Interleukin-6 in SARS-CoV-2 induced disease: Interactions and therapeutic applications.

Interleukin-6 (IL-6) is a multi-tasking cytokine that represents high activity in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and cancer. High concentration of this pleiotropic cytokine accounts for hyperinflammation and cytokine storm, and is related to multi-organ failure in patients with SARS-CoV-2 induced disease. IL-6 promotes lymphopenia and increases C-reactive protein (CRP) in such cases. However, blockade of IL-6 is not a full-proof of complete response. Hypoxia, hypoxemia, aberrant angiogenesis and chronic inflammation are inter-related events occurring as a response to the SARS-CoV-2 stimulatory effect on high IL-6 activity. Taking both pro- and anti-inflammatory activities will make complex targeting IL-6 in patient with SARS-CoV-2 induced disease. The aim of this review was to discuss about interactions occurring within the body of patients with SARS-CoV-2 induced disease who are representing high IL-6 levels, and to determine whether IL-6 inhibition therapy is effective for such patients or not. We also address the interactions and targeted therapies in cancer patients who also have SARS-CoV-2 induced disease.

Introduction

Coronavirus disease 2019 (COVID-19) is a condition resulted from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [1]. SARS-CoV-2 induced disease occurs due to excessive responses from host immune system and is manifested by progressive hypoxemia [1] and pneumonia [2]. The outcomes are shortness of breath, chest thickness and respiratory failure [3]. Patients with severe SARS-CoV-2 induced disease require face mask or nasal cannula for supplemental O2, while critical cases may need invasive approaches, such as mechanical ventilation [4], [5].

Interleukin (IL)−6 is a pleiotropic cytokine that was discovered in the year 1986 [6]. It takes both pro- and anti-inflammatory functions [7]. Adaptation to the intense training during exercise is a result of anti-inflammatory and metabolic activity of this cytokine [8]. By contrast, IL-6 takes pro-inflammatory activity when there is no control over its production, so a surge in release of this cytokine can be a backbone for pathogenesis of arthritis diseases and conditions like cytokine release syndrome (CRS) [2]. IL-6 at homeostatic levels is responsible for resolution of tissue lesions [9], but its amplification induces cytokine storm [10]. IL-6 is a key cytokine related to severity and mortality of SARS-CoV-2 induced disease [11]. The current literature was designed in order to discuss about IL-6 activity and targeting this cytokine in patients with SARS CoV-2 induced disease. We also have a discussion over IL-6 roles in patients with SARS CoV-2 induced disease who also have cancer.

SARS-CoV-2 induced disease

The crisis of SARS-CoV-2 induced disease: a global pandemic of virus infected event

SARS-CoV-2 induced disease triggered a world pandemic at December 2019 [12]. The number of people affected from this disease worldwide as of May 2nd, 2020 was over 3.400.000 [12], and it reached to over 150 million until May 2, 2021 [13]. The world death rate from the virus surpassed from 641,000 at July 25, 2020 [14] to over 3 million at May 2021 [13]. Severe symptoms are developed in 15–20% of cases [1], and the rate of mortality is about 60% in patients with critical conditions [3]. Morality from this disease is higher in males [15] and older individuals [16]. Other factors that increase the rate of death from SARS-CoV-2 induced disease are comorbidities, such as hypertension, diabetes and obesity.

SARS-CoV-2 identity, and the staging, phases and scaling of SARS-CoV-2 induced disease

SARS-CoV-2 is a virus of single RNA strain family that causes infection mainly in respiratory system [9]. SARS-CoV-2 contains particles that are ranged from 50 nm to 200 nm. These particles are comprised of three glycoproteins including spike (S), membrane (M) and envelope (E). S protein is related closely to the invasive capacity of the virus, and is a basis for developing neutralizing antibodies against the virus [17]. The S protein contains subunits 1 (S1) and 2 (S2). S1 acts for attaching to host cellular receptors, and S2 is responsible for infusion of the virus to cell membrane [18]. M protein is contributed to the formation of envelope and virus budding [17]. E proteins are small membranous proteins that act as potential ion channels, and their presence is vital for viral pathogenicity and important for drug targets and vaccines [19].

Three clinical stages are considered for SARS-CoV-2 with the stage III representing severe systemic inflammatory syndrome along with severe respiratory failure. Patients at this stage display extremely high levels of inflammatory markers. Infection raised by SARS-CoV-2 follows two overlapping phases: (1) high replication of virus [9] in which viral load reaches a peak concentration before day 5, represented by a peak of 7.11 × 108 RNA copies per throat swab at day 4 [20], and (2) mitigating immune responses from host [9]. Ordinal scaling system can be used for assessment of clinical improvement in patients with SARS-CoV-2 induced disease, which includes seven points, marked from death (point 1) to discharge from hospital (point 7) [4].

Interleukin-6 receptor/ligand

IL-6 is a four helical [21] and pleiotropic pro-inflammatory cytokine with a molecular weight of 26 kD that includes 185 amino acids [22]. There are two forms of IL-6 receptors (IL-6Rs) and two types of IL-6-mediated signaling. IL-6Rs are soluble (sIL-6R) and membrane-bound (mIL-6R) [3], and IL-6-related signaling are trans-signaling and classic cis-signaling. mIL-6R is selectively expressed on surface of immune cells. In the classic signaling, IL-6 binds with mIL-6R forming a complex that further binds to the transmembrane protein glycoprotein 130 (gp-130) to complete the signal transduction, namely taking a pro-inflammatory role. In the trans-signaling IL-6 interacts with sIL-6R and the complex further binds to the gp-130. The final complex interactions transduce signaling within cytosol through janus kinas (JAK) and signal transducer and activator of transcription 3 (STAT3). In the cytosol, gp-130-bounded JAK is contributed to the phosphorylation of STAT3, and the phosphorylated STAT3 further translocate into the nucleus to take function as transcription activator [23]. sIL-6R is expressed by endothelial cells (ECs). Such cells do not express mIL-6R [24]. Immune cells including monocytes, macrophages, T cells and neutrophils are the main population of cells expressing mIL-6R [7], [24]. Activated STAT3 promotes transcription of target genes, such as vascular endothelial growth factor (VEGF) and hypoxia inducible factor (HIF)−1α [25]. VEGF is an inflammatory and regulatory cytokine with pro-angiogenic activities [26], [27]. HIF-1α is a key mediator during the severe or acute phase of hypoxia [28] ( Fig. 1a, b).

