The signalling cascade from initial stimulus to downstream effector — and where therapeutic intervention is possible at each node.
Interleukin-6 (IL-6) is a pleiotropic cytokine with both pro-inflammatory and acute-phase regulatory roles, produced by macrophages, T-cells, fibroblasts, and endothelial cells. IL-6 signals through two mechanisms: classic signalling via membrane-bound IL-6 receptor alpha (mIL-6R, expressed on hepatocytes, leucocytes, and megakaryocytes) and trans-signalling via soluble IL-6R (sIL-6R), which enables IL-6 responsiveness in cells lacking mIL-6R expression. Both complexes engage gp130, the ubiquitously expressed signal transducer, activating JAK1/2 and downstream STAT3, MAPK, and PI3K pathways. Trans-signalling is considered the predominantly pro-inflammatory arm, driving endothelial activation and non-haematopoietic cell recruitment.
In rheumatoid arthritis, IL-6 is central to the systemic manifestations of the disease — driving acute phase protein production (CRP, fibrinogen, serum amyloid A), stimulating hepatic hepcidin synthesis (causing anaemia of chronic disease), and promoting osteoclast activation and joint destruction via RANKL induction. IL-6-driven STAT3 activation shifts the T-cell differentiation balance toward pathogenic Th17 cells while suppressing regulatory T-cell generation, amplifying autoimmune inflammation. In giant cell arteritis, IL-6 is a key mediator of granulomatous vascular inflammation, with elevated baseline IL-6 levels predicting relapse risk.
Tocilizumab blocks IL-6 signalling by binding IL-6Ralpha (both membrane-bound and soluble), preventing IL-6 from engaging either receptor form and therefore inhibiting both classic and trans-signalling. This differs mechanistically from ligand-targeting antibodies. Sarilumab similarly targets IL-6Ralpha. Siltuximab targets the IL-6 ligand itself and is licensed specifically for Castleman disease, where IL-6 overproduction by tumour cells is the primary driver. Tocilizumab's role in cytokine release syndrome (CRS) management — including CAR-T cell therapy-related and COVID-19-associated CRS — reflects the therapeutic importance of acute IL-6 pathway blockade in hyperinflammatory states.
Upstream blockade vs downstream blockade — understanding the distinction is critical for treatment selection and sequencing.
Clinically actionable insights for treatment selection and sequencing
Interleukin-6 (IL-6) is a pleiotropic cytokine with both pro-inflammatory and acute-phase regulatory roles, produced by macrophages, T-cells, fibroblasts, and endothelial cells.
IL-6 signals through two mechanisms: classic signalling via membrane-bound IL-6 receptor alpha (mIL-6R, expressed on hepatocytes, leucocytes, and megakaryocytes) and trans-signalling via soluble IL-6R (sIL-6R), which enables IL-6 responsiveness in cells lacking mIL-6R expression.
Both complexes engage gp130, the ubiquitously expressed signal transducer, activating JAK1/2 and downstream STAT3, MAPK, and PI3K pathways.
Trans-signalling is considered the predominantly pro-inflammatory arm, driving endothelial activation and non-haematopoietic cell recruitment.
Guidelines, trials, clinical briefs, podcasts and CPD connected to this pathway.
Get Infectious Disease Drug Science updates, related trials and education resources in your ClinicaliQ preferences.
Complete the reflective questions and self-assessment to claim your CPD certificate for this molecular mechanism hub.
Go to CPD Centre →