COVID-19: Molecular and Cellular Response

Abstract

In late December 2019, a vtiral pneumonia with an unknown agent was reported in Wuhan, China. A novel coronavirus was identified as the causative agent. Because of the human-to-human transmission and rapid spread; coronavirus disease 2019 (COVID-19) has rapidly increased to an epidemic scale and poses a severe threat to human health; it has been declared a public health emergency of international concern (PHEIC) by the World Health Organization (WHO). This review aims to summarize the recent research progress of COVID-19 molecular features and immunopathogenesis to provide a reference for further research in prevention and treatment of SARS coronavirus2 (SARS-CoV-2) infection based on the knowledge from researches on SARS-CoV and Middle East respiratory syndrome-related coronavirus (MERS-CoV).

Full text

COVID-19: Molecular and
Cellular Response
Shamila D. Alipoor
1
, Esmaeil Mortaz
2,3
*, Hamidreza Jamaati
4
, Payam Tabarsi
2
,
Hasan Bayram
5
, Mohammad Varahram
6
and Ian M. Adcock
7,8
1
Molecular Medicine Department, Institute of Medical Biotechnology, National Institute of Genetic Engineering and
Biotechnology (NIGEB), Tehran, Iran,
2
Clinical Tuberculosis and Epidemiology Research Center, National Research Institute
of Tuberculosis and Lung Diseases, Shahid Beheshti University of Medical Sciences, Tehran, Iran,
3
Department of
Immunology, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran,
4
Chronic Respiratory
Diseases Research Center, National Research Institute of Tuberculosis and Lung Diseases (NRITLD), Shahid Beheshti
University of Medical Sciences, Tehran, Iran,
5
Department of Pulmonary Medicine, Koc University School of Medicine, Koc
University Research Center for Translational Medicine (KUTTAM), Istanbul, Turkey,
6
Mycobacteriology Research Center,
National Research Institute of Tuberculosis and Lung Diseases (NRITLD), Shahid Beheshti University of Medical Sciences,
Tehran, Iran,
7
National Heart and Lung Institute, Imperial College London and the NIHR Imperial Biomedical Research
Centre, London, United Kingdom,
8
Priority Research Centre for Asthma and Respiratory Disease, Hunter Medical Research
Institute, University of Newcastle, Newcastle, NSW, Australia
In late December 2019, a vtiral pneumonia with an unknown agent was reported in
Wuhan, China. A novel coronavirus was identified as the causative agent. Because of the
human-to-human transmission and rapid spread; coronavirus disease 2019 (COVID-19)
has rapidly increased to an epidemic scale and poses a severe threat to human health; it
has been declared a public health emergency of international concern (PHEIC) by the
World Health Organization (WHO). This review aims to summarize the recent research
progress of COVID-19 molecular features and immunopathogenesis to provide a
reference for further research in prevention and treatment of SARS coronavirus2
(SARS-CoV-2) infection based on the knowledge from researches on SARS-CoV and
Middle East respiratory syndrome-related coronavirus (MERS-CoV).
Keywords: COVID-19, cytokines, IL-6, IL-8, ACE2
INTRODUCTION
Coronavirus disease 2019 (COVID-19) is caused by a member of the Coronaviridae family. It was
initially reported in Wuhan, China and has expanded rapidly in the world and is now at pandemic
level (Lippi and Plebani, 2020). The novel b-CoV was named as “SARS-CoV-2”by the International
Virus Classification Commission (Wu F. et al., 2020).
Coronaviruses are enveloped positive-stranded RNA viruses, which replicate in the host cell
cytoplasm. They possess a 5’capped RNA and also contain the longest RNA among all RNA viruses
with a length of ~30kbp containing 14 open reading frames (ORFs) (Marra et al., 2003;Perlman and
Netland, 2009). Coronaviruses are classified into four genera (a,b,g, and d) based on phylogeny (Su
et al., 2016).
The first human coronavirus was described in 1965 from patients with the common cold for
which there is still no vaccine. Counting SARS coronavirus2 (SARS-Cov2), there are currently seven
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org February 2021 | Volume 11 | Article 5630851
Edited by:
Rachel L. Roper,
The Brody School of Medicine at East
Carolina University, United States
Reviewed by:
Teneema Kuriakose,
St. Jude Children’s Research Hospital,
United States
Monica Miranda-Saksena,
Westmead Institute for Medical
Research, Australia
*Correspondence:
Esmaeil Mortaz
emortaz@gmail.com
Specialty section:
This article was submitted to
Virus and Host,
a section of the journal
Frontiers in Cellular
and Infection Microbiology
Received: 17 May 2020
Accepted: 08 January 2021
Published: 11 February 2021
Citation:
Alipoor SD, Mortaz E, Jamaati H,
Tabarsi P, Bayram H, Varahram M and
Adcock IM (2021) COVID-19:
Molecular and Cellular Response.
Front. Cell. Infect. Microbiol. 11:563085.
doi: 10.3389/fcimb.2021.563085
REVIEW
published: 11 February 2021
doi: 10.3389/fcimb.2021.563085

human coronaviruses strains known to infect humans which
belong to the aand bcoronavirus genera (Table 1)(Su et al.,
2016;Rithanya and Brundha, 2020).
Coronaviruses gained notoriety with the outbreak of Severe
Acute Respiratory Syndrome (SARS) in 2002-2003. This led to
the isolation and identification of HCoV-NL63 and HCoV-
HKU1. Further, the emergence of the middle east respiratory
syndrome coronavirus (MERS-CoV) in 2012 revealed that these
pathogens frequently cross the species and can pose a serious risk
to human health.
Bats are the main reservoirs of a large variety of viruses
especially human a- and b-coronaviruses (Hu et al., 2015). Some
SARS-like coronavirus (SL-CoVs) isolated from the bats have
high genomic sequence similarity and receptor usage compared
to SARS-CoV. This suggests that the spread of a bat coronavirus
to man presents a major global health risk (Ge et al., 2013;
Menachery et al., 2015;Yang X. L. et al., 2015). In the case of the
novel b-CoV strain, genome sequencing revealed (Lu et al., 2020)
the new virus has 81% identity with the sequence of the bat-
derived SARS.
The majority of COVID 19 cases (about 80%) are
asymptomatic or show mild symptoms but a low percentage
experience severe respiratory failure (Surveillances, 2020).
Interestingly, the viral load in asymptomatic patients was
similar to that in symptomatic patients. In a study in china it
was reported that in people with normal CT scans and no clinical
symptoms, who were in close contact with confirmed virus-
positive patients; nasal and throat swab tests were positive on
days 7, 10, and 11 after contact (Zou et al., 2020). Furthermore,
SARS-CoV-2 RNA was detectable in stool, saliva and urine
samples as well as in gastrointestinal tissue in patients with
COVID-19 in China. Thus, the digestive tract should also be
considered as a route of infection (Xiao et al., 2020). These
findings emphasize the potential of viral transmission of
asymptomatic patients and indicate the urgent need for
strategies revolving around case detection and isolation
(Surveillances, 2020).