Fig. 1

Interleukin (IL)−6 receptor/ligand interactions. IL-6 interacts with two types of IL receptors (IL-6Rs): (a) Interaction with membranous form directs classic cis-signaling pathway, whereas interaction with the soluble form (i.e. sIL-6R) promotes trans-signaling pathway (b). The trans-membrane protein gp-130 acts as a subunit for IL-6R, transducing signals toward cytosol. IL-6/IL-6R/gp-130 interactions transduce signals via janus kinas (JAK) and signal transducer and activator of transcription 3 (STAT3). STAT3 is further transferred toward the nucleus in order to promote activation of target gens, such as vascular endothelial growth factor (VEGF) and hypoxia inducible factor (HIF)−1α. Inhibition of membrane-bound and soluble IL-6Rs by agents like acetolizumab is of therapeutic importance in cancer patients and cases with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induced disease. (c) Interleukin (IL)−6 amplifier in patients with SARS-CoV-2 induced disease. The activity of NF-κβ is induced by IL-6/STAT-3. NF-κβ, in turn, promotes IL-6 transcription and its release toward the extracellular milieu. This mechanism of implication will intensify the severity of condition in patients with high activity of IL-6, such what seen in SARS-CoV-2 induced disease. (d) IL-6 inducible effect on SARS-CoV-2 cellular entry. SARS-CoV-2 interacts with angiotensin-converting enzyme 2 (ACE2) receptor in order to enter the cellular cytosol. Recognition of the ACE2 by the virus and its membrane infusion is mediated by cathepsin L. IL-6 induces the activity of ACE2 and cathepsin L, thus facilitating cellular entry of the virus.

Key note IL-6 acts via mIL-6R and sIL-6R to promote JAK/STAT3 mediated signal transduction.

Interleukin-6 in SARS CoV-2 induced disease

Exuberant generation of pro-inflammatory cytokines and repressed anti-viral innate immune responses are features of SARS CoV-2 induced disease [29]. IL-6 is associated with a severe disease [1], and is considered as a reliable factor for distinguishing the prognosis and severity of SARS CoV-2 induced disease [30]. High levels of IL-6 is used as a good predictor of oxygen requirement, intubation or mortality in the affected patients [31]. Gorham and colleagues in a retrospective study evaluated IL-6 concentrations between survivors and non-survivors in patients admitted to intensive care unit (ICI). The authors noticed a big gap in IL-6 levels between the two groups, represented by considerably higher rate in non-survivors (720 pg/mL) compared with survivors (336 pg/mL). They suggested repeated evaluation of IL-6 as a marker of poor prognosis in critically ill patients SARS CoV-2 induced disease [32]. Varchetta and colleagues also noticed that higher level of serum IL-6 in deceased versus survivors from SARS CoV-2 induced disease [33]. The pro-inflammatory activity of this cytokine is linked to the pneumonia [2]. Overproduction of cytokines promotes pulmonary fibrosis [5] and respiratory and multi-organ failure [30]. Negative relation between IL-6 with partial oxygen pressure in arterial blood (PaO2) and peripheral oxygen saturation (SpO2) is indicative of respiratory failure [9]. Elevated level of IL-6 represents a tight relation with the patient requirement for mechanical ventilation [34]. Down regulation of factors, such as miR-451a is contributed to the higher IL-6R protein levels in patients with SARS CoV-2 induced disease [35].

Key note Evaluation of IL-6 is of prognostic value in patients with SARS-CoV-2 induced disease.

Interleukin-6 amplifier in patients with SARS-CoV-2 induced disease

Generation of IL-6 follows a mechanism of amplification. To explain, there is a synergistic cross-connections between STAT3 with nuclear factor of kappa β (NF-κβ) [6]. IL-6 amplifier is described by co-activation of STAT3 and NF-κβ in non-immune cells and hyperactive NF-κβ signaling, which result in autoimmune and inflammatory diseases [6], [36]. The concentration of IL-6 is determined by the activity of NF-κβ [37]. IL-6 is a NF-κβ target [6] in which the activity of the latter results in the induction of IL-6 expression [36], [38], [39]. IL-6 amplifier occurring in the context of SARS-CoV-2 induced disease may be an explanation for occurrence of CRS in patients with a severe disease. IL-6 amplifier is, in fact, a mechanism for enhancing the release of several pro-inflammatory cytokines, such as IL-6 [40], mediated by NF-κβ hyperactivation. Recruitment of myeloid and lymphoid cells toward lesion area is also induced by IL-6 amplifier. IL-6 amplifier can occur due to aging, which indicates the higher rate of mortality from SARS-CoV-2 induced disease in aged individuals [36] (Fig. 1c).

Key note IL-6 amplifier explains the severity of CRS in patients with SARS-CoV-2 induced disease, particularly in aged individuals.

The activity of interleukin-6 in viral entry into the lung

Elevated cytokine levels are reflective of viral load and lung damage in patients with SARS CoV-2 induced disease [41]. The activity of IL-6 is needed for SARS CoV-2 intruding respiratory cells. There are two mechanisms for such action: (1) IL-6 inducible effect on cathepsin L, and (2) IL-6 inducible effect on upregulation of angiotensin-converting enzyme 2 (ACE2) receptor [42]. Cathepsin L cleaves S protein of the SARS-CoV-2 virus. The viral S protein interacts with ACE2 for cellular attachment and entry [43] (Fig. 1d).

Key note IL-6 activity helps cellular entry of SARS CoV-2 virus.

