SUMMARY Porphyromonas gingivalis is a low-abundance oral anaerobic bacterium implicated in periodontitis, a polymicrobial inflammatory disease, and the associated systemic conditions. However, the mechanism by which P. gingivalis contributes to inflammation and disease has remained elusive. Here we show that P. gingivalis, at very low colonization levels, triggers changes to the amount and composition of the oral commensal microbiota leading to inflammatory periodontal bone loss. The commensal microbiota and the complement pathway were both required for P. gingivalis-induced bone loss as germ-free mice or conventionally raised C3a and C5a receptor deficient mice did not develop bone loss after inoculation with P. gingivalis. These findings demonstrate that a single, low-abundance species can disrupt host-microbial homeostasis to cause inflammatory disease. The identification and targeting of similar low-abundance pathogens with community-wide impact may be important for treating inflammatory diseases of polymicrobial etiology.

Aging is linked to increased susceptibility to chronic inflammatory diseases several of which, including periodontitis, involve neutrophil-mediated tissue injury. Here, we found that aging-associated periodontitis was accompanied by diminished expression of Del-1 (EDIL3), an endogenous inhibitor of LFA-1 integrin-dependent neutrophil adhesion, and by a reciprocal increase in IL-17 expression. Consistently, IL-17 inhibited gingival endothelial cell expression of Del-1, thereby promoting LFA-1-dependent neutrophil recruitment. Young Del-1-deficient mice developed spontaneous periodontitis featuring excessive neutrophil infiltration and IL-17 expression; disease was prevented in Del-1–LFA-1 and Del-1–IL-17 receptor double-deficient mice. Locally administered Del-1 inhibited IL-17 production, neutrophil accumulation, and bone loss. Therefore, Del-1 suppresses LFA-1-dependent neutrophil recruitment and IL-17-triggered inflammatory pathology and may thus be a promising therapeutic for inflammatory diseases.

Leukocyte adhesion deficiency Type I (LAD-I), a disease syndrome associated with frequent microbial infections, is caused by mutations on the CD18 subunit of β2 integrins. LAD-I is invariably associated with severe periodontal bone loss, historically attributed to lack of neutrophil surveillance of the periodontal infection. Here, we challenge this dogma by showing that the cytokine IL-17 plays a major role in the oral pathology of LAD-I. Defective neutrophil recruitment in LAD-I patients, or in LFA-1 (CD11a/CD18)-deficient mice that exhibit the LAD-I periodontal phenotype, was associated with excessive production of predominantly T cell-derived IL-17 in the periodontal tissue. The pathological elevation of IL-17 in the LFA-1–deficient periodontal tissue derived also from innate lymphoid cells. Strikingly, local treatment with anti-IL-17 (or anti-IL-23) in LFA-1-deficient mice not only blocked inflammatory periodontal bone loss but also caused a reduction in the total bacterial burden, suggesting that the IL-17-driven pathogenesis of LAD-I periodontitis leads to dysbiosis. Our findings therefore support an IL-17-targeted therapy for this condition.

Mammalian biological processes such as inflammation, involve regulation of hundreds of genes controlling onset and termination. MicroRNAs (miRNAs) can translationally repress target mRNAs and regulate innate immune responses. Our model system comprised primary human keratinocytes, which exhibited robust differences in inflammatory cytokine production (interleukin-6 and tumor necrosis factor-␣) following specific Toll-like receptor 2 and 4 (TLR-2/TLR-4) agonist challenge. We challenged these primary cells with Porphyromonas gingivalis (a Gram-negative bacterium that triggers TLR-2 and TLR-4) and performed miRNA expression profiling. We identified miRNA (miR)-105 as a modulator of TLR-2 protein translation in human gingival keratinocytes. There was a strong inverse correlation between cells that had high cytokine responses following TLR-2 agonist challenge and miR-105 levels. Knock-in and knock-down of miR-105 confirmed this inverse relationship. In silico analysis predicted that miR-105 had complementarity for TLR-2 mRNA, and the luciferase reporter assay verified this. Further understanding of the role of miRNA in host responses may elucidate disease susceptibility and suggest new anti-inflammatory therapeutics.

The Toll-like receptor (TLR)4 is the major sensor for bacterial lipopolysaccharide and its two common co-segregating polymorphisms, Asp299Gly and Thr399Ile, which occur at a frequency of between 6 and 10%, have been associated with infectious diseases, LPS hypo-responsiveness and cardiovascular disease. Porphyromonas gingivalis is a Gram-negative bacterium implicated in chronic periodontitis and is a known TLR4 and TLR2 agonist. We obtained two gingival epithelial cell primary cultures from subjects heterozygous for the TLR4 polymorphism Asp299Gly and compared response characteristics with similar cells from patients (four) with the wild-type TLR4 genes. Cytokine responses and transcriptome profiles of gingival epithelial cell primary culture cells to TNFa challenge were similar for all primary epithelial cell cultures. P. gingivalis challenge, however, gave markedly different responses for Asp299Gly heterozygous and wild-type epithelial cell cultures. The epithelial cells heterozygous for the TLR4 polymorphism Asp299Gly were functionally hypo-responsive, evidenced by differences in BD-2 mRNA expression, mRNA response profile by microarray analysis and by pro-inflammatory and chemokine cytokines at the protein and mRNA level. These findings emphasize variance in human epithelial cell TLRs, linked with Asp299Gly carriage, which results in a hypo-responsive epithelial cell phenotype less susceptible to Gram-negative diseases and associated systemic conditions.