Since Dec 2019, when the first case of disease was reported,
there have been over 80 M confirmed cases and 1.7 M deaths
have been reported across 235 countries (https://www.who.int/
emergencies/diseases/novel-coronavirus-2019). Given its
properties and rapid spread, there is an emergent need to
expand our knowledge of the molecular features and immune
pathogenesis of COVID-19. This review summarizes recent
findings on the potential mechanisms and clinical features of
COVID-19 and relies on our knowledge of SARS-CoV and
MERS-CoV. This work aims to provide a reference for further
research in the prevention and the treatment of SARS-CoV-
2 infection
VIROLOGY AND GENOME
The SARS-CoV-2 genome consist of 29,903nt (nucleotides) and
has been assigned GenBank accession number MN908947. RNA
from the virus is closely related to two bat derived SARS-like
coronaviruses SL-CoVZC45 and SL-CoVZXC21, with a
nucleotide identity of 88.1%, but is more distant from SARS-
CoV (about 79%) and MERS-CoV (about 50%) (Lu et al., 2020;
Wu F. et al., 2020).
The genome of SARS-Cov-2, similar to other CoVs, contains
ten open reading frames (ORFs). The first ORF covers two-
thirds of the viral RNA, which encodes polypeptides of the viral
replicase-transcriptase complex (Fehr and Perlman, 2015). The
remaining ORFs translate four main structural proteins: spike
(S), envelope (E), nucleocapsid (N), and membrane (M)
proteins. The genome is packaged into a helical nucleocapsid
surrounded by a host-derived lipid bilayer (Fehr and
Perlman, 2015).
STRUCTURAL DETERMINANTS IN THE
PATHOGENICITY OF SARS-COV2
A key to tackling the new pandemic is a clear understanding of
the factors that determine the virus pathogenicity including the
mechanism underlying receptor recognition, cell entry, and the
processing of viral proteins inside cells.
Virus Receptor
Receptor recognition is crucial in viral infectivity, pathogenesis
and cell tropism and can be considered as the primary
determinant of virus pathogenicity.
Corona viruses use a variety of receptors to enter the cells.
Dipeptidyl protease 4 (DPP4) or CD26 is used by MERS-CoV
TABLE 1 | Human Corona virus overview.
Name Discovery/location Group Receptor Symptom
HCOV-229E 1966 aAminopeptidase N
(hAPN, CD13)
Common cold
HCoV-OC43 1967 ß 9-O-acetylated sialic
acid
Common cold
SARS-CoV 2003/china ß ACE2/CD209L Severe respiratory illness
HCoV-NL63 2004/Netherlands aACE2 Lower respiratory tract infection, croup and bronchiolitis in children and immunocompromised
patients
HCoV-HKU1 2005/HongKong ß 9-O-acetylated sialic acid Chronic pulmonary disease in Hong Kong
MERS-CoV 2012/Middle East ß DPP4 Severe respiratory illness
Sars-Cov2 2019/China ß ACE2 Severe respiratory illness
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(Wang et al., 2013) and angiotensin converting enzyme 2 (ACE2)
by SARS-CoV (Jeffers et al., 2004) and NL63-Cov (Hofmann
et al., 2005). CD209L is also used by SARS-CoV as an alternative
receptor (Jeffers et al., 2004)(Table 1). Based on viral genome
analysis of the new virus and high similarity to SARS-CoV, it is
likely that SARS-CoV-2 uses ACE2 as its receptor for cellular
invasion (Xu H. et al., 2020;Wan Y. et al., 2020). Further
evidence for ACE2 being the SARS-CoV-2 receptor is provided
by the fact that Baby Hamster Kidney fibroblasts (BHk) cells
transfected with human ACE2 become susceptible to SARS-
CoV-2 infection (Walls et al., 2020).
The ACE2 enzyme is involved in the renin–angiotensin–
aldosterone system (RAAS) activation. The RAAS includes a
cascade of vasoactive peptides that regulate the blood volume
and systemic vascular resistance in a prolonged manner
(Dronavalli et al., 2008). ACE2 is an exo-peptidase that
catalyzes the conversion of angiotensin (Ang I) I to the
nonapeptide Ang-(1-9) or the conversion of angiotensin II to
the heptapeptide Ang-(1-7) (Dronavalli et al., 2008).
Previous studies demonstrated a positive correlation between
the expression pattern of ACE2 and the infectivity of SARS-CoV.
In addition, some ACE2 variants interact with the SARS-CoV-2
or NL635 S-proteins with a lower-affinity (Mathewson et al.,
2008). Therefore, the pattern of ACE2 expression in different
tissues can determine tropism, susceptibility, symptoms, and
outcome of SARS-CoV-2 infection (Chen J. et al., 2020). ACE2
is expressed on the mucosa of oral cavity and is highly enriched
in epithelial cells of the tongue which highlights the role of the
oral cavity for infection with SARS-CoV-2 (Xu H. et al., 2020).
ACE2 is also highly expressed in vascular endothelial cells of the
heart and the kidney, and it affects cardiac function (Luft, 2014;
Badae et al., 2019).
COVID-19 has a higher mortality rate in males compared to
females (Xie et al., 2020) despite both genders having a similar
infection rate (The Sex, Gender, and COVID-19 Project, 2020).
There are differences in the reported male-biased mortality
between countries ranging from 59%–75% of total mortality
(Pradhan and Olsson, 2020;Jin et al., 2020). Biological (immune
responses) and behavioral factors (e.g., smoking, mask wearing
and other lifestyle habits) may be responsible for placing men at
a greater risk of infection by SARS-CoV-2 or the consequences of
COVID-19 infection (Galasso et al., 2020). However, in a few
countries such as India, Nepal, Vietnam, and Slovenia the
COVID-19 case fatality rate is higher in women than men
(The Sex, Gender, and COVID-19 Project, 2020). These
differential findings may be due to the incomplete data in the
case identification by sex or demographic factors such as co-
morbidities that increase the health risk for women in these
regions (Dehingia and Raj, 2020).
A recent study reported that Asian males have higher
expression of ACE2 (Zhao et al., 2020). It was also previously
reported that German cases showed mild clinical symptoms for
SARS without severe illness (Cao et al., 2020). However, the
pattern of ACE2 expression and function in different populations
remains to be investigated (Cao et al., 2020). For example, the
rate of COVID 19 related death is significantly higher among the
Afro-Caribbean and South-East Asian people. Data to date
indicates that the confirmed cases of COVID-19 in black
counties is more than 3-fold-, and the rate of death is 6-fold-,
higher than that in white counties in the USA (Yancy, 2020;
Thebault R and Williams, 2020). This stark difference in the rate
of disease and outcome in the black population may be explained
in part, but not fully, by concomitant comorbidities (Giudicessi
et al., 2020). Comorbidities such as hypertension, metabolic
syndrome, diabetes, and cardiovascular disease (CVD) are the
main risk factors for worse outcome and mortality in patients
with COVID-19 (Bansal, 2020) and these are prevalent in Afro-
Caribbean and south-East Asian populations (Owczarek
et al., 2018).
Spike Glycoprotein (S Protein)
The viral spike glycoprotein (S protein) binds to the host cellular
receptor and is therefore, considered as the main determinant of
cell tropism and pathogenesis (Kuba et al., 2010). In corona
viruses; the spike protein is a large transmembrane protein and is
highly glycosylated. Spike proteins assemble as homo-trimers on
the virion surface to form a crown-like appearance or “corona”
(Belouzard et al., 2012).
