Immune-based therapeutic approaches in COVID-19

Coronavirus disease 2019 (COVID-19) is a viral disease caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), a member of the Coronaviridae family. On March 11, 2020 the World Health Organization (WHO) has named the newly emerged rapidly-spreading epidemic as a pandemic. Besides the risk-reduction measures such as physical and social distancing and vaccination, a wide range of treatment modalities have been developed; aiming to fight the disease. The immune system is known as a double-edged sword in COVID-19 pathogenesis, with respect to its role in eliminating the pathogen and in inducing complications such as cytokine storm syndrome. Hence, immune-based therapeutic approaches have become an interesting field of COVID-19 research, including corticosteroids, intravenous immunoglobulins (IVIG), interferon therapy, and more COVID-19-specific approaches such as anti-SARS-CoV-2-monoclonal antibodies. Herein, we did a comprehensive review on immune-based therapeutic approaches for COVID-19. Data availability statement Not applicable.


Introduction
Coronavirus disease 2019 (COVID- 19), a viral disease caused by a member of the Coronaviridae family, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has caused one of the greatest global pandemics, as declared by the World Health Organization (WHO) on March 11, 2020 [1]. Lung epithelium damage, hypercoagulability, and vascular leak lead to an important clinical manifestation, named acute respiratory distress syndrome (ARDS). Patients with previously diagnosed hypertension, cardiovascular disease, and diabetes are highly susceptible to ARDS [2]. As of 20 March 2022, more than 468 million confirmed cases and more than 6 million deaths have been reported [3]. As the main route of COVID-19 transmission is via respiratory droplets, social distancing is one of the most important measures for controlling the spread of the virus [4]. In addition to social distancing, developing effective vaccines could be a potent tool in limiting the disease spread and lowering the disease burden [1]. Amongst a variety of treatment approaches, immunotherapy has become an interesting option for COVID-19, although, the treatment results closely depend on choice of the right patient and right timing of drug administration [5]. Herein, we did a comprehensive review on immune-based therapeutic approaches for COVID-19. respiratory tract involvement, and ARDS/ multi organ dysfunction syndrome (MODS) [7]. The pathophysiological, clinical, and immunological characteristics of these stages are summarized in Table 1.
The pathological manifestations of the disease are mostly noticeable in the lung tissue, including hyaline membrane formation, accumulation of serous, exudate (mostly monocytes and macrophages), and fibrin in alveoli, and hemorrhagic infarction [20,21]. Moreover, inclusion bodies, proliferation, and detachment has been spotted in pneumocyte II cells [22]. Studies on the alveolar septa shows edema, hyperemia, mononuclear cell infiltration, and hyaline thrombose formation [20]. On the other side, some extrapulmonary manifestations have been spotted in spleen (shrinkage, fewer number of T cells), heart (necrosis, leukocyte infiltration), liver (cellular degeneration, hepatomegaly), gall bladder (discoloration to dark-red, volume increase), and kidneys
The extent of abnormalities in chest X-ray (CXR) imaging is of diagnostic and prognostic importance and indicates the severity of the disease [23]. In more than 80% of the cases, the early stage of the disease is characterized by a bilateral multi-lobar ground-glass opacity, mainly distributed in middle-lower lungs [24][25][26]. In progressive and peak phases, these abnormalities increase in size, along with the appearance of interlobar septal thickening and consolidations. After the 14th day of illness, the radiological abnormalities gradually disappear [23]. In some cases, despite the viral clearance and resolution of the symptoms, some long-term sequelae such as progressive fibrotic abnormalities develop [27].
Laboratory indicators such as serological and molecular indices could be helpful in COVID-19 diagnosis, which are summarized in Table 2.
Management of COVID-19 patients involves both general supportive care and pharmacological treatment [8]. Supportive measures include, bed rest and temperature control in patient's environment. Furthermore, monitoring oxygen saturation, liver and kidney function, electrolyte balance, and blood analyses such as complete blood count, c-reactive protein (CRP), and coagulation state are carried out [32]. In some cases, high-flow oxygen or oxygen-hydrogen mixtures (H 2 /O 2 : 66.65/33.39%) and rehydration therapy including IV fluid administration could be helpful [8,33]. The effectiveness of some antiviral drugs such as lopinavir/ritonavir, oseltamivir, and ribavirin have been reported [34][35][36]. Arbidol, chloroquine, and interferon (IFN)-α had been used in Wuhan outbreak and in vitro studies demonstrated their effect on viral load reduction [37][38][39]. Management of critically ill patients, include respiratory and circulatory supportive care, oxygen therapy and intubation in some cases, besides the preventive and therapeutic measures for secondary complications and infections [8].

