Factor XII truncation accelerates activation in solution

ESSENTIALS: During contact system activation, factor XII is progressively cleaved by plasma kallikrein. We investigated the role of factor XII truncation in biochemical studies. Factor XII contains naturally occurring truncating cleavage sites for a variety of enzymes. Truncation of factor XII primes it for activation in solution through exposure of R353. SUMMARY: BACKGROUND: The contact activation system and innate immune system are interlinked in inflammatory pathology. Plasma kallikrein (PKa) is held responsible for the stepwise processing of factor XII (FXII). A first cleavage activates FXII (into FXIIa); subsequent cleavages truncate it. This truncation eliminates its surface‐binding domains, which negatively regulates surface‐dependent coagulation. OBJECTIVES: To investigate the influence of FXII truncation on its activation and downstream kallikrein‐kinin system activation. METHODS: We study activation of recombinant FXII variants by chromogenic assays, by FXIIa ELISA and western blotting. RESULTS: We demonstrate that FXII truncation primes it for activation by PKa in solution. We demonstrate this phenomenon in three settings. (i) Truncation at a naturally occurring PKa‐sensitive cleavage site, R334, accelerates FXIIa formation in solution. A site‐directed mutant FXII‐R334A displays ~50% reduced activity when exposed to PKa. (ii) A pathogenic mutation in FXII that causes hereditary angioedema, introduces an additional plasmin‐sensitive cleavage site. Truncation at this site synergistically accelerates FXII activation in solution. (iii) We identify new, naturally occurring cleavage sites in FXII that have so far not been functionally linked to contact system activation. As examples, we show that non‐activating truncation of FXII by neutrophil elastase and cathepsin K primes it for activation by PKa in solution. CONCLUSIONS: FXII truncation, mediated by either pathogenic mutations or naturally occurring cleavage sites, primes FXII for activation in solution. We propose that the surface‐binding domains of FXII shield its activating cleavage site, R353. This may help to explain how the contact system contributes to inflammatory pathology.