The S protein is composed of an extracellular and a
transmembrane domain (TM), as well as a short cytoplasmic
tail region (CP) (Figure 1A). The extracellular domain of the S
protein is composed of two subunits (S1 and S2) which are
responsible for host-cell receptor recognition and membrane
fusion, respectively. S1 binds to ACE2 via the receptor-binding
domain (RBD) (Bosch et al., 2003).
A three-dimensional (3-D) atomic-scale map of the RBD of
SARS-CoV-2 in complex with human ACE2 has been obtained
(https://www.rcsb.org/structure/6VYB) (Figure 1B)(Shang
et al., 2020a). This analysis shows that the RBD of the SARS
CoV-2 S protein is more compact and binds human ACE-2 four
times more strongly than the SARS-CoV S protein (Shang et al.,
2020a). The RBD is the most variable part of the coronavirus
genome. Six RBD amino acids are critical for ACE2 binding (Tai
et al., 2020). Five of these six residues differ between SARS-CoV-
2 and SARS-CoV, which accounts for the higher binding affinity
of the SARS-CoV-2 S protein for human ACE2 (Andersen
et al., 2020).
Even though the SARS-CoV-2 RBD has a higher binding
affinity than SARS-CoV RBD for human ACE2, the binding
affinity of the entire SARS-CoV-2 S protein for human ACE2 is
lower than, or comparable to, that of the SARS-CoV S protein
(Shang et al., 2020b). This paradox can be explain based on the
dynamic state of the RBD. Cryo-electron microscopy studies
captured two states of the RBD: either buried (lying state) or
exposed (standing state), illustrating an inherently flexible RBD
readily recognized by its receptor (Yuan et al., 2017)
(Figure 1C).
The RBD in the SARS-CoV S protein is mostly in the
standing-up state, however, it is mostly in the lying-down state
in SARS-CoV-2 and is not exposed and unable to bind ACE2.
Thus, although the SARS-CoV-2 RBD is more potent, it is less
exposed than the SARS-CoV RBD and the overall entry
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efficiencies of SARS-CoV-2 and SARS-CoV are comparable
(Shang et al., 2020b). The hidden RBD may also be responsible
for inefficient immune response and prolonged recovery time as
well as for the long incubation period. Many SARS-CoV-2
patients develop low levels of neutralizing antibodies and suffer
prolonged illness.
Proteolytic Processing
Host protease activation is the other significant determinant of
coronavirus infection and pathogenesis. Coronavirus entry is
tightly regulated by the expression and activation of host proteases.
The presence of a proteolytic cleavage site within the S protein
mediates membrane fusion and viral infectivity. Sequence
variation around this cleavage site impacts severely on cellular
tropism and pathogenesis of CoVs (Kido et al., 2012;Coutard
et al., 2020). In the case of H5N1 hemagglutinin HA, insertion of
a multi-basic motif created a typical furin-like cleavage site in the
S protein which was responsible for the hyper-virulence of the
virus during the Hong Kong 1997 outbreak (Claas et al., 1998).
MERS-CoV also uses furin to enter the cells (Millet and
Whittaker, 2014).
In the case of the influenza viruses, the highly pathogenic
forms contain a furin-like cleavage site that can be cleaved by
different host proteases allowing the virus to infect a broad range
of cells (Kido et al., 2012). The low-pathogenicity forms are only
cleaved by trypsin-like proteases and so virus infectivity is
limited to the respiratory and/or intestinal organs due to tissue
distribution of the activating protease(s) (Sun et al., 2010;
Coutard et al., 2020).
Interestingly, a furin-like cleavage site has been identified in
the SARS-CoV-2 S protein that is absent in the other SARS-like
CoVs and may be responsible for its high pathogenicity (Coutard
et al., 2020).
Sequence analysis of the S protein in SARS-CoV-2 reveals the
presence of a four amino acid residue insertion; “RRAR”;
producing a furin-sensitive motif at the boundary between the
S1 and S2 subunits (Figure 1). Interestingly this furin cleavage
site is conserved among all the isolated and sequenced SARS-
CoV-2 virions (Shang et al., 2020a) and its abrogation reduced the
efficiency of viral entry (Shang et al., 2020b;Letko et al., 2020).
Processing of the S1/S2 site occurs during viral packaging in
infected cells, presumably by furin in the Golgi compartment
(Shang et al., 2020b;Letko et al., 2020).
The S protein is further cleaved by host proteases at the S2′
site located immediately upstream of the fusion peptide. This
cleavage is proposed to activate the protein for membrane fusion
A
BC
FIGURE 1 |(A) Schematic of genome encoding spike protein in SARS-CoV2. SP, signal peptide; NTD, N-terminal domain; RBD, receptor-binding domain; RBM,
receptor-binding motif; FP, fusion peptide; HR1 and HR2, heptad repeat regions 1 and 2; TM, transmembrane; CP, cytoplasmic domain. (B) Schematic drawing of
the structure of coronavirus spike. S1, receptor-binding subunit (homotrimers); S2, membrane fusion subunit; TM, transmembrane anchor; IC, intracellular tail.
(C) SARS-CoV-2 spike ectodomain structure (open state) (details provided at https://www.rcsb.org/structure/6VYB).
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via extensive irreversible conformational changes (Letko
et al., 2020).
Transmembrane serine protease2 (TMPRSS2) and lysosomal
cathepsins both have cumulative effects with furin on activating
SARS-CoV-2 entry. However, the furin pre-activation in
producer cells allows SARS-CoV-2 to be less dependent on
target cell proteases and increases its ability to enter cells with
relatively low expressions of TMPRSS2 or lysosomal cathepsins
(Shang et al., 2020b;Hoffmann et al., 2020). On the other hand,
prior cleavage at the S1/S2 site increases cleavage at the S2′site
(Kuba et al., 2010).
TMPRSS2 activity is important for SARS-CoV-2 cell entry
(Braun and Sauter, 2019). The expression of TMPRSS2 and
TMPRSS4 in ACE2+ mature enterocytes in the human small
intestine facilitates virus entry into these cells. This property
makes the intestine a potential site for SARS-CoV-2 infection,
which may progress to a systemic disease (Zang et al., 2020).
It is suggested that “camostat mesylate”, a TMPRSS2 inhibitor
approved for clinical use, may block viral entry and could be
considered as a potential treatment option (Hoffmann et al.,
2020)(Figure 2). However, in the case of SARS-CoV, the virus
can use endosomal cysteine proteases including cathepsins B
and L for S protein priming in TMPRSS2 negative cells
(Simmons et al., 2005).
MECHANISM OF VIRAL CELL ENTRY
A key to preventing the spread of SARS CoV-2 and the resultant
pandemic is to gain a clear understanding of its mechanism of
cell entry. The virus surface S protein mediates this process by
binding to ACE2. Cleavage of the S glycoprotein between the S1
and S2 domains starts during viral packaging. This process is
completed by the type II transmembrane serine protease
TMPRSS2 (Kreye et al., 2020) which results in activation of the
S2 subdomain.
