such as SARS-CoV-1 S protein that requires activation by cathepsin L [89], or MERS-CoV that requires furin-mediated activation of the S protein [98].
Fig. 1 Schematic representation of the possible effects of chloroquine on the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication cycle. SARS-CoV2, like other human coronaviruses, harbours three envelope proteins, the spike (S) protein (180–220 kDa), the membrane (M) protein (25–35 kDa) and the envelope (E) protein (10–12 kDa), which are required for entry of infectious virions into target cells. The virion also contains the nucleocapsid (N), capable of binding to viral genomic RNA, and nsp3, a key component of the replicase complex. A subset of betacoronaviruses use a hemagglutinin-esterase (65 kDa) that binds sialic acids at the surface of glycoproteins. The S glycoprotein determines the host tropism. There is indication that SARS-CoV-2 binds to angiotensin-converting enzyme 2 (ACE2) expressed on pneumocytes [85,99]. Binding to ACE2 is expected to trigger conformational changes in the S glycoprotein allowing cleavage by the transmembrane protease TMPRSS2 of the S protein and the release of S fragments into the cellular supernatant that inhibit virus neutralisation by antibodies [100]. The virus is then transported into the cell through the early and late endosomes where the host protease cathepsin L further cleaves the S protein at low pH, leading to fusion of the viral envelope and phospholipidic membrane of the endosomes resulting in release of the viral genome into the cell cytoplasm. Replication then starts and the positive-strand viral genomic RNA is transcribed into a negative RNA strand that is used as a template for the synthesis of viral mRNA. Synthesis of the negative RNA strand peaks earlier and falls faster than synthesis of the positive strand. Infected cells contain between 10 and 100 times more positive strands than negative strands. The ribosome machinery of the infected cells is diverted in favour of the virus, which then synthesises its non-structural proteins (NSPs) that assemble into the replicase-transcriptase complex to favour viral subgenomic mRNA synthesis (see the review by Fehr and Perlman for details [101]). Following replication, the envelope proteins are translated and inserted into the endoplasmic reticulum and then move to the Golgi compartment. Viral genomic RNA is packaged into the nucleocapsid and then envelope proteins are incorporated during the budding step to form mature virions. The M protein, which localises to the trans-Golgi network, plays an essential role during viral assembly by interacting with the other proteins of the virus. Following assembly, the newly formed viral particles are transported to the cell surface in vesicles and are released by exocytosis. It is possible that chloroquine interferes with ACE2 receptor glycosylation, thus preventing SARS-CoV-2 binding to target cells. Chloroquine could also possibly limit the biosynthesis of sialic acids that may be required for cell surface binding of SARS-CoV-2. If binding of some viral particles is achieved, chloroquine may modulate the acidification of endosomes thereby inhibiting formation of the autophagosome. Through reduction of cellular mitogen-activated protein (MAP) kinase activation, chloroquine may also inhibit virus replication. Moreover, chloroquine could alter M protein maturation and interfere with virion assembly and budding. With respect to the effect of chloroquine on the immune system, see the elegant review by Savarino et al. [11]. ERGIC, ER-Golgi intermediate compartment.
Already in 2007, some of us emphasised in this journal the possibility of using chloroquine to fight orphan viral infections [10]. The worldwide ongoing trials, including those involving the care of patients in our institute [90], will verify whether the hopes raised by chloroquine in the treatment of COVID-19 can be confirmed.