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Immunological Investigations
A Journal of Molecular and Cellular Immunology
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/iimm20
COVID-19 Infection: Concise Review Based on the
Immunological Perspective
Parisa Lotfinejad , Zahra Asadzadeh , Shiva Najjary , Mohammad Hossein
Somi , Khalil Hajiasgharzadeh , Ahad Mokhtarzadeh , Afshin Derakhshani ,
Elmira Roshani & Behzad Baradaran
To cite this article: Parisa Lotfinejad , Zahra Asadzadeh , Shiva Najjary , Mohammad Hossein
Somi , Khalil Hajiasgharzadeh , Ahad Mokhtarzadeh , Afshin Derakhshani , Elmira Roshani &
Behzad Baradaran (2020): COVID-19 Infection: Concise Review Based on the Immunological
Perspective, Immunological Investigations, DOI: 10.1080/08820139.2020.1825480
To link to this article: https://doi.org/10.1080/08820139.2020.1825480
Published online: 28 Sep 2020.
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COVID-19 Infection: Concise Review Based on the
Immunological Perspective
Parisa Lotnejad
a,b,c
, Zahra Asadzadeh
a
, Shiva Najjary
a
,
Mohammad Hossein Somi
d
, Khalil Hajiasgharzadeh
a
, Ahad Mokhtarzadeh
a
,
Afshin Derakhshani
a
, Elmira Roshani
a,e
, and Behzad Baradaran
a,b
a
Immunology Research Center, Tabriz University of Medical Sciences, Tabriz, Iran.;
b
Department of Immunology,
Tabriz University of Medical Sciences, Tabriz, Iran;
c
Student Research Committee, Tabriz University of Medical
Sciences, Tabriz, Iran;
d
Liver and Gastrointestinal Diseases Research Center, Tabriz University of Medical
Sciences, Tabriz, Iran;
e
Department of Biochemistry, School of Medicine, Urmia University of Medical Sciences,
Urmia, Iran
ABSTRACT
The outbreak of coronavirus disease 2019 (COVID-19) has posed
a serious threat to public health. There is an urgent need for discovery
methods for the prevention and treatment of COVID-19 infection.
Understanding immunogenicity together with immune responses are
expected to provide further information about this virus. We hope that
this narrative review article may create new insights for researchers to
take great strides toward designing vaccines and novel therapies in the
near future. The functional properties of the immune system in COVID-
19 infection is not exactly claried yet. This is compounded by the many
gaps in our understanding of the SARS-CoV-2 immunogenicity proper-
ties. Possible immune responses according to current literature are
discussed as the rst line of defense and acquired immunity. Here, we
focus on proposed modern preventive immunotherapy methods in
COVID-19 infection.
KEYWORDS
Coronavirus; 2019 novel
coronavirus; COVID-19; host
immune response; vaccine
development
Introduction
At the end of December 2019, an outbreak of the novel coronavirus disease named severe
acute respiratory syndrome coronavirus 2 (SARS-CoV2) that is also known as the 2019
novel coronavirus (2019-nCoV) and coronavirus disease 2019 (COVID-19) was reported in
Wuhan, Hubei Province, China. According to the first reports, the disease began in the
Huanan seafood market through animal-to-human transmission and it rapidly spread and
affected many individuals around the world, which led the World Health Organization
(WHO) to announce a global emergency in late January 2020. Then on March 11th, 2020,
WHO declared a global pandemic (Organization, W.H. 2020; Park et al. 2020). Based on
research done, R
0
is between 2 and 3.5, meaning each positive case can infect two to three
persons. At the WHO’s first meeting on the prevalence of this disease the mortality rate of
COVID-19 patients was estimated at 2 to 3% (Chen et al. 2020). Recent studies have shown
that people over the age of 60 are at greater risk than children, who are said to be less likely
to become infected by the virus (Q. Li et al. 2020b). Since the outbreak of the COVID-19
CONTACT Behzad Baradaran baradaranb@tbzmed.ac.ir Immunology Research Center, Tabriz University of Medical
Sciences, Tabriz, Iran
IMMUNOLOGICAL INVESTIGATIONS
https://doi.org/10.1080/08820139.2020.1825480
© 2020 Taylor & Francis Group, LLC
worldwide, there have been 607,166 cases reported in more than 180 countries worldwide,
of which 27,674 have died from the disease. Coronaviruses belong to a large family of
respiratory viruses called Coronaviridae that in humans generally cause mild respiratory
infections, such as those found in the common cold. However, in 2003 and 2012, two
human coronavirus infections comprising severe acute respiratory syndrome (SARS) in
China and the Middle East respiratory syndrome (MERS) in Saudi Arabia, caused fatal
endemics. These diseases belong to the genus Betacoronaviruses and are zoonotic that cause
a fatal infection of the lower respiratory tract as well as extra-pulmonary manifestations
(Drosten et al. 2003; Zaki et al. 2012). The genome sequence of COVID-19 is 75% similar to
that of the SARS-CoV and also 88% resembles the two bat coronaviruses, batSLCoVZC45
and bat-SL-CoVZXC21. This virus has a genome size of ~30 kb that encodes structural
proteins including nucleocapsid protein (N), an envelope protein (E), spike protein (S),
a membrane protein (M), as well as non-structural protein. COVID-19 infection can cause
severe respiratory syndrome with a clinical presentation very similar to SARS-CoV leading
to ICU admission and possibly patient death (Lu et al. 2020). Remodeled phylogenetic tree
from coronavirus strains of different animals displayed that pangolin CoV, bat RaTG13,
and SARS-CoV-2 genomes formed a monophyletic cluster, indicating that pangolins may
be considered as SARS-CoV-2 intermediate hosts. Although the RaTG13 bat virus genome
sequence has more similarity to SARS-CoV-2, some pangolin CoVs display strong similar-
ity to SARS-CoV-2 in the receptor-binding domain (RBD), including all six key RBD
residues that we can hypothesize that pangolins probably are intermediate hosts of the
virus that jumped to humans (Andersen et al. 2020; Bezerra et al. 2020). However,
phylogenetic analyses and a special amino acid sequence in the S gene of SARS-CoV-2
did not support the hypothesis of SARS-CoV-2 arising directly from the pangolin-CoV
-2020 (Sen et al. 2020; Tiwari et al. 2020). Clinical manifestations of COVID-19 infection
include fever, dry cough, fatigue, myalgia, shortness of breath, and in some cases a decrease
in the number of leukocytes. In a study by Huang and colleagues on 41 confirmed cases,
they reported that about 63% of these patients had lymphopenia and that the cytokine storm
could be related to the severity of the illness. Also, some studies showed that severe COVID-
19 disease includes pathological intravascular clotting leading to end-organ ischemia
including strokes (Oxley et al. 2020), and severe heart disease (myocarditis) that can be
fatal (Guo et al. 2020). Early studies demonstrate that the mechanism of human cell entry of
this virus is very similar to SARS-CoV (Huang et al. 2020). In a study on the modeling of
spike protein receptors for SARS-CoV-2, Xu and colleagues stated that angiotensin-
converting enzyme 2 (ACE2) probably acts as a receptor for this novel virus (X. Xu et al.