Mechanism of action
Corticosteroids are a class of substances, either synthetic or naturally expressed in adrenal cortex, which have a wide range of effects on the immune system, inflammation, stress response, metabolism, body fluids, and electrolytes [41]. Corticosteroids induce the production of lipocortin, which inhibits phospholipase A2, an essential enzyme for the production of inflammatory mediators [42].
In most COVID-19 cases, mortality is a result of exaggerated immune response against the infection. Corticosteroids control such responses and subsequently, decrease the time on mechanical ventilations, period of residence in intensive care unit (ICU), and mortality rate in COVID-19 patients [43]. Fig. 3 illustrates the mechanism of corticosteroids' effect of COVID-19.

Administration indications and clinical findings
During the SARS outbreak in 2003, corticosteroids, alone or in combination with ribavirin, were extensively used [44,45]. In the current pandemic, a variety of studies have shown the efficacy of corticosteroids in different stages of COVID-19 infection; the results of these studies are summarized in Table 3.
Based on the WHO guidelines on the administration of corticosteroids, there are two recommendations. First recommendation states that systemic corticosteroids are favored in comparison to non-systemic corticosteroids, which is the choice in severe and critically ill COVID-19 patients regardless of their hospitalization status [46]. Severe COVID-19 infection applies to patients with SpO2 < 94% on room air at sea level, respiratory rate of > 30 breaths per minute, PaO2/-FiO2 < 300 mm Hg, or pulmonary infiltrates > 50%. Critical COVID-19 infection applies to patients with conditions requiring life-sustaining procedures (mechanical ventilation, etc.) such as ARDS, multiple organ failure, cardiac failure, exaggerated inflammation, and septic shock [47].
The second recommendation states that it is better not to use corticosteroids in treatment of non-severe COVID-19 patients. Though, in cases of previously initiated treatment for other clinical conditions, corticosteroids should not be discontinued. Corticosteroid therapy, though beneficial in some cases, can cause susceptibility to complicated infections and should be taken into consideration while using this modality [46].

Challenges
Although the short-term use of corticosteroids has been helpful, high-dose corticosteroids can delay the viral clearance [40]. Based on the current guidelines, the administration of corticosteroids should be limited due to their wide range of side effects. Some of these include increase in mortality rates, diabetes, avascular necrosis, femoral head osteonecrosis, psychosis, and induction of lung injury and shock [66].

Mechanism of action
Intravenous immunoglobulin (IVIG) is a mixture of human immunoglobulins against microbial infections obtained from the recovered patients; it is administered as an immunomodulatory agent in autoimmune diseases and infections, and as replacement therapy in immunodeficiencies. In viral infections, administration of IVIG induces antibody-dependent cellular cytotoxicity (ADCC) through binding to viruses and inducing phagocytosis via binding to FCγR receptors; IVIG prevents the viral entry to the host cell via blocking viral surface proteins and modulates inflammatory reactions following the blockage of FCγR IIa and FCγR IIIb on leukocytes [67]. Fig. 4 summarizes these mechanisms.

Administration Indications and Clinical Findings
IVIG administration in SARS patients has improved patient's survival, shortened the viremia period, and led to early discharge [68]. In addition, IVIG can reduce the mortality rate in patients affected with MERS-CoV [69].
Several studies have been conducted to assess the effectiveness of IVIG for COVID-19, which are summarized in Table 4.
High dose IVIG is indicated in all acute severe COVID-19 patients (aimed to reduce post-infection cytokine storm and prevent thrombosis), all patients presented with or developed autoimmune disorders such as Guillain-Barre syndrome, children with multisystem inflammatory syndrome, and patients with primary and secondary immunodeficiencies succumbing to acute COVID-19. Low dose IVIG is indicated in unvaccinated patients with autoimmune disorders or primary and secondary immunodeficiencies for protection against exposure to the virus. IVIG is also helpful in neurological disorders as a result of COVID-19's inflammatory sequelae, including cognitive dysfunction, neuralgia, insomnia, and autonomic nervous system involvement [70].

