Can we use interleukin-6 (IL-6) blockade for coronavirus disease 2019 (COVID-19)-induced cytokine release syndrome (CRS)?

The newly emerging coronavirus disease 2019 (COVID-19), first reported in Wuhan, China, has swept across 202 countries with stunning mortality. The World Health Organization (WHO) has declared this deadly outbreak a pandemic, with tremendous ramifications impacting every life. By March 27, 2020, the number of deaths had climbed to 23,495 among 512,701 confirmed cases in WHO reports [1]. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel beta-coronavirus, has been identified as the pathogen for COVID-19 [2]. This strain has been the third most lethal pathogenic human coronavirus, following severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses in 2003 and 2012, respectively. SARS-CoV-2 _targets the lung and likely other organs as well, leading to multiorgan damage by binding to the angiotensin-converting enzyme 2 (ACE2) receptor [2], a cell surface protein highly expressed in the lung, heart and kidney [3].

Clinical data from Wuhan, China, showed that approximately 17.7–32.0% of patients require intensive care unit (ICU)-level care, with approximately 9.5–12.0 days from symptom onset to multiorgan dysfunctions, namely, acute respiratory distress syndrome (ARDS) (67%), acute kidney injury (29%), acute cardiac injury (23%), and liver dysfunction (29%) [[4], [5], [6]]. The mortality of critically ill patients is as high as 49.0–61.5% [4,5]. Evidence suggests that CRS might play a major role in severe COVID-19. Inflammatory cytokines and chemokines, including interleukin-6 (IL-6), interleukin-1β (IL-1β), induced protein 10 (IP10) and monocyte chemoattractant protein-1 (MCP-1) were significantly elevated in COVID-19 patients, and some were more commonly seen in severe patients than in nonsevere patients (Table 1 ). In COVID-19 patients with elevated inflammatory cytokines, postmortem pathology has revealed tissue necrosis and interstitial macrophage and monocyte infiltrations in the lung, heart and gastrointestinal mucosa [7,8]. Moreover, severe lymphopenia with hyperactivated proinflammatory T cells [8] and decreased regulatory T cells [9] is commonly seen in critically ill patients, suggesting dysregulated immune responses.

CRS refers to an uncontrolled and overwhelming release of proinflammatory mediators by an overly activated immune system [10]. CRS is a common immunopathogenesis underlying many pathological processes, such as ARDS, sepsis, graft-versus-host disease (GvHD), macrophage activation syndrome (MAS) induced by rheumatic diseases, and primary and secondary hemophagocytic lymphohistiocytosis (HLH) [11]. Recently, CRS has also been reported to be a complication of immunotherapies, such as chimeric antigen receptor (CAR) T cell therapies [12]. Previous experience with SARS and MERS has also revealed florid CRS in critically ill patients (Table 1). Studies have shown that ARDS occurs in some SARS patients despite a diminishing viral load, suggesting that an exuberant host immune response rather than viral virulence is possibly responsible for tissue pathologies. Therefore, antiviral therapy alone may be inadequate [13]. Corticosteroids, one of the most widely utilized anti-inflammatory agents, are still commonly prescribed in treating COVID-19 patients (72.2% in the ICU setting) [14]. However, as outlined in the Chinese guidelines of COVID-19 [15], physicians need to be cautious of steroid use due to its nebulous benefits in the setting of viral respiratory infection. Several studies even reported inferior outcomes of SARS patients treated with corticosteroids [16]. Another concern of corticosteroids is their short- and long-term adverse effects. More than half of SARS patients treated with corticosteroids suffer from joint pain and bone marrow abnormalities [17]. Other therapies aiming to dampen excessive serum inflammatory mediators, such as plasmapheresis or continuous renal replacement therapy (CRRT), either require specific equipment or lack documented efficacy [18]. Thus, there is still an unmet need for the treatment of COVID-19-induced CRS [19]. In the past decade, immunotherapy has made great strides in managing CRS of various etiologies, including autoimmunity, malignancy and CAR T cell therapies (Table 2 ). We propose herein that attenuating the detrimental host immune response by immunomodulators may be a beneficial addition to antiviral therapy.