2020a; 2020b). Previous studies have also demonstrated that ACE2 is also a receptor for
HCoV-NL63 and SARS-nCoV (Wu et al. 2009). Another study by Zhou and colleagues on
virus infection showed that 2019-nCoV requires the ACE2 receptor to enter HeLa cells
(Zhou et al. 2020). In our search strategy, all of the articles were retrieved from PubMed,
Google Scholar, Scopus, and Web of Science until June 5, 2020, and the following search
terms were used: (coronavirus) and (2019 novel coronavirus OR COVID-19 OR SARS-CoV
-2) and (immune response) and (Innate response) and (acquired immunity). Most of the
articles we have used including original articles after the corona outbreak, the details of
these articles have been carefully studied and reviewed. All the gathered data have been
meticulously selected and studied from original articles published after the coronavirus
outbreak. At present, the diagnosis of the COVID-19 infection is based on epidemiological
2P. LOTFINEJAD ET AL.
history, clinical manifestations, chest radiography, and immune detection technology, but
confirmation still relies on nucleic acid examination strategies (Carter et al. 2020).
Currently, the number of affected people is on the rise, and there is an ongoing worldwide
effort by scientists to develop effective interventions to control and prevent it, as well as to
find an effective treatment or vaccine (Pormohammad et al. 2020). This is a narrative review
article on modern immunological approaches and current therapeutic or preventive immu-
notherapy methods in COVID-19.
Proposed diagnostic methods for COVID-19
The current outbreak of infections by SARS-CoV-2 has increased wide public health
concerns. Therefore, rapid, easy, and precise diagnosis of suspected COVID-19 is central
in the treatment of patients. Clinical diagnosis of COVID-19 is commonly based on
epidemiological history, clinical manifestations, and several auxiliary examinations, such
as nucleic acid detection, CT scan, and immune detection technology (Millán-Oñate et al.
2020). The first action for the clinical diagnostic is to recognize exposure history or close
touch with certified patients in the past two weeks. While the number of patients with
uncertain exposure history due to the fast and widespread of the infection is developing (Q.
Li et al. 2020b). In the primary phase of the disease, the total leukocytes number reduced or
is normal, with lessened lymphocyte numbers or high or normal monocytes. It is necessary
to pay attention to the status that the definite value of lymphocyte is lower than 0.8 × 10
9
/L,
or the count of CD4 and CD8 T cells are remarkably reduced, which usually suggests
repeating the blood routine changes 3 days later (Jin et al. 2020). The symptoms are
different in patients and primary hematology examination may not present the obvious
diagnosis of COVID-19 infection. So, auxiliary examinations are vital for the diagnosis of
COVID-19 (Dai et al. 2020). Nucleic acid examination strategies such as reverse transcrip-
tion-polymerase chain reaction (qRT-PCR) are regarded as an efficient method for con-
firming the diagnosis in clinical cases of COVID-19 (Udugama et al. 2020). While the
output of nucleic acid examination depends on multiple rate-limiting factors, including
accessibility and amount of the examination kits in the affected region. More remarkably,
the quality, constancy, and reproducibility of the examination kits are discussible. The effect
of methodology, disease stages, specimen collection strategies, nucleic acid extraction
strategies, and the amplification system are all important agents for the precision of test
results (Chu et al. 2020; Zhou et al. 2020). These reasons suggest that Computed
Tomography (CT) imaging is also considered as an accurate method for COVID-19
infection in the current status (Dai et al. 2020). CT and radiography have appeared as
a fundamental strategy in COVID-19 detection and diagnosis. A large number of COVID-
19 cases have a similar characteristic on CT images comprising ground-glass opacities in the
early phase and pulmonary consolidation in the late phase (Ai et al., 2020). It is found that
chest CT had a less rate of mistake in the diagnosis of COVID- 19 and may be helpful as
a standard strategy for the fast diagnosis of COVID- 19 to optimize the patient’s control.
However, due to the low specificity of CT scan for detecting the cause of COVID-19-related
pneumonia, this method shows incomplete clinical performance for proper COVID-19
disease diagnosis. Moreover, it is found that a combination of RT-PCR and CT has
a superior sensitivity than RT-PCR alone or CT alone or a combination of two RT-PCR
tests. Hence, the combination of RT-PCR and CT with superior sensitivity is an appropriate
IMMUNOLOGICAL INVESTIGATIONS 3
model to detect COVID-19 infection (Lu et al. 2020). Another most largely used strategy is
the serological test for the detection of antibodies against viral proteins.