The S2 subdomain then mediates the fusion of the viral
genome with the host cell membrane to create a pore allowing
the viral RNA and RNA-associated proteins to gain access to the
cytoplasm. Another possibility is that ACE2/SARS-CoV-2
complex undergoes endocytosis. The rapid endocytosis of
SARS-CoV-2 occurs through clathrin-mediated endocytosis
(Millet and Whittaker, 2014). However, it is not completely
clear how the viral genome of SARS-CoV-2 gains access to the
cytoplasm (Figure 3).
However; after entering the cell, the viral RNA is released into
the cytoplasm and translates viral proteins followed by viral
genome replication (Kuba et al., 2010). The newly formed
envelope glycoproteins are inserted into the membrane of the
endoplasmic reticulum or Golgi, forming nucleocapsids by
assembling the genomic RNA and nucleocapsid protein. Viral
particles then incorporate into the endoplasmic reticulum-Golgi
intermediate compartment (ERGIC) and the vesicles containing
the virus particles fuse with the plasma membrane to release the
virus (Chockalingam et al., 2016)(Figure 3).
IMMUNOLOGICAL OUTCOME AND
PATHOLOGICAL EFFECT OF COVID-19
During viral infections, the innate immune system acts as a first
line defense to prevent viral invasion or replication. This innate
immune response utilizes pattern recognition receptors (PRRs)
to detect specific viral components such as viral RNA. Following
viral entry into the cell, single stranded RNA viruses are
recognized by PRRs such as Toll-like receptor TLR7 and TLR8,
RIG-I (retinoic acid-inducible gene I)-like receptors (RLRs), and
the NOD-like receptor, CARD-containing-2 (NLRC2), that are
expressed by airway epithelial cells and innate immune cells
including alveolar and tissue macrophages. RLRs are also able to
detect double-stranded (ds) RNA structures (Streicher and
Jouvenet, 2019). Sensing of ssRNA by PRRs results in the
FIGURE 2 | Blocking SARS coronavirus2 (SARS–CoV2) cell entry. SARS-CoV-2 binds to ACE2 and then uses the transmembrane Serine Protease 2 (TMPRSS2)
for S protein priming and entry to cell. A TMPRSS2 inhibitor can blocked viral entry by binding to and inhibition of enzyme activity.
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production of the Type-I and -III antiviral interferons (IFNs)
and chemokines. The activation of this IFN-mediated antiviral
response is the first major defense mechanism against viral
infections (Streicher and Jouvenet, 2019).
Although a rapid and well-coordinated immune response is
necessary for a potent defense against viral infection, an excessive
inflammatory response may lead to tissue damage at the systemic
level. The massive production of cytokines and chemokines
detected during COVID-19 infection, the so-called “cytokine
storm”, is mainly responsible for the broad and uncontrolled
tissue damage observed. The cytokine storm resembles the
cytokine release syndrome (CRS) and results in plasma leakage,
vascular permeability and disseminated vascular coagulation.
These excessive proinflammatory host responses are major
factors in the pathological outcomes such as acute lung injury
(ALI) and acute respiratory distress syndrome (ARDS) seen in
severe SARS-CoV-2 infected patients (Bhaskar et al., 2020).
In addition to the dominant manifestation of respiratory
symptoms, some patients have severe cardiovascular damage
and individuals with underlying cardiovascular disease (CVD)
have an increased risk of death (Zheng Y.Y. et al., 2020). The
mechanism underling the acute myocardial injury might be
related to the high expression of ACE2 in the CVS (de Abajo
et al., 2020).
On the other hand, the combination of cytokine storm
together with respiratory dysfunction and hypoxaemia may be
a mechanism by which COVID-19 results in damage to
myocardial cells (Zheng Y.Y. et al., 2020).
Patients also show lymphopenia and pneumonia with
characteristic pulmonary ground-glass opacity changes on
chest CT (Wu et al., 2020a;Zhang et al., 2020). Other forms of
severity including myocarditis, arrhythmia, cardiogenic shock as
well as acute kidney injury have been reported in 10%–30% of
ICU patients with confirmed COVID-19 (Murthy et al., 2020). In
particular, neurological manifestations (Mao et al., 2020a) and
pneumonia were reported in most hospitalized COVID-19
patients (Huang et al., 2020).
The Pathogenicity of COVID-19
Induced Pneumonia
Among the critically ill patients admitted to ICU, severe disease
with respiratory failure due to mucus plugs; severe pneumonia
and ARDS is reported in 60%–70% of patients whilst sepsis and
septic shock is seen in 30% of patients (Halacli et al., 2020).
SARS-CoV-2 viruses preferentially infect type II epithelial
cells within the lungs. In addition, some in vitro studies, ex-vivo,
and in silico studies suggest that mucociliary and goblet cells are
also primary target cells for infection (Hui et al., 2020;Sungnak
et al., 2020;Mason, 2020). Upon infection, type II epithelial cells
undergo programmed cell death as a part of the virus replicative
cycle (Anderson et al., 2010). Since these cells are the main player
of surfactant production, the reduced surfactant in the alveoli
FIGURE 3 | The mechanism of SARS-CoV-2 cell entry and replication. The coronavirus spike (S) protein binds to angiotensin converting enzyme 2 (ACE2)
receptors. Cleavage of the S glycoprotein between the S1 and S2 domains, which begins during viral packaging by furin in the Golgi compartments, is completed by
the protease trans-membrane serine Protease 2 (TMPRSS2) and lysosomal Cathepsin which enables cell membrane-viral fusion and viral RNA release. This process
may either create a pore allowing the viral RNA and RNA-associated proteins to gain access to the cytoplasm (1) or, alternatively, the ACE2/SARS-CoV-2 complex
may be internalized by endocytosis and be uncoated in the acidic lysosomal environment to enable release of the single stranded viral RNA (ssRNA) into the cytosol
(2). The viral genome is then replicated and translated into viral proteins by the host cell machinery. The newly formed envelope glycoproteins are processed within
the Golgi. Further assembling of the genomic RNA and nucleocapsid protein results in the formation of viral particles which are released via vesicular exocytosis.
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leads the alveoli to collapse which subsequently facilitate
pneumonia and ARDS in the severe patients (Ayala-Nunez
et al., 2019). In addition, viral infection of the airway epithelial
cells can cause high levels of virus-induced pyroptosis
with associated vascular leakage. Pyroptosis is a form of
programmed cell death which can be triggered by cytopathic
viruses (Chen et al., 2019).
In SARS patients, despite a decrease in nasopharyngeal viral
titers 10–15 days after the onset of symptoms, the pathological
outcomes and alveolar damage continue to worsen (Peiris et al.,
2003). This suggests that much of the pathological damage seen
is due to the host immune response to infection. It is likely that
similar events are occurring with SARS-CoV-2. However, the
viral nasopharyngeal load may be very different from that in the
lungs as autopsy samples showed a high concentration of virus in
different organs including the lung and intestine (Gu et al., 2005;
Farcas et al., 2005).
It was also demonstrated that a cytokine storm and its
subsequently inflammatory events trigger endothelial activation and
induce endotheliitis as well as progressive microvascular pulmonary
thrombosis in lung which lead to the disseminated intravascular
coagulation (DIC) and impaired lung microcirculation (Luks and
Swenson, 2020).