A better understanding of the pathogenesis underlying CRS may facilitate the design of novel immunotherapies. The immunologic mechanism of CRS induced by coronaviruses is not fully elucidated, and existing data are largely derived from SARS coronavirus (SARS-CoV), a close counterpart of SARS-CoV-2. It is believed that delayed kinetics of virus clearance are the trigger. The delayed type I interferon (IFN) response plays a pivotal role in the process of SARS. In the initial phase, SARS-CoV evades pattern recognition receptors (PRRs) and antagonizes the type I IFN response by inducing double-membrane vesicles that lack PRRs, mRNA capping and proteins that inhibit PRR downstream cascades [20,21]. The dampened type I IFN in airway and alveolar epithelial cells results in rapid viral replication. Plasmacytoid dendritic cells (pDCs) and macrophages are exceptions, with a full response to SARS-CoV, launching a delayed but robust type I IFN response and releasing other inflammatory cytokines against SARS-CoV [21,22]. Consequently, the activation of type I IFN signaling cascades induces extensive IFN-stimulated gene (ISG) expression and attracts inflammatory monocyte-macrophages (IMMs), neutrophils, dendritic cells and natural killer cells to the lung. This process amplifies the innate response, forming a cytokine-driven vicious cycle [21]. The virus-specific T cell immune response is indispensable for virus clearance, an essential step in protecting mice from lethal SARS-CoV infection [23]. Both regulatory T cells and naïve T cells negatively regulate the activated innate immune responses by cell-cell interactions [24]. Exuberant production of cytokines, such as type I IFN, diminishes T cell responses by inducing T cell apoptosis to aggravate CRS and lymphopenia, as observed in SARS patients [21,25]. The overwhelming proinflammatory cytokines and chemokines cause localized pulmonary injury characterized by diffuse alveolar damage with epithelial and endothelial apoptosis, dysregulated coagulation and pulmonary fibrinolysis. They may also leak into systemic circulation to cause extrapulmonary manifestations and eventually multiple organ dysfunction syndrome [26,27].

Among the excessive cytokines produced by activated macrophages, IL-6 is one of the key cytokines. Elevated IL-6 levels were observed in patients with SARS and were correlated with disease severity (Table 1) [28]. IL-6 activates its downstream Janus kinase (JAK) signal by binding the transmembrane (cis-signaling) or soluble form (trans-signaling) of the IL-6 receptor (IL-6R) and interacting with membrane-bound gp130 [29]. Excessive IL-6 signaling leads to a myriad of biological effects that contribute to organ damage, such as maturing naïve T cells into effector T cells, inducing vascular endothelial growth factor (VEGF) expression in epithelial cells, increasing vessel permeability [30], and reducing myocardium contractility [31].

The elevated cytokine levels may also be responsible for the lethal complications of COVID-19. As shown in Table 1, patients with COVID-19, SARS or MERS presented distinct cytokine profiles. Patients with COVID-19 presented elevated T helper 2 cytokines (interleukin-4) in addition to T helper 1 cytokines compared to those in patients with SARS or MERS. There are many potential therapies _targeting the host immune system that may be effective for COVID-19, such as inflammatory cytokine blockade (IL-6, IL-1, and IFN), stem cell therapy, immune cell depletion, transfusion of convalescent plasma and artificial extracorporeal liver support [32], among which we believe IL-6 blockade is a promising strategy for COVID-induced CRS. We noticed that elevated IL-6 levels were consistently reported in several studies of COVID-19 [[33], [34], [35], [36]] and might serve as a predictive biomarker for disease severity [37]. A large retrospective cohort study found that IL-6 levels were correlated with mortality in patients with COVID-19 [6]. Mechanistically, IL-6 is essential for the generation of T helper 17 (Th17) cells in the dendritic cell-T cell interaction [30]. The excessive IL-6 may explain the overly activated Th17 cells observed in COVID-19 patients, as reported by Xu et al. [8]. Although clinical data of IL-6 blockade in virus infection-related CRS are unavailable, animal studies of SARS-CoV have demonstrated that inhibiting nuclear factor kappa-B (NF-κB), a key transcription factor of IL-6, or infecting animals with SARS-CoV lacking the coronavirus envelope (E) protein, a strong stimulus to NF-κB signaling, increased animal survival, with reduced IL-6 levels [38]. Interestingly, we noticed that the E proteins of SARS-CoV-2 (Ref sequence QHD43418.1) and SARS-CoV (Ref sequence NP_828854.1) share 95% homology. Since the E protein is the determinant of virulence and mediates the host immune reaction to coronavirus [39,40], it is reasonable to speculate that both viruses elicit a similar immune response. Hence, _targeting IL-6 may be effective for COVID-induced CRS.