Challenges
Some serious adverse effects such as hemolytic anemia, acute lung injury, thrombosis, cardiac arrhythmia, meningitis, and renal impairment and lack of sufficient data concerning this therapeutic method have restricted its use in COVID-19 patients; therefore, more studies are necessary in this regard [67].

Mechanism of action
Expression of interferon-I (IFN-I) family, including IFN-α and IFN-β, is upregulated following the viral attachment to cell surface receptors and induction of pattern recognition receptors (PRRs). Higher levels of IFN-α and IFN-β, subsequently induce Janus kinase (JAK) signaling pathway and interferon-stimulated genes, providing the first line of antiviral defense [81]. In comparison to other coronaviruses such as SARS-CoV-1, SARS-CoV-2 shows higher efficacy in proliferation and infection with shorter duration to peak levels and higher number of viral particles at the time of peak, which can be a result of insufficient IFN-I response [82][83][84]. In addition, disease severity can be associated with insufficient innate IFN-I levels, which results in reduction in IFN-stimulated genes expression. Therefore, patients with low IFN-I signaling levels have a poor prognosis [85]. A study by Hadjadj et al. demonstrated that expression of IFN-stimulated genes is upregulated when patients are subjected to IFN-α stimulation, stating that the downstream components of the signaling pathway are not impaired [86]. Fig. 5 illustrates these mechanisms.

Administration indications and clinical findings
Administration of IFN-I in earlier stages of the disease can reduce the later-coming immunopathologies. Although administration of IFN-I can improve disease outcomes, it can over-activate inflammatory responses, exacerbating the condition. Therefore, IFN-III can be a substitute for IFN-I with similar antiviral characteristics and less toxicity [87]. A study by Jagannathan et al. on administration of pegylated (PEG) IFN-λ1a in mild to moderate COVID-19 patients showed that it can shorten viral shedding and duration of the symptomatic phase, if administered in a period of 72 h after diagnosis [88]. IFN-λ can inhibit the tissue repair as well as the damaging effects of neutrophils on lungs; of note, outcomes of using IFN-λ depend on location, timing, and duration of administration [89].
There is evidence on early administration of interferon in patients receiving glucocorticoids that shorten the duration of hospital stay and symptomatic stage, suggesting a therapeutic synergy [90].
Several studies have been conducted on administration of interferons in COVID-19 patients, some of the most recent of which are summarized in Table 5.
According to the COVID-19 Treatment Guidelines, based on the results from clinical trials, and with respect to the lack of thorough evidence on the occurrence of adverse effects in some patient groups, the Panel recommend against the administration of interferons (α, β, and λ) in hospitalized patients unless in the setting of a clinical trial [33].

Challenges
The immune system imbalance and the resulting immunopathology has restricted the administration of interferons in COVID-19 patients; in patients receiving interferons, careful patient monitoring during the treatment is crucial [87].

Mechanism of action
Monoclonal antibodies target different molecules that are involved in COVID-19 pathogenesis; amongst which there are some of the viral surface proteins such as spike (S) protein and contributors to the cytokine storm syndrome (CSS) such as IL-6, TNF, and IL-1β [107]. The S protein has two subunits; S1 is involved in attachment to ACE2 by means of a receptor binding domain (RBD) and help of the N-terminal domain (NTD) through recognition of sugar moieties, and S2 is involved in fusion of the viral particle. Neutralizing any of these targets can be helpful for the prevention of viral infection [107][108][109]. CSS is an uncontrolled inflammation especially in critical COVID-19 patients, leading to fever, ARDS, multiple organ failure, and death. Targeting the inflammatory factors involved in CSS can reduce the COVID − 19 mortality rate [110][111][112][113]. Fig. 6 illustrates these mechanisms.