Overall, the main histological findings in the lungs represents
patchy necrosis; hyaline membrane formation and hyperplasia of
type II pneumocystis that represent diffuse alveolar damage and
injury to the gas-exchange surfaces (Anderson et al., 2010). The
pathological lung damages in the novel viral disease may be due
to either directly viral destruction of alveolar and bronchial
epithelial cells or a cytokine storm (Xu Z. et al., 2020).
However, respiratory distress in COVID-19 patients may be
due to the viral access to CNS and induced damage in the
respiratory centers of the brain, making it more complicated to
manage these patients (Baig et al., 2020).
The Neurological Pathogenicity of
SARS-CoV-2
36.4% of patients with COVID-19 develop neurological
symptoms (Mao et al., 2020a;Mao et al., 2020b)andthe
neuro-invasive properties of SARS CoV-2 is now accepted.
The retrospective studies as well as case reports from different
region in the world; indicate that Covid-19 affects CNS in several
ways and leads to a broad spectrum of neurological symptoms
from a simple headache to more serious encephalitis (Sheraton
et al., 2020).
Autopsy reports have revealed edema and partial neuronal
degeneration in brain tissue in severe patients (Channappanavar
and Perlman, 2017) and the presence of SARS-CoV-2 RNA in
the cerebrospinal fluid (CSF) was confirmed by genome
sequencing (Babaha and Rezaei, 2020). However, the
pathophysiological characteristics of SARS-CoV-2-associated
encephalitis are not fully understood.
The new virus may induces nerve damage through the several
mechanisms including direct infection or immune injury (Wu
et al., 2020b) which may result in edema and alterations in
consciousness (Ye et al., 2020).
Coronaviruses are able to reach to the CNS via the synapse
connected routes and retrograde transport (Perlman et al., 1990;
Ye et al., 2020). In the case of murine models of SARS or MERS-
CoVs infection, intranasal infection enables the virus to access
the brain via the olfactory nerves and rapidly infect specific brain
areas such as the brain stem and thalamus (Netland et al., 2008)
and cause to critical neuronal histopathological changes in these
areas. Since SARS-CoV-2 is mainly spread through the
respiratory system, retrograde transport through the olfactory
nerve may be the main route for transfer of virus to the central
nervous system (CNS) (Mori, 2015;Desforges et al., 2019). The
occurrence of loss of smell or hyposmia during the early phase of
COVID-19 infections should be taken into consideration for the
involvement of the CNS (Baig et al., 2020).
Another pathway proposed for the entry of SARS-CoV-2 into
the nervous system is via the blood circulation and disruption of
the blood-brain barrier (BBB). Noroviruses use different
mechanisms for disruption of BBB including direct infection of
BBB endothelial cells or enhancing BBB permeability by
alteration the expression of matrix metalloproteinases (MMPs)
and tight junction proteins (Al-Obaidi et al., 2018). Interestingly,
the integrity of the epithelial–endothelial barrier is critically
compromised in critical COVID-19 cases (Li H. et al., 2020).
A Trojan horse mechanism has also been proposed to be used
by SARS-CoV-2 to reach the CNS (Park, 2020). Here, ACE2-
expressing CD68+CD169+ macrophages act as the carrier cell
and may contribute to viral spread in COVID-19 patients and
induce an enhanced inflammatory response during SARS-CoV-2
infection (Park, 2020).
The highly specialized functions and limited regenerative
capacity of neurons means that chronic and latent CNS SARS-
CoV-2 infection may have long-term detrimental consequences
(Andries and Pensaert, 1980). SARS-CoV-2 could trigger CNS
degeneration and may be a potential trigger of future
neurodegenerative diseases such as Parkinson’sdiseaseor
multiple sclerosis (Serrano-Castro et al., 2020). Considering the
large number of patients involved globally, the risk of future
neurological disorders is worrying and the clinicians should pay
more attention to the neurologic symptoms in COVID-19
patients especially in the early phase of infection.
CELLULAR AND MOLECULAR
MECHANISMS OF DISEASE
Based on data from the hospitalized patients, the majority of
COVID 19 cases (about 80%) are asymptomatic or show mild
symptoms while the rest experience severe respiratory failure
(Surveillances, 2020). This may be explained by the fact that the
onset and development of COVID-19 depends upon virus
infectivity and the strength of the individual’s immune
response. The viral factors include virus type, titer, viability
and mutations, whilst the host immune factors including age,
gender and genetics (such as HLA genes) which together
determine the duration and severity of the disease (Rithanya
and Brundha, 2020).
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SARS-CoV-2 tends to have a long incubation period: 5–15
days on average. This long incubation period is due to its ability
to escape from host immune detection at the early stages of
infection (Prompetchara et al., 2020). At the initiation of
infection, inhaled SARS-CoV-2 binds to ACE2 on nasal
epithelial cells and starts replicating. Despite the low viral
burden and local propagation of virus, the virus can be
detected by nasal swabs by RT-PCR (Mason, 2020). There is a
low immune response at this step. The virus then replicates and
migrates through the conducting airways and a more robust
innate immune response is triggered. At this time, the disease
COVID-19 clinically manifests itself and early markers of the
innate immune response such as CXCL10 are detectable
(Mason, 2020).
CXCL10 is known as a disease marker in SARS (Xu Z. et al.,
2020) and its expression is significantly increased in alveolar type
II cells in response to both SARS-CoV (Qian et al., 2013) and to
influenza (Wang J. et al., 2011). Higher levels of interleukin IL-6
and IL-10, and lower levels of CD4+T and CD8+T are also
observed in patients with COVID-19 and these correlate with the
severity of disease (Wan S. et al., 2020).
In the remainder of the review, we discuss the potential
mechanisms by which SARS-CoV-2 modulates the host
immune system predominantly based on the strategies used by
SARS-CoV and MERS.
Innate Immune Responses
The innate immune response is the first line of defense against
viral infection and has a determinant role in protective or
destructive responses upon infection (Kindler et al., 2016).
Upon viral infection, type I interferon (IFN) responses and its
downstream cascade are initiated in order to control viral
replication and induce an effective adaptive immune response
(Prompetchara et al., 2020).
In the case of SARS-COV and MERS; the virus modulate anti-
viral IFN responses by using different strategies that interfere
with IFN production and its downstream signaling pathways
(Kindler et al., 2016). This dampening procedure is closely
associated with disease severity. In two MERS-CoV patients
with different severities, the type I IFN response was
significantly lower in the patient with a poor outcome (death)
than in the patient who recovered (Shokri et al., 2019). IFN
production requires the phosphorylation and activation of IFN-
regulatory factor3 (IRF3) which is inhibited following infection
with SARS-CoV (Kindler et al., 2016)(Figure 4).
Nuclear translocation of IRF3 and its subsequent stimulation
of IFN bexpression is inhibited by viral ORF4a, ORF4b, and
ORF5 proteins (Yang et al., 2013;Yang Y. et al., 2015). In
addition, the MERS-CoV accessory protein 4a blocks IFN
induction through direct interaction with double-stranded
RNA (Niemeyer et al., 2013). Based on the genomic similarity
between SARS-CoV-2, SARS-CoV and MERS-CoV it is likely
that SARS-CoV-2 utilizes similar strategies to modulate the type
I IFN response (Dandekar and Perlman, 2005).
