Complement C1 and HMGB1 alarmin molecular crosstalk in inflammation ; Dialogue entre le complément C1 et l'alarmine HMGB1 dans l'inflammation

C1s protease is a central component in the initiation of the classical pathway of the complement system. It was originally believed to exclusively target proteins C2 and C4 in this proteolytic cascade. However, recent discoveries have highlighted the presence of constitutively active free C1s in certain pathologies, suggesting a broader role for this protease beyond complement activation. Among the non-canonical targets identified for C1s is the HMGB1 protein, initially described as a nuclear protein involved in chromatin condensation and gene expression.Recent studies have shown that HMGB1 can also be localized in different cellular compartments and plays a crucial role in inflammation when released into the extracellular environment. The main objective of this thesis project was to elucidate the role of C1s cleavage of HMGB1 in modulating the inflammatory response. Our work has shown that HMGB1 digestion fragments have distinct effects from the whole protein on complement activation and macrophage cytokine responses.In particular, we confirmed that the whole protein activates the classical complement pathway when bound to a surface and promotes M1 macrophage polarization in response to LPS. In contrast, fragment f2 is capable of activating the classical complement pathway, even when in solution, while fragment f3 inhibits the secretion of pro-inflammatory cytokines in cell studies. In addition, we explored the impact of cysteine redox state on the effects of HMGB1 and its fragments using mimetic mutants. HMGB1 digestion is restricted when the protein is in disulfide form, suggesting an important role of the disulfide bridge in access to the C1s digestion site. The redox forms of the whole protein do notappear to affect its ability to activate complement, while oxidized fragment f2 may lose its ability to activate it in solution. These results reveal that C1s cleavage of HMGB1 acts as an inflammation timer, orchestrating the inflammatory response through the transition from a pro-inflammatory amplification ...