Serological diagnosis is particularly central for patients with mild to moderate illness who
may present late, beyond the first 2 weeks of illness beginning. Serological diagnosis also is
becoming a vital tool to understand the extent of COVID-19 in the community and to
recognize individuals who are immune and potentially “protected” from becoming infected
(Sethuraman et al. 2020). As a conclusion, although so far, the most commonly used and
reliable test for diagnosis of COVID-19 has been the RT-PCR test performed using
nasopharyngeal swabs or other upper respiratory tract specimens, but it is suggested to
use a combination of multiple diagnosis strategies to confirm the final results.
Immunogenicity and immunopathology of SARS-CoV-2
One of the main anti-viral immunity is presenting viral antigens by antigen presentation
cells (APCs). APCs recognize the viral antigens and present them to the host CD8+ cytotoxic
T lymphocytes (CTLs). This way is general in many pathogen eliminations but little is
known about immunogenicity and immunopathogenesis of SARS-CoV-2. Based on pre-
vious studies that have been done on SARS-CoV and MERS-CoV along with our limited
information, we can only propose some of the possible mechanisms involved. Liu et al. have
revealed that MHC class I and partially MHC class II are involved in the presentation of
viral peptides of SARS-CoV. They revealed that N1 peptide (related to the protein of
coronavirus nucleocapsid) was detected as the SARS-CoV immunodominant epitope with
binding features to HLA-A*2402 allele of MHC class I (Liu et al. 2010). But what’s
interesting is that some MHC molecules may have the potential role in disease susceptibility
for example in one study on the Vietnamese population, the results indicated that HLA-
DRB1*1202 as predominant allele displayed the direct correlation with SARS-CoV suscept-
ibility (Keicho et al. 2009). This is while some other molecules have a protective role against
SARS-CoV as HLA-Cw1502, HLA-A*0201, and HLA-DR0301 alleles (Wang et al. 2011).
Parallel to these data, it has been found that susceptibility to MERS-CoV is related to the
expression of HLA-DQB1*02:0 and HLA-DRB1*11:01 in individuals (Hajeer et al. 2016).
However, the pathogenesis of SARS-CoV-2 has not been determined yet. It is observed that
to entry to the host cells MERS-CoV uses particular receptor as Dipeptidyl peptidase-4
(DPP-4) which also is known as CD26. The interaction of the MERS-CoV spike protein
binding site (receptor binding domain) with the DPP-4 domain on the host cells initiates
virus internalization (Raj et al. 2013). But it seems that SARS-CoV and SARS-CoV-2 use
another mechanism as told above, ACE2 receptor (Li et al. 2003). ACE2 is expressed by type
II alveolar cells (AT2 cells) (Wiener et al. 2007) and also nasal epithelial cells (Sungnak et al.
2020). Law et al. have demonstrated that monocytes and dendritic cells do not express the
ACE2 receptor (Law et al. 2005) on the other hand, Tseng et al. have suggested that ACE2
receptor could be identified in monocyte-derived macrophages and dendritic cells at the
protein level but not detected as surface markers (Tseng et al. 2005). The others have
pointed out that the expression of ACE2 receptor is absent on T-lymphocytes (Lu et al.
2020), now whether SARS-CoV-2 can infect immune cells is not exactly understand yet. It is
not clear that which receptors on immune cells are exactly involved and maybe SARS-CoV
-2 uses another cellular internalize system such as antibody-dependent enhancement
(ADE). In ADE pre-existing antibodies bound to viral particles, instead of viral
4P. LOTFINEJAD ET AL.
neutralization, ADE facilitates internalizing of the virus into the host cells and subsequently
leading to sustained viral infection (Tetro 2020). Another challenge about SARS-CoV-2 is
whether SARS-CoV-2 could develop protective antibodies that are not fully understood.
Maybe antibodies against coronavirus don’t remain for a long time or they are producing at
a low level or maybe the recovered patients are still virus carriers (Lan et al. 2020).
Functions of viral proteins
Viruses commonly generate proteins that interfere with the immune system to either stop
a response or augment one as part of their pathogenicity. Several viral proteins alter
different parts of the immune response pathways to interrupt the immune system and
support their viral evasion and pathogenesis. Also, viral proteins can regulate other cellular
agents that may also interrupt the immune response to increase pathogenesis (Schoeman
and Fielding 2019; Wei et al. 2010). The most central structural proteins of CoV are spike
(S) protein (trimeric), membrane (M) protein, envelop (E) protein, and the nucleocapsid
(N) protein (Prajapat et al. 2020). The S protein has a vital function in virus entry into the
host. Moreover, this protein triggers the immune response of the host cell toward CoV (Li
2016). The E protein plays a significant function in viral morphogenesis, particularly during
assembly and egress. E proteins oligomerization results in ion channel development.
E protein also functions as a virulence factor (Venkatagopalan et al. 2015). Preservation
of the form of the viral envelope is the most significant role of the M protein. It also involves
in the development and secretion of virus-like particles (Schoeman and Fielding 2019).
M protein also has a role in the sensitization of the host by the virus. The M protein of
SARS-CoV stimulates the nuclear factor kappa pathway and IFN-beta pathway, through
a Toll-like receptor-based pathway (Wang and Liu 2016). Formation and preservation of
the RNP complex are the most central activities of the N protein. Furthermore, it controls
the replication and transcription of viral RNA, and in the host, it stops protein translation
via EF1α-mediated activity, change of host cell metabolism, host cell cycle, and apoptosis
(McBride et al. 2014).
