Interleukin-6: Biology, signaling and strategies of blockade.

Interleukin-6 (IL-6) is one of the most important inflammatory cytokines. IL-6 is unique in signaling via a membrane bound and a soluble receptor. Intriguingly, these two pathways strongly differ in their biologic consequences. While classic IL-6 signaling via the membrane bound receptor is mainly regenerative and protective, IL-6 trans-signaling via the soluble IL-6R is rather pro-inflammatory. Intracellular signaling of IL-6 in response to receptor activation is through STAT-dependent and STAT-independent signaling modules, which are regulated by a complex regulatory network. The complex biology of IL-6 has consequences for therapeutic targeting of this cytokine. We hypothesize that specific inhibition of the trans-signaling pathway may be superior to global blockade of IL-6 activity with help of antibodies directed against IL-6 or IL-6R.