Interspecies regulation of virulence through gingipain inhibition by a Tannerella forsythia serpin

Periodontitis is driven by dysbiotic microbial communities in which Porphyromonas gingivalis (Pg) acts as a keystone pathogen through its caspase-like fold (Clan CD) proteases, gingipains. Tannerella forsythia (Tf), another member of the periodontal pathogenic community, uniquely produces the broad-spectrum serpin termed miropin, yet its biological significance as an inhibitor in periodontal inflammation remains unclear. Here, we demonstrate that miropin directly inhibits the lysine-specific gingipain Kgp and the calpain-like protease Tpr produced by Pg, forming irreversible covalent inhibitory complexes. While recombinant native miropin impairs Pg growth, an engineered variant targeting both lysine- and arginine-specific gingipains entirely abolishes bacterial growth in a defined medium. In vivo, recombinant miropin reduces Pg virulence and, when co-infected with wild-type Tf, prevents lethal infection and significantly reduces alveolar bone loss, but not when co-infected with a miropin-deficient mutant. Protection is associated with inhibition of gingipain activity and normalization of the RANKL/OPG ratio in periodontal tissues. These findings identify miropin as a potent interspecies virulence modulator and challenge the prevailing view of Tf as an exclusively pathogenic member of the oral microbiome.