The rst line of defense against the virus
There are lots of undiscovered information about the ability of SARS-CoV-2 to elicit innate
or acquired immune responses. As previously discussed, SARS-CoV-2 shares genome
sequence similarity with both MERS-CoV and SARS-CoV (Hajeer et al. 2016; Jin et al.
2020; Lu et al. 2020). Then we hypothesized that the host immunity may trigger similar
responses against SARS-CoV-2. In coronavirus family, it is demonstrated that their genome
as pathogen-associated molecular patterns (PAMPs) are recognized by pattern recognition
receptors (PRRs) such as Toll-like receptors (TLRs) 3, 7, 8 and cytosolic PRRs as RIG-I and
MDA5. By these recognition receptors, the innate immunity senses the invasion of the virus,
so this begins downstream signaling and activate transcription factors to induce type
I interferon (IFN-I) production (Totura et al. 2015; Zalinger et al. 2015). But SARS-CoV
and MERS-CoV can inhibit type I interferon gene expression which will be explained in
detail in the virus evasion part. Therefore, it is thought that SARS-CoV-2 may also use
similar strategies to attenuate innate immune mechanisms. It is demonstrated that type III
IFNs also is known as interferon lambdas (IFNλs), have a critical role in the innate
IMMUNOLOGICAL INVESTIGATIONS 5
immunity of the respiratory tract. IFN lambdas family is comprised of IFNλ1/IL-29, IFNλ2/
IL-28A, IFNλ3/IL-28B, IFNλ4 interferons which bind to heterodimeric receptors FNLR1
and IL10RB then initiate downstream signaling. Signal transduction induces interferon-
stimulated genes (ISGs) genes expression, involved in the restriction of viral replication, and
protects epithelial surfaces of lung and intestinal lumen from viral invasion (Kotenko et al.
2003). For example, in a mouse model of SARS-CoV disease, the results showed that in
knockout mice that lack receptors for IFNAR1 0/0 and IL28Rα 0/0, SARS-CoV was detected
in feces examination of infected mice, supporting the hypothesis that IFN-λ has an
important role in defense against SARS-CoV infection in epithelial cells of both respiratory
and gastrointestinal tracts (Mordstein et al. 2010). Not only bronchial and alveolar epithelial
cells but also plasmacytoid DCs, smooth muscle cells and fibroblast cells of the respiratory
tract produce high levels of IFN-λ, suggesting that when the first defense barrier of innate
immunity is breached, further sources of IFNλ would support this defect (Coccia et al. 2004;
Lauterbach et al. 2010; Okabayashi et al. 2011; Spann et al. 2014). The outcome of a viral
infection depends on the balance between virus and host immunity. Studies have demon-
strated that COVID-19 infection severity and increased risk of death are associated to host
immunity state and an immunocompromised status (Xia et al. 2020). It has displayed that
several innate immunity factors such as NOD-like receptor protein 3 (NLRP3), NLRP6, IgA
antibody along with IL-23, and IL-10 can alter the homeostasis of intestinal microbiota.
This imbalanced microbiota would lead to an increase in the damage of mucosal immunity
and facilitate second infection (Ray and Dittel 2015). These results suggesting that immune-
depletion with refractory hypoxemia along with gut microbiota imbalance maybe are
potentially high risk of COVID-19. The first organized mechanism of defense against
infection or tissue damage is inflammation which is the result of innate and adaptive
immune responses. Preliminary studies have demonstrated that in COVID-19 infection,
inflammation, and acute respiratory distress syndrome (ARDS) occurred as a result of
a cytokine storm (D. Wang et al. 2020a). Cytokine storm is described as excessive levels of
pro-inflammatory and anti-inflammatory cytokines/chemokines that can lead to extensive
tissue damage or organ failure (D’Elia et al. 2013). A recent study on COVID-19 cases in
China has shown that in severe cases, in comparison with moderate cases, neutrophil and
CD4
+
/CD8
+
lymphocyte levels were significantly lower and also renal, hepatic, and respira-
tory failure were more frequently complications occurred in severe cases. The cytokine
profile indicated increased levels of IFN-γ, IL-1ra, IL-2ra, IL-6, IL-10, IL-18, HGF (hepa-
tocyte growth factor), MCP-3 (monocyte chemotactic protein-3), MIG (monokine induced
gamma interferon), M-CSF (macrophage colony-stimulating factor), G-CSF (granulocyte
colony-stimulating factor), MIG-1a (macrophage inflammatory protein 1 alpha), CTACK
(cutaneous T-cell-attracting chemokine) and IP-10 (interferon-gamma induced protein 10)
demonstrating that cytokine storm occurred resulting COVID-19 infection. Serial changes
analysis of IL-1ra, IP-10, and MCP-3 at different days after illness onset in severe cases
displayed that the continuous elevated concentration of these cytokines was related to
worsening outcomes (Yang et al. 2020). In a similar study elevated levels of TNF-α, IL-
2 R, and IL-10 in the majority of COVID-19 severe cases patients were higher than did those
in moderate cases, indicating cytokine storm is likely to be associated with the severity of the
disease (D. Wang et al. 2020a) (Figure 1).