Administration indications and clinical findings
Monoclonal antibodies can be beneficial in the treatment as well as pre-exposure and post-exposure prophylaxis [33]. Administration of monoclonal antibodies cause a better overall survival in hospitalized patients [114]. A variety of clinical and preclinical studies have been carried out to evaluate the effect of administration of monoclonal antibodies in different stages of COVID-19 infection. Table 6 summarizes results of recently conducted clinical trial in this regard.
Among these antibodies, some have been approved by the Food and Drug Administration (FDA). Bamlanivimab plus etesevimab and casirivimab plus imdevimab combination therapies are two of the approved treatment regimens. Though, administration of these products has been paused in the US due to lower susceptibility of the Omicron (B.1.1.529) variant. Sotrovimab is also authorized for administration in both SARS-CoV-1 and different variants of SARS-CoV-2. Combination therapy using tixagevimab plus cilgavimab has also been approved and is used in different variants of COVID-19, including the Omicron variant. This combination can also be used as a pre-exposure prophylaxis and has received an Emergency Use Authorization (EUA) from the FDA in this matter.
The COVID-19 Treatment Guidelines panel recommends administration of single dose sotrovimab 500 mg IV in non-hospitalized mild to moderate infection as soon as possible. Administration of bamlanivimab plus etesevimab and casirivimab plus imdevimab combination therapy is not recommended in omicron variant outbreaks. The panel also recommends against the use of monoclonal antibodies in cases of hospitalized severe infection, and also in immunocompromised patients (due to risk of resistance) [33].

Challenges
The rapid emergence of various SARS-CoV-2 new variants calls for the need to developing antibodies against the new epitopes and developing tools for timely prediction of the emergence of new variants [107].

Mechanism of action
Mesenchymal stem cells (MSCs) are plastic-adherent stem cells with an in vitro differential potential and an ability to express a variety of markers such as CD105, CD90, and CD73, and lack of CD11b, CD14, CD19, CD34, CD45, CD79α, and HLA-DR [145]. MSCs could be an appropriate treatment candidate due to the easy isolation from the donor tissue, as well as lack of expression of HLA markers, rapid proliferation, proper homing capacity of the target site, and persistence in the target lung tissue [146].
The mechanisms involving these cells include immunomodulation and paracrine secretion of cytokines such as, IL-37, keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), lipoxin A4, and angiopoietin-1. In addition, they can induce ATP production in alveoli and enhance phagocytosis by means of mitochondrial transfer. In ARDS cases, it can improve the condition by inducing lung permeability, fluid clearance in alveoli, and lung repair in both epithelial and endothelial tissues [146]. Fig. 7 illustrates these mechanisms.

Administration indications and clinical findings
Since MSCs express low levels of ACE2 and transmembrane protease serine type 2 (TMPRSS2), they are resistant to SASRS-CoV-2 infection regardless of their source [147]. This therapeutic approach has been well tolerated in COVID-19 patients according to the results of the clinical trials. Table 7 summarizes the results of recent studies in this field.
Though promising results have been observed regarding this treatment modality, the COVID-19 Treatment Guideline Panel recommends against the administration of MSCs, unless it is in a clinical trial setting, due to limitation of data [33].

Challenges
Although recent studies have shown promising results for using MSCs, we still face some problems. Some of such barriers include variability of MSC sources and the need to identify the most suitable source and standardization of the methods of handling MSCs, both in preparation and administration [146].