Cytokine storm, cytokine release syndrome and interleukin-6 in patients with SARS-CoV-2 induced disease

Cytokine storm is a condition by which a wide range of immune active molecules, such as chemokines and cytokines are released at extremely high concentrations from immune system. This abundant release of immune-related molecules is potentially triggering multi-organ failure or even death [21]. Patients encountering a severe stage SARS-CoV-2 induced disease display hyperinflammation, which is controllable by drugs prescribed for autoimmune diseases [44]. SARS CoV-2-related cytokine storm induces CRS [10]. Cytokine storm and CRS are systemic inflammatory syndromes that occur in the context of autoimmune diseases, cancer and therapy [24]. Chimeric antigen receptor T-cell (CAR-T) therapy is an example of therapeutic modalities that has CRS as an adverse event [2]. Here, activation of innate immunity and dysregulation of adaptive immunity cause secretion of pro-inflammatory cytokines and chemokines [30]. CRS resulted from CAR-T therapy is mirroring clinical characteristics of severe SARS-CoV-2 induced disease [45].

SARS-CoV-2 virus stimulates CD4+ T cells and their conversion into pathogenic T helper (Th)−1 cells. The cells secrete different cytokines [45] that activate inflammatory monocytes for further release of IL-6 within the lung environment [21], [46]. Thus, high concentration of IL-6 in plasma and alveoli of patients with SARS-CoV-2 induced disease is interpreted as a marker of poorer clinical outcomes [47]. IL-6 is one of the earliest cytokines activated during inflammation [7], produced during both acute and chronic inflammation [21] and its activity is important for innate and adaptive immunity [15]. However, a surge in the secretion of this cytokine can promote a state of hyperinflammation that is harmful for host cells [15], [44]. IL-6 is a biomarker of hyperinflammation [44], and its concentration is considered as an independent marker of a severe disease [9]. High IL-6 level secreted from monocytes reduced weeks after recovery [33] ( Fig. 2).

Fig. 2

Cytokine storm in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Patients experience a hyperinflammatory state called cytokine storm. SARS-CoV-2 stimulates the activity of CD4+ T cells and their conversion into T helper (Th)−1 cells. The pathogenic Th1 cells release a number of factors including granulocyte-macrophage colony-stimulating factor (GM-CSF) that act for stimulation of inflammatory monocytes and macrophages. The inflammatory monocytes and macrophages intrude lung environment through leaky vessels mediated by vascular endothelial growth factor (VEGF) released from endothelial cells (ECs) of blood vessels. Among a number of factors released from monocytes, over-release of interleukin (IL)−6 takes important roles in the excessive cytokine release. High presence of IL-6 in the area will turn the tissue nearby into developing hypoxia and further damages to lung alveoli and capillary bed, manifested by EC disruption. This will hamper efficient delivery of oxygen, thereby causing shortness of breath, chest thickness and respiratory failure. Fibrotic lungs seen in imaging systems is a result of tissue damage in lung parenchyma. The hypoxic environment also facilitates promotion of coagulopathy-related events.

Lung is the main organ or cytokine storm in patients with SARS-CoV-2 induced disease [21]. High infiltration of macrophages and neutrophils into the lung and their increased presence within peripheral blood is correlated with severe lung damages in such patients [46]. A rise in the level of cytokines and infiltration of lungs with immune cells cause acute injury of lung capillary and epithelia/alveoli [1], which resulted in the inflammation and pneumonia [29]. The inflammation raised by cytokine storm spreads via systemic circulation within the whole body, causing serious infections and multiple organ dysfunctions [46]. Severe respiratory involvement or severe SARS-CoV-2 induced pneumonia is delineated by O2 saturation (SaO2) of 92% or lower [12], and the partial pressure of O2 (PaO2)/fraction of inspired O2 (FiO2) ≤300 mm Hg [48], [49].

Key notes IL-6 is a biomarker of hyperinflammation. IL-6 concentration can be assessed for monitoring the severity of SARS-CoV-2 induced disease.

Interleukin-6 in relation with hypoxia/hypoxemia in patients with SARS-CoV-2 induced disease

Hypoxia is a common condition occurring within the site/s of SARS-CoV-2-related inflammation [50]. Hypoxia is a feature of severe stage or phase III of SARS-CoV-2 induced disease [51]. Here, pro-inflammatory cytokines are induced by hypoxia [50], thereby exacerbating damages occurring due to inflammatory-related events. In patients with SARS-CoV-2 induced disease there is a thrombo-inflammatory feedback loop inside lung microvasculature, delineated by promotion of local hypoxia that further acts for exacerbating EC disruption and activation of coagulating-related events, which resulted in thrombosis and hemorrhage [52]. Normal T cell function occurs in healthy body when the fraction of O2 is about 5%, low concentration of which may cause T cell dysfunction [53]. Sabaka and colleagues in a report have made a link between IL-6 level with development of hypoxemia in patients with SARS-CoV-2 induced disease. The authors set the IL-6 concentration of 24 pg/mL or higher as a predictor of hypoxemia with high specificity (89%) and sensitivity (100%) [54].

Key note IL-6-mediates hypoxemia in SARS-CoV-2 induced disease.

Interleukin-6 in vasculopathy of SARS-CoV-2 induced disease

ECs play key roles for regulation of fibrinolysis, determination of vascular permeability and maintaining normal vessel homeostasis. SARS-CoV-2 induces local dysfunction of EC activity [52]. Ackermann and colleagues evaluated angiogenic pattern in patients with SARS-CoV-2 induced disease, and noticed higher rate of new vessel growth in this disease (by 2.7 times) compared with that for influenza. The type of angiogenesis which was predominant in patients with SARS-CoV-2 induced disease was intussuseptive angiogenesis [55]. EC dysfunction is linked to the development of thromboinflammatory processes in lung microvasculature [52].