The fate of Natural killer cells (NK-cells) in COVID-19 patients is another mysterious
outcome of COVID-19 infection. NKG2A is considered as an inhibitory receptor which
6P. LOTFINEJAD ET AL.
promotes the exhaustion of NK-cells in chronic infections (Li et al. 2013). Increased
expression of the NKG2A receptor on NK-cells and CD8 + T-lymphocytes is demonstrated
as an exhaustion index (André et al. 2018). In a recent study, it is shown that the expression
of NKG2A receptors is increased on NK-cells and CD8 + T-lymphocytes of COVID-19
patients in comparison with healthy people. Analysis intracellular cytokines of NK-cells and
CD8 + T-lymphocytes displayed decreased levels of IL-2, IFN-γ, TNF-α, and CD107a with
reduced granzyme B production. On the other hand, the follow-up of recovered patients
after antiviral therapy revealed a reduced percentage of NKG2A expression. These findings
may indicate that NKG2A expression is associated with functional exhaustion of NK-cells
and CD8 + T-lymphocytes and disease progression of COVID-19 in the early stage of
infection (Zheng et al. 2020). It is demonstrated that anti-PD-1 (as PD-1-inhibitor of
CD8 + T-lymphocytes) and anti-TIGIT (as TIGIT-inhibitor of NK-cells) prevent
CD8 + T-lymphocytes and NK-cells exhaustion and restore T-lymphocytes response in
viral infection and tumors (Barber et al. 2006; Zhang et al. 2018). NKG2A is also considered
as a novel immune checkpoint target (Haanen and Cerundolo 2018) so, targeting the
NKG2A receptor may restore NK-cells and CD8 + T- lymphocyte’s immune responses
and prevent exhaustion, therefore eliminate viral infection of COVID-19. Similar to any
Figure 1. In respiratory RNA viruses; recognition of pathogen-associated molecular patterns by TLR3, 7
and 8 and cytosolic PRRs such as MDA5 and RIG-I initiates subsequent downstream signaling cascades.
Virus is recognized by these receptors, consecutively triggers transcription factors signaling which finally
induce expression of interferon-stimulated genes (ISGs) and different cytokines. The over-reacted
immune response against the persistent virus leads to releasing large amounts of cytokines which
defined as “cytokine storm”.
IMMUNOLOGICAL INVESTIGATIONS 7
other disease, there is a possibility of sequela after healing from COVID-19. Secondary
complications including pulmonary embolism or persistence of RNA virus in patient
specimens for weeks after onset of symptoms should be discriminated from the recurrence
of COVID-19. Immunosuppressive remedies, patient genetics, and pathological settings
possibly will affect on impair viral clearance and lead to SARS-CoV-2 reactivation (Ling
et al. 2020; Vittori et al. 2020). In some cases also it is demonstrated that consequences after
recovery in patients are related to the long-term presence of the SARS-CoV-2 which can
lead to other complications such as pulmonary inflammation, hypoxia, and pneumonitis
(Batisse et al. 2020).
Acquired immunity responses during COVID-19 infection
An effective immune response with activating CD8
+
cytotoxic T-lymphocytes is required to
eliminate virus infection. Therefore activated specific T-lymphocytes are critical for eradication
of COVID-19 and patient recovery (Li et al. 2008). Recent studies reported a substantial
reduction in the lymphocyte levels in COVID-19 cases. But it has not been determined yet
which factors influence on the decreased number of lymphocytes, some reports suggest that
maybe SARS-CoV-2 targets T-lymphocytes and suppress immune function by the destruction
of these cells (Chen et al. 2020; Huang et al. 2020; Liu et al. 2017; D. Wang et al. 2020a). Lung
autopsy analysis of dead patients from COVID-19 infection, displayed edema and hyaline
membrane formation along with inflammatory infiltration of immune cells, indicating that
acute respiratory failure has been occurred because of over-activated infiltrated immune cells (Z.
Xu et al. 2020b). In a recent study, the relationship between lymphocyte levels and COVID-19
cases outcome was evaluated. The results displayed that after standard treatment in patients who
had a negative test for COVID-19, clearance of virus occurred and reduced-lymphocytes such as
CD3 + T-lymphocyte, CD4 + T-lymphocyte, CD8 + T-lymphocyte, and B-lymphocyte returned
close to normal levels with a significant change compared to the first day of admission. These
results suggest that increased T-lymphocytes count to normal levels has a critical effect on the
clearance of viruses and also it is maybe an indicator of the recovery of COVID-19 cases (Diao
et al. 2020). In a comparative study between severe and moderate cases of COVID-19, the
number of both CD4+ and CD8 + T-lymphocytes was significantly decreased in severe cases of
patients. Although there was a decreased number of CD8 + T-lymphocytes, the ratio of HLA-
DR expression on CD8 + T-lymphocytes in severe cases of COVID-19 was more than 35% in
comparison with moderate cases (D. Wang et al. 2020a). HLA-DR expression on T-lymphocytes
is known as activation marker and it is demonstrated that CD8
+
HLA-DR
+
T-lymphocytes are
considered as regulatory T-lymphocytes subset. They exert their inhibitory effect on neighbor-
ing T-lymphocytes by the CTLA-4 signaling pathway (Arruvito et al. 2014). So, this might be
a reason for the worsening condition of COVID-19 severe cases compared with moderate
patients. On the other hand, a previous study about SARS-CoV infection has revealed that
CD8 + T-lymphocytes are the most proportion of infiltrated inflammatory cells in the pulmon-
ary interstitium which play a critical role in the virus elimination (Arruvito et al. 2014).
Chu et al. have shown that MERS-CoV has a distinct capacity to invade T-lymphocytes
of peripheral blood and some lymphoid organs which lead to the activation of both intrinsic
and extrinsic apoptosis pathways. These results may indicate that MERS-CoV tendency to
invade T-lymphocytes and induce apoptosis can be attributed to its high pathogenicity
(Chu et al. 2016). Then probably lymphopenia in COVID-19 cases is due to apoptosis.
8P. LOTFINEJAD ET AL.
