Interleukin-6 trans-signalling as a selectively targetable driver of neurodegeneration.

Interleukin-6 (IL-6) exerts protective and pathogenic effects in the central nervous system through distinct receptor-signalling modes. Classical signalling via membrane-bound IL-6 receptor (IL-6R) is often associated with homeostatic and reparative functions, whereas trans-signalling, mediated by soluble IL-6R, expands IL-6 responsiveness to gp130-expressing cells and may promote chronic inflammation. Emerging evidence implicates dysregulated IL-6 trans-signalling in amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. Here, we review mechanisms linking IL-6 trans-signalling to immune, glial, neuronal, and vascular dysfunction in neurodegeneration. We highlight key knowledge gaps and discuss whether selective targeting of trans-signalling can limit inflammatory pathology while preserving beneficial classical IL-6 functions.