Interaction between IL-6 with sIL-6R expressed on ECs is contributed to the acute EC activation [24], [52]. Secretion of VEGF is also induced by IL-6 in SARS-CoV-2 induced disease [56]. Increased expression of VEGF is associated with vascular abnormality, delineated by leakiness, hyper permeability and pulmonary dysfunction [24]. High expression of VEGF is also contributed to acute lung injury in patients with SARS-CoV-2 induced disease. Vascular leakiness, lung inflammation and hypoxia are inter-related events occurring as a response to high VEGF activity in these patients, which is indicative of the clinical significance of using VEGF inhibitors, such as bevacizumab [57].

Key note IL-6 mediates aberrant angiogenesis in SARS-CoV-2 induced disease.

Interleukin-6 in relation with acute and chronic inflammation

IL-6 is a marker of both acute and chronic inflammation [21]. IL-6 which shows a rise in patients with SARS-CoV-2 induced disease possesses the ability to regulate switching from acute toward chronic inflammation [7]. This means that in patients with persistent SARS-CoV-2 related lesions there is a possibility of developing chronic inflammation. Thus, untreated lesions remained in patients affected from SARS-CoV-2 seemingly promote a chronic inflammatory event, which may increase the risk of cancer development later in life.

Key note IL-6 links SARS-CoV-2 with chronic inflammation.

Interleukin-6 in obese patients with SARS-CoV-2 induced disease

Obesity is considered as a bona fide risk factor for mechanical ventilation in patients with SARS-CoV-2 [58], which increases the rate of mortality [59]. Obesity influences innate and adaptive immunity thus enhancing infection severity. Obese individuals are more prone for promoting dysfunctional ECs and lung and renal diseases, which is indicative of experiencing a more severe condition in obese patients affected from SARS-CoV-2 [60]. Obesity is a state of low-grade chronic inflammation characterized by high secretion of pro-inflammatory cytokines from adipose tissue [61]. IL-6 is a known mediator of inflammation in obese patients [62]. IL-6 signaling is induced by obesity [25], and its high concentration is linked with NK cell dysfunction [62]. Therefore, the severity of SARS-CoV-2 induced disease is presumably higher in obese individuals.

Key note Obesity increases IL-6 levels and is linked to a more severe SARS-CoV-2 induced disease.

Relation between interleukin-6 with checkpoints in SARS-CoV-2 induced disease

Expression of programed death-1 receptor (PD-1), programmed death ligand 1 (PD-L1) and cytotoxic T lymphocyte associated antigen-4 (CTLA-4) is increased on surface of T cells in patients with SARS-CoV-2 induced disease [63]. Xu an colleagues, however, reported no considerable difference in the rate of PD-1 expression on T cells between a critical vs. severe disease [64]. Reduced IL-6 level is accompanied by normalizing the expressions of checkpoints T-cell immunoglobulin and mucin domain-3 (TIM-3) and PD-1, and restoring the fraction of CD8+ T cells [33]. A point here is the impact of plasma IL-6 which acts as an unfavorable prognosticator. Application of immune checkpoint inhibitors (ICIs) for patients with SARS-CoV-2 induced disease can aggravate the condition, mediated via intensifying immune hyperactivation. It is also of noting that pneumonitis occurring as a side effect of ICI can be mistakenly interpreted as an outcome of SARS-CoV-2 induced disease [47].

Key note Intensification of immune hyperactive state hampers the efficacy of ICI in SARS-CoV-2 induced disease.

Interleukin-6 in relation with other biomarkers in patients with SARS-CoV-2 induced disease

The impact of interleukin-6 on T cell count

CD4+CD8+ T cell and natural killer (NK) cell count in peripheral blood is reduced in patients with SARS-CoV-2 induced disease [46], [65]. It was reported a reduced number of circulatory lymphocytes in 85% of severe/critical cases [3]. Reduction of circulatory T cells is also more pronounced in critical compared with severe cases, delineated by a state of severe immunosuppression in critically ill patients [64]. This is indicative of a particular reduction in the number of T cells in cases at intensive care unit (ICU). Diao and colleagues found a negative correlation between CD4+ T cell, CD8+ T cell and total T cell concentrations of respective 400, 300 and 800 per µL with survival of patients with SARS-CoV-2 induced disease [66]. Varchetta and colleagues reported enriched hyperactive T cells along with a rise in the fraction of terminally differentiated but functionally exhausted NK cells in patients with severe SARS-CoV-2 induced disease. Such alterations were deemed as poor prognostic markers [33]. Hyperactive T cells is a possible reason for T cell depletion or lymphopenia in these patients [14]. Lymphopenia occurs more in older patients compared to the children, indicating a reason for higher rate of mortality among aged individuals with SARS-CoV-2 induced disease [16].

IL-6 suppressive effect on T cell activity is a possible explanation for lymphopenia in patients with SARS-CoV-2 induced disease [67]. Belaid and colleagues evaluated circulatory IL-6 level and T cell count in patients from North Africa. They noticed a considerable rise in the IL-6 level in severe cases or dead individuals, and the rise in the rate of this cytokine was negatively related with T cell count within peripheral blood. Th17 cells show a rise in blood of patients with SARS-CoV-2 induced disease, whereas the number of Th2 cells was reduced. The authors directed a statistical link between IL-6 level (a rate higher than 106.44 pg/mL) and CD8+ T cell number (a rate lower than 150 cells per µL) with mortality in such cases [65]. In line, Diao and colleagues reported a negative correlation between T cell number with serum IL-6 level. The authors noticed reduction of IL-6 concentration along with the restoration of T cell counts upon disease resolution [66]. Yang and colleagues investigated the clinical significance of immune-inflammatory index in patients with SARS-CoV-2 induced disease and noticed a positive relation between high IL-6/lymphocyte ratio with adverse outcomes and severity of the condition [67].

Key notes IL-6 is negatively correlated with T cell counts in patients with SARS-CoV-2 induced disease. Measuring IL-6/lymphocyte ratio is of clinical importance.