Although this is just a hypothesis and it is not yet defined by which mechanism the COVID-
19 leads to lymphocyte reduction. Diao et al. have been suggested that lymphocyte reduc-
tion in COVID-19 cases is not due to direct virus infection, exactly contrary to what Chu
et al. have claimed about MERS-CoV infection. In Diao et al. study the association between
T-lymphocytes count and IL-2, IL-4, IL-6, IL-10, TNF-α and IFN-γ cytokines level in
COVID-19 patients demonstrated that IL-6, IL-10, and TNF-α cytokines level were sig-
nificantly increased in severe cases of COVID-19 patients. Moreover, they demonstrated
that the concentration of these three cytokines was negatively associated with CD4+ and
CD8 + T-lymphocytes number. After the patient’s recovery, serum levels of IL-6, IL-10, and
TNF-α were significantly decreased in recovered patients as compared with their illness
period while the number of CD4+ and CD8 + T-lymphocytes came close to normal level
during the recovery period. This negative association suggests that a reduced number of
T-lymphocytes is probably the impact of high cytokines level of IL-6, IL-10, and TNF-α on
regulation T-lymphocytes survival, proliferation, and function (Diao et al. 2020). Taken
together, these findings indicate that the immune system is attempting to remove the virus
by all means but similar to other inflammation, like what happens in a long-term infection
maybe T-lymphocytes differentiate to the exhausted phenotype (Wherry and Kurachi
2015). This phenomenon was tested and results demonstrated that PD-1 and Tim-3 on
T-lymphocytes during viral infection development were higher in severe cases of COVID-
19 patients in comparison with moderate cases (Diao et al. 2020) which indicates
T-lymphocytes are exposed to persistent viral antigens or inflammatory signals (McLane
et al. 2019; Mescher et al. 2006) (Figure 2).
What are the mechanisms of SARS-CoV-2 immune evasion?
The immune response has a significant role in several steps of infection and disease. The
initial and most vital step in a host’s response to viral infection is the detection of the viral
factor through the immune system. Recognition of viral infection stimulates the generation
of antiviral proteins, the expansion of cytokines, and the recruitment of immune cells to the
infection site. The innate immune system is the first step of protection against pathogens. It
extends several strategies to detect invading viruses, and next signaling pathways are
targeted at the primary inhibition of infection (Woodland 2018). Although the SARS-
CoV and MERS-CoV outbreak in last years resulted in a global epidemic, the more recently
detected severe acute respiratory syndrome, COVID-19 leads to an extreme case-fatality
rate and can cause severe respiratory disease (Xia et al. 2020). According to their genetic
plasticity, SARS-CoV and MERS-CoV have expanded numerous mechanisms to avoid the
innate immune response. Formerly several molecular strategies of innate immune evasion
by +ssRNA viruses have been described (Nelemans and Kikkert 2019). According to
achieved information about SARS-CoV and MERS-CoV, collected clinical and experimen-
tal evidence on these viruses, and their genomic similarity with SARS-CoV-2, we can
assume and even anticipate how SARS-CoV-2 can be detected by the host immune system
and how this virus can evade from the immune responses (Prompetchara et al. 2020). In this
part, we will focus on the immune evasion of this virus using experience gained from the
outbreak of SARS-CoV and MERS-CoV.
One of the proofs why +ssRNA viruses can cause infection in humans is possibly their
capability in the innate immune evasion, especially the IFN response evasion. As earlier
IMMUNOLOGICAL INVESTIGATIONS 9
mentioned, IFNs are considered as the key factors during the antiviral innate response. The
IFN response is regarded as vital for the regulation of coronavirus infection (Nelemans and
Kikkert 2019). For SARS-CoV and MERS-CoV, the response to viral infection via type I IFN
is stopped. These coronaviruses use different mechanisms to inhibit the signaling leading to
type I IFN generation and the signaling downstream of IFNAR. Also, IFN treatments
enhanced the results of SARS-CoV and MERS-CoV infection in mice and non-human
primates (Bordi et al. 2020). It is considered that SARS-CoV-2 uses the same mechanisms to
regulate the host innate immune response, particularly in inhibiting the type I IFN response
but extra new strategies may be detected.
Studies of SARS-CoV pathogenesis have explained various mechanisms of immune
escape used by the virus to promote infection and transmission. Studies on a SARS
mouse model represent that dysregulated type I interferon and inflammatory monocyte-
macrophage responses lead to fatal pneumonia (Channappanavar et al. 2016). It is found
that Nsp1 prevents interferon signaling in SARS-CoV infected cells by stopping STAT1
phosphorylation and preventing host gene expression via inactivating the ribosome’s
translation function (Jauregui et al. 2013). Hu et al. indicated that SARS Coronavirus
Nucleocapsid prevents the generation of Type I IFN via Interfering with TRIM25-based
RIG-I Ubiquitination. The SARS-CoV N protein has an important role in the viral life cycle
Figure 2. COVID-19 is taken up by ACE2-expressing AT2 cells, the influx of neutrophils and monocytes
leads to production of cytokines. Infiltration of immune cells into alveoli with excessive pro-inflammatory
and anti-inflammatory cytokines production results in lung edema and hyaline membrane formation.
10 P. LOTFINEJAD ET AL.
and the interaction of virus-host. They suggested that the interaction between the
C-terminus of the N protein and the SPRY domain of TRIM25 stopped TRIM25-based
RIG-I ubiquitination, which leads to blockade of IFN production. They also indicated that
the MERS-CoV N protein interacts with TRIM25 and prevents RIG-I signaling and there-
fore results in suppression of IFN secretion (Hu et al. 2017). Recently, Shi et al. revealed that
open reading frame-9b, a protein encoded by SARS-CoV inhibits innate immunity through
affecting mitochondria and the MAVS/TRAF3/TRAF6 signalosome. Their results demon-
strate that SARS-CoV ORF-9b regulates host cell mitochondria and mitochondrial activity
to facilitate host innate immune evasion (Shi et al. 2014).
