4.1. IL-6
IL-6 plays multiple biological roles, which include enhancing the synthesis of inflammatory response proteins, such as C-reactive protein (CRP), regulating both T and B immune cells to increase immune responses, and promoting the production of vascular endothelium growth factor (VEGF), which aggravate the inflammatory damage by inducing angiogenesis and impairing the vascular integrity. In addition to its immune and inflammatory regulatory activities, IL-6 also upregulates the activation of the coagulation pathway, increasing the thrombotic events (D’Alessandro et al., 2020).
The proinflammatory function of IL-6 is mediated through a series of cell signaling pathways. IL-6 binds to its receptor (IL-6R) to initiate intra cellular signaling. Two forms of IL-6R have been found: membrane IL-6 receptor (mIL-6R) and soluble IL-6 receptor (sIL-6R). The binding of IL-6 to both receptor results in the dimerization and activation of the glycosylated type I membrane protein of 130–150 kDa (gp130). IL-6/mIL-6R medicated activation of gp130 induces IL-6 classic signaling pathway; while IL-6/sIL-6R induced the activation of gp130 leads to activation of IL-6 trans-signaling pathway (Ebihara, Matsuda, Nakamura, Matsuda & Murakami, 2011). Classic IL-6 signaling is generally regarded as anti-inflammation and protective pathway, while IL-6 trans-signaling is mainly pro-inflammatory pathway (Garbers, Aparicio-Siegmund & Rose-John, 2015). Blockade of IL-6 trans-signaling, not the full blockade of IL-6 signaling, prevents inflammation (Barkhausen et al., 2011).
The dimerization of signaling receptor gp130 mediates the activation of Janus kinases (JAKs) and subsequent activation of phosphatase Src homology domains containing tyrosine phosphatase-2 (SHP-2), the ras/raf/MAPK pathway, signal transducer and activator of transcription factor-3 STAT-3) (Wang S, Zhang W 2016), and PI3K/Akt (Zegeye et al., 2018), which are translocated into the nucleus to activate target genes (Figure 1).
IL-6 has been closely associated with SARS-CoV-2 induced cytokine storm. IL-6 is one of the key cytokines that is frequently reported to be increased in COVID-19 patients with hypercytokinemia (Kaman, Azmy, Chichra, Britto-Leon & Price, 2021). The level of IL-6 is valuable as a prognostic parameter for the disease severity in COVID-19 patients (Liu et al., 2020). Increasing IL-6 concentrations are associated with the requirement of ventilatory support, the progression of ARDS, and the risk of death (Herold et al., 2020; Wu et al., 2020a). Although a positive correlation between IL-6 and CRP exists in COVID-19 patients, IL-6 appears to be a better predictor for the disease progression, since IL-6, not CRP, levels were significantly lower in survivors than in non-survivors of all age groups of COVID-19 patients, suggesting that IL-6 is predictive of in-hospital mortality after SARS-CoV-2 infection (Santa Cruz et al., 2021; Zhang et al., 2020).
Since IL-6 functions through its receptor and the monoclonal antibody against IL-6R has been developed to prevent the activation of IL-6R by IL-6. Tocilizumab is a monoclonal antibody against IL-6R that has been approved by the US FDA for the treatment of cytokine release syndrome. It is also rational to expect tocilizumab to be capable of attenuating the cytokine storm induced by SARS-CoV-2 (Pelaia, Calabrese, Garofalo, Bruni, Vatrella & Pelaia, 2021).
Primary data using tocilizumab demonstrated some benefits for critical COVID-19 patients. Tocilizumab can rapidly resolve the symptoms. In severe COVID-19 patients, fever returned to normal on the first day after tocilizumab administration; other symptoms were improved remarkably within a few days: 75.0% of patients reduced their requirement of oxygen intake within 5 d; and the percentage of lymphocytes in peripheral blood returned to normal in 52.6% of patients on the fifth day after treatment (Xu et al., 2020).
In severe to critical COVID-19 disease with hypercytokinemia in ICU, tocilizumab significantly reduced inflammatory response illustrated by the decrease in CRP, decreased the requirement of respiratory support, and lowered the mortality (Nasa et al., 2020). The cases with unfavorable outcomes after tocilizumab treatment were linked to the failure in reducing the level of CRP (Luo, Liu, Qiu, Liu, Liu & Li, 2020). In addition, since IL-6 is the main inducer of CRP production in the liver during the acute phase response (Sproston, El Mohtadi, Slevin, Gilmore & Ashworth, 2018), tocilizumab seemed to be more effective in patients with markedly elevated CRP (> 200 mg/L) . Systemic review of the online data suggests that addition of tocilizumab to the standard of care might reduce mortality in severe COVID-19; however, the inconsistency in the effectiveness exists among patients with reduced severity (Boregowda, Perisetti, Nanjappa, Gajendran, Kutti Sridharan & Goyal, 2020; Gorgolas Hernandez-Mora et al., 2021).
The severity of COVID-19 is an important parameter that affects the effectiveness of tocilizumab, since tocilizumab given to severe, not critical (requirement of intubation or ICU admission), SARS-CoV-2 pneumonia patients increased the requirement of intubation and elevated the mortality. In contrast, in patients with COVID-19 pneumonia and life-threatening acute respiratory distress syndrome (ARDS) requiring ventilatory support, intravenous administration of tocilizumab significantly improve the patients’ respiratory conditions and downregulate the inflammatory markers (Conrozier et al., 2020; Toniati et al., 2020). However, two phase III clinical trials include an earlier one by Roche (Roche, 2020) and a recent report (Rosas et al., 2021), the use of tocilizumab in hospitalized patients with severe COVID-19 pneumonia did not result in significantly better clinical status or lower mortality.
In conclusion, the effectiveness of tocilizumab might be subjected to various factors, such as the severity of the disease and the inflammatory status. The value of tocilizumab in the treatment of COVID-19 should be further evaluated. One side effect of tocilizumab is that its administration in COVID-19 patients significantly increased IL-6 levels (Antwi-Amoabeng, Kanji, Ford, Beutler, Riddle & Siddiqui, 2020). Tocilizumab only blocks IL-6R without effects on the production of IL-6. Blockade of IL-6R might reduce the IL-6 consumption that leads to an increase in IL-6 (Nishimoto, Terao, Mima, Nakahara, Takagi & Kakehi, 2008). The concern is whether higher IL-6 levels after tocilizumab treatment in COVID-19 patients would have adverse effects or exacerbate the CNS inflammation due to impermeability of BBB to tocilizumab (Zhang et al., 2020).
Since it is the trans IL-6 pathway that has been associated with inflammation, the specific antagonists for this pathway might increase the efficacy and reduce the side effects. The development of regulators of trans-IL-6 signaling pathway may hold better promise for the treatment of COVID-19.