Macrophage activation and invasion by Porphyromonas gingivalis is modulated by PPAD and accessory fimbriae subunits

Background: Porphyromonas gingivalis is a master manipulator of host immune responses in periodontium. Peptidyl arginine deiminase (PPAD), a recently described virulence factor of P. gingivalis, responsible for both host- and bacterium-derived proteins and peptides citrullination, also plays an important role in hijacking immune responses. While PPAD’s modification of fimbrial subunits (FimCDE) affects TLR2 signaling in fibroblasts, its impact on immune cells remains unclear. Methods: Human monocyte-derived macrophages (MDMs) were stimulated with wild-type or mutant P. gingivalis strains and isolated fimbriae. Inflammatory responses were assessed by measuring cytokine expression and secretion, transcriptional changes using RNA-seq and Pi3K/Akt pathway activation, while bacterial invasion through flow cytometry, fluorescent microscopy and intracellular survival assay. Results: PPAD-modified fimbriae enhanced MDM proinflammatory responses, such as PGE2- related gene expression as well as cytokines secretion, while fimbriae from accessory subunit mutants failed to induce inflammation. PPAD modification of accessory fimbrial subunits was found to protect P. gingivalis from killing by macrophages. PPAD and accessory fimbriae subunits were involved in complex immune evasion strategies, upregulating genes associated with viral infections or interactions with T cells. Conclusions: These findings highlight the importance of protein citrullination in TLR2-dependent signaling and provide insight on how P. gingivalis subverts host immune responses.