Relation between interleukin-6 and granulocyte-macrophage colony-stimulating factor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a known mediator of cytokine storm [68]. High level of GM-CSF in patients with SARS-CoV-2 induced disease induces production of IL-6 [45]. Inter-relations between IL-6 with GM-CSF is indicative of using GM-CSF inhibitors, such as mavrilimumab as an approach for combating systemic hyperinflammation in patients with severe SARS-CoV-2 induced disease [68].

Key note Inter-relations between GM-CSF with IL-6 amplifies inflammatory state in patients with SARS-CoV-2 induced disease.

Interleukin-6 in relation with C-reactive protein

Elevation of the concentration of C-reactive protein (CRP) is another marker of hyperinflammation [12]. Broman and colleagues in a study evaluated the number of inflammatory markers in patients with SARS-CoV-2 induced disease and reported the importance of IL-6 and CRP as the biomarkers most relevant to the severity of the disease in hospitalized patients [69]. The effect of IL-6 inhibitors tocilizumab or sarilumab on mortality of patients with SARS-CoV-2 induced disease. Gavalli and colleagues in this study found that IL-6 inhibition had no conspicuous impact on reducing the risk of mortality. By contrast, patients with high CRP levels showed a considerable reduced risk of mortality [49]. This is indicative of the inter-relations between IL-6 and CRP with the importance of the evaluation for CRP in patients underwent IL-6 inhibitor therapy. The normal value for CRP is <6 mg/L, but in patients with SARS-CoV-2 induced disease it reaches a rate of over 100 mg/L [12]. Levels >75 mg/L of this protein is defined by irreversible tissue damage [15]. The CRP level defined for SARS-CoV-2 related hyperinflammation by Manson and colleagues is a level greater than 150 mg/L or a rate higher than 50 mg/L doubled within 24 h [70]. The rate defined for CRP by Gavalli and colleagues as a feature of hyperinflammation was ≥ 100 mg/L [49]. Cytokines like IL-6 promote an inflammatory event which stimulates production of CRP predominantly by liver cells [7]. IL-6 also induces CRP release from immune cells, so inhibition of this cytokine can normalize CRP levels [34].

Key note CRP level is a predictor of response to IL-6 inhibitor therapy.

Interleukin-6 in relation with ferritin

Ferritin is a protein combined with iron that its elevated level is associated with inflammation. An increase in the blood concentration of ferritin along with strikingly high level of IL-6 is associated with higher mortality from SARS-CoV-2 induced disease [59]. The rate of ferritin defined for SARS-CoV-2 related hyperinflammation by Manson and colleagues is a level higher than 1500 μg/L [70]. Gavalli and colleagues defined the ferritin level of 900 ng/mL or higher as the rate considered for hyperinflammation [49]. There is no study published yet referring directly to the inter-related IL-6/ferritin activity, but it seems that the level of ferritin is influenced positively from high SARS-CoV-2 induced IL-6 level.

Key note Ferritin level is seemingly induced by IL-6 surge and hyperinflammation in SARS-CoV-2 induced disease.

High interleukin-6 level and multi-organ dysfunction

Besides the impact of high IL-6 on lungs and lung injury, there are issues in regard with damages in other organs. Inflammatory events related to the high IL-6 level is a systemic phenomenon, thus causing damages in other organs [18]. Besides, The SARS-CoV-2 virus is not restricted to lungs. The virus is also detected in other organs including liver, pharynx, heart, brain and kidneys although at low concentrations, indicating the organotropism of SARS-CoV-2 [71]. One of the organs exposed to the harm effects of inflammatory surge is liver [18]. Effenberger and colleagues reported that in patients with SARS-CoV-2 induced disease systemic IL-6 was correlated with liver damage, delineated by high aspartate aminotransferase (AST) [72]. Diarrhea is the most frequent gastrointestinal symptom in such patients [73]. It was found a high level of IL-6 in patients experiencing diarrhea [74]. High IL-6 level can also cause intrarenal inflammation [75].

Hypertension is one of the most frequent comorbidities in patients with SARS-CoV-2 induced disease, experienced by 56.6% of cases. The presence of hypertension is, in fact, implying a pro-inflammatory state, which is seen in various infectious diseases [76]. IL-6 level is described to be related to a hypertensive state [77]. Increased serum level of IL-6 in hypertensive patients is related to either higher blood pressure and/or end-organ lesion [78]. Inflammation also perpetuates a hyper-coagulative state, mediated through EC damages that cause thrombosis seen in patients with SARS-CoV-2 induced disease. Interactions between SARS-CoV-2 with platelets promote activation of these cells, and the active cells further stimulate the release of inflammatory mediators, such as IL-6 [79]. Fibrinogen level is elevated in response to high IL-6 release, and it forms fibrin clots after conversion into fibrin in the presence of thrombin [79].

Several patients affected by SARS-CoV-2 induced disease die for virus-related myocarditis. Inflammation and IL-6 play a key role [80]. The negative effects of SARS-CoV-2 on myocardial tissue are magnified by the co-occurrence of type 2 diabetes [81] and are very often controlled by the anti-inflammatory role of heparin [82]. Systemic hyperinflammation can activate coronary ECs for promoting myocardial injury and ischemia. Myocardial damage is a result of upregulation of leukocyte adhesion molecules from activated ECs, which cause leukocyte transmigration into the myocardium. IL-6 takes central role in the transmigration of leukocytes toward peripheral tissues, such as myocardium [83]. A rise in the IL-6 level can cause hypoxemia [54], an outcome of which is myocardial ischemia [84]. Thus, inhibition of IL-6 can be protective against myocardial damage and ischemia. Augmented IL-6 signaling and the resultant cytokine storm can cause tachycardia and left ventricular dysfunction [85]. Fibrosis is developed in a bed of inflammation. IL-6 hyperactivity is linked to the pulmonary fibrosis [86], which is an outcome of SARS-CoV-2 induced disease [87]. Besides, myocardial inflammation and fibrosis is reported in over 78% of chronic cases [88], and due to the link between IL-6 with development of myocardial fibrosis [89] it is presumable to speculate also the impact of SARS-CoV-2 on IL-6 for promoting cardiac fibrosis ( Fig. 3).