Conclusion
COVID-19 pandemic has caused considerable morbidities and mortalities over the past couple of years. Involvement of the immune system as a double-edged sword, both in constraining the disease and in causing complications such as CSS, has made immune-based approaches a great candidate in combating the disease, amongst which corticosteroids, IVIG, interferon therapy, and monoclonal antibodies are reviewed in this article. Each of these therapeutic approaches are beneficial in specific clinical settings and disease stages to gain the most improvement in clinical conditions with the least adverse effects (susceptibility to infections, etc.). Corticosteroids are advised to be considered in severe or critically ill patients due to their variety of potential side effects. IVIG has shown promising results in trials, though its use is limited and more studies need to be conducted due to the risk of multisystem adverse events. Interferon therapy though considered beneficial in a variety of infectious diseases, including COVID-19, it can cause a range of immune system imbalances; therefore, its usage is currently limited. Monoclonal antibodies are a targeted therapeutic modality and have shown promising results in patients in a variety of disease stages. Nevertheless, due to the rapid emergence of new variants they might lose their efficacy in different outbreaks based on the prevalent variants, hence, it is important to develop antibodies against new epitopes. Overall, in each clinicobiological phase in COVID-19 pathogenesis, some therapeutic agents are indicated. In the asymptomatic phase, monoclonal antibodies can be used; for instance, as mentioned before, bamlanivimab has been used in medical staff as a prophylaxis agent (Table 6). In the propagating phase, different agents can be suitable depending on disease severity. Patients with mild disease, respond to monoclonal antibodies such as bamlanivimab, the combination of bamlanivimab plus etesevimab, MW33, regdanvimab, and sotrovimab (Table 6). In moderately ill patients, monoclonal antibodies (e.g, tocilizumab, itolizumab, bamlanivimab, the combination of bamlanivimab plus etesevimab, MW33, and sotrovimab [ Table 6]), interferon therapy (Table 5), and based on results of some trials (Table 4), IVIG can be beneficial. In severely ill patients and patients in complicating phase of the disease, corticosteroids, IVIG, interferons, and monoclonal antibodies can be beneficial. For the administration of monoclonal antibodies, the virus strain is an important factor and it should be taken into consideration in deciding the drug of choice. Although a wide range of pre-clinical and clinical studies have been conducted regarding immune-based approaches, more studies are

Conflict of interest statement
The authors report no conflict of interest.

Data availability
No data was used for the research described in the article.

Acknowledgments
Not applicable.

Disclosure of interest
The authors report no conflict of interest.

Consent to participate
Not applicable.

Authors Contribution
AM conceptualized the title and prepared the first draft. NY conceptualized the title, edited and revised the manuscript and finalized the draft. NR conceptualized the title, critically revised the manuscript, finalized the draft, and supervised the project. All the authors have read and approved the final draft of the manuscript. Pilot study Constant rise of PaO2/FiO2 in first 7 days, 3 of the 5 patients survived and were extubated on day 9, the method was relatively well-tolerated [151] Perinatal tissue ARDS secondary to COVID-19 Case series 7 patients showed clinical improvement, 6 of the 7 patients enrolled survived, reduction in levels of TNF-α, IL-8, CRP, IFN-γ, and IL-6, and remarkable signs of radiologic recovery was observed [152] Umbilical cord Severe COVID-19 Randomized controlled trial Lower incidence of progression, mortality, shorter time to clinical improvement in treatment group, reduction in levels of IL-6 and CRP [153] Menstrual blood Severe/critical COVID-19 patients Exploratory clinical trial Lower mortality (7.69% vs. 33.33%) in treatment group in comparison with controls, improvement in SpO2, dyspnea and radiological findings [154] Umbilical cord Severe/critical COVID-19 patients Pilot study Improvement in oxygenation index, radiological findings, and lymphocyte count, lower mortality in comparison with historical rate (6.25% vs. 45.4%) [155] N/A (ACE2-MSCs) Severe COVID-19 pneumonia Pilot study Clinical improvement, reduction in levels of CRP, over-activated cytokine secreting cells, TNF-α, and increase in levels of peripheral lymphocyte, regulatory DC, IL-10 [156] Umbilical cord Critical COVID-19 Randomized controlled trial 2.5 times higher survival rate in treatment group in comparison with controls, no significant difference in length of stay in ICU and ventilator usage, reduction in IL-6 levels [157] Umbilical cord Severe COVID-19 Randomized controlled phase 2 trial Improvement in radiological findings [158] Umbilical cord ARDS secondary to COVID-19 Randomized controlled phase 1/2a trial Significant reduction I. Inflammatory cytokines, improvement in patient survival (91% vs. 42%) and time to recovery (P = 0.03) [159]