**7. Plausible mechanism of tocilizumab against COVID-19**

According to a study, by the team of Haiming Wei [25], after the SARS-CoV-2 infection, CD4 + T lymphocytes are activated to become pathogenic T helper cells, generating GM-CSF (Granulo Macrophage Colony Stimulating Factor]. This leads to severe inflammatory storm created by CD14 + CD16+ inflammatory monocytes with elevated expression of IL-6. These excessive immune cells usually invade the pulmonary circulation and cause damage to the immune system, thus leading to functional disability of lungs and mortality. Therefore, drugs like Tocilizumab are administered to prevent the cytokine storm. Tocilizumab has yielded effective results as an IL-6R antagonist.

Excessive stimulation of IL-6 can cause CRS [Cytokine Release Syndrome] in hospitalized patients. The higher the level of CRS, higher is the serum peak concentration of IL-6. IL-6 binds to its receptor IL-6R and a complex is formed. IL-6R then binds to the signal transducer glycoprotein 130 (gp-130) to cause signal transduction. Two types of IL-6R are there, one is the Soluble form (sIL-6R) and the other is Membrane bound form [mIl-6R]. In classical signal transduction pathway, IL-6 binds to mIL-6R [transmembrane integral protein], and forms a complex, which then prohibits the connection of IL-6R with gp130 [integral membrane protein]. Thus no cytokine storm is produced. In the trans-signaling pathway, binding of Tocilizumab to sIl-6R, prevents the binding of IL-6R to gp130 [present on the membrane of monocytes, macrophages, dendritic cells] and thus hinders release of inflammatory storm. JAK/STAT tyrosine kinase system mediates one pathway, while Ras/mitogen-activated protein kinase (MAPK)/ NF-κB-IL-6 pathway mediates the other. Tocilizumab [humanized anti-IL-6R monoclonal antibody], is thus considered a potential drug in COVID-19 treatment [26, 27].