However, sequence analysis showed several changes to
SARS-CoV-2 that indicate that SARS-CoV-2 is more sensitive
to type I IFN (Lokugamage et al., 2020). ORF3b from SARS-CoV
encodes a 154 amino acid (aa) protein that suppress the type I
IFN responses by inhibition of IRF3 phosphorylation. In SARS-
CoV-2 a premature stop codon in ORF3b results in a truncated
20aa protein which lacks an equivalent function to that seen with
SARS-CoV (Lokugamage et al., 2020). SARS-CoV-2 has similar
viral replication kinetics to SARS-CoV in Vero cells (Lokugamage
et al., 2020) but is much more sensitive to type I IFN pre-treatment
and a significant reduction in viral replication is seen following
type I IFN treatment (Lokugamage et al., 2020b). Together these
observations suggest that type I IFN may be a potential treatment
for COVID-19 as seen in animal models of SARS-CoV and
MERS-CoV infection (Channappanavar et al., 2019). In
contrast, a delayed induction of type I IFN compromises the
early viral control and drives the influx of hyper-inflammatory
neutrophils and monocytes-macrophages which lead to a mass
production of pro-inflammatory cytokines and may evoke a
cytokine storm (Channappanavar and Perlman, 2017).
A cytokine storm leads to an uncontrolled systemic
inflammatory response that may trigger a severe attack on the
body by its own immune system. As with SARS-CoV and MERS-
CoV infection, this may lead to ARDS, multiple organ failure and
death in severe cases of SARS-CoV-2 infection (Channappanavar
and Perlman, 2017;Tabarsi, 2020). In severe COVID-19
patients, serum levels of proinflammatory cytokines including
IL-2, IL6, IL-7, IL-10, G-CSF, IP-10, MCP-1, MIP-1aand TNFa
are highly elevated (Xu Z. et al., 2020). Furthermore, increased
total blood neutrophils and decreased total blood lymphocytes in
patients within ICU compared with patients not in ICU care
correlated with disease severity and death (Prompetchara
et al., 2020).
The previous studies and clinical evidence also strongly
indicate the importance of the NLR family pyrin domain
containing 3 (NLPR3) inflammasome in the pathologic effects
of severe COVID-19 (Shah, 2020). In patients with reduced
immune fitness whose innate immune system is unable to clear
the virus, NLRP3 activation may occur (Freeman and Swartz,
2020). A sustained and unregulated NLRP3-dependent
inflammatory response leads to the severe clinical symptoms
including necrosis, fever, release of damage-associated molecular
patterns (DAMP) and severe inflammation (van den Berg and Te
Velde, 2020). Direct activation of NLRP3 induces pyroptosis and
cell death in human cells. Additionally, hyper-activation of
NLRP3 may result in coagulopathy, neutrophil infiltration,
Th17 and macrophage activation and a cytokine storm in
severe COVID-19 patients (Merad and Martin, 2020). Genetic
variations in host inflammasome pathways may also influence
disease outcome and may be responsible for the heterogeneous
response of patients with COVID-19 and the array of clinical
severity (Freeman and Swartz, 2020).
Adaptive Immune Responses
Corona viruses including SARS and MERS have ability to infect
immune cells which plays a key role in the disease pathogenesis
(Dandekar and Perlman, 2005). In addition, MERS-CoV induces
rapid apoptosis of macrophages by the limiting of the early
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induction of IFN (Shokri et al., 2019). In the case of SARS–CoV,
infection of lymphocytes has been proposed to play the major
role in viral-induced pathogenicity. SARS-CoV-infected
lymphocytes, similar to feline infectious peritonitis virus
(FIPV)-infected macrophages in domestic cats, might transport
the virus to distant organs resulting in systemic infection (de
Groot-Mijnes et al., 2005).
Upon virus entry, immunogenic peptides are presented to T
cells in association with human leukocyte antigen (HLA) on the
surface of antigen presenting cells (APC). Polymorphisms in the
HLA system may influence the susceptibility and outcome of
SARS-CoV infection (Yuan et al., 2014). Some HLA alleles
including HLA-B*4601, HLA-B*0703, HLA-DRB1*1202 HLA-
DRB4*01010101, and HLA-Cw*0801 are associated with
susceptibility to SARS-CoV infection (Wang S.-F. et al., 2011).
Whereas, the HLA-DR0301, HLA-Cw1502, and HLA-A*0201
alleles are protective against severe SARS infection (Hajeer et al.,
2016). Furthermore, polymorphisms within the MBL (mannose-
binding lectin) gene are also associated with susceptibility to
SARS-CoV infection (Tu et al., 2015).
In the case of MERS-CoV infection, HLA-DRB1*11:01 and
HLA-DQB1*02:0 alleles increased the risk of infection (Wang
S-F. et al., 2011). These alleles should be assessed in COVID-19
patients (Li X. et al., 2020). In addition, antigen presentation via
the class I and II major histocompatibility complex (MHC) was
down-regulated in MERS-CoV infection. This would markedly
diminish T cell activation in response to the virus (Shokri et al.,
2019). During viral infection, antigen presentation triggers
both humeral and cellular immunity mediated by virus-specific
B and T cells.
B Cell Responses
B cell profiling by RNA-seq showed a specific pattern of new B
cell-receptor changes (IGHV3–23 and IGHV3–7) in COVID-19
patients. It is also indicated that the number of naïve B cells is
decreased while the number of peripheral blood plasma cells was
remarkably increased (Wen et al., 2020).
With SARS infection, the profile of neutralizing antibodies are
predominantly IgM and IgG. Specific B and T cells epitopes are
commonly mapped against the structural S and N proteins
(Berry et al., 2010;Liu et al., 2017). SARS-CoV infection
induces seroconversion 4 days after the onset of disease and
SARS-specific IgM antibodies are found in most patients up to
week 12 post-infection while the IgG antibody can persist for
much longer. Indeed, long lasting specific IgG was detected up to
2 years post infection (Li et al., 2003).
In COVID-19, SARS-CoV-2 specific antibodies may be
produced to neutralize the virus. In one patient, serology
reports showed the peak of specific IgM at day 9 after the
onset of the disease and switching to IgG by week 2 (Zhou P.
FIGURE 4 | The innate immune modulation mechanisms by SARS-CoV and Middle East respiratory syndrome-related coronavirus (MERS-CoV). Double stranded
(ds)RNA, a by-product of RNA virus replication in the cytoplasm, is sensed by the pattern recognition receptors such as retinoic-acid inducible gene I (RIG-I) and
melanoma differentiation-associated protein 5 (MDA5) which subsequently leads to activation of the kinase TANK Binding Kinase 1 (TBK1). These kinases then
phosphorylate interferon (IFN) regulatory factor 3 (IRF3) and traffic to the nucleus to active the transcription of IFNs. Viral proteins actively modulate this pathway.
Open reading frame (ORF)3b, nucleocapsid (N), and non-structural protein (NSP)1 affect the signal transduction pathway that activates IRF3. In addition, the papain-
like protease (PLP) blocks the phosphorylation of IRF3 and its activation. Viral protein 4a suppress the activation of RIG-I/MDA5 signaling and blocks the induction of
IFNs through interaction with dsRNA.
Alipoor et al. COVID-19 and Response
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et al., 2020). Interestingly, sera from five patients with confirmed
COVID-19 show some cross-reactivity with SARS-CoV.