Tocilizumab is a recombinant humanized monoclonal anti-IL‐6R antibody. It binds both soluble and membrane‐bound IL‐6R to inhibit IL‐6‐mediated cis- and trans-signaling [41]. Tocilizumab has been approved by the U.S. Food and Drug Administration for the treatment of severe CAR T cell‐induced CRS (Table 2) [12]. As mentioned earlier, CRS is the most severe adverse effect induced by CAR T cell therapy, with an incidence of 50–100% [41]. It is believed that binding of the CAR T cell receptor to its antigen induces the activation of bystander cells to release massive a mounts of interferon γ (IFN-γ) and tumor necrosis factor-α (TNF-α), which further activate innate immune cells, including macrophages and endothelial cells, to secrete IL-6 and other inflammatory mediators [42]. IL-6 is a central mediator of toxicity in CRS, and its level correlates with the severity of CAR T cell‐induced CRS [12,43]. Clinically, severe cases of CAR-T induced CRS present with fever, hypoxia, acute renal failure, hypotension, and cardiac arrhythmia that often warrants ICU admission [12]. Tocilizumab showed promising efficacy in severe CRS. After one or two doses of tocilizumab, 69% of patients responded within 14 days, for whom fever and hypotension resolved within hours, and vasopressors could be weaned quickly in several days [10,41]. The effect of tocilizumab has also been reported in CRS related to several other conditions, such as sepsis, GvHD and MAS [[44], [45], [46]]. Moreover, tocilizumab is safe for both pediatric and adult patients, as no adverse reactions have been reported in a retrospective analysis of patients with CAR T cell-induced CRS [41]. The most common serious adverse effect is infections in patients with rheumatoid arthritis, in which chronic therapy is maintained for a longer period of time (3.11–3.47/100 person-years with 8 mg/kg tocilizumab every 4 weeks) [47]. Moreover, a possible correlation between tocilizumab and medication-related osteonecrosis of the jaws was reported in patients with osteoporosis [48].

