Unique Lipids from a Common Human Bacterium Represent a New Class of Toll-Like Receptor 2 Ligands Capable of Enhancing Autoimmunity

Abstract

Recent reports suggest that commensal bacteria may play a down-regulatory role in autoimmune disease. In the present studies, we demonstrate that phosphorylated dihydroceramides, uniquely structured lipids derived from the common human oral bacterium Porphyromonas gingivalis and from bacteria commonly found in the gastrointestinal tract and other organs, are capable of enhancing autoimmunity. We have previously reported that these lipids have proinflammatory effects on human fibroblasts in vitro and, in preliminary studies, have recovered these lipids from surgically removed human carotid atheroma, suggesting that they may play a role in human inflammatory disease. To investigate whether these lipids have functional effects on autoimmunity, we administered phosphorylated dihydroceramides to mice with the murine model of multiple sclerosis, experimental allergic encephalomyelitis (EAE). We find that these lipids, and particularly the phosphoethanolamine dihydroceramide (PE DHC) fraction, significantly enhanced EAE. Mechanistically, PE DHC enhances EAE in mice lacking natural killer T cells, fails to enhance EAE in Toll-like receptor 2 (TLR2)-deficient mice and, in vitro, induces dendritic cell interleukin-6 secretion in a TLR2-dependent manner.

Macrophage Depletion Abates Porphyromonas gingivalis–Induced Alveolar Bone Resorption in Mice

“…In addition to P. gingivalis proteins and LPS penetrating gingival tissue, P. gingivalis phosphorylated dihydoceramide lipids have been isolated from gingival tissue and subgingival plaque of chronic periodontitis patients (52)(53)(54). Whole P. gingivalis cells and certain components, including the RgpA-Kgp complexes and lipids, induce secretion of a number of proinflammatory cytokines from epithelial and fibroblast cells in vitro, such as IL-1, IL-6, IL-8, PGE 2 , macrophage chemoattractant protein (MCP)-1a, and ICAM-1 (50,54,55).…”

Free Lipid A Isolated from Porphyromonas gingivalis Lipopolysaccharide Is Contaminated with Phosphorylated Dihydroceramide Lipids: Recovery in Diseased Dental Samples

“…Our previous studies indicated that hydrolysis of plaque or gingival tissue lipid extracts yields substantial levels of 3-OH iso C 17:0 , which could reflect the presence of bacterial lipids, including dihydroceramide lipids or free lipid A species. We now conclude that 3-OH iso C 17:0 previously recovered from lipid extracts of diseased dental samples (26,29) does not reflect the presence of phosphorylated lipid A of P. gingivalis.…”

“…In summary, the levels of monophosphorylated lipid A species in all dental samples from periodontitis disease sites were negligible compared with the phosphorylated dihydroceramide lipids. Therefore, the previously reported recovery of 3-OH iso C 17:0 in lipid extracts of diseased teeth and subgingival plaque (26,29) does not result from the presence of free lipid A species. The Tri-Reagent method was previously shown to extract LPS constituents from P. gingivalis that contain diphosphorylated penta-acylated lipid A species as well as monophosphorylated tetra-and penta-acylated lipid A species, whereas the MgCl 2 /…”

“…However, LPS or lipid A of P. gingivalis has not been demonstrated in diseased periodontal tissues. Instead, we reported that a unique fatty acid constituent of both lipid A and LPS of P. gingivalis, called 3-hydroxy isobranched C 17:0 (3-OH iso C 17: 0), exists in lipid products recovered predominantly from organic solvent extracts of subgingival plaque, periodontally diseased tissues, and diseased teeth, with little recovered in aqueous extracts of these samples (26,29). Since LPS partitions essentially only into the aqueous phase with this extraction technique (39), we concluded that LPS of P. gingivalis and other related Bacteroidetes bacteria (33), including Prevotella intermedia and Tannerella forsythia, is not present to a significant extent in diseased periodontal tissues.…”

“…This observation is important because the phosphorylated dihydroceramide lipids of P. gingivalis are reported to engage Toll-like receptor 2 (TLR2) when promoting dendritic cell secretion of interleukin-6 (IL-6) (29) and inhibiting osteoblast function and mineral deposition in vivo and in vitro (43). Other reports indicate that P. gingivalis mediates bone loss in experimental animals in a TLR2-dependent manner (14,15), and a recent report shows that P. gingivalis modulates osteoclastogenesis in vitro through TLR2 engagement, resulting in differential induction of NFATc1 and NF-B (47).…”