Reports of MERS-CoV stimulating type I IFN in a late and weak manner indicates that
MERS-CoV has also developed to evade the host immune system. Different researches
demonstrated the development of particular interferon antagonists against the innate
immune response by MERS-CoV (Niemeyer et al. 2013). It is reported that MERS-CoV
M, ORF4a, ORF4b, and ORF5 proteins are potent IFN antagonists. In addition, studies
indicated that MERS-CoV accessory protein ORF4a, ORF4b, and ORF5 stop stimulation of
type I IFN and NF-kappaB signaling pathways (Chafekar and Fielding 2018). As mentioned
above, SARS-CoV and MERS-CoV have several mechanisms to evade immune responses
(Masters 2006). It is also demonstrated that SARS-CoV and MERS-CoV can stimulate the
generation of double-membrane vesicles without PRRs. So, they can replicate in these
vesicles and escape the host immune response (Knoops et al. 2008). In the case of adaptive
immune evasion, after infection of macrophages or dendritic cells with MERS-CoV, antigen
presentation through MHC class I and MHC class II was decreased and leads to reduced
activation of T-lymphocytes (Frieman et al. 2008). Additionally, it is indicated that virus
infection affects the differentiation and function of DCs, stops the following the adaptive
immune response, and facilitates the virus evasion of the adaptive immune response (Shen
et al. 2020). In conclusion, these findings support the immune evasion strategies of SARS-
CoV and MERS-CoV. So, these findings enable us to notice the pathogenicity of extremely
contagious coronavirus; SARS-CoV-2.
Vaccine and potential immunotherapy-based strategies
Previous coronavirus epidemics and effective treatments against other respiratory viruses
can guide on COVID-19 treatment strategies. Currently, there are no effective treatments or
approved vaccines for COVID-19 and all countries are working to prevent further spread of
COVID-19 by using prevention measures (Guo et al. 2019). Treatment options currently
used for patients, especially those with severe symptoms, include mechanical ventilation
and ICU admission (Cheng and Shan 2020). From beginning the outbreak of COVID-19 in
China and its spread to various parts of the world until now, many researchers have started
clinical trials to combat the disease. These investigations include the combination of HIV
drug lopinavir and ritonavir (which reduces SARS and MERS replication) and remdesivir.
Although these drugs have been effective on MERS, SARS, and SARS-CoV-2 coronaviruses
in vitro, their safety, and efficacy in vivo must be further evaluated before using them (M.
Wang et al. 2020a; 2020b). A clinical trial has also been conducted by Chinese researchers
using ritonavir-boosted lopinavir and interferon-alpha 2b on patients hospitalized with
COVID-19 (ChiCTR2000029308) (Cao et al. 2020). In addition, the National Medical
Products Administration of China has approved the use of Favilavir as a treatment for
IMMUNOLOGICAL INVESTIGATIONS 11
coronavirus. This clinical trial is reported to be ongoing in 70 patients in Shenzhen, and its
efficacy has been determined with minimal side effects (https://www.clinicaltrialsarena.
com/). In addition to drugs, various new vaccines in the biotech industry are being tested
by pharmaceutical companies and research centers around the world to begin clinical trials.
Chinese researchers at Inovio Pharmaceuticals have reported that the first COVID-19
vaccine in China is expected to enter clinical trials in late April. In another study, research-
ers are testing an mRNA-1273 vaccine against SARS-CoV-2 that targets the S protein
(NCT04283461). Similar research is exploring a single-dose vaccine, which is based on an
influenza vaccine (https://www.clinicaltrialsarena.com). Most recently, a group of research-
ers at the Hong Kong University of Science and Technology (HKUST) conducted research
to help scientists to develop effective vaccines and identified a set of SARS-CoV-derived
epitopes that can be used as potential targets for the SARS-CoV-2 (Ahmed et al. 2020).
Although many researchers are trying to find an effective treatment for the disease, finding
a drug or vaccine or a method that could be used against SARS-CoV-2 may take months or
even years. One of the alternative options that can be used for antiviral treatment is
immunotherapy. The inflammatory response is reported to have a fundamental role in
coronavirus-induced lung injury (G. Li et al. 2020a; 2020b). According to studies, cytokine
storms have a particular role in the development of the severity of lower respiratory tract
infections such as SARS and influenza (Dunning et al. 2018; Huang et al. 2005). For
example, in 2003 following the outbreak of SARS, corticosteroids were widely used to
control pulmonary infection, however, current WHO guidelines do not recommend their
use because research has shown that corticosteroids do not reduce the mortality of the
patients and make the virus more stable (Arabi et al. 2018; Huang et al. 2020; Russell et al.
2020). In a study by Wang et al., they indicated that CD147 (Basigin) can bind to S protein
in SARS-CoV-2, thereby facilitate viral invasion into host cells. They concluded that
targeting the CD147-SP and using anti-CD147 antibody could inhibit viral invasion.
Therefore, this route could be used as a new target for the development of antiviral
treatments against SARS-CoV-2 (Ulrich and Pillat 2020). Moreover, there are other alter-
native strategies currently being evaluated, some of which include immunomodulation with
chloroquine, monoclonal antibodies (mAbs), and immunoglobulins (Park et al. 2020). The
combination of mAbs such as baricitinib with remdesivir or lopinavir has also been
suggested as a perfect therapeutic option for COVID-19 (Cohen 2020; Stebbing et al.
2020). Studies of affected individuals with SARS-CoV have demonstrated that these patients
display potent neutralizing antibodies (NAbs) against the virus (Yang et al. 2004). It has
therefore been suggested that the use of SARS- CoV specific mAbs for SARS-CoV-2 may be
effective given the high similarity between SARS-CoV-2 and SARS-CoV. It has also been
revealed that CR3022, a SARS-CoV specific mAb, can bind to SARS-CoV- receptor-binding
domain (RBD), which can be used as a potential therapeutic factor for COVID-19.