Given the efficacy of tocilizumab in CRS and the pivotal role of IL-6 in COVID-19, we propose to repurpose tocilizumab to treat severe cases of COVID-19. Regarding its clinical use, we suggest taking the following factors into consideration and hope that future clinical trials will be able to address them. 1) Diagnosis criteria. There is currently no consensus in diagnosing CRS in COVID-19. Early diagnosis of CRS in COVID-19 patients and prompt initiation of immunomodulatory treatment may be beneficial, as suggested by the experience in HLH [49]. Prompt screening of COVID-19 patients with Hscore, a diagnostic score for HLH, may help to discriminate patients with CRS [50].2) Disease severity grading system. Experience with immunotherapy-triggered CRS suggests that tocilizumab is indicated only for severe cases, while the risk benefit assessment favors symptomatic management for mild cases [10]. This approach is rationalized by the worry that aggressive antiinflammation therapy may negate the effect of therapeutic biologicals, such as CAR T cells. This principle is not shared in viral infections, such as COVID-19, in which timely intervention in mild or moderate patients may prevent progression. A disease severity grading system may provide an objective tool to assess the most appropriate timing to initiate tocilizumab treatment. Currently, the Chinese guidelines for COVID-19 grade patients into mild, moderate, severe and critical by vital signs, radiographic findings and complications [51]. It is currently unclear which population may benefit the most from the treatment. 3) Combined antiviral treatment. Based on experience with corticosteroids, immunosuppressive agents may delay virus clearance. Combining immunomodulators with antiviral agents may add further benefit. Preliminary results from clinical trials of several antiviral treatments are expected to be available soon (remdesivir [NCT04252664, NCT04257656], favipiravir [ChiCTR2000029600, NCT04310228] and chloroquine [ChiCTR2000029609, NCT04286503]). 4) Secondary infection. Infection is a common adverse effect associated with immunomodulators such as tocilizumab. Critically ill COVID-19 patients are susceptible to secondary infection and may have an increased risk of comorbid chronic infections, such as hepatitis B and tuberculosis [5]. It is unclear to what degree tocilizumab contributes to secondary infection. Hence, the goal of treatment is to prevent or attenuate life-threatening inflammation while minimizing the potential of secondary infection. For this reason, prophylactic antibiotics may be indicated, and bacteriologic and fungal assessments are of great importance. For patients with secondary infection or coexisting chronic infection, the utilization of tocilizumab should be cautious. 5) Cytokine measurement. Cytokine levels may serve as biomarkers for risk stratification and prognosis. A previous cohort study suggested that IL-6 levels were significantly elevated in COVID-19 patients but varied considerably among both ICU and non-ICU patients [34]. This observation raises the question of whether IL-6 blockade is effective only in patients with elevated serum IL-6 levels. If so, IL-6 measurement may be an indispensable part of the grading system. Moreover, the IL-6 level alone may not be sufficient to reflect its functional downstream effects [52]. An assay that distinguishes functional IL-6 from total IL-6 may provide a refined approach to guide therapeutic decisions. C-reactive protein (CRP), an acute-phase inflammatory protein synthesized by IL-6-dependent hepatic biosynthesis, is a reliable marker of IL-6 bioactivity and is used to predict CRS severity and monitor IL-6 blockade efficacy for patients with CAR T cell-induced CRS [10,12]. The CRP level in virus-induced CRS remains to be determined. Most studies suggested that elevated CRP levels were associated with severe COVID-19 [37,53,54], with a few exceptions [35]. Nevertheless, future studies on biomarkers are needed for the purpose of risk stratification and therapeutic effect monitoring. There is also a battery of biological agents available that _target various critical molecules in the inflammatory network (Table 2), such as IL-1, IL-18, TNF, and IFN, or Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling. These agents may also be beneficial, and if so, routine inflammatory cytokine measurement is warranted.

Notably, SARS-CoV, MERS-CoV and SARS-CoV-2 were all considered to have originated in bats, a nature reservoir of various coronavirus species with high genomic diversity [55,56]. It is unclear how many bat coronaviruses are directly or indirectly transmissible to humans and how many have the potential to cause disease, especially for those that share the viral spike sequence and are capable of using the human ACE2 receptor for entry [55]. Thus, it is highly likely that a novel bat coronavirus could cause future epidemics. For future epidemic preparedness and to reduce mortality in COVID-19 patients, global effort is needed to promote novel therapy to treat virus-induced CRS during the COVID-19 outbreak. Potential therapies available for CRS are summarized in Table 2. We hope that this assessment will spur future clinical trials on COVID-19-induced CRS. Utilizing biologicals such as tocilizumab to treat virus-induced CRS is a new field. Many other therapeutic options, including hydroxychloroquine combined with azithromycin (NCT04322123, NCT04321278) [57], mesenchymal stem cell therapy (NCT04269525, NCT04252118) and convalescent plasma (NCT04292340), have moved into clinical trials for COVID-19. We look forward to seeing additional exciting progress and clinical evidence in this area.