Autoinhibitory regulation of S100A8/S100A9 alarmin activity locally restricts sterile inflammation

Abstract

Autoimmune diseases, such as psoriasis and arthritis, show a patchy distribution of inflammation despite systemic dysregulation of adaptive immunity. Thus, additional tissue-derived signals, such as danger-associated molecular patterns (DAMPs), are indispensable for manifestation of local inflammation. S100A8/S100A9 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, to our knowledge, a mechanism of autoinhibition in mice and humans 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 hiding the TLR4/MD2-binding site within the tetramer interphase, thus preventing undesirable systemic effects. Loss of this autoinhibitory mechanism in vivo results in TNF-α-driven fatal inflammation, as shown by lack of tetramer formation in crossing S100A9-/- mice with 2 independent TNF-α-transgene mouse strains. Since S100A8/S100A9 is the most abundant DAMP in many inflammatory diseases, specifically blocking the TLR4-binding site of active S100 dimers may represent a promising approach for local suppression of inflammatory diseases, avoiding systemic side effects.

Inflammation in osteoarthritis: is it time to dampen the alarm(in) in this debilitating disease?

“…© 2018 British Society for Immunology, Clinical and Experimental Immunology, 195:[153][154][155][156][157][158][159][160][161][162][163][164][165][166] …”

The alarmin S100A9 hampers osteoclast differentiation from human circulating precursors by reducing the expression of RANK

“…In this context, it should be mentioned again that the physiologically relevant form of S100A8 and S100A9 is the heterodimer, which is short-lived in activity and for this reason is not suitable for cell culture studies in which a bolus of recombinant factors is added. Homodimers, which do not inactivate by themselves by oligomer formation, are therefore adequate S100-based damage-associated molecular patterns to investigate fully and constitutively active heterodimers (31). Interestingly however, in this study, we introduce the concept that S100A9 is not only involved in the promotion of osteoclast differentiation and activation during sterile inflammation.…”

“…Under high-calcium conditions, such as those present in the extracellular milieu and culture medium, S100A8/A9 heterodimers quickly tetramerize into (S100A8/A9) 2 , thereby losing their TLR4 signaling capacity. Therefore, the S100A9 homodimers, which cannot tetramerize, are widely accepted experimental stimuli that closely mimic S100A8/A9 heterodimer activity (31). In this study, we used 1 mg/ml of S100A9 to stimulate.…”

S100A8/A9 modulates inflammatory collateral tissue damage during intraperitoneal origin systemic candidiasis

“…The resolution of infection is an active process involving reprogramming of cells and modulation of mediators (10) such as the S100A8/A9 heterodimeric complex (calprotectin) also known as alarmin or DAMP (11–13). Here, we used an experimental model for IAC to study the contribution of S100A8/A9 to ICTD.…”