Fig. 3

Serge of interleukin (IL)−6, cytokine storm and multi-organ injury.

Key note High IL-6 level and related systemic inflammatory events causes multi-organ dysfunction in SARS-CoV-2 induced disease.

Interleukin-6 in diabetic patients with SARS-CoV-2 induced disease

Among the leading causes of death in patients with SARS-CoV-2 induced disease, type-2 diabetes is one of the most important one. Several reports have shown that patients affected by type 2 diabetes may die easily than non-diabetic patients [90], [91] and also vaccination seems to be less effective in those patients with poor metabolic control [92]. Inflammation related type 2 diabetes [93] has a pivotal role and SARS-CoV-2 might enhance the IL-6 response [91]. The level of IL-6 is higher in hospitalized hyperglycemic patients with SARS-CoV-2 induced disease; in addition, responses to the IL-6 inhibitor tocilizumab therapy is depended greatly on glucose control. Hyperglycemic patients treated with tocilizumab are reported to have higher IL-6 levels compared with normoglycemic cases [94].

Key note Hyperglycemia is a barrier for IL-6 targeted therapy in patients with SARS-CoV-2 induced disease.

Interleukin-6 in patients with SARS-CoV-2 induced disease who also have cancer

Cancer is the second cause of death after cardiovascular disease worldwide [95]. Estimations have shown that about 80% of all types of cancers are derived from a number of solid organs including prostate, breast, ovary, colon and lung [96]. Tumor microenvironment (TME) of solid cancers is a dynamic niche that represents a number of cells and signaling with pro- or anti-inflammatory activities [97]. For a cancer like pancreas the extensive infiltration of immunosuppressive T cells is a barrier in cancer immunotherapy [98]. IL-6 is released from a number of cells in this milieu, and it takes pro-tumor activities, as shown in the Fig. 4.

Fig. 4

Interleukin (IL)−6 mediated immunosuppressive tumor microenvironment (TME). IL-6 is released from a number of pro-tumor cells in TME including cancer-associated fibroblasts (CAFs), cancer cells, macrophage type 2 (M2) cells and endothelial cells (ECs). High release of this cytokine into the TME mediates conversion of cancer cells into cancer stem cells (CSCs), promotes natural killer (NK) cell dysfunction, M2 polarity and suppresses dendritic cell (DC) differentiation. The outcomes of such modality in the TME are tumor aggression and therapy resistance.

As discussed, patients with SARS-CoV-2 induced disease represent cytokine storm, which is a devastating condition causing a more severe disease and multi-organ dysfunction. Cancer patients represent an immunosuppressive TME, which is a way for evading from tumor-killing immune cells and for promoting resistance [99], [100] and metastasis [101], rendering them low- or non-responsive to immunotherapy [102]. Thus, immunosuppression is an unfavorable outcome in cancer patients, but is that favoring patients with SARS-CoV-2 induced disease? This is quite a complicated question in deed, and response to such question could be both ‘yes’ or ‘no’. Immunosuppression in cancer patients can reduce the extent of cytokine storm caused by SARS-CoV-2 [103]. Thus, when observing from this perspective, cancer patients affected from SARS-CoV-2 presumably represent lower severity of symptoms related to the cytokine storm particularly in lung. Patients receiving IL-6 inhibitor agents may promote immunosuppression as a potential adverse event of these drugs [44], so it is presumable that anti-IL-6 therapy which is designated for treatment of patients with SARS-CoV-2 induced disease may favor progression of cancer due to promoting immunosuppression. However, the story has another side. In fact, in order to promote tumor progression, IL-6 must take anti-inflammatory activity. IL-6 is suppressed by tocilizumab in order to reduce the extent of inflammation. IL-6 acts for impairing the differentiation of pro-inflammatory Th1 cells in TME. In mice receiving IL-6 blockade therapy it was found restoration of impaired anti-tumor activity of CD4+ T cells and recovering anti-tumor activity of CD8+ T cells [104]. Cancer cells are another cell types that are affected from IL-6. IL-6 induces cancer cells to release factors with immunosuppressive activities including transforming growth factor (TGF)-β and lactate [105]. Besides its immunosuppressive role [106], TGF-β is a key mediator of fibrosis [107] and promoter of metastasis [108], [109]. An increase in the level of IL-6 occurs upon exposure to high-dose radiation [110], so radiation therapy may not be suggested in cancer patients who also have active SARS-CoV-2 virus.

Belaid and colleagues reported reduced blood number of Th2 cells in patients with SARS-CoV-2 induced disease [65]. Roncati and colleagues noticed the cytological signals from circulatory Th2 cells in such cases, which need intensive care [111]. In addition, patients with severe SARS-CoV-2 induced disease under the influence of factors like IL-6 promote a hyperactive Th1 cellular state rendering systemic anti-viral response [14] due to high state of immune activation. The hyperactive cells will turn into an exhausted state, which resulted in the reduction of their anti-tumor activity. Taken together, it seems that cancer patients are at the higher risk of experiencing damages occurring due to SARS-CoV-2 compared with non-tumor cases who are under the influence of such virus. Another interpretation is that SARS-CoV-2 may change tumor ecosystem into a more aggressive phenotype ( Fig. 5).

