Aims

The paradigm in IL-6 biology is that classical (cis) IL-6 signaling, mediated by IL-6 binding to the membrane form of IL-6 receptor (mIL-6R), is crucial for homeostatic functions. Conversely, the complex of the soluble form of IL-6R (sIL-6R) and IL-6 binding to cell-membrane expressed gp130, known as IL-6 trans-signaling, is thought to mediate local inflammation. In both cases, IL-6 signaling is believed to be driven through the formation of a hexameric complex consisting of 2 trimers of IL-6/IL-6R/gp130.  Our aim was to challenge this paradigm and evaluate whether under certain conditions trans-signaling drives the production of acute phase proteins.  These experiments would thus also provide insights into IL-6 signaling complex formation, exploring if a trimeric versus a tetrameric complex is sufficient to drive IL-6 responses.

Methods & Results

25F10 is an anti-mouse IL-6R monoclonal antibody (mAb) that binds to both sIL-6R and mIL-6R yet the unique property of inhibiting only trans signaling events.  Using Biacore, 25F10 does not prevent the binding of IL-6 to the IL-6R or the recruitment of gp130 to the receptor/ligand complex. Mapping the epitope of 25F10 through mutation of the mouse IL-6R, revealed that 25F10 binds IL-6R at site IIb, potentially preventing the formation of the hexameric signalling complex.  Data obtained from in vivo blockade of IL-6 trans-signaling in mouse models of inflammation show that under certain inflammatory conditions 25F10 blocks the production of acute phase proteins, i.e., serum amyloid A, previously believed to be produced via classical IL-6 signaling only.

Conclusions

These data challenge the existing paradigm that only classical (cis) signaling can induce the production of acute phase proteins.  In addition, our data suggest new insights into the biology of the IL-6 signaling complex formation be exploiting the unique mechanism of action of 25F10.