Capsular K-antigen Coats Outer Membrane Vesicles of Porphyromonas gingivalis

Periodontal disease is an inflammatory disorder that arises from dysbiosis of the subgingival microbiota, with Porphyromonas gingivalis acting as a keystone pathogen in the shift from health to disease. P. gingivalis employs multiple strategies to subvert host immune defenses, and its capsular K-antigen serves as a key virulence determinant. Here, a pre-adsorbed antiserum (pAds106) was generated by removing nonspecific antibodies using cells from a K-antigen–null mutant (W83ΔPG0106), resulting in exceptional specificity for the P. gingivalis K1-antigen. Immunofluorescence analysis revealed that the K-antigen preferentially coats outer membrane vesicles (OMVs), rather than attaching to the bacterial cell surface. This localization was further confirmed by ELISAs of density gradient ultracentrifuge-purified OMVs, with background signal detected in OMVs derived from K-antigen-deficient strains, non-K1-strains, and other oral Bacteroidetes. K-antigen–coated OMVs exhibited higher hydrophilicity and elicited weaker inflammatory responses compared to K-antigen–deficient OMVs, consistent with previously reported properties of encapsulated strains. Importantly, the antiserum detected K-antigen-coated OMVs in subgingival plaque from periodontal patients, suggesting that K-antigen is actively produced at diseased sites. These findings revise the prevailing view that K-antigen solely encapsulates the bacterial cell body and suggest that K-antigen–coated OMVs produced by P. gingivalis play distinct roles in immune evasion during periodontal disease.