Fig. 5

Interleukin (IL)−6 in cancer patients who also have SARS-CoV-2 induced disease. The activity of anti-tumor T helper 1 (Th1) cells is reduced in tumors. By contrast, pro-tumor Th2 cells are active and promote anti-inflammatory and immunosuppressive signals. The resultant immunosuppression is indicative of lower extent of cytokine storm, and presumably lower extent of SARS-CoV-2 related damages in cancer patients. Th1 cells represent a hyperactive state in response to the factors like IL-6 released as a response to the severe SARS-CoV-2 induced disease, rendering systemic antiviral responses. However, a hyperactive cell type generally shows an exhausted state, which means the final decapitation for T cell-based immunity. Signals from T cells seen in patients with SARS-CoV-2 induced disease may be an explanation for further immunosuppression. Taken together, it seems that cancer patients are at the higher risk of experiencing damages due to SARS-CoV-2 compared with non-tumor patients who have active SARS-CoV-2 induced disease. Another interpretation is that SARS-CoV-2 may change tumor ecosystem into a more aggressive phenotype. Star indicates a hyperactive state, and the question mark represents uncertainty of the SARS-CoV-2 action on Th2 cell activity.

Key note SARS-CoV-2 seemingly strengthens cancer aggression, and IL-6 is a mediator of this event.

Interleukine-6 targeting strategies against SARS-CoV-2

Inhibitors of IL-6 or antagonists of IL-6R can be used as immunomodulatory agents for patients with SARS-CoV-2 induced disease [1].

Tocilizumab for treatment of SARS-CoV-2 induced disease

Tocilizumab (also called atlizumab [the trade name: Actemra]) is a humanized monoclonal antibody (mAb) [2] and a drug tested for treatment of severe SARS-CoV-2 induced disease [45]. Tocilizumab binds specifically to both mIL-6R and sIL-6R and suppresses further signal transduction [3]. Tocilizumab competes for IL-6Rs in order to block them and the result is to leave free IL-6 within plasma [5]. The free IL-6 is unable to cause immune damage on cells of target, thereby reducing the rate of inflammatory responses [3]. Tocilizumab is administered for patients with arthritis and CAR-T-induced CRS [5].

Perrone and colleagues reported that tocilizumab was seemingly more effective in cases who did not need mechanical respiratory support [2]. Guaraldi and colleagues also found reduction of the risk for invasive mechanical ventilation (IMV) in tocilizumab-treated patients with severe pneumonia [5]. Galvn-Roman and coworkers evaluated serum IL-6 in patients treated with tocilizumab, and designated 30 pg/mL or higher levels of this cytokine as the best predictor of IMV [112]. Biran and colleagues found that in patients who required ICU, administration of tocilizumab attenuated the rate of mortality. Tocilizumab represents clinical efficacy in patients with CRP of ≥ 15 mg/dl [30]. Xu and colleagues reported reduced CRP activity in 16/19 and increase in the number of circulatory lymphocytes in 10/19 of patients treated with tocilizumab [3].

Soin and colleagues performed a phase 3 trial in Indian population receiving 6 mg/kg tocilizumab and noticed no considerable difference in the primary or secondary endpoints among standard care patients receiving tocilizumab. The authors represent no essence for routine administration of tocilizumab in patients with SARS-CoV-2 induced disease [113]. Stone and colleagues in a study evaluated the effect of tocilizumab in hospitalized patients. Tocilizumab had no effect on preventing death or intubation of moderately ill patients. However, severe infections were lower in tocilizumab vs. placebo [114]. The study was continued by Salama and colleagues evaluating hospitalized patients with SARS-CoV-2 related pneumonia. Tocilizumab reduced the death rate or the need for mechanical ventilation (12% vs. 19.3% in placebo) at 28 days after therapy, but as stated by authors no improvement in survival was seen [115]. In line with these studies, Rosas and colleagues performed a work by investigating tocilizumab effects on patients with severe SARS-CoV-2 related pneumonia. Evaluations after 28 days showed no superior activity of tocilizumab over placebo in terms of the clinical status and reduced mortality [116]. Campochiaro and colleagues evaluated the efficacy of 400 mg tocilizumab in severe cases. They noticed 15% mortality rate and 69% clinical improvement in tocilizumab-treated patients. As compared to the respective 33% and 69% in standard care patients, the outcomes were more favorable for tocilizumab therapy but were not significant statistically. The authors also noticed rather similar serious adverse events related to the tocilizumab therapy (25%) compared with the standard care group (27%) [12]. Finally, Salvarani and colleagues also noticed no benefit of using tocilizumab over standard care for reducing the risk of disease progression in SARS-CoV-2 related pneumonia [117]. From the outcomes of these studies it could be asserted that tocilizumab therapy is safe. The application of this IL-6 inhibitor although is effective for patients with SARS-CoV-2 induced disease particularly in cases who do not need mechanical ventilation, its efficacy is moderate in cases with a severe condition ( Table 1).

Table 1

Interleukin (IL)−6 antagonists for patients with SARS-CoV-2 induced disease.

Dose and number of patients (n)	Clinical outcomes	Safety	Ref.
Tocilizumab
8 mg/kg up to 800 mg/per dose, n = 180	reduced lethality	no specific toxicity	[2]
8 mg/kg up to 800 mg/per dose, n = 60	no benefit over standard care for reducing the risk of disease progression	adverse events in 23.3% of patients compared with 11.1% in standard care	[117]
4–8 mg/kg up to 800 mg/per dose, n = 20	reduced CRP reactivity in 84.2% of cases	no adverse reactions	[3]
	circulatory lymphocytes returned to normal in 52.6% of cases
	all patients were discharged from hospital
6 mg/kg, n = 91	disease progression (14 days after therapy): 9% vs. 13% in standard care group.	adverse events in 36% of patients vs. 25% in standard care	[113]
8 mg/kg, n = 161	disease progression (14 days after therapy): 18% vs. 14.9% in placebo	serious infections: 8.1% vs. 17.3% in placebo	[114]
8 mg/kg, n = 249	mechanical ventilation or death at day 28 after therapy: 12% vs. 19.3% in placebo.	severe adverse events: 15.2% vs. 19.7% in placebo	[115]
8 mg/kg, n = 294	death at day 28 after therapy: 19.7% vs. 19.4% in placebo	serious adverse events: 34.9% vs. 38.5% in placebo	[116]
400 mg, n = 32	clinical improvement and mortality: 69% and 15% vs. 61% and 33% in standard care	similar pulmonary thrombosis and infection rates between tocilizumab and standard care groups	[12]
Sarilumab
400 mg, n = 28	clinical improvement at day 28 after therapy: 61% vs. 64% in standard care group.	similar pulmonary thrombosis and infection rates between sarilumab (1 case, 21%) and standard care (1 case, 18%) groups	[48]
200 and 400 mg, n = 159 and 173	no advantage over placebo in terms of clinical efficacy.	similar safety profile with that for placebo	[4]
8 mg tocilizumab (n = 353) or 400 mg sarilumab (n = 48)	Both drugs showed improved survival at day 90 after therapy	–	[11]
CRP, C-reactive protein

