Inhibition of C3 and IgG proteolysis enhances phagocytosis of Porphyromonas gingivalis

In the face of an apparently competent immune response to Porphyromonas gingivalis, it is unclear how P. gingivalis evades the immune response and persists in human periodontitis. Particularly germane may be its ability to resist phagocytosis by degrading and not binding serum opsonins. In our study, the resistance by invasive (W83 and A7436) and noninvasive (ATCC 33277) P. gingivalis strains to phagocytosis by human neutrophils was compared with their C3- and IgG-proteolytic activity. The ability of opsonic human serum antibody to inhibit C3 proteolysis was also evaluated. Our results indicate that the more phagocytosis-resistant invasive strains accumulate less ¹²⁵I-C3 than the noninvasive strain; moreover, invasive strains degrade complement C3 in a dose-dependent manner inhibitable by rabbit antiserum or adult periodontitis serum. Opsonization and C3 accumulation on strain A7436 were both facilitated by pretreatment with rabbit antiserum, certain adult periodontitis sera, protease inhibitors (p-chloromercuriphenylsulfonic acid, Nα-p-tosyl-L-lysine chloromethyl ketone, diisopropylfluorophosphate) heat (60°C 15 min), and were Mg²⁺ dependent. The sera from 13 human subjects with or without periodontitis were assayed for antibody titers to P. gingivalis (ELISA units), opsonic activity (% of PMN engaged in phagocytosis) and enhancement of C3 accumulation. Statistically significant associations were observed between % of PMN engaged in phagocytosis and % C3 accumulation, between % of PMN engaged in phagocytosis and ELISA units and between % C3 accumulation and ELISA units. Degradation of purified rabbit IgG, but not specific antibody-containing rabbit IgG by P. gingivalis A7436 was observed, and was inhibited by diisopropyl fluorophosphate (DFP) or cold (2°C). Our data suggest that C3 and IgG cleavage by P. gingivalis proteases are inhibitable by antibody and are contributory factors in, but are not the sole determinants of, phagocytosis resistance.