Local restriction of sterile inflammation by auto-inhibitory regulation of S100A8/S100A9-alarmin activity

Autoimmune diseases, like psoriasis or arthritis, show a patchy distribution of inflammation despite systemic dysregulation of adaptive immunity. Thus, additional tissue-derived signals like Danger-Associated Molecular Pattern molecules (DAMPs) are indispensable for manifestation of local inflammation. S100A8/100A9-complexes are the most abundant DAMPs in many autoimmune diseases. However, regulatory mechanisms locally restricting DAMP-activities are barely understood. We now unravel for the first time a novel mechanism of auto-inhibition in mice and man restricting S100-DAMP activity to local sites of inflammation. Combining protease degradation, pull-down assays, mass spectrometry and targeted mutations we identified specific peptide sequences within the second calcium-binding EF-hands triggering TLR4/MD2-dependent inflammation. These binding sites are free when S100A8/S100A9-heterodimers are released at sites of inflammation. Subsequently, S100A8/S100A9-activities are locally restricted by calcium-induced (S100A8/S100A9)2-tetramer formation now hiding the TLR4/MD2-binding site within the tetramer interphase thus preventing undesirable systemic effects. Loss of this auto-inhibitory mechanism in vivo results in TNFa-driven fatal inflammation as shown by lack of tetramer formation crossing S100A9-/- mice with two independent TNFa-transgene mouse strains. Since S100A8/S100A9 is the most abundant DAMP in many inflammatory diseases, specifically blocking of the TLR4-binding site of active S100-dimers represents an innovative approach for local suppression of inflammatory diseases avoiding systemic side effects. Human blood monocytes stimulated with MRP8 (S100A8) - MRP14 (S100A9) homo or hetero dimers or LPS were used for RNA extraction and hybridization on Illumina microarrays.