Srilumab (kevzara)

In patients treated with sarilumab lung consolidation lower than 17% is a predictive marker of clinical improvement. Della-Torre and colleagues evaluated the efficacy of 400 mg sarilumab in patients with severe SARS-CoV-2 related pneumonia and compared the outcomes with that of standard care group. There were no significant differences in clinical improvement in sarilumab-treated patients, but they showed faster recovery, depicted by shorter time for reaching the designated lung consolidation criteria at CT scan (10 days vs. 24 days for standard care group) [48].

Lescure and colleagues noticed sharper difference between 400 mg sarilumab with placebo in the seven-point ordinal scaling of the first two weeks compared to the second two weeks, but the clinical efficacy was not beyond what seen for placebo. There were also superior survival benefits for sarilumab only in patients at ICU or under extensive respiratory support, which is indicative of the benefit of immunomodulation only for patients more serious disease. Here, a single dose of the drug was used, and it was only effective for the first two weeks, not beyond that time. As mentioned in this study, the ordinal scaling is effective mainly for patients with lung involvement, not in cases with multi-organ issues [4]. Sinha and colleagues reported the high benefit from sarilumab or tocilizumab when treatment was started early in the disease course [118]. In regard with the superior activity of anti-IL-6 therapy for critically ill patients, Gordon and colleagues carried out a study evaluating the efficacy of tocilizumab or sarilumab, and noticed an improvement in 90-day survival in treated patients [11] (Table 1).

Key note Patients with Severe/critical SARS-CoV-2 induced disease are the most desired cases for sarilumab therapy, whereas tocilizumab is effective particularly for cases who do not need mechanical ventilation.

Potential challenges for targeting interleukin-6 in patients with SARS-CoV-2 induced disease

One of the challenges raised in the context of IL-6 and its targeting in patients with SARS-CoV-2 induced disease is that this cytokine is not the sole mediator of hyperinflammation. Other molecules, such as tumor necrosis factor (TNF)-α, interferon (IFN)-γ, monocyte chemoattractant protein (MCP)−1 and other IL family including ILs 1 (1β), 2, 7, 8, 10, 12, 16, & 18 are also released, and are able to promote a widespread damage [15], [45], [47]. IL-1, for instance, is a highly active pro-inflammatory cytokine [119] that is believed to precede IL-6 release. Due to the inducible effect of IL-1 on release of IL-6 and sIL-6R, it is presumable that IL-1 is the main actor in initiation of CRS [120]. This indicates that targeting IL-6 is not a full-proof for quelling the inflammatory phase of SARS-CoV-2 induced disease [4]. Other cytokines are also interfering with the outcomes. Leisman and colleagues in a systematic review evaluated serum level of IL-6 in three groups of patients. There was lower IL-6 concentration in patients with severe/critical condition (36.7 pg/mL) compared with patients with acute respiratory distress syndrome (460 pg/mL), sepsis (983.6 pg/mL) and CRS (3110.5 pg/mL). The interpretations of this systematic review will put into question the importance of SARS-CoV-2 related cytokine storm in organ dysfunction. The outcomes can also be interpreted in another way in which the involvement of other factors are also important for promotion of cytokine storm [121]. However, it is important to note that the activity of IL-6 in severe SARS-CoV-2 induced disease is central compared with the network of other cytokines [15], and is a better representative of systemic inflammation [72].

Another challenge could possibly be that inhibitors of pro-inflammatory cytokines used for controlling hyperinflammation adversely cause immunosuppression. CIGB-258, for instance, is a peptide that exerts anti-inflammatory activity, mediated via augmenting the activity of Tregs and further suppression of CD4+ effector T cells. Such immunosuppression may negatively impact the outcomes [44]. Secondary infections, however, are not increased significantly in patients with a severe disease who are receiving anti-IL-6 drugs compared with the standard care group. This is a result of systematic review by Han and colleagues [122]. The interpretation is that although anti-inflammatory agents pose a suppressive immune activity, their potential harm to the body is not beyond what seen for patients receiving standard care regimens.

Key note Blockade of IL-6 is not a full-proof of complete response.

Conclusions and future perspectives

Screening for hyperinflammation is a strategy in patients with SARS-CoV-2 induced disease. When the presence of hyperinflammation is approved, immunosuppression could be a suggested strategy, as indicated elsewhere [123]. Immunosuppression strategies, however, are not suggested for patients who also have cancer, due to progressing toward more aggressive or metastatic disease. IL-6 inhibitors show moderate response in patients with SARS-CoV-2 induced disease and are effective for reducing adverse effects related to the CAR-T therapy. IL-6 is linked positively with hypoxia, hypoxemia, chronic inflammation and aberrant angiogenesis, and careful evaluation for persistent inflammation or inflammatory-related events is suggested in order to reduce undesired devastating conditions in the future. Serum IL-6 is proposed to be considered in management and treatment of SARS-CoV-2 induced disease [119], and it is suggested to evaluate the rate of this cytokine particularly in cancer patients who also have SARS-CoV-2 induced disease.

CRediT authorship contribution statement

Collection and revision of information. KM., Conceptualization, JM and KM. Writing, original draft preparation, review and editing, JM, and KM. Two authors have read and agreed to publish of the manuscript.

Conflict of interest statement

None.