Interleukin-6, its role in fibrosing conditions

Interleukin-6 is a classic pro-inflammatory cytokine needed to mount an effective immune response. It is secreted by a wide array of cell types, however, its target cells are more restricted, due to the fact that very few cells, except lymphocytes and hepatocytes, express the functional membrane IL-6 receptor. This therefore limits the amount of cells that can respond to IL-6. Transsignalling, the shedding of the membrane bound form of the IL-6 receptor (sIL-6R) into the local microenvironment, greatly increases the range of cells that can respond e.g. as part of a wound healing response necessary to restore the homeostatic balance. Therefore, tight regulation of IL-6 signalling must occur to stop an inappropriate wound healing response occurring. This review focusses on the role of IL-6 in inflammation and fibrosing conditions, with a particular emphasis on systemic sclerosis (SSc), a chronic autoimmune disease in which a classical hallmark of fibrosis occurs. This fibrosis, in particular the skin and internal organs, leads to contractures and internal organ failure respectively with potential fatal consequences. In this review we will discuss the biology of IL-6 in the context of fibrosing conditions such as SSc and argue why molecular targeting of IL-6 is a promising therapeutic target.