Furthermore, all sera from patients were able to neutralize
SARS-CoV-2 in an in vitro plaque assay, suggesting a possible
successful mounting of the humoral responses (Zhou P
et al., 2020).
In a recent study of 285 patients with mild to severe disease,
96.8% of tested patients achieved seroconversion of IgG or IgM
within 20 days after the onset of symptoms with the titer
plateauing within 6 days after seroconversion. Moreover, 100%
of patients had positive virus-specific IgG approximately 17–19
days after symptom onset. In addition, 94.1% patients were
positive for virus-specific IgM approximately 20–22 days after
symptom onset (Liu et al., 2020;Long et al., 2020). Antibody
titers in the severe group were higher than those with milder
disease. Interestingly, a number of individuals with negative
nucleic acid results and no symptoms had positive IgG and/or
IgM tests (Long et al., 2020). This highlights the importance of
serological testing as a complement to the RT-PCR test in
surveying for asymptomatic patients in close contact with
other (Liu et al., 2020;Long et al., 2020).
T Cell Responses
During viral infection, T helper (Th) cells play an important role
in the adaptive immunity. The cytokine microenvironment
generated by antigen presenting cells directs T cell responses.
In SARS-CoV infection a strong Th1 cell response and higher
levels of neutralizing antibodies were observed in the mild-
moderate group, while in the fatal group a higher level of
Th2 cytokines (IL-4, IL-5, IL-10) was detected (Li et al., 2008).
Current evidence indicates that a Th1 response is crucial for the
successful control of SARS-CoV and MERS-CoV and this may
also be essential for SARS-CoV-2 (Li et al., 2008).
The balance between naïve and memory T cells is crucial to
controlling infection. Naïve T cells are responsible for the defense
against new, previously unrecognized infection by the
production of cytokines. In contrast, memory T cells promote
antigen-specific immune responses. Disruption of the balance in
favor of naïve T cells could strongly promote hyper
inflammation. On the other hand, the reduction in memory T
cells could contribute to COVID-19 relapse, which is reported in
a number of recovered cases of COVID-19 (Zhou L. et al., 2020;
Yuan et al., 2020;Chen D. et al., 2020).
In severe patients with COVID 19, CD8+ T cell responses
were more frequent and robust than CD4+ T cell responses and
an early rise in CD8+ T cell numbers was correlated with disease
severity (Prompetchara et al., 2020). CD8+ T cells contained a
large number of cytotoxic granules which may have induced
severe immune injury in the patients (Xu Z. et al., 2020).
Additionally, a recent report of a 50 year old male with covid-
19whodiedshowedasignificantly reduced number of
peripheral blood CD4+ and CD8+ T cells. These cells had a
hyper-activated appearance and were double positive for HLA-
DR and CD38 (Xu Z. et al., 2020).
COVID-19 also induces T-cell exhaustion (Diao et al., 2020).
T cell exhaustion is a state of T cell dysfunction that arises during
many chronic infections as well as during persistent viral
infections (Wherry, 2011). Peripheral blood T cells from
COVID-19 patients expressed high levels of the exhaustion
markers PD-1 and Tim-3 (Diao et al., 2020).
Peripheral blood total counts of T cells, CD4+, and CD8+ T
cells were significantly lower in ICU patients than non-ICU cases
with COVID-19 and counts were negatively correlated with
patient survival (Diao et al., 2020). It seems that the cytokine
storm may promote apoptosis or necrosis of T cells and thereby
reduce their numbers (Xi-zhi and Thomas, 2017). Furthermore,
the levels of TNF-a, IL-6, and IL-10 were significantly increased
in infected patients and raisedevenmoreinpatientswho
required ICU treatment. Interestingly, the concentrations of
these cytokines were negatively correlated with total T cell
counts (Diao et al., 2020).
Besides suppression of T cell proliferation, IL-10 can induce T
cell exhaustion by increased levels of PD-1 and Tim-3 (Brooks
et al., 2006). Inhibition of IL-10 also reduced the degree of T cell
exhaustion in animal models of chronic infection (Ejrnaes et al.,
2006). TNF-ais a pro-inflammatory cytokine that promotes T
cell apoptosis via the TNF receptor TNFR-1 signaling pathway.
The expression of TNFR-1 is increased in aged T cells (Aggarwal
et al., 1999). IL-6 contributes to host defense by stimulating acute
phase responses or immune reactions in response to infections
and tissue injury. Deregulated and continual synthesis of IL-6
plays a pathological role in chronic inflammation and infection
(Jones and Jenkins, 2018). The level of IL-6 in severe patients
continue to increase over time and is higher in non-survivors
(Zhou F et al., 2020).
It is suggested that the different mortality rates in COVID-19
patients is due to the difference in the response to infection
(Bienvenu et al., 2020). Analysis of viral RNA in COVID-19
patients indicated that the males show delayed viral clearance
(Xu K. et al., 2020;Zheng S. et al., 2020). Furthermore, it is
reported that male patients had higher plasma levels of innate
immune cytokines including IL-8 and IL-18 along with activated
non-classical monocytes. In contrast, female patients had a more
robust T cell activation during SARS-CoV-2 infection. A poor T
cell response may be responsible for the worse outcome in male
patients, while in female patients, higher levels of innate immune
cytokines were associated with worse disease (Takahashi
et al., 2020).
COVID-19 AND POST
INFECTION IMMUNITY
Another question regarding this new disease is that whether the
infection induces persistent immune memory that could protect
the recovered individual against reinfection. The durability of
protective antibodies induced by SARS-CoV-2 or the antibody
titers that will protect against reinfection is still unclear. It is
reported that antibodies disappear rapidly after recovery
particularly in patients with mild disease. For example, IgG has
a half-life of approximately 21 days in SARS-CoV-2 patients.
However; even in the absence of specific serum antibodies, the
presence of memory B and T cells may be maintained (Cox and
Brokstad, 2020).
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In a recent study, COVID-19 induced memory lymphocytes
with an antiviral protective immune function. This longitudinal
study assessed the immune response in patients who recovered
from mild symptomatic COVID-19. These recovered individuals
developed SARS-CoV-2-specific IgG antibody and neutralizing
plasma in addition to virus-specific memory B and T cells that
had ability to expand over three months following the onset of
symptoms (Rodda et al., 2020). Furthermore, following antigen
re-exposure the memory T cells secreted IFN-gand were able to
clonally expand whilst memory B cells expressed antibodies
capable of neutralizing the virus (Rodda et al., 2020).
Studies of patients infected with SARS-CoV in 2003 suggested
that the infection induced durable T cell responses lasting for up
to 6 years but no prolonged memory B cells. Importantly, these T
cells could cross-react with the SARS-CoV-2 virus after 17 years,
but it is not clear that whether they can provide protection
against COVID-19 (Le Bert et al., 2020). On the other hand,
recent reports demonstrated that T cell reactivity against SARS-
CoV-2 exists in many unexposed people. It is hypothesized that
this might be due to immunity to common cold coronaviruses
that could influence COVID-19 disease severity (Sette and
Crotty, 2020).
POTENTIAL PHARMACOLOGIC
STRATEGIES IN THE TREATMENT
AND PROTECTION
At present, there are no specific antiviral drugs or a vaccine
available to treat COVID-19. Currently, broad-spectrum
antiviral drugs like nucleoside analogues and HIV-protease
inhibitors are being used to attenuate viral infection (Lu,
2020). Other treatment regimens include a combination of
oral oseltamivir, lopinavir and ritonavir and intravenous
administration of ganciclovir for 3–14 days (Chen N. et al.,
2020). There is an ongoing clinical trial for examination of the
safety and efficacy of lopinavir-ritonavir and IFN-2b in patients
with COVID-19 (Lu, 2020;Habibzadeh and Stoneman, 2020).
Although the antivirals remdesivir and chloroquine controlled
SARS-CoV-2 infection in vitro, these have shown variable results
in the clinic (Wang et al., 2020). EIDD-2801 is a new drug used
to control and treat seasonal and pandemic influenza virus
infections. It has high therapeutic efficacy in humans and may
be considered as a potential drug for the treatment of COVID-19
infection (Toots et al., 2019).
Therapies based on the ACE2 receptor have raised concerns
about the use of Renin-Angiotensin-Aldosterone System (RAAS)
inhibitors that may alter ACE2 function and expression.
Angiotensin II receptor blockers (ARBs) and other RAAS
inhibitors increased ACE2 expression in the lung in animal
models (Gurwitz, 2020). Data from cohort and cross sectional
studies in patients with heart failure or cardiovascular disease
showed that the effect of these inhibitors on ACE2 is not uniform
even in response to a given drug class (Shearer et al., 2013). In
addition, recent evidence suggests that ACE inhibitors do not
affect COVID-19 infection or severity (Bryan Williams, 2020).
Angiotensin receptor 1 (AT1R) blockers, such as losartan,
have also been suggested as a therapeutic approach for
reducing the severity and mortality of SARS-CoV-2 infections
(Gurwitz, 2020).
Interestingly, human recombinant soluble ACE2 (hrsACE2)
blocked SARS-CoV-2 infections in engineered human tissues,
which suggests promise as a treatment capable of stopping early
infection of the novel coronavirus. In this regard, APN01
developed by APEIRON is currently in clinical trials (Zoufaly
et al., 2020). SARS-CoV-2 uses the serine protease TMPRSS2 for
S protein priming. It is suggested that previously approved
TMPRSS2 inhibitors such as nafamostat mesylate and
camostat mesylate (Hoffmann et al., 2020)willblockviral
entry and may be a possible treatment option (Tay et al., 2020).
The central role of cytokine dysregulation in the pathogenicity
of seriously ill COVID-19 patients has indicated the potential of
cytokine-targeted therapy in managing disease progression
(Rahmati M. 2020). Drugs such as IL-6 inhibitors (tocilizumab,
sarilumab, and siltuximab) or IL1-binhibitors are possible
interventions in this area.
Tocilizumab (Actemra) is a humanized anti-IL-6 receptor
antibody that was successful in the treatment of rheumatoid
arthritis (RA) and juvenile idiopathic arthritis (Burmester et al.,
2016;Yokota et al., 2016). Whether tocilizumab restores T cell
counts in COVID-19 patients by suppressing IL-6 signaling
needs to be investigated (Diao et al., 2020;Atal and Fatima,
2020). Interleukin-1 targeting with a high-dose of anakinra in
patients with COVID-19 was safe and showed clinical
improvement in 72% of patients in a retrospective cohort
study (Cavalli et al., 2020). Anakinra is a recombinant
interleukin-1 (IL-1) receptor antagonist that has anti-
inflammatory and immunomodulatory effect used to treat of
inflammatory arthritides (Furst, 2004).
The NLRP3 inflammasome with its downstream pathways is
also an attractive target for therapy of COVID-19 (Pontali et al.,
2020;Jamilloux et al., 2020;Soy et al., 2020). The first clinical trial
study of using tranilast (an NLRP inflammasome inhibitor)
to treat COVID-19 is ongoing and registered in the Chinese
clinical trial registry (http://www.chictr.org.cn/showprojen.aspx?
proj=49738).
SARS-CoV-2 is more sensitive to type I IFN in vitro and in
vivo than SARS suggesting that type I IFNs may be a potential
treatment for protection against COVID19. Interestingly,
Bacillus Calmette–Guerin (BCG) vaccination may reduce the
mortality associated with COVID-19. BCG vaccine stimulates
the production of IFNs by the innate immune system in a TLR2-
dependent manner (Ades, 2014;Kativhu and Libraty, 2016) and
may provide protection against COVID19. IFN-bmay prove
efficacious as a vaccine adjuvant to boost immune cell function
and INF production (Ades, 2014).
Remdesivir an antiviral compound that was firstly introduced
for Ebola virus (Wang et al., 2020) and was successful in
shortening the time to recovery in adults hospitalized with
COVID-19 (Lu, 2020;Beigel et al., 2020;Harapan et al., 2020).
There are ongoing clinical trials in a number of countries for
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remdesivir as a potential treatment for COVID-19. Remdesivir,
made by Gilead, is currently going through the FDA approval
process and is authorized in the United States for use under an
Emergency Use Authorization (EUA). Currently, the drug is
given intravenously through daily infusions in the hospital.
However, an inhaled formulation given through a nebulizer,
may potentially allow for easier administration outside the
hospital at earlier stages of the disease.
These approaches would be valuable to investigate for
COVID-19 and open a window for finding new ways to
protect against and treat this deadly epidemic. There are
currently 109 ongoing clinical trials for COVID-19 registered
on clinicaltrials.gov. These include numerous pharmacological
and biological approaches and the use of natural products.
CONCLUSION
The COVID-19 pandemic is rapidly spreading across the world
and has caused more deaths compared with SARS or MERS.
Despite the high rate of mortality and morbidity, no medication
or vaccine has been consistently shown to be effective. SARS-
CoV-2 belongs to the Coronaviridae family which was also
responsible for previous widespread outbreaks of SARS‐CoV in
2002 and MERS‐CoV in 2008. The rapid spread, induction of
severe infection, cross-species transmission and unpredicted
behavior of coronaviruses result in them being a continuous
threat to human health. This is important due to the existence
of many animal reservoirs for CoVs and the lack of an
approved treatment.
There is an imperative need to design and develop effective
therapeutic and preventive strategies. This will require using the
accumulated knowledge of how the host immune response system
responds to SARS and MERS but importantly will require analysis
of immune responses within the lungs of COVID-19 patients.
Techniques such as single cell sequencing and cellular indexing of
transcriptomes and epitopes by sequencing (CITE-seq) will
tremendously increase our understanding of disease pathology
and enable knowledge-based decisions for the adoption of new
therapeutic immune modalities.
AUTHOR CONTRIBUTIONS
SA wrote first draft. EM, HJ, PT, IMA, MV, and HB have revised
equally the first draft. All authors contributed to the article and
approved the submitted version.
FUNDING
IMA is financially supported by the British Heart Foundation
(PG/14/27/30679), the Dunhill Medical Trust (R368/0714), the
Welcome Trust (093080/Z/10/Z), the EPSRC (EP/T003189/1),
the Community Jameel Imperial College COVID-19 Excellence
Fund (G26290) and by the UK MRC (MR/T010371/1).
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Conflict of Interest: The authors declare that the research was conducted in the
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