However, other SARS-CoV specific mAbs such as m396 and CR3014 cannot bind to the
S protein in SARS-CoV-2, indicating that a specific level of similarity is necessary between
SARS-CoV, RBD, and SARS-CoV-2 for cross-reactivity (Tian et al. 2020).
Altogether, vaccination could be used to stop infection or to decrease disease severity,
viral shedding, and transmission, therefore helping to govern outbreaks of SARS-CoV-2.
Numerous vaccination strategies were established against SARS-CoV and experienced in
animals, including an inactivated virus, a live-attenuated virus, viral vectors, subunit
vaccines, recombinant proteins, and DNA vaccines (Graham et al. 2013; Roper and Rehm
12 P. LOTFINEJAD ET AL.
2009). Similar strategies have been used for the expansion of experimental MERS-CoV
vaccines (Du and Jiang 2015). From the experience of former vaccine researches on
SARS-CoV and MERS-CoV, it is demonstrated that the S-protein on the surface of the
virus is an appropriate target for vaccine design. This S-protein interacts with the receptor
angiotensin I-converting enzyme 2 (ACE2) (Sanche et al. 2020). The full structure of the
S-protein from SARS-CoV-2 is now accessible, and it is being investigated for vaccine
targets (Du et al. 2009).
Conclusions
Beside researches based on the virology perspective of COVID-19, discovering, and
identification of immune base procedures are essential in exploring and developing
effective treatment of patients. Identifying the potential mechanisms that the SARS-
CoV-2 uses to cause pathogenesis in patients is essential to improve the patient’s
condition. When a person encounters the virus via the respiratory system, the virus
entry occurs by binding to the ACE2 receptor on alveolar cells. The onset of the
immune response following virus entry occurs by initiate inflammatory response of
innate immunity. Because of uncontrolled cytokine secretion, “cytokine storm” hap-
pens which can lead to ARDS (di Mauro Gabriella et al. 2020). Immediate diagnosis
and prevention are effective in preventing the spread of the disease in the early stages
of the epidemic infection so discovery the novel, safe, rapid, and precise methods are
necessary. Currently, researchers are working on different types of vaccines that may
eventually propose potential vaccines against SARS-CoV-2. The immunopathogenesis
and development of SARS-CoV-2 are associated with the host’s immune system and
virus characteristic features. These properties are associated with severity, the period of
infection, and viral clearance. Therefore, more studies are needed to understand the
nature of the host immune response and also the effect that the virus has on the
immune system. The present management of COVID-19 infection remains just sup-
portive and there is no certain approved treatment. (Channappanavar and Perlman
2017; Law et al. 2005). Apart from common medications in the treatment of viral
infections which have been suggested for control of COVID-19 infection, several
immune-related therapies are displayed promising results and have been suggested
for the management of patients including corticosteroids, intravenous immunoglobu-
lins (IVIG), blockade of cytokines, Inhibition of Janus kinases and convalescent plasma
therapy (Seguin et al. 2016). Therefore, most of them require approval and therapeutic
efficacy in animals and humans to be effectively used against SARS-CoV-2. In this
manuscript, we explained the immunogenicity features of SARS-CoV-2 and recently
proposed diagnostic methods. For a better understanding of how does the immune
system combats this virus, we discussed possible host immune responses according to
recent literature. In this narrative review, we focus on proposed modern preventive
immunotherapy methods in COVID-19 infection. Since this manuscript is provided
based on the results of recent articles about COVID-19, then we hope that this review
may create new insights for researchers to take great strides toward designing vaccines
and novel therapies in the near future.
IMMUNOLOGICAL INVESTIGATIONS 13
Acknowledgments
The authors would like to thank the staff of the Immunology Research Center of Tabriz University of
Medical Sciences for the sincere collaboration and support.
Disclosure statement
The authors declare that there is no conflict of interest.
Funding
This study received no specific grant from any funding bodies.
ORCID
Parisa Lotfinejad http://orcid.org/0000-0001-6142-9036
Zahra Asadzadeh http://orcid.org/0000-0002-4118-6476
Shiva Najjary http://orcid.org/0000-0001-5088-0016
Mohammad Hossein Somi http://orcid.org/0000-0002-0770-9309
Khalil Hajiasgharzadeh http://orcid.org/0000-0003-4593-4803
Ahad Mokhtarzadeh http://orcid.org/0000-0002-4515-8675
Afshin Derakhshani http://orcid.org/0000-0002-3243-233X
Elmira Roshani http://orcid.org/0000-0002-9783-5156
Behzad Baradaran http://orcid.org/0000-0002-8642-6795
Data Availability Statement
Data sharing not applicable to this article as no datasets were generated or analysed during the current
study. https://ind01.safelinks.protection.outlook.com/?url=https:%2F%2Fauthorservices.taylorandfrancis.
com%2Fdata-sharing-policies%2Fdata-availability-statements%2F%23:~:text%3DAs%2520a%2520data%
2520availability%2520statement%2Canonymized%2520version%2520of%2520the%2520manuscript.%
26text%3DThe%2520data%2520that%2520support%2520the%2Creference%2520number%2520%
255Breference%2520number%255D&data=02%7C01%7Csathyan.dhanasekaran%40integra.co.in%
7Ca61cc99bb6194ceccbb008d85f9ee080%7C70e2bc386b4b43a19821a49c0a744f3d%7C0%7C0%
7C637364483269538189&sdata=n%2FbXrm4%2BFzCCjf1hxEMWLMkx3j5WOzq5loZbu1WELQ8%
3D&reserved=0
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