Porphyromonas gingivalis Sphingolipids Regulate Inflammation

Objectives: It is known that Porphyromonas gingivalis (Pg) produces sphingolipids, and these molecules can permeate gingival tissues in both health and periodontal disease. Recent studies show that synthesis of sphingolipids by other members of the Bacteroidetes is essential for intestinal homeostasis and capable of interference with immune response. Our group has recently determined that the absence of sphingolipid synthesis by Pg leads to a hyperinflammatory response from host cells, suggesting that synthesis of sphingolipids by Pg is immunosuppressive. Further, Pg is able to deliver sphingolipids to host cells using outer membrane vesicles (OMVs). The goal of this study was to gain a better understanding of the role of one group of the Pg sphingolipids, phosphoglycerol-dihydroceramide (PG-DHC), and to begin to characterize the impact this class of sphingolipids has on the ability of Pg to elicit an immune response.
Methods: Pg parent strain W83 (WT) and PG-DHC enriched mutant (DsphK) were added either directly to, or co-cultured with human macrophage-like THP-1 cells using a transwell insert to prevent bacteria-cell contact. Separately, THP1 cells were stimulated with purified OMVs from WT Pg and the DsphK mutant. Supernatant fluids were collected after 2h, 6h and 24h, and analyzed by multiplex immunoassay to determine the inflammatory host response.

Results: While sphingolipid null Pg (DSPT) elicited a significantly more robust immune response from THP-1 cells than observed with WT, the levels of cytokines and chemokines elicited by the Pg DsphK mutant were comparable to WT, either when added directly to, or when separated from THP-1 cells by transwell inserts. In addition, purified OMVs from DsphK mutant caused either similar or lower host inflammatory response than the OMVs from WT Pg.

Conclusions: Our data suggest that Pg PG-DHCs synthesis is linked to suppression of the host inflammatory response, thus